Andronov bifurcation and excitability in semiconductor lasers with optical feedback

We experimentally investigate the dynamical behavior of a semiconductor laser with optical feedback. We show that noise plays an important role close to the instability threshold, while determinism controls the so-called coherence collapse regime. We identify the bifurcation which is at the origin of the low frequency fluctuations. It is the result of a collision between a stable fixed point and a saddle point (Andronov's bifurcation). We provide experimental proof that the laser with optical feedback behaves as an excitable medium.