Radiation trapping in laser diodes

Abstract

This paper is concerned with the problem of cavity geometries which exhibit anomalously low radiative efficiencies. While the ideas presented apply to any type of cavity, the experiments and discussion refer specifically to the type of cavity of importance in commercial injection lasers, a Fabry-Perot cavity with cleaved end facets and sawed sidewalls. In this study, it is shown that the lowering of the external efficiency is probably associated with the excitation of internally circulating modes, analogous to those found in lasers with four cleaved sides. A dual-cavity diode structure is employed to demonstrate that it is the sidewall loss which controls the excitation of the internally circulating modes. To determine the geometric conditions for internal mode excitation in single and double heterojunction diodes, an extensive study was made of the combination of length, width, and facet reflectivity which lead to efficiency decreases. It is assumed that the internal modes are excited when the distributed sidewall loss for the internal modes is less than the distributed Fabry-Perot end loss for the externally radiated modes. The conditions for internal mode excitation and accompanying efficiency decrease are predicted by either assigning an empirically determined attenuation factor to the sawed side or assuming that the loss is associated with diffraction from small mirror segments at the sidewall which have been formed during the sawing operation. These predictions may be used to design Fabry-Perot sawed sidewall injection lasers so as to avoid efficiency loss through the excitation of internal modes.