Low Loss Terahertz Fiber Devices And Their Properties Analysis

With the development of terahertz（THz）technology,the THz system is no longer satisfied with the bulky free-space propagation.It is important to study the THz waveguide with low transmission loss for using the THz technology wildly.The material absorption loss of the terahertz porous core fiber can be reduced due to the integration of a high core air-filling ratio.THz fiber devices based on porous core fiber are important components which promote terahertz technology step into applied research.The fiber devices have potential applications in THz communication and sensing systems.In this topic,two different high birefringence THz fibers based on porous-core and a THz fiber polarization splitter based on dual suspended-core were studied.The characteristics of these three structures were numerically analyzed.In this dissertation,the research background of terahertz technology is summarized at first,including the definition,application,generation,detection and transmission of THz waves.Then the state-of-art of THz fiber devices is introduced,which reveals the significance of this topic.The light guiding mechanism and numerical analysis method of optical fiber are explored.The finite-element method（FEM）and FEM based COMSOL Multiphysics software are introduced in detail.The steps of using this software for characteristic analysis of fiber are outlined.We propose a kind of square porous-core photonic crystal fiber（PCF）for polarization-maintaining terahertz wave guidance.An asymmetry is introduced by implementing rectangular array air holes in the porous core of the PCF,and ultrahigh birefringence and low effective material loss（EML）can be achieved simultaneously.The properties of THz wave propagation are analyzed numerically in detail.The numerical results indicate that the proposed fiber offers a high birefringence of 0.063 and a low EML of 0.081cm^-1 at 1 THz.Moreover,a very low flattened dispersion profile is observed over a wide frequency domain of 0.85–1.9 THz.The zero flattened dispersion can be controlled.It is predicted that this PCF would be used potentially in polarization maintaining and dispersion management of THz waves.We report a highly birefringent THz fiber that contains rectangular air slots in the suspended core.The proposed fiber can provide a high birefringence of the order of 10^-2 over a wide frequency region in single-mode operation.An ultrahigh birefringence of 0.078,low effective material loss of 0.39 dB/cm,and low confinement loss of 1.25×10^-5 dB/cm are obtained simultaneously at 0.95 THz.Moreover,numerical results indicate that a low dispersion with small variation can be achieved,and the power profile envelope has a Gaussian shape,which is important for coupling with common THz sources.The calculated coupling efficiency is as high as～95%for y-polarization mode.It is anticipated that the proposed suspended-core fiber would be useful in THz polarization maintaining wave guidance and bacteria detection applications.We propose a new kind of terahertz polarization splitter based on suspended dual-core fiber.Firstly,the basic theory of the double-core directional coupler is introduced.Then the numerical analysis of the THz polarization splitter is carried out.Each core of the fiber is formed with two adherent subwavelength polymer tubes,which is helpful for a high modal birefringence.The proposed splitter can completely separate two orthogonal polarization modes at a specific splitting length,due to the difference of coupling length between x and y polarizations.A 1.184 cm long splitter with a low transmission loss of 0.6 dB and a bandwidth of 0.05 THz with the extinction ratio better than-10 dB can be achieved at a center frequency of 1 THz.Moreover,different from other THz polarization splitters,an efficient rule exists for the design of the proposed polarization splitter with a high extinction ratio for any desired operation frequency.