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 birefrin gence with low frequency signal. The SOM can be used as a polarization modu lator in a heterodyne detection scheme to measure the ellipticity induced o n a linearly polarized laser beam. Its operation makes use of the strain pr oduced on a glass window by two blocked PZTs, thus enabling a careful contr ol of the stress and of the anisotropy induced on the isotropic glass. A se nsitivity 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 aris es from the drift of the spurious birefringences present in the optical sys tem. Through the use of an active feedback system this quasi-static anisotr opy can be compensated, enabling much longer integration times without degr adation of the sensitivity.