Chinese Optics Letters
Co-Editors-in-Chief
Zhizhan Xu
Contents 3 Issue (s), 65 Article (s)
Vol. 21, Iss.3—Mar.1, 2023 • pp: 030501- Spec. pp:
Vol. 21, Iss.2—Feb.1, 2023 • pp: 020601- Spec. pp:
Vol. 21, Iss.1—Jan.1, 2023 • pp: 010002- Spec. pp:
Research ArticlesVol. 21, Iss.3-Mar..1,2023
Diffraction, Gratings, and Holography
Binary diffractive lens with subwavelength focusing for terahertz imaging
Ran Ning, Dayong Wang, Lu Rong, Jie Zhao, Yunxin Wang, and Shufeng Lin

The converging lens is one of the key components in high-resolution terahertz imaging. In this Letter, a binary diffractive lens is proposed for the scanning imaging system working at 278.6 GHz, in which a convergent beam with a waist diameter of 0.65 mm is generated, and 1 mm lateral imaging resolution is realized. This low-cost terahertz lens, constituted by concentric rings with different radii, is optimized by stimulated annealing algorithm and fabricated by three-dimensional printing. Compared with the conventional transmissive convex lens, higher resolution and enhanced imaging quality are achieved via smaller focal spot of the illumination beam. This type of lens would promote terahertz imaging closer to practical applications such as nondestructive testing and other scenarios.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 030501 (2023)
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Diffraction, Gratings, and Holography
Self-healing of holographically generated moiré lattice wave fields
Siwei Tang, Chunlei Shang, Zhaofeng Liu, Chengzhen Lu, Yangjian Cai, Yuanmei Gao, and Zengrun Wen

Self-healing in optics generally refers to the ability to reconstruct itself and restore the original state after encountering obstacles in the propagation of the light field. In this research, we observe the processes of the wave fields from perfect to defect in front of the focal plane of the 4f system, finally returning to an intact situation after the plane. According to simulations and experimental results, there is a minimum self-healing distance for the moiré lattice field that positively associates with the radius of the defect (obstacle) in the nondiffracting transmission range. Furthermore, it is observed that the defect self-healing is a process of “repairing the center and then repairing the edges.” These findings can be applied in areas such as optical imaging, capture, and information processing.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 030502 (2023)
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Fiber Optics and Optical Communications
Speckle backpropagation for compensation of nonlinear effects in few-mode optical fibers
Pavel S. Anisimov, Evgeny D. Tsyplakov, Viacheslav V. Zemlyakov, and Jiexing Gao

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

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 030601 (2023)
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Fiber Optics and Optical Communications
Fiber communication receiver models based on the multi-head attention mechanism
Yubin Zang, Zhenming Yu, Kun Xu, Minghua Chen, Sigang Yang, and Hongwei Chen

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

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 030602 (2023)
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Imaging Systems and Image Processing
Convenient noncooperative speckle-correlation imaging method
Shuyue Zhu, Wenjun Yi, Meicheng Fu, Junli Qi, Mengjun Zhu, Xin Chen, Hongyu Zhang, Junyi Du, Ping Wang, Ju Liu, and Xiujian Li

For speckle-correlation-based scattering imaging, an iris is generally used next to the diffuser to magnify the speckle size and enhance the speckle contrast, which limits the light flux and makes the setup cooperative. Here, we experimentally demonstrate a non-iris speckle-correlation imaging method associated with an image resizing process. The experimental results demonstrate that, by estimating an appropriate resizing factor, our method can achieve high-fidelity noncooperative speckle-correlation imaging by digital resizing of the raw captions or on-chip pixel binning without iris. The method opens a new door for noncooperative high-frame-rate speckle-correlation imaging and benefits scattering imaging for dynamic objects hidden behind opaque barriers.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031101 (2023)
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Integrated Optics
Flexible high-resolution thin micropolarizers for imaging polarimetry
Yan Wu, Yang Yang, Yue Yin, Linmao Dai, Xiaochun Li, Huihui Huang, and Shuangchun Wen

More durable (with high impact force), lighter, and more compact flexible azo dye micropolarizers are attractive candidates for low-cost, simple polarization imaging systems. The liquid crystal polymer (LCP), as an emerging material developed by photo-alignment technology, is a potential material for organizing the long-range ordered structure of azo dyes. However, little research has been done on LCP aligned azo dyes. This paper points out and solves a key problem that restricts the fabrication of high-precision arrays in guest (azo dye)-host (LCP) systems: the doping of dyes leads to disorder of the LCP during curing. After solving the problem, the relationship between the thickness of the LCP and the extinction ratio of the polarizing film was investigated, which effectively improved the extinction ratio. Alignment of azo dye molecules in the range of 2 µm (0°–180°) and arrays of micropolarizers (0°, 45°, 90°, -45°) with 8 µm × 8 µm pixel pitch was achieved by laser direct writing technology. The bending cycle test demonstrates the mechanical stability of the ultrathin flexible polarizer. The flexible patterned polarizer with robust chemical and mechanical stabilities provides a flexible way to capture the polarization of the light and highly integrated advanced flexible optoelectronic devices.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031301 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Continuous-wave three-wavelength operation of a diode-pumped Tm:YVO4 laser on the 3H43H5 and 3F43H6 transitions
Jiaqun Zhao, Yuantong Liu, Ping Cheng, and Rui Yu

A diode-pumped continuous-wave Tm:YVO4 laser operating on the H34→H35 and F43→H36 transitions was demonstrated for the first time, to the best of our knowledge. An a-cut Tm:YVO4 crystal with 1.5% (atomic fraction) Tm3+ ion concentration was used to characterize the laser behavior. A common commercial laser diode with a central wavelength of 790 nm and a bandwidth of 3.2 nm was utilized as a pump source. With an output coupler for the H34→H35 and F34→H36 transitions, simultaneous three-wavelength laser operation was achieved. The laser emissions at 2292 and 2363 nm in π-polarization and at 2108 nm in σ-polarization were realized. With an incident pump power of 22 W, the total output power of 1.17 W at 2292, 2363, and 2108 nm was obtained. The output power at 2292 and 2363 nm was measured to be 750 mW, and the output power at 2108 nm was measured to be 420 mW.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031401 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Polarization domains and self-mode-locked pulses in an erbium-doped fiber laser
Peiyun Cheng, Mengmeng Han, Yueqing Du, and Xuewen Shu

We have observed various polarization domains and a giant self-mode-locked pulse in a 130 m long erbium-doped fiber laser without any mode-locking devices. By adjusting the intracavity polarization controller, we investigated the evolution process of the polarization domain with the varying cavity birefringence. When the birefringence was close to zero, the polarization domains split into multidomains, and finally a giant self-mode-locked pulse formed for the first time. We analyzed that the generation of the self-mode-locked pulse was related to the multiple subdomains ascribed to the strong coherent cross coupling between the orthogonal polarization light components in the long fiber cavity.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031402 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Serrated periodic electrode for high energy efficiency and large bandwidth acousto-optic modulators
Ji Wu, Li Liang, Kefeng Tu, Kunying Li, Zi Wang, and Guoqiang Lü

In an acousto-optic modulator, the electrode shape plays an important role in performance, since it affects the distribution of the acoustic field. The acousto-optic modulator based on the conventional rectangular electrode has the problems of low energy efficiency and small modulation bandwidth due to an imperfect acoustic field. In this paper, a new serrated periodic electrode has been proposed for using acousto-optic modulator transducers. The proposed electrode has the following advantages. By using serrated periodic electrodes to suppress the sidelobes, the collimation of the acoustic field in the direction perpendicular to the light incidence is improved. This makes the acousto-optic modulator have a stable diffraction efficiency fluctuation and high energy efficiency. In addition, the electrode has a large divergence angle in the direction of light incidence, so a large bandwidth can be obtained. The simulations and experiments demonstrate that the serrated periodic electrode has an increased bandwidth and high energy efficiency.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031403 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
All-fiber-based ultrastable laser with long-term frequency stability of 1.1 × 10-14
Yafeng Huang, Di Hu, Meifeng Ye, Yating Wang, Yanli Li, Ming Li, Yinnan Chen, Qiuzhi Qu, Lingke Wang, Liang Liu, and Tang Li

We demonstrate an ultrastable miniaturized transportable laser system at 1550 nm by locking it to an optical fiber delay line (FDL). To achieve optimized long-term frequency stability, the FDL was placed into a vacuum chamber with a five-layer thermal shield, and a delicate two-stage active temperature stabilization, an optical power stabilization, and an RF power stabilization were applied in the system. A fractional frequency stability of better than 3.2×10-15 at 1 s averaging time and 1.1×10-14 at 1000 s averaging time was achieved, which is the best long-term frequency stability of an all-fiber-based ultrastable laser observed to date.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031404 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Beam homogenization structure for a laser illuminator design based on diode laser beam combining technology
Jinliang Han, Jun Zhang, Xiaonan Shan, Yawei Zhang, Hangyu Peng, Li Qin, and Lijun Wang

With the rapid development of laser technology, laser as the light source of night vision illuminating can realize long-distance and clear imaging, which has been widely used in laser active illuminating field. A high-power diode laser with a wavelength of 808 nm was designed as the laser active illuminating source, and the output power of no less than 100 W was obtained by spatial beam multiplexing, polarization multiplexing, and high efficiency fiber coupling techniques. In view of the beam homogenization of illuminating source, a novel beam homogenization system based on waveguide is proposed in this work. A square spot with a horizontal divergence angle of 40°, a vertical divergence angle of 10°, and an illuminating power ratio of 4:1 was obtained by a collimating lens. Comparing with the traditional circular illuminating beam, the square illuminating beam can match the illuminating angle of CCD camera better, and the energy utilization rate is higher. In addition, by optimizing the structure of waveguide and collimating lens, the illuminating angle can be changed to meet the illuminating requirements under different conditions theoretically.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031405 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Subharmonic mode locking of a Q-switched Nd:YAG laser
Alexey Gribanov, Mikhail Yakovin, Dmitry Yakovin, and Mikhail Mosin

A Q-switched Nd:YAG laser has been actively mode-locked at a subharmonic frequency for the first time, to the authors’ knowledge. The laser operation mode is provided by a combination of a traveling wave acousto-optic modulator and a spherical cavity mirror. The dynamics of laser generation is investigated. Pulses with a duration of 70 ps and a peak power of about 10 MW were obtained. Also presented are new results on obtaining high-power (∼60 kW) picosecond tunable radiation in the ∼620 nm region based on frequency conversion of a superluminescent parametric generator pumped by such a laser.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031406 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Transverse mode interaction-induced Raman laser switching dynamics in a silica rod microresonator
Xueying Jin, Qinglin Fang, Xin Xu, Yu Yang, Haoran Gao, and Haojie Xia

We investigate the mechanisms to realize the Raman laser switching in a silica rod microresonator with mode-interaction-assisted excitation. The laser switching can be triggered between two whispering gallery modes (WGMs) with either the same or distinct mode families, depending on the pumping conditions. The experimental observations are in excellent agreement with a theoretical analysis based on coupled-mode equations with intermodal interaction terms involved. Additionally, we also demonstrate switching of a single-mode Raman laser and a wideband spectral tuning range up to ∼32.67 nm by selective excitation of distinct mode sequences. The results contribute to the understanding of Raman lasing formation dynamics via interaction with transverse mode sequences and may extend the microcavity-based Raman microlasers to potential areas in switchable light sources, optical memories, and high sensitivity sensors.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031407 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Cylindrical vector beam generator on photonic crystal cavity integrated with metal split ring nanoresonators
Yingke Ji, Xin Xie, Liang Fang, Yisong Zhu, Jianlin Zhao, and Xuetao Gan

We propose a chip-integratable cylindrical vector (CV) beam generator by integrating six plasmonic split ring resonators (SRRs) on a planar photonic crystal (PPC) cavity. The employed PPC cavity is formed by cutting six adjacent air holes in the PPC center, which could generate a CV beam with azimuthally symmetric polarizations. By further integrating six SRRs on the structure defects of the PPC cavity, the polarizations of the CV beam could be tailored by controlling the opening angles of the SRRs, e.g., from azimuthal to radial symmetry. The mechanism is governed by the coupling between the resonance modes in SRRs and PPC cavity, which modifies the far-field radiation of the resonance mode of the PPC cavity with the SRR as the nano-antenna. The integration of SRRs also increases the coupling of the generated CV beam with the free-space optics, such as an objective lens, promising its further applications in optical communication, optical tweezer, imaging, etc.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 033601 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
High color saturation and angle-insensitive ultrathin color filter based on effective medium theory
Xinting Li, Yang Li, Chao Li, Song Gao, and Wenjing Yue

An ultrathin angle-insensitive color filter enabling high color saturation and a wide color gamut is proposed by relying on a magnesium hydride-hydrogenated amorphous silicon (MgH2-a-Si:H) lossy dielectric layer. Based on effective medium theory, the MgH2-a-Si:H layer with an ultrathin thickness can be equivalent to a quasi-homogeneous dielectric layer with an effective complex refractive index, which can be tuned by altering the thickness of MgH2 to obtain the targeted value of the imaginary part, corresponding to the realization of high color saturation. It is verified that the proposed color filter offers highly enhanced color saturation in conjunction with a wide color gamut by introducing a few-nanometer thick MgH2 layer. As the MgH2-a-Si:H layer retains the advantages of high refractive index and tiny thickness, the proposed color filter exhibits large angular tolerance up to ±60°. In addition, MgH2 with an unstable property can interconvert with Mg under a dehydrogenation/hydrogenation reaction, which empowers the proposed color filter with dynamically tunable output color. The proposed scheme shows great promise in color printing and ultracompact display devices with high color saturation, wide gamut, large angular tolerance, and dynamic tunability.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 033602 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Azimuthal vector beam illuminating plasmonic tips circular cluster for surface-enhanced Raman spectroscopy
Lu Zhang, Chao Meng, Hao Yang, and Wending Zhang

Noble metallic nanostructures with strong electric near-field enhancement can significantly improve nanoscale light–matter interactions and are critical for high-sensitivity surface-enhanced Raman spectroscopy (SERS). Here, we use an azimuthal vector beam (AVB) to illuminate the plasmonic tips circular cluster (PTCC) array to enhance the electric near-field intensity of the PTCC array, and then use it to improve SERS sensitivity. The PTCC array was prepared based on the self-assembled and inductive coupled plasmon (ICP) etching methods. The calculation results show that, compared with the linearly polarized beam (LPB) and radial vector beam excitations, the AVB excitation can obtain stronger electric near-field enhancement due to the strong resonant responses formed in the nanogap between adjacent plasmonic tips. Subsequently, our experimental results proved that AVB excitation increased SERS sensitivity to 10-13 mol/L, which is two orders of magnitude higher than that of LPB excitation. Meanwhile, the PTCC array had excellent uniformity with the Raman enhancement factor calculated to be ∼2.4×108. This kind of vector light field enhancing Raman spectroscopy may be applied in the field of sensing technologies, such as the trace amount detection.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 033603 (2023)
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Nonlinear Optics
Characteristic extraction of soliton dynamics based on convolutional autoencoder neural network
Congcong Liu, Jiangyong He, Pan Wang, Dengke Xing, Jin Li, Yange Liu, and Zhi Wang

