Determination of the thermal lensing in a broad area semiconductor laser amplifier

Abstract

The local refractive index in the active medium of semiconductor lasers is affected by heat generated from the injection of carriers, non-radiative recombination and absorption of photons. Temperature gradients arising from generation and diffusion of the heat induce a refractive index distribution and contribute to the formation of a positive lens in the lateral dimension of the active area. The thermal lens may be approximated by the coefficient γ which relates the local refractive index across the laser diode to the effective refractive index as: n(x) = n 0 (1-γ2X2/2), (1) where n 0 is the effective refractive index and x the lateral coordinate. This thermal lens significantly influences the propagation of lateral optical modes through the amplifier. Using the amplifier as part of a diode laser with internal or external resonator, the thermal lens may also determine the properties of the lateral modes generated and hence may effect beam quality.