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The goal of this work was an investigation of Ho:YAP (Ho:YAlO3) crystal as an active medium of resonantly longitudinally pumped multi-watt microchip laser operating at 2.1 ¹m spectral region. Three Ho:YAP crystals,a"-cut, b"-cut, and c"-cut Pbnm, with the same dimensions (7mm long, 3mm in diameter) and Ho-doping concentration (1.06 at.% Ho/Y) were compared. Resonator mirrors were deposited directly on the crystals faces. The output coupler transmission for desired laser emission wavelength range 2.1 µm was 11% and T = 3% @ 1.94 µm. The pumping mirror was highly reflecting at 2.1 µm and T = 89% @ 1.94 µm. Samples were fixed in air-cooled Cu-heatsink and longitudinally pumped by a CW Tm-fibre laser with the maximum output power amplitude of 12W @ 1939.2nm behind a focusing lens (f = 200 mm). The laser output power, emission wavelength, and output beam profile were measured in respect to incident pumping power. All three lasers had similar input-output power characteristics with the laser threshold close to 1.5W and slope efficiencies reaching quantum limit in respect to the incident pumping power. The best result (slope efficiency 79 %, laser threshold 1.54W, max output power 8.2W in an almost diffraction-limited, linearly polarized beam) was reached for microchip laser using b"-cut Ho:YAP crystal. Laser emission wavelength was 2119nm for a"-cut and b"-cut Ho:YAP and 2132nm for c"-cut Ho:YAP-based microchip laser. The designed lasers can serve as compact wavelength converters for laser radiation and could be used to expand capabilities of current Tm-fibre lasers used in medicine and industry preserving the overall system efficiency.
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