Research On Key Techniques Of Common-path Dual-wavelength Off-axis Digital Holography

Owning to its high accuracy,non-destruction and full-field,digital holography(DH)is wildly used as a 3D quantitative imaging technique in micro-nano manufacturing,material engineering,biomedical engineering,and so on.The conventional DH usually uses a single wavelength,and while the optical path difference(OPD)induced by the object is longer than the wavelength used in single-wavelength DH(SWDH),a numerical phase unwrapping algorithm should be applied to retrieve the unwrapped phase.However,all single-wavelength unwrapping algorithms suffer from intense computation and fail to handle phase objects with sharp edges,such as steps,grooves.Different from SWDH,dual-wavelength DH(DWDH)can not only retrieve the actual phase easily via simple calculation,but also measure phase objects with sharp edges successfully.Therefore,DWDH attracts more and more attention from the research community.However,most of DWDH is unstable and complicated in optical setup,while the simultaneous achievement of high measurement speed and high accuracy is still a challenging task in DWDH.Those drawbacks limit the development of DWDH.The subject "Research on Key Techniques of Common-path Dual-wavelength Off-axis Digital Holography" is to develop novel methods of common-path DWDH with orthogonal carrier based on divided spectrum common-path interference,which is highly stable and can record the information of dual-wavelength simultaneously with low crosstalk by the techniques of flipping field of view(FOV),polarization modulation and reflective filtering.Then,based on the division of the complex-amplitude,division multiplexing and square-filtering are integrated into DWDH to speed up the phase retrieval.Furthermore,a direct linear programming algorithm(DLPA)by combining the advantages of both the differential syntheticwavelength and the additive synthetic-wavelength for the phase reconstruction of DWDH is proposed to produce the unwrapped phase with lower noise level.Finally,the multiplexed multiplication and division of the complex-amplitude is proposed as a novel theory and method for DWDH with high accuracy,high efficiency and high stability.This study has promising prospect and high practical value in 3D discontinuous topography measurement.The main contents of this research are described as follows:(1)To reduce the redundant calculation in the phase retrieval of DWDH,after analyzing division-based phase retrieval algorithm by using specimen-free hologram in SWDH,a phase retrieval algorithm based on division for DWDH is proposed.The carrier and system aberration could be recorded by a specimen-free hologram,so that a simple division operation could remove the carrier and phase aberration and the operation of carrier and phase aberration removal is avoided.Therefore,the complexity of the phase retrieval algorithm for DWDH is reduced,and this algorithm provides theoretical basis for the following study.(2)To reduce the complexity and optical alignment of the optical setup,benefitting from multiply-window interference technique,a flipping FOV based DWDH with orthogonal carrier is proposed.Two perpendicularly arranged RRs are used to produce orthogonal carrier in two wavelengths,so that one shot can record the information of both wavelengths.Therefore,the grating or point diffraction is unnecessary,while the utilization of spectrum is improved and the high-quality phase retrieval with low crosstalk can be achieved.To relieve the constraint on the shape of object and improve the utilization of the input FOV,common-path DWDH based on reflective filtering is proposed.Reflective filtering is used to generate reference beam from a part of the object beam,which archives full-field imaging by sacrificing some light and provides novel technique for stable and low-crosstalk imaging.(3)After analyzing the information in real images of two wavelengths,a DWDH phase retrieval algorithm based on division-multiplexing is proposed to simplify and speed up the phase retrieval process.The DWDH phase retrieval algorithm based on division-multiplexing can obtain the actual phase directly.Afterwards,the critical sampling is integrated into divisionmultiplexing,which can reduce the data amount and achieve video-rate phase retrieval even for megapixel holograms.To simplify phase retrieval algorithm further,the square-filtering is used to recombine the spectrum components,and a fast DWDH phase retrieval algorithm based on square-filtering is proposed,which requires only once Fourier-based operation on the square of the dual-wavelength off-axis hologram.The fast DWDH phase retrieval algorithm based on square-filtering would sacrifice the quality of the retrieved phase to speed up the phase retrieval further.These three phase retrieval algorithms lay down theoretical foundations for real-time phase reconstruction in DWDH.(4)To solve the noise amplification in DWDH,after analyzing SNR(signal-noise ratio)of 4 measured results,a direct linear programming algorithm(DLPA)for phase unwrapping of DWDH is proposed to reduce the noise in the unwrapped phase,which combines the advantages of both the differential synthetic-wavelength and the additive synthetic-wavelength.The whole process of DLPA is clear and simple.A DWDH straight phase retrieval algorithm based on multiplication and division multiplexing,which can retrieve unwrapped differential phase and wrapped additive phase directly,can be integrated into DLPA.Therefore,the boundary condition in DLPA is reduced into 1D,which can speed up the phase unwrapping.Therefore,this research lays the theoretical foundation for big-scale and high precision 3D topography measurement.