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The use of an ultra low expansion cavity plays a crucial role in laser stabilization, and in atomic or ion clocks. We propose an easy method of precise monitoring of optical path distance in Fabry-Perot interferometer. The spacing of mirrors of the Fabry-Perot interferometer in ambient air represents the optical path distance referenced to stable optical frequency of the femtosecond mode-locked laser. With the help of highly selective optical filter it is possible to get only a few of separate spectral components of laser comb. Optical path distance is transfered to optical frequency of the comb component and through the repetition frequency of the laser to the radio-frequency domain. Repetition frequency of the laser can be monitored with the uncertainty referenced to the any local oscillator or through the GPS to the atomic clock standard. By using this mehod we are able to measure and lock the Fabry-Perot cavity to a selected single component of optical frequency comb an to measure the optical path distance directly in rf domain.

We present an experimental arrangement of an interferometric system designed to operate with full compensation for varying refractive index of air in the measuring axis. The concept is based on a principle where the wavelength of the laser source is derived not from an optical frequency of the stabilized laser but from a fixed length being a base-plate or a frame of the whole measuring setup. This results into stabilization of the wavelength of the laser source in atmospheric conditions to mechanical length of suitable etalon made of a material with very low thermal expansion. The ultra-low thermal expanding glass ceramic materials available on the market perform thermal expansion coefficients on the level 10 -8 which significantly exceeds the limits of uncertainty posed by indirect evaluation of refractive index of air through Edlen formula. To verify the concept a tracking of laser frequency following the drift of refractive index has been performed.

We describe in this paper a pilot experiment of a white-light fringe analysis with a low-cost color CCD camera. The used detection technique employs the phase-crossing algorithm which identifies the zero optical path difference as the point where the phase difference between the red, green and blue part of the white-light interference fringe becomes equal to zero. An experimental arrangement is based on superluminescent LED diode. The experimental setup is designed to be a crucial part of the complex system for automatic contactless diagnostic and calibration of gauge blocks.