Optimisation of the key SOA parameters for amplification and switching

Wireless Sensor Networks (WSN) are composed of small, low cost, resource-constrained computing nodes equipped with low power wireless transceivers. Generally, they are embedded in their environment to perform some specific monitoring and/or control function. Unlike wired networks that have dedicated routers for network connectivity and message forwarding, every node in a WSN can act as a router in a multi-hop network. A WSN can offer a cheap, applicationspecific solution in a variety of situations including military and disaster response scenarios, where other approaches are not viable. Due to their unattended nature and deployment in possibly hostile environmental conditions, there are many challenges in ensuring that a WSN is formed effectively and survives long enough to fulfil its function. Securing a WSN against attack is a particular challenge. Traditional encryption mechanisms are resource hungry and are not sufficient alone to provide a complete solution. This project is concerned with secure routing protocols. Formal methods are used to model and analyse the design of existing protocols and to demonstrate some previously unreported weaknesses

Optical signal processing via two-photon absorption in a semiconductor microcavity for the next generation of high-speed optical communications network

Due to the introduction of new broadband services, individual line data rates are expected to exceed 100 Gb/s in the near future. To operate at these high speeds, new optical signal processing techniques will have to be developed. This paper will demonstrate that two-photon absorption in a specially designed semiconductor microcavity is an ideal candidate for optical signal processing applications such as autocorrelation, sampling, and demultiplexing in high-speed wavelength-division-multiplexed (WDM) and hybrid WDM/optical time-division-multiplexed networks

High Speed All Optical Switching and Encryption using Ultrafast Devices

The next generation of fiber-optic communication system demands ultra-high speed data processing and switching components. Conventional electro-optical parts have reached their bottleneck both speed-wise and efficiency-wise. The idea of manipulating high speed data in all-optical domain is gaining more popularity. In this PhD dissertation, I showed the design and performance analysis of two kinds of ultra-fast all-optical latches, Set-Reset latch and D-flip-flop, based on two different schemes: (1) cross gain and phase modulation (XGM and XPM) in quantum dot semiconductor optical amplifiers (QD-SOA) and (2) two-photon absorption (TPA) in bulk semiconductor optical amplifiers. Design and simulation of a scheme to realize high speed all-optical encryption and decryption using key-stream generators and XOR gates based on QD-SOA are included in this dissertation. We also proposed and simulated all-optical Boolean logic functions with improved output quality using binary phase shift keyed signal based on QD-SOA. A fiber ring laser system with charcoal nano-particles as saturable absorber inside the cavity has been designed and experimentally demonstrated. This fiber ring laser system can generate optical pulse train @ 20Gb/s with improved stability and smaller pulse width comparing with the system without nano-particles in the cavity

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

Employing wavelength diversity to improve SOA gain uniformity

In this paper, we propose a wavelength diversity technique for the semiconductor optical amplifier (SOA) to improve the gain uniformity for ultra-high speed optical routers. In such routers, fast SOA gain recovery is required to ensure the minimum gain standard deviation and thus leading to reduction in the system power penalty. The SOA is modeled using a segmentation technique and the detailed theoretical analysis for the model is presented. A direct temporal analysis of the impact of the signal wavelength on the SOA gain is investigated. The SOA gain profile when injected with a burst of input Gaussian pulses for a single wavelength and the proposed wavelength diversity technique are investigated. The operation principle is simulated and the results show a reduction in the gain standard deviation (at 1 mW input power) of 13.1, 11, 8.1, 6.2 and 4.8 dB for the data rates of 10, 20, 40, 80 and 160 Gb/s, respectively