Theory of direct and indirect effect of two-photon absorption on nonlinear optical losses in high power semiconductor lasers

The effect of the transverse laser structure on two-photon absorption (TPA) related effects in high-power diode lasers is analysed theoretically. The direct effect of TPA is found to depend significantly on the transverse waveguide structure, and predicted to be weaker in broad and asymmetric waveguide designs. The indirect effect of TPA, via carrier generation in the waveguide and free-carrier absorption, is analysed for the case of a symmetric laser waveguide and shown to be strongly dependent on the active layer position. With the active layer near the mode peak, the indirect effect is weaker than the direct effect due to the population of TPA-created carriers being efficiently depleted by their diffusion and capture into the active layer, whereas for the active layer position strongly shifted towards the p-cladding, the indirect effect can become the dominant power limitation at very high currents. It is shown that for optimizing a laser design for pulsed high power operation, both TPA related effects and the inhomogeneous carrier accumulation in the waveguide caused by diffusive current need to be taken into account.