HIGH-PRECISION BALANCED MODE POLARIMETRY WITH A PULSED-LASER BEAM

We apply balanced mode polarimetry to study an amplifying atomic medium endowed with optical rotation and linear dichroism. The medium is prepared by pulsed excitation with a pump laser just before application of a collinear probe laser pulse. We discuss how displacements of the probe beam and small modifications of the angle of incidence, i.e. geometrical effects, can affect the measurements. We demonstrate double subtraction of such instrumental defects. First, we measure the difference between the two imbalances of the polarimeter corresponding to the amplified probe pulse and a consecutive unamplified reference pulse. Second, we insert and remove a half wave plate just in front of the polarimeter: this enables us to discriminate true polarisation effects from geometrical effects. Under such conditions the zero of the rotation of the plane of polarisation induced by the amplifying atomic medium acquires a real physical meaning with an accuracy better than 10(-6) rad. This result has important implications for future atomic parity violation measurements. Finally we consider near extinction polarimetry from the point of view of geometrical defects and suggest an adaptation of half wave plate switching as a possible way of improving measurements in the near extinction operation.