Study On Microstructured Low Loss THz Fiber And Fiber Sensing Technology

The terahertz (THz) propagation fibers and directional coupling devices are important components in THz propagation and sensing system which is similar to silica optical fiber. However, almost all materials are highly absorbent in the THz region making designs of the low-loss, isolated from disturbing of surrounding and easy operating THz fibers and fiber directional coupling devices challenging.The Surface Plasmon Resonance (SPR) sensors based on microstructured optical fiber (MOF) have advantages of high sensitivity, label-free, and rapid detection. Nevertheless, there are usually two principal difficulties in the development of the highly-sensitive MOF-based SPR sensors. One of the problems is that it is hard to realize phase matching between the core-guided mode and surface plasmon mode. The second problem is the difficulty to increase plasmon penetration depth into the analyte.In this dissertation, based on the simulation analyzing method, we focus on the research of several kinds of key sensing devices based on microstructure, including THz fiber, polarization splitter, SPR sensor and single-polarization single-mode fiber. The main contents of this dissertation are as follows:1. The low-loss suspended hollow-core THz fiberLow-loss transmission of THz waves can be realized in the sub-wavelength THz fiber, while the THz propagation is susceptible to high perturbation from any environmental contact on the surface. To conquer this difficulty, a suspended hollow-core THz fiber is designed in this dissertation, the dielectric strips with equal width are used to construct the fiber, and the majority space in the fiber is filled with air. The suggested configuration forms an effectively high-index core and low-index cladding. The transmission loss of the THz fiber can be tuned by adjusting the period of the dielectric strips, which is in contrast to the original suspended core THz fiber, where the low loss transmission can only be achieved by reducing the strip width and core size. The proposed suspended hollow-core THz fiber can also be applied to achieve highly-birefringent operation and two-core operation. By preserving the thickness of horizontal dielectric strips while tuning the thickness of the vertical dielectric strips, low-loss highly-birefringent operation can be achieved, the birefringence of the fiber can be on the order of10-2. The previously reported THz polarization splitters generally have long device length, and high transmission loss. The proposed suspended hollow-core can be applied to construct two-core THz fiber, which shows strong polarization-dependent coupling characteristics. Low-loss THz polarization splitter can be achieved based on the configuration.2. The low-loss crossed structure THz fiberThe microstructured optical fiber based on total-reflection theory generally requires the core size larger than the cladding strips to supply light guiding mechanism. We reported in here a novel kind of THz fiber which does not have evident core and cladding regions. The refraction indexes along the radial direction in the so called cross-structure THz fiber shows an effectively gradient reduction, which is why the effectively total-reflection guidance can be achieved. Compared to the suspended core THz fiber with the same transmission loss, the thickness of the dielectric strip of this fiber can be twice thick and the inner diameter of the dielectric cirque can be even larger. The configuration also shows the merits of simple structure, easy fabrication and wide operational bandwidth. By simply adjusting the thickness of one direction, highly-birefringent transmission with loss loss can be achieved. Two-core THz fiber can be constructured by increasing the strips at one direction. It’s found the proposed two-core THz fiber shows single-polarized coupling characteristics, that is, one of the polarization state do not coupling with each other, whereas another state show low loss coupling. Such phenomenon is totally different with the conventional two-core optical fibers.3. The single polarization and single mode fiberBy simply adjusting the angle between the two dielectric layers crossed in the cross-structure fiber, it’s found the difference between the losses of the two polarizations can be increased, which is applied to achieve single-polarization optical fiber working at the near-infrared band. The structure avoids the polarization coupling in conventional optical fiber and contains high air-filling fraction, which will be advantageous for sensing application to ensure strong interaction with liquid or gas filled in the air-holes, as a result, high sensitivity can be achieved.4. The SPR sensor based on MOF with two layers of annular-shaped holesIn this dissertation, we propose for the first time a novel SPR sensor design based on MOF with two layers of annular-shaped holes. In the proposed structure, a small fiber core is applied to extending the energy of the core mode to the first air-hole layer, so that the effective refractive index of the core-guided mode can be reduced. The resonance wavelength can be tuned across the C+L-band and the low operation frequency of the sensor increases plasmon penetration depth into the analyte, thus enhancing sensor sensitivity. In the first layer of holes, the sector ring arms, used as supporting stripes, with increased wideness along the radius can tune the resonance depth and guarantee the Gaussian intensity distribution of a core-guided mode, which facilitating the excitation by standard Gaussian laser sources.