Method Of Dual-Wavelength Digital Holography For Micro-Structured Surface Topography

Digital holography is a novel imaging technique that can quantitatively recoverboth the intensity and the phase information of the object under inspection. Digitalholography has been gaining wide attention thanks to the high-speed update of thecomputer and the image sensor technology. The extensive application ofmicro-structure objects needs high precision, fast non-destructive measurements tomeasure the surface topography information of them. Digital holographicmeasurement can obtain the surface topography of micro structure of objects with nodamage, non-contact, high resolution, fast processing characteristics. By recording thehologram, we can obtain the phase map, which can quickly and accurately reflect thethree-dimensional topography of the surface of the object. These unique advantagesare just right for the topography measurement of microstructure objects.In the numerical reconstruction, the periodic nature of the phase information,causing phase being wrapped, results in a so-called phase imaging ambiguity forobjects with optical path lengths greater than the wavelength. A conventionalapproach to removing the2ambiguity is to apply phase-unwrapping algorithms, butmost of the algorithms require subjective intervention, or take advantage of a priorknowledge of the structure of the object. Furthermore, these phase-unwrappingmethods could not be applied to obtain phase information of the objects which have ahigh aspect ratio. In order to avoid numerical unwrapping, dual-wavelengthphase-imaging digital holography can be used.This paper focuses on the dual-wavelength digital holography. First, the basictheory of dual-wavelength phase unwrapping is discussed. In dual-wavelength phaseunwrapping, two wavelengths are used to produce a longer wavelength called beatwavelength, and subtraction of the two phase images corresponding to twowavelengths can yield a new phase map for the beat wavelength. By properlychoosing closer two wavelengths, the beat wavelength can be free of the2π phasediscontinuities. Second, dual-wavelength digital holography is explored by numericalsimulation. The result demonstrates that dual-wavelength phase unwrapping iseffective and contribute to the fast and efficient3D imaging by digital holography.Third, the experimental setup of a dual-wavelength digital holography is set up. Twolasers of the different wavelengths are used to make phase contrast imaging to twophase-only acrylic plates etched with micro grooves. The results are in goodagreement with the result given by profilometer and confocal microscopy. Itdemonstrates that the3D imaging result by using dual-wavelength digital holography is effective. Dual-wavelength digital holography as a fast and effective3D imagingtechnology can be used for the topography measurement of microstructure objects.