383 research outputs found
Tecnologias coerentes para redes Ăłpticas flexĂveis
Next-generation networks enable a broad range of innovative services with
the best delivery by utilizing very dense wired/wireless networks. However,
the development of future networks will require several breakthroughs in
optical networks such as high-performance optical transceivers to support a
very-high capacity optical network as well as optimization of the network
concept, ensuring a dramatic reduction of the cost per bit.
At the same time, all of the optical network segments (metro, access,
long-haul) need new technology options to support high capacity, spectral
efficiency and data-rate flexibility. Coherent detection offers an opportunity
by providing very high sensitivity and supporting high spectral efficiency.
Coherent technology can still be combined with polarization multiplexing.
Despite the increased cost and complexity, the migration to dual-polarization
coherent transceivers must be considered, as it enables to double the spectral
efficiency. These dual-polarization systems require an additional digital signal
processing (DSP) subsystem for polarization demultiplexing. This work seeks
to provide and characterize cost-effective novel coherent transceivers for
the development of new generation practical, flexible and high capacity
transceivers for optical metro-access and data center interconnects. In this
regard, different polarization demultiplexing (PolDemux) algorithms, as well
as adaptive Stokes will be considered.
Furthermore, low complexity and modulation format-agnostic DSP techniques
based on adaptive Stokes PolDemux for flexible and customizable
optical coherent systems will be proposed. On this subject, the performance
of the adaptive Stokes algorithm in an ultra-dense wavelength division multiplexing
(U-DWDM) system will be experimentally evaluated, in offline
and real-time operations over a hybrid optical-wireless link. In addition, the
efficiency of this PolDemux algorithm in a flexible optical metro link based
on Nyquist pulse shaping U-DWDM system and hybrid optical signals will be
assessed. Moreover, it is of great importance to find a transmission technology
that enables to apply the Stokes PolDemux for long-haul transmission
systems and data center interconnects. In this work, it is also proposed
a solution based on the use of digital multi-subcarrier multiplexing, which
improve the performance of long-haul optical systems, without increasing
substantially, their complexity and cost.As redes de telecomunicaçÔes futuras permitirão uma ampla gama de serviços
inovadores e com melhor desempenho. No entanto, o desenvolvimento das
futuras redes implicarå vårios avanços nas redes de fibra ótica, como transcetores
óticos de alto desempenho capazes de suportar ligaçÔes de muito
elevada capacidade, e a otimização da estrutura da rede, permitindo uma
redução dråstica do custo por bit transportado.
Simultaneamente, todos os segmentos de rede Ăłtica (metropolitanas, acesso
e longo alcance) necessitam de novas opçÔes tecnológicas para suportar
uma maior capacidade, maior eficiĂȘncia espetral e flexibilidade. Neste contexto,
a deteção coerente surge como uma oportunidade, fornecendo alta
sensibilidade e elevada eficiĂȘncia espetral. A tecnologia de deteção coerente
pode ainda ser associada à multiplexação na polarização. Apesar de um
potencial aumento ao nĂvel do custo e da complexidade, a migração para
transcetores coerentes de dupla polarização deve ser ponderada, pois permite
duplicar a eficiĂȘncia espetral. Esses sistemas de dupla polarização requerem
um subsistema de processamento digital de sinal (DSP) adicional para desmultiplexagem
da polarização. Este trabalho procura fornecer e caracterizar
novos transcetores coerentes de baixo custo para o desenvolvimento de uma
nova geração de transcetores mais prĂĄticos, flexĂveis e de elevada capacidade,
para interconexĂ”es Ăłticas ao nĂvel das futuras redes de acesso e metro.
Assim, serĂŁo analisados diferentes algoritmos para a desmultiplexagem da
polarização, incluindo uma abordagem adaptativa baseada no espaço de
Stokes.
