A R TIC LE Random-access distributed fiber sensing

Optical sensing offers an attractive solution to the societal concern for prevention of natural and human-generated threats and for efficient use of natural resources. The unprecedented properties of optical fibers make them ideal for implementing a ’nervous system’ in structural health monitoring: they are small, low-cost and electrically and chemically inert. In particular, the nonlinear interaction of stimulated Brillouin scattering allows for the distributed measurement of strain and temperature with tens of km range. In this work, a novel, radar-inspired technique for random-access Brillouin scattering-based sensors is shown, making a significant step towards a real optical sensing nerve. The method selectively addresses each fiber segment as a distinct sensing element in a synaptic neuronal system. The measurement principle relies on phase-coding of both the Brillouin pump and signal waves by a high-rate, pseudo-random bit sequence. Temperature measurements with 1 cm resolution are reported. The measurement range is scalable to several km. LETER A R IC LE Random-access distributed fiber sensing Avi Zadok1,*, Yair Antman1, Nikolay Primerov2, Andrey Denisov2, Juan Sancho2,3, and Luc Thévenaz2 The precision localization of disturbances has been a mainstay of radar systems since WW-II. A widely employed radar technique relies on the transmission of long sequences of short pulses [1], and their subsequent processing by a pre-designed matched filter at the receiving end. The filtering procedure compresses the sequence of pulses to a temporally-narrow impulse response function, which provides high resolution together with a large signal to noise ratio [1]. Matched filters in radars are realized by correlating the received radar echoes against a replica of the transmitted sequence, which was stored as a reference. The correlation, in turn, is implemented either through ratio-frequency, electrical analogue mixing or via digital signal processing. Correlation coding has been introduced to Rayleigh scattering-based optical time-domain reflectometry, as early as 1989 [2]. In our work, we carry over the principle of match-filtering long pulse sequences to the realm of fiberoptic sensing [3–7], and use it for the unambiguous probing of a random locale. Stimulated Brillouin scattering (SBS) is an ideally suited platform for such random-access sensing system: not only is it inherently dependent on both strain and temperature [8–12], it also allows for the realization of correlation-based matched filtering of pulse sequences directly in the optical domain, as will be described in detail. In stimulated Brillouin scattering (SBS), a relatively intense pump wave interacts with a counter-propagating signal wave, which is detuned in frequency [8]. The combination of the two waves generates an intensity beating pattern, whose frequency equals the difference Ω between the two optical frequencies. Through electrostriction, the intensity beat introduces an acoustic wave, which in turn leads to a traveling grating of refractive index variations, due to the photo-elastic effect. The traveling grating can couple optical power between the pump and signal waves. Effective coupling, however, requires that Ω should closely match a fiber-dependent parameter known as the Brillouin frequency shift ΩB ∼ 2π ·11 GHz [8]. The bandwidth of SBS driven by a continuous pump wave is only 30 MHz, as decreed by the relatively long phonon lifetime τ ∼ 5 ns [8]. The value of ΩB varies with both temperature and strain [9–12], hence an assessment of the local ΩB may provide a distributed measurement of either of these two parameters [9–12]. SBS-based distributed measurements are commercialized for over 15 years. Most available systems rely on Brillouin optical time domain analysis (B-OTDA) techniques, in which the pump and / or signal waves are pulsed [13]. The power gain of the signal is observed as a function of both Ω and the time of arrival, signifying position. The spatial resolution of B-OTDA is fundamentally restricted to about 1 meter [14]: the use of pulses shorter than τ both reduces the SBS interaction length and broadens its linewidth [14]. Numerous techniques have been proposed in recent years for the enhancement of B-OTDA resolution [15, 16], such as the pre-excitation of the acoustic wave [17,18], dark [19] and π-phase [20, 21] pump pulses, repeated measurements with pump pulses of different widths [22], differentiation of the probe power [23] and many more. Such methods considerably increase the complexity of the B-OTDA instrument. The range and signal-to-noise ratios of long-reach B-OTDAs have been improved considerably by encoding 1 Faculty of Engineering, Bar-Ilan University, Ramat-Gan 52900, Israel 2 Ecole Polytechnique Fédérale de Lausanne, Institute of Electrical Engineering, SCI-STI-LT Station 11, 1015 Lausanne, Switzerland 3 Permanent address: iTEAM Institute, Universidad Politécnica de Valencia, 46022 