Nand, Anbhawa (2007) Intragrating sensing using chirped optical fibre Bragg gratings. PhD thesis, Victoria University.

Abstract

This thesis describes a study of intragrating position (localised heat source) and strain measurement sensor systems. The design, development and performance of intensity (power) reflection spectrum based intragrating sensing systems employing chirped fibre Bragg gratings (CFBGs) are investigated. Intragrating sensing is the process of obtaining a continuous profile of a measurand over the grating length from either the amplitude or phase, or both the amplitude and phase of its measured reflection response. Techniques for intragrating sensing employing conventional fibre Bragg gratings have been reviewed and analysed in order to design an intragrating sensor which overcomes the problems associated with the existing sensor systems. The inability to determine the position of the disturbance, direction of the strain gradient and the broadening of the spectra together with the reduction in reflectance complicates the grating inverse reconstruction technique to recover the disturbance profile and is also computationally expensive. Thus the selection of CFBGs is desirable because each Bragg wavelength in the broadband reflection spectrum from a CFBG corresponds to particular local position along the grating. Thus a change in the reflection intensity spectrum, due to a disturbance, at a particular wavelength can be quantified to a local position in the grating and hence the spatial distribution of the measurand can be evaluated. The thesis consists of four major sections: fabrication of CFBGs, sensor calibration, intragrating position and strain measurements. Techniques for the production and controlling the spectral response of CFBGs suitable for intragrating sensing application were investigated using two fabrication techniques; the prism interferometer and the scanning phase mask system. The modelling and experimental results showed that fabrication constraints exist for designing a suitable sensor for intragrating measurements with the prism technique, and thus most of the gratings used for this research were fabricated with the scanning phase mask system. The analysis of results for strain/temperature characterisation, showed that CFBGs fabricated in hydrogenated standard telecommunications fibre have a temperature coefficient approximately 20% higher than standard FBGs in the 1550 nm region with identical strain coefficients. Thus a simple technique for strain-independent temperature measurement is proposed and demonstrated using a sensor head with a combination of a standard FBG and a CFBG. This is an additional finding of the research. The intragrating position measurements within a CFBG sensor were investigated as a function of grating strength. Five linear CFBGs of chirp rate 20 nm/cm, approximate length of 15 mm with varying levels of reflectance (6-53%), fabricated with the scanning phase mask technique were employed for the determination of the centre position of a localised heat source within a CFBG. An iterative approach in conjunction with a fast Fourier transform (FFT) was implemented for solving the inverse problem of obtaining the nonuniform temperature distribution from the measured intensity reflection spectrum. The extracted temperature distribution was characterised by the three parameters of amplitude, width and the centre position. The precision of the inferred centre position of the localised heat source was investigated as a function of grating strength. It was demonstrated that the centre position root mean square (rms) error generally improves with grating strength up to approximately 30% peak reflectance and then decreases with further increase in grating strength for both the high and moderate temperature amplitudes which were investigated. A position rms error below 0.03 mm and a repeatability rms error of 0.005 mm were obtained for the centre position measurements with the CFBG intragrating sensor for grating strengths in the range 20-30%. The position rms error reported is the best to date for CFBG based intragrating sensing systems. The intragrating strain measurements were investigated using three embedded CFBGs sensors to determine nonuniform strain distribution near a stress concentration within a notched aluminium specimen subjected to an axial tensile force. The strain distribution was determined from the analysis of the intensity reflection spectrum through the use of an integration method which did not require an initial strain distribution hypothesis. The best case performance was obtained with the sensor having a reflectance of around 23%. The shape of the recovered strain distributions were in reasonable agreement with that predicted by the finite element method (FEM) modelling however some discrepancies were noted. These are most likely due to incomplete transfer of surface strain within the specimen through 3-layers (host/adhesive/fibre) to the fibre core. A repeatability rms error of less than 4 me [mu epsilon] in the extracted strain profiles was obtained.

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