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

Abstract

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.