Valencia, Spain * Corresponding author: e-mail: Avinoam.Zadok@biu.ac. il © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim L2 LASER & PHOTONICS REVIEWS A. Zadok et al.: Random-access distributed fiber sensing pulse sequences [24–27], however these methods did not address high resolution measurements. An alternative solution path is that of Brillouin optical correlation-domain analysis (B-OCDA). Rather than rely on pulses, it is based on the synchronous frequency modulation of constant-magnitude pump and signal waves, nominally detuned by Ω, by a common sine wave [28, 29]. The instantaneous difference in frequency between the two counterpropagating waves remains fixed at particular fiber locations only, known as correlation peaks, whereas the frequency difference elsewhere is oscillating [28]. Consequently, effective SBS amplification is confined to the narrow correlation peaks. However, the unambiguous measurement range is restricted to the separation between periodic correlation peaks. When simple sine-wave frequency modulation is used, the measurement range is on the order of a few hundred resolution points. The method had recently been extended to multiple modulation frequencies, resulting in 27 cm resolution over 1.5 km range [30]. B-OCDA also allows for dynamic strain analysis, monitoring vibrations of frequencies as high as 200 Hz [31]. In this work, we propose and demonstrate a new paradigm for SBS-based distributed sensing, which significantly extends its state of the art in terms of range versus resolution trade-off. The random-access sensing principle is illustrated in Fig. 1. Similarly to matched-filtering processing in radars, both Brillouin pump and signal waves are jointly phase-modulated by a common binary pseudo random bit sequence (PRBS), whose symbol duration T is much shorter than τ [32]. The modulation phase within each symbol assumes a value of either 0 or π , with equal probabilities. Suppose at first that the phase modulation is synchronized so that the two waves are of equal phases at their respective entry points. The instantaneous driving force for the SBS acoustic field generation is proportional to the product of the pump wave envelope and the complex conjugate of the signal wave envelope [8]. We may therefore distinguish between the dynamics of the SBS-induced acoustic field in two different regions. In the vicinity of the fiber center the pump and signal are correlated, hence their phase difference is constant and the driving force for the acoustic field generation keeps a steady non-zero value. Consequently, the acoustic field is allowed to build up to its steady state value, and permits the interrogation of the local Brillouin shift. The width of the correlation peak is on the order of Δz = 2 vgT , where vg is the group velocity of light in the fiber [32]. In all other locations, the driving force for the acoustic field is randomly alternating in sign on every symbol duration T τ . The acoustic field magnitude thus averages out to a zero expectation value, and the SBS interaction outside the correlation peak is largely inhibited [32]. The profile of the acoustic field that is generated in the fiber is equivalent to the impulse response function of a matchfiltered, spread-time radar sequence ( [1], see Fig. 1). The phase modulation broadens the power spectral density of the pump wave, and elevates the threshold of depletion due to spontaneous Brillouin scattering. The SBS interaction subject to the above modulation scheme is both localized and stationary, so that measureFigure 1 (online color at: www.lpr-journal.org) The principle of random access distributed fiberoptic sensing using SBS. a) Illustration of binary phase modulated Brillouin probe wave complex envelope, propagating in the positive z direction (top row). The sign of the optical field randomly alternates in between symbols through binary phase modulation. Bottom row: the SBS pump wave complex envelope, comodulated by the same binary phase sequence, and propagating in the opposite direction. b) The instantaneous driving force for the generation of the SBS acoustic field is proportional to the inner product between the pump envelop, and the complex conjugate of the signal envelope. Therefore, a constant driving force prevails at discrete peak locations only (center), in which the two replicas of the modulation sequence are in correlation. Elsewhere, the driving force is oscillating. c) The magnitude of the resulting acoustic field, obtained by temporal integration over the driving force (see supplementary information item 1 for more detail). The Brillouin interaction performs a match filtering operation directly in the optical layer. ment noise can be averaged out over detection intervals of choice. The separation between adjacent correlation peaks equals 2 MvgT , where M is the PRBS length. The unambiguous measurement range can be made arbitrarily long, while retaining the above high resolution Δz [32]: PRBS generators readily provide se