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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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°.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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°.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.