Resolving 1 million sensing points in an optimized differential time-domain Brillouin sensor

Abstract

A differential pulse-width pair (DPP) Brillouin distributed fiber sensor is implemented to achieve centimetric spatial resolution over distances of several kilometers. The presented scheme uses a scanning method in which the spectral separation between the two probe sidebands is kept constant, while the optical frequency of the pump is swept to scan the Brillouin spectral response. Experimental results show that this method avoids detrimental temporal distortions of the pump pulses, which in a standard implementation prevent the DPP method from operating over mid-to-long distances. Such a novel scanning procedure allows the resolving, for the first time in pure time-domain Brillouin sensors, of 1,000,000 sensing points, i.e., 1 cm spatial resolution over 10 km in a conventional acquisition time. Brillouin-based distributed optical fiber sensors have become an established sensing technology due to their ability to measure strain/temperature changes along tens of kilometers of fiber, making them very suitable to monitor large civil infrastructures. In particular, in Brillouin optical time-domain analysis (BOTDA), the interaction of two counter-propagating signals, a pulsed pump, and a continuous-wave (CW) probe signal allows for measuring the temperature-and straindependent Brillouin gain/loss response of the sensing fiber with a spatial resolution given by the pump pulse width [1]. The probe wave is most commonly generated by making use of double-sideband (DSB) intensity modulation to mitigate the depletion of the pump pulse power, particularly in long-range schemes [2]. However, it has been recently proven that DSB schemes also bring massive spectral and temporal distortion on the pump pulse at high probe powers, inducing severe deformations in the Brillouin gain and loss spectra, and ultimately impairing the Brillouin frequency shift (BFS) determination [3]. Such a distortion results from asymmetric gain and loss, affecting the pump pulse in the conventional probe sideband frequency scanning, and scales directly with the probe wave power. To operate in a safe range, i.e., within a power range that secures negligible spectral distortions, the probe power in such a BOTDA scheme is limited to roughly −5 dBm per sideband [3], thus limiting the best attainable signalto-noise ratio (SNR) and measurand precision [4].On the other hand, the non-instantaneous (∼10 ns) response time of the acoustic wave generated at each fiber location imposes another limitation to the conventional BOTDA sensing technique: it impacts the spatial resolution, which cannot be reliably improved below 1 m, using an isolated interrogating pulse. To overcome this limitation and achieve submetric spatial resolution, different approaches have been proposed based on frequency, correlation [5][6][7][8][9], or time-[10-12] domain approaches. All of these techniques aim at increasing the number of resolved sensing points in the fiber. Frequency-and correlation-domain approaches allow very sharp spatial resolutions (in the order of millimeters or a few...

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