Above-threshold longitudinal profiling of carrier nonpinning and spatial modulation in asymmetric cavity lasers

Abstract

Longitudinal variations of photon and carrier density along the length of a Fabry–Perot quantum well laser are simulated and compared to experimental carrier density measurements performed using a side spontaneous emission scanning technique. Both simulation and measurement demonstrate the existence of carrier nonpinning along the cavity of the laser, with increasing carrier density from the low reflectivity facet toward the high reflectivity facet. Measured carrier densities increased monotonically above their threshold value with injection current, at all points along the laser, seemingly contradicting the condition of net round-trip gain pinning at threshold. This behavior is attributed to current spreading into the low-gain regions aside the active region causing a current-dependent increase in spontaneous emission, and thus an apparent increase in measured active region carrier density. Periodic spatial modulation of the carrier density profile, consistent with the hypothesis of beam steering induced by beating and locked lateral modes, was also observed.