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the resonance frequency of a Fabry-Perot cavity. Indeed, this approach takes advantage of the recent progress in frequency metrology and should result in an unsurpassed sensitivity along with an impressive dynamic range. The tests performed by J.L. Hall and coworkers lead to an equivalent sensitivity δn = δν/ν ≃ 1 × 10 − 17 (see Table I), which appeared to be limited by various experimental effects, including vibrations, perfectible optical isolation and birefringence noise of the cavity mirrors. 

In fact, J.L. Hall’s proposal is similar to the method used in large active ring lasers aimed at precisely measuring the Earth rotation rate [22, 23] thanks to the Sagnac effect. These rings have areas of tens to hundreds of square meters, and exhibit a relative frequency sensitivity on the order of 10 − 18 − 10 − 20 $_{/}$√$_{Hz.}$ 

In this paper, we present promising sensitivity tests performed on an apparatus which is based on the same idea but aims at measuring small directional anisotropies of the propaga- tion medium. Indeed, we have built a ring cavity, so that one can distinguish two propagation directions. A laser light beam is frequency-stabilized to the clockwise cavity resonance, and injected into the cavity both clockwise and counterclockwise. The counterclockwise error signal yields the value of the directional anisotropy. 

Our first goal is to measure the magneto-electric directional anisotropy of light which 

