Yamagiwa Masatomo, Minamikawa Takeo, Trovato Clément, Ogawa Takayuki, Ibrahim Dahi Ghareab Abdelsalam, Kawahito Yusuke, Oe Ryo, Shibuya Kyuki, Mizuno Takahiko, Abraham Emmanuel, Mizutani Yasuhiro, Iwata Tetsuo, Yamamoto Hirotsugu, Minoshima Kaoru, Yasui Takeshi
Opt Express. 2018 Oct 1;26(20):26292-26306. doi: 10.1364/OE.26.026292.
Digital holography (DH) is a promising method for non-contact surface topography because the reconstructed phase image can visualize the nanometer unevenness in a sample. However, the axial range of this method is limited to the range of the optical wavelength due to the phase wrapping ambiguity. Although the use of two different wavelengths of light and the resulting synthetic wavelength, i.e., synthetic wavelength DH, can expand the axial range up to several hundreds of millimeters, its axial precision does not reach sub-micrometer. In this article, we constructed a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an optical-comb-referenced frequency synthesizer, enabling us to generate multiple synthetic wavelengths within the range of 32 µm to 1.20 m. A multiple cascade link of the phase images among an optical wavelength ( = 1.520 µm) and 5 different synthetic wavelengths ( = 32.39 µm, 99.98 µm, 400.0 µm, 1003 µm, and 4021 µm) enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the axial range ( = 2.0 mm) to the axial resolution ( = 34 nm), achieves 5.9 × 10, which is larger than that of previous synthetic wavelength DH. Such a wide axial dynamic range capability will further expand the application field of DH for large objects with meter dimensions.
数字全息术（DH）是一种用于非接触表面形貌测量的很有前景的方法，因为重建的相位图像可以可视化样品中的纳米级不均匀性。然而，由于相位包裹模糊性，该方法的轴向范围限于光波长范围。尽管使用两种不同波长的光以及由此产生的合成波长，即合成波长数字全息术，可以将轴向范围扩展到数百毫米，但它的轴向精度达不到亚微米级。在本文中，我们构建了一个锁相于光学频率梳的可调谐外腔激光二极管，即光学频率梳参考频率合成器，使我们能够在32微米至1.20米的范围内产生多个合成波长。在一个光学波长（λ = 1.520微米）和5个不同的合成波长（λ = 32.39微米、99.98微米、400.0微米、1003微米和4021微米）之间的相位图像的多重级联链路，能够以34纳米的轴向分辨率对毫米尺寸的反射阶梯表面进行形状测量。轴向动态范围定义为轴向范围（L = 2.0毫米）与轴向分辨率（δ = 34纳米）之比，达到5.9×10⁴，这比以前的合成波长数字全息术的轴向动态范围更大。如此宽的轴向动态范围能力将进一步扩大数字全息术在米级大型物体上的应用领域。
