The Internet has become an irreplaceable aspect of our daily life. It is used every day by billions of people around the world for various functions such as business, study, and entertainment. Hence, an unabated rise in the demand for higher and faster data traffic has been experienced through the last few decades. This demand for bandwidth is further fuelled by the introduction of bandwidth intensive applications such as ultra-high-definition video streaming, real time online gaming and cloud services making the realization of higher capacity and performance optical networks a necessity. Today’s telecommunication systems are static, with pre-provisioned links requiring an expensive and time-consuming reconfiguration process. The state-of-the-art approach (wavelength division multiplexing - WDM), entailing multiple lasers emitting differing wavelengths (each modulated) multiplexed together (on a 50 GHz grid), cannot meet the growing demands. Hence, future networks need to be flexible and programmable, allowing for resources to be directed, where the demand exists, thus improving network efficiency. A cost-effective solution is to utilise the legacy fibre infrastructure more efficiently by reducing the size of the guard bands and allowing closer optical carrier spacing, thereby increasing the overall spectral efficiency. However, such a scheme imposes a stringent transmitter requirement in terms of wavelength stability, noise properties and cost-efficiency, which would not be met with the incumbent laser-array based transmitters. An attractive alternative would be to employ an optical frequency comb (OFC), which generates multiple phase-correlated optical carriers with a precise frequency separation. The reconfigurability of such a multi-carrier transmitter would enable tuning of channel spacing, number of carriers and emission wavelengths, according to the dynamic network demands. This thesis focusses on the externally injected gain-switched laser-based OFC (GSL-OFC) technique. Advances to the state of the art are achieved via a detailed static and dynamic characterisation of lasers, which is then used for enhancing the comb generation process. Specifically, initial efforts are devoted to the use of different laser structures for OFC generation. This aspect is then furthered by incorporating the concept of photonic integration to reduce the cost, power consumption and footprint of the multi-carrier transmitter. Self and externally seeded photonic integrated circuits are used to generate combs that are then fully characterized to verify their employability in optical networks.