Structure Design And Property Study Of High-Birefringence/Broad-bandwidth Microstructured Terahertz Fiber

Terahertz (THz) electromagnetic radiation is normally defined as lying in thespecial region which is just between the microwave and optical frequency. It isbelieved to exhibit lots of peculiar properties and many potential applications due tothis special band region. However, most of the THz systems and THz-communicationexperiments are performed in the free space, which inevitably exhibits many kinds ofproblems, such as the stupendous volume, the difficult alignment, the uncertainabsorption loss influenced by the surroundings, etc. Therefore, it is needed toinvestigate proper waveguides to propagate THz waves. The polymer has thecharacterizations of diversiform fabricated methods, low moulding temperature, lowcost in material and light weight. What’s more, in the THz region, it also has lowabsorption loss and low dispersion. Besides, microstructured optical fibers have manyfancy merits such as tunable bandwidth and dispersion, endless single-mode, highnonlinearity, high birefringence and so on, which is unaccessible by single-modefibers. Therefore, to develop proper microstructured polymer optical fibers, as onekind of THz waveguides, which have the functions of polarization-maintaining, broadbandwidth and flattened dispersion is quite important for the new generation of THztransmission technology. The main details are described as follows:(1) A kind of low-loss, broadband, single-mode and polarization-maintainingmicrostructured polymer porous fiber is designed and optimized for THz-wavepropagation. The theory simulations based on full vector finite element method areused to analyze the single mode property, birefringence and transmission loss of thisporous fiber. Numerical simulations show that, by rotating the major axis of theelliptical air-hole, the birefringence can be enhanced and there exists an optimalrotating angle at30. The birefringence, in a wide single-mode frequency range, ashigh as0.0445can be received at this optimal angle. Besides, low-loss THz-waveguidance can be obtained as a result of effective reduction of the material absorptionin such a porous fiber. The effective mode loss, which depends on the material of Topascyclic olefin copolymer (COC), in the single mode range from0.73to1.18THz, can be well suppressed no larger than0.20and0.15dB/cm, respectively for thex-and the y-polarization modes. Owing to these analyses, it demonstrates this porousfiber is very useful for single-mode polarization-maintaining and low-loss THz-waveguidance.(2) By intentionally combing the porous fiber and the air-core photonic bandgapfiber, a novel kind of broadband, low-loss, dispersion flattened porous-core photonicbandgap fiber is proposed for THz-wave propagation. Its broad bandwidth isperformed by use of the plane wave expansion method. The THz-wave propagationbandwidth of this fiber is enhanced efficiently compared with the traditional air-corebandgap fiber. Numerical simulations are employed by the full vector finite elementmethod to analyze the transmission loss and dispersion properties. Owing to theTopas COC, the transmission loss of this photonic bandgap fiber can be furtherreduced. Besides, this simple structure fiber processes the advantages of flatteneddispersion. The frequency range for the dispersion among±2.5ps/THz/cm is aslarge as170GHz, which is acceptable in THz remote monitoring and communication.