Importance of self-induced carrier-density modulation in semiconductor lasers

Semiconductor lasers have a built-in mechanism for modulating the carrier density at multiples of the longitudinal-mode spacing. This mechanism is believed to be relatively unimportant for solitary laser diodes since the mode spacing typically exceeds 50 GHz. A general numerical model capable of including self-saturation, cross saturation, and four-wave mixing occurring due to both interband and intraband effects is presented, and the self-induced carrier-density modulation is shown to play an important role in solitary laser diodes. In particular, it can severely degrade the gain margin and the mode-suppression ratio in single-mode semiconductor lasers when the operating current is increased. Degradation depends on the linewidth enhancement factor and the laser length and can be especially severe when the cavity length exceeds 1 mm.<>