Characterization and modeling of a femtosecond optical parametric oscillator suitable for tunable pump-probe spectroscopy

An improved cavity configuration continuous-wave mode-locked optical parametric oscillator (OPO) suitable for two-frequency femtosecond pump-probe spectroscopy is described. The OPO is based on intracavity pumping of a KTiOPO₄ (KTP) crystal in a colliding pulse mode-locked dye laser and produces femtosecond pulses at a 90 MHz repetition rate at average powers of 22 mW at 620 nm and 1-2 mW in the infrared. A model is synthesized from existing laser and parametric oscillator theories, and numerical simulations reproduce some qualitative aspects of the parametric oscillator´s behavior. The theory accurately predicts the renormalization properties of the pump pulses in the presence of a nonlinear intracavity loss, and the existence of megahertz intensity oscillations on the output power envelope for excessively self-phase modulated pump pulses