Lateral mode discrimination and control in high-power single-mode diode lasers of the large-optical-cavity (LOC) type

A comprehensive study of lateral mode discrimination and control in weak-laterally-confining large-optical-cavity (LOC)-type structures is presented. The analysis centers on two types of CDH-LOC laser structures: type A, which supports only the fundamental lateral mode in both the passive and active regimes; and type B, which supports several lateral modes in the passive regime and only the fundamental mode in the active regime. The transverse confinement factor
\Gamma
is peaked in the center of the structure and varies significantly across the lasing region for both device types. In the passive regime it is found that the effective-index (lateral) profile is a W-shaped waveguide for type A devices and a positive-index waveguide for type B devices. A discussion and analysis of losses in W-guides is also presented. Under carrier injection (i.e., active regime) the evolution of W-guides in CDH-LOC structures is presented as a function of increasing current density up to lasing threshold. For both type A and type B devices the effective-index profiles and corresponding lateral far-field patterns are analyzed as a function of threshold mode gain. Carrier-induced bulk-index depressions are found to be in the -0.02 to -0.04 range depending on the value of the threshold mode gain. The corresponding antiguiding parameter,
R = k_{o} \deltan/\deltag
, takes values in the -3 to -4 range, which imply values between 6 and 8 for the linewidth enhancement factor α. It is found that by controlling the threshold mode gain (i.e., changing the device length and/or its facet(s) reflectivity) devices of the same cross-sectional geometry can be made to lase either multimode (spatially) or in the fundamental mode.