Influence of reflected radiation waves caused by large mode field and large refractive index mismatches on splice loss evaluation between elliptical-hole lattice core holey fibers and conventional fibers

When the mode field and refractive index mismatches between two spliced fibers are small, the splice loss is generally evaluated by calculating an overlap integral without reflection waves. A single-polarization circular-hole holey fiber with a core consisting of an elliptical-hole lattice (EC-CHF) has a strikingly different mode field caused by elliptical holes in the core region from those of conventional single-mode fibers (SMFs) and, thus, reflected radiation modes may significantly appear in splicing an EC-CHF to conventional SMFs. We study the influence of reflected radiation modes on the splice loss evaluation of optical fibers with large mode field and large refractive index mismatches through numerical analyses using a bidirectional eigenmode propagation method and a three-dimensional finite-element method

Analysis of Multiple Waveguide Discontinuities Using Propagation Operator Method and Beam Propagation Method

We apply an analysis scheme combining propagation operator method (POM) and beam propagation method (BPM) to waveguide discontinuity problems. In this approach, finite element method (FEM) with higher adaptability to waveguide geometry is utilized for discretizing waveguide cross section. While BPM based on FEM (FE-BPM) can efficiently analyze waveguides with long propagation distance, it encounters serious degradation of accuracy when waveguides with discontinuous structure has to be analyzed. The presented method overcomes this defect by applying POM which can analyze discontinuous facets and sufficiently cover various mode coupling

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

Full-vectorial analysis of optical waveguide discontinuities using a propagation operator method based on the finite element scheme discontinuities using a propagation operator method based on the finite element scheme

We propose an efficient numerical method for the full-vectorial analysis of three-dimensional (3-D) optical waveguide discontinuities. In this method, the finite element method with higher adaptability and flexibility is employed to discretize the waveguide cross section. In order to calculate the square root of the characteristic matrix, the Denman-Beavers iterative scheme is used. Applying this method to 3-D strongly guiding waveguide discontinuity problems, the modal reflectivities of the fundamental TE-like and TM-like modes are calculated. These results show unique vector properties and significantly differ from those of scalar analysis because various mode couplings between the field components occur at the discontinuity facet and they cannot be ignored

A Study on Topology Optimization of Plasmonic Waveguide Devices Using Function Expansion Method and Evolutionary Approach

We propose a novel topology optimization method for plasmonic devices. Plasmonic devices that have a great potential to downsize various optical devices beyond the diffraction limit attract a lot of attention. In order to develop high-performance plasmonic devices, a novel design theory is expected to be established instead of the conventional theory for dielectric waveguide devices. In this paper, we employ the function expansion method to express a device structure in the design region and optimize the design variables by using several evolutionary approaches, which do not require the sensitivity analysis. The validity and usefulness of this approach are demonstrated through the design examples of optical diode and optical circulator

Design of tapered polarization splitter based on EC-CHFs by full-vectorial FE-BPM using coordinate transformation

We propose and design a tapered polarization splitter (PS) using the single-polarization property of an elliptical-hole core circular-hole holey fiber (EC-CHF). It is reported that polarization splitting without any crosstalk is enabled using the PS with EC-CHF. In this paper, we show that the mode coupling is enhanced with a tapered PS structure, and the full waveguide separation at the input and output ends can be simultaneously realized. We also propose a full-vectorial finite element beam propagation method using coordinate transformation to efficiently analyze a waveguide whose structure varies along the longitudinal direction. In this approach, computational accuracy and efficiency are greatly improved. (C) 2020 Optical Society of Americ

Topology optimal design for optical waveguides using time domain beam propagation method

New topology optimal design approach for optical waveguide devices using a time domain beam propagation method (TD–BPM) is presented. A sensitivity analysis method for topology optimization using TD–BPM is formulated based on an adjoint variable method (AVM). A density method is used as a way to represent refractive index distribution. As design examples, a loss–reduced bending waveguide and a reflector are designed. It is confirmed that our design approach can surely enhance the performance of optical waveguide devices

Design of polarization splitter and rotator using function-expansion based topology optimization considering two-layer structure

Function expansion based topology optimization method for optical waveguide devices has been proposed as an automatic design method which can produce an optimal device structure having an arbitrary topology. In this paper, we aim to extend the function expansion based topology optimization method to the design problems of three-dimensional photonic devices with structural variation in the depth direction. We confirm the effectiveness of our approach through the design example of the polarization splitter and rotator which utilizes structural asymmetry in the depth direction

Ultra-small shape-simplified optical diode derived from topology optimal design in plasmonic waveguide

We present in this paper an ultra-small plasmonic optical diode, which converts a symmetric mode into an asymmetric one and reflects backward propagation of the symmetric one, with a metal-insulator-metal (MIM) structure. A profile of an optical diode is derived from a topology optimal structure obtained in our previous study, and it is simplified so as to reduce insertion loss and difficulty in fabrication. An optimal profile is found out using a differential evolution (DE) which is one of the evolutionary algorithms, and optimization is carried out taken into account fabrication tolerance. According to the results of numerical simulation by 2D finite element method (FEM), the optimized plasmonic optical diode has insertion loss of <0.5 dB, reflection of <-20 dB in the forward propagation, and backward transmission of <-20 dB over C-band, and it is tolerant of ±5 nm boundary deviation. In addition, this device has an extremely small functional region (<1.5 μm)

Efficient Shape and Topology Optimization Based on Sensitivity Analysis for Optical Waveguide Devices Utilizing Full-Vectorial BPM

We present a new design approach of shape and topology optimization utilizing the full-vectorial beam propagation method (FV-BPM) so as to achieve efficient design for high-refractive-index contrast optical waveguides and devices based on gradual mode-evolution. In our design approach, material distribution is represented via normalized density parameters, and these design parameters are updated with sensitivity computed based on an adjoint variable method (AVM). The sensitivity analysis method and the design procedure are offered in the specific case where the density method, the AVM, and the FV-BPM based on an alternative direction implicit method (AIDM) are employed. The applicability of our design approach is numerically studied by designing high-refractive-index contrast photonic components which induce polarization rotation: a TM0-to-TE1 mode order converter and a polarization rotator. The results of the design examples indicate that our approach has potential to be suitable for efficient initial design of optical devices based on gradual mode-evolution