Dissertations / Theses on the topic 'Optical waveguides'

Optical waveguide bends are key building blocks in many integrated optical components. Accurate numerical modelling of these bends is of great practical value to the design of the integrated optical technology. Analyzed in this thesis are the propagation characteristics of optical waveguide bends based on the method of lines (MoL), not only for its good numerical performances (accuracy, speed of computation and minimal memory requirements), but also for its high suitability for the analysis of waveguide structures. This thesis gives the detailed formulation for the calculation of the waveguide-bending radiation loss and the transition loss due to modal mismatch at the junctions. Besides, the 1D and 2D spatial field distribution algorithms are also covered in the formulation. The code based on the formulation has been implemented successfully. To validate the code, we applied it to three typical waveguide structures appearing in other literatures and compared the results. The comparison shows that our code works very well, and can be used not only for the lossless dielectric media, but also for the lossy media at the optical frequency. This thesis also explores the application of the developed code to metal waveguide bends. The numerical results of the propagation characteristics of the metal waveguide bends at the optical frequencies are presented for what is believed to be the first time.

Bandwidth demands in global telecommunication infrastructures continue to rise and new optical techniques are needed to deal with massive data flows. Generating high bandwidth signals (> 40 GHz) using conventional modulation techniques is hindered by material limitations and fabrication complexities. Similarly, controlling such high bandwidths in both the temporal and spectral domain becomes more problematic using conventional electronic processes. Advances in electro-optic organic materials, fibers/micro-fluidics integration, and nonlinear optics have significant potential for higher bandwidth modulation and temporal/spectral control. The work presented in this dissertation demonstrates the use of various nonlinear optical effects in new photonic device and system designs towards the generation and manipulation of highspeed optical pulses. First, an all fiber-based system utilizing an integrated carbon disulfide-filled liquidcore optical fiber (i-LCOF) and co-propagating pulses of comparable temporal lengths is presented. The slow light effect was observed in 1-meter of i-LCOF, where 18 ps pulses were delayed up to 34 ps through the use of stimulated Raman scattering. Delays greater than a pulse width indicate a potential application as an ultrafast controllable delay line for time division multiplexing in multi-Gb/s telecommunication systems. Similarly, an optically tunable frequency shift was observed using this system. Pulses experienced a full spectral bandwidth shift at low peak pump powers when utilizing the Raman-induced frequency shift and slow light effects. Numerical simulations of the pulse-propagation equations agree well with the observed shifts. Included in our simulations are the contributions of both the Raman cross-frequency shift and slow light effects to the overall frequency shift. These results make the system suitable for numerous applications including low power wavelength converters. Second, a silica/electro-optic (EO) polymer phase modulator with an embedded bowtie antenna is proposed for use as a microwave radiation receiver. The detection of high-frequency electromagnetic fields has been heavily studied for wireless data transfer. Recently there has been growing interest in the field of microwave photonics. We present the design and optimization of a waveguide with an EO polymer core and silica/sol-gel cladding. The effect of electrodes on the insertion losses and poling efficiency are also analyzed, and conditions for low-loss and high poling efficiency are established. Experimental results for a fabricated device with microwave-response between 10 - 14 GHz are presented. Finally, we present the design for a fast optical switch incorporating silicon as the passive waveguide structure and EO polymer as the active material. The design uses a simple directional coupler with coplanar electrodes and promises to have low cross-talk and high switching speed (on the order of nanoseconds). An initial design for a 1x2 switch is fabricated and tested, and future optimization processes are also presented.

In this thesis, we study optical to microwave conversion and generation of ultrashort electrical pulses by the use of optical rectification at telecommunication wavelengths, λ = 1550 nm. By using optical rectification, an electromagnetic pulse is generated in a completely passive semiconductor waveguide. This pulse is coupled in a microwave transmission line with periodically loaded ground electrodes to create a velocity-matched structure. The optical waveguide and the microwave transmission line form the optical rectification device. Although in theory, the width of the electrical pulse in a travelling wave structure is limited only by the duration of the optical excitation pulse, imperfections in the velocity matching will attenuate and disperse most of the electrical pulse. The calculated effective optical refractive index of the rectification devices, nopt - 3.30, matches the measured effective microwave index in one of our structures namely DevO68 (nmw = 3.30). If the structure is slightly velocity-mismatched, losses as high as 14 dB/mm at frequencies of 1 THz will affect the propagation of the electrical pulse. The optical rectification device was fabricated using conventional photolithography techniques and e-beam lithography techniques. The advantages of e-beam lithography are: better pattern definition, perfect alignment and easier lift-off process. The only disadvantage is the cost associated with running the e-beam writer and maybe the time it takes to complete a pattern. The semiconductor material system of choice for the rectification devices is GaAs / AlGaAs due to its well-known large nonlinear coefficient. The use of GaAs/AlGaAs with light at λ = 1550 nm, presents serious absorption effects. The absorption effects mask the pure optical rectification signal and therefore must be minimised. The most significant absorption effect at λ = 1550 nm is two-photon absorption (TPA), which in more than one experiment gave us pulses of a few nanosecons duration. Our rectification device is engineered to minimise TPA, and this is the perhaps the hardest challenge in the design of the device. This also maybe the reason why there is not rectification devices such as ours reported in the literature working at λ = 1550 nm. The reason why we wanted to work with GaAs/AlGaAs is the potential integration of the rectification device in optoelectronic systems. In the final rectification device, we could observe a clear polarization dependence of the generated signal indicating optical rectification. The signal detected was small in magnitude, ~75 dBm and on top of an offset signal which is believed to be TPA. Nevertheless, we proved that an optical rectification signal could be generated and detected by experimental means. Finally, Q-switched diode lasers in Al-quaternary material were fabricated and evaluated as possible sources for the rectification devices. The lasers produced a pulse train ranging from 1 GHz to 2 GHz depending on the bias current. We reckon that our measurement set-up is not ideal to characterize the rectification signal but is the simplest set-up capable of giving us an indicative result. The time domain observation of the optical rectification signal has still to be done and the integration of a photoconductive switch to the optical rectification device seems to be the most obvious solution to achieve this.

The linear and nonlinear optical properties of a spin-coated polydiacetylene, [5,7-dodecadiyn-1,12-diol-bis(n-butoxy-carbonyl-methyl-urethane)], or poly(4BCMU), were measured to predict its performance in all-optical devices at 1.319 μm. Material requirements for all-optical devices were identified and figures-of-merit noted. A two-photon absorption figure of merit was verified by numerical simulation of a waveguide device. The refractive index and waveguide loss in spin-coated poly(4BCMU) films were measured. A photo-induced bleaching was observed, and its effect on linear and nonlinear optical properties was quantified. Fabrication of integrated-optical structures using this photobleaching process was demonstrated. The nonlinear refractive index and absorption were measured at 1.319 μm with 60 picosecond laser pulses, using poly(4BCMU) strip-loaded channel waveguides. A novel pulse-modulated interferometer was developed for measuring the intensity-dependent refractive index. The fast electronic contribution was found to be n₂ = (4.8 ± 2.7) x 10⁻⁸ cm²/MW, an a slower thermal contribution of n₂(T) = -(7.9 ± 4.5) x 10⁻¹¹ cm²/MW was measured. The thermal index change was shown to limit the duty cycle of operation for a poly(4BCMU) device. The two-photon absorption coefficient was also measured, yielding γ < 0.25 cm/GW. These values were used to estimate performance of a poly(4BCMU) all-optical device using standard figures-of-merit. For this specific waveguide, the figures-of-merit indicated poor performance. If waveguide losses were neglected, (by assuming improved fabrication for example), and assuming the nonlinearity does not saturate at intensities below the damage threshold, the figures-of-merit improve to useful levels. The limit on duty cycle imposed by thermal effects appears to restrict operation to GHz frequencies of slower.

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.

The nonlinear optical properties of a semiconductor-doped glass (SDG) channel waveguide were measured on a picosecond time-scale; namely, fluence-dependent changes in the absorption and the refractive index as well as the relaxation time of the nonlinearity. Slower, thermally-induced changes in the refractive index were also observed. The saturation of the changes in the absorption and the refractive index with increasing optical fluence is explained using a plasma model with bandfilling as the dominant mechanism. The fast relaxation time of the excited electron-hole plasma (20 ps) is explained using a surface-state recombination model. A figure of merit for a nonlinear directional coupler fabricated in a material with a saturable nonlinear refractive index is presented. The measured nonlinear change in the refractive index of the SDG saturates below the value required to effect fluence-dependent switching in a nonlinear directional coupler. Experiments with a channel-waveguide directional coupler support this prediction. However, absorption switching due to differential saturation of the absorption in the two arms of the directional coupler was observed.

The objective of this work presented here is to extend the capabilities of siloxane waveguide technology in the field of biochemical sensing. Recent advances in the integration of polymeric optical waveguides with electronics onto standard printed circuit boards (PCBs) allow the formation of cost-effective lab-on-achip modules suitable for mass production. This technology has been primarily designed for on-board data communication. The focus of this research is to investigate the possibility of realising a Siloxane polymer based lab-on-chip sensor. Different siloxane-polymer-based optical waveguide sensor structures have been designed and analysed from the aspect of biochemical sensing. An evanescent-wave absorption sensor based on mode-selective asymmetric waveguide junctions is proposed for the first time. The device mitigates the common optical effect of spurious response in absorption sensors due to the analyte transport fluid. Head injury is the leading cause of death in the population of people under 40 years. Currently, 3 out of 5 deaths in emergency rooms are due to severe brain injuries in the developed world. Researchers at the Neurosciences Critical Care Unit (NCCU) at Addenbrooke's Hospital have managed to correlate biochemical changes with the severeness of the injury and the likelihood of patient recovery. Considerable progress has been made to develop a lab-on-chip sensor capable of continuously monitoring glucose, lactate and pyruvate concentrations in the brain fluid, hence the contribution to the current trend in the advancement of portable lab-on-chip technologies for the deployment of point-of-care diagnostic tools. A novel recognition layer has been developed based on porphyrin in combination with glucose, lactate and pyruvate oxidase for measuring all the analytes, enabling fast and reversible chemical reactions to be monitored by optical interrogation. The operational wavelength of the developed recognition layer is 425 nm, which required the formation of polymer features that were beyond the fabrication capabilities at the time. Through considerable process development and the adoption of nanoimprinting lithography, siloxane polymer based optical waveguides were fabricated allowing the realisation of highly sensitive optical sensors. Based on the results that are presented here, it can be concluded the functionalization of siloxane polymer waveguide have a potential for realising biochemical sensors in the future. The new fabrication technique will allow the formation of more robust and complex lab-on-chip sensors based on this material.

The effect of an externally applied magnetic field on the propagation characteristics of a plasmon-polariton wave supported by an infinitely wide thin metal waveguide was investigated. In order to do so, the dispersion relation was derived, from Maxwell's equations, enabling accurate modelling of the situation of interest. The general dispersion relation, including the constraint equation, for magneto-plasmons was derived in general, and then, specifically for a magnetic field applied along three orthogonal cartesian axes. The losses in the metal were included in the dispersion equation so that a better understanding of the influence of an externally applied magnetic field may be provided. The dispersion relation is used as the basis of a software model of magneto-plasmons in thin metal films. This model is validated against specific cases in the literature with and without an externally applied magnetic field. The specific formulations in the literature were deemed to be incorrect, and have been corrected and the results have been interpreted. The model is then used to simulate thin gold films bounded by silicon dioxide at an infrared wavelength. The modelling results include the effect of the externally applied magnetic field on the propagation constant and the corresponding field components for all three Cartesian orientations of externally applied magnetic field. The results from these simulations are presented and interpreted. (Abstract shortened by UMI.)

