Abstract
Distributed measurement of forward stimulated Brillouin scattering (FSBS) attracted substantial attention for its ability to probe media surrounding optical fibers. Currently, all techniques extract the information from the FSBS-induced local energy transfer among distinct optical tones, this transfer being fundamentally sensitive to intensity perturbations imposed by nonlinear effects. Instead, here we propose to extract the local FSBS information by measuring the frequency shift of a short optical pulse subject to the phase chirp modulation caused by harmonic FSBS oscillation. In full contrast with existing techniques, the optical pulse is much shorter than the period of the acoustic oscillation, enabling ultrashort spatial resolutions, and its frequency shift is precisely probed by a standard Brillouin optical time-domain analyzer. The proposed technique is validated in both remote and integrally distributed sensing configurations, demonstrating spatial resolutions of 0.8 m and 2 m, respectively, substantially outperforming state-of-the-art techniques.
Highlights
Forward stimulated Brillouin scattering (FSBS) designates the acousto-optic physical process in which two co-propagating light waves couple via the compressive oscillations in their hosting medium of finite cross-section [1,2,3]
Note that forward stimulated Brillouin scattering (FSBS) is weak in the 200 m single-mode fibers (SMF) as the thick layer of acrylate coating (∼125 μm) severely dampens acoustic waves [2,10], so that only the 30 m bare fiber is suitable for sensing the surrounding material. This is by far a commonly used approach to evaluate the performance of a distributed FSBS sensor [14,15,16,17], though most of the FUT is practically insensitive. This may prevent some detrimental effects accumulating with FSBS interaction, which may turn a critical limiting factor for integrally distributed sensors where all fiber positions are subject to strong FSBS
The power of all optical waves sent into the FUT are carefully adjusted to maximize the signal-to-noise ratio (SNR) and FSBS efficiency, while remaining lower than the threshold of detrimental nonlinear effects, such as backward amplified spontaneous Brillouin scattering for the activating pulse [17] and modulation instability (MI) for the reading pulse [26]
Summary
Forward stimulated Brillouin scattering (FSBS) designates the acousto-optic physical process in which two co-propagating light waves couple via the compressive oscillations in their hosting medium of finite cross-section [1,2,3]. FSBS has drawn significant attention as a candidate to diversify the quantities probed by fiber sensors, by identifying the substance of a surrounding medium via its acoustic impedance [9] This method exploits oscillating transverse acoustic waves that convey information from the fiber surroundings to the light confined in the fiber core, requiring no structural modification of standard fibers. To mitigate the issue of the fragility of a bare fiber, a thin polyimide coating has been proposed as a promising protective
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