Highly-efficient Ho:KY(WO4)2 thin-disk lasers at 2.06 µm

The recent advances in the development of Holmium monoclinic double tungstate thin-disk lasers are reviewed. The thin-disk is based on a 250-μm-thick 3 at. % Ho:KY(WO₄)₂ active layer grown on a (010)-oriented KY(WO₄)₂ substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce pump geometry, the continuous-wave Ho:KY(WO₄)₂ thin-disk laser generates an output power of 1.01 W at 2057 nm corresponding to a slope efficiency η of 60% and a laser threshold of only 0.15 W. The thin-disk laser is passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror. In this regime, it generates an average output power of 0.551 W at ~2056 nm with η = 44%. The best pulse characteristics are 4.1 μJ / 201 ns at a repetition rate of 135 kHz. The laser performance, beam quality and thermo-optic aberrations of such lasers are strongly affected by the Ho³⁺ doping concentration. For the 3 at.% Ho³⁺-doped thin-disk, the thermal lens is negative (the sensitivity factors for the two principal meridional planes are -1.7 and -0.6 m^-1/W) and astigmatic. The Ho:KY(WO₄)₂ epitaxial structures are promising as active elements in mode-locked thin-disk lasers at ~2060 nm.