Continuous-wave (CW) Tm-laser operation is achieved for the first time using the thin disk laser concept and an epitaxial active element based on the monoclinic KLu(WO 4 ) 2 crystal with 15 at.% thulium doping of the overgrown layer, which is only 80 μm thick. A single double pass of the pump radiation from the diode-laser ensures efficient pumping due to the large absorption of the active layer. For output coupling between 0.4% and 2.8%, the slope efficiency in the CW regime is in the 7 -11% range and the laser threshold amounts to 1.5 -2.5 W of absorbed pump power. The laser emission spectra are centered at ∼ 1850, 1915, and 1940 nm for output couplers of 2.8%, 1.6%, and 0.4% transmission, respectively. In all cases the emission spectra are "structured", consisting of a number (typically 5-10) of narrow emission lines spread irregularly over 15 -30 nm. Physical cavity length, mm Average output power of the epitaxial Tm:KLuW/KLuW(010) laser in the QCW regime versus physical cavity length. The incident average pump power is 2.8 W, the output coupler transmission is T = 1.6%, and the duty cycle is 18%

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We report a high-efficiency room-temperature 5%Tm:KLu(WO4)2 laser relied upon the gain slab-shaped crystal of volume less one and half cubic millimeter. Continuous-wave output power of 10.4 W at 1907 nm for 34.2 W of diode pump power has been demonstrated, with optical and slope efficiencies being of 31 and 43%, respectively. Thermal coefficient of optical path was evaluated also, TCOP (E || Np) ∼ –2×10-6.

The thermal lensing effect is characterized in slab-shaped monoclinic Tm:KLu(WO 4 ) 2 crystal cut for light propagation along the N m optical indicatrix axis (the light polarization is E N p ). The optical power of the thermal lens was measured directly by a modified probe beam technique. Alternatively, it was calculated on the basis of measured material parameters, including the thermal expansion coefficients along the optical indicatrix axes (α p = 3.1, α m = 11.1 and α g = 13.1(10 −6 K −1 )) and the principal thermo-optic coefficients (dn p /dT = −13.3, dn m /dT = −5.9, dn g /dT = −9.0(10 −6 K −1 ) at 1.95 µm). The two methods are in good agreement and confirm the defocusing action of the thermal lens; the 'effective' thermo-optic coefficient for N m -cut crystal is −2.1 ± 0.5 × 10 −6 K −1 . The fractional head load η h for slab-shaped Tm:KLu(WO 4 ) 2 crystal is 0.33 ± 0.03 (as determined from ISO-standard laser calorimetry).

We report on the first holmium (Ho) monoclinic double tungstate thin-disk laser. It is based on a 250 μm thick 3 at. % Ho:KY(WO) active layer grown on a (010)-oriented KY(WO) substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce (single double-pass) pump geometry, the CW Ho:KY(WO) thin-disk laser generated 1.01 W at 2057 nm, corresponding to a slope efficiency η of 60% and a laser threshold of only 0.15 W. Implementing a double-bounce (second double-pass) for the pump, the output of this laser was scaled to 1.57 W with η=55%. The maximum stimulated emission cross section σ of the Ho ions in the epitaxial layer reaches 2.5×10 cm at 2056.5 nm for E‖N. The Ho:KY(WO) epitaxial structures are promising for multi-watt mode-locked thin-disk lasers at ∼2.06  μm.