System Of Extremely Small Optical Rotation Angles Detection

Detection of extremely small optical rotation angles is wildly used, such as Verdetconstant measurement, detection of liquid concentration, high-voltage current measurement,CPT and sensitive nuclear magnetometer. Some methods are used for detecting the opticalrotation of the probe beam. Faraday modulator is a good candidate for the most sensitivemagnetometers. We designed a measuring system, including a Faraday modulator and aphoto-detector, to detect a very small optical rotation. The total measuring system consists ofa laser, a Faraday modulator, a photo-detector, and a lock-in amplifier. To test the sensitivityof the system, we built another system to generate the small rotation angle.There are five chapters in this paper. In the first chapter we mainly introduce theapplication background of measurement of plane polarization light rotation. In the secondchapter we introduce the detection method based on Faraday modulator and opticalpolarimeter, and show the difference between them. The third chapter presents the relatedcontent about system building. In the fourth chapter we introduce the test of the system of thesmall optical rotation angles detection. The last part shows the conclusions of our work andthe image of the improvement of the system.We carried out the experiment without temperature control and magnetic shield. Somefactors which influence the detection system have been analyzed experimentally to find thebest working setup of the system. The rotation-generating system can drive probe beampolarization by2°due to the Faraday effect under1.5kHz modulation frequency. Recentlywe can detect an angle of magnitude at10-6rad and the SNR of the output of the lock-inamplifier is above728.1Hz1/2. The sensitivity of the angle detection is better than110-8rad/Hz1/2. To make sure the Faraday modulator work stability, we monitor thefrequency and amplitude of the modulating signal by observing the second harmonic signalreceived by the photo-detector.