High-power continuous wave mid-infrared fluoride glass fiber lasers

The generation of high-power coherent radiation extending beyond the so-called near-infrared region of the electromagnetic spectrum has been a challenging task for the scientific community over the last decades. To that purpose, although fiber lasers are featuring several advantages namely in terms of their ruggedness and beam quality, their initial development has been mainly relying on fused silica possessing a wavelength operation window restricted to the near-IR. The advent of fluoride glass fibers allowing for significant doping levels of lanthanides along with superior optical and physical properties has, however, changed the deal. Nowadays, doped double-clad fluoride glass fibers with minimum background losses on the order or even below 10dB/km near 2.6 µm are commercially produced, which has fueled the development of watt-level monolithic fiber lasers covering the wavelength range between 2.7 and 3.9µm thanks to three trivalent lanthanide ions, Er³⁺, Ho³⁺, and more recently Dy³⁺. In this chapter, we first review from an historical perspective of the main developments that have led to such achievement. A special attention is also paid to the various strategies that have been deployed to alleviate the so-called bottleneck issue that is plaguing several mid-infrared (MIR) emission bands from those ions. Then the main engineering challenges that are specific to the power scaling issues related to MIR generation from fluoride glass fibers are addressed. Finally, future perspective for these sources along with their most promising application fields are discussed.