Optical Orthogonal Frequency Division Multiplexing (OOFDM) has been considered as a promising technique for future high-capacity optical networks to satisfy the exponentially increasing end-users' demands for broadband services. The aim of this thesis is to extensively explore, theoretically and experimentally, the feasibility of implementing the OOFDM technique in cost-sensitive Intensity Modulation and Direct Detection (IMDD) network scenarios: Multi-Mode Fibre (MMF)-based Local Area Networks (LANs) and Single-Mode Fibre (SMF)-based access networks. In the theoretical exploration part, special attention is given to investigating the feasibility of the use of the Adaptively Modulated Optical OFDM (AMOOFDM) technique for upgrading installed I Gb/s MMF links to I OGb/s and above. The impact of a wide range of different parametcrs of various components involved in the AMOOFDM modems on the transmission performance of AMOOFDM signals is thoroughly explored, in single-channel, un-amplified, MMF-based, IMDD transmission links. Practically available optimum component parameters are identified, based on which the AMOOFDM modems are optimised. The validity of the identified optimum parameters and the feasibility of the optimised AMOOFDM modems are also statistically verified for implementation in the vast majority of installed MMF links. It is shown that, based on practically available components>, 50Gb/s AMOOFDM signal transmissiono ver 300m is achievablei n 99.5% of already installed IMDD MMF links; whilst by employing components that may be available in the future, AMOOFDM is capable of supporting 10OGb/s signal transmission over 150m in 99.5% of already installed IMDD MMF links. In addition, it is also statistically confirmed that, the optimised AMOOFDM modems have excellent performance flexibility and great robustness to various fibre- and/or system implementation-related impairments. Furthermore, this thesis identifies that the 3dB optical bandwidth of MMFs can be used to simply and accurately estimate the transmission performance of IMDD AMOOFDM signals. Finally, a simple and high-speed synchronization technique has been proposed and verified in OOFDM systems. The techniqueo ffers the following advantages:lo w complexity for correlation calculation, fast tracking speed of Sampling Clock Offset (SCO), high accuracy of estimated SCO. In the experimental research part, in real-time OOFDM transceivers, an advanced channel estimation technique for high speed OOFDM systems is proposed, implemented and verified. The technique has a number of features: high accuracy, low complexity, small pilot bandwidth usage, excellent stability and buffer-free data flow. The real-time end-toend transmission of 3Gb/s 16- Quadrature Amplitude Modulation (QAM)-encoded OOFDM signals over 75km MetroCorTm SMFs, is achieved with negative power penalties of -2dB in Directly Modulated DFB Laser (DML)-based, IMDD systems with neither inline optical amplification nor chromatic dispersion compensation. Experimental investigations also show that, it is feasible to transmit 5.25Gb/s 128-QAM-encoded OOFDM real-time signals over 25km MetroCorTm SMFs and 500m MMFs. The highest ever spectrale fficiency of 5.25 bit/s/Hz is demonstrated successfully in simple IMDD realtime OOFDM systems.