Study On Scattering Characteristics And Measuring Key Technologies Of Targets In Terahertz Range

The research of target scattering characteristics in the terahertz band,which involves the electromagnetic field theory,radar system,measurement technology and so on as the basis of terahertz radar,possesses great military and civilian value.China's research in this field is still in its infancy.Considering the big gap between China and the developed countries in Europe and America it is difficult for us to carry out this kind of work.In the calculation,large electrical size electromagnetic scattering algorithm needs to be further optimized for the high-frequency algorithm and computer hardware constraints.In the technical field,the lack of dielectric parameter data of material in the terahertz band and the limitation of the practical terahertz source and detection technology in measurement capability of the target bring great difficulty in the study of space target scattering.In this paper,we launched a series of work for calculation and experimental measurement of the radar cross section?RCS?of the terahertz radar for the simple radar target beginning with the work on the theoretical calculation of dielectric parameter.In addition,we carried out some research of dual-wavelength and multi-wavelength lasers used for generating high-efficiency broadband terahertz sources based on differential frequencies that can be used in measurement system,and the main research results are as follows:1.One generic multi-beam interference model is proposed,and a method to inverting the dielectric parameters in the terahertz range is established based on the model and Kramers-Kronig?K-K?dispersion relation between the reflectance spectrum and the phase shift.Dielectric parameters in the terahertz range of non-metallic materials such as silicon,polyethylene,quartz and wave-absorbing material,metal materials such as stainless steel and silver are obtained utilizing far infrared Fourier spectroscopy?FTIR?and terahertz time domain spectroscopy?THz-TDS?as the basis of experimental measurement equipment,and combining multi-beam interference model and K-K relationship inversion of dielectric parameter theory.The feasibility of the method is verified,which provides theoretical and experimental reference for the acquisition of dielectric parameters in the terahertz frequency band.2.The expressions of the backscattering field of smooth metal sphere,perfect conductor and the dielectric cylinder at the terahertz band are deduced.The RCS curves of some standard targets are calculated.The back-scattering process of good conductor metal spheres,which can be used as radar standard targets,are obtained based on the transmission mechanism of plane wave in free space and the theory of electromagnetic wave superposition.At the same time,the relationship between the RCS and the wavelength is obtained.The backscatter field distribution of the good conductor and the dielectric cylinder is deduced.The backscatter expression of the target is derived based on Kirchhoff approximation theory,and the RCS curves of metal plates with different roughness and cones and cylinders with different bottom diameters are calculated.3.A high-resolution near-field terahertz RCS measurement system with automatic control is presented.We build a set of laboratory radar cross section measurement system based on high-power carbon dioxide laser pumped terahertz laser and equipped with high sensitivity low temperature Si-Bolometer terahertz detector.The system SNR reach up to 24dB,and the system is calibrated by smooth metal sphere.It is verified that the system relative error between measurement value and the theoretical value is less than 3dB.For the first time,we measured some standard targets?such as round plate,cone and cylinder and so on?by the system,which laid the foundation for the RCS measurement of complex targets.4.Multiple sets of multi-wavelength lasers are realized based on optical parametric oscillation and stimulated Raman scattering,which can be utilized for generating terahertz radiation.Intracavity optical parametric oscillators?IOPO?are presented based on GYSGG/KTA and GYSGG/KTP and the eye-safe dual-wavelength laser was obtained which could be used to detect the gas and the terahertz radiation by nonlinear difference frequency.High-order Stokes wave is observed in a KTP crystal based on stimulated Raman scattering?SRS?pumped by an Nd:YLF laser.Output spectra including the fundamental wave at 1047nm and six Stokes wavelengths at 1077nm,1110nm,1130nm,1143nm,1164nm,1180nm based on two Raman frequency shift at 267.4cm^-1 and 693.0cm^-1 are obtained simultaneously.This work provides a reference for solid multi-wavelength lasers.