Cryogenic monocrystalline silicon Fabry–Perot cavity for the stabilization of laser frequency

A 1.6 kg silicon monocrystal was used to make a Fabry–Perot optical cavity operated at cryogenic temperatures. High-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 helium–neon laser was then locked to a transmission resonance at liquid-helium temperatures, and the laser frequency tracked the cavity resonance with error fluctuations at the level of 10 Hz/√Hz in the bandwidth dc to 1 Hz. Implications of the combined set of data, thermal expansion plus frequency-tracking fluctuations, for using such a system as a frequency standard are discussed.