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

摘要

Digital holography (DH) is a technique to reconstruct the amplitude and phase images of a sample by calculating thewavefront propagation from the interference image. Although DH enables three-dimensional shape measurement basedon the phase images, axial dynamic range of a single-optical-wavelength DH is limited to less than a full or half opticalwavelength due to phase wrapping ambiguity. To extend the axial range over the optical wavelength, synthesizedwavelength DH has been proposed. In this method, DH is performed at two different wavelengths, and then synthesizedwavelengths between them are used. However, use of a single longer synthesized wavelength degrades the axialresolution because the axial dynamic range is limited by the phase noise. To extend the axial dynamic range, one has toincrease the axial range while maintaining the axial resolution of sub-wavelength. One promising approach to do it iscascade linking between multiple synthetic wavelengths with different orders. In this paper, we present multicascadelinkedsynthetic wavelength DH using an optical-comb-referenced frequency synthesizer (OFS). OFS is a tunableexternal cavity laser diode phase-locked to an optical frequency comb, and is effectively used for multiple syntheticwavelengths within the range of 32 um to 1.20 m. A multiple cascade link of the phase images among an opticalwavelength and 5 different synthetic wavelengths enables the shape measurement of a reflective millimeter-sizedstepped surface with the axial resolution of 34 nm.