Photoelastic waveguides and their effect on stripe-geometry GaAs/Ga1−xAlxAs lasers

Abstract

This paper describes a new type of waveguide mechanism found in oxide-insulated stripe-geometry GaAs/Ga1−xAlxAs lasers. The photoelastic waveguide is formed by small variations of the refractive index of the active layer beneath the stripe contact of the laser. The refractive-index variations are caused by the effect of a strain field in the semiconductor surrounding the stripe window. The large photoelastic coefficients of III-V semiconductors cause significant changes in refractive index for relatively small strains. The strain field in the semiconductor is caused by the high compressive stress in the oxide film; it builds up to its highest value beneath the edges of the stripe window. The maximum calculated compressive strain in the active layer 2-μm beneath a stripe window 20 μm wide opened in an oxide film 2700 Å thick is 3×10−4. This strain reduces the dielectric constant of the active layer by approximately 10−2. The sign of the oxide stress, the stripe width, and the active layer depth determine whether a waveguide or antiwaveguide is formed. Experimental measurements have been carried out on two types of photoelastic waveguide at 1.15μm wavelength. The results confirm the theoretical calculations. These simply fabricated waveguides may be suitable for integrated optic applications. The effect of the photoelastic waveguide on operating stripe-geometry lasers has been determined by controlled experiments, which clearly indicate that it is the predominant builtin waveguide mechanism. The results also show that a reasonably strong builtin photoelastic waveguide is essential if well-behaved (’’kink’’-free) stripe lasers are to be made with stripe widths of about 20 μm.