Continuous-wave Terahertz Computed Tomography Based On Bessel Beam

The Terahertz(THz)band is the only electromagnetic band that has not yet been fully developed.Due to its unique and important characteristics such as penetrability,low energy and hydrophilicity,THz imaging technology has become a research hotspot in recent years,and has played an important role in national security,security,anti-terrorism,radar communication,biomedical imaging and other important fields.THz computed tomography(CT)obtains the projection data at different angles by rotating the sample.Reconstruction algorithm is used to obtain the spatial distribution of the absorption coefficient of the object.Continuous wave(CW)terahertz imaging system is simpler than terahertz time domain pulse imaging.At present,the focal depth of CW THz CT is limited by Gaussian beams,and the data acquisition rate is limited by the point-by-point scanning mode.In recent years,THz light field regulation method represented by the THz Bessel beam has gradually become a research hotspot.It has a large focal depth and no diffraction characteristics,and can be used to extend the depth of focus in a CW THz CT system.In the reconstruction algorithm,the traditional filtered back projection algorithm(FBP)requires enough angle projection data to obtain better reconstruction quality,and severe strip artifacts will occur when the projection data is not enough.The a algebraic reconstruction technique(ART)is different from the traditional reconstruction algorithm.By solving the projection equation,the image can be reconstructed accurately with a small amount of projection data.Therefore,the data acquisition time of CW THz CT is greatly reduced.In the early stage,the research status of THz imaging and THz CT was fully investigated,and the application of terahertz light field control technology represented by Bessel beam in THz CT was studied.The basic principle of CT is elaborated,and the difference between the analytical class algorithm represented by the FBP algorithm and ART algorithm is compared.Aiming at the problem of slow data acquisition rate,an automatic data acquisition program was developed.The CW THz CT method based on ART algorithm was studied.The reconstruction was compared by simulation with FBP algorithm and algebraic iterative algorithm.The simulation results show that ART algorithm can reconstruct the image more accurately than the projection data of the filtered back projection algorithm,and can reduce image artifacts to a large extent,thereby improving the image quality.Aiming at the problem of short focal depth of Gaussian beam,a CW THz Bessel beam characteristic experimental study is carried out.Firstly,the principle of THz Bessel beam is theoretically analyzed,and the axicon with different apex angles and different materials is designed.Based on the FIR-295 carbon dioxide pumped continuous terahertz laser,the characteristics of the Bessel beam under different experimental parameters were experimentally studied in the 2.52 THz band.An improved algebraic iterative algorithm based on compressed sensing theory is presented.The simulation results of the two algorithms are compared by the image quality evaluation function.It can be seen that under a small amount of projection data,the improved algebraic iterative algorithm can eliminate image artifacts to a greater extent,and the reconstructed image has better fidelity,and is more suitable for experimental research of incomplete projection data.CW THz CT experiments based on Bessel and Gaussian beams were carried out using FIR-295 CW THz source and goley single-point detector,and tomography was performed on water pipes at different depths of focus.The algebraic iterative algorithm and the improved algorithm are used to reconstruct the comparison.The effectiveness of the algorithm and the large depth of focus of THz Bessel beam are verified,which provides a new idea for CW THz CT.