Aurélien Coillet


Kerr frequency combs

For my first post-doctoral position, I have been working at the FEMTO-ST Institute. This position was funded by the « Nextphase » European Research Council Starting Grant obtained by Yanne Chembo in November 2011, and whose main goal is to generate 10 GHz microwave with ultra low phase noise. This microwave will result from the beating note of the lines of an optical Kerr comb in a whispering gallery mode (WGM) resonator. Since I was the first post-doctorant of this project, my mission was to produce the frequency combs and to study their dynamics and different regimes.


Photography of a CaF2 WGM resonator. Green light is coupled into the resonator by evanescent coupling thanks to the optical microfiber next to it. You can find many other photographies in the gallery.

Whispering Gallery Mode Resonators

WGM resonators are dielectric disks or spheres. When light enter them tangentially, it undergoes multiple total internal reflections and can't leave the cavity: light is trapped in the resonator. In wave optics, this is described as resonating modes just below the surface of the disk. Only absorption and diffusion can limit the photon life-time in the cavity, and the lower the losses, the higher the quality factor.

The home-made resonators reach quality-factor in the 109 range, and a finesse of 20000. This means that a photon will travel 20000 round trips in the cavity before being absorbed by the material or scattered by the surface. This phenomenom implies a high field increase, allowing for interesting nonlinear effects.

Scheme of a WGM resonator. The traject of ligh (red) follows the outer perimeter of the disk, undergoing total internal reflections. In such an object, only specific wavelengths can exist, and the transmission spectrum is composed of equidistant frequency lines.

Kerr Combs

The high optical intensity in the resonator allows for interesting nonlinear phenomena, and especially, four wave mixing (FWM) can occur. This processus converts two photons of frequencies α and β in two other photons with frequencies γ and δ. For instance, with two same frequency resonating photons, one can create two other symetrical frequencies in neighbours resonances. When cascading this operation, we obtain a set of equidistant spectral lines: this is a Kerr comb.

Principle of the generation of a Kerr comb through four wave mixing.

Theses combs present complex dynamics and different interesting regimes depending on the resonator's parameters and pump settings. Our first goal is therefore to study these dynamics and to find the most interesting parameters for each application. These applications are diverses: microwave generation at the FSR frequency, multiplexing in optical telecommunication, spectroscopy,...

Experimental realisation of a Kerr comb in a MgF2 WGM resonator.


More details can be found in the following links: