We report the fabrication of a barium magnesium fluoride (${\mathrm{Ba}\mathrm{Mg}\mathrm{F}}_4$) whispering-gallery-mode (WGM) microresonator demonstrating an intrinsic quality factor ($Q_0$) of about $2\times {10}^9$ at $1.55\text{μ}\mathrm{m}$, corresponding to a loaded quality factor ($Q_L$) of about $1\times {10}^9$ under critical coupling. At $398\mathrm{nm}$, the resonator exhibits a quality factor ($Q_L$) of $1.25\times {10}^8$ measured in the undercoupled regime, highlighting its potential for ultraviolet (UV) photonics. This performance was achieved in a 2.23-$\mathrm{mm}$-diameter resonator produced via a multistage process combining single-point diamond turning, chemomechanical polishing, and high-temperature annealing, yielding a surface roughness of approximately 1-$\mathrm{nm}$ RMS. Photoluminescence measurements under UV excitation revealed broadband visible emission, confirming the high optical quality of the crystal. When pumped in the near-infrared range, the resonator acted as a nonlinear element enabling the formation of Kerr optical frequency combs with a spectral width of about 30 nm and a line spacing of 42 GHz in the normal-dispersion regime. When incorporated into a fiber-laser cavity, it supported the generation of a frequency comb with a bandwidth of about 20 nm and a mode spacing of approximately 1 GHz at a pump power threshold of 86 mW. These results establish ${\mathrm{Ba}\mathrm{Mg}\mathrm{F}}_4$ WGM microresonators as a promising platform for nonlinear photonics across a broad spectral range, from the UV to the near-infrared.

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• Frequency combs & self-phase locking
• Integrated optics
• Nonlinear optics
• Nonlinear optical materials
• Nonlinear waves
• Sample preparation
• Whispering gallery mode resonators