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Telecommunication networks based on optical fiber technology have become a major information-transmission system, satisfying the growing demand for bandwidth due to increased internet traffic and other applications such as video on demand, etc. Fiber Bragg gratings (FBGs), in recent years, have emerged as critical components for enabling high-capacity transmission since their response can be tailored to meet the needs of specific applications. FBGs are currently the focus of intense research interest in both the fiber communications and sensing fields. Optical fiber Bragg grating structures in single-mode fiber (SMFBGs) have been studied extensively since the discovery of photosensitivity in germanium-doped silica fiber. They have been used in numerous applications ranging from wavelength-selective filtering in wavelength-division-multiple-access (WDMA) systems to temperature and strain sensing. To a lesser extent, Bragg gratings in multimode fibers have also received attention because of easy coupling with light sources. Most of the MMFBGs related research work has demonstrated the formation of a Bragg grating in a graded-index MMF and briefly reported the measured transmission spectrum. So far, there are few theoretical studies on Bragg gratings in multimode fibers. In this thesis, we investigate Bragg gratings in multimode optical fibers both theoretically and experimentally. A comprehensive numerical model for MMFBGs has been established and the corresponding computer simulation software (MMFBG simulator combined with mode solver) developed. The optical properties of MMFBGs were systematically studied for the first time using our own MMFBG numerical software package. It effectively assists the design modeling for MMFBG-based optical devices. Bragg gratings in multimode fiber were also investigated experimentally. Our theoretical simulation results show good agreement with experiments and offer the insightful explanations for the underlying physics of the device. First, the guided modes were modeled and simulated for step index multimode fibers and graded index multimode fibers with emphasis on parabolic fiber structure. These are popular, standard and commercially available MM fibers, and employed throughout our experiments. This allows us for the simulation of fiber characteristics such as cut-off wavelength, mode effective index, propagation constants and optical field distribution. It also allows for calculation of mode coupling coefficients by overlap integral between any chosen guided modes. Therefore, it serves as a powerful model for the design and analysis of optical fibers. Second, the generalized MMFBG coupled mode theory formalism is derived. The physical mechanism of the behavior of MMFBGs is studied and discussed. The general solution to the MMF Bragg grating problem is achieved by Runge-Kutta, Newton-Raphson and shooting numerical methods. Our theoretical treatment, in particular, offers the advantages which can deal with not only self-coupling but also more complicated cross-coupling interactions and can solve arbitrary large number of mode coupling problems throughout the entire spectra simultaneously for multimode FBGs, thus allowing for a precise and quantitative study of MMFBGs. Such an intensive multimode fiber Bragg grating physical modeling and simulations have not been reported previously. It provides an effective means for the design and analysis of optical fiber devices based on Bragg gratings. Third, the optical properties of multimode FBGs were studies experimentally. Numerical predications of the grating spectral characteristics under fabrication and experimental condition are calculated. The results of the numerical calculations are compared with experimentally measured spectra of multimode gratings written by ultraviolet irradiation of deuterium-sensitized fiber with grating reflectivities ranging from 78% to 99.39%. Good agreement is obtained between the theoretical simulations and the experimental results. Thus, we provide quantitative explanations for the observed experimental phenomena. These explanations give both physical insight and a more complete understanding of the nature of the interaction between the wave propagation and multimode fiber gratings. Furthermore, the spectral simulation of the actual experiments prepares a theoretical guidance for the advanced experimental investigation and also presents a step toward MMFBG device design. Finally, the optical properties of MMFBGs were also studied theoretically. To our knowledge, this is the first detailed analysis and thorough investigation on grating characteristics in MMF. It is demonstrated that the transmission and reflection spectra of fiber Bragg gratings in multimode optical fibers strongly depend on the length of grating, index modulation, period of grating, mode excitation condition and physical structure of MMF. The simulation results