Atmospheric turbulence-induced random fluctuations in the refractive index can lead to the degradation of the polarization of polarized light. In accordance with the unified theory of coherent polarization, a comprehensive investigation was undertaken to explore the variation in the degree of polarization (DOP) of laser beams propagating through atmospheric turbulence channels under diverse weather conditions. This investigation involved both theoretical analyses and experimental validations, providing a multifaceted approach to understanding the dynamics of laser beam propagation in atmospheric turbulence. To this end, numerical simulations were performed to analyze the polarization-maintaining characteristics of laser beams with varying wavelengths, turbulence intensities, and initial DOP values. To validate the simulation results for various weather scenarios, three experimental links with different propagation distances were constructed. The experimental results demonstrated that as the turbulence intensity increased, the average DOP of the beam continuously decreased until it reached a threshold value. Furthermore, the polarization fluctuations exhibited a distance-threshold effect, wherein the polarization parameters tended to saturate beyond a critical propagation distance.

In the connection between the last kilometer access network and the backbone network, the dual-hop hybrid free-space optical-radio frequency (FSO-RF) relay system is considered to be an effective solution to increase the capacity and coverage of the wireless communication system. However, the transmission performance of FSO link is greatly affected by atmospheric turbulence, pointing errors and weather such as fog and snow, and the RF access network has low rate and relatively high delay. To ensure communication quality, a hybrid FSO/RF-terahertz (THz) relay system has been designed. Decode-and-forward relay is selected, and the hybrid FSO/RF communication based on adaptive combining scheme is considered before the relay node, and THz link is used after the relay node to access the users. Using the statistical characteristics of different links obtained, expressions for the outage probability and average bit error rate of the system were derived. The effects of different parameters on the performance of hybrid FSO/RF-THz relay system is studied, and the first hop adaptive combining scheme is compared with the single-threshold switching scheme and the single-link FSO system. The analysis results indicate that the hybrid FSO/RF-THz relay system can provide better outage and bit error rate performance by using the adaptive combining scheme, which can improve the system communication rate while taking into account the communication reliability.

The dynamic non-stationarity of atmospheric turbulence poses a significant challenge to adaptive optics (AO) systems, often inducing severe servo-lag errors. To mitigate this, the present study proposes an empirical datadriven deep spatiotemporal predictive control strategy specifically designed to compensate for such latencyinduced errors. By establishing three free-space optical transmission links at varying propagation distances, authentic atmospheric wavefront datasets were collected under a multitude of meteorological conditions. Leveraging the phase structure function, the evolutionary dynamics of distorted wavefronts were characterized from a physical perspective. To address the inherent nonlinearities in wavefront evolution, a comparative study was performed across various spatiotemporal modeling frameworks, including FCNN, ConvLSTM, and several attention-augmented variants. The deep learning prediction model extracts spatiotemporal variation features from four consecutive historical frames to achieve advance prediction of wavefront phase at future time points, thereby driving the deformable mirror for correction. Experimental results demonstrate that the attentionenhanced spatiotemporal fusion network (ConvLSTM-CBAM) exhibits the most robust generalization and highest prediction accuracy under complex conditions. Compared to the traditional passive prediction mode (t-1), the proposed method improves the average Structural Similarity Index Measure (SSIM) to 95%, reduces the average residual wavefront RMS error to 0.046 um, and enhances the Strehl Ratio (SR) up to 83%. This approach offers a novel method for wavefront prediction in dynamic atmospheric environments.

Abstract The propagation of an optical beam through turbulent media induces intensity scintillation, significantly degrading the beam quality and limiting the improvement in communication system performance. Based on Rytov perturbation theory and the spectral method, we derived the spectral filter function and the variance of the log-amplitude fluctuations for vortex beams under weak turbulence conditions. The trends associated of spectral filter function and intensity scintillation under different parameters were analyzed and subsequently validated through field experiments. Our findings indicate that the log-amplitude fluctuation variance of vortex beams is smaller than that of Gaussian beams and other beam models, and decreases with increasing topological charge. Overall, the results demonstrate that the intensity scintillation of vortex beams is less affected by turbulence, and the spectral method offers a more straightforward approach than the direct method. The physical mechanism of vortex beam scintillation effect can also be obtained from the analysis of spectral filtering functions. This study not only fills current gaps in beam propagation theory, but also provides an effective exploration of the statistical fluctuation characteristics of beams after propagation through turbulence.

