M. Shah Alam

Design Optimization of Equiangular Spiral Photonic Crystal Fiber for Large Negative Flat Dispersion and High Birefringence

The design of a residual dispersion compensating fiber in an equiangular spiral photonic crystal fiber (ES-PCF) structure is presented in the wavelength range 1350-1650 nm. A step-by-step design optimization is demonstrated and a maximum optimized value of average dispersion - 393 ps/nm km with a flattened dispersion profile with a very high birefringence is reported. It has been found that for the ES-PCF, a very high birefringence of 0.0278 at the wavelength 1550 nm can be obtained by employing an elliptical air hole as defect in the core region. An efficient full-vectorial finite-element method with vector edge element is employed in this study. The proposed ES-PCF can be an excellent candidate to be used in a wavelength-division-multiplexing optical fiber transmission system for residual chromatic dispersion compensation as well as maintaining single polarization.

A Highly Nonlinear Spiral Photonic Crystal Fiber for Tailoring Two Zero Dispersion Wavelengths in the Visible Region

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Dispersion and nonlinearity properties of a chalcogenide As 2 Se 3 suspended core fiber

A highly nonlinear suspended core fiber (SCF) has been proposed, where a geometrical design parameter called suspension factor (SF) has been used for dispersion tailoring in the infrared region (1.2 μm to 2.8 μm). We have investigated the effect of different suspended conditions of the SCF core on group velocity dispersion, fiber nonlinearity, and power distributions. Peak effective nonlinearity (∼500 W−1 m−1 at 1200 nm with eight air holes) can be varied significantly with SF. The effect of SF on tailoring the zero dispersion wavelengths has been observed. The SF can also be utilized to control mode overlap between core mode and air hole mode for different sensing applications.

Design of a spiral silica photonic crystal fiber for nonlinear applications in visible region

A silica spiral photonic crystal fiber is presented here for tailoring two zero dispersion wavelengths (ZDWs) in the visible region. The proposed fiber has two ZDWs (523.1 and 716.8 nm) along with a very high nonlinearity parameter (1060 W−1 km−1 at 500 nm) around the visible region. The proposed design shows improvement over the group dispersion control and air holes collapsibility of highly air filled hexagonal photonic crystal fiber (HPCF), and low damage threshold of the soft glass photonic crystal fiber. Besides, the low air filling fraction (≈43%) of the proposed design reduces the probability of sustaining higher order modes in the fiber and also ensures easy fabrication due to fewer air holes.

Ultra-Broadband Confinement in Deep Sub-Wavelength Air Hole of a Suspended Core Fiber

We demonstrate low loss (0.4043 dB/Km at 1.55 μm) deep sub-wavelength broadband evanescent field confinement in low index material from near IR to mid IR wavelengths with the aid of an specialty optical fiber whilst achieving at least 1.5 dB improvement of figure of merit over the previous design. Plane strain analysis has been conducted to foresee fiber material dependent fabrication challenges associated with such nanoscale feature due to thermal stress. Size dependence of air hole is explained rigorously by modifying the existent slot waveguide model. We report significant improvement of field intensity, interaction length, bandwidth and surface sensitivity over the conventional free standing nanowire structure. The effect of metal layer thickness on surface plasmon resonance sensitivity is explored as well. A method to obtain strong evanescent field in such structure for medical sensing is also demonstrated. The proposed technique to enhance sub-wavelength confinement is expected to be of potential engineering merits for optical nanosensors, atomic scale waveguide for single molecule inspection and ultra-low mode volume cavity.

An Extremely Large Mode Area Microstructured Core Leakage Channel Fiber With Low Bending Loss

A technique of mitigating the reduction of mode area in bent large mode area (LMA) fibers by employing higher index rods in one region of the core along with lower index microstructured rods at the opposite region has been presented in this paper. The effective area is further increased by lowering the index difference between the background and some selected rods in the cladding. Apart from this, a very large ratio of higher order mode confinement loss to the fundamental mode confinement loss is achieved to ensure the single modedness. The ingenious techniques have enabled the proposed design to exhibit effective area over 4000 μm 2 allied with very low bending loss at the wavelength of 1.064 μm.

A Tunable Photonic Double Heterostructure Cavity on Ferroelectric Barium Titanate

We designed a photonic double heterostructure cavity on rectangular lattice with an incorporated slot waveguide, which shows high quality factor for both TE and TM modes despite an absence of complete bandgap. The confinement capability of the cavity has been studied using the finite difference time domain method. The ferroelectric barium titanate (BaTiO3) is used as the base material of the proposed cavity. The dispersive and absorptive natures of the material have been incorporated in the calculations. The presence of the slot waveguide improved the quality factor of the cavity, confirming the existing results. Despite the absorptive nature of BaTiO3, the finite extent of the photonic crystal and absence of complete bandgap, the calculated quality factor compares favorably with values reported previously in the literature.

A square lattice photonic crystal fiber based surface plasmon resonance sensor with high sensitivity

A surface plasmon resonance (SPR) sensor design based on four-core square lattice photonic crystal fiber (SL-PCF) has been proposed in this work. The fundamental coupling properties between the core guided light and surface plasmon polaritons are investigated here by using the finite element method (FEM). It is found that the phase matching phenomenon plays an important role in mode coupling. Through optimization of the design and numerical investigations, it is found that the use of SL-PCF with four cores yields high average sensitivity of 7432 nm/RIU in the sensing range of 1.43 to 1.50. Our study suggests that the use of SL-PCF with four cores is a promising possibility for surface plasmon resonance sensor design.

Birefringence Properties of Side-Hole Optical Fibers

An analysis of birefringence properties of side-hole optical fibers has been presented in this work by using a finite element method. The birefringence caused by the thermal stress due to the different thermal expansion coefficients of core and cladding and also the geometrical birefringence are simultaneously considered. The validity of the calculations are checked for circular fibers with circular or elliptical core by comparing with the existing results. The influences of core ellipticity, air-hole radius and position, as well as the thermal expansion coefficient of core and cladding materials are discussed. The change of refractive index in x and y polarized light due to photo-elastic effect is also calculated. The birefringence increases with the core ellipticity as the stress in and around the core gets higher with the increase in the core ellipticity and pitch length.

Bend-insensitive single-mode photonic crystal fiber with ultralarge effective area for dual applications

Abstract. A novel photonic crystal fiber (PCF) having circular arrangement of cladding air holes has been designed and numerically optimized to obtain a bend insensitive single mode fiber with large mode area for both wavelength division multiplexing (WDM) communication and fiber-to-the-home (FTTH) application. The bending loss of the proposed bent PCF lies in the range of 10−3 to 10−4  dB/turn or lower over 1300 to 1700 nm, and 2 × 10−4  dB/turn at the wavelength of 1550 nm for a 30-mm bend radius with a higher order mode (HOM) cut-off frequency below 1200 nm for WDM application. When the whole structure of the PCF is scaled down, a bending loss of 6.78×10−4  dB/turn at 1550 nm for a 4-mm bend radius is obtained, and the loss remains in the order of 10−4  dB/turn over the same range of wavelength with an HOM cut-off frequency below 700 nm, and makes the fiber useful for FTTH applications. Furthermore, this structure is also optimized to show a splice loss near zero for fusion-splicing to a conventional single-mode fiber (SMF).