Tunable laser operation of Tm³⁺:KY₃F₁₀ near 1.9 µm via upconversion pumping at 1064 nm

We experimentally demonstrate, for the first time to our knowledge, that upconversion pumping can be employed with a 1064-nm fiber laser to achieve tunable laser operation with 8 at. % Tm³⁺:KY₃F₁₀ crystal near 1.9 µm, in addition to the previously explored upconversion pumping of the 2.3-µm laser transition. Two different sets of cavity optics were used to investigate lasing at 1.9 and 2.3 µm, corresponding to the laser transitions ³F₄−³H₆ and ³H₄−³H₅, respectively. In the case of the ³F₄−³H₆ laser transition, an x-cavity laser oscillator was constructed with a 2.3% output coupler and as high as 142 mW of output power was obtained at an incident pump power of 1.9 W at 1064 nm. Tunable laser operation could be obtained between 1849 and 1994 nm by using CaF₂ and suprasil prisms. The highest slope efficiency of 29% with respect to average absorbed pump power was obtained with a 5.5% output coupler. For the case of 1.9 µm lasing, it was further demonstrated that the nonlinear absorption of the crystal at 1064 nm depends on the intracavity laser intensity as well as the pump intensity. The ³H₄−³H₅ laser transition was also investigated with the same crystal in a z-cavity configuration under upconversion pumping, giving as high as 130 mW of output power at the central wavelength of 2344 nm with 1.6 W of incident pump power.