Rozprawy doktorskie na temat „Coupled waveguides”

Soliton states in three coupled optical waveguide systems were studied: two linearly coupled waveguides with quadratic nonlinearity, two linearly coupled waveguides with cubic nonlinearity and Bragg gratings, and a quadratic nonlinear waveguide with resonant gratings, which enable three-wave interaction. The methods adopted to tackle the problems were both analytical and numerical. The analytical method mainly made use of the variational approximation. Since no exact analytical method is available to find solutions for the waveguide systems under study, the variational approach was proved to be very useful to find accurate approximations. Numerically, the shooting method and the relaxation method were used. The numerical results verified the results obtained analytically. New asymmetric soliton states were discovered for the coupled quadratically nonlinear waveguides, and for the coupled waveguides with both cubic nonlinearity and Bragg gratings. Stability of the soliton states was studied numerically, using the Beam Propagation Method. Asymmetric couplers with quadratic nonlinearity were also studied. The bifurcation diagrams for the asymmetric couplers were those unfolded from the corresponding diagrams of the symmetric couplers. Novel stable two-soliton bound states due to three-wave interaction were discovered for a quadratically nonlinear waveguide equipped with resonant gratings. Since the coupled optical waveguide systems are controlled by a larger number of parameters than in the corresponding single waveguide, the coupled systems can find a much broader field of applications. This study provides useful background information to support these applications.

La integració de tots els components bàsics per a la generació, manipulació i detecció de llum en xips òptics està impulsant avenços científics i tecnològics, per exemple, en el desenvolupament de tecnologies de la informació o de dispositius de detecció per a les tecnologies quàntiques. Degut a la seva flexibilitat, escalabilitat i la possibilitat d’observar directament l’evolució de la funció d’ona utilitzant senzilles tècniques de tractament d’imatges, les estructures fotòniques integrades són una plataforma ideal per a la simulació quàntica, és a dir, per emular fenòmens quàntics que apareixen en altres branques de la física. A més, aquestes analogies òptiques-quàntiques també permeten dissenyar circuits fotònics integrats amb propietats excepcionals. En aquesta tesi aprofitem propietats no trivials de la física quàntica per dissenyar nous dispositius fotònics integrats amb funcionalitats avançades i rendiments millorats, així com nous simuladors fotònics. Específicament, explotem les similituds entre les equacions de Helmholtz i de Schrödinger, que permeten reproduir la dinàmica temporal d’una partícula atrapada en un potencial periòdic amb l’evolució espacial de la llum propagant-se en guies d’ona acoblades, per aplicar transformacions supersimètriques i processos adiabàtics així com explorar geometries topològiques no trivials en sistemes de guies d’ona òptiques acoblades. En aquesta línia, la primera part de la tesi està dedicada a introduir els conceptes físics i matemàtics que descriuen les guies d’ona òptiques acoblades, les analogies òptiques-quàntiques i la supersimetria en òptica. La segona part de la tesi engloba el disseny de nous dispositius fotònics integrats combinant l’aplicació de transformacions supersimètriques per manipular modes espacials amb tècniques de passatge adiabàtic per introduir la robustesa. Primer presentem un nou mètode per a la multiplexació de modes espacials basat en guies d’ona supersimetriques, que filtren els modes, en combinació amb la tècnica de passatge adiabàtic espacial que es fa servir per transmetre eficient i robustament els modes escollits entre guies. De manera similar, mantenint-nos en la idea d’aplicar protocols d’enginyeria quàntica per dissenyar nous dispositius fotònics amb rendiments millorats, proposem connectar de manera adiabàtica estructures supersimètriques al llarg de la distància de propagació. En particular, aquesta tècnica l’utilitzem per dissenyar guies d’ona còniques, filtres de modes, divisors de feixos i interferòmetres, eficients i robustos. Finalment, la tercera part de la tesi està dedicada a la simulació de diferents fenòmens quàntics utilitzant sistemes fotònics. Per començar aquesta part, explorem els efectes que les transformacions supersimètriques indueixen en sistemes amb propietats topologies no trivials, les quals estan intrínsecament lligades a les simetries internes del sistema. Amb aquest objectiu, considerem el sistema més simple amb propietats topològiques no trivials i demostrem en sistemes de guies d’ona acoblades com la protecció topològica d’un estat pot ser suspesa i restablerta utilitzant transformacions supersimètriques. A més, per accedir a aquestes fases topològiques no trivials, un element clau és la introducció de camps artificials gauge (AGF) que controlen la dinàmica de partícules no carregades que d’una altra manera eludeixen la influència dels camps electromagnètics estàndards. En aquesta línia, investiguem la possibilitat d’induir AGF utilitzant llum amb moment orbital angular en comptes de manipular la geometria del sistema. Específicament, mesurem l’efecte de gàbia d’Aharonov-Bohm que està lligat amb la presència d’un camp magnètic. Aquesta tècnica permet accedir a diferent règims topològics en una sola estructura, un pas important per a la simulació quàntica utilitzant sistemes fotònics.
La integración de todos los componentes básicos para la generación, manipulación y detección de luz en chips ópticos está impulsando avances científicos y tecnológicos, por ejemplo, en el desarrollo de tecnologías de la información o en los dispositivos de detección para las tecnologías cuánticas. Debido a su flexibilidad, escalabilidad y a la posibilidad de observar directamente la evolución de la función de onda utilizando senzillas técnicas de trata, las estructuras fotónicas son ideales para la simulación cuántica, es decir, para emular fenómenos cuánticos que aparecen en otras ramas de la física. Es más, estas analogías ópticas-cuánticas también permiten diseñar nuevos circuitos fotónicos integrados con propiedades excepcionales. En esta tesis, aprovechamos propiedades no triviales que emergen de la física cuántica para diseñar nuevos dispositivos fotónicos integrados con funcionalidades avanzadas y rendimientos mejorados, así como nuevos simuladores fotónicos. Específicamente, explotamos las similitudes entre las ecuaciones de Helmholtz y de Schrödinger, que permiten reproducir la dinámica temporal de una particula atrapada en un potencial periódico con la evolución espacial de la luz propagándose en guías de onda, para aplicar transformaciones supersimétricas y procesos adiabáticos así como explorar geometrías topológicas no triviales en sistemas de guías de onda ópticas acopladas. La primera parte de la tesis está dedicada a introducir los conceptos matemáticos y físicos que describen las guías de onda ópticas acopladas, las analogías ópticas-cuánticas y la supersimetria óptica. La segunda parte de la tesis engloba el diseño de nuevos dispositivos fotónicos integrados basados en combinar transformaciones supersimétricas para manipular los modos espaciales con las técnicas adiabáticas para introducir robustez. Primero presentamos un nuevo método para la multiplexación de modos espaciales basado en guías de onda supersimétricas, que filtran los modos, en combinación con la técnica de pasaje adiabático espacial que se usa para transmitir de manera eficiente y robusta los modos escogidos entre guías. De manera similar, manteniéndonos en la idea de aplicar protocolos de ingeniería cuántica para diseñar nuevos dispositivos fotónicos con rendimientos superiores, proponemos conectar de manera adiabática estructuras supersimétricas a lo largo de la propagación. En particular, ésta técnica la utilizamos para diseñar guías de onda cónicas, filtros modales, divisores de haz e interferómetros. Finalmente, la tercera parte de la tesis está dedicada a la simulación de diferentes fenómenos físicos utilizando sistemas fotónicos. Para empezar, exploramos los efectos que las transformaciones supersimétricas inducen en sistemas con propiedades topológicas no triviales, las cuales están intrínsecamente ligadas a las simetrías internas del sistema. Con este objetivo, consideramos el sistema más simple con propiedades topológicas no triviales y demostramos en un sistema de guías de onda acopladas cómo la protección topológica de un estado puede ser suspendida y restablecida utilizando transformaciones supersimétricas. Además, para acceder a las fases topológicas no triviales, un elemento clave es la introducción de campos artificiales de gauge (AGF) que controlan la dinámica de partículas no cargadas que de otra manera eluden la influencia de los campos electromagnéticos. Es esta línea, investigamos la posibilidad de inducir AGF utilizando luz con momento orbital angular en lugar de manipular la geometría del sistema. Específicamente, medimos el fenómeno de jaula de Aharonov-Bohm que está ligado a la presencia de un campo magnético sintético. Esta técnica permite acceder a diferentes regímenes topológicos en una sola estructura, un paso importante para la simulación cuántica utilizando sistemas fotónicos.
The integration of all the basic components for light generation, manipulation and detection in optical chips is boosting scientific and technological advances, for instance, in the development of information technology and data communications or of sensing devices for quantum technologies. Due to its flexibility, scalability and of the possibility of directly observing the wavefunction evolution using simple imaging techniques, integrated photonic structures are an ideal playground for quantum simulation i.e., for emulating quantum phenomena appearing in other branches of physics. Moreover, these quantum-optical analogies also allow to design novel integrated photonic circuits with exceptional properties. In this context, in this thesis we harness non-trivial properties stemming from quantum physics to design novel integrated photonic devices with advanced functionalities and enhanced performances as well as to engineer novel photonic simulators. Specifically, we exploit the similarities between the Helmholtz and the Schrödinger equations, which allow to mimic the temporal dynamics of a single particle trapped in a lattice potential with the spatial evolution of a light beam propagating in an array of optical waveguides, to apply supersymmetric (SUSY) transformations and adiabatic passage processes as well as to explore non-trivial topological geometries in systems of coupled optical waveguides. In this vein, the first part of the thesis is devoted to introduce the mathematical concepts and physical ideas behind coupled optical waveguides, quantum-optical analogies and optical SUSY. After that, the second part of the thesis encompasses the design of novel integrated photonic devices by combining the spatial modal content manipulation offered by SUSY transformations with the robustness supplied by adiabatic passage techniques. In this regard, we start by presenting a novel method for mode division (de)multiplexing rooted on SUSY waveguides, which provide the mode filtering capabilities, in combination with a Spatial Adiabatic Passage protocol, which is used to efficiently and robustly transfer the desired modes between waveguides. Similarly, keeping on the idea of applying quantum engineering protocols to design novel photonic devices with enhanced performances, we also propose to connect, in an adiabatic fashion, SUSY structures along the propagation direction. In particular, this technique is used to engineer efficient and robust tapered waveguides, mode filters, beam splitters and interferometers. Finally, the third part of the thesis is dedicated to the photonic simulation of different phenomena. We explore first the effect that SUSY transformations induce in systems with non-trivial topological properties, which are intrinsically connected with the system's internal symmetries. To this aim, we consider the simplest system with non-trivial topological properties and demonstrate in waveguide arrays how the topological protection of a targeted state can be suspended and reestablished by applying SUSY transformations. Moreover, to access these non-trivial topological phases, a key step is the introduction of Artificial Gauge Fields (AGF) controlling the dynamics of uncharged particles that otherwise elude the influence of standard electromagnetic fields. To this end, we investigate the possibility of inducing AGF by injecting light beams carrying Orbital Angular Momentum, rather than manipulating the geometry of the system. Specifically, we measure the Aharonov-Bohm caging effect, which is directly related with the presence of a synthetic magnetic flux, in an array of coupled optical waveguides. This technique paves the way towards accessing different topological regimes in one single structure, representing an important step forward for quantum simulation in photonic structures.

Lorsqu'un défaut est introduit dans un cristal phononique, des états apparaissent dans les bandes interdites et se localisent au niveau des défauts. Ils décroissent rapidement loin du défaut. Par conséquent, il est possible de localiser et de guider la propagation des ondes en concevant des défauts dans un cristal phononique parfait. Le guide d’onde à résonateurs couplés, fondé sur le couplage d'une séquence de cavités, présente simultanément un fort confinement des ondes et une faible vitesse de groupe ; il peut être utilisé pour concevoir des circuits plutôt arbitraires. En outre, la propagation des ondes élastiques dans une matrice solide peut être contrôlée en remplissant des cavités d'un fluide, sur la base des systèmes couplés fluides-solides. Ils ont des applications essentielles pour la réduction des vibrations et l’isolation acoustique. Dans cette thèse, les ondes acoustiques et élastiques se propageant dans les guides d’ondes à résonateurs couplés périodiques et apériodiques sont étudiées. L’interaction fluide-solide dans les cristaux phononiques fluide / solide est étudiée. Les travaux sont menés en combinant simulation numérique, analyse par modèles théoriques et investigation expérimentale
When a defect is introduced into a phononic crystal, states localized at the defect appear in the band gaps. They decay rapidly far away from the defect. Therefore, it is possible to localize and guide wave propagation by designing defects in the perfect phononic crystal. Coupled-resonator waveguides based on the coupling effect between a sequence of defect cavities have simultaneously strong wave confinement and low group velocity, and can be used to design rather arbitrary circuits. Furthermore, the propagation of elastic waves in a solid matrix can be controlled through changing fluid fillings based on fluid-solid interaction. Thus, they have essential applications in vibration reduction and noise isolation. In this thesis, the acoustic and elastic waves propagating in both periodic and aperiodic coupled-resonator waveguides are investigated. The fluid-solid interaction in fluid/solid phononic crystals is studied. The work is conducted by combining numerical simulations, theoretical model analysis and experimental investigations

The characteristics of slotted circular waveguides with different dimensions, including cutoff frequencies of TE and TM modes, impedance and modal field distributions will be analyzed using the generalized spectral domain approach. The Method of Moment will be applied, basis functions that include the edge conditions will be used and a computer program will be developed. Obtained results will be presented for different number, depth and thickness of coupling slots, and compared with available data to demonstrate the accuracy and the efficiency of the approach. Plots of the electric and magnetic field lines corresponding to the dominant as well as a number of higher order modes will be presented for quadruple ridge case.

There exists substantial applications motivating the study of nonlinear longitudinal wave propagation in layered (or laminated) elastic waveguides, in particular within areas related to non-destructive testing, where there is a demand to understand, reinforce, and improve deformation properties of such structures. It has been shown [76] that long longitudinal waves in such structures can be accurately modelled by coupled regularised Boussinesq (cRB) equations, provided the bonding between layers is sufficiently soft. The work in this thesis firstly examines the initial-value problem (IVP) for the system of cRB equations in [76] on the infinite line, for localised or sufficiently rapidly decaying initial conditions. Using asymptotic multiple-scales expansions, a nonsecular weakly nonlinear solution of the IVP is constructed, up to the accuracy of the problem formulation. The asymptotic theory is supported with numerical simulations of the cRB equations. The weakly nonlinear solution for the equivalent IVP for a single regularised Boussinesq equation is then constructed; constituting an extension of the classical d'Alembert's formula for the leading order wave equation. The initial conditions are also extended to allow one to separately specify an O(1) and O(ε) part. Large classes of solutions are derived and several particular examples are explicitly analysed with numerical simulations. The weakly nonlinear solution is then improved by considering the IVP for a single regularised Boussinesq-type equation, in order to further develop the higher order terms in the solution. More specifically, it enables one to now correctly specify the higher order term's time dependence. Numerical simulations of the IVP are compared with several examples to justify the improvement of the solution. Finally an asymptotic procedure is developed to describe the class of radiating solitary wave solutions which exist as solutions to cRB equations under particular regimes of the parameters. The validity of the analytical solution is examined with numerical simulations of the cRB equations. Numerical simulations throughout this work are derived and implemented via developments of several finite difference schemes and pseudo-spectral methods, explained in detail in the appendices.

