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 ${\tau }_1$ and ${\tau }_2$, respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of $T\approx {\tau }_1+{\tau }_2$. 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 ${\tau }_1\ne {\tau }_2$, 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 $T=10.3\mathrm{ns}$ are observed experimentally.

© 2016 Optical Society of America

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