Data traffic has dramatically increased over the last decades driven by emerging media-rich applications and services, essential for the modern information society. Wavelength Division Multiplexing (WDM) effectively enabled a continual scaling of fibre optical network capacities. Nevertheless, the relentless global traffic growth shows no sign of abating, and forces optical transport networks to evolve towards higher capacities, performance and flexibility to keep meeting the demand for bandwidth. Advanced modulation formats and multicarrier modulation techniques, such as Nyquist WDM and all optical Orthogonal Frequency Division Multiplexing (OFDM), allow capacity scaling and improved spectral efficiency by encoding information in the optical carrier amplitude, phase and polarization and by minimizing spectral guards between neighbouring channels. The implementation of these techniques, however, imposes stringent requirements on the multi-carrier optical sources in the transmitters, in terms of wavelength stability, good noise properties and cost efficiency. Optical frequency comb sources are key candidates that simultaneously generate multiple phase correlated optical carriers with a stable and constant frequency separation. This thesis is focused on externally injected gain switched optical frequency comb sources (GSOFCS). Several advances on the state of the art of these GS-OFCS are presented that further enhance their potential for network deployment. Firstly, a highly flexible GS-OFCS that can be software reconfigured is proposed and fully characterized for flexible optical networks. Secondly, two novel configurations are experimentally demonstrated for broadband GS-OFCS generation, thus, expanding their bandwidth coverage. Thirdly, this work also studies the need for the de-multiplexing of comb sources and, in order to yield further compactness and cost-efficiency, a detailed characterization of two photonic integrated devices for GS-OFCS generation and de-multiplexing is reported. Finally, the integrated GS-OFCS is implemented into two spectrally efficient transmission systems employing multi-level amplitude and phase modulation formats, which prove the quality and relevancy of these integrated devices for future optical networks.