Mode guidance parallel to the junction plane of double-heterostructure GaAs lasers

We investigate the origins of mode confinement, parallel to the junction plane, of stripe-geometry double-heterostructure GaAs injection lasers. Based upon the kind of control of the mode extent by the stripe width, we establish a distinction between two types of lasing mode guidance; (i) well-behaved, where a single mode substantially fills the entire active region, and (ii) filamentary, where the mode size is much smaller than the width of the active region and its location is random. For low-order well-behaved modes, it is shown, at least in principle, that gain alone (no refractive index effects are required) can explain not only the confinement but also the approximate waveguide dimensions for which leakage losses, connected with the penetration of the mode into the regions outside the active guide, become important. When only this postulated gain-guiding prevails in the case of higher-order modes, we find that their spatial character is markedly different from the Hermite-Gaussian distributions which have been seen for GaAs lasers. The introduction of a positive incremental index, which may reasonably be associated with the gain and/or with a thermal mechanism, restores agreement with observation. If any incremental index within the active region is negative, then although there is a defocusing effect, confinement may still be maintained by the gain. In the case of filamentary lasing we analyze four focusing mechanisms; two are connected with local built-in gain (loss) and refractive index variations, while two are current dependent and are related to the saturation of gain-associated refractive index and the free carrier effect. While all four processes appear to be reasonable candidates for providing the necessary confinement in filamentary lasing, the latter two simultaneously violate a condition necessary for stability of a filament. We conclude that imperfections in lasers are likely to be related to filamentary lasing and that such behavior may not be intrinsic to such devices.