Electrical and Electronic Engineering - Theses
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ItemOptical add-drop multiplexers and amplifier placement methods for metropolitan WDM ring network( 2002)The tremendous growth in broadband communication services, brought about by the phenomenal expansion of the Internet, has triggered an unprecedented demand for bandwidth in telecommunication networks. Optical fibre communication technology has kept up with the growing bandwidth demand by exploiting wavelength division multiplexing (WDM) technology. WDM is now beginning to expand from a network core technology towards the metropolitan and access networks with the goal of bringing the benefit of cost and network efficiencies of the optical networking revolution to the end users. The advent of metropolitan WDM networks has motivated the work in this thesis to develop new devices and design methods in order to reduce cost and improve network performance. This thesis explores the design of new optical add-drop multiplexers (OADMs), which are critical devices enabling the realisation of metropolitan WDM ring networks. Three types of OADMs are investigated corresponding to different network requirements, namely fixed, reconfigurable, and bidirectional OADMs. Three new fixed single-channel OADMs are proposed and demonstrated. The devices use a multi-port optical circulator (OC) and one or two fibre Bragg gratings (FBGs), depending on crosstalk requirements. The OADMs have a simple and compact configuration, low insertion loss, and excellent crosstalk performance. Two new reconfigurable multi-channel OADMs (RM-OADMs) are also proposed and demonstrated in this thesis. The devices employ multi-port OCs, FBGs, and optical switches (OSWs). By effectively using 8- port OCs, the new RM-OADMs significantly reduce component count and insertion loss, and achieve good crosstalk characteristics. One of the new RM-OADMs potentially has the lowest insertion loss among existing RM-OADMs. This thesis also presents a new bidirectional OADM (BOADM) with gain for use in single-fibre bidirectional networks. With the use of multi-port OCs, FBGs, and a single amplifier element, the new structure can add and/or drop channels in both directions and achieves more than 16 dB of bidirectional gain. The BOADM utilises FBGs and light absorbers to remove Rayleigh backscattered and reflected light. All of the proposed OADMs have the potential to be commercially deployed in metropolitan WDM networks. This thesis also investigates new amplifier placement methods to reduce the number of amplifiers in metropolitan WDM rings. Two amplifier placement methods for single-fibre ring networks are developed based on integer programming techniques. The first method describes the amplifier placement problem exactly and uses a nonlinear programming solver to obtain a solution. The second method approximates some requirements in the problem and employs a linear programming solver to derive the amplifier placement solution. Both logical-meshed and logical-hubbed ring networks are investigated. A new amplifier placement method for self-healing WDM rings is also developed in this thesis. The new method is based on iteratively solving an amplifier placement problem for a ring network under different link failure scenarios. The solution provides a minimum number of amplifiers required to operate the self-healing ring under a normal, or any single link or single node failure conditions. Amplifier placement solutions for different network architectures including logical-hubbed unidirectional, logical-meshed unidirectional, and bidirectional self-healing rings are presented.
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ItemAn evaluation of system penalties from polarisation mode dispersion( 2007)Polarisation mode dispersion (PMD) arises from the polarisation properties of an optical signal interacting with the non-degenerate polarisation modes of an optical fibre. When an optical signal is launched into a fibre, the power is split between the polarisation modes. The amount of power contained in each mode depends on the relationship between the launch state of polarisation and the axes of the fibre's polarisation modes. Assuming that some optical power is contained in each polarisation mode, the difference in propagation constants between the modes causes pulse broadening to data bits contained within an optical signal. The inter-symbol interference that results from PMD-induced pulse broadening is the first-order manifestation of PMD. However, in deployed optical systems, the coordinate axes of the fibre's polarisation modes vary with length, and are sensitive to mechanical stresses and temperature variations. The resulting all-order PMD causes more complicated signal distortions, which are stochastic in nature. This thesis investigates the impact of first- and all-order PMD in optical systems using the PMD-induced system penalty. To achieve this, tools are first developed to aid the investigation. Firstly, a novel channel characteristic is introduced, for monitoring PMD-induced penalties. Secondly, the channel characteristic is used to develop a new theoretical framework, to interrelate different methods for monitoring PMD to the PMD-induced penalty. The techniques investigated include degree of polarisation, RF spectral power monitoring and eye closure. The framework is