Thermal-noise limited laser stabilization to a crystalline   whispering-gallery-mode resonator
  We have stabilized an external cavity diode laser to a whispering gallery mode resonator formed by a protrusion of a single-crystal magnesiumdifluoride cylinder. The cylinder's compact dimensions (<1 cm^3) reduce the sensitivity to vibrations and simplify the stabilization of its temperature in a compact setup. In a comparison to an ultrastable laser used for precision metrology we determine a minimum Allan deviation of 20 Hz, corresponding to a relative Allan deviation of 6*10^-14, at an integration time of 100 ms. This level of instability is compatible with the limits imposed by fundamental fluctuations of the material's refractive index at room temperature. 
J. Alnis, A. Schliesser, C. Y. Wang, J. Hofer, T. J. Kippenberg, and   T. W. H\"ansch
physics.optics