A rigorous modal solution approach, based on the numerically efficient finiteelement method (FEM), has been used to design and characterize a photonic crystal fiber (PCF) with a porous air core, which has the potential for use for low-loss guidance of terahertz (THz) waves. Here, for the first time, it is reported that a large fraction of the power that is also well confined in the waveguide can be guided in the low-loss air holes, thus to reduce the overall modal loss. This novel PCF design can readily be fabricated by use of a range of techniques including stack-and-draw, extrusion, and drilling.

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An improved design of Teflon photonic crystal fiber (PCF) with a porous air-core is presented for low-loss terahertz guidance. Optimization of total power confinement in the air-holes, together both in the cladding and core regions, is carried out for both quasi-TE and quasi-TM polarizations by using a full-vectorial finite element method (FEM). To achieve the polarization maintenance, modal birefringence is enhanced by destroying the circular symmetry with the introduction of unequal size air-holes in the first ring.

This is the unspecified version of the paper.This version of the publication may differ from the final published version. In this work, it is shown that the differential loss between the TE-and TM-polarized fundamental modes in a highly birefringent photonic crystal fiber (PCF) can be enhanced by bending the fiber. As a result, a design approach for single-mode single-polarization operation has been developed and is discussed. A rigorous full-vectorial H-field-based finite element approach, which includes the conformal transformation and the perfectly matched layer, is used to determine the single-polarization properties of such a highly birefringent PCF by exploiting its differential bending losses. Permanent

A rigorous modal solution approach based on the numerically efficient finite element method has been used to design a tapered Photonic Crystal Fibre with a large mode area that could be efficiently coupled to an optical fiber. We report here for the first time that the expanded mode area can be stabilized against possible fabrication tolerances by introducing a secondary surrounding waveguide with larger air-holes in the outer ring. A full-vectorial H-field approach is employed to obtain mode field areas along the tapered section and the Least Squares Boundary Residual (LSBR) method is used to obtain the coupling coefficients to a butt-coupled fiber.