With advances in emerging material technologies, intelligent reflecting surface (IRS)-assisted vehicular networks have been gaining growing interest. By adaptively shaping the wireless propagation environment, IRSs can improve vehicular network quality of service (QoS). However, most IRS-assisted vehicular network studies are limited to individual RF or VLC frameworks, while only a few investigate IRS-assisted aggregated VLC-RF vehicular networks that combine wide RF coverage with high VLC data rates. In this paper, aggregated VLC-RF vehicular networks are supported by both optical IRSs (OIRSs) and RF IRSs, and a resource allocation scheme is developed to improve the total achievable rate. First, we establish a system model for IRS-assisted aggregated VLC-RF vehicular networks, and then formulate a problem to maximize the total achievable rate. Furthermore, we decompose the maximization of the total achievable rate into five subproblems and solve them iteratively via an efficient alternating optimization scheme based on block coordinate descent (BCD). Moreover, simulation results validate the convergence and efficiency of our algorithm, while highlighting the effects of crucial parameters on system performance, providing valuable insights for resource allocation in IRS-assisted aggregated VLC–RF vehicular networks.

This paper systematically analyzes the propagation, transformation, and accumulation mechanisms of multi-source noise and device non-idealities within the complete signal chain from the transmitter through the channel to the receiver, focusing on wireless optical coherent communication systems from a signal propagation perspective. It establishes the stepwise propagation process of signals and noise from the transmitter through the atmospheric turbulence channel to the coherent receiver, clarifying the coupling mechanisms and accumulation patterns of various noise sources within the propagation chain. From a signal propagation viewpoint, the study focuses on analyzing the impact mechanisms of factors, such as Mach-Zehnder modulator nonlinear distortion, atmospheric turbulence effects, 90 degrees mixer optical splitting ratio imbalance, and dual-balanced detector responsivity mismatch, on system bit error rate performance and constellation diagrams under conditions of coexisting multiple noises. Simultaneously, by introducing differential and common-mode processes, the propagation and suppression characteristics of additive noise at the receiver end within the balanced detection structure were analyzed, revealing the dominant properties of different noise components under varying optical power conditions. Simulation results indicate that within the range of weak turbulence and engineering parameters, the impact of modulator nonlinearity on system bit error rate is relatively minor compared to channel noise. Atmospheric turbulence dominates system performance degradation through the combined effects of amplitude fading and phase perturbation, causing significant constellation spreading. Imbalanced optical splitting ratios and mismatched responsivity at the receiver weaken common-mode noise suppression, leading to variations in effective signal gain and constellation stretching/distortion. Under different signal light power and local oscillator light power conditions, the system noise exhibits distinct dominant characteristics.

This article proposes a laser-based, non-contact vibration signal transmission system for rotating machinery. The non-coaxial misalignment between the optical and rotational axes is modeled, revealing that the detector's effective photosensitive area and received optical power vary periodically with shaft speed, thereby introducing harmonics at integer multiples of the rotational frequency. Beam non-uniformity and wavefront aberration manifest as additive noise, which jointly degrade the signal quality. A compact joint time-frequency analysis is achieved by projecting the signal with the fractional Fourier transform. Experiments show that the optimal transform order p=0.92 is identified using the criterion that maximizes the rate of spectral entropy change, demonstrating high-resolution feature extraction. A C#-MATLAB mixed-programming platform completes the system by providing data storage, offline processing, and web-based publishing.

After passing through non Kolmogorov atmospheric turbulence, vortex beam will generate various turbulence effects. This article derives expressions for the intensity distribution, beam broadening, and drift variance of vortex beams based on Rytov perturbation theory and Huygens Fresnel diffraction principle. The impacts of varying topological charges and spectral power-law exponents on the intensity distribution, beam broadening, and beam wander characteristics were investigated. Simultaneously conducting field experiments verified the trends of beam broadening and drift. The results show that: (1) As the topological charge increases, the beam spreading increases and the beam drift variance decreases. (2) As the power law exponent increases, the beam spread decreases and the beam drift variance increases. This study lays a certain foundation for the propagation of vortex beams in random media and provides support for the theoretical framework construction of subsequent propagation characteristics.

