Study on Dual-wavelength Digital Holography and Experimental System Optimization
|School||Beijing University of Technology|
|Keywords||digital holography dual-wavelength phase unwrapping phase-contrastimaging 3D imaging|
Digital holography is the technology of the combination of the traditional opticalholography, computer technology and photoelectric detection technology withadvantages of fast, real-time, full field, non-contact and quantitative phase-contrastimaging. Digital holography has been gaining wide attention because of theseadvantages. Digital holographic measurement becomes a hot research area, and hasbroad application prospects in various fields such as the topography measurement ofmicrostructure objects, biomedical imaging detection and diagnosis.In the numerical reconstruction, the periodic nature of the phase informationmay cause phase being wrapped for objects with optical path lengths greater than thewavelength, and results in a so-called phase imaging ambiguity. A conventionalapproach to removing the2ambiguity is with numerical phase-unwrappingalgorithms. However, the phase-unwrapping algorithms could not be applied to obtainphase information of the objects if they have a high aspect ratio. In order to avoidnumerical unwrapping, dual-wavelength phase-imaging digital holography can beused.This paper focuses on the dual-wavelength digital holography used in thetopography measurement of microstructure objects and phase imaging. Firstly thebasic theory of dual-wavelength phase unwrapping is discussed. Two wavelengths areused to produce a longer wavelength called beat wavelength in dual-wavelength phaseunwrapping, and subtraction of the two phase images corresponding to twowavelengths can yield a new phase map for the beat wavelength. And the beatwavelength can be free of the2π phase discontinuities by choosing closer twowavelengths properly. Secondly dual-wavelength digital holography is explored bynumerical simulation. Comparing numerical unwrapping method and dual-wavelengthphase unwrapping, the result demonstrates that dual-wavelength digital holographycontributes to the fast and efficient3D imaging. Thirdly, the experimental setup of adual-wavelength digital holography is built. Two lasers of the different wavelengthsare used to make phase contrast imaging to a phase-type grating recorded in aFe:Cu:LiNbO3crystal. According to its reconstruced image, the grating constant andthe refractive index modulation are obtained. Then optical setup of dual-wavelengthdigital holography is redesigned, and is used to make phase contrast imagings for one phase-only acrylic plates etched with micro grooves and the phase-type grating in aFe:Cu:LiNbO3crystal. The experimental results demonstrate dual-wavelength digitalholography is effective and practicable for phase imaging in the topographymeasurement of microstructure objects and internal phase structure.