Evaluating spectral characteristics of narrow-linewidth fiber lasers in coherent beam combining system via using CDC metric

In this manuscript, complex degree of coherence (CDC) is proposed to depict the spectral characteristics of narrow-linewidth fiber lasers in coherent beam combining (CBC) system. Both the theoretical and experimental demonstrations are shown that the CDC of individual narrow-linewidth fiber laser is directly proportional to its combining efficiency of CBC system. Compared with the multiplicity of typical ‘spectral linewidth’ concepts, such as full wave at half maximum (FWHM), spectral power ratio (SPR) and RMS value, CDC can be directly acted as a unified metric to evaluate the spectrally and temporally coherent capacity of narrow-linewidth fiber lasers with different spectral distributions. Finally, the impacts of spectral distribution and delay time on CDC are also theoretically and experimentally investigated.