Prewett, Adam Keith; (1993) Digital optical fibre sensing systems and architectures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis investigates the options for realising passive optical fibre sensing systems based on digital modulation techniques with the aim of overcoming some of the systems problems, such as low multiplexing gain potential and high lead- and crosstalk-sensitivities, which plague analogue intensity-modulated schemes. The investigation is principally directed towards process control applications and concentrates on requirements for either simple threshold sensing or high precision measurement. After a review of the many noise sources affecting optical fibre sensing systems, the thesis considers the sensor capacity limitations of Time-Division Multiplexed (TDM) and Frequency-Division Multiplexed (FDM) systems using conventional parallel fibre networks in support of ON/OFF sensors. The main limitation of such systems is the high cost of the optical power splitting components (invariably fused or waveguide couplers for large networks) which to date has prevented such technology displacing conventional electronic sensing in most civil applications. An extensive study is therefore conducted into systems which do not require optical power splitting components. Transmissive and reflective systems capable of independently determining the states of a large number of ON/OFF sensors serially deployed along a single bus fibre are investigated. A transmissive Coherence-Domain Multiplexed (CDM) system, based on a low cost video-disc laser diode and capable of supporting in excess of 100 clip-on sensors, is demonstrated. The design of digital optical sensors for multi-level and high precision measurement is covered by comparing the various pulse-modulation formats in terms of optical power efficiency and sensor/system complexity. Sensor complexity is minimised by using intrinsic (in-fibre) modulation mechanisms and to this end, a novel intrinsic incremental digital temperature/strain sensor with a potential measurement dynamic range of 40 dB is demonstrated. The systems potential of this sensor is then investigated.

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