Applications of multiple quantum well saturable absorbers in long distance ultrafast optical fibre transmission systems
Optically non-linear multiple quantum wells (MQWs) can be used for regeneration in periodically amplified optical fibre transmission systems, or as the basis of all-optical switches with applications including wavelength conversion. This thesis investigates the use of InGaAsP/InGaAsP MQW devices for regeneration and wavelength conversion in 10 Gb/s to 40Gb/s fibre transmission systems. Computer modelling is used to investigate signal propagation in systems employing soliton signal pulses, and to show that regeneration with suitable MQW devices can result in significant performance improvements. MQW-based wavelength conversion is also modelled. Three non-linear mechanisms are considered: excitonic absorption bleaching (EAB), absorption changes due to the transient electric field changes which occur in devices incorporating MQWs in reverse biased p-i-n junctions, and non-linear polarisation rotation (NPR). EAB-based devices are ion-implanted to increase the speed of the non-linearity, such that they are suitable for ultra-fast operation. High contrast devices are realised through incorporation of non-linear MQWs in asymmetric Fabry-Perot (AFP) cavities. Characterisation experiments are developed to measure the magnitudes and temporal dynamics of optical non-linearities, and experimental results presented for devices based on each mechanism. EAB recovery faster than 6ps and 9dB non-linear reflectivity change are achieved in devices using an ion-implanted MQW in an AFP cavity. Wavelength conversion of a low repetition rate pulse stream is demonstrated in p-i-n and ion-implanted EAB devices. Ion-implanted EAB devices are also used to demonstrate conversion of high repetition rate signals representative of those used in systems applications. Together with the systems modelling, the experimental results form the basis of a final evaluation of the suitability of each of the three mechanisms for use at 10Gb/s to 40Gb/s. The analysis considers fundamental limitations of the mechanisms, and the effort required for further development. Ion-implanted EAB-based devices are concluded to be most promising.