The role of laser coherence length in continuous-wave coherent laser radar

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Abstract
The effect of laser coherence length on the performance of continuous-wave coherent laser radar (lidar) is examined. A laboratory lidar investigation using semiconductor lasers with linewidths of the order of 1 MHz has been carried out for ranges much shorter than, comparable with and much longer than the laser coherence length (about 50m). The signal strength, spectrum and fluctuation statistics are all shown to be sensitive to the various effects resulting from limited laser coherence. Different laser line-broadening mechanisms are considered, including phase diffusion and jitter, and the fundamental difference between single- and multiple-mode sources is examined. Limited laser coherence is also shown to give rise to an excess noise contribution, and this can be severely detrimental to the lidar system performance.

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