Degradation of mode-suppression ratio due to longitudinal-mode beating in semiconductor lasers

Abstract

Gain nonlinearities play an important role in determining the static and dynamic characteristics of semiconductor lasers. Most of the theoretical work to date considers primarily the nonlinear gain arising from intraband effects, for which the total carrier density remains essentially constant. Recently it has been shown that beating of the total carrier density at the longitudinal-mode-spacing frequency also gives rise to important effects, even when the longitudinal-mode spacing is much greater than the reciprocal carrier lifetime. In this paper we show that the nonlinear gain arising from such interband effects can lead to degradation of the side-mode suppression ratio in single-frequency lasers. We utilize a general multi-longitudinal-mode model that includes self- and cross-saturation, as well as four-wave mixing, arising from both intraband and interband sources. Particular attention is paid to the influence of the linewidth enhancement factor and the laser length on the mode suppression ratio. Our results suggest that relatively long distributed feedback lasers (length >500 μm) are likely to operate in a single longitudinal mode only over a limited current range.