Efficient femtosecond 50 MHz repetition rate mid-IR source up to 17 µm by difference-frequency generation in AgGaSe2

We present an efficient coherent source widely tunable in the mid-infrared (mid-IR) spectral range consisting of a novel femtosecond Yb-fiber laser operating at ~50 MHz repetition rate, a synchronously-pumped OPO (SPOPO) and difference-frequency generation (DFG) in AgGaSe2. With an average input power of 5 W for ~260 fs pump pulses at 1034 nm, the SPOPO outputs are tunable from ~1710 to 1950 nm (signal) and from 2200 to 2600 nm (idler) with pulse durations between 200 and 250 fs over the entire tuning range. After temporally overlapping signal and idler through a delay line, the two beams are spatially recombined with a dichroic mirror and focused to a beam diameter of ~75 μm. For DFG we employ an uncoated 2-mm-thick AgGaSe2 nonlinear crystal cut for type-I interaction at θ=57°. The generated femtosecond mid-IR pulses are continuously tunable between 5 and 17 μm with average power of up to 69 mW at 6 μm and more than 1 mW at 17 μm. Their spectra and autocorrelation traces are measured up to 12 μm and 8 μm, respectively, and indicate that the input spectral bandwidth and pulse duration are maintained to a great extent in the nonlinear frequency conversion processes. The DFG pulse width measured at 7.2 μm amounts to ~300 fs (FWHM). The measured spectral bandwidth supports ~150 fs Gaussian pulse durations across the entire DFG tuning range. For the first time mid-IR pulses with energy exceeding 1 nJ are generated at such high repetition rates.