Abstract
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.
Highlights
In the high repetition rate regime (∼100 MHz), devices based on nonlinear frequency conversion, in particular synchronously-pumped optical parametric oscillators (OPOs), are unrivalled in their ability to generate kilowatt-level pulse trains of quasi-continuous-wave radiation across 1–12 μm, with pulse durations able to reach as short as a few optical cycles
Femtosecond OPOs cannot generally reach typical conversion efficiencies of their picosecond and cw counterparts, primarily due to more pronounced temporal walk-off effects scitation.org/journal/app between the pump, signal, and idler pulses, which propagate with mismatched group velocities through the nonlinear crystal
While this does not pose a major problem for relatively narrowband picosecond lasers, a transform-limited 100 fs pulse at 1064 nm spans a full-width-half-maximum (FWHM) spectral bandwidth of ∆λ ∼ 12 nm, far broader than the pump acceptance bandwidth afforded by most crystals longer than a few millimeters
Summary
Mid-infrared (mid-IR) femtosecond laser sources are of great interest across many research fields, for applications including frequency comb generation, pump-probe spectroscopy, and materials science. In the high repetition rate regime (∼100 MHz), devices based on nonlinear frequency conversion, in particular synchronously-pumped optical parametric oscillators (OPOs), are unrivalled in their ability to generate kilowatt-level pulse trains of quasi-continuous-wave (cw) radiation across 1–12 μm, with pulse durations able to reach as short as a few optical cycles.. The long crystal provides additional dispersive broadening of the interacting pulses by increasing the effective interaction length from ∼1 mm (for ∼100 fs pulses) to ∼8 mm (for ∼500 fs pulses) due to temporal walk-off, leading to enhanced down-conversion efficiency from the pump to OPO output By deploying this approach, we achieve substantially lower oscillation thresholds and higher conversion efficiencies as compared to conventional femtosecond OPOs. The ability to rapidly tune across the mid-IR using pump wavelength variation is exploited to perform transmission spectroscopy of methane in the 3.3 μm spectral region. We explore the possible merits of applying the same technique to alternative nonlinear crystals in the mid-IR
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