  Optical Fabry-Perot cavities always show a non-degeneracy of two orthogonal
polarisation states. This is due to the unavoidable birefringence of dielectric
mirrors whose effects are extremely important in Fabry-Perot based
high-accuracy polarimeters. We have developed and present here a theory of the
polarisation state dynamics in a birefringent Fabry-Perot resonator, and we
validate it through measurements performed with the polarimeter of the PVLAS
experiment. The measurements are performed while a laser is frequency-locked to
the cavity, and provide values for the finesse of the cavity and for the phase
difference between the two orthogonal polarisation components introduced by the
combination of the two cavity mirrors (equivalent wave-plate). The theoretical
formulas and the experimental data agree well showing that the consequences of
the mirror birefringence must be taken into account in this and in any other
similar experiment.
