Microresonator-based optical frequency combs
NPL’s time and frequency group is a key player in the recently established UK initiative to develop miniature optical atomic clocks with accuracy close to those of national standards. For most applications of these clocks, miniature frequency combs will be required to convert the optical output frequency to the microwave and/or radiofrequency domain. Optical frequency combs generated by parametric frequency conversion in optical microresonators appear to be a promising technology to meet this need, but significant scientific challenges remain to be addressed before they can be used as part of an optical clock. Foremost amongst these is the need to achieve self-referencing of the comb, which requires a low-noise, octave-spanning spectrum to be achieved, together with appropriate means to control the two degrees of freedom of the comb. Other challenges include achieving suitable comb mode spacings, matching the spectral coverage to optical clock wavelengths and achieving low parametric thresholds to minimize power consumption. Within this EngD project two different microresonator technologies will be investigated for frequency comb generation: CO2-laser-machined microresonators made from amorphous materials such as quartz rods and a chip-based technology such as silicon nitride. A key goal will be to achieve self-referenced and fully stabilized frequency combs based on one of these technologies. There may also be possibilities to explore the application of microresonator-based optical frequency combs to chip-based optical sensing and spectroscopy.