A microwave temperature control system dedicated to a cryogenic sapphire-spaced Fabry - Pérot optical frequency reference

The extremely high frequency stability attainable with a cryogenic Fabry-Perot cavity is limited by temperature-induced length fluctuations of the sapphire spacer. Presents a method of measuring the temperature in the sapphire spacer at cryogenic temperatures. The temperature is detected by monitoring the AC susceptibility at microwave frequencies. Two microwave detection circuits are proposed and the noise limitations of each discussed. Shows that an optical frequency reference with an Allan variance of a few times 10\+-17\- for integration times up to 100 s can be achieved using this system. (Abstract quotes from original text)