Chinese Optics Letters
Co-Editors-in-Chief
Zhizhan Xu
Contents 5 Issue (s), 54 Article (s)
Vol. 20, Iss.12—Dec.1, 2022 • pp: 120501- Spec. pp:
Vol. 20, Iss.11—Nov.1, 2022 • pp: 110501- Spec. pp:
Vol. 20, Iss.10—Oct.1, 2022 • pp: 100002- Spec. pp:
Vol. 20, Iss.9—Sep.1, 2022 • pp: 090501- Spec. pp:
Vol. 20, Iss.8—Aug.1, 2022 • pp: 080601- Spec. pp:
Research ArticlesVol. 20, Iss.12-Dec..1,2022
Diffraction, Gratings, and Holography
Generating large topological charge Laguerre–Gaussian beam based on 4K phase-only spatial light modulator
Ruijian Li, Yuan Ren, Tong Liu, Chen Wang, Zhengliang Liu, Jie Zhao, Rusheng Sun, and Ziyang Wang

The resolution of the spatial light modulator (SLM) screen and the encoding algorithm of the computer-generated hologram are the primary limiting factors in the generation of large topological charge vortex beams. This paper attempts to solve these problems by improving both the hardware and the algorithm. Theoretically, to overcome the limitations of beam waist radius, the amplitude profile function of large topological charge Laguerre–Gaussian (LG) beam is properly improved. Then, an experimental system employing a 4K phase-only SLM is set up, and the LG beams with topological charge up to 1200 are successfully generated. Furthermore, we discuss the effect of different beam waist radii on the generation of LG beams. Additionally, the function of the LG beam is further improved to generate an LG beam with a topological charge as high as 1400. Our results set a new benchmark for generating large topological charge vortex beams, which can be widely used in precise measurement, sensing, and communication.

Chinese Optics Letters
Dec. 10, 2022, Vol. 20 Issue 12 120501 (2022)
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Imaging Systems and Image Processing
End-to-end optimization of a diffractive optical element and aberration correction for integral imaging
Xiangyu Pei, Xunbo Yu, Xin Gao, Xinhui Xie, Yuedi Wang, Xinzhu Sang, and Binbin Yan

In the integral imaging light field display, the introduction of a diffractive optical element (DOE) can solve the problem of limited depth of field of the traditional lens. However, the strong aberration of the DOE significantly reduces the final display quality. Thus, herein, an end-to-end joint optimization method for optimizing DOE and aberration correction is proposed. The DOE model is established using thickness as the variable, and a deep learning network is built to preprocess the composite image loaded on the display panel. The simulation results show that the peak signal to noise ratio value of the optimized image increases by 8 dB, which confirms that the end-to-end joint optimization method can effectively reduce the aberration problem.

Chinese Optics Letters
Dec. 10, 2022, Vol. 20 Issue 12 121101 (2022)
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Nanophotonics, Metamaterials, and Plasmonics
High-efficiency monolayer metallic metasurface for modulation of orbital angular momentum
Peijun Liu, Yanan Fu, Xi Xie, Changjun Min, Yuquan Zhang, and Xiaocong Yuan

The optical vortex beam has widely been studied and used because of its unique orbital angular momentum (OAM). To generate and control OAM in compact and integrated systems, many metallic metasurface devices have been proposed, however, most of them suffer from the low efficiency. Here, we propose and experimentally verify a high-efficiency monolayer metallic metasurface composed of semicircular nano-grooves distributed with detour phase. The metasurface can generate single or an array of OAM with spin-sensitive modulation and achieve the maximum efficiency of 60.2% in theory and 30.44% in experiment. This work has great potential in compact OAM detection and communication systems.

Chinese Optics Letters
Dec. 10, 2022, Vol. 20 Issue 12 123601 (2022)
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Optical Materials
Crystal growth and spectral properties of (Yb0.15Lu0.85xY0.85-0.85x)3Al5O12 single crystals
Ruifeng Tian, Mingyan Pan, Lu Zhang, and Hongji Qi

Four single crystals (Yb0.15Lu0.85xY0.85-0.85x)3Al5O12 (x = 0, 0.25, 0.5, 1) were grown by the Czochralski method. The correlation of the host atom Lu:Y ratios with the density and the luminescence properties were revealed. The density increases linearly with increasing of Lu3+ content, which will improve the gamma ray cut-off ability. The integrated intensity of the X-ray excited luminescence spectrum increases exponentially with the increasing Y:Lu ratio, while the decay time becomes even shorter with the increasing Lu3+ content. These results will provide a basis to balance the comprehensive properties to match different application requirements.

Chinese Optics Letters
Dec. 10, 2022, Vol. 20 Issue 12 121601 (2022)
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Physical Optics
Symmetry detection of rotating patterns based on rotational Doppler effect of light
Fang Han, Weijie Wang, Tong Liu, Yuan Ren, Zhengliang Liu, and Song Qiu

We propose a method for detecting the symmetry of rotating patterns based on the rotational Doppler effect (RDE) of light. The basic mechanisms of the RDE are introduced, and the spiral harmonic distribution of rotating patterns is analyzed. By irradiating the rotating pattern using a superimposed optical vortex and analyzing the amplitude of the RDE signal, the spiral harmonic distribution of the pattern can be measured, and then its symmetry can be detected. We demonstrate this method experimentally by using patterns with different symmetries and shapes. As the method does not need to receive the scattered light completely and accurately, it promises potential application in detecting symmetrical rotating objects at a long distance.

Chinese Optics Letters
Dec. 10, 2022, Vol. 20 Issue 12 122601 (2022)
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Research ArticlesVol. 20, Iss.11-Nov..1,2022
Diffraction, Gratings, and Holography
Measuring the topological charge of optical vortices with a single plate
Jingyin Zhao, Yunxia Jin, Fanyu Kong, Dongbing He, Hongchao Cao, Wang Hao, Yubo Wu, and Jianda Shao

Measuring the topological charge (TC) of optical vortex beams by the edge-diffraction pattern of a single plate is proposed and demonstrated. The diffraction fringes can keep well discernible in a wide three-dimensional range in this method. The redundant fringes of the diffracted fork-shaped pattern in the near-field can determine the TC value, and the orientation of the fork tells the handedness of the vortex. The plate can be opaque or translucent, and the requirement of the translucent plate for TC measurement is analyzed. Measurement of TCs up to ±40 is experimentally demonstrated by subtracting the upper and lower fringe numbers with respect to the center of the light. The plate is easy to get, and this feasible measurement can bring great convenience and efficiency for researchers.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 110501 (2022)
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Fiber Optics and Optical Communications
Security enhancement for OFDM-UWOC system using three-layer chaotic encryption and chaotic DFT precoding
Huan Deng, Zihao Du, Jianmin Xiong, Xingqi Yang, Yan Hua, and Jing Xu

