Theory of intracavity frequency doubling in passively mode-locked femtosecond lasers

The authors construct a quantitative theoretical model of an intracavity frequency-doubled and passively mode-locked laser, supported by experimental observations with a colliding pulse mode-locked femtosecond dye laser. The major findings are that for second-harmonic conversion efficiencies consistent with continuing laser operation (<5%): (1) a stable mode-locking regime always exists, although it narrows somewhat with increasing conversion efficiency; and (2) the duration of the fundamental pulses can always be preserved, even in the femtosecond time domain, by readjusting saturable gain and saturable loss under the same conditions. Both the model and observations contrast with previous studies of actively mode-locked and synchronously mode-locked lasers containing intracavity frequency-doubling crystals