Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives

Optical fiber sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements. The fiber is used as sensing element, and a value for temperature and/or strain can be obtained from any point along the fiber. After explaining the principle and presenting the standard implementation, the latest developments in this class of sensors will be introduced, such as the possibility to measure with a spatial resolution of 10 cm and below while preserving the full accuracy on the determination of temperature and strain.

Brillouin Distributed Fibre Sensing: State of the Art and Perspectives

Luc Thévenaz

Developed societies require more and more information for their safety and for their economic development. Recent disasters due to landslides, fires in tunnels and collapses of bridges are a source of serious concern on the part of the public which seeks for more safety and for an efficient prevention of these frequent recurrences of dangers. Sensing in adverse environment and extreme conditions requires dedicated techniques. Quite recent technologies may offer novel valuable solutions and give rise to a strenuous research effort. Optical technologies are an essential actor owing to their tremendous capability to transmit and process a high density of information. Optical fibre is a key component for these technologies and its potential for optical processing and for collecting information as sensing probe is still widely unexplored. The development and the popularity of telecommunications have entirely screened the fact that optical fibres may be efficiently used for sensing purposes. In this paper we present applications of a novel technique using optical fibres to monitor large structures for safety purpose. The fibre is used as sensing element and can provide distributed measurements of quantities like temperature or strain. In other words a value of temperature and/or strain can be obtained for any point along the fibre. This is made possible by using a nonlinear optical effect in the fibre called Stimulated Brillouin Scattering (SBS). Optical fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements. The Brillouin interaction causes the coupling between optical and acoustic waves when a resonance condition is fulfilled. It turns out that this resonance condition is strain and temperature-dependent, so that determining the resonance frequency directly provides a measure of temperature or strain. The resonance frequency is an intrinsic property of the material that may be observed in any silica fibre. This is very attractive since the bare fibre itself acts as sensing element without any special processing or preparation on the fibre. Standard optical cables may thus be used, resulting in a low-cost sensing element that may be left in the structure. Since the optical effect only depends on the fibre material, it is absolutely stable in time and independent of the instrument. Different measurements performed over a long-term period are thus fully comparable. The latest developments in this class of sensors will be shown, such as the possibility to measure with a spatial resolution of 10 cm and below, while preserving the full accuracy on the determination of temperature and strain. Illustrative examples of site measurements will be presented and future prospects will be discussed.

Materials Research Society of Singapore2009

La diffusion Brillouin dans les fibres optiques

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.

EPFL1997

Recent progress towards centimetric spatial resolution in distributed fibre sensing

Sang Hoon Chin, Stella Foaleng Mafang, Nikolay Primerov, Kwang-Yong Song, Luc Thévenaz

Optical fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements. The fibre is used as sensing element and a value for temperature and/or strain can be obtained from any point along the fibre. While classical configurations have practically a spatial resolution limited by the phonon lifetime to 1 meter, novel approaches have been demonstrated these past years that can overcome this limit. This can be achieved either by the prior activation of the acoustic wave by a long lasting pre-pumping signal, leading to the optimized configuration using Brillouin echoes, or by probing a classically generated steady acoustic wave using a ultra-short pulse propagating in the orthogonal polarization of a highly birefringent fibre. These novel configurations can offer spatial resolutions in the centimetre range, while preserving the full accuracy on the determination of temperature and strain.

The International Society for Optical Engineering2010

Brillouin Distributed Fibre Sensing: State of the Art and Perspectives

Luc Thévenaz

Materials Research Society of Singapore2009

La diffusion Brillouin dans les fibres optiques

EPFL1997