On the distinctive features of gain coupled DFB lasers and DFB lasers with second-order grating

The two types of distributed-feedback (DFB) lasers with gain coupling, i.e., true gain coupled lasers (with either a gain or loss grating) and second-order index coupled lasers, are shown to be equivalent mathematically and to some extent also physically. The operation of these DFB lasers, partly based on the overlap of the standing wave pattern in the optical power density with the periodic gain/loss variation, is addressed. It is shown in detail how this special feature leads to important modifications in the expressions for the modal loss or threshold gain, the threshold gain difference, the efficiency and the linewidth