Nonlinear Effects in CW Mode-Locked Solid-State Lasers with   Semiconductor Saturable Absorbers
  The influence of nonlinear properties of semiconductor saturable absorbers on ultrashort pulse generation was investigated. It was shown, that linewidth enhancement, quadratic and linear ac Stark effect contribute essentially to the mode locking in cw solid-state lasers, that can increase the pulse stability, decrease pulse duration and reduce the mode locking threshold 
V. L. Kalashnikov, D. O. Krimer