Stable single- and dual-wavelength fiber laser mode locked and spectrum shaped by a Fabry-Perot saturable absorber.

We show that the spectral response of a Fabry-Perot semiconductor saturable absorber depends on laser beam focusing. We used this effect to achieve stable passive mode-locked operation in a simple and environmentally stable fiber laser by introducing spectral filtering and forcing near-resonant operation. By reducing the free spectral range and increasing the nonlinear response of the Fabry-Perot saturable absorber, we obtained dual-wavelength mode-locked operation.