| Measurement of plane polarization light rotation was developed in recent years. It was used to research the optical properties of the material by measuring the rotation of the plane polarized light when it pass the material. Presently, this technology is widely used in many fields, such as non-destructive measurement, the detection of solution concentration and the measurement of some optical quantity. In areas of general application, this technology has become mature gradually. However, for some research areas with a requirement of higher precision, the current level of technology is also far from the requirements. Therefore, the incessant improvement of the accuracy of this technique is significant for these areas.An experimental system of measuring the small rotation of the plane polarized light with double modulation was designed on the basic of a great deal of surveys. The experiment was improved a little on the basic of the classical experiment with the method of double modulation, i.e. the light path and the signal are both modulated. With this method the small rotation of the plane polarized light was accurate detected. This design not only can increase the measured signal effectively but also can eliminate the effect introduced by the misalignment angle between polarizer and analyzer. This design also can reduce the effect of background light. The sensitivity of the system was in the level of 4. 15×10-8rad. This paper describes the experiment detailed. It includes the applications of this technique, the design of the experiment, the construction and test of the system and the noise analysis. Finally, the improvement of the design and the estimation of the resolution of the final experiment system are put forward, and the sensitivity of the final system should be about 2. 81×10-10rad.
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