This research project explored the potential of forming an integrated optics technology based on silicon core waveguides suitable for application in sensors and communications in the wavelength range 1.2 to 1.6 mum. Integrated optics has evolved around the use of compounds such as lithium niobate and III-V semiconductors due to their available electro-optic properties. By contrast silicon has received relatively little attention as its indirect band gap has prevented the fabrication of light sources in the material and its centrosymmetric crystal structure means that it has no useful linear electro-optic effect. The lack of a demonstrated low loss integrated optical waveguide compatible with single mode optical fibres has been a further limitation. However, these major drawbacks in silicon waveguide technology may be more than offset by the potential advantages of forming silicon integrated optical devices using well established silicon microelectronics fabrication methods. The project focused research on waveguiding in silicon-on-insulator (SOI) structures with the aim of developing a practical low loss waveguide in these structures and understanding the various loss mechanisms. In principle the optical absorption of pure crystalline silicon over the wavelength range of interest allows waveguides with losses less than 0.1 dB/cm to be formed. SOI material formed by ion implantation has been developed for microelectronic applications and provides a commercial source of a silicon planar waveguide structure with high quality interfaces and low defect density. The project studied waveguides based on this material. Initially planar waveguides with silicon thickness from 0.57 to 7.3 microns and buried oxide thickness of 0.07 to 0.4 microns were studied. Fabrication methods and structures were identified which allowed multi-microns planar SOI waveguides to be formed with losses less than the benchmark of 1 dB/cm. For these structures a buried oxide thickness of 0.4 microns was found to be sufficient to prevent substrate leakage loss. It has been concluded that the predominate loss mechanism is scattering of light at the silicon to buried oxide interface. Rib waveguides were formed in SOI following the insight into loss mechanisms gained in the planar waveguide studies. Optical rib waveguides with widths from 2.73 to 7.73 microns were formed in SIMOX (Separation by IMplantation of OXygen) based SOI structures consisting of a 4.32 micron thick surface silicon layer and a 0.398 micron buried oxide layer. The effect of waveguide width, bend radius, Y-junction splitting and interface roughness on loss and mode characteristics were studied at wavelengths of 1.15 and 1.523 microns. The experimental results support the hypothesis that certain rib dimensions can lead to single mode waveguides even though planar SOI waveguides of similar multi-micron dimension are multimode. The propagation losses of waveguides 3.72 microns wide were found to be 0.0 dB/cm and 0.4 dB/cm for the TE and TM modes respectively when measured at 1.523 microns. The measurement uncertainty was estimated to be +/-0.5 dB/cm. These results are thought to be the lowest loss measurements for silicon integrated optical waveguides reported to date. During the course of the project other researchers have demonstrated useful electro-optic properties in silicon semiconductor junctions based on the free carrier plasma dispersion effect and room temperature electroluminescence in silicon based junctions. The combination of these developments with the practical waveguide structure demonstrated in this project now makes the possibility of developing a practical silicon based integrated optics technology a reality.

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.

A series of fluorinated copolymers were prepared and used to fabricate optical waveguides by photolithography. Reproducible syntheses were established for a buffer material, 4-fluoro-styrene-co-pentafluoro-styrene-co-[4-styren-4'-yl-2,3,5,6-tetrafluorophenylether] (bufTxx), and guiding material, Styrene-co-penta-fluorostyrene-co-[4-styren-4'-yl-2,3,5,6-tetrafluorophenyl-ether] (terTxx), as well as their precursors. The polymers were characterized by attenuated total reflection Fourrier transform infrared spectroscopy (ATR FT-IR), gel permeation chromatography (GPC), proton nuclear magnetic resonance spectroscopy (1H NMR), and thermal analyses. Freestanding waveguide structures and multilayer buried waveguide devices were prepared on silicon substrates. The guiding polymer behaved as a negative resist, facilitating photoinitiated lithographic printing. These structures were investigated by optical microscopy, field-emission gun scanning electron microscopy (FEG SEM) and prism coupling analysis. A 632 nm HeNe laser was successfully end-coupled to printed waveguide structures from an optical fiber.
Une série de copolymères fluorés a été préparée et employée pour fabriquer des guides d'ondes optiques par photolithographie. Des synthèses reproductibles ont été établies pour un matériel tampon, le 4-fluorostyrène-co-pentafluorostyrène-co-[4-(styrèn-4'-yl)-2,3,5,6-tétrafluorophényléther] (bufTxx), pour le matériel de guidage, du styrène-co-pentafluorostyrène-co-4-(styrèn-4'-yl)-2,3,5,6-tétra-fluoro-phényléther] (terTxx), ainsi que leurs précurseurs. Les polymères ont été caractérisés par spectroscopie infrarouge par transformé de Fourier de type réflexion totale atténué (ATR FT-IR), par chromatographie sur gel perméable (GPC), par spectroscopie de résonance magnétique nucléaire de proton (1H RMN), ainsi que par des analyses thermiques. Des motifs de guides d'ondes indépendants et des dispositifs de guide d'ondes à plusieurs couches ont été préparés sur des substrats de silicium. Le polymère agissant comme guide d'ondes a des propriétés de résistances négatives, facilitant une impression lithographique photoinitiée. Ces motifs ont été étudiés par microscopie optique, par microscopie électronique de balayage utilisant un canon à émission de champ (FEG SEM) et par analyse de couplage par prisme. Un laser de 632 nanomètres (HeNe) a été couplé avec succès aux motifs de guide d'ondes imprimées d'une fibre optique.

