Ultrastable Optical Cavities – Designs, Applications and Challenges

Lasers with long coherence times and narrow bandwidth are an essential tool for precision measurements, like quantum sensors and optical clocks. At present, most of these systems obtain their stability from Fabry-Perot reference cavities, thus relating the fractional frequency instability to fractional length fluctuations of the cavity, with the best systems now achieving instabilities down to 4 ×10−17. However, as macroscopic devices these cavities are susceptible to many perturbing influences and, ultimately, limited by fundamental thermodynamic fluctuations. In this tutorial I will present techniques to decouple cavities from ambient noise, address ways to reduce the influence of thermal Brownian noise, like novel coatings and the related challenges, and discuss the transfer of the stability to other wavelengths. An outlook towards more reliable, maintenance-free and robust cavity setups for, e.g. applications in transportable optical clocks and in Space will be given.