Single-mode stabilization by traps in semiconductor lasers

Recently it has been observed that certain single-transverse-mode semiconductor lasers continue to emit light predominantly in a particular longitudinal mode even after the optical gain peak has shifted by one or more mode spacings due to changes in bias current or temperature [1], [2]. The purpose of this paper is to show that this type of mode stabilization can be caused by the saturable optical absorption resulting from deep-level states or traps, which have been observed in AlGaAs laser structures [3]-[7]. The mode-selection mechanism is due to the spatial variation in the optical loss created by the standing-wave pattern of the single predominant mode. This loss pattern results in a lower average loss for the creating mode and a higher loss for all other modes. It is possible that laser devices with greatly improved single-mode stability can be made by introducing traps of the proper type and density during fabrication.