allow us to deeply comprehend and visualize the more sophisticated behavior within a multimode fiber grating, and will also allow us to confidently predict and evaluate the performance of more complex structure MMFBGs. It provides the fundamental principles for designing the targeted spectrum performance and settles the theoretical rationale for realizing the practical applications. Overall, the comprehensive numerical model and MMFBG solver package developed in this thesis opens a clear and broad window for understanding MMFBG mechanisms from the physical point of view. Various simulation results and spectral characteristics have been researched and discussed under both ideal and experimental conditions for the purpose of experimental analysis and device design. The results of our study indicate that a new class of potential applications based on MMFBGs can be expected in optical fiber sensors and advanced communication systems. / Thesis / Master of Applied Science (MASc)

La mesure de l'indice de réfraction a été étudiée depuis qu'Ernest Abbé aie initialement conçu un réfractomètre en 1869, appelé le réfractomètre d'Abbé. Depuis lors, de nombreux réfractomètres ont été développés tels que le réfractomètre à prisme optique ainsi que le réfractomètre à fibre optique, en raison de leurs applications étendues pour la détection de divers paramètres physiques, biologiques et chimiques. Récemment, un grand nombre de chercheurs ont mis au point des réfractomètres basés sur des fibres optiques, exploitant des mécanismes tels que la résonance des plasmons de surface (SPR), les interférences multimodes, les fibres à réseaux de Bragg (FBG), les fibres à réseaux à longues périodes (LPG), les fibres optiques coniques et la fibre multimode à gaine dénudée. Les capteurs fibrés sont avantageux grâce à leur immunité contre les interférences électromagnétiques, passivité électrique au niveau de la sonde de détection et potentiel de mesure in situ à long terme. Cette thèse concerne le développement de modèles complets fonctionnels et précis pour les réfractomètres à fibres optiques basés sur la modulation d'intensité optique, en particulier la réfractométrie à fibre multimode à gaine dénudée ainsi que les systèmes hybrides associant fibres monomode et multimode, et un réfractomètre hybride tout fibré utilisant des fibres à cristaux photoniques. L'objectif clé de ce travail est de caractériser les performances de ces réfractomètres à fibres optiques basés sur la modulation d'intensité en termes de réponse en puissance, de sensibilité, de résolution et de dynamique de mesure. Les résultats de simulation qui sont corroborés expérimentalement démontrent que la très grande sensibilité obtenue dans la zone II (c'est-à-dire le régime de détection typiquement utilisé pour mesurer l'indice du milieu supérieur à l'indice de gaine mais inférieur ou égal à l'indice du coeur) pour tous les trois réfractomètres. Cependant, la sensibilité dans la Zone (c’est-à-dire le régime de détection pour lequel l’indice du milieu à mesurer est supérieur à celui du coeur) est très faible. Ainsi, un refractomètre fibré hybride monomode-multimode est utilisé pour améliorer la sensibilité dans la Zone III. D'autre part, la sensibilité pour la zone I (c'est-à-dire le régime de détection pour mesurer l’indice du milieu inférieur à l'indice de la gaine) a été améliorée en augmentant l'absorption des ondes évanescentes à l'aide du réfractomètre hybride tout fibré à base de fibres à cristaux photoniques à coeur solide. En termes d'application réelle du réfractomètre à fibre pour la détection biochimique, une preuve de concept pour un capteur du gaz méthane a été démontrée en utilisant les supramolécules de cryptophane-A qui permettent de piéger les molécules du méthane. Le cryptophane-A incorporé dans un film hôte à base de styrène acrylonitrile (SAN) est appliqué sur la zone dénudée du capteur comme une région fonctionnalisée. L'indice de réfraction de cette couche sensible augmente proportionnellement avec l'augmentation de la concentration du méthane, ce qui induit une variation de la puissance optique transmise dans le capteur fibré. / Refractive index measurement has been studied since Ernest Abbé initially designed a refractometer in 1869, which is named the Abbé refractometer. Since then, numerous types of refractometers have been developed by employing either the optical prism-based refractometer or the optical fiber-based refractometer, due to their wide-ranging applications such as for sensingvarious physical, biological and chemical parameters. Recently, a large number of researchers have been developing refractometers based on optical fibers, exploiting mechanisms such as surface plasmon resonance (SPR), multimode interference, fiber Bragg gratings (FBG), long period gratings (LPG), tapered optical fibers, and striped-cladding multimode fibers (MMF), for their advantages in immunity against electromagnetic interference, electrical