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Although experimental efforts have been active for about 30 years now, adirect laboratory observation of vacuum magnetic birefringence, an effect dueto vacuum fluctuations, still needs confirmation. Indeed, the predictedbirefringence of vacuum is $\Delta n = 4.0\times 10^{-24}$ @ 1~T. One of thekey ingredients when designing a polarimeter capable of detecting such a smallbirefringence is a long optical path length within the magnetic field and atime dependent effect. To lengthen the optical path within the magnetic field aFabry-Perot optical cavity is generally used with a finesse ranging from ${\calF} \approx 10^4$ to ${\cal F} \approx7\times 10^5$. Interestingly, there is adifficulty in reaching the predicted shot noise limit of such polarimeters. Wehave measured the ellipticity and rotation noises along with a Cotton-Moutonand a Faraday effect as a function of the finesse of the cavity of the PVLASpolarimeter. The observations are consistent with the idea that the cavitymirrors generate a birefringence-dominated noise whose ellipticity is amplifiedby the cavity itself. The optical path difference sensitivity at $10\;$Hz is$S_{\Delta{\cal D}}=6\times 10^{-19}\;$m$/\sqrt{\rm Hz}$, a value which webelieve is consistent with an intrinsic thermal noise in the mirror coatings.

Intrinsic mirror noise in Fabry-Perot based polarimeters: the case for the measurement of vacuum magnetic birefringence