Optical Orthogonal Frequency Division Multiplexing (OOFDM) has been widely considered as one of the strongest contenders for high-speed Next-Generation PONs (NG- PONs) to satisfy the continuously increasing bandwidth requirements from individual and business users. From telecommunication equipment vendor's and network operator's point of view, cost-effectiveness is a key factor for practical mass deployment of OOFDM-based NG-PONs in future. As the electro-optic convertors involved in OOFDM transceivers take the majority of the OOFDM transceiver cost, the convertors, therefore, play critical roles in determining the system performance and cost. The thesis aims to theoretically investigate the feasibility of using low-cost semiconductor laser devices including Directly Modulated DFB Lasers (DMLs), Semiconductor Optical Amplifiers (SOAs) and Reflective SOAs (RSOAs) for converting real-valued electrical OFDM signals into the optical domain for transmission over simple Intensity Modulation and Direct Detection (IMDD) NG-PON systems. The validity of various theoretical models employed in the thesis is also rigorously verified by comparing with real-time experimental measurements. For DML-based OOFDM IMDD PON systems, special attention is first dedicated to extensive investigations of the impact of various physical mechanisms on the system performance to maximize the achievable optical power budget, transmission capacity and reach. The physical mechanisms include OOFDM signal Extinction Ratio (ER), DML- induced frequency chirp and subcarrier intermixing upon direct detection in the receiver. Results show that the low ER of a DML-modulated OOFDM signal is the predominate factor limiting the achievable optical power budget, and the subcarrier intermixing effect upon square-law photon detection in the receiver reduces the optical power budget by at least 1dB. The use of a 0.02nm bandwidth optical Gaussian bandpass filter in the transmitter with a 0.01 nm wavelength offset with respect to the optical carrier wavelength is proposed to enhance the 11.25Gb/s OOFDM signal ER by approximately 1.24dB. This results in a 7dB optical power budget improvement at a total channel BER of 1x 10'3. Moreover, by adopting a DML-modulated carrier suppressed Single Sideband (SSB) OOFDM technique in NG-PONs, 30Gb/s carrier suppressed SSB OOFDM signal transmission over 80km SMF without in-line optical amplification is achievable, which doubles the performance corresponding to conventional Double Sideband (DSB) OOFDM I signals without carrier suppression. It is also shown that 10Gb/s carrier suppressed SSD OOFDM signal transmission over 1200km SMF incorporating in-line Erbium Doped Fibre Amplifiers (EDFAs) is feasible. Given the fact that Wavelength-Division Multiplexing (WDM) has been widely deployed in practice, and that RSOAs/SOAs have advantages of compactness, capability of monolithic integration with electrical components and wide wavelength coverage of typically >100nm, in this thesis, RSOAs/SOAs are proposed to facilitate colourless transmitters for OOFDM IMDD WDM-PONs. Explorations show that, under optimized SOA operating conditions, 30Gb/s OOFDM signal transmission over 80km SMF without in-line optical amplification and dispersion compensation is feasible at 1550nm; and colourless transmissions of SOA-intensity modulated OOFDM signals of 30Gb/s over 60km SMF are also achievable for a wide wavelength range from 1510nm to 1590nm. Moreover, numerical investigations are also undertaken of the performance comparisons between RSOA and SOA intensity modulators, and detailed discussions are also made of the influence of RSOA rear-facet reflectivity on the achievable RSOA-modulated OOFDM signal performance. Results indicate that the maximum transmission performance of 30Gb/s over 60km SMF under optimum RSOA operating conditions is independent of rear-facet reflectivity, and the performance is very similar to that corresponding to SOAs; whilst under un-optimised operating conditions, the RSOA intensity modulators outperform considerably the SOA intensity modulators in transmission performance, which decreases significantly with reducing RSOA rear-facet reflectivity and optical input power. Finally, to simplify the network architecture and simultaneously improve the wavelength control functionality and system performance, wavelength reused bidirectional colourless OOFDM IMDD WDM"PONs are proposed, which incorporate a SOA intensity modulator and a RSOA intensity modulator in the optical line terminal and optical network unit, respectively. Results show that 10Gb/s downstream and 6Gb/s upstream over 40km SMF transmission of conventional DSB OOFDM signals are feasible. In particular, the aforementioned transmission performances can be further improved to 23Gb/s downstream and 8Gb/s upstream over 40 km SMF when SSB-Subcarrier Modulation (SCM) is introduced in the downstream systems.