passivity at the sensing probe, and capability to long term in-situ measurement. This thesis concerns the development of comprehensively functional and accurate models for optical fiber refractometers based on optical intensity modulation, in particular for stripped-cladding MMF refractometry as well as hybrid systems involving a combination of single-mode-multimode fiber refractometery and the all-fiber hybrid refractometer using photonic crystal fibers. A key objective of this work is to characterize the performance of these intensity-based optical fiber refractometers in terms of their power response, sensitivity, resolution, and dynamic range. The simulation results which are corroborated experimentally demonstrate very high sensitivity being obtained in Zone II (i.e. the sensing regime typically employed for measuring a sensing medium index higher than the cladding index but less than or equal to the core index) for all three types of refractometers. However, the sensitivity in Zone III (i.e. the sensing regime for which the sensing medium index is higher than the core index) is very low. A hybrid single-mode fiber - multimode fiber configuration is used to improve the sensitivity in Zone III. On other hand, the sensitivity for Zone I (i.e. the sensing regime typically employed for measuring a sensing medium index lower than the cladding index) has been improved by increasing evanescent wave absorption using the all-fiber hybrid refractometer based on solid-core photonic crystal fibers. As a further potential of the fiber refractometer for applications in biochemical sensing, the proof-of-concept for a methane gas sensor has been demonstrated using supramolecular cryptophane-A which enables to trap the methane molecules. Cryptophane-A incorporated into a functionalized film of StyreneAcrylonitrile (SAN) host is applied to a de-cladded region of the sensor as the sensitive region. The refractive index of this functionalized layer increases proportionally with increasing methane concentration, subsequently inducing variations in the transmitted optical power along the fiber sensor.

This thesis investigates the generation and radio-over-fibre (RoF) transport of unlicensed 60 GHz millimetre-wave (mm-wave) frequency band. The investigated benefits of transmission schemes applicable for the mm-wave generation include optical carrier suppression (OCS), optical frequency multiplication (OFM) and remote heterodyne detection (RHD). For the in-door cabling of the mm-wave transmission, a low-cost polymer optical fibre (POF) along with bend-insensitive single mode fibre (BI-SMF) has been investigated for short-range networks. Transporting mm-wave generated signals over POF and BI-SMF cables based on OCS scheme showed results with the highest spectral efficiency and least inter-symbol interference over a 2.5 Gbit/s data delivery. Based on this thesis analysis, OCS simulation of POF showed the most reliable power penalty performance and receiver sensitivity at 30-m whilst the BI-SMF fiber produced equal observations at 150-m and more. In observing the free space links of delivering the RoF signal, the attenuation on the received signal power for both POF and BI-SMF was insignificant but expected, as the simulation assumed complete and total collimation of the light beams onto the aperture of the photodetector. OCS scheme for mm-wave generation and transport was explored based on the cost effectiveness of using one external modulator compared to other generation schemes that utilised more than one external modulator. OFM scheme was simulated to transport LTE and Wi-Fi signals along with 60 GHz RF band through both SMF and MMF-POF/BI-SMF cables. OFM transport scheme produced the highest attenuation on LTE, Wi-Fi and mm-wave signals carrying 100 Mbit/s data as simulated POF lengths increased. The best performance POF length was observed at 10-m. The application of offset launch technique at the coupling of SMF and POF showed insignificant improvement on signal bandwidth. The free space OFM transmission also demonstrated negligible change to the received signal power. This reinforces the attributes of deploying OWC system in an in-door environment. In other investigation, the simulated successful delivery of mm-wave signal using RHD scheme modulated and transported 10 Gbit/s data signal over POF and BI-SMF cables. Additional observed unrecorded result also showed BI-SMF cable maintained a 2% reduction of received power for 450-m fiber cable from 150-m. The attributes to RHD includes its low operating power system application and delivery of localised 60 GHz signal for uplink RoF transmission. The conceptualised design of Gigabit data delivery for indoor customer applications either through POF or BI-SMF cable, transporting various wireless channels has been presented in this thesis for the design of a robust next generation Broadband access network to reinforce the fiber-inside-the-home (FiTH) deployment.

