Whispering gallery mode lasing in a fiber-coupled sub-millimeter Yb:CaF2 crystalline microcavity

We report on the fabrication and characterization of a Yb:CaF₂ whispering-gallery mode (WGM) microcavity. A disk with a radius of 0.56 mm was successfully fabricated from a 5% Yb:CaF₂ bulk crystal, exhibiting ultra-high optical quality factors (Q) of $2.8\times {10}^8$ at 1550 nm and $3\times {10}^6$ at 1064 nm. The 1064 nm Q factor of $3\times {10}^6$ corresponds to an estimated absorption coefficient of $2.8\times {10}^{-4}$ cm⁻¹. Leveraging the three-level energy structure of Yb³⁺ ions, efficient 1064 nm laser emission was generated under 976 nm diode laser pump excitation. We investigated laser performance under two distinct coupling configurations: free-space pumping and angled-polished fiber (APF) coupling. The APF coupling method demonstrated better efficiency, achieving a low laser threshold of 3 mW and output power exceeding the mW level. These results demonstrate the promise of Yb:CaF₂ crystalline microcavities for compact, efficient, and potentially tunable microlaser sources.