In this work we propose the apodization or windowing of the coupling coefficients of the unit cells conforming a coupled resonator device as a mean to reduce the level of secondary sidelobes in the case of SCISSOR configuration [7] or reducing the passband ripples in the case of CROW configuration [8]. This technique is regularly employed in the design of digital filters [18] and has been applied as well in the design of other photonic devices such as corrugated waveguide filters [9] and fiber Bragg gratings [19]. We also propose a novel technique for the apodization of coupled resonator structures by applying a longitudinal offset between resonators in order to modify the power coupling constant, which alleviates the technical requirements required for the production of these devices. We will demonstrate the design, fabrication and characterization of CROW structures employing the apodization through the aforementioned technique.
Doménech Gómez, JD. (2013). Apodized Coupled Resonator Optical Waveguides: Theory, design and characterization [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/32278
TESIS

Thesis (S.M.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and the Woods Hole Oceanographic Institution); and, (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.
Includes bibliographical references (p. 149-151).
Despite the great achievements obtained with fast-field and parabolic equation models, normal mode programs still remain a very efficient, simple and practical tool for describing ocean acoustics in range-independent environments. Numerical implementations of wave-theory solutions for range-dependent acoustic problems can be classified as: normal-mode techniques (adiabatic or coupled modes); parabolic-approximation techniques (narrow- or wide-angle parabolic equations solved by split-step or finite-difference techniques); and finite-element/finite- difference solutions of the full wave equation. The mode techniques provide approximate field solutions if implemented in the adiabatic approximation, while complete wave theory solutions can be obtained by including full mode coupling. Parabolic approximations to the elliptic wave equation have been extensively studied over the past 10 years([15], [23]). The advantage of using a parabolic wave equation is that it can be efficiently solved by noniterative forward marching techniques. However, any form of the parabolic equation is an approximate wave equation derived under the assumptions of: (1) forward propagation only, and (2) that energy is propagating within a limited angular spectrum around the main propagation direction. The last category of models based on finite-difference and finite-element solutions of the full wave equation([22]) is well suited for providing solutions for propagation in general range-dependent environments.
(cont.) The existing codes, however, are extremely computer intensive. My thesis focuses on a two-dimensional two-way coupled modes model, and then expend it to a three-dimensional coupled modes model for two-dimensional, range- dependent waveguides. Numerical examples of two-dimensional and three-dimensional problems are presented, and comparisons with the results from analytical solution, as well as from COUPLE are also considered.
by Wenyu Luo.
S.M.

The following thesis is comprised of four main areas of work. These are centred around the experimental observation of phenomena associated with both linear and non-linear optics in silicon photonic-wires. As a comparison, I also discuss a similar coupled-waveguide system; dual-core hollow-core photonic crystal fibre. To introduce the reader to this work, the first chapter will recap some undergraduate level theory; a general introduction to optical waveguides. It is not intended to be a complete theoretical picture, as many beautiful texts on optics already exist [1–3]. This chapter concerns itself only with the aspects of optics with which the author was intimately aware of throughout the completion of this thesis. Thereafter, the chapters become specific to the particular experiments undertaken. Each one follows a simple framework: examination of the relevant theory, extending upon that already discussed in the first chapter, a literature review and finally a discussion of the work I completed within this thesis. Chapter 2 is the only chapter not related to silicon based photonics. Here I discuss dual-core hollow-core photonic crystal fibres; including guidance mechanisms, fabrication methods and the numerical modelling techniques employed in my work. I will compare these numerical results to experimental results taken by colleagues at the university of Bath. Chapter 3 analyses linear propagation in arrays of silicon photonic wires. I extend the simple picture of light propagating in waveguides to discuss the di↵erent types of dispersion inherent in this system and how dispersion tailoring can be achieved; with reference to the other literature on this topic. Experimental results are examined and discussed. Chapters 4 and 5 discuss non-linear propagation in silicon photonic wire arrays; modulation instability and spatio-temporal solitons respectively. In each case I extend the ideas on non-linearity presented in Chapter 1 to explain both modulation instability and optical solitons. Detailed descriptions of the experiments undertaken, and associated numerical modelling completed are then discussed. Whilst the work I present is incomplete, I will discuss subsequent work performed by my colleagues at the University of Bath based on my initial work. Finally, Chapter 6 draws together my conclusions.

Coupled waveguides, such as directional couplers and grating-assisted directional couplers, have many applications in integrated optics, optical communication and nonlinear optics. In this thesis, we investigate two important applications of them in optical integration and nonlinear frequency conversion. The work presented in this thesis is composed of two main parts. In the first part, we present a compact double-grating coupler between silicon-on-insulator (SOI) waveguides. This type of waveguide coupler has potential applications in multi-layer photonic integrated circuits. In the second part, we investigate nonlinear frequency conversion, such as second harmonic generation, in directional couplers.
The double-grating coupler consists of two gratings: one acting as an outcoupler to couple the guided light from a waveguide to its radiation mode, and the other acting as an incoupler to couple the light in the radiation mode to the guided mode of another waveguide. The coupling between these two waveguides is therefore achieved. Since the coupling does not require field overlap between the guided modes of the two waveguides, the separation between the two waveguides can be large. This advantage leads to promising applications in optical integration in multi-layer photonic integrated circuits. A transfer efficiency of 29% is achieved between two SOI waveguides, and further optimization is possible by using blazed-gratings and Bragg reflectors.
In the second part of this thesis, we investigate a new theory for nonlinear frequency conversion in directional couplers. As is well known, when a harmonic oscillator is driven at resonance by an external harmonic force, the largest possible oscillation amplitude is obtained. Coupled mode theory shows that similar phenomena occur when the nonlinear effect takes place in a waveguide directional coupler. We show that by considering the amplitude of the second harmonic signal in the coupled waveguides as an oscillator and the nonlinear polarization as an external harmonic force, a resonant condition can be found, resulting in highly efficient power transfer from pump to signal. Similar resonant effects also occur in other nonlinear frequency conversion effects. The proposed phenomena can also be understood by phase-matching between the supermodes of directional couplers. On one hand, the new theory has fundamental significance for nonlinear optics, on the other hand, it also demonstrates that waveguide directional couplers can be found in a new application area besides integrated optics and optical communication. We also report the first experimental observation of continuous-wave second-harmonic generation in waveguide directional couplers. We employ a GaAs/AlGaAs system and observe four resonance peaks in a ∼15nm spectral range, with a maximal conversion efficiency of 1.6%W-1cm-2. This new configuration has the potential to open a new range of applications for nonlinear frequency conversion.

