Lateral-mode analysis of gain-guided and index-guided semiconductor-laser arrays

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Abstract
The lateral-mode behavior of a semiconductor-laser array is analyzed after including the effect of injected charge carriers on the active-layer dielectric constant. The wave equation is solved self-consistently and the lowest-threshold lateral supermode is obtained by treating the whole array as one unit. The resulting mode profile incorporates the effects of gain guiding, carrier-induced index antiguiding, and built-in index guiding. The analysis is therefore applicable to gain-guided as well as index-guided arrays; near- and farfields for both kinds of arrays are obtained and compared to those obtained without gain-loss considerations using the plane-wave diffraction theory or the coupled-mode theory. For gain-guided arrays, carrier-induced antiguiding plays an important role and leads to a transition from twin-lobe to three-lobe farfields for closely spaced, narrow array elements. For weakly index-guided arrays the in-phase or out-of-phase coupling between neighboring elements can occur depending on whether the gain is laterally homogeneous or not. The calculated results are in qualitative agreement with the reported experimental observations. The present analysis is useful to understand the role of various guiding mechanisms as well as to provide guidelines for the device optimization.