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

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 110601 (2022)
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Fiber Optics and Optical Communications
Visible light communication system at 3.59 Gbit/s based on c-plane green micro-LED
Guoqiang Li, Runze Lin, Haichao Guo, Pengfei Tian, and Nan Chi

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

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 110602 (2022)
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Infrared and Terahertz Photonics
Inversely-designed terahertz metadevice with ultrafast modulation of double electromagnetically induced transparency windows
Siyang Hu, Cheng Xiang’ai, Weibao He, Yuze Hu, Mingyu Tong, and Zhongjie Xu

Terahertz metasurfaces have great applications for efficient terahertz modulation, but there are still problems in designing terahertz metadevices in terms of complexity and inefficiency. Herein, we demonstrate an inversely-designed terahertz metasurface with double electromagnetically induced transparency (EIT)-like windows by incorporating a particle swarm optimization (PSO) algorithm with the finite-difference time-domain method. We prepared and tested the metadevices, and the experimental terahertz signals are close to the designed results. By hybridizing amorphous germanium film with the inversely-designed metasurface, two EIT-like windows, including transmission and slow-light effect, exhibit ultrafast modulation behavior in 25 ps excited by a femtosecond laser. The modulation depths of transmission in two transparency windows are 74% and 65%, respectively. The numerical simulations also illustrate the ultrafast dynamic process and modulation mechanism, which match well with the experiment results. Our work thus offers opportunities for designing other objective functions of the terahertz metadevice.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 113701 (2022)
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Instrumentation, Measurement, and Optical Sensing
Improving the sensitivity of DC magneto-optical Kerr effect measurement to 10−7rad/Hz
Junying Ma, Feng Gu, Ying Xu, Jiaming Le, Fanlong Zeng, Yizheng Wu, and Chuanshan Tian

A high-sensitivity DC magneto-optical Kerr effect (MOKE) apparatus is described in this Letter. Via detailed analysis on several dominating noise sources, we have proposed solutions that significantly lower the MOKE noise, and a sensitivity of 1.5×10-7 rad/Hz is achieved with long-term stability. The sensitivity of the apparatus is tested by measuring a wedge-shaped Ni thin film on SiO2 with Ni thickness varying from 0 to 3 nm. A noise floor of 1.5×10-8 rad is demonstrated. The possibility of further improving sensitivity to 10-9 rad via applying AC modulation is also discussed.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 111201 (2022)
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Integrated Optics
Fabrication and characterization of on-chip silicon spherical-like microcavities with high Q-factors
Hailong Han, Hao Li, Lixing You, and Xiaoping Liu

An effective way to fabricate high-quality (Q) silicon microcavities on-chip is proposed and studied. Our fabrication technique consists of two significant steps: (1) patterning a special silicon micro-pillar by Bosch processes and (2) subsequent reflow of the pillar into a spherical-like microcavity using a laser pulse at 532 nm. Its shape and surface roughness are characterized using a scanning electron microscope and an atomic force microscope. The root-mean-square roughness of the surface is about 0.6 nm. A representative value for the loaded Q-factors of our silicon spherical-like microcavities is on the order of 105.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 111301 (2022)
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Lasers, Optical Amplifiers, and Laser Optics
Intense, wideband optical waveform generation by self-balanced amplification of fiber electro-optical sideband modulation
Yuzhuo Wang, Yizun He, Lingjing Ji, Jiangyong Hu, Xing Huang, Yudi Ma, Liyang Qiu, Kaifeng Zhao, and Saijun Wu

We demonstrate a simple method to obtain accurate optical waveforms with a gigahertz-level programmable modulation bandwidth and a watt-level output power for wideband optical control of free atoms and molecules. Arbitrary amplitude and phase modulations are transferred from microwave to light with a low-power fiber electro-optical modulator. The sub-milliwatt optical sideband is co-amplified with the optical carrier in a power-balanced fashion through a tapered semiconductor amplifier (TSA). By automatically keeping TSA near saturation in a quasi-continuous manner, typical noise channels associated with pulsed high-gain amplifications are efficiently suppressed. As an example application, we demonstrate interleaved cooling and trapping of two rubidium isotopes with coherent nanosecond pulses.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 111401 (2022)
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Lasers, Optical Amplifiers, and Laser Optics
Highly efficient dual-wavelength acousto-optically Q-switched Tm,La:CaF2 laser
Qianqian Hao, Wenxin Liu, Yuqian Zu, Yangxiao Wang, Jie Liu, and Liangbi Su

A highly efficient milli-joule-level Q-switched Tm,La:CaF2 laser is experimentally demonstrated. By employing an acousto-optic modulator, the diode-pumped pulsed lasers are stably operated at repetition rates ranging from 500 Hz to 10 kHz. Dual-wavelength operation of 1881.7 nm and 1888.5 nm is achieved with slope efficiency of 64.7%. Up to 1.89 mJ of pulse energy is obtained at a pulse width of 100 ns, corresponding to a peak power of 18.88 kW. These results verified that the Tm,La:CaF2 crystal could be a promising candidate for achieving highly efficient and high-energy pulsed lasers.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 111402 (2022)
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Light-Matter Interaction
High-fidelity parametric amplification of Ince–Gaussian beams
Ding Yan, Zhiyuan Zhong, Tong Qi, Hongying Chen, and Wei Gao

Ince–Gaussian (IG) beams, as eigenfunctions of the paraxial wave equation in elliptical coordinates, are attracting increasing interest owing to their propagation-invariant and full-field properties. Optical amplification via parametric interactions can further expand their application areas, yet it is rarely studied. In this work, we report on a high-fidelity parametric amplifier for IG beams. The nonlinear transformation of the spatial spectra of the signal and associated influences on the beam profiles of the amplified signal, under different pump structures, were theoretically and experimentally investigated. By using a perfect flattop beam as the pump, we show that the transverse structure of IG signals is well maintained, and the distortion induced by radial-mode degeneration is overcome during amplification. This proof-of-principle demonstration paves the way for a mode-independent and distortion-free amplifier of arbitrary structured light and has great significance in relevant areas, such as quantum optics, tunable infrared-laser generation, and image amplification.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 113801 (2022)
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Microwave Photonics
Weak RF signal detection with high resolution and no blind zone based on ultra-simple multi-mode optoelectronic oscillation
Xin Zhang, Tao Pu, Yunshan Zhang, Hua Zhou, Lin Lu, Jin Li, Xiangfei Chen, and Jilin Zheng

