PPLN based optical parametric oscillators pumped by high power lasers around 1 µm are well established sources for generating light in the 3-5 µm spectral region, of interest for a wide range of scientific, commercial and military applications. We have been investigating optical parametric amplification (OPA), or difference-frequency generation (DFG), single-pass alternatives to conventional resonant OPOs. This avoids the need for a cavity and the corresponding design constraints that this can impose; such as, fixed repetition rates, sensitive alignment and/or poor output beam qualities at high average power levels. In this paper, we review recent results on high average power ( > 6 W) nanosecond pulse generation in the 3.3-3.5 μm region at MHz repetition rates, employing Yb:fibre and Er:fibre master oscillator power amplifiers (MOPA) systems pumping PPLN OPAs. We use focused Gaussian beam theory to validate the experimental results. We will also discuss spectral extension further into the mid-infrared, using different nonlinear crystal and alternative rare-earth doped fibre MOPA and Raman shifted fibre laser combinations. Ongoing work aimed at the power scaling of the mid-infrared light in both the nanosecond pulsed and continuous wave regimes will be presented.
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