Wide axial dynamic range digital holography using multicascade-linked synthetic wavelengths and optical wavelength

Digital holography (DH) is a technique to reconstruct the amplitude and phase images of a sample by calculating the wavefront propagation from the interference image. Although DH enables three-dimensional shape measurement based on the phase images, axial dynamic range of a single-optical-wavelength DH is limited to less than a full or half optical wavelength due to phase wrapping ambiguity. To extend the axial range over the optical wavelength, synthesized wavelength DH has been proposed. In this method, DH is performed at two different wavelengths, and then synthesized wavelengths between them are used. However, use of a single longer synthesized wavelength degrades the axial resolution because the axial dynamic range is limited by the phase noise. To extend the axial dynamic range, one has to increase the axial range while maintaining the axial resolution of sub-wavelength. One promising approach to do it is cascade linking between multiple synthetic wavelengths with different orders. In this paper, we present multicascadelinked synthetic wavelength DH using an optical-comb-referenced frequency synthesizer (OFS). OFS is a tunable external cavity laser diode phase-locked to an optical frequency comb, and 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 and 5 different synthetic wavelengths enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm.