Stimulated Brillouin Scattering (SBS) shows the lowest threshold among all non-linear processes observed in optical fibers . It is also strongly dependent on local physical parameters of the fiber, since the scattered light experiences a frequency downshift v@h respect to the incident light proportional to the acoustic velocity within the fiber, this latter being function of temperature and strain. SBS is therefore naturally used to achieve distributed sensors measuring these quantities, and numerous contributions in this field have been presented in the past few years [ 1,2,3 1. In this paper, we discuss the fundamental limitations of the SBS analysis as a distributed sensing method when the spatial resolution is in the meter range. We also present a novel experimental configuration that reaches the best performances achievable for this kind of sensors. The basic configuration of a distributed Brillouin sensor is simple: a strong light pulse, hereafter called pump, is launched into the fiber. It crosses a weak CW lightwave, called signal or probe wave, that propagates in the backward direction. SBS occurs when pump and probe overlap, resulting in an amplification of the probe wave provided that the difference between the two frequencies lies within the Brillouin gain spectrum (BGS). This BGS shows a Lorentzian distribution centered on the Brillouin shift vg that is the quantity to determine. To obtain the BGS and thus determinevB , one simply measures the amplification of the Stokes wave while making a frequency scan. Let&(v) be the gain spectrum; the net amplification of the signal wave after interaction with the pump pulse is given by 1, = IO egBfv~pL ) where the intensities are I0 for the incident signal wave,lg for the amplified signal wave andIp for the pump, respectively, and L = Tvs is the equivalent pump pulse length, with T the temporal width and ZJ~ the group velocity of the pump light. The spatial resolution for distributed measurements is directly related to the pulse 1engthL. As shown in equation 1, the amplification of the Stokes waves exponentially decreases when L is getting shorter. This can be compensated by increasing proportionally the pump intensity, so that the product ZpL remains constant, leaving a sufficient intensity for the measurement. Figure 1 shows the experimental setup, based on a configuration developped in our laboratory [ 4 1. Its main original feature is the presence of a single laser source that is modulated through an Mach-Zehnder electro-optic modulator (EOM) to generate both pump and probe lightwaves. This gives to the system an inherent stability, as far as frequency drifts of the laser are concerned. In addition, access to a single fibe end is required to perform the measurements, what is an obvious advantage in the field. On-site measurements have been so far performed using a 150 mW Nd:YAG laser at 1319 nm, leading to a 3 m best resolution [ 5 1. To improve this figure, it was necessary to boost the intensity of the pump wave, what can be ideally performed using an...

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On-site distributed measurements using a sensor based on stimulated Brillouin scattering are presented. Long fibre length can be used, so that a dense 2D or 3D measurement of strain or temperature can be obtained in large structure.

Long-range distributed strain and temperature measurements along an optical fiber is presented, using a novel optical sensor based on stimulated Brillouin scattering. The optical effect only depends on the fiber material, so that the bare fiber itself acts as sensing element without any special fiber processing or preparation. The sensor accuracy is ± 1°C for temperature and ± 20 µε for deformation. The spatial resolution is 1 meter and the sensor range is more than 20 km.Successful monitoring of a concrete dam element has been performed using an embedded standard cabled fiber. The temperature dynamics of lake waters have been also observed by simply laying a cable over the lake bed.

Long-range distributed strain and temperature measurements along an optical fiber is presented, using a novel optical sensor based on stimulated Brilloum scattering. The optical effect only depends on the fiber material, so that the bare fiber itself acts as sensing element without any special fiber processing or preparation. The sensor accuracy is 1°C for temperature and 20 ic for deformation. The spatial resolution is 1 meter and the sensor range is more than 20 km. Successful monitoring of a concrete dam element has been performed using an embedded standard cabled fiber. The temperature dynamics of lake waters have been also observed by simply laying a cable over the lake bed.