Spatial, spectral, and temporal resolved measurements of semiconductor injection lasers - Part I. Homostructure lasers: Mode formation

Spatial, spectral, and temporal (SST) resolved measurements are carried out on homostructure junction lasers at room temperature. Two distinctly different mechanisms of transverse-mode formation are illustrated. One is attributed to anomalous interaction of spectral modes where their formations take place within a few nanoseconds of each other and is characterized by spectral separation of a few tenths of a nanometer with the eventual domination by the long wavelength mode/s. In the other, a second mode at shorter wavelength appears only above a current I0. The delay of this second mode is well-fitted by the equation with τ of the order of 27 ns. The separation of the two spectral modes increases during the pulse and finally settles to a magnitude an order higher than the first case. It is experimentally observed that a cavity-width reduction mechanism is present as the laser emission evolves during a pulse. Evidence of a non-linear gain versus injected carrier concentration due to the switching of power from one mode to another is discussed. The results have been explained in terms of changes in the cavity brought about by refractive index variation, gain distribution, or saturable absorption.