Linnik白光干涉仪自动对焦及光程差最小化

众所周知由于两个干涉臂光路不匹配和参考镜面与被测表面的离焦,要得到相干长度非常短的Linnik白光干涉仪的干涉条纹是非常困难的.本文提出了一种自动调节的方法来解决这个问题.为了实现参考镜面和被测表面的对焦,在商用DVD读取头的基础上,对其像散法进行改进,具体方法是对和信号SS设定一个阈值,通过此阈值对归一化后的FES曲线(NFES)进行裁剪,从而获得一个与离焦距离成单调关系的曲线(TNFES),其过零点对应的就是焦点.经过实验证明,改进后的自动对焦系统的动态范围为190 μm,平均灵敏度70 mV/μm,平均标准偏差0.041 μm,分辨率4.4 nm,不确定度55 nm.此外为了最小化两个干涉臂的光程差,本文采用均方根RMSFC算法来计算成像在CCD上的干涉条纹的对比度,通过找到其最大值来最小化光程差.实验证明本文提出的自动方法可以有效地获得Linnik白光干涉仪的干涉条纹.

Automated Method of Focusing and Minimizing OPD in Linnik White Light Interferometry

It is difficult, as we know, to search for interference fringes in Linnik white light interferometry with an extremely short coherence length because of the optical path mismatch of two interference arms and out of focus of the reference mirror and the test surface. An automated method to tackle this problem is presented. The determination of best foci of the reference mirror and the test surface is implemented by the astigmatic method based on a modified commercial DVD pickup head. The astigmatic method is improved by setting a threshold value in the Sum Signal (SS) to truncate the Normalized Focus Error Signal (NFES). The Truncated NFES (TNFES) has a monotonic relationship with the displacement of the test surface and the zero crossing point identifies true focus. The developed autofocus system is confirmed experimentally with a dynamic range of 190 μm, average sensitivity of 70 mV/μm, average standard deviation of 0.041 μm, displayed resolution of 4.4 nm and accuracy of 55 nm. The minimization of Optical Path Difference (OPD) of two interference arms is carried out by finding the maximum fringe contrast of the image captured by a CCD camera with the Root Mean Square Fringe Contrast (RMSFC) function. Experimental tests show that the automated method can be effectively utilized to search for interference fringes in Linnik white light interferometry.