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Automated determination of best focus and minimization of optical path difference in Linnik white light interferometry

机译：在Linnik白光干涉仪中自动确定最佳聚焦并最小化光程差

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摘要

It is difficult 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 the defocus of the reference mirror and the test surface. We present an automated method to tackle this problem in this paper. 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 embedded in the interference system. The astigmatic method is improved by setting a threshold value in the sum signal to truncate the normalized focus error signal (NFES). The truncated NFES has a monotonic relationship with the displacement of the test surface, which removes the position ambiguity of the test surface during the autofocus process. The developed autofocus system is confirmed experimentally with a dynamic range of 190 (mu)m, average sensitivity of 70 mV/(mu)m, average standard deviation of 0.041 (mu)m, displayed resolution of 4.4 nm, and accuracy of 55 nm. The minimization of the optical path difference 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. The RMSFC function, combined with a 4 X 4 pixel binning of the CCD camera, is recommended to improve the computational efficiency. Experimental tests show that the automated method can be effectively utilized to search for interference fringes in Linnik white light interferometry.
机译：由于两个干涉臂的光路不匹配以及参考镜和测试表面的散焦，很难在具有相干长度非常短的Linnik白光干涉术中搜索干涉条纹。在本文中，我们提出了一种自动方法来解决此问题。参考像镜和测试表面的最佳焦点的确定是通过像散方法实现的，该方法基于嵌入在干涉系统中的改进的商用DVD拾取头。通过在和信号中设置阈值以截断归一化聚焦误差信号（NFES），可以改善散光方法。截短的NFES与测试表面的位移具有单调关系，从而消除了自动聚焦过程中测试表面的位置歧义。实验证实了开发的自动对焦系统的动态范围为190μm，平均灵敏度为70 mV /μm，平均标准偏差为0.041μm，显示分辨率为4.4 nm，精度为55 nm 。通过找到具有均方根条纹对比度（RMSFC）函数的CCD摄像机捕获的图像的最大条纹对比度，可以使两个干涉臂的光程差最小。建议将RMSFC功能与CCD摄像机的4 X 4像素合并相结合，以提高计算效率。实验测试表明，该自动方法可以有效地用于搜索Linnik白光干涉仪中的干涉条纹。

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