Recent progress in distributed Brillouin scattering fiber sensors (Invited) Moshe Tur , Avi Motil, Ido Sovran, and Arik Bergman Faculty of Engineering, Tel-Aviv University, Tel-Aviv 6997801, Israel. tur@post.tau.ac.il Abstract— An overview of recent progress in the field of distributed fiber-optic sensors based on stimulated Brillouin scattering (SBS) is presented. SBS sensors provide distributed measurements of temperature and strain, in which every segment of standard fiber serves as a sensing element. The fundamental measurement configuration was first proposed in the late 1980's, and commercial systems are available since the 1990's. The technology is being adopted by the oil and gas, energy, civil engineering and aerospace sectors. Examples of several recent and significant advances include the extension of the measurement range beyond 100 km; cm-scale spatial resolution over km-scale lengths; range and spatial resolution improvement, using elaborate radar coding techniques; the use of polarization- maintaining fibers to differentiate between strain and temperature; dynamic measurements of fast (~millisecond) events; and finally performance modeling and the emergence of guidelines and figures of merit. Keywords—Optical fiber sensors, Brillouin, distributed sensing. I. INTRODUCTION Brillouin distributed sensing, invented in the late 1980's, has been finally adopted commercially for many applications, mainly in the monitoring of long-range pipe lines, as well as in power and communication lines [1–3]. Distances of many tens of kilometers are reached, with primarily static measurements, having spatial resolution of the order of 1m. Recent years have also seen an impressive progress in all aspects of Brillouin distributed sensing. This paper aims at reviewing these advances. It starts with a basic introduction to Brillouin distributed sensing (Sec. II), followed by a discussion of the relevant important specifications and characteristics and how they have been improved (Sec. III). It also discusses dynamic Brillouin distributed sensing, which is an emerging promising application area for this technology. II. BRILLOUIN DISTRIBUTED SENSING Brillouin scattering [4] in optical fibers is a nonlinear process, where a propagating guided wave interacts with longitudinal acoustic waves to produce back-scattered waves. To be efficient, energy and momentum conservation laws must be obeyed, resulting in a narrowband process, where an incident wave of frequency ν pump gives rise to two back– scattered waves: a lower frequency (Stokes) one and a higher frequency (anti-Stokes) one, whose frequencies are given by: c nV pump a B B pump Stokes anti B pump Stokes / 2 ; ; ν ν ν ν ν ν ν ν = + = - = - . (1) B ν is called the Brillouin Frequency Shift (BFS), n is the refractive index, c is the speed of light and a V is the speed of sound along the fiber. For pump waves at 1550nm, propagating in a standard single mode optical fiber, B ν is very close to 11 GHz and the two back-scattered bands are quite narrow: ~30MHz for a CW pump wave, Fig. 1 [1-3]. This natural linewidth is the inverse of the acoustic wave (acoustic phonon) lifetime in the fiber. Such a narrow linewidth requires the pump laser to be even narrower (<1MHz), i.e., highly coherent, for maximum interaction. Through its dependence on a V , the BFS, B ν , is sensitive to both temperature (1MHz/ 0 C) and strain (50MHz/1000με), thereby making Brillouin scattering a sensing technology [1-3]. Fig. 1: The Brillouin Gain Spectrum and Brillouin Frequency Shift Quite a few schemes have been developed for Brillouin- based optical fiber sensing. In a Brillouin Optical Time Domain Reflectometer (BOTDR) [5] the pump wave, injected from one end of the fiber, interacts with the ever-present thermally induced acoustic waves in the fiber (i.e., acoustic phonons) to generate a spontaneous Brillouin back-scattered wave. The optical spectrum of this frequency-shifted wave is then analyzed to find the frequency location of its peak, from which the BFS is deduced. Much like in radar systems, pulsing the pump wave makes it possible to map the BFS along the fiber, achieving true distributed sensing on a standard single mode optical fiber. While BOTDRs need access to only one side of the measured fiber, the optical power level of the backscattered wave is weak, and averaging of multiple measurements is required to reach a satisfactory Signal-to- Noise Ratio (SNR). This may cause long sensing times of up to several minutes. Much stronger signals can be achieved if both ends of the fiber are accessible. In the so-called Stimulated Brillouin Scattering (SBS) sensing process, two counter-propagating This work was supported by the Israel Science Foundation under Grant 1380/12. 978-1-4799-0162-3/14/$31.00 ©2014 IEEE