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Max Planck Junior Research Group for Novel Applications of Ultra-High-Q Optical Microcavities

Final Activity Report Summary - JRG-UHQ (Max Planck Junior Research Group for Novel Applications of Ultra-High-Q Optical Microcavities)

The development of optical frequency combs allowed for measuring fundamental constants with previously unattained precision within the last years. A frequency comb is a laser source, which consists not only of one frequency (continuous wave laser), but consists of a large number of discrete frequencies that are equally spaced (like the teeth of a comb). In case that the comb's frequencies are known, it is possible to use it to measure the frequency of unknown light sources for example the light emitted from certain atomic transitions. Just like a ruler is used to measure lengths, a frequency comb is the perfect tool for measuring optical frequencies of light with high precision.

In our research group at the Max-Planck-Institute of Quantum Optics (MPQ) it has been possible to demonstrate a completely novel and simple method for frequency comb generation. This technique was proposed in the IRG proposal and is based on four wave mixing (parametric oscillations). While previously mode-locked lasers (complex and bulky systems) have been used for comb generation, we could show that it is possible to generate frequency combs in a micron size resonator made of fused quartz that sits on a silicon microchip. These resonators, which have a diameter in the range of the size of a human hair can store photons for comparably long times, which leads to extremely high circulating light intensities and to nonlinear optical effects that enable the frequency comb generation. Since this new comb generation process is entirely different from conventional frequency combs, we proved that the spectrum generated with our system indeed constitutes a frequency comb.

This experiment has been performed by comparing our comb to a conventional frequency comb. In a second step, we have shown that it is possible to fully control and stabilise the frequency comb, which is important to use it as tool for example in the field of frequency metrology. To achieve this control a novel method has been developed to change the spacing of the frequency comb teeth. Since the spacing depends on the length of the path that the light has to travel within the resonator and this path depends on the temperature of the resonator, we have shown that it is possible to stabilise and control the comb spacing extremely fast by varying the laser power sent into the resonator.