Separating the Siamese twins: using a π-shifted Sagnac interferometer to control the relative weight/influence of circular and linear birefringence on the loop transmission facilitating their measurement.

We demonstrate a novel method to measure circular birefringence (CB) and linear birefringence (LB) present simultaneously in the device under study. By using a π-shifted Sagnac interferometer, the scheme eliminates the dependence on incoming polarization and on the orientation angle of the linear birefringence. Moreover, due to different handedness symmetry/response of CB and LB to counter-propagating waves, the technique allows us to control the relative influence of the two birefringences leading to a requirement of only two measurements to determine both of them. Thus, comparing to Stokes polarimeters and other methods, our scheme has advantages when characterizing media containing both birefringences. Our findings are experimentally confirmed.