The continued growth of the internet driven by the demand for media rich content and escalating IP traffic has been fuelling the massive growth in demand for bandwidth to handle very high data rates. In order to meet this demand for capacity, optical multiplexing techniques such as wavelength division multiplexing (WDM), optical time division multiplexing (OTDM) and hybrid WDM/OTDM systems need to be utilised. The development of a source of wavelength tunable picosecond optical pulses with excellent temporal and spectral purity at high repetition rates is extremely important for use in such high-speed optical communication systems. The technique of gain switching a commercially available laser diode and self seeding it is shown to be one of the simplest and most cost effective methods of generating transform limited, wavelength tunable pulses with a high Side Mode Suppression Ratio (SMSR). This thesis examines the use of optical gain-switching for the generation of optical pulses that may be used in high-speed OTDM and WDM/OTDM systems. This work specifically deals with the investigation and characterisation of the SMSR and non-linear chirp of optical pulses generated using the gain-switching technique, and outlines how these characteristics, that may degrade system performance, can be optimised such as that optimum performance is attained. In particular, the work demonstrates the development of an optical pulse source with duration < 4ps, SMSR > 30 dB, jitter < 800 fs, and extinction ratio > 30 dB, that would be suitable for use in OTDM systems operating at 80 Gbit/s.