To form the cavity, the 5.5 m long, commercially available, single-mode, Dy-zirconium fluoride fiber employed had fiber Bragg gratings directly written into it, through the fiber’s polymer coating, using femtosecond laser inscription. Pumping was in-band, by a fiber-coupled, laser-diode-pumped, Er-fluoride fiber laser, operating at 2.83 µm, providing up to 25 W, of which up to 19 W was coupled to the laser via a 1 m length of undoped fluoride fiber. At the highest output powers, a 0.4% RMS stability was achieved, with a detector resolution limited linewidth of less than 70 pm. The authors infer that increased efficiency should be possible through active fiber length optimization, but note that further power-scaling, although possible, would be more difficult and would need significant reduction in the cavity losses and improvements in the fused splice technology employed. However, this work as it stands represents a major milestone in developments in compact, efficient, high-power mid-infrared sources.
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