Area expansion for aspherical shape measurement with phase-detection wavelength-scanning interferometry

利用外腔可调谐半导体激光器和PZT反射镜作为光源和相移器，提出了亚奈奎斯特干涉法的概念，研究了相位检测波长扫描干涉法（PWSI）的新的可能性。利用该方法，可以实现半波长以上台阶和镜面非球面等各种工程表面的轮廓测量。该方法成功地抑制了PWSI的误差源，特别是CCD芯片盖板玻璃等光学元件的背反射影响，实验证明，在波长漂移为8nm时，PWSI的精度优于50nm。由于该技术是基于逐像素检测的，因此高度测量也可以在条纹密度的亚奈奎斯特条件下进行，其中条纹间距小于CCD像素的间距。通过该特征，非球面的可测量区域可以扩大两倍以上，并且通过仿真研究表明，可以轻松测量包括台阶和非球面形式在内的复杂形状，例如菲涅耳透镜。该技术现已推广到多孔径综合干涉测量，用于大型光学元件的轮廓评估。本技术也被应用到轮廓术使用相移数字全息术，其中相位斜率由于波长变化被检测到的数字重建的物体表面上，这可能会导致更简化和准确的形状测量技术。

We investigated the new possibilities of the phase-detection wavelength-scanning interferometry (PWSI) with the concept of the sub-Nyquist interferometry, in which an external-cavity tunable laser diode and a PZT-mirror were used as the light source and the phase-shifter. With the present method, the profile measurements of various engineering surfaces including steps higher than the half-wavelength and specular aspheric forms are realized. Accuracy of this method has proved to be better than 50 nm with 8 nm of wavelength-excursion by experiments because the error sources of the PWSI, especially the influence of back-reflections by optics including the cover glass of the CCD chip, was successfully suppressed. Because this technique is based on pixel by pixel detection, the height measurement can also be conducted under the sub-Nyquist condition of fringe density, in which fringes spacing is less than a pitch of CCD pixel. The measurable area for aspherical surfaces can be extended more than twice by this feature, and it was shown that the complex shapes including the steps and aspherical form, such as the Fresnel lens, can be easily measured by the simulation researches. This technique is now extended to the multiple-aperture synthetic interferometry for profile evaluations of large optics. The present technique is also applied to the profilometry using the phase-shifting digital holography in which the phase-slope due to the wavelength-change is detected on the digitally reconstructed object surfaces, which might lead to more simplified and accurate shape measurement techniques.