A MICROWAVE TEMPERATURE CONTROL-SYSTEM DEDICATED TO A CRYOGENIC SAPPHIRE-SPACED FABRY-PEROT OPTICAL FREQUENCY REFERENCE

The extremely high frequency stability attainable with a cryogenic all -sapphire Fabry-Perot cavity is limited by temperature-induced length fluctuations of the sapphire spacer. This paper presents a method of m easuring the temperature in the sapphire spacer at cryogenic temperatu res. The two systems described are analysed to have precisions of 750 and 75 nK over measurement times of 1 s. The temperature is detected b y monitoring the AC susceptibility at microwave frequencies. The tempe rature coefficient of the susceptibility originates from the paramagne tic contribution of residual chromium in the sapphire. Two microwave d etection circuits are proposed and the noise limitations of each discu ssed. The optimal doping levels of the paramagnetic ion are found to b e commonly available as residuals in commercially available sapphire c rystals. We show that an optical frequency reference with an Allan var iance of a few times 10(-17) for integration times up to 100 s can be achieved using this system.