Generation of anomalous and normal dispersion frequency combs in an MgF₂ WGM microresonator via self-Injection Locking

Recent progress in microresonator frequency combs has revealed their great potential for applications such as precision spectroscopy, distance measurement, microwave generation, and optical communications. In this context, whispering-gallery-mode (WGM) resonators based on magnesium fluoride (MgF₂) crystals are attractive owing to their low thermo-optic coefficient and ultrahigh-Q properties. These characteristics ensure excellent thermal stability, while the coexistence of multiple mode families allows access to both anomalous and normal dispersion within a single resonator.

We demonstrate the generation of bright and dark Kerr combs in a single MgF₂ WGM microresonator using self-injection locking techniques. The resonator was mechanically polished, with a diameter of approximately 7.5 mm, corresponding to a free spectral range of about 8.8 GHz. Using a commercial distributed feedback (DFB) laser near 1550 nm, we observed stable comb generation without active temperature control or rapid frequency scanning. By tuning the laser current, two distinct comb states were obtained: a bright soliton and a dark pulse comb. These combs originate from different mode families within the same resonator, highlighting the multimode nature of the over-moded MgF₂ WGM platform. To our knowledge, this is the first demonstration of both anomalous and normal dispersion Kerr combs generated in a single MgF₂ resonator via self-injection locking in the telecom wavelength band.