Computer model of an injection laser with asymmetrical gain distribution

The power outputs from the two ends of some LPE-grown (Al, Ga)As double-heterostructure proton-delineated stripe-geometry lasers are observed to be different for a given drive current. The emissive portions of the facets (mirrors) of these devices are identical in appearance, i.e., they are free of processing debris, cleaving artifacts, and inadvertently present dielectric coatings. In addition, there is no consistent evidence from n-side electroluminescent viewing of the stripe region that internal reflecting surfaces are present, so that an explanation of the output power asymmetry based upon effectively different facet reflectivities seems ruled out. Significant differences in reflectivities because of differences in the angles of incidence that the cavity traveling waves make with the two mirrors also appear implausible. It is the purpose of this paper to demonstrate the nonobvious proposition that an isotropic optical loss, asymmetrically located between the identical output facets of a laser, can cause different powers to be emitted from the two ends. The study is based upon a computer model of an injection laser. Optical losses which are either power-dependent or power-independent can promote output asymmetries, but only power-dependent losses of the type which increase as the power (current) is increased seem capable of accounting for experimental observations. The physical origin of the power-dependent asymmetry of the outputs is thought to be the presence of local or nonuniform kinking (beam displacements). Several suggestions for correcting this device deficiency are discussed. The presence of internal losses can cause the light-current curves to exhibit "soft" nonlasing to lasing transition regions which are typical of many lasers. Under these circumstances, a lasing threshold is difficult to define and the large-signal roundtrip gain remains substantially below unity at operating levels. In the absence of loss regions the model produces light-current curves suggestive of ideal injection lasers with large-signal roundtrip gain very close to unity.