High sensitive measurement of optical rotation based on photo-elastic modulation

Abstract

In order to realize the continuous and stable, high speed, high precise and high sensitive measurement of optical rotation, and considering the application advantages of photo-elastic polarization modulation technology with high modulation frequency, high modulation purity, high modulation accuracy and good modulation stability, a new scheme about the measurement of optical rotation based on photo-elastic modulation is presented. Probe laser orderly passes through a polarizer, the rotation sample to be measured, a photo-elastic modulator, and a analyzer, and finally reaches the detector, this system uses less optical devices than any others previously reported, so it considerably reduces the measurement error that may be introduced by the optical devices. In the detecting of light path, the polarization axes of the polarizer and analyzer are respectively adjusted with respect to the photo-elastic modulator's fast axis directions 0° and 45°, the optical arrangements make the rotation angle to be measured appear in the alternating current signal, and the photo-elastic modulator's residual birefringence only appears in the odd harmonics. Consequently, the second harmonic signal of photo-elastic modulation is selected as the object to study, which effectively avoids the influence of residual birefringence of the photo-elastic modulator on optical rotation measurement, and efficiently improves the accuracy of optical rotation measurement. What is more, the detector output signal is separated into two parts, the direct current and alternating current signal. The alternating current signal is amplified, then outputs by a lock-in amplifier, which enhances the measurement sensitivity further. A ingenious verification test experiment is done, firstly, the probe laser is modulated into circularly polarized light, and then precisely rotates the polarizer to replace the optical rotation sample. The results show that the new scheme is feasible, this experiment gives the proportion coefficient of the measurement system, the sensitivity of optical rotation measurement increasing up to 3.15× 10-7 rad, and the measurement precision exceeding 0.3%. Therefore, in this scheme achieved is a high sensitive and precise measurement of optical rotation, and it is expected to be applied to the high sensitive and precise rotation measurement. The verification test experiment designed by us can also provide a outstanding calibration reference for high sensitive rotation measurement system.