Fabrication of femtosecond laser written depressed-cladding waveguides in Tm³⁺:BaY₂F₈ crystal and laser operation near 2 μm

To our knowledge, we report the first waveguide laser operation of Tm³⁺:BaY₂F₈ crystal near the wavelength of 2 μm. Femtosecond laser inscription was employed to fabricate two waveguides, each in the form of a circular depressed cladding, with diameters of 30 μm and 50 μm. For the 50-μm-diameter waveguide, the propagation loss was determined as 0.22 dB/cm at the wavelength of 731 nm. A two-mirror resonator consisting of a butt-coupled input high reflector and a 10-cm-radius curved output coupler housed the Tm³⁺:BaY₂F₈ crystal waveguide during lasing experiments. By using a continuous-wave Ti³⁺:sapphire laser at 781 nm, waveguide lasing was obtained at 1.86 μm. By using a 6% output coupler, as high as 73 mW of output power was obtained with the 50-μm-diameter waveguide at an incident pump power of 680 mW, corresponding to a slope efficiency of 13.1%. Lasing threshold pump powers as low as 23 mW were further achieved with a 2% output coupler by using the 30-μm-diameter waveguide. These results show that the Tm³⁺:BaY₂F₈ crystal possesses an important potential in the development of compact, efficient, femtosecond laser written waveguide lasers near 2 μm.