Além disso, são propostas técnicas de DSP independentes do formato de
modulação e de baixa complexidade baseadas na desmultiplexagem de Stokes
adaptativa para sistemas Ăłticos coerentes flexĂveis. Neste contexto, o desempenho
do algoritmo adaptativo de desmultiplexagem na polarização
baseado no espaço de Stokes é avaliado experimentalmente num sistema
U-DWDM, tanto em anĂĄlises off-line como em tempo real, considerando um
percurso Ăłtico hibrido que combina um sistema de transmissĂŁo suportado
por fibra e outro em espaço livre. Foi ainda analisada a eficiĂȘncia do algoritmo
de desmultiplexagem na polarização numa rede Ăłtica de acesso flexĂvel
U-DWDM com formatação de pulso do tipo Nyquist. Neste trabalho foi
ainda analisada a aplicação da técnica de desmultiplexagem na polarização
baseada no espaço de Stokes para sistemas de longo alcance. Assim, foi
proposta uma solução de aplicação baseada no uso da multiplexagem digital
de mĂșltiplas sub-portadoras, tendo-se demonstrado uma melhoria na eficiĂȘncia
do desempenho dos sistemas Ăłticos de longo alcance, sem aumentar
significativamente a respetiva complexidade e custo.Programa Doutoral em Engenharia Eletrotécnic
A survey on OFDM-based elastic core optical networking
Orthogonal frequency-division multiplexing (OFDM) is a modulation technology that has been widely adopted in many new and emerging broadband wireless and wireline communication systems. Due to its capability to transmit a high-speed data stream using multiple spectral-overlapped lower-speed subcarriers, OFDM technology offers superior advantages of high spectrum efficiency, robustness against inter-carrier and inter-symbol interference, adaptability to server channel conditions, etc. In recent years, there have been intensive studies on optical OFDM (O-OFDM) transmission technologies, and it is considered a promising technology for future ultra-high-speed optical transmission. Based on O-OFDM technology, a novel elastic optical network architecture with immense flexibility and scalability in spectrum allocation and data rate accommodation could be built to support diverse services and the rapid growth of Internet traffic in the future. In this paper, we present a comprehensive survey on OFDM-based elastic optical network technologies, including basic principles of OFDM, O-OFDM technologies, the architectures of OFDM-based elastic core optical networks, and related key enabling technologies. The main advantages and issues of OFDM-based elastic core optical networks that are under research are also discussed
Wavelength tunable transmitters for future reconfigurable agile optical networks
Wavelength tuneable transmission is a requirement for future reconfigurable agile optical networks as it enables cost efficient bandwidth distribution and a greater degree of transparency. This thesis focuses on the development and characterisation of wavelength tuneable transmitters for the core, metro and access based WDM networks.
The wavelength tuneable RZ transmitter is a fundamental component for the core network as the RZ coding scheme is favoured over the conventional NRZ format as the line rate increases. The combination of a widely tuneable SG DBR laser and an EAM is a propitious technique employed to generate wavelength tuneable pulses at high repetition
rates (40 GHz). As the EAM is inherently wavelength dependant an accurate characterisation of the generated pulses is carried out using the linear spectrogram
measurement technique. Performance issues associated with the transmitter are investigated by employing the generated pulses in a 1500 km 42.7 Gb/s circulating loop
system. It is demonstrated that non-optimisation of the EAM drive conditions at each operating wavelength can lead to a 33 % degradation in system performance. To achieve
consistent operation over a wide waveband the drive conditions of the EAM must be altered at each operating wavelength.
The metro network spans relatively small distances in comparison to the core and therefore must utilise more cost efficient solutions to transmit data, while also
maintaining high reconfigurable functionality. Due to the shorter transmission distances, directly modulated sources can be utilised, as less precise wavelength and chirp control can be tolerated. Therefore a gain-switched FP laser provides an ideal source for wavelength tuneable pulse generation at high data rates (10 Gb/s). A self-seeding scheme that generates single mode pulses with high SMSR (> 30 dB) and small pulse duration is demonstrated. A FBG with a very large group delay disperses the generated pulses and subsequently uses this CW like signal to re-inject the laser diode negating the need to tune the repetition rate for optimum gain-switching operation.
The access network provides the last communication link between the customerâs premises and the first switching node in the network. FTTH systems should take advantage of directly modulated sources; therefore the direct modulation of a SG DBR tuneable laser is investigated. Although a directly modulated TL is ideal for reconfigurable access based networks, the modulation itself leads to a drift in operating frequency which may result in cross channel interference in a WDM network. This effect is investigated and also a possible solution to compensate the frequency drift through simultaneous modulation of the lasers phase section is examined
Approaching the non-linear Shannon limit
We review the recent progress of information theory in optical communications, and describe the current experimental results and associated advances in various individual technologies which increase the information capacity. We confirm the widely held belief that the reported capacities are approaching the fundamental limits imposed by signal-to-noise ratio and the distributed non-linearity of conventional optical fibres, resulting in the reduction in the growth rate of communication capacity. We also discuss the techniques which are promising to increase and/or approach the information capacity limit
Investigation of performance issues affecting optical circuit and packet switched WDM networks
Optical switching represents the next step in the evolution of optical networks. This thesis describes work that was carried out to examine performance issues which can occur in two distinct varieties of optical switching networks.
