远程多光束激光相干场成像拍频误差校正研究

多光束激光相干成像技术是地基观测空间目标的重要方式,各光束的稳定性和性能一致性直接决定着系统成像质量,目前针对系统频率稳定性对成像质量的影响以及激光源频漂的补偿已有一定研究,然而针对多光束发射系统,由驱动放大噪声及声光移频噪声引起的各光束间独立的频漂还需进一步抑制.基于此,本文提出了动态解调和置信区间解调两种抑制方法,理论仿真了动态解调对缓慢频漂抑制的可行性,同时实验证明了置信区间解调法对成像效果的提升,并在200 m和1.2 km的湍流环境中对该解调方法进行了验证.研究表明,置信区间解调法对于各拍频间独立漂移有较好的实时补偿效果,能够有效抑制发射阵列中由声光调制及驱动放大引入的频率噪声,对水平距离1.2 km外的25 mm目标成像角分辨率达到4μrad.本研究为未来远程大功率发射阵列成像中的频率噪声抑制提供了较好的技术方案.

Beat frequency error rectifying in multi-beam laser coherent remote tmaging

Coherent imaging with a multi-beam laser is considered as a key technique in ground based imaging. The image quality is directly determined by stability and consistency of each beam in transmitter. Although the stabilities of laser frequency and the drifting compensation methods have been studied previously, they mostly focused on the laser source. In most cases, especially in large transmitter array, however, transmitted beams are always disturbed by different influential factors, such as frequency drift induced by acoustic-optical modulation (AOM) and high power driven amplification. Therefore this kind of frequency drifting needs further rectification. Aiming at this problem, in this paper we propose two new methods called dynamic demodulation and dependence range demodulation. Firstly, the dynamic demodulation takes the whole drifting frequency drift as a changing procedure. It is believed that the beat frequency drifted at any position still carries the target information, so the system demodulates the signal at that drifted position. According to this method, the response speed of the demodulation system should be very high. But in a real system this acquisition is too high to be satisfied. It cannot work as quickly as expected. In computer simulation some slow varying drifts are induced at the beat frequency and the variation is distributed only in three parts of spatial frequency of transmitter interfering array. Simulation results show that this method may well compensate for slow drifting beat frequency. While its response speed is often limited by hardware system. On the other hand, for the dependence range demodulation, the beat drifting range is considered as a useful district, in which all the beat energy is added and demodulated at a preset position. An experiment is carried out to verify this method. The result demonstrates that it can well restrict the beat frequency drift within 100 Hz, which often happens in the procedure of AOM and driving amplification. Besides the laboratory setup research, the field experiments in 200 m and 1.5 km range are also carried out. The dependence range demodulation is proved to be well performed as well. The resolution of the 25 cm simulated target in 1.5 km reaches 0.008 rad. In the consideration of real system, the imaging range is further expanded and the amplifier power is stronger. The field experiments reveal that this demodulation method is applicable in such a condition. Therefore the research in this article provides some new techniques for the remote high resolution imaging in multi-beam laser interfering imaging.