Calculation of optical-waveguide grating characteristics using Green's functions and the Dyson's equation

We present a method for calculating the transmission spectra, dispersion, and time delay characteristics of optical-waveguide gratings based on Green's functions and Dyson's equation. Starting from the wave equation for transverse electric modes we show that the method can solve exactly both the problems of coupling of counter-propagating waves (Bragg gratings) and co-propagating waves (long-period gratings). In both cases the method applies for gratings with arbitrary dielectric modulation, including all kinds of chirp and apodisation and possibly also imperfections in the dielectric modulation profile of the grating. Numerically, the method scales as O(N) where N is the number of points used to discretize the grating along the propagation axis. We consider optical fiber gratings although the method applies to all 1D optical waveguide gratings including high-index contrast gratings and 1D photonic crystals.Comment: 16 pages including 16 figure

Fixing of volume holograms in ferroelectric K_(1-y)Li_yTa_(1-x)Nb_xO_3

We report on the fixing of photorefractive volume holograms in potassium lithium tantalate niobate with ionic gratings and also with ferroelectric domain-reversed gratings. A diffraction efficiency of 55% is obtained with domain reversal in a 2-mm-thick ferroelectric phase K1–yLiyTa1–xNbxO3 crystal doped with Co, V, and Ti. We measured the decay rate of the domain gratings and also of the initial electron gratings and ion gratings. The domain grating decay agrees with Vogel–Fulcher fits. The activation energies for ionic and electronic conductivity are 0.76 and 0.12 eV, respectively

Curved Gratings as Plasmonic Lenses for Linearly Polarised Light

The ability of curved gratings as sectors of concentric circular gratings to couple linearly polarized light into focused surface plasmons is investigated by theory, simulation and experiment. Curved gratings, as sectors of concentric circular gratings with four different sector angles, are etched into a 30-nm thick gold layer on a glass coverslip and used to couple linearly-polarised free space light at nm into surface plasmons. The experimental and simulation results show that increasing the sector angle of the curved gratings decreases the lateral spotsize of the excited surface plasmons, resulting in focussing of surface plasmons which is analogous to the behaviour of classical optical lenses. We also show that two faced curved gratings, with their groove radius mismatched by half of the plasmon wavelength (asymmetric configuration), can couple linearly-polarised light into a single focal spot of concentrated surface plasmons with smaller depth of focus and higher intensity in comparison to single-sided curved gratings. The major advantage of these structures is the coupling of linearly-polarised light into focused surface plasmons with access to and control of the plasmon focal spot, which facilitates potential applications in sensing, detection and nonlinear plasmonics.Comment: 15 pages and 12 figure

Advanced fibre Bragg grating structures: design and application

This thesis presents experimental and computational work on a variety of advanced fibre Bragg grating structures covering long dispersion compensating chirped Bragg gratings, superstructured Bragg gratings for identical multiple channel operation, Bragg gratings for pulse-shaping applications and Bragg gratings for add-drop applications in high bit-rate systems. Development of the fabrication-technique developed and analysed as a part of this work has led to a number of experimental 'firsts', including the meter-long Bragg gratings with dispersion-characteristics designed to compensate simultaneous linear and higher order dispersion. Upon transfer of this technology to our industrial partners, a number of field-trial experiments utilising gratings written using this fabrication technique have been successfully performed. Some of the requirements identified from customers led to the discovery of the importance and understanding of high-quality reflection and time-delay profiles. Another product of the high flexibility provided by the developed fabrication technique have led to demonstrations of superstructured Bragg gratings for a number of exciting applications such as multiple-channel filters obtained through a periodic sinc modulation of the refractive index-profile in fibre Bragg gratings and pulse-reshaping from a soliton to square-pulse with applications in high-speed demultiplexing. Additionally, it is discussed how uniform apodised Bragg gratings filters for application in dense WDM networks, despite their near ideal spectral performance, suffer from non-linear phase attributes in the stop-band, that could limit their use in high bit-rate systems (10Gbit/s and above). Linear phase-filters for dispersion-free filtering are proposed and demonstrated as a solution to this problem for bit-rates up to 40Gbit/s and channel spacings as narrow as 25GHz

Performance Testing of a Novel Off-plane Reflection Grating and Silicon Pore Optic Spectrograph at PANTER

An X-ray spectrograph consisting of radially ruled off-plane reflection gratings and silicon pore optics was tested at the Max Planck Institute for extraterrestrial Physics PANTER X-ray test facility. The silicon pore optic (SPO) stack used is a test module for the Arcus small explorer mission, which will also feature aligned off-plane reflection gratings. This test is the first time two off-plane gratings were actively aligned to each other and with a SPO to produce an overlapped spectrum. The gratings were aligned using an active alignment module which allows for the independent manipulation of subsequent gratings to a reference grating in three degrees of freedom using picomotor actuators which are controllable external to the test chamber. We report the line spread functions of the spectrograph and the actively aligned gratings, and plans for future development.Comment: Draft Version March 19, 201

Theory and simulation of subwavelength high contrast gratings and their applications in vertical-cavity surface-emitting laser devices

This work intends to fully explore the qualities and applications of subwavelength gratings. Subwavelength gratings are diffraction gratings with physical dimensions less than the wavelength of incident light. It has been found that by tailoring specific dimension parameters, a number of different reflection profiles can be attained by these structures including high reflectivity or low reflectivity with broad and narrow spectral responses. In the course of this thesis the physical basis for this phenomenon will be presented as well as a mathematical derivation. After discussion of the mechanics of the reflection behavior, the methods used in modeling subwavelength gratings and designing them for specific functions will be explored. Following this, the fundamentals of vertical-cavity surface-emitting lasers (VCSELs) will be discussed, and the applications of subwavelength gratings when used with these lasers will follow. Several devices, both theoretical proposals and fabricated examples, will be presented in addition to the available performance measurements. Finally, the fabrication challenges that restrict subwavelength gratings from adoption as standard components in VCSEL design will be considered with regard to ongoing fabrication research

Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: static and dynamic studies

The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other