激光相干雷达中光单次下变频技术

在相干激光雷达远程测距中，为提高雷达的探测距离和距离分辨力，通常使用大时宽带宽积的信号如调频信号对光载波进行调制，并在接收端进行脉冲压缩处理，为降低接收端数字信号处理的数据量以提高运算的实时性，需要将接收信号下变频至合适的频段。传统的外差式雷达接收机需对光信号和电信号分别进行一次下变频，导致系统结构较复杂，且受器件非理想化的限制，下变频过程中会引入额外的噪声，此外还存在镜像频率噪声干扰的问题，导致解调信号信噪比降低。提出了一种将光信号直接下变频至脉冲压缩所需频段的方案，该方案使用正交解调的方式进行，能够简化系统结构同时抑制镜频噪声。首先将本振光进行频移并分为两束，通过控制相位使两本振光相互正交。将信号光分为两束并分别与两路本振光在光电探测器表面进行混频，接着对电信号进行采集，通过相关算法对幅相不平衡进行矫正。经仿真和实验，该方法能够在有效简化激光相干雷达接收机系统结构的同时避免镜频噪声干扰，在10 GSps采样率下，相较于外差式接收机，解调信号信噪比提高了约3 dB。

Single down conversion technology of light in laser coherent radar

In coherent LiDAR remote ranging,in order to improve the detection range and range resolution of the LiDAR,a large time broadband product signal such as a frequency modulation signal is usually used to modulate the optical carrier,and pulse compression processing is performed at the receiving end. To reduce the data required for digital signal processing at the receiving end and improve the real time performance of the calculation,it is necessary to down convert the received signal to an appropriate frequency band. The traditional heterodyne radar receiver requires a single down conversion of the optical and electrical signals,resulting in a more complex system structure and by the limitations of the device non idealisation,additional noise is introduced during the downconversion process. In addition,there is the problem of image frequency noise interference,which leads to the degradation of the SNR of the demodulated signal. In this paper,a scheme is proposed to downconvert the optical signals to the frequency band required for pulse compression,which is carried out using orthogonal demodulation to simplify the system structure and suppress mirror frequency noise. Firstly,the local oscillator light is frequency shifted and divided into two beams,and the two local oscillators are made orthogonal to each other by controlling the phase. The signal light is divided into two beams and mixed with two local oscillators on the surface of the photodetector. Then,the electrical signals are collected and the amplitude phase imbalance is corrected through relevant algorithms. Through simulation and experiments,this method can effectively simplify the structure of the laser coherent radar receiver system while avoiding mirror frequency noise interference. At a sampling rate of 10 GSps,the SNR of demodulated signal is improved by about 3 dB compared to a heterodyne receiver.