EFFECT OF CROSS SATURATION ON FREQUENCY FLUCTUATIONS IN A NEARLY SINGLE-MODE SEMICONDUCTOR-LASER

The effect of side-mode cross saturation on the frequency noise of the main mode is investigated by considering the Langevin rate equations which contain the nonlinear gain with both the self-saturation and cross-saturation terms. When cross saturation is stronger than self saturation, the frequency noise of the main mode is found to be significantly enhanced in the low-frequency regime (< 1 GHz). An increase of more than 20 dB is predicted due to a side mode suppressed by 15-20 dB. The enhanced frequency noise manifests as broadening of the laser linewidth and may affect the performance of coherent optical communication systems.