Novel Hollow Core Anti-resonant Terahertz Waveguide

In recent years,THz technology has been rapidly developed.But due to the lack of low loss THz waveguide,THz signal mainly propagates through free space transmission.This results in high transmission loss due to the existence of water vapor and impurities in the air.Moreover,the lack of effective THz waveguide makes THz system huge,complex,and poor stable.The lack of low loss THz waveguide has become a key bottleneck problem to restrict the application of THz technology.Hollow core anti-resonant THz waveguide has the advantages of low transmission loss,large transmission bandwidth,large structural design freedom and easy fabrication.Besides light guidance through hollow core can break through the limitation of material loss.As a result,hollow core anti-resonant THz waveguide has become one of the research hotspots in the field of THz waveguide.Supported by National Natural Science Foundation of China(NSFC),this thesis carries out theoretical and experimental research on several novel hollow core anti-resonant THz waveguide by the innovation of theory,material and preparation technology so as to meet the rapid developing requirement of THz application field.The main innovative achievements are listed as below:1.The triangular air hole is introduced into the hollow core anti-resonant THz waveguide to improve the anti-deformation performance for the first time.A pentagram anti-deformation hollow core anti-resonant THz waveguide with acute angle structure is proposed and developed.Considering the fabrication feasibility,the photosensitive resin commonly used in 3D printing is selected as the fabrication material.The influence of structural parameters and deformation on the transmission characteristics of the waveguide is systematically studied.The waveguide sample is fabricated by 3D printing technology,and its characteristics are measured by THz-TDS system.The measured minimum loss of the waveguide is 0.041 cm^-1 at the frequency of 0.848 THz,which accords with the design characteristics.Compared with the circular and rectangular air holes,the proposed hollow core anti-resonant THz waveguide structure with triangular air holes has the best anti-deformation ability.2.A double pentagon nested anti-deformation hollow core anti-resonant THz waveguide is proposed and developed.On the basis of retaining five triangle holes as the cladding structure,a double pentagonal nested THz waveguide is designed by using two pentagons nested with each other.A 25 cm waveguide sample is fabricated by 3D printing technology,and measured by THz-TDS system.The measured lowest loss is 0.011 cm^-1 at 0.995 THz,and the average transmission loss is 0.07 cm^-1 in the frequency range from 0.2 to 1 THz.The experimental results are in good agreement with the design characteristics.Compared with the pentagram anti-deformation hollow core anti-resonant THz waveguide,the improved double-pentagonal nested THz waveguide not only reduces the waveguide size by about 40%,but also maintains low transmission loss and good anti-deformation ability.These results are more beneficial to the miniaturization and integration of THz system.3.An effective single-mode hollow core anti-resonant THz waveguide with semielliptical tube cladding and its mode filter are proposed and developed.The high order mode extinction ratio of 6.01 is obtained by fitting the cladding semielliptical tube area and optimizing the wall thickness of the core area.The waveguide is fabricated by 3D printing with photosensitive resin.The experimental results show that the mode field at the output end of the waveguide is similar to the Gaussian distribution of the fundamental mode,which proves that the waveguide can achieve efficient single-mode propagation.The minimum loss of 0.009 cm^-1 is obtained at 0.82 THz.In the frequency range from 0.2 to 1 THz,the average loss of the perfect cascaded waveguide is 0.048 cm^-1.Even though there is a slight dislocation between the cascaded waveguide,the average loss is very close to the value of the perfect cascaded waveguide.The mode filter based on this hollow core anti-resonant THz waveguide structure can filter the HE₃₁ mode by two schemes.4.An elliptical core polarization maintaining THz waveguide is proposed and developed.The influence of the structure parameters on the transmission characteristics is systematically studied.The results show that the wall thickness mainly affects the position of the transmission window and the high birefringence region in the transmission spectrum,but has no direct effect on the loss and birefringence.The effect of core size and ellipticity is similar.The larger the core area is,the lower the loss of the X and Y polarization modes and the birefringence are.The waveguide sample is fabricated by 3D printing with photosensitive resin. The experimental results show that the birefringence can reach the order of 10^-4 in the high birefringence region of 0.2-0.24 THz and 0.27-0.29 THz.The minimum loss measured by the X and Y polarization modes is 0.016 cm^-1 and 0.020 cm^-1, respectively.5.A polarization-maintaining hollow core THz waveguide with two parallel plates embedded in the cladding is proposed and developed.On the basis of different core diameters in X and Y directions,different wall thicknesses are introduced to further improve the birefringence.The birefringence of the optimized waveguide is higher than the order of 10^-3 in the frequency range of 0.45-0.53 THz.THz waveguide samples are fabricated by 3D printing with white and transparent photosensitive resins respectively.The experimental results show that the highest birefringence is4.91×10^-3 and 3.43×10^-3 in the frequency range from 0.2 to 1 THz,respectively.Compared with the elliptical core THz waveguide,the birefringence value is increased by an order of magnitude.