Fiber-coupled and monolithic cryogenic optical resonators

We have developed a fiber coupled cryogenic optical sapphire resonator for the frequency stabilization of a Nd:YAG laser. With this method a frequency instability of 2/spl middot/10/sup -15/ for /spl tau/=1 hour integration time has been achieved. This is among the lowest values ever obtained for optical oscillators. We also present a new approach to test the time-independence of the fine structure constant, based on the use of a single monolithic resonator. Preliminary results on the development of suitable resonators are described.