Topics
Contents Fiber optics and optical communications, 403 Article(s)
Coexistence of noise-like pulse and dark pulse in an Er/Yb co-doped fiber laser
Jing Li, Chuncan Wang, and Peng Wang

The coexistence of a noise-like pulse (NLP) and a dark pulse was experimentally demonstrated in a net-anomalous dispersion Er/Yb co-doped fiber (EYDF) laser, for the first time, to our knowledge. The cavity was mode-locked by nonlinear polarization rotation (NPR) technique. Meanwhile, a Sagnac loop with a section of polarization-maintaining fiber (PMF) was used as a comb filter to enable multiwavelength pulse operation. When the PMF length was 0.3 m, an asymmetric two-peak spectrum with central wavelengths of 1565.3 and 1594.2 nm was obtained by adjusting polarization controllers (PCs). It is a composite state of NLP and dark pulse due to the cross-phase modulation between the two different wavelength components along orthogonal polarization axes. The two pulses are synchronized with a repetition rate of 7.53 MHz. By adjusting the PC in the Sagnac loop, the spectral ranges of NLPs and dark pulses can be tuned from 1560 to 1577.8 nm and from 1581.8 to 1605.4 nm, respectively. In addition, the pulse characteristics were investigated by incorporating the PMF with different lengths, where the coexistence patterns can be generated when the PMF lengths were 0.2 and 0.3 m. A longer PMF can lead to a narrowband comb filtering, which causes a larger loss and is not favorable for stable operation of the coexistence regime. This fiber laser demonstrates an interesting operation regime and has significant potential for numerous practical applications.

Chinese Optics Letters
May. 15, 2024, Vol. 22 Issue 5 051403 (2024)
MLSE-assisted decision feedback equalizer for error-propagation suppression in high-speed IM-DD transmission systems
Jiahao Zhou, Jing Zhang, Xue Zhao, Rui Wang, Jinjiang Li, Shaohua Hu, Qianwu Zhang, Qi Yang, Bo Xu, and Kun Qiu

We propose a trellis-compressed maximum likelihood sequence estimation (TC-MLSE)-assisted sliding-block decision feedback equalizer (DFE) to suppress the error propagation resulting from the DFE in high-speed systems. We use an out-of-range detector to detect the end of burst errors from the DFE and activate the optional TC-MLSE to correct burst errors. We conduct experiments to transmit a 201-Gbit/s PAM-8 signal. The results show that the proposed method achieves a bit error rate of 3.65 × 10-3, which is close to that of MLSE. The optional MLSE is only activated when needed and processes 11.4% of the total symbols. Moreover, the proposed method compresses the maximum length of burst errors from 19 to 5.

Chinese Optics Letters
May. 17, 2024, Vol. 22 Issue 5 050604 (2024)
Preparation of panda-shaped photonic crystal fibers with and without silver wire
Junbo Lou, Yonghui Yang, Qiang Qu, and Shuguang Li

A method of preparing panda-shaped photonic crystal fibers (PCFs) based on secondary drawing technology is proposed in this paper. The secondary drawing can not only reduce fiber diameter but also reduce core size by adding a glass sleeve. Silver-filled and non-silver-filled panda PCFs are prepared. The two ends of silver-filled panda PCF are connected with a broadband light source and a spectrometer, respectively, and the surface plasmon resonance phenomenon is detected. The secondary drawing technology provides a meaningful reference for the preparation of PCF in the future, and the prepared silver-filled panda PCF can be prepared into an optical fiber filter.

Chinese Optics Letters
May. 08, 2024, Vol. 22 Issue 5 050603 (2024)
Joint modulation format identification and OSNR monitoring based on Stokes vector distribution features for digital coherent optical receivers
Zhengyu Pu, Lin Jiang, Lianshan Yan, Anlin Yi, Haijun He, Wei Pan, and Bin Luo

For joint modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring, a simple and intelligent optical communication performance monitoring method is proposed, and the feasibility is demonstrated by digital coherent optical communication experiments. The experiment results show that for all modulation formats, including 28 GBaud polarization division multiplexing (PDM) QPSK/8-QAM/16-QAM/64-QAM, 100% MFI accuracies are achieved even at OSNR values lower than the corresponding theoretical 20% forward error correction limit, as well as the high accuracies for OSNR monitoring. Furthermore, the proposed scheme has a reasonable monitoring level when chromatic dispersion and fiber nonlinear effects are varied.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 050602 (2024)
Robust thin-film lithium niobate modulator on a silicon substrate with backside holes
Mai Wang, Lu Qi, Haohua Wang, Ziliang Ruan, Gengxin Chen, Bin Chen, Shengqi Gong, Kaixuan Chen, and Liu Liu

Recently, Mach–Zehnder modulators based on thin-film lithium niobate have attracted broad interest for their potential for high modulation bandwidth, low insertion loss, high extinction ratio, and high modulation efficiency. The periodic capacitively loaded traveling-wave electrode is optimally adopted for ultimate high-performances in this type of modulator. However, such an electrode structure on a silicon substrate still suffers from the velocity mismatch and substrate leakage loss for microwave signals. Here, we introduce a thin-film lithium niobate modulator structure using this periodic capacitively loaded electrode on a silicon substrate. Backside holes in the silicon substrate are prepared to solve robustly the above difficulties. The fabricated device exhibits an insertion loss of 0.9 dB, a halfwave-voltage–length product of 2.18 V·cm, and an ultra-wide bandwidth well exceeding 67 GHz for a 10-mm-long device. Data transmissions with rates up to 112 Gb/s are demonstrated. The proposed structure and fabrication strategy are compatible for other types of monolithic and heterogeneous integrated thin-film lithium niobate modulators on a silicon substrate.

Chinese Optics Letters
May. 14, 2024, Vol. 22 Issue 5 050601 (2024)
Low-cost, large-coverage, and high-flexibility coherent PON for next-generation access networks: advances, challenges, and prospects [Invited]
Sizhe Xing, Junwen Zhang, Wangwei Shen, An Yan, Guoqiang Li, Aolong Sun, Ji Zhou, Dong Guo, Jianyang Shi, Ziwei Li, Chao Shen, and Nan Chi

Increasing bandwidth requirements have posed significant challenges for traditional access networks. It is difficult for intensity modulation/direct detection to meet the power budget and flexibility requirements of the next-generation passive optical network (PON) at 100G and beyond considering the new requirements. This is driving researchers to develop novel optical access technologies. Low-cost, wide-coverage, and high-flexibility coherent PON is emerging as a strong contender in the competition. In this article, we will review technologies that reduce the complexity of coherent PON (CPON), enabling it to meet the commercial requirements. Also, advanced algorithms and architectures that can enhance system coverage and flexibility are also discussed.

Chinese Optics Letters
On the CoverApr. 18, 2024, Vol. 22 Issue 4 040604 (2024)
Picosecond gain-switched polymer fiber random lasers
Wenyu Du, Sen Gao, Xiaojuan Zhang, Siqi Li, Yan Kuai, Zhiqiang Wang, Zhigang Cao, Feng Xu, Yu Liu, Lin Xu, Junxi Zhang, Kang Xie, Benli Yu, and Zhijia Hu

Random lasers are a type of lasers that lack typical resonator structures, offering benefits such as easy integration, low cost, and low spatial coherence. These features make them popular for speckle-free imaging and random number generation. However, due to their high threshold and phase instability, the production of picosecond random lasers has still been a challenge. In this work, we have developed three dyes incorporating polymer optical fibers doped with various scattering nanoparticles to produce short-pulsed random fiber lasers. Notably, stable picosecond random laser emission lasting 600 ps is observed at a low pump energy of 50 µJ, indicating the gain-switching mechanism. Population inversion and gain undergo an abrupt surge as the intensity of the continuously pumped light nears the threshold level. When the intensity of the continuously pumped light reaches a specific value, the number of inversion populations in the “scattering cavity” surpasses the threshold rapidly. Simulation results based on a model that considers power-dependent gain saturation confirmed the above phenomenon. This research helps expand the understanding of the dynamics behind random medium-stimulated emission in random lasers and opens up possibilities for mode locking in these systems.

Chinese Optics Letters
Apr. 17, 2024, Vol. 22 Issue 4 040603 (2024)
High-security multi-constellation shaping modulation with asymmetric encryption
Lei Jiang, Bo Liu, Jianxin Ren, Xiangyu Wu, Rahat Ullah, Yaya Mao, Shuaidong Chen, Yilan Ma, Lilong Zhao, and Feng Tian

This Letter proposes a high-security modulation scheme for optical transmission systems. By using multi-constellation shaping and asymmetric encryption, the information security can be enhanced and quantum computer cracking can be effectively resisted. Three-dimensional (3D) carrier-less amplitude phase modulation is utilized to superposition and transmit 3D signals. Experimental verification is conducted using a seven-core weakly coupled fiber platform. The results demonstrate that the proposed scheme can effectively protect the system from any illegal attacker.

Chinese Optics Letters
Apr. 26, 2024, Vol. 22 Issue 4 040602 (2024)
Neural network equalization based on delta-sigma modulation
Bo Liu, Jianxin Ren, Xiangyu Wu, Shuaidong Chen, Yaya Mao, and Li Zhao

We propose a neural network equalization delta-sigma modulation (DSM) technique. After performing DSM on the multi-order quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal at the transmitting end, neural network equalizer technology is used in the digital signal processing at receiving end. Applying this technology to a 4.6 km W-band millimeter wave system, it is possible to achieve a 1 Gbaud 8192-QAM OFDM signal transmission. The data rate reached 23.4 Gbit/s with the bit error rate at 3.8 × 10-2, lower than soft-decision forward-error correction threshold (4 × 10-2).

Chinese Optics Letters
Apr. 26, 2024, Vol. 22 Issue 4 040601 (2024)
LP modes exchange based on multiplane light conversion
Yanan Zhong, Chuxuan Lin, Juncheng Fang, Ting Lei, and Xiaocong Yuan

Data exchange between different mode channels is essential in the optical communication network with mode-division multiplexing (MDM). However, there are challenges in realizing mode exchange with low insert loss, low mode crosstalk, and high integration. Here, we designed and fabricated a mode exchange device based on multiplane light conversion (MPLC), which supports the transmission of LP01, LP11a, LP11b, and LP21 modes in the C-band and L-band. The simulated exchanged mode purities are greater than 85%. The phase masks were fabricated on a silicon substrate to facilitate the integration with optical systems, with an insert loss of less than 2.2 dB and mode crosstalk below -21 dB due primarily to machining inaccuracies and alignment errors. We carried out an optical communication experiment with 10 Gbit/s OOK and QPSK data transmission at the wavelength of 1550 nm and obtained excellent performance with the device. It paves the way for flexible data exchange as a building block in MDM optical communication networks.

Chinese Optics Letters
Mar. 22, 2024, Vol. 22 Issue 3 030602 (2024)
Simple real-time high-sensitivity heterodyne coherent optical transceiver at intraplane satellite communication
Yuanzhe Qu, Qianwu Zhang, Yanyi Wang, Yanhao Chen, Lewei Gong, Ziyue Liu, Junjie Zhang, Yingchun Li, Jian Chen, and Yingxiong Song

In this paper, we demonstrate a high-sensitivity and real-time heterodyne coherent optical transceiver for intraplane satellite communication, without digital-to-analog converter (DAC) devices and an optical phase lock loop (OPLL). Based on the scheme, a real-time sensitivity of -49 dBm is achieved at 5 Gbps QPSK. Because DAC is not needed at the transmitter, as well as OPLL at the receiver, this reduces the system cost. Furthermore, the least required Rx ADC bit-width is also discussed. Through theoretical analysis and experimental results, our cost-effective transceiver satisfies the scenario and could be a promising component for future application.

Chinese Optics Letters
Mar. 21, 2024, Vol. 22 Issue 3 030601 (2024)
Fast mode decomposition for few-mode fiber based on lightweight neural network
Jiajia Zhao, Guohui Chen, Xuan Bi, Wangyang Cai, Lei Yue, and Ming Tang

In this paper, we present a fast mode decomposition method for few-mode fibers, utilizing a lightweight neural network called MobileNetV3-Light. This method can quickly and accurately predict the amplitude and phase information of different modes, enabling us to fully characterize the optical field without the need for expensive experimental equipment. We train the MobileNetV3-Light using simulated near-field optical field maps, and evaluate its performance using both simulated and reconstructed near-field optical field maps. To validate the effectiveness of this method, we conduct mode decomposition experiments on a few-mode fiber supporting six linear polarization (LP) modes (LP01, LP11e, LP11o, LP21e, LP21o, LP02). The results demonstrate a remarkable average correlation of 0.9995 between our simulated and reconstructed near-field light-field maps. And the mode decomposition speed is about 6 ms per frame, indicating its powerful real-time processing capability. In addition, the proposed network model is compact, with a size of only 6.5 MB, making it well suited for deployment on portable mobile devices.

Chinese Optics Letters
Feb. 22, 2024, Vol. 22 Issue 2 020604 (2024)
Lens-free wavefront shaping method for a diffuse non-line-of-sight link in visible light communication
Chaoxu Chen, Xiaomeng Zhou, Ziwei Li, Chao Shen, Junwen Zhang, Jianyang Shi, and Nan Chi

In this Letter, we propose and experimentally demonstrate a lens-free wavefront shaping method that utilizes synchronized signal block beam alignment and a genetic algorithm (SSBGA) for a diffuse non-line-of-sight (NLOS) visible light communication (VLC) system. The proposed method effectively controls the position and mobility of visible light beams by partitioning spatial light modulator pixels and manipulating beams to converge at distinct spatial positions, thereby enhancing wavefront shaping efficiency, which achieves a significant 23.9 dB optical power enhancement at +2 mm offset, surpassing the lens-based continuous sequence (CS) scheme by 21.7 dB. At +40° angle, the improvement reaches up to 11.8 dB and 16.8 dB compared to the results with and without lens-based CS, respectively. A maximum rate of 5.16 Gbps is successfully achieved using bit-power loading discrete multi-tone (DMT) modulation and the proposed SSBGA in an NLOS VLC system, which outperforms the lens-based CS by 1.07 Gbps and obtains a power saving of 55.6% during the transmission at 4 Gbps. To the best of our knowledge, this is the first time that high-speed communication has been realized in an NLOS VLC system without a lens.

Chinese Optics Letters
Feb. 22, 2024, Vol. 22 Issue 2 020603 (2024)
All-optical suppression on the impact of Sun outage in laser satellite communication systems by using a nonlinear semiconductor optical amplifier
Xiaoliang Li, Rongke Liu, and Feng Fan

We introduce an all-optical approach, optical parametric amplification (OPA) processor to suppress the impact of Sun outage in laser satellite communication systems, which is implemented by only one nonlinear semiconductor optical amplifier driven by both electrical and optical pumps. The optimized OPA processor, with a current of 539 mA and a pump-to-signal ratio of 16 dB, could significantly improve the signal quality by 3.5 dB in experiments for the elevation angle of Sun radiation of 0 rad. The signal quality improvement is observed in the whole range of the elevation angle, confirming the effectiveness of the proposed OPA processor in the field of Sun radiation mitigation.

Chinese Optics Letters
Feb. 20, 2024, Vol. 22 Issue 2 020602 (2024)
Real-time UWOC miniaturized system based on FPGA and LED arrays and its application in MIMO
An Huang, Hongxi Yin, Yanjun Liang, Jianying Wang, and Zhongwei Shen

In order to alleviate the impact of turbulence on the performance of underwater wireless optical communication (UWOC) in real time, and achieve high-speed real-time transmission and low cost and miniaturization of equipment, a 2×2 real-time multiple-input and multiple-output (MIMO) high-speed miniaturized UWOC system based on a field-programmable gate array (FPGA) and a high-power light-emitting diode (LED) array is designed in this Letter. In terms of multiplexing gain, the imaging MIMO spatial multiplexing and high-order modulation for the first time are combined and the real-time high-speed transmission of PAM-4 signal based on the LED array light source in 12 m underwater channel at 100 Mbps rate is implemented, which effectively improves the throughput of the UWOC system with a high-power commercial LED light source. In light of diversity gain, the system employs the diversity of repeated coding scheme to receive two identical non-return-to-zero on-off keying (NRZ-OOK) signals, which can compensate the fading or flickering sublinks in real time under the bubble-like simulated turbulence condition, and has high robustness. To our knowledge, this is the first instance of a high rate and long-distance implementation of a turbulence-resistant real-time MIMO miniaturized UWOC system based on FPGA and high-power LED arrays. With spatial diversity or spatial multiplexing capabilities, its low cost, integrity, and high robustness provide the system with important practical prospects.

Chinese Optics Letters
Feb. 22, 2024, Vol. 22 Issue 2 020601 (2024)
4.096 Tbit/s multidimensional multiplexing signals transmission over 1000 km few mode fiber
Yu Zhang, Chen Wang, Kaihui Wang, Junjie Ding, Bowen Zhu, Lei Shen, Lei Zhang, Ruichun Wang, Changkun Yan, Bo Liu, and Jianjun Yu

We experimentally transmit eight wavelength-division-multiplexing (WDM) channels, 16 quadratic-amplitude-modulation (QAM) signals at 32-GBaud, over 1000 km few mode fiber (FMF). In this experiment, we use WDM, mode division multiplexing, and polarization multiplexing for signal transmission. Through the multiple-input–multiple-output (MIMO) equalization algorithms, we achieve the total line transmission rate of 4.096 Tbit/s. The results prove that the bit error rates (BERs) for the 16QAM signals after 1000 km FMF transmission are below the soft-decision forward-error-correction (SD-FEC) threshold of 2.4×10-2, and the net rate reaches 3.413 Tbit/s. Our proposed system provides a reference for the future development of high-capacity communication.

Chinese Optics Letters
Dec. 29, 2023, Vol. 22 Issue 1 010602 (2024)
Vector bending sensor based on chirped long-period grating with off-axis micro-helix taper
Jian Zhou, Xuelan He, Hongzhou Chen, Ziyang Xiong, Jing Yang, Chunying Guan, and Libo Yuan

Vector bending sensing has been consistently growing in many fields. A low-cost and high sensitivity vector bending sensor based on a chirped long-period fiber grating (LPFG) with an off-axis micro helix taper is proposed and experimentally demonstrated. The grating is composed of several sections of single-mode fiber with gradually larger lengths, and the off-axis micro helix tapers with fixed lengths when they are fabricated by using the arc discharge technology. The large refractive index modulation in the micro-helix taper greatly reduces the sensor size. The total length of the sensor is only 4.67 mm. The micro-helix taper-based LPFG can identify the bending direction due to the asymmetric structure introduced by the micro helix. The experimental results show that the transmission spectra of the sensor have distinct responses for different bending directions, and the maximum bending sensitivity is 14.08 nm/m-1 in the range from 0.128 m-1 to 1.28 m-1. The proposed bending sensor possesses pronounced advantages, such as high sensitivity, small size, low cost, and orientation identification, and offers a very promising method for bend measurement.

Chinese Optics Letters
Jun. 05, 2023, Vol. 21 Issue 6 060604 (2023)
Quasiperiodic photonic crystal fiber [Invited]
Exian Liu, and Jianjun Liu

In the fields of light manipulation and localization, quasiperiodic photonic crystals, or photonic quasicrystals (PQs), are causing an upsurge in research because of their rotational symmetry and long-range orientation of transverse lattice arrays, as they lack translational symmetry. It allows for the optimization of well-established light propagation properties and has introduced new guiding features. Therefore, as a class, quasiperiodic photonic crystal fibers, or photonic quasicrystal fibers (PQFs), are considered to add flexibility and richness to the optical properties of fibers and are expected to offer significant potential applications to optical fiber fields. In this review, the fundamental concept, working mechanisms, and invention history of PQFs are explained. Recent progress in optical property improvement and its novel applications in fields such as dispersion control, polarization-maintenance, supercontinuum generation, orbital angular momentum transmission, plasmon-based sensors and filters, and high nonlinearity and topological mode transmission, are then reviewed in detail. Bandgap-type air-guiding PQFs supporting low attenuation propagation and regulation of photonic density states of quasiperiodic cladding and in which light guidance is achieved by coherent Bragg scattering are also summarized. Finally, current challenges encountered in the guiding mechanisms and practical preparation techniques, as well as the prospects and research trends of PQFs, are also presented.

Chinese Optics Letters
Fast Track , On the CoverMay. 03, 2023, Vol. 21 Issue 6 060603 (2023)
Security enhanced underwater visible light communication system based on chaotic phase scrambling and conjugate frequency hopping
Zhilan Lu, Chao Shen, Jianyang Shi, and Nan Chi

We propose an encryption technique for underwater visible light communication (UVLC) based on chaotic phase scrambling (PS) and conjugate frequency hopping (CFH). The technique is experimentally tested using an 8-level pulse amplitude modulation (PAM-8) and a 1.2 m underwater link. The security key of the phase scrambling code is generated according to a logistic map, and the frequency hopping is achieved by adding the same zero frequency points to the signal spectrum. The maximum transmission rate of 2.1 Gbit/s is measured with bit-error-rate (BER) below 7% the hard-decision forward error correction (HD-FEC) threshold of 3.8×10-3.

Chinese Optics Letters
Jun. 13, 2023, Vol. 21 Issue 6 060602 (2023)
Optical scrambler using WGM micro-bottle cavity
Pengfa Chang, Chen Wang, Tao Jiang, Longsheng Wang, Tong Zhao, Hua Gao, Zhiwei Jia, Yuanyuan Guo, Yuncai Wang, and Anbang Wang

An optical scrambler using a whispering-gallery-mode (WGM) micro-bottle cavity to scramble a complex optical signal to generate an uncorrelated output is proposed. We experimentally demonstrated this micro-cavity scrambler by using chaotic laser light as the incident signal and studied the influence of the coupling state. Experiments achieved full scrambling with a low cross correlation of 0.028 between the output and the input. Results indicate that the scrambling effect originates from the interference among numerous WGMs in the bottle cavity. It is believed that the micro-bottle cavity with an efficient scrambling function can become a promising candidate for encryption.

Chinese Optics Letters
Jun. 06, 2023, Vol. 21 Issue 6 060601 (2023)
Low-resolution optical transmission using joint shaping technique of signal probability and quantization noise
Hongyu Huang, Zhenming Yu, Liang Shu, Kaixuan Sun, Feifei Yin, and Kun Xu

In this paper, we propose and experimentally demonstrate a joint shaping technique to improve the performance of a low-resolution transmission system for the first time, to the best of our knowledge. The joint shaping technique combines probabilistic shaping (PS) and error feedback noise shaping (EFNS). In the 40-Gbaud intensity-modulation direct-detection (IM/DD) experimental transmission system, a bit-error-rate (BER) of 3.8×10-3 can be achieved easily with the joint shaping at the physical number of bits (PNOB) of 3. In the 30-Gbaud dual polarization (DP) coherent experimental transmission system, a BER below 1×10-3 is easily obtained with a 3-bit quantizer by using joint shaping. The optimization of the shaping degree is also analyzed.

Chinese Optics Letters
Apr. 17, 2023, Vol. 21 Issue 5 050602 (2023)
High gain optical amplification and lasing performance of the Bi/P co-doped silica fiber in the O-band
Jinmin Tian, Mengting Guo, Fan Wang, Cheng Wu, Lei Zhang, Meng Wang, Yafei Wang, Jun Chen, Chunlei Yu, and Lili Hu

In this Letter, the optical amplification characteristics of the home-made Bi/P co-doped silica fiber were systematically explored in the range of 1270–1360 nm. The maximum gain of 24.6 dB was obtained in the single-pass amplification device, and then improved to 38.3 dB in the double-pass amplification device for -30 dBm signal power. In addition, we simultaneously investigated the laser performance of the fiber with the linear cavity. A slope efficiency of 16.4% at ∼1313 nm was obtained with a maximum output power of about 133 mW under the input pump power of 869 mW at 1240 nm. As far as we know, it is the first laser reported based on the bismuth-doped fiber in China.

Chinese Optics Letters
May. 10, 2023, Vol. 21 Issue 5 050601 (2023)
W-band radio-over-fiber transmission system with delta-sigma modulation and direct detection
Zhou Ju, Jiaxuan Liu, and Jianjun Yu

We experimentally built a W-band photonics-aided millimeter-wave radio-over-fiber transmission system and demonstrated the delivery of up to 8192-ary quadrature amplitude modulation (QAM) signal. Discrete multitone signals are converted into 1-bit data streams through delta-sigma modulation and then modulated onto a 76.2 GHz carrier. An envelope detector is used at the receiver side for direct detection. The results prove that our proposed system can support 2048QAM and 8192QAM transmission while meeting the hard decision forward error correction threshold of 3.8×10-3 and the soft decision forward error correction threshold of 4.2×10-2, respectively. We believe this cost-effective scheme is a promising candidate for future high-order QAM millimeter-wave downlink transmission.

Chinese Optics Letters
Mar. 24, 2023, Vol. 21 Issue 4 040602 (2023)
Comparative analysis of temporal-spatial and time-frequency features for pattern recognition of φ-OTDR
Fanran Meng, Wenxiang Zhang, Xiaojun Liu, Fei Liu, and Xian Zhou

The phase-sensitive time-domain reflectometer (φ-OTDR) has been popularly used for events detection over a long period of time. In this study, the events classification methods based on convolutional neural networks (CNNs) with different features, i.e., the temporal-spatial features and time-frequency features, are compared and analyzed comprehensively in φ-OTDR. The developed CNNs aim at distinguishing three typical events: wind blowing, knocking, and background noise. The classification accuracy based on temporal-spatial images is higher than that based on time-frequency images (99.49% versus 98.23%). The work here sets a meaningful reference for feature extraction and application in the pattern recognition of φ-OTDR.

Chinese Optics Letters
Mar. 09, 2023, Vol. 21 Issue 4 040601 (2023)
Improve the throughput of M-to-1 free-space optical systems by employing uniquely decodable codes
Yatian Li, Tianwen Geng, and Shijie Gao

This paper utilizes uniquely decodable codes (UDCs) in an M-to-1 free-space optical (FSO) system. Benefiting from UDCs’ nonorthogonal nature, the sum throughput is improved. We first prove that the uniquely decodable property still holds, even in optical fading channels. It is further discovered that the receiver can extract each source’s data from superimposed symbols with only one processing unit. According to theoretical analysis and simulation results, the throughput gain is up to the normalized UDC’s sum rate in high signal-to-noise ratio cases. An equivalent desktop experiment is also implemented to show the feasibility of the UDC-FSO structure.

Chinese Optics Letters
Mar. 14, 2023, Vol. 21 Issue 3 030603 (2023)
Fiber communication receiver models based on the multi-head attention mechanism
Yubin Zang, Zhenming Yu, Kun Xu, Minghua Chen, Sigang Yang, and Hongwei Chen

In this paper, an artificial-intelligence-based fiber communication receiver model is put forward. With the multi-head attention mechanism it contains, this model can extract crucial patterns and map the transmitted signals into the bit stream. Once appropriately trained, it can obtain the ability to restore the information from the signals whose transmission distances range from 0 to 100 km, signal-to-noise ratios range from 0 to 20 dB, modulation formats range from OOK to PAM4, and symbol rates range from 10 to 40 GBaud. The validity of the model is numerically demonstrated via MATLAB and Pytorch scenarios and compared with traditional communication receivers.

Chinese Optics Letters
Nov. 14, 2022, Vol. 21 Issue 3 030602 (2023)
Speckle backpropagation for compensation of nonlinear effects in few-mode optical fibers
Pavel S. Anisimov, Evgeny D. Tsyplakov, Viacheslav V. Zemlyakov, and Jiexing Gao

We propose an alternative approach to compensation of intermodal interactions in few-mode optical fibers by means of digital backpropagation. Instead of solving the inverse generalized multimode nonlinear Schrödinger equation, we accomplish backpropagation of the multimode signals with help of their near-field intensity distributions captured by a camera. We demonstrate that this task can successfully be handled by a deep neural network and provide a proof of concept by training an autoencoder for backpropagation of six linearly polarized (LP) modes of a step-index fiber.

Chinese Optics Letters
Nov. 14, 2022, Vol. 21 Issue 3 030601 (2023)
Temperature-sensing scheme based on a passively mode-locked fiber laser via beat frequency demodulation
Jian Luo, Haoran Wang, Xun Cai, Zhengqian Luo, and Hongyan Fu

In this paper, we propose a temperature-sensing scheme utilizing a passively mode-locked fiber laser combined with the beat frequency demodulation system. The erbium-doped fiber is used in the laser ring cavity to provide the gain and different lengths of single-mode fibers inserted into the fiber ring cavity operate as the sensing element. Different temperature sensitivities have been acquired in the experiment by monitoring the beat frequency signals at different frequencies. The experimental results indicate that the beat frequency shift has a good linear response to the temperature change. The sensitivity of the proposed sensor is about -44 kHz/°C when the monitored beat frequency signal is about 10 GHz and the ratio of the sensing fiber to the overall length of the laser cavity is 10 m/17.5 m, while the signal-to-noise ratio (SNR) of the monitored signal is approximately 30 dB. The proposed temperature-sensing scheme enjoys attractive features such as tailorable high sensitivity, good reliability, high SNR, and low cost, and is considered to have great potential in practical sensing applications.

Chinese Optics Letters
Oct. 12, 2022, Vol. 21 Issue 2 020603 (2023)
OCT-based partial data-carrying subcarriers precoding and ISFA-enabled DMT-VLC system
Yi Liu, Ming Chen, Pingyuan Liang, Taoyun Zhou, Shan Qiu, and Yun Cheng

To overcome the unbalanced signal-to-noise ratio (SNR) among data-carrying subcarriers (SCs) induced by the imperfect frequency response of optoelectronic devices and various interferences, a channel-independent partial data-carrying SCs precoding (PDSP) method based on orthogonal circular matrix transform (OCT) is proposed and experimentally investigated in an intra-symbol frequency average (ISFA)-enabled discrete multi-tone (DMT) visible light communication (VLC) system. After transmission over 1.9 m free space, at the optimal bias current of 100 mA, the experimental results show that the bit error ratio (BER) performance can be improved by up to an order of magnitude with conventional full data-carrying SCs precoding (FDSP) and PDSP scheme, compared to that without a precoding scheme. Moreover, the BER performance can further be enhanced when the ISFA algorithm with optimal taps is employed. Compared with the FDSP scheme, the proposed PDSP scheme owns a similar BER performance and a significant reduction in required multiplication and addition operations, and it may be a good option to efficiently combat the unbalanced impairments of DMT-VLC transmission systems.

Chinese Optics Letters
Nov. 24, 2022, Vol. 21 Issue 2 020602 (2023)
50 m/187.5 Mbit/s real-time underwater wireless optical communication based on optical superimposition
Yongxin Cheng, Xingqi Yang, Yufan Zhang, Chao Zhang, Hao Zhang, Zhijian Tong, Yizhan Dai, Weichao Lü, Xin Li, Haiwu Zou, Zejun Zhang, and Jing Xu

In this paper, an optical pulse amplitude modulation with 4 levels (PAM-4) using a fiber combiner is proposed to enhance the data rate of a field-programmable gate-array-based long-distance real-time underwater wireless optical communication system. Two on–off keying signals with different amplitudes are used to modulate two pigtailed laser diodes, respectively, and the generated optical signals are superimposed into optical PAM-4 signals by a fiber combiner. The optical PAM-4 scheme can effectively alleviate the nonlinearity, although it reduces the peak-to-peak value of the emitting optical power by 25%. A real-time data rate of 187.5 Mbit/s is achieved by using the optical PAM-4 with a transmission distance of 50 m. The data rate is increased by about 25% compared with the conventional electrical PAM-4 in the same condition.

Chinese Optics Letters
Oct. 09, 2022, Vol. 21 Issue 2 020601 (2023)
Simultaneous determination of polarization states and mode coefficients of fiber LP modes with four-channel off-axis holography
Zhigang Han, Junbo Li, Siliang Liu, Fangxin Li, Zhenying Yang, Qi Wang, Jiuduo Rui, Hua Shen, and Rihong Zhu

Four-channel off-axis holography is proposed to simultaneously understand the polarization states and the mode coefficients of linearly polarized (LP) modes in few-mode fiber. Far-field off-axis holograms in the four polarization directions of the fiber laser were acquired at the same moment through a four-channel holographic device. The weights, the relative phase differences, and the polarization parameters of the vector fiber laser mode can be solved simultaneously. The simulated and experimental mode analysis of the laser output by 1060-XP fiber with 6 LP modes at 632.8 nm is conducted, which shows that the similarity of the total intensity distribution of the laser before and after mode analysis is above 0.97. The mode polarization states, the mode weights, and the relative phase differences of the few-mode laser can be determined simultaneously in a single shot by four-channel off-axis holography.

Chinese Optics Letters
Dec. 08, 2023, Vol. 21 Issue 12 120601 (2023)
Polarization and magneto-optical characteristics of Tb:YAG crystal-derived silica fiber via laser-heating drawing technique
Hao Shi, Jianxiang Wen, Beibei Xing, Yanhua Luo, Xiaobei Zhang, Fufei Pang, and Tingyun Wang

Fiber quarter-wave plates and magneto-optical fibers are important components that greatly affect the sensitivity of fiber-optic current sensors. A Tb:YAG crystal-derived silica fiber (TYDSF) was fabricated using a CO2 laser-heating drawing technique. The linear birefringence of TYDSF was measured as 6.661×10-6 by a microscope birefringence measurement instrument. A fiber quarter-wave plate was fabricated by TYDSF at 1310 nm, which produced circularly polarized light with a polarization extinction ratio of 0.34 dB. Additionally, the linear birefringence of TYDSF was decreased by 22% by annealing at 750°C for 7 h, and the Verdet constants of annealed TYDSF were measured to be 9.83, 6.67, and 3.48 rad/(T·m) at 808, 980, and 1310 nm, respectively.

Chinese Optics Letters
Nov. 05, 2023, Vol. 21 Issue 11 110601 (2023)
Optical fiber-based magnetically-tuned graphene mechanical resonator
Hongqian Cao, Zengyong Liu, Danran Li, Zhenda Lu, Ye Chen, and Fei Xu

In this study, an optical fiber-based magnetically-tuned graphene mechanical resonator (GMR) is demonstrated by integrating superparamagnetic iron oxide nanoparticles on the graphene membrane. The resonance frequency shift is achieved by tuning the tension of the graphene membrane with a magnetic field. A resonance frequency tunability of 23 kHz using a 100 mT magnetic field is achieved. The device provides a new way to tune a GMR with a non-contact force. It could also be used for weak magnetic field detection in the future with further improvements in sensitivity.

Chinese Optics Letters
Aug. 28, 2022, Vol. 21 Issue 1 010601 (2023)
Spectrally sliced heterodyne coherent receiver with halved electrical bandwidth
Qi Wu, Yixiao Zhu, Ziyu Cheng, Longjie Yin, and Weisheng Hu

A spectrally sliced heterodyne coherent receiver (SHCR) employing four balanced photodetectors and analog-to-digital converters with half of the signal bandwidth is proposed to complete the signal reception and field recovery. We first numerically characterize the performance of SHCR compared with an intradyne coherent receiver and then validate the principle of the SHCR in a proof-of-concept single-polarization experiment. A 60 GBaud 16-quadrature amplitude modulation transmission is experimentally demonstrated over 80 km standard single-mode fiber with a bit-error-rate of 8.5×10-4 below the 7% hard-decision forward error correction threshold of 3.8×10-3. The SHCR offers a low-cost, hybrid-free, and channel-skew-tolerant candidate for data center interconnects.

Chinese Optics Letters
Jun. 15, 2022, Vol. 20 Issue 9 090601 (2022)
Optimized blind equalization for probabilistically shaped high-order QAM signals
Fengchu Cao, Mingyi Gao, Pengfei Wang, Xiaodi You, and Gangxiang Shen

Probabilistically shaped (PS) high-order quadrature amplitude modulation (QAM) signals are attractive to coherent optical communication due to increased spectral efficiency. However, standard digital signal processing algorithms are not optimal to demodulate PS high-order QAM signals. Therefore, a compromise equalization is indispensable to compensate the residual distortion. Meanwhile, the performance of conventional blind equalization highly depends on the accurate amplitude radius and distribution of the signals. The PS high-order QAM signals make the issue worsen because of indistinct amplitude distributions. In this work, we proposed an optimized blind equalization by utilizing a peak-density K-means clustering algorithm to accurately track the amplitude radius and distribution. We experimentally demonstrated the proposed method in a PS 256-QAM coherent optical transmission system and achieved approximately 1 dB optical signal-to-noise ratio improvement at the bit error rate of 1×10-3.

Chinese Optics Letters
May. 19, 2022, Vol. 20 Issue 8 080601 (2022)
Single-wall carbon nanotube assisted all-optical wavelength conversion at 2.05 µm
Zonghui Tao, Wanzhuo Ma, Lei Du, Xin Li, Yan Lou, Tianshu Wang, and Huilin Jiang

We fully demonstrate the special requirements of a mid-infrared all-optical wavelength converter. The construction mechanism of a 2.05 µm all-optical wavelength converter based on the single-wall carbon nanotube (SWCNT) is proposed. Systematic experiments are carried out, and the converter device is successfully developed. With the assistance of SWCNT-coated microfiber, the conversion efficiency up to -45.57 dB is realized, and the tuning range can reach 9.72 nm. The experimental results verify the correctness of the proposed mechanism and the feasibility of the converter device so that it can be a new technical approach for all-optical wavelength conversion beyond 2 µm. We believe the research can extend the application of this composite waveguide in the field of all-optical communication.

Chinese Optics Letters
Apr. 29, 2022, Vol. 20 Issue 6 060602 (2022)
Direct phase control method for binary phase-shift keying space coherent laser communication
Weijie Ren, Jianfeng Sun, Peipei Hou, Ronglei Han, Hongyu He, Haisheng Cong, Chaoyang Li, Longkun Zhang, and Yuxin Jiang

An optical phase locking method based on direct phase control is proposed. The core of this method is to synchronize the carrier by directly changing the phase of the local beam. The corresponding experimental device and the supporting algorithm were configured to verify the feasibility of this method. Phase locking can be completed without cycle skipping, and the acquisition time is 530 ns. Without an optical preamplifier, a sensitivity of -34.4 dBm is obtained, and the bit error rate is 10-9 for 2.5 Gbit/s binary phase-shift keying modulation. The measured standard deviation of the phase error is 5.2805°.

Chinese Optics Letters
Apr. 29, 2022, Vol. 20 Issue 6 060601 (2022)
Real-time reception of NHS-OFDM signal with SPA-enhanced channel estimation for intensity-modulated direct-detection systems
Gang Chen, Ming Chen, Wen Chen, Lede Yin, Aitao Deng, Yuxin Cai, and Dengqiao Wang

In this Letter, we have experimentally verified a low-complexity subcarrier pairwise-averaging (SPA)-enhanced channel estimation (CE) method for small-size fast Fourier transform (FFT) non-Hermitian symmetric orthogonal frequency-division multiplexing (NHS-OFDM) transceivers. Compared with intra-symbol frequency averaging (ISFA), more than 20% look-up tables and 10% logic power consumption can be saved. The least-square (LS), ISFA, and SPA CE methods are compared by offline and real-time digital signal processing approaches. The results show that the receiver sensitivity of the SPA NHS-OFDM transmission system with 64/128-point FFT can be improved by more than 1 dB at the bit error rate of 3.8 × 10-3 compared to the LS.

Chinese Optics Letters
Mar. 25, 2022, Vol. 20 Issue 5 050601 (2022)
Michelson interferometer composite cavity fiber laser sensor with radio frequency interrogation
Xu Wang, Enlai Wang, and Fang Wang

A Michelson interferometer (MI) composite cavity fiber laser sensing system based on radio frequency (RF) interrogation is proposed and experimentally demonstrated. The system down-converts the traditional MI light frequency detection to RF detection, which improves the stability of the system. The optic fiber MI is placed in the laser resonator to form a composite cavity structure, which greatly improves the sensitivity of beat frequency signal demodulation.

Chinese Optics Letters
Feb. 16, 2022, Vol. 20 Issue 4 040603 (2022)
Highly efficient and stable coupling of kilowatt-level continuous wave laser into hollow-core fibers
Yulong Cui, Wei Huang, Zhiyue Zhou, Hao Li, Meng Wang, Zilun Chen, and Zefeng Wang

Fiber gas lasers based on gas-filled hollow-core fibers (HCFs) perfectly combine the advantages of fiber lasers and gas lasers and have obtained fast development in the past years. However, stable and efficient coupling of high-power pump lasers into the HCFs is one of the key problems to be solved. In this paper, we study the coupling of high-power continuous wave fiber lasers into anti-resonant HCFs through an end-cap. By optimizing the splicing parameters, a maximum laser power of 1167 W was injected into the 1-m-long HCFs, and 1021 W was obtained at the output end, giving a total transmission efficiency of ∼87.5%. A more than 1 h test showed the stability of such a coupling method. Meanwhile, the laser beam quality was well maintained. This work opens new opportunities for stable and highly efficient coupling of high-power lasers into HCFs, which is significant for its applications in many other fields besides high-power fiber gas lasers, such as high-power laser delivering.

Chinese Optics Letters
Feb. 14, 2022, Vol. 20 Issue 4 040602 (2022)
Ultra-small interferometric fiber optic gyroscope with an integrated optical chip
Kejun Shang, Ming Lei, Haowei Li, Tianqi Zhang, Xiaozhi Yu, Qiang Xiang, Yonglin Na, and Lizhe Zhang

This study proposes a novel interferometric fiber optic gyroscope (IFOG) based on an integrated optical chip, application-specific integrated circuit, and small-diameter sensing coil. The overall size and weight of the prototype are 30 mm×30 mm×30 mm and 68 g, respectively, making it the smallest closed-loop IFOG, to the best of our knowledge. A static experiment shows that the bias stability of the integrated IFOG is very close to the theoretical accuracy limit determined by the fiber coil and can satisfy the requirements of near-navigation-grade compact inertial navigation systems.

Chinese Optics Letters
Feb. 16, 2022, Vol. 20 Issue 4 040601 (2022)
Impacts of the measurement equation modification of the adaptive Kalman filter on joint polarization and laser phase noise tracking
Shaohua Hu, Jing Zhang, Qun Liu, Linchangchun Bai, Xingwen Yi, Bo Xu, and Kun Qiu

Kalman filtering (KF) has good potential in fast rotation of state of polarization (RSOP) tracking. Different measurement equations cause the diverse RSOP tracking performances. We compare the conventional KF (CKF) and the modified KF (MKF), which have different measurement equations. Semi-theoretical analysis indicates the lower conditional variances of measurement residuals and process noise of MKF. Compared with CKF, the MKF has >3 dB optical signal-to-noise ratio (OSNR) improvement at the 10 MHz scrambling rate in simulation. For MKF, more significant tracking speed improvement exists for lower OSNR. MKF can be smoothly combined with an adaptive algorithm, which outperforms adaptive CKF throughout the simulations.

Chinese Optics Letters
Dec. 13, 2021, Vol. 20 Issue 2 020603 (2022)
Mode field switching in narrow linewidth mode-locked fiber laser
Shaokang Bai, Yaqiong Lu, and Zuxing Zhang

Acousto-optic interaction can be used for ultrafast optical field control in passively mode-locked fiber lasers. Here, we propose the use of an intracavity acousto-optic mode converter (AOMC) with combination of a few-mode fiber Bragg gratings (FM-FBG) to achieve narrow linewidth mode-locked pulse output with switchable transverse mode and wavelength in a ring fiber laser. Due to the selectivity of the FM-FBG to the input mode, the output mode and wavelength can be adjusted in the mode-locked fiber laser based on a semiconductor saturable absorption mirror. In experiments, by adjusting the acoustic frequency imposed in the AOMC, the wavelength of mode-locked pulses was switched from 1551.52 nm to 1550.21 nm, retaining the repetition rate of 12.68 MHz. At the same time, the mode conversion from the LP01 to the LP11 mode in the FM-FBG transmission port was achieved. This laser may find application in mode-division multiplexing systems.

Chinese Optics Letters
Dec. 02, 2021, Vol. 20 Issue 2 020602 (2022)
High-accuracy mode recognition method in orbital angular momentum optical communication system
Lin Zhao, Yuan Hao, Li Chen, Wenyi Liu, Meng Jin, Yi Wu, Jiamin Tao, Kaiqian Jie, and Hongzhan Liu

Vortex optical communication has been a hot research field in recent years. A key step is mode recognition in the orbital angular momentum (OAM) free-space optical (FSO) communication system. In this article, we propose an OAM mode recognition method based on image recognition technology, which uses the interferogram between the vortex beam and the Gaussian beam to identify the OAM mode. In order to resist the influence of atmospheric turbulence on the recognition accuracy, we added a Gaussian smoothing filter into the recognition process. Moreover, we used random phase screens to generate interferogram sets at distances of 1 km and 2 km. The verification result shows that the proposed scheme produces high identification accuracy for the distorted optical field. The average accuracy can reach 100% and 87.78% under the conditions of medium- and strong-turbulence levels, respectively. It is anticipated that these results might be helpful for improving the reliability of the OAM-FSO communication system in the future.