In this article, we use a convolutional autoencoder neural network to reduce data dimensioning and rebuild soliton dynamics in a passively mode-locked fiber laser. Based on the particle characteristic in double solitons and triple solitons interactions, we found that there is a strict correspondence between the number of minimum compression parameters and the number of independent parameters of soliton interaction. This shows that our network effectively coarsens the high-dimensional data in nonlinear systems. Our work not only introduces new prospects for the laser self-optimization algorithm, but also brings new insights into the modeling of nonlinear systems and description of soliton interactions.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 031901 (2023)
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Optoelectronics
High-uniformity 2 × 64 silicon avalanche photodiode arrays with silicon multiple epitaxy technology
Tiancai Wang, Peng Cao, Hongling Peng, Chuanwang Xu, Haizhi Song, and Wanhua Zheng

In this paper, high-uniformity 2×64 silicon avalanche photodiode (APD) arrays are reported. Silicon multiple epitaxy technology was used, and the high performance APD arrays based on double-layer epiwafers are achieved for the first time, to the best of our knowledge. A high-uniformity breakdown voltage with a fluctuation of smaller than 3.5 V is obtained for the fabricated APD arrays. The dark currents are below 90 pA for all 128 pixels at unity gain voltage. The pixels in the APD arrays show a gain factor of larger than 300 and a peak responsivity of 0.53 A/W@M = 1 at 850 nm (corresponding to maximum external quantum efficiency of 81%) at room temperature. Quick optical pulse response time was measured, and a corresponding cutoff frequency up to 100 MHz was obtained.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 032501 (2023)
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Optoelectronics
High-stability 4H-SiC avalanche photodiodes for UV detection at high temperatures
Xingye Zhou, Yuanjie Lü, Hongyu Guo, Xubo Song, Yuangang Wang, Shixiong Liang, Aimin Bu, and Zhihong Feng

In this work, high-stability 4H-SiC avalanche photodiodes (APDs) for ultraviolet (UV) detection at high temperatures are fabricated and investigated. With the temperature increasing from room temperature to 150°C, a very small temperature coefficient of 7.4 mV/°C is achieved for the avalanche breakdown voltage of devices. For the first time, the stability of 4H-SiC APDs is verified based on an accelerated aging test with harsh stress conditions. Three different stress conditions are selected with the temperatures and reverse currents of 175°C/100 µA, 200°C/100 µA, and 200°C/500 µA, respectively. The results show that our 4H-SiC APD exhibits robust high-temperature performance and can even endure more than 120 hours at the harsh aging condition of 200°C/500 µA, which indicates that 4H-SiC APDs are very stable and reliable for applications at high temperatures.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 032502 (2023)
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Spectroscopy
Advances in multipass cell for absorption spectroscopy-based trace gas sensing technology [Invited]
Yahui Liu, and Yufei Ma

In the field of absorption spectroscopy, the multipass cell (MPC) is one of the key elements. It has the advantages of simple structure, easy adjustment, and high spectral coverage, which is an effective way to improve the detection sensitivity of gas sensing systems such as tunable diode laser absorption spectroscopy. This invited paper summarizes the design theory and the research results of some mainstream types of MPCs based on two mirrors and more than two mirrors in recent years, and briefly introduces the application of some processed products. The design theory of modified ABCD matrix and vector reflection principle are explained in detail. Finally, trends in its development are predicted.

Chinese Optics Letters
Mar. 01, 2023, Vol. 21 Issue 3 033001 (2023)
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Research ArticlesVol. 21, Iss.2-Feb..1,2023
Fiber Optics and Optical Communications
50 m/187.5 Mbit/s real-time underwater wireless optical communication based on optical superimposition
Yongxin Cheng, Xingqi Yang, Yufan Zhang, Chao Zhang, Hao Zhang, Zhijian Tong, Yizhan Dai, Weichao Lü, Xin Li, Haiwu Zou, Zejun Zhang, and Jing Xu

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

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 020601 (2023)
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Fiber Optics and Optical Communications
OCT-based partial data-carrying subcarriers precoding and ISFA-enabled DMT-VLC system
Yi Liu, Ming Chen, Pingyuan Liang, Taoyun Zhou, Shan Qiu, and Yun Cheng

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

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 020602 (2023)
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Fiber Optics and Optical Communications
Temperature-sensing scheme based on a passively mode-locked fiber laser via beat frequency demodulation
Jian Luo, Haoran Wang, Xun Cai, Zhengqian Luo, and Hongyan Fu

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

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 020603 (2023)
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Infrared and Terahertz Photonics
NH3 laser THz emission under optical pumping by “long” (∼100 µs) CO2 laser pulses
Andrey Alekseevich Ionin, Igor Olegovich Kinyaevskiy, Yuri Mikhailovich Klimachev, Andrey Yurievich Kozlov, Darya Ivanovna Kormashova, Andrey Alexandrovich Kotkov, Yury Alekseevich Mityagin, Sergey Alexandrovich Savinov, Adilya Maratovna Sagitova, Dmitry Vasilievich Sinitsyn, and Maxim Vladimirovich Ionin

Terahertz (THz) NH3 lasing with optical pumping by electron-beam-sustained discharge “long” (∼100 µs) CO2 laser pulses was obtained. The NH3 laser emission pulses and the “long” pulses of the CO2 pump laser were simultaneously measured with nanosecond response time. The NH3 lasing duration and its delay with respect to the pump pulse were measured for various CO2 laser pulse energies. For the CO2 laser pump line 9R(30), three wavelengths of 67.2, 83.8, and 88.9 µm were recorded. For the CO2 laser pump line 9R(16), only a single NH3 laser line with a wavelength of 90.4 µm was detected.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023701 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Wavelength-tunable multi-point pump semiconductor disk laser based on an intra-cavity transmission grating
Jian Feng, Cunzhu Tong, Yufei Zhao, Andreas Popp, Berthold Schmidt, Lijie Wang, Bo Meng, Huanyu Lu, Yanjing Wang, Xin Zhang, and Lijun Wang

We report a wavelength-tunable multi-point pump scheme of the semiconductor disk lasers (SDLs). By designing an external cavity of SDL with an intra-cavity transmission grating, multiple pump gain regions share the same resonator. The effect of the intra-cavity grating on the output laser power, wavelength, and beam quality was investigated. The emission wavelength could be tuned over a bandwidth of ∼18 nm. With multi-point pumping, we achieve the laser output power with almost no loss, and further improvement is limited by the thermal effect. The changes in the beam are due to the mode selectivity by the intra-cavity grating.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021401 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Passively Q-switched operation of a 1.94 µm thulium-doped solid-state laser based on MXene V2CTx
Jinhe Yuan, Jiarui Li, Zhengyang Wu, Shuangcheng Li, Jie Han, and Linjun Li

MXene V2CTx has great practicability because it is not easy to degrade under ambient conditions. In this paper, a V2CTx saturable absorber (SA) was firstly applied to a passively Q-switched (PQS) laser, to the best of our knowledge. The V2CTx-SA was prepared by the spin-coating method. The linear absorption of the V2CTx-SA in the 1000–2200 nm region and the nonlinear absorption near 2 µm were studied. With the V2CTx-SA, a typical PQS operation at 1.94 µm was realized in a Tm:YAlO3 laser. The minimum pulse width produced by the PQS laser was 528 ns, and the peak power, repetition rate, and average output power were 10.06 W, 65.9 kHz, and 350 mW, respectively. Meanwhile, the maximum pulse energy was 6.33 µJ. This work demonstrates that the V2CTx can be used as an effective SA to obtain nanosecond pulses with high peak power and high repetition rate simultaneously.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021402 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Continuously tunable CW single-frequency Nd:YAP/LBO laser with dual-wavelength output
Pixian Jin, Yi Cui, Jing Su, Huadong Lu, and Kunchi Peng

