spectral density of this signal, obtained with a 3251 spectrum analyzer from Aeroflex. In our present experimental conditions, this spectrum represents fairly well the frequency detuning noise of the laser beam to the cavity. The smallest noise density is obtained at frequencies on the order of a few hundred Hz. The shot-noise limit can be evaluated from the average reflected power; it is situated about 15 dB below the lowest noise density. We will see in the next section that the anisotropy signal appears as a disturbance of the laser frequency, hence the better noise suppression, the smaller anisotropy detected. In this section, we describe the experimental tests we have performed in order to evaluate the sensitivity of our apparatus. Since its present configuration aims at measuring frequency differences between the two propagation directions, we mimicked such an anisotropy with a broadband electro-optic modulator (bEOM, not shown on Fig. 1) placed on the clockwise beampath just before the cavity. The bEOM is fed with a sinusoidal signal of amplitude V FM at frequency f FM , giving birth to a phase modulation of amplitude ˜ βV FM at frequency f FM , hence a frequency modulation of amplitude δν FM = ˜ βV FM f FM at the same frequency f FM . Here ˜ β = 0 . 85 mrad/V is the bEOM modulation depth, which has been accurately measured at this wavelength in a separate experiment with a Michelson interferometer. The laser frequency modulation acts as a perturbation and is corrected as such by the servo-loop placed on the cw propagation direction, provided its frequency is within the loop bandwidth. This is definitely the case since the modulation frequency is in the 100-Hz range while the servo-loop bandwidth is in the 100-kHz range. By contrast, the efficient suppression of the frequency modulation on the cw beam creates a discrepancy between the laser and the cavity resonance on the ccw direction, which is detectable by analyzing the ccw error signal at frequency f FM . Using a lock-in amplifier we extract the f FM -spectral component δV on the error signal. We present on Fig. 4 the results of our measurements for different frequency modulation amplitudes and two modulation frequencies f FM of 217 and 276 Hz, situated in the region where the noise spectral density is at its lowest (see Fig. 3). The experimental points fit nicely on a line over more than 6 orders of magnitude. The frequency sensitivity is 500 µ Hz after a measurement time τ ≃ 1000 s, which is equivalent to a birefringence sensitivity δn ≃ $^{2}$× 10 − $^{18}$. Assuming that the noise is white in the region 6 IV. SENSITIVITY MEASUREMENTS