The measurement of physical fields using optical fibres and fibre gratings

Abstract

In this thesis a number of novel optical fibre and fibre Bragg grating sensors have been investigated for the purpose of measuring physical fields, particularly strain, temperature and pressure. These sensors are based on using short coherence length light source.

Two laboratory prototypes of interferometric fibre pressure sensors have been demonstrated using a hollow-glass microsphere and optical fibres, respectively. These are intended for ultimate application as strain sensors for smart structures.

A novel white-light polarimetric interrogator is demonstrated. The device consists of two equal lengths of highly-birefringent fibre, spliced with their polarization axes orthogonal. The dispersion effect on the responsivity of white-light interferometry is evaluated using a simplified transfer function. Using Jones matrix and Poincaré sphere analysis methods, the device performance has been discussed. Experimental results are presented by thermally-scanning the device for the interrogation of sensing interferometer.

A fibre Bragg grating sensor for measuring pressure, strain and temperature is evaluated using measured data from over twenty different fibre gratings including Bragg gratings in highly-birefringent fibres. Intensive experimental results on thermally-induced decay of fibre gratings are presented for the evaluation of their potential in a wide variety of sensors and multiplexed systems.

A first demonstration of a fibre grating interrogation system using an acousto-optic tunable filter is presented. The system involves frequency-shift-keying of the RF drive to the filter to track the measurand-induced wavelength shifts of multiplexed fibre gratings. An equivalent linear model has been developed to describe the operation of a wavelength tracking loop. Experimental results for both open-loop and close-loop operations are presented for strain and temperature measurements.

Three novel techniques to resolve the thermal effect in strain measurement using fibre gratings are demonstrated. The first method involves using surface-mounted fibre grating pairs to compensate for thermal effects. The second technique uses two superimposed fibre gratings (single sensing element) to simultaneously measure strain and temperature. Last but not least by using a chirped Bragg grating in a tapered optical fibre as a strain sensing element, strain sensing can be made to be temperature-independent. Both theoretical analysis and experimental work are presented for these approaches.

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