Blue-band frequency comb and photodarkening in silica whispering gallery microresonators

Abstract

In contrast to the generation of optical frequency combs in the near and mid-infrared wavelengths, pushing optical frequency combs into the visible and ultraviolet is equally valuable, for example for direct applications in miniature optical clocks and spectroscopy. Currently, the blue-band is the shortest wavelength region for the generation of Kerr frequency combs, which have been realized using a magnesium fluoride resonator. However, direct comb generation from silica resonators would significantly simplify both the fabrication and integration processes. Here, we demonstrate for the first time, a blue-band Kerr frequency comb directly generated in a silica whispering gallery resonator (WGR) with a silica tapered fiber for optical coupling. A Kerr frequency comb with 13 comb lines is observed using a laser centered around 462 nm with a launched pump power of 30 mW. Significantly, the transmission through the coupling tapered fiber and the optical quality of the whispering gallery modes (WGMs) degrade quickly when pumped with the blue light. This leads to annihilation of the frequency comb, photodarkening being the expected cause. These observations appear to be a significant limiting factor for nonlinear optical processes in silica WGRs and provide a baseline for future nonlinear optics research in WGRs in the blue region.