We present a continuously tunable high-power continuous wave (CW) single-frequency (SF) Nd:YAlO3/lithium triborate (Nd:YAP/LBO) laser with dual-wavelength output, which is implemented by combining an optimized and locked etalon with an intracavity nonlinear loss. The obtained output powers of the stable SF 1080 and 540 nm lasers are 2.39 and 4.18 W, respectively. After the etalon is locked to an oscilating mode of the laser, the wideband continuous frequency tuning and long-term stable single-longitudinal-mode operation of the laser are successfully realized, which can be well used for the applications of quantum information and quantum computation. To the best of our knowledge, this is the first realization of the continuously tunable high-power CW SF 1080/540 nm dual-wavelength laser.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021403 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser
Hongye Li, Xin Tian, Hao Li, Baiyi Wu, Xiaofan Zhao, Meng Wang, Chenhui Gao, Binyu Rao, Xiaoming Xi, Zilun Chen, Zefeng Wang, and Jinbao Chen

We fabricate a pair of fiber Bragg gratings (FBGs) by a visible femtosecond laser phase mask scanning technique on passive large-mode-area double-cladding fibers for multi-kilowatt fiber oscillators. The bandwidth of high-reflection (HR) and low-reflection (LR) FBG is ∼1.6 nm and 0.3 nm, respectively. The reflection of the HR-FBG is higher than 99%, and that of the LR-FBG is about 10%. A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs, a record output power of 5027 W located in the signal core is achieved with a slope efficiency of ∼82.1%, and the beam quality factor M2 is measured to be ∼1.6 at the maximum power. The FBGs are simply fixed on a water cooling plate without a special package, and the thermal efficiency of the HR-FBG and the LR-FBG is 2.76°C/kW and 1°C/kW, respectively. Our research provides an effective solution for robust high-power all-fiber laser oscillators.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021404 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Widely tunable 2 µm optical vortex from a Tm:YAP laser
Jingjing Zhou, Changsheng Zheng, Bin Chen, Ning Zhang, Qinggang Gao, Hangqi Yuan, Daiwen Jia, Zhanxin Wang, Shande Liu, Yuping Zhang, Huiyun Zhang, and Yongguang Zhao

In this paper, we report on a wide wavelength tuning optical vortex carrying orbital angular momentum (OAM) of ±ħ, from a thulium-doped yttrium aluminum perovskite (YAP) laser employing a birefringent filter. The OAM is experimentally found to be well maintained during the whole wavelength tuning process. The Laguerre–Gaussian (LG0,+1) mode with a tuning range of 58 nm from 1934.8 to 1993.0 nm and LG0,-1 mode with a range of 76 nm from 1920.4 to 1996.6 nm, are, respectively, obtained. This is, to the best of our knowledge, the first experimental implementation of wavelength tuning for a scalar vortex laser in the 2 µm spectral range, as well as the broadest tuning range ever reported from the vortex laser cavity. Such a vortex laser with robust structure and straightforward wavelength tuning capability will be an ideal light source for potential applications in the field of optical communication with one additional degree of freedom.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021405 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Hundred-watt level high-order mode all-fiberized random distributed feedback Raman fiber laser with high mode purity
Yang Li, Xiulu Hao, Yi An, Yi Zhu, Tianfu Yao, Xianglong Zeng, and Pu Zhou

An all-fiberized random distributed feedback Raman fiber laser (RRFL) with LP11 mode output at 1134 nm has been demonstrated experimentally, where an intracavity acoustically induced fiber grating is employed for modal switching. The maximum output power of LP11 mode is 93.8 W with the modal purity of 82%, calculated by numerical mode decomposition technology based on stochastic parallel-gradient descent algorithm. To our best knowledge, this is the highest output power with high purity of LP11 mode generated from the RRFL. This work may pave a path towards advanced fiber lasers with special temporal and spatial characteristics for applications.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021406 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Laser frequency locking method for Rydberg atomic sensing
Kai Yang, Ruiqi Mao, Qiang An, Zhanshan Sun, and Yunqi Fu

Based on the Rydberg cascade electromagnetically induced transparency, we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature. The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator, realizing real-time feedback for both lasers. We studied the effect of the different probe and coupling laser powers on the error signal. In addition, the Allan variance and a 10 kHz amplitude-modulated signal are introduced to evaluate the performance of the laser frequency stabilization. In principle, the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021407 (2023)
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Microwave Photonics
Single-lane 200 Gbit/s photonic wireless transmission of multicarrier 64-QAM signals at 300 GHz over 30 m
Hongqi Zhang, Lu Zhang, Zuomin Yang, Hang Yang, Zhidong Lü, Xiaodan Pang, Oskars Ozolins, and Xianbin Yu

Recently, wireless communication capacity has been witnessing unprecedented growth. Benefits from the optoelectronic components with large bandwidth, photonics-assisted terahertz (THz) communication links have been extensively developed to accommodate the upcoming wireless transmission with a high data rate. However, limited by the available signal-to-noise ratio and THz component bandwidth, single-lane transmission of beyond 100 Gbit/s data rate using a single pair of THz transceivers is still very challenging. In this study, a multicarrier THz photonic wireless communication link in the 300 GHz band is proposed and experimentally demonstrated. Enabled by subcarrier multiplexing, spectrally efficient modulation format, well-tailored digital signal processing routine, and broadband THz transceivers, a line rate of 72 Gbit/s over a wireless distance of 30 m is successfully demonstrated, resulting in a total net transmission capacity of up to 202.5 Gbit/s. The single-lane transmission of beyond 200 Gbit/s overall data rate with a single pair of transceivers at 300 GHz is considered a significant step toward a viable photonics-assisted solution for the next-generation information and communication technology (ICT) infrastructure.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023901 (2023)
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Microwave Photonics
Linearized microwave downconversion link based on fast and intelligent impairment equalization for noncooperative systems
Zhiyu Chen, Xin Zhong, Lin Jiang, Jiaxin Xu, Jingxian Liu, Yan Pan, and Tao Zhou

We experimentally demonstrated the use of intelligent impairment equalization (IIE) for microwave downconversion link linearization in noncooperative systems. Such an equalizer is realized based on an artificial neural network (ANN). Once the training process is completed, the inverse link transfer function can be determined. With the inverse transformation for the detected signal after transmission, the third-order intermodulation distortion components are suppressed significantly without requiring any prior information from an input RF signal. Furthermore, fast training speed is achieved, since the configuration of ANN-based equalizer is simple. Experimental results show that the spurious-free dynamic range of the proposed link is improved to 106.5 dB · Hz2/3, which is 11.3 dB higher than that of a link without IIE. Meanwhile, the training epochs reduce to only five, which has the potential to meet the practical engineering requirement.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023902 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Understanding wide field-of-view flat lenses: an analytical solution [Invited]
Fan Yang, Sensong An, Mikhail Y. Shalaginov, Hualiang Zhang, Juejun Hu, and Tian Gu

Wide field-of-view (FOV) optics are widely used in various imaging, display, and sensing applications. Conventional wide FOV optics rely on complicated lens assembly comprising multiple elements to suppress coma and other Seidel aberrations. The emergence of flat optics exemplified by metasurfaces and diffractive optical elements (DOEs) offers a promising route to expand the FOV without escalating complexity of optical systems. To date, design of large FOV flat lenses has been relying upon iterative numerical optimization. Here, we derive, for the first time, to the best of our knowledge, an analytical solution to enable computationally efficient design of flat lenses with an ultra-wide FOV approaching 180°. This analytical theory further provides critical insights into working principles and otherwise non-intuitive design trade-offs of wide FOV optics.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023601 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Metasurfaces enabled dual-wavelength decoupling of near-field and far-field encoding
Jun Liu, Xiaoshu Zhu, Juanzi He, Yifan Zhou, Mingqian Shi, Zhaofu Qin, Shuming Wang, and Zhenlin Wang

