Measuring the Phase Velocity of Light in a Magnetic Field with the PVLAS   Detector
  A method is suggested for measuring the velocity of light in a magnetic field using the PVLAS detector. It is proposed to modulate the linear polarization of the input light. In that case the feedback signal necessary to keep the high finesse Fabry-Perot cavity on resonance is a measure of the change in the speed of light for the two orthogonal polarizations. 
A.C.Melissinos