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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. 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][4][5][6][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][9][10][11][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. LETTER ARTICLE RandomIn 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 ...

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A novel technique for the localization of stimulated Brillouin scattering (SBS) interaction is proposed, analyzed and demonstrated experimentally. The method relies on the phase modulation of two counterpropagating optical waves by a common pseudo-random bit sequence (PRBS), these waves being spectrally detuned by the Brillouin frequency shift. The PRBS symbol duration is much shorter than the acoustic lifetime. The interference between the two modulated waves gives rise to an acoustic grating that is confined to narrow correlation peaks, as short as 1.7 cm. The separation between neighboring peaks, which is governed by the PRBS length, can be made arbitrarily long. The method is demonstrated in the generation and applications of dynamic gratings in polarization maintaining (PM) fibers. Localized and stationary acoustic gratings are induced by two phase modulated pumps that are polarized along one principal axis of the PM fiber, and interrogated by a third, readout wave which is polarized along the orthogonal axis. Using the proposed technique, we demonstrate the variable delay of 1 ns-long readout pulses by as much as 770 ns. Noise due to reflections from residual off-peak gratings and its implications on the potential variable delay of optical communication data are discussed. The method is equally applicable to the modulation of pump and probe waves in SBS over standard fibers. ©2012 Optical Society of America References and links1. R. W. Boyd, Nonlinear Optics, 3rd edition, (Academic Press, 2008). 2. T. Kurashima, T. Horiguchi, and M. Tateda, "Distributed-temperature sensing using stimulated Brillouin scattering in optical silica fibers," Opt. Lett. 15(18), 1038-1040 (1990). 3. T. Horiguchi, T. Kurashima, and M. Tateda, "A technique to measure distributed strain in optical fibers," IEEE Photon. Technol. Lett. 2(5), 352-354 (1990). 4. X. Bao, D. J. Webb, and D. A. Jackson, "22-km distributed temperature sensor using Brillouin gain in an optical fiber," Opt. Lett. 18(7), 552-554 (1993). 5. M. Niklès, L. Thévenaz, and P. A. Robert, "Simple distributed fiber sensor based on Brillouin gain spectrum analysis," Opt. Lett. 21(10), 758-760 (1996). 6. T. Horiguchi and M. Tateda, "Optical-fiber-attenuation investigation using stimulated Brillouin scattering between a pulse and a continuous wave," Opt. Lett. 14(8), 408-410 (1989). 7. A. Fellay, L. Thevenaz, M. Facchini, M. Nikles, and P. Robert, "Distributing sensing using stimulated Brillouin scattering: Toward ultimate resolution," in

We experimentally demonstrate a novel technique to process broadband microwave signals, using all-optically tunable true time delay in optical fibers. The configuration to achieve true time delay basically consists of two main stages: photonic RF phase shifter and slow light, based on stimulated Brillouin scattering in fibers. Dispersion properties of fibers are controlled, separately at optical carrier frequency and in the vicinity of microwave signal bandwidth. This way time delay induced within the signal bandwidth can be manipulated to correctly act as true time delay with a proper phase compensation introduced to the optical carrier. We completely analyzed the generated true time delay as a promising solution to feed phased array antenna for radar systems and to develop dynamically reconfigurable microwave photonic filters. 5293-5300 (1996). 5. Y. Liu, J. Yang, and J. Yao, "Continuous true-time-delay beamforming for phased array antenna using a tunable chirped fiber grating delay line," Photon.

Abstract:We propose and experimentally demonstrate new architectures to realize multi-tap microwave photonic filters, based on the generation of a single or multiple dynamic Brillouin gratings in polarization maintaining fibers. The spectral range and selectivity of the proposed periodic filters is extensively tunable, simply by reconfiguring the positions and the number of dynamic gratings along the fiber respectively. In this paper, we present a complete analysis of three different configurations comprising a microwave photonic filter implementation: a simple notch-type Mach-Zehnder approach with a single movable dynamic grating, a multi-tap performance based on multiple dynamic gratings and finally a stationary grating configuration based on the phase modulation of two counter-propagating optical waves by a common pseudo-random bit sequence (PRBS).

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