Indoor positioning technology is very important in modern intelligent navigation systems, but the accuracy of traditional indoor visible light positioning methods is reduced due to signal occlusion, signal reflection, and environmental noise. In this paper, a visible light localization method based on a convolutional neural network optimized by thermal map is proposed. The discrete fingerprint coordinates are transformed into continuous probability distribution through Gaussian kernel function and trained combined with the characteristics of light intensity, so as to achieve accurate visible light localization. The adaptive federated integrated navigation algorithm is adopted, which combines the autonomous advantages of visible light positioning technology and inertial navigation system. The experimental results show that the average positioning error of the single visible light positioning method based on the thermal map optimization convolutional neural network is 5.8 cm while the average positioning error of the adaptive federated integrated navigation algorithm combined with inertial navigation is reduced to 3.7 cm. The integrated navigation algorithm can effectively overcome the environmental interference of inertial navigation, such as cumulative drift error and visible light signal interruption, and enhance the robustness of the system in complex environments.

Abstract Expressions for the heterodyne detection efficiency of the partially coherent electromagnetic Gaussian–Schell model (EGSM) in atmospheric turbulence were derived. In this study, the signal and local oscillator beams were configured in five different polarization states: linear (LP), left/right-handed circular (LHCP/RHCP), and left/right-handed elliptical (LHEP/RHEP). The results show that as the spatial coherence length increases, the detection efficiency rises rapidly before gradually saturating. A dynamic “critical spatial coherence length” exists, which varies with propagation distance. Furthermore, the detection efficiency decreases with increasing zenith angle, turbulence intensity, and detector aperture. The influence of inner turbulence scale is significant, while that of the outer turbulence scale is negligible. Finally, the partially coherent heterodyne detection experimental system we constructed revealed that, although polarization matching is most efficient under perfectly matched conditions, the relative attenuation of circular and elliptical polarization is relatively small under turbulent conditions.

Visible light communication (VLC) is widely regarded as a key enabler for future vehicular networks, thanks to its extremely large unlicensed bandwidth and non-interference with existing radio frequency (RF) communication networks. With the goal of maximizing the benefits of both RF and VLC technologies, aggregated VLC–RF vehicular networks, in which any vehicle can be served by both RF and VLC access points (APs) concurrently, have recently become a more robust and promising approach for enhancing vehicle-to-everything (V2X) applications and improving the quality-of-service (QoS) of vehicular networks. This paper focuses on the joint spectrum reuse and power allocation problem in aggregated VLC–RF vehicular networks with delayed channel state information (CSI) feedback, where vehicle-to-vehicle (V2V) links opportunistically reuse the RF spectrum allocated to vehicle-to-infrastructure (V2I) links. Specifically, we focus on maximizing the total V2I achievable rate to support high-rate content delivery, and guaranteeing the required reliability of V2V links tasked with exchanging safety-critical information. Furthermore, the sum V2I achievable rate maximization problem is decomposed into four subproblems, which are iteratively solved through an efficient block coordinate descent (BCD)-based alternating optimization algorithm. Moreover, simulation results validate the convergence and efficiency of the proposed algorithm while highlighting the impact of critical parameters on system performance, providing valuable insights for resource allocation in aggregated VLC–RF vehicular networks.

This study is based on the Richards–Wolf vector diffraction integral theory and investigates the field evolution and angular momentum characteristics of circularly polarized vortex beams focused by a high-numerical aperture (NA) lens. By simulating the focusing process of left- and right-handed circularly polarized vortex beams with different topological charges, the intensity, phase, spin angular momentum (SAM), and orbital angular momentum (OAM) density distributions in the focal plane are analyzed in connection with the theoretical formulations. On this basis, the focusing characteristics of fractional-order circularly polarized vortex beams are further examined. Moreover, the effects of different NA values on the focal intensity distribution, angular momentum conversion, and depth of focus are compared. The results indicate that the topological charge l plays a crucial role in modulating the focal field. As the NA increases, SAM is more readily converted into OAM, while the depth of focus decreases. Additionally, fractional topological charges lead to phase singularity splitting and dark-core expansion during focusing. This study provides a theoretical foundation for understanding the spin–orbit angular momentum interaction in high-NA lens systems and offers insights for applications in vector beam control, high-resolution imaging, and optical manipulation.

