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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).