Abstract

Femtosecond laser sources in the mid-infrared (mid-IR) are of great interest for a wide range of applications including spectroscopy, material processing, and frequency combs [1]. With the limited availability of ultrafast lasers in the mid-IR, nonlinear frequency conversion processes offer one of the most promising techniques for the generation of tunable high-power femtosecond pulses across ~1-12 µm, a region which encompasses the important molecular fingerprints beyond 2 µm. In parametric interaction involving femtosecond pulses, the dispersion properties of the nonlinear material results in different group velocities among the pump, signal and idler pulses, leading to temporal walk-off, thus limiting the useful interaction length. The group-velocity-mismatch (GVM) imposes an upper limit on the crystal interaction length (typically 1-2 mm), also maximizing the phase-matching acceptance bandwidth to accommodate the broadband pump pulses for the attainment of highest conversion efficiency in the femtosecond parametric conversion process. The effects of GVM can be overcome when the dispersion properties of the nonlinear material are such that the pump and idler group velocities are identical at certain wavelengths satisfying the phase-matching condition. Under such circumstances, long crystal lengths can be deployed to attain high conversion efficiencies [2]. Here we report on the exploitation of zero-GVM condition in a long (42-mm) MgO:PPLN crystal for femtosecond optical parametric generation (OPG) at 10 MHz repetition rate, providing broadband mid-IR idler pulses with excellent characteristics at high conversion efficiency.

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