Influence of ridge geometry on lateral mode stability of (Al,In)GaN laser diodes

Height and width of the ridge forming the laser diode waveguide determine threshold current density and lateral mode stability. We measure the optical near-field of the laser mode and simulate the two-dimensional mode distribution including waveguide losses and optical gain. The simulations show that weak guiding and not current spreading is the major cause for increased threshold current densities in weakly guided laser diodes. The near-field measurements show fundamental and higher order modes for nominally identical ridge laser diodes. We demonstrate that asymmetric losses in the waveguide bias the lateral mode competition towards higher order modes for an intermediate range of index guiding. This asymmetric damping is specific for (Al,In)GaN laser diodes due to absorption introduced in the p-waveguide by magnesium doping. This competition of lateral modes can also be seen spectrally as two longitudinal mode combs of different optical gain. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.