Generation of Kerr combs in MgF2 and CaF2 microresonators

We report on a study of hyper-parametric oscillation in high-Q calcium fluoride and magnesium fluoride optical microresonators, and show that, magnesium fluoride microresonators generate wider Raman-free Kerr frequency combs at telecom wavelengths, in contrast with calcium fluoride ones. We observed 30 nm wide frequency combs in magnesium fluoride microresonators pumped with only 2 mW of continuous wave 1543 nm light.