Optical pulse processing towards Tb/s high-speed photonic systems

Due to the continued growth of high-bandwidth services provided by the internet, there is a requirement to operate individual line rates in excess of 100 Gb/s in next generation optical communications systems. Thus, to implement these high-speed optical networks all-optical processing techniques are necessary for pulse shaping and pulse routing. Two sub-systems (pulse generation and wavelength conversion), which exploit optical processing techniques are explored within this thesis. Future systems will require high-quality pulse sources and this thesis develops the pulse generation technique of gain switching to provide simple and cost efficient pulse sources. The poor pulse quality typically associated with gain switching is enhanced by developing all-optical methods. The main attribute of the first pulse generation scheme presented is its wavelength tunability over 50 nm. The novelty of the second scheme lies in the ability to design a grating which has a nonlinear chirp profile exactly opposite to the gain-switched pulses. This grating used in conjunction with the gain-switched laser generates transform limited pulses suitable for 80 Gb/s systems. Furthermore the use of a vertical microcavity-based saturable absorber to suppress detrimental temporal pulse pedestals of a pulse source is investigated. Next generation networks will require routing of data in the optical domain, which can be accomplished by high-speed all-optical wavelength converters. A semiconductor optical amplifier (SOA) is an ideal device to carry out wavelength conversion. In this thesis pulses following propagation through an SOA are experimentally characterised to examine the temporal and spectral dynamics due to the nonlinear response of the SOA. High-speed wavelength conversion is presented using SOA-based shifted filtering. For the first time 80 Gb/s error-free performance was obtained using cross phase modulation in conjunction with blue spectral shifted filtering. In addition an important attribute of this work experimentally examines the temporal profile and phase of the SOA-based shifted filtering wavelength converted signals. Thus the contribution and effect of ultrafast carrier dynamics associated with SOAs is presented

Pattern effect mitigation techniques for all-optical wavelength converters based on semiconductor optical amplifiers

All-optical wavelength converters (AOWC) are considered key to overcome wavelength blocking issues in next generation transparent networks. The focus of this book is on semiconductor optical amplifiers (SOA), a mature nonlinear element with very favorable nonlinear characteristics, and on a discussion of various filter configurations as well as on their adaptations for providing optimum performance matched to the nonlinear element working in high-speed all-optical wavelength converters

An Optical Grooming Switch for High-Speed Traffic Aggregation in Time, Space and Wavelength

In this book a novel optical switch is designed, developed, and tested. The switch integrates optical switching, transparent traffic aggregation/grooming, and optical regener-ation. Innovative switch subsystems are developed that enable these functionalities, including all-optical OTDM-to-WDM converters. High capacity ring interconnection between metro-core rings, carrying 130 Gbit/s OTDM traffic, and metro-access rings carring 43 Gbit/s WDM traffic is experimentally demonstrated. The developed switch features flexibility in bandwidth provisioning, scalability to higher traffic volumes, and backward compatibility with existing network implementations in a future-proof way

Characterisation and optimisation of the semiconductor optical amplifier for ultra-high speed performance

This research is in the area of high speed telecommunication systems where all- optical technologies are being introduced to meet the ever increasing demand for bandwidth by replacing the costly electro-optical conversion modules. In such systems, all-optical routers are the key technologies capable of supporting networks with high capacity/bandwidth as well as offering lower power consumption. One of the fundamental building blocks in all-optical routers/networks is the semiconductor optical amplifier (SOA), which is used in for clock extraction, wavelength conversion, all-optical gates and optical processing. The SOAs are perfect for optical amplification and optical switching at a very high speed. This is due to their small size, a low switching energy, non-linear characteristics and the seamless integration with other optical devices. Therefore, characterisation of the SOA operational functionalities and optimisation of its performance for amplification and switching are essential and challenging. Existing models on SOA gain dynamics do not address the impact of optical propagating wavelength, the combined input parameters and their adaptation for optimised amplification and switching operations. The SOA operation is limited at high data rates > 2.5 Gb/s to a greater extent by the gain recovery time. A number of schemes have been proposed to overcome this limitation; however no work has been reported on the SOA for improving the gain uniformity. This research aims to characterise the boundaries conditions and optimise the SOA performance for amplification and switching. The research also proposes alternative techniques to maximise the SOA gain uniformity at ultra-high speed data rates theoretically and practically. An SOA model is been developed and used throughout the research for theoretical simulations. Results show that the optimum conditions required to achieve the maximum output gain for best amplification performance depends on the SOA peak gain wavelength. It is also shown that the optimum phase shift of 180º for switching can be induced at lower input power level when the SOA biasing current is at its maximum limit. A gain standard deviation equation is introduced to measure the SOA gain uniformity. New wavelength diversity technique is proposed to achieve an average improvement of 7.82 dB in the SOA gain standard deviation at rates from 10 to 160 Gb/s. Other novel techniques that improved the gain uniformity employing triangular and sawtooth bias currents, as replacements for the uniform biasing, have been proposed. However, these current patterns were not able to improve the SOA gain uniformity at data rates beyond 40 Gb/s. For that reason, an optimised biasing for SOA (OBS) pattern is introduced to maximise the gain uniformity at any input data rates. This OBS pattern was practically generated and compared to the uniform biased SOA at different data rates and with different input bit sequences. All executed experiments showed better output uniformities employing the proposed OBS pattern with an average improvement of 19%

Linearization techniques to suppress optical nonlinearity

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is shown the implementation of the linearization techniques such as feedforward and pre-distortion feedback linearization to suppress the optical components nonlinearities caused by the fibre and semiconductor optical amplifier (SOA). The simulation verified these two linearization techniques for single tone direct modulation, two tone indirect modulation and ultra wideband input to the optical fibre. These techniques uses the amplified spontaneously emission (ASE) noise reduction in two loops of SOA by a feed-forward and predistortion linearizer and is shown more than 6dB improvement. Also it investigates linearization for the SOA amplifier to cancel out the third order harmonics or inter-modulation distortion (IMD) or four waves mixing. In this project, more than 20 dB reductions is seen in the spectral re-growth caused by the SOA. Amplifier non-linearity becomes more severe with two strong input channels leading to inter-channel distortion which can completely mask a third adjacent channel. The simulations detailed above were performed utilizing optimum settings for the variable gain, phase and delay components in the error correction loop of the feed forward and Predistortion systems and hence represent the ideal situation of a perfect feed-forward and Predistortion system. Therefore it should be consider that complexity of circuit will increase due to amplitude, phase and delay mismatches in practical design. Also it has describe the compatibility of Software Defined Radio with Hybrid Fibre Radio with simulation model of wired optical networks to be used for future research investigation, based on the star and ring topologies for different modulation schemes, and providing the performance for these configurations