Towards an ultra-stable optical sapphire cavity system for testing   Lorentz invariance
  We present a design for an ultra-stable cryogenically cooled sapphire optical cavity system, with fractional frequency stability better than 1 x 10^-16 at one second integration time. We plan to use such ultra-stable cavities to perform a test of the isotropy of light propagation at the 10^-20 level. 
M. Nagel and A. Peters
physics.optics hep-ph