Differential internal multi-reflection method for nano-displacement measurement

Abstract

By utilizing the internal-reflection effect at air–glass boundary, we have proposed a novel method of nano-displacement measurement. Excellent linearity of the reflectance versus the angle of incidence in internal reflection is achieved by making use of a differential detection scheme. Thereby, the target's displacement which causes the change of incident angle can be determined accurately by measuring reflectance. Using a differential detector such as quadrant detector, a compact system based on the above method has been developed, with advantages of simple configuration and low costs compared with interferometers and traditional confocal methods (CMs). The resolution of this system is about 2nm in a wide measurement range. Factors such as the initial angle of incidence, the number of reflections, and the polarization state of the light determine the resolution and measurement range. It is very suitable for many potential applications, especially in the automatic focus of the state-of-art microscopy, in which the traditional problem of focus drift may be solved. In addition, theoretical analysis and experimental results of a prototype sensor are presented.