Nonlinearly limited saturable-absorber mode locking of an erbium fiber laser

We describe an erbium fiber laser that is passively mode locked by a novel, precision antireflection-coated semiconductor saturable-absorber mirror that incorporates an additional two-photon absorber. It is shown that passive mode locking evolves from a Q?switching instability. The results are achieved by use of saturable absorbers that provide a large (15%) nonlinear (saturable) loss. Exploiting two-photon absorption can substantially reduce the peak power of the Q?switched pulses, which results in improved reliability of the laser. Moreover, two-photon absorption can be used to produce an optimal stability range for saturable-absorber mode locking.