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Digital holography (DH) is a promising method for non-contact surfacetopography because the reconstructed phase image can visualize the nanometerunevenness in a sample. However, the axial range of this method is limited tothe range of the optical wavelength due to the phase wrapping ambiguity.Although the use of two different wavelengths of light and the resultingsynthetic wavelength, i.e., synthetic wavelength DH, can expand the axial rangeup to a few tens of microns, this method is still insufficient for practicalapplications. In this article, a tunable external cavity laser diodephase-locked to an optical frequency comb, namely, an optical-comb-referencedfrequency synthesizer, is effectively used for multiple synthetic wavelengthswithin the range of 32 um to 1.20 m. A multiple cascade link of the phaseimages among an optical wavelength (= 1.520 um) and 5 different syntheticwavelengths (= 32.39 um, 99.98 um, 400.0 um, 1003 um, and 4021 um) enables theshape measurement of a reflective millimeter-sized stepped surface with theaxial resolution of 34 nm. The axial dynamic range, defined as the ratio of themaximum axial range (= 0.60 m) to the axial resolution (= 34 nm), achieves1.7*10^8, which is much larger than that of previous synthetic wavelength DH.Such a wide axial dynamic range capability will further expand the applicationfield of DH for large objects with meter dimensions.

Multicascade-linked synthetic wavelength digital holography using an optical-comb-referenced frequency synthesizer