Effect of stress on the polarization of stimulated emission from injection lasers

Abstract

The role of stress on the polarization of stimulated emission is discussed in this paper. The stimulated emission from injection lasers is usually polarized in the TE mode. However, in the presence of a sufficiently high stress in the active region, TM polarized stimulated emission at threshold can be observed. We find that for some InGaAsP buried heterostructure lasers with a kink in the light-current characteristic, the kink represents the onset of a TM polarized stimulated emission. X-ray diffraction measurements show that the active region of these devices are under stress. We show that the change in polarization characteristics associated with the light-current kinks can be due to (i) higher optical gain of the TM emission than TE in the presence of stress and (ii) larger reflectivity of the higher-order TM modes. This mechanism is supported by the observation of higher-order TM modes than TE and the observation that the TM gain is unsaturated above TE threshold. We also find that the TM kinks occur at lower currents as the external pressure is increased. In the absence of stress in the active region, stimulated emission in higher-order TE modes associated with light-current kinks is observed in buried heterostructure lasers with large active region. Our results show that the observation of stimulated TM emission in real index guided laser structures is indicative of internal stress in the active region of these devices.