35 W Core-Pumped Holmium Fiber Laser

Holmium fiber lasers offer several unique characteristics that make them interesting from research and commercial perspectives. First, their broad emission around 2.1 μm, which coincides with an existing atmospheric transmission window, enables applications such as LIDAR, remote sensing and free-space optical communication [1]. Defence applications are also possible as 2.1 μm is considered an “eye-safer” wavelength, which lowers the threat of collateral damage. The small quantum defect present in in-band pumped setups theoretically enables efficient power-scaling as the thermal load of the fiber can be kept low. Some challenges arise due to using silica as fiber material, mainly the increasing intrinsic absorption of this material beyond 2 μm, OH- absorption [2] and the limited solubility of rare-earth ions. The latter problem can be addressed by introducing sufficiently high alumina co-doping, as there seems to be a direct link between the molar ratio of Al/Ho and slope efficiency [3]. The fiber used in this contribution has been prepared inhouse by MCVD and nanoparticle doping. The H<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf>3+ doping concentration is 3850 mol ppm and the Al/Ho ratio is 49. The high doping concentration leads to an NA of 0.24 and a small 6.4 μm diameter core to maintain single-mode operation. The cladding diameter is 125 μm to facilitate simple splicing to standard telecom fibers.