Spectral output of 1.3 μm InGaAsP semiconductor diode lasers

Theories of the spectral output of diode lasers assume that the devices are uniform along the length of the active region. Spatially-resolved degree of polarisation measurements are presented which show that devices are not uniform along the length and that significant scattering exists along the length of the active region. The spectral output of lasers is found to be correlated with the amount of scattering in the active region. Calculations to support and explain the observed correlation are presented. For the devices tested, it is found that gain guided devices tend to exhibit less scattering than index guided devices and tend to operate more multi-longitudinal mode. Based on these measurements it is suggested that differences in the spectral properties of index and gain guided lasers may be explained on the basis of the light scattering characteristics of the active region.