High-pulse-energy cryogenic Ho:YLF laser pumped by a Tm:Fiber laser

High-energy 2-μm holmium lasers have a range of important applications including LIDARS for remote sensing and atmospheric science. In addition, such lasers are ideal for pumping nonlinear conversion stages based on ZnGeP₂ for generating radiation in the 3-5 μm and 8-12 μm atmospheric transmission windows. Holmium lasers can be pumped at 800 nm by co-doping with Tm or at 1.9 μm by using Tm:fiber lasers [1-5]. The latter approach enables highly efficient end-pumping by optimizing the spatial overlap between the pump and the laser mode. Holmium lasers are quasi three-level systems at room temperature. The population of electrons in the lower laser level can be drastically reduced by cryogenic cooling, and the laser then becomes a quasi four-level system, with potential for higher gain, increased energy extraction efficiency, and reduced bleaching of the pump absorption process, compared to room-temperature operation. We here demonstrate a cryogenically cooled Q-switched Ho:YLF laser, which operates at 1-20 Hz repetition rate and has a pulse energy to pump power efficiency of up to 5.5 J/kW.