  Although experimental efforts have been active for about 30 years now, a
direct laboratory observation of vacuum magnetic birefringence, an effect due
to vacuum fluctuations, still needs confirmation. Indeed, the predicted
birefringence of vacuum is $\Delta n = 4.0\times 10^{-24}$ @ 1~T. One of the
key ingredients when designing a polarimeter capable of detecting such a small
birefringence is a long optical path length within the magnetic field and a
time dependent effect. To lengthen the optical path within the magnetic field a
Fabry-Perot optical cavity is generally used with a finesse ranging from ${\cal
F} \approx 10^4$ to ${\cal F} \approx7\times 10^5$. Interestingly, there is a
difficulty in reaching the predicted shot noise limit of such polarimeters. We
have measured the ellipticity and rotation noises along with a Cotton-Mouton
and a Faraday effect as a function of the finesse of the cavity of the PVLAS
polarimeter. The observations are consistent with the idea that the cavity
mirrors generate a birefringence-dominated noise whose ellipticity is amplified
by 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 we
believe is consistent with an intrinsic thermal noise in the mirror coatings.