This thesis presents the realization of a characterization method for optical waveguides in terms of propagation loss. The method employs a highly linear video camera aided by a personal computer running image processing and analysis software. After a brief theoretical overview, the design of the experimental set-up and its realization are introduced. Several optical waveguides are examined using the present method and loss values are obtained for various channel waveguides built on silicon substrates, with the experimental results detailed for two such wafers. The advantages and limitations of the method are also briefly discussed.

A theoretical and experimental study on the optical trapping and propulsion of latex, gold aggregate and colloidal gold particles with average radius of 1.5µm, 250nm and 10nm respectively, in the evanescent region of an illuminated ion-exchange channel waveguide is documented in this thesis. Optimisation of light-induced forces exerted on a particle on a waveguide relies on two important factors, firstly a maximisation of the intensity and intensity gradient present in the guide-cover interface and secondly, an optimisation of the polarizability of a particle. To this end, a transcendental equation was established and was used to generate design curves for the normalised waveguide thickness required for achieving a maximum gradient force on the guide-cover interface of a waveguide for a specific set of indices. A study based on Mie theory for the investigation of morphology dependent resonance exhibited by a spherical particle is described. The dependence of resonances on particle radius, index of the sphere with respect to the surrounding medium, absorption, plasmon resonance and symmetry of the incident beam has been investigated. In particular, a simplification of the Mie model was carried out to derive Rayleigh expressions of cross sections from which particle polarizability originates. The validity of the Rayleigh model was assessed with respect to the limiting particle radius. Based on a semi-classical approach, a derivation of light-induced forces applying to a Rayleigh sphere in the cover region of a waveguide is detailed. The three main optical force components produced are (i) a forward scattering and absorption force due to the intensity of the incident radiation which accounts for propulsion of particles, (ii) a transverse gradient force due to an intensity gradient generated by a decaying evanescent field and finally (iii) a lateral gradient force which arises from the near-Gaussian intensity distribution on a channel waveguide. A comparison of the relative magnitude of each component is described, with additional forces due to gravity, buoyancy and Brownian motion studied. Factors affecting the propulsion of a gold nanoparticle were investigated. It was shown that the particle velocity is linearly dependent upon the waveguide modal power, increases with a wavelength closer to plasmon resonance in the case of a Rayleigh gold particle, is stronger for TM polarized light, increases with a larger change in the waveguide refractive index and is maximum for a minimum modesize. For the first time, under the action of light-induced forces generated on the surface of an optical waveguide, colloidal gold particles are propelled in the direction of wave propagation reaching at a maximum velocity of 10µm/s for a modal power of 500mW at l=1.047µm. Results obtained will be useful for future applications in particle sorting, fluorescence sensing and surface enhanced Raman sensing of chemical species.

Waveguide polaritons are the quasi-particles arising from the strong coupling of quantum well excitons to the photonic mode of a waveguide. These are complimentary to the polaritons observed in semiconductor microcavities which in the two decades following their first observation have been a rich source of interesting physical phenomena such as parametric scattering, condensation, superfluidity and solitons. Whilst waveguide polaritons are a complimentary scheme they do have a number of important advantages over microcavities, firstly the use of total internal reflection to confine the photonic mode in principle affords lower losses and the reduced mode volume increases the coupling to quantum well excitons. Additionally the thin structure more naturally lends the system towards to fabrication of complex polaritonic devices. The waveguide polariton scheme was first investigated in the late 1980s and early 1990s however the lack of direct access to the dispersion hindered progress. Recently however advancements in photonics have led to the development of integrated grating couplers which are used in this thesis to couple light in and out of the waveguide structure. The relationship between the emission angle from these grating couplers and the internal wavevector is exploited in Chapter 3 to make the first unambiguous observations of waveguide polaritons by a direct observation of the characteristic anti-crossing dispersion indicative of the strong coupling regime. In the second half of Chapter 3 the design of the waveguide device was improved by adding further quantum wells to increase the Rabi-splitting and reduce the effect of absorption in the tail of the exciton line. It is then shown that the strong coupling regime is preserved in this device up to 100 K. In Chapter 4 it is shown that the interactions between polaritons inherited from the exciton component leads to an optical nonlinearity which causes the defocusing of high intensity beams travelling through the waveguide. This nonlinearity can be described as a negative nonlinear refractive index which can support the generation of single or pairs of dark-spatial solitons depending on the initial conditions. Finally this nonlinearity is also shown to persist to 100 K suggesting the possibility of future polaritonic devices operating at higher temperatures. In Chapter 5 it is shown that the curvature of the polariton dispersion in the anti-crossing regions gives rise to a massive group velocity dispersion which causes the dilation of injected pulses as they propagate along the waveguide. At high particle densities within the pulse this group velocity can be balanced against the optical nonlinearity arising from inter-particle interactions to form bright temporal solitons. Finally due to the comparable nonlinear-, diffractive- and dispersive-length scales it is shown that this system support the formation of a hereto unobserved hybrid of a spatially-dark and temporally-bright soliton. In this thesis waveguide polaritons are reintroduced as a complimentary system to microcavities and the first observations made of their formation and interactions. This thesis lays the foundation for future studies into waveguide polaritons and showcases their nonlinear properties through the study of spatio-temporal solitons.

