Noise Suppression in OCDMA Networks using Nonlinear Optical Devices

Optical code division multiple access (OCDMA) is a multiplexing technique that has a number of inherent advantages that make it suitable for use in passive optical networks, such as allowing subscribers to transmit information in an asynchronous fashion over a single optical fibre. This form of multiplexing can provide a higher degree of flexibility and simplicity in comparison to other techniques. However, due to the asynchronous nature of transmission, OCDMA networks suffer from multiple access interference (MAI) and optical beat noise which severely impairs system performance. A number of solutions have been proposed to mitigate these noise sources. Increasing the optical code lengths used can reduce the level of optical beat noise, however this is generally at the expense of transmission speed and increased transmitter complexity. MAI suppression can be achieved through the use fibre-based nonlinear thresholders or optical time-gating. One problem with these solutions is the requirement of long lengths of nonlinear fibre that are susceptible to changes in environmental conditions. Therefore, this thesis focuses on the development and testing of a nonlinear optical receiver based on semiconductor devices for the suppression of noise in OCDMA systems. The nonlinear optical process of two-photon absorption (TPA) in a commercially available 1.3 micron Fabry-P´erot laser is investigated as a method for optical thresholding in an OCDMA system. It is shown that the use of a saturable absorber (SA) directly before the TPA-based detector can provide additional suppression of MAI noise. However, the level of beat noise that is present on the optical signal can be increased due to the nonlinear responses of both devices. As a result, a gain-saturated semiconductor optical amplifier (SOA) is demonstrated as a method for the reduction of optical beat noise. It is shown that error-free performance can be achieved in an optical testbed designed to simulate an OCDMA system using an SA-SOA-TPA-based receiver. The performance improvement due to the suppression of MAI and beat noise using an SA-SOA receiver is examined in relation to a current fibre-based thresholding technique; a Mamyshev filter. It is shown that the SA-SOA receiver can offer a similar level of improvement when compared to the performance of a Mamyshev filter

SYSTEM AND METHOD FOR PERFORMING OPTICAL CODE DIVISION MULTIPLE ACCESS COMMUNICATION USING BPOLAR CODES

An optical encoding and decoding system which performs code-division multiple access (CDMA) communication in the incoherent, or direct detection, optical domain using bipolar +1/-1 codes. The present invention uses code modu lation and detection principles that permit all-optical imple mentation of the bipolar. +1/-1. code and correlation detec tion that have been developed for the radio frequency (RF) systems. This is possible in spite of the non-negative, or unipolar, +1/0, nature of the incoherent optical system that only detects and processes the signal intensity. The unipolar optical system of the present invention is equivalent to the bipolar RF system in that the correlation properties of the bipolar codes is completely preserved. The optical CDMA system can be realized both in time or frequency domain encoding with all-optical components

Hybrid pulse interval modulation-code-division multiple-access for optical wireless communications.

The work in this thesis investigates the properties of the IR diffuse wireless link with regard to: the use of sets of signature sequences with good message separation properties (hence providing low BER), the suitability of a hPIM-CDMA scheme for the IR diffuse wireless systems under the constraint of eye safety regulations (i.e. when all users are transmitting simultaneously), the quality of message separation due to multipath propagation. The suitability of current DS-CDMA systems using other modulation techniques are also investigated and compared with hPIM-CDMA for the performances in power efficiency, data throughput enhancement and error rate.A new algorithm has also been proposed for generating large sets of (n,3,1,1)OOC practically with reduced computation time. The algorithm introduces five conditions that are well refined and help in speeding up the code construction process. Results for elapsed computation times for constructing the codes using the proposed algorithm are compared with theory and show a significant achievement. The models for hPIM-CDMA and hPPM-CDMA systems, which were based on passive devices only, were also studied. The technique used in hPIM-CDMA, which uses a variable and shorter symbol duration, to achieve higher data throughput is presented in detail. An in-depth analysis of the BER performance was presented and results obtained show that a lower BER and higher data throughput can be achieved. A corrected BER expression for the hPPM-CDMA was presented and the justification for this detailed. The analyses also show that for DS-CDMA systems using certain sets of signature sequences, the BER performance cannot be approximated by a Gaussian function

Influência dos formatos de modulação no desempenho dos sistemas ts-ocdma.

