Generation Of Terahertz Diffraction-free Beam With Long Distance And Its Application In Imaging

Terahertz waves behold several unique properties,such as high permeability for non-metallic materials and low energy,which make them suitable for imaging and non-destructive testing.While traditional imaging system formed by the Gaussian beam does the imaging unevenly,the imaging system will have a much more longer focal depth by introducing the non-diffracting beam into it.Within the non-diffracting distance,the quality of imaging maintains consistently good.This thesis combines the research of two technologies,terahertz wave and structured beam.Based on the theory of single axicon to produce bessel beam,imagining technology and generation of terahertz diffraction-free beam are researched.In the research process,the method of theoretical derivation,numerical simulation and experimental test are all taken into consideration,and terahertz bessel beam with a length of 1000 mm was successfully generated by the structure of the axicon-lens-axicon structure,which has a delivery distance at the same while.Then the Bessel beam was introduced into the imaging system for practical application,and both two-dimensional imaging and three-dimensional tomography were performed.The main core work of the full text is:(1)Generation of long-distance terahertz bessel beams.The background and theoretical basis of terahertz bessel beam generation are introduced.The disadvantages of single axicon to generate bessel beam are pointed out,and the structure of axicon-lens-axicon is proposed to generate bessel beam.Based on the structure,the theoretical derivation is carried out,the experimental structure is simulated by Matlab and FDTD solutions.Finally,the axicon is produced by 3D printing technology.The experiment successfully produced a long-distance terahertz bessel beam with a certain delivery distance,which is in agreement with the simulation results.(2)2D imaging systems based on terahertz bessel beams.In the terahertz active imaging system,bessel beam is introduced to replace the traditional focused Gaussian wave,and the transmission and reflection imaging system are built respectively.A metal resolution plate is used as the sample to be tested.The two imaging systems achieved two-dimensional imaging of the sample metal resolution plate and maintained a consistent imaging effect over a distance.(3)CT system based on terahertz bessel beam.Firstly a traditional CT system was built.Then the focusing lens used to get the beam onto the sample was replaced by an axicon to form a terahertz bessel CT system for three-dimensional imaging research.Several samples were tested here,including metal struts,scroll painting,silk cocoons,scroll 3D printed bamboo slips.Through the Matab algorithm,the built CT system successfully realized the three-dimensional reconstruction of the sample,and further realized the reading of the scrolling information without open it.This thesis takes the terahertz bessel beam as the main theme,and studies the production technology and application of terahertz imaging.The experimental results show that the improved structure can produce the expected long-distance terahertz beam successfully,and the detection and imaging of large depth of focus can be realized after the introduction of the bessel beam.Terahertz bessel beams do provide an effective method for nondestructive detection and imaging.