The metasurface is a platform with a small footprint and abundant functionalities. With propagation phase and geometric phase, polarization multiplexing is possible. However, different response behaviors of propagation phase and geometric phase to wavelength have not been fully employed to widen the capabilities of metasurfaces. Here, we theoretically demonstrate that metasurfaces can achieve near-field and far-field decoupling with the same polarization at two wavelengths. First, we found a set of pillars whose propagation phase difference between two wavelengths covers the full range of 2π. Then, by rotating pillars to control the geometric phase, the phase at both wavelengths can cover the full range of 2π. Finally, by means of interference principle, arbitrary independent coding for the near field and far field of dual wavelengths is realized. In addition, when the far-field function is focusing, the focused spot is close to the diffraction limit, and, when the NA of the lens is very small, the final output focal length is four times of initial input focal length. This work circumvents the strong wavelength-dependent limitation of planar devices and paves the way toward designing multi-wavelength and multi-functional metadevices for scenarios such as AR applications, fluorescence microscopy, and stimulated emission depletion microscopy.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023602 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Transmissive angle-multiplexed meta-polarizer based on a multilayer metasurface
Chenxi Pu, Zhuo Wang, Shulin Sun, Lei Zhou, and Qiong He

Metasurfaces have exhibited great capabilities to control electromagnetic waves, and many multifunctional metasurfaces were recently proposed. However, although angle-multiplexed meta-devices were successfully realized in reflection geometries, their transmission-mode counterparts are difficult to achieve due to the additional requirements. Here, we design and fabricate a transmissive angle-multiplexed meta-polarizer in the microwave regime based on a multilayer metasurface. Coupled-mode-theory analyses reveal that the device exhibits distinct angle-dependent transmissive responses under excitations with different polarizations, and such differences are further enhanced by multiple scatterings inside the device. Microwave experimental results are in good agreement with numerical simulations and theoretical analyses.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023603 (2023)
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Nonlinear Optics
Multi-color laser generation in periodically poled KTP crystal with single period
Sujian Niu, Zhiyuan Zhou, Jingxin Cheng, Zheng Ge, Chen Yang, and Baosen Shi

Frequency conversion based on three-wave mixing is a critical nonlinear optic application, extending the frequency range of existing lasers and realizing frequency-transduced detectors in a wavelength range that lacks an effective detector. Phase matching is vital for effective frequency conversion. The advantages of quasi-phase matching (QPM) over birefringent phase matching are a lack of walk-off effect, a maximum nonlinear coefficient, and phase matching in the entire transparency window. Herein, using different types and orders of QPM, four kinds of effective frequency doubling processes are realized in a periodically poled potassium titanyl phosphate (KTP) crystal with a single period, and three kinds of frequency doubling processes are experimentally verified. We also show a feasible way to construct an RGB color generator based on two different QPM processes. This study significantly expands the feasible frequency conversion of existing lasers to different wavelengths, providing an effective method for multi-color laser generation based on periodically poled KTP crystals.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021901 (2023)
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Nonlinear Optics
Nonlinear optical limiting effect of graphene dispersions at 3.8 µm
Zhengnan Wu, Yuangang Lu, Jianqin Peng, Jian Huang, and Chongjun He

We investigate the nonlinear optical limiting effect of novel graphene dispersions and graphene dispersions in carbon tetrachloride at a wavelength of 3.8 µm. The transmittances of graphene dispersions in carbon tetrachloride under two different concentrations (0.004 and 0.008 mg/mL) and two different solution thicknesses (10 and 20 mm) are measured. The influences of concentration and thickness on the optical limiting effect of graphene dispersion are analyzed. Theoretical analysis of the experimental data shows that the main optical limiting mechanism of the new graphene dispersions is Mie scattering. The limiting capacity coefficient of the new graphene dispersion in carbon tetrachloride reaches 0.405 and the minimum transmittance reaches 0.292 when the concentration is 0.004 mg/mL and the thickness is 20 mm. The graphene dispersions in carbon tetrachloride can be used to achieve good nonlinear optical limiting effects at the wavelength of 3.8 µm.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021902 (2023)
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Optical Design and Fabrication
Flexible omnidirectional reflective film for CO2 laser protection
Wenling Chen, Chao Liu, Yuqi Zou, Zhihe Ren, Yuanzhuo Xiang, Fanchao Meng, Yinsheng Xu, Chong Hou, Sheng Liang, Lüyun Yang, and Guangming Tao

In this Letter, we presented a flexible omnidirectional reflective film made of polymer substrates and multiple alternating layers of two chalcogenide glasses for full-angle CO2 laser protection. The structure parameters of the device were simulated for theoretical prediction of best device structure. The reflector was fabricated by alternate thermal evaporation of two chalcogenide glasses with large refractive index contrast. The reflectivity was greater than 78% at 10.6 µm. The flexible reflective film can provide an effective solution for full-angle CO2 laser protection of the moving targets, such as laser operators and mobile optical components, with potential applications for wearable laser protective clothing.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 022201 (2023)
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Optical Materials
Nonlinear ionization control by temporally shaped fs+ps double-pulse sequence on ZnO
Weiyi Yin, Juan Song, Xiangyu Ren, Qian Yao, Xian Lin, and Ye Dai

We designed a femtosecond (fs) + picosecond (ps) double-pulse sequence by using a Mach–Zehnder-like apparatus to split a single 120 fs pulse into two sub-pulses, and one of them was stretched to a width of 2 ps by a four-pass grating system. Through observing the ripples induced on the ZnO surface, we found the ionization rate appeared to be higher for the sequence in which the fs pulse arrived first. The electron rate equation was used to calculate changes of electron density distribution for the sequences with different delay times. We suggest that using a temporally shaped fs+ps pulse sequence can achieve nonlinear ionization control and influence the induced ripples.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 021602 (2023)
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Physical Optics
Full description of dipole orientation in organic light-emitting diodes
Lingjie Fan, Maoxiong Zhao, Jiao Chu, Tangyao Shen, Minjia Zheng, Fang Guan, Haiwei Yin, Lei Shi, and Jian Zi

Considerable progress has been made in organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency, among which dipole orientation has a remarkable effect. In most cases, the radiation of the dipoles in OLEDs is theoretically predicted with only one orientation parameter to match with corresponding experiments. Here, we develop a new theory with three orientation parameters to fully describe the relationship between dipole orientation and power density. Furthermore, we design an optimal test structure for measuring all three orientation parameters. All three orientation parameters could be retrieved from non-polarized spectra. Our theory provides a universal plot of dipole orientations in OLEDs, paving the way for designing more complicated OLED devices.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 022601 (2023)
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Spectroscopy
Optical emission spectrometric diagnosis of laser-induced plasma and shock front produced at moderate pressure
Xin Peng, Minsun Chen, and Hao Liu

We present a non-contact optical investigation of laser-induced plasma at moderate Ar pressure ranging from 1 to 100 Pa. The significant shock front and spatial fractionation among the different charged ions are demonstrated at the pressure of 20 Pa. The collisions between Si IV ions and ambient Ar atoms generate distinct and excited Ar II ions, fresh Si III ions, and electrons at the dense layer. The electron density peaks at the position of the shock front, indicating that the collision that yields electrons is dominant over the recombination process in the region of the shock layer and its immediate vicinity.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023001 (2023)
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X-Ray Optics
Damage resistance of B4C reflective mirror irradiated by X-ray free-electron laser
Jinyu Cao, Shuhui Li, Yajun Tong, Ming Tang, Wenbin Li, Qiushi Huang, Huaidong Jiang, and Zhanshan Wang

In this paper, a simple theoretical model combining Monte Carlo simulation with the enthalpy method is provided to simulate the damage resistance of B4C/Si-sub mirror under X-ray free-electron laser irradiation. Two different damage mechanisms are found, dependent on the photon energy. The optimum B4C film thickness is determined by studying the dependence of the damage resistance on the film thickness. Based on the optimized film thickness, the damage thresholds are simulated at photon energy of 0.4–25 keV and a grazing incidence angle of 2 mrad. It is recommended that the energy range around the Si K-edge should be avoided for safety reasons.

