With assured applications in the biomedical market, compact, high-average-power, short pulse (sub-ns) sources operating in the water window around 2.9 µm are an active topic of current research. In this joint academic-industrial partnership, a commercial optical parametric amplifier was used to seed ~1 ns, 7 mJ, 2.8 µm pulses at 5 kHz repetition rate into a two-stage Er:ZrF₄ fiber amplifier chain. The bidirectionally-pumped amplifiers had core diameters of 85 µm and 115 µm, with optimized lengths of 1.9 m and 1.7 m respectively. GaAs amplifier pump diodes, operating around 940 nm (70 W first amplifier) and 960 nm (2 x 60 W diodes, second amplifier) where absorption is lower, were used to reduce the heat load along the fibers. The amplifiers were simply passively cooled on a metal plate.

The generated peak power was ~1 MW, with a pulse energy of 1.02 mJ (a record for fluoride-based systems at this wavelength), achieving an output intensity of 10 GW/cm² with a symmetrical beam, with M² of 4.1. Consequently, nonlinear spectral broadening in the amplifiers was observed, with spectra lying between 2.7 and 3.0 µm. Importantly, no output facet damage was recorded, and end capping was not deployed. The authors estimate a damage threshold of 20 GW/cm², hence further energy scaling is possible and propose that 5 mJ pulses should be achievable with a 180 µm core diameter third amplifier, adding further to the attractiveness of this unique compact source for the vast array of biomedical applications.

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