Intensity Modulation and Direct Detection (IMDD) Optical Orthogonal Frequency Division Multiplexing (OOFDM) is considered as one of the most competitive candidates for high-speed, cost-effective and flexible Next Generation Passive Optical Networks (NGPONs). For practical implementation of the technique, five technical challenges originating from inherent OFDM properties and/or IMDD system characteristics have to be solved successfully. The challenges include: i) insufficient utilization of Multi-Mode Fibre (MMF) frequency response; ii) improvement in IMDD OOFDM transmission capacity in Single- Mode Fibre (SMF)-based systems; iii) simplification of OOFDM transceiver configurations; iv) the employment of low-cost transceiver components to achieve the desired system performance; v) effective compensation of directly modulated DFB laser (DML)-induced positive frequency chirp. The present PhD dissertation research is dedicated to addressing the aforementioned challenges. For fully utilizing the system frequency response of a MMF transmission link, an Adaptively Modulated OOFDM (AMOOFDM) modem using Subcarrier Modulation (SCM) (AMOOFDM-SCM) is proposed, which consists of two AMOOFDM modems in parallel with one operating at the baseband and the other being modulated onto an intermediate Radio Frequency (RF) carrier. Extensive investigations show that, compared with AMOOFDM, AMOOFDM-SCM not only enhances the transmission capacity versus reach performance by a factor of approximately 2, but also considerably improves the system flexibility and performance robustness. When use is made of the AMOOFDM-SCM technique in SMF-based transmission links, the intermixing effect induced by direct detection in the receiver is identified to be a dominant factor limiting the maximum achievable AMOOFDM-SCM performance. To maximize the link performance through mitigating the intermixing effect, three AMOOFDM-SCM modem designs of different complexity levels are proposed by applying Single Sideband (SSB) modulation and/or spectral gapping in AMOOFDM-SCM. It is shown that these AMOOFDM-SCM designs can support >60Gb/s signal transmission over at least 20km, which is >1.5 times higher than that supported by the AMOOFDM modems. I In the above-mentioned three AMOOFDM-SCM modems, two Inverse Fast Fourier Transform (IFFT)/ Fast Fourier Transform (FFT) operations are required in the transmitter/receiver. To reduce the transceiver complexity and system cost, three simplified AMOOFDM-SCM modems are proposed, each of which requires a single IFFT/FFT operation. These designs not only significantly simplify the AMOOFDM-SCM modem configurations but also offer extra network features such as input/output reconfigurability without compromising the transmission performance. To relax the requirements on parameters of key transceiver components such as Digital-to- Analogue Converters (DACs) and Analogue-to-Digital Converters (ADCs), a reduction in Peak-to-Average Power Ratio (PAPR) associated with an OFDM signal is necessary. To achieve such an objective, AMOOFDM using Phase Modulation (PM) (AMOOFDM-PM) is proposed and explored in IMDD SMF systems. AMOOFDM-PM utilises an electrical OFDM signal to modulate the phase of a RF carrier prior to performing optical intensity modulation. Compared to AMOOFDM, AMOOFDM-PM can considerably reduce the PAPRs of OFDM signals and simultaneously lower the minimum requirements on quantization bits and sampling rates of DACs/ADCs. To effectively compensate the DML frequency chirp effects, detailed investigations of dynamic negative power penalty characteristics of OOFDM signal transmission are undertaken in DML-based IMDD systems incorporating MetroCor fibres with negative dispersion parameters. Excellent agreement between numerical simulations and real-time experimental measurements is obtained over a wide diversity of system conditions. The physical mechanism underpinning the occurrence of negative power penalties is the reduction in subcarrier intermixing impairments due to the compensation between DML positive frequency chirp and MetroCor negative chromatic dispersion. It is also shown that the negative power penalty is independent of both cyclic prefix and signal modulation format, and, more importantly, controllable when adaptive modulation and/or appropriate adjustments of DML operating conditions are applied. Finally, for reducing the DML frequency chirp, a simple and effective chirp compensation technique is also proposed, which utilizes an electrical analogue circuit and an optical phase modulator. The electrical analogue circuit produces a phase signal mimicking the original phase of the DML-modulated optical signal, and the optical phase modulator driven by the generatedp hases ignal compensatesth e DML frequency chirp. In DMLII based IMDD AMOOFDM PON systems, the technique can almost completely alleviate the DML frequency chirp effect and simultaneously improve the transmission capacity by approximately 25% for transmission distances in a range of 30-80km. In addition, the technique is also robust to variations in DML operating conditions. The results presented in the thesis can provide valuable technical solutions for further improving the OOFDM transmission capacity, performance robustness and system flexibility, as well as simultaneously reducing the transceiver complexity and system cost for cost-sensitive NG-PONs.