Research On Imaging Method Of Continuous-Wave Terahertz Off-Axis Digital Holography

The frequency range of the terahertz(THz)radiation spans 0.1-10 THz,and the corresponding wavelength is 30 μm-3 mm.THz radiation are non-ionizing radiation with unique characteristics of microwave and infrared radiation.THz imaging and spectroscopy have been widely used in non-destructive testing,safety scanning,biomedical imaging and etc.Digital holography is a promising interferometric imaging method that can obtain the morphology and refractive index distribution of two-dimensional thin samples without damage and marking.The unique penetrability and non-ionization of THz radiation are combined with digital holography to form THz digital holography(TDH)for full-field phase imaging.The digital hologram is recorded by THz array detector,and then the quantitative amplitude and phase information of the measured object are reconstructed by numerical calculating of the diffraction propagation.The morphology and refractive index of objects can be extracted from the phase image.This thesis is about the continuous-wave(CW)off-axis TDH imaging method,the main research works of this thises are as follows:1.The theory of CW TDH is discussed in detail.The principle of the wavefront recording geometries and the reconstruction method of the off-axis Fresnel THz digital hologram are studied.Three known numerical propagation algorithms include the Fresnel algorithm,the convolution algorithm and the angular spectrum algorithm.For the lensless Fourier transform TDH,two reconstruction algorithms,i.e.,the single inverse Fourier transform algorithm and the angular spectrum algorithm,are analyzed.And the tilted-plane correction method is presented to correct the anamorphism induced by non-parallel between the object and the hologram planes.2.We proposed a common path off-axis TDH method.This phase imaging approach is based on the specially designed Fresnel’s mirrors.The wavefront downstream from the object is split by two independently tilted mirrors.An off-axis hologram is recorded by interference between the partitioned object.The contrast of the interference fringes is ～ 0.75.The quantitative full-field phase contrast imaging of the THz test target,the cicada wings and the polythene plastic plate are obtained,The experimental resolution is 325 μm..Furthermore,the influence on the reference beam and the reconstructed results is investigated by simulation,including the position of the sample in the illumination scope,and the high-frequency component diffracted by the object.The results show that placing the object in half of the illumination scope is conducive to obtain the reconstruction results with pure background.In addition,a CW TDH transmission imaging system based on the Mach-Zehnder interferometer was built with our system,and the time stability of our imaging system is improved by three times.3.We proposed a CW THz lensless Fourier transform digital holography(TLFTDH)imaging method based on the reflective focusing point source.A spherical reference beam is produced by an off-axis parabolic mirror with 60° off-axis angle.The object and the point source are kept in the same plane and equidistant from the recording plane.A single inverse Fourier transform of the digital hologram is adopt to diffraction propagation calculation of the digital hologram.The experimental resolution is 346.2μm.The dehydration process of an aquatic plant leaf is monitored for the first time at the THz band.Rapid variations in water content and the morphology are measured with a time interval of 0.6 s and a total time of 5 min from a series of reconstructed amplitude and phase images.4.The reconstructed image is improved based on the hologram stitching method.The synthetic aperture is demonstrated to achieve the enhancement of resolution from391.5 μm to 290 μm.The Sub-pixel image registration and image stitching algorithms are presented to expand the field-of view(FOV)by 2.4 times.Furthermore,a divergent spherical wave as the reference beam is formed through a concave mirror,which is coplanar with the sample center.And a reflective TLF-TDH imaging system is designed and build.Because the object plane is not parallel to the recording plane,the error is eliminated by a tilted plane correction based on coordinate transformation,and the actual morphology distribution of metal coin is reconstructed.Finally,the research works of this thesis are summarized.A common path TDH apporoach is proposed,which improves the time stability of the off-axis Fresnel digital holographic imaging system.A TLF-TDH imaging approach with reflective focusing point source is proposed and applied for both transmissive and reflective modes.By using this approach,the change of morphology and water content of aquatic plant leaves is quantitatively monitored during its rapid dehydration with sub-second time resolution;and the surface morphology of metal products is obtained.The synthetic aperture method of hologram,the sub-pixel image registration and image stitching of reconstructed image are proposed.The lateral resolution of TLF-TDH is improved by35% and the FOV of reconstructed image is expanded by 2.4 times.Finally,the possible research work in the future is briefly prospected.