Frequency comb generation in crystalline MgF2 whispering-gallery mode resonators

Ultrahigh-Q whispering gallery mode (WGM) resonators, with the ability to confine and store optical energy in small volumes enable optical frequency comb generation via four-wave mixing (FWM) as first demonstrated in fused silica microtoroids. In contrast to fused silica, resonators made from crystalline materials such as CaF₂, MgF₂ and quartz feature exceptionally low loss over a broad spectral window, ranging from the UV (ca. 160 nm) to the mid-infrared (ca. 8 micron). Moreover, the millimeter sizes of crystalline resonators enable the repetition rates of the combs to be in the range of few tens of gigahertz. These repetition rates are of special interest to astrophysical spectrometer calibration, and are not directly accessible with conventional frequency combs based on mode-locked lasers. In this paper, the authors demonstrate optical frequency comb generation in crystalline MgF₂ resonators, which due to their intrinsic material properties allow for thermal self-locking.