In the last few years, interest in polymer optical fibers (POF) has increased because of their low cost, easy handling and good flexibility even at large diameters. Moreover, optical cables do not have the problem of electromagnetic interference, which gives, for instance, the problem of cross-talk in copper telephone cables. In the usage of current communication and computer systems the yield has gained a big importance and it has seen from studies that light scattering loss is the only loss, which cannot be eliminated entirely. Besides, this loss causes its attenuation loss intrinsically and determines the lower limit of loss in the POF. In this work, the importance and the dependencies of light scattering were studied, and calculations were done in order to find more appropriate polymer for using as core material of POFs. For this aim, a computer program that calculates the light scattering loss of several amorphous polymers and plots the graph of isotropic scattering loss versus isothermal compressibility and total attenuation loss versus wavelength was written.

Because of their compatibility with the planar concept of integrated optics, grating couplers offer the most satisfactory means of coupling light into thin film optical waveguides. The purpose of this dissertation has been to study the behaviour, both theoretically and experimentally, and fabrication of grating couplers in nonlinear waveguides. A theory of nonlinear grating couplers is presented based on a coupled-mode approach. The dependence of coupling efficiency on incident beam intensity, beam size, beam position, incident angle, chirp rate, and waveguide losses have been examined all in the presence of nonlinearities in the waveguide. It is reported that, in the presence of nonlinearities, the coupling efficiency decreases with increasing incident power. Different ways of optimizing the coupling efficiency at high incident power levels are presented. These include adjusting the beam size, the coupling angle, and chirping the grating. A new technique is reported for fabrication of regular period, chirped, and curved photoresist gratings. The experimental arrangement is essentially based on Lloyd's mirror fringes and is characterized by its stability, simplicity, and versatility. We also report on successful use of Reactive Ion-Beam Etching (RIBE) with C₂F₆ gas in producing very smooth and deep gratings with high aspect ratios in different waveguide structures. Experimental coupling efficiencies of up to 40% are reported in polystyrene waveguides using etched grating couplers. Experiments are reported in support of the theoretical findings of this dissertation using a polystyrene waveguide with thermal nonlinearity.

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.

Second Harmonic Generation has traditionally been restricted to crystals; however, due to the development of highly sophisticated fabrication technology, poling and phase matching techniques, it has now become feasible in glass fibres and waveguides which are widely used in nonlinear optics. For instance, silica glass based Photonic Crystal Fibres (PCF) exhibit long coherence lengths and controllable optical properties such as chromatic dispersion despite low second order nonlinearity. Given such benefits, the numerical modelling and analysis of optical waveguides accurately and efficiently has become vital for the advancement of nonlinear optics. Hence, this thesis focuses on enhancing the Second Harmonic Generation in optical waveguides through the use of different structures and different materials, and demonstrating the same by numerical simulations. In this thesis, the accurate and numerically efficient Finite Element based Beam Propagation Method has been employed to investigate the evolution of Second Harmonic Generation in highly nonlinear SF57 soft glass Equiangular Spiral PCFs. Further, the H-field based Finite Element Method has been employed for the stationary analysis of Photonic Crystal Fibres. It is shown here that the second harmonic output power in highly nonlinear

Integrated optics present a potentially low cost and higher performance alternative to electronics in optical communication systems. Surface plasmon waveguides (SPWGs) offer a new approach for manipulating light in integrated optical chips. SPWGs provide several advantages over dielectric waveguides. In this study, a fabrication process for SPWGs is developed. SPWGs are fabricated with various lengths and bend radii to allow for study of absorption and bending losses in the waveguides at telecommunications wavelengths (~1550nm). Finite-element method models of straight, bent, and optically coupled waveguides are developed and analyzed.

