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

利用亚奈奎斯特干涉的概念，研究了用外腔可调谐激光二极管和pzt反射镜作为光源和移相器的相位检测波长扫描干涉（PWSI）的新可能性。利用该方法，实现了半波长以上台阶和镜面非球面等多种工程表面的轮廓测量。实验证明，该方法成功地抑制了PWSI的误差源，特别是CCD芯片覆盖玻璃等光学器件的背反射影响，精度优于50 nm，波长偏移为8 nm。由于该技术是基于逐像素检测，因此也可以在条纹密度的亚奈奎斯特条件下进行高度测量，其中条纹间距小于CCD像元的一个间距。利用这一特性可以将非球面的测量面积扩大一倍以上，并通过仿真研究表明，包括台阶和非球面在内的复杂形状，如菲涅耳透镜，可以很容易地测量。该技术现已扩展到多孔径合成干涉测量中，用于大口径光学系统的轮廓评估。该技术还可应用于相移数字全息的轮廓测量，在数字重建的物体表面上检测波长变化引起的相斜率，从而使形状测量技术更加简化和精确。

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.