Haiping Mei

Orbital angular momentum (OAM) modes can increase the capacity of free-space optical communication, but turbulence-induced modal crosstalk limits their practical use. Atmospheric turbulence simulators offer controlled test environments; however, their short propagation distances make it unclear how convection-driven spatial heterogeneity and weak scintillation affect OAM spectra and adaptive-optics (AO) correction. Here, we investigate the propagation of vortex beams through Rayleigh–Benard (RB) turbulence and evaluate AO compensation under physically controlled conditions. The RB turbulence produces strong phase distortions with weak scintillation, leading to progressive OAM modal degradation without pronounced bimodal features in the power spectrum. As the vertical temperature difference increases, modal power is redistributed into an hourglass-shaped pattern associated with the large-scale circulation of RB convection, with stronger perturbations near the boundaries and weaker perturbations in the central region. We further implement a hardware-parameterized AO framework that incorporates wavefront sensing, phase reconstruction, and deformable-mirror correction. The results show that AO effectively suppresses modal crosstalk and improves recovery of the OAM power spectrum under boundary-driven turbulence. These findings clarify how convective flow structures perturb OAM modes and provide quantitative guidance for assessing turbulence-compensation strategies in OAM-based free-space optical communication.

To address the challenges of weak target features, severe background interference, and strict real-time constraints in micro-UAV detection under complex and degraded imaging conditions, this paper proposes an efficient real-time detection framework termed EDN-YOLO. Built upon Yolov10, the proposed method improves detection performance from two complementary perspectives: efficient global context modeling and stable multi-scale feature fusion, without introducing significant computational overhead. On the MMAUD dataset, EDN-YOLO achieves an mAP50 of 0.923, outperforming the Yolov10n baseline by 2.9% with only 8.6 GFLOPs. On the degraded MMAUD dataset, it attains an mAP50 of 0.747, yielding a 5.5% improvement over the baseline and a notably higher Recall. Accuracy–efficiency trade-off analyses further demonstrate that EDN-YOLO consistently occupies the favorable high-accuracy and low-complexity region under both clear and degraded conditions, highlighting its strong potential for real-time UAV deployment. Extensive experiments validate the effectiveness and practicality of the proposed approach for micro-UAV detection tasks.

This study presents a comprehensive numerical framework for Background-Oriented Schlieren (BOS) to systematically evaluate its performance and reconstructive capabilities under complex flow conditions. This framework integrates two stages: forward modeling, using ray tracing to simulate image degradation, and inverse processing, using optical flow and a conjugate gradient algorithm to extract displacements and reconstruct phase information. This method is first validated using turbulent flow fields in the Johns Hopkins Turbulence Database, where the reconstructed phase screens closely match the original data, with relative errors below 4% and structural similarity indices above 0.75 in all cases, providing a possible restoration method for degraded flow field images. It is then applied to shock wave fields with varying Mach numbers; this method achieves meaningful reconstruction at short ranges but fails under long-range imaging due to severe wavefront distortions. However, even in degraded conditions, the extracted optical flow fields preserve structural features correlated with the underlying shock patterns, indicating potential for BOS-based target recognition. These findings highlight both the capabilities and limitations of BOS and suggest new pathways for extending its use beyond traditional flow visualization.

CO2 is an important greenhouse gas(GHG) that affects the Earth's atmosphere, and continuous measurement of CO2 helps understand its sources and sinks. The passive laser heterodyne spectrometer(LHS) has been used in meteorology and environmental monitoring recently, especially in the measurement of the radiation properties of the Earth and the densities of atmospheric greenhouse gases. As well known, the passive LHS uses sunlight, the heterodyne efficiency is low, the heterodyne signal processing is complex, and it cannot be measured without sunlight. Therefore, it is difficult for passive LHS to achieve continuous measurement of the GHG of interest. In order to improve the application field of LHS, a 1.57 μm active LHS is investigated and built by using a tunable DFB laser, acoustic-optic frequency shifter and fiber amplifier. It adopts the reflected laser after amplified and frequency shifted as the input light and mixed with seed laser, which possesses the advantage of high coherence efficiency and continuous observation day and night. The CO2 absorption spectrum measurement in the range of 2.2 km is realized, and the signal-to-noise ratio is about 286. The densities of CO2 are 419.15 ~ 424.48ppmv during the experiment. The research will make up for the deficiency of the passive laser heterodyne spectrometer.

Background-oriented schlieren (BOS) is widely used for visualizing transparent flow fields, but traditional displacement extraction algorithms struggle in large-scale flow fields with complex, noisy backgrounds. This paper presents robust-IRR model, based on IRR-PWCNet, which improves robustness through physics-based noise augmentation. Experimental results demonstrate that robust-IRR reduces endpoint error (EPE) by 43.06% in noisy remote sensing backgrounds compared to other methods, while maintaining high computational efficiency. Moreover, it reduces the error by 11.72% to 14.20% at different PSNR levels compared to the IRR without noise enhancement. Furthermore, to reconstruct the flow field and wavefront shapes, we apply a bi-conjugate gradient (BiCG) method with ASDI-based initial values, achieving a 7.47% error reduction over using BiCG or ASDI alone. The proposed method offers a robust and efficient solution for high-precision flow field measurement, visualization, and wavefront reconstruction in complex environments.

The application of ultra-high-bandwidth free-space optical (FSO) communication has become a widely researched hotspot. In remote areas without network coverage, the low altitude platform (LAP), such as airships, drones, etc., can provide a low-cost and rapid deployment emergency FSO communication platform covering hundreds of kilometers and has immeasurable application prospects. In our work, we experimentally demonstrated a line rate of 112 Gbit/s FSO communication at a distance of 104.8 km over Qinghai Lake. By adopting the methods of polarization multiplexing (PM), high-order quadrature amplitude modulation (n-QAM), automatic gain optical amplification (AGOA), etc., we have achieved a long-haul and high-bandwidth laser communication in atmospheric link with the optimum bit error rate (BER) of 3E-3 (zero-BER after error-correction) for a time-length of 50 s. It is particularly emphasized that the AGOA effectively can suppress the fluctuation of received optical power caused by atmospheric turbulence scintillation effect in Qinghai Lake, which solved the difficulty of detection-demodulation with ultra-high-speed laser signals. In addition, the efficient soft decision forward error correction (SD-FEC) also improved the stability of the communication link, which played a crucial role in the success of this experiment. Our design of non-adaptive optical (AO) system is very suitable for application in small-scale LAP and will potentially become an important foundation of the large-capacity nonterrestrial network (NTN) based on LAP. To the best of our knowledge, this experiment verified the feasibility of single-wavelength (1556.15 nm) 112 Gbit/s high-bandwidth laser signal transmission with a long-haul of 104.8 km in low-altitude horizontal atmospheric link for the world’s first time. To a large extent, our research findings also have great application value in the field of low-altitude economy.

All roads lead to Rome. In this article we propose a novel theoretical framework to demonstrate vector beams whose degree of polarization does not change on atmospheric propagation. Inspired by the Fresnel equations, we derive the reflected and refracted field of vector beams propagating through a phase screen by employing the continuity of electromagnetic field. We generalize the conventional split-step beam propagation method by considering the vectorial properties in the vacuum diffraction and the refractive properties of a single phase screen. Based on this vectorial propagation model, we extensively calculate the change of degree of polarization (DOP) of vector beams under different beam parameters and turbulence parameters both in free-space and satellite-mediated links. Our result is that whatever in the free-space or satellite-mediated regime, the change of DOP mainly fluctuates around the order of 10⁻¹³ to 10⁻⁶, which is almost negligible.