Measurement of the thermal expansion coefficient of an all-sapphire optical cavity

We present measurements of the frequency-temperature dependence of an all-sapphire Fabry-Perot optical cavity to be used as an optical frequency reference. Measurements were made by tracking the frequency of the cavity relative to a second high-stability cryogenic sapphire-spaced cavity-a technique with impressive resolution for measurements of the coefficient of thermal expansion (CTE). Measurements presented here cover the temperature range of 11 K to 26 K. We find that the CTE of the all-sapphire cavity for these temperatures is given by /spl alpha//sub cavity/=(7.7/spl plusmn/0.9)/spl times/10/sup $/ -/sup 13/T/sup (3.23/spl plusmn/0.05/).