Facet phases and sub-threshold spectra of DFB lasers: spectral extraction, features, explanations and verification

The sub-threshold spectra of distributed feedback (DFB) lasers are heavily influenced by the phase of the internal grating with respect to the end facets. In this paper, we document features commonly observed in sub-threshold spectra and explain these features as manifestations of the facet phases. We extract estimates of facet phases by fitting a probability-amplitude transfer-matrix model to spectra from six truncated-well DFB lasers, and use the probability-amplitude model to document, isolate, and explain the sub-threshold spectral dependence on facet phase. To verify the accuracy of the approach that we have taken, we compare estimates of the facet phases from the fits to independent measurements of the facet phases using a scanning photoluminescence method. The results from the two methods are compared and are found to be in agreement. The agreement validates our use of the probability-amplitude model in this paper to explain laser facet phase phenomena.