Simulation analysis of isolation between laser communication ground test equipments

The distance between a laser communication ground test platform and the terminal under test is far less than the actual communication distance due to space limitations. As a result, the backscattered stray light generated by the test platform optical device will enter the terminal under test, and the signal will seriously affect the performance of the terminal under test. Aiming at this problem, we research the isolation relationship between the tested terminal and the test platform based on the optical interference problem. The Cassegrain and off-axis three-mirror optical antenna are designed respectively. According to astigmatic transmission model, the stray light analysis software is employed to analyze the influence of optical antenna's structure and surface roughness on the isolation. The results of the analysis show that the isolation when applying the off-axis three-mirror optical antenna is significantly higher than that applying the Cassegrain optical antenna, and that this isolation increases with a decrease in the roughness of the optical surface. When the optical surface's roughness reaches 0.892 nm, the isolation is −86.22 dB. Finally, the relationship between the ABg model and the Harvey model parameters is derived. According to calculation formula of the roughness and TIS, the ABg model parameters with roughness of 0.7 nm and 0.5 nm are theoretically obtained. The isolation between the terminals is −94.39 dB and −97.3 dB, achieving an isolation rating of −90 dB.