Development of highly scattering distributed fibre optic sensing for structural health monitoring

In this thesis, fibre optic sensing has been investigated as an important technique for structural health monitoring. Distributed fibre optic sensing based on Rayleigh scattering is a fibre optic sensing technique to achieve the spatially continuous strain monitoring for critical locations for the structures. However, the Rayleigh backscattering intensity in commercial optical fibres is low which is a limitation to Rayleigh scattering based fibre optic sensing. In recent years, methods to improve the intensity of the backscattered light in optical fibres have been proposed. By doping nanoparticles into the optical fibre, the backscattered light increases dramatically. Then, the signal-to-noise ratio may increase which would be beneficial for strain measurement with this Rayleigh scattering based method for structural health monitoring. The main research question is ’how can the enhancement of light scattering used in distributed fibre optic sensing be an advantage for structural health monitoring’. The aim of this research is to develop the enhancement of light scattering in the distributed fibre optic sensing as an advantage for structural health monitoring. Gold spherical nanoparticles were chosen as the contrast agents for backscattered light enhancement. The spectral characteristics (light intensity, spectral shift, etc.) have been investigated in detail in this thesis. In this dissertation, firstly, a model of light scattering by gold nanoparticles at optical fibre interfaces was proposed to overcome the difficulty of manufacturing nanoparticle doped optical fibre in an optical laboratory. Gold nanoparticle liquids were dropped to the optical fibre interfaces to evaluate the backscattered light levels from the nanoparticles. Secondly, a model of light scattering by gold nanoparticles in the core of the optical fibres was proposed and an optimisation of light scattering enhancement by gold nanoparticles in fused silica optical fibres was investigated. By comparing the models of light scattering by gold nanoparticles in the core of the optical fibres and at optical fibre interfaces, the relationship between them has been built to evaluate the light scattering level in the optical fibre from the results obtained from the optical fibre interfaces. Then, the characteristics of the backscattered light spectra from the nanoparticle doped optical fibres and the characteristics of the spectral shift under axial strain were investigated. The backscattered light spectral shifts have been compared with the cases of commercial optical fibres and fibre Bragg gratings. A case study of strain acquisition of gold nanoparticle doped distributed optical fibre sensing based on backscattering was investigated with different typical gauge lengths and spectral ranges. Different noise levels were applied to the spectra to analyse the influence on the strain acquisition with signal-to-noise ratio improvement. Lastly, due to the use of gold as the material for nanoparticles, plasmon resonance is induced by gold nanoparticles. The plasmon resonance based gold nanoparticle doped optical fibre strain sensing was studied to make it a potential auxiliary strain detection method along with distributed fibre optic sensing based on Rayleigh scattering.