湍流大气中随机粗糙表面激光回波空间相干性仿真

Simulation of spatial coherence of laser echo light field from random rough surface in turbulent atmosphere

根据广义Huygens-Fresnel原理,推导了von Karman湍流谱条件下激光回波复相干度的理论解析式;基于湍流相位屏分步传输算法和随机粗糙目标表面模型,实现了激光回波光场的仿真计算.首先通过镜面反射回波光场的仿真分析,验证了算法的正确性;然后基于1.1 km的均匀传输路径,综合分析了随机粗糙目标表面特性和路径湍流强度对回波光场复相干度的影响.结果表明:回波光场的空间相干性随目标表面高度均方根的增大而降低,随目标表面相关长度的减小而降低;当表面相关长度远小于大气相干长度时,回波相干性会被严重破坏.该研究可为目标表面特性或利用已知表面获取路径湍流状态的相干探测提供有益的参考.

According to the generalized Huygens-Fresnel principle, we derive the analytical formula for the complex degree of coherence of the echo light field under the von Karman atmospheric turbulence spectrum condition.Based on split-step beam propagation method of the turbulent phase screen and the target surface model, the fold pass propagation simulation of the laser in the turbulent atmosphere is realized. The dynamic speckle characteristics on the image plane are consistent with the experimental phenomenon. Firstly, the simulation results of the complex degree of coherence and phase structure function of the mirrored reflection echo light field are compared with the theoretical values, which verifies the correctness of the algorithm. Based on this, the complex degree of coherence of the echo light field reflected by the optical rough surface is calculated and analyzed. The results show that on a double-path turbulent flow path of 1.1 km, in other words, it transmits2.2 km in unfolded mode, the spatial coherence of the echo light field is very sensitive to the root mean square value of height. When the root-mean-square value of height is close to the wavelength, the coherence is seriously degraded. When the correlation length of the target surface is much larger than the atmospheric coherence length, the coherence length of the echo light field is relatively close to the set spatial coherence length. When the correlation length of the target surface is close to the atmospheric coherence length, the influence of the rough surface of the target on the beam coherence cannot be ignored. When the correlation length of the target surface is much smaller than the atmospheric coherence length, the target surface characteristics have a dominant influence on the echo coherence, the spatial coherence of the light field is seriously degraded, and the echo is close to incoherent light. Considering the smooth target reflection surface, the greater the strength of turbulence, the faster the complex coherence decreases with space. The atmospheric coherence diameter rcan be calculated further according to the complex degree of coherence. For the Pearson correlation coefficient the simulation value and theoretical value are both 0.998, which indicates that the atmospheric coherence diameter calculated by the complex degree of coherence has a high correlation with the theoretical value. This research provides a theoretical basis for the coherent detection scheme of echoes from rough surfaces in the turbulent atmosphere. The simulation algorithm extracts the target surface features by analyzing the variation of the complex coherence of laser echo signals in the turbulent atmosphere with the spatial distance, and also provides a method of using the known target surface to obtain path turbulence information.