Weak RF signal detection with high resolution and no blind zone based on directly modulated multi-mode optoelectronic oscillation has been proposed. The high-sensitivity optical modulators and optical filters are avoided because multi-mode oscillation is obtained based on directly modulating the semiconductor laser at the relaxation oscillation frequency. For the directly modulated optoelectronic oscillator, the detection characteristics such as gain for the RF signal, resolution, noise floor, and sensitivity are firstly analyzed. The experimental results are consistent with the simulated results. For the RF signal of unknown frequency, it can be detected out and amplified by tuning the bias current and delay time of the loop. There is no blind zone within 1–4.5 GHz. The system provides a maximum gain of 17.88 dB for the low-power RF signal. The sensitivity of the system can reach as high as -95 dBm. The properties such as gain dynamic range and power stability are also investigated. The system has potential for weak RF signal detection, especially for the RF signal with unknown frequency.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 113901 (2022)
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Nanophotonics, Metamaterials, and Plasmonics
Fast dual-beam alignment method for stimulated emission depletion microscopy using aggregation-induced emission dye resin
Miao Zhao, Fengming Liu, Yang Yu, Xinjun Guo, Hao Ruan, and Jing Wen

A stimulated emission depletion is capable of breaking the diffraction limit by exciting fluorescent molecules with a solid Gaussian beam and quenching the excited molecules with another donut beam through stimulated emission. The coincidence degree of these two beams in three dimensions will significantly influence the spatial resolution of the microscope. However, the conventional alignment approach based on raster scanning of gold nanoparticles by the two laser beams separately suffers from a mismatch between fluorescence and scattering modes. To circumvent the above problems, we demonstrate a fast alignment design by scanning the second beam over the fabricated sample, which is made of aggregation-induced emission (AIE) dye resin. The relative positions of solid and donut laser beams can be represented by the fluorescent AIE from the labeled spots in the dye resin. This design achieves ultra-high resolutions of 22 nm in the x/y relative displacement and 27 nm in the z relative displacement for fast spatial matching of the two laser beams. This study has potential applications in scenarios that require the spatial matching of multiple laser beams, and the field of views of different objectives, for example, in a microscope with high precision.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 113601 (2022)
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Nanophotonics, Metamaterials, and Plasmonics
Fabrication of lithium niobate metasurfaces via a combination of FIB and ICP-RIE
Chunyan Jin, Wei Wu, Lei Cao, Bofeng Gao, Jiaxin Chen, Wei Cai, Mengxin Ren, and Jingjun Xu

Lithium niobate (LN) metasurfaces have emerged as a new platform for manipulating electromagnetic waves. Here, we report a fabrication technique for LN nano-grating metasurfaces by combining focused ion beam (FIB) milling with inductively coupled plasma reactive ion etching (ICP-RIE). Steep sidewalls with angles larger than 80° are achieved. Sharp quasi-bound states in the continuum are observed from our metasurfaces. The measured transmission spectra show good agreement with the numerical simulations, confirming the high quality of the fabricated metasurfaces. Our technique can be applied to fabricate the LN metasurfaces with sharp resonances for various applications in optical communications, on-chip photonics, laser physics, sensing, and so on.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 113602 (2022)
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Nonlinear Optics
Interactions of the second-order solitons with an external probe pulse in the optical event horizon
Jifang Rong, Yiwu Ma, Meng Xu, and Hua Yang

We demonstrate manipulating the interactions of a second-order soliton with a weak probe pulse under the condition of group velocity match and group velocity mismatch (GVMM). During these interactions, the second-order soliton acting as an effective periodic refractive-index barrier leads to the polychromatic scattering of the probe pulse, which is represented as unequally spaced narrow-band sources with adjustable spectral width. In the case of GVMM, almost all the spectral components of the narrow-band sources meet the nonlinear frequency conversion relationship by using the wavenumber-matching relationship due to the robustness of the second-order soliton under moderate high-order-dispersion perturbations, so this case is more conducive to the study of the soliton wells. In addition, different transmission states of a soliton well are demonstrated under different probe pulse properties in the fiber-optical analog of the event horizon. When the power of the probe pulse is strong enough, a dispersive wave can be generated from the collision of two fundamental solitons split from the two second-order solitons. These interesting phenomena investigated in this work as a combination of white- and black-hole horizons can be considered as promising candidates for frequency conversion and broadband supercontinuum generation.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 111901 (2022)
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Nonlinear Optics
Sum-frequency generation of a laser and its background in an on-chip lithium-niobate microdisk
Zhenzhong Hao, Li Zhang, Jie Wang, Fang Bo, Feng Gao, Guoquan Zhang, and Jingjun Xu

Lithium-niobate microcavities have not only the ability to resonantly enhance light–matter interaction but also excellent nonlinear optical properties, thereby providing an important platform for nonlinear optical investigations. In this paper, we report the observation of multi-peak spectra in the near infrared range in lithium-niobate microcavities on a chip under the pump of a 1550 nm continuous laser. Such a multi-peak spectrum was attributed to the sum-frequency of the pump laser and its background. The conversion efficiencies of the sum-frequency processes are of the order of 61.5%W-1. The influences of the phenomenon on nonlinear processes were further discussed.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 111902 (2022)
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Optoelectronics
Progress on photovoltaic AlGaN photodiodes for solar-blind ultraviolet photodetection
Xu Liu, and Shengjun Zhou

Solar-blind ultraviolet photodetectors (SBPDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. Aluminum gallium nitride (AlGaN), a kind of representative III-nitride semiconductor, has promising prospects in solar-blind photodetection owing to its tunable wide bandgap and industrial feasibility. Considering the high defect density in the AlGaN epilayer directly grown on a sapphire substrate, employing an AlN/sapphire template turns out to be an effective method to achieve a high-quality AlGaN epilayer, thereby enhancing the SBPD performances. In recent years, a variety of remarkable breakthroughs have been achieved in the SBPDs. In this paper, the progress on photovoltaic AlGaN-based SBPDs is reviewed. First, the basic physical properties of AlGaN are introduced. Then, fabrication methods and defect annihilation of the AlN/sapphire template are discussed. Various photovoltaic SBPDs are further summarized, including Schottky barrier, metal-semiconductor-metal, p-n/p-i-n and avalanche photodiodes. Furthermore, surface modification and photoelectrochemical cell techniques are introduced. Benefitting from the development of fabrication techniques and optoelectronic devices, photovoltaic AlGaN photodiodes exhibit a promising prospect in solar-blind ultraviolet photodetection.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 112501 (2022)
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Physical Optics
High-speed three-dimensional shape measurement with inner shifting-phase fringe projection profilometry
Shichao Yang, Hanlin Huang, Gaoxu Wu, Yanxue Wu, Tian Yang, and Fei Liu

Fringe projection profilometry (FPP) has been extensively studied in the field of three-dimensional (3D) measurement. Although FPP always uses high-frequency fringes to ensure high measurement accuracy, too many patterns are projected to unwrap the phase, which affects the speed of 3D reconstruction. We propose a high-speed 3D shape measurement method using only three high-frequency inner shifting-phase patterns (70 periods), which satisfies both high precision and high measuring speed requirements. Besides, our proposed method obtains the wrapped phase and the fringe order simultaneously without any other information and constraints. The proposed method has successfully reconstructed moving objects with high speed at the camera’s full frame rate (1700 frames per second).

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 112601 (2022)
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Physical Optics
Second-order cumulants ghost imaging
Huan Zhao, Xiaoqian Wang, Chao Gao, Zhuo Yu, Shuang Wang, Lidan Gou, and Zhihai Yao

Ghost imaging (GI) is a technique to retrieve images by correlating intensity fluctuations. In this Letter, we present a novel scheme for GI referred to as second-order cumulants GI (SCGI). The image is retrieved from fluctuation information, and resolution may be enhanced compared to traditional GI. We experimentally performed SCGI image reconstruction, and the results are in agreement with theoretical predictions.

Chinese Optics Letters
Nov. 10, 2022, Vol. 20 Issue 11 112602 (2022)
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Research ArticlesVol. 20, Iss.10-Oct..1,2022
Fiber Optics and Optical Communications
Portable system integrated with time comparison, ranging, and communication
Qiongqiong Zhang, Chengkai Pang, Yurong Wang, Guangyue Shen, Lei Yang, Zhaohui Li, Haiyan Huang, and Guang Wu

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

Chinese Optics Letters
Oct. 10, 2022, Vol. 20 Issue 10 100601 (2022)
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Fiber Optics and Optical Communications
High gain E-band amplification based on the low loss Bi/P co-doped silica fiber
Jinmin Tian, Mengting Guo, Fan Wang, Chunlei Yu, Lei Zhang, Meng Wang, and Lili Hu

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

Chinese Optics Letters
Oct. 10, 2022, Vol. 20 Issue 10 100602 (2022)
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Integrated Optics
Experimental investigation on the unbalanced Mach–Zehnder interferometer on lithium niobate thin film
Xuerui Sun, Yinan Wu, Chuanyi Lu, Yuting Zhang, Hao Li, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen

We design and fabricate an unbalanced Mach–Zehnder interferometer (MZI) via electron beam lithography and inductively coupled plasma etching on lithium niobate thin film. The single unbalanced MZI exhibits a maximum extinction ratio of 32.4 dB and a low extra loss of 1.14 dB at the telecommunication band. Furthermore, tunability of the unbalanced MZI by harnessing the thermo-optic and electro-optic effect is investigated, achieving a linear tuning efficiency of 42.8 pm/°C and 55.2 pm/V, respectively. The demonstrated structure has applications for sensing and filtering in photonic integrated circuits.

Chinese Optics Letters
Oct. 10, 2022, Vol. 20 Issue 10 101301 (2022)
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Microwave Photonics
Delivery of 40 Gbit/s W-band signal over 4600 m wireless distance employing advanced digital signal processing
Bowen Zhu, Yanyi Wang, Weiping Li, Feng Wang, Jiaxuan Liu, Miao Kong, and Jianjun Yu

We experimentally built a photonics-aided long-distance large-capacity millimeter-wave wireless transmission system and demonstrated a delivery of 40 Gbit/s W-band 16-ary quadrature amplitude modulation (QAM) signal over 4600 m wireless distance at 88.5 GHz. Advanced offline digital signal processing algorithms are proposed and employed for signal recovery, which makes the bit-error ratio under 2.4×10-2. To the best of our knowledge, this is the first field-trial demonstration of >4 km W-band 16QAM signal transmission, and the result achieves a record-breaking product of wireless transmission capacity and distance, i.e., 184 (Gbit/s)·km, for high-speed and long-distance W-band wireless communication.

Chinese Optics Letters
Oct. 10, 2022, Vol. 20 Issue 10 103901 (2022)
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Nanophotonics, Metamaterials, and Plasmonics
Broadband angular momentum cascade via a multifocal graphene vortex generator
Zhongwei Jin, Guiyuan Cao, Hongtao Wang, Han Lin, Baohua Jia, and Cheng-Wei Qiu

Light beams carrying orbital angular momentum (OAM) have inspired various advanced applications, and such abundant practical applications in turn demand complex generation and manipulation of optical vortices. Here, we propose a multifocal graphene vortex generator, which can produce broadband angular momentum cascade containing continuous integer non-diffracting vortex modes. Our device naturally embodies a continuous spiral slit vortex generator and a zone plate, which enables the generation of high-quality continuous vortex modes with deep depths of foci. Meanwhile, the generated vortex modes can be simultaneously tuned through incident wavelength and position of the focal plane. The elegant structure of the device largely improves the design efficiency and can be fabricated by laser nanofabrication in a single step. Moreover, the outstanding property of graphene may enable new possibilities in enormous practical applications, even in some harsh environments, such as aerospace.

Chinese Optics Letters
Oct. 10, 2022, Vol. 20 Issue 10 103602 (2022)
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Ultrafast Optics: Fundamentals and Applications
Layer-dependent photoexcited carrier dynamics of WS2 observed using single pulse pump probe method
Lin Zhang, Jiamin Liu, Hao Jiang, Honggang Gu, and Shiyuan Liu

Understanding the ultrafast carrier dynamics and the mechanism of two-dimensional (2D) transition metal dichalcogenides (TMDs) is key to their applications in the field of optoelectronic devices. In this work, a single pulse pump probe method is introduced to detect the layer-dependent ultrafast carrier dynamics of monolayer and few-layer WS2 excited by a femtosecond pulse. Results show that the ultrafast carrier dynamics of the layered WS2 films can be divided into three stages: the fast photoexcitation phase with the characteristic time of 2–4 ps, the fast decay phase with the characteristic time of 4–20 ps, and the slow decay phase lasting several hundred picoseconds. Moreover, the layer dependency of the characteristic time of each stage has been observed, and the corresponding mechanism of free carrier dynamics has been discussed. It has been observed as well that the monolayer WS2 exhibits a unique rising time of carriers after photoexcitation. The proposed method can be expected to be an effective approach for studying the dynamics of the photoexcited carriers in 2D TMDs. Our results provide a comprehensive understanding of the photoexcited carrier dynamics of layered WS2, which is essential for its application in optoelectronics and photovoltaic devices.

