An optical vernier technique for in situ measurement of the length of long Fabry-Perot cavities

摘要

We propose a method for in situ measurement of the length of kilometre-sized Fabry-Perot cavities in laser gravitational wave detectors. The method is based on the vernier, which occurs naturally when the laser beam incident on the cavity has asideband. By changing the length of the cavity over several wavelengths we obtain a set of carrier resonances alternating with sideband resonances. From the measurement of the separation between the carrier and a sideband resonance we determine the length of the cavity. We apply the technique to the measurement of the length of a Fabry-Perot cavity in the Caltech 40m interferometer and discuss the accuracy of the technique.