Highly sensitive frequency metrology for optical anisotropy measurements.

In this paper we present a novel apparatus aimed at measuring very small birefringences and anisotropies and based on frequency metrology and not on polarimetry as usual. In our experiment, a very high finesse resonant cavity is used to convert the phase difference into a resonance frequency difference, which can then be measured with very high accuracy. We describe the setup and present the results of experimental tests, which exhibited a sensitivity delta n approximately = 2x10(-18), allowing for the measurement of long-predicted magnetoelectro-optical effects in gases. Since the shot-noise limited sensitivity of our apparatus lies well below the state-of-the-art sensitivity, frequency metrology appears as a promising technique for small birefringence measurements.