La propagation d'ondes dans des guides d'ondes couplés peut être décrite par la théorie des modes couplés. Ce formalisme est mathématiquement analogue à l'équation de Schrödinger dans l'approximation des ondes tournantes (RWA) qui décrit la dynamique quantique du transfert de population entre des états atomiques couplés. Ces analogies ont été précédemment étudiées dans des systèmes de guides couplés possédant un indice effectif constant. Ces systèmes sont parfaitement analogues aux systèmes quantiques couplés en résonance. Nous étendons ce type d'études à des systèmes optiques possédant un paramètre de contrôle supplémentaire via la modulation longitudinale des constantes de propagation. Ces configurations sont analogues aux systèmes quantiques couplés avec une excitation variable et non résonnante. Le transfert de population entre les états atomiques est contrôlé par les fréquences de Rabi et le désaccord de fréquence entre le laser couplant les niveaux et la fréquence de transition. Et en optique, le transfert de lumière entre les guides est ajusté et contrôlé grâce aux constantes de couplage et de propagation des modes.Dans cette thèse, nous exploitons ces analogies pour réaliser des transferts robustes et adiabatiques de la lumière entre différents guides. Le premier système étudié est basé sur le processus, récemment développé en physique quantique, de STIRAP à deux états (two-state STImulated Raman Adiabatic Passage). Cette analogie est mise en œuvre pour la première fois en optique grâce à une structure composée de deux guides, pour lesquels les constantes de couplage et de propagation sont modulées longitudinalement pour réaliser un diviseur de faisceau 50:50 large bande. Ce composant est d’abord étudié théoriquement puis démontré expérimentalement. Les expériences sont réalisées avec des structures de guide d'ondes reconfigurables et accordables, générés par la technique d'illumination latérale d'un cristal photo réfractif. Cette plateforme expérimentale innovante permet de générer des structures de guides qui peuvent être facilement effacés et reconfigurés pour tester différents systèmes.Une seconde analogie, inspirée du processus de Passage Adiabatique Rapide (RAP), est ensuite théoriquement étudiée et expérimentalement réalisée. Nous exploitons un système à deux guides couplés, où le désaccord entre les constantes de propagation s’annule au centre de la propagation. Une telle modulation du désaccord entre les constantes de propagation et de la constante de couplage permet d’effectuer un transfert total de la lumière entre les guides (réalisation d’un coupleur directionnel large bande et robuste).Le processus de RAP décrit précédemment s'applique à des systèmes à deux états. Nous démontrons théoriquement et numériquement que les analogies optiques du RAP peuvent être étendues à des systèmes contenant N guides (N>2) grâce à une technique quantique appelée élimination adiabatique. Elle consiste en l'élimination formelle du (des) guide(s) d'ondes intermédiaire(s). Cela réduit le système à un formalisme mathématique où seuls les deux guides externes restent, et où, par conséquent, le RAP peut être appliqué. Cette technique permet un transfert de lumière entre les guides externes sans excitation du (des) guide(s) intermédiaires. Contrairement à la technique du STIRAP conventionnel, l’élimination adiabatique fonctionne de manière symétrique, et pour un nombre de guides N pair ou impair. L’élimination adiabatique requiert un désaccord important des constantes de propagation.Ce travail contribue ainsi à démontrer que les analogies entre la mécanique quantique et l’optique guidée sont des outils puissants pour concevoir de nouvelles structures photoniques large bande et robustes. Elles peuvent être étendues à la réalisation future de nouvelles fonctions utilisant des guides en régime non linéaire, la propagation non linéaire induisant la variation spatiale (dépendante de l'intensité de la lumière)
The propagation of optical waves in coupled waveguides can be described by the coupled-mode theory. This formalism is mathematically analogous to the one for the quantum dynamics of the population of coupled atomic states, which is described by the Schrödinger equation within the so called Rotating Wave Approximation (RWA). This analogy has been pointed out in recent years. It was applied to coupled waveguide systems with constant effective refractive indices, which are fully analogous to resonant coupled quantum systems. This work extends this kind of studies to optical systems possessing an additional control parameter: the longitudinal modulation of the waveguide propagation constants. This approach is analogous to quantum systems with time dependent non-resonant excitation. As the population transfer between atomic states can be controlled by means of the Rabi frequencies and the laser frequency detunings, the light transfer between coupled waveguides can be controlled by means of the coupling constants and the propagation constants.Several such analogies are studied and exploited in this thesis for the demonstration of robust (broadband) adiabatic light transfer and splitting. The first system is based on the recently introduced quantum process known as two-state STImulated Raman Adiabatic Passage (two-state STIRAP). This is implemented for the first time in classical optics with a set of two evanescently coupled waveguides with proper longitudinal modulation of the mode propagation constants and of the coupling coefficient between them. A broadband 50:50 beam splitter based on two-state STIRAP is theoretically proposed and experimentally demonstrated. The experiments are performed using reconfigurable and tuneable waveguide structures that are optically induced by a lateral illumination technique of a nonlinear photorefractive crystal. This experimental platform provides versatile guiding structures that can be erased and reconfigured to test various systems depending on the considered analogy.A second quantum analogy based on the process of Rapid Adiabatic Passage (RAP) is theoretically studied and experimentally implemented for a set of two waveguides for which the longitudinally varying detuning crosses zero at half propagation distance. The modulation of the detuning and of the coupling constant provide a very robust and highly achromatic mechanism for full light transfer between the waveguides (broadband directional coupler). This is analogous to the RAP-based robust inversion of two-level quantum systems.The above RAP-like transfer applies to systems containing only two waveguides. It is also shown theoretically and numerically that the same functionality can be obtained in systems containing N waveguides with N>2. This relies on a technique called adiabatic elimination. It consists in the formal elimination of the N-2 internal waveguide(s) who reduces the system to an effective two-waveguide system where RAP can be applied. This is relevant because it permits a light transfer between the outer waveguides without excitation of the middle one(s). In contrast to the already known technique based on the conventional STIRAP process, the technique studied here works in a symmetric way and for an odd and even total number of guides N. Adiabatic elimination is achieved by a strong detuning between the two outer waveguides and the remaining one(s). It can be concluded that the analogies of all the classical optical systems studied in this work with corresponding non-resonant quantum systems and processes give powerful tools to design new broadband photonic structures. Moreover, the present studies can pave the way for dealing with future novel functionalities in nonlinear optical waveguide systems, which involve in a natural way a spatial light-intensity-dependent variation of the waveguide propagation constants and detuning

MSc
Thesis (MSc (Physics))--University of Stellenbosch, 2009.
The propagation of electromagnetic radiation in homogeneous or periodically modulated media can be described by the coupled mode equations. The aim of this study was to derive analytical expressions modeling the solutions of the coupled-mode equations, as alternative to the generally used numerical and transfer-matrix methods. The path integral formalism was applied to the coupled-mode equations. This approach involved deriving a path integral from which a generating functional was obtained. From the generating functional a Green’s function, or propagator, describing the nature of mode propagation was extracted. Initially a Green’s function was derived for the propagation of modes having position independent coupling coefficients. This corresponds to modes propagating in a homogeneous medium or in a uniform grating formed by a periodic variation of the index of refraction along the direction of propagation. This was followed by the derivation of a Green’s function for the propagation of modes having position dependent coupling coefficients with the aid of perturbation theory. This models propagation through a nonuniform inhomogeneous medium, specifically a modulated grating. The propagator method was initially tested for the case of propagation in an arbitrary homogeneous medium. In doing so three separate cases were considered namely the copropagation of two modes in the forward and backward directions followed by the counter propagation of the two modes. These more trivial cases were used as examples to develop a rigorous mathematical formalism for this approach. The results were favourable in that the propagator’s results compared well with analytical and numerical solutions. The propagator method was then tested for mode propagation in a periodically perturbed waveguide. This corresponds to the relevant application of mode propagation in uniform gratings in optical fibres. Here two case were investigated. The first scenario was that of the copropagation of two modes in a long period transmission grating. The results achieved compared well with numerical results and analytical solutions. The second scenario was the counter propagation of two modes in a short period reflection grating, specifically a Bragg grating. The results compared well with numerical results and analytical solutions. In both cases it was shown that the propagator accurately predicts many of the spectral properties of these uniform gratings. Finally the propagator method was applied to a nonuniform grating, that is a grating for which the uniform periodicity is modulated - in this case by a raised-cosine function. The result of this modulation is position dependent coupling coefficients necessitating the use of the Green’s function derived using perturbation theory. The results, although physically sensible and qualitatively correct, did not compare well to the numerical solution or the well established transfer-matrix method on a quantitative level at wavelengths approaching the design wavelength of the grating. This can be explained by the breakdown of the assumptions of first order perturbation theory under these conditions.

