Kinks in the light/current characteristics and near-field shifts in (GaAl)asheterostructure stripe lasers and their explanation by the effect of self focusing on a built-in optical waveguide

Abstract

Measurements of the light/current characteristics and the near-and far-field patterns of apparently similar anodically insulated stripe-geometry (GaAl) As-heterostructure lasers show marked differences between individual devices in the linearity of the light output and the centrality of the near-field pattern as a function of current. Comparison of the near-and far-field patterns indicates the presence, even close to threshold, of a built-in optical waveguide. An approximate theoretical model has been developed to explain the effects. This considers the behaviour of an optical waveguide formed by the combination of a constant parabolic profile of real relative permittivity and a variable parabolic profile of complex relative permittivity associated with the injected carriers, which may be offset with respect to each other. The distribution of optical intensity, the distribution of injected carriers and the characteristics of the optical waveguide are derived in a self-consistent way as a function of current. It is shown that as the current is increased the conventional gain-cum-dielectric-guided behaviour gives way to a condition where the filament is driven towards one or other side of the stripe, even with infinitesimal asymmetry present. At high current a self-focused mode of operation may take over, but this can only be reached by a discontinuous process. The values of the various parameters involved in the treatment can be chosen to give fairly accurate modelling of the observed behaviour, and only relatively small variations in the values are required to explain the performance variations actually measured. The region of asymmetric behaviour and self-focused operation can be moved to higher current by increasing the strength of the constant component of the waveguide.