Abstract
Whispering-gallery mode (WGM) microresonators made from magnesium fluoride (MgF2) have long been studied as a platform for the generation of optical frequency combs [1]. They support high-finesse whispering gallery modes which allow access to optical nonlinear effects with only tens of milliwatts of optical power. Coherent frequency combs are typically generated at wavelengths where the resonator exhibits a negative group velocity dispersion, which allows for the realization of localised structures known as temporal cavity solitons. While there have been many demonstrations of optical frequency combs in these devices, practical application of MgF2 microresonators have been limited. This is in part due to the fact that WGM resonators typically support hundreds of spatial modes that interact in the form of avoided mode crossings, hindering the excitation of coherent soliton combs. Here, we consider a photonic belt resonator (PBR) - a WGM resonator which supports propagating optical modes in a small ridge-like waveguide only several microns across [2]. This photonic belt provides optical confinement to only a few spatial modes, thereby considerably simplifying the comb generation process and removing spectral defects. A microscope image of the fabricated resonator is shown in Fig. 1 (Left), consisting of three PBRs fabricated on a cylinder of MgF2. The resonators were fabricated using diamond turning techniques with ultra-fine machining tools and polished to a finesse of 2\times 10^{4} .