The aim of this thesis is to explore innovative technical solutions of utilising Optical Orthogonal Frequency Division Multiplexing (OOFDM) in intensity modulation and direct detection (IMDD) based future access networks to provide multi-service capability with a minimum 1 Gb/s per user. This thesis extensively investigates and analyses the feasibility and performance of adaptively modulated optical orthogonal frequency division multiplexing multiple access passive optical networks (AMOOFDMA PONs) upstream transmission systems by numerically simulating AMOOFDMA PONs using experimentally determined parameters. OOFDM transceivers incorporating reflective semiconductor optical amplifiers (RSOAs) and distributed feedback (DFB) lasers are utilised in the transceivers and intensity modulation and direct detection (IMDD) transmission systems are also employed to achieve a low complexity, high speed and large bandwidth PON as a solution for next generation access networks. Numerical simulations has also being undertaken to improve overall AMOOFDMA PON performance and power budget by incorporating optical band-pass filters (OBPFs) at the output of optical network units (ONUs). A major challenge of making PONs spectrally efficient has been addressed in this thesis by investigating the AMOOFDMA PON with ONUs on a single upstream wavelength. The performance of the single upstream wavelength AMOOFDMA PON is compared to the multiple wavelength AMOOFDMA PON. Another major challenge in AMOOFDMA PONs namely improving system capacity has also been addressed by implementing multiband transmission in an AMOOFDMA PON. Results show that for a multiple upstream OOFDMA IMDD PON system over 25 km single mode fibre (SMF) can achieve an aggregated data rate of 11.25 Gb/s and the minimum wavelength spacing between ONUs are independent of the number of ONUs. Results also show that a single upstream wavelength AMOOFDMA IMDD PON with multiband incorporated at the ONUs can achieve a aggregated line rate of 21.25 Gb/s over 25 km SMF.