In underwater blue-green optical communication, blue-green light is attenuated by seawater absorption, scattering, and ocean turbulence, and the scattering coefficients of blue-green light are different for different depths of seawater; that is, the transmission attenuation of blue-green light varies in different depths of seawater. We calculated the scattering coefficients and albedo of seawater at different depths and analyzed the received optical power of the detector and bit error rate when the blue-green light was line-of-sight (LOS) transmitted in different ways (horizontal, vertical, and oblique range) with the same distance in the same seawater depth range under the effect of absorption and scattering of seawater. Numerical results show that under the effect of absorption and scattering of seawater, the transmission attenuation of blue-green light is large in seawater with large albedo. Therefore, blue-green light in different depths of seawater in the same way LOS transmission of the same distance, the power of light required in the albedo of large seawater is large.

The cross-spectral density matrix for elliptical Gaussian partially coherent beams propagating along a slant path in non-Kolmogorov turbulence is derived based on the generalized Huygens-Fresnel principle and the quadratic approximation of the Rytov phase structure function. The spectral degree of cross-polarization (SDCP) between two distinct points of elliptical Gaussian partially coherent beams is analyzed for different source parameters, turbulence inner and outer scales, and zenith angles, and is compared with the classical degree of polarization (CDOP) when the two points coincide. The results show that the variations in the SDCP of an elliptical Gaussian partially coherent beam are more pronounced. As the relative distance between two points grows, the SDCP exhibits a "self-recovering" evolutionary trend with the off-axis distance initially declining, then rising, and ultimately stabilizing at a constant value. This stable value decreases with higher amplitude ratio and zenith angle but increases with longer coherence length and larger turbulence inner scale. Additionally, the impact of atmospheric turbulence on the SDCP becomes more significant over longer transmission distances. Finally, Monte Carlo simulations were used to validate the theoretical results. It is noted that the results have established a sound theoretical basis on the topic of improving the performance of free-space optical communication.

In adaptive optics systems, most methods rely on reconstruction techniques centered on regional or global orthogonal bases, which struggle to accommodate the multi-scale characteristics of atmospheric turbulence wavefronts. This paper adopts a hybrid basis wavefront reconstruction method based on mutual information sorting, combining Zernike modes with Daubechies wavelet modes for mutual information calculation and sorting. The modes with the highest correlation are selected for reconstruction, effectively reducing the scale of the reconstruction matrix while considering both global and local features. The reconstruction results show that when the number of modes is 20, the root mean square (RMS) of the wavefront residual error of the hybrid basis reconstruction drops to 0.14 rad, outperforming 0.19 rad of the Zernike mutual information method and 0.33 rad of the Zernike expansion method. The peak-to-valley (PV) value after wavefront correction converges to 0.057 μm at the 39th iteration, demonstrating a faster convergence speed and smaller residual error; the RMS value converges to 0.027 μm at the 77th iteration after correction.

Vortex beams have broad application prospects in fields such as information modulation, target detection and recognition due to their carrying orbital angular momentum. Firstly, the relevant theories of vortex beam and bidirectional reflection distribution function were introduced. Combining scattering theory and differential panel method, the expression of backscattering intensity of vortex beam irradiated on composite targets (sphericalcylinder combination, spherical-cone combination, spherical-frustum combination) was derived, and simulation analysis was carried out. Finally, the theoretical analysis was verified through experiments. The research results indicate that for the same target, the peak range of the backscattered light power curve of the vortex beam is wider, which is conducive to multi angle detection. For targets with significant differences in edge regions, the peak difference in the backscattered light power curve of vortex beams is greater, which is beneficial for target recognition. When the materials of the combination are different, the peak value of the backscattered light power curve of the vortex beam changes with the material, but the trend of change is basically the same.