Investigating Novel Optical Fibres for More Advanced Distributed Optical Fibre Sensing

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.

Coherent Rayleigh time domain reflectometry

Xin Lu

Owing to the advantages of the optical fibre, such as lightweight, small size, low cost and immunity to electromagnetic interferences, the fibre sensors have been applied in diverse fields and the scientific research on fibre sensing keeps improving to meet new industrial demands. In this thesis, a novel distributed fibre sensor based on coherent Rayleigh scattering is investigated to realise best accuracy measurements. Coherent Rayleigh scattering is found to be essentially assimilated to a random walk process, thus its statistical properties are theoretically analysed based on the mature theory on random walk. Then the spectral characteristics of the Rayleigh backscattered light is investigated for different incident pulse shapes. The spectral distribution of the trace amplitude turns out to be identical to the spectrum of the incident pulse. Meanwhile, a theoretical model is proposed to describe the coherent Rayleigh trace in time and frequency domains. Experiments are carried out to validate the theoretical analysis and the proposed model. Then the statistical properties and the visibility of the coherent Rayleigh traces obtained by experiment and simulation are investigated for different detection bandwidth. The minimum 3 dB bandwidth required to resolve all features of a coherent Rayleigh intensity trace is determined to be 3 times larger than the bandwidth of the incident rectangular pulse. Then coherent Rayleigh scattering is applied to distributed sensing using the optical time domain reflectometry technique. The working principle is explained by the restorability of the Rayleigh trace in time domain and by a spectral shift in frequency domain. The temperature sensitivity is calculated over a wide range from 300 K down to 4.5 K based on the thermo-optic effect and the thermal expansion of the silica fibre. The experimental results in a standard fibre with acrylate coating and a fibre specially coated with ORMOCER® confirm the high temperature sensitivity (over 1 GHz/K) and demonstrate a temperature resolution in the milliKelvin range even under cryogenic conditions. The fibre with the ORMOCER® coating exhibits a higher temperature sensitivity at room temperature than the acrylate coated fibre due to the extra strain induced by the temperate change in the coating and a more stable sensing behaviour because the polymers forming the coating stabilise the mechanical and thermal responses of the fibre. The sensors based on coherent Rayleigh scattering are also applied to retrieve the phase birefringence distribution along polarisation-maintaining and standard single mode fibres. The minimum detectable birefringence reaches 3×10¿7 when the spatial resolution is 2 m and even smaller value can be resolved using longer optical pulses. In addition, this birefringence measurement is capable of distributed temperature and strain sensing in a PANDA and an elliptical-core polarisation-maintaining fibres since the birefringence is sensitive to environmental variations. The experimental results demonstrate that this method is one order of magnitude more sensitive than a fibre sensor based on Brillouin scattering. Moreover, discriminated temperature and strain sensing is realised by a combination of this birefringence measurement and a standard COTDR sensor. This thesis is concluded by a discussion of further research directions on coherent Rayleigh scattering based distributed sensing.

EPFL2016

Extreme temperature sensing using Brillouin scattering in optical fibers

Alexandre Fellay

Stimulated Brillouin scattering in silica-based optical fibers may be considered from two different and complementary standpoints. For a physicist, this interaction of light and pressure wave in a material, or equivalently in quantum theory terms between photons and phonons, gives some glimpses of the atomic structure of the solid and of its vibration modes. For an applied engineer, the same phenomenon may be put to good use as a sensing mechanism for distributed measurements, thanks to the dependence of the scattered light on external parameters such as the temperature, the pressure or the strain applied to the fiber. As far as temperature measurements are concerned, Brillouin-based distributed sensors have progressively gained wide recognition as efficient systems, even if their rather high cost still restricts the number of their applications. Yet they are generally used in a relatively narrow temperature range around the usual ambient temperature; in this domain, the frequency of the scattered light increases linearly with the increase in temperature. The extension of this range toward higher (up to 800°C) and lower (down to 1 K) temperature is the main aim of this thesis. In both cases, our measurements on various fiber samples show that the aforementioned linearity does not hold. Most notably, the characteristics of Brillouin scattering at low and very low temperature are strikingly different from those observed under normal conditions; they are directly related to the disordered nature of the silica that constitutes the fiber, what motivates the large place we here devote to the physics of amorphous solids. Despite the observed nonlinearities, our results demonstrate the feasibility of thermometry based on Brillouin scattering over the full investigated temperature range. We can thus swap in the last part of this work the physicist's point of view for that of an engineer and present the setup and the performances of a sensor, or rather of a whole family of sensors we have tested and improved in the course of the last few years; moreover, some real measurement examples are given. Finally, we propose some possibilities of further enhancement, that remain right now at the preliminary stage of their development.

EPFL2003

Coherent Rayleigh time domain reflectometry

Xin Lu

EPFL2016

Extreme temperature sensing using Brillouin scattering in optical fibers

Alexandre Fellay

EPFL2003