Multiplexed and distributed optical fibre sensors

Abstract

This thesis presents three novel optical fibre sensor systems which monitor optical path lengths. The systems have been used to measure strain in an optical fibre. All three systems make several measurements at different locations along a fibre, allowing the spatial distribution of a measurand to be obtained.

For the first time, incoherent optical frequency domain reflectometry is used together with time division multiplexing to measure the optical path length of an array of fibre sections. Sensing sections are 5m long and are defined by broadband optical reflectors. A closed loop interrogation system is demonstrated to monitor the sensors in real time with an accuracy of 2.1µ.epsilon/√Hz, in good agreement with the theoretically predicted value.

Simultaneous monitoring of multiple fibre Bragg grating sensors, several millimetres in length, is also demonstrated by simultaneously generating multiple passbands in a single acousto-optic tunable filter. This is the only technique demonstrated to simultaneously monitor multiple gratings using a single wavelength-tunable device.

The first distributed Bragg grating sensor to measure arbitrary strain profiles is also demonstrated. Low-coherence interferometry selects the interrogation position and a tunable filter measures the local wavelength. Two configurations of the technique are presented, which have achieved spatial resolutions of 300µm and real-time strain measurements with 5.4µε/√Hz accuracy, showing good agreement with theoretically predicted values. The only grating sensor network to be both distributed and multiplexed is presented together with the first results.

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