Gain−induced guiding and astigmatic output beam of GaAs lasers

We report on experimental evidence for gain−induced guiding and refractive−index antiguiding along the junction plane of stripe−geometry double−heterostructure GaAs lasers. It had previously been found that carrier diffusion out of the active region leads to a gain profile along the junction plane that can be approximated by a parabolic variation. This results in a lowest−order mode having a Gaussian profile, a cylindrical phase front, and a constant radius of curvature. This curvature accounts for the astigmatism always observed in the output beam from stripe−geometry GaAs lasers, where gain guiding dominates index guiding. Experimental determinations of the far−field diffraction angle, ϑ, and the beam width, 2w, at the laser mirror in the junction plane enable us to calculate the parameters characterizing the gain distribution responsible for mode confinement, as well as a negative refractive−index increment which would tend to defocus the mode. The negative index increment, as measured between the center and the edges of the stripe, appears to be related to a competition between a negative free−carrier effect and a positive thermal focusing mechanism. Above threshold, changes in ϑ and w with current imply changes in the gain−guiding mechanism. The theory implies in particular a decrease in gain at the center of the stripe and an increase at the edges with an increase in current.