Fibre based multi-wavelength lasers are highly useful optical components with significant applications as cheap and compact signal sources for a variety of applications. Fibre multi-wavelength sources are of special interest due to their low cost-per-wavelength and their compatibility with the normal silica-based optical fibres used for transmission in modern optical networks. Multi-wavelength laser sources based on the Semiconductor Optical Amplifier represent the next stage of evolution in multi-wavelength sources for fibre optics; compact, energy-efficient, and simple to build. They also overcome the limitations of homogenous gain media, thereby reducing the occurrence of mode-competition and suppression and opening a variety of new avenues for the development of multi-wavelength sources.