Transverse mode selection in injection lasers with widely spaced heterojunctions

Abstract

This paper applies the theory of transverse mode selection in injection lasers with thick optical cavities to the important class of large-optical-cavity (LOC) heterojunction lasers where the mode-guiding region is divided into a thin active recombination region and a wider passive region. It is shown that for reasonable values of the refractive-index discontinuity at the heterojunctions, the fundamental mode is not excited at threshold in devices with a mode-guiding layer of the order of several micrometers. The order of the preferred mode increases with the thickness of the mode-guiding region; but for values above a few micrometers, the differences in the gain coefficient at threshold become so small that the mode-selection process is affected by differences in the facet reflectivity for the different modes; this correction favors the highest possible TE mode. The experimental results are shown to be consistent with the theoretical predictions of the order of the mode at threshold. In the representative diodes studied, a fit of the theory and experiments can be obtained with a four-layer model of the cavity geometry. The parameters are the measured widths of the waveguide and of the active region and the dielectric discontinuities at the heterojunctions which were adjusted. The results indicate that the excited mode which was observed was not necessarily the highest cavity mode that could propagate.