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

利用亚奈奎斯特干涉术的概念，利用外腔可调谐激光二极管和PZT反射镜作为光源和移相器，研究了相位检测波长扫描干涉术的新可能性。利用该方法，实现了阶跃高于半波长和镜面非球面的各种工程表面的轮廓度测量。实验证明，该方法在波长漂移8 nm的情况下，测量精度优于50 nm，因为成功地抑制了PWSI的误差源，特别是包括CCD芯片盖玻璃在内的光学元件的背向反射的影响。由于该技术是基于逐个像素检测的，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，其中条纹间距小于一个CCD像素的节距。利用这一特性可以将非球面的测量面积扩大两倍以上，仿真研究表明，菲涅耳透镜等复杂形状的非球面可以很容易地测量出阶梯和非球面形状。这项技术现已扩展到用于大型光学系统轮廓评估的多孔径合成干涉术。该技术也被应用于相移数字全息术的轮廓术中，在数字再现的物体表面上检测由于波长变化引起的相位斜率，这可能导致更简单和准确的面形测量技术。

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.