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 τ1 and τ2, respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of T ≈ τ1 + τ2. When τ1 = τ2, 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 τ1 ≠ τ2, asymmetric SWs are obtained with a tunable duty cycle of τ1/(τ1 + τ2). High-order symmetric SWs with a period of T = (τ1 + τ2)/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.