ON THE DISTINCTIVE FEATURES OF GAIN-COUPLED DFB LASERS AND DFB LASERSWITH 2ND-ORDER GRATING

First, the two types of DFB lasers with gain coupling, true gain coupl ed 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 base d on the overlap of the standing wave pattern in the optical power den sity with the periodic gain/loss variation, is addressed. It is furthe r shown in detail how this special feature also leads to important mod ifications in the expressions for the modal loss or threshold gain, th e threshold gain difference, the efficiency and the line-width.