Optical clocks for space applications
The CHRONOS (Route to on-chip resonating optical clocks via nonlinear optics) project investigated novel operating principles for the control of mode-phase-locking in microcavity driven optical frequency combs (OFCs). The objective was the construction of precision optical clocks with ultra-high repetition rates. Project work was based on a novel mode-locking principle for high-repetition-rate (> 200 GHz) lasers embedding a non-linear high-quality resonator in a fibre laser cavity. Such a scheme delivers a stable, fully mode-locked pulse train, providing a simple and effective way to lock the phase of an OFC induced by four-wave mixing in microresonators. Remarkably, the design consists of a simple fibre cavity. The fibre is the active element that provides the gain necessary to the lasing, while the microresonator is responsible for the wave mixing of the modes. Design of the control parameters for the nested cavity laser has been accomplished. The laser scheme, which forms the basis of the most recent generation of cross-correlated photon pairs, has been published in Optics Express and Nature Communications. The experimental study of the laser targeted a more general set of non-linear filters, leading to the demonstration of an alternative mode-locking regime. These results have been recently presented at international conferences and are now being prepared for publication. The detailed study of the four-wave mixing dynamic developed allowed a study on the stability of optical beams, which has been published in Physical Review Letters, with interesting consequences for optical waveguides direct laser writing. An innovative approach for the noise characterisation of the laser has been developed and published. The investigation not only had the scientific-technological values discussed above, but demonstrated the potential to develop an exceptionally simple general-purpose test bench, easy to manage and highly versatile, for a general class of optical technologies. The project was theoretically and experimentally challenging: it aimed at the definition of a novel class of optical clocks enabled by the use of advanced approaches in non-linear science, creating new, fundamental and interdisciplinary knowledge. As a result, it has improved the competitiveness of the European photonics industry, through the development of cost-effective photonics ultrafast sources for a wide range of applications.
Keywords
Optical clocks, laser, CHRONOS, nonlinear optics, optical frequency combs, resonators
