CRYOGENIC OPTICAL RESONATORS - A NEW TOOL FOR LASER FREQUENCY STABILIZATION AT THE 1 HZ LEVEL

We demonstrate a laser system with a frequency instability of 0.7 Hz f or integration times of 20 s, corresponding to 2.3 x 10(-15) fractiona l instability. This is the lowest shea-term instability of any current oscillator. The system uses diode-pumped miniature Nd:YAG lasers stab ilized to sapphire Fabry-Perot reference cavities at liquid helium tem perature. Cryogenic optical resonators are promising for precision exp eriments, such as high-resolution spectroscopy, flywheel oscillators f or optical frequency standards, and displacement measurements. As an e xample, we report a measurement of the ultralow thermal expansion of a sapphire cavity at 1.9 K.