Investigation of the regenerative and cascadability properties of optical signal processing devices at high bit-rates
This thesis investigates the use of Semiconductor Optical Amplifiers (SOA) for 10 and 40Gb/s all-optical 3R signal regeneration and wavelength conversion for application to advanced high-speed all-optical WDM networks. Detailed experimental characterisation of the magnitude and the time scales of inter-band nonlinear effects in SOA is carried out. The regenerative properties of SOA gates are theoretically investigated and related to the SOA physical parameters by means of deriving the SOA gate transfer function. A novel configuration for all-optical regeneration based on Polarisation Switching in an SOA-assisted Sagnac Interferometer (PSSI) is proposed and used to demonstrate, for the first time, multi-channel simultaneous 3R regeneration. This scheme allows to significantly enhance the switching frequency of the SOA, demonstrating error-free regeneration of 40Gb/s signal with long PRBS sequences using an SOA with carrier lifetime exceeding 250ps. The reshaping and retiming ability of the scheme are also assessed experimentally at bit-rate of 40Gb/s, to show the largest distortion tolerance range published to date for regeneration. The scheme is also used to demonstrate 40 to lOGb/s demultiplexing. The cascadability of optical regenerators and wavelength converters is also investigated experimentally. The impact of varying the inter-regenerator spacing in transmission with cascaded wavelength conversion and 3R regeneration over transoceanic distances, is experimentally investigated for the first time, using a novel reconfigurable fibre loop. These results show that a trade-off exists between the transmission signal Q-factor and the inter-regenerator spacing, which depends on the regenerator transfer function characteristics, and thus can be predicted from the SOA parameters. In 40Gb/s transmission with optical regeneration it was demonstrated that the use of an optical regenerator before the electrical receiver increases the power margin and maximum error-free transmission distance at 40Gb/s in excess of 100km. Finally a novel concept for multi-channel wavelength conversion and regeneration is presented utilising an integrated SOA array in a novel configuration to demonstrate, for the first time, simultaneous regenerative wavelength conversion of 1 OGb/s signals. This also shows the potential for large scale monolithic integration for optical processing applications in WDM networks.