Chinese Optics Letters
Oct. 08, 2021, Vol. 20 Issue 2 020601 (2022)
Visible light communication system at 3.59 Gbit/s based on c-plane green micro-LED
Guoqiang Li, Runze Lin, Haichao Guo, Pengfei Tian, and Nan Chi

Visible light communication (VLC) based on the micro light emitting diode (micro-LED) has attracted increasing attention owing to its high bandwidth, low power consumption, and high security. Compared with semi-polar or non-polar micro-LEDs, the commercial polar micro-LED has the advantages of low cost and more mature epitaxy technique. In this study, green micro-LEDs with different indium tin oxide (ITO) sizes are fabricated based on the commercial c-plane LED epitaxial wafer. The transmission performance of 80, 100, and 150 µm devices has been studied in detail. A partial pre-equalization scheme is utilized to increase data rates. Finally, the VLC system with a 100 µm green micro-LED as the transmitter could achieve a maximum data rate of 3.59 Gbit/s. Such a result will be beneficial to promote the further development of low-cost, high-speed VLC devices in the future.

Chinese Optics Letters
Jul. 14, 2022, Vol. 20 Issue 11 110602 (2022)
Security enhancement for OFDM-UWOC system using three-layer chaotic encryption and chaotic DFT precoding
Huan Deng, Zihao Du, Jianmin Xiong, Xingqi Yang, Yan Hua, and Jing Xu

Security is one of the key issues in communications, but it has not attracted much attention in the field of underwater wireless optical communication (UWOC). This Letter proposes a UWOC encryption scheme with orthogonal frequency division multiplexing (OFDM) modulation, based on the three-layer chaotic encryption and chaotic discrete Fourier transform (DFT) precoding. The three-layer chaotic encryption processes are bit stream diffusion, in-phase/quadrature encryption, and time-frequency scrambling. With multi-fold data encryption, the scheme can create a keyspace of 9.7×10179, effectively resisting brute force attacks and chosen-plaintext attacks. A 3 Gbit/s encrypted OFDM signal is successfully transmitted over a 7 m water channel.

Chinese Optics Letters
Editors' PickJul. 05, 2022, Vol. 20 Issue 11 110601 (2022)
High gain E-band amplification based on the low loss Bi/P co-doped silica fiber
Jinmin Tian, Mengting Guo, Fan Wang, Chunlei Yu, Lei Zhang, Meng Wang, and Lili Hu

A home-made low loss Bi/P co-doped silica fiber was fabricated using the modified chemical vapor deposition (MCVD) technique combined with the solution doping method, where the background loss at 1550 nm was as low as 17 dB/km. We demonstrated for the first time, to the best of our knowledge, an all-fiber amplifier using the home-made Bi/P co-doped fiber achieving broadband amplification in the E-band. The amplifying performance was evaluated and optimized with different pumping patterns and fiber length. A maximum net gain at 1355 nm close to 20 dB and a minimum noise figure of 4.6 dB were obtained for the first time, to the best of our knowledge, using two 1240 nm laser diodes under bidirectional pumping with the input pump and signal powers of 870 mW and -30 dBm, respectively.

Chinese Optics Letters
Jun. 25, 2022, Vol. 20 Issue 10 100602 (2022)
Portable system integrated with time comparison, ranging, and communication
Qiongqiong Zhang, Chengkai Pang, Yurong Wang, Guangyue Shen, Lei Yang, Zhaohui Li, Haiyan Huang, and Guang Wu

We demonstrate a portable system integrated with time comparison, absolute distance ranging, and optical communication (TRC) to meet the requirements of space gravitational wave detection. A 1 km free-space asynchronous two-way optical link is performed. The TRC realizes optical communication with 7.7×10-5 bit error rate with a Si avalanche photodiode single-photon detector, while the signal intensity is 1.4 photons per pulse with the background noise of 3×104 counts per second. The distance measurement uncertainty is 48.3 mm, and time comparison precision is 162.4 ps. In this TRC system, a vertical-cavity surface-emitting laser diode with a power of 9.1 µW is used, and the equivalent receiving aperture is 0.5 mm. The TRC provides a miniaturization solution for ultra-long distance inter-satellite communication, time comparison, and ranging for space gravitational wave detectors.

Chinese Optics Letters
Jun. 25, 2022, Vol. 20 Issue 10 100601 (2022)
Spectrally U-shaped profile of beam propagation factor in all-solid photonic bandgap fiber
Xiao Chen, Liangjin Huang, Yi An, Huan Yang, Zhiping Yan, Yisha Chen, Xiaoming Xi, Zhiyong Pan, and Pu Zhou

We found the beam quality factor M2 of the fundamental mode as a function of wavelength is U-shaped in the working photonic bandgap (PBG) of an all-solid PBG fiber (AS-PBGF) for the first time, to the best of our knowledge, and our simulation results also match well with the phenomenon. The normal band that is near the high-frequency edge of the third PBG integrates the lowest M2 and single-mode operation simultaneously, while the other two edge regions suffer from anomalous variation of M2 versus wavelength. The general applicability of this finding can be further extended to other PBGs and also other representative structures in the AS-PBGF field.

Chinese Optics Letters
Nov. 23, 2021, Vol. 20 Issue 1 010602 (2022)
Harmonic mode-locked fiber laser based on microfiber-assisted nonlinear multimode interference
Renyan Wang, Liang Jin, Jiazhu Wang, Shangzhi Xie, Xiaohui Li, Yingtian Xu, He Zhang, Xin Zhao, and Xiaohui Ma

A novel harmonic mode-locked fiber laser based on nonlinear multimode interference (NL-MMI) in a microfiber-assisted ultrafast optical switch is proposed in this Letter. The microfiber-assisted ultrafast optical switch can be obtained by tapering the splicing point of the graded-index multimode fiber (GIMF) and single-mode fiber, which not only helps to shorten the self-imaging period in GIMF to relax the strict requirement of NL-MMI on the length of multimode fiber, but also improves the harmonic order. In the experiment, with the waist diameter of ∼15 µm, the repetition rates of the fiber laser can be stably locked at 285 MHz, corresponding to the 16th-order harmonic mode-locking, with the pulse duration of 1.52 ps. Our results provide novel insight into the design of a high-repetition-rate laser and the application of microfibers in the mode-locking device.

Chinese Optics Letters
Oct. 09, 2021, Vol. 20 Issue 1 010601 (2022)
Compact vector twist sensor using a small period long period fiber grating inscribed with femtosecond laser
Fangcheng Shen, Xuewen Shu, Kaiming Zhou, Haiming Jiang, Hongyan Xia, Kang Xie, and Lin Zhang

We propose and demonstrate a sensitive vector twist sensor based on a small period long period fiber grating (SP-LPFG) fabricated with a femtosecond (fs) laser. The fabricated SP-LPFG is compact in size (2.8 mm) and shows strong polarization dependent peaks in its transmission spectrum due to the vectorial behavior of high-order cladding modes. Twist sensing is realized by monitoring the polarization dependent peaks, since the polarization of input light changes with fiber twist. The proposed sensor can be interrogated by the peak intensity and wavelength, with high twist sensitivity that reaches 0.257 dB/deg and 0.115 nm/deg, respectively.

Chinese Optics Letters
Sep. 10, 2021, Vol. 19 Issue 9 090601 (2021)
Recent advance in hollow-core fiber high-temperature and high-pressure sensing technology [Invited]
Zhe Zhang, Yingying Wang, Min Zhou, Jun He, Changrui Liao, and Yiping Wang

The pure-silica hollow-core fiber (HCF) has excellent thermostabilities that can benefit a lot of high-temperature sensing applications. The air-core microstructure of the HCF provides an inherent gas container, which can be a good candidate for gas or gas pressure sensing. This paper reviews our continuous efforts to design, fabricate, and characterize the high-temperature and high-pressure sensors with HCFs, aiming at improving the sensing performances such as dynamic range, sensitivity, and linearity. With the breakthrough advances in novel anti-resonant HCFs, sensing of high temperature and high pressure with HCFs will continuously progress and find increasing applications.

Chinese Optics Letters
Jul. 10, 2021, Vol. 19 Issue 7 070601 (2021)
Machine-vision-based acquisition, pointing, and tracking system for underwater wireless optical communications
Jiaming Lin, Zihao Du, Chuying Yu, Wenmin Ge, Weichao Lü, Huan Deng, Chao Zhang, Xiao Chen, Zejun Zhang, and Jing Xu

Due to the proliferation of underwater vehicles and sensors, underwater wireless optical communication (UWOC) is a key enabler for ocean exploration with a strong reliance on short-range bandwidth-intensive communications. A stable optical link is of primary importance for UWOC. A compact, low-power, and low-cost acquisition, pointing, and tracking (APT) system is proposed and experimentally demonstrated to realign the optical link within 0.04 s, even when the UWOC transmitter and receiver are in relative motion. The system successfully achieves rapid auto-alignment through a 4 m tap water channel with a relatively large number of bubbles. Furthermore, the required minimum illumination value is measured to be as low as 7.1 lx, implying that the proposed APT scheme is robust to dim underwater environments. Meanwhile, mobility experiments are performed to verify the performance of the APT system. The proposed system can rapidly and automatically align moving targets in complex and unstable underwater environments, which can potentially boost the practical applications of UWOC.

Chinese Optics Letters
May. 10, 2021, Vol. 19 Issue 5 050604 (2021)
Secure orthogonal time-frequency multiplexing with two-dimensional encryption for optical-wireless communications
Jie Zhong, Ji Zhou, Shecheng Gao, and Weiping Liu

This paper firstly, to the best of our knowledge, proposed two-dimensional (2D) encryption based on the Arnold transformation for implementing a secure DC-biased optical orthogonal time-frequency multiplexing (DCO-OTFM) in optical-wireless communications (OWCs). The encrypted data is transformed to the particular 2D matrix and decrypted by the only key to get the correct information. Meanwhile, the number of keys in 2D encryption is enormous, which prevents eavesdroppers from exhaustively searching secret keys rapidly to find the right decryption. Numerical results demonstrate that the secure DCO-OTFM based on 2D encryption can effectively prevent signal decryption from the eavesdropper, which has good secure performance for applying in OWC.

Chinese Optics Letters
May. 10, 2021, Vol. 19 Issue 5 050603 (2021)
Improved optical coupling based on a concave cavity lens fabricated by optical fiber facet etching
Kanglin Li, Jiangbing Du, Weihong Shen, Jiacheng Liu, and Zuyuan He

We report a low-fabrication-complexity and wideband fiber lens, which is formed by fiber facet etching and filling high refractive index UV adhesive. The optical field can be significantly shrunk by the facet lens so as to obtain improved optical coupling. Numerical simulations were carried out for different coupling conditions, on both fundamental mode and high-order mode, for a nine-mode fiber. The fundamental mode area can be reduced from 152.17 to 12.57 μm2, and the coupling loss between the fiber lens and a photonic waveguide can be reduced to -2.9 dB with over 1000 nm 3 dB bandwidth.

Chinese Optics Letters
May. 10, 2021, Vol. 19 Issue 5 050602 (2021)
Multi-focus optical fiber lens based on all-dielectric metasurface
Xingyu Zhang, Chunying Guan, Keda Wang, Lin Cheng, Jing Yang, Jinhui Shi, Hongchao Liu, Zhihai Liu, and Libo Yuan

A multi-focus optical fiber lens is numerically demonstrated based on an all-dielectric metasurface structure. The metasurface consists of an array of rectangular silicon resonators with varying widths in order to obtain the required phase distribution. The core diameter of the multimode fiber is large enough to contain sufficient resonance units. The spatial distribution of the dielectric resonators is dictated by spatial multiplexing, including interleaving meta-atoms and lens aperture division, to achieve multi-focus properties. The proposed optical fiber metalens can produce two or three focal points along the longitudinal direction with high focusing efficiency. The size of every focal point is close to the diffraction limit, and the relative intensity on each focus can be controlled by adjusting the number of the respective resonators. The proposed optical fiber lens will have a great potential in the fields of integrated optics and multifunctional micro/nano devices.

Chinese Optics Letters
May. 10, 2021, Vol. 19 Issue 5 050601 (2021)
Influence of the sparsity of random speckle illumination on ghost imaging in a noise environment
Long Pan, Chenjin Deng, Cuiping Yu, Shuai Yue, Wenlin Gong, and Shensheng Han

The influence of the sparsity of random speckle illumination on traditional ghost imaging (GI) and GI via sparsity constraint (GISC) in a noise environment is investigated. The experiments demonstrate that both GI and GISC obtain their best imaging quality when the sparsity of random speckle illumination is 0.5, which is also explained by some parameters such as detection of the signal to noise ratio and mutual coherence of the measurement matrix.

Chinese Optics Letters
Apr. 10, 2021, Vol. 19 Issue 4 041103 (2021)
Thermo-mechanical dynamics of nanoimprinting anti-reflective structures onto small-core mid-IR chalcogenide fibers [Invited]
Christian R. Petersen, Mikkel B. Lotz, Christos Markos, Getinet Woyessa, David Furniss, Angela B. Seddon, Rafael J. Taboryski, and O. Bang

Thermal nanoimprinting is a fast and versatile method for transferring the anti-reflective properties of subwavelength nanostructures onto the surface of highly reflective substrates, such as chalcogenide glass optical fiber end faces. In this paper, the technique is explored experimentally on a range of different types of commercial and custom-drawn optical fibers to evaluate the influence of geometric design, core/cladding material, and thermo-mechanical properties. Up to 32.4% increased transmission and 88.3% total transmission are demonstrated in the 2–4.3 μm band using a mid-infrared (IR) supercontinuum laser.

Chinese Optics Letters
Editors' PickMar. 10, 2021, Vol. 19 Issue 3 030603 (2021)
Tunable phase-shifted fiber Bragg grating based on a microchannel fabricated by a femtosecond laser
Li Zeng, Xiaoyan Sun, Zikun Chang, Youwang Hu, and Ji’an Duan

A phase-shifted fiber Bragg grating (PS-FBG) based on a microchannel was proposed and realized by combining the point-by-point scanning method with chemical etching. The PS-FBG is composed of a fiber Bragg grating (FBG) and a microchannel through the fiber core. The microchannel can introduce phase shift into the FBG. What is more important is that it exposes the fiber core to the external environment. The phase shift peak is sensitive to the liquid refractive index, and it shows a linear refractive index response wavelength and intensity sensitivity of 2.526 nm/RIU and -111 dB/RIU, respectively. Therefore, such gratings can be used as sensors or tunable filters.

Chinese Optics Letters
Mar. 10, 2021, Vol. 19 Issue 3 030602 (2021)
Classification of interference-fading tolerant Φ-OTDR signal using optimal peak-seeking and machine learning [Invited]
Yixin Zhang, Tong Zhou, Zhewen Ding, Yanqing Lu, Xuping Zhang, Feng Wang, and Ningmu Zou

A simple and effective interference-fading tolerant method for phase-sensitive optical time-domain reflectometry (Φ-OTDR) using optimal peak-seeking is proposed. This method can reconstruct the vibration signal with high fidelity under the premise of using only an ordinary single-mode sensing fiber without changing the structure of the traditional Φ-OTDR system. Based on the data after interference suppression, we applied different machine learning models to recognize the invasive events category. The promising results show potential applications of Φ-OTDR equipment and future implementation with machine learning algorithms.

Chinese Optics Letters
Mar. 10, 2021, Vol. 19 Issue 3 030601 (2021)
10 Gb/s free space optical interconnect with broadcasting capability enabled by a silicon integrated optical phased array
Quan You, Daigao Chen, Xi Xiao, and Shaohua Yu

We have proposed and experimentally demonstrated a reconfigurable free space optical interconnect with broadcasting capability based on an eight-channel silicon integrated optical phased array. By using the silicon integrated beam steering and broadcasting device, 10 Gb/s on–off keying data is transmitted over 15 cm in free space for up to three receivers located in three different cards. The experimental results show that the optical phased array can be used with broadcasting capability provided to multi-receivers in the card to card optical interconnects, which can significantly reduce device size, system complexity, and total costs.

Chinese Optics Letters
Dec. 10, 2021, Vol. 19 Issue 12 120602 (2021)
Robust integration of nitrogen-vacancy centers in nanodiamonds to optical fiber and its application in all-optical thermometry
Ce Bian, Minxuan Li, Wei Cao, Manli Hu, Zhiqin Chu, and Ruohui Wang

In this Letter, we developed a robust method for integrating nanodiamonds (NDs) to optical fiber. The NDs, containing nitrogen-vacancy (NV) centers, were uniformly mixed with UV adhesive before coating the end surface of a multimode fiber as a hemispherical film. The excitation and collection efficiency of NV fluorescence can be enhanced by increasing the thickness of UV adhesive film and additional aluminum film deposition. The fiber-based quantum sensor was also experimentally demonstrated for all-optical thermometry application. The variation of the refractive index of UV adhesive under different temperatures will also affect the NV collection efficiency by changing the light confinement. The demonstrated facile integration approach paves the way for developing fiber-based quantum thermometry and magnetometry.

Chinese Optics Letters
Editors' PickDec. 10, 2021, Vol. 19 Issue 12 120601 (2021)
Multi-distributed probabilistically shaped PAM-4 system for intra-data-center networks
Mengli Liu, Mingyi Gao, and Junchi Ke

Probabilistically shaped (PS) pulse amplitude modulation (PAM) is a promising technique for intra-data-center networks due to its superior performance, for which a low-complexity and cost-effective distributed matching method is critical. In this work, we propose an energy-level-assigned method to yield PS-PAM-4 signals with various bit rates based on variable probabilistic distributions. We experimentally demonstrate the proposed method in a 25 Gbaud PS-PAM-4 transmission over a bandwidth of approximately 10 GHz. Compared to a uniform PAM-4 system, the proposed multi-distributed PS-PAM-4 system approaches the hard decision threshold at a wide range of received optical power for different applications.

Chinese Optics Letters
Nov. 10, 2021, Vol. 19 Issue 11 110604 (2021)
Gain and laser performance of heavily Er-doped silica fiber fabricated by MCVD combined with the sol-gel method
Qiubai Yang, Yan Jiao, Chunlei Yu, Chongyun Shao, Fengguang Lou, Shikai Wang, Lei Zhang, Qiuhong Yang, and Lili Hu

In this work, a heavily Er-doped fiber with an 8 µm core diameter and a numerical aperture of 0.13 was prepared by the modified chemical vapor deposition (MCVD) technique combined with the sol-gel method. The background loss and absorption coefficient at 1530 nm were measured to be 20 dB/km and 128 dB/m, respectively. Thanks to the sol-gel method, the fiber showed a good doping homogeneity, which was confirmed through unsaturable absorption measurement. The net gains of three 25, 45, and 75-cm-long fibers were measured in the range of 1520 to 1600 nm, and the highest gain reached above 23 dB at both 1530 and 1560 nm in 25 and 75-cm-long fibers, respectively. The short-cavity laser performance was measured using centimeter-scale fibers. The maximum output power of 12 mW was demonstrated in a 6.5-cm-long active fiber with a slope efficiency of 20.4%. Overall, the prepared heavily Er-doped silica fiber is a promising item to be applied in a high-repetition-rate or single-frequency fiber laser.

Chinese Optics Letters
Nov. 10, 2021, Vol. 19 Issue 11 110603 (2021)
Signal transmission of 4 GHz beyond the system bandwidth in UV-C LED communication based on temporal ghost imaging
Mengyin Jin, Zeyuan Qian, Xinwei Chen, Xugao Cui, Ke Jiang, Xiaojuan Sun, Dabing Li, and Pengfei Tian

Due to the bandwidth limitation of the ultraviolet-C (UV-C) optical communication system and strong channel attenuation, it is difficult to transmit high-frequency signals. In this paper, the temporal ghost imaging (TGI) algorithm was first applied to the UV-C communication experimentally, and we realized the transmission of a 4 GHz signal through 95.34 MHz system bandwidth. The study indicates that the TGI algorithm can significantly improve the signal-to-noise ratio (SNR) compared with the on–off keying method. Our research provides a new approach for alleviating transmission frequency limitation due to poor SNR and insufficient hardware bandwidth.

Chinese Optics Letters
Nov. 10, 2021, Vol. 19 Issue 11 110602 (2021)
Compensation of turbulence-induced wavefront aberration with convolutional neural networks for FSO systems
Min’an Chen, Xianqing Jin, Shangbin Li, and Zhengyuan Xu

To reduce the atmospheric turbulence-induced power loss, an AlexNet-based convolutional neural network (CNN) for wavefront aberration compensation is experimentally investigated for free-space optical (FSO) communication systems with standard single mode fiber-pigtailed photodiodes. The wavefront aberration is statistically constructed to mimic the received light beams with the Zernike mode-based theory for the Kolmogorov turbulence. By analyzing impacts of CNN structures, quantization resolution/noise, and mode count on the power penalty, the AlexNet-based CNN with 8 bit resolution is identified for experimental study. Experimental results indicate that the average power penalty decreases to 1.8 dB from 12.4 dB in the strong turbulence.

Chinese Optics Letters
Nov. 10, 2021, Vol. 19 Issue 11 110601 (2021)
Generation of nanosecond cylindrical vector beams in two-mode fiber and its applications of stimulated Raman scattering
Wending Zhang, Lu Zhang, Chao Meng, and Feng Gao

We present the generation of the nanosecond cylindrical vector beams (CVBs) in a two-mode fiber (TMF) and its applications of stimulated Raman scattering. The nanosecond (1064 nm, 10 ns, 10 Hz) CVBs have been directly produced with mode conversion efficiency of ~18 dB (98.4%) via an acoustically induced fiber grating, and then the stimulated Raman scattering signal is generated based on the transmission of the nanosecond CVBs in a 100-m-long TMF. The transverse mode intensity and polarization distributions of the first-order Stokes shift component (1116.8 nm) are consistent with the nanosecond CVBs pump pulse.

Chinese Optics Letters
Jan. 10, 2021, Vol. 19 Issue 1 010603 (2021)
Wavelength synchronization technology for UDWDM-PON transmitter based on injection locking
Ju Wang, Ting Jia, Shuaishuai Wang, Tianyu Li, Chuang Ma, Tianyuan Xie, Yang Yu, and Jinlong Yu

We propose a concept of wavelength synchronization to ensure the stability of ultra-dense channels in an ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) transmitter. A mode-locked laser is used to provide wavelength references for users. By injection locking the semiconductor laser, the separation of the wavelength reference is realized in an optical line terminal. The downlink and uplink wavelength references are interlaced and distributed to facilitate the synchronization of uplink carriers. In the optical network unit, the uplink optical carriers are filtered by injection locking semiconductor lasers, which achieve wavelength synchronization for the uplink users. In this Letter, an adaptive wavelength synchronization transmitter for UDWDM-PON is realized with a channel spacing of 5 GHz.

Chinese Optics Letters
Jan. 10, 2021, Vol. 19 Issue 1 010602 (2021)
Physical layer data encryption using two-level constellation masking in 3D-CAP-PON
Shuaidong Chen, Bo Liu, Yaya Mao, Jianxin Ren, Xiumin Song, Rahat Ullah, Delin Zhao, Lei Jiang, Shun Han, Jianye Zhao, Jiajia Shen, and Xueyang Liu

A novel physical layer data encryption scheme using two-level constellation masking in three-dimensional (3D) carrier-less amplitude and phase modulation (CAP) passive optical network (PON) is proposed in this Letter. The chaotic sequence generated by Chua’s circuit model realizes two-level encryption of displacement masking and constellation rotation for 3D constellations. We successfully conduct an experiment demonstrating 8.7 Gb/s 3D-CAP-8 data transmission over 25 km standard single-mode fiber. With two-level constellation masking, a key space size of 2.1 × 1085 is achieved to bring about high security and good encryption performance, suggesting broad application prospects in future short-range secure communications.

Chinese Optics Letters
Jan. 10, 2021, Vol. 19 Issue 1 010601 (2021)
Digital-analog hybrid optical phase-lock loop for optical quadrature phase-shift keying
Shaowen Lu, Yu Zhou, Funan Zhu, Jianfeng Sun, Yan Yang, Ren Zhu, Shengnan Hu, Xiaoxi Zhang, Xiaolei Zhu, Xia Hou, and Weibiao Chen

We analyze a feasible high-sensitivity homodyne coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK). A fourth-power phase-lock loop based on a digital look-up table is used. Considering the non-negligible loop delay, we optimize the loop natural frequency. Without error correction coding, a sensitivity of ?37 dBm/?35 dBm is achieved, while the bit error rate is below 10?9 at 2.5 Gbaud/5 Gbaud rate. For the QPSK communication system, the bit rate is twice the baud rate. The loop natural frequency is 0.647 Mrad/s, and the minimized steady-state phase-error standard deviation is 3.83°.

Chinese Optics Letters
Sep. 10, 2020, Vol. 18 Issue 9 090602 (2020)
Augmented communications: spectral efficiency and security enhanced visible light communications by design
Monette H. Khadr, and Hany Elgala

A plethora of physical-layer techniques aim to enhance the performance of communication systems in several ways. Spectral efficiency and security are on the top of the list of enhancements; however, both are isolated and antagonistic islands of research. Augmented communication (ACom) is introduced in this Letter as the first technique that aims to combine these two enhancements in visible light communications (VLCs). The dividends of the proposed concept are demonstrated via simulations and the performance is experimentally validated. Results show that ACom can simultaneously provide the high spectral efficiency and the resistance to eavesdropping, while introducing minimal signal-to-noise ratio penalties.

Chinese Optics Letters
Sep. 10, 2020, Vol. 18 Issue 9 090601 (2020)
DOE effect on BER performance in MSK space uplink chaotic optical communication
Mi Li, Yuan Chen, Yuejiang Song, Cheng Zeng, and Xuping Zhang

With the increasing demand for space optical communication security, space chaotic optical communication has attracted a great amount of attention. Compared with traditional space optical communication, a chaotic optical communication system has a higher bit error rate (BER) for its complex system design. In order to decrease the BER of space chaotic optical communication systems, we introduce two diffractive optical elements (DOEs) at a transmitting terminal (Tx). That is because the commonly used reflective optical antenna at Tx blocks the central part of the transmission beam, which leads to a great amount of power consumption. Introducing the DOEs into the optical subsystem at Tx can reshape the transmission beam from a Gaussian distribution to a hollow Gaussian distribution so that the block of the secondary mirror in the reflective optical antenna can be avoided. In terms of the DOE influence on communication quality, we give a BER model based on a minimum-shift-key (MSK) space uplink chaotic optical communication system to describe the DOE function. Based on the model, we further investigate the effect of the DOEs through analyzing the BER relationship versus basic system parameters based on the BER model. Both different mismatch conditions of chaotic systems and different atmospheric turbulence conditions are considered. These results will be helpful for the scheme design of space uplink chaotic optical communication systems.

Chinese Optics Letters
Jul. 10, 2020, Vol. 18 Issue 7 070601 (2020)
Polarization scrambling characteristic analysis based on density of polarization states statistics
Shangchun Wang, and Zhengyong Li

We report a new method to deeply analyze the scrambling characteristic of polarization scramblers based on density of polarization states (DPS) statistics that makes it possible to describe the DPS distribution in detail on the whole Poincaré sphere, thus easy to locate accurately the nonuniform areas of defective polarization scramblers, which cannot be realized by existing methods. We have built a polarization scrambling system to demonstrate the advantages of our method compared with others by experiments and suggested effective evaluation indexes whose validity is well confirmed by applying to a commercial scrambler. Our conclusions are valuable for accurately analyzing and diagnosing the performance of any polarization scrambler, and quality evaluation of polarization controllers or other polarization devices.

Chinese Optics Letters
Jun. 10, 2020, Vol. 18 Issue 6 060604 (2020)
All-optical inverter based on carbon nanotube-polyvinyl alcohol thin film
Zhengpeng Shao, Kan Wu, and Jianping Chen

All-optical light control plays an important role in optical signal processing and communications. In this Letter, we demonstrate an all-optical inverter using carbon nanotube (CNT)-polyvinyl alcohol (PVA) thin film and obtain a long-time stable output due to the environmental insensitivity of the device. The thermo-optic effect in the CNT-PVA thin film generates a thermal lens and modifies the beam propagation in the thin film. The obtained all-optical inverter has a front (trailing) time constant of ~55 μs (44 μs) for 1550 nm signal pulses and ~7 kHz response bandwidth.

Chinese Optics Letters
On the CoverJun. 10, 2020, Vol. 18 Issue 6 060603 (2020)
Matching theory based user-grouping for indoor non-orthogonal multiple access visible light communication heterogeneous networks
Zhengxi Chen, Zhitong Huang, and Yuefeng Ji

This Letter proposes a model of indoor visible light communication (VLC) heterogeneous networks entirely based on LEDs with different specifications and applies non-orthogonal multiple access (NOMA) to it because of the narrow modulation bandwidth of LEDs. Moreover, a user-grouping scheme that is based on matching theory is proposed to improve the network achievable sum rate. Simulation results indicate that when each NOMA cluster contains 6 users, the proposed scheme has a 49.54% sum-rate enhancement compared with the traditional user-grouping scheme. As the number of users in each NOMA cluster increases, the proposed scheme performs better at the cost of computational complexity.

Chinese Optics Letters
Jun. 10, 2020, Vol. 18 Issue 6 060602 (2020)
Tuning of group delay with stimulated Raman scattering-induced dispersion in gas-filled optical fiber
Haihong Bao, Wei Jin, and Hoi Lut Ho

We report the first demonstration of group delay tuning with stimulated Raman scattering-induced dispersion in a hydrogen-filled hollow-core optical fiber. A pump laser induces a sharp refractive index change near the S0(0) Raman transition of hydrogen molecules, enabling the control of the group velocity of signal pulses around the Stokes wavelength. Experiments with an 80-m-long hollow-core fiber filled with 2.5 bar hydrogen achieved continuous tuning of the pulse delay up to 1.42 ns by varying the Raman amplification from 0 to 10 dB. The tunable pulse delay is realized by changing the pump power as well as the hydrogen pressure. This work provides a new technique for controlling the pulse propagation in optical fibers with high flexibility.

Chinese Optics Letters
Jun. 10, 2020, Vol. 18 Issue 6 060601 (2020)
Deep learning for position fixing in the micron scale by using convolutional neural networks
Hongye Li, Hu Liang, Qihao Hu, Meng Wang, and Zefeng Wang

We propose here a novel method for position fixing in the micron scale by combining the convolutional neural network (CNN) architecture and speckle patterns generated in a multimode fiber. By varying the splice offset between a single mode fiber and a multimode fiber, speckles with different patterns can be generated at the output of the multimode fiber. The CNN is utilized to learn these specklegrams and then predict the offset coordinate. Simulation results show that predicted positions with the precision of 2 μm account for 98.55%. This work provides a potential high-precision two-dimensional positioning method.

Chinese Optics Letters
May. 10, 2020, Vol. 18 Issue 5 050602 (2020)
Uniformly high-speed semi-open loop polarization control and PMD suppression
Bochen Wang, and Zhengyong Li

We propose and demonstrate a novel scheme of semi-open-loop polarization control (SOL-PC), which controls the state of polarization (SOP) with high accuracy and uniform high speed. For any desired SOP, we first adjust the initial SOP using open-loop control (OLC) based on the matrix model of a three-unit piezoelectric polarization controller, and quickly move it close to the objective one. Then closed-loop control (CLC) is performed to reduce the error and reach precisely the desired SOP. The response time is three orders faster than that of the present closed-loop polarization control, while the average deviation is on par with it. Finally, the SOL-PC system is successfully applied to realize the suppression of the polarization mode dispersion (PMD) effect and reduce the first-order PMD to near zero. Due to its perfect performance, the SOL-PC energizes the present polarization control to pursue an ideal product that can meet the future requirements in ultrafast optical transmission and quantum communication.

Chinese Optics Letters
Editors' PickMay. 10, 2020, Vol. 18 Issue 5 050601 (2020)
Applications of weakly-coupled few-mode fibers [Invited]
Huiyuan Liu, He Wen, and Guifang Li

Space-division multiplexing (SDM) has attracted significant attention in recent years because larger transmission capacity is enabled by more degrees of freedom (DOFs) in few-mode fibers (FMFs) compared with single-mode fibers (SMFs). To transmit independent information on spatial modes without or with minor digital signal processing (DSP), weakly-coupled FMFs are preferred in various applications. Several cases with different use of spatial DOFs in weakly-coupled FMFs are demonstrated in this work, including single-mode or mode-group-multiplexed transmission, and spatial DOFs combined with time or frequency DOF to improve the system performance.

Chinese Optics Letters
On the CoverApr. 10, 2020, Vol. 18 Issue 4 040601 (2020)
Hollow-core photonic quasicrystal fiber with high birefringence and ultra-low nonlinearity
Zhuang Huo, Exian Liu, and Jianjun Liu

A hollow-core fiber based on photonic quasicrystal arrays is theoretically proposed for high-quality light wave propagation with high polarization maintaining performance and low nonlinearity. This fiber, called hollow-core photonic quasicrystal fiber (HC-PQF), can simultaneously realize a high birefringence that reaches 1.345 × 10-2 and a small nonlinear coefficient of 1.63 × 10-3 W-1·km-1 at a communication wavelength of 1.55 μm due to the air-filled core and unique quasiperiodic fiber structure. To further demonstrate the controllability of the nonlinear coefficient and the application of sensor and polarization-maintaining fiber, the nonlinearity is investigated by filling different inert gases in the fiber core while the birefringence keeps a high order of 10?2. In the wavelength range λ ∈ [1.53 μm, 1.57 μm], the dispersion is near zero and flattened. The HC-PQF is expected to be used for applications in optical communication, high power pulse transmission, polarization beam splitters, etc.

Chinese Optics Letters
Mar. 10, 2020, Vol. 18 Issue 3 030603 (2020)
Tunable fiber-tip lens based on thermo-optic effect of amorphous silicon
Ping Li, Jing Liu, Peng Huang, Xingyu Zhang, Jinhui Shi, Libo Yuan, and Chunying Guan

We theoretically demonstrate a compact all-dielectric metasurface fiber-tip lens composed of sub-wavelength amorphous silicon on the end face of a multimode fiber. The full 2π phase control was realized by varying the widths of resonant units. The tunable focal length is achieved by using the thermal-optic effect of amorphous silicon. The focal length increases from 309 μm to 407 μm when the temperature changes by 300 K. The temperature controlled all-fiber integrated lens is compact and with high efficiency and provides an excellent platform of a fiber-tip lab. Meanwhile, the proposed fiber lens does not have any structural changes during dynamic tuning, which improves the durability and repeatability of the devices.

Chinese Optics Letters
Mar. 10, 2020, Vol. 18 Issue 3 030602 (2020)
Resonator fiber optic gyros with light time-division input and multiplexing output in clockwise and counterclockwise directions
Zongfu Hu, Zhiguo Jiang, Jinfang Wang, and Su Mei

A resonator fiber optic gyro with the light time-division input and multiplexing output (TDM-RFOG) in the clockwise (CW) and counterclockwise (CCW) directions is proposed. The light time-division input in the CW and CCW directions can effectively suppress the backscattering induced noise. The TDM-RFOG is implemented with the 2 × 2 Mach–Zehnder interferometer (MZI) optical switch. The response time of the fiber ring resonator is analyzed, and it is demonstrated by experiments that light time-division input in the CW and CCW directions can reduce the backscattering induced noise. The suppression effectiveness of backscattering induced noise in the TDM-RFOG is determined by the extinction ratio of the optical switch, so a closed loop is designed to adjust the phase shift difference between the two arms of the MZI optical switch to control the extinction ratio. The method using two arms of the MZI optical switch with twin 90° polarization-axis rotated splices is proposed to make the extinction ratio along both slow and fast axes greater than ?70 dB.

Chinese Optics Letters
Mar. 10, 2020, Vol. 18 Issue 3 030601 (2020)
Analysis of detection error for spot position in fiber nutation model
Xueqiang Zhao, Yunpeng Zhang, Minjie Huang, Peipei Hou, Ren Zhu, Jianfeng Sun, Xia Hou, and Weibiao Chen

The influences of nutation trail accuracy, simplification of coupling model, spot position jitter, and power variation of incident light on the detection error are analyzed theoretically. Under the condition of satisfying the requirements, the nutation radius is less than 1.13 μm, the accuracy of the nutation trail is less than 0.04 μm, and the detection range is [?5 μm, +5 μm]. The nutation frequency is 160 times spot position jitter frequency and 100 times intensity jitter frequency of incident light. The analysis is of great significance for determining nutation radius and frequency in the tracking system based on fiber nutation.

Chinese Optics Letters
Feb. 10, 2020, Vol. 18 Issue 2 020604 (2020)
Stable RF transfer over a fiber-optic ring with DSBCS modulation and a DSB RF signal
Pingyang Shi, Guiling Wu, Liang Hu, Qi Li, and Jianping Chen

We propose a radio frequency (RF) transfer technique with passive phase noise compensation over a fiber-optic ring. By adopting different frequencies and same wavelength transmission and double sideband (DSB) with carrier suppression (DSBCS) modulation, the impact of backscattering can be effectively suppressed. A stable RF signal can be obtained via frequency mixing at an arbitrary access site along the fiber-optic ring. As the two directional transmissions adopt the same fiber and same wavelength from the same laser, the bidirectional propagation symmetry can be maximally guaranteed. We experimentally demonstrate 2 GHz RF signal transfer along a 100 km standard single-mode fiber-optic ring.

Chinese Optics Letters
Feb. 10, 2020, Vol. 18 Issue 2 020603 (2020)
Highly efficient excitation of LP01 mode in ring-core fibers by tapering for optimizing OAM generation
Chunxiang Zhang, Fufei Pang, Huanhuan Liu, Lifei Chen, Junfeng Yang, Jianxiang Wen, and Tingyun Wang

We have demonstrated the highly efficient excitation of the linearly polarized mode (LP01) in ring-core fibers (RCFs) by tapering the spliced point between the RCF and the standard single-mode fiber (SMF) to optimize all-fiber orbital angular momentum (OAM) generation. The tapering technique has been investigated theoretically and experimentally. Before tapering, only 50% of light can be coupled from SMFs to RCFs. The modal interference spectrum with an extinction ratio (ER) of ~9 dB is observed, showing that higher-order modes are excited in RCF. By tapering the spliced point, 90% of light is coupled, and the ER is minimized to be ~2 dB, indicating that the higher-order modes are effectively suppressed by tapering. Such tapered spliced points of RCF–SMF are further applied for all-fiber OAM generation. The efficiencies of OAM+1 and OAM?1 generation are found to be enhanced by approximately 11.66% and 12.41%, respectively, showing that the tapered spliced point of the RCF–SMF is a feasible way to optimize OAM generation.

Chinese Optics Letters
Feb. 10, 2020, Vol. 18 Issue 2 020602 (2020)
Dual-channel microfluidic sensor based on side-hole fiber with two long-period fiber gratings
Yu Zhang, Yongzhi Li, Chao Liu, Zhihai Liu, Yaxun Zhang, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan

We propose and demonstrate a dual-channel microfluidic sensor based on a side-hole fiber (SHF) with two long-period fiber grating (LPFG) structures. There are two air holes in the SHF, which are natural microfluidic channels. We fabricate two LPFGs (long-period gratings LPG-A and LPG-B) in the SHF with the resonance wavelengths of 1268.7 nm and 1385.8 nm, respectively. Results show that the refractive index sensitivities of LPG-A and LPG-B are ?76.0 nm/RIU and ?71.1 nm/RIU, respectively. One can measure the refractive index of liquid samples in two channels simultaneously. The proposed dual-channel microfluidic sensor has advantages of good linearity response, fluidic technology compatibility, and easy light input/output coupling and system integration, which helps the sensor to have a potential application in environmental detection and food safety detection.

Chinese Optics Letters
Feb. 10, 2020, Vol. 18 Issue 2 020601 (2020)
Single scattering turbulence model based on the division of effective scattering volume for ultraviolet communication
Tao Shan, Jianshe Ma, Tianfeng Wu, Zanqiu Shen, and Ping Su

In this Letter, a single scattering turbulence model in a narrow beam case for ultraviolet (UV) communication is proposed based on the division of the effective scattering volume. This model takes the variation of atmospheric scattering, absorption, and turbulence in different paths into account. Meanwhile, the applicable transceiver configurations of this model are provided by analyzing path loss error caused by the single scattering assumption in the UV channel. Furthermore, we investigate the effect of turbulence on the probability density function of the arriving power in both coplanar and non-coplanar scenarios. The averaging effect of multipath propagation on the arriving power’s fluctuations is presented. Then, the bit-error-rate performance is also studied. This work provides an efficient way for UV turbulence channel estimation.

Chinese Optics Letters
Dec. 10, 2020, Vol. 18 Issue 12 120602 (2020)
Near-navigation-grade interferometric fiber optic gyroscope with an integrated optical chip
Kejun Shang, Ming Lei, Qiang Xiang, Yonglin Na, Lizhe Zhang, and Huaiyong Yu

We present a near-navigation-grade interferometric fiber optic gyroscope (IFOG) based on an integrated optical chip. The chip comprises a light source, a photodiode, and a 3 dB coupler within an area of 48 mm2. By interrogating with an integrated optical modulator and a small-diameter sensing coil, the IFOG is realized. This allows for a significant reduction in size, weight, power consumption, and cost. Preliminary performance data of a gyro prototype exhibits 0.018 deg/h bias instability.

Chinese Optics Letters
Dec. 10, 2020, Vol. 18 Issue 12 120601 (2020)
Visible light communication for Vehicle to Everything beyond 1 Gb/s based on an LED car headlight and a 2 × 2 PIN array
Chaofan Wang, Guoqiang Li, Fangchen Hu, Yiheng Zhao, Junlian Jia, Peng Zou, Qiuyi Lu, Jiang Chen, Zhongya Li, and Nan Chi

Visible light communication (VLC) shows great potential in Internet of Vehicle applications. A single-input multi-output VLC system for Vehicle to Everything is proposed and demonstrated. A commercial car headlight is used as transmitter. With a self-designed 2 × 2 positive-intrinsic-negative (PIN) array, four independent signals are received and equalized by deep-neural-network post-equalizers, respectively. Maximum-ratio combining brings high signal-to-noise ratio and data rate gain. The transmission data rate reaches 1.25 Gb/s at 1 m and exceeds 1 Gb/s at 4 m. To the best of our knowledge, it is the first-time demonstration of beyond 1 Gb/s employing a commercial car headlight.

Chinese Optics Letters
Nov. 10, 2020, Vol. 18 Issue 11 110602 (2020)
Dual-services generation using an integrated polarization multiplexing modulator
Feng Zhao, Jianjun Yu, and Jingling Li

We proposed and demonstrated a new scheme for simultaneous generation of independent wired and wireless signals employing an integrated polarization multiplexing modulator. In the experimental system, a 10 Gb/s wired signal is imposed on the original optical carrier with one polarization, while a wireless signal with a bit rate larger than 4 Gb/s is carried on the generated millimeter wave of 76.44 GHz, which has polarization orthogonal to the wired signal. The dual services are successfully delivered over a 15 km standard single mode fiber; the power penalties of the wired and wireless signals are around 0.4 dB and 1.5 dB, respectively.

Chinese Optics Letters
Nov. 10, 2020, Vol. 18 Issue 11 110601 (2020)
Hybrid tilted fiber gratings-based surface plasmon resonance sensor and its application for hemoglobin detection
Yarien Moreno, Qingguo Song, Zhikun Xing, Yuezhen Sun, and Zhijun Yan

We proposed a hybrid tilted fiber gratings (polarizing grating and tilted fiber Bragg grating)-based surface plasmon resonance (SPR) sensor. The hybrid tilted fiber grating, consisting of a polarizing grating and tilted fiber Bragg grating (TFBG), is fabricated in a single-mode fiber in series by using a UV-inscription technique, in which the TFBG could generate a dense cladding mode resonance to excite SPR and the polarizing grating could filter out the S-polarization cladding mode of the TFBG. Such proposed hybrid tilted fiber gratings could greatly simplify the interrogation system of the TFBG-based SPR sensor. The experiment results showed that the hybrid tilted fiber gratings-based SPR sensor has the refractive index sensitivity of 522.8 nm/RIU. Finally, by using the proposed sensor, we have achieved the hemoglobin concentration detection within a sensing range from 0.1 mg/mL to 1.0 mg/mL and the sensitivity of 8.144 nm/(mg/mL).

Chinese Optics Letters
Oct. 10, 2020, Vol. 18 Issue 10 100601 (2020)
Stimulated Brillouin scattering by the interaction between different-order optical and acoustical modes in an As2Se3 photonic crystal fiber
Weiqing Gao, Liang Chen, Wenhui Jiang, Zhengxiong Zhang, Xiu Zhang, Panyun Gao, Kang Xie, Wei Zhang, Yong Zhou, Meisong Liao, Takenobu Suzuki, and Yasutake Ohishi

Brillouin gain spectra (BGS) in an As2Se3 photonic crystal fiber (PCF) are investigated numerically. The profiles of the BGS are simulated by calculating the characteristics of different-order optical and acoustic waves in the PCFs with different core diameters. For the small-core PCF, there are two peaks in BGS, but there is only one peak for the large-core PCF. We also reveal that in the small-core PCF, the difference of Brillouin frequency shift between the LP01 and LP11 modes is obvious, while it is not obvious in the large-core PCF. The Brillouin threshold increases with the core diameter increasing.

Chinese Optics Letters
Jan. 10, 2020, Vol. 18 Issue 1 010602 (2020)
Polarization in D-shaped fiber modulated by magneto-optical dichroism of magnetic fluid
Rende Ma, Luxue Wan, Xiuhao Liu, Xiaoping Li, Junfeng Jiang, and Yunjie Xia

The polarization of a D-shaped fiber is modulated after immersing it in magnetic fluid (MF) and applying a magnetic field. Theoretical analysis predicts that magneto-optical dichroism of MF plays a key role in light polarization modulation. During light polarization modulation, the evanescent wave polarized parallel to the magnetic field has greater loss than its orthogonal component. Light polarization of a D-shaped fiber with a wide polished surface can be modulated easily. High concentration MF and a large magnetic field all have great ability to modulate light polarization.

Chinese Optics Letters
Jan. 10, 2020, Vol. 18 Issue 1 010601 (2020)
Random Bragg-gratings-based narrow linewidth random fiber laser with a π-phase-shifted FBG
Shuaijie Miao, Wentao Zhang, and Ying Song

A new structure of short-cavity random fiber laser (RFL) with narrow linewidth lasing is proposed. A π-phase-shifted fiber Bragg grating (FBG) loop mirror was used in the RFL for spectral filtering and to suppress multi-mode oscillation due to the narrow transmission window. The random feedback of the RFL was implemented by a randomly dispersed weak reflection FBG array. The high gain of the erbium-doped fiber and a half-open cavity design results in a low lasing threshold. The linewidth of the laser was 225 Hz with 58 dB side-mode-suppression ratio. The laser threshold was 4.5 mW, and the optical signal-to-noise ratio was up to 63 dB.

Chinese Optics Letters
Sep. 10, 2019, Vol. 17 Issue 9 090605 (2019)
Misalignment measurement of optical vortex beam in free space
Xin Yang, Shibiao Wei, Shanshan Kou, Fei Yuan, and En Cheng

The misalignment of optical vortex (OV) beams, including transversal displacement and tilt, occurs in many situations, including on reflection or refraction at an interface between two different media and in propagation and tracking systems for optical communications. We propose a reliable method to determine and subsequently eliminate tilt and transversal displacement in an OV beam. An experimental setup was established to verify the proposed method, and the experimental results showed good agreement with those of the numerical simulations. Using the measured misalignments, the initial orbital angular momentum spectrum can be recovered in free space.

Chinese Optics Letters
Sep. 10, 2019, Vol. 17 Issue 9 090604 (2019)
Optical-fiber-based powerful tools for living cell manipulation [Invited]
Xiaotong Zhang, Shitai Yang, and Libo Yuan

By using a specialty optical fiber, a series of powerful microparticle manipulation tools, including optical tweezers, a micro-optical hand, and an optical gun, are developed and demonstrated. In this paper, a review of our research activities on the optical manipulation of microparticles is presented. In particular, we will describe a kind of specialty optical fiber designed and fabricated for building optical trapping and manipulating tools. The performances of annular core fiber-based optical tweezers, a multicore fiber-based micro-optical hand, and a coaxial dual waveguide fiber-based optical gun are demonstrated as examples of applications and discussed in detail. The fiber can be used in cell manipulation in life science and drug response in medicine.

Chinese Optics Letters
Editors' PickSep. 10, 2019, Vol. 17 Issue 9 090603 (2019)
Beam wander relieved optical switch using Bessel beams in turbulent atmosphere
Youpeng Xie, Ting Lei, Chuanwu Yang, Luping Du, and Xiaocong Yuan

The digital micro-mirror device (DMD)-based optical switch has the advantages of high-speed channels reallocation, miniaturization, stability, and large capacity for short reach optical communication in the datacenter. However, thermal turbulent atmosphere in the datacenter would cause perturbations and channel crosstalk for the optical switch. The self-healing optical beams such as the Bessel beams have the non-diffraction property to mitigate the turbulence issue. Here, we propose and demonstrate a Bessel beams enabled DMD-based optical switch to improve the stability and performance of optical communication in turbulent atmosphere. We statistically characterize the beam wanders of the Gaussian and Bessel beams in turbulent atmosphere at temperatures of 60°C and 80°C. We build the two-channel optical switch communication system and measure the bit error rate of the 15 Gbit/s on–off keying signals transmitted by the Gaussian and Bessel beams at temperatures of 60°C and 80°C, respectively. The optical switch using the Bessel beams shows lower bit error rates with weaker fluctuations compared with the Gaussian beams. The DMD-based optical switch using the Bessel beams has the potential for practical optical communication applications in the datacenter.

Chinese Optics Letters
Sep. 10, 2019, Vol. 17 Issue 9 090602 (2019)
Thermal expanded core ultraviolet fiber for optical cavity mode matching
Xin-Xia Gao, Jin-Ming Cui, Yun-Feng Huang, Chuan-Feng Li, and Guang-Can Guo

We have demonstrated a mode matching method between two different fibers by a hybrid thermal expanded core technique, which can be applied to match the modes of fiber-based Fabry–Pérot cavities. Experimentally, this method has achieved an expansion of the ultraviolet fiber core by 3.5 times while keeping fundamental mode propagation. With the experiment parameters, the fundamental mode coupling efficiency between the fiber and micro-cavity can reach 95% for a plano-concave cavity with a length of 400 μm. This method can not only have potential in quantum photonics research but also can be applied in classical optical fields.

Chinese Optics Letters
Sep. 10, 2019, Vol. 17 Issue 9 090601 (2019)
Hybrid OCS/OBS interconnect in intra-data-center network
Qian Kong, Yafeng Zhan, and Peng Wan

We report on a data center network (DCN) architecture based on hybrid optical circuit switching (OCS) and optical burst switching (OBS) interconnect for dynamic DCN connectivity provisioning. With the combination of the centralized and distributed control of the software-defined optical networks, the proposed interconnect can achieve unprecedented flexibility in dealing with both mice and elephant flow in the DCN. Numerical simulation is employed to investigate the performance of the proposed architecture. The results show that the OBS module has preferable performance in dealing with a larger burst packet, and the throughput is constrained by the capacity of the server random access memory module.

Chinese Optics Letters
Aug. 10, 2019, Vol. 17 Issue 8 080605 (2019)
10 Gb/s two-dimensional free-space optical code division multiple access wiretap channel with spatial diversity
Jianhua Ji, Jianjia Zhang, Bing Wu, Ke Wang, and Ming Xu

We experimentally demonstrate a 10 Gb/s free-space optical wiretap channel based on a spatial-diversity scheme and optical code division multiple access. In weak and middle turbulence cases, the bit error rate of a legitimate user can be decreased, and physical layer security can be simultaneously enhanced.

Chinese Optics Letters
Aug. 10, 2019, Vol. 17 Issue 8 080604 (2019)
A Q-switched erbium-doped fiber laser based on ZrS2 as a saturable absorber
Ping Hu, Yan Huang, Fangfang Liu, Ying Liu, Liping Guo, Xiaolu Ge, and Xiaojuan Liu

A passively Q-switched erbium-doped fiber (EDF) laser is proposed and demonstrated utilizing a zirconium disulfide (ZrS2)-based saturable absorber (SA). ZrS2 nanosheets are prepared, whose modulation depth, saturation intensity, and nonsaturable absorbance are measured to be 14.7%, 0.34 MW/cm2, and 17.4%, respectively. Then, a Q-switched EDF laser is implemented by the ZrS2-SA. The pulse repetition rate varies from 40.65 to 87.1 kHz when the pump power changes from 55 to 345 mW. The shortest pulse width is 1.49 μs with pulse energy of 33.5 nJ. As far as we know, this is the shortest pulse width obtained by a ZrS2-SA so far.

Chinese Optics Letters
Aug. 10, 2019, Vol. 17 Issue 8 080603 (2019)
Demonstration of high-dimensional free-space data coding/decoding through multi-ring optical vortices
Shiyao Fu, Yanwang Zhai, Heng Zhou, Jianqiang Zhang, Ci Yin, and Chunqing Gao

Multi-ring optical vortices are a kind of structured beams that carry orbital angular momentum (OAM) and have concentrical doughnut intensity distributions. In this Letter, we present both theoretically and experimentally a scheme that employs the dimensions of both the OAM states and radial index of multi-ring optical vortices simultaneously to accomplish high-dimensional free-space data coding/decoding transmissions. Such a scheme can further improve the coding efficiency when limited OAM states are available. In the proof-of-concept experiment, 64-ary coding/decoding employing 16 OAM states and four radial indices are present. Also, the coding/decoding performance when facing various atmosphere turbulences is evaluated. Furthermore, a 64 × 64 gray image (totally, 32,768 bits) is also transmitted through multi-ring optical vortices coding in free-space successfully, showing good transmission performance.

Chinese Optics Letters
Aug. 10, 2019, Vol. 17 Issue 8 080602 (2019)
Vernier effect of fiber interferometer based on cascaded PANDA polarization maintaining fiber
Shun Liu, Ping Lu, Enci Chen, Wenjun Ni, Deming Liu, Jiangshan Zhang, and Zhenggang Lian

This Letter shows the Vernier effect based on two segments of PANDA polarization maintaining fiber (PMF), whose lengths are 28 and 23 cm, respectively. The two PMFs are spliced together, and the angle between the fast axes is set to 45°. This cascaded PMF is inserted in a Sagnac loop to form an interferometer that can generate the Vernier effect. The spectrum consists of finesse fringe and envelope and realizes simultaneous measurement of strain and temperature. The envelope can provide strain and temperature sensitivities of 58.0 pm/με and 1.05 nm/°C. The finesse fringe provides sensitivities of 5.9 pm/με and 1.36 nm/°C.

Chinese Optics Letters
Aug. 10, 2019, Vol. 17 Issue 8 080601 (2019)
125 μm fiber based all-optical ultrasound probes for pulse-echo imaging
Yinlong Zhang, Yizhi Liang, Long Jin, and Baiou Guan

All-optical ultrasound probes that contain a photoacoustically-based ultrasound generator paired with a photonic acoustic sensor provide a promising imaging modality for diagnostic and MRI-compatible applications. Here, we demonstrate the fabrication of a fiber-based all-optical ultrasound probe and its applications in pulse-echo ultrasound imaging. The ultrasound generator is fabricated on a 125 μm multimode optical fiber by forming a light-absorbing multiwalled carbon nanotube (MWCNT)-polydimethylsiloxane (PDMS) composite coating on its distal end. A peak-to-peak acoustic pressure of 0.95 MPa was achieved with laser irradiation at 2.46 μJ by chemically functionalizing the fiber surface to enable a strong adsorption. Ultrasound reception was performed by a fiber-laser ultrasound sensor that translates ultrasound pressure into differential lasing-frequency changes. By linearly scanning the probe, ex vivo two- and three-dimensional imaging of a segment of swine trachea was demonstrated by detecting the echo ultrasound signals and reconstructing the acoustic scatterers. The probe presents axial and lateral resolutions at 150 and 62 μm, respectively. The small-sized, side-looking all-fiber ultrasound probe presents a promising approach for assembling an interventional endoscopy.

Chinese Optics Letters
Editors' PickJul. 10, 2019, Vol. 17 Issue 7 070604 (2019)
Traffic estimation based on long short-term memory neural network for mobile front-haul with XG-PON
Min Zhang, Bo Xu, Xiaoyun Li, Yi Cai, Baojian Wu, and Kun Qiu

A novel predictive dynamic bandwidth allocation (DBA) method based on the long short-term memory (LSTM) neural network is proposed for a 10-gigabit-capable passive optical network in mobile front-haul (MFH) links. By predicting the number of packets that arrive at the optical network unit buffer based on LSTM, the round-trip time delay in traditional DBAs can be eliminated to meet the strict latency requirement for MFH links. Our study shows that the LSTM neural network has better performance than feed-forward neural networks. Based on extensive simulations, the proposed scheme is found to be able to achieve the latency requirement for MFH and outperforms the traditional DBAs in terms of delay, jitter, and packet loss ratio.

Chinese Optics Letters
Jul. 10, 2019, Vol. 17 Issue 7 070603 (2019)
Performance improvement in double-ended RDTS by suppressing the local external physics perturbation and intermodal dispersion
Jian Li, Yang Xu, Mingjiang Zhang, Jianzhong Zhang, Lijun Qiao, and Tao Wang

We propose and experimentally demonstrate a novel Raman-based distributed fiber-optics temperature sensor (RDTS) for improving the temperature measurement accuracy and engineering applicability. The proposed method is based on double-ended demodulation with a reference temperature and dynamic dispersion difference compensation method, which can suppress the effect of local external physics perturbation and intermodal dispersion on temperature demodulation results. Moreover, the system can omit the pre-calibration process by using the reference temperature before the temperature measurement. The experimental results of dispersion compensation indicate that the temperature accuracy optimizes from 5.6°C to 1.2°C, and the temperature uncertainty decreases from 16.8°C to 2.4°C. Moreover, the double-ended configuration can automatically compensate the local external physics perturbation of the sensing fiber, which exhibits a distinctive improvement.

Chinese Optics Letters
Jul. 10, 2019, Vol. 17 Issue 7 070602 (2019)
Intensity detecting refractive index sensors with long-period fiber gratings written in STS structure
Zuliang Ruan, Li Pei, Jingjing Zheng, Jianshuai Wang, Liangying Wu, Tigang Ning, and Ji Wang

A refractive index intensity detecting sensor with long-period grating written in the single-mode–thin-core–single-mode fiber (STS) structure is proposed and optimized theoretically. The sensor is composed of two single-mode fibers connected by a section of long-period fiber grating fabricated on thin-core fiber. After optimization and benefitting from the phase matching point, the loss peak of the structure can reach 62.8 dB theoretically. The wavelength of the characteristic peak is fixed at the phase matching point, so intensity detection can be achieved. The sensitivity can reach 272.5 dB/RIU. The structural optimization in this Letter provides a reference for the fabrication of an easy-made all-fiber sensor without extra cladding.

Chinese Optics Letters
Jul. 10, 2019, Vol. 17 Issue 7 070601 (2019)
Microwave photonic filter design and optimization based on stimulated Brillouin scattering using a directly modulated pump
Kai Hu, Lilin Yi, Wei Wei, and Weisheng Hu

We propose a general guideline on the design of a stimulated Brillouin scattering (SBS)-based microwave photonic filter (MPF) using a directly modulated pump. Filter gain profiles and passband ripples with waveform repetition periods of the driving current ranging from 2 to 100 ns are measured after the transmission of different fiber lengths. The results show that the filter performance has nothing to do with the fiber length, and the digital-to-analog converter bandwidth requirement for the driving current is no more than 500 MHz. Therefore, the low cost, flexible reconfiguration, and miniaturization characteristics make an SBS filter using a directly modulated pump a promising choice as an MPF.

Chinese Optics Letters
Jun. 10, 2019, Vol. 17 Issue 6 060603 (2019)
Bidirectional carrier aggregation in next-generation radio-over-fiber heterogeneous network based on FBMC
Gour Chandra Mandal, Rahul Mukherjee, Khaleda Mallick, Paulomi Mandal, and Ardhendu Sekhar Patra

We have projected and verified a bidirectional intra-/inter-radio-access-technology carrier-aggregation method for a next-generation heterogeneous mobile network supported by filter bank multicarrier (FBMC). Successful transmission of intra/inter-band carrier aggregation between five broadband FBMC signals and three bands 4G long-term-evolution-advanced signal over 50 km single-mode fiber plus 10 m free-space is successfully broadcasted by employing an incoherent light-injection scheme in downlink. In uplink, two intra-bands carrier-aggregated wireless local area network Institute of Electrical and Electronics Engineers 802.11g signal is carried over the equal distance. High receiver sensitivity, low error vector magnitude, and clear constellation diagrams show successful delivery of different wireless services for different consumers. Therefore, the proposed hybrid system should become a potential solution for a future mobile front-haul network because of its low latency and high capacity.

Chinese Optics Letters
Jun. 10, 2019, Vol. 17 Issue 6 060602 (2019)
High-accuracy optical time delay measurement in fiber link [Invited]
Xiangchuan Wang, Shupeng Li, Xin Jiang, Jintao Hu, Min Xue, Shangzhe Xu, and Shilong Pan

Optoelectronic components and subsystems such as optically controlled phased array antennas, distributed radar networks, interferometric optical fiber hydrophones, and high-speed optoelectronic chips demand high-accuracy optical time delay measurement with large measurement range and the capability for single-end and wavelength-dependent measurement. In this paper, the recent advances in the optical time delay measurement of a fiber link with high accuracy are reviewed. The general models of the typical time delay measurement technologies are established with the operational principle analyzed. The performance of these techniques is also discussed.

Chinese Optics Letters
Editors' PickJun. 10, 2019, Vol. 17 Issue 6 060601 (2019)
Study on polarization spectrum and annealing properties of 45°-tilted fiber gratings
Xi Guo, Zhikun Xing, Huabao Qin, Qizhen Sun, Deming Liu, Lin Zhang, and Zhijun Yan

We have investigated the whole polarization-extinction-ratio (PER) spectrum and annealing properties of 45°-tilted fiber gratings (45°-TFGs). Experimental results show the PER spectrum of 45°-TFGs is a Gaussian-like profile and covers a 540 nm bandwidth from 1260 to 1800 nm, in which the bandwidth with PER greater than 10 dB is over 250 nm. The output polarization distribution of 45°-TFGs was analyzed by employing a bulk linear polarizer, and the results show a perfect figure “8”, which indicates that the 45°-TFG is a type of linear polarizer. Moreover, the annealing property of 45°-TFGs was measured up to 700°C, in which the PER of the grating started to decrease at 300°C and reached the minimum at 700°C. Based on these results, the 45°-TFGs can be used as an ultra-wide bandwidth in-fiber polarizing device.

Chinese Optics Letters
May. 10, 2019, Vol. 17 Issue 5 050601 (2019)
FP cavity and FBG cascaded optical fiber temperature and pressure sensor
Hui Li, Qingchao Zhao, Shaodong Jiang, Jiasheng Ni, and Chang Wang

A fiber Bragg grating (FBG) and Fabry–Perot (FP) cavity cascaded fiber sensing system was manufactured for temperature and pressure sensing. Temperature sensing as high as 175°C was performed by an FBG for the linear variation of an FBG wavelength with temperature. After the temperature was sensed, the demodulation system can find the original FP cavity length and its pressure and cavity length correlation coefficient; thus, the ambient pressure would be calculated. The sensing pressure can be as high as 100 MPa with a repeatability of 1/10,000 and high stability. This kind of fiber sensor has been used in the Shengli Oil Field.

Chinese Optics Letters
Apr. 10, 2019, Vol. 17 Issue 4 040603 (2019)
Impact of optical–electrical conversion responsivity in sub-sampled photonic analog-to-digital converter
Fan Yang, Weiwen Zou, Lei Yu, Shaofu Xu, and Jianping Chen

This Letter investigates the impact of the photodiode (PD) saturation in a sub-sampled photonic analog-to-digital converter (PADC) with two individual pulse lasers. It is essentially proved that when the optical power to the saturated PD increases, the optical–electrical conversion (OEC) responsivity and digitized output power of the PADC decrease. If femtosecond pulses are employed for the PADC sampling clock, the time-stretching process in a dispersive medium is necessary to decrease the impact of the PD saturation. In contrast, when the sampling clock with picosecond pulses is utilized, the PD saturation is more tolerable, and thus, the OEC responsivity can be improved by an increase of the optical power to the PD no matter if the time-stretching process is employed.

Chinese Optics Letters
Apr. 10, 2019, Vol. 17 Issue 4 040602 (2019)
A tutorial on laser-based lighting and visible light communications: device and technology [Invited]
Yujian Guo, Omar Alkhazragi, Chun Hong Kang, Chao Shen, Yuan Mao, Xiaobin Sun, Tien Khee Ng, and Boon S. Ooi

This tutorial focuses on devices and technologies that are part of laser-based visible light communication (VLC) systems. Laser-based VLC systems have advantages over their light-emitting-diode-based counterparts, including having high transmission speed and long transmission distance. We summarize terminologies related to laser-based solid-state lighting and VLC, and further review the advances in device design and performance. The high-speed modulation characteristics of laser diodes and superluminescent diodes and the on-chip integration of optoelectronic components in the visible color regime, such as the high-speed integrated photodetector, are introduced. The modulation technology for laser-based white light communication systems and the challenges for future development are then discussed.

Chinese Optics Letters
Apr. 10, 2019, Vol. 17 Issue 4 040601 (2019)
Visible light positioning: moving from 2D planes to 3D spaces [Invited]
E. W. Lam, and T. D. C. Little

The global navigation satellite system (GNSS) is a well-established outdoor positioning system with industry-wide impact due to the multifaceted applications of navigation, tracking, and automation. At large, however, is the indoor equivalent. One hierarchy of solutions, visible light positioning (VLP) with its promise of centimeter-scale accuracy and widespread coverage indoors, has emerged as a viable, easy to configure, and inexpensive candidate. We investigate how the state-of-the-art VLP systems fare against two hard barriers in indoor positioning: the need for high accuracy and the need to position in the three-dimensions (3D). We find that although most schemes claim centimeter-level accuracy for some proposed space or plane, those accuracies do not translate into a realistic 3D space due to diminishing field-of-view in 3D and assumptions made on the operating space. We do find two favorable solutions in ray–surface positioning and gain differentials. Both schemes show good positioning errors, low-cost potential, and single-luminaire positioning functionality.

Chinese Optics Letters
Mar. 10, 2019, Vol. 17 Issue 3 030604 (2019)
Polarization de-multiplexing using a modified Kalman filter in CO-OFDM transmissions
Yang Jiang, Xingwen Yi, Shaohua Hu, Xiatao Huang, Wei Tang, Wenjing Zhou, Xinning Huang, Jing Zhang, and Kun Qiu

We propose the modified Kalman filter (MKF) using the received signal for observation and constructing an inverse process of the conventional Kalman filter (CKF) for polarization de-multiplexing in coherent optical (CO) orthogonal frequency-division multiplexing (OFDM) transmissions. The MKF can avoid the convergence error problem in CKF without matrix inverse operation and has a faster converging speed and a much larger tolerance to the process and measurement noise covariance, about two orders of magnitude more than those of CKF. We experimentally demonstrate the 12 Gbaud OFDM signal transmission over 480 km standard single-mode fiber. The performance of MKF and CKF outperforms pilot-aided polarization de-multiplexing with better accuracy and nonlinearity tolerance.

Chinese Optics Letters
Mar. 10, 2019, Vol. 17 Issue 3 030603 (2019)
2080  nm long-wavelength, high-power dissipative soliton resonance in a dumbbell-shaped thulium-doped fiber laser
Hua Wang, Tuanjie Du, Yanhong Li, Jinhai Zou, Kaijie Wang, Fuyong Zheng, Junfeng Fu, Jihai Yang, Hongyan Fu, and Zhengqian Luo

We demonstrate a 2080 nm long-wavelength mode-locked thulium (Tm)-doped fiber laser operating in the dissipative soliton resonance (DSR) regime. The compact all-fiber dumbbell-shaped laser is simply constructed by a 50/50 fiber loop mirror (FLM), a 10/90 FLM, and a piece of large-gain Tm-doped double-clad fiber pumped by a 793 nm laser diode. The 10/90 FLM is not only used as an output mirror, but also acts as a periodical saturable absorber for initiating DSR mode locking. The stable DSR pulses are generated at the center wavelength as long as 2080.4 nm, and the pulse duration can be tunable from 780 to 3240 ps as the pump power is increased. The maximum average output power is 1.27 W, corresponding to a pulse energy of 290 nJ and a nearly constant peak power of 93 W. This is, to the best of our knowledge, the longest wavelength for DSR operation in a mode-locked fiber laser.

Chinese Optics Letters
Mar. 10, 2019, Vol. 17 Issue 3 030602 (2019)
Photonic microwave filters with ultra-high noise rejection [Invited]
Xihua Zou, Peixuan Li, Wei Pan, and Lianshan Yan

As a key figure-of-merit for high-performance microwave filters, the out-of-band noise rejection is of critical importance in a wide range of applications. This paper overviews the significant advances in photonic microwave filters (PMFs) having ultra-high rejection ratios for out-of-band noise suppression over the last ten years. Typically, two types of PMFs, the bandpass and bandstop ones, are introduced with fundamental principles, detailed approaches, and then cutting-edge results for noise rejection. Ultra-high noise rejection ratios of ~80 dB and >60 dB have been demonstrated for single-passband and single-stopband PMFs, respectively, which are comparable with the state-of-the-art electronic filters operating in stringent conditions. These PMFs are also characterized by wide frequency coverage, low frequency-dependent loss, and strong immunity to electromagnetic interference due to the intrinsic features from the advanced photonics technology.

Chinese Optics Letters
Mar. 10, 2019, Vol. 17 Issue 3 030601 (2019)
A reliable sunlight communication system
Haichao Guo, Tao Shan, Li Li, Li Zhang, Xiaojun Li, and Han Gao

A sunlight communication system is proposed that uses Sr2Si5N8:Eu2+ phosphors to concentrate sunlight signals in strong background light noise; thus, a wide spectrum sunlight communication system is converted into a narrow spectrum one. A communication method is proposed to enable compression to the dark line H-α (656.28 nm) spectrum. A 50% solar energy conversion efficiency is achieved with a 0.3 μs code delay, a 0.2 μs code rise time (20%–80%), and a 96% optical transmittance. Experimental results show that phosphors enhance the sunlight intensity 1.5 times with the same distance. This method has immense potential in future long-distance sunlight communication.

Chinese Optics Letters
Dec. 10, 2019, Vol. 17 Issue 12 120605 (2019)
Package-level passive equalization technology enabling DML-based 112 Gbps/λ PAM4 transmission
Zhike Zhang, Jinye Li, Zeping Zhao, Jianguo Liu, and Xingjun Wang

We demonstrate a package-level passive equalization technology in which the wire-bonding-induced resonance effect is used to compensate for the limited gain strength within the Nyquist frequency. The corresponding gain strength under various inductance and capacitance combinations could be quantitatively determined using a numerical simulation. With the increase in the Nyquist frequency, the capacitance shows a greater effect on the gain strength than the inductance. Therefore, the parasitic capacitance should be decreased to realize the desired gain strength at a higher Nyquist frequency. With this equalization technology, gain strength of 5.8 dB is obtained at 22 GHz, which can compensate for the limited bandwidth for the 112 Gbps pulse amplitude modulation (PAM4) signal. The experimental results show that 112 Gbps/λ PAM4 transmission based on a directly modulated laser (DML) module can be realized with a bit error rate of 1 × 10 3 at a received optical power of 3 dBm.

Chinese Optics Letters
Dec. 10, 2019, Vol. 17 Issue 12 120604 (2019)
Investigation on temperature characteristics of weak fiber Bragg gratings in a wide range
Xiaojun Fan, Junfeng Jiang, Xuezhi Zhang, Kun Liu, Shuang Wang, and Tiegen Liu

A weak fiber Bragg grating (WFBG) is an ideal quasi-distributed optical fiber sensor. Special attention should be paid to the spectrum and sensing performance of the WFBG at extreme temperatures due to its poor reflection intensity. In this Letter, the temperature characteristics of the WFBG from 252.75°C to 200.94°C are experimentally investigated. Five WFBGs with reflectivity from ~0.25% to ~10% are used in the experiments. The reflectivity variations and wavelength shifts at different temperatures are studied. Experimental results show that the WFBG can survive and work at extreme temperatures, but the performance is affected significantly. The reflectivity is affected significantly by both cryogenic temperature and high temperature. The temperature responses of Bragg wavelengths in the wide temperature range are also obtained.

Chinese Optics Letters
Dec. 10, 2019, Vol. 17 Issue 12 120603 (2019)
Ultra-broadband highly efficient mode converter at 1 μm fabricated by a line-focused CO2 laser
Xin Tian, Hao Li, Le Liu, Meng Wang, Xiaoming Xi, and Zefeng Wang

We report here an ultra-broadband linearly polarized (LP) LP01 LP11 mode converter operating at 1 μm based on a long period fiber grating (LPFG) fabricated in a conventional two-mode fiber (TMF) by a line-focused CO2 laser. The measured 3 dB bandwidth is about 240 nm, which is the broadest bandwidth for such fiber mode converters. The maximum conversion efficiency between the LP01 and LP11 modes is >99% over the range of 1000 nm to 1085 nm, almost covering the whole emission band of Yb3+, which is useful for further power scaling of high-power fiber lasers operating at the 1 μm band.

Chinese Optics Letters
Dec. 10, 2019, Vol. 17 Issue 12 120602 (2019)
Propagation characteristics of orbital angular momentum modes at 810 nm in step-index few-mode fibers
Zhouxiang Wang, Yuchen Xie, Shuangyin Huang, Han Zhou, Rui Liu, Zhifeng Liu, Min Wang, Wenrong Qi, Qianqian Tian, Lingjun Kong, Chenghou Tu, Yongnan Li, and Huitian Wang

In free-space or in optical fibers, orbital angular momentum (OAM) multiplexing for information transmission has been greatly developed. The light sources used were well coherent communication bands, and the fibers used were customized. Here, we use an 810 nm femtosecond laser to generate optical vortices carrying OAM and then feed them into two kinds of commercial step-index few-mode fibers to explore the transmission characteristics of OAM modes. We also propose a method without multiple-input multiple-output digital signal processing to identify the input OAMs. It is of great guiding significance for high-dimensional quantum information experiments via the OAMs as a degree of freedom, using the light generated by the spontaneous parametric down-conversion as the source and the commercial fibers for information transmission.

Chinese Optics Letters
Dec. 10, 2019, Vol. 17 Issue 12 120601 (2019)
Improvement on refractive index sensing by exploiting the tapered two-mode fibers
Fang Fang, Bing Sun, Zuxing Zhang, Jing Xu, and Lin Zhang

The spectral characteristics and sensitivities of a tapered two-mode fiber sandwiched between two single-mode fibers are systematically investigated. Theoretical calculations reveal that a dispersion turning point (DTP) appears when the group effective refractive index (RI) difference between the fundamental mode and the higher-order mode equals zero; as a result, ultrahigh RI sensitivities can be achieved. Furthermore, the location of the DTP is strongly dependent on the tapering condition. Then, we experimentally demonstrate high sensitivities of the RI sensor with the waist diameter of ~4 μm by means of immersing it in a flow cell filled with glycerol solution. In further tracking of the resonant wavelength shift around the DTP, it is found that the proposed RI sensor exhibits a sensitivity of 1.81 × 104 nm/RIU and a limit of detection down up to 3.29 × 10 5 RIU in a liquid glycerol solution.

Chinese Optics Letters
On the CoverNov. 10, 2019, Vol. 17 Issue 11 110604 (2019)
Saturation behavior of a one-pump fiber optical parametric amplifier in the presence of the fourth-order dispersion coefficient and dispersion fluctuation
N. Othman, K. G. Tay, N. S. Mohd Shah, R. Talib, H. Pakarzadeh, and N. A. Cholan

The influence of the fourth-order dispersion coefficient on the behavior of parametric gain and saturation power of a one-pump fiber optical parametric amplifier over a signal wavelength span in the presence of fiber random dispersion fluctuations was investigated. The output signal power for the parametric gain calculation was obtained by numerically solving the three-coupled amplitude equations. Based on the calculations of the parametric gain over a variation of the signal wavelength, it is found that the saturation power behavior is dependent on the behavior of parametric gain. The manipulations of signal wavelength and the fourth-order dispersion coefficient changed the phase-matching condition, thereby affecting the resulting parametric gain and saturation power.

Chinese Optics Letters
Nov. 10, 2019, Vol. 17 Issue 11 110603 (2019)
Power-efficient heterodyne radio over fiber link with laser phase noise robustness
Yuancheng Cai, Xiang Gao, Yun Ling, Bo Xu, and Kun Qiu

A pre-coding-assisted power detection scheme for radio over fiber downlink is presented. This scheme can eliminate laser phase noise while avoiding high energy-consuming electrical carrier required in conventional power and/or envelope detection schemes. Theoretical analysis and experimental verification are performed. 0.625 Gbaud pre-coded quadrature phase-shift keying or 16 quadrature amplitude modulation signals can both be recovered by power detection without electrical carrier assistance at the receiver after 75 km fiber transmission. Not only robust against the laser phase noise, an improvement of about 5 dB in receiver sensitivity can also be achieved, as compared with the conventional power detection scheme.

Chinese Optics Letters
Nov. 10, 2019, Vol. 17 Issue 11 110602 (2019)
Hollow-core fiber-based Raman probe extension kit for in situ and sensitive ultramicro-analysis
Hao Cai, Xingtao Yu, Qian Chu, Zhiqiang Jin, Bo Lin, and Guanghui Wang

A metal-lined hollow-core fiber-based Raman probe extension kit is proposed in this Letter for in situ and sensitive ultramicro-analysis. A hollow-core fiber can confine light and fluid samples in its hollow core, with enhanced light–sample interaction. By using a homemade light coupling device with a glass window for liquid isolation, a 3.5-cm-long hollow-core fiber could mount on and connect to a Raman probe, with perfect light coupling efficiency. After full filling the hollow-core fiber chamber with a volume of 13 μL by using a syringe pump, it can act as an extension kit for an ordinary Raman probe and be used as a ultramicro-analysis tool for the sample of microfluidic chips. In order to enhance its sensitivity, a gold film coated fiber tip is inserted into the capillary, which can double the Raman signal received by reflecting pump light and Raman light. Finally, a detection limit of 5% for ethanol solution and an enhancement factor of two compared with direct detection of bulk sample volume are demonstrated. Above all, our device can be utilized as a Raman probe extension kit, which is suitable for rapid, sensitive, and in situ measurements for a few microliter level samples.

Chinese Optics Letters
Nov. 10, 2019, Vol. 17 Issue 11 110601 (2019)
All-fiber few-mode erbium-doped fiber amplifier supporting six spatial modes
Zhenzhen Zhang, Cheng Guo, Liang Cui, Yichi Zhang, Cheng Du, and Xiaoying Li

Using the few-mode erbium-doped fiber (FM-EDF) with a simple two-layer erbium-doped structure, we demonstrate an all-fiber FM-EDF amplifier. The gain equalization among the six spatial modes supported by the FM-EDF is achieved when only the pump in the fundamental mode (LP01) is applied. When the signals in six spatial modes are simultaneously amplified, the average modal gain is about 15 dB, and differential modal gain is about 2.5 dB for the signal at 1550 nm.

Chinese Optics Letters
Oct. 10, 2019, Vol. 17 Issue 10 100604 (2019)
Analyzing OAM mode purity in optical fibers with CNN-based deep learning
Tianying Lin, Ang Liu, Xiaopei Zhang, He Li, Liping Wang, Hailong Han, Ze Chen, Xiaoping Liu, and Haibin Lü

Inspired by recent rapid deep learning development, we present a convolutional-neural-network (CNN)-based algorithm to predict orbital angular momentum (OAM) mode purity in optical fibers using far-field patterns. It is found that this image-processing-based technique has an excellent ability in predicting the OAM mode purity, potentially eliminating the need of using bulk optic devices to project light into different polarization states in traditional methods. The excellent performance of our algorithm can be characterized by a prediction accuracy of 99.8% and correlation coefficient of 0.99994. Furthermore, the robustness of this technique against different sizes of testing sets and different phases between different fiber modes is also verified. Hence, such a technique has a great potential in simplifying the measuring process of OAM purity.

Chinese Optics Letters
Oct. 10, 2019, Vol. 17 Issue 10 100603 (2019)
OFDM-based visible light communication with rotated polarity modulation aided complex color shift keying
Jianxun Xu, Ming Jiang, and Yufa Chen

High-rate techniques, such as optical orthogonal frequency division multiplexing (OFDM) and color shift keying (CSK), have been proposed for visible light communication (VLC). To fully exploit their advantages, in this Letter, we design a modulation scheme called rotated polarity modulation (RPM) aided complex CSK (CCSK) for OFDM-based VLC systems and derive its theoretical bit error rate and an optimal scaling factor. Analytical and simulation results show that in comparison to the existing schemes, the new RPM-CCSK-OFDM system offers an improved link performance and data rate under a modest complexity. It can also be applied to VLC systems equipped with different types of LED devices, thus enabling flexible deployments.

Chinese Optics Letters
Oct. 10, 2019, Vol. 17 Issue 10 100602 (2019)
A compact complex-coefficient microwave photonic filter with continuous tunability
Jinwang Qian, Zhennan Zheng, Mingzheng Lei, Chunqi Song, Shanguo Huang, and Xinlu Gao

A complex-coefficient microwave photonic filter with continuous tunability is proposed and demonstrated, which has a compact structure and stable performance without splitting the optical path and tuning the polarization state. By only controlling the DC biases of the modulator, the amplitudes and the phases of the filter taps can both be tuned. The phase difference between the two filter taps covers a full 360° range from 10 GHz to 32 GHz. Frequency responses of the proposed filter are measured within 10–20 GHz with different center frequencies.

Chinese Optics Letters
Oct. 10, 2019, Vol. 17 Issue 10 100601 (2019)
Application of Kramers–Kronig receiver in SSB-OFDM-RoF link
Yuancheng Cai, Yun Ling, Xiang Gao, Bo Xu, and Kun Qiu

We propose and investigate the use of a Kramers–Kronig (KK) receiver in a single sideband orthogonal frequency division multiplexing radio over fiber (SSB-OFDM-RoF) link based on an optical remote heterodyne solution. This scheme is effective in eliminating the signal-to-signal beating interference introduced by square-law detection of a photo-detector in an SSB-OFDM-RoF link. We extensively study the influences of different carrier-to-signal power ratios (CSPRs), laser linewidths, and transmission distances on our proposed scheme. It is proved that the KK-based receiver can reduce optimal CSPR by more than 5 dB and provide about 1.1 dB gain over the conventional mixer-based receiver scheme with CSPR of 11 dB after 75 km fiber transmission.

Chinese Optics Letters
Jan. 10, 2019, Vol. 17 Issue 1 010605 (2019)
Switchable multi-wavelength erbium-doped fiber laser based on a four-mode FBG
Yong Zhou, Panyun Gao, Xiu Zhang, Peng Wang, Liang Chen, and Weiqing Gao

We propose a switchable multi-wavelength erbium-doped fiber laser based on a four-mode fiber Bragg grating (FBG). The four-mode FBG is fabricated in a hydrogen pre-loaded fiber by the phase mask method, which can support four linearly polarized modes around 1550 nm. The five operation wavelengths are at 1547.5, 1546.8, 1546.0, 1545.1, and 1544.2 nm, respectively. Through adjusting the polarization state and the lateral offset coupling in cavity, the laser can be switched into the operation state of single, dual, or triple wavelengths. The proposed laser has the advantages of simple configuration, stable operation, and easy adjustment.

Chinese Optics Letters
Jan. 10, 2019, Vol. 17 Issue 1 010604 (2019)
High-efficiency DBA-BOTDA with optimized SNR by multiple bandwidths pump modulation
Huiliang Ma, Xinhong Jia, Kai Lin, and Cong Xu

For Brillouin optical time-domain analysis (BOTDA) based on distributed Brillouin amplification (DBA), constant Brillouin response achieved by the exponentially variable bandwidth/intensity pump modulation suffers from the much lower pumping efficiency for long-range sensing, which counterbalances the merit of DBA. In this Letter, pump modulation by multiple constant bandwidths was proposed and demonstrated. The ~98.9 km sensing with ~5 m spatial resolution and no use of optical pulse coding (OPC) was achieved by ~8 dBm Brillouin pump, which is lower by ~9 dB in theory by comparison with exponentially increased bandwidth modulation. Compared with traditional DBA-BOTDA, signal-to-noise ratio (SNR) enhancement with >4.6 dB was obtained. The flattened standard deviation (STD) of Brillouin frequency shift (BFS) (less than ~2 MHz) along the whole fiber was demonstrated.

Chinese Optics Letters
Jan. 10, 2019, Vol. 17 Issue 1 010603 (2019)
Doping radius effects on an erbium-doped fiber amplifier
Md. Ziaul Amin, Khurram Karim Qureshi, and Md. Mahbub Hossain

In an erbium-doped fiber amplifier (EDFA), erbium ions act as a three-level system. Therefore, much higher pump energy is required to achieve the population inversion in an erbium-doped fiber (EDF). This higher pump energy requirement complicates the efficient design of an EDFA. However, efficient use of the pump power can improve the EDFA performance. The improved performance of an EDFA can be obtained by reducing the doping radius of the EDF. A smaller doping radius increases pump–dopant interactions and subsequently increases the pump–photon conversion efficiency. Decreasing the doping radius allows a larger proportion of dopant ions, which are concentrated near the core, to interact with the highest pump intensity. However, decreasing the doping radius beyond a certain limit will bring the dopant ions much closer and introduce detrimental ion–ion interaction effects. In this Letter, we show that an optimal doping radius in an EDF can provide the best gain performance. Moreover, we have simulated the well-known numerical aperture effects on EDFA gain performance to support our claim.

Chinese Optics Letters
Jan. 10, 2019, Vol. 17 Issue 1 010602 (2019)
High-sensitivity FBG microphone with static pressure equalization and optimized response
Faxiang Zhang, Chang Wang, Xiaolei Zhang, Shaodong Jiang, Jiasheng Ni, and Gangding Peng

We present a high-sensitivity fiber Bragg grating (FBG)-based microphone with a flat response and static pressure equalization. A very high dynamic sensitivity of the microphone is achieved by a pressure sensing structure based on a carbon fiber diaphragm. Static pressure equalization is realized by a balance structure with a capillary glass tube. The resonances of the sensor are suppressed by air damping structures, and a broadened flat response is achieved. An acoustic-solid interaction model was used to analyze the response characteristics of the microphone. An experimental prototype was produced and tested, and the results are well consistent with the design. The tested sensitivity was 10 dB re 1 pm/Pa from 10 Hz to 2.5 kHz with a fluctuation of less than ±1.5 dB. Combined with the phase-generated-carrier-based coherent detection scheme, the microphone can achieve a sound resolution of the milli-pascal level. The static pressure sensitivity is measured to be 0.27 pm/atm, which is 100 dB lower than the dynamic sensitivity.

Chinese Optics Letters
Jan. 10, 2019, Vol. 17 Issue 1 010601 (2019)
Comparative study on reduced impacts of Brillouin pump depletion and nonlinear amplification in coded DBA-BOTDA
Kai Lin, Xinhong Jia, Huiliang Ma, Cong Xu, Xuan Zhang, and Lei Ao

The impacts of Brillouin pump depletion and nonlinear amplification in coded long-range Brillouin optical time-domain analysis (BOTDA) based on distributed Brillouin amplification (DBA) were studied. The error of Brillouin frequency shift (BFS) due to Brillouin pump depletion was compared for DBA-BOTDA using non-cyclic and cyclic coding. For non-cyclic coding, significant over- and under-shoots of BFS were found in the range with larger BFS variation, such as hot spot. The impact of Brillouin pump depletion can be reduced considerably by cyclic coding. Furthermore, to compensate the BFS error due to nonlinear amplification, a simple and effective log linearization was proposed and demonstrated.

Chinese Optics Letters
Sep. 10, 2018, Vol. 16 Issue 9 090604 (2018)
Broad bandwidth SOA-based multiwavelength laser incorporating a bidirectional Lyot filter
Abdul Hadi Sulaiman, Muhammad Zamzuri Abdul Kadir, Nelidya Md Yusoff, Noran Azizan Cholan, Fairuz Abdullah, Ahmad Fauzi Abas, Mohammed Thamer Alresheedi, and Mohd Adzir Mahdi

We demonstrate a broad bandwidth multiwavelength laser based on a bidirectional Lyot filter and a semiconductor optical amplifier with a mechanism of intensity-dependent loss as the flatness agent. A wide bandwidth of a multiwavelength spectrum of 32.9 nm within a 5 dB uniformity is obtained under optimized polarization parameters. For this case, the number of generated lasing lines is 329 with a fixed wavelength separation of 0.1 nm. The power stability of this multiwavelength laser is less than 1.35 dB within 200 min time frame. This shows that the bidirectional Lyot filter provides an alternative option for multiwavelength generation in laser systems.

Chinese Optics Letters
Sep. 10, 2018, Vol. 16 Issue 9 090603 (2018)
Intensity modulated silver coated glass optical fiber refractive index sensor
Zahra Samavati, Alireza Samavati, A. F. Ismail, Mukhlis A. Rahman, and M. H. D. Othman

Miniature optical fiber sensors with thin films as sensitive elements could open new fields for optical fiber sensor applications. Thin films work as sensitive elements and a transducer to get response and feedback from environments, in which optical fibers act as a signal carrier. A novel Ag coated intensity modulated optical fiber sensor based on refractive index changes using IR and UV-Vis (UV-visible) light sources is proposed. The sensor with an IR light source has higher sensitivity compared to a UV-Vis source. When the refractive index is enhanced to 1.38, the normalized intensity of IR and UV-Vis light diminishes to 0.2 and 0.8, respectively.

Chinese Optics Letters
Sep. 10, 2018, Vol. 16 Issue 9 090602 (2018)
An interferometric phase shift fiber Bragg grating sensing system with greatly reduced background phase noise
Lina Ma, Yu Chen, Zhengliang Hu, Jinxing Huang, Jun Wang, and Shiqing Li

We presented an interferometric phase shift fiber Bragg grating (FBG) sensor, which inherited the advantages of FBG sensors, and, at the same time, the greatly reduced full-width-at-half-maximum bandwidth brought longer coherent length, higher sensitivity, and lower phase noise. Experiments show that at least a 7 dB reduction of phase noise can be achieved compared to FBG sensors interrogated by interferometer with the same optical path difference.

Chinese Optics Letters
Sep. 10, 2018, Vol. 16 Issue 9 090601 (2018)
Large-mode-area neodymium-doped all-solid double-cladding silicate photonic bandgap fiber with an index step of ∼0.5%
Meng Li, Longfei Wang, Shuai Han, Chunlei Yu, Danping Chen, Wei Chen, and Lili Hu

A large-mode-area neodymium-doped silicate photonic bandgap fiber was theoretically designed and experimentally demonstrated. The relative index step between the high-index rods and the background glass was ~0.5%, which is the lowest cladding index difference reported on rare-earth-doped all-solid photonic bandgap fibers to our knowledge. An output power of 3.6 W with a slope efficiency of 31% was obtained for a 100-cm-long fiber.

Chinese Optics Letters
Aug. 10, 2018, Vol. 16 Issue 8 080601 (2018)
Broadband photonic ADC for microwave photonics-based radar receiver
Jiaqian Yang, Shangyuan Li, Xuedi Xiao, Dexin Wu, Xiaoxiao Xue, and Xiaoping Zheng

A broadband photonic analog-to-digital converter (ADC) for X-band radar applications is proposed and experimentally demonstrated. An X-band signal with arbitrary waveform and a bandwidth up to 2 GHz can be synchronously sampled and processed due to the optical sampling structure. In the experiment, the chirp signal centered at 9 GHz with a bandwidth of 1.6 GHz is sampled and down-converted with a signal-to-noise ratio of 7.20 dB and an improved noise figure. Adopting the photonic ADC in the radar receiver and the above signal as the transmitted radar signal, an X-band inverse synthetic aperture radar system is set up, and the range and cross-range resolutions of 9.4 and 8.3 cm are obtained, respectively.

Chinese Optics Letters
Jun. 10, 2018, Vol. 16 Issue 6 060605 (2018)
MISO visible light communication system utilizing hybrid post-equalizer aided pre-convergence of STBC decoding
Liang Qiao, Xingyu Lu, Shangyu Liang, Jiao Zhang, and Nan Chi

In this Letter, we propose a modified hybrid linear and nonlinear post-equalizer to aid pre-convergence of space–time block coding (STBC) decoding in the formulated multiple-input-single-output (MISO) visible light communication (VLC) model. Meanwhile, we present a channel estimation algorithm, as the existing method is suboptimal. Experiments demonstrate that a data rate of 1 Gbit/s is easily achieved in 1.3 m indoor free space transmission with the bit error rate (BER) limited to 3.8×10 3. Correspondingly, the Q factor can be improved to 3.13 dB compared to the pure linear equalizer case.

Chinese Optics Letters
Jun. 10, 2018, Vol. 16 Issue 6 060604 (2018)
4 × 20 GHz silica-based AWG hybrid integrated receiver optical sub-assemblies
Chaoyi Li, Junming An, Jiashun Zhang, Liangliang Wang, Jianguang Li, Yue Wang, Xiaojie Yin, Hongjie Wang, and Yuanda Wu

Both the 4×20 GHz coarse wavelength division multiplexing and LAN-WDM receiver optical sub-assemblies (ROSAs) were developed. The ROSA package was hybrid integrated with a planar lightwave circuit arrayed waveguide grating (AWG) with 2% refractive index difference and a four-channel top-illuminated positive-intrinsic-negative photodetector (PD) array. The output waveguides of the AWG were designed in a multimode structure to provide flat-top optical spectra, and their end facet was angle-polished to form a total internal reflection interface to realize vertical coupling with a PD array. The maximum responsivity of ROSA was about 0.4 A/W, and its 3 dB bandwidth of frequency response was up to 20 GHz for each transmission lane. The hybrid integrated ROSA would be a cost-effective and easy-assembling solution for 100 GbE data center interconnections.

Chinese Optics Letters
Jun. 10, 2018, Vol. 16 Issue 6 060603 (2018)
DFB fiber laser sensor for simultaneous measurement of acoustic and magnetic fields
Rui Ma, Wentao Zhang, Wenzhu Huang, and Fang Li

A novel distributed feedback (DFB) fiber laser sensor, which can measure acoustic and magnetic fields simultaneously, is proposed. The magnetic field can be measured by detecting the change of resonant frequency of the fiber laser, and the acoustic pressure can be measured by detecting the phase shift of the fiber laser. Both of the signals can be simultaneously demodulated in the frequency domain without affecting each other. Experimental studies show that the acoustic pressure sensitivity of this sensor is about 130 dB (0 dB re 1 pm/μPa) and the sensor has a good linearity with a magnetic field sensitivity of 0.57 Hz/mT.

Chinese Optics Letters
Jun. 10, 2018, Vol. 16 Issue 6 060602 (2018)
Flexible polarization demultiplexing method based on an adaptive process noise covariance Kalman filter
Jun Ge, Lianshan Yan, Anlin Yi, Yan Pan, Lin Jiang, Liangliang Dai, Wei Pan, and Bin Luo

A flexible polarization demultiplexing method based on an adaptive Kalman filter (AKF) is proposed in which the process noise covariance has been estimated adaptively. The proposed method may significantly improve the adaptive capability of an extended Kalman filter (EKF) by adaptively estimating the unknown process noise covariance. Compared to the conventional EKF, the proposed method can avoid the tedious and time consuming parameter-by-parameter tuning operations. The effectiveness of this method is confirmed experimentally in 128 Gb/s 16QAM polarization-division-multiplexing (PDM) coherent optical transmission systems. The results illustrate that our proposed AKF has a better tracking accuracy and a faster convergence (about 4 times quicker) compared to a conventional algorithm with optimal process noise covariance.

Chinese Optics Letters
Jun. 10, 2018, Vol. 16 Issue 6 060601 (2018)
Investigation on the pre-compensation and post-„compensation cascaded multi-channel-chirped fiber Bragg gratings for a repeaterless transmission system
K. Khairi, H. Fong Kok, Z. Lambak, M. I. Abdan, M. A. Farhan, M. H. Othman, M. L. H. Jamaluddin, S. A. Mohamad Rofie, Tee Din Chai, K. Chia Ching, A. Mohd. Ramli, M. A. Mokhtar, S. A. Syed Ahmad, M. Mokhtar, S. B. Ahmad Anas, and M. A. Mahdi

We experimentally designed dispersion-managed repeaterless transmission systems with a pre-compensation and post-compensation technique using multi-channel-chirped fiber Bragg gratings. The repeaterless transmission link supports a single channel (1548.51 nm) with a 10 Gbps repeaterless transmission system over 300 km standard single-mode fiber (SSMF). In the system design, two distributed Raman amplifiers (DRAs) were used to improve the signal level propagated along the 300 km SSMF. The co-propagating DRA provided 15 dB on–off gain and the counter-propagating produced 32 dB on–off gain at the signal wavelength. The experiment results show that the post-compensation configuration achieves an optimal performance with a bit error rate at 1×10 9.

Chinese Optics Letters
Apr. 10, 2018, Vol. 16 Issue 4 040607 (2018)
Regenerated fiber Bragg grating for fiber laser sensing at high temperatures
Yumin Zhang, Dandan Rong, Lianqing Zhu, Mingli Dong, and Fei Luo

Thermally regenerated low-reflectivity fiber Bragg gratings (RFBGs), as one mirror of a resonant cavity, have been introduced as linear-cavity fiber lasers combining with fiber saturable absorbers. The output of lasing presents an optical signal-to-noise ratio of 50 dB and temperature sensitivity coefficient of 15.36 pm/°C for the heating process and 15.46 pm/°C for the cooling process. The lasing wavelength variation and power fluctuation at 700°C are less than 0.02 nm and 0.21 dB, respectively. The RFBG-based fiber laser sensing has displayed good linearity for both the temperature rising and cooling processes, and favorable stability at high temperatures.

Chinese Optics Letters
Apr. 10, 2018, Vol. 16 Issue 4 040606 (2018)
Investigation on an all-optical intensity modulator based on an optical microfiber coupler
Yang Yu, Qiang Bian, Nan Zhang, Yang Lu, Xueliang Zhang, and Junbo Yang

An all-optical intensity modulator based on an optical microfiber coupler (OMC) is presented. The modulator works at 1550 nm wavelength and is modulated directly by heating the coupling region with 980 nm pump light injected through the coupling port of the OMC. The OMC is controlled to have at least a 30 mm long coupling region with diameter smaller than 8 μm, and the uniform waist region diameter is about 3 μm. This is helpful to ensure the optical modulation function based on the light induced thermal effect in the coupling region, while pump light is injected. The modulation response is measured to show good linearity when the 980 nm pump light has a lower intensity (with power below 2.5 mW), which proves that the OMC acts as an all-optical modulator. The bandwidth of the modulator can be at 0.2–50 kHz with the average power of the intensity-modulated pump light about 2 mW, which can be further improved by optimizing the design of the coupler. The demonstrated modulator may have potential value for the application in an all-optical integration system.

Chinese Optics Letters
Apr. 10, 2018, Vol. 16 Issue 4 040605 (2018)
SER estimation method for 56 GBaud PAM-4 transmission system
Aadil Raza, Kangping Zhong, Salman Ghafoor, Saeed Iqbal, Muhammad Adeel, Shahid Habib, Muhammad Fasih Uddin Butt, and Chao Lu

In this Letter, we have proposed a generalized Gaussian probability density function (GGPDF)-based method to estimate the symbol error ratio (SER) for pulse amplitude modulation (PAM-4) in an intensity modulation/direct detection (IM/DD) system. Furthermore, a closed form expression of SERGGD for PAM-4 has been derived. The performance of the proposed method is evaluated through simulation as well as experimental work. The fitting of probability density functions of the received signal is applied via GGPDF and shape parameters P1 and P2 associated with different PAM-4 levels are determined. The optimum single value of shape parameter P is then calculated to estimate the SER. The mathematical relationship of P with different received optical powers and receiver bandwidths has been determined and verified. The proposed method is a fast and accurate method to estimate SER of a PAM-4 system, which is more reliable and in agreement with the error counting method.

Chinese Optics Letters
Apr. 10, 2018, Vol. 16 Issue 4 040604 (2018)
50 GHz optical true time delay beamforming in hybrid optical/mm-wave access networks with multi-core optical fiber distribution
Álvaro Morales, Idelfonso Tafur Monroy, Fredrik Nordwall, and Tommi Sørensen

We propose and experimentally validate an optical true time delay beamforming scheme with straightforward integration into hybrid optical/millimeter (mm)-wave access networks. In the proposed approach, the most complex functions, including the beamforming network, are implemented in a central office, reducing the complexity and cost of remote antenna units. Different cores in a multi-core fiber are used to distribute the modulated signals to high-speed photodetectors acting as heterodyne mixers. The mm-wave carrier frequency is fixed to 50 GHz (V-Band), thereby imposing a progressive delay between antenna elements of a few picoseconds. That true time delay is achieved with an accuracy lower than 1 ps and low phase noise.

Chinese Optics Letters
On the CoverApr. 10, 2018, Vol. 16 Issue 4 040603 (2018)
Simultaneous measurement of axial strain and temperature based on twisted fiber structure
Xinyue Jiang, Ping Lu, Yuan Sun, Hao Liao, Deming Liu, Jiangshan Zhang, and Hui Liao

In this Letter, an alternative solution is proposed and demonstrated for simultaneous measurement of axial strain and temperature. This sensor consists of two twisted points on a commercial single mode fiber introduced by flame-heated and rotation treatment. The fabrication process modifies the geometrical configuration and refractive index of the fiber. Different cladding modes are excited at the first twisted point, and part of them are coupled back to the fiber core at the second twisted point. Experimental results show distinct sensitivities of 34.9 pm/με with 49.23 pm/°C and 36.19 pm/με with 62.99 pm/°C for the two selected destructive interference wavelengths.

Chinese Optics Letters
Apr. 10, 2018, Vol. 16 Issue 4 040602 (2018)
Resonator integrated optic gyro based on multilevel laser frequency lock-in technique
Ziwen Pan, Chengfei Zhang, Chengfeng Xie, Yongqiu Zheng, Haotian Li, Jun Tang, and Jun Liu

The resonator integrated optic gyros (RIOGs) based on the Sagnac effect have gained extensive attention in navigation and guidance systems due to their predominant advantages: high theoretical accuracy and simple integration. However, the problems of losing lock and low lock-in accuracy are the bottlenecks, which restrict the development of digital RIOGs. Therefore, a multilevel laser frequency lock-in technique has been proposed in this Letter to address these problems. The experimental results show that lock-in accuracy can be improved one order higher and without losing lock in a variable temperature environment. Then, a digital miniaturized RIOG prototype (18 cm × 18 cm × 20 cm) has been produced, and long-term (1 h) bias stability of 26.6 deg/h is successfully demonstrated.

Chinese Optics Letters
Apr. 10, 2018, Vol. 16 Issue 4 040601 (2018)
Thermal expanded core technique applied to high power fiber mode field adapter
Fanlong Dong, Xinhai Zhang, and Feng Song

A mode field adapter (MFA) fabricated by the thermal expanded core (TEC) technique is investigated. Firstly, the mode field characteristics of the TEC large mode area fiber (LMAF) are analyzed. Compared with the single-mode fiber (SMF), the mode field diameter of the LMAF enlarged slower than that of the SMF. Secondly, the mode field characteristics of the different fibers with TEC treatment are discussed. Thirdly, the transmission efficiency of the MFA fabricated by the SMF and LMAF is also investigated. Finally, we used the 6/125 μm SMF and 15/130 μm LMAF to fabricate an MFA with transmission efficiency of 92% and the handling power as high as 100 W.

Chinese Optics Letters
Mar. 10, 2018, Vol. 16 Issue 3 030602 (2018)
Wideband signal detection based on high-speed photonic analog-to-digital converter
Guang Yang, Weiwen Zou, Ye Yuan, and Jianping Chen

This Letter demonstrates the effectiveness of a high-speed high-resolution photonic analog-to-digital converter (PADC) for wideband signal detection. The PADC system is seeded by a high-speed actively mode locked laser, and the sampling rate is multiplied via a time-wavelength interleaving scheme. According to the laboratory test, an X-band linear frequency modulation signal is detected and digitized by the PADC system. The channel mismatch effect in wideband signal detection is compensated via an algorithm based on a short-time Fourier transform. Consequently, the signal-to-distortion ratio (SDR) of the wideband signal detection is enhanced to the comparable SDR of the single-tone signal detection.

Chinese Optics Letters
Mar. 10, 2018, Vol. 16 Issue 3 030601 (2018)
Advanced modulation formats for underwater visible light communications [Invited]
Nan Chi, and Meng Shi

In this paper, we present a detailed comparison of applying three advanced modulation formats including carrierless amplitude and phase modulation (CAP), orthogonal frequency division multiplexing (OFDM), and discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S OFDM) in underwater visible light communication (UVLC) systems. Cascaded post-equalization schemes are suggested to compensate the system impairments. For the first time, a two-level post-equalizer is presented to mitigate the nonlinear effect and improve the system performance of UVLC. The first post-equalization is based on a novel recursive least square Volterra. These modulation formats are all experimentally demonstrated with corresponding digital signal processing (DSP) algorithms. The experimental results show that single carrier modulations including CAP and DFT-S OFDM can outperform OFDM. Our experiment results show that up to 3 Gb/s over a 1.2 m underwater visible light transmission can be achieved by using DFT-S OFDM 64QAM and CAP-64. The measured bit error rate is well under the hard decision-forward error correction (HD-FEC) threshold of 3.8×10 3.

Chinese Optics Letters
Dec. 10, 2018, Vol. 16 Issue 12 120603 (2018)
Switching response of dual-output Mach–Zehnder modulator in channel-interleaved photonic analog-to-digital converter
Lei Yu, Weiwen Zou, Guang Yang, Xinwan Li, and Jianping Chen

This Letter theoretically and experimentally studies the response of photonic switching in a channel-interleaved photonic analog-to-digital converter (PADC) with high sampling rate and wide input frequency range. A figure of merit (FoM) is introduced to evaluate the switching response of the PADC when a dual-output Mach–Zehnder modulator (MZM) serves as the photonic switch to parallelize the sampled pulse train into two channels. After the optimization of the FoM and utilization of the channel-mismatch compensation algorithm, the system bandwidth of PADC is expanded and the signal-to-distortion ratio is enhanced.

Chinese Optics Letters
Dec. 10, 2018, Vol. 16 Issue 12 120602 (2018)
Thermal characteristics of Fabry–Perot cavity based on regenerated fiber Bragg gratings
Yumin Zhang, Yue Ren, Mingli Dong, Fanyong Meng, and Lianqing Zhu

The Letter reports the thermal stability and strain response of Fabry–Perot (FP) cavity under different high temperatures. The FP cavity was made by thermal regeneration of two identical cascaded fiber Bragg gratings (FBGs). It is demonstrated that the FP cavity is capable of measuring temperatures from 300°C to 900°C with a temperature sensitivity of 15.97 pm/°C. The elongation of the fiber was observed through the drifted Bragg wavelength at 700°C or above when weight was loaded. The elongation was further inferred by the slight change in the interference spectra of the FP cavity at 900°C.

Chinese Optics Letters
Dec. 10, 2018, Vol. 16 Issue 12 120601 (2018)
High resolution inclinometer based on vertical pendulum and fiber Bragg grating Fabry–Perot cavity
Peng Cui, Wentao Zhang, and Ying Song

In this paper, an inclinometer based on a vertical pendulum and a fiber Bragg grating Fabry–Perot cavity (FBG-FP) is proposed. A low-damping rotation structure is used to reduce the mechanical frictions of the pendulum system and induce a wavelength shift of FBG-FPs. We find that the sensitivity can be maximized by optimizing the parameters of the inclinometer. Using a high-resolution demodulation system, a sensitivity of 179.9 pm/(°), and a resolution better than 0.02″ can be achieved. Experiments also show that the proposed inclinometer has good linearity and repeatability.

Chinese Optics Letters
Nov. 10, 2018, Vol. 16 Issue 11 110603 (2018)
Wideband tunable optoelectronic oscillator based on a single-bandpass microwave photonic filter and a recirculating delay line
Yongchi Xu, Huanfa Peng, Rui Guo, Huayang Du, Guoqing Hu, Lixin Zhu, and Zhangyuan Chen

A wideband tunable optoelectronic oscillator (OEO) based on a tunable single-bandpass microwave photonic filter (MPF) and a recirculating delay line is proposed and experimentally demonstrated. The MPF is formed by cascading a finite impulse response filter and an infinite impulse response filter, which can enhance the quality factor of the MPF and suppress the side modes of the OEO. The frequency response of the tunable MPF is theoretically analyzed. By placing the MPF into the OEO, tunable microwave signals from 10.3 GHz to 26.7 GHz and a 100 MHz step tunability are experimentally demonstrated. The phase noise is 113 dBc/Hz@10 kHz. The results agree well with the theory.

Chinese Optics Letters
Nov. 10, 2018, Vol. 16 Issue 11 110602 (2018)
In-fiber integrated optics: an emerging photonics integration technology [Invited]
Xiaotong Zhang, Tingting Yuan, Xinghua Yang, Chunying Guan, Jun Yang, Zhihai Liu, Hongchang Deng, and Libo Yuan

In-fiber integrated optics is an attempt to use silica fiber as a substrate, integrating various optical paths or optical components into a single fiber, to build a functional optical device or component, and to realize a micro optical system, achieving various functions. In-fiber integrated optics is expected to be a new branch of photonics integration. This integration technique enables convenient light beams control and manipulation inside in one fiber. It also provides a research platform with micro and nano scale for interaction between light wave and microfluidic materials. In this review, we briefly summarize the main ideas and key technologies of the in-fiber integrated optics by series integration examples.

Chinese Optics Letters
Nov. 10, 2018, Vol. 16 Issue 11 110601 (2018)
Performance comparison among three different Stokes vector direct-detection receivers
Xiaojie Shen, Jiahao Huo, Xian Zhou, Kangping Zhong, Jinhui Yuan, Jiajing Tu, Keping Long, Changyuan Yu, Alan Pak Tao Lau, and Chao Lu

Stokes vectors direct detection (SV-DD) is an effective solution for short-reach optical communications. In this Letter, we investigate two-dimensional-modulation direct-detection systems based on three Stokes vector receivers (SVRs). The influences of three key factors including the states-of-polarization (SOP), the splitting ratio of the coupler, and the excess loss (EL) are studied in detail. It is shown that the splitting ratio for achieving optimum performance will be changed with SOP and EL conditions. Among these SVRs, the 3×3 coupler-based receiver with its optimal splitting ratio shows the best bit error rate performance and stability against the change of SOP.

Chinese Optics Letters
Oct. 10, 2018, Vol. 16 Issue 10 100605 (2018)
All-fiber linear polarization and orbital angular momentum modes amplifier based on few-mode erbium-doped fiber and long period fiber grating
Jianfei Xing, Jianxiang Wen, Jie Wang, Fufei Pang, Zhenyi Chen, Yunqi Liu, and Tingyun Wang

A few-mode erbium-doped fiber (FM-EDF) is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm, respectively. The refractive index difference is 0.98%, numerical aperture (NA) is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating (LPFG) is fabricated, which can convert LP01 mode to LP11 mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum (OAM) is also generated by combining the LPFG and polarization controller (PC). Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP11 mode and first-order OAM beams, is built up. Its on-off gain of the LP11 mode beam is 37.2 dB at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP11 mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme, which can be used in the optical communication fields.

Chinese Optics Letters
Oct. 10, 2018, Vol. 16 Issue 10 100604 (2018)
Plasmonic gold nanoshell induced spectral effects and refractive index sensing properties of excessively tilted fiber grating
Binbin Luo, Huafeng Lu, Shenghui Shi, Mingfu Zhao, Jiao Lu, Yajie Wang, and Xin Wang

We demonstrate a fiber refractive index (RI) sensor based on an excessively tilted fiber grating (ExTFG) immobilized by large-size plasmonic gold nanoshells (GNSs). The GNSs are covalently linked on ExTFG surface. Experimental results demonstrate that both the intensity of the transverse magnetic (TM) and transverse electric (TE) modes of ExTFG are significantly modulated by the localized surface plasmon resonance (LSPR) of GNSs due to the wide-range absorption band. The wavelength RI sensitivities of the TM and TE modes in the low RI range of 1.333–1.379 are improved by ~25% and ~14% after GNSs immobilization, respectively, and the intensity RI sensitivities are ~599%/RIU and ~486%/RIU, respectively.

Chinese Optics Letters
Oct. 10, 2018, Vol. 16 Issue 10 100603 (2018)
Passive phase noise compensation for fiber-optic radio frequency transfer with a nonsynchronized source
Tiancheng Lin, Guiling Wu, Hongwei Li, Guoyong Wang, and Jianping Chen

We propose a passive compensation fiber-optic radio frequency (RF) transfer scheme with a nonsynchronized RF stable source during a round-trip time, which can avoid high-precision phase-locking and efficiently suppress the effect of backscattering only using two wavelengths at the same time. A stable frequency signal is directly reproduced by frequency mixing at the remote site. The proposed scheme is validated by the experiment over a 40 km single mode fiber spool using nonsynchronized common commercial RF sources. The influence of the stability of nonsynchronized RF sources on the frequency transfer is investigated over different length fiber links.

Chinese Optics Letters
Oct. 10, 2018, Vol. 16 Issue 10 100602 (2018)
A polarization-independent torsion sensor based on the near-helical long period fiber grating
Zhihang Han, Cuiting Sun, Xiren Jin, Hang Jiang, Chong Yao, Shuo Zhang, Weiliang Liu, Tao Geng, Feng Peng, Weimin Sun, and Libo Yuan

A torsion sensor based on the near-helical (NH) long period fiber grating (LPFG) is fabricated by using a high frequency pulsed CO2 laser. Each groove of the NH-LPFG is spirally written in the four sides of a single-mode fiber. The NH-LPFG has a helical periodic vertical index modulation. This is different from the screw-type index modulation of the common helical LPFGs (H-LPFGs) fabricated in a twisted fiber. The torsion and temperature characteristics of the NH-LPFG are experimentally investigated. The temperature sensitivity is about 0.0668 nm/°C. The torsion sensitivity is 0.103 nm/(rad/m) and independent of the polarization state of incident light.

Chinese Optics Letters
Oct. 10, 2018, Vol. 16 Issue 10 100601 (2018)
Passively Q-switched S-band thulium fluoride fiberlaser with multi-walled carbon nanotube
H. Ahmad, and S. A. Reduan

A stable, passively Q-switched thulium fluoride fiber laser (TFFL) using a multi-walled carbon nanotube (MWCNT)-based saturable absorber (SA) for operation in the S-band region is proposed and demonstrated. The proposed TFFL has a central lasing wavelength of 1486.4 nm and an input power range of 87.1–126.6 mW. The output pulses have a repetition rate and pulse width range of 30.1–40.0 kHz and 9.0–3.2 μs, respectively, with a maximum pulse energy of 28.9 nJ. This is the first time, to the author’s knowledge, of the successful demonstration of a passively Q-switched S-band TFFL using an MWCNT-based SA.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010609 (2018)
Suppression of backscattering induced noise by the sideband locking technique in a resonant fiber optic gyroscope
Ning Liu, Yanxiong Niu, Lishuang Feng, Hongchen Jiao, and Xiao Wang

Polarization fluctuation induced noise and backscattering-induced noise are the dominant noises in resonant fiber optic gyroscopes. This Letter proposes a new method to suppress the carrier and backscattering induced noise by the sideband locking technique. Besides choosing an optimized modulation depth and different clockwise and counterclockwise modulation frequencies, the sideband is locked to the cavity resonance. With the proper modulation frequency, the carrier frequency component locates at a position far away from the resonant frequency, and then it is suppressed by the cavity itself, which can be taken as a bandpass filter. The amplitude of the carrier frequency can be suppressed by 20–25 dB additionally by the cavity and the total intensity suppression ratio can reach 115.74 dB. The backscattering induced noise can be eliminated for the adoption of different frequencies. The method can realize a stable and high suppression ratio without high requirements for parameter accuracy or device performance.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010608 (2018)
fs-level laser–RF synchronization with a fiber-loop optical-microwave phase detector
Shangyu Si, Liwen Feng, Yanying Zha, Gang Zhao, Fangming Liu, Senlin Huang, and Kexin Liu

In order to improve the precision of the laser–radio-frequency (RF) synchronization system from sub-picosecond to femtosecond (fs), a synchronization system between a picosecond laser and a 1.3 GHz RF generator has been developed based on a fiber-loop optical-microwave phase detector (FLOM-PD). Synchronization with fs-level (3.8 fs) rms jitter, integrated from 10 Hz to 1 MHz, is achieved for the first time, to the best of our knowledge, in this kind of configuration. This system will be used for the superconducting RF accelerator at Peking University.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010607 (2018)
Demodulation method combining virtual reference interferometry and minimum mean square error for fiber-optic Fabry–Perot sensors
Xinwang Gui, Michael Anthony Galle, Li Qian, Weilong Liang, Ciming Zhou, Yiwen Ou, and Dian Fan

We propose a cavity length demodulation method that combines virtual reference interferometry (VRI) and minimum mean square error (MMSE) algorithm for fiber-optic Fabry–Perot (F-P) sensors. In contrast to the conventional demodulating method that uses fast Fourier transform (FFT) for cavity length estimation, our method employs the VRI technique to obtain a raw cavity length, which is further refined by the MMSE algorithm. As an experimental demonstration, a fiber-optic F-P sensor based on a sapphire wafer is fabricated for temperature sensing. The VRI-MMSE method is employed to interrogate cavity lengths of the sensor under different temperatures ranging from 28°C to 1000°C. It eliminates the “mode jumping” problem in the FFT-MMSE method and obtains a precision of 4.8 nm, corresponding to a temperature resolution of 2.0°C over a range of 1000°C. The experimental results reveal that the proposed method provides a promising, high precision alternative for demodulating fiber-optic F-P sensors.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010606 (2018)
Robust fiber-based frequency synchronization system immune to strong temperature fluctuation
Xi Zhu, Bo Wang, Yichen Guo, Yibo Yuan, Romeo Gamatham, Bruce Wallace, Keith Grainge, and Lijun Wang

In order to make the fiber-based frequency synchronization system suitable for the use of large-scale scientific and engineering projects in which the ambient temperature of the fiber links change dramatically, we design a non-harmonic frequency dissemination system immune to strong temperature fluctuation. After the lab tests, in which the ambient temperature of the fiber fluctuates 40°C/day and 20°C/h, respectively, the relative frequency stabilities of this system reaches 4.0 × 10 14/s and 3.0 × 10 16/104 s. It is demonstrated that the proposed non-harmonic scheme shows a strong robustness to complicated working environment with strong temperature fluctuation.

Chinese Optics Letters
On the CoverJan. 10, 2018, Vol. 16 Issue 1 010605 (2018)
Polarization-maintained coupled optoelectronic oscillator incorporating an unpumped erbium-doped fiber
Tianhua Du, Dan Zhu, and Shilong Pan

A polarization-maintained coupled optoelectronic oscillator (COEO) with its performance significantly improved by a short-length unpumped erbium-doped fiber (EDF) is reported and experimentally investigated. A 10 GHz optical pulse train with a supermode suppression ratio of 61.8 dB and a 10 GHz radio frequency signal with a sidemode suppression ratio of 94 dB and a phase noise of 121.9 dBc/Hz at 10 kHz offset are simultaneously generated. Thanks to saturable absorption of the 1 m unpumped EDF, which introduces relatively large cavity loss to the undesired modes and noise, the supermode suppression ratio and the phase noise are improved by 9.4 and 7.9 dB, respectively.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010604 (2018)
Effect of power scale of 974 and 633 nm lasers on the induced loss at 633 nm of Yb3+/Al3+ co-doped silica fiber
Fenghou Xie, Chongyun Shao, Fengguang Lou, Meng Wang, Chunlei Yu, Suya Feng, and Lili Hu

Induced loss at 633 nm is tested in Yb3+/Al3+ co-doped silica fiber by a core pumped with a 974 nm laser and probed with a 633 nm laser. The fiber is prepared by the modified chemical vapor deposition method combined with solution doping. Different power scales of pump light and probe light are used in the tests. It is found that there is a dynamic equilibrium between photobleaching induced by 633 nm probe light and photodarkening (PD) induced by 974 nm pump light. For the first time to our knowledge, the effect of 633 nm probe laser power on an induced loss test of Yb3+/Al3+ co-doped silica fiber is studied quantitatively. It suggests that as long as the 633 nm probe light power is less than 0.2 mW, the induced loss is mainly contributed by the PD effect of pumping light, and the deviation of induced loss is less than 5%.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010603 (2018)
Generation of cylindrical vector beams in a mode-locked fiber laser using a mode-selective coupler
Yu Cai, Jie Wang, Jiaojiao Zhang, Hongdan Wan, Zuxing Zhang, and Lin Zhang

We experimentally obtain cylindrical vector beams (CVBs) in a passively mode-locked fiber laser based on nonlinear polarization rotation. A mode-selective coupler composed of both a single-mode fiber (SMF) and a two-mode fiber (TMF) is incorporated into the cavity to act as a mode converter from LP01 mode to LP11 mode with broad spectral bandwidth. CVBs in different mode-locked states including single-pulse, multi-pulse, and bound pulse are obtained, for the first time to our best knowledge. The ultrafast CVBs with different operation states have potential applications in many fields such as laser beam machining, nanoparticle manipulation, and so on.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010602 (2018)
Design and fabrication of wavelength tunable AWGs based on the thermo-optic effect
Pei Yuan, Yue Wang, Yuanda Wu, Junming An, and Xiongwei Hu

In this Letter, a 16 channel 200 GHz wavelength tunable arrayed waveguide grating (AWG) is designed and fabricated based on the silicon on insulator platform. Considering that the performance of the AWG, such as central wavelength and crosstalk, is sensitive to the dimension variation of waveguides, the error analysis of the AWG with width fluctuations is worked out using the transfer function method. A heater is designed to realize the wavelength tunability of the AWG based on the thermo-optic effect of silicon. The measured results show that the insertion loss of the AWG is about 6 dB, and the crosstalk is 7.5 dB. The wavelength tunability of 1.1 nm is achieved at 276 mW power consumption, and more wavelength shifts will gain at larger power consumption.

Chinese Optics Letters
Jan. 10, 2018, Vol. 16 Issue 1 010601 (2018)
Frequency-doubled triangular shape lightwave generation with a flexible modulation index
Jing Li, Ze Hao, Li Pei, Tigang Ning, and Jingjing Zheng

An approach for full duty frequency-doubled triangle shape lightwave generation is proposed and demonstrated. It requires a dual-parallel Mach–Zehnder modulator (DP-MZM) driven by a sinusoidal signal. A stop band filter is coupled to filter out two undesired sidebands. By tuning the bias voltage applied to the DP-MZM, the output optical intensity with a full duty cycle triangle shape profile can be obtained. It is found that the required modulation index is no longer a fixed one. It can vary within a range, without degrading the target waveform. The principle is analyzed by theory and evaluated by simulation. A proof-of-concept experiment is also conducted. Good agreements between theoretical prediction and experimental results have been found. This approach might be attractive due to the feature of a variable modulation index, which insures simple operation in practice.

Chinese Optics Letters
Sep. 10, 2017, Vol. 15 Issue 9 090603 (2017)
High temperature-sensitivity sensor based on long period fiber grating inscribed with femtosecond laser transversal-scanning method
Xinran Dong, Zheng Xie, Yuxin Song, Kai Yin, Dongkai Chu, and Ji’an Duan

We propose a high temperature-sensitive long period fiber grating (LPFG) sensor fabricated by using the femtosecond laser transversal-scanning method. The femtosecond pulses scan over the whole fiber core and some part of the cladding region; the modified regions are more extended. It is found that the LPFG-I fabricated by the transversal-scanning method shows higher temperature sensitivity and better temperature uniformity than that of LPFG-II written by the femtosecond laser point-by-point method. The LPFG-I with a temperature sensitivity of 75.96 pm/°C in the range of 25°C–400°C is measured. Moreover, in the range from 400°C to 800°C, a higher temperature sensitivity of 148.64 pm/°C and good linearity of 0.99 are achieved, while the temperature sensitivity of LPFG-II is only 95.55 pm/°C. LPFG-I exhibits better temperature characteristics, which, to the best of our knowledge, has the highest sensitivity in silica fiber temperature sensors.

Chinese Optics Letters
Sep. 10, 2017, Vol. 15 Issue 9 090602 (2017)
Q-switched fiber laser using carbon platinum saturable absorber on side-polished fiber
H. Ahmad, H. Hassan, R. Safaei, K. Thambiratnam, and I. S. Amiri

A 1550 nm Q-switched fiber laser using a carbon platinum saturable absorber deposited on side-polished fiber (SPF) is proposed and demonstrated. The SPF is approximately 2 mm with a polarization-dependent loss (PDL) of 0.4 dB and an insertion loss of 2.5 dB. A stable Q-switched output spectrum is obtained at 1559.34 nm with a peak power of ~6 mW, a pulse width of 1.02 μs, pulse energy of 5.8 nJ, average output power of 0.76 mW, and a repetition rate of 131.6 kHz taken at a pump power of 230.0 mW. A signal-to-noise ratio of 49.62 dB indicates that the Q-switched pulse is highly stable.

Chinese Optics Letters
Sep. 10, 2017, Vol. 15 Issue 9 090601 (2017)
Generation and time jitter of the loose soliton bunch in a passively mode-locked fiber laser
Zhenhong Wang, Zhi Wang, Yan'ge Liu, Ruijing He, Guangdou Wang, Guang Yang, and Simeng Han

We report a regime of the loose soliton bunch in an erbium-doped passively mode-locked fiber laser. In this state, every soliton bunch consists of multiple pulses. The amount of multiple pulses inside the soliton bunch increase as the pump power rises. Moreover, the temporal average pulse-to-pulse separation decreases in general with the increase of the pump power. Further, the spatial-temporal sequences based on the dispersive Fourier transformation technique show that pulse-to-pulse interactions and time jitter can result in pulse forking inside the soliton bunch. Finally, we theoretically demonstrate the soliton bunch with different pulse-to-pulse separations.

Chinese Optics Letters
Aug. 10, 2017, Vol. 15 Issue 8 080605 (2017)
Investigation on microwave photonic filter group delay performance
Bijuan Chen, Erwin H. W. Chan, Xinhuan Feng, Xudong Wang, and Bai-Ou Guan

Investigation on the group delay performance of microwave photonic filters is presented. Analysis and simulation results show symmetrical distribution on the delayed optical signals in the impulse response is required to obtain a constant group delay performance. Experimental results for both symmetrical and asymmetrical tap distribution microwave photonic notch filters are presented showing the group delay response with <±25 ps and around 1 ns ripples, respectively.

Chinese Optics Letters
Aug. 10, 2017, Vol. 15 Issue 8 080604 (2017)
Impact of Brillouin amplification on the spatial resolution of noise-correlated Brillouin optical reflectometry
Mingjiang Zhang, Xiaoyi Bao, Jing Chai, Yongning Zhang, Ruixia Liu, Hui Liu, Yi Liu, and Jianzhong Zhang

To obtain high spatial resolution over a long sensing distance in Brillouin optical correlation domain reflectometry (BOCDR), a broad laser spectrum and high pump power are used to improve the signal-to-noise ratio (SNR). In this Letter, we use a noise-modulated laser to study the variation of the Brillouin spectrum bandwidth and its impact on the coherent length of BOCDR quantitatively. The result shows that the best spatial resolution (lowest coherent length) is achieved by the lowest pump power with the highest noise-modulation spectrum. Temperature-induced changes in the Brillouin frequency shift along a 253.1 m fiber are demonstrated with a 19 cm spatial resolution.

Chinese Optics Letters
Aug. 10, 2017, Vol. 15 Issue 8 080603 (2017)
C-band real-time 400/300  Gb/s OOK bidirectional interconnection over 20  km multicore fibers
Zhenhua Feng, Honglin Ji, Ming Tang, Lilin Yi, Lin Gan, Lei Xue, Qiong Wu, Borui Li, Jiajia Zhao, Weijun Tong, Songnian Fu, Deming Liu, and Weisheng Hu

We experimentally demonstrate a real-time quasi-full-duplex 400G/300G optical interconnection over 20 km multicore fibers (MCFs), using 10G-class transponders operated in the C-band. Optical delay interferometer (ODI)-based optical frequency equalization is applied to mitigate chirp and dispersion induced impairments, so that the tolerance to inter-symbol interference (ISI) can be enhanced, thus enabling 4×25 Gb/s on-off keying (OOK) transmission per core over severely limited bandwidth channels. Real-time bit error ratio (BER) performances of the bidirectional 400 Gb/s transmission are measured without using extra digital signal processing (DSP) or electrical equalization, which ensures low complexity and less power consumption.

Chinese Optics Letters
Aug. 10, 2017, Vol. 15 Issue 8 080602 (2017)
Bandwidth-efficient visible light communication system based on faster-than-Nyquist pre-coded CAP modulation
Nan Chi, Jiaqi Zhao, and Zhixin Wang

With the rapid development of the light-emitting diode (LED) industry, interest in visible light communication (VLC) is growing. The limited bandwidth of commercial LEDs is one of the main challenges to achieve high-speed VLC. In this Letter, a kind of bandwidth-efficient VLC system based on carrierless amplitude and phase (CAP) modulation is proposed, where a simple differential faster-than-Nyquist (FTN) pre-coding scheme is employed to compress the spectrum and further improve the overall system baud rate. The system is experimentally demonstrated with a data rate of 1.47-Gb/s over 1.5 m free space transmission. The results indicate that an improvement of 80 Mbaud is achieved by FTN-CAP4 at 20% subband overlap and 40 Mbaud rate improvement by FTN-CAP16 at 7.5% subband overlap. Compared with traditional CAP, the FTN pre-coded CAP shows a better performance in spectral efficiency (SE) and intercarrier interference resistance. To the best of our knowledge, it is the first time to employ FTN pre-coded CAP in indoor high-data-rate VLC systems.

Chinese Optics Letters
Aug. 10, 2017, Vol. 15 Issue 8 080601 (2017)
Designing broadband fiber optic parametric amplifier based on near-zero single ZDW PCF with ultra-flat nature
Partha Sona Maji, and Ritwick Das

We propose a broadband fiber optic parametric amplifier (FOPA) based on a near-zero ultra-flat dispersion profile with a single zero-dispersion wavelength (ZDW) by using a selective liquid infiltration technique. The amplifier gain and bandwidth is investigated for a variety of fiber lengths, pump power, and operating wavelengths. It is observed that sufficient peak gains and broader bandwidths can be achieved with a small negative anomalous dispersion (β2≤0) and a positive value of the 4th-order dispersion parameter (+ β4) around the pump. We can optimize an FOPA with a bandwidth of more than 220 nm around the communications wavelength.

Chinese Optics Letters
Jul. 10, 2017, Vol. 15 Issue 7 070606 (2017)
Spectrum-profile-identification-based wavelength division multiplexing method for a fiber Bragg grating sensor
Yuheng Pan, Junfeng Jiang, Weijia Lu, Huijia Yang, Kun Liu, Shuang Wang, Hui Wang, and Tiegen Liu

A new wavelength division multiplexing method for fiber Bragg grating (FBG) sensors based on spectrum profile identification is proposed. In this method, FBGs and tilted FBG (TFBG) sensors are cascaded in a single fiber in one sensing channel. The different spectrum profiles enable the cross-correlation method to demodulate the wavelength. Therefore, the different types of sensors can occupy the same central wavelength band. Using this method, the multiplexing capacity is optimized. Experiment results demonstrate the feasibility of this method and it is useful for applications where large numbers of FBGs are needed.

Chinese Optics Letters
Jul. 10, 2017, Vol. 15 Issue 7 070605 (2017)
Impact of external optical feedback on the beat notes of dual-polarization fiber lasers
Kuanglu Yu, Linghao Cheng, Yiqin Lao, Yunbo Li, Bai-Ou Guan, Chongqing Wu, and Yao Zhao

We present a theoretical analysis and an experimental study of the impacts of external optical feedback on dual-frequency fiber lasers. The external optical feedback can effectively suppress the phase noise of the beat notes of dual-frequency fiber lasers, provided some requirements are satisfied. The polarization of the optical feedback is important for the fiber laser’s stability, and it can also tune the beat note frequency. A side effect of external optical feedback, as demonstrated in the experiments, is lowering the sensitivity of the dual-frequency fiber laser-based sensors, although such degradation is not obvious.

Chinese Optics Letters
Jul. 10, 2017, Vol. 15 Issue 7 070604 (2017)
Frequency-octupled phase-coded signal generation based on carrier-suppressed high-order double sideband modulation
Xuan Li, Shanghong Zhao, Zihang Zhu, Kun Qu, Tao Lin, and Shilong Pan

An approach for photonic generation of a frequency-octupled phase-coded signal based on carrier-suppressed high-order double sideband modulation is proposed and experimentally demonstrated. The key component of the scheme is an integrated dual-polarization quadrature phase shift keying modulator, which is used to achieve the carrier-suppressed high-order double sideband modulation. At the output of the modulator, two fourth-order optical sidebands are generated with the optical carrier suppressed. After that, a Sagnac loop incorporating a fiber Bragg grating and a phase modulator is employed to separate the two optical sidebands and phase modulate one sideband with a binary coding signal. The approach features large carrier frequency tuning range for the generated phase-coded signal from several megahertz to beyond the W-band. A proof-of-concept experiment is carried out. The 2 Gbit/s phase-coded signals with frequencies of 16.48, 21.92, and 29.76 GHz are generated.

Chinese Optics Letters
Jul. 10, 2017, Vol. 15 Issue 7 070603 (2017)
Luminescent properties of Tb3+-doped poly(methyl methacrylate) fiber
P. Miluski, M. Kochanowicz, J. Zmojda, and D. Dorosz

Lanthanide-doped polymers are very attractive, since they can be used for luminescent optical fiber fabrications. This Letter presents the terbium-ions-doped poly(methyl methacrylate) fiber fabrication and spectroscopic characterization. The measured excited state (D45) lifetime of 0.741 ms confirms that a used organometallic can be used to obtain an intense luminescence in a polymeric fiber. The luminescence spectrum shape modification versus the fiber length is also investigated.

Chinese Optics Letters
Jul. 10, 2017, Vol. 15 Issue 7 070602 (2017)
Simple capillary-based extrinsic Fabry–Perot interferometer for strain sensing
Xiaobei Zhang, Haiyang Shao, Haiyang Pan, Yong Yang, Huawen Bai, Fufei Pang, and Tingyun Wang

A simple capillary-based extrinsic Fabry–Perot structure is presented and fabricated, using the manual welding method with the strain sensing characteristics investigated in detail. Strain sensitivities of 4.2, 2.8, and 2.4 pm/με are obtained experimentally with the interferometer length around 50 μm and the inner diameter of 75, 50, and 25 μm, respectively. The underlying physics of the strain sensitivity of this device is negatively correlated with the interferometer length and positively correlated with the capillary inner diameter, which provides two simple parameters to tailor the strain sensitivity.

