Spectral linewidth in microcavity surface-emitting lasers

The Schawlow-Townes expression for the laser linewidth predicts a substantial cw linewidth enhancement in microcavity lasers, in which a large fraction of spontaneous emission is directed into the lasing mode, in contrast with conventional semiconductor lasers, in which the lasing mode accepts only a tiny fraction of spontaneously emitted photons. By performing a theoretical analysis of rigorous solutions of the wave equation in the surface-emitting cavity and of the band structure in the active region, it is shown that the increase in the linewidth is much slower than the increase in the spontaneous-emission factor p because of reductions in the total spontaneous emission rate, the threshold carrier density, and the linewidth enhancement factor and an increased slope of the light-current characteristic obtained for the microcavity laser in steady state. Also much smaller driving currents in a microcavity laser are required to achieve the same power output compared with conventional semiconductor lasers, resulting in a linewidth of the order of several hundred MHz at moderate driving currents for the former. We also discuss the various factors influencing the linewidth in microcavity and strained quantum well lasers and the relation between lasing threshold and spectral linewidth in both macroscopic and microscopic cavity lasers.