Study On The Principle And Application Of Magneto - Optical Modulation Polarimeter

Though a certain crystal, the polarization plane of linearly polarized light is ro-tated around its optical axis, which is called the optically active phenomenon. We callthe instrument as polariscope which is used to measure the rotation angle. Mostly, theprinciple of polariscope is to find the position of light disappeared. This method is de-signed to compensate for the rotation angle of polarization plane, by a stepper motorwhich is connected with the polarizer.Based on the principle of traditional polariscope, we analyze the frequency spec-trum of the polariscope’s output signal and design two different methods to measurethe optical rotation angle in the theory. One method is sure to measure the small anglewhich measure range is limited, and the other method is sure to measure the bigger an-gle which is not a monotonic relationship at the small angle. By simulated the phasedifferences between the output signal and the reference signal, a method is proposed todistinguish the positive angle and the negative angle which contribute to the separationof the enantiomers in the chemical analysis.We introduce a new method to measure Faraday rotation angle by analysis of therelationship among the signal frequency spectrum, Faraday rotation angle and opticalrotation angle. The method is sample, safe and effective using a crystal which has theknown optical rotation angle.We design a system of a magneto-optical modulated polariscope, which uses asolenoid as the modulator and a semiconductor laser as light source. The laser passesthrough a polarizer, a modulator, a material to be measured and an analyzer one by one.A photoelectric converter converts the light signal to electric signal. Passing amplify-ing circuit, the signal is acquired and processed by digital signal processor (DSP) tomeasure the optical rotation angle. We analyze the negative influence caused by powerfrequency noise and discrete Fourier transformation (DFT) leakage. Measuring threestandard optical tubes, the result shows that the measurement accuracy is less than0.05degree and the standard deviation of multiple measurements is less than0.1%. TheFaraday rotation angles under different magneto are measured by changing the circuitswhich produce the magneto. The result that the linear fitting degree of the voltage andFaraday rotation angle is better than99%proves the feasibility of the method.