Excitability and coherence resonance in lasers with saturable absorber

We show that a laser with a saturable absorber, described by the Yamada mod el, displays excitability just below threshold. A small perturbation, for e xample, a small input pulse, can trigger a single high output pulse, after which the system relaxes back to the off state. In order to study possible applications, such as pulse reshaping and clock recovery, approximate expre ssions are given for the excitability threshold and the delay between input and output pulses. Under the influence of optical noise, the system displa ys coherence resonance: below threshold the laser produces pulse trains wit h minimal jitter for a particular optimal noise level. This all-optical coh erence resonance allows direct experimental verification. [S1063-651X(99)02 712-9].