Dispersion engineering of MgF2 belt resonators for frequency comb generation

Whispering-gallery-mode resonators fabricated from magnesium fluoride produce some of the highest finesse microresonator devices, allowing optical frequency combs to be excited with low optical powers. They are typically fabricated using diamond-point turning and polishing, which, unfortunately, often result in a large mode volumes which supports multiple spatial modes. These modes interact, leading to complex dispersion properties. Here, we explore fabrication using ultra-sharp diamond point tools, allowing high resolution resonator structures to be defined, which require little polishing. This allows us to fabricate few-mode resonators which are amenable to dispersion engineering, and produce clean and broadband frequency comb spectra.