Rong-Jin Yu

Highly birefringent rectangular lattice photonic crystal fibres

The dispersive and birefringent properties of rectangular lattice photonic crystal fibres with circular air-holes are investigated. The fibres have twofold rotational symmetry and present the split of the fundamental cladding mode and the lifted degeneracy of the two polarization states, which lead to large birefringence and special behaviour of group-velocity walk-off. The properties are closely tied to the underlying radiation states of the rectangular lattice. In addition, the technique of fabricating the proposed fibres with the stack-and-draw procedure is presented.

Analysis of photonic bandgaps in modified honeycomb structures

By introducing an additional air hole into the center of each unit cell of the conventional honeycomb lattice, a new kind of structure that can support air guidance of light with low-order bandgaps is proposed. Numerical investigation indicates that, due to the generation of the lower order bandgaps, larger air-guiding photonic bandgaps (PBGs) than those of triangular structures can be achieved in the new structures. Furthermore, the proposed structures also provide additional freedom in tailoring PBGs.

Proposal for Ultralow Loss Hollow-Core Plastic Bragg Fiber With Cobweb-Structured Cladding for Terahertz Waveguiding

The transmission losses of a terahertz (THz) wave in a hollow-core Bragg fiber with cobweb cladding are analyzed. The numerical results show that the losses in the wavelength range of 70-1000 mum are below 8.5times10-5 dB/cm. The lowest loss of 9.03times10-6 dB/cm or 2.08times10-6 cm-1 is obtained for Fiber A at 90mum. The losses in this new class of Bragg fibers are reduced to far lower than those of other fibers for a THz wave reported so far in the literature

Polarization properties of elliptical-hole rectangular lattice photonic crystal fibres

We explore the polarization properties of elliptical-hole rectangular lattice photonic crystal fibres. Analysis of the cladding fundamental space-filling modes indicates that the anisotropic properties of the cladding are closely tied to the underlying radiation states of the pure lattices, which are determined by the ellipticity of the elliptical hole, the width and height ratio of the rectangular lattice and the air-filling fraction. Furthermore, optimization of the fibre core by inserting a centre hole is also presented. The achievement of high birefringence and a broad single-mode region in the fibres is demonstrated numerically.

Confinement losses and optimization in rectangular-lattice photonic-crystal fibers

Confinement losses in highly birefringent rectangular-lattice photonic-crystal fibers (RLPCFs) are investigated by the multipole method. The losses are strongly polarization dependent, which is due to the anisotropic properties of the cladding. The feature is applied to construct a new kind of single-polarization single-mode photonic-crystal fibers (PCFs), which is based on large differential loss between the two polarizations of the fundamental mode. Furthermore, it is found that the birefringence of RLPCFs is determined mainly by the air holes near the defect core. Consequently, a sandwiching (between triangular structures) rectangular configuration that has the property of reducing leakage loss while simultaneously retaining the birefringence of RLPCF is proposed.

Coupling characteristics of dual-core rectangular lattice photonic crystal fibres

We analyse the coupling characteristics of polarized lights in highly birefringent dual-core rectangular-lattice photonic crystal fibres (PCFs) by the plane-wave expansion method. The design of polarization independent coupling devices based on the PCFs is investigated. We also propose a new kind of polarization splitter based on the proposed PCFs, the length of which can be optimized by appropriately selecting the radii of the defect centre air holes. The coupling properties of the proposed PCF polarization splitter are verified by the full vectorial beam propagation method.

Wavelength-selective coupling of dual-core photonic crystal fiber and its application

We investigate the coupling characteristics of photonic crystal fiber composed of two cores with different index profiles. The index curves of the fundamental modes of the two cores can cross at a desired wavelength by choosing appropriate index profiles of the cores. As a result, wavelength-selective coupling is achieved. The designed coupler can be applied as an optical fiber bandstop filter with 26-nm bandwidth.

Polarization and leakage properties of rectangular lattice photonic crystal fibres with doped cores

Based on the analysis of birefringence and leakage properties in rectangular lattice photonic crystal fibres (RLPCFs), we present a simple technique of improving the optical properties of the fibres by the introduction of a doped core. The advantage of a doped core is firstly investigated by the comparison of the mode profiles of RLPCFs with a pure silica core, an additional air-hole in the core and a down-doped silica core. The influence of doping level on that of birefringence and leakage losses of the fibres is also explored. Our numerical simulation indicates, for PCFs with a anisotropic cladding, increased birefringence and reduced leakage losses can be obtained by a down-doped core. The proposed technique also has the merit of enlarging the mode field without the reduction of birefringence, therefore, coupling light to the fibre will become easier.