La diffusion Brillouin dans les fibres optiques:étude et application aux capteurs distribués

Among all non-linear optical effects observed in single-mode optical fibres, stimulated Brillouin scattering (SBS) is of particular importance since it has numerous practical implications. SBS occurs when laser waves are scattered through the interaction of light with hypersonic acoustic waves in the medium. It manifests through the generation of a backscattered Stokes wave that carries most of the input optical power once a definite intensity level is reached within the fibre core. It limits the maximum optical power that can be transmitted through an optical fibre and therefore can cause a severe penality for fibre optics telecommunications. The backscattered Stokes wave is down shifted in frequency with respect to the incident lightwave frequency. This frequency shift – often called the Brillouin frequency shift – depends on the fibre parameters and on the wavelength of the incident light. It is directly proportional to the acoustic velocity and ranges from 12 to 13 GHz for silica fibres at a 1.3 µm wavelength. The Brillouin frequency shift is also very sensitive to environmental quantities changing the acoustic velocity such as temperature and strain. This feature makes SBS very interesting for temperature and strain monitoring in optical fibres and has been used in the design of fibre optics sensors. In the present work, a novel method to measure SBS characteristics in single-mode optical fibres has been developed. It is based on the so-called pump and probe technique using two counterpropagating optical waves within the test fibre. It basically presents the originality to require only one laser source and relies on an electro-optic modulator to generate the probe signal by microwave modulation of the pump light. The inherent high stability of the experimental set-up has made possible the characterisation of different fibres in terms of SBS parameters with a higher accuracy than the traditional two lasers set-up. The influence of dopant concentration in silica fibres on the SBS properties has been fully characterised. Furthermore it has been determined that the presence of core dopants decreases the acoustic velocity resulting in a smaller Brillouin frequency shift. The temperature and strain effects on the SBS characteristics have been extensively investigated. The problem of the evolution of the polarisation of the interacting lightwaves has been studied and a model clarified the so far unexplained behaviour in low birefringence fibres. A new measurement procedure has been defined to achieve polarisation-independent Brillouin gain determination. The second facet of the present thesis deals with the concept of distributed fibre optics sensors based on SBS. The determination of the Brillouin frequency shift gives access to the temperature or strain experienced by the fibre, while a modified OTDR technique provides the information on the position. Here again the use of a single laser source together with an electro-optic modulator brings several advantages. Besides the convenient flexibility in the generation of the probe signal, it makes possible to pulse the optical signals to localise the interaction within the test fibre. The overall temperature or strain distribution of the test fibre can be carried out by measuring the Brillouin frequency shift at any locations throughout the test fibre. A sensor has been experimentally achieved and its performances can be summarised as follow: spatial resolution less than 10 meters over more than 10 kilometres, with a resolution on the Brillouin frequency determination of 1 MHz, that corresponds to a ±1°C temperature resolution or to a 2x10-5 strain resolution. The dynamic range can be increased up to 30 kilometres to the detriment of the spatial resolution. The ultimate performances of the SBS distributed fibre optics sensors have been investigated in terms of spatial resolution and dynamic range. It turns out that these sensors are eventually dedicated to distributed measurements over several tens of kilometres with a spatial resolution limited to the meter range. Finally two practical applications of such sensors are described: the measurement of the strain distribution in installed fibre optics cables for telecommunications and the temperature monitoring of electrical energy distribution cables. On site measurements using a Brillouin sensor have been performed for the first time thanks to the high stability and reliability of the sensor

Method for measuring the Brillouin scattering in an optical fibre and device for implementing this method

This device for measuring Brillouin scattering includes a single laser source (11), a modulator (12) placed on the path of the light emitted by the source (11) and designed to generate a pumping signal and a test signal which are injected to one end (13) of the fibre (10), a mirror (15) arranged at the other end (14) of the fibre in order to reflect the said signals, and an electronic unit (16) for producing time encoding of these signals in order to offset them in time relative to one another so that they cross at a point in the fibre after reflection at the end (14). A detector (20) connected to a detection fibre (18) that is coupled to the fibre (10) by a coupler (19) allows opto-electrical conversion of the two signals and determination of the value of the Brillouin scattering in the fibre.; Determination of physical parameters external to the fibre, such as temperature, on the basis of the value of the Brillouin scattering in this fibre

Brillouin Gain Spectrum Measurement using a Single Laser Source

Brillouin gain spectrum measurements in fibres are presented, where a single source is used to generate the pump and probe signals, giving highly reliable results

Multi-hydrogenated compounds monitoring in optical fibre manufacturing process by photoacoustic spectroscopy

Sub-ppm hydrogen chloride (HCl) and water vapour (H2O) monitoring using photoacoustic spectroscopy in optical fibre manufacturing is reported. The development and performance of a sensor based on an acoustic resonant configuration is described, and on-site measurements are presented. Two DFB lasers emitting in the 1370nm and 1740nm range were used for the detection of H2O and HCl, respectively. A detection limit (defined for a SNR=3) of 60ppb for HCl and 40ppb for H2O was achieved. Contamination sources of the carrier gas used for the fibre preform manufacturing are identified and discusse

Monitoring of large structures for safety issues using Brillouin distributed sensing

Brillouin time-domain analysis in optical fibres is a novel technique making possible a distributed measurement of temperature and strain over long distance and will deeply modify our view about monitoring large structures, such as dams, bridges, tunnels and pipelines

Fast measurement of local PMD with high spatial resolution using stimulated Brillouin scattering

Abstract Local beat length with a 21.5cm spatial resolution is measured in one second along a single mode fibre using the polarisation dependence of stimulated Brillouin scattering, in a non-destructive and simple way. Introduction It has been demonstrated over the past two decades that polarisation mode dispersion (PMD) may limit the ultimate data rate through an optical fibre. The basic reason for PMD is the presence of intrinsic or induced birefringence within the fibre. Measuring the local birefringence would thus provide key information to localize the fibre segments mainly contributing to PMD, so that an efficient action could be undertaken for cable upgrade. A simple solution based on Rayleigh reflectometry using polarized light, the socalled POTDR techniqu

Overall polarization dispersion after propagation through different fibres

Polarization dispersion is one of the effect which can give rise to data rate limitation. An actual optical link can be installed using different appended fibres, manufactured by different producers, processing techniques or batches. Knowing the polarization dispersion of each individual fibre, it could be difficult to forecast the overall dispersion of such an heterogeneous link. Fortunately recent models make the prediction of the global polarization dispersion possible. An experimental verification of this model using two different methods is reported, so that the predicted dispersion of two appended non-similar fibres could be compared with actual measurements

Applications de la diffusion Brillouin stimulée dans les fibres optiques monomodes

L'exploitation de la diffusion Brillouin stimulée dans les fibres optiques a connu un regain d'intérêt ces dernières années, surtout en raison de la récente disponibilité de sources laser adéquates. Ces nouvelles sources puissantes, compactes et très cohérentes, tels les lasers Nd:YAG pompés par des diodes laser, permettent à présent de mettre confortablement en évidence les effets de cette diffusion