confirmed experimentally, and then used to demonstrate that the novel channel characteristic is the most practical method for monitoring PMD-induced penalties. Finally, the impact of the penalty measurement is recognised as a factor that influences the PMD-induced penalty. As a result, a new theoretical basis is derived to quantify the differences between the various methods of measuring system penalties, including eye closure penalties, Q-penalties, power penalties and optical signal-to-noise ratio penalties. This basis is also experimentally verified. In the second half of the thesis the tools that have been developed are used to experimentally investigate the system impact of first- and all-order PMD on two different systems. The first system does not employ PMD compensation, while the second system is compensated for PMD, using a maximum likelihood sequence estimation-based electronic equaliser. In both cases, the work extends previous investigations of PMD-induced penalties, by investigating the impact of first- and all-order PMD over many launch states of polarisation. The results from the study of PMD-induced penalties in the PMD uncompensated system are used to derive a new PMD-induced penalty model, which is then critically compared to existing models. The new model is shown to better represent both first- and all-order PMD-induced system penalties. The impact of all-order PMD in the two optical systems is reported in detail, and it is shown that in most circumstances first-order PMD is the dominant cause of system penalty. In addition, the results provide experimental evidence for previous simulation based studies of all-order PMD, and some new findings are reported.
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ItemTechnologies for millimetre-wave fibre-radio systems( 1999)The recent advances in mobile wireless technology have increased the need for more transmission bandwidth to accommodate for future telecommunication services. The millimetre-wave (mm-wave) hybrid fibre-radio system is one of the potential technologies for provision of these broadband services. With the advent of low-loss and high bandwidth optical fibre in telecommunication networks, it is an ideal medium for distributing the broadband mm-wave information. In addition to the provision of broadband services and high capacity, the hybrid fibre-radio system also enables customer mobility. This thesis investigates the performance of different architectures for distributing radio signals over fibre in mm-wave fibre-radio applications. In particular, networks incorporating radio signal distribution as mm-wave frequency and as baseband data over fibre are investigated. There have been many different downlink implementations incorporating mm-wave radio signal distribution over fibre for mm-wave fibre-radio system, however there has been little focus on the uplink path. For the first time a simple mm-wave uplink path in a mm-wave fibre-radio system is presented in this thesis which incorporates direct modulation of a multi-section mode-locked distributed-Bragg reflector (DBR) laser at the base station. The performance of the multi-section laser as a mm-wave optical transmitter is presented, and a detailed characterisation of the multi-section laser stabilised via hybrid mode-locking at fundamental and subharmonic frequencies is carried out. The uplink path implementation using the multi-section laser is further extended to incorporate multi-channel transmission. Efficient multi-channel transmission incorporating the multi-section laser requires the separation of the stabilisation and modulation functions of the drive signal applied to the laser. Two different stabilisation techniques namely fundamental hybrid and subharmonic synchronous mode-locking are considered for multichannel operation, and their performance investigated. A detailed theoretical analysis is then presented that quantifies the impact of fibre chromatic dispersion-induced rf power penalties when using the multi-section laser to transmit mm-wave frequencies over fibre. The model is also used to study the performance of the laser as a function of its operating conditions. The thesis also presents a detailed investigation of the implementation and performance of a mm-wave hybrid fibre-radio system with baseband data delivery over optical fibre, a technique which has not been previously reported. The first realisation of a mm-wave fibre-radio system with baseband data delivery over the optical fibre network is presented. This system incorporates a novel modulation scheme for simultaneous transmission of baseband digital data and remote local oscillator (LO) signal delivery. The modulation scheme exhibits several unique features including the use of a single dual-electrode Mach-Zehnder modulator and the delivery of the LO signal is such a way that it is not affected by fibre dispersion. A full-duplex mm-wave fibre-radio system comprising broadband baseband data transmission is implemented with custom-designed mm-wave diplexers and antennas. A theoretical model is also developed to quantify the performance of the baseband data modulation scheme. The analysis is used to determine the sensitivity of the modulator input parameters on the system performance and good agreement is obtained between the model and measurements of the fibre-radio link incorporating baseband data modulation.
