Excitability in optical systems close to Z₂-symmetry

We report theoretically and experimentally on excitability in semiconductor ring lasers in order to reveal a mechanism of excitability, general for systems close to Z<sub>2</sub>-symmetry. The global shapes of the invariant manifolds of a saddle in the vicinity of a homoclinic loop determine the origin of excitability and the features of the excitable Pulses. We show how to experimentally make a semiconductor ring laser excitable by breaking the Z<sub>2</sub>-symmetry in a controlled way. The experiments confirm the theoretical predictions