Non-reciprocal emission with high single-mode operation in multisection complex-coupled distributed feedback cavities

Abstract

We investigate theoretically the threshold properties of complex-coupled distributed feedback (CC-DFB) lasers in which the index and gain gratings are purposely out of phase by π/2. Such a phase difference between the two coupling constants is liable to produce a non-reciprocity in emission and leads to the same modal selectivity as in purely gain-coupled or, on the contrary, purely-index-coupled Bragg reflectors. Furthermore, by distributing non-uniformly the whole complex coupling coefficient and by varying the ratio of gain and index coupling along the cavity, the threshold properties of these particular devices can be strongly modified with respect to conventional complex-coupled structures: single-mode operation can be notably enhanced while maintaining a low threshold gain, the intra-cavity field profile of the lasing mode (which is related to spatial hole burning) is found to be well affected and the ratio of output optical powers emitted by each facet (the non-reciprocity) can be chosen with almost no effects on the threshold net gain and gain margin of the device.