The effect of delayed optical feedback on semiconductor ring lasers

Semiconductor ring lasers (SRLs) have been investigated due to their interesting two-mode dynamical properties. The study of the bistable behavior between the clockwise (CW) and the counterclockwise (CCW) propagating modes in a tandem of two coupled SRLs has demonstrated appealing features for applications in ultrafast all-optical switching and optical memories. Solitary SRLs are found to operate bidirectionally up to about twice the threshold, where unidirectional operation starts. In the bidirectional operation starting at threshold, the two counterpropagating modes are both continuous wave with equal power. As the injected current is increased, a new regime appears through a Hopf-bifurcation and the intensities of the two counter propagating modes undergo alternate sinusoidal oscillations with frequency ~100 MHz. At still higher current values the SRL operates in a bistable unidirectional regime that is most interesting for switching and memory applications. Like any other semiconductor laser, SRLs are sensitive to spurious reflections, originating e.g. from the wafer edges. Through such time-delayed feedback, the CW mode will be injected into the CCW mode and vice versa. The aim of this work is to investigate the impact of such double optical feedback on the stability of the unidirectional regime. We investigate both short and long time delays and consider that the feedback can be both symmetric and asymmetric for the two modes.