Characterization of the absolute frequency stability of an individual   reference cavity
  We demonstrated for the first time the characterization of absolute frequency stability of three reference cavities by cross beating three laser beams which are independently locked to these reference cavities. This method shows the individual feature of each reference cavity, while conventional beatnote measurement between two cavities can only provide an upper bound. This method allows for numerous applications such as optimizing the performance of the reference cavity for optical clockwork. 
T. Liu, Y. N. Zhao, V. Elman, A. Stejskal, and L. J. Wang