Optical and opto-electronic components play important roles in both classical and quantum information processing technologies. Despite fundamental differences in these technologies, both stand to benefit greatly from moving away from bulky, individually packaged components, toward a scalable platform that supports dense integration of low power consumption devices. Planar photonic circuits, composed of devices etched in a thin slab of high refractive index material, are considered an excellent candidate, and have been used to realize many key components, including low-loss waveguides, light sources, detectors, modulators, and spectral filters. In this dissertation, a novel triple-microcavity structure was designed, externally fabricated, and its linear and nonlinear optical properties were thoroughly characterized. The best of the structures exhibited both high four-wave mixing conversion efficiencies and low threshold optical bistability, which are relevant to frequency conversion and all-optical switching applications. The device consisted of three coupled photonic crystal (PC) microcavities with three nearly equally spaced resonant frequencies near telecommunication wavelengths (λ ~ 1.5 μm), with high quality factors (~ 10⁵, 10⁴ and 10³). The microcavity system was coupled to independent input and output PC waveguides, and the cavity-waveguide coupling strengths were engineered to maximize the coupling of the input waveguide to the central mode, and the output waveguide to the two modes on either side. A novel and sophisticated measurement and analysis protocol was developed to characterize the devices. This involved measuring and modelling the linear and nonlinear transmission characteristics of each of the modes separately with a single tunable laser, as well as the frequency conversion efficiency (via stimulated four-wave mixing) when two tunable lasers pumped two of the modes, and the power generated in the third mode was monitored. Comparisons of the entire set of model and experimental results led to the conclusion that this structure can be used to achieve both low-power-threshold optical switching and high efficiency four-wave-mixing-based frequency conversion. The advantages of this structure over others in the literature are its small footprint, multi-mode functionality and independent input and output channels. The main disadvantage that requires further refinement, has to do with its sensitivity to fabrication imperfections.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

El objetivo de esta tesis es la investigación de estructuras y técnicas de acoplo para minimizar las pérdidas de acoplo entre guías dieléctricas y cristales fotónicos planares. En primer lugar se ha estudiado el modelado del acoplo entre guías dieléctricas y guías en cristal fotónico así como la influencia de los principales parámetros del cristal en la eficiencia de acoplo. Se han obtenido expresiones cerradas para las matrices de reflexión y transmisión que caracterizan totalmente el scattering que ocurre en el interfaz formado entre una guía dieléctrica y una guía en cristal fotónico. A continuación y con el fin de mejorar la eficiencia de acoplo desde guías dieléctrica de anchura arbitraria, se ha propuesto como contribución original una técnica de acoplo basada en la introducción de defectos puntuales en el interior de una estructura de acoplo tipo cuña realizada en el cristal fotónico. Diferentes soluciones, incluida los algoritmos genéticos, han sido propuestas con el objetivo de conseguir el diseño óptimo de la configuración de defectos. Una vez conseguido un acoplo eficiente desde guías dieléctricas a guías en cristal fotónico, se ha investigado el acoplo en guías de cavidades acopladas. Como contribución original se ha propuesto una técnica de acoplo basada en la variación gradual del radio de los defectos situados entre cavidades adyacentes. Además, se ha realizado un riguroso análisis en el dominio del tiempo y la frecuencia de la propagación de pulsos en guías acopladas de longitud finita. Dicho estudio ha tenido como objetivo la caracterización de la influencia de la eficiencia del acoplo en los parámetros del pulso. Finalmente, se han presentado los procesos de fabricación y resultados experimentales de las estructuras de acoplo propuestas.
Sanchis Kilders, P. (2005). Coupling techniques between dielectric waveguides and planar photonic crystals [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/1854
Palancia

An approach to integrated frequency-comb filtering is presented, building from a background in photonic crystal cavity design and fabrication. Previous work in the development of quantum information processing devices through integrated photonic crystals consists of photonic band gap engineering and methods of on-chip photon transfer. This work leads directly to research into coupled-resonator optical waveguides which stands as a basis for the primary line of investigation. These coupled cavity systems offer the designer slow light propagation which increases photon lifetime, reduces size limitations toward on-chip integration, and offers enhanced light-matter interaction. A unique resonant structure explained by various numerical models enables comb-like resonant clusters in systems that otherwise have no such regular resonant landscape (e.g. photonic crystal cavities). Through design, simulation, fabrication and test, the work presented here is a thorough validation for the future potential of coupled-resonator filters in frequency comb laser sources.
Ph.D.
Doctorate
Optics and Photonics
Optics and Photonics
Optics

La tesis estudia cinco formulaciones del método de modos acoplados para analizar guías con medios magnéticos en su interior. La formulación indirecta, de tipo general, se ha aplicado al análisis de desfasadores toroidales obteniéndose resultados similares a los obtenidos mediante un método numérico puro como es el de diferencias finitas. La tesis introduce igualmente una formulación del método de adaptación que junto con el método de modos acoplados permite analizar discontinuidades simples. La combinación de los métodos anteriores con la matriz de dispersión generalizada ha permitido analizar discontinuidades en guías de onda con ferritas transversalmente magnetizadas con discontinuidades en las tres direcciones del sistema de coordenadas.

V současnosti jsme svědky stále zvyšujících se nároku na rychlost přenosu a zpracování signálu a kapacitu pamet’ových zařízení. Proto se pozornost výzkumných pracovníku zaměřuje k plně optickým zařízením, která by mohla splnit zmíněné požadavky. Jednou z intenzívně zkoumaných možností je využití mikroprstencových optických rezonátoru. Při výzkumu je nutné využít numerických metod, které simulují šíření optického záření v dané struktuře. K tomuto účelu existuje celá rada metod, které se liší v efektivitě výpočtu, použitých aproximacích, i možnostech použití. Cílem této práce bylo vyvinout dvě jednoduché a praktické numerické metody pro modelování šíření pulzního záření v nelineárních vlnovodných strukturách. Přítom bylo požadováno, aby, na rozdíl od obecně známé a často využívané metody konečných diferencí v časové oblasti (FD-TD), bylo možné metody snadno aplikovat při studiu nelineárních struktur založených na mikroprstencových rezonátorech. Proto vyvinuté metody používají některé aproximace, zejména aproximaci pomalu proměnné obálky. Výhodou metod je vysoká rychlost a skromné požadavky na výpočetní zdroje. Obě metody vycházejí ze zkutečnosti, že naprostá většina nelineárních struktur založených na mikroprstencových rezonátorech se skládá ze dvou základních prvku: obyčejných vlnovodu a vlnovodných vazebních clenu. První metoda řeší vázané parciální diferenciální rovnice, které popisují šíření obálky pulzu ve struktuře. Přitom je použito tzv. „up-wind“ schéma vhodné pro parciální diferenciální rovnice popisující šíření vln. Druhá metoda vychází z první; rozdíl je v popisu vazby mezi dvěma vlnovody. Pokud se v první metodě uvažuje realistická vazba rozložená na určité délce, pak druhá metoda je založena na představě vazby nacházející se v jednom místě. Díky tomu je možné integrovat příslušné rovnice a dosáhnout výrazného urychlení výpočtu. Kvazianalytický charakter druhé metody umožňuje dále snadnou klasifikaci různých typu ustálených řešení. Vzhledem k těmto vlastnostem byla druhá metoda využita k výzkumu samovolné generace optických pulzu ve strukturách skládajících se z vázaných prstencových rezonátoru. Obě metody, které byly vyvinuty během této práce, představují rychlé a fyzikálně názorné alternativy k metodě FD-TD, a tak lze očekávat, že mohou hrát důležitou roli při výzkumu nelineárních vlnovodných struktur.

Nonlinear phenomena originating from the distributed coupling process were observed when distributed couplers, such as prisms and gratings, were used to couple light into nonlinear ZnS thin film waveguides. The efficiency of the nonlinear distributed coupling process was found to depend on two independent parameters, the angle of the incident beam and the power of the incident beam. Depending on the detuning of the incident angle, from the optimum incident angle at low powers, either optical limiting, power-dependent switching, or power-dependent bistability of the coupling efficiency, and thereby of the in-coupled power, was observed. At zero detuning, a twenty-fold decrease of the coupling efficiency with increasing powers was measured. At a nonzero detuning of the incident angle, power-dependent switching at milliwatt powers was observed. At larger angular detunings, corresponding to the angular width. FWHM, of the coupling peak at low powers, power-dependent bistability was observed, and the width of the bistability loop was found to increase with increasing detunings. All-optical beam scanning via a nonlinear grating coupler was also demonstrated, utilizing a control-signal beam configuration, where the signal beam scanned through an angle of 0.5° when the power of the control beam was varied. The observed nonlinearity in ZnS was positive and of thermal origin. The power-induced change in the refractive index was found to be 0.01 and a relaxation time of 10 μsec was measured. Problems with the long-term stability of the nonlinear distributed coupling process were traced to the occurrence of desorption and adsorption of water vapor in the ZnS films.

High skirt selectivity and extended out-of-band rejection is a major challenge for the successful progress of in-line microwave filters. This thesis presents novel filter realizations with improved performance, compatible with the standard single thin all-metal insert in a split-block housing and therefore maintaining the low-cost fabrication characteristics. In addition, significant filter performance improvement is achieved. The synthesis procedure implemented for the filter concept consists of a few steps. Some preliminary steps are a rigorous characterization of a double-ridge coaxial waveguide, and the modelling of an equivalent circuit model for the parallel coupled ridge waveguide devised in the filter concept. From these elements, a full wave electromagnetic analysis shows that parallel-coupled asymmetric ridge waveguides produce strongly dispersive coupling which introduces a transmission zero. Later on this property is extended to parallel-coupled asymmetric ridge waveguide resonators, where it is demonstrated that it is possible to independently control the coupling coefficient and the frequency of the transmission zero. This allows the realization of pseudo-elliptic narrowband in-line bandpass filters in E-plane technology. A general synthesis procedure for high order filters is outlined and numerical and experimental results are presented for validation. The elements employed for the synthesis procedure of the bandpass prototypes are also applied to investigate structures suitable for different applications. In particular, stopband and dual stopband filters are presented with numerical and experimental results. Finally, the study of a microwave chemical/biochemical sensing device for the characterization and detection of cells in chemical substances and cells in solution in micro-litre volumes is also reported.

The purpose of this project is to examine a novel approach for coupling RF-power from a transmitter chain in a MiniLink radio. The coupling is achieved by inserting a probe into the microstrip to waveguide transition of the radio.The main purpose of the coupler is to be used for decoupling of output power to an external detector.Today the RF-detector is often placed on the PA chip, this is limiting the number of designs available on the market for HighPower versions of radios. Also an off chip coupler before the waveguide transition will have the drawback of higher insertion loss and also by larger dimensions.An alternative use of the coupler could be to design an RF-loop channel from Tx to Rx in a radio.Simulations in Ansoft HFSS and ADS have been used for designing and testing the tolerance of the design for product variations on the 23 GHz waveguide.Two different designs are handled in this report, one where the probe for coupling of output power is placed where the force is as strongest and one where the probe is placed where it is preferable considering the design on the rest of circuit board.Both designs were drawn and simulated in HFSS with satisfying results. A design that meets the requirements was created by simulations. First, simulations were made for establishing what variables that affect the interesting outcome values such as coupling level between probe and u-strip, insertion loss et cetera, then an optimize simulation was made to get the best values on those variables.The consequence of lack of directivity was examined with ADS also here with a satisfying result.
Uppsatsnivå: C

The properties of all-optical nonlinear waveguide devices are investigated. In particular, the nonlinear directional coupler (NLDC) and nonlinear Mach-Zehnder interferometer (NLMZ) are analyzed using perturbation theory. The perturbation theory provides differential equations that describe the amplitude of the waveguide modes as a function of the propagation distance. To be practical, these waveguide devices require nonlinear phase shifts of π or more. Therefore, the theoretical investigation of these devices emphasizes their fabrication in bulk and multiple-quantum-well (MQW) gallium arsenide (GaAs). For the first time, absorption, carrier diffusion, and thermal effects are included in the theoretical investigation of the NLMZ and NLDC. The nonlinear dependence of the coupling terms, which has been neglected in all previous work, is shown to be significant for semiconductor based NLDC's. The effects of carrier diffusion on the nonlinear response of a GaAs waveguide is demonstrated using a self-consistent numerical method. The effects are heavily dependent on the waveguide geometry, and, therefore, should be included in the analysis of nonlinear semiconductor waveguide devices. However, if the diffusion length is large compared to the mode width, carrier diffusion simplifies the investigation since the nonlinear absorption and index change are uniform across the mode. This important conclusion is used in the models for the NLMZ and NLDC. The theoretical models predict the NLMZ and NLDC should work in bulk and MQW GaAs. To demonstrate that the required nonlinear phase shifts for the NLMZ and NLDC are indeed possible in bulk and MQW GaAs, the first experimental observation of electronic optical bistability in a MQW GaAs strip-loaded waveguide is recounted. This original research illustrated that phase shifts in excess of 2π are possible in MQW GaAs waveguides and, therefore, the future of all-optical waveguide devices in semiconductors is optimistic.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004.
Includes bibliographical references (leaves 98-103). Also available in electronic version. Access restricted to campus users.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 136-140).
Semiconductor optical devices are important in the photonics industry due to their significant advantages in size, weight, and power consumption (SWAP) and to their capability for photonic integration. However, these devices traditionally suffer from low fiber coupling efficiency and have been limited to relatively low power applications. This thesis explores the potential of the slab-coupled optical waveguide (SCOW) semiconductor gain medium for use in high power optical amplifiers and external cavity lasers. The thesis begins by introducing the SCOW concept and describing the benefits of utilizing a low optical confinement design for high power operation. Detailed analysis and measurements of the output power, gain, and noise properties of slab-coupled optical waveguide amplifiers (SCOWAs) and slab-coupled optical waveguide external cavity lasers (SCOWECLs) are also presented. It will be shown that these devices not only exhibit Watt class output power with high coupling efficiency (> 90 %) but also demonstrate the capability for low noise operation.
by William Loh.
S.M.

Ce travail de thèse présente une étude théorique, numérique et expérimentale de l’intégration sur un guide d’onde diélectrique de chaînes de nanoparticules d’or supportant des résonances « plasmon de surface localisé ». Les guides d’onde à plasmon de surface localisé procurent un confinement sub-longueur d’onde de la lumière, ce qui permet d’envisager la réalisation de composants optiques ultra-compacts. Cependant, leurs pertes optiques élevées restreignent leur application à de courtes distances de propagation, contrairement aux guides d’onde diélectriques. Une combinaison judicieuse des deux types de guide doit donc permettre de bénéficier de leurs avantages respectifs. Dans un premier temps, nous avons étudié théoriquement les propriétés des chaînes des nanoparticules grâce à un modèle analytique basé sur l’approximation de dipôles ponctuels couplés, que nous avons développé. Cette étude a permis de déterminer la forme et les dimensions des nanoparticules qui ont ensuite été introduites dans un logiciel de FDTD pour simuler le couplage entre la chaîne de nanoparticules et le guide diélectrique (SOI ou en Si3N4). De cette étude numérique, nous avons déduit les géométries des structures à fabriquer. Les structures réalisées ont été caractérisées à l’aide d’un banc de transmission résolue spectralement, mis en place pendant cette thèse, et d’un système de mesures en champ proche optique en collaboration avec le LNIO (Troyes). Pour la première fois, nous avons montré expérimentalement les propriétés d’une chaîne courte de nanoparticules intégrée sur un guide SOI, ainsi que le phénomène de guides couplés entre une chaîne longue de nanoparticules et un guide SOI. Une valeur record de la constante de couplage a été obtenue, et ce, aux longueurs d’onde des télécoms (proche infrarouge). L’énergie lumineuse transportée par le mode TE du guide SOI peut ainsi être entièrement transférée au guide plasmonique en 4 ou 5 nanoparticules, soit une distance de propagation de moins de 600 nm. Nous avons également étudié les propriétés de réseaux de Bragg à base de plasmon de surface localisé en confrontant les résultats de mesures de transmission résolue spectralement aux résultats théoriques d’un modèle analytique basé à la fois sur l’approximation de dipôle ponctuel en régime quasi-statique et la théorie des modes couplés. Ces travaux ouvrent la voie à des applications de pinces optiques, de capteurs ou de spaser, qui bénéficieront de l’intégration de nanoparticules métalliques dans les circuits photoniques
This PhD work presents a theoretical, numerical and experimental study of the integration of a gold nanoparticle chain supporting "localized surface plasmon resonances" on a dielectric waveguide. The localized surface plasmon allows a sub-wavelength confinement of light which could lead to the achievement of ultra-compact optical components. However, the high level of optical losses restricts their application to short propagating distances unlike dielectric waveguides. A judicious combination of both types of guides should therefore allow taking profit of their respective advantages. Firstly, we have theoretically studied the properties of nanoparticles chains using an analytical model that we have developed following the coupled dipoles approximation. This has helped us to determine the shape and size of nanoparticles, which have been further used in a FDTD software, to simulate the coupling between the chain and the dielectric waveguide (SOI or Si3N4). Using this numerical study, we have deduced the geometries of structures to be fabricated. The realized structures have been characterized using a spectrally resolved transmission set-up, built during this thesis, and an optical near field measurement set-up (collaboration LNIO Troyes). For the first time, we have experimentally shown the properties of short nanoparticle chains integrated on a SOI waveguide as well as the existence of a coupled waveguide phenomenon between long nanoparticle chains and SOI waveguides. A record value has been obtained for the coupling constant at telecom wavelengths (near infrared). The light energy carried by the TE mode of the SOI waveguide can be completely transferred into the plasmonic waveguide via the first 4 or 5 nanoparticles of the chain, which means a distance of less than 600 nm. We have also studied the properties of Bragg gratings based on localized surface plasmon. Experimental results from spectrally resolved transmission measurements have been compared to theoretical results obtained from an analytical model based on the point dipole approximation in quasi-static regime, on one hand, and using the coupled mode theory, on the other hand. This work opens the way for applications to optical tweezers, sensors or spasers, which will benefit from the integration of metal nanoparticles in photonic circuits

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 75-78).
As silicon photonics enters mainstream technology, we find ourselves in need of methods to seamlessly transfer light between the optical fibers of global scale telecommunications networks and the on-chip waveguides used for signal routing and processing in local computing networks. Connecting these components directly results in high loss from their unequal sizes. Therefore, we employ a coupler, which acts as an intermediary device to reduce loss through mode and index matching, and provide alignment tolerance. This thesis presents a potential fiber-to-waveguide coupler design for use in integrating such networks. A quadratic index stack focuses incident light from a fiber in one plane, while a planar lens and linear taper do likewise in the perpendicular plane. Once the mode is sufficiently compressed, the light then enters and propagates through the waveguide. We performed simulations using the beam propagation method and finite difference time domain, among other modeling techniques, to optimize coupling efficiency and gain an understanding of how varying certain parameters affects coupler performance. The simulation results were then incorporated into a mask layout for fabrication and measurement.
by Trisha M. Montalbo.
S.M.

In our work an optical bus-coupler is proposed, which enables easy bidirectional connection between two waveguides without interrupting the bus using a core-to-core coupling principle. With bended waveguides the coupling ratio can be tuned by adjusting the overlap area of the two cores. In order to ensure large overlap areas at short coupling lengths, the waveguides have rectangular cross sections. To examine the feasibility of this coupling concept a simulation was performed, which is presented in this paper. Due to multimode waveguides, used in short range data communication, a non-sequential ray tracing simulation is reasonable. Simulations revealed that the bending of the waveguide causes a redistribution of the energy within the core. Small radii push the main energy to the outer region of the core increasing the coupling efficiency. On the other hand, at excessive lowered bend radii additional losses occur (due to a coupling into the cladding), which is why an optimum has to be found. Based on the simulation results it is possible to derive requirements and design rules for the coupling element.

The aim of this project is to fabricate highly efficient grating couplers in thin-film silicon-on-insulator (SOI) wafers, which have a silicon (Si) thickness of the order of 1 mum. These thin-film waveguides allow the development of higher speed Si optical modulators, sensors and vertical surface coupling for Si light emitting diodes (LEDs), Hence, SOI rectangular and blazed grating couplers were fabricated where the buried oxide layer in SOI was designed as a reflective layer. The former gratings were fabricated by electron beam lithography followed by reactive ion etching, while the latter gratings were fabricated by angled argon ion beam etching. Both types of grating were designed at the diffraction order of -1, for a wavelength of 1.3 mum. The fabricated rectangular gratings have grating heights of 0.14, 0.23, 0.30 and 0.44 mum and a pitch of 0.40 mum whereas the sawtooth blazed gratings have a grating depth of 0.08 mum and a period of 0.38 mum To our knowledge, no Si blazed gratings with a pitch of less than 500 nm have been fabricated before. The SOI rectangular grating couplers yield a maximum output efficiency of 71 +/- 5 % towards the superstrate, while the blazed grating couplers produce an output efficiency of 84 +/- 5 % towards the substrate. These experimental output efficiencies are the highest yet reported in SOI for each grating profile, respectively. In addition, an optical loss of 0.15 +/- 0.05 dB/cm of Unibond SOI was measured for the first time. Furthermore, the experimental output efficiencies of the grating couplers with various grating heights were found to be consistent with perturbation theory. Thus, our aim of designing and fabricating an highly efficient thin film SOI waveguide grating coupler has been achieved. These grating couplers may enhance the applications of integrated optics in Si, and may allow the development of devices such as those mentioned above.

Photonic crystals (PhCs) are periodic dielectric structures that exhibit a photonic bandgap, i.e., a range of wavelength for which light propagation is forbidden. The special band structure related dispersion properties offer a realm of novel functionalities and interesting physical phenomena. PhCs have been manufactured using semiconductors and other material technologies. However, InP-based materials are the main choice for active devices at optical communication wavelengths. This thesis focuses on two-dimensional PhCs in the InP/GaInAsP/InP material system and addresses their fabrication technology and their physical properties covering both material issues and light propagation aspects. Ar/Cl2 chemically assisted ion beam etching was used to etch the photonic crystals. The etching characteristics including feature size dependent etching phenomena were experimentally determined and the underlying etching mechanisms are explained. For the etched PhC holes, aspect ratios around 20 were achieved, with a maximum etch depth of 5 microns for a hole diameter of 300 nm. Optical losses in photonic crystal devices were addressed both in terms of vertical confinement and hole shape and depth. The work also demonstrated that dry etching has a major impact on the properties of the photonic crystal material. The surface Fermi level at the etched hole sidewalls was found to be pinned at 0.12 eV below the conduction band minimum. This is shown to have important consequences on carrier transport. It is also found that, for an InGaAsP quantum well, the surface recombination velocity increases (non-linearly) by more than one order of magnitude as the etch duration is increased, providing evidence for accumulation of sidewall damage. A model based on sputtering theory is developed to qualitatively explain the development of damage. The physics of dispersive phenomena in PhC structures is investigated experimentally and theoretically. Negative refraction was experimentally demonstrated at optical wavelengths, and applied for light focusing. Fourier optics was used to experimentally explore the issue of coupling to Bloch modes inside the PhC slab and to experimentally determine the curvature of the band structure. Finally, dispersive phenomena were used in coupled-cavity waveguides to achieve a slow light regime with a group index of more than 180 and a group velocity dispersion up to 10^7 times that of a conventional fiber.
QC 20100712

Recent advances in computing technology have highlighted deficiencies with electrical interconnections at the motherboard and card-to-backplane levels. The CPU speeds of computing systems are drastically increasing with on-chip local clock speeds expected to approach 6 GHz by 2010. Yet, card-to-backplane communication speeds have been unable to maintain the same pace. At speeds beyond a few gigahertz the implementation of electronic interconnects gets increasingly complex, thus, alternative optical interconnection techniques are being extensively researched to relieve the expected CPU to data bus bottleneck. Despite the advantages afforded by optical interconnects there are still demands for improved packaging, enhanced signal tapping, and reduced cost expenditures. In this dissertation, we present a novel array waveguide evanescent coupling (AWEC) technology for card-to-backplane applications. The interconnection scheme is based on waveguide directional coupling between a backplane waveguide and a flexible waveguide connected to the access card or daughter board. To gain access to the shared bus media, coupling of evanescent waves is exploited to tap optical signals from the backplane waveguide to the corresponding card waveguide. The approach results in the elimination of micro-mirror out of plane deflectors and local waveguide termination obstacles present in other reported optical interconnect schemes. Most importantly, the AWEC method can yield efficient multi-drop bus architectures, not possible through free-space, fiber, or traditional guided wave approaches, that only achieve point-to-point topologies. The AWEC concept for optical interconnection was introduced through coupled mode theory, numerical simulations and BeamPROP aided CAD models. Subsequent experimental waveguide analysis was performed and shown to reasonably agree with the simulation results. Likewise, a high-resolution, cost-effective, and rapid prototyping approach for AWEC fabrication has been formulated. Significantly, when compared to other soft lithographic methods, the novel vacuum assisted microfluidic (VAM) technique results in improved waveguide structures, polymer background residue elimination and lower propagation losses. Moreover, experimental results show that our evanescent coupling approach facilitates high-speed coupling between card and backplane waveguides at speeds of 10 Gbps per channel; currently limited only by our testing electronics. In addition, satisfactory eye diagram performance comparable to that of a conventional fiber link, was also observed for the AWEC, alluding to possible aggregate speeds of 100 Gbps. Similarly, we implemented an elementary AWEC shared bus architecture and demonstrate a microprocessor-to-memory interconnect prototype through the proposed AWEC link. Notably, we expect that the AWEC scheme will be significant for high-speed optical interconnects in advanced computing systems.

This thesis analyzes different light localization phenomena in waveguide arrays. We report on the observation of quasi-Bloch oscillations, a new type of dynamic localization in the spatial evolution of light in curved, coupled optical waveguides. The delocalization and final relocalization of an optical beam in a waveguide array is shown by spatially resolving the optical intensity at various propagation distances. Through comparison to different structures, quasi-Bloch oscillations are shown to be robust beyond the nearest-neighbor tight-binding approximation.

碩士
國立交通大學
電子工程系所
92
In this research, we investigate the ARROW-based coherently coupled bending waveguides. The transfer matrix method and effective index method are used to analyze the rib-ARROW structures. The beam propagation method is used to simulate the propagation performance of the ARROW-based bending waveguides. Due to the adjustable characteristics of waveguide width and rib depth, we propose flexible bending devices based on rib-ARROW’s structures. When the total bending angles are designed as 27.2°, 36.8° and 91.2°, respectively, the corresponding waveguide widths are 5.0 mm, 4.0 mm and 4.0 mm, respectively. Furthermore, the corresponding device transmissions and device areas are 0.828 (0.82 dB), 0.746 (1.27 dB), 0.610 (2.15 dB) and 1.32 mm2, 1.24 mm2, 9.54 mm2, respectively.

Whether over micron-long or kilometer-long distances, periodic phenomena can strongly affect both the propagation and the confinement of optical pulses. Periodicities can be engineered through the structural design of optical waveguides, or they may manifest self-consistently from induced nonlinear polarizations. In light of recent developments in fabrication technologies for semiconductor waveguides, polymeric materials, and optical fiber, we show that both strongly- and weakly-nonlinear channels are promising for new devices and systems in optical communications. This thesis proposes and discusses applications of guided wave periodicities in the framework of photonic crystals (coupled-resonator optical waveguides as well as transverse Bragg resonance waveguides and amplifiers), nonlinear phenomena in photorefractive semiconductors, and the nonlinear evolution of temporal solitons in dispersion-managed fibers.

Coupled-resonator optical waveguides (CROWs) are composed of a periodic array of electromagnetic resonators, typically on the micron or sub-micron length scales. A photon in such a waveguide sees a periodic potential, and according to the Floquet-Bloch theorems, has a wavefunction that reflects this periodicity. CROWs have a unique dispersion relationship compared to other semiconductor waveguides, and can be used to slow down the speed of propagation, enhance nonlinear interactions such as second-harmonic generation and four-wave mixing, and form frozen soliton-type field distributions that use the optical Kerr nonlinearity to stabilize themselves against decay via adjacent-resonator or waveguide-resonator coupling.

In optical fibers that possess the optical Kerr nonlinearity in addition to group-velocity dispersion, it is possible to propagate pulses with envelopes that "breathe" with distance, typically at kilometer or longer length scales. Such waveforms are characterized by a set of parameters, e.g., amplitude, chirp, etc., that vary in a periodic manner as the pulse propagates. Borrowing an idea from field theory, e.g., of classical pendulums, or quantum-mechanical elementary particles, the pulse envelope may be viewed as a particle traversing a trajectory in a phase space defined by its characteristic parameters. Distinct, non-overlapping trajectories are assigned as symbols of a multilevel communication code. Since it is the periodicity, arising from the Kerr nonlinearity, that generates this diversity in phase-space, there is no analog of this multiplexed system in linear optical transmission links. The overall bit-rate can be improved several fold above the current limits.

Coupled-Resonator Optical Waveguides (CROWs) are chains of resonators in which light propagates by virtue of the coupling between the resonators. The dispersive properties of these waveguides are controllable by the inter-resonator coupling and the geometry of the resonators. If the inter-resonator coupling is weak, light can be engineered to propagate slowly in these structures. The small group velocities possible in CROWs may enable applications in and technologies for optical delay lines, interferometers, buffers, nonlinear optics, and lasers. This thesis reports on achieving and controlling the optical delay in passive and active CROWs. Both theoretical and experimental results are presented. Transfer matrices, tight-binding models, and coupled-mode approaches are developed to analyze and design a variety of coupled resonator systems in the space, frequency, and time domains. Although each analytical method is fundamentally different, in the limit of weak inter-resonator coupling these approaches are consistent with each other. From these formalisms, simple expressions for the delay, loss, bandwidth, and a figure of merit are derived to compare the performance of CROW delay lines. Using a time-domain tight-binding model, we examine the resonant gain enhancement and spontaneous emission noise in amplifying CROWs to find that the net amplification of a propagating wave does not always vary with the group velocity but instead depends on the termination and excitation of the CROW. CROWs in the form of high-order (> 10) weakly coupled passive polymer microring resonators were fabricated and measured. The measured transmission, group delay, and dispersive properties of the CROWs agreed with the theoretical results. Delays in excess of 100 ps and slowing factors of about 25 over bandwidths of about 20 GHz were observed. The main limitation of the passive CROWs was the optical losses. To overcome the losses and to enable electrical integration, we demonstrated active CROWs in the form of current injection InP-InGaAsP Fabry-Perot laser arrays. Even though the losses could be completely compensated, the transmission spectra and signal-to-noise ratio depended strongly on the injection current and resonator position. The signal-to-noise ratio degraded rapidly away from the input. Our results highlight possible avenues to operate laser arrays as loss-compensated or amplifying CROWs.

碩士
國立交通大學
光電工程學系
99
We present an analytical way to study the directional couplers (DCs) based on coupled resonant optical waveguides (CROWs) in the photonic crystal slab (PCS). It holds potential for combining the applications of slow-light propagation, nonlinear optical processes and optical signal coupling in integrated photonic circuits. From the analytical equations derived by the extended tight-binding theory (TBT), we can obtain the dispersion relations and the electromagnetic (EM) mode distribution of a single PCS-CROW and the PCS-DCs. In the dielectric-rod structures, we find that the dispersion curves of the opposite-type PCS-DCs never cross and the frequency difference of them remains constant. Additionally, the dispersion relation of the alternating-type PCS-DCs with larger defects possess a crossing point, which will shift to the smaller wavevector and the higher frequency by increasing the defect radius. At this crossing point, the energy in one waveguide will never transfer into the other one and is also called the decoupling point. On the other hand, in the air-hole structures, we know that the dispersion curves of both the opposite-type and alternating-type PCS-DCs have a decoupling point nearly fixed at a certain wavevector. Moreover, as increasing the wavevector, the frequency difference between the curves of the opposite-type PCS-DCs increases, and that of the alternating-type PCS-DCs increases and then decreases. In conclusion, the dielectric-rod structure can be used to form the demultiplexers, and the air-hole structures can be used to create the beam splitters. All of these theoretical analyses from the TBT agree well with the numerical ones using the plane-wave expansion method, and give the design rules for these kind of structures.