基于弹光调制的微小旋光度测量系统设计与实现

Design and Implementation of The System for Small Optical Rotation Measurement Using the Photoelastic Modulation

针对高精度微小旋光度测量需求,本文提出一种基于弹光调制器的高分辨率微小旋光度测量方法及实验验证方案。该方法原理是当起偏器、检偏器相对弹光调制器分别45°和0°放置时,待测微小旋光度与弹光调制信号的二次谐波近似成线性关系。为验证该方法的实验效果,本文采用双光电管的平衡探测方法设计了一个低噪声前置光电转电路,并利用锁相放大技术从弹光调制信号中提取谐波分量,在上述基础上实现了高分辨率旋光度测量。实验结果表明,基于弹光调制器的新型旋光度测量方法可实现高达10-6rad的高分辨率旋光度测量;并且本文设计的双光电管平衡探测电路可明显提高信号的信噪比,而基于锁相放大技术提取弹光调制信号谐波分量的方案也有助于实现高分辨率旋光度检测。

For satisfying the demand of high precision small optical rotation measurement, an new method is proposed and experimental researched, which is using the photoelastic modulator（PEM）. Its measurement mechanism is that when the polarizer is set at 45° to PEM and analyzer is set at 0° to PEM, the magnitude of the optical rotation is linearly dependent with the PEM＇s second harmonic. For verifying the effect of this mechanism, a low noise photoelectric conversion circuit is designed that using dual photodiode, and the second harmonic is detected by a lock-in amplifier. Finally, a high resolution rotation measurement is experimentally realized. Our experimental results show that the a high precision small optical rotation measurement that reaching up to 10^-6 rad based on the PEM is possible. Besides, the dual photodiode-based photoelectric conversion circuit design could improve the signal-to-noise ratio. And the lock-in amplifier circuit used for processing the harmonic signal of PEM also contributes to high resolution small optical rotation measurement.