Highly-efficient Tm-fiber Laser Resonantly Pumped by 1.7 μm Laser Diode

CW laser diode emitting at 1.7 μm was used to unidirectionally pump a thulium-doped fiber laser. Cladding-pumped fiber laser resonator consisted of dichroic (high reflectivity at 2 μm, high transparency at 1.7 μm) free-space plane mirror, placed between two lenses used to collimate and to focus the pumping radiation, and Fresnell reflection of active fiber end which serves as an output coupler. For absorbed power 11.5 W output power 5.2 W was reached at wavelength 1950 nm with slope efficiency 70 %. The goal of our work is to attract attention to the possibility of pumping thulium fiber lasers resonantly using laser diodes that are already readily available today. Thanks to resonant pumping, compared to cross-relaxation-assisted pumping at 0.8 μm, higher efficiency and lower heat generation can be achieved, which can help to reach high mean powers from thulium fiber lasers. An important fact is that both the generated laser radiation and pumping radiation belong to the eye-safe spectral region, which reduces the risk of vision damage.