Stress-optic modulator: a novel device for high sensitivity linear birefringence measurements

In this paper we present a device, called the stress-optic modulator (SOM), that allows us to perform high sensitivity measurements of linear birefringence with low frequency signal. The SOM can be used as a polarization modulator in a heterodyne detection scheme to measure the ellipticity induced on a linearly polarized laser beam. Its operation makes use of the strain produced on a glass window by two blocked PZTs, thus enabling a careful control of the stress and of the anisotropy induced on the isotropic glass. A sensitivity of 3 x 10(-8) Hz(-1/2) for an ellipticity signal at the frequency of 1 Hz is obtained. For long term operation of the device a drawback arises from the drift of the spurious birefringences present in the optical system. Through the use of an active feedback system this quasi-static anisotropy can be compensated, enabling much longer integration times without degradation of the sensitivity.