Single-Mode Fiber-Based THz Directional Coupler

近二十幾年來，兆赫波(100GHz-10THz)的產生及偵測技術都有很大的進步。然而，由於許多物質在此波段都有很強的吸收，要如何有效的傳遞兆赫波仍是一個大問題。目前絕大部分有關兆赫波的實驗，都是靠架在光學桌上的反射鏡來傳遞兆赫波。這種固定、無法變動的架構，在應用上有相當多的限制。若有像optical fiber這樣的東西來傳遞兆赫波，兆赫波就能更廣泛的被運用。 因此，我們去年利用次波長光纖傳遞兆赫波，這種波導可塑性大，製作簡單，成本便宜，且損耗低。而這回，我們首先改善次波長光纖兩個缺點，並建立一結合寬頻的兆赫波源與Martin-Puplett偏振式干涉儀的系統後，進一步研究次波長光纖，發現改良過的次波長光纖損耗更低。有趣的是，次波長光纖具類似帶通濾波器的效果，且不同線徑的光纖會在相異的特定頻率有最小的損耗。所以，若想傳遞某特定頻率的兆赫波，只要經由選擇適當的線徑，便可將損耗降到最低。 甚者，將次波長光纖應用在方向耦合器上，我們成功地結合次波長光纖與聚乙烯薄膜，建立全世界第一個兆赫波光纖方向耦合器。特別的是，這種情況下，反對稱模態是截止的，故有別於一般傳統的方向耦合器，其耦合比例與耦合長度有很大的關係；但此耦合器的耦合比例可高達47%，與耦合長度及波長無關，極適合做為3dB power divider。 相信這些兆赫波光纖的研究，對於往後的應用，如:兆赫波光纖感測、兆赫波光纖影像系統、兆赫波內視鏡、兆赫波通訊等將有很大的幫助。The scientific investigation about the terahertz phenomena was intensively performed in the past twenty years out of the rapid development of the terahertz generation and the detection technique. Among all the setups of THz application systems nowadays, most of them are constructed with planar or curved metal reflectors fixed on an optical table. Such system is very rigid and vulnerable to environmental disturbances. With the perspective on a controllable, reliable, and flexible THz system for various applications, a low-loss THz waveguide is indeed essential. Last year, we proposed and demonstrated a subwavelength THz fiber for terahertz wave guiding with a low attenuation constant (~ cm-1). In this thesis, we successfully established a single-mode fiber-based THz directional coupler. We first modified two major shortcomings of the subwavelength fiber and established a system composed of a photoconductive antenna as a broadband source and a Martin-Puplett interferometer to further investigate its spectral characteristics. Interestingly, we found that the characteristics of the subwavelength fiber are like a band-pass filter and the attenuation of THz waves could be on the order of or less than cm-1 at a specific wavelength which depends on the fiber diameter. It implies that the THz attenuation at the operating frequency can be greatly minimized by the proper design of the diameter of the subwavelength fiber. Moreover, we applied the subwavelength fiber to the THz directional coupler. By using PE films and subwavelength fibers, we established a single-mode fiber-based THz directional coupler. The anti-symmetric mode in the established coupler was found to be cutoff. The coupling ratio is thus independent of the coupling length and the wavelength, which is much different from the traditional directional coupler. This subwavelength directional coupler with a coupling ratio up to 47% is expected to be good for the application of the 3dB power divider. We believe that these will be very versatile for the terahertz applications such as fiber sensing, fiber imaging, endoscope, terahertz communication, and so on.Contents Chapter 1 Introduction 1 1.1 Background of Terahertz Technology 2 1.2 Organization of the Thesis 4 Chapter 2 Construction of a Martin-Puplett Interferometer 6 2.1 Principle of the Martin-Puplett Interferometer 7 2.1.1 Basics of the MPI 7 2.1.2 The Interferogram and theSpectrum 8 2.2 Experimental results of the MPI 10 2.2.1 Photoconductive Antenna 10 2.2.2 Experimental Setup 10 2.2.3 Measurement of the Spectrum 13 2.3 Discussion 15 Chapter 3 Investigation of Subwavelength Terahertz Fiber 17 3.1 Review of the Low-Loss Subwavelength THz Fiber 18 3.1.1 Basic Model of the Subwavelength THz Fiber 18 3.1.2 The Low-Loss Subwavelength PE Fiber 21 3.2 The Subwavelength Terahertz Fiber Measurement 24 3.2.1 Fiber Preparation and Experimental Setup 25 3.2.2 Measurement of Fiber Attenuation 28 3.2.3 Discussion 33 3.3 Study of the Loss Mechanisms 34 3.3.1 Radiation Loss out of Diameter Variation 34 3.3.2 Adiabatic Approximations 36 3.3.3 Discussion 41 Chapter 4 Single-Mode Fiber-Based THz Directional Coupler 49 4.1 Principle of the THz Directional Coupler 50 4.1.1 Modes of Two Parallel Fibers 50 4.1.2 Anti-symmetric mode Cutoff 53 4.2 THz Directional Coupler Measurement 53 4.2.1 Preparation for Directional Coupler 53 4.2.2 Measurement of the THz Directional Coupler 55 Chapter 5 Conclusions 61 5.1 Summary 61 5.2 Future Works 6