Low-threshold, high-repetition-frequency femtosecond optical parametric oscillator based on chirped-pulse frequency conversion

We report a quasi-phase-matched optical parametric oscillator that incorporates a chirped nonlinear crystal and uses prechirped pulses matched to the crystal chirp to improve the conversion efficiency and reduce the operational threshold. A 20-mm crystal of aperiodically poled KTiOPO4 is phase matched to stretched Ti:sapphire pump pulses. The Ti:sapphire laser produces 104-MHz output pulses at 850 nm that are stretched from 190 to 900 fs with an average output power of 750 mW. The system has demonstrated a pump depletion of more than 80%, a signal slope efficiency of 35%, and a threshold of 14.4 mW. The cavity showed tuning from 1194 to 1455 nm over a length range of 130 μm. The approach described demonstrates the potential of using chirped-pulse–chirped-crystal quasi-phase matching in long nonlinear crystals as a method to reduce ultrafast optical parametric oscillator thresholds.