The design, simulation, and initial fabrication of a novel ultra-compact 2x2 silicon multimode-interference device evanescently coupled to a dual germanium metal-semiconductor-metal (MSM) photodetector is presented. For operation at the standard telecom wavelength of 1.5 µm, the simulations demonstrate high-speed operation at 30 GHz, low dark current in the nanoamp range, and external quantum efficiency of 80%. Error analysis was performed for possible tilt error introduced by hybrid integration of the MSM layer on top of the MMI waveguides by use of surface mount technology (SMT) and direct wafer bonding.

Thesis (PhD) -- Stellenbosch University, 2001. / Some digitised pages may appear illegible due to the condition of the original hard copy. / ENGLISH ABSTRACT: A new technique for the design of multimode devices in overmoded waveguide is presented. The technique applies the principle of growth-based design and uses a conceptual functional block representation of the design structure to provide necessary flexibility to the design algorithms. Two growth based design strategies are proposed and evaluated. The first uses a generalized synthesis-oriented scanning technique, and the second uses an evolutionary strategy. The techniques provide reliable solutions to a variety of multimode design problems. In order to facilitate sufficiently fast numerical analysis, novel enhancements of the mode matching technique are developed and the use of surrogate models is investigated. In addition, to allow physical evaluation of the finished devices, original techniques of measuring multimode devices are formulated and utilised. Two practical problems are used to evaluate the performance of the design procedures. The first is the design of overmoded waveguide chokes for microwave heating facilities, and the second is the design of multimode horns for antenna and spatial power combining applications. Various examples of each type of problem are presented with measurements of manufactured solutions. / AFRIKAANSE OPSOMMING: ’n Nuwe tegniek vir die ontwerp van multimodusstelsels binne multimodus golfleier word voorgestel. Die tegniek maak gebruik van die beginsel van groei-georienteerde ontwerp en ontgin ’n konsepsuele funksionele module-voorstelling van die ontwerpstruktuur om die nodige buigsaamheid aan die ontwerpsalgoritmes te verleen. Twee groei-georienteerde ontwerpstrategiee word aangebied en geevalueer. Die eerste is gebasseer op ’n veralgemeende sintese-georienteerde skandeertegniek, en die tweede maak gebruik van ’n evolusie-strategie. Die tegniek verskaf betroubare oplossings vir ’n verskeidenheid van multimodusontwerpsprobleme. Ten einde ’n numeriese analise-tegniek daar te stel wat vinnig genoeg is, word oorspronklike verbeterings van die modal-pas metode ontwikkel en surrogaatmodelle is ook ondersoek. Verder, vir fisiese evaluasie, word oorspronklike meettegnieke vir multimodusstelsels geformuleer en gebruik. Twee praktiese probleme word gebruik om die ontwerpprosedures te evalueer. Die eerste is die ontwerp van multimodus golfleierdrywingsdempers vir mikrogolfverhitting, en die tweede is die ontwerp van multimodus horings vir antenna- en ruimtelike drywingskombineerdertoepasings. Verskeie voorbeelde van elke tipe probleem word gegee met metings van gei'mplementeerde oplossings.