Chinese Optics Letters
Oct. 10, 2022, Vol. 20 Issue 10 100002 (2022)
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Research ArticlesVol. 20, Iss.9-Sep..1,2022
Diffraction, Gratings, and Holography
Double-groove rectangular gratings for high-efficiency wideband vertical coupling in planar-integrated optical systems
Guoqing Ma, Changhe Zhou, Yongfang Xie, Ge Jin, Rongwei Zhu, Jin Zhang, Junjie Yu, and Guohai Situ

A planar-integrated optical system (PIOS) represents powerful optical imaging and information processing techniques and is a potential candidate for the realization of a three-dimensional (3D) integrated optoelectronic intelligent system. Coupling the optical wave carrying information into a planar transparent substrate (typically fused silica) is an essential prerequisite for the realization of such a PIOS. Unlike conventional grating couplers for nano-waveguides on the silicon-on-insulator platform, the grating couplers for PIOS enable to obtain a higher design freedom and to achieve much higher coupling efficiency. By combining the rigorous coupled wave algorithm and simulated annealing optimization algorithm, a high-efficiency asymmetric double-groove grating coupler is designed for PIOS. It is indicated that, under the condition of the normal incidence of TE polarization, the diffraction efficiency of the -1st order is over 95%, and its average value is 97.3% and 92.8% in the C and C+L bands. The simulation results indicate that this type of grating coupler has good tolerance and is expected to be applied in optical interconnections, waveguide-based augmented reality glasses, and planar-integrated 3D interconnection optical computing systems.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 090501 (2022)
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Fiber Optics and Optical Communications
Spectrally sliced heterodyne coherent receiver with halved electrical bandwidth
Qi Wu, Yixiao Zhu, Ziyu Cheng, Longjie Yin, and Weisheng Hu

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

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 090601 (2022)
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Imaging Systems and Image Processing
Subwavelength resolved terahertz real-time imaging based on a compact and simplified system
Zhiyong Tan, Wenjian Wan, Chang Wang, and Juncheng Cao

A real-time imaging system based on a compact terahertz laser is constructed by employing one off-axis parabolic mirror and one silicon lens. Terahertz imaging of water, water stains, leaf veins, human hairs, and metal wire is demonstrated. An imaging resolution of 68 µm is achieved. The experiments show that this compact and simplified imaging system is suitable for penetration demonstration of terahertz light, water distribution measurement, and imaging analysis of thin samples.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091101 (2022)
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Imaging Systems and Image Processing
Dispersion control of broadband super-Rayleigh speckles for snapshot spectral ghost imaging
Pengwei Wang, Zhentao Liu, Jianrong Wu, Xia Shen, and Shensheng Han

Snapshot spectral ghost imaging, which can acquire dynamic spectral imaging information in the field of view, has attracted increasing attention in recent years. Studies have shown that optimizing the fluctuation of light fields is essential for improving the sampling efficiency and reconstruction quality of ghost imaging. However, the optimization of broadband light fields in snapshot spectral ghost imaging is challenging because of the dispersion of the modulation device. In this study, by judiciously introducing a hybrid refraction/diffraction structure into the light-field modulation, snapshot spectral ghost imaging with broadband super-Rayleigh speckles was demonstrated. The simulation and experiment results verified that the contrast of speckles in a broad range of wavelengths was significantly improved, and the imaging system had superior noise immunity.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091102 (2022)
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Infrared and Terahertz Photonics
Role of the interlayer interactions in ultrafast terahertz thermal dynamics of bilayer graphene
Tingyuan Jia, Shaoming Xie, Zeyu Zhang, Qinxue Yin, Chunwei Wang, Chenjing Quan, Xiao Xing, Juan Du, and Yuxin Leng

Bilayer graphene, which is highly promising for electronic and optoelectronic applications because of its strong coupling of the Dirac–Fermions, has been studied extensively for the emergent correlated phenomena with magic-angle manipulation. Due to the low energy linear type band gap dispersion relationship, graphene has drawn an amount of optoelectronic devices applications in the terahertz region. However, the strong interlayer interactions modulated electron-electron and electron-phonon coupling, and their dynamics in bilayer graphene have been rarely studied by terahertz spectroscopy. In this study, the interlayer interaction influence on the electron-electron and the electron-phonon coupling has been assigned with the interaction between the two graphene layers. In the ultrafast cooling process in bilayer graphene, the interlayer interaction could boost the electron-phonon coupling process and oppositely reduce the electron-electron coupling process, which led to the less efficient thermalization process. Furthermore, the electron-electron coupling process is shown to be related with the electron momentum scattering time, which increased vividly in bilayer graphene. Our work could provide new insights into the ultrafast dynamics in bilayer graphene, which is of crucial importance for designing multi-layer graphene-based optoelectronic devices.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 093701 (2022)
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Instrumentation, Measurement, and Optical Sensing
White light interferometry with spectral-temporal demodulation for large-range thickness measurement
Yunlong Zhu, Zhuoran Li, Xu Lu, Yonggui Yuan, and Jun Yang

Film thickness measurement can be realized using white light interferometry, but it is challenging to guarantee high precision in a large range of thicknesses. Based on scanning white light interferometry, we propose a spectral-temporal demodulation scheme for large-range thickness measurement. The demodulation process remains unchanged for either coatings or substrate-free films, while some adjustments are made according to the estimated optical thickness. Experiments show that the single-point repeatabilities for 500 nm SiO2 coating and 68 µm substrate-free Si film are no more than 0.70 nm and 1.22 nm, respectively. This method can be further developed for simultaneous measurement of surface profile and film thickness.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091201 (2022)
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Lasers, Optical Amplifiers, and Laser Optics
Investigations on beam quality improvement of an NCPM-KTA-based high energy optical parametric oscillator using an unstable resonator with a Gaussian reflectivity mirror [Invited]
Jun Meng, Chen Li, Zhenhua Cong, Zhigang Zhao, Shang Wang, Gaoyou Liu, and Zhaojun Liu

Beam quality improvements by a large margin for signal and idler beams of a high energy 100 Hz KTiOAsO4 (KTA) non-critical phase matching (NCPM) optical parametric oscillator (OPO) were demonstrated using an unstable resonator configuration instead of a plane-parallel one. Theoretically, influences of cavity lengths and transmission of an output coupler on the OPO conversion efficiency for both were numerically simulated. For OPO based on an unstable resonator with a Gaussian reflectivity mirror, the maximum pulse energies at the signal (1.53 µm) and idler (3.47 µm) were about 75 mJ and 26 mJ, respectively. The corresponding beam quality factors of the signal were Mx2 = 9.8 and My2 = 9.9, and Mx2 = 11.2 and My2 = 11.5 for the idler. As a comparison, 128 mJ of signal and 48 mJ of idler were obtained with the plane-parallel resonator, and the M2 factors of the signal were Mx2 = 39.8 and My2 = 38.4, and Mx2 = 32.1 and My2 = 31.4 for the idler. Compared with a plane-parallel cavity, over eight times and three times brightness improvements were realized for the signal and idler light, respectively.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091401 (2022)
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Optical Materials
Nd3+-doped silica glass and fiber prepared by modified sol-gel method
Yinggang Chen, Zhiquan Lin, Yafei Wang, Meng Wang, Lei Zhang, Yan Jiao, Hehe Dong, Shikai Wang, Chunlei Yu, and Lili Hu

