| As a new imaging technology,the pulsed terahertz imaging technology can not only realize the clear imaging of the internal defects of most non-polar and non-metallic materials,but also obtain the refractive index and absorption coefficient of the sample through the terahertz wave signal of the sample,which makes the pulsed terahertz imaging technology have broad application prospects in the fields of non-destructive testing and biomedicine.Affected by the short focal depth of the pulsed terahertz imaging system,when detecting the internal defects of thick samples whose thickness exceeds the focal depth range,in order to obtain images with good lateral and longitudinal resolution,multiple two-dimensional scans of thick samples are required,resulting in a lot of time and reduced detection efficiency.In this paper,aiming at the problem of short depth of focus in the focused lens pulsed terahertz system,the Bessel beam is combined with the pulsed terahertz system to carry out the research of long focal depth imaging technology based on Bessel beam.The main research work of this paper is as follows:1)The model of pulsed terahertz long depth of focus imaging system based on Bessel beam is constructed.Based on the study of the principle of conventional pulsed terahertz imaging system based on Gaussian beam,the transmission characteristics of Gaussian beam and the limitations of its imaging technology are analyzed.The model of pulsed terahertz imaging system based on Bessel beam is constructed.The non-diffracting characteristics and generation methods of Bessel beam are analyzed theoretically.On this basis,a pulsed terahertz Bessel beam imaging system is designed based on the axicon method.2)The non-diffracting characteristics of pulsed terahertz Bessel beams based on biconical axicons are studied.Based on the diffraction integral theory and geometric optics theory,the optical field distribution and non-diffracting distance of Bessel beam generated by axicon are analyzed.A biconical axicon structure is designed to successfully generate a terahertz Bessel beam with a non-diffracting distance of 110.18 mm,which is 82.63 mm wider than the traditional axicon non-diffracting distance.3)The influence of the errors in the processing and assembly process of the biconical axicon on the non-diffracting characteristics of the terahertz Bessel beam is analyzed.In the machining error,the influence of vertex alignment error and axis angle error on non-diffracting characteristics is analyzed,and the allowable range of machining error of biconical axicon is given.In the assembly error,the eccentric tilt error and the eccentric translation error are analyzed emphatically.The results show that the eccentric tilt error will not affect the non-diffracting characteristics,while the eccentric translation error will cause the axial position of the Bessel beam to shift,which must be corrected during assembly.4)Pulsed terahertz long depth of focus imaging based on Bessel beam is studied.A pulsed terahertz Bessel beam transmission detection device was built to successfully generate a terahertz Bessel beam with a non-diffracting distance of 110 mm.On this basis,the imaging resolution of the pulse terahertz imaging system based on Bessel beam is experimentally analyzed.The results show that compared with the Gaussian beam imaging system,the lateral resolution can be maintained at 0.25 lp/mm in the non-diffracting region.The system is applied to the detection of internal defects in modified polypropylene pore samples.The results show that the imaging system can accurately detect the depth position and size of defects in pore samples.The average error between the detection results of defect depth and the actual measurement results is 0.205 %,which verifies the feasibility of the system in the detection of actual thick samples. |
