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.