Chinese Optics Letters
Feb. 01, 2023, Vol. 21 Issue 2 023401 (2023)
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Research ArticlesVol. 21, Iss.1-Jan..1,2023
Diffraction, Gratings, and Holography
Electrically switchable structural patterns and diffractions in a dual frequency nematic liquid crystal
Zhenpeng Song, Ziyang Li, Xiaohu Shang, Chaoyi Li, Lingling Ma, Yanqing Lu, and Bingxiang Li

Electrically driven structural patterns in liquid crystals (LCs) have attracted considerable attention due to their electro-optical applications. Here, we disclose various appealing reconfigurable LC microstructures in a dual frequency nematic LC (DFNLC) owing to the electroconvection-induced distortion of the LC director, including one-dimensional rolls, chevron pattern, two-dimensional grids, and unstable chaos. These patterns can be switched among each other, and the lattice constants are modulated by tuning the amplitude and frequency of the applied electric field. The electrically switchable self-assembled microstructures and their beam steering capabilities thus provide a feasible way to tune the functions of DFNLC-based optical devices.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 010501 (2023)
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Fiber Optics and Optical Communications
Optical fiber-based magnetically-tuned graphene mechanical resonator
Hongqian Cao, Zengyong Liu, Danran Li, Zhenda Lu, Ye Chen, and Fei Xu

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

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 010601 (2023)
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Imaging Systems and Image Processing
Self-filtering illumination source and application in fluorescence imaging
Feifei Qin, Fan Shi, Xumin Gao, Jiabin Yan, Ziqi Ye, Yulong Su, Jianwei Fu, and Yongjin Wang

To date, fluorescence imaging systems have all relied on at least one beam splitter (BS) to ensure the separation of excitation light and fluorescence. Here, we reported SiO2/TiO2 multi-layer long pass filter integrated GaN LED. It is considered as the potential source for imaging systems. Experimental results indicate that the GaN LED shows blue emission peaked at 470.3 nm and can be used to excite dye materials. Integrating with a long pass filter (550 nm), the light source can be used to establish a real-time fluorescence detection for dyes that emit light above 550 nm. More interestingly, with this source, a real-time imaging system with signature words written with the dyes, such as ‘N J U P T’, can be converted into CCD images. This work may lead to a new strategy for integrating light sources and BS mirrors to build mini and smart fluorescence imaging systems.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011101 (2023)
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Imaging Systems and Image Processing
Vibration measurement with frequency modulation single-pixel imaging
Wenxin Zhang, Yuxiu Tao, Yangkang Wu, Fu Zhu, Wenchao Cai, Ning Liu, Qiang Zhao, and Ping Xue

Single-pixel imaging can reconstruct the image of the object when the light traveling from the object to the detector is scattered or distorted. Most single-pixel imaging methods only obtain distribution of transmittance or reflectivity of the object. Some methods can obtain extra information, such as color and polarization information. However, there is no method that can get the vibration information when the object is vibrating during the measurement. Vibration information is very important, because unexpected vibration often means the occurrence of abnormal conditions. In this Letter, we introduce a method to obtain vibration information with the frequency modulation single-pixel imaging method. This method uses a light source with a special pattern to illuminate the object and analyzes the frequency of the total light intensity signal transmitted or reflected by the object. Compared to other single-pixel imaging methods, frequency modulation single-pixel imaging can obtain vibration information and maintain high signal-to-noise ratio and has potential on finding out hidden facilities under construction or instruments in work.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011102 (2023)
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Instrumentation, Measurement, and Optical Sensing
Simultaneous temperature and magnetic field measurements using time-division multiplexing
Haobin Lin, Ce Feng, Yang Dong, Wang Jiang, Xuedong Gao, Shaochun Zhang, Xiangdong Chen, and Fangwen Sun

Nitrogen-vacancy color centers can perform highly sensitive and spatially resolved quantum measurements of physical quantities such as magnetic field, temperature, and pressure. Meanwhile, sensing so many variables at the same time often introduces additional noise, causing a reduced accuracy. Here, a dual-microwave time-division multiplexing protocol is used in conjunction with a lock-in amplifier in order to decouple temperature from the magnetic field and vice versa. In this protocol, dual-frequency driving and frequency modulation are used to measure the magnetic and temperature field simultaneously in real time. The sensitivity of our system is about 3.4 nT/Hz and 1.3 mK/Hz, respectively. Our detection protocol not only enables multifunctional quantum sensing, but also extends more practical applications.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011201 (2023)
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Instrumentation, Measurement, and Optical Sensing
Preliminary study on direct measurements and diagnostics for chemical reaction dynamics of NOx by using laser wavelength modulation spectroscopy
Yongjian Li, Shuai Zhang, Jinyi Li, Xiaotao Yang, Yunfei Meng, Xu Liu, and Zhenhui Du

Studies on the kinetics of gas-phase chemical reactions currently rely on calculations or simulations and lack simple, fast, and accurate direct measurement methods. We developed a tunable laser molecular absorption spectroscopy measurement system to achieve direct measurements of such reactions by using wavelength modulated spectroscopy and performed online measurements and diagnostics of molecular concentration, reaction temperature, and pressure change during the redox reaction of ozone with nitrogen oxides (NOx) with 0.1 s temporal resolution. This study provides a promising diagnostic tool for studying gas-phase chemical reaction kinetics.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011202 (2023)
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Integrated Optics
Fabrication and photo-response of monolithic 90° hybrid-photodetector array chip for QPSK detection
Han Ye, Qin Han, Shuai Wang, Feng Xiao, Fan Xiao, Yimiao Chu, and Liyan Geng

A quadrature phase-shift keying (QPSK) coherent photodetector chip consisting of a 4×4 multimode interference 90° optical hybrid and a four-channel evanescent photodetector array is designed and fabricated with its photo-response in the L-band characterized. The metal organic chemical vapor deposition regrowth method is adopted to realize active–passive monolithic integration. The chip exhibits a low dark current below 100 nA for each photodetector in the array, a low excess loss of 0.85 dB, a common mode ratio rejection better than 13.6 dB, and a phase deviation within ±10° over the 40 nm wavelength span.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011301 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
All-solid-state far-UVC pulse laser at 222 nm wavelength for UVC disinfection
Qihui Luo, Jian Ma, Miao Wang, Tingting Lu, and Xiaolei Zhu