Large-size Al3+/Nd3+ co-doped silica glass with 5000 ppm Nd3+ and 50,000 ppm Al3+ doping concentrations was prepared by the modified sol-gel method combined with high-temperature melting and molding technology. Electron probe micro-analyzer tests indicated that high doping homogeneity was achieved with this sample preparation method. The spectral properties of the Nd3+ ions were evaluated. Nd3+-doped silica fiber (NDF) with a core-to-clad ratio of 20/125 μm was drawn from the preform with the Al3+/Nd3+ co-doped silica glass as the core. In the laser oscillation experiment, a maximum output power of 14.6 W at 1.06 μm with a slope efficiency of 39.6% was obtained from the NDF pumped by a commercial 808 nm laser diode. To the best of our knowledge, this is the highest laser power reported for an NDF operated at 1060 nm and prepared by a non-chemical vapor deposition method. In the master oscillator power amplifier experiment, a maximum power of 16.6 W corresponding to a slope efficiency of 30.5% at 1061 nm was also demonstrated. The laser performance of the NDF exhibited the great advantages and potential of the modified sol-gel method in fabricating Nd3+-doped silica glass for a new type of NDFs like large mode area fibers and fibers with large diameter ratio of core/cladding.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091601 (2022)
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Optical Materials
Dynamic coloration of polymerized cholesteric liquid crystal networks by infiltrating organic compounds
Yu Cao, Li Chong, Ke-Hui Wu, Lu-Qian You, Sen-Sen Li, and Lu-Jian Chen

We demonstrate the dynamic coloration of polymerized cholesteric liquid crystal (PCLC) networks templated by the “wash-out/refill” method in the presence of organic compounds. The reflection colors were modulated by two key approaches, that is, the injection of mutually soluble organic fluids into a microfluidic channel and the diffusion of volatile organic compounds (VOCs). The reversible tuning of reflected colors with central wavelengths between ∼450 nm and ∼600 nm was achieved by alternative injection of nematic liquid crystal E7 (nav = 1.64) and benzyl alcohol (n = 1.54) using syringe pumps. The fascinating iridescence with reflection centers from ∼620 nm to ∼410 nm was presented from the volatilization and diffusion of alcohol as a model VOC. Additionally, the flow velocity of fluid and the diffusion time were adjusted to explore the underlying mechanism for the dynamic coloration of cholesteric networks. This work is expected to extend the study of PCLCs as a dynamically tunable optofluidic reflector, visually readable sensor, or compact anti-counterfeit label in response to organic compounds.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091602 (2022)
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Optical Materials
Structural and optical properties evolution in pressure-induced amorphization of metal-organic framework ZIF-8
Xin Huang, Jin He, Yiguang Jiang, Zhuocheng Chen, Xing Duan, and Long Zhang

Zeolitic imidazolate framework-8 (ZIF-8), a metal-organic framework (MOF) with a non-centrosymmetric crystal structure, exhibits nonlinear optics (NLO) properties and can act as the nanoporous matrix of guest molecules. Amorphization of ZIF-8 can be achieved by pressure or high temperature. Both crystalline and amorphous states have their inherent features for optical applications. The effects of the crystalline-amorphous transition on the structural and optical properties under pressure were investigated in detail. Amorphization leads to the destruction of the ZIF-8 lattice structure, collapse of pores, and the change of spatial symmetry, which in turn alters the NLO properties of ZIF-8 and the luminescence properties of the guest Eu cations. Our results establish the structure–optical properties relationship in the amorphization process and provide new clues in designing novel MOFs optical materials.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 091603 (2022)
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Optoelectronics
Generation of Lommel beams through highly scattering media
Shijie Tu, Qiannan Lei, Yangjian Cai, and Qian Zhao

Lommel beams have been potential candidates for optical communication and optical manipulation, due to their adjustable symmetry of transverse intensity distribution and continuously variable orbital angular momentum. However, the wavefront of the Lommel beam is scrambled when it transmits through highly scattering media. Here, we explore the construction of Lommel beams through highly scattering media with a transmission matrix-based point spread function engineering method. Experimentally, various Lommel beams with different parameters were generated through a ZnO scattering layer by use of a digital micromirror device. The construction of Lommel beams under high scattering is expected to benefit the optical applications behind highly scattering media.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 092501 (2022)
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Quantum Optics and Quantum Information
Loss-tolerant measurement device independent quantum key distribution with reference frame misalignment
Jipeng Wang, Zhenhua Li, Zhongqi Sun, Tianqi Dou, Wenxiu Qu, Fen Zhou, Yanxin Han, Yuqing Huang, and Haiqiang Ma

Reference frame independent and measurement device independent quantum key distribution (RFI-MDI-QKD) has the advantages of being immune to detector side loopholes and misalignment of the reference frame. However, several former related research works are based on the unrealistic assumption of perfect source preparation. In this paper, we merge a loss-tolerant method into RFI-MDI-QKD to consider source flaws into key rate estimation and compare it with quantum coin method. Based on a reliable experimental scheme, the joint influence of both source flaws and reference frame misalignment is discussed with consideration of the finite-key effect. The results show that the loss-tolerant RFI-MDI-QKD protocol can reach longer key rate performance while considering the existence of source flaws in a real-world implementation.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 092701 (2022)
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Spectroscopy
Reconstruction and fitting of second-harmonic signals by wavelength modulation spectroscopy method based on fast Fourier transform
Linquan Lai, Yue Chen, Kongtao Chen, Jiale Tang, Kaiwen Yin, Fuqiang Jia, Dun Qiao, Yuanlong Fan, Kang Li, and Nigel Copner

Conventional wavelength modulation spectroscopy (WMS) is vulnerable to the influence of low-frequency noise. Accuracy of the method highly depends on the performance of the costly lock-in amplifier. In this article, we report a new and effective method for reconstructing second-harmonic signals through WMS based on fast Fourier transform (FFT). This method is less disturbed by low-frequency noise because it does not use a low-frequency ramp wave. Formulation and detection procedures were presented. The discrete second-harmonic waveform can be obtained by continuously changing the DC signal and FFT analysis in this method. Second-harmonic waveforms acquired by the two means are generally consistent. The experimental study validates the obtained gas concentration from 5% to 30%, showing a good linear relationship by the proposed method. The maximum relative error on concentration extraction is 2.87%; as for conventional WMS, this value is 4.50%. The developed measurement method may have potential in computed tomography.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 093001 (2022)
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Ultrafast Optics and Attosecond/High-Field Physics
Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air
Hengyi Zheng, Fukang Yin, Tie-Jun Wang, Yaoxiang Liu, Yingxia Wei, Bin Zhu, Kainan Zhou, and Yuxin Leng

