Nanometer-precision surface metrology of millimeter-sized stepped objects using full-cascade-linked synthetic-wavelength digital holography using a line-by-line full-mode-extracted optical frequency comb

Digital holography (DH) is a powerful tool for the surface profilometry of objects with sub-wavelength precision. In this article, we demonstrate full-cascade-linked synthetic-wavelength DH for nanometer-precision surface metrology of millimeter-sized stepped objects. 300 modes of optical frequency comb (OFC) with different wavelengths are sequentially extracted at a step of mode spacing from a 10GHz-spacing, 3.72THz-spanning electro-optic modulator OFC. The resulting 299 synthetic wavelengths and a single optical wavelength are used to generate a fine-step wide-range cascade link covering within a wavelength range of 1.54 mu m to 29.7 mm. We determine the sub-millimeter and millimeter step differences with axial uncertainty of 6.1 nm within the maximum axial range of 14.85 mm. (c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement