Enhancement of Efficiency in Femtosecond Optical Parametric Oscillators Using Group-Velocity-matching in Long Nonlinear Crystals

The authors describe the exploitation of group-velocity-matching in femtosecond optical parametric oscillators (OPOs) for enhanced down-conversion efficiency into the mid-infrared (mid-IR). We demonstrate the concept in a femtosecond OPO based on a long MgO:PPLN crystal, for the first time, by utilizing group-velocity-matching between pump and idler pulses. Taking advantage of the wide phase-matching bandwidth when pumped near 1 µm enables the use of a 42-mm-long crystal, resulting in an oscillation threshold as low as 5 mW, pump depletion of 78%, and an idler quantum conversion efficiency up to 48% into the mid-IR. Using 80–100 fs pump pulses tunable across 997–1070 nm, we have generated idler radiation across 3132–4273 nm (2340–3193 cm−1) with spectral bandwidth of 140–180 nm, providing up to 65 mW of average power at 80 MHz repetition rate. The near-IR signal is tunable across 1392–1568 nm, with up to 76 mW of average power in transform-limited pulses of ∼400–600 fs duration without dispersion compensation. Preferential operation at group-velocity-matched wavelengths leads to intrinsically high passive power stability with <1% rms fluctuation over 1 h for both signal and idler. With the capability for rapid pump tuning in the mid-IR, this OPO represents a viable source for spectroscopic applications, which we demonstrate using CH4 gas. Extension to other quasi-phase-matched nonlinear crystals is also discussed.