This thesis considers the use of square wave frequency modulation (SWFM) as the most suitable modulation technique for transmitting DC-connected wideband analogue data over optical fibre. SWFM has been chosen as the most suitable modulation technique as it offers greater signal-to-noise ratio performance compared to analogue modulation and all other pulse time modulation techniques. In addition, the SWFM complexity and transmission bandwidth overhead are much lower than digital modulation techniques, such as pulse code modulation. To overcome restricted drift performance of standard SWFM due to the poor thermal characteristics of key components, this thesis introduces a novel digital demodulator within the SWFM transmitter and receiver, dramatically improving performance with a single fibre system. Previous approaches to overcoming this thermal drift problem have required the use of a secondary data path to transmit a control signal. This new approach offers performance capability equivalent to the previous two-fibre systems.