Generation of mid-infrared femtosecond pulses tunable in the 3-10 μm range from an optical parametric amplifier*

In this work we present an optical parametric amplifier (OPA), pumped by an amplified femtosecond Yb:KGW laser and directly generating mid-infrared (MIR) pulses in a broad tuning range, thus allowing to cover the entire vibrational spectrum between 2.5 and 10 μm. Avoiding the usual difference-frequency generation (DFG) stage to access the MIR region simplifies the setup while enabling high conversion efficiencies. The two-stage design employs a beta barium borate (BBO) crystal for the first stage, which is pumped at 0.515 μm and amplifies a near-infrared (NIR) seed in the 1.1-1.7 μm. Instead, the second stage exploits either periodically poled lithium niobate (PPLN), which is optimal for the CH/OH bonds stretching region (3-5 μm), or LiGaS₂ (LGS), which allows the extension of the tunability range to the fingerprint region (up to 10 μm). The system has been characterized by measuring the spectra of the pulses, the signal and idler output powers, the second stage gain and by calculating the transform limited durations. We anticipate applications of this source to ultrafast vibrational spectroscopy, Fourier-transform IR microscopy and photothermal IR microscopy.