The purpose of this thesis is the theoretical study of interference and correlation effects in multimode and continuum mode quantum systems. We are concerned with interference effects in multiport devices which in a sense are generalised Mach-Zehnder interferometers. It is shown how these multimode devices can be employed for the study of negative result and interaction free measurements. Interference and coherence effects are also studied in relation to the radiation fields generated by atoms through the process of spontaneous emission. Besides first order interference, higher order coherence effects are investigated with the aid of Glauber's photodetection theory and it is found that detectors that lie in spacelike regions may display nonclassical correlations under certain conditions. It is well known that the vanishing of field commutators between regions that cannot be connected by subluminal signals reflects the locality of quantum field theory. But is it possible that these spacelike regions exhibit correlations that violate Bell type inequalities? This is the main question and principal concern of the thesis and the answer is affirmative, nonclassical correlations between spacelike regions are indeed possible. A scheme of four detectors that lie in spacelike points was also studied. In this case we do not consider the radiation field but a free scalar field in vacuum state. Nevertheless the virtual quanta of this field may induce nonclassical correlations if the intervals between the detectors are spacelike but small enough. The fundamental reason for this fact is the nonvanishing of the Feynman propagator outside the light cone. Since this propagator is decaying expotentially with the distance it is demonstrated that for large spacelike intervals field correlations obey classical inequalities. We should also note that different inertial observers will agree on the violation or not of these inequalities since the results are manifestly Lorentz invariant.

Compact high power fiber lasers and the vertical-external-cavity surface-emitting lasers (VECSELs) are promising candidates for high power laser sources with diffraction-limited beam quality and are currently the subject of intensive research and development. Here three large mode area fiber lasers, namely, the photonic crystal fiber (PCF) laser, the multicore fiber (MCF) laser, and the multimode interference (MMI) fiber laser, as well as the VECSEL are modeled and designed.For the PCF laser, the effective refractive index and the effective core radius of the PCF are investigated using vectorial approaches and reformulated. Then, the classical step-index fiber theory is extended to PCFs, resulting in a highly efficient vectorial effective-index method for the design and analysis of PCFs. The new approach is employed to analyze the modal properties of the PCF lasers with depressed-index cores and to effectively estimate the number of guided modes for PCFs.The MCF laser, consisting of an active MCF and a passive coreless fiber, is modeled using the vectorial mode expansion method developed in this work. The results illustrate that the mode selection in the MCF laser by the coreless fiber section is determined by the MMI effect, not the Talbot effect. Based on the MMI and self-imaging in multimode fibers, the vectorial mode expansion approach is employed to design the first MMI fiber laser demonstrated experimentally.For the design and modeling of VECSELs, the optical, thermal, and structural properties of common material systems are investigated and the most reliable material models are summarized. The nanoscale heat transport theory is applied for the first time, to the best of my knowledge, to design and model VECSELs. In addition, the most accurate strain compensation approach is selected for VECSELs incorporating strained quantum wells to maintain structural stability. The design principles for the VECSEL subcavity are elaborated and applied to design a 1040nm VECSEL subcavity that has been demonstrated for high power operation of VECSELs where near diffraction-limited output over 20 W is obtained. Physical modeling of the VECSEL is also discussed and used to compare VECSEL subcavity designs on the laser level.

Silicon-on-insulator (SOI) technology has become increasingly attractive because of the strong light confinement, which significantly reduces the footprint of the photonic components, and the possibility of monolithically integrating advanced photonic waveguide circuits with complex electronic circuits, which may reduce the cost of photonic integrated circuits by mass production. This thesis is dedicated to numerical simulation and experimental performance measurement of passive SOI waveguide devices. The thesis consists of two main parts. In the first part, SOI curved waveguide and corner turning mirror are studied. Propagation losses of the SOI waveguide devices are accurately measured using a Fabry-Perot interference method. Our measurements verify that the SOI corner turning mirror structures can not only significantly reduce the footprint size, but also reduce the access loss by replacing the curved sections in any SOI planar lightwave circuit systems. In the second part, an optical 90o hybrid based on 4 × 4 multimode interference (MMI) coupler is studied. Its quadrature phase behavior is verified by both numerical simulations and experimental measurements.