The temporal evolutions of electron density and plasma diameter of 1 kHz femtosecond laser filament in air are experimentally investigated by utilizing a pump-probe longitudinal diffraction method. A model based on scalar diffraction theory is proposed to extract the spatial phase shift of the probe pulse from the diffraction patterns by the laser air plasma channel. The hydrodynamic effect on plasma evolution at 1 kHz filament is included and analyzed. The measured initial peak electron density of ∼1018 cm-3 in our experimental conditions decays rapidly by nearly two orders of magnitude within 200 ps. Moreover, the plasma channel size rises from 90 µm to 120 µm as the delay time increases. The experimental observation is in agreement with numerical simulation results by solving the rate equations of the charged particles.

Chinese Optics Letters
Sep. 10, 2022, Vol. 20 Issue 9 093201 (2022)
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Research ArticlesVol. 20, Iss.8-Aug..1,2022
Biophotonics
A novel needle probe for deeper photoacoustic viscoelasticity measurement
Daoqian Yang, Zhongjiang Chen, and Da Xing

We present for the first time, to the best of our knowledge, a needle probe for photoacoustic viscoelasticity (PAVE) measurements at a depth of 1 cm below the sample surface. The probe uses a gradient index rod lens, encased within a side-facing needle (0.7 mm outer diameter), to direct excitation light (532 nm) and detection light (1325 nm) focused on the sample, collecting and directing the returned detection light in a spectral domain low coherence interferometry system, which allows for obtaining optical phase differences due to photoacoustic oscillations. The feasibility of needle probe for PAVE depth characterization was investigated on gelatin phantoms and in vitro biological tissues. The experimental results in an in vivo animal model predict the great potential of this technique for in vivo tumor boundary detection.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081701 (2022)
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Fiber Optics and Optical Communications
Optimized blind equalization for probabilistically shaped high-order QAM signals
Fengchu Cao, Mingyi Gao, Pengfei Wang, Xiaodi You, and Gangxiang Shen

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

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 080601 (2022)
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Imaging Systems and Image Processing
FAANet: feature-aligned attention network for real-time multiple object tracking in UAV videos
Zhenqi Liang, Jingshi Wang, Gang Xiao, and Liu Zeng

Multiple object tracking (MOT) in unmanned aerial vehicle (UAV) videos has attracted attention. Because of the observation perspectives of UAV, the object scale changes dramatically and is relatively small. Besides, most MOT algorithms in UAV videos cannot achieve real-time due to the tracking-by-detection paradigm. We propose a feature-aligned attention network (FAANet). It mainly consists of a channel and spatial attention module and a feature-aligned aggregation module. We also improve the real-time performance using the joint-detection-embedding paradigm and structural re-parameterization technique. We validate the effectiveness with extensive experiments on UAV detection and tracking benchmark, achieving new state-of-the-art 44.0 MOTA, 64.6 IDF1 with 38.24 frames per second running speed on a single 1080Ti graphics processing unit.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081101 (2022)
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Instrumentation, Measurement, and Optical Sensing
Four-quadrant demodulation fiber sensor for wavelength monitoring of a wavelength phase-shifting interferometer
Xiaoqin Shan, Xingyue Fan, Zhigang Han, Jin Wang, Fengrui Li, and Rihong Zhu

Real-time monitoring of wavelength is important for high-speed wavelength phase-shifting interferometry. In this paper, a wavelength sensor based on a polarization-maintaining fiber interferometer with four-quadrant demodulation was proposed. We built the wavelength sensing system with resolution better than 0.005 pm and 0.1 ms sampling interval and measured the response time of the tuned wavelength at 35 ms in the phase-shifting process of a commercial wavelength phase-shifting free-space interferometer, as well as the wavelength drift velocity of 0.01 pm per second in the hysteresis process. The optical fiber wavelength sensor with four-quadrant demodulation provides a real-time wavelength sensing scheme for high-speed wavelength phase-shifting interferometers.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081201 (2022)
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Instrumentation, Measurement, and Optical Sensing
Zero field optically pumped magnetometer with independent dual-mode operation
Shaowen Zhang, Jixi Lu, Ying Zhou, Fei Lu, Kaifeng Yin, Di Zhan, Yueyang Zhai, and Mao Ye

We propose a dual-mode optically pumped magnetometer (OPM) that can flexibly switch between single-beam modulation mode and double-beam DC mode. Based on a 4 mm×4 mm×4 mm miniaturized vapor cell, the double-beam DC mode achieves a sensitivity of 7 fT/Hz1/2 with probe noise below 4 fT/Hz1/2 and working bandwidth over 65 Hz. This mode is designed to precisely measure the noise floor of a mu-metal magnetic shield. The single-beam modulation mode (sensitivity 20 fT/Hz1/2) exhibits bandwidth characteristics suitable for biomagnetic measurements. Thus, our design is suitable for a miniaturized OPM with multiple functions, including magnetic-shield background noise measurement and medical imaging.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081202 (2022)
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Instrumentation, Measurement, and Optical Sensing
Wideband Rydberg atom-based receiver for amplitude modulation radio frequency communication
Kai Yang, Zhanshan Sun, Ruiqi Mao, Yi Lin, Yi Liu, Qiang An, and Yunqi Fu

Based on Autler–Townes splitting and AC Stark shifts, we present a Rydberg atom-based receiver for determining the amplitude modulation (AM) frequency among a wideband carrier range utilizing a cesium atomic vapor cell. To verify this approach, we measured the signal-to-noise ratio and the data capacity with a 10 kHz AM frequency in the carrier range from 2 GHz to 18 GHz. Without changing the lasers, the working band can be easily extended to a higher range by optimizing the feed antenna and experimental configurations.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081203 (2022)
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Integrated Optics
Temperature insensitive multi-channel light amplification systems on SOI platform
Junhu Zhou, Jie You, Hao Ouyang, Runlin Miao, Xiang’ai Cheng, and Tian Jiang

