Ultrafast all-optical signal processing using semiconductor optical amplifiers

As the bit rate of one wavelength channel and the number of channels keep increasing in the telecommunication networks thanks to the advancement of optical transmission technologies, switching is experiencing the transition from the electrical domain to the optical domain. All-optical signal processing, including wavelength conversion, optical logic gates and signal regeneration, etc, is one of the most important enabling technolgies to realize optical switching, including optical circuit switching, optical burst switching and optical packet switching. Semiconductor optical ampli??ers (SOAs) are very promising in all-optical signal processing because they are compact, easy to manufacture and power e??cient. It is therefore very important to develop numerical models for the SOAs to understand their behaviour in di??erent system con??gurations, especially when the interacting pulse duration becomes shorter and shorter with increasing bit rate, where several e??ects that are neglected in previous models have to be accounted for. To investigate high-speed SOA-based all-optical signal processing systems, in this thesis we develop a comprehensive model, which includes both inter- and intra-band carrier dynamics, gain dispersion and group velocity dispersion, in this thesis. Polarization dependent e??ects can also be taken into account through introducing an imbalance factor f. Finite-di??erence beam propagation method is employed to solve the numerical model. Mode-locking lasers o??er a lot of applications in all-optical signal processing systems. In this thesis we investigate a novel mode-locked laser based on nonlinear polarization rotation in an SOA. The pulse narrowing process is demonstrated numerically, achieving good agreement with our experimental results. The pulse performance is largely determined by the ultrafast SOA gain dynamics and the cavity dispersion. The laser can produce a pulse train of sub-picosecond pulse width at a repetition rate of 28 GHz, which is limited by the carrier lifetime, for a moderate SOA current level. For higher currents instabilities occur in the laser. vi One of the drawbacks of the SOA-based devices is the relatively long gain recovery time which results in strong pattern e??ects for high bit rate operaion. In this thesis we extensively investigate a very high bit rate wavelength converter based on a single SOA and an optical bandpass ??lter. The enhancement in operation speed is based on ??ltering an amplitude- and phase-modulated signal. We study the underlying working principle and perform detailed analysis of the high-speed wavelength converter, which leads to optimization rules for high-speed SOA-based wavelength conversion. Moreover, both inverted and non-inverted wavelength conversion at much higher bit rate(1 T b/s), is also predicted. Furthermore, genetic algorithm is introduced in the optimization of the transfer function of the OBF following the SOA. Through optimization, eye opening of more than 33 dB is shown for non-inverted wavelength conversion. The optimized ??lter can be experimentally implemented through a combination of asymmetric Mach-Zehnder interferometer and a Gaussian ??lter. Enlightened by the working principle of the wavelength converter, we proposed and demonstrated experimentally a novel optical logic gate with a very simple structure: an SOA followed by an OBF. This logic gate can realize AND, OR and XOR gate functions based on the same setup but with di??erent operation conditions. This novel device can be integrated.