Continuous-wave Terahertz Reflective Off-axis Digital Holography

The terahertz(THz)band,the frequencies range of 0.1-10 THz corresponding wavelengths of 30 ?m-3 mm,lies between the microwave and the infrared regions.It has unique transmission and imaging characteristics such as penetration,non-destructive and non-ionization.THz imaging plays an increasingly important role in biomedical imaging,security,non-destructive testing and other fields.As a combination of digital holography and THz imaging,continuous-wave(CW)THz digital holography records the interference of the object beam and the reference beam,and reconstructs the complex amplitude information of object wavefront,It is an important development direction of THz imaging.The paper studies the reflective CW THz digital holographic imaging.The main contents are as follows:The research on speckle noise suppression of CW THz digital holography is studied in chapter 3.Firstly,the causes of THz speckle noise and its characteristics are introduced,and the principle of multi-polarization and micro-displacement of the object to suppress the speckle noise are analyzed.An experimental system for suppressing speckle noise of CW THz digital holography by linearly polarized THz wave in different polarization directions is built.The polarization characteristics of THz source and optical element are studied,and the feasibility is verified by experiments.Finally,a method of reducing the speckle noise of THz digital holography based on slightly moving the object is studied,multiple holograms with different speckle patterns are obtained,and the speckle noise is suppressed by average superposition of reconstructed amplitude images.The imaging of hidden objects based on reflective CW THz digital holography with the large field of view is studied in chapter 4.The principle of expanding the field-of-view by subpixel image registration and image fusion algorithm are explained firstly.An imaging system based on reflective CW THz off-axis digital holography is built,and the object is hidden behind the polytetrafluoroethylene plate.The experimental results demonstrate that the THz digital holography has the ability to acquire the amplitude,phase and morphological information of the hidden object.At the same time,the influence of the thickness and refractive index of the covered materials for the imaging quality are compared.In addition,the multi-layer imaging of hidden object based on CW THz digital holography is realized,and the information of the bottom measured object and the surface covered plate are obtained simultaneously.Finally,multiple holograms are recorded by moving the object,then the field-of-view is expanded by sub-pixel image registration and image fusion algorithm.