Characterization of the absolute frequency stability of an individual reference cavity.

We demonstrated for the first time (to our knowledge) the characterization of absolute frequency stability of three reference cavities by cross beating three laser beams that are independently locked to these reference cavities. This method shows the individual feature of each reference cavity, while conventional beat note measurement between two cavities can provide only an upper bound. This method allows for numerous applications such as optimizing the performance of the reference cavity for optical clockwork.