Chinese Optics Letters
Jul. 10, 2017, Vol. 15 Issue 7 070601 (2017)
Study on a v-band signals and pulses generator with a tunable optical carrier to sideband ratio
Hongyao Chen, Tigang Ning, Jing Li, Li Pei, Jin Yuan, and Xiaodong Wen

This Letter demonstrates a filterless v-band signals and pulses generator scheme with a tunable optical carrier to sideband ratio (OCSR). Through complete theoretical analysis, the mathematical expression of the OCSR that contains the extinction ratio, phase modulation index, and bias angle is obtained. It is found that the OCSR has a wide tuning range, from 50 to 70 dB. By careful adjustment, a 60 GHz millimeter-wave signal with the OCSR at 50.72 dB and the electrical spurious suppression ratio at 36.6 dB can be achieved. Moreover, the discussions of using optimum OCSR to generate a Nyquist pulse or triangular-shaped pulse are also presented in this Letter.

Chinese Optics Letters
Jun. 10, 2017, Vol. 15 Issue 6 060605 (2017)
Optical phase remodulation for Rayleigh noise mitigation in 10  Gb/s/channel WDM passive optical networks
Jing Xu, Yang Qiu, and Ning Deng

We investigate the principles of optical phase remodulation and demonstrate its application in a future-proof 10 Gb/s/channel wavelength-division-multiplexed (WDM) passive optical network to realize a colorless optical network unit and bidirectional transmission over a single fiber. The modulation depth of downstream differential phase-shift keying is properly reduced to facilitate phase remodulation and Rayleigh noise mitigation. For both downstream and upstream 10 Gb/s signals, error-free transmission via a 20 km single-mode fiber is demonstrated without dispersion compensation operation.

Chinese Optics Letters
Jun. 10, 2017, Vol. 15 Issue 6 060604 (2017)
Experimental demonstration of 60  Gb/s optical OFDM transmissions at 1550  nm over 100  m OM1 MMF IMDD system with central launching
Jian Chen, Ling Fang, Qianwu Zhang, Junjie Zhang, Yingxiong Song, Yingchun Li, and Min Wang

Record-high 60 Gb/s optical orthogonal frequency division multiplexing (OFDM) transmissions over intensity modulation and direct-detection (IMDD)-based 100 m optical mode (OM1) multi-mode fiber (MMF) links are experimentally demonstrated, utilizing 10 GHz electro-absorption modulated laser intensity modulators at a single 1550 nm wavelength. Adaptive bit loading and a simple central launching scheme of the proposed scheme show an effective way for combating the channel fading and simplifying the system structure. It shows good potential in short reach data center interconnections.

Chinese Optics Letters
Jun. 10, 2017, Vol. 15 Issue 6 060603 (2017)
Pump induced birefringence in dual-polarization fiber grating lasers
Linghao Cheng, Yunbo Li, Yizhi Liang, Hao Liang, and Bai-Ou Guan

Birefringence is critical in dual-polarization fiber-laser-based fiber-optic sensing systems, as it directly determines the beat frequency between the two polarizations. A study of pump induced birefringence in dual-polarization fiber lasers is presented here, which shows that the pump induced birefringence is a result of the interplay among pump induced refractive index change, laser dynamics, and anisotropy inside fiber lasers. For erbium-doped fiber lasers, pumping at 1480 nm is better than pumping at 980 nm in lower pump induced birefringence. Moreover, injection at 532 nm for an adequately long enough time can permanently reduce anisotropy and, hence, reduce pump induced birefringence.

Chinese Optics Letters
Jun. 10, 2017, Vol. 15 Issue 6 060602 (2017)
Seven-core photonic liquid crystal fibers for simultaneous mode shaping and temperature sensing
Min Liu, Bingyue Zhao, Xu Yang, and Jingyun Hou

Through doping liquid crystals into the core region, we propose a kind of seven-core photonic crystal fiber (PCF) which can achieve mode shaping and temperature sensing simultaneously in the communication window of 1.1–1.7 μm. To the best of our knowledge, this is the first time that the function of seven-core PCFs as temperature sensors is investigated. By using the full vectorial finite element method, the characteristics of the fiber with the temperature, such as the effective mode area, the waveguide dispersion, and the confinement loss, are analyzed. This kind of PCF can be competitive in providing temperature sensing in multi-core PCF lasers.

Chinese Optics Letters
Jun. 10, 2017, Vol. 15 Issue 6 060601 (2017)
Experimental demonstration of indoor interfered visible light communication system employing successive interference cancellation
Miaofeng Li, Xiang Li, Chao Yang, Qi Yang, and Shaohua Yu

Multipath interference induced power fading occurs when the transmission path lengths from the light emitting diodes to a single receiver are different in a visible light communication system. To solve this problem, we apply a QR-decomposition-based channel equalizer (QR-CE) to achieve successive interference cancellation for a discrete Fourier transform spreading (DFT-S) orthogonal frequency division multiplexing (OFDM) signal. We experimentally demonstrate a 200 Mb/s DFT-S OFDM over a 2 m free-space transmission. The experimental results show that a DFT-S OFDM with QR-CE attains much better bit error rate performance than a DFT-S OFDM with conventional CEs. The impacts of several parameters on a QR-CE are also investigated.

Chinese Optics Letters
May. 10, 2017, Vol. 15 Issue 5 050604 (2017)
Highly linear W-band receiver front-end based on higher-order optical sideband processing
Haojie Wang, Shangyuan Li, Xiaoping Zheng, Xiaoxiao Xue, Hanyi Zhang, and Bingkun Zhou

A highly linear W-band receiver front-end based on higher-order optical sideband (OSB) processing is proposed and experimentally demonstrated. Two-tone analysis shows that by manipulating higher-order OSBs, the third-order intermodulation distortion (IMD3) introduced by optoelectronic components (mainly modulators) in the receiver front-end can be further suppressed, and a 9 dB improvement of the ratio of the fundamental and IMD3 can be attained. In the experiment, the spurious-free dynamic range of the W-band receiver front-end is up to 122.1 dB·Hz2/3, with improvement by 9 dB compared with that of only processing the five OSBs.

Chinese Optics Letters
May. 10, 2017, Vol. 15 Issue 5 050603 (2017)
Symbol error rate performance analysis of soft-decision decoded MPPM free space optical system over exponentiated Weibull fading channels
Duan Zhou, Tian Cao, Yintang Yang, Jianxian Zhang, Ping Wang, and Bensheng Yang

The symbol error rate (SER) performance of a multipulse pulse-position modulation (MPPM) free space optical (FSO) system under the combined effect of turbulence-induced fading modeled by exponentiated Weibull (EW) distribution and pointing errors with a soft-decision detector is investigated systematically. Particularly, the theoretical conditional SER (CSER) of soft-decision decoded MPPM is derived. The corresponding closed-form CSER is obtained via curve fitting with the Levenberg–Marquardt method. The analytical SER expression over the aggregated fading channels is then achieved in terms of Laguerre integration. Monte Carlo simulation results are also offered to corroborate the validity of the proposed SER model.

Chinese Optics Letters
May. 10, 2017, Vol. 15 Issue 5 050602 (2017)
Artificial neural-network-based visible light positioning algorithm with a diffuse optical channel
Heqing Huang, Aiying Yang, Lihui Feng, Guoqiang Ni, and Peng Guo

Visible light positioning becomes popular recently. However, its performance is degraded by the indoor diffuse optical channel. An artificial neural-network-based visible light positioning algorithm is proposed in this Letter, and a trained neural network is used to achieve positioning with a diffuse channel. Simulations are made to evaluate the proposed positioning algorithm. Results show that the average positioning error is reduced about 13 times, and the positioning time is reduced about two magnitudes. Moreover, the proposed algorithm is robust with a different field-of-view of the receiver and the reflectivity of the wall, which is suitable for various positioning applications.

Chinese Optics Letters
May. 10, 2017, Vol. 15 Issue 5 050601 (2017)
Demonstration of a 400  kbps real-time non-line-of-sight laser-based ultraviolet communication system over 500  m
Kun Wang, Chen Gong, Difan Zou, Xianqing Jin, and Zhengyuan Xu

We extend the transmission range of non-line-of-sight ultraviolet communication to 500 m in a real-time system experiment using a 200 mW solid-state 266 nm laser, where the data rate can reach 400 kbps at a frame error rate lower than 10 5 in the real-time system test. The results can beat the best record so far, in terms of both the data rate and transmission distance.

Chinese Optics Letters
Apr. 10, 2017, Vol. 15 Issue 4 040602 (2017)
Temperature-insensitive optical Fabry–Perot flow measurement system with partial bend angle
Huijia Yang, Junfeng Jiang, Shuang Wang, Yuheng Pan, Xuezhi Zhang, and Tiegen Liu

A flow measurement system consisting of an optical fiber Fabry–Perot (F-P) sensor and an elbow tube is proposed and demonstrated to realize flow measurements and eliminate thermal disturbance. Two F-P sensors are symmetrically mounted on the inner-wall surface of the elbow of 90° in order to eliminate the effect of thermal disturbance to the flow measurement accuracy. Experimental results show that the absolute phase difference is the square root of the fluid flow. It is consistent with the theoretical analysis, which proves that the flow measurement method can measure flow and eliminate the influence of thermal disturbance simultaneously.

Chinese Optics Letters
Apr. 10, 2017, Vol. 15 Issue 4 040601 (2017)
Influence of the nonlinear propagation effect on the optical signal-to-noise ratio of 400G optical fiber communication systems
Huaqiang Qin, and Xiaosheng Xiao

The influence of the nonlinear propagation effect on three 400 Gb/s/ch (400G) optical fiber communication systems with typical modulation formats, dual-carrier 16-quadrature amplitude modulation (16QAM), single-carrier 16QAM (single-16QAM), and four-carrier quadrature phase-shift keying, are investigated. The received optical signal-to-noise ratio (OSNR), affected by the nonlinear interference noise together with the amplified spontaneous emission noise, are compared with three 400G systems and a standard 100 Gb/s/ch system by numerical simulations. Both single channel and multichannel cases are considered. Single-16QAM is found to have the best OSNR among those modulation formats.

Chinese Optics Letters
Mar. 10, 2017, Vol. 15 Issue 3 030604 (2017)
Ultra-compact silicon-arrayed waveguide grating routers for optical interconnect systems
Guangyi Song, Jun Zou, and Jianjun He

We present compact silicon-arrayed waveguide grating routers (AWGRs) with three different channel spacings of 20, 6.4, and 3.2 nm for optical interconnect systems. The AWGR with the 20 nm channel spacing shows a low loss of 2.5 dB and a low crosstalk of 20 dB and has a footprint of only 0.27 mm×0.19 mm. The AWGR with the channel spacing of 6.4 nm has loss ranging from 3 to 8 dB, and the crosstalk is 18 dB. As for the 3.2 nm channel spacing, the loss is about 4 dB, and the crosstalk is 12 dB.

Chinese Optics Letters
Mar. 10, 2017, Vol. 15 Issue 3 030603 (2017)
Carrier-less amplitude and phase modulated visible light communication system based on a constellation-shaping scheme
Zhixin Wang, Mengjie Zhang, Siyuan Chen, and Nan Chi

In this Letter, we propose a novel constellation-shaping carrier-less amplitude and phase (CAP) modulation scheme to alleviate the systematic nonlinearity in visible light communication (VLC) systems. A simple geometric transformation shaping method is employed to convert the normal square lattice constellation into multiple circular constellations. The feasibility and performance are investigated and experimentally demonstrated by a 1.25 Gb/s CAP-modulated VLC system. The results indicate that the circular constellation has better resistance to systematic nonlinearity compared with a rectangular constellation. The dynamic range of input signal peak-to-peak values promotes 20% at a low bias voltage nonlinear area and 50% at a high bias voltage nonlinear area. To the best of our knowledge, this is the first time constellation-shaping CAP has ever been reported in indoor high data rate VLC systems.

Chinese Optics Letters
Mar. 10, 2017, Vol. 15 Issue 3 030602 (2017)
Dispersion compensation using high-positive dispersive optical fibers
Mohammad Hadi, Farokh Marvasti, and Mohammad Reza Pakravan

The common and traditional method for optical dispersion compensation is concatenating the transmitting optical fiber by a compensating optical fiber having a high-negative dispersion coefficient. In this Letter, we take the opposite direction and show how an optical fiber with a high-positive dispersion coefficient is used for dispersion compensation. Our optical dispersion compensating structure is the optical implementation of an iterative algorithm in signal processing. The proposed dispersion compensating system is constructed by cascading a number of compensating sub-systems, and its compensation capability is improved by increasing the number of embedded sub-systems. We also show that the compensation capability is a trade-off between the transmission length and bandwidth. We use the simulation results to validate the performance of the introduced dispersion compensating module. Photonic crystal fibers with high-positive dispersion coefficients can be used for constructing the proposed optical dispersion compensating module.

Chinese Optics Letters
Mar. 10, 2017, Vol. 15 Issue 3 030601 (2017)
Temperature-insensitive refractive index sensor based on Mach–Zehnder interferometer with two microcavities
Yinfeng Wang, Sumei Wang, Lan Jiang, Hua Huang, Liuchao Zhang, Peng Wang, Lingya Lv, and Zhitao Cao

We propose a temperature-insensitive refractive index (RI) fiber sensor based on a Mach–Zehnder interferometer. The sensor with high sensitivity and a robust structure is fabricated by splicing a short photonic crystal fiber (PCF) between two single-mode fibers, where two microcavities are formed at both junctions because of the collapse of the PCF air holes. The microcavity with a larger equatorial dimension can excite higher-order cladding modes, so the sensor presents a high RI sensitivity, which can reach 244.16 nm/RIU in the RI range of 1.333–1.3778. Meanwhile it has a low temperature sensitivity of 0.005 nm/°C in the range of 33°C–360°C.

Chinese Optics Letters
Feb. 10, 2017, Vol. 15 Issue 2 020603 (2017)
Air turbulence effects on performance of optical wireless communication with crosstalk in server backplane
D. Elmakias, D. Bykhovsky, and S. Arnon

Free space optical interconnections (FSOIs) are anticipated to become a prevalent technology for short-range high-speed communication. FSOIs use lasers in board-to-board and rack-to-rack communication to achieve improved performance in next generation servers and are expected to help meet the growing demand for massive amounts of inter-card data communication. An array of transmitters and receivers arranged to create an optical bus for inter-card and card-to-backplane communication could be the solution. However, both chip heating and cooling fans produce temperature gradients and hot air flow that results in air turbulence inside the server, which induces signal fading and, hence, influences the communication performance. In addition, the proximity between neighboring transmitters and receivers in the array leads to crosstalk in the received signal, which further contributes to signal degradation. In this Letter, the primary objective is to experimentally examine the off-axis crosstalk between links in the presence of turbulence inside a server chassis. The effects of geometrical and inter-chassis turbulence characteristics are investigated and first-and second-order statistics are derived.

Chinese Optics Letters
Feb. 10, 2017, Vol. 15 Issue 2 020602 (2017)
Generation of passively Q-switched fiber laser at 1  μm by using MoSSe as a saturable absorber
H. Ahmad, S. A. Reduan, S. N. Aidit, and Z. C. Tiu

Passively Q-switched ytterbium-doped fiber lasers (YDFLs) incorporating a molybdenum sulfide selenide (MoSSe)-based saturable absorber (SA) are demonstrated. The modulation depth and saturation intensity of the MoSSe-based SA are measured to be approximately 25.0% and 0.002 MW/cm2, respectively, using the twin detector technique. The YDFL’s output has a center wavelength of 1038.5 nm with a top pulse width and energy of 1.2 μs and 18.9 nJ, respectively, at a pump power of 333 mW. The MoSSe-based SA has a good linear optical response, providing significant opportunity for use in applications over an ultra-broadband spectrum, particularly spectroscopy and biomedical diagnostics.

Chinese Optics Letters
Feb. 10, 2017, Vol. 15 Issue 2 020601 (2017)
Pulse reference-based compensation technique for intensity-modulated optical fiber sensors
Qiang Bian, Zhangqi Song, Yuzhong Chen, and Xueliang Zhang

We propose a compensation technique based on pulse reference for intensity-modulated optical fiber sensors that can compensate the power fluctuation of the light source, the change of optical components transmission loss, and the coupler splitting ratio. The theoretical principle of this compensation technique is analyzed and a temperature sensor based on fiber coating-covered optical microfiber is carried out to demonstrate the compensation effect. The system noise is measured to be 0.0005 dB with the temperature sensitivity reaching 0.063 dB/°C, and the output drift is 0.006 dB in 2 h at room temperature. The output shows a slight variation (0.0061 dB) when the light source and the common light path suffer a 3 dB attenuation fluctuation.

Chinese Optics Letters
Dec. 10, 2017, Vol. 15 Issue 12 120603 (2017)
Real-time measurement of nano-particle size using differential optical phase detection
Qing Li, He Huang, Feng Lin, and Xingkun Wu

We demonstrate a size sensing technique for nano-particles using optical differential phase measurement by a dual fiber interferometer through phase-generated carrier (PGC) demodulation. Nano-particle diameters are obtained from the differential phase shift as a result of adding an optical scattering perturbation into two-beam interference. Polystyrene nano-particles with diameters from 200 to 900 nm in a microfluidic channel are detected using this technique to acquire real-time particle diameters. Compared with amplitude sensing with over 10 mW of laser irradiance, particle sizing by PGC phase sensing can be achieved at a laser power as low as 1.18 mW. We further analyze major sources of noise in order to improve the limits of detection. This sensing technique may find a broad range of applications from the real-time selection of biological cell samples to rare cell detection in blood samples for early cancer screening.

Chinese Optics Letters
Dec. 10, 2017, Vol. 15 Issue 12 120602 (2017)
Performance enhancement of Brillouin optical correlation domain analysis based on frequency chirp magnification
Bin Wang, Xinyu Fan, Jiangbing Du, and Zuyuan He

We propose a method to enhance the performance of Brillouin optical correlation domain analysis (BOCDA) with a broad chirp span of optical sources based on the frequency chirp magnification technique. In BOCDA systems, the number of effective sensing points is proportional to the chirp span of the light source, which is normally limited by the characteristics of the laser diode. We demonstrate a chirp span of 126 GHz with the proposed method, to double the effective sensing points of BOCDA. By combining with differential measurement schemes, a spatial resolution of <10 cm over a 1 km range is achieved.

Chinese Optics Letters
Dec. 10, 2017, Vol. 15 Issue 12 120601 (2017)
Theoretical study of stimulated Raman scattering in long tapered fiber amplifier
Chen Shi, Xiaolin Wang, Pu Zhou, and Xiaojun Xu

A model that is based on the propagation equation and coupled mode theory is introduced in order to describe stimulated Raman scattering (SRS) effects in long tapered fiber amplifiers. Based on the presented model, fiber amplifiers with uniform and long tapered fibers are theoretically and numerically simulated. It can be drawn from the results of our simulations that the long tapered fiber has the advantage in suppressing SRS when applied in fiber laser amplifiers. Our results can provide guidance in the designing of system configuration in long tapered-fiber-based fiber laser systems.

Chinese Optics Letters
Nov. 10, 2017, Vol. 15 Issue 11 110605 (2017)
Adaptive multiple-input multiple-output mode switching for indoor visible light communication system with orthogonal frequency division multiplexing modulation
Xinyue Guo, Xin Li, and Ruyi Huang

We propose and experimentally demonstrate an adaptive multiple-input multiple-output (MIMO) mode switching scheme for an indoor visible light communication system combined with orthogonal frequency division multiplexing modulation. Only requiring 1 bit feedback from the receiver, the MIMO mode at the transmitter switches between spatial multiplexing and transmit diversity adapting to the channel correlation. In such a way, we can take advantage of both spatial multiplexing and transmit diversity, where the spatial multiplexing benefits for its multiplexing gain in the low channel correlation environment, and the transmit diversity is robust to high channel correlation. Experimental results validate the performance improvement over the pure spatial multiplexing or transmit diversity system. With the increasing of the channel correlation, the measured bit error rates of the proposed system are below the 7% pre-forward error correction (pre-FEC) limit of 3.8 × 10 3 when the transmitted data rate is 50 Mb/s, and below the 20% pre-FEC threshold of 5.0 × 10 2 when the transmitted data rate is raised up to 100 Mb/s.

Chinese Optics Letters
Nov. 10, 2017, Vol. 15 Issue 11 110604 (2017)
Cascaded fiber Sagnac loop-based microwave photonic multiband bandpass filter with a selectable passband frequency
Jun Gu, Fei Wang, Youxi Lu, Mengmeng Peng, Lun Shi, and Chang-hee Lee

A passband frequency-electable microwave photonic multiband bandpass filter based on a cascaded fiber Sagnac loop is presented and experimentally demonstrated. A broadband light is sliced by the cascaded fiber Sagnac loop, which serves as a spectrum slicer. After broadband light passes through the spectrum slicer, the sliced broadband light will have the varied periodical spectral characteristics that will cause a transmitted spectral distribution with uniform or multiple periods, then will be modulated by an optical phase modulator and delayed by a dispersive medium. Therefore, a frequency-band-selectable microwave photonic multiband bandpass filter with a suppressed baseband is achieved that can be switched between the single-passband, dual-passband, and triple-passband state. The presented filter exhibits a maximal out-of-band rejection ratio of about 30 dB.

Chinese Optics Letters
Nov. 10, 2017, Vol. 15 Issue 11 110603 (2017)
High OSNR and simple configuration dual-wavelength fiber laser with wide tunability in S+C+L band
Ting Feng, Mingming Wang, Dongliang Ding, and X. Steve Yao

A simple configuration dual-wavelength fiber laser, by combining the first-order Brillouin laser and the residual pump laser, is proposed and experimentally demonstrated. A 1 km long single-mode fiber is used as the stimulated Brillouin scattering gain medium pumped by a narrow linewidth tunable laser source (TLS). Through simply adjusting the TLS output power, power-equalized dual-wavelength lasing can be achieved with a high optical signal to noise ratio (OSNR) of >80 dB. With the good tunability of the TLS, the dual-wavelength fiber laser has a tunable range of ~130 nm, and simultaneously the beat frequency of the two lasing wavelengths can be tuned from 10.1875 to 11.0815 GHz with the tunable range of 0.8940 GHz. The high stability of the dual-wavelength operation is experimentally verified by the measured beat frequency fluctuation of ≤6 MHz in 1 h and power fluctuation of ≤0.03 dB in 2 h. The temporal characteristics of the fiber laser are also investigated experimentally. The fiber laser will find good applications in fiber sensing and microwave photonics areas.

Chinese Optics Letters
Nov. 10, 2017, Vol. 15 Issue 11 110602 (2017)
Performance study of optical triangular-shaped pulse generation with full duty cycle
Ze Hao, Jing Li, Chuangye Wang, and Jin Yuan

A scheme of triangular-shaped pulses with full duty cycle generation is proposed and analyzed. Benefiting from the feature of orthogonal polarization, two approximate sinusoidal signals on different polarization states can be combined without inducing coherent interference. By tuning the time mismatching between the two signals, an approximately triangular-shaped profile can be obtained in the optical intensity. It is found that the modulation index β is no longer a fixed one, but it can be aligned within a proper range of 0.92 to 1.57. To evaluate the proposal, impacts of radio frequency voltage fluctuation, bias voltage drift, and time mismatching are discussed. Within the defined fitting error (η≤3%), the tolerable range of the modulation index, time mismatching, and voltage have been found, which insures a simple operation in practice.

Chinese Optics Letters
Nov. 10, 2017, Vol. 15 Issue 11 110601 (2017)
Four-element division algorithm to focus coherent light through a turbid medium
Longjie Fang, Cheng Zhang, Haoyi Zuo, Jianhua Zhu, and Lin Pang

Aiming to overcome the low converging rate and susceptibility to the environment in focusing the coherent light through the turbid medium, four-element division algorithm (FEDA) optimization is proposed. Full levels of comparisons with the currently employed element-based algorithms, stepwise sequential algorithm (SSA), and continuous sequential algorithm (CSA) show that FEDA only takes one third of the measurement time to find the optimized solution, which means that FEDA is promising in practical applications, such as for deep tissue imaging.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 102901 (2017)
Coordinate difference homogenization matching method for motion correction in 3D range-intensity correlation laser imaging
Liang Sun, Xinwei Wang, Pengdao Ren, Pingshun Lei, Songtao Fan, Yan Zhou, and Yuliang Liu

This Letter proposes a coordinate difference homogenization matching method to solve motion influence in three-dimensional (3D) range-intensity correlation laser imaging. Firstly, features and feature pairs of gate images are obtained by speeded-up robust figures and bi-directional feature matching methods. The original mean value of the feature-pair coordinate differences is calculated. Comparing the coordinate differences with the original mean value, the wrong feature pairs are removed, and then an optimized mean value is updated. The final feature-pair coordinates are re-registered based on the updated mean value. Thus, an accurate transformation is established to rectify motion gate images for 3D reconstruction. In the experiment, a 3D image of a tower at 780 m is successfully captured by our laser gated imaging system on a pan–tilt device.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 102802 (2017)
Imaging process and signal-to-noise ratio improvement of enhanced self-heterodyne synthetic aperture imaging ladar
Guo Zhang, Jianfeng Sun, Yu Zhou, Zhiyong Lu, Guangyuan Li, Guangyu Cai, Mengmeng Xu, Bo Zhang, Chenzhe Lao, Hongyu He, and Liren Liu

This Letter gives the general construction of an enhanced self-heterodyne synthetic aperture imaging ladar (SAIL) system, and proposes the principle of image processing. A point target is reconstructed in the enhanced self-heterodyne SAIL as well as in down-looking SAIL experiments, and the achieved imaging resolution of the enhanced self-heterodyne SAIL is analyzed. The signal-to-noise ratio (SNR) of the point target final image in the enhanced self-heterodyne SAIL is higher than that in the down-looking SAIL. The enhanced self-heterodyne SAIL can improve the SNR of the target image in far-distance imaging, with practicality.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 102801 (2017)
Quantum dot quantum cascade photodetector using a laser structure
Fengjiao Wang, Ning Zhuo, Shuman Liu, Fei Ren, Shenqiang Zhai, Junqi Liu, Jinchuan Zhang, Fengqi Liu, and Zhanguo Wang

We report on a quantum dot quantum cascade detector (QD-QCD), whose structure is derived from a QD cascade laser. In this structure, more ordered InAs QD layers formed in the Stranski–Krastanow growth mode on a thin GaAs buffer layer are incorporated into the active region. This QD-QCD can operate up to room temperature with a peak detection wavelength of 5.8 μm. A responsivity of 3.1 mA/W at 160 K and a detectivity of 3.6 × 108 Jones at 77 K are obtained. The initial performance of the detector is promising, and, by further optimizing the growth of InAs QDs, integrated QD-quantum cascade laser/QCD applications are expected.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 102301 (2017)
Polarization-independent two-dimensional beam steering using liquid crystal optical phased arrays
Liang Wu, Xiangru Wang, Caidong Xiong, Ziqiang Huang, Rusheng Zhuo, Jiarui Rao, and Qinggui Tan

A polarization-independent nonmechanical laser beam steering scheme is proposed to realize continuous two-dimensional (2D) scanning with high efficiency, where the core components are two polarization-dependent devices, which are called liquid crystal optical phased arrays (LC-OPAs). These two one-dimensional (1D) devices are orthogonally cascaded to work on the state of azimuthal and elevation steering, respectively. Properties of polarization independence as well as 2D beam steering are mathematically and experimentally verified with a good agreement. Based on the experimental setup, linearly polarized beams with different polarization angles are steered with high accuracy. The measured angular deviations are less than 5 μrad, which is on the same order of the accuracy of the measurement system. This polarization-independent 2D laser beam steering scheme has potential application for nonmechanical laser communication, lidar, and other LC-based systems.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101601 (2017)
Graphene oxide-COOH as a new saturable absorber for both Q-switching and mode-locking fiber lasers
Fengyan Zhao, Yishan Wang, Yonggang Wang, Hushan Wang, and Yajun Cai

Graphene oxide carboxylic acid (COOH), a novel two-dimensional (2D) layered material with its unique optical and electronic properties, is discovered to exhibit the saturation of optical absorption under laser illumination. Applying the liquid-phase exfoliation method, we prepare graphene oxide-COOH dispersions with deionized water and fabricate graphene oxide-COOH polyvinyl alcohol polymer composite film. We further obtain stable Q-switching pulse and mode-locked laser operation with a 22.7 MHz repetition rate and a 1.5 ps pulse duration by incorporating the graphene oxide-COOH-based saturable absorbers into the all-fiber erbium-doped fiber laser cavity. The experimental results show that the proposed graphene oxide-COOH material can act as an effective absorber for pulsed fiber lasers, which demonstrate potential applications in the area of ultrafast optics.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101402 (2017)
Switchable Brillouin frequency multiwavelength and pulsed fiber laser
S. J. Tan, H. Haris, and S. W. Harun

Switchable single and double Brillouin multiwavelength and pulsed laser is successfully demonstrated. Brillouin spacing can be switched from single (0.08 nm) to double spacing (0.16 nm) or vice versa by swapping the ports of the coupler in the proposed configuration. The proposed configuration can also be used to produce pulsed laser by inserting a home-made carbon nanotubes saturable absorber into the laser cavity. The proposed system is very versatile and flexible as it can be used as a multiwavelength laser or pulsed laser to cater for different types of applications.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101401 (2017)
Design of a grating by a joint optimization method for a phase-shifting point diffraction interferometer
Meng Zheng, Ke Liu, Lihui Liu, and Yanqiu Li

A grating is an important element of a phase-shifting point diffraction interferometer, and the grating constant and duty cycle have a great impact on the interferometer, so the design of a grating becomes significant. In order to measure the projection objective with a numerical aperture of 0.2, we present a joint optimization method of a pinhole and grating based on scalar diffraction and the finite difference time domain method. The grating constant and the film thickness are selected, and the duty cycle of the grating is optimized. The results show that in the grating processing the material chromium is adopted, the thickness is 200 nm, and the grating constant is 15 μm. When the duty cycle is 55%, the interference fringe contrast is the greatest. The feasibility of the design result is further verified by experiment.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101203 (2017)
Using single wavelength light to improve the synchronization accuracy of the White Rabbit system
Xu Yuan, and Bo Wang

We demonstrate a new synchronization method for the White Rabbit system. Signals are transmitted in a single mode fiber in both directions with the same light wavelength. Without the complex calibration process of the fiber asymmetry parameter, the new method reduces the effect of chromatic dispersion and improves the synchronization accuracy. The experiment achieves timing synchronization accuracy below 200 ps over 50 km fiber constructed by different companies’ fiber spools. The proposed method would make White Rabbit technology immune to the chromatic dispersion of fiber links and can be applied to long distance synchronization.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101202 (2017)
Large-range displacement measurement using sinusoidal phase-modulating laser diode interferometer
Ming Zhang, Chang Ni, Yu Zhu, Leijie Wang, Chuxiong Hu, and Jinchun Hu

A signal processing method of realizing a large-range displacement measurement in a sinusoidal phase-modulating laser diode interferometer is proposed. The method of obtaining the dynamic value of the effective sinusoidal phase-modulating depth is detailed, and the residual amplitude modulation is also taken into account. Numerical simulations and experiments are carried out to compare this method with the traditional one. We prove that, with this method, the sinusoidal phase-modulating laser diode interferometer can realize a centimeter-level displacement measurement range with high precision, which is much better than the traditional method.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101201 (2017)
Compressed sensing in synthetic aperture photoacoustic tomography based on a linear-array ultrasound transducer
Xiangwei Lin, Naizhang Feng, Yawei Qu, Deying Chen, Yi Shen, and Mingjian Sun

Photoacoustic tomography (PAT) has the unique capability of visualizing optical absorption inside several centimeters-deep biological tissue with a high spatial resolution. However, single linear-array transducer-based PAT suffers from the limited-view challenge, and thus the synthetic aperture configuration is designed that still requires multichannel data acquisition hardware. Herein, a feasible synthetic aperture PAT based on compressed sensing reconstruction is proposed. Both the simulation and experimental results tested the theoretical model and validated that this approach can improve the image resolution and address the limited-view problem while preserving the target information with a fewer number of measurements.

Chinese Optics Letters
On the CoverOct. 10, 2017, Vol. 15 Issue 10 101102 (2017)
Simple and effective method to improve the signal-to-noise ratio of compressive imaging
Yao Zhao, Qian Chen, Guohua Gu, and Xiubao Sui

This Letter presents a simple and effective method to improve the signal-to-noise ratio (SNR) of compressing imaging. The main principles of the proposed method are the correlation of the image signals and the randomness of the noise. Multiple low SNR images are reconstructed firstly by the compressed sensing reconstruction algorithm, and then two-dimensional time delay integration technology is adopted to improve the SNR. Results show that the proposed method can improve the SNR performance efficiently and it is easy to apply the algorithm to the real project.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 101101 (2017)
Minimizing the effects of unmodulated light and uneven intensity profile on the holographic images reconstructed by pixelated spatial light modulators
Jhen-Si Chen, Jia Jia, and Daping Chu

A simple and effective approach is proposed to minimize the effect of unmodulated light and uneven intensity caused by the pixelated structure of the spatial light modulator in a holographic display. A more uniform image is produced by purposely shifting the holographic images of multiple reconstructed lights with different incident angles from the zero-diffraction-order and overlapping those selected different orders. The simulation and optical experimental results show that the influence of the zero-diffraction-order can be reduced, while keeping the good uniformity of the target images by this new approach.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 100901 (2017)
Self-seeded quantum-dash laser based 5  m–128  Gb/s indoor free-space optical communication
M. A. Shemis, A. M. Ragheb, E. Alkhazraji, M. A. Esmail, H. Fathallah, S. Alshebeili, and M. Z. M. Khan

We demonstrate an indoor 5 m free-space optical wireless coherent communication in mid L-band (1606.7 nm) by employing a tunable self-seeded InAs/InGaAlAs/InP quantum-dash (Qdash) laser as a subcarrier generator for 128 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) modulation signal. The bare Qdash laser diode displays ~6 nm self-locked Fabry–Perot mode tunability with ~30 dB side mode suppression ratio (SMSR) and ~10 dBm mode power across the tuning range, thus encompassing ~10 modes with an achievable capacity of 1.28 Tb/s (10×128 Gb/s) and potentially qualifying the source requirements for future access networks.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 100604 (2017)
Radio-frequency arbitrary waveform generation based on dispersion compensated tunable optoelectronic oscillator with ultra-wide tunability
Anle Wang, Jianghai Wo, Jin Zhang, Xiong Luo, Xin Xu, Daoming Zhang, Pengfei Du, and Lan Yu

Photonic generation of radio-frequency (RF) arbitrary microwave waveform with ultra-wide frequency tunable range based on a dispersion compensated optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. Dispersion compensation scheme and specially designed fiber Bragg grating (FBG)-based Fabry–Perot (F-P) filters are employed in the OEO loop to realize a frequency tunable range of 3.5–45.4 GHz. An optimization process provided by the combination of an erbium-doped fiber amplifier (EDFA) and FBG is employed to improve the signal-to-noise ratio (SNR) of final RF signals. The generation of linear-frequency and phase-coded microwave waveforms, with a tunable carrier frequency ranging from 4 to 45 GHz and tuned chirping bandwidths or code rates, is experimentally demonstrated.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 100603 (2017)
Femtoseconds soliton mode-locked erbium-doped fiber laser based on nickel oxide nanoparticle saturable absorber
A. Nady, M. H. M. Ahmed, A. A. Latiff, C. H. Raymond Ooi, and S. W. Harun

We demonstrate a femtosecond mode-locked erbium-doped fiber laser (EDFL) using a nickel oxide (NiO) as a saturable absorber (SA). NiO nanoparticles are hosted into polyethylene oxide film and attached to fiber ferrule in the laser cavity. The NiO-SA shows a 39% modulation depth with a 0.04 MW/cm2 saturation intensity. Our ring laser cavity based on erbium-doped active fiber with managed intracavity dispersion has the ability to generate ultrashort pulses with a full width at half-maximum (FWHM) of around 2.85 nm centered at 1561.8 nm. The pulses repeat at a frequency of 0.96 MHz and duration of 950 fs.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 100602 (2017)
Tunable single-longitudinal-mode SOA-based fiber laser based on the spectral narrowing effect in a dispersion-shifted fiber
Zhenkun Shen, Lin Wang, Yuan Cao, Xudong Wang, Xinhuan Feng, and Bai-Ou Guan

A tunable single-longitudinal-mode (SLM) semiconductor optical amplifier (SOA)-based fiber laser based on a dispersion-shifted fiber (DSF) is proposed and successfully demonstrated. SLM operation is obtained due to the spectral narrowing effect resulting from inverse four-wave mixing in a DSF. A tunable optical filter performs wavelength selection function. By inserting a length of DSF in the laser cavity, SLM lasing can possibly be obtained when laser oscillation is stably established after traveling through the DSF many roundtrips. Stable tunable SLM oscillation with a signal-to-noise ratio as high as 65 dB over a wavelength range of about 35 nm is achieved experimentally, and each spectral linewidth is less than 6.5 kHz.

Chinese Optics Letters
Oct. 10, 2017, Vol. 15 Issue 10 100601 (2017)
Investigation of clustering effects on erbium-doped fiber laser performance
Md. Ziaul Amin, and Khurram Karim Qureshi

To design a compact erbium-doped fiber laser, a high-concentration erbium-doped fiber (EDF) is needed. However, increasing the erbium ion (Er3+) concentration can reduce the EDF performance via the Er3+-Er3+ interaction. In this Letter, we investigate the Er3+-Er3+ interaction effect by designing a tunable erbium-doped fiber-ring laser (EDFRL). This is the first time (to the best of our knowledge) that someone has considered different numbers of ions per cluster and simulated the EDFRL output power degradation due to ion–ion interaction. If the number of ions in the cluster is increased, the lasing output power will decrease accordingly. The most dominant effect is seen in the 1530 nm wavelength region, where the EDF shows a higher signal absorption compared to the other wavelength region. Moreover, a comparison has been done for lasing performance analysis with different dopant ion concentrations. The comparison results show that a higher dopant concentration is advantageous for longer-wavelength lasing.

Chinese Optics Letters
Jan. 10, 2017, Vol. 15 Issue 1 010601 (2017)
Adaptive modulation system for liquid crystal phase modulator
Bin Zhang, Shiyu Liu, Xianzhu Tang, and Jian'gang Lu

An adaptive modulation system for a liquid crystal (LC) phase modulator is demonstrated. The phase retardation may be modulated by resetting the driving voltage automatically by matching the measured and ideal transmittance of an LC cell sandwiched by crossed polarizers. By using this system, an LC phase modulator can get a low error function of 0.25% in a short modulation time, which is less than the 10% obtained using a conventional modulation method.

Chinese Optics Letters
Sep. 10, 2016, Vol. 14 Issue 9 090604 (2016)
Simultaneous optical signal to noise ratio and symbol rate estimation with blind chromatic dispersion compensation for auxiliary amplitude modulation optical signals
Junyu Wei, Jiangyi Qin, Zhiping Huang, and Xiaojun Guo

Based on the peak to valley ratio (PTVR) of the average magnitude difference function (AMDF), we present a novel optical signal to noise ratio (OSNR) and symbol rate (SR) estimation method for commonly used auxiliary amplitude modulations (AAMs). Moreover, it is demonstrated that the influence of chromatic dispersion (CD) on the method can be mitigated by maximizing the PTVR of the AMDF with additional tunable dispersion compensators. The results of simulations show that the OSNR estimation error can be kept within 0.8 dB in the wide OSNR range of (12, 32) dB, while the SR estimation error is below 0.079% for four widely used 10 Gsymbol/s AAM signals.

Chinese Optics Letters
Sep. 10, 2016, Vol. 14 Issue 9 090603 (2016)
Indoor imaging visible light positioning with sampled sparse light source and mobile device
Heqing Huang, Lihui Feng, Guoqiang Ni, and Aiying Yang

Indoor visible light positioning becomes attractive due to the increasing demands of location-based services. This Letter proposes an indoor imaging visible light positioning scheme with a sampled sparse light source, image sensor, and gyro. An indoor positioning cellular with a single reference light source and an off-the-shelf mobile device is demonstrated. Experimental results show that the 3-dimensional positioning error is only several centimeters even with a rotated, rolled, and pitched mobile device. The proposed scheme is convenient and cost effective because the transmitter takes advantage of the existing lighting infrastructure and the receiver is a commercial mobile phone without any extra accessories.

Chinese Optics Letters
Sep. 10, 2016, Vol. 14 Issue 9 090602 (2016)
Non-linear polarization orthogonality loss in a semiconductor optical amplifier
Wei Zhang, Zhengyong Li, Haiyang Wang, Xiangkong Zhan, Zhongbo Song, and Chongqing Wu

Polarization-based optical communications are attracting more attention recently, where the crucial points are polarization features and their measurements. Based on the Müller matrix method, we obtain measurable expressions for the polarization-dependent gain (PDG) and the loss of polarization orthogonality (LPO), while give the boundary of the LPO for any PDG devices. We experimentally demonstrate that non-linear LPO can be created in a semiconductor optical amplifier and find that the LPO will slightly skim over the boundary near the threshold of the injected current. Furthermore, an empirical formula is achieved to gauge the LPO-induced power penalty, which is proven to be valid in differential polarization shift-keying transmission by executing a bit error rate measurement. Our conclusions are applicable to non-orthogonal polarization cases and valuable to polarization-related communications, even orbital angular momentum multiplexing.

Chinese Optics Letters
Sep. 10, 2016, Vol. 14 Issue 9 090601 (2016)
Proposal of an InP-based few-mode transmitter based on multimode interference couplers for wavelength division multiplexing and mode division multiplexing applications
Zhaosong Li, Dan Lu, Bing Zuo, Song Liang, Xuliang Zhou, and Jiaoqing Pan

An InP-based monolithically integrated few-mode transmitter aiming at the combination of wavelength division multiplexing (WDM) and mode division multiplexing (MDM) technologies is proposed. The core elements of the proposed transmitter are mode converters and a wavelength-mode division multiplexer that are all based on multimode interference (MMI) couplers. Simulations show that the wavelength-mode division multiplexer has a large fabrication tolerance of 30 and 0.5 μm for the length and the width of the device, respectively. A low loss below 0.26 dB for the passive parts of the transmitter is obtained in the whole C-band wavelength range.

Chinese Optics Letters
Aug. 10, 2016, Vol. 14 Issue 8 080601 (2016)
Simultaneous frequency transfer and time synchronization over a 430  km fiber backbone network using a cascaded system
Qin Liu, Shenglong Han, Jialiang Wang, Zitong Feng, Wei Chen, Nan Cheng, Youzhen Gui, Haiwen Cai, and Shensheng Han

Based on dense wavelength-division multiplexing technology, frequency transfer and time synchronization are simultaneously realized over a compensated cascaded fiber link of 430 km, which is a part of the Beijing–Shanghai optical fiber backbone network. The entire cascaded system consists of two stages with fiber links of 280 and 150 km, respectively. To keep high symmetry and low noise, specific bi-directional erbium-doped fiber amplifiers are used to compensate the large optical attenuation of each fiber link. When the compensation servo is active in every stage, the cascaded system achieves the stability of 1.94×10 13 at 1 s and 1.34×10 16 at 104 s, for frequency transfer. It is also verified that the actual results of the cascaded system are in good agreement with the theoretical ones calculated from error theory. Simultaneously, after calibration of each stage, time synchronization is also realized. The final accuracy of the whole system is within 94 ps.

Chinese Optics Letters
Jul. 10, 2016, Vol. 14 Issue 7 070602 (2016)
United block sequence mapping-based visible light positioning for dense small cell deployment
Runmei Zhao, Zhitong Huang, Yuxin Liu, and Yuefeng Ji

Visible light positioning (VLP) is an emerging candidate for indoor positioning, which can simultaneously meet the requirements for accuracy, cost, coverage area, and security. However, intercell interference caused by light intensity superposition limits the application of VLP. In this Letter, we propose a united block sequence mapping (UBSM)-based VLP that utilizes superposition to integrate the multidimensional information from dense small cells into 2D information. The experimental result shows that UBSM-based VLP can achieve an accuracy of 1.5 cm with a 0.4 m row spacing and 0.35 m column spacing of LED lights.

Chinese Optics Letters
Jul. 10, 2016, Vol. 14 Issue 7 070601 (2016)
Reduction of FWM noise in WDM-based QKD systems using interleaved and unequally spaced channels
Yongmei Sun, Yishan Lu, Jianing Niu, and Yuefeng Ji

To extensively deploy quantum key distribution (QKD) systems, copropagating with classical channels on the same fiber using wavelength division multiplexing (WDM) technology becomes a critical issue. We propose a user-based channel-interleaving WDM scheme with unequal frequency spacing (UFS-iWDM) to reduce the impairment on the quantum channels induced by four-wave mixing (FWM), and theoretically analyze its impact on quantum bit error rate (QBER). Numerical simulation results show that a UFS-iWDM can significantly reduce the FWM noise and improve QBER compared with the corresponding WDM scheme with equal frequency spacing (EFS), especially in the case of nonzero dispersion shifted fiber.

