THz Anti-resonant Reflecting Pipe Waveguide

在兆赫波領域的應用上，為了建立一個彈性、穩固、低損耗的兆赫波系統，發展出一個低損耗的兆赫波波導是必要的。 過去，許多建立在微波及光波上的波導模型都曾被提出來應用在兆赫波領域上。但是，由於在兆赫波波段下，介電損耗和有限的金屬導電性造成了很大的材料吸收。如何建立一個低損耗的兆赫波波導是一大挑戰。 因為在兆赫波下透明的材料當中，最容易取得的是乾空氣，一個合理的可行方式是將兆赫波傳導在空氣中來達成低損耗的兆赫波波導。有兩種不同的方式來達成這個目的: 次波長型態和中空管型態。 上次，我們利用釣魚線成功地完成了次波長型態的兆赫波波導。低損耗係數在接近0.3THz的地方可以低於0.01cm-1。但是，利用次波長形態傳導兆赫波不利於在彎曲的情況下傳導，環境的干擾也會對其造成比較大的影響。 因此，轉而利用中空管型態，我們成功地提出並驗證了一種很簡單的塑膠管就可以傳導兆赫波。跟過去的中空管型態有很大的不同，我們不採用高反射的包覆面，而是採用單一層均勻且具有相對較低折射率的結構。這種波導呈現了抗共振反射式波導的本質。利用一般市面上可得的鐵氟龍管，我們確認了這種波導可以將兆赫波傳導在管子中間空氣的部份。其損耗係數可以達到或低於0.001 cm-1，耦合效率可以達到80%，頻寬超過200 GHz。 相信這種簡單且隨處可得的兆赫波管型波導可以更加精簡往後的兆赫波應用，如兆赫波內視鏡、生物檢測。With the perspective on a flexible, robust and low-cost THz system for various applications, it is essential to develop a low-cost waveguide. Several waveguide solutions have been proposed according to either microwaves or photonics technologies, but the unacceptably high absorption loss in dielectrics and finite metal conductivity at THz range are still challenging the waveguide design. Since the most transparent medium for THz waves broadly available is dry air, guiding THz waves in air is feasible to provide low attenuation THz waveguide delivery. Two different types of THz waveguides have been proposed based on this concept: the sub-wavelength type, and the air-core type. Last time, we have proposed a sub-wavelength type waveguide, such as a fishing line, can successfully guide THz waves with a low attenuation constant less than 0.01cm-1 in the frequency range near 0.3THz. However, THz wave guided on sub-wavelength fiber is sensitive in bending loss and disturbance of surroundings. In this thesis, we successfully proposed and demonstrated a very simple plastic pipe, which confines THz waves within the air-core region. Very different from previous air-core waveguides to design a high-reflection-coated cladding layer, the circular plastic pipe walls are a single-layer structure with a relatively low but uniform refractive index. Our study on the attenuation spectra indicated its anti-resonant reflecting waveguiding nature. Using commercially available Teflon pipes, we confirmed that terahertz waves are successfully guided in the air-core region, with attenuation constant on the order of or less than 0.001 cm-1, coupling efficiency up to 80%, and bandwidth over 200 GHz. We believe that these simple and broadly available THz pipe waveguides can ease and impact many THz applications such as endoscopic imaging and biosensing.致謝 I要 IIbstract IIIontents Ⅳigure Contents Ⅵhapter 1 Introduction 1.1 An Introduction of Terahertz Waveguide………………………..2.2 Organization of the Thesis………………………………………4eference………………………………………………………….....6hapter 2 Design of the Plastic Air-Core Waveguide 11.1 Review of the Low-Loss THz Air-Core Microstructure Fiber....12.2 Comparison between the Commercially available plastic Pipes for THz delivery……………………………………………….…..........15.2.1 Sample Preparation and Experimental Setup……….....15.2.2 Attenuation Constants of Commercially Available Pipes………………………………………………………….....16.3 Optical Constants of Teflon…………………………………......20.3.1 Terahertz Time Domain Spectroscopy (THz-TDS) Setup……………………………………………..……………..20.3.2 Calculated Method of Optical Constant form THz-TDS……………………………………………………….21.3.3 Measured Optical Constants of Teflon……………..….23eference………………………………………………………...….28hapter 3 THz Anti-Resonant Reflecting Teflon Pipe Waveguide 31.1 Measurement of Waveguide Attenuation……………………….33.1.1 Experimental Setup…………………………………....33.1.2 Results and Discussion………………………………...36.2 Measured Intensity Distributions of the Mode Pattern………....42.2.1 Experimental Setup…………………………………....42.2.2 Results and Discussion…………………………..…….43.3 Measured Bending Losses of the Teflon Pipe………………......46.3.1 Experimental Setup…………………………………....46.3.2 Results and Discussion………………………………...48eference…………………………………………………………....50hapter 4 Conclusions 52.1 Summary………………………………………………………..52.2 Future Works…………………………………………………....54eference…………………………………………………………....56ppendix A11. The Finite Difference Frequency-Domain (FDFD) Mode Solver…………………………………………………………....A12. Modal Characteristics of THz Pipe Waveguide………………..A3eference…………………………………………………………....A