  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 a few tens of microns, this method is still insufficient for practical
applications. In this article, a tunable external cavity laser diode
phase-locked to an optical frequency comb, namely, an optical-comb-referenced
frequency synthesizer, is effectively used for multiple synthetic wavelengths
within the range of 32 um to 1.20 m. A multiple cascade link of the phase
images among an optical wavelength (= 1.520 um) and 5 different synthetic
wavelengths (= 32.39 um, 99.98 um, 400.0 um, 1003 um, and 4021 um) 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
maximum axial range (= 0.60 m) to the axial resolution (= 34 nm), achieves
1.7*10^8, which is much 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.
