Phase noise measurement of laser sources in coherent lidar system

In coherent lidar systems, the phase noise of the laser source determines the maximum operation range and performance. To measure the laser phase noise of the lidar system, we studied three related indicators: the field spectrum, the frequency-modulation noise spectrum, and the phase-error variance. We established a delayed self -homodyne laser phase noise measurement system with an optical coherent receiver, and completed the measurement of phase noise in a MHz-level-linewidth laser. In addition, we proposed an indicator named effective coherent accumulation period (ECAP) of the coherent lidar system, in order to evaluate the coherent accumulation effect. The ECAP of the laser under test is calibrated, within the measured phase noise lev and tens-of-meters operation range. We experimentally verified the impact of effective coherent accumulation period in two typical coherent lidar systems. In FMCW ranging lidar, laser phase noise gradually accumulates with the increase of coherence accumulation time, resulting in a decrease in ranging accuracy. In micro-Doppler sensing lidar, this factor leads to a reduction in signal-to-noise ratio and spectrum broadening. The experimental results of this article can provide a reference for the model selection of laser sources in coherent lidar and the evaluation of coherent accumulation performance.