Semiconductor racetrack resonator coupled to an S-bent waveguide: Influence of the coupling coefficients on the unidirectional operation

Semiconductor ring lasers have attracted remarkable interest as laser sources in photonic integrated circuits. They result in bidirectional laser owing to the rotation symmetry between the two counter-propagating modes of the ring cavity. This symmetry can be broken incorporating in a racetrack resonator an S-bend waveguide, which generates an un-balanced loss mechanism and a nonreciprocal gain between clockwise and counter-clockwise direction beams. The propa-gating field along the resonator in the undesirable direction is evanescently coupled to the S element in correspondence of two coupling regions and converted into the preferred one. In this work, we examined how the field coupling coefficients of the couplers impact the resonator unidirectionality. In nu-merical simulations, we changed the coupler gap distance and the coupler length of the directional couplers to scan the full range of variability of the coefficients. The simulated performances of the resonator are discussed in term of the extinction ratio between the clockwise and the counter-clockwise modes as well as the power truly circulated in the two directions of the resonator net of all losses. The finite-difference time-domain method within Synopsys RSoft© suite was used to simu-late the evolution of the counterpropagating field along the racetrack.