Precise monitoring of ultra low expansion Fabry-Perot cavity length by the use of a stabilized optical frequency comb

The use of an ultra low expansion cavity plays a crucial role in laser stabilization, and it is essential in atomic or ion clocks. In this work we present a method of mirror distance monitoring in an evacuated Fabry-Perot cavity (FPC) made from an ultra low expansion material (Zerodur expansion class 0). The FPC was placed into a temperature-stabilized stainless steel chamber and the FPC temperature was set to remain close to the point where the expansion coefficient of the cavity material is the lowest. The precise distance of the FPC mirrors in vacuum was indirectly proportional to the optical frequency of a laser. One of the frequency comb component chosen by FBG grating was locked to the FPC of 400 MHz free spectral range. The 100 MHz repetition frequency of the femtosecond laser comb was monitored with counter referenced to a crystal oscillator with short term stability < 10⁻¹² and long term stability locked to a GPS.