Frequency Comb Generation in Optical Microresonators via Soliton Formation
Dr. John Jost (Laboratory of Photonics and Quantum Measurements, Lausanne, Schweiz)

Frequency combs, i.e. spectra of equidistant laser lines, are enabling tools in precision spectroscopy and optical frequency metrology [1].
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/microwave-frequencies and thereby provide and absolute frequency reference in the optical domain. 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 [2, 3].
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 [4, 5]. 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.

We have demonstrated that one route to low-noise frequency combs are via the formation of temporal dissipative optical solitons in high-Q Magnesium Fluoride [6] and Silicon Nitride microresonators. The formation of solitons is enabled by the balance between anomalous resonator dispersion and Kerr-nonlinearity. The solitons produced can be very stable and ultra-short in some cases with durations on the order of ~200 fs. These ultrashort pulses can be used for microwave generation or spectrally broadened to close to what is necessary for self-referencing.

[1] S.T. Cundiff and J. Ye. “Colloquium: Femtosecond optical frequency combs”, Reviews of Modern Physics, 75, 325-342 (2003) [2] T.J. Kippenberg, et. al. “Microresonator-based optical frequency combs”, Science, 332, 555-559 (2011) [3] D. J. Moss, et. al. "New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear optics," Nature Photonics 7, 597-607 (2013).
[4] D. J. Jones, et. al. “Carrier–envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis”, Science 288, 635-639 (2000)
[5 ] R. Holzwarth, et. al. “Optical frequency synthesizer for precision spectroscopy”, Phys. Rev. Lett. 85 (11), 2264 -2267 (2000) [6] T. Herr, et. al. “Soliton mode-locking in optical microresonators”, Nature Photonics (in press), arXiv: 1211.0733 (2012)