Preparation Of Low-Loss Terahertz Hollow Waveguides Based On Polymer Tubes

With the development of terahertz(THz)wave in imaging,communication,sensing and other application fields,problems aroused from THz wave transmission have gradually attracted widespread attentions.In comparison with wireless transmission in free space,bending,low-loss and stable transmission of THz wave is expected to be achieved through the waveguide.Among various types of THz waveguides,hollow waveguide has a simple structure,easy preparation,low cost,and thus possesses a broad application prospect.In this thesis,the hollow waveguide structures were designed according to the related theories.Based on different polymer tubing materials,the waveguide samples were prepared and the comprehensive transmission performances to THz wave were explored.According to the anti-resonance theory,a dielectric tube hollow waveguide was designed for broadband transmission of THz wave.Polycarbonate(PC)dielectric tube hollow waveguides with different wall thicknesses were prepared via the piston-type controllable drop-liquid coating process.With optimization of the fabrication procedure,the thickness of the dielectric layer was increased to 80 ?m,from the upper limit of 16?m possibly achieved through the traditional process.The analysis of the infrared(IR)transmission spectrum shows that the dielectric layer is of high uniformity.It is confirmed by the terahertz time domain spectroscopy(THz-TDS)measured that the PC dielectric tube hollow waveguide has the ability to transmit THz wave in a wide frequency band of 1.9 THz.For the metal/dielectric hollow waveguide,the optimized thickness range of the polypropylene(PP)dielectric layer was calculated to be 300-350 ?m,based on the ray model theory and COMSOL finite element analysis.With the help of self-developed silver-plating process outside the polymer tube,the Ag/PP dielectric hollow waveguide was fabricated.The results of THz-TDS measurement disclose that the low-loss transmission window of the waveguide is located near the frequency of 300 GHz.By a300-GHz wave source,the experimental straight transmission loss is measured to be1.05 d B/m.The bending transmission loss reaches 1.34 d B/m with the bending angle of 270° and the bending radius of 15 cm.The transmission loss has a linear relationship with the bending angle.In the research of the metal-coated hollow waveguide,the minimum thickness of the metal coating was calculated according to the skin-depth theory.AcrylonitrileButadiene-Styrene(ABS)tubing was used as the structural tube for constructing an ABS/Ag-coated hollow waveguide.Samples with inner diameters of 2.3 mm,3.2 mm,and 4.2 mm were prepared by the silver-plating process inside the polymer tube.The micro-morphology analysis demonstrates the thickness of the Ag coating meets the theoretical requirements.The ABS/Ag-coated waveguide samples with different bore size were used to deliver THz wave.The waveguide samples with 2.3 mm,3.2 mm,and4.2 mm bore can achieve bendable and low-loss transmission of 300-GHz terahertz wave.Among them,the 3.2-mm-bore waveguide has a straight transmission loss of1.02 d B/m at 300 GHz,and a bending transmission loss of 1.56 d B/m at the bending angle of 150°(bending radius 10 cm).