title: Square-wave oscillations in a semiconductor ring laser subject to counter-directional delayed mutual feedback.
abstract: 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 τ1and 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.
authors.forename: Song-Sui
authors.initials: SS
authors.lastname: Li
authors.forename: Xiao-Zhou
authors.initials: XZ
authors.lastname: Li
authors.forename: Jun-Ping
authors.initials: JP
authors.lastname: Zhuang
authors.forename: Gabor
authors.initials: G
authors.lastname: Mezosi
authors.forename: Marc
authors.initials: M
authors.lastname: Sorel
authors.forename: Sze-Chun
authors.initials: SC
authors.lastname: Chan