Fiber-coupled cryogenic optical resonators

Summary form only given. We have developed fiber coupled cryogenic optical sapphire resonators for laser frequency stabilization. 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. The development of cryogenic optical resonators (COREs) is motivated by (i) the need for more stable reference resonators for high-resolution spectroscopy and (ii) improved tests of special and general relativity provided a sufficiently high long term stability can be reached. In previous work the laser beams were coupled into the COREs through windows in the cryostat, and on time scales longer than tens of minutes, the drift of the laser beam alignment limited the stability of the lock. We have therefore proceeded to implement coupling via an optical fiber whose end is stably positioned relative to the CORE.