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Title: Optical fibre sensing by time domain reflectometry
Author: Saunders, Charles T. W.
ISNI:       0000 0004 2701 8657
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2006
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This thesis considers cost effective optical time domain rcflectometry for distributed fibre sensors, using new developments in light sources, detectors, fibres and computational power, suitable to interrogate distributed fibre sensors from 20m to 10km, within a £5,000 hardware budget. The characteristics of 200μm core diameter polymer clad silica (PCS) fibre and 980μm core diameter PMMA POF (polymethyl methacrylate plastic optical fibre) were theoretically evaluated including damage thresholds, optimum sensitisation (evanescent field attenuation and micro-bending) and launch conditions for optimum performance as a distributed sensor. Rayleigh backscattered signals, forward-propagating power and Fresnel reflections for different fibre types at different distances along the fibre were considered. PCS fibre allows solutes and gases ready access to the core-cladding interface and is preferred for distributed optical fibre sensors (DOFS) of 1000m. 50-125 graded index multimode fibre is preferred as a sensor of mechanical measurands for DOFS 10km long. The higher backscattering coefficient of PMMA POF returns the highest reflected signals for DOFS of up to 30m. Test beds of simulated distributed optical fibre sensors built from single- and multimode silica, hybrid and PCS fibres were assembled for interrogation by visible and NIR wavelengths using mechanical measurands and misaligned splices as point losses, to determine the relationships between launch pulse power, pulse width and backscattered power and the factors determining spatial resolution and dynamic range. Commercial analogue OTDRs (optical time domain reflectometers) and custom-built photon counting OTDRs (ν-OTDRs) were used to probe the fibre sensors. This enabled the design, custom build and evaluation of an OTDR-based DOFS system where the light source may easily be changed for one of a different power or wavelength. The performance of intrinsic or modified fibres in applications of chemical and pH sensing was evaluated: A simulated distributed PMMA POF was demonstrated to sense pH to a resolution of ±1 pH when placed in aqueous solutions of 4.2 x 10⁻⁴ M methyl red between pH 2.89 and 9.70 and probed with 648nm light using a ν-OTDR. An undyed PCS fibre was used to sense aqueous methyl red when probed with 657nm light from a ν-OTDR. An undyed PCS fibre was used to detect 6.5 x 10⁻⁴ M methyl red in ethanol using 657nm light from a ν-OTDR by modifying the cladding refractive index from 1.401 to 1.370. A PCS fibre dyed with 4.4 x 10⁻⁶ M chlorophyll a in ethanol solution then dried was probed by ν-OTDR at 657nm, returning a 4.0dB peak on the trace indicating detection by Fresnel reflection by light in the cladding. A slope of -2.0dB/m on the trace indicated evanescent field absorption due to the 662nm absorption peak. An avalanche photodiode (APD) detection system with inherent stability suitable for long term monitoring of Rayleigh back-scattered signals was designed and built. The modules included an active quench and recharge circuit capable of 20MHz count rate with a novel quenching circuit bias arrangement to provide immunity from spurious triggering, and a Peltier cooler circuit to regulate the APD to ± 0.1°C. A dynamic bias control system based around a PCI-6602 (NI) counter-timer card was designed, built, evaluated and shown by calculation to limit the tolerance on 2.0V excess bias to ± 0.048V. The tighter control of the excess bias stabilises quantum efficiency, resolution and dark count.
Supervisor: Scully, Patricia Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available