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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081101 (2022)
DOI:10.3788/COL202220.081101
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081401 (2022)
DOI:10.3788/COL202220.081401
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081701 (2022)
DOI:10.3788/COL202220.081701
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 080601 (2022)
DOI:10.3788/COL202220.080601
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081201 (2022)
DOI:10.3788/COL202220.081201
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081301 (2022)
DOI:10.3788/COL202220.081301
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081403 (2022)
DOI:10.3788/COL202220.081403
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081202 (2022)
DOI:10.3788/COL202220.081202
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.

Aug. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 8 081402 (2022)
DOI:10.3788/COL202220.081402
Jiahao Li, Mengwei Cao, Weili Liang, Yilin Zhang, Zhenwei Xie, and Xiaocong Yuan

We introduce a simple one-dimensional (1D) structure in the design of 1D color splitters (1D-CSs) with RGB unit cells for color imaging and propose a single-to-double-layer design in 1D-CSs. Based on inverse design metasurfaces, we demonstrate numerically a single-layer 1D-CS with a full-color efficiency of 46.2% and a double-layer 1D-CS with a full-color efficiency of 48.2%; both of them are significantly higher than that of traditional color filters. Moreover, we demonstrate a 1D-CS that has application value by evaluating the double-layer 1D-CS’s performances in terms of incident angle sensitivity, polarization angle sensitivity, and assembly tolerance.

Jul. 10, 2022
Chinese Optics Letters
Vol.20 Issue, 7 073601 (2022)
DOI:10.3788/COL202220.073601
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