Mode property of a kind of Terahertz (THz) polymer tubes including a circular polymer tube (CPT) and an elliptical polymer tube (EPT) are investigated in this paper. Low-loss property of the THz polymer tube is achieved due to the ring-structure cross section of the polymer tube where the central air core traps a large part of the mode power of the fundamental guiding mode, where the air is almost lossless for THz wave. Effective indexes, mode profile, single mode property, and relative absorption loss of both the CPT and the EPT are presented. Simulation results show that the CPT with a cross section of a circular ring structure exhibits better loss property than a conventional solid polymer fiber. The EPT with a cross section of an elliptical ring structure can achieve a high birefringence (up to the order of 0.01), which is due to the effect of the continuity of the electric displacement vector in the normal direction of the interfaces between the polymer materials and the air in the cross section.
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