We demonstrate a laser system with a frequency instability of 0.7 Hz for integration times of 20 s, corresponding to 2.3×10^-15 fractional instability. This is the lowest short-term instability of any current oscillator. The system uses diode-pumped miniature Nd:YAG lasers stabilized to sapphire Fabry-P\'erot reference cavities at liquid helium temperature. Cryogenic optical resonators are promising for precision experiments, such as high-resolution spectroscopy, flywheel oscillators for optical frequency standards, and displacement measurements. As an example, we report a measurement of the ultralow thermal expansion of a sapphire cavity at 1.9 K.
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Seel et al. (1997) studied this question.
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