临界角法检焦系统的设计

Design of focus error detection system based on critical angle method

利用临界角法检焦具有分辨力高、损失光能小、结构简单、系统调试容易的特点设计了亚微米级检焦系统。介绍了临界角法离焦检测的基本原理,通过合理假设,利用菲涅尔公式和高斯光学公式得到了离焦误差信号的计算公式。实验采用单光路临界角法,利用He-Ne激光器、临界角棱镜、四象限光电探测器、信号采样电路、数据采集卡等元器件组成离焦检测系统,实现离焦信号的提取;通过数字滤波、归一化处理等技术得到离焦误差信号(FES),以此获得FES的大小和变化趋势与离焦量的关系曲线。实验表明,临界角法探焦系统静态分辨力<15nm、线性范围可达±4μm,满足亚微米级检焦系统的设计要求。

A focus error detection system in sub-micrometer grade is designed based on critical angle method with high resolution and low light losses. The principle of defocus detection by critical angle method is introduced and the expression of defocus is deduced with Gauss optical formula based on Fresnel formula and a reasonable hypothesis. The detection system consists of He-Ne laser, critical angle prism, four element photodetector, signal sampling circuit and data acquisition card, etc. , the error of defocus is detected by single light path critical angle method. Moreover,a digital filter and the normalization technology are used to extract the Focus Error Signal（FES） to obtain the defocus curve. Experimental results indicate that the focus resolution is less than 15 nm and static linearity range is ±4 μm, which satisfies the focus error detection system requirements in sub-micrometer grade and provides references for engineering application of focus error detection system with critical-angle method.