Enhancement of an Optical Fiber Sensor: Source Separation Based on Brillouin Spectrum

Abstract

International audienceDistributed optical fiber sensors have gained an increasingly prominent role in structural-health monitoring. These are composed of an optical fiber cable in which a light impulse is launched by an opto-electronic device. The scattered light is of interest in the spectral domain: the spontaneous Brillouin spectrum is centered on the Brillouin frequency, which is related to the local strain and temperature changes in the optical fiber. When coupled with an industrial Brillouin optical time-domain analyzer (B-OTDA), an optical fiber cable can provide distributed measurements of strain and/or temperature, with a spatial resolution over kilometers of 40 cm. This paper focuses on the functioning of a B-OTDA device, where we address the problem of the improvement of spatial resolution. We model a Brillouin spectrum measured within an integration base of 1 m as the superposition of the elementary spectra contained in the base. Then, the spectral distortion phenomenon can be mathematically explained: if the strain is not constant within the integration base, the Brillouin spectrum is composed of several elementary spectra that are centered on different local Brillouin frequencies. We propose a source separation methodology approach to decompose a measured Brillouin spectrum into its spectral components. The local Brillouin frequencies and amplitudes are related to a portion of the integration base where the strain is constant. A layout algorithm allows the estimation of a strain profile with new spatial resolution chosen by the user. Numerical tests enable the finding of the optimal parameters, which provides a reduction to 1 cm of the 40-cm spatial resolution of the B-OTDA device. These parameters are highlighted during a comparison with a reference strain profile acquired by a 5-cm-resolution Rayleigh scatter analyzer under controlled conditions. In comparison with the B-OTDA strain profile, our estimated strain profile has better accuracy, with centimeter spatial resolut ion

Parametric Inversion of Brillouin Spectra to Enhance the Accuracy of Distributed Strain Measurement

“…A first method proved the possibility of enhancement of the spatial sampling (Buchoud et al, 2013). However, this method missed some local strain variations that is crucial for the soil displacement quantification.…”

“…However, as the BOTDA integrates over 1 meter at each z, it is possible to mix non-uniform local strain within w (Buchoud et al, 2013) inducing a broadening and asymmetric Brillouin spectrum. In this previous work, the assumption was made that the measured spectrum at z k is the combination of local stimulated Brillouin spectra S e (v) within w centered on local Brillouin frequencies :…”

“…Thus, the detection and localization of strain gradients into structures will be upgraded. To fulfil such goal, a model proposed in Buchoud et al, 2013 explains the link between the hidden information inside Brillouin spectra and the local Brillouin frequencies. A parametric inverse methodology is proposed to estimate these local frequencies.…”