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 2 -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 2 -symmetry in a controlled way. The experiments confirm the theoretical predictions.
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