Chinese Optics Letters
Jun. 10, 2016, Vol. 14 Issue 6 060602 (2016)
Sidelobe suppression analysis of microwave photonic filter based on spectrum-shaped optical frequency combs
Yang Yu, Shangyuan Li, Xiaoping Zheng, Hanyi Zhang, and Bingkun Zhou

A finite impulse-response microwave photonic filter is typically achieved based on spectrum-shaped optical frequency combs and a dispersive element. We propose an analytical model to describe the amplitude responses of the sidelobes. The model shows that the sidelobe suppression ratio is limited by the spectrum structure of the optical combs. By taking Gaussian-profiled combs as an example, it is both theoretically and experimentally proved that the suppression ratio can be improved by optimizing the spectral power range, which is defined as the ratio of the maximum tap weight to the minimum tap weight.

Chinese Optics Letters
Jun. 10, 2016, Vol. 14 Issue 6 060601 (2016)
Direct-detection OFDM-QPSK system performance improvement enabled by DFT spread and receiver-based ISFA algorithm
Jie Su, Xinying Li, and Jianjun Yu

We experimentally demonstrate a direct-detection orthogonal-frequency-division-multiplexing quadrature-phase-shift-keying (OFDM-QPSK) system that is capable of delivering a 32 Gbaud OFDM-QPSK signal over 7 km single-mode fiber-28 (SMF-28). Intra-symbol frequency-domain averaging (ISFA) channel response estimation is applied to suppress in-band noise, while discrete Fourier transform-spread (DFT-spread) is used to reduce the peak-to-average power ratio (PAPR) of the transmitted OFDM signal. With the aid of ISFA-based channel estimation and PAPR reduction enabled by DFT-spread, the bit-error ratio of the system after 7 km SMF-28 transmission can be improved from 2×10 3 to error-free when the received optical power is 8.5 dBm.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050608 (2016)
Multi-hop relay-based maritime visible light communication
Hyeong-Ji Kim, Samrat Vikramaditya Tiwari, and Yeon-Ho Chung

This Letter presents a multi-hop relay visible light communication (VLC) system for maritime applications. Maritime VLC systems suffer from limited coverage distance due inherently to the usage of light-emitting diodes and photodetectors. The proposed system employs a multiple of decode-and-forward relays to extend coverage distance in maritime environments. The multi-hop relay based maritime VLC is analyzed under a maritime channel modeled by the JONSWAP spectrum and gamma–gamma distribution. It is found that the use of relays in maritime environments can extend the coverage distance significantly and also improve the performance. In addition, the performance of the system is analyzed using various combining techniques at the receiver to enhance the performance. The maximal ratio combining technique is found to provide superior link quality in maritime environments.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050607 (2016)
Temperature-compensated dual-polarization fiber grating laser sensors
Jianming Zhu, Long Jin, Yizhi Liang, Linghao Cheng, and Bai-Ou Guan

We report on temperature compensation for beat-frequency-encoded dual-polarization fiber laser sensors based on a cleave-rotate-splice method. By cleaving the laser cavity into two segments with comparable lengths, aligning them with a rotated angle of 90°, and then fusion splicing the two halves, the temperature sensitivity in terms of beat-frequency variation can be greatly reduced from 1.99 to 0.30 MHz/°C (or by 84.9%). In contrast, the sensitivity to point loaded mass hardly changes. We also find that the beat-frequency fluctuation decreases from ±30 to ±25 kHz as a result of the temperature compensation.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050606 (2016)
Coherent ONU based on 850  μm-long cavity-RSOA for next-generation ultra-dense access network
Guang Yong Chu, Adolfo Lerín, Iván N. Cano, Victor Polo, and Josep Prat

In this Letter, an efficient bidirectional differential phase-shift keying (DPSK)—DPSK transmission for a ultra-dense wavelength division-multiplexed passive optical network is proposed. A single distributed feedback laser at the optical network unit (ONU) is used both as the local laser for downlink coherent detection and the optical carrier for uplink. Phase-shift keying is generated using a low-cost reflective semiconductor optical amplifier (RSOA) at the ONU. The RSOA chip has the bandwidth of 4.7 GHz at the maximum input power and bias current. For uplink transmission, the sensitivity of the RSOA chip reaches 48.2 dBm at the level of bit error rate=10 3 for back-to-back, and the penalty for 50 km transmission is less than 1 dB when using polarization diversity.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050605 (2016)
Refractive index and temperature sensing characteristics of an optical fiber sensor based on a tapered single mode fiber/polarization maintaining fiber
Zaihang Yang, Hao Sun, Tingting Gang, Nan Liu, Jiacheng Li, Fei Meng, Xueguang Qiao, and Manli Hu

An evanescent field optical fiber sensor based on a short section of polarization maintaining fiber spliced with a tapered single mode fiber is proposed and experimentally investigated. We mainly focus on the refractive index (RI) and temperature sensing characteristics of this compact device. The transmission spectrum of the resonance wavelength, induced by the interference between the excited low order cladding modes and core modes, shows the quadratic function relationships with RI and linear relationships with temperature. Thus, the proposal of this simple-to-fabricate, compact, and low cost sensor shows its possible potential in the sensitive detection field.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050604 (2016)
Combined nonlinear effects for UV to visible wavelength generation in a photonic crystal fiber
Binbin Yan, Jinhui Yuan, Xinzhu Sang, Kuiru Wang, and Chongxiu Yu

We experimentally study the combined nonlinear effects, including four-wave mixing, stimulated Raman scattering, soliton dynamics, and cross-phase modulation by coupling femtosecond pulses around 850 nm into the normal dispersion region near the zero-dispersion wavelength in the fundamental mode of a homemade silica photonic crystal fiber. The nonlinear optical dynamics at different stages are demonstrated, and the discrete ultraviolet (UV) to visible wavelengths widely separated from the pump wave are generated by the interaction of several nonlinear effects involved. The UV to visible wavelengths can be used as short pulse sources for multiphoton ionization, fluorescence spectroscopy, and biochemical imaging.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050603 (2016)
Dual-polarization fiber grating laser-based laser Doppler velocimeter
Zeyuan Kuang, Linghao Cheng, Yizhi Liang, Hao Liang, and Bai-Ou Guan

A dual-frequency laser Doppler velocimeter implemented by a dual-polarization fiber grating laser is proposed, with the two laser frequencies produced by the two orthogonally polarized laser outputs of the fiber grating laser. The reflected laser outputs from a moving target experience the Doppler frequency shift, which is shown to be linearly related to the velocity and the beat note frequency difference between the laser outputs and the reflected light. With a digital frequency demodulation scheme, a high sensitivity of 0.64 MHz/(m/s) and a velocity resolution of less than 0.5% of the velocity for velocity measurement are demonstrated, which shows that the proposed laser Doppler velocimeter is capable of measurement of wide range of velocity.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050602 (2016)
Fiber in-line Fabry–Pérot interferometer for simultaneous measurement of reflective index and temperature
Xiaoqi Ni, Ming Wang, and Dongmei Guo

A fiber in-line Fabry–Pérot interferometer is presented. The sensing head consists of a micro ellipsoidal air cavity and a small section of solid-core photonic crystal fiber. The reflective index (RI) and temperature can be interrogated simultaneously through a fast Fourier transform and by tracing the dip wavelength shift of the reflective spectrum. Experimental results show that the RI amplitude and wavelength sensitivities are 5.30/RIU and 8.46×10 1 nm/RIU in the range from 1.34 to 1.43, and the temperature amplitude and wavelength sensitivities are 6.8×10 4/°C and 2.48×10 3 nm/°C in the range from 15°C to 75°C, respectively. Easy fabrication, a simple system, and simultaneous measurement make it appropriate for dual-parameter sensing application.

Chinese Optics Letters
May. 10, 2016, Vol. 14 Issue 5 050601 (2016)
Flexible multidimensional modulation formats based on PM-QPSK constellations for elastic optical networks
Zilong He, Wentao Liu, Bailin Shen, Xue Chen, Xiqing Gao, Sheping Shi, Qi Zhang, Dongdong Shang, Yongning Ji, and Yingfeng Liu

We demonstrate flexible multidimensional modulation formats, polarization multiplexed k-symbol check quadrature phase shift keying (PM-kSC-QPSK), based on PM-QPSK constellations for elastic optical networks. The experimental results show a significant optical signal noise ratio (OSNR) tolerance improvement for PM-2SC-QPSK and PM-4SC-QPSK over PM-QPSK in both back-to-back and 500 km transmission scenarios at the expense of spectral efficiency reduction. This flexible modulation method can be used in elastic optical networks to provide a trade-off between the spectral efficiency and OSNR tolerance.

Chinese Optics Letters
Apr. 10, 2016, Vol. 14 Issue 4 040602 (2016)
Microwave photonic phase shifter with spectral separation processing using a linear chirped fiber Bragg grating
Wensheng Zhai, Xinlu Gao, Wenjing Xu, Mingyang Zhao, Mutong Xie, Shanguo Huang, and Wanyi Gu

A stable and broadband microwave photonic phase shifter based on the combined use of a linear chirped fiber Bragg grating and optical single-sideband (OSSB) modulation is proposed and experimentally demonstrated. The quality of the radio frequency (RF) signal is improved by the spectral separation delay processing. The theoretical fundamentals of the scheme are explained and the phase shift can be controlled linearly by the wavelength of the light source. In the experiment, a full 360° phase shift with a 10 GHz bandwidth can be achieved and tuned dynamically, continuously, and stably.

Chinese Optics Letters
Apr. 10, 2016, Vol. 14 Issue 4 040601 (2016)
100  Gb/s all-optical clock recovery based on a monolithic dual-mode DBR laser
Biwei Pan, Liqiang Yu, Lu Guo, Limeng Zhang, Dan Lu, Xin Chen, Yue Wu, Caiyun Lou, and Lingjuan Zhao

We experimentally demonstrate all-optical clock recovery for 100 Gb/s return-to-zero on–off keying signals based on a monolithic dual-mode distributed Bragg reflector (DBR) laser, which can realize both mode spacing and wavelength tuning. By using a coherent injection locking scheme, a 100 GHz optical clock can be recovered with a timing jitter of 530 fs, which is derived by an optical sampling oscilloscope from both the phase noise and the power fluctuation. Furthermore, for degraded injection signals with an optical signal-to-noise ratio as low as 4.1 dB and a 25 km long distance transmission, good-quality optical clocks are all successfully recovered.

Chinese Optics Letters
Mar. 10, 2016, Vol. 14 Issue 3 030604 (2016)
Simultaneous refractive index and temperature measurements by using dual interference in an all-fiber Mach–Zehnder interferometer
Xu Yan, Haiwei Fu, Huidong Li, and Xueguang Qiao

A Fourier analysis applied to the Mach–Zehnder interferometer (MZI) transmission spectrum for simultaneous refractive index (RI) and temperature measurements is proposed and experimentally demonstrated in this Letter. In the fast Fourier transform (FFT) spectrum of the MZI transmission spectrum, several frequency components are generally observed, which means that the transmission spectrum of the MZI is formed by the superposition of some dual-mode interference (DMI) spectra, and each frequency component represents different core-cladding interferences. We can select some dominant frequency components in the FFT spectrum of the MZI transmission spectrum to take the inverse FFT (IFFT). Then, the corresponding DMI patterns can be obtained. Due to the shift of the wavelength of these DMI spectra with changes in the environmental parameters, we can use the coefficient matrix of these DMI spectra for multi-parameter sensing. In this Letter, two DMI patterns are separated from the resultant transmission spectrum of the MZI. As the RI and temperature change, the shifts of the two DMI patterns with respect to the RI and temperature will be observed. The sensitivities of the RI and temperature are 137.1806 nm/RIU (RI unit) and 0.0860 nm/°C, and 22.9955 nm/RIU and 0.0610 nm/°C for the two DMIs. Accordingly, it can be used to simultaneously measure RI and temperature changes. The approach can eliminate the influence of multiple interferences and improve the accuracy of the sensor.

Chinese Optics Letters
Mar. 10, 2016, Vol. 14 Issue 3 030603 (2016)
Dual-output modulation in time-wavelength interleaved photonic analog-to-digital converter based on actively mode-locked laser
Huajie Zhang, Weiwen Zou, Guang Yang, and Jianping Chen

This Letter demonstrates the application of dual-output modulation in a photonic analog-to-digital converter (PADC) with a high sampling rate and resolution. The PADC is time-wavelength interleaved and based on an actively mode-locked laser. According to theoretical analysis, the dual-output PADC system shows a better linearity for achieving a higher dynamic range. In the experiment, third-order distortion is significantly suppressed by ~40 dB when the dual-output modulator is used and the effective number of bits of the PADC has reached 9.0 bits below 0.2 GHz and 6.4 bits at 6.1 GHz in our PADC with a sampling rate of 20 GS/s.

Chinese Optics Letters
Mar. 10, 2016, Vol. 14 Issue 3 030602 (2016)
Modeling and analyzing the temperature sensitivity of radiation-induced attenuation in a Ge-P co-doped fiber
Jing Jin, Chunjing Liu, Jixun Liu, and Yunxia Hou

A model for the temperature sensitivity of radiation-induced attenuation (RIA) is investigated. The RIA spectra in a germanium (Ge) and phosphorous (P) co-doped fiber ranging from 825 to 1600 nm at different temperatures are measured and decomposed according to the configurational coordinate model. It is found that there is a linear relationship between the parameters of the color center absorption band and temperature. The model is verified at 850, 1310, and 1550 nm by both simulation and experiment. This work will be useful to research on the applications of optical fiber sensors in a complicated space environment.

Chinese Optics Letters
Mar. 10, 2016, Vol. 14 Issue 3 030601 (2016)
Investigating the light stability of solid-solution-based AgCl-AgBr and AgBr-TlI crystals
Alexandr S. Korsakov, Alexandr E. Lvov, Dmitry S. Vrublevsky, and Liya V. Zhukova

For the development of mid-infrared fiber-optical elements, one needs light-stable, flexible materials that are transparent within this spectral band. Solid solutions of silver and monadic thallium halides prove to be the most suitable crystalline media for these needs. We study the light stability variation of high-purity AgCl1 xBrx (0≤x≤1) and Ag1 xTlxBr1 xIx (0≤x≤0.05) crystals by measuring the optical transmission change as functions of the composition and UV exposure time. The former is executed in a broad spectral range from 6500 350 cm 1, within which we choose three wavelengths to trace the transmission change. For thallium-monoiodide-containing samples, an effect is observed that we assumed to be translucence.

Chinese Optics Letters
Feb. 10, 2016, Vol. 14 Issue 2 020603 (2016)
Temperature-insensitive refractive index sensor based on an optical fiber extrinsic Fabry–Perot interferometer processed by a femtosecond laser
Pengfei Liu, Lan Jiang, Sumei Wang, Zhitao Cao, and Peng Wang

An optical fiber extrinsic Fabry–Perot interferometer (EFPI) is designed and fabricated for refractive index (RI) sensing. To test the RI of liquid, the following two different methods are adopted: the wavelength tracking method and the Fourier-transform white-light interferometry (FTWLI). The sensitivities of sensors with cavity lengths of 288.1 and 358.5 μm are 702.312 nm/RIU and 396.362 μm/RIU, respectively, by the two methods. Our work provides a new kind of RI sensor with the advantages of high sensitivity, mechanical robustness, and low cross sensitivity to temperature. Also, we provide a new method to deal with gold film with a femtosecond laser.

Chinese Optics Letters
Feb. 10, 2016, Vol. 14 Issue 2 020602 (2016)
Pilot-added carrier-phase recovery scheme for Nyquist M-ary quadrature amplitude modulation optical fiber communication system
Wenbo Zhang, Dongwei Pan, Xiaofei Su, Xiaoguang Zhang, Lixia Xi, and Xianfeng Tang

A new two-stage carrier-phase recovery scheme using a combination of an optical pilot-aided algorithm with the crossed constellation transformation algorithm for either square-framed or non-square-framed M-level-quadrature amplitude modulation (QAM) Nyquist systems is proposed. It is verified in 32- and 128-QAM systems that it can provide high linewidth tolerance with little complexity.

Chinese Optics Letters
Feb. 10, 2016, Vol. 14 Issue 2 020601 (2016)
750  Mb/s monochromatic LED-based real-time visible light communication system employing a low-complexity cascaded post-equalizer
Jiabin Luo, Yi Tang, Huiping Jia, Qingwei Zhu, and 唯 薛

We experimentally demonstrate a 750 Mb/s real-time visible light communication (VLC) system based on non-return-to-zero on–off keying modulation by employing single commercially available monochromatic light-emitting diodes. To enhance the 3-dB bandwidth of the VLC link, we propose a low-complexity cascaded post-equalizer based on NPN transistors. With two different frequency-selecting networks in our post-equalizer, the highest achieved 3-dB bandwidth of the VLC link is 370 MHz. The highest achieved data rate is 750 Mb/s at a communication distance of 170 cm with a bit error rate below 1×10 6, which is far below the forward error correction limit (3.8×10 3). Based on our monochromatic VLC system, a wavelength-division multiplexing VLC system could be designed with a higher data rate.

Chinese Optics Letters
Dec. 10, 2016, Vol. 14 Issue 12 120604 (2016)
1.5  μm, 8 × 12.5  Gb/s of hybrid-integrated TOSA with isolators and ROSA for 100  GbE application
Zeping Zhao, Yu Liu, Zhike Zhang, 向飞 陈, Jianguo Liu, and Ninghua Zhu

Compact transmitter and receiver optical sub-assemblies (TOSA and ROSA) are fabricated in our laboratory and have an aggregated capacity of 100 Gb/s. Specially, directly modulated laser (DML) drivers with two layers of electrical circuit boards are designed to inject RF signals and bias currents separately. For all the lanes, the 3 dB bandwidth of the cascade of the TOSA and ROSA exceeds 9 GHz, which allows the 12.5 Gb/s operation. With the 12.5 Gb/s × 8-lane operation, clear eye diagrams for back-to-back and 30-km amplified transmission with a dispersion compensation fiber are achieved. Low cost and simple processing technology make it possible to realize commercial production.

Chinese Optics Letters
Dec. 10, 2016, Vol. 14 Issue 12 120603 (2016)
Acoustic emission detection using intensity-modulated DFB fiber laser sensor
Tan Yang, Ying Song, Wentao Zhang, and Fang Li

The bonded distributed feedback (DFB) fiber laser (FL) acoustic emission sensor and the intensity response of the DFB-FL to external acoustic emissions are investigated. The dynamic sensitivity of the DFB-FL is calibrated by a referenced piezoelectric receiver. In the DFB-FL we used here, the minimum detectable signal is 2×10 6 m/s at 5 kHz. Using wavelet packet technology, the collected signals are analyzed, which confirms that an intensity-modulated DFB-FL sensor can be used to detect acoustic emission signals.

Chinese Optics Letters
Dec. 10, 2016, Vol. 14 Issue 12 120602 (2016)
Multilayer-core fiber with a large mode area and a low bending loss
Youchao Jiang, Guobin Ren, Yudong Lian, Yu Liu, Huaiqing Liu, Haisu Li, Wenhua Ren, Wei Jian, and Shuisheng Jian

We present a single-mode multilayer-core fiber with a large mode area (LMA) and a low bending loss in this Letter. A low equivalent core-cladding refractive index difference is achieved by exploiting the multilayer structure. The multilayer structure has a better bending performance than a traditional step-index core and this structure also contributes to realizing different curved refractive index profiles that have a better bending performance. An index trench is also introduced to dramatically reduce the bending loss. The experimental results show that, at a wavelength of 1550 nm, the mode area of the fabricated fiber is about 215.5 μm2 and the bending loss is 0.58 dB/turn at a 10 mm bending radius. The LMA and excellent bending performance can be obtained simultaneously with the proposed fiber.

Chinese Optics Letters
Dec. 10, 2016, Vol. 14 Issue 12 120601 (2016)
Inter-satellite range-finding method with high precision and large range based on optoelectronic resonance
Bin Chen, Jinlong Yu, Ju Wang, Tianyu Li, Wenrui Wang, Yang Yu, and Tianyuan Xie

As microsatellite technology develops, precise inter-satellite measurement shows its significance in distributed satellite systems. In this Letter, a novel technique is proposed for measuring the inter-satellite distance, which adopts optoelectronic resonance and has a function of self-referencing. Resonance cavities have a high spectral purity and a high oscillation frequency. By utilizing the accumulative amplification principle to convert the measurement of distance to that of the frequency, high accuracy is achieved. In the experiment, this measuring scheme has a large measuring range between 1 and 6 km (can be potentially larger), and the accuracy is better than 1.5 μm. The relative accuracy reaches the level of 10 10.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110608 (2016)
Free-space optical communication using patterned modulation and bucket detection
Tianyi Mao, Qian Chen, Weiji He, Yunhao Zou, Huidong Dai, and Guohua Gu

In this Letter, free-space optical (FSO) communication using patterned modulation and bucket detection is introduced to improve the bit error rate (BER) performance in complex and noisy environments. The scattered light is averaged in this communication structure. Second-order correlation, wavelet normalization, and compressed sensing are combined in the reconstruction algorithm. A signal with N bits is reconstructed well from much less than N measurements. Numerical simulations and experiments are performed without the narrowband optical filters used in traditional FSO communication. It can also be employed in real networks where secure communication is required. This provides the great opportunity to pave the way for real applications of FSO communication.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110607 (2016)
Circularly polarizing fibers with a triple-lobe stress region
Yueyu Xiao, Jinkui Yan, Lei Peng, and Mingjue Tang

A fiber structure with a circularly polarizing property is proposed. The fiber consists of a round core and a triple-lobe stress region in the cladding, which is compatible with single-mode fibers, and it can be fabricated using conventional metal chemical vapor deposition processes. By coating a layer with a high-refractive index, the twist pitch required for resonant coupling is increased to hundreds of micrometers, and the mature pit-in-jacket drawing technique can be used. The polarization filtering properties of the fabricated circularly polarizing fibers are measured experimentally, showing high distinction ratios in a broad frequency region.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110606 (2016)
High performance fiber optic gyroscope with a radiation-tolerant and temperature-stable scale factor
Yuanhong Yang, and Fuling Yang

A simple wavelength-independent scale factor model is established for a closed-loop interferometer fiber optic gyroscope (IFOG) and a method to keep the scale factor radiation tolerant and temperature stable in a high performance IFOG for space application is proposed. The half-wave voltage (Vπ) of the multifunction gyro chip at different wavelengths and temperatures is measured and the radiation-independent inherent parameter of the modulator is picked out and found be proportional to the temperature and slightly wavelength dependent. An experimental IFOG is developed and the scale factor is measured at different temperatures and under Co60 irradiation, respectively. Less than a 10 ppm scale factor instability is achieved within the 40°C to +60°C temperature range and more than 100 krad(Si) total radiation dose.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110605 (2016)
Experimental demonstration of an adaptive orthogonal frequency division multiplexing visible light communication system
Xinyue Guo, and Xin Li

We propose and experimentally demonstrate a simple visible light communication system by combining an adaptive transmission technique with orthogonal frequency division multiplexing. In the adaptive transmission scheme, power allocation is performed in such a way as to maximize the channel capacity under electric power constraints, and adaptive modulation is then introduced to achieve the maximum data rate given a target bit error rate. Experimental results show that spectrum efficiency can be greatly improved through this adaptive transmission technique.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110604 (2016)
High sensitivity side-hole fiber magnetic field sensor based on surface plasmon resonance
Sijun Weng, Li Pei, Jianshuai Wang, Tigang Ning, and Jing Li

A high sensitivity magnetic fluid (MF) filled side-hole fiber magnetic field sensor based on surface plasmon resonance (SPR) is designed and investigated. Within the side-hole fiber, the water-based MF is injected into the two side holes, and both of the holes are coated with gold to realize the measurement of the magnetic field by using the SPR effect. The refractive index of the MF changes with different external magnetic fields, resulting in the resonant wavelength moving to other wavelengths. Furthermore, when the external magnetic field increases from 30 to 210.9 Oe, the magnetic field sensitivity can reach to 1.063 nm/Oe.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110603 (2016)
High order modulation format identification based on compressed sensing in optical fiber communication system
Bingxiang Hui, Xianfeng Tang, Na Gao, Wenbo Zhang, and Xiaoguang Zhang

We propose a method of modulation format identification based on compressed sensing using a high-order cyclic cumulant combined with a binary tree classifier. Through computing the fourth-order cyclic cumulant of the pretreated band signal, which is obtained by compressed sensing with the sampling rate much less than the Nyquist sampling value, the feature vector for classification is extracted. Simulations are carried out in the optical coherent fiber communication system with different modulation formats of multiple phase-shift keying and multiple quadrature amplitude modulation. The results indicate that this method can identify these modulation formats correctly and efficiently. Meanwhile, the proposed method is insensitive to laser phase noise and signal noise.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110602 (2016)
Decision-free radius-directed Kalman filter for universal polarization demultiplexing of square M-QAM and hybrid QAM signals
Yanfu Yang, Qun Zhang, Yong Yao, Guoliang Cao, Kangping Zhong, Xian Zhou, Alan Pak Tao Lau, and Chao Lu

We propose and experimentally demonstrate a universal and blind polarization-state tracking scheme for M-quadrature amplitude modulation (QAM) signals based on a decision-free radius-directed linear Kalman filter (RD-LKF). The polarization tracking performance is investigated through simulation and experiments for both quadrature phase-shift keying (QPSK) and 16-QAM signals. The influence of the filter parameters on the static polarization demultiplexing performance and dynamic tracking capability are discussed via simulation. The optimization strategy of the filter parameters in modulation-format-independent scenarios is proposed and simulations are carried out to evaluate the polarization demultiplexing penalty for QPSK, 16QAM, and hybrid QPSK/16QAM signals. Finally, the proposed decision-free RD-LKF is experimentally compared to the respective algorithms of constant modulus algorithm and multimodulus algorithm for QPSK and 16-QAM and the advantage of ultrafast polarization tracking ability is confirmed.

Chinese Optics Letters
Nov. 10, 2016, Vol. 14 Issue 11 110601 (2016)
Crosstalk-aware RCSA for spatial division multiplexing enabled elastic optical networks with multi-core fibers
Ruijie Zhu, Yongli Zhao, Hui Yang, Haoran Chen, Jie Zhang, and Jason P. Jue

In this Letter, we propose two crosstalk-aware routing, core, and spectrum assignment (CA-RCSA) algorithms for spatial division multiplexing enabled elastic optical networks (SDM-EONs) with multi-core fibers. First, the RCSA problem is modeled, and then a metric, i.e., CA spectrum compactness (CASC), is designed to measure the spectrum status in SDM-EONs. Based on CASC, we propose two CA-RCSA algorithms, the first-fit (FF) CASC algorithm and the random-fit (RF) CASC algorithm. Simulation results show that our proposed algorithms can achieve better performance than the baseline algorithm in terms of blocking probability and spectrum utilization, with FF-CASC providing the best performance.

Chinese Optics Letters
Oct. 10, 2016, Vol. 14 Issue 10 100604 (2016)
All-fiber electro-optic modulator based on D-shaped twin-core fiber
Jianshuai Wang, Li Pei, Sijun Weng, Liangying Wu, Tigang Ning, and Jing Li

In a D-shaped twin-core fiber (DTCF), the central core is insensitive to the variation of the external environment, while the other core is highly sensitive. As an electro-optic polymer coated on a DTCF, the coupling between the two cores varies with voltages applied to the polymer. Based on this, a superior all-fiber modulator is proposed that bears little coupling loss, prohibits mode mismatch, and provides a more stable working circumstance. A half-wave driving voltage (Vπ) of 1.26 V is achieved. Moreover, a high modulation depth of 40 dB can be realized for a voltage of 2.7 V at a 1550 nm wavelength.

Chinese Optics Letters
Oct. 10, 2016, Vol. 14 Issue 10 100603 (2016)
Raman scattering enhancement characteristic of NbCl5- and Nb2O5-doped silica fibers
Pengxiang Yang, Zhenyi Chen, Na Chen, Shupeng Liu, Bo Lu, and Tingyun Wang

Two kinds of Nb-doped silica fibers, an NbCl5-doped fiber and an Nb2O5-doped fiber, are fabricated and characterized in this Letter. First, the refractive index profiles of both fibers are obtained, and then their Raman spectra are measured with 785 nm exciting light. The Nb-doped fibers’ Raman spectra are compared with a conventional GeO2-doped single-mode silica fiber that is prepared with the same method and under the same conditions. As a result, the Raman gain coefficients of the Nb-doped silica fiber core are obtained. The experimental results show that Nb2O5 doping can enhance the Raman scattering intensity of the optical fibers.

Chinese Optics Letters
Oct. 10, 2016, Vol. 14 Issue 10 100602 (2016)
Carrier phase estimation scheme for faster-than-Nyquist optical coherent communication systems
Chengcheng Li, Dongwei Pan, Yiqiao Feng, Jiachuan Lin, Lixia Xi, Xianfeng Tang, Wenbo Zhang, and Xiaoguang Zhang

We propose a modified-Viterbi and Viterbi phase estimation (VVPE) carrier phase recovery scheme that shows an effective capability of reducing the frequent and accumulated cycle slips induced by inter-symbol interference (ISI) in a faster-than-Nyquist (FTN) optical coherent communications system. In a 28-Gbaud FTN polarization-division multiplexed quadrature phase-shift keying optical communication system, the comparison of the proposed modified-VVPE scheme and the conventional VVPE scheme is carried out. It is proved that the proposed modified-VVPE scheme can effectively overcome the challenge of ISI induced error carrier phase estimation, which leads to a better bit error ratio performance.

Chinese Optics Letters
Oct. 10, 2016, Vol. 14 Issue 10 100601 (2016)
Field-programmable gate array-based large-capacity sensing network with 1642 ultra-weak fiber Bragg gratings
Zhi Wang, Hongqiao Wen, Chenyuan Hu, Wei Bai, and Yutang Dai

A field-programmable gate array (FPGA)-based large-capacity sensing network with ultra-weak fiber Bragg gratings (FBGs) is proposed and experimentally studied. The demodulation system is constructed to interrogate 1642 serial time-division-multiplexing FBGs with a peak reflectivity of about 40 dB and equal separations of 2.5 m. Two semiconductor optical amplifiers and an InGaAs linear sensor array controlled by an FPGA are introduced to the demodulation system to achieve fast, precise, and flexible interrogation. The low crosstalk and spectral distortion are investigated through both theoretical analysis and experiments.

Chinese Optics Letters
Jan. 10, 2016, Vol. 14 Issue 1 010601 (2016)
Diversity-combining technique for discreet Hartley transform-based asymmetrical clipping optical orthogonal frequency-division multiplexing
Zhuo Cai, Ji Zhou, and Yaojun Qiao

In this Letter, we propose a scheme that integrates a diversity-combining technique with asymmetrical clipping optical orthogonal frequency-division multiplexing (ACO-OFDM) based on a discreet Hartley transform (DHT). Simulations are demonstrated for the DHT-based ACO-OFDM system with a diversity-combining technique. The simulation results indicate that when the optimal weighting factor is chosen, the bit error rate (BER) performance is improved by about 2 dB when the binary phase-shift keying (BPSK) is modulated and about 3 dB when the 16 pulse-amplitude modulation is modulated. Additionally, experiments are presented to verify the feasibility of diversity-combining DHT-based ACO-OFDM. In the transmission experiments, a BPSK-modulated DHT-based ACO-OFDM with a diversity-combining receiver is realized, which improves the BER performance by 1.5 and 1.3 dB for a span length of back-to-back and a 50 km standard single-mode fiber, respectively.

Chinese Optics Letters
Oct. 30, 2015, Vol. 13 Issue Suppl. S20605 (2015)
Research on the temperature sensing mechanism of a photonic crystal fiber filled with liquid
Yanyun Jin, Hongbing Li, Shanglin Hou, and Ronghua Cui

A new distributed temperature sensor based on stimulate Brillouin scattering (SBS) is designed by filling alcohol in a photonic crystal fiber (PCF). Since the refractive index of alcohol varies with temperature, which results in the effective mode area of the PCF varying with temperature, the output power of a pulse in fiber varies with temperature. In order to achieve a precision of 1°C for the new design, the dynamic range of power measurement equipment is only 42 dB, which is far less than the dynamic range of the frequency measurement equipment of the traditional measurement system. Some influence factors in the new system are analyzed.

Chinese Optics Letters
Oct. 30, 2015, Vol. 13 Issue Suppl. S20604 (2015)
Magnification of divergence angle in a ground test of space laser communication
Yuan Hu, Huilin Jiang, Shoufeng Tong, Lizhong Zhang, and Dewen Cheng

For a long communication distance, the divergence angle of laser transmission by a space laser communication system must be sufficiently narrow to solve the energy problem. However, it needs to be magnified during the ground test before the satellite launches. This way, the two communication terminals can be constantly in the coverage of each other’s laser beam even when the tracking accuracy decreases because of the atmosphere. This Letter presents methods to magnify the divergence angle for laser transmission. The diffraction intensity distributions with different methods and their influence on communication are discussed. We provide the optimal scheme in the ground test of a certain space laser communication. The result represents the accumulated experience for the space laser communication ground test.

Chinese Optics Letters
Oct. 30, 2015, Vol. 13 Issue Suppl. S20603 (2015)
Multiwavelength Tm-doped fiber laser based on nonlinear polarization rotation
Lei Li, Bin Zhang, Ke Yin, and Jing Hou

We demonstrate a multiwavelength Tm-doped fiber laser employing polarization-maintaining undoped fiber (PMF) based on nonlinear polarization rotation. The PMF and a polarization-sensitive isolator between two polarization controllers are used to induce the intensity-dependent loss and frequency-dependent loss to alleviate mode competition caused by homogeneous gain broadening in an active fiber. An up to four wavelengths laser operating in a cavity is observed without the PMF. After the PMF is inserted into the cavity, stable dual-wavelength operation is achieved. The output had an average power distribution. The spectral fluctuation at each wavelength is smaller than 0.9 dB.

Chinese Optics Letters
Oct. 30, 2015, Vol. 13 Issue Suppl. S20602 (2015)
Microwave signal generation based on frequency multiplication with fiber ring loop modulation and microwave photonic filters
Shiwei Zhang, Hao Chen, Zuowei Xu, Congxian Wu, Kaijun Che, and Hongyan Fu

In this Letter, microwave generation based on frequency multiplication with fiber ring loop modulation and microwave photonic filters is proposed and experimentally demonstrated. High-order microwave harmonics can be generated and enhanced by the fiber ring loop modulation formed by incorporating a Mach–Zehnder modulator (MZM) into a fiber ring structure. Cascaded fiber ring based microwave photonic filters (FRMPFs) are adopted to filter out the desirable harmonics. In our work, with a driving signal of 2.835 GHz, the fourth-order harmonic with a frequency of 11.34 GHz is generated by using two cascaded FRMPFs.

Chinese Optics Letters
Oct. 30, 2015, Vol. 13 Issue Suppl. S20601 (2015)
MB-OFDM UWB over fiber system with direct detection
Jing He, Xuejie Wen, and Lin Chen

We experimentally demonstrate the multiband orthogonal frequency-division multiplexing ultra-wideband (MB-OFDM UWB) over fiber system with direct detection. Different sub-carrier modulation formats (quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (QAM)) are investigated in the MB-OFDM UWB over fiber system. The experimental results show that a 3.84 Gb/s 16 QAM-encoded MB-OFDM UWB signal can be successfully transmitted over 70 km standard single-mode fiber without chromatic dispersion compensation.

Chinese Optics Letters
Jan. 19, 2015, Vol. 13 Issue s1 S10604 (2015)
Single polarization and poling optimized twin-hole optical fiber
Lin Huang, and Guobin Ren

We investigate optimal twin-hole poling optical fiber configurations, including distances between core and each electrode, and poling voltage based on the two-dimensional charge dynamics model. We propose a poled fiber with optimized .(2) as well as single-polarization property. Small distance between core and anode -guarantees the poled fiber with large .(2) in fiber core and large polarization-dependent loss. A maximum .(2) in the core region either outside or inside nonlinear layer can be realized by appropriately selecting edge-to-edge distance between core and cathode. The maximum .(2) in the core region can be even larger by increasing the poling voltage.

Chinese Optics Letters
Jan. 16, 2015, Vol. 13 Issue s1 S10603 (2015)
Research on optical true-time delay of multiple optical carriers in phased array antenna
Mingfei Wei, Min Wang, Xiaomin Zhang, Chun Wang, and Fang Yang

In order to improve the bandwidth and ability to resist electromagnetic interference of phased array antenna, we use the real-time delay technology of optical fiber. By using phase control principle of optically controlled phased array, we deduce the relation formula of time delay and phase control. Based on multiple optical carriers and optical switch delay technology, we analyze design method of optical fiber time delay system and give the results of the experimental test. From the results, we find that the system can improve ability of phased array antenna about phase control and resisting electromagnetic interference in broadband condition.

Chinese Optics Letters
Jan. 16, 2015, Vol. 13 Issue s1 S10602 (2015)
Performance analysis of LDPC code based on Watermark scheme in high-speed optical communication system
Wenxue He, Wenbo Zhang, Chao Li, Lixia Xi, Xiaoguang Zhang, and Yaojun Qiao

We study the decoding performance of low-density parity-check (LDPC) code based on Watermark scheme under different percentages of water-mark bits over 100 Gb/s differential quadrature phase shift keying (DQPSK) high-speed optical communication system. We also find the optimum percentage of water-mark bits according to the performance comparison. Simulation result shows an improvement of 0.5 dB net code gain by using the optimum Watermark scheme at a post-forward error correction bit error rate (BER) of 10-9, comparing with the traditional log-likelihood ratios belief propagation decoding algorithm. Also, for the same BER, there is a decrease in the number of iterations used in LDPC decoding.

Chinese Optics Letters
Jan. 26, 2015, Vol. 13 Issue s1 S10601 (2015)
Three-dimensional model of thermal-induced optical phase shifts in rotation sensing
Xuyou Li, Weiwei Ling, Yanhui Wei, and Zhenlong Xu

By studying the thermal-induced phase shift mechanism of an interferometric fiber-optic gyroscope (IFOG) sensing coil, a novel generalized expression based on a three-dimensional (3D) model is proposed. Compared with the traditional pure Shupe effect model, the simulation results show that the new 3D model, including elastic strain and the elasto-optical effect, can describe the thermal effect of the coils more accurately. Experiments with temperature change rates between 40°C and 70°C are performed to verify the effectiveness of the proposed generalized expression. The results of our work can guide researchers in identifying countermeasures to reduce the thermal-induced bias error in IFOG.

Chinese Optics Letters
Sep. 10, 2015, Vol. 13 Issue 9 090603 (2015)
Crystals based on solid solution of Ag1 − xTlxBr1 − xIx for the manufacturing of IR fibers
Alexandr Korsakov, Liya Zhukova, Dmitrii Salimgareev, and Vladislav Zhukov

For the development of fiber optics for the range from 0.2 to 50.0 μm, one needs light-stable, nonhygroscopic, ductile crystals that would be transparent within this spectral range and have a lack of cleavage, and from which the flexible infrared (IR) fibers are extruded. The crystals based on solid solutions of silver and monadic thallium halides meet the conditions listed above. Consequently, by differential thermal and x ray analyses, we study the TlBr–TlI phase diagram using the crystals with optimal compositions, which we grow ourselves. We also manufacture light-stable nanocrystalline IR fibers that are transparent at longer wavelengths compared with AgCl–AgBr fibers.

Chinese Optics Letters
Sep. 10, 2015, Vol. 13 Issue 9 090602 (2015)
Global transmission diagrams for evanescent waves in a nonlinear hyperbolic metamaterial
Munazza Zulfiqar Ali, Ashfaq Ahmad Bhatti, Qamar ul Haque, and Shahzad Mahmood

A theoretical model of a nonlinear hyperbolic metamaterial is presented in the form of a stack of subwavelength layers of linear plasmonic and nonlinear dielectric materials. A broad picture of the properties of evanescent waves (high-k modes) in this stack is investigated by plotting global transmission diagrams. The presence of nonlinearity strongly modifies these diagrams. The emergence and modification of nonlinear evanescent waves is observed. Some signatures of nonlinear phenomenon such as formation of orbits and trajectories around fixed points are also seen in our work.

Chinese Optics Letters
Sep. 10, 2015, Vol. 13 Issue 9 090601 (2015)
Quasi-optical single-sideband modulation with continuous carrier-to-sideband ratio tunability
Jing Li, Tigang Ning, Li Pei, Jingjing Zheng, Jian Sun, Yueqin Li, and Jin Yuan

A quasi-optical single-sideband (quasi-OSSB) modulation approach with a tunable carrier-to-sideband ratio (CSR) is proposed and demonstrated. By simply tuning the polarizing angle, a continuously tunable CSR can be obtained. Since the upper sideband is highly suppressed during the CSR’s tuning, quasi-OSSB modulation signals with extremely small interference are generated. An experiment is undertaken for verification. It is found that the target CSR can be continuously tuned over a wide range, which can be used to improve the receiver sensitivity of the fiber links.

Chinese Optics Letters
Aug. 10, 2015, Vol. 13 Issue 8 080606 (2015)
High-speed phosphorescent white LED visible light communications without utilizing a blue filter
Honglei Li, Yini Zhang, Xiongbin Chen, Chunhui Wu, Junqing Guo, Zongyu Gao, and Hongda Chen

We experimentally demonstrate a high-speed phosphorescent white light emitting diode (LED) visible light communication (VLC) system without utilizing an optical blue filter. Here, the white light response is equalized by using the proposed analog equalizers. The 3 dB bandwidth of the VLC link could be extended from 3 to 132 MHz, which allows 330 Mbit/s non-return-to-zero on–off keying (NRZ-OOK) data transmission with a bit error ratio (BER) of 7.2×10 10 and 672 Mbit/s 64-quadrature amplitude modulation (64-QAM) data transmission with a BER of 3.2×10 3. These resultant BERs are less than the forward error correction (FEC) limit of 3.8×10 3. The VLC link distance is 1 m using a single 1 W LED. The transmitter and receiver modules are integrated to a compact size. Furthermore, the relationships between the signal performance and illumination level or optical power are investigated and analyzed.

Chinese Optics Letters
Aug. 10, 2015, Vol. 13 Issue 8 080605 (2015)
Distributed fiber temperature sensor using a laser diode modulated by a pseudo-random bit sequence
Jing Chai, Mingjiang Zhang, Yi Liu, Lan Li, Weipeng Xu, and Yuncai Wang

We propose and demonstrate a scheme to measure the distribution of temperature along an optical fiber based on pseudo-random sequence modulation. In our experimental work, current modulation with a pseudo-random bit sequence (PRBS) is applied to a laser diode that serves as the Brillouin pump light and reference light. Because of the independence of the spatial resolution on the PRBS length, the measurement range can be extended while maintaining high spatial resolution using a long PRBS length. Temperature-induced changes in a Brillouin frequency shift of 250 m fiber sections are clearly observed with 54 cm spatial resolution by this method.

Chinese Optics Letters
Aug. 10, 2015, Vol. 13 Issue 8 080604 (2015)
Simplified model and experimental validation for ultraviolet single-scattering channels
Peng Wang, Hongming Zhang, and Zhengyuan Xu

For nonline-of-sight ultraviolet communication links, a simple and concise parametric expression (PE) of channel path loss is valuable for link performance analysis in typical scenarios. It is observed that the light energy in the scattering volume can be approximated using a line integral. Combining curve fitting for the scattering phase function and the mean value theorem of integrals, we propose a simple but highly accurate PE. It matches well with the Monte Carlo simulations for typical LED-based communication with small beam divergence (<45°); when the beam divergence is smaller than 10°, their differences are less than 1 dB in most geometrical conditions. The proposed PE also shows good consistency with our outdoor experimental measurements, and the reported experimental results in the literature.

Chinese Optics Letters
Aug. 10, 2015, Vol. 13 Issue 8 080603 (2015)
56 Gb/s direct-detection polarization multiplexing multi-band CAP transmission
Zhixin Wang, Li Tao, Yiguang Wang, and Nan Chi

Carrierless amplitude and phase (CAP) modulation is generating increasing interest for short-reach optical communications. Polarization multiplexing (PolMux) is a good way to improve the CAP system’s data rate further with the limited bandwidth of electrical devices. In this Letter, we experimentally demonstrate a 56 Gb/s direct-detection PolMux CAP system over a 15 km fiber link for the first time, to our best knowledge. Two-band CAP modulation with different modulated orders for each band is employed. An optical filter at the receiver is utilized to realize the polarization separation. No extra digital signal processing for polarization-dependent distortion is required.

Chinese Optics Letters
Aug. 10, 2015, Vol. 13 Issue 8 080602 (2015)
Preparation and characterization of a liquid level sensor based on plastic fibers
Huixin Zhang, Lishuang Feng, Yulong Hou, Shan Su, Jia Liu, Wenyi Liu, Jun Liu, and Jijun Xiong

Preparation and characterization of a liquid level sensor based on macro-bending coupling of fibers are demonstrated in this Letter. The sensitive component can be obtained through a twisting and twining structure of transparent cladding plastic fibers. The difference in light power originating from the surrounding media in the fibers is tested. The light power loss for different tested media and the fiber bending magnitude are investigated. The sensing measurements show that the coupling light power in the passive fiber decreases in accordance with increasing liquid level, whereas it exhibits a steady tendency in the case of the active fiber.

