Parity-time symmetric gratings in 1550  nm distributed-feedback laser diodes: insight on device design rules

We build on former results from our work on parity-time symmetric gratings implemented in 1550 nm distributed-feedback laser diodes to address the design issues raised by the first trends we observed. These laser diodes are of a complex-coupled nature, with modulations of both real and imaginary parts of the effective index, with a relative phase related to the parity-time symmetry. The unidirectionality of the photonic behavior in the reflection mode is dependent on the level of extra losses incurred by the metallic grating used to implement fixed loss modulation onto a nearly uniform gain, either as a first-order or a third-order grating. The observed behavior suggests that facets play a large role in setting the desired unidirectional lasing operation points, with preferential emission on one side. We explore this issue, which is of generic interest for the coupling of parity-time symmetric structures to open space.