Recently, there has been a dramatic increase in the amount of data transmission within short range local area networks (LAN). Multimode fibre (MMF) is widely used in local area networks because of its coupling and alignment along with the low cost of related components. Graded index MMF has become common due to the reduction in pulse spreading; however, as demands for high bandwidth increase towards a future gigabit rate network, the typical MMF using conventional transmission methods will not be suitable. Meanwhile, this increasing demand for high speed data transmission will soon reach the Shannon capacity limit of single mode fibres. After multiple input and multiple output (MIMO) technology was successfully used in wireless communication, the researcher realised that the same idea could also be applied to an optical fibre network. Optical MIMO techniques are gaining interest in order to create parallel channels over orthogonal modes in a MMF or a few mode fibre (FMF). This approach could lead to a significant increase in the bandwidth distance product and be employed in the next 40Gb/s or even 100Gb/s optical fibre transmission systems. Generally speaking, optical MIMO appears to be the best solution to the bandwidth limitation problem in either short distance MMF or long distance FMF systems. This thesis focuses on designing a simple, cost-effective, and energy efficient optical MIMO system based on MMFs. This proposed system can be realised by combining radial offset launching and annular multi-segment detectors. First, in the initial work, we performed a theoretical and numerical study of the key impairments of MMFs, and the mode propagation in an MMF was analysed mathematically. The variation in electrical field intensity for linearly polarised (LP) modes in the core region of an MMF and the analytical solutions for power coupling coefficients in either radial offset launching or centre launching were presented. In addition, the modal time delays, impulse response, and transfer function were all introduced. Subsequently, the near field intensity pattern (NFP) was simulated at the output facet of the MMF, which indicated that the overall NFP suffered from blurring when it contained mode mixing, and that the intensity pattern was particularly sensitive to the random phase. According to the spatial distribution of the NFP, the annular detector can be exploited more efficiently. All of the results were calculated and plotted using the MATLAB program. Secondly, the optical MIMO model in the multimode fibre was briefly summarised, including the MIMO channel matrix H expression, a mathematical expression of optical MIMO capacity, MIMO channel estimation and an equalization method. Two metrics can be used to characterise the MIMO channel performance: condition number and crosstalk at each receiver. The numerical results demonstrated that the new type of annular multi-segment detector exhibits superior performance compared to the conventional multiple single mode fibre (SMF) detectors, making them attractive for future optical MIMO systems. Finally, the core work of this thesis can be divided into two parts: the modelling of a 10Gb/s intensity modulation direct detection (IM-DD) optical MIMO MMF system; and the modelling of an advanced 10Gb/s coherent differential phase shift keying (DPSK) MIMO FMF system. In both simulation systems, the important transmission parameters of intra-group mode mixing, modal dispersion, chromatic dispersion, and mode attenuation were considered and discussed in detail. In the IM-DD optical MIMO system, the optimization of the transceiver can be based upon the laser spot size and the power flux distribution emitted by the transmitter. Results from the simulation showed that the intra-group mode mixing had a limited impact on system performance, and due to its inability to compensate for linear impairments, the IM-DD optical MIMO was not favourable for long distance transmission systems. Nevertheless, the new type of optical fibre FMF seems to be the most promising candidate for use in long haul transmission systems. Therefore, the well-known DPSK modulation format in conjugation with the coherent detection deployed in FMF was studied. Both heterodyne and intradyne detection schemes were analysed followed by mathematical derivation and numerical simulation; the results illustrated that similar system performances can be achieved in both schemes. Meanwhile, the coherent DPSK simulation results also demonstrated that the linear impairments were almost compensated by the frequency domain MIMO equalization process, which resulted in system performance being independent to transmission distance for up to 10km. This advantage proved that the coherent optical DPSK MIMO system can be employed in long haul networks. As with an IM-DD optical MIMO system, optimization of a coherent MIMO system was also possible. However, in contrast to the optimization of an IM-DD MIMO system, a trade-off had to be made between sufficient spatial diversity at the transceiver and differential modal delay caused by modal dispersion; consequently, the numerical results showed that the proposed coherent optical DPSK MIMO gained reasonable good results without using any active device, such as a spatial light modulator and a mode converter. In conclusion, this proposed optical MIMO system provided easy implementation and integration and is feasible for use in future optical communication systems.