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ItemBroadband millimetre-wave fibre-radio access networks( 2000)Millimetre-wave (mm-wave) fibre-radio networks are emerging as a promising technology for providing customer access to future interactive broadband multimedia services. At mm-wave frequencies, propagation effects through air limit radio cell sizes so an implementation of a mm-wave fibre-radio network would require large numbers of simple, low-cost antenna base stations (BSs). To achieve this goal, it is advantageous to share some of the electronic functionality and processing at a central office (CO) rather than at each BS, with the connection between the CO and BSs provided by an optical fibre network The integration of the optical and radio technologies poses a new set of technical challenges. These include mm-wave generation schemes using optics, effects of fibre distribution on radio signals, implementation of low cost BSs, network architectures that provide lower cost links with improved system performance, and system modelling techniques. This thesis advances each of these areas. The optical chirp associated with modulation is shown to be able to improve fibre link performances that are usually limited by chromatic dispersion. Furthermore, a novel modulation scheme generating an optical single sideband- with-carrier (OSSB-WC) spectrum and a novel frequency tripling self-heterodyning scheme are shown to also significantly reduce the effects of fibre dispersion. These optical sources are also inc1uded in demonstrations of a number of novel bi-directional links. A descriptive analysis is used to determine the expected system performance using each technique. This same analysis is extended to predict component specifications and system algorithms that are necessary to achieve certain levels of system performance in a variety of mm-wave fibre-radio link scenarios. The thesis also examines the optical-radio interface and determines suitable techniques that can be used to transport different radio modulation formats, inc1uding multilevel QAM and spread spectrum signals. The physical optoelectronic interface is also considered and a novel system function and hybrid integrated circuit package design is described. A major contribution of this thesis is a detailed investigation into wavelength-division-multiplexing (WDM) based mm-wave fibre-radio networks. Simple uni-directional ring/bus and star-tree architectures are proposed and implemented and the deterioration effects of linear and homodyne crosstalk are also considered so that the limitations of each architecture can be determined. The final major contribution of this thesis is to examine the modelling of mm-wave fibre-radio systems, which is especially important for dimensioning networks. One critical parameter that does require consideration in the models is the deterioration of signal quality from intermodulation distortion in multi-carrier systems. This thesis extends conventional fixed load models that have been developed for low frequency fibre-radio networks and applied them to mm-wave systems. This fixed load model was the first attempt to link physical layer limitations to system performance. While the model derived is simple and fast to simulate, it does not model the behaviour of users in the system. In this thesis, the first model to link physical layer limitations and user behaviour has been implemented and the significant changes in the expected system performance are shown. The model can also be used to predict the required component specifications and for examining system algorithms that are necessary to achieve certain levels of system performance in a variety of mm-wave fibre-radio link scenarios.
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ItemFeedforward compensation of directly modulated semiconductor lasers for analogue fibre communications systems( 1991)Optical fibre communication has become very attractive in the field of telecommunication. Recently, subcarrier multiplexing has emerged as a method for providing a way for each customer to directly access voice, data, and video transmissions. Although digital transmission is becoming very popular in telecommunications, analogue transmission still has its attractive features due to its low cost and compatibility to recent customer electronics. Directly modulated semiconductor lasers are important optical sources in optical communication systems. In analogue fibre communications, nonlinear distortion and intensity noise in these directly modulated semiconductor laser are main obstacles. The thesis first presents experimental characterization of semiconductor lasers, including light-current characteristics, static longitudinal mode spectrum, and small-signal frequency response. Since the linearity of semiconductor lasers plays an important role in analogue fibre communications, distortion characterization of semiconductor lasers has been investigated in considerable detail. This thesis gives details of distortion characteristics, particularly in experimental work. The main work presented in this thesis is an optical feedforward compensation scheme. This feedforward system was successfully built for experimental investigation. Since nonlinear distortion in semiconductor lasers is a severe problem in analogue fibre-communications, the feedforward compensation has been employed to compensate the distortion. Reduction of harmonic distortion and intermodulation distortion was achieved by feedforward compensation. The thesis describes the optical feedforward system in considerable detail. A simultaneous reduction of intensity noise and distortion was achieved using the feedforward technique. This provided a great improvement in signal-to-noise ratio in analogue subcarrier multiplexed systems. The experimental results are presented in this thesis. Also, the theoretical calculation of noise limitations in the feedforward system is presented in this thesis.