On the thermal noise limit of ultrastable optical cavities

We consider two methods of lowering thermal noise, which limits the attainable frequency stability of lasers stabilised to a high-Q external Fabry – Perot cavity. These are cavity lengthening and use of high-Q mechanical materials in the production of its constituent parts. The results of numerical simulation are outlined for the horizontal and vertical suspension systems of the cavity body and its sensitivity to vibrations in these cases. The progress in the development of ultrastable laser systems using cryogenic silicon cavities is also discussed. (paper)