Improving the accuracy of optical rotation measurement based on optical null methods by curve-fitting

An optical rotation measuring system using optical null methods is set up. The system is based on a step-motor rotating stage structure and a modulated laser, which both are controlled by digital signal processor. By introducing a curve-fitting method, the optical rotation angle is obtained from the step difference between the crossed positions in light signal curves with or without sample. The principle of this method is studied, and the system performance is investigated experimentally. We achieved an accuracy of the order of 10(-3) degree in at least ±45° range, which is an improvement compared with the resolution of 0.01° for step-motor rotation stage.