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

Investigation of code reconfigurable fibre Bragg gratings for Optical Code Division Multiple Access (OCDMA) and Optical Packet Switching (OPS) Networks

This thesis documents my work in the telecommunication system laboratory at the Optoelectronics Research Centre, towards the implementation of code reconfigurable OCDMA and all-optical packet switching nodes based on fibre Bragg grating (FBG) technology. My research work involves characterizing the performance of various gratings, specifically high reflectivity, short chip duration, long code sequences, multiple phase level and tunable superstructured fiber Bragg gratings (SSFBGs), by using the recently proposed Frequency-Resolved Optical Gating technique based on Electro-Absorption Modulator (EAM-FROG). This technology can obtain the complex code profile along the grating, making it a powerful method to understand the thermally-induced code-reconfigurable grating. Efforts have been made to improve the grating design to achieve better system performance. Three different types of FBGs optical encoder/decoder, e.g. conventional discrete phaseshift SSFBGs, code-reconfigurable gratings, and novel continuous phase-shift SSFBGs, have been investigated comparatively, as well as their performance in various optical coding/decoding systems. This thesis also discusses the possibility of reducing multiple access interference (MAI) using a Two-Photon Absorption (TPA) process. The advanced grating devices enable the improvement of system performance. A dynamically reconfigurable optical packet processing system and a 16-channel reconfigurable OCDMA/DWDM system with 50GHz DWDM intervals has been demonstrated.These results highlight the feasibility of FBG-based optical coding/decoding techniques, with improved system flexibility and sustainability