Research On Terahertz Ptychography Technology Based On Diffraction-free Beam

The unique properties of terahertz radiation,such as high penetration and safety,make it widely used in imaging.The ptychography is a new computational holography technology,and its feasibility has been experimentally verified in the X-ray band,visible light band and terahertz band.In order to increase the imaging distance of ptychography and improve the utilization of probe beams,this article introduces the diffraction-free beam into the ptychography imaging system.The specific research consists:(1)The terahertz imaging technology of different principles and the terahertz diffraction-free beam used for imaging were introduced.The research progress of ptychography was introduced separately from the visible light band and the terahertz band.Difficult to determine the imaging distance and low utilization rate of the emission source were the limitations of terahertz ptychography technology(2)Completed the ptychography algorithm programming combined with the plane wave angular spectrum theory.To improve the utilization of terahertz beams in ptychography,Gaussian beam were used to replace traditional diffracted beam,and a simulation experiment of ptychography with Gaussian beam as a probe was launched.The simulation experiment successfully restored the phase object,and the depth resolution reached ?/4.Using 3D printing technology to manufacture the phase object and imaging lens,the terahertz ptychography experiment with Gaussian beam as a probe was carried out at a frequency of 0.1 THz.The imaging distance and detection distance were 62 mm and 50 mm respectively,and the depth resolution of the experiment had reached ?/2.Analyzed the reasons for the difference between this result and the simulation experiment result.(3)To increase the imaging distance of ptychography.The simulation experiment of ptychography with Bessel beam as probe was carried out and the depth resolution of ?/4was obtained,which proved the feasibility of using Bessel beam as a probe.A 3D printed axicon lens was used to generate a single-cascade Bessel beam,and the ptychography experiment with a single-cascade Bessel beam as a probe was carried out at a frequency of0.1 THz.The imaging distance and detection distance were 72 mm and 50 mm respectively,and the depth resolution of the experiment had reached ?/2.Analyzed the reasons for the difference between this result and the simulation experiment result.(4)To further increase the imaging distance of ptychography,the double-cascade Bessel beam generated by the multiple cascaded lens-axicon doublets was studied.The double-cascade Bessel beam produced by it and the triple-cascade Bessel beam produced by the existing system in the laboratory were used as probe.At a frequency of 0.1 THz,the imaging distance of 175 mm,the detection distance of 50 mm,ptychography experiment with the double-cascade Bessel beam as a probe was carried out.Depth resolution reached?/4.At a frequency of 0.1 THz,the imaging distance of 500 mm,the detection distance of50 mm,ptychography experiment with the triple-cascade Bessel beam as a probe was carried out.Depth resolution reached ?/4.