We review our development of wavelength tuneable, high average power, picosecond-pulse, mid-infrared sources in the three micron widow for proposed application in tissue ablation studies and with the objective of source simplification and exploring the potential of all-fiber integration. Initial systems were based on difference frequency generation (DFG) of two synchronous master oscillator power fiber amplifier (MOPFA) schemes. The generated idler was tuneable over the range 3.28–3.45 μ m, delivering greater than 3 W of average power, with a maximum pump to total DFG power conversion efficiency of 78 % . By simplifying the seed sources through synchronised in-line modulation of cw diode laser sources, more than 6 W was generated 3.31–3.48 μ m with similar efficiency and with near diffraction limited beam quality ( M^2=1.4 ). In an improved and significantly simplified experimental configuration a source emitting around 3 μ m was developed employing a novel χ^(3)/χ^(2) cascaded nonlinear conversion architecture. Picosecond pulses from a 1.064 μ m mode-locked Yb:fiber pump laser were used to generate 1.65 μ m signal pulses through χ^(3) based four-wave mixing in a polarisation preserving photonic crystal fiber (PCF). The output of the PCF was then directly focused into a periodically poled lithium niobate crystal, generating idler radiation around 3 μ m, with peak powers of ∼ 0.5 kW, via χ^(2) -based three-wave mixing between the pump and signal pulses.
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