The efficient use of a transmission channel can be achieved by means of Code Division Multiple Access (CDMA) technology. This multiplexing technique uses spectral spreading to allow multiple users to share the transmission medium asynchronously. CDMA systems applied to the optical medium are known as OCDMA systems and these can be classified as coherent or incoherent, depending on how the user code is applied to the optical signal. The present work performs a comprehensive analysis on the influence of digital modulation techniques on a coherent OCDMA system with Time-Spread (TS-OCDMA). In this context, the simulation variables were approximated to values of practical systems, so that the results of this work can be a reference for a real implementation. Thus, the performances of OCDMA systems with different digital modulations are compared in terms of bit error rate (BER).FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas GeraisTrabalho de Conclusão de Curso (Graduação)A utilização eficiente de um canal de transmissão pode ser alcançada por meio da tecnologia de acesso múltiplo por divisão de código (CDMA -Code Division Multiple Access). Essa técnica de multiplexação utiliza o espalhamento espectral para permitir que múltiplos usuários possam compartilhar o meio de transmissão de forma assíncrona. Os sistemas CDMA aplicados ao meio óptico são chamados de sistemas OCDMA e estes podem ser classificados como coerentes ou incoerentes, dependendo de como o código é aplicado ao sinal óptico. O presente trabalho realiza uma análise detalhada sobre a influência das técnicas de modulações digitais em um sistema coerente OCDMA com codificação de temporal de fase (Optical Time-Spread - TS-OCDMA). Nesse contexto, objetivou-se aproximar as variáveis das simulações a valores de sistemas práticos, de forma que os resultados deste trabalho possam ser referência para uma implementação real. Assim, os desempenhos dos sistemas OCDMA com diferentes modulações digitais são comparados em termos de taxa de erro de bit (BER – Bit Error Rate)

Indoor infrared wireless PPM systems

Optical wireless communication systems offer several advantages over their radio frequency counter parts. For example, cost, wavelength re-usability and a large unregulated bandwidth are among the advantages. The limitations of this technique include the optical power safety issue, multipath dispersion and interference from ambient light and background radiation. In this project, pulse position modulation is used for receiver sensitivity improvement, and code division multiple access is exploited to provide multiple access facilities. The limitations of an optical wireless system are studied and methods for performance improvement are suggested

Optical Switching for Scalable Data Centre Networks

This thesis explores the use of wavelength tuneable transmitters and control systems within the context of scalable, optically switched data centre networks. Modern data centres require innovative networking solutions to meet their growing power, bandwidth, and scalability requirements. Wavelength routed optical burst switching (WROBS) can meet these demands by applying agile wavelength tuneable transmitters at the edge of a passive network fabric. Through experimental investigation of an example WROBS network, the transmitter is shown to determine system performance, and must support ultra-fast switching as well as power efficient transmission. This thesis describes an intelligent optical transmitter capable of wideband sub-nanosecond wavelength switching and low-loss modulation. A regression optimiser is introduced that applies frequency-domain feedback to automatically enable fast tuneable laser reconfiguration. Through simulation and experiment, the optimised laser is shown to support 122×50 GHz channels, switching in less than 10 ns. The laser is deployed as a component within a new wavelength tuneable source (WTS) composed of two time-interleaved tuneable lasers and two semiconductor optical amplifiers. Switching over 6.05 THz is demonstrated, with stable switch times of 547 ps, a record result. The WTS scales well in terms of chip-space and bandwidth, constituting the first demonstration of scalable, sub-nanosecond optical switching. The power efficiency of the intelligent optical transmitter is further improved by introduction of a novel low-loss split-carrier modulator. The design is evaluated using 112 Gb/s/λ intensity modulated, direct-detection signals and a single-ended photodiode receiver. The split-carrier transmitter is shown to achieve hard decision forward error correction ready performance after 2 km of transmission using a laser output power of just 0 dBm; a 5.2 dB improvement over the conventional transmitter. The results achieved in the course of this research allow for ultra-fast, wideband, intelligent optical transmitters that can be applied in the design of all-optical data centres for power efficient, scalable networking

