Generation Of Terahertz Structured Beams With Application In Terahertz Imaging

Terahertz(THz)wave is electromagnetic wave that falls between the infrared and microwave bands.Terahertz(THz)radiation has great potential for imaging applications due to its ability to penetrate most dielectric materials is better than the infrared wave.THz wave provides higher imaging resolution compared with microwave.Moreover,the images of lowdensity materials obtained with THz-wave have high contrast compared with the ones obtained with X-ray.In summary,THz imaging can be used as the complement of the existing imaging technologies and applied in the fields of non-destructive testing and security screening.The longer the depth of field and the finer the resolution means the better the performance of the imaging system.It is difficult to satisfy for the conventional continuouswave THz imaging based on the Gaussian beam.It is limited by a tradeoff between Rayleigh distance and beam width.In recent years,the development of THz structured beams provide the effective method to overcome the above problems.For example,the THz zero-order Bessel beam and the THz accelerating Airy beam do not diverge,which can keep the beam width un-changed in the non-diffraction range.Furthermore,if part of these two kinds of beam is obstructed or distorted the beam reconstructs itself in the propagation.Hence,both of beams can be introduced to THz imaging to extend the depth of field and retain the resolution.In this dissertation,the generation of zero-order Bessel beam and the generation of accelerating Airy beam in the THz range is investigated.Moreover,the extension of the depth of field in continuous-wave THz imaging systems induced by non-diffraction beam and the influence of the reconstruction methods used in THz CT is investigated.The main work of the dissertation can be listed as follows:Firstly,the generation of zero-order Bessel beam and accelerating Airy beam in the THz range is simulated numerically based on the angular spectrum theory.The diffractive element,1D cubic phase plate(CPP)and 2D CPP are designed and fabricated by 3D printing technology,which is suitable for 0.3 THz.The diffractive elements are applied to generate zero-order Bessel beam which is compared with the ones generated by axicons.The results demonstrate that the diffractive element can produce the zero-order Bessel beam effectively and efficiently.1D CPP and 2D CPP are combined with corresponding lenses to generate the 1D accelerating Airy beam and 2D accelerating Airy beam.Secondly,we have constructed a conventional continuous-wave THz transmission imaging system with a Gaussian beam and investigated its depth of field and resolution.Besides,transmission images are filtered in the frequency domain to remove the noise induced by the relative translation between sample and detector.Then the zero-order Bessel beam and accelerating Airy beam are introduced to the continuous-wave THz transmission imaging system by the diffraction element and 2D CPP.For investigating the influence of two kinds of non-diffraction beam to the transmission imaging,the depth of field and the resolution are also demonstrated and analyzed.Third,a conventional continuous-wave THz reflection imaging system with a Gaussian beam is designed.Moreover,the diffraction element introduces zero-order Bessel beam to the continuous-wave THz reflection imaging system.Likewise,the depth of field and resolution is investigated and compared in two reflection imaging systems with Gaussian beam and zero-order Bessel beam.The results demonstrate the THz reflection imaging system with zero-order Bessel beam has the advantages of longer depth of field and the antiinterference ability.Fourth,a conventional continuous-wave THz CT system with a Gaussian beam is designed and used to obtain the projection of the sample.Then the projection is dealt by the convex algorithm based on compress sensing and other reconstruction algorithms to obtain the reconstruction images.The results indicate the convex algorithm based on compress sensing can remove the streaking artifacts effectively without introducing other distortions.Besides,the imaging process of this continuous-wave THz CT is simulated numerically.In the end,the zero-order Bessel beam is introduced to THz CT imaging system.The images of the sample obtained by the THz CT system with a Gaussian beam and zero-order Bessel beam are compared.Then the imaging processes of two THz CT systems with a Gaussian beam and a zero-order Bessel beam are investigated.