Highly efficient Tm-fiber laser resonantly pumped by a 1.7  µm laser diode

A CW laser diode emitting at 1.7 µm was used to unidirectionally pump a thulium-doped fiber laser. A cladding-pumped fiber laser resonator consisted of a 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 Fresnel reflection of an active fiber end that serves as an output coupler. For an absorbed power of 11.5 W, an output power of 5.2 W was achieved at a wavelength 1950 nm with a slope efficiency of 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.