Cascaded stimulated Brillouin laser and Brillouin-Kerr optical frequency comb in high-Q MgF disk resonators.

Backward stimulated Brillouin scattering (SBS) in optical microcavities has been widely used in nonlinear optics and microwave photonics. Compared with glass material microcavity, magnesium fluoride crystal microcavity has the advantages of small absorption coefficient, fewer defects and larger nonlinear coefficient, moreover, it usually has a narrow gain bandwidth of tens of megahertz. Here, we design a high precision machining system to produce ultra-smooth surface magnesium fluoride crystal disk cavities with a diameter of about 5 mm, Q value exceeding 10, FSR matching material Brillouin gain. By simply modulating the pump wavelength and coupling power, we observe SBS phenomena with a 13.47 GHz Brillouin frequency shift near 1.55µm and cascaded stimulated Brillouin lasers (SBL) of up to 12 orders. In addition, the Brillouin-Kerr optical frequency comb in this device is demonstrated, observing nearly 300 comb lines spanning across a spectral window of 250 nm. Our research provides a way to fabricate high-Q crystal microcavities and demonstrates the potential of these devices in applications such as microwave sources and nonlinear optics.