Temperature measurement and stabilization in a birefringent whispering   gallery resonator
  Temperature measurement with nano-Kelvin resolution is demonstrated at room temperature, based on the thermal dependence of an optical crystal anisotropy in a high quality whispering gallery resonator. As the resonator's TE and TM modes frequencies have different temperature coefficients, their differential shift provides a sensitive measurement of the temperature variation, which is used for active stabilization of the temperature. 
D.V. Strekalov, R.J. Thompson, L.M. Baumgartel, I.S. Grudinin, and N.   Yu
physics.optics physics.ins-det