Masashi Eguchi

Single-mode single-polarization holey fiber using anisotropic fundamental space-filling mode.

We present the single-mode single-polarization regime of a circular-hole holey fiber consisting of a core with large elliptical holes. The elliptical holes in the core, which produce large anisotropies, split the fundamental mode into two orthogonally polarized fundamental modes, often referred to as slow and fast modes. This fiber can guide only one polarization state of the fundamental mode when a fundamental space-filling mode index of the cladding region is designed to lie between these indices of the slow and fast modes of the core region. We demonstrate one design example of this fiber and show that the single-polarization regime can be achieved over a wide wavelength range.

Vectorial wave analysis of side-tunnel type polarization-maintaining optical fibers by variational finite elements

Vectorial wave analysis of side-tunnel type polarization-maintaining optical fibers is presented using the vector H -field finite-element method, which is applicable to the fibers having arbitrarily cross-sectional shape. First, to improve the accuracy of solutions, several techniques are investigated such as the zero-extrapolation method, curvilinear elements, and an improved virtual-boundary method. After checking the accuracy of solutions, the relation between the shape of the fibers and their polarization-mode properties, such as the axial propagation constants, the modal birefringence, the polarization-mode dispersion, and the magnetic-field distribution, is studied in detail. Also studied are the optical fields of side-tunnel fibers, which have never been investigated, and the mechanism for the phenomenon that the propagation constants of the fundamental modes come extraordinarily close to those of the first higher-order modes is clarified.

Design of Polarization Splitter With Single-Polarized Elliptical-Hole Core Circular-Hole Holey Fibers

We propose a novel polarization splitter based on coupled elliptical-hole core circular-hole holey fibers (EC-CHFs). Utilizing the single-polarization nature of the EC-CHFs, in the proposed polarization splitter, two orthogonally polarized waves couple only to different EC-CHFs and the crosstalk-free polarization splitting is realized. In addition, the coupling length for two orthogonally polarized waves can be independently designed and this splitter is easy to design.

Accurate finite-element analysis of dual-mode highly elliptical-core fibers

A scalar finite-element method is used for investigating propagation characteristics of dual-mode highly elliptical-core fibers. The dispersion property, polarization modal birefringence, and spatial modal birefringence are calculated. To improve the accuracy of solutions, isoparametric curvilinear elements are introduced. The applicability of the scalar finite-element method for highly elliptical-core fibers is assessed by comparing the results obtained with the vector finite-element method. An approximate simple approach, in which an elliptical-core is replaced by an appropriate rectangular-core, is also examined. >

Design of single-polarization elliptical-hole core circular-hole holey fibers with zero dispersion at 1.55 μm

The dispersion properties of single-mode single-polarization elliptical-hole core circular-hole holey fibers (EC-CHFs) are investigated. In these fibers, the single-polarization regime can be easily achieved by the anisotropic fundamental-space filling modes (FSMs) in the core region and isotropic FSMs in the cladding region. We show that there is a remarkable difference between the mode fields of EC-CHFs having two mutually orthogonal directions of elliptical holes, although their dispersion properties are very close to each other. We also demonstrate that, for small air holes, the single-polarization EC-CHFs with zero dispersion at 1.55 μm are easily designed by the FSMs, whereas, in the case of a large lattice pitch, tuning zero-dispersion wavelength is needed for designed EC-CHFs by using the FSMs. Moreover, our rough bending-loss estimations based on the weakly guiding fiber approximation show that low bending losses can be achieved in the EC-CHFs.

Finite-element modal analysis of large-core multimode optical fibers.

Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method hasbeen used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable fornonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations.

Squeezed lattice elliptical-hole holey fiber with extremely high birefringence

A novel highly flat elliptical-hole holey fiber with a squeezed lattice is introduced for achieving extremely high birefringence. The fiber consists of two kinds of elliptical holes with the same major axis length and different minor axis lengths for the core and cladding regions. Numerical results demonstrate a modal birefringence exceeding 0.065.

Single-Polarization Elliptical-Hole Lattice Core Photonic-Bandgap Fiber

In photonic bandgap (PBG) fibers, light is confined by a photonic bandgap caused by a periodic structure of air holes in the cladding regions. The doubly degenerate fundamental mode in ideal PBG fiber structures becomes slightly nondegenerate in actually produced fibers, and this causes polarization instability and polarization mode dispersion. Here, to avoid these problems, we propose a novel absolutely single-polarization PBG fiber structure with an elliptical-hole lattice core. A PBG fiber with a single-polarization bandwidth of 420 nm is numerically demonstrated. Furthermore, based on the proposed fiber structure, we report another single-polarization PBG fiber that has two absolutely single-polarization bands being orthogonal to each other.

Geometrical birefringence in square-lattice holey fibers having a core consisting of a multiple defect

Geometrical birefringence in circular- and elliptical-hole square-lattice holey optical fibers having a core consisting of a multiple defect is investigated. The effect of unidirectional extension of the core area on the birefringence of these holey fibers (HFs) is discussed. We demonstrate that, as expected, a high birefringence can be induced in circular-hole HFs by unidirectional extension of the core area. In contrast, the maximum birefringence of elliptical-hole HFs, the holes of which have their major axes parallel to the long axis of the core, is achieved in a core structure that has a single defect. We also found that for elliptical holes having their major axes orthogonal to the long axis of the core, an exchange of the fast and slow modes occurs between the two orthogonally polarized fundamental modes by birefringence compensation.

Study on Crosstalk-Free Polarization Splitter With Elliptical-Hole Core Circular-Hole Holey Fibers

We proposed a novel polarization splitter based on single-polarized elliptical-hole core circular-hole holey fibers (EC-CHFs). We employed the full-vector finite element beam propagation method to demonstrate the polarization splitter which is designed by the large hole EC-CHFs (air filling fraction in core is 36.73%) and small hole EC-CHFs (air filling fraction in core is 4.08%) can completely split an arbitrarily polarized light beam into two orthogonal polarization states without any crosstalk. In addition, we also calculated the tolerance and wavelength dependence of this kind of polarization splitter.