Influence of surface roughness on the optical mode profile in GaN-based violet ridge waveguide laser diodes

We investigate the influence of the epitaxial layer roughness on the far field profile of the optical mode in gallium nitride based, c plane ridge waveguide laser diodes. Occasionally, we observe long range growth instabilities leading to a periodical modulation of the surface. Amplitude and period of this surface roughness is typically on the order of a few 10 nm and 20 mu m, respectively. Using different characterization techniques, we investigate the influence of the surface roughness on the vertical mode profile along the fast axis in the far field, in particular the contribution of light scattering at the rough waveguide interfaces, as well as that of substrate modes.