A μrad Accuracy and nW Detection Sensitivity Four-Quadrant Heterodyne Coherent Angular Measurement System

In gravitational wave measurement and inter-satellite laser communication systems, the relative rotation and motion between the transmitter and receiver terminals introduces small angular deviations over a link distance of thousands of kilometers, which need to be measured with high accuracy and sensitivity. The heterodyne coherent angle measurement has a higher measurement accuracy and detection sensitivity compared with the traditional direct detection technique, which performs angle measurement through the phase of a beat frequency signal. In this paper, we propose a four-quadrant heterodyne coherent angle measurement technique with μrad accuracy and nW-level detection sensitivity. A mathematical model of a differential wavefront sensing (DWS) angle solution was formulated, and a Monte Carlo simulation system was built for performance testing. An experimental system was devised to assess the accuracy and sensitivity of the heterodyne coherent measurement method and to compare the performance with that of the direct detection method. The experimental results showed that for azimuth and pitch axes, the accuracy of the heterodyne coherent angle measurement was 2.54 μrad and 2.85 μrad under the same signal power of −16 dBm, which had a 5-fold improvement compared with direct detection. The sensitivity of the heterodyne coherent detection was −50 dBm at the 20 μrad accuracy threshold, which was a 1000-fold improvement compared with direct detection. This research is of great significance for the phase measurement and tracking system in the field of gravitational wave detection and has a guiding role in system design work in the field of inter-satellite laser communication.