  Digital holography (DH) is a powerful tool for surface profilometry of
objects with sub-wavelength precision. In this article, we demonstrate
full-cascade-linked synthetic-wavelength DH (FCL-SW-DH) for nanometer-precision
surface metrology of millimeter-size 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 (EOM-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 um to 29.7 mm. We determine the
0.1000mm-stepped surface with axial uncertainty of 6.1 nm within the maximum
axial range of 14.85 mm.