This thesis describes an investigation of polarisation conversion effects in gallium arsenide optical waveguides. The research was carried out with the aims of predicting, preventing and harnessing such effects. Experimental results are presented to demonstrate changes in the polarisation state of light propagating in passive deep-etched waveguides. The results are described by established modelling techniques. The effect due to process-dependent features of waveguide cross-section geometry, in particular asymmetry resulting form non-vertical etching, is investigated. The polarisation angles of hybrid waveguide modes are measured, and a novel technique is presented for the measurement of the differences been the effective indices of orthogonally polarised modes. The measurements obtained are used to analyse the evolution of elliptical polarisation states during propagation, and to provide an account of the physical origin of the polarisation conversion. Details of the nature of the optical modes predicted by rigorous numerical method simulations are demonstrated by the experimental results, while quantitative agreement between the simulated and measured data is shown. A simplified account of the behaviour is provided using a coupled-mode formulation. The influence of the linear electrooptic effect in modifying the polarisation conversion behaviour is explored experimentally, and is described using established theory. Waveguide designs are obtained which prevent unintended polarisation conversion in the presence of identified causes, while maintaining the main waveguide parameters of material composition, optical mode size and shape, electrooptic performance, and fabrication process. The polarisation behaviour in waveguides fabricated to these designs is evaluated, and the expected performance benefits are confirmed. A novel waveguide device which provides electrooptic control and switching of the optical polarisation state is presented. The device is capable of converting any input polarisation state into an arbitrary output state using the linear electrooptic effect. A working design is obtained and the fabrication of devices is described. Experimental results are presented which demonstrate the concept. Further developments of the polarisation controller device are proposed, including the realisation of the potential for switching speeds at frequencies of tens of GHz.

This thesis describes a number of optical spectroscopic measurements in novel one and two dimensional photonic crystals patterned in a slab waveguide. Two dimensional photonic crystals have also been embedded in a ridge waveguide to confine light to the plane containing the photonic crystal. Linear characterization of these structures revealed a photonic band gap. Non linear measurements revealed pulse· compression, negative differential transmission and showed that the ridge waveguide nonlinear response dwarfs that of the photonic crystal. Pump-probe spectroscopy is used to study the nonlinear response of AlGaAsllnAlGaAs MQW one dimensional photonic crystals. The modulation of the reflectivity spectra due to the refractive index change produced by two and three photon induced free-carriers was measured. Pump-induced blue-shifts in the wavelength of photonic resonances were measured. These were followed by rapid decay - 25 ps. The blue-shifts in the photonic resonances were as large as 15 nm. The lifetime of free-carrier nonlinearities in one dimensional photonic crystals was found to depend on the photonic crystal parameters, i.e. period, air fill factor and etch depth. It was found that the free-carrier nonlinearities lifetime varied from 8 to 33.5 ps. Ultrafast tuning of the photonic resonances was obtained via the optical Kerr effect and the optical (AC) Stark effect. Decay-less tuning was observed. The response time was measured to be at FWHM - 300-400 fs.

Thesis (M.S.) -- University of Maryland, College Park, 2005.
Thesis research directed by: Dept. of Electrical and Computer Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In nonlinear optical waveguide structures, injection of an optical beam above a given threshold intensity can be theoretically shown to emit spatial solitons. An all-optical switch based on this phenomenon is designed and computer simulations show that the phenomenon persists despite detrimental effects such as saturation of the nonlinearity and nonlinear absorption. In the cylindrical geometry of an optical fiber, a soliton ring can be emitted from the core and fragment into filaments via a transverse modulational instability. The inclusion of temporal dispersion can destabilize spatially stable solutions, both for normal and anomalous group velocity dispersion. For a saturating nonlinearity, the temporal dispersion can be used to generate trains of nonlinearly self-bound light packets from a continuous sub-emission-treshold input field.