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 Z2-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 fea- tures of the excitable pulses. We show how to experimentally make a semiconductor ring laser excitable by breaking the Z2-symmetry in a controlled way. The experiments confirm the theoretical predictions. 
Stefano Beri, Lilia Mashall, Lendert Gelens, Guy Van der Sande, Gabor   Mezosi, Marc Sorel, Jan Danckaert and Guy Verschaffelt
physics.optics physics.bio-ph