Time variations in the far-field diffraction patterns of spatial modes from electron-beam-pumped semiconductor lasers

Abstract

Detailed observations of the far-field radiation patterns from electron-beam-pumped GaAs and CdSe lasers reveal angular deviations of the patterns from the laser axis at angles greater than 25°. Moreover, these angular deviations are observed to decrease monotonically as a function of time during each output pulse. A simple model for the near-field laser modes predicts such angular deviations. Furthermore, the model allows the time-varying angular deviations to be accounted for in terms of a time-varying refractive index difference between the pumped and unpumped regions of the laser cavity. Two major sources of sufficient index variations are identified, viz., heating by the electron beam and index changes related to the presence of gain. The relative magnitudes of the two effects have been determined for GaAs and CdSe. The inherent time dependence of these effects proves adequate to account for the observed ``angular tuning'' of the spatial modes. Spatial variations in the refractive index have consequences other than angular tuning of the far-field modes. In particular, it is found that the diffraction losses associated with the spatial modes are strongly dependent on the refractive index difference. The impact on laser dynamics of the consequent time-varying total cavity loss is discussed.