Precise determination of the refractive index of air in Fabry-Perot cavity by means of the optical frequency comb

We experimentally demonstrated a method for monitoring of cavity length changes based on femtosecond laser comb. Our pilot experiment based on separation of frequency comb lines by fiber Bragg grating produces an unlimited variety of Fabry-Perot cavity lengths to be locked on. We experimentally demonstrated that the Fabry-Perot cavity length could be locked to one chosen femtosecond frequency comb line. Tracking the optical path distance of FPC to optical component of frequency comb can be directly measured by means of counter of repetition frequency.