A reconstruction technique for long-range stimulated Brillouin scattering distributed fibre-optic sensors: experimental results

Abstract

The experimental validation of a numerical technique for temperature/strain profile reconstruction based on Brillouin optical-fibre time-domain analysis (BOTDA) sensors is presented. In this approach, we search directly for the Brillouin frequency shift profile along the fibre that matches the measured data. The algorithm is based on a harmonic expansion of the unknown profile, whose coefficients are determined by means of a multidimensional minimization. Experimental measurements have been carried out in order to reveal the influence of nonlocalities in Brillouin measurements, and to prove the capability of the proposed algorithm to compensate for these effects.

Long-range Brillouin optical time-domain analysis sensor employing pulse coding techniques

“…In BOTDA sensors, a single-mode optical fiber is used as a distributed amplifier, where a pump wave transfers part of its energy to a counter-propagating probe signal through the interaction with acoustic phonons [1][2][3]. The probepump interaction actually creates a periodic perturbation of the refractive index of the fiber, thus reflecting part of the pump signal by Bragg diffraction and reinforcing the acoustic wave.…”

“…In BOTDA sensors, the spatial resolution is obtained by using a pulsed pump signal (launched at the fiber input, z = 0) and the local BFS is measured by sweeping the frequency offset within a range of some hundreds of MHz around the BFS. Thus, in order to reconstruct the Brillouin gain spectrum along the fiber, variations on the CW probe signal intensity ( I CW ) are measured at the fiber input (z = 0) as a function of time, t, and for different frequency offset, ν [3]:…”

“…Neglecting pump depletion, the expression for the pump intensity simplifies to I P 0 exp(−αξ ), where I P 0 is the input pump intensity [3]. From equations (2) and (3) we can note that when using short pulses, a short pump-probe interaction length takes place; as a consequence, the CW-intensity contrast decreases, reducing the SNR of the measurement [5,6].…”

“…Although equation (3) points out that the pump power could be increased in order to extend the sensing range, the maximum pump and probe powers are however limited by pump depletion [3] and modulation instability [7], effects which induce distortion in the reconstructed BGS. In fact, when using high pump power, the local pump intensity along the fiber, I P (ξ , ν), depends on the Brillouin interaction at every preceding position, inducing significant deviation of the measured BGS peak frequency with respect to the real local value [3]. This feature leads to systematic errors in the strain and temperature measurements, which increase with the distance and optical power.…”

“…The typical detection technique used in SBS-based sensors refers to the so-called Brillouin optical time-domain analysis (BOTDA) technique [1,3], which is based on the use of an optical pulse acting as a pump signal and a counter-propagating continuous-wave (CW) probe beam. Thus, by measuring variations of the Brillouin gain spectrum (BGS) [4] along an optical fiber, distributed strain and temperature measurements can be carried out.…”