Square-wave oscillations in a semiconductor ring laser subject to counter-directional delayed mutual feedback

Square-wave (SW) switching of the lasing direction in a semiconductor ring laser (SRL) is investigated using counter-directional mutual feedback. The SRL is electrically biased to a regime that supports lasing in either counter-clockwise (CCW) or clockwise (CW) direction. The CCW and CW modes are then counter-directionally coupled by optical feedback, where the CCW-to-CW and CW-to-CCW feedback are delayed by tau(1) and tau(2), respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of T approximate to tau(1) + tau(2). When tau(1) = tau(2), symmetric SWs with a duty cycle of 50% are obtained, where the switching time and electrical linewidth of the SWs can be reduced to, respectively, 1.4 ns and 1.1 kHz by strengthening the feedback. When tau(1). tau(2), asymmetric SWs are obtained with a tunable duty cycle of tau(1)/(tau(1) + tau(2)). High-order symmetric SWs with a period of T = (tau(1) + tau(2))/n can also be observed for some integer n. Symmetric SWs of order n = 13 with a period of T = 10.3 ns are observed experimentally. (C) 2016 Optical Society of America