A 222 nm all-solid-state far-ultraviolet C (UVC) pulse laser system based on an optical parametric oscillator (OPO) and second-harmonic generation (SHG) using β-Ba2BO4 (BBO) crystals was demonstrated. Pumped by a Nd:Y3Al5O12 laser with a repetition rate of 100 Hz at 355 nm, the maximum signal laser pulse energy of 1.22 mJ at 444 nm wavelength was obtained from the BBO-OPO system, corresponding to a conversion efficiency of 27.9%. The maximum output pulse energy of 164.9 µJ at the 222 nm wavelength was successfully achieved, corresponding to an SHG conversion efficiency of 16.2%. Moreover, the tunable output wavelength of UVC light from 210 nm to 252.5 nm was achieved.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011401 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
InAs/GaAs quantum dot laterally coupled distributed feedback lasers at 1.3 μm
Wenfu Yu, Xuyi Zhao, Shixian Han, Antian Du, Ruotao Liu, Chunfang Cao, Jinyi Yan, Jin Yang, Hua Huang, Hailong Wang, and Qian Gong

We report the InAs/GaAs quantum dot laterally coupled distributed feedback (LC-DFB) lasers operating at room temperature in the wavelength range of 1.31 µm. First-order chromium Bragg gratings were fabricated alongside the ridge waveguide to obtain the maximum coupling coefficient with the optical field. Stable continuous-wave single-frequency operation has been achieved with output power above 5 mW/facet and side mode suppression ratio exceeding 52 dB. Moreover, a single chip integrating three LC-DFB lasers was tentatively explored. The three LC-DFB lasers on the chip can operate in single mode at room temperature, covering the wavelength span of 35.6 nm.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011402 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Directly modulated 25 Gbaud/s tunable in-series DFB laser array for WDM systems
Zhenxing Sun, Yaguang Wang, Rulei Xiao, Leilei Wang, Yangyang Gong, Yi-Jen Chiu, and Xiangfei Chen

In this Letter, we proposed and experimentally demonstrated a directly modulated tunable laser based on the multi-wavelength distributed feedback (DFB) laser array. The lasers are placed in series to avoid the usage of an optical combiner and additional power loss. A three-section design is utilized to reduce the interference from other lasers and improve the electro-optic response bandwidth. Besides, the reconstruction-equivalent-chirp technique is used to simplify the grating fabrication and precisely control the grating phase. We realized 12 channels with 100 GHz spacing with high side mode suppression ratios of above 50 dB. The output power of all the channels is above 14 mW. The 3 dB electro-optic bandwidth is above 20 GHz at a bias current of 100 mA for all four lasers. A 25 Gb/s data transmission over a standard single-mode fiber of up to 10 km is demonstrated for all 12 channels, and 50 Gb/s data per wavelength is obtained through the four-level pulse amplitude modulation. The proposed directly modulated tunable in-series DFB laser array shows the potential for a compact and low-cost light source for wavelength division multiplexing (WDM) systems, such as next-generation front-haul networks and passive optical networks.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011403 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Design of an optical slot waveguide amplifier based on Er3+-doped tellurite glass
Ning Wei, Xiaobo Li, Jiajing He, Yongtao Fan, Yaping Dan, and Jun Wang

The idea of a slot waveguide amplifier based on erbium-doped tellurite glass is first theoretically discussed in this work. Choosing the horizontal slot for low propagation loss, the TM mode profile compressed in the insertion layer was simulated, and the gain characteristics of the slot waveguide amplifier were calculated. Combining the capacity to confine light locally and the merits of tellurite glass as an emission host, this optimized amplifier shows enhanced interactions between the electric field and erbium ions and achieves a net gain of 15.21 dB for the 0.01 mW input light at 1530 nm, implying great promise of a high-performance device.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011404 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Generation of 12th order harmonic mode-locking in a Nd-doped single-mode all-fiber laser operating at 0.9 µm
Bin Zhang, Ping Li, Zhaojun Liu, Ming Li, Jing Liu, Haoxu Zhao, Qiongyu Hu, and Xiaohan Chen

Based on the Nd-doped single-mode fiber as the gain medium, an all-fiber 12th harmonic mode-locked (HML) laser operating at the 0.9 µm waveband was obtained for the first time, to the best of our knowledge. A mandrel with a diameter of 10 mm was employed to introduce bending losses to suppress mode competition at 1.06 µm, which resulted in a suppression ratio of up to 54 dB. The 1st–12th order HML pulses with the tunable repetition rate of 494.62 kHz–5.94 MHz were obtained in the mode-locked laser with a center wavelength of ∼904 nm. In addition, the laser has an extremely low threshold pump power of 88 mW. To the best of our knowledge, this is the first time that an HML pulse has been achieved in a 0.9 µm Nd-doped single-mode all-fiber mode-locked laser with the advantages of low cost, simple structure, and compactness, which could be an ideal light source for two-photon microscopy.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011405 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Wideband tunable REC-DFB laser array using thin-film heaters on the submount
Pan Dai, Zhuo Chen, Zhenxing Sun, Hantian Ge, Ji Dai, Jun Lu, Feng Wang, Rulei Xiao, Hua Tong, Rongrong Dou, and Xiangfei Chen

A wideband wavelength-tunable 4×5 distributed feedback (DFB) semiconductor laser array based on the reconstruction-equivalent-chirp (REC) technique using a simple tuning scheme is demonstrated. It consists of 20 DFB lasers with 4×5 matrix interleaving distributions, two-level cascaded Y-branch optical combiners, and one active semiconductor optical amplifier (SOA), all in-series integrated on one chip. Unlike the traditional thermal-electric cooler (TEC)-based wavelength-tuning scheme, the tunable 4×5 REC-DFB laser array achieves a faster and broader continuous wavelength-tuning range using TaN thin-film heaters integrated on the AlN submount. By changing the injection current of the TaN resistor from 0 to 190 mA, the proposed tunable laser achieves a wavelength-tuning range of ∼2.5 nm per channel and a total tuning of over 50 nm. This study opens up new avenues for realizing cost-effective and wide-tuning-range semiconductor lasers.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011406 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Improvement of bandwidth in a 100 kHz swept laser source with phase controllable signal driving
Zhiwei Yang, Xu Wu, Jihong Pei, and Shuangchen Ruan

Swept source optical coherence tomography (SS-OCT) is a new noninvasive technique for assessing tissue. Although it has advantages, such as being label-free, noninvasive, and with high resolution, it also has drawbacks: there has been no in-depth research into identifying the driving of swept source. Based on preliminary research, we demonstrate a novel driving modulation method of a fiber Fabry–Perot tunable filter ranging phase adjustable as a tool for making bandwidth compensation of a swept laser source. This novel method is analyzed in detail; a swept laser source with a sweep rate of 100.5 kHz over a range of 152.25 nm and at a center wavelength of 1335.45 nm is demonstrated.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011407 (2023)
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Lasers, Optical Amplifiers, and Laser Optics
Room temperature continuous-wave operation of a dual-wavelength quantum cascade laser
Yanjiao Guan, Ruixuan Sun, Ning Zhuo, Xiyu Lu, Jinchuan Zhang, Shenqiang Zhai, Junqi Liu, Shuman Liu, Lijun Wang, and Fengqi Liu

We report on the design and fabrication of a dual-wavelength switchable quantum cascade laser (QCL) by optimizing the design of a homogeneous active region and combining superposed distributed feedback gratings. Coaxial, single-mode emissions at two different wavelengths were achieved only through adjusting the bias voltage. Room temperature continuous-wave operation with output powers of above 30 mW and 75 mW was realized for single-mode emission at 7.61 µm and 7.06 µm, respectively. The simplified fabrication process and easy wavelength control of our designed dual-wavelength QCL make it very attractive for developing miniature multi-species gas sensing systems.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 011408 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Generation of tunable superchiral spot in metal-insulator-metal waveguide
Tao Zhuang, Haifeng Hu, and Qiwen Zhan

