Reflectivity measurements of internal cavity defects in semiconductor laser diodes

The presence of defects inside the cavity of a semiconductor laser can have a very strong influence on the emission spectrum. The resulting output spectrum can become highly modulated even in the presence of a single defect. By carefully positioning a limited number of defects, for example by etching slots into a ridge waveguide laser it is even possible to achieve quasisingle longitudinal-mode operation. Here we show that by a straightforward Fourier analysis of a sub-threshold laser spectrum we can extract the complex reflectivities of an internal scatterer.