Slow optical switching in which lightpaths are requested, provisioned and torn down when no longer required is known as optical circuit switching (OCS). Services enabled by OCS
include wavelength routing, dynamic bandwidth allocation and protection switching. With network elements such as reconfigurable optical add/drop multiplexers (ROADMs) and
optical cross connects (OXCs) now being deployed along with the generalized multiprotocol label switching (GMPLS) control plane this represents the current state of the art in commercial networks. These networks often employ erbium doped fiber amplifiers (EDFAs) to boost the optical signal to noise ratio of the WDM channels and as channel configurations change, wavelength dependent gain variations in the EDFAs can lead to channel power
divergence that can result in significant performance degradation. This issue is examined in detail using a reconfigurable wavelength division multiplexed (WDM) network testbed and results show the severe impact that channel reconfiguration can have on transmission
performance.
Following the slow switching work the focus shifts to one of the key enabling technologies for fast optical switching, namely the tunable laser. Tunable lasers which can switch on the nanosecond timescale will be required in the transmitters and wavelength converters of optical packet switching networks. The switching times and frequency drifts, both of commercially available lasers, and of novel devices are investigated and performance issues which can arise due to this frequency drift are examined. An optical packet switching transmitter based on a novel label switching technique and employing one of the fast tunable lasers is designed and employed in a dual channel WDM packet switching system. In depth
performance evaluations of this labelling scheme and packet switching system show the detrimental impact that wavelength drift can have on such systems
Novel optical transmitters for high speed optical networks
The objective of this thesis is to investigate the performance of novel optical transmitter lasers for use in high speed optical networks. The laser technology considered is the discrete mode laser diode (DMLD) which is designed to achieve single wavelength operation by etching features on the surface of the ridge waveguide. This leads to a simplified manufacturing process by eliminating the regrowth step used in conventional approaches, presenting an economic approach to high volume manufacture of semiconductor lasers. Two application areas are investigated in this work.
The bit rate in next generation access networks is moving to 10 Gbit/s. This work characterises the performance of DMLDs designed for high speed operation with the objective of identifying the limitations and improving performance to meet the specifications for uncooled operation at 10 Gbit/s.
With the deployment of advanced modulation formats the phase noise of the laser source has become an important parameter, particularly for higher order formats. DMLDs were developed for narrow linewidth operation. The linewidth of these devices was characterised and a value as low as 70 kHz was demonstrated. Transmission experiments were also carried out using a coherent transmission test bed and the performance achieve is compared with that of an external cavity laser
Digital electronic predistortion for optical communications
The distortion of optical signals has long been an issue limiting the performance of
communication systems. With the increase of transmission speeds the effects of distortion
are becoming more prominent. Because of this, the use of methods known from digital
signal processing (DSP) are being introduced to compensate for them.
Applying DSP to improve optical signals has been limited by a discrepancy in digital signal
processing speeds and optical transmission speeds. However high speed Field
Programmable Gate Arrays (FPGA) which are sufficiently fast have now become available
making DSP experiments without costly ASIC implementation possible for optical
transmission experiments.
This thesis focuses on Look Up Table (LUT) based digital Electronic Predistortion (EPD) for
optical transmission. Because it is only one out of many possible implementations of EPD,
it has to be placed in context with other EPD techniques and other distortion combating
techniques in general, especially since it is possible to combine the different techniques.
Building an actual transmitter means that compromises and decisions have to be made in
the design and implementation of an EPD based system. These are based on balancing the
desire to achieve optimal performance with technological and economic limitations. This
is partly done using optical simulations to asses the performance.
This thesis describes a novel experimental transmitter that has been built as part of this
research applying LUT based EPD to an optical signal. The experimental transmitter
consists of a digital design (using a hardware description language) for a pair of FPGAs and
an analogue optical/electronic setup including two standard DAC integrated circuits. The
DSP in the transmitter compensated for both chromatic dispersion and self phase
modulation.
We achieved transmission of 10.7 Gb/s non-return-to-zero (NRZ) signals with a +4 dBm
launch power over 450 km keeping the required optical-signal-to-noise-ratio (OSNR) for a
bit-error-rate of 2x10^{-3} below 11 dB. In doing so we showed experimentally, for the first
time, that nonlinear effects can be compensated with this approach and that the
combination of FPGA-DAC is a viable approach for an experimental setup
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