The effect of field-interference in modelling gain-coupled DFB lasers

Transfer matrix method models for distributed feedback (DFB) lasers have generally neglected the effects of field interference, presumably, because of the computational complexity of the problem. Instead, the forward- and backward-propagating components of the field are treated as independent and the interference considerations are averaged out. These models are inappropriate for gain-coupled DFB structures, where the gain material distribution in the device varies significantly over spatial distances on the order of the wavelength of light and the details of spatial distribution of the longitudinal photon density becomes crucial. In this paper the consequences of considering the spatial distribution of the photon density due to the interference between the counter-propagating components of the field are presented using a new photon rate equation. The results from this more involved and more accurate calculation show that the effective coupling strength of a laser is larger than that predicted from previous averaged models.