|
As sources of short, high-amplitude light pulses, self-pulsing lasers are central to many applications, including telecommunications and neuromorphic photonic computing. We consider an excitable semiconductor micropillar laser subject to delayed optical feedback. The microlaser alone displays an all-or-none response to external perturbations. In the presence of feedback, a first excitable pulse can regenerate itself after a delay time, thus resulting in a train of pulses with repetition rate close to the delay time. Several pulse trains can be triggered and sustained simultaneously. Although they can seem independent on the timescale of the experiment, recent work showed that all pulsing patterns correspond to very long transient towards one of the stable periodic solutions of the system. Only stable solutions corresponding to equidistant light pulses in the feedback cavity were observed.
We demonstrate experimentally and numerically that stable periodic solutions corresponding to non-equidistant pulses can also exist and be stable. A bifurcation analysis of a suitable mathematical model unveils the conditions on the timescales of the gain and absorption variables for such solutions to exist. We show that the long-term timing between non-equidistant pulses is fixed by the system parameters and does not depend on the initial timing between the pulse trains. Moreover, the bifurcation analysis demonstrates that, for a given number of coexisting pulses in the feedback cavity, only one configuration is stable, corresponding either to equidistant pulses or non-equidistant pulses with a fixed interpulse timing. The latter originates from a period doubling bifurcation and can also be viewed as a symmetry breaking phenomenon of the time-shift symmetry sustained by the system.
Our results provide a better understanding of pulsing dynamics in an excitable laser with delayed feedback. Because the only ingredients here are excitability and feedback, we believe our results may be of interest beyond the scope of laser dynamics.
|
