A circuit model for absorption grating (AG) gain coupled (GC) distributed feedback (DFB) semiconductor laser diodes (LD) is presented,based on the rate equations of carrier densities in active region and absorption grating,as well as the photon density.The relation of photon lifetime to cavity loss and facet loss,and the relation of light power to average photon density are derived.As an example,the self pulsation in AG GC DFB LD is studied.