Ion implanted quantum well saturable absorbers for high bit rate optical communications

This thesis concerns the realization of ultrafast semiconductor saturable absorbers for all- optical signal regeneration in high bit rate long-distance optical transmission systems. The saturable absorbers used for this application should have strong non-linear response to input light and require low saturation energy, to suppress low-power noise accumulated in long distance periodically amplified optical systems at minimum cost. In addition, the saturable absorbers should have ultrafast recovery time for operation at high bit rates. Semiconductor saturable absorbers are designed and fabricated. Multiple quantum wells and integration in a Fabry-Perot cavity are used to increase the contrast ratio. Operation with incident light normal to the plane of the wells makes the saturable absorbers polarization insensitive. The technique of high energy implantation is adopted to reduce the recovery time. Ultrafast (ps) response is demonstrated in a number of devices. The semiconductor saturable absorbers produced are passive and require no post-implantation fabrication. Experimental tests at high bit rates indicate that the saturable absorbers available improve the transmission of zero-bits but degrade the transmission of one-bits. The combination of saturable absorbers with a passive fibre element that provides suppression of amplitude fluctuations of one-bits to form a complete 2R all-optical regenerator is demonstrated. Using this configuration, error-free transmission distance enhancement by a factor > 3.5 is achieved in a 10 Gb/s recirculating loop configuration.