Localized saturation of loss grating in complex-coupled DFB laser diodes

The longitudinal dependence of the standing-wave envelope in a complex-coupled DFB laser is experimentally investigated. We have observed localized clamping of the luminescence for unstable operating conditions. This local clamping is a consequence of the absorber saturation with increasing injection current and of the short period of the standing-wave envelope that is characteristic for multimode emission. The period can be used as a criterion to distinguish between regions of multimode emission and optical instabilities.