Nanoparticle- (NP-) doped optical fibres show the potential to increase the signal-to-noise ratio and thus the sensitivity of optical fibre strain detection for structural health monitoring. In this paper, our previous experimental/simulation study is extended to a design study for strain monitoring. 100 nm spherical gold NPs were randomly seeded in the optical fibre core to increase the intensity of backscattered light. Backscattered light spectra were obtained in different wavelength ranges around the infrared C-band and for different gauge lengths. Spectral shift values were obtained by cross-correlation of the spectra before and after strain change. The results showed that the strain accuracy has a positive correlation with the relative spectral sensitivity and that the strain precision decreases with increasing noise. Based on the simulated results, a formula for the sensitivity of the NP-doped optical fibre sensor was obtained using an aerospace case study to provide realistic strain values. An improved method is proposed to increase the accuracy of strain detection based on increasing the relative spectral sensitivity, and the results showed that the error was reduced by about 50%, but at the expense of a reduced strain measurement range and more sensitivity to noise. These results contribute to the better application of NP-doped optical fibres for strain monitoring.

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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. @en

The colour of the ground layers of a painting has an influence on its visual appearance. In addition to the commonly used white ground layers, other colour ground layers have been used, for example, the grey ground layer used in Peter Paul Rubens’s painting Portrait of Clara Serena Rubens helps the colour transition of the skin tones. Understanding the effects caused by the colours of the ground layers is of significance for both technical art history and conservation. Optical non-destructive testing (NDT) techniques are useful tools for the investigation of paintings, for example, optical coherence tomography (OCT) can be used to study the surface and subsurface layers non-destructively. In this work, the interaction of light with paint and ground layers is modelled to supplement OCT measurements of paintings with ground layers. A previously described near-infrared light range OCT system provides high spatial and depth resolution measurements. A four-flux model has been developed for analysing the light interaction in the paint and ground layers. This model considers forwards-propagating collimated light, backwards-propagating collimated light, forwards-propagating diffuse light and backwards-propagating diffuse light. The model uses the optical material properties, including refractive index (RI), absorption and layer thickness, as input. This paper describes the construction of the model and an evaluation of its performance by comparison with OCT data.

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Nanoparticle (NP) doping of optical fibres can be used to increase the intensity of the backscattered light used for distributed strain sensing and has shown the advantages of high precision strain detection and multiplex sensing experimentally. However, the backscatter spectral characteristics of NP-doped optical fibres have not been described even though they are quite different from the spectra from fibre Bragg gratings (FBGs) or commercial single mode fibres. In this paper, gold NPs, used as the contrast agent in the optical fibre to increase the intensity of the backscattered light, were investigated from the aspect of their spectra. A single scattering model with Mie theory and an effective refractive index (RI) model were used to evaluate the backscattered light spectra and the Monte Carlo Method was used for seeding NPs. The results showed that the strain responsivity of gold-NP doped fibres with low volume ratio doping (single scattering restriction) are close to FBGs and commercial fibres. High volume ratios of gold NP doping increase the imaginary part of the RI of the optical fibre, which has a significant influence on the spectra in the wavenumber domain. These theoretical insights may promote the future engineering design of NP-doped fibre sensors.@en

The plasmon resonance spectral peak of a gold spherical nanoparticle (NP) will shift when the NP shape is changed from sphere to spheroid. This may be used as a novel strain detection method with gold NPs embedded in a medium of different refractive index (RI). Applying a strain to the external medium will cause a change in the shape of the NP from spherical to spheroidal. In our previous work, it was found that when the RI change of the medium surrounding the NPs is close to zero, the shape change induced plasmon resonance spectral peak shift will become important. In order to obtain only the wavelength shift values caused by the shape change of the NPs, the RI of medium surrounding the gold NPs is set at a constant of 1.45 and the RI of the gold NP is assumed unchanged. The T-matrix method is used to calculate the scattered light and light extinction by the NP morphing. The diameters of the gold NPs are set from 100 nm to 400 nm, with the size interval at 10 nm, to cover a wide size range for typical sizes of gold spherical NPs. The spectra of the light scattering and light extinction were calculated on the Delft University high performance computing cluster. The results show that the plasmon resonance spectral peak shift is related to the size of the NPs. Larger sizes of gold NPs have larger peak shift values, but there is an inflection point around 200 nm and the bandwidth of the resonance peak becomes larger which will cause a difficulty in precisely locating the peak.

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Strain-based structural health monitoring (SHM) relies on high performance strain sensing methods. Gold nanoparticle (NP) doped fibre optic sensors not only have the potential to increase the intensity of the backscattered signal to increase the signal to noise ratio but also have plasmon resonance peaks in the visible light range. The spectral peak shift of the plasmon resonance may be used for strain sensing. In this paper, the spectral peak shift of the plasmon resonance of an optical fibre containing gold NPs under axial strain was analysed. A modified Lorentz-Drude (LD) model with the T-matrix method was used and the spectral peak shifts of spheroidal NPs under strain were calculated. An approximate analytical expression was derived for faster calculation. The modelling presented in this paper shows that the ratio of the change of the peak wavelength to the strain can be related to the refractive index (RI) change of the optical fibre under strain, the shape change of the gold NP, and the RI change of the gold NP. The peak shift was also observed experimentally in an optical adhesive containing gold NPs under compression. The peak shifts were analysed at different RI of the optical fibres, 1.35, 1.45, 1.55 and 1.65 respectively, in order to cover the range of RI of fused silica and some polymer materials. The results confirm experimentally that the applied axial strain can induce the peak wavelength shift by the NPs. By choosing a different optical fibre or the properties of the NPs, the wavelength change ratio has the potential to be tuned, which may be used for highly sensitive strain sensing.

