The Quality Improvement Of Continuous-wave Terahertz Ptychography

Terahertz(THz)waves are located in the microwave and infrared radiations of the electromagnetic spectrum,with hydration absorption and penetrating without ionization.Terahertz imaging technology is developing rapidly in the fields of biomedical imaging,non-destructive testing(NDT)and security inspection.THz phase-contrast imaging is more suitable for weakly absorbing biological samples,which can not only obtain the internal information such as refractive index and thickness of the transmitted samples,but also acquire the topography of the reflective object.Therefore,it has become one of the most important research field of terahertz imaging.Ptychography is a lensless coherent diffraction imaging technique,in which the complex amplitude distribution of the sample and the probe function can be retrieved from a set of diffraction patterns originating from overlapping sample illumination areas.This approach has a compact layout and unlimited field-of-view.Continuous-wave(CW)terahertz ptychography can not only release the unique characteristics of terahertz wave,but also reflect the advantage of ptychography.The reconstructed image quality and resolution still need to be improved,otherwise it will be difficult to apply in biomedical field.The inaccuracy of the illumination probe's parameters would increase computational cost and degrade the reconstruction quality.Therefore,it is urgently needed which probe position correction for CW terahertz ptychography.It has a compact layout without crosstalk from the illumination probe function,which will be a method of NDT in the future.The main contents of this paper are as follows:The research status of terahertz phase-contrast and ptychography imaging was fully surveyed,focusing on the reflective terahertz imaging,position correction for ptychography and reflective ptychography.The recording and reconstruction process of ptychography were described in detail and three ptychographical iterative engine were introduced.A method for probe position correction for CW terahertz ptychography was proposed.Firstly,the principle of this method was introduced,and then the effectiveness was verified through simulation.In the case of priori probe function,the magnitude of the translation errors was calculated.Finally,the CW terahertz wave ptychography was built based on a 2.52 THz optically pumped far-infrared gas laser and a PY-III pyroelectric array detector.The quality of the reconstructed images were compared with using the probe position estimation and the causes of the error distribution with a periodicity equal were analyzed.The CW terahertz reflective ptychography was proposed.Firstly,the tilted plane correction(TPC)was introduced and its necessity was proved.Secondly,verification experiments were carried out in the visible band,including two kinds of reflective ptychography system based on semiconductor laser and PCO scientific SCMOS camera.The feasibility of the setup and algorithm was verified by using the USAF resolution target.After analyzing the particularity of terahertz devices,the CW terahertz ptychography by oblique illumination was built based on the 3.1 THz optically pumped laser and INO uncooled microbolometer,to obtained the topography of the steel ruler and Polish one-cent coin.