Frequency comb generation in crystalline MgF<inf>2</inf> whispering-gallery mode resonators

Abstract

Ultra-high-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) [1] as first demonstrated in fused silica microtoroids [2,3]. In contrast to fused silica, resonators made from crystalline materials [4–9] such as CaF 2 , MgF 2 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 [10, 11], and are not directly accessible with conventional frequency combs based on mode-locked lasers.