The role of the lateral hypothalamic neural outputs in motivated behaviour

Comprendre comment le cerveau traite l’information est l’une des questions les plus intrigantes auxquelles les neurosciences modernes sont confrontées. L’étude actuelle vise à caractériser comment l’hypothalamus latéral (HL) traite l’information vers des cibles cérébrales en aval afin de guider des réponses comportementales appropriées. L’HL est une zone du cerveau qui régule des comportements vitaux tels que les fonctions autonomes et endocriniennes, l’équilibre homéostatique, la régulation du métabolisme et les cycles veille-sommeil. De plus, des études récentes soulignent son importance dans le traitement de l’aversion et de la récompense. L’HL envoie des projections neurales à de nombreux noyaux cérébraux connus pour traiter des signaux qui jouent un rôle important pour guider et orchestrer des réponses comportementales appropriées. Des expériences visant a déterminer les connections afferentes et efférentesde l’HL ont démontré que trois noyaux cérébraux importants reçoivent des projections importantes de l’HL. Il s’agit de l’habenula latéral (HbL), de l’aire tegmentale ventrale (ATV) et du noyau raphe dorsal (NRD). L’HbL est le principal centre de déception du cerveau : son activité augmente spécifiquement lorsqu’un animal est soumis à des stimuli aversifs ou en l’absence de récompenses attendues, jouant un rôle important dans la signalisation d’une erreur de prédiction de punition. L’ATV dopaminergique est le principal centre de récompense du cerveau. Il joue un rôle important dans l’encodage de la valeur des récompenses, l’apprentissage du renforcement et la motivation. Le NRD est le principal centre de sérotonine jouant un rôle important dans le traitement des émotions et les réponses adaptatives. Pour examiner spécifiquement la contribution des sorties neuronales de l’HL chez les souris en mouvement libre, nous avons utilisé une technique d’imagerie avancée du calcium – système de photométrie à fibres. La photométrie à fibres est une approche puissante qui combine des indicateurs de calcium codés génétiquement et des fibres optiques multimodes pour monitorer l’activité neuronale chez les animaux en mouvement libre, ce qui est essentiel pour comprendre comment des groupes spécifiques de neurones sont impliqués dans le contrôle ou la réponse à une action ou à un stimulus. Dans le premier chapitre, je présente un protocole qui a été développé pour une détection fiable des signaux de calcium à l’aide d’un système de photométrie multifibre. Le protocole détaille les composantes d’un système de photométrie multifibres, la méthode pour accéder aux structures profondes du cerveau pour délivrer et collecter la lumière, et une méthode pour prendre en compte les artefacts de mouvement avant et pendant les enregistrements. En outre,je présente un algorithme de traitement des signaux enregistrés qui tient compte des sources communes d’artefacts qui sont inévitables pendant les enregistrements. Dans le deuxième chapitre, je présente les résultats de l’étude du rôle fonctionnel de trois sorties neurales de l’HL vers le NRD, l’ATV et l’HbL. En utilisant le protocole décrit dans le premier chapitre, l’activité dans les voies HL→NRD, HL→ ATV et HL→HbL a été simultanément enregistrée lors de réponses comportamentales dans des contextes d’aversion et de récompense. Nous avons constaté que l’activité à ces trois sorties neurales de l’HL augmentait avec des stimuli et des signaux prédictifs de stimuli aversifs. L’activité neuronale augmente également lors des réponses comportementales motivées spontanées et diminue lors de l’immobilité comportementale. L’activation optogénétique indépendante des terminaisons axonales de l’HL au niveau de l’HbL, l’ATV ou le NRD était suffisante pour augmenter la mobilité, mais a eu des effets différents dans d’autres tests comportementaux. Dans l’ensemble, nous proposons que l’HL envoie des signaux complémentaires aux cibles en aval pour traiter les informations engagées pour promouvoir des comportements motivés. En annexe, je présente un ensemble d’analyse de données python qui a été développé pour traiter tous les enregistrements de photométrie à fibre optique présentés dans l’étude actuelle. Cet ensemble permet de combiner, de stocker et d’analyser les enregistrements de plusieurs souris, essais et différentes expériences avec diverses mesures, événements comportementaux et stimuli de manière standardisée.Understanding how brain processes information is the one of the most intriguing questions that modern neuroscience faces. The current study aims to characterize how the lateral hypothalamus (LH) processes information to downstream brain targets to guide proper behavioral responses. The LH is a brain area that regulates vital behaviors such as autonomic and endocrine functions, homeostatic balance, regulation of metabolism, and sleep-wake cycles. Moreover, recent studies point out its importance in aversive and appetitive processing. The LH sends neural projections to many brain nuclei known to process signals that play important roles to guide and orchestrate proper behavior responses. Tracing experiments demonstrated that three important brain nuclei receive significant inputs from the LH, the lateral habenula (LHb), the ventral tegmental area (VTA), and the dorsal raphe nucleus (DRN). The LHb is the main disappointment center of the brain: its activity specifically increases when an animalis presented an aversive stimuli or in the absence of expected rewards, playing an important role in signaling punishment prediction error. The dopaminergic VTA is the main brain reward center. It plays important roles in reward-value encoding, reinforcement learning and motivation. The DRN is the main serotonin center playing and important role in emotion processing and adaptive responses. To specifically examine the contribution of LH neural outputs in freely moving mice, we used an advanced calcium imaging technique – fiber photometry system. Fiber photometry is a powerful approach that combines genetically encoded calcium indicators and multimode optical fibers to monitor neuronal activity in freely moving animals, which is critical to understand how specific groups of neurons play in directing or responding to an action or a stimulus. In the first chapter, I present a protocol that was developed for reliable detection of calcium signal using a camera-based multi-fiber photometry system. The protocol details the components of a multi-fiber photometry system, a method to access deep brain structures to deliverand collect light, and a method to account for motion artifacts before and during recordings. Additionally, I present an algorithm for processing of recorded signals that accounts common sources of artefacts that are inevitable during recordings. In the second chapter, I present results of the investigation of the functional role of three LH outputs to the DRN, VTA, and LHb. Using the protocol described in the first chapter, activity in the LH→DRN, LH→ VTA and LH→LHb pathways were simultaneously recorded during motivated responses in aversive and appetitive contexts. We found that these three LH neural outputs increased activity with aversive stimuli and cues predicting them. The neural activity also increased at onsets of spontaneous motivated behavior responses and decreased duringbehavioral immobility. Independent optogenetic activation of axon terminals in LHb, VTA,or DRN was sufficient to increase mobility, but had different effects in other behavioural tests. Altogether, we propose that LH sends complementary signals to the downstream targets to process information engaged in motivated behaviors. In the annex, I present a data analysis python package that was developed to process all fiber photometry recordings presented in the current study. The package allow to combine, store, and analyze recordings from multiple mice, trials, and different experiments with various measurements, behavioural events, and stimuli in a standardized way