The chiral feature of an optical field can be evaluated by the parameter of g-factor enhancement, which is helpful to enhance chiroptic signals from a chiral dipole. In this work, the superchiral spot has been theoretically proposed in metal-insulator-metal waveguides. The g-factor enhancement of the superchiral spot can be enhanced by 67-fold more than that of circularly polarized light, and the spot is confined in the deep wavelength scale along each spatial dimension. Moreover, the position of the superchiral spot can be tuned by manipulating the incident field. The tunable superchiral spot may find applications in chiral imaging and sensing.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 013601 (2023)
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Nanophotonics, Metamaterials, and Plasmonics
Modulation of epsilon-near-zero wavelength and enhancement of third-order optical nonlinearity in ITO/Au multilayer films
Bin Guo, Zhongshuai Zhang, Yanyan Huo, Shuyun Wang, and Tingyin Ning

We report the modulation of epsilon-near-zero (ENZ) wavelength and enhanced third-order nonlinearity in indium tin oxide (ITO)/Au multilayer films. The samples consisting of five-layer 40 nm ITO films spaced by four-layer ultrathin Au films of different thickness, i.e., ITO(40 nm)/[Au(x)/ITO(40 nm)]4, were prepared by magnetron sputtering at room temperature. The ENZ wavelength in the multilayer films is theoretically calculated and experimentally confirmed. The nonlinear refractive index and nonlinear absorption coefficient of the samples of x=0, 2, 3, 4 nm were determined using the Z-scan method at a wavelength of 1.064 µm. The large nonlinear refractive index n2=1.12×10-13 m2/W and nonlinear absorption coefficient β=-1.78×10-7 m/W in the sample of x = 4 nm are both four times larger than those in the single-layer ITO film. The large optical nonlinearity due to the ENZ enhancement and carrier concentration is discussed. The results indicate that the ITO/Au multilayer films are promising for advanced all-optical devices.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 013602 (2023)
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Special Issue on Optical Metasurfaces: Fundamentals and Applications
Photo-reconfigurable and electrically switchable spatial terahertz wave modulator [Invited]
Hongguan Yu, Huacai Wang, Zhixiong Shen, Shina Tao, Shijun Ge, and Wei Hu

Spatial terahertz wave modulators that can arbitrarily tailor the electromagnetic wavefront are in high demand in nondestructive inspections and high-capacity wireless communications. Here, we propose a liquid crystal integrated metadevice. It modulates the terahertz wave based on the adjustable electromagnetically induced transparency analog when spatially changing the environmental refractive index. The functions of the device can be arbitrarily programmed via photo-reorienting the directors of liquid crystals with a digital micromirror device-based exposing system. The thin liquid crystal layer can be further driven by an electric field, and thus the function can be rapidly switched. Amplitude modulation and the lens effect are demonstrated with modulation depths over 50% at 0.94 THz.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 010002 (2023)
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Special Issue on Optical Metasurfaces: Fundamentals and Applications
Effects of propagation phase on the coupling of plasmonic optical modes
Wanxia Huang, Yabo Zhang, Yuan Pei, Maosheng Wang, Fenghua Shi, and Kuanguo Li

The temporal coupled-mode theory (TCMT) has made significant progress in recent years, and is widely applied in explaining a variety of optical phenomena. In this paper, the optical characteristics of the metasurface composed of nano-bars and nano-rings are simulated. The simulation results are well explained by TCMT under the coupled basis vector. However, when the structural asymmetry is large, the fitting of results shows that the total radiation loss is not conservative, in contradiction to the requirement of traditional TCMT. We solved this inconsistency by introducing the propagation phase into the near-field coupling term of TCMT. The studies show that, unlike the local mode near the exceptional point which corresponds to the radiation loss of the bright mode, the global mode near the diabolic point is closely related to the propagation phase. Furthermore, the structure near the diabolic point shows characteristic cross-coupling with the change of period. This study proposes a new theoretical framework for comprehending the interaction of light and matter and offers some guiding implications for the application of TCMT to a variety of related domains.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 010003 (2023)
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Special Issue on Optical Metasurfaces: Fundamentals and Applications
Plasmonic nanostructure characterized by deep-neural-network-assisted spectroscopy
Qi'ao Dong, Wenqi Wang, Xinyi Cao, Yibo Xiao, Xiaohan Guo, Jingxuan Ma, Lianhui Wang, and Li Gao

The lateral geometry and material property of plasmonic nanostructures are critical parameters for tailoring their optical resonance for sensing applications. While lateral geometry can be easily observed by a scanning electron microscope or an atomic force microscope, characterizing materials properties of plasmonic devices is not straightforward and requires delicate examination of material composition, cross-sectional thickness, and refractive index. In this study, a deep neural network is adopted to characterize these parameters of unknown plasmonic nanostructures through simple transmission spectra. The network architecture is established based on simulated data to achieve accurate identification of both geometric and material parameters. We then demonstrate that the network training by a mixture of simulated and experimental data can result in correct material property recognition. Our work may indicate a simple and intelligent characterization approach to plasmonic nanostructures by spectroscopic techniques.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 010004 (2023)
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Special Issue on Optical Metasurfaces: Fundamentals and Applications
Photon pair generation from lithium niobate metasurface with tunable spatial entanglement
Jihua Zhang, Jinyong Ma, Dragomir N. Neshev, and Andrey A. Sukhorukov

The two-photon state with spatial entanglement is an essential resource for testing fundamental laws of quantum mechanics and various quantum applications. Its creation typically relies on spontaneous parametric downconversion in bulky nonlinear crystals where the tunability of spatial entanglement is limited. Here, we predict that ultrathin nonlinear lithium niobate metasurfaces can generate and diversely tune spatially entangled photon pairs. The spatial properties of photons including the emission pattern, rate, and degree of spatial entanglement are analyzed theoretically with the coupled mode theory and Schmidt decomposition method. We show that by leveraging the strong angular dispersion of the metasurface, the degree of spatial entanglement quantified by the Schmidt number can be decreased or increased by changing the pump laser wavelength and a Gaussian beam size. This flexibility can facilitate diverse quantum applications of entangled photon states generated from nonlinear metasurfaces.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 010005 (2023)
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X-Ray Optics
X-ray volumetric quantitative phase imaging by Foucault differential filtering with linear scanning
Young-Sung Park, Jieun Hong, and Jaeho Choi

Non-interferometric X-ray quantitative phase imaging (XQPI), much simpler than the interferometric scheme, has provided high-resolution and reliable phase-contrast images. We report on implementing the volumetric XQPI images using concurrent-bidirectional scanning of the orthogonal plane on the optical axis of the Foucault differential filter; we then extracted data in conjunction with the transport-intensity equation. The volumetric image of the laminate microstructure of the gills of a fish was successfully reconstructed to demonstrate our XQPI method. The method can perform 3D rendering without any rotational motion for laterally extended objects by manipulating incoherent X-rays using the pinhole array.

Chinese Optics Letters
Jan. 01, 2023, Vol. 21 Issue 1 013401 (2023)
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Topics GeneralDiffraction, Gratings, and HolographyBiophotonicsIntegrated OpticsInstrumentation, Measurement, and Optical Sensing Special Issues
Ultrafast Optics: fundamentals and applications (2022)
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Editor (s): Shian Zhang, Shengye Jin, Olivier Faucher
Special Issue on Optical Metasurfaces: Fundamentals and Applications (2022)
Call for Papers
Editor (s): Tao Li, Cheng Zhang, Yongmin Liu
Lithium Niobate Based Photonic Devices (2021)
Published
Editor (s): Feng Chen, Yuping Chen
Metal Halide Perovskite and Their Applications (2021)
Published
Editor (s): Yupeng Zhang, Han Zhang
Soft-matter Photonics (2020)
Published
Editor (s): Zhi-gang Zheng, Wei Hu, Chen-hui Peng
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