Whereas a laser is a source of exactly one precise defined frequency, a frequency combs has hundreds of such well defined frequencies with a precise spacing between the individual lines. Commonly such combs are generated through the use of a third order (Kerr) nonlinearity. Second oder nonlinearities can be much stronger and we are studying the comb generation in a second order nonlinear crystal, such as Lithium Niobate. The trick is that we send both an optical as well as a microwave signal into the crystal and if everything is perfectly resonant a comb will be formed. We posted this on arXiv (2018).
Narrow linewidth lasers
High quality resonators can be used to reduce the linewidth of fibre lasers. We have shown that by just placing a WGM resonator within the loop of a fibre laser, the linewidth can be reduced to sub kHz. Standard fiber ring lasers are cheap, but suffer from unstable lasing behavior and a relative large linewidth. Utilizing a millimeter sized diamond turned CaF2 WGM resonator as a passive filtering element in an erbium-doped fiber ring laser we achieve single mode lasing. This system sustains single mode lasing without the usage of active stabilization techniques which we characterized with a three-cornered-hat measurement to have a linewidth below 13kHz, Optics Letters (2010). These measurements were limited by the reference lasers used. Currently we have build an identical WGM system and together with a commercial system have measures the relative frequency stability to be below a 700Hz and Optics Express (2014).