Research on fiber coupling characteristics of linearly polarized modes in coherent free space optical communication systems

This study examines the influence of relative motion-induced angular deviations on fiber coupling and communication performance in high-speed coherent free-space optical systems. Simulations reveal that higher-order modes experience a steeper drop in coupling efficiency than the fundamental mode as the incident angle grows. Using a spatial light modulator (SLM) and fast steering mirror (FSM) to generate higher-order linearly polarized (LP) modes and emulate angular tilt, the coupling behavior was experimentally validated. Within 0-1.2 mrad deflection, the LP 01 mode's bit error rate (BER) increased from 4.978 × 10 −13 to 1.302 × 10 −6 , accompanied by an error vector magnitude (EVM) rise from 8.65% to 10.73%. In comparison, the LP 31 and LP 03 modes reached EVMs of 11.89% and 16.92%, respectively, underscoring their heightened sensitivity to angular misalignment. These findings underscore the critical role of mode selection and tolerance design in high-capacity spatial optical receivers under angular mismatch.