Chinese Optics Letters
Aug. 10, 2015, Vol. 13 Issue 8 080601 (2015)
Generation of ultra-flat optical frequency comb using a balanced driven dual parallel Mach–Zehnder modulator
Jian Wang, Haiwen Cai, Dijun Chen, and Ronghui Qu

We propose and experimentally demonstrate an ultra-flat optical frequency comb (OFC) generator by a balanced driven dual parallel Mach–Zehnder modulator. Five- and seven-tone OFC with exactly equal intensity can be generated theoretically. Experimentally obtained five- and seven-tone OFC with flatness of 0.6 and 1.26 dB are demonstrated, respectively, which agrees well with the theoretical results.

Chinese Optics Letters
Jun. 10, 2015, Vol. 13 Issue 6 060604 (2015)
Theoretical study of all-optical RZ-OOK to NRZ-OOK format conversion in uniform FBG for mixed line-rate DWDM systems
Oskars Ozolins, and Vjaceslavs Bobrovs

In this work we study all-optical multi-channel return-to-zero (RZ)–on-off keying (OOK) to nonreturn-to-zero (NRZ)–OOK format conversion in single uniform fiber Bragg grating (FBG) for mixed line-rate dense wavelength-division multiplexing systems using mathematical simulations. Forty and 20 Gbit/s RZ–OOK signals with 33% and 50% duty cycles are converted to NRZ–OOK signals in single uniform FBG with 21% reflectivity. Impact of amplitude noise from FBG contrast profile on modulation format conversion efficiency is also studied.

Chinese Optics Letters
Jun. 10, 2015, Vol. 13 Issue 6 060603 (2015)
Radiation-induced change of OH content in Yb-doped silica glass
Shuang Liu, Shupei Zheng, Ke Yang, and Danping Chen

Yb-doped silica glasses containing low, medium, and high content of OH are prepared through nanoporous glass sintering technology. High-OH sample exhibits better X-ray irradiation resistivity than low- and medium-OH samples. After irradiation, OH content of low- and medium-OH samples increases 37.5% and 11%, respectively; in contrast, OH content of high-OH sample decreases dramatically. The different OH content changes among the samples are discussed regarding the proposed inter-conversion reactions involving Si-H and Si-OH during the irradiation.

Chinese Optics Letters
Jun. 10, 2015, Vol. 13 Issue 6 060602 (2015)
10  Gb/s 16-quadrature amplitude modulation signal delivery over a wireless fiber system by using a directly modulated laser for electrical/optical conversion
Lun Zhao, and Jianguo Yu

We propose and experimentally demonstrate a novel scheme to realize electrical/optical (E/O) conversion on the receiver side of a wireless fiber integration system at the W band. At the receiver, a directly modulated laser (DML) is used to realize E/O conversion. The received 85 GHz wireless millimeter-wave (mm-wave) signal is first down-converted into a 10 GHz electrical intermediate-frequency (IF) signal to overcome the insufficient bandwidth of the subsequent DML. Then, two cascaded electrical amplifiers (EAs) are employed to boost the electrical IF signal before it is used to drive a DML. By using this scheme, we transmit a 10 Gb/s 16 quadrature amplitude modulation (16QAM) signal over a 10 m wireless link, and then deliver it over a 2 km single-mode fiber-28 (SMF-28) wire link with a bit error ratio (BER) that is less than the hard-decision forward error correction threshold of 3.8×10 3. Our experimental results show that the DML is good device to be used for the E/O conversion of a 16QAM signal.

Chinese Optics Letters
Jun. 10, 2015, Vol. 13 Issue 6 060601 (2015)
Hybrid fiber interferometer for simultaneous measurement of displacement and temperature
Fei Meng, Zhongbao Qin, Qiangzhou Rong, Hao Sun, Jiacheng Li, Zaihang Yang, Manli Hu, and Honggao Geng

A hybrid fiber interferometer sensing configuration for displacement and temperature measurements is proposed and experimentally demonstrated that is constructed by splicing a short section of polarization maintaining optical fiber to an end-cleaved single mode optical fiber with a tapering structure. The reflected spectrum changes with the variation of displacement and temperature. The sensing configuration uses the method of wavelength and intensity modulations for displacement and temperature measurements, respectively, to which the sensitivities are 0.01392 nm/μm, 0.0214 dBm/μm, 0.09136 nm/°C, and 0.15795 dBm/°C. Experimental results show that displacement and temperature can be measured simultaneously by demodulating the reflected spectrum.

Chinese Optics Letters
May. 10, 2015, Vol. 13 Issue 5 050603 (2015)
Magnetostrictive composite material-based polarimetric heterodyning fiber-grating laser miniature magnetic field sensor
Wei He, Linghao Cheng, Qiang Yuan, Yizhi Liang, Long Jin, and Bai-Ou Guan

A novel fiber-optic magnetic field sensor is demonstrated based on a dual-polarization fiber-grating laser, which is embedded in an epoxy resin-bonded magnetostrictive composite material with doped Terfenol-D particles. A simple structure is designed to convert the magnetic field-induced strain to transversal stress, which is applied to the fiber laser to produce beat note frequency changes for measurement purposes. The response of the proposed sensor is measured, and shows quite a good directivity and linearity with a sensitivity of 10.5 Hz/μT to the magnetic field. It also shows a large measurable range up to about 0.3 T.

Chinese Optics Letters
May. 10, 2015, Vol. 13 Issue 5 050602 (2015)
Tunable negative coefficient microwave photonic filter based on a polarization modulator and a polarization beam interferometer
Yanbing Jin, Erwin H. W. Chan, Xinhuan Feng, Xudong Wang, and Bai-ou Guan

A new tunable microwave photonic notch filter with negative coefficients is presented. It is based on a polarization modulator and a polarization beam interferometer. Experimental results are presented that embody the new concept.

Chinese Optics Letters
May. 10, 2015, Vol. 13 Issue 5 050601 (2015)
Performance analysis of non-line-of-sight ultraviolet communication through turbulence channel
Tao Liu, Peng Wang, and Hongming Zhang

The bit error rate performance of non-line-of-sight ultraviolet communication through atmospheric turbulence is studied. The communication performance degradation under different strengths of turbulence is evaluated. Particularly, under strong turbulence conditions, the communication distance can be shortened by 30%, or at a given distance the communication rate can be reduced by half than the counterpart of no turbulence.

Chinese Optics Letters
Apr. 10, 2015, Vol. 13 Issue 4 040601 (2015)
Mode-locked 2  μm fiber laser with a multi-walled carbon nanotube as a saturable absorber
S. M. Azooz, F. Ahmad, H. Ahmad, S. W. Harun, B. A. Hamida, S. Khan, A. Halder, M. C. Paul, M. Pal, and S. K. Bhadra

We propose and demonstrate a passively mode-locked fiber laser operating at 1951.8 nm using a commercial thulium-doped fiber (TDF) laser, a homemade double-clad thulium–ytterbium co-doped fiber (TYDF) as the gain media, and a multi-walled carbon nanotube (MWCNT) based saturable absorber (SA). We prepare the MWCNT composite by mixing a homogeneous solution of MWCNTs with a diluted polyvinyl alcohol (PVA) polymer solution and then drying it at room temperature to form a film. The film is placed between two fiber connectors as a SA before it is integrated into a laser ring cavity. The cavity consists of a 2 m long TDF pumped by a 800 nm laser diode and a 15 m long homemade TYDF pumped by a 905 nm multimode laser diode. A stable mode-locking pulse with a repetition rate of 34.6 MHz and a pulse width of 10.79 ps is obtained when the 905 nm multimode pump power reaches 1.8–2.2 W, while the single-mode 800 nm pump power is fixed at 141.5 mW at all times. To the best of our knowledge, this is the first reported mode-locked fiber laser using a MWCNT-based SA.

Chinese Optics Letters
Mar. 10, 2015, Vol. 13 Issue 3 030602 (2015)
Method for measurement of fusion-splicing-induced reflection in a photonic bandgap fiber-optical gyro
Xiaobin Xu, Zhihao Zhang, Zuchen Zhang, Jing Jin, and Ningfang Song

We propose a method for the real-time measurement of the reflection at both splicing points between a photonic bandgap fiber coil and conventional fiber during the process of fusion splicing in a photonic bandgap fiber optical gyroscope (PBFOG), using the interference among the secondary waves, which arise from the fusion splicing points and the mirror face produced by intentionally cutting the bear end of the coupler. The method is theoretically proven and experimentally verified in a practical PBFOG, and it is significant for inline examination of the fusion splicing quality and evaluation of the PBFOG performance.

Chinese Optics Letters
Mar. 10, 2015, Vol. 13 Issue 3 030601 (2015)
Fifty-five km reach, bidirectional, 400  Gb/s (40 × 10  Gb/s), channel-reuse DWDM–PON employing a self-wavelength managed tunable laser
Zhiguo Zhang, Xu Jiang, Jiahe Wang, Xue Chen, and Liqian Wang

We experimentally demonstrate a channel-reuse bidirectional 10 Gb/s/λ long-reach dense wavelength-division multiplexing passive optical network (DWDM-PON) and an optical beat-noise-based automatic wavelength control method for a tunable laser used in a colorless optical network unit (where λ=wavelength). A 55 km, bidirectional, 400 Gb/s (40×10 Gb/s) capacity channel-reuse transmission with 100 GHz channel spacing is achieved. The transmission performance is also measured with different optical signal to Rayleigh backscattering noise ratios and different central wavelength shifts between upstream and downstream in the channel-reuse system.

Chinese Optics Letters
Feb. 10, 2015, Vol. 13 Issue 2 020603 (2015)
Spin effect on a single-mode single-polarization optical fiber
Minning Ji, Gang-Ding Peng, and Yanhua Luo

The spin effect on a single-mode single-polarization optical fiber is investigated theoretically and numerically. To get a practical single elliptically polarized fiber the normalized spin rate must be in the range of 0.2–0.35. A single elliptically polarized fiber with a normalized spin rate around 0.224 is demonstrated. It has a broad band from 1.530 to 1.558 μm, in which the low-leakage elliptically polarized eigenmode loss is kept within 1.2 dB while the high-leakage elliptically polarized eigenmode loss is greater than 20 dB. This fiber can be used as an elliptical polarizer or applied in current sensing.

Chinese Optics Letters
Feb. 10, 2015, Vol. 13 Issue 2 020602 (2015)
Wide dynamic range experiments on a resonator fiber optic gyro based on closed-loop frequency locking technique
Yongqiu Zheng, Yongfeng Ren, Panlong An, Chengqun Chu, Xiaofeng Li, Chenyang Xue, Jun Liu, and Shubin Yan

Wide dynamic range is an important index of the resonator fiber optic gyro (RFOG). The dynamic range is related to the parameters of the fiber ring resonator (FRR). After adopting the appropriate parameters, the resonant curve of a FRR and the synchronous demodulated curve are measured. Based on the closed-loop frequency locking technique, the wide dynamic range is obtained, while the linearity is guaranteed. The experiment’s results show that the dynamic range is ±480 deg/s with less than 1% nonlinearity, and that the bias stability is 0.04 deg/s over 2000 s. This Letter demonstrates the scheme for a RFOG with a wide dynamic range.

Chinese Optics Letters
Feb. 10, 2015, Vol. 13 Issue 2 020601 (2015)
Fabrication and sensing characteristics of intrinsic Fabry–Perot interferometers in fiber tapers
Xiaobei Zhang, Jiabao Xiong, Fan Gu, Jinlong Li, Wenyuan Wang, Fufei Pang, and Tingyun Wang

This Letter presents intrinsic Fabry–Perot interferometers in the fiber tapers fabricated by the femtosecond laser micromachining technique. The sensing of temperatures as high as 1000°C based on the fiber device is characterized, with a sensitivity of 15.28 pm/°C. A nearly linear refractive index sensing is also obtained by using the fringe visibility to characterize, with a sensitivity of 73.05 dB/RIU. These intrinsic Fabry–Perot interferometers in fiber tapers may be useful in applications of high-temperature and linear refractive index sensing.

Chinese Optics Letters
Dec. 10, 2015, Vol. 13 Issue 12 120602 (2015)
Combination of light-emitting diode positioning identification and time-division multiplexing scheme for indoor location-based service
Xiang Liu, Aiying Yang, Yu Wang, and Lihui Feng

A combination of light-emitting diode (LED) identification and a time-division multiplexing scheme is proposed in this Letter for indoor location-based service. With the scheme, the arrangement of white LED lamps and the structure of a data frame are designed to realize high-accuracy indoor positioning and location-based payload data transmission simultaneously. The results of the experiment demonstrate that the indoor positioning accuracy is 10 cm and 2 Mb/s data transmission with high signal quality is realized.

Chinese Optics Letters
Dec. 10, 2015, Vol. 13 Issue 12 120601 (2015)
Improved modulation format identification based on Stokes parameters using combination of fuzzy c-means and hierarchical clustering in coherent optical communication system
Longxue Cheng, Lixia Xi, Donghe Zhao, Xianfeng Tang, Wenbo Zhang, and Xiaoguang Zhang

In this Letter, we develop the Stokes space-based method for modulation format identification by combing power spectral density and a cluster analysis to identify quadrature amplitude modulation (QAM) and phase-shift keying (PSK) signals. Fuzzy c-means and hierarchical clustering algorithms are used for the cluster analysis. Simulations are conducted for binary PSK, quadrature PSK, 8PSK, 16-QAM, and 32-QAM signals. The results demonstrate that the proposed technique can effectively classify all these modulation formats, and that the method is superior in lowering the threshold of the optical signal-to-noise ratio. Meanwhile, the proposed method is insensitive to phase offset and laser phase noise.

Chinese Optics Letters
Oct. 10, 2015, Vol. 13 Issue 10 100604 (2015)
Novel distributed passive vehicle tracking technology using phase sensitive optical time domain reflectometer
Zhaoyong Wang, Zhengqing Pan, Qing Ye, Bin Lu, Zujie Fang, Haiwen Cai, and Ronghui Qu

A novel distributed passive vehicle tracking technology is proposed and demonstrated. This technology is based on a phase-sensitive optical time domain reflectometer (Φ-OTDR) that can sense and locate vibrations. Two algorithms, dynamic frequency-space image and 2D digital sliding filtering, are proposed to distinguish a car’s moving signals from severe environmental noises and disturbances. This technology is proved effective by field experiments for tracking a single car and multiple cars. This work provides a new distributed passive way for real-time vehicle tracking and this technology will be extremely important for traffic controlling and public safety in modern society.

Chinese Optics Letters
Oct. 10, 2015, Vol. 13 Issue 10 100603 (2015)
1.2  Gbit/s visible light transmission based on orthogonal frequency-division multiplexing using a phosphorescent white light-emitting diode and a pre-equalization circuit
Xingxing Huang, Siyuan Chen, Zhixin Wang, Yiguang Wang, and Nan Chi

We propose a two-cascaded, constant-resistance, symmetrical bridged-T amplitude equalizer for a high-speed visible light communication (VLC) system. With the pre-equalization circuit, the 3 dB bandwidth of the VLC system can be extended from 12 to 235 MHz using a commercially available phosphorescent white light-emitting diode (LED), a blue filter, and a low-cost PIN photodiode. The data rate is 1.20 Gbit/s, exploiting 16-quadrature amplitude modulation-orthogonal frequency-division multiplexing with a 300 MHz modulation bandwidth over 50 cm of free-space transmission under the pre-forward error correction limit of 3.8×10 3. To our knowledge, this is the highest 3 dB bandwidth and the highest data rate ever achieved by using a pre-equalization circuit and white LED in a VLC system.

Chinese Optics Letters
Oct. 10, 2015, Vol. 13 Issue 10 100602 (2015)
FFPI-FBG hybrid sensor to measure the thermal expansion and thermo-optical coefficient of a silica-based fiber at cryogenic temperatures
Litong Li, Dongsheng Zhang, Xiaoyan Wen, and Sisi Peng

In this Letter, a sensor consisting of a fiber Bragg grating (FBG) and a fiber Fabry–Perot interferometer (FFPI) sensor is developed to measure the coefficient of thermal expansion (CTE) and the thermo-optical coefficient (TOC) of a silica-based optical fiber at cryogenic temperatures. The FFPI is fabricated by welding together two acid-etched fibers. The temperature performance of the FFPI-FBG hybrid sensor is studied in the temperature range of 30–273 K. The CTE and TOC of the optical fiber at cryogenic temperatures are derived analytically and verified by experiments.

Chinese Optics Letters
Oct. 10, 2015, Vol. 13 Issue 10 100601 (2015)
Dynamical phase alignment of sampling clock in optical performance monitoring system
Aiying Yang, Chaochao Liu, and Lihui Feng

In an optical performance monitoring system for high-speed optical communications, the ultra-short optical pulse from the laser is divided into two parts. One is to sample high-speed optical communication signal and the other as the sampling clock in an analog/digital converter (ADC) after detecting using a photon-detector. We propose a simple method based on variance calculation to align the phase of sampling clock in an ADC. The experiments demonstrate that, with the proposed method, the eye diagram or constellation diagram of high-speed optical communication signal can be reconstructed by the optical performance monitoring system.

Chinese Optics Letters
Dec. 29, 2014, Vol. 13 Issue 1 010606 (2015)
Scheme for generation of flat top and high signal-to-noise ratio optical frequency comb
Jianrui Li, Jiachuan Lin, Xiaoguang Zhang, Lixia Xi, Xianfeng Tang, and Yaojun Qiao

We demonstrate a new scheme for generation of optical frequency comb (OFC) based on cascade modulators, 23 comb lines within 0.5 dB spectral power variation are obtained. An optical finite impulse response (FIR) filter is introduced for suppression of amplified spontaneous emission noise. It is shown that carrier-to-noise-ratio of the OFC generated by this scheme can be as high as 38.8 dB with 12 dB improvement by using a 16-tap FIR filter, and the error vector magnitude performances of loaded Nyquist-16 quadrature amplitude -modulation signal is improved from 14.20% to 7.44%.

Chinese Optics Letters
Jan. 05, 2015, Vol. 13 Issue 1 010605 (2015)
Compensation of chromatic dispersion for full-duplex ROF link with vector signal transmission using an optical phase shifter
Chao Gao, Shanguo Huang, Jinghua Xiao, Xinlu Gao, Qian Wang, Yongfeng Wei, Wensheng Zhai, Wenjing Xu, and Wanyi Gu

We demonstrate the theory of chromatic dispersion (CD)-induced constellation rotation (CR) in a radio-over-fiber (ROF) link and a method for compensation. We also propose a 60 GHz full-duplex ROF system with vector signal transmission including no CD effect. The evaluation of 5 Gb/s 16 quadrature amplitude modulation signal transmission shows that the CD-induced CR can be entirely overcome due to the proposed method which is simply implemented through an optical phase shifter. The proposed ROF schedule is not only applicable for V-band (57–64 GHz) but also fits for W-band (75–110 GHz), or any other bandwidth.

Chinese Optics Letters
Jan. 05, 2015, Vol. 13 Issue 1 010604 (2015)
Security enhancement for double-random phase encryption using orthogonally encoded image and electronically synthesized key data
In-Ho Lee, and Myungjin Cho

We propose a security-enhanced double-random phase encryption (DRPE) scheme using orthogonally encoded image and electronically synthesized key data to cope with the security problem of DRPE technique caused by fixed double-random phase masks for encryption. In the proposed scheme, we adopt the electronically synthesized key to frequently update the phase mask using a spatial light modulator, and also employ the orthogonal encoding technique to encode the image and electronically synthesized key data, which can enhance the security of both data. We provide detailed procedures for encryption and decryption of the proposed scheme, and provide the simulation results to show the encryption effects of the proposed scheme.

Chinese Optics Letters
Dec. 12, 2014, Vol. 13 Issue 1 010603 (2015)
Design of hybrid-core PCF with nearly-zero flattened dispersion and high nonlinearity
Xi Liu, Lihong Han, Xiaoyu Jia, Jinlong Wang, Fangyong Yu, and Zhongyuan Yu

We present a new structure of nearly-zero flattened dispersion and highly nonlinear photonic crystal fiber (PCF) in the telecommunication window. This fiber design is comprised of a hybrid-core region with -bismuth-doped region in the center and three lower bismuth-doped regions in the first ring that enables dispersion control while maintaining a high nonlinear coefficient. Numerical analysis results show that the proposed PCF is achieved with a nonlinear coefficient of about 3301 W-1 km-1, a dispersion value of about 0.5537 ps/(nm·km) at 1550 nm, and nearly-zero flattened dispersion fluctuating within 2.5 ps/(nm·km) ranging from 1.496 to 1.596 μm.

Chinese Optics Letters
Dec. 30, 2014, Vol. 13 Issue 1 010602 (2015)
Multichannel wavelength multicasting for two QPSK signals based on FWM in SOA
Jun Qin, Yuefeng Ji, Hongxiang Wang, Danshi Wang, Min Zhang, and Guo-Wei Lu

We experimentally demonstrate multichannel wavelength multicasting for two nonreturn-to-zero quadrature phase-shift keying (NRZ-QPSK) channels based on four-wave mixing (FWM) in semiconductor optical amplifier (SOA). Through the interaction with the two pumps in SOA, the input two 25 Gb/s NRZ-QPSK channels are successfully simultaneously multicast to five and two new wavelengths, respectively. All the multicast channels are with a power penalty less than 2.5 dB at a bit error rate (BER) of 10-3. A characterization of the system performance using conversion efficiency and BER as figures-of-merit in terms of pump and signal powers is also presented. The results indicate that the pump and signal powers can be optimized to eliminate the introduced deleterious nonlinear components. The wavelengths of the two NRZ-QPSK channels and the two pumps need to be specified to avoid the crosstalk induced by high-order FWM.

Chinese Optics Letters
Dec. 12, 2014, Vol. 13 Issue 1 010601 (2015)
Beam quality and photodarkening comparison of tandem-pumped and directly diode-pumped ytterbium-doped fiber amplifiers
Hailong Yu, Xiaolin Wang, Pu Zhou, Hu Xiao, and Jinbao Chen

Compared with the conventional diode-pumped ytterbium-doped fiber amplifiers (YDFAs), tandem-pumped YDFAs are regarded as a better solution for high power scaling. In this letter, we compare and analyze the two types of pumping scheme with respect to beam quality (BQ). The numerical model adopted by us is based on steady-state rate equations with consideration of transverse mode competition. The results show that tandem pumping is not only suitable for high power scaling but also has an advantage over direct diode pumping in BQ. For instance, the power fraction of fundamental mode in tandem pumping at 1030 nm is about 4% higher than its corresponding value in direct diode pumping at 976 nm under the condition of counter-propagating pumping configuration with the same fiber parameters except cladding diameter. Mode selection by controlling dopant distributions and coiling the fibers is also simulated and discussed. Moreover, the simulation results show that the tandem-pumped YDFAs have lower photodarkening rate than the conventional YDFAs at part area of gain fiber, but there is no obvious difference between them from the mean perspective of entire gain fiber.

Chinese Optics Letters
Sep. 10, 2014, Vol. 12 Issue s2 S20604 (2014)
Experimental study on mode instabilities in all-fiberized high-power fiber amplifiers
Rumao Tao, Pengfei Ma, Xiaolin Wang, Pu Zhou, and Zejin Liu

We report the experimental study on mode instabilities (MI) in large mode area step-index fibers by testing a 30/400 mm step-index fiber in a single-pass co-pumped all-fiberized amplifier, delivering up to ~550 W of extracted output power without MI. The pump power is increased well above the MI threshold to study the temporal dynamics of MI in detail, which are characterized by using both high-speed camera measurement with ~2200 frames per second and photodiode traces. The experimental results are compared with the theoretical results. The MI frequency component is seen to appear on top of system noise, such as electric noise, which shows that system noise may influence the onset of MI. The beam quality of the fiber amplifier is measured, which is ~1.4 before the onset of MI, and degrades gradually to ~2.1 after the onset of MI.

Chinese Optics Letters
Oct. 15, 2014, Vol. 12 Issue s2 S20603 (2014)
Piezoelectric strain coefficient measurement based on elasto-optic effect of fiber
Tao Qin, Zhengyong Li, Chongqing Wu, and Zhi Wang

Based on elasto-optic effect theory, we present a simplified model for the fiber squeezer assisted with a piezoceramic actuator (PZT), and it is experimentally demonstrated with reasonable approximation. Results show that there is a quadratic polynomial relationship between the arccosine reciprocal value of light intensity outputted from polarization analyzer and the reciprocal value of applied voltage for the PZT. Using this formula, the key parameters of the PZT such as the piezoelectric strain coefficient are further obtained. Our conclusions are significant for accurate measurement and polarization control.

Chinese Optics Letters
Nov. 03, 2014, Vol. 12 Issue s2 S20602 (2014)
Rotation function development of four-core fiber optic tweezers
Shang Gao, and Libo Yuan

We demonstrate a new four-core fiber optic tweezers by polishing the end face of a four-core fiber into a truncated pyramid that is asymmetric to fiber cores. Optical intensity propagation and distribution are modeled numerically, and rotation function is developed by means of calculating three-dimensional trapping force and torque in ray optics regime. Simulation results show our tweezers is a feasible approach that can trap and rotate microscopic spheroid objects stably.

Chinese Optics Letters
Oct. 17, 2014, Vol. 12 Issue s2 S20601 (2014)
A broadband chaotic light transmitter
Xinyu Dou, Chenguang Wu, Xiaolei Chen, Hongxi Yin, Qingchun Zhao, Yang Hao, and Nan Zhao

A broadband- and photonic-integrated convenient chaotic-light transmitter with both optical feedback and mutual optical-coupling techniques are proposed. Both numerical simulation and experimental results show that the bandwidth of the presented transmitter is much higher than that of the traditional transmitter with only the optical feedback or the mutual optical-coupling; and the influence of optical feedback strength and mutual optical coupling strength on the bandwidth is also investigated numerically.

Chinese Optics Letters
Mar. 25, 2014, Vol. 12 Issue s1 S10610 (2014)
Low-cost fiber-tip Fabry-Perot interferometer and its application for high temperature sensing
Xiaogang Jiang, Daru Chen, Jie Shao, Gaofeng Feng, and Junyong Yang

We propose, fabricate, and demonstrate a compact fiber-tip Fabry-Perot interferometer of microcavity for high-temperature sensing. The microcavity which can be used for temperature or pressure sensing is fabricated by using arc discharge at the end of a multimode fiber, which is processed with chemical etching. Advantages of the sensor based on the fiber-tip Fabry-Perot interferometer are small size, easy fabrication, low cost and may have potential applications in space-limited and high-temperature environment.

Chinese Optics Letters
Mar. 20, 2014, Vol. 12 Issue s1 S10609 (2014)
A high-performance compact vibration sensor based on fiber Bragg grating
Xiaolei Zhang, Faxiang Zhang, Shujuan Li, Meng Wang, Lujie Wang, Zhiqiang Song, Zhihui Sun, Haifeng Qi, Chang Wang, and Gangding Peng

A high-performance compact vibration sensor based on fiber Bragg grating (FBG) is designed. The acceleration sensitivity of the FBG vibration sensor is measured to be larger than 30 pm/g. From 10 Hz to 250 Hz, a quite flat frequency-response curve can be obtained with additional damping, which suppresses the resonance peak effectively. A phase-generated carrier (PGC) demodulation technique realized by compact reconfigurable input and output (RIO) system is applied in our sensing system.

Chinese Optics Letters
Mar. 04, 2014, Vol. 12 Issue s1 S10608 (2014)
Large mode area and nearly zero flattened dispersion photonic crystal fiber by diminishing the pitch of the innermost air-holes-ring
Jiajia zhao, Jin Hou, Chunyong Yang, Zhiyou Zhong, Yihua Gao, and Shaoping Chen

In this study, we propose that by diminishing only the pitch of the innermost air-holes-ring of a HF1 photonic crystal fiber, both an effective mode area up to 100 μm2 at 1.55 μm wavelength and nearly zero dispersion of 0.2 ± 1 ps/(km·nm) within a spectrum range of 1.23–1.65 μm can be achieved simultaneously. Because only one parameter is needed to be tuned in the proposed design scheme, the fiber would be easier to be fabricated compared to other fibers using either multiple changing parameters or additional kinds of materials and would have potential applications in optical communications.

Chinese Optics Letters
Mar. 03, 2014, Vol. 12 Issue s1 S10607 (2014)
A dither-free bias voltage and driver signal amplitude control technique for optical 8PSK generator
Yupeng Zhu, Yanfu Yang, Bingyang Yan, Jiechang Zhong, and Yong Yao

We propose a dither-free 8-phase-shift-keying (8PSK) generator control scheme for adjusting the bias voltage of Mach–Zehnder modulator (MZM) and the electrical driver signals of two cascaded phase modulators (PMs). The control module includes a delay-line interferometer (DLI), a balanced photo-detector (BPD), and asynchronous sampling data processing. Both analysis and simulation results show that the bias voltage and the driver signal amplitude control can be achieved independently between these cascaded modulators with only one tap point and one control configuration. Finally, the influence of the bit number of A/D converter (ADC) for asynchronous sampling is also discussed for the practical implementation.

Chinese Optics Letters
Mar. 02, 2014, Vol. 12 Issue s1 S10606 (2014)
A novel reconfigurable modulator implementation for generating optical eight-ary PSK/QAM signals
Bingyang Yan, Yanfu Yang, Yupeng Zhu, Jiechang Zhong, Jianchao Cao, Bingxiong Wu, and Yong Yao

We propose a reconfigurable 8PSK/8QAM modulator implementation by employing two cascaded QPSK modulators with an interferometer in between. With simple control of bias voltage of the interferometer, a flexible switching between 8PSK/8QAM can be achieved. The transmission performance of the generated 8PSK/8QAM signals over different link scenarios is compared via numerical simulations. The results reveal that the proposed implementation has the capability of maximizing transmission distance in the case of ynamic optical network.

Chinese Optics Letters
Mar. 02, 2014, Vol. 12 Issue s1 S10605 (2014)
All-dielectric metameric filters for optically variable devices
Yikun Bu, Rong Guo, Yankai Li, Zengyou Meng, and Nan Chen

In order to increase the anti-counterfeiting performance of optically variable devices, the innovative interference security image structures based on metamerism have been developed. In this letter, we show a pair of all-dielectric metameric filters offering a hidden image effect with the color shift at a specific angle of observation. These filters are designed by two materials TiO2/SiO2 based on the different angle color target optimization. The 6-layer- and 9-layer stacks are achieved and the performance of prototype filters prepared by remote plasma sputtering is shown. The color difference index of the experiment is up to 1.19, which shows good metameric matching effect.

Chinese Optics Letters
Mar. 02, 2014, Vol. 12 Issue s1 S10604 (2014)
Measurement of Er3+-doped concentration in optical fiber by using fiber Bragg grating Fabry-rerot cavity ring-down spectrum
Haiyan Chen, Lilin Chen, Cong Chen, Meng Wang, Qi Li, and Kaiqiang Huang

We propose and experimentally demonstrate a novel approach to measure the Er3+ concentration in Er3+-doped silica fiber by fiber Bragg grating Fabry-Perot (FBG-FP) cavity ring-down spectrum. The relationship between the cavity ring-down time and the Er3+-doped concentration is derived. The results demonstrate that the cavity ring-down time is a function of the temperature of FBG, and an Er3+-doped concentration of 0.3 × 1025 m-3 at the FBG operation temperature of 25℃ is obtained, which is consistent with the commercial Er3+-doped silica fiber parameter. The results obtained have theoretical guidance and develop a new method to measure the ion doped concentration in solid matter.

Chinese Optics Letters
Mar. 04, 2014, Vol. 12 Issue s1 S10603 (2014)
The network test tools based on SDN
Song Yang, Xiaoguang Zhang, Lixia Xi, and DongHe Zhao

Based on Software defined network (SDN), we brought up the NetWork Test Tools for communication networks. It is an innovative application of SDN, which can support the application traffic test.

Chinese Optics Letters
Feb. 28, 2014, Vol. 12 Issue s1 S10602 (2014)
Analysis and application of fiber bending in high-speed optical communication
Song Yang, Xiaoguang Zhang, Lixia Xi, and Donghe Zhao

Fiber Bending causes the bending loss, which mainly reduces the optical signal noise ratio (OSNR) of the optical transmission signal. When the fiber is bent to a certain degree, it will cause an interrupt signal. In this paper, we do the theoretical analysis and simulation for fiber bending, and present some engineering application examples.

Chinese Optics Letters
Feb. 26, 2014, Vol. 12 Issue s1 S10601 (2014)
Data-aided channel estimation and frequency domain equalization of minimum-shift keying in optical transmission systems
Dingxin Xie, Jing He, Lin Chen, Jin Tang, and Ming Chen

We demonstrate a digital optical communication system based on minimum shift keying (MSK) signal transmission with coherent detection. 5-Gb/s MSK signal can transmit over a 160-km standard single mode fiber (SSMF) without phase compensation. At the receiver, we use data-aided channel estimation and frequency domain equalization (FDE) techniques in the digital signal processing (DSP) algorithm, then analyze its performance characteristics compared with quadrature phase shift keying (QPSK) format. The simulation results show that the MSK format will be a potential candidate for next-generation access network.

Chinese Optics Letters
Apr. 04, 2014, Vol. 12 Issue 4 040604 (2014)
20-Gbps low cost WDM-OFDM-PON downstream transmission with tunable f ilter and linear APD module
Hao He, Jun Li, Meihua Bi, and Weisheng Hu

This letter presents a low cost solution for wavelength division multiplexed orthogonal frequency division multiplexed passive optical network (WDM-OFDM-PON) with widely tunable optical filter and linear small form-factor pluggable (SFP) module. With 9-nm tunable range from 1551 to 1560 nm, the tunable filter can support up to 10-channel 100-GHz spacing WDM PON system. A linear avalanche photodiode (APD) based SFP+ module is designed for optical OFDM signal demodulation, which can provide better receiver performance compared with limiting APD module. Experimental results show that ~34 dB power budget can be achieved in 4×5-Gbps WDM-OFDM-PON system, which can satisfy the transmission requirements of next generation PON system.

Chinese Optics Letters
Apr. 04, 2014, Vol. 12 Issue 4 040603 (2014)
Hybrid approach for loss recovery mechanism in OBS networks
Ramesh P. G. V., and Prita Nair

This letter reports a study of a hybrid burst assembly and a hybrid burst loss recovery scheme (delay-based burst assembly and hybrid loss recovery (DBAHLR)) which selectively employs proactive or reactive loss recovery techniques depending on the classification of traffic into short term and long term, respectively. Traffic prediction and segregation of optical burst switching network flows into the long term and short term are conducted based on predicted link holding times using the hidden Markov model (HMM). The hybrid burst assembly implemented in DBAHLR uses a consecutive average-based burst assembly to handle jitter reduction necessary in real-time applications, with variations in burst sizes due to the non-monotonic nature of the average delay handled by additional burst length thresholding. This dynamic hybrid approach based on HMM prediction provides overall a lower blocking probability and delay and more throughput when compared with forward segment redundancy mechanism or purely HMM prediction-based adaptive burst sizing and wavelength allocation (HMM-TP).

Chinese Optics Letters
Apr. 04, 2014, Vol. 12 Issue 4 040602 (2014)
63-Tb/s (368×183.3-Gb/s) C- and L-band all-Raman transmission over 160-km SSMF using PDM-OFDM-16QAM modulation
Chao Li, Ming Luo, Xiao Xiao, Jie Li, Zhixue He, Qi Yang, Zhu Yang, and Shaohua Yu

By using PDM-OFDM-16QAM modulation, all-Raman amplification, coherent detection, and 7% forward error correction (FEC) threshold, we successfully demonstrate 63-Tb/s (368×183.3-Gb/s) signal over 160-km standard single mode fiber (SSMF) transmission in the C- and L-bands with 25-GHz channel spacing. 368 optical channels with bandwidth spacing of 25 GHz are generated from 16 external cavity laser sources. After 160-km SSMF transmission, all tested bit error rate (BER) are under 3.8×10-3, which can be recovered by 7% FEC threshold. Within each channel, we achieve the spectral efficiency of 6.85 bit/s/Hz in C/L band.

Chinese Optics Letters
Apr. 04, 2014, Vol. 12 Issue 4 040601 (2014)
Ultra-high-speed single red–green–blue light-emitting diode-based visible light communication system utilizing advanced modulation formats
Nan Chi, Yuanquan Wang, Yiguang Wang, Xingxing Huang, and Xiaoyuan Lu

We report a 3.75-Gb/s visible light communication (VLC) system thatusessingle-carrier frequency-domain equalization (SC-FDE) based on a single red–green–blue (RGB) light-emitting diode (LED), with the measured bit error rates (BERs) under a pre-forward-error-correction threshold of 3.8 \times 10-3. The fundamental characteristics of an RGB-LED-based VLC system are measured. We also compare SC-FDE with OFDM in terms of peak-to-average power ratio and BER performance, which shows that SC-FDE outperforms the OFDM modulation scheme.

Chinese Optics Letters
Jan. 08, 2014, Vol. 12 Issue 1 010605 (2014)
160-Gb/s NRZ-DQPSK optical transmission system employing QC-LDPC code
Sha Li, Chongxiu Yu, Zhe Kang, Gerald Farrell, and Qiang Wu

A quasi-cyclic low-density parity check (QC-LDPC) code is constructed by an improved stability of the shortest cycle algorithm for 160-Gb/s non-return zero differential quadrature phase shift keying (NRZ-DQPSK) optical transmission system with the fiber-based optical parametric amplifier (FOPA). The QC-LDPC code with stability of the shortest cycle reduces the bit error ratio (BER) to 10-14 and restrains the error floor effectively.

Chinese Optics Letters
Jan. 08, 2014, Vol. 12 Issue 1 010604 (2014)
Dispersion compensation properties of dual-concentric core photonic crystal fibers
Lihong Han, Liming Liu, Zhongyuan Yu, Huijie Zhao, Xin Song, Jinhong Mu, Xiu Wu, Junjie Long, and Xi Liu

The dispersion compensation properties of dual-concentric core photonic crystal fibers are theoretically investigated in this letter. The effects of geometric structure on the dispersion properties of dual-concentric core photonic crystal fibers are carefully studied by finite element method. The first layer of holes around the core area is enlarged in a new manner with the near-core point fixed. Considering the tradeoff among several parameters, results show that the dispersion compensation wavelength and strength can be tuned to desired values by constructing an appropriate design of the geometric structure of photonic crystal fibers.

Chinese Optics Letters
Jan. 08, 2014, Vol. 12 Issue 1 010603 (2014)
Optical f iber sensor based on the short-range surface plasmon polariton mode
Xiaoyan Wang, Fang Liu, Ao Liu, Boyu Fan, Kaiyu Cui, Xue Feng, Wei Zhang, and Yidong Huang

An optical fiber sensor for ultrathin layer sensing based on short-range surface plasmon polariton (SRSPP) is proposed, and the sensing characteristics are theoretically analyzed. Simulation results indicate that even for a detecting layer much thinner than the vacuum wavelength, a resolution as high as 3.7\times 10-6 RIU can be obtained. Moreover, an average thickness-detection sensitivity of 6.2 dB/nm is obtained, which enables the sensor to detect the thickness variation of the ultrathin layer up to tens of nanometers. The sensitive region of thickness could be adjusted by tuning the structure parameters.

Chinese Optics Letters
Jan. 08, 2014, Vol. 12 Issue 1 010602 (2014)
Secure and noise-free holographic encryption with a quick-response code
Zhenbo Ren, Ping Su, Jianshe Ma, and Guofan Jin

In holographic encryption, double random-phase encoding in the Fresnel domain (DRPEiFD) is a prevalent encryption method because it is lensless and secure. However, noises bring adverse effects during decryption. In this letter, we introduce quick-response (QR) coding during encryption to resist noises. We transform the original information into a QR code and then encrypt the code as a hologram through DRPEiFD. To retrieve the input, we decrypt the hologram in the opposite manner to the encryption and subsequently obtain a QR code with noises. By scanning this code with proper applications in smartphones, we can obtain a noise-free retrieval. Numerical experiments and images scanned by a smartphone are shown to validate our proposed method.

Chinese Optics Letters
Dec. 25, 2013, Vol. 12 Issue 1 010601 (2014)
Research on characterization of frequency chirp in RSOA
Junfang Shang, Jian Chen, Rujian Lin, Yingxiong Song, and Yingchun Li

Light chirp is a major issue in optical fiber links. This letter deals with precise characterizations of the frequency chirp parameters of reflective semiconductor optical amplifiers (RSOAs). The RSOA chirp properties are represented by transient and adiabatic chirps, whose parameters are characterized utilizing a ratio between the phase and the amplitude modulation depths of the RSOA when modulated with sine waves. Utilizing a high-resolution optical spectrum analyzer, a RSOA linewidth enhancement factor \alpha and an adiabatic factork are obtained experimentally, based on which the influence of chirp parameters on the transmission performance of non-return-to-zero (NRZ) signals can be analyzed.

Chinese Optics Letters
Sep. 29, 2013, Vol. 11 Issue s2 S20607 (2013)
Packaging multi-wavelength DFB laser array using REC technology
Yi Ni, Xuan Kong, Xiaofeng Gu, Xiangfei Chen, Guanghui Zheng, and Jia Luan

Packaging of Distributed feedback (DFB) laser array based on reconstruction-equivalent-chirp (REC) technology is a bridge from chip to system, and influences the practical process of REC chip. In this letter, DFB laser arrays of 4 channel @1 310 nm and 8 channel @1 550 nm are packaged. Experimental results show that both 4 channel @1 310 nm and 8 channel @1 550 nm have uniform wavelength spacing and average side mode suppression ratio (SMSR)>35 dB. When I=35 mA, we get the total output power 1 mW of 4 channel @1 310 nm, and 227 \mu W of 8 channel @1 550 nm, respectively. The high frequency characteristic of the packaged chips is also demonstrated, and the requirements of 4 \times 10 G or even 8\ teims 10 G system can be reached. we demonstrate the practical and low cost performance of REC technology and indicates its potential application in the future fiber-to-the-home (FTTH).

Chinese Optics Letters
Sep. 15, 2013, Vol. 11 Issue s2 S20606 (2013)
Comparative study on self-pulsation ef fect in Erbium-Ytterbium co-doped f iber laser under bi-directional and unidirectional cladding-pumping regime
Jun Liu, Deyuan Shen, Xiaoqi Zhang, and Dianyuan Fan

Self-pulsation effects of cladding-pumped Erbium-Ytterbium co-doped fiber laser (EYDFL) at around the lasing threshold are investigated. It is demonstrated that laser output of the Erbium-Ytterbium co-doped fiber under the bi-directional pumping regime is more stable than that under the unidirectional pumping one due to the relatively uniform pumping of the fiber. Mechanisms of self-pulsations in the laser system are discussed and possible techniques to avoid self-pulsing and stabilize the laser are proposed.

Chinese Optics Letters
Aug. 20, 2013, Vol. 11 Issue s2 S20605 (2013)
Octagonal dual-concentric-core photonic crystal fiber for C-band dispersion compensation with low conf inement loss
Wenbin Liang, Ningliang Liu, Zhihua Li, and Peixiang Lu

A novel kind of octagonal dual-concentric-core photonic crystal fiber (PCF) is calculated with the finite element method (FEM) and proposed for broadband compensation. In order to realize highly negative dispersion, the hole diameter of the second ring is reduced relative to conventional PCFs. Numerical simulation results show that when the diameters of large holes and small holes are 0.77 \mu m and 0.5 \mu m, respectively, and the pitch of the adjacent rings is 1.1 μm, the negative dispersion can achieve about –840 ps.nm-1.km-1 at 1 550 nm and the dispersion slope can match with single mode fiber (SMF-28) perfectly. This PCF can compensate the positive dispersion and also its slope of about 50 times its length of SMF-28 fiber, which is suitable for C-band optical fiber telecommunication as a dispersion compensation fiber.

Chinese Optics Letters
Aug. 20, 2013, Vol. 11 Issue s2 S20604 (2013)
Research on ef fects of active fiber length and dopant concentration on 976-nm Yb-doped fiber amplifiers
Ruixing Wang, Jianqiu Cao, Ying Liu, Shaofeng Guo, and Jinbao Chen

In this letter, the theoretical model and rate equations of Ytterbium-doped double-clad fiber amplifiers are introduced. The output performance of fiber amplifiers is analyzed through numerical simulations considering three main factors: active fiber length, pump power, and dopant concentration. It is found that the output signal power experiences a stable growth and then decreases rapidly as the active fiber length becomes longer. It is also revealed that the dopant concentration shows similar effects as active fiber length on the output signal power of fiber amplifiers.

Chinese Optics Letters
Jul. 17, 2013, Vol. 11 Issue s2 S20603 (2013)
Fiber Bragg grating sensing system based on code division multiple access
Wei Li

A novel fiber Bragg grating (FBG) sensing system based on code division multiple access (CDMA) technology is proposed. CDMA is used to separate each reflected sensor. Simulation of experimental results indicates the CDMA technology combines with optical fiber grating sensing system together successfully. Furthermore, the system using semiconductor optical amplifier (SOA) is experimented. The experimental results show that theory and simulation are correct.

