Effect of enhanced Rayleigh scattering on the random fiber laser efficiency

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Optical spectral probing for epithelial tissue characterisation

Philippe Thueler

In this thesis, a new method has been developed, which determines the optical coefficients spectra (absorption and scattering) of small volumes of tissue (1 mm3) by the measurement of spatially resolved reflectance between 480 and 950 nm. Firstly, an algorithm has been developed to determine the optical coefficients form reflectance measurements: It is based on the optical propagation model introduced by Bevilacqua [Bevilacqua 99 JOSA]. This model takes the first two moments of the phase function into account through the γ parameter, allowing to accurately simulate the propagation of photons close to the source by the Monte Carlo method. These simulations are then fitted to the reflectance to obtain the optical coefficients. To obtain any value of absorption, scattering or γ parameter, the simulations are interpolated by 3D B-splines. A complete robustness to noise study of the algorithm has been presented. A theoretical study of the γ parameter in tissue over broad wavelengths has been reported. Tissue was simulated with Mie theory for a fractal distribution of sphere diameters. For such a distribution, the γ spectrum is almost flat, instead of being wavy like in the case of mono-disperse sphere diameter. The correlation of the mean value of γ with the fractal power (or dimension) of the distribution was made and showed an analytical form of hyperbolic tangent. It allowed deepening the interpretation of the γ parameter as being an indicator of the relative number of small scatterers (much smaller than the wavelength, e.g. of Rayleigh type) in comparison of bigger ones (their size being comparable to the wavelength). The assessment of the set-up and the algorithm was made by measuring the reflectance of a solution containing 5 different diameters of polystyrene spheres simulating the scattering and Nigrosin colorant as absorber. It showed a good agreement between measurements and prediction. In order to built a better comprehension of absorption and scattering at the tissue scale, a first part of in vivo investigations has been made on an animal model. The skin of the mouse back was altered with Freund's adjuvant and TPA and the optical coefficients were measured on treated and control sites. The effect of alteration (epidermis and dermis thickening, oil vesicle appearing, inflammatory cells infiltration, etc.) was clearly correlated with optical coefficients variations. The scattering spectrum always showed the better contrast. The absorption spectrum, especially in the haemoglobin peak region (between 550 and 600 nm) also showed significant information, in terms of oxy and deoxy-haemoglobin concentration and saturation. Sensitivity and specificity of the "diagnosis" was evaluated. The study of stomach epithelium in vivo on 29 human subjects was made using a small optical fibre probe (

EPFL2001

Investigating Novel Optical Fibres for More Advanced Distributed Optical Fibre Sensing

Malak Mohamed Hossameldeen Omar Mohamed Galal

The ever-growing need for distributed optical fibre sensors (DOFS) in numerous fields and applications demands continuous research towards the advancement of the existing sensing systems. The convenience in the field of DOFS lies in the several degrees of freedom that it offers, such as the scattering mechanism, the interrogation technique, the medium for propagation, and the sensing fibre itself. Even though a vast amount of research is oriented towards the fabrication of novel fibres, in many cases the real benefit of such fibres is not really exploited. Therefore, in this thesis, we use and characterise multiple novel optical fibres with the aim of harnessing their full potential by understanding their ultimate strengths and limitations. To that effect, we perform efficient distributed optical fibre sensing by investigating different scattering mechanisms in different scattering media by means of several interrogation techniques. The thesis is divided into two groups of chapters depending on the type of scattering mechanism and medium. In the first group of chapters, we work on the investigation of the most prominent scattering mechanism in silica-core single-mode optical fibres (SMFs), namely Rayleigh scattering. We interrogate a novel reflection-enhanced fibre (REF) based on fibre Bragg gratings using one of the most widely-utilised interrogation techniques for Rayleigh scattering, which is phase-sensitive optical time-domain reflectometry (f-OTDR). So far, no theoretical expression has been presented to relate the most crucial parameters of Rayleigh-based sensing systems. We, therefore, develop a model, as a figure-of-merit for Rayleigh-based systems, addressing this concern, and we confirm it with experimental results. By performing a distributed temperature measurement as a form of comparison between the REF and an SMF, we yield a 6× lower experimental uncertainty for the REF when compared to the SMF, and this lower uncertainty is directly attributed to the 6× higher signal-to-noise ratio that the REF offers. The model as well as the guidelines for utilising REFs to maximum effect will aid the scientific community in better understanding Rayleigh-based sensing systems and employing them more efficiently.The focus then shifts to the most prominent scattering mechanism in a gaseous medium of high density (at 1 atm for instance), namely Brillouin scattering. The analysis is carried out using several novel hollow-core optical fibres particularly anti-resonant fibres (HC-ARFs) which we fill with gas. We demonstrate for the first time to the best of our knowledge a Brillouin gain measurement in gas-filled HC-ARFs and highlight the square dependence of the Brillouin gain on the gas pressure. We perform a comparison between three hollow-core fibres of different dimensions which we fill with two different gases, and we indicate the trade-offs in terms of Brillouin gain coefficients, gas filling times as well as gas pressure limitations. Additionally, we conduct for the first time to our knowledge a distributed temperature measurement using a gas-filled hollow-core conjoined-tube anti-resonant fibre (HC-CAF) and show a higher sensitivity to temperature change which is about 2× greater than the sensitivity of conventional silica fibres. The results indicate the great potential of gas-filled hollow-core fibres and pave the way for researchers to employ them as promising candidates for lasing,sensing and imaging applications.

EPFL2022

Comparison of scattering and reflection SFG: a question of phase-matching

Hilton Barbosa De Aguiar, Kailash Chandra Jena, Sylvie Roke, Rüdiger Scheu

We present a comparison between sum frequency scattering (SFS) and reflection mode sum frequency generation (R-SFG). We have used scattering theory to describe both scattering experiments as well as reflection mode experiments. The interfacial vibrational spectrum of nanoscopic oil droplets dispersed in water was probed with SFS as well as with R-SFG. Spectra recorded in phase-matched R-SFG mode and spectra recorded with SFS from the same sample are different, which shows that different interfaces are measured. Scattering spectra at different scattering angles agree with nonlinear light scattering theory. We further present experiments with polymer films aimed at quantifying the comparative strength of R-SFG and SFS experiments.