Investigation On The Hollow Waveguide Devices With Annular Grating Cladding At Terahertz Frequency

Terahertz (THz) radiation (0.1–10 THz, 1 THz =1012Hz) occupies a large of the electromagnetic spectrum between the infrared and microwave bands and was called the THz gap in the portion of the electromagnetic spectrum. THz radiation has the vast applications in physics, chemistry, biology, medicine, materials, information and national security due to the wide bandwidth, low energy, high coherence, high resolution and good confidentiality etc. THz radiation source, propagation and efficient collector are the three difficulties in THz science and technology. For the low loss, low group velocity dispersion solide medium suiting to THz propagation is substantially scarce in the natural worlds; the key technology of THz application is to research and design materials for guiding THz wave. Until recently, the loss coefficient of developed waveguide is over than 1 dB/m, and the propagation length is less than 1 m. In this dissertation, based on the investigation of THz hollow waveguide with annular grating cladding, we have made detailed theoretical, finite element method simulated and experimental study on several new types of hollow waveguide. Some important and valuable results which bring forth some new ideas are accomplished and listed as the followings:1. A novel hollow waveguide cladding configuration with embedded dual annular grating (EDAGWG) is proposed and designed. Cylindrical polyethylene pipe was embedded with dual annular grating in the inner and outer surfaces, to form a coaxial waveguid. When the fill factor of the grating approaches to 1, EDAGWG is same to dielectric-lined cylindrical metallic waveguides; when approaches to 0, that is regarding as a Bragg filber with few claddings. The guiding mechanism of EDAGWG is dependence on the polarization and high reflectivity caused by the enhanced efficient permittivity to confine the THz wave in the air core. The full-vector finite element method (FEM) has been applied to estimate and analyse the propagation loss, dispersion and mode distribution of EDAGWG.. THz time domain system (THz-TDS) is used to estimate the performance of EDAGWG, for the fundmental mode of EDAGWG with bore diameter of 3 mm,the measured minimum attenuation constant is 0.62 dB/m. The theoretical losses for this waveguide are agreed well with experimental values.2. A dielectric-lined annular grating hollow waveguide with terminate metal film is proposed not to guide THz wave with low loss, but also greatly reduces the difficulties of processing. Full-vector FEM is used to simulate and analyse the mode distribution and attenuation constant of dominant mode.The influence factors of propagation loss of the hollow waveguide with grating-cladding include grating thickness, insertion depth, fill factor, refrective index of cladding dielectric, mode characteristics and the wavelength, etc. The interactions of parameters are considered to reduce the propagation loss. The optimal thickness and insertion depth of grating for HE11 mode is 6-17μm and 20-25μm at 2 THz, respectively. THz-TDS is used to measure the performances of inner metallic grating-lined waveguide (IMG) and hollow waveguide with embedded grating cladding (EMG); the experimental results show the attenuation of fundamental mode in EMG is smaller than that of IMG.3. An effective medium model of cylindrical dielectric pipe with embedded annular grating is established and effective permittivity and permeability independance on wavelength. Based on the full-vector FEM and perturbation method, the distributions of mode and attenuation constants of hollow waveguide with annular grating cladding were simulated and analyzed, and the dependance of attenuation constant and effective refractive index depending on effective parameters are discussed. Optimal combination of geometry and optical parameters is presented, and agree well with the results of fill-vector FEM.4. THz dual-wire-grid polarizer: mirror symmetry copper wire-grid was etched on both surfaces of polyethylene film substrate with low-loss. Approximate coupled wave theory is used to optimize the geometry parameters and thickness of core dielectric. A dual-wire-grid polarizer with suitable thickness of core dielectric can be regarding as a succedaneum of free-standing wire-grid polarizer. The performances of the polarizer were measured using a THz-TDS, and the measured results show the transmittance is more than 70% and below 2% in the parallel and perpendicular directions of the polarizer, respectively. The mean extinction ratio is better than 33 dB in the region of 0.5-3THz, which is higher than the conventional free-standing polarizer in the high frequency region.In brief, a THz hollow waveguide with embedded dual annular grating cladding can confine THz wave to propagate in the air-core by the polarization and enhanced effective refractive index in the cladding, with high quality mode, low loss and low group velocity dispersion, it is possible for the transimission length over than 1 m. therefore, it is feasible to apply the hollow waveguide with embedded annular grating cladding in THz imaging, sensing and endoscope of medical diagnosis.