A 1.6 kg silicon monocrystal was used to make a Fabryndash;Perot optical cavity operated at cryogenic temperatures. Highhyphen;resolution thermal expansion measurements were made as the silicon cooled to 4.2 K in order to characterize the cavity as a length reference standard. A heliumndash;neon laser was then locked to a transmission resonance at liquidhyphen;helium temperatures, and the laser frequency tracked the cavity resonance with error fluctuations at the level of 10 Hz/sqrt;Hz in the bandwidth dc to 1 Hz. Implications of the combined set of data, thermal expansion plus frequencyhyphen;tracking fluctuations, for using such a system as a frequency standard are discussed.
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