We present a theoretical analysis of a novel multi-channel light amplification photonic system on chip, where the nonlinear Raman amplification phenomenon in the silicon (Si) wire waveguide is considered. Particularly, a compact and temperature insensitive Mach–Zehnder interferometer filter working as demultiplexer is also exploited, allowing for the whole Si photonic system to be free from thermal interference. The propagation of the multi-channel pump and Stokes lights is described by a rigorous theoretical model that incorporates all relevant linear and nonlinear optical effects, including the intrinsic waveguide optical losses, first- and second-order frequency dispersion, self-phase and cross-phase modulation, phase shift and two-photon absorption, free-carriers dynamics, as well as the inter-pulse Raman interaction. Notably, to prevent excessive drift of the transmission window of the demultiplexer caused by ambient temperature variations and high thermo-optical coefficient of Si, an asymmetric waveguide width is adopted in the upper and lower arms of each Mach–Zehnder interferometer lattice cell. A Chebyshev half-band filter is utilized to achieve a flat pass-band transmission, achieving a temperature sensitivity of 1.4 pm/K and over 100 K temperature span. This all-Si amplifier shows a thermally robust behavior, which is desired by future Si-on-insulator (SOI) applications.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081301 (2022)
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Lasers, Optical Amplifiers, and Laser Optics
High-power narrow-linewidth diode laser pump source based on high-efficiency external cavity feedback technology
Jinliang Han, Jun Zhang, Xiaonan Shan, Hangyu Peng, Yawei Zhang, and Lijun Wang

In this research, the highly efficient external cavity feedback technology based on volume Bragg grating (VBG) is studied. By using the structure of a fast axis collimating lens, the beam transformation system, a slow axis collimating lens, and VBG, the divergence angle of the fast and slow axes of the diode laser incident on the VBG is reduced effectively, and the feedback efficiency of the external cavity is improved. Combined with beam combining technology, fiber coupling technology, and precision temperature control technology, a high-power and narrow-linewidth diode laser pump source of kilowatt class is realized for alkali metal vapor laser pumping. The core diameter of the optical fiber is 1000 µm, the numerical aperture is 0.22, the output power from the fiber is 1013 W, the fiber coupling efficiency exceeds 89%, and the external cavity efficiency exceeds 91%. The central wavelength is 852.052 nm (in air), which is tunable from 851.956 nm to 852.152 nm, and the spectral linewidth is 0.167 nm. Research results can be used for cesium alkali metal vapor laser pumping.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081401 (2022)
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Lasers, Optical Amplifiers, and Laser Optics
Dynamics of multi-state in a simplified mode-locked Yb-doped fiber laser
Zexin Zhang, Jinrong Tian, Youshuo Cui, Yunfeng Wu, and Yanrong Song

The dispersive Fourier transform technique provides feasibility of exploring non-repetitive events and the buildup process in ultrafast lasers. In this paper, we report a new buildup process of dissipative solitons in a simplified mode-locked Yb-doped fiber laser, which includes more complex physics stages such as the Q-switching stage, raised and damped relaxation oscillation stages, noise-like stage, successive soliton explosions stage, and soliton breathing stage. Complete evolution dynamics of noise-like pulse and double pulse are also investigated with dispersive Fourier transform. For the noise-like pulse dynamics process, it will only experience the Q-switching and relaxation oscillation stages. In the case of dissipative soliton and noise-like pulse, the double pulse buildup behavior is manifested as the replication of individual pulses. A weak energy migration occurs between two pulses before reaching steady state. Meanwhile, real-time mutual conversion of the dissipative soliton and noise-like pulse has been experimentally observed, which appears to be instantaneous without extra physical processes. To the best of our knowledge, this is the first report on these physical phenomena observed together in a mode-locked fiber laser. The results further enrich the dynamics of mode-locked fiber lasers and provide potential conditions for obtaining intelligent mode-locked lasers with controllable output.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081402 (2022)
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Lasers, Optical Amplifiers, and Laser Optics
Transfer of laser frequency from 729 nm to 1.5 µm with precision at the level of 10−20
Pengcheng Fang, Huanyao Sun, Yan Wang, Yanqi Xu, and Qunfeng Chen

By using a self-reference transfer oscillator method, two individual 1560 nm lasers with about 1.2 GHz frequency difference were phase locked to a 729 nm ultra-stable laser at two preset ratios. By measuring the beat frequency of the two 1560 nm lasers, fractional instabilities of 2×10-17 at 1 s and 2×10-20 at 10,000 s averaging time were obtained, and the relative offset compared with the theoretical value was 4.2×10-21±4.5×10-20. The frequency ratio of them was evaluated to a level of 1.3×10-20 in one day’s data acquisition. This work was a preparation for remote comparison of optical clocks through optical fiber links. The technique can also be used to synthesize ultra-stable lasers at other wavelengths.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081403 (2022)
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Microwave Photonics
Stimulated-Brillouin-scattering-based arbitrarily phase coded microwave waveform transmitter with anti-dispersion transmission
Sha Zhu, Kunpeng Zhai, Wei Li, and Ning Hua Zhu

We focus on photonic generation and transmission of microwave signals in this work. Based on dual-pumped stimulated Brillouin scattering, a single-sideband (SSB) optical signal with high sideband rejection ratio is obtained. Combined with a phase-modulated optical carrier, an arbitrarily phase coded microwave signal is generated after photoelectric conversion. The SSB modulation can eliminate the fiber-dispersion-induced power dispersion naturally, and the phase modulation of the optical carrier can achieve arbitrary phase encoding and suppress background noise. The proposed scheme can achieve both generation and anti-dispersion transmission of arbitrarily phase coded signals simultaneously, which is suitable for one-to-multi long-distance radar networking.

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 083901 (2022)
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Nonlinear Optics
Statistical study on rogue waves in Gaussian light field in saturated nonlinear mediaOn the Cover
Ziyang Chen, Fuqiang Li, and Cibo Lou

The spatial rogue waves (RWs) generated by a wide Gaussian beam in a saturated nonlinear system are experimentally observed. Our observations show that RWs are most likely to occur when Gaussian light evolves to the critical state of filament splitting, and then the probability of RWs decreases with voltage fluctuations. The occurrence probability of RWs after splitting is related to the nonlinear breathing phenomenon of optical filament, and the statistics of RWs satisfy the long-tailed L-shaped distribution. The experiment proves that the presence of high-frequency components and the aggregation of low-frequency components can serve as a prerequisite for the occurrence of extreme events (EEs).

Chinese Optics Letters
Aug. 10, 2022, Vol. 20 Issue 8 081901 (2022)
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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)
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Editor (s): Tao Li, Cheng Zhang, Yongmin Liu
Lithium Niobate Based Photonic Devices (2021)
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Editor (s): Feng Chen, Yuping Chen
Metal Halide Perovskite and Their Applications (2021)
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Editor (s): Yupeng Zhang, Han Zhang
Soft-matter Photonics (2020)
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Deep learning-based scattering removal of light field imagingEditors' Pick
Chinese Optics Letters, Vol. 20, Issue 4, 041101 (2022)
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