  Excitable systems with delayed feedback are important in areas from biology
to neuroscience and optics. They sustain multistable pulsing regimes with
different number of equidistant pulses in the feedback loop. Experimentally and
theoretically, we report on the pulse-timing symmetry breaking of these regimes
in an optical system. A bifurcation analysis unveils that this originates in a
resonance phenomenon and that symmetry-broken states are stable in large
regions of the parameter space. These results have impact in photonics for e.g.
optical computing and versatile sources of optical pulses.
