Six-wave mixing theory of tilted-grating, broad-area semiconductor diode lasers with suppressed filamentation

Modern semiconductor diode lasers are appealing sources of laser radiation for many applications. However, an important disadvantage of semiconductor diode lasers as laser sources for the widest range of applications is that they are limited in output power. The output power of typical narrow-stripe semiconductor diode lasers is limited by thermal runaway effects that cause catastrophic facet damage to the device. To address this problem, researchers have developed broad-area diode lasers that can achieve increased output power by virtue of their larger emission area on the facet. In practice, however, the loss of lateral confinement that produces single-mode lasing in narrow-stripe semiconductor diode lasers leads to complex, multi-mode lasing in broad-area semiconductor diode lasers. The lateral spatial distribution of laser energy is further complicated by an optical nonlinearity in the semiconductor laser gain region, due to the dependence of the refractive index on the free-charge carrier density. This nonlinearity is responsible for the self-deformation of the lateral field distribution, so that in the lateral direction, the active region has areas of higher and lower light intensity, often called filaments. This highly-nonuniform lateral field distribution results in poor beam quality and hot-spot facet damage at relatively low output power for typical broad-area semiconductor diode lasers.;This dissertation proposes and analyzes a specific approach to suppress filamentation in broad-area semiconductor diode lasers, with the general goal of increasing the laser power that can be produced while retaining good beam quality. The analysis approach is based on a six-wave mixing methodology for the evaluation of filamentation in general broad-area semiconductor diode laser structures. This methodology is first applied to examine the growth of filamentation in typical broad-area semiconductor diode laser designs. A specific design modification to a broad-area laser is then proposed, based on the incorporation of a tilted grating (not at Bragg-resonant conditions) into the active area. The original analysis approach is extended to allow the analysis of this modified broad-area laser design. This modified analysis approach is then applied to evaluate the effectiveness of the new design and determine the specific design parameters that give the best performance in suppressing filamentation. Results of the analysis show that a properly-selected tilted grating design should be completely successful in suppressing filamentation as the semiconductor diode laser pump current is increased---at least within the parameter space in which the analysis is valid. Finally, some limitations of the present six-wave mixing analysis are discussed and suggestions are made for future research.