Spectral broadening of microresonator based frequency combs for self-referencing

Frequency combs, i.e. spectra of equidistant laser lines, are enabling tools in precision spectroscopy and optical frequency metrology. Conventionally, frequency combs are generated using mode-locked lasers, where the mode-locking of the optical comb lines implies the generation of a train of ultra-short optical pulses. Frequency combs can link optical frequencies to radio-frequencies (RF) and thereby provide and absolute frequency reference in the optical domain. An essential prerequisite for this RF-to-optical link is self-referencing, which can be achieved via e.g. f-2f or 2f-3f interferometry. Necessary for these self-referencing schemes is a minimal optical comb bandwidth of an octave (f-2f) or two thirds of an octave (2f-3f). In conventional frequency combs the broad spectrum is not directly provided by the mode-locked laser but achieved via external spectral broadening in a non-linear medium. A novel approach of frequency comb generation is based on continuously driven, Kerr-non-linear, optical microresonators where cascaded four-wave mixing leads to the emergence of an equidistant frequency comb spectrum. So far however, self-referencing of microresonator combs has not been possible as no system was capable of generating sufficiently broad spectra while maintaining the low-noise level required for metrology operation.