Chinese Optics Letters
Jul. 17, 2013, Vol. 11 Issue s2 S20602 (2013)
A wideband tunable phase shifter based on orthogonal optical single-sideband
Zhihu Wei, Rong Wang, Tao Pu, Guodan Sun, and Tao Fang

We present a novel technique to generate an orthogonally polarized optical single sideband (OSSB) generated by a tunable bandpass filter (TBF).When the OSSB passes through the other polarization modulation (PolM) which is polarization dependent, the phase shift of the optical carrier and first-order sideband is different under different bias. As a result, a wideband tunable phase shifter is realized by adjusting the bias applied to the polarization modulator.

Chinese Optics Letters
Jul. 17, 2013, Vol. 11 Issue s2 S20601 (2013)
Calibrating an optical fiber humidity sensor and applying it in real-time monitoring of relative humidity in fresh concrete
Xiaokang Zhang, Zhuoyong Deng, and Hanlang Xu

A doubly cladding single-mode fiber humidity sensor is fabricated by agarose. The sensor has an insertion loss of -0.08 dB and a power change of -17.83 dB. The responses of the sensor to a relative humidity (RH) range form 30% to 100% at a temperature range form 25 to 34 oC are validated. The experiments demonstrate that the absorbability of agarose gel to moisture decreases with increasing RH in measured gas. We propose a calibration method that uses lookup tables and construct a corresponding calibration matrix. Using the sensor, we conduct real-time monitoring of RH in fresh concrete during its hardening process.

Chinese Optics Letters
Sep. 03, 2013, Vol. 11 Issue 9 090604 (2013)
Effective SNR improvement using a high-power pulsed pump in an ytterbium-doped fiber amplifier
Jing Liu, Lixin Xu, Anting Wang, Chun Gu, Qing Liu, Ankun Wei, Teng Xu, and Hai Ming

A high-power pulsed pump method is proposed to obtain a high-energy output with an improved signaltonoise ratio (SNR) during pulse amplification. Based on numerical analysis, the ytterbium-doped fiber amplifiers are compared under different pumping conditions. At the same signal gain, the output using the high-power pulsed pump shows great SNR improvement and reduced output signal distortion, compared with a continuous-wave pump and a low-power pulsed pump. By adjusting the pump parameters, the amplifier can achieve the optimal output SNR without sacrificing the signal gain. We believe that the high-power pulsed pump scheme is very suitable for the high-energy nanosecond pulse amplification, which has a high SNR requirement.

Chinese Optics Letters
Aug. 30, 2013, Vol. 11 Issue 9 090603 (2013)
Theoretical analysis of the performance of a single side band multicarrier source based on a re-circulating frequency shifter influenced by an optical bandpass filter
Wan Meng, Lixia Xi, Xiaoguang Zhang, Jiachuan Lin, Feng Tian, Hao Tu, and Xia Zhang

An optical bandpass filter (OBF) is an important element whose properties considerably influence the output performance of a single side band multicarrier source based on a re-circulating frequency shifter. The influence of the OBF in the loop on the output spectrum is theoretically analyzed. Numerical simulations and experiments are also carried out. Results show that the steepness and deepness of a nonflat-top filter influence the stability of the spectrum of an output multicarrier. To obtain multicarrier output with tone to noise ratio (TNR) >26 dB and error vector magnitude (EVM) <0.25, the steepness ratio of the filter should be greater than 0.75 and deepness should be larger than 0.99.

Chinese Optics Letters
Aug. 28, 2013, Vol. 11 Issue 9 090602 (2013)
Experimental validation of domain-level gradient-based and hierarchical PCE-based routing and control in heterogeneous optical networks
Rui Lu, Xiaoping Zheng, Nan Hua, Wangyang Liu, and Xiaohui Chen

A domain-level gradient-based routing (DLR) algorithm for heterogeneous optical networks with synchronous digital hierarchy and optical transport network domains is proposed and experimentally validated. This algorithm classifies domains into groups with incremental levels on the basis of domain-level partitioning, and guides paths level by level along a gradient on the basis of interdomain routing tree evolution. The proposed algorithm is implemented in the hierarchical path computation element-based control architecture for connection provisioning. Testbeds with commercial and emulated nodes are established to verify the feasibility and performance of the algorithm. Experimental and emulation results show that DLR effectively performs in terms of network blocking probability, real time characteristics, and scalability.

Chinese Optics Letters
Aug. 22, 2013, Vol. 11 Issue 9 090601 (2013)
Design and demonstration of room division multiplexing-based hybrid VLC network
Zhitong Huang, and Yuefeng

A room division multiplexing (RDM)-based hybrid visible light communication (VLC) network for realizing indoor broadband communication within a multi-room house is presented. The downlink information is transmitted by light-emitting diode lamps, whereas the uplink information is transmitted through WiFi. RDM is introduced to improve the VLC network throughput; in addition, the associated signaling localization and active handoff mechanisms are designed for implementation. The experimental platform demonstrates the effectiveness of the proposed hybrid architecture, along with the RDM and active handoff mechanisms.

Chinese Optics Letters
May. 31, 2013, Vol. 11 Issue 6 060603 (2013)
Enhanced fourth-power algorithm for phase estimation with frequency separation in direct-detection optical OFDM systems
Min Kong, Jiangnan Xiao, Ze Dong, Liuqingqing Yang, Lin Chen, and Jianjun Yu

The optical orthogonal frequency division multiplexing (OFDM) signal is affected by impairments introduced by electrical filters and optical chromatic dispersion. An enhanced fourth-power algorithm for phase estimation with frequency separation is used to estimate and compensate the phase rotation of OFDM subcarriers. The performance of the proposed phase estimation algorithm is evaluated on a 4-Gb/s OFDM signal at different frequencies. Experimental results using the proposed algorithm show a 1.8-dB received power sensitivity improvement at a bit error rate of 1×10-4 after a 100-km standard single-mode fiber transmission, compared with the conventional technique.

Chinese Optics Letters
May. 31, 2013, Vol. 11 Issue 6 060602 (2013)
Joint nonlinear electrical equalization in coherent optical PDM DFT-spread-OFDM systems
Zhiyuan Huang, Fan Zhang, and Zhangyuan Chen

We propose a joint nonlinear electrical equalization approach in coherent optical discrete-Fourier-transform spread orthogonal-frequency-division-multiplexing (DFT-spread-OFDM) systems with polarization division multiplexing (PDM). This method is based on an adaptive Volterra series expansion for nonlinear distortions of two orthogonal polarizations. The nonlinear electrical equalization is validated through numerical simulation of 100-Gb/s quadrature phase shift keying and 200-Gb/s 16 quadrature amplitude modulation PDM DFT-spread-OFDM systems.

Chinese Optics Letters
May. 30, 2013, Vol. 11 Issue 6 060601 (2013)
Metal-organic-coordinated supramolecular material filled hollow core photonic crystal fiber for temperature sensor
Jiarong Zheng, Yongqin Yu, Chenlin Du, Peiguang Yan, Mei Pan, Jishun Wang, and Zhuoye Liu

A novel hollow core photonic crystal fiber (HCPCF) temperature sensor that is based on intensity modulation and metal-organic-coordinated Co-cage supramolecular material filled in the air holes of the fiber is introduced. Temperature-dependent character of the structure in the range of 90 to 190 oC has been investigated. The numerical analyses show that the confinement loss is both decreased linearly with temperature at 1 064 and 1 550 nm. For a 6.5-cm long HCPCF, the sensitivity of temperature is experimentally determined to be 9.5 10?2 dB/oC at 1 064 nm, and 7.9 \times 10?2 dB/oC at 1 550 nm, respectively.

Chinese Optics Letters
Dec. 20, 2012, Vol. 10 Issue s2 S20611 (2012)
PMD mitigation by LDPC codes with polarization scramblers
Lixia Xi, Lei Yang, Haoran Chen, Dahai Han, and Xiaoguang Zhang

Low-density parity-check (LDPC) codes combined with polarization scramblers are used to mitigate polarization mode dispersion (PMD) in 40-Gb/s optical fiber system. The simulations are performed to compare the correction performance of LDPC codes and Reed–Solomon (RS) code used in this PMD mitigation scheme. Results show that LDPC codes can achieve a better performance than the RS code with the same redundancy. The scrambling speed of polarization scramblers for LDPC codes system is also discussed, and the optimal speed of 10 MHz is obtained.

Chinese Optics Letters
Jan. 01, 1900, Vol. 10 Issue s2 S20609 (2012)
Novel methods for the simulation of impairments in optical network
Song Yang, Xiaoguang Zhang, Lixia Xi, and Wenyu Peng

The practical and effective methods to implement the impairments in optical network are put forward and applied in testing the equipment reliability and transmission quality under the condition of bit errors. The results show that the proposed methods are effective in equipment reliability testing.

Chinese Optics Letters
Nov. 23, 2012, Vol. 10 Issue s2 S20608 (2012)
Vector projection method used for calibration of the polarimeter module
Wenbo Zhang, Qingyue Di, Lixia Xi, and Xiaoguang Zhang

To solve the problem of calibrating the polarimeter module, we design a vector projection (VP) method which is based on the theorem of vector projection and the principle of calibration. The calibration matrix can be calculated by VP method with the given data. The experimental result shows that, in comparison with the common iterative algorithm, VP method has very high computational efficiency and accuracy. The measured error of the degree of polarization (DOP) is less than 3%.

Chinese Optics Letters
Nov. 13, 2012, Vol. 10 Issue s2 S20607 (2012)
Long-term stable f iber–based picoseconds optical synchronization system in SG-II
Xiaochao Wang, Wei Fan, Guoyang Li, Peng Zhang, Shengjia Zhang, and Xuechun Li

A fiber-based precision synchronization triggering system using fiber pulse stacker combined with high-speed electronics processing technology is presented. The relative timing jitter between two laser pulses achieved with this system is 4.09 ps (rms) in 2 h. The impact of the optical pulse amplitude fluctuation on the timing jitter is effectively reduced by high-speed analog-digital conversion and the reliability of the synchronization measurement system is confirmed.

Chinese Optics Letters
Nov. 09, 2012, Vol. 10 Issue s2 S20606 (2012)
Demodulation errors ef fects on f iber-optic current sensor
Yuan Mou, Damin Zhang, Peng Pang, Dongning Sun, Weilin Xie, Yi Dong, and Weisheng Hu

Harmonic imbalance and phase deviation affect the linearity and sensitivity of fiber-optic current sensor (FOCS). The effects of these two factors are theoretically analyzed. Both the harmonic imbalance and phase deviation must be restricted within a tolerable range of variation. By experimentally optimizing these two factors, the scale factor (SF) error of the current sensor within the range of 0.5% is demonstrated and implemented.

Chinese Optics Letters
Nov. 09, 2012, Vol. 10 Issue s2 S20605 (2012)
Ef fects of fill factor of fiber combiner on the all-fiber coherent combining
Jie Li, Haichuan Zhao, Zilun Chen, Hang Zhou, Haihuan Chen, Xuanfeng Zhou, and Xiaojun Xu

A new approach for an all-fiber coherent beam combining by active phasing a fiber combiner is presented, and effect of fill factor of the fiber combiner on the all-fiber coherent combining is also numerical analyzed. Experimental results show that the power in the bucket (PIB) increases more than a factor of 2.35. The results validate the feasibility of coherent combining using a fiber combiner. The PIBs with different fill factors are calculated and the results show that the coherent combining is reduced with the increasing fill factor.

Chinese Optics Letters
Sep. 23, 2012, Vol. 10 Issue s2 S20603 (2012)
All f iber 1.55- \mu m SLM DBR f iber laser with 10-Gb/s NRZ-OOK transmission over 100-km f iber link
Suchun Feng, Kangping Zhong, Nan Jia, Qi Li, and Wanjing Peng

The 10-Gb/s non-return-to-zero (NRZ) on-off-keying (OOK) data transmission over a 100-km transmission fiber link employing an all fiber 1.55-\mu m single longitudinal mode (SLM) distributed Bragg reflector (DBR) fiber laser as the signal source is demonstrated. The experimental result is comparable with that of the transmission system by using a commercially semiconductor distributed feedback (DFB) laser.

Chinese Optics Letters
Sep. 23, 2012, Vol. 10 Issue s2 S20602 (2012)
High-speed polarization-shift keying: experimental and numerical investigation
Zhengyong Li, Zhihao Wang, Jian Wang, Xiangzhi Yu, Lanlan Liu, and Chongqing Wu

Polarization-shift keying (PolSK), which is performed based on two orthogonal polarization states, is investigated in detail by experimental and numerical simulation for data rates of 10 and 40 Gb/s. Using a general Mach-Zehnder intensity modulator, a new type of PolSK transmitter is presented and demonstrated by polarization coupling technology. And then the PolSK transmission is experimentally demonstrated in a 50-km single mode fiber system, whose eye diagrams show that the PolSK transmitter works well at 10 Gb/s and the power fluctuation is ~8%. Further simulation reveals that the PolSK has great potential in long distance transmission with data rate of 40 Gb/s, while its error-free distance goes beyond 2 100 km.

Chinese Optics Letters
Sep. 23, 2012, Vol. 10 Issue s2 S20601 (2012)
Mth-power-law method to compensate laser linewidth of 100 Gb/s PDM-CO-OFDM systems
Xuejun Liu, Haiying Liang, Bo Dai, and Bo Lan

Laser linewidth is the important determinant of applying coherent optical orthogonal frequency division multiplexing (CO-OFDM) in optical transmission systems. The laser linewidth impairments in 100-Gb/s polarization division multiplexing CO-OFDM (PDM-CO-OFDM) system without optical dispersion compensation are compensated by the phase drift compensator (PDC) based onMth-power-law method located at the receiver. PDC is more effective to compensate the phase drift due to laser linewidth. Simulating results show that the maximum Q factor can be increased by almost >10.0 dB for back-to-back (BtoB). For the 100-kHz linewidth system of 800-km system, a benefit of about 4.9 dB is possible for the maximum Q factor.

Chinese Optics Letters
Jun. 20, 2012, Vol. 10 Issue s1 S10609 (2012)
Switchable dual-wavelength erbium-doped fiber laser based on polarization-maintaining fiber Sagnac loop mirror and fiber Bragg gratings
Ou Xu, and Suchun Feng

A switchable dual-wavelength erbium-doped fiber laser is proposed and experimentally demonstrated, which is constructed by a polarization-maintaining fiber Sagnac loop mirror (PMF-SLM) and two FBGs with different wavelengths. Wavelength switching operation is achieved by properly adjusting the polarization controller (PC) in the PMF-SLM. Stable single- or dual-wavelength lasing output can be realized. The maximum amplitude variation for every lasing wavelength is less than 1 dB, and the signal-to-amplified spontaneous emission (ASE) ratio is about 35 dB.

Chinese Optics Letters
Jun. 20, 2012, Vol. 10 Issue s1 S10608 (2012)
Design of the photonic crystal f iber-based surface plasmon resonance sensors
Yang Peng, Jing Hou, Zhihe Huang, Bin Zhang, and Qisheng Lu

The photonic crystal fiber-based surface plasmon resonance (SPR) refractive index sensor is demonstrated by using post-processing technique of photonic crystal fiber (PCF). The coupling of PCF mode field and SPR can be controlled by the air holes collapsing in PCF. The effects of metal film thickness and air hole diameter on the sensor at different wavelength are simulated by using finite element method. The simulation results are analyzed by using the modes matching theory. The amplitude based and spectrum based detection methods are discussed respectively. Refractive index sensitivity 1 700 nm/per refractive index unit (RIU) can be achieved for an aqueous analyte.

Chinese Optics Letters
Jun. 20, 2012, Vol. 10 Issue s1 S10607 (2012)
Spectrum consecutiveness based routing and spectrum allocation in flexible bandwidth networks
Ying Wang, Jie Zhang, Yongli Zhao, Junyan Liu, and Wanyi Gu

In flexible bandwidth optical networks, the dynamic lightpath setup and teardown inevitably lead to spectrum fragmentation which blocks the new connections. In order to effectively allocate the spectrum along a better route, a notion of available spectrum consecutiveness is introduced and three dynamic routing and spectrum assignment (RSA) algorithms are proposed in this letter accordingly. The novel algorithms retain the spectrum consecutiveness as much as possible when establishing a lightpath and reduce the spectrum fragmentation. Simulation results indicate that our proposed spectrum consecutiveness based RSA algorithms achieve lower blocking probability and higher adaptability to more line rates mixture.

Chinese Optics Letters
May. 16, 2012, Vol. 10 Issue s1 S10606 (2012)
Modif ied LDPC decoding rule for pulse position modulation
Yongsheng Wu, Aiying Yang, Yunan Sun, and Qiuling Zhao

In this letter, a modified grouped bit-flipping decoding algorithm for low-density parity-check (LDPC) coded pulse position modulation (PPM) signals is proposed. The improvement in performance is observed by Monte Carlo simulations. The coding gain is more than 1 and 2 dB for the order numbers of PPM of 4 and 16 respectively.

Chinese Optics Letters
May. 09, 2012, Vol. 10 Issue s1 S10605 (2012)
All-f iber nanosecond pulsed f iber amplif ier
Feng Yu, Tingwu Ge, Jingjing Dai, Wenqi Zhang, and Zhiyong Wang

An all fiber structure nanosecond pulse is realized through two stage amplifiers, which has 8.78 W in average power, 1 064 nm in wavelength, 11 kHz in repetition frequency, 125 ns in impulse duration, 0.8 mJ in single pulse power, 5.3 kW in peak power. The optic-optic conversion efficiency is 55.6%. Meanwhile, we setup another system, a nanosecond pulse is realized through just one stage amplifier, which has 10.10 W in average power, 1 064 nm in wavelength, 11 kHz in repetition frequency, 150 ns in impulse duration, 0.92 mJ in single pulse power, 6 kW in peak power. The optic-optic conversion efficiency is 67%. The only one stage of amplifier simplifies the system structure and reduces the cost.

Chinese Optics Letters
Apr. 25, 2012, Vol. 10 Issue s1 S10604 (2012)
System performance influenced by channel plan for coherent PDM-QPSK/OOK hybrid transmission systems
Xiaogang Yi, Yan Li, Jian Wu, Kun Xu, Xiaobin Hong, Hongxiang Guo), and Jintong Lin

The impact of channel occupancies on polarization division multiplexed quadrature phase-shift keying (PDM-QPSK)/on-off keying (OOK) hybrid systems is investigated numerically over a dispersion-managed transmission link. In addition to keep large residual dispersion per span, we find that suitable channel occupancy also help to improve the nonlinear tolerance of such hybrid systems significantly. About 2.4-dBm more nonlinear threshold (NLT) and 75% increase of transmission reach can be obtained under suitable channel plan for 112-Gb/s PDM-QPSK/OOK hybrid systems.

Chinese Optics Letters
May. 09, 2012, Vol. 10 Issue s1 S10603 (2012)
Supercontinuum generation in web-like microstructure optical fiber
Jie Shu, Peiguang Yan, Jian Zhao, Junqing Zhao, Shuangchen Ruan, Huifeng Wei, and Jie Luo

A novel web-like microstructure optical fiber (MOF) is fabricated which serves as a bundle of suspended submicron-thickness membranes as planar waveguides and their Y-type connection nodes as the secondary cores. The polarization dependent visible continuum coverage from 350 to 1 000 nm is generated from a 20-cm-long membrane of 0.55-μm-thickness pumped by 200-fs laser at 800 nm. We also obtain narrowband signal of blue dispersive wave around 410 nm with the up-conversion efficiency routinely beyond 20% in the Y-type secondary core in another smaller web-like MOF.

Chinese Optics Letters
Apr. 18, 2012, Vol. 10 Issue s1 S10602 (2012)
Theoretical analysis of temperature sensor based on dual-core fiber
Daru Chen, Gufeng Hu, Baojin Peng, Genzhu Wu, and H. Y. Tam

A novel temperature sensor based on a dual-core fiber (DCF) is proposed and theoretically analyzed. The DCF-based temperature sensor is simply formed by splicing a segment of DCF to two segments of single mode fibers, where the DCF is used as the sensing element. The mode coupling between two fiber cores of the DCF is sensitive to the temperature-induced index change of the silica in the DCF. Simulations show that there is a linear relationship between the temperature of the DCF and the wavelength shift of the output spectrum of the DCF-based temperature sensor when the broadband light is injected into one fiber core of the DCF. Temperature sensors based on DCFs with different parameters for temperature sensing are also investigated.

Chinese Optics Letters
Apr. 18, 2012, Vol. 10 Issue s1 S10601 (2012)
Microball lens integrated fiber probe for optical frequency domain imaging
Jae-Ho Han, and J. U. Kang

An integrated microball lens fiber catheter probe is demonstrated, which has better lateral resolution and longer working distance than a corresponding bare fiber probe with diverging beam for Fourier domain optical coherence tomography (FDOCT). Simulation results are shown to gain the effect of the distance between the microball lens and the bare fiber to the focusing plane and beam width. The freedom of modifying the working distance and lateral resolution is shown. This is achieved by changing the gap distance between the single-mode fiber and the microball lens within the packaged surgical needle catheter without using an additional beam expander having a fixed length. The probe successfully acquired cross-sectional images of ocular tissues from an animal sample with the proposed miniaturized imaging probe.

Chinese Optics Letters
Jul. 11, 2011, Vol. 9 Issue 9 090608 (2011)
Flextensional fiber Bragg grating-based accelerometer for low frequency vibration measurement
Jinghua Zhang, Xueguang Qiao, Manli Hu, Zhongyao Feng, Hong Gao, Yang Yang, and Rui Zhou

The intelligent structural health monitoring method, which uses a fiber Bragg grating (FBG) sensor, is a new approach in the field of civil engineering. However, it lacks a reliable FBG-based accelerometer for taking structural low frequency vibration measurements. In this letter, a flextensional FBG-based accelerometer is proposed and demonstrated. The experimental results indicate that the natural frequency of the developed accelerometer is 16.7 Hz, with a high sensitivity of 410.7 pm/g. In addition, it has a broad and flat response over low frequencies ranging from 1 to 10 Hz. The natural frequency and sensitivity of the accelerometer can be tuned by adding mass to tailor the sensor performance to specific applications. Experimental results are presented to demonstrate the good performance of the proposed FBG-based accelerometer. These results show that the proposed accelerometer is satisfactory for low frequency vibration measurements.

Chinese Optics Letters
Jul. 11, 2011, Vol. 9 Issue 9 090607 (2011)
Proposal of metal bellows-based fiber Bragg grating accelerometer
Jinghua Zhang, Xueguang Qiao, Manli Hu, Zhongyao Feng, Hong Gao, Yang Yang, and Rui Zhou

A metal bellows-based fiber Bragg grating (FBG) accelerometer is proposed and experimentally demonstrated. The optical fiber (containing the FBG) is pre-tensioned, and the two ends of the optical fiber are fixed directly from the shell to the inertial mass. In this design, the FBG is uniformly tensioned to obtain a constant strain distribution over it. By employing this configuration, the FBG always has a sharp reflection characteristic with no broadening in its reflection spectrum during wavelength shifting. Dynamic vibration measurements show that the proposed FBG accelerometer has a wide frequency response range (5–110 Hz) and an extremely high sensitivity (548.7 pm/g). The two important indicators of FBG accelerometer can be tuned by the addition of mass to tailor the sensor performance to specific applications, identifying it as a good candidate for structural health monitoring.

Chinese Optics Letters
Jun. 30, 2011, Vol. 9 Issue 9 090606 (2011)
New transfer matrix method for long-period fiber gratings with coupled multiple cladding modes
Guodong Wang, and Yunjian Wang

A new transfer matrix method for long-period fiber gratings with coupled multiple cladding modes is proposed and numerically characterized. The transmission spectra of uniform and non-uniform longperiod fiber gratings are numerically characterized. The theoretical results excellently agree with the experimental measurements. Compared with commonly used methods, such as using the fourth-order adaptive step size control of the Runge-Kutta algorithm in solving the coupled mode equation, the new transfer matrix method exhibits a faster calculation speed.

Chinese Optics Letters
Jul. 11, 2011, Vol. 9 Issue 9 090605 (2011)
Eye-safe, single-frequency pulsed all-fiber laser for Doppler wind lidar
Yuan Liu, Jiqiao Liu, and Weibiao Chen

A single-frequency pulsed erbium-doped fiber (EDF) laser with master-oscillator power-amplifier configuration at 1 533 nm is developed. A short-cavity, erbium-doped phosphate glass fiber laser is utilized as a seeder laser with a linewidth of 5 kHz and power of 40 mW. The seeder laser is modulated to be a pulse laser with a repetition rate of 10 kHz and pulse duration of 500 ns. The amplifier consists of two pre-amplifiers and one main amplifier. The detailed characteristics of the spectrum and linewidth of the amplifiers are presented. A pulse energy of 116 1J and a linewidth of 1.1 MHz are obtained. This laser can be a candidate transmitter for an all-fiber Doppler wind lidar in the boundary layer.

Chinese Optics Letters
Jul. 12, 2011, Vol. 9 Issue 9 090604 (2011)
Demultiplexer based on an of f-axis holographic lens with low monochromatic aberrations
Sensen Li, Shou Liu, Xiangsu Zhang, and Xuechang Ren

A novel scheme is proposed, in which the aberrations in the off-axis holographic lenses used as demultiplexers are reduced to a low enough level for relatively small channel spacing. The scheme includes optimizing the recording and reconstruction geometries and collimating the reconstruction wave with a gradient-index lens. A demultiplexer operated in the 1 550-nm band with 5-nm channel spacing and ?\infty-dB crosstalk is obtained using the scheme. The channel spacing can be decreased to 2 nm by etching the cladding of the output fibers to a smaller size.

Chinese Optics Letters
Jun. 30, 2011, Vol. 9 Issue 9 090603 (2011)
Polarization and modal field properties of quinquangular-core photonic crystal fibers
Xuyou Li, Hanrui Yang, Qiuli Zheng, Jinhui Hao, and Wei Hong

A kind of highly birefringent quinquangular-core photonic crystal fiber (Q-PCF) structure is proposed and analyzed by full-vector finite element method (FEM). The modal field, effective index, and birefringence properties are investigated. From the numerical results, it is found that the birefringence of the new polarization-maintaining PCFs is at least five times larger than that of the standard highly birefringent hexagonal PCFs (H-PCFs) with the same hole pitch, hole diameter, and whole hole area as that of the new PCFs at 1 550 nm. Moreover, the modal field of the new PCFs could be better restricted than that of the standard highly birefringent H-PCFs; hence, the loss of fibers could be reduced.

Chinese Optics Letters
Jun. 21, 2011, Vol. 9 Issue 9 090602 (2011)
Wavelength-selective switch based on a high f ill-factor micromirror array
Sihua Li, Zhujun Wan, Jing Xu, Shaolong Zhong, and Yaming Wu

A novel design and fabrication approach for a high fill-factor micro-electro-mechanical system (MEMS) micromirror array-based wavelength-selective switch (WSS) is presented. The WSS is composed of a polarization-independent transmission grating and a high fill-factor micromirror array. The WSS is successfully demonstrated based on the fabricated high fill-factor micromirror array. Test results show that the polarization-dependent loss (PDL) is less than 0.3 dB and that the insertion loss (IL) of the wavelength channel is about –6 dB. The switching function between the two output ports of WSS is measured. The forward switching time is recorded to be about 0.5 ms, whereas the backward switching time is about 7 ms.

Chinese Optics Letters
Jun. 16, 2011, Vol. 9 Issue 9 090601 (2011)
Label swapping and packet transmission of DPSK-labeled PPM signal in optical label switching
Yan Shi, Qi Zhang, Chongxiu Yu, Xiangjun Xin, Cang Jin, and Rui Zhou

A label swapping scheme of an optical labeled signal with differential phase shift keying (DPSK) for label at 2.5 Gb/s and pulse position modulation (PPM) for payload at 40 Gb/s is demonstrated by simulation. Power penalties of ?1.8 and ?0.8 dB are achieved for both the payload and label over 80-km single mode fiber (SMF) transmission. This labeling scheme allows the use of four-wave mixing (FWM) in semiconductor optical amplifier (SOA) to perform label erasure, with advantages of transparence for bit rate, high processing rate, simple architecture, and low cost. Label swapping is demonstrated with appropriate penalties of ?3.5 and 0.8 dB for PPM payload and new DPSK label, respectively. To further prove the effectiveness of the proposed scheme, label swapping in the case of using 10-Gb/s DPSK label is also investigated with the power penalties of 6 and 2 dB for PPM payload and new DPSK label.

Chinese Optics Letters
Jun. 03, 2011, Vol. 9 Issue 8 080602 (2011)
High channel-count comb f ilter based on multi-concatenated sampled chirped fiber Bragg gratings
Hongwei Yuan, Jun Zhou, Weitao Ma, Yongtang Sun, and Tiefeng Xu

A high channel-count comb filter based on multi-concatenated sampled chirped fiber Bragg gratings (MC-SCFBGs) is proposed and optimally designed by using several chirped gratings with different fundamental grating periods, instead of non-grating sections of SCFBGs. The numerical simulations of the reflection spectra show that the channel spacing and the channel bandwidth in MC-SCFBGs are smaller than those in multi-concatenated chirped fiber Bragg gratings (MC-CFBGs) and that the spectral bandwidth of MC-SCFBGs can be greatly broadened by increasing the cascade number of the grating sections in each sampling period.

Chinese Optics Letters
Jun. 03, 2011, Vol. 9 Issue 8 080601 (2011)
Impact of the distribution pattern of fast polarization scrambler on the performance of PMD mitigation with FEC for DQPSK optical system
Dahai Han, Lixia Xi, Minliang Li, Haoran Chen, and Feifei Liu

The combination of fast polarization scrambler (FPS) and forward error correction (FEC) is one of the methods to mitigate the polarization mode dispersion (PMD) in high-speed optical fiber communication systems. The effect of the different distribution patterns of FPS on PMD mitigation with FEC is investigated. A comparison of the bit error rates (BERs) between two cases where the distributed FPS is absent and present along the fiber is carried out by simulation. A novel representation called the "ring chart" to assess the performance of different distribution patterns intuitively is proposed. The results show that the distribution pattern is an impact factor for this PMD mitigation scheme.

Chinese Optics Letters
Jun. 16, 2011, Vol. 9 Issue 7 070604 (2011)
Coupled-mode analysis for single-helix chiral fiber gratings with small core-of fset
Li Yang, Linlin Xue, Jue Su, and Jingren Qian

Using conventional coupled-mode theory, a set of coupled-mode equations are formulated for single-helix chiral fiber long-period gratings. A helical-core fiber is analyzed as an example. The analysis is simple in mathematical form and intuitive in physical concept. Based on the analysis, the polarization independence of mode coupling in special fiber gratings is revealed. The transmission characteristics of helical-core fibers are also simulated and discussed.

Chinese Optics Letters
May. 26, 2011, Vol. 9 Issue 7 070603 (2011)
Impairment-aware QoS provisioning in dual-header OBS networks
Yuqi Fan, Bin Wang, and Lian Li

Signal degradation due to physical impairments may result in unacceptable bit-error rates of received signals at the destination. Based on earlier work, we study the impairment-aware quality of service (QoS) provisioning problem in dual-header optical burst switching (OBS) networks that employ two control packets for each data burst. At an OBS node, the proposed algorithm schedule bursts for transmission by searching for available resources using admission control and preemption. The algorithm also verifies signal quality. Simulation results show that this algorithm is effective in providing QoS support in OBS networks while considering physical impairment effects.

Chinese Optics Letters
May. 18, 2011, Vol. 9 Issue 7 070602 (2011)
Frequency-swept coherently detected spectral amplitude code for flexible implicit optical label switching
Yongsheng Cao, Fushen Chen, and Zhigao Yang

A new optical label switching system with coherently detected implicit spectral amplitude code (SAC) labels is proposed in this letter. The implicit SAC labels are recognized using a frequency-swept local light source oscillator. Intensity modulation payloads of 625 Mb/s and 1.25 Gb/s are considered. Label and payload bit error rate (BER) performances are assessed and compared by simulations. The results reveal that, at a BER value of 10^{-9}, -32.4-dBm label received power can be obtained. In addition, 8.3-dB optical signal-to-noise ratio (OSNR) is obtained when carrying a payload of 625 Mb/s. The label BER value hardly reaches 10^{-9} if the payload bit rate is at 1.25 Gb/s; however, a high payload bit rate only has little influence on received payload quality at a BER value of 10^{-9}. Finally, a payload of 1.25 Gb/s could obtain -28.2 dBm received power and 9.5-dB OSNR.

Chinese Optics Letters
May. 18, 2011, Vol. 9 Issue 7 070601 (2011)
Optimized optical phase conjugation configuration for fiber nonlinearity compensation in CO-OFDM systems
Lujiao Li, Yaojun Qiao, and Yuefeng Ji

The optimized optical phase conjugation (OPC) configuration is proposed for the 40-Gb/s CO-OFDM system. The proposed configuration for nonlinear cancellation is systematically depicted in transmission links with lumped amplification. Numerical simulations are performed to demonstrate effectiveness. Simulation results show that mid-span spectral inversion (MSSI) can partially compensate for nonlinear distortions. Moreover, its optimized configuration can further improve system performances and increase nonlinear compensation effectiveness. Compared with MSSI, the maximal Q factor, nonlinear threshold, and transmission distance of optimized OPC configuration increase by over 1.6 dB, 2 dB, and 2 times, respectively.

Chinese Optics Letters
May. 12, 2011, Vol. 9 Issue 6 060604 (2011)
Improved large-mode-area Bragg fiber
Peiguang Yan, Jian Zhao, Shuangchen Ruan, Junqing Zhao, Geguo Du, Huifeng Wei, Xiang Li, and Shengping Chen

A novel large-mode-area Bragg fiber (BF) is proposed for selectively suppressing the amplified spontaneous emission (ASE) of Yb. Confinement loss can be effectively lowered by adding a layer of F-doped glass near the core of this fiber. The BF can achieve effective suppression of ASE of Yb when the bend radius is 0.15 m at wavelength lower than 1.13 \mu m in theory, and eliminate LP11 mode in mode competition in wavelength range of 1.15-1.2 \mu m.

Chinese Optics Letters
May. 06, 2011, Vol. 9 Issue 6 060603 (2011)
Analysis of IPACT network performance considering channel switch latency for WDM-EPON
Zhao Zhou, Shilin Xiao, Min Zhu, and Meihua Bi

In wavelength-division multiplexing (WDM) ethernet passive optical networks (EPONs), to realize the statistical multiplexing of upstream wavelength resources, some optical tunable components are introduced in the optical network units. However, the switch latency (SL) of these tunable components constrains the performance of WDM-EPON. In this letter, we extend the mathematical model of the WDM interleaved polling with adaptive cycle time (IPACT) scheme by additionally considering the SL conditions. We also investigate the effect of channel SL on network performance. The simulation results show that the performance of WDM-IPACT-SL deteriorates as the SL increases.

Chinese Optics Letters
May. 06, 2011, Vol. 9 Issue 6 060602 (2011)
Radiation-induced effects in polarization-maintaining optical fibers for interferometric gyroscope
Xueqin Wang, Chunxi Zhang, Jing Jin, and Ningfang Song

Radiation-induced attenuation (RIA) in four types of polarization-maintaining optical fibers for interferometric fiberoptic gyroscope (IFOG) at 1310 nm is measured. The measurements are conducted during and after steady-state γ-ray irradiation using a 60Co source in order to observe significantly different RIA behavior and recovery kinetics. Mechanisms involving dopants and manufacturing process are introduced to analyze the RIA discrepancy as well as to guide the choice and hardening of optical fibers during the design of IFOG. Medium-accuracy IFOG using Ge–F-codoped fiber and pure silica core fiber can survive in the space radiation environment.

Chinese Optics Letters
May. 06, 2011, Vol. 9 Issue 6 060601 (2011)
OSNR-aware lightpath provisioning in distributed optical network
Huifeng Bai, and Yuefeng Ji

An optical signal-to-noise ratio (OSNR) aware lightpath provisioning mechanism (OSNR-LPM) is proposed for distributed optical networks. This OSNR-LPM takes the OSNR value of the lightwave along the lightpath into consideration when establishing the lightpath for the connection request using resource reservation protocol-traffic engineering (RSVP-TE). Moreover, the OSNR-LPM makes full advantages of the OSNR monitoring function in each node and assigns the lightwave by judging the OSNR value carried by the signaling message in order to guarantee the reliable establishment of the lightpath in the data plane.The simulation results show that the OSNR-LPM outperforms other lightpath assignment mechanisms in terms of real blocking probability in the data plane.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 5 050605 (2011)
Highly efficient fiber-to-chip evanescent coupling based on subwavelength-diameter optical fibers
Xiaowei Shen, Xinwan Li, Linjie Zhou, Zehua Hong, Xiaocao Yu, Ying Zhang, and Jianping Chen

A novel, compact, and highly efficient fiber-to-chip evanescent coupling structure is proposed based on a subwavelength-diameter fiber. The coupling structure is characterized by a large misalignment tolerance and easy fabrication. The dependence of coupling efficiency on various parameters is calculated and analyzed. The simulation results show that a coupling efficiency as high as 95% can be obtained within a coupling length of <4 \mum.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 5 050604 (2011)
Design procedure for photonic crystal fibers with ultra-flattened chromatic dispersion
Huizhen Xu, Jian Wu, Yitang Dai, Cong Xu, and Jintong Lin

A simple design procedure is used to generate photonic crystal fibers (PCFs) with ultra-flattened chromatic dispersion. Only four parameters are required, which not only considerably saves the computing time, but also distinctly reduces the air-hole quantity. The influence of the air-hole diameters of each ring of hexagonal PCFs (H-PCF, including 1-hole-missing and 7-hole-missing H-PCFs), circular PCFs (C-PCF), square PCFs (S-PCF), and octagonal PCFs (O-PCF) is investigated through simulations. Results show that regardless of the cross section structures of the PCFs, the 1st ring air-hole diameter has the greatest influence on the dispersion curve followed by that of the 2nd ring. The 3rd ring diameter only affects the dispersion curve within longer wavelengths, whereas the 4th and 5th rings have almost no influence on the dispersion curve. The hole-to-hole pitch between rings changes the dispersion curve as a whole. Based on the simulation results, a procedure is proposed to design PCFs with ultra-flattened dispersion. Through the adjustment of air-hole diameters of the inner three rings and hole-to-hole pitch, a flattened dispersion of 0+-0.5 ps/(nm.km) within a wavelength range of 1.239–2.083 \mum for 5-ring 1-hole-missing H-PCF, 1.248–1.992 \mum for 5-ring C-PCF, 1.237–2.21 \mum for 5-ring S-PCF, 1.149–1.926 \mum for 5-ring O-PCF, and 1.294–1.663 \mum for 7-hole-missing H-PCF is achieved.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 5 050603 (2011)
Fiber-optic Fabry Perot strain sensor based on graded-index multimode fiber
Tian Zhao, Yuan Gong, Yunjiang Rao, Yu Wu, Zengling Ran, and Huijuan Wu

By using a graded-index multimode fiber (GI-MMF) with a relatively flat index profile and high refractive index of the fiber core, a microextrinsic fiber-optic Fabry?P′erot interferometric (MEFPI) strain sensor is fabricated through chemical etching and fusion splicing. Higher reflectance of the microcavity is obtained due to the less-curved inner wall in the center of the fiber core after etching and higher index contrast between the GI-MMF core and air. The maximum reflection of the sensor is enhanced 12 dB than that obtained by etching of the Er- or B-doped fibers. High fringe contrast of 22 dB is obtained. The strain and temperature responses of the MEFPI sensors are investigated in this experiment. Good linearity and high sensitivity are achieved, with wavelength-strain and wavelength-temperature sensitivities of 7.82 pm/\mu \varepsilon and 5.01 pm/?C, respectively.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 5 050602 (2011)
Experiment on 60-GHz MMW transmission performance in an optical fiber and wireless system
Qi Tang, Lin Chen, Jiangnan Xiao, and Zizheng Cao

We experimentally investigate the transmission performance of 60-GHz signals over standard single-mode fiber (SSMF) and wireless links at different bit rates. Experimental results show that in a transmission of over 10-km SSMF and 1.3-m wireless link, bit rate reaches up to 5 Gb/s and bit error rate (BER) is less than 10^{-4}. The main limiting factor in such radio-over-fiber (ROF) systems is intersymbol interferences caused by the so-called walk-off effect when BER is below 10^{-8}. In addition, a transmission of over 20-km SSMF without chromatic dispersion compensation is brie°y investigated. For a BER of 10?8, the optical penalty is 2 dB.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 5 050601 (2011)
Rerouting end-face-TIR capable rays to significantly increase evanescent wave signal power
Jianjun Ma, Yasser Chiniforooshan, Huacai Chen, Jiahua Chen, Wojtek J. Bock, and Andrea Cusano

The critical findings associated with end-face total internal reflection (TIR) phenomemon we proved before are reported. In particular, these findings reveal that the end-face-TIR capable rays experience enormous mode mixing when encountering a roughened end face. As a result, 94% of the overall detectable power is contributed by this effect. With a smooth fiber end face, this figure is mere 52%. We interpret the mechanism behind these unusual phenomena and its significance for the performance enhancement of fiber optic evanescent wave sensor.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 4 040603 (2011)
Novel scheme enabling broadcast signal transmission in WDM passive optical network
Xuejiao Ma, and Chaoqin Gan

We present a novel method for providing broadcast signal transmission in a wavelength division multiplexing passive optical network (WDM-PON). An unmodulated optical carrier for downstream transmission and a pair of unmodulated single-side band subcarriers are utilized for broadcast delivery and upstream transmission, respectively. System performance at 2.5-Gb/s down/upstream and 2.5-Gb/s broadcast transmission is also investigated.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 4 040602 (2011)
Dispersion flattened photonic crystal fiber with high nonlinearity for supercontinuum generation at 1.55 \mum
Long Zheng, Xia Zhang, Xiaomin Ren, Huifang Ma, Lei Shi, Yamiao Wang, and Yongqing Huang

A robust design for a photonic crystal fiber (PCF) based on pure silica with small normal dispersion and high nonlinear coefficient for its dual concentric core structure is presented. This design is suitable for flat broadband supercontinuum (SC) generation in the 1.55-μm region. The numerical results show that the nonlinear coefficient of the proposed eight-ring PCF is 33.8 W?1.km?1 at 1550 nm. Ultraflat dispersion with a value between -1.65 and -0.335 ps/(nm.km) is obtained ranging from 1375 to 1625 nm. The 3-dB bandwidth of the SC is 125 nm (1496–1621 nm), with a fiber length of 80 m and a corresponding input peak power of 43.8 W. The amplitude noise is considered to be related to SC generation. For practical fabrication, the influence of the random imperfections of airhole diameters on dispersion and nonlinearity is discussed to verify the robustness of our design.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 4 040601 (2011)
Effects of high-order dispersions on dark-bright vector soliton propagation and interaction
Hong Li, and Dongning Wang

The dynamics of dark-bright vector solitons is investigated in a birefringent fiber with the high-order dispersions, and their effects on vector soliton propagation and interaction are analyzed using the numerical method. The combined role of the high-order dispersions, such as the third-order dispersion (TOD) and the fourth-order dispersion (FOD), may cause various deformation of the vector soliton and enhance interaction. These effects depend strictly on the sign of the high-order dispersions. Results indicate that the disadvantageous effects can be reduced effectively via proper mapping of the high-order dispersions.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 3 030604 (2011)
All-optical reconfigurable multi-logic gates based on nonlinear polarization rotation effect in a single SOA
Lilin Yi, Weisheng Hu, Hao He, Yi Dong, Yaohui Jin, and Weiqiang Sun

We demonstrate an all-optical reconfigurable logic gate based on dominant nonlinear polarization rotation accompanied with cross-gain modulation effect in a single semiconductor optical amplifier (SOA). Five logic functions, including NOT, OR, NOR, AND, and NAND, are realized using 10-Gb/s on-off keying signals with flexible wavelength tunability. The operation principle is explained in detail. By adjusting polarization controllers, multiple logic functions corresponding to different input polarization states are separately achieved using a single SOA with high flexibility.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 3 030603 (2011)
Electronic compensator for 100-Gb/s PDM-CO-OFDM long-haul transmission systems
Xuejun Liu1, Yaojun Qiao, and Yuefeng Ji

We study an electronic compensator (EC) as a receiver for a 100-Gb/s polarization division multiplexing coherent optical orthogonal frequency division multiplexing (PDM-CO-OFDM) system without optical dispersion compensation. EC, including electrical dispersion compensation (EDC), least squares channel estimation and compensation (LSCEC), and phase compensation (PC), is used to compensate for chromatic dispersion (CD), phase noise, polarization mode dispersion (PMD), and channel impairments, respectively. Simulations show that EC is highly effective in compensating for those impairments and that the performance is close to the theoretical limitation of optical signal-to-noise rate (OSNR), CD, and PMD. Its robustness against those transmission impairments and fiber nonlinearity are also systematically studied.

Chinese Optics Letters
Jul. 09, 2021, Vol. 9 Issue 3 030602 (2011)
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