Nonlinearity in power-output–current characteristics of stripe-geometry injection lasers

Abstract

The origins and behaviors of kinks in power‐output–injection‐current (I‐L) curves of stripe‐geometry lasers are theoretically investigated. Laser output power is calculated as a function of injection current by solving the carrier diffusion equation and Maxwell’s equation. Transverse modes along the junction plane are assumed to be guided by the injected‐carrier‐induced gain profile. At low current densities, it is shown that the gain profile is nearly parabolic, resulting in the transverse mode very close to the lowest Hermite‐Gaussian distribution. However at high current densities the gain profile is deformed by the stimulated recombination of carriers near the center of the stripe. The resultant mode becomes deformed and penetrates into the low‐gain or lossy region. Consequently the mode gain is reduced, and the power saturation (kink) appears in the I‐L curves. If there exists an asymmetry in the injection current density profile, it is shown that the optical intensity peak moves along the junction plane and the beam direction shifts from the normal to the laser facets with the increase of the current density. Such theoretical results are qualitatively in good agreement with recent experimental observations and demonstrate that kinks originate from the gain profile deformation and the resultant mode deformation. Investigating the dependence of the output power at which a kink appears on various device parameters such as the stripe width, the amount of current spreading, and the degree of asymmetry, we conclude that the scatter of the output power experimentally observed is caused by the small fluctuations of such device parameters.