Radiation trapping in laser diodes

A careful study of (AlGa)As-GaAs heterojunction lasers has shown that in certain cases, the external differential quantum efficiencies are anomalously low. Evidence suggests that the decrease in external efficiency is due to the trapping of radiation in internally circulating modes. Data indicates that the excitation of the internal modes and corresponding loss of efficiency appears when the effective Fabry-Perot cavity end loss equals the internal mode losses at the sawed sides. The existence of the internal mode and its effect on the external differential quantum efficiency is demonstrated in a novel diode structure where this mode can be turned on and off at will, leading to a corresponding decrease and increase in external quantum efficiency. A model of the sawed sides based upon small diffracting facets imbedded in a high loss medium leads to simple expressions which seem to describe the onset of internal modes. Agreement has been obtained between predictions of the model and measurements on diodes. The model can be employed to design sawed-side, high peak power Fabry-Perot cavity lasers so as to avoid both catastrophic facet damage and the loss of efficiency through the excitation of internal modes.