李昕淼, 梅海平, 张骏昕, 李艳玲, 邓涵凌, 陶志炜. 深度湍流条件下激光往返传输漂移特性的实验探究[J]. 红外与激光工程, 2024, 53(5): 20230719. DOI: 10.3788/IRLA20230719
引用本文: 李昕淼, 梅海平, 张骏昕, 李艳玲, 邓涵凌, 陶志炜. 深度湍流条件下激光往返传输漂移特性的实验探究[J]. 红外与激光工程, 2024, 53(5): 20230719. DOI: 10.3788/IRLA20230719
Li Xinmiao, Mei Haiping, Zhang Junxin, Li Yanling, Deng Hanling, Tao Zhiwei. Experimental study on the laser beam wander characteristics of fold path propagation under deep turbulence condition[J]. Infrared and Laser Engineering, 2024, 53(5): 20230719. DOI: 10.3788/IRLA20230719
Citation: Li Xinmiao, Mei Haiping, Zhang Junxin, Li Yanling, Deng Hanling, Tao Zhiwei. Experimental study on the laser beam wander characteristics of fold path propagation under deep turbulence condition[J]. Infrared and Laser Engineering, 2024, 53(5): 20230719. DOI: 10.3788/IRLA20230719

深度湍流条件下激光往返传输漂移特性的实验探究

Experimental study on the laser beam wander characteristics of fold path propagation under deep turbulence condition

  • 摘要: 为探究深度湍流条件下激光往返传输的漂移特性,基于修正的Rytov理论,推导了深度湍流条件下光束漂移角方差表达式。搭建了折返路径激光成像探测系统,实验获取了1 km和7 km传输路径上不同时段激光光斑的回波图像和大气相干长度。利用回波图像计算了漂移角均方差及其各向异性因子并与理论对比。结果表明:漂移角均方差和大气相干长度随时间变化具有明显周期性,两者变化同步,但趋势相反。1 km传输路径上,漂移角均方差在2~24 μrad之间波动。而7 km传输路径上漂移角均方差在4~26 μrad之间波动。通过对比Kolmogorov 理论、Von Karman理论、Exponential理论和修正的Rytov理论模型下漂移角均方差与实测值的相对偏差发现:基于Von Karman理论的模型与1 km传输路径上漂移角均方差的实测值更为吻合,平均相对偏差约为18.20%,基于修正Rytov折射率功率谱理论的模型与7 km传输路径上漂移角均方差的实测值更为吻合,平均相对偏差约为21.09%。在1 km和7 km传输路径上,漂移特性均随着大气相干长度的减小而趋于各向同性。相关实验结果进一步揭示了深度湍流下往返路径传输激光漂移的机理,对于长距离空间光通信维持稳定通信链路具有一定参考价值。

     

    Abstract:
      Objective  Affected by atmospheric turbulence, the laser produces scintillation, beam wander, and fluctuation of the angle-of-arrival. Beam wander causes the beam to deviate from the target detector and reduces the efficiency of energy transmission, affecting applications such as laser tracking, laser positioning, and optical communications, especially over long distances. Under weak fluctuation conditions, the traditional theory of laser atmospheric propagation can be relatively accurately described. There is still a lack of reliable experimental evidence as to whether the theory is accurate in several kilometers of propagation under deep turbulence near the ground. Most of the current theories and simulations are based on existing assumptions, and there is often a certain deviation from the actual situation, especially under deep turbulence, where the theory is not yet perfect. The distances of the beam wander experiments are all relatively short and do not support the theory associated with long distance propagation. Therefore, it is of great significance to summarize the relevant theories of beam wander under deep turbulence and carry out long-distance laser propagation experiments to reveal the differences between the theoretical model and the reality. The relevant data provide an experimental basis for evaluating the model accuracy or error, and optimizing or correcting the model.
      Methods  Firstly, the theory of the variance of the drift angle for different refractive index power spectral models in the moderately to strongly turbulent region is summarized and derived. Then, the experiment system of fold path laser propagation (Fig.1) was built to collect the echo spot data using a large-aperture telescope and the atmospheric coherence length by an atmospheric coherence length meter. Four days of experiments were carried out on the 1 km propagation path and three days on a 7 km propagation path. The daily variation of the standard deviation of drift angle is obtained by calculating the echo spot data and studied in comparison with the theory. Simultaneously, the variation of turbulence anisotropy during the experiment period is analyzed.
      Results and Discussions   On the 1 km and 7 km propagation paths, the daily variations of the standard deviation of drift angle and the atmospheric coherence length show some periodicity, with synchronized changes but opposite trends (Fig.2). The standard deviation of drift angle decreases with increasing atmospheric coherence length, showing good agreement with the theoretical variation curve (Fig.3). The Rytov variance is used to classify the turbulence into three turbulence states of weak, medium and strong. Thereinto, the 1 km propagation path passes through three turbulence states, and the degree of proximity to the different theoretical models varies in different states (Tab.2). The relative deviation of turbulence in different states is further analyzed. On the 1 km propagation path, the measured values are closest to the theory based on the Von Karman refractive index model, with an average relative deviation of about 18.20%. While on the 7 km propagation path, the theory based on the modified Rytov shows a good agreement with the measured values, with an average relative deviation of about 21.09%. On the 1 km and 7 km propagation path, the anisotropy factor R of the laser beam wander gradually converges to 1 as atmospheric coherence length decreases (Fig.6), which means the beam wander tends to be isotropic. The anisotropy factor can reflect the contribution of thermal convection and transverse wind to the turbulent energy injection, and its specific influence mechanism needs to be further explored. The appropriate model and method of simulation needs to be chosen according to the specific situation, which will help to improve the accuracy and reliability.
      Conclusions  In this paper, the experimental system of laser fold path propagation is decomposed into laser propagation and spot image propagation, and formulas for the variance of the drift angle under moderate to deep turbulence are summarized and derived. The measured values in the 1 km propagation path are closer to the theoretical values based on the Von Karman refractive index model, and the average value of the relative deviation over four days is 18.2%. The measured values on the 7 km propagation path are closer to the theoretical values based on the modified Rytov theoretical refractive index model, and the average value of the relative deviation over three days is 21.09%. The anisotropy factor R of the laser beam wander tends to 1 as r_0 decreases, which means the beam wander tends to be isotropic. Through this experiment, we have a clearer understanding of the laser echo beam wander characteristics under different propagation conditions, the range of the standard deviation of drift angle intervals and their deviation from the theoretical expectations, which is of great significance for the model selection and engineering application evaluation. The relevant data have a certain reference value for revealing the target-in-loop laser propagation mechanism under deep turbulence conditions, and provide indispensable experimental data for optoelectronic systems involving laser aiming, positioning, coupling, and other applications.

     

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