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Optical fibre backscatter reflectometry is an important technique for Structural Health Monitoring (SHM). In recent years, increasing the intensity of backscattered light in backscatter reflectometry has shown the advantage of improving the signal detection in shape sensing and temperature detection due to the increase of signal to noise ratio and this approach could potentially be used to improve the performance of an SHM system. Doping nanoparticles (NPs) is a method to increase the intensity of backscattered light in distributed fibre optic sensing. The increased intensity of light backscattered by the NPs needs to be investigated to design suitable optical sensing fibres with NPs for backscatter reflectometry. In this work NPs were added to refractive index matching liquid and tested with commercial NP suspensions experimentally between the tips of two optical fibres. An estimate of the intensity of backscattered light from the NPs in this structure was performed by simulation to give a better understanding of the expected levels of intensities of scattered light from NPs in this distributed fibre optic sensing configuration. We present analytical models based on Mie theory and the Monte Carlo Method. Simulated results are presented, for a broad bandwidth Gaussian spectra shape incident light with a central wavelength around 1550 nm, to match the experimental conditions in this work. The novelty is in developing this model for scattered light by NPs at optical fibre interfaces and the evaluation of the possibility of detection by the calculated scattered intensity levels.

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This study forms a part of the research in using nanoparticles (NPs) to increase the intensity of light scattering signal in the optical fibres. Increasing the intensity of the backscattered light signal in the optical fibres shows the potential to increase the signal-to-noise ratio in order to improve the sensitivity of the backscatter reflectometry. Doping NPs into the optical fibres can greatly increase the scattered light. However, it is not easy to manufacture NP-doped optical fibres to test different designs. To overcome this problem, in our former work we used the method of dropping refractive index matching liquid containing gold NPs at the optical fibres end tips to investigate the intensity change of the scattered light from the interfaces. In this paper, some new initial experimental results for the scattered light between the optical fibre end tips are shown. Gold NPs have been mixed into the optical adhesive (Norland) and is then dropped and cured at the optical fibre end tips. A backscatter reflectometer (LUNA ODiSI-B) was used in the experiment to measure the intensity of scattered light distribution between the optical fibre end tips. We investigated 4 cases of light scattering between the optical fibre end tips: (i) the backscattered light intensity distribution in the case of the air gap between the optical fibre end tips; (ii) the backscattered light intensity distribution with optical adhesive between the optical fibre end tips; (iii) the backscattered light intensity distribution with optical adhesive containing gold NPs (gold nanopowder (<100 nm), Sigma Aldrich) between the optical fibre end tips before curing process and (iv) the backscattered light intensity distribution with optical adhesive containing gold NPs between the optical fibre end tips after the curing process. Our initial findings are that the scattered light by gold NPs at the optical fibre interfaces can be detected by the backscatter reflectometer. By obtaining the differential signal between the distributed light scattering by cured optical adhesive containing gold NPs and only optical adhesive between the optical fibre end tips, the light scattered by the gold NPs has be determined.

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A conventional distributed fiber optic sensing system offers close to linear sensitivity along the fiber length. However gold nanoparticles (NP) have been shown to be able to enhance the contrast ratio to improve the quality of signal detection. The challenge in improving the contrast of reflected signals is to optimise the nanoparticle doping concentration over the densed sensing length to make best use of the distributed fiber sensing hardware. In this paper, light enhancement by spherical gold NPs in the optical fibers was analyzed by considering the size-induced NP refractive index changes. This was achieved by building a new model to relate backscattered light from a gold NP suspension between the optical fiber end tips and backscattered light from gold NPs in the core of the optical fiber. The paper provides a model to determine the optimized sizes and concentrations of NPs for sensing at different desired penetration depths in the optical fiber.

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This study forms the first part of research into enhancing the forward and back scattering of light in an optical fibre using nanoparticles (NPs). This approach has the potential to enhance the sensitivity of optical fibre sensing by increasing the signal-to-noise ratio. The work described in this paper is focused on understanding the scattering of light by a suspension of NPs in refractive index matching liquid. It was noted early in the experimental work that rheological effects related to the viscosity and flow of the liquid affect the scattered light measured and therefore these effects are considered in the analysis. Gold nanoparticles in the tens to hundreds of micrometre size range were selected as the scattering particles based on their optical properties. These are suspended in a refractive index liquid with a similar refractive index to the optical fibre core. Effort was needed to transfer the NPs from their aqueous sodium citrate solution to the paraffin based solution. We investigated two types of interaction with the optical fibre: (i) dropping the NP suspension onto the end of a single-mode optical fibre and (ii) using the NP suspension as an interface between two single-mode optical fibres. It was noted that the surface tension of the liquid, the diameter of the fibre and the spacing between the fibres in case (ii) influence the reflected and transmitted light. In case of excess liquid, droplets flowed down the fibre and interestingly in case (ii) modified the reflected and forward transmitted light as it passed across the fibre interface. Our initial findings are that the influence of the gap between two optical fibres decreased after dropping refractive index liquid into the gap after fibre collimation, which is a beneficial result for understanding the influence of scattered light from a liquid containing NPs. Note, the position between the two fibres can also change due to the weight of the droplet and the fibre ends had to be re-collimated to investigate the influence of the moving droplets. These results will be expanded by additional experiments and modelling of the scattering from the nanoparticales and droplets.@en