152 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 1, JANUARY 1, 2014 Extending the Real Remoteness of Long-Range Brillouin Optical Time-Domain Fiber Analyzers Marcelo A. Soto, Xabier Angulo-Vinuesa, Sonia Martin-Lopez, Sang-Hoon Chin, Juan Diego Ania-Casta˜ non, Pedro Corredera, Etienne Rochat, Member, IEEE, Miguel Gonzalez-Herraez, and Luc Th´ evenaz, Senior Member, IEEE Abstract—The real remoteness of a distributed optical fiber sen- sor based on Brillouin optical time-domain analysis is considerably extended in this paper using seeded second-order Raman ampli- fication and optical pulse coding. The presented analysis and the experimental results demonstrate that a proper optimization of both methods combined with a well-equalized two-sideband probe wave provide a suitable solution to enhance the signal-to-noise ra- tio of the measurements when an ultra-long sensing fiber is used. In particular, the implemented system is based on an extended optical fiber length, in which half of the fiber is used for sensing purposes, and the other half is used to carry the optical signals to the most distant sensing point, providing also a long fiber for dis- tributed Raman amplification. Power levels of all signals launched into the fiber are properly optimized in order to avoid nonlinear effects, pump depletion, and especially any power imbalance be- tween the two sidebands of the probe wave. This last issue turns out to be extremely important in ultra-long Brillouin sensing to pro- vide strong robustness of the system against pump depletion. This way, by employing a 240 km-long optical fiber-loop, sensing from the interrogation unit up to a 120 km remote position (i.e., corre- sponding to the real sensing distance away from the sensor unit) is experimentally demonstrated with a spatial resolution of 5 m. Furthermore, this implementation requires no powered element in the whole 240 km fiber loop, providing considerable advantages in situations where the sensing cable crosses large unmanned areas. Index Terms—Brillouin scattering, distributed optic fiber sen- sor, distributed Raman amplification, optical pulse coding, strain and temperature measurements. Manuscript received April 29, 2013; revised October 14, 2013 and November 8, 2013; accepted November 18, 2013. Date of current version De- cember 6, 2013. This work was supported in part by the Spanish Ministry of Sci- ence and Innovation through projects TEC2009–14423-C02–01 and TEC2009– 14423-C02–02 and the Comunidad de Madrid through Project FACTOTEM-2. The work of S. Martin-Lopez was supported by the Spanish Ministry of Science and Innovation through a “Juan de la Cierva” Contract. The work of M. A. Soto and L. Th´ evenaz was supported by the Swiss Commission for Technol- ogy and Innovation Project 13122.1. The work of M. Gonzalez-Herraez was supported by the European Research Council through Starting Grant U-FINE Grant 307441. M. A. Soto and L. Th´ evenaz are with the Swiss Federal Institute of Technology of Lausanne, Institute of Electrical Engineering, 1015 Lausanne, Switzerland (e-mail: marcelo.soto@epfl.ch; luc.thevenaz@epfl.ch). X. Angulo-Vinuesa, J. D. Ania-Casta˜ non, and P. Corredera are with the Insti- tuto de ´ Optica, Consejo Superior de Investigaciones Cient´ıficas, Madrid 28006, Spain (e-mail: xabier.angulo@focustech.eu; juan.diego@io.cfmac.csic.es; pcorredera@io.cfmac.csic.es). S. Martin-Lopez and M. Gonzalez-Herraez are with the Departamento de Electr´ onica, Universidad de Alcal´ a, Edificio Polit´ ecnico, Madrid 28871, Spain (e-mail: sonia.martinlo@uah.es; miguelg@depeca.uah.es). S.-H. Chin and E. Rochat are with Omnisens SA, 1110 Morges, Switzerland (e-mail: sanghoon.chin@omnisens.com; etienne.rochat@omnisens.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2013.2292329 I. INTRODUCTION T HE demand on distributed fiber optic sensors based on Brillouin optical time-domain analysis (BOTDA) [1]–[3] has been increasing over the past years in many application fields due to their unique capability to monitor distributed strain [1] and temperature [2] changes over many tens of km of optical fiber with metric spatial resolution. BOTDA sensors make use of two counter-propagating optical waves, a pulsed pump wave and a continuous-wave (CW) probe signal, which interact with an acoustic wave generated within the fiber through stimulated Brillouin scattering (SBS) [3]. Since pump and probe signals must propagate in counter-propagating directions along the op- tical fiber, the BOTDA interrogation unit requires access to both fiber ends. Therefore, BOTDA sensors can only exploit the en- tire sensing fiber whenever the fiber ends are at close distance, typically in bi-dimensional or three-dimensional configurations, e.g., for monitoring civil structures; however, for applications in which a linear fiber configuration is required, e.g., for long pipelines or offshore monitoring, the real remoteness of the sen- sor, and hence, the useful sensing distance, is restricted to half the fiber length. A potential scheme to increase the remoteness of the sensor is the use of a single-end-access BOTDA [4]–[6]; however, such schemes are highly subject to optical noise and provide limited sensing performance. A more suitable alternative to monitor lin- ear structures along ultra-long distances is to double the length of the optical fiber connected to a standard two-end-access BOTDA sensing unit, while only half of the fiber is used for sensing pur- poses [7]. This way, the first half of the optical fiber can be used as a distributed sensing element, while the second half is only dedicated to transfer the probe signal up to the most distant point in the sensing fiber. In this paper this fiber arrangement will be hereafter called “linear sensing fiber configuration.” Although the sensing length is not increased in this fiber configuration, the real remoteness of the sensor can be doubled [7], allow- ing the BOTDA system to sense critical points located at much longer distances. Unfortunately, with such a scheme the probe power reaching the receiver is highly attenuated, and therefore, no sensing capabilities or eventually an extremely low sensing performance should be expected at long distances. Some techniques have been recently proposed in the liter- ature to extend the sensing length of BOTDA sensors operat- ing with the standard fiber configuration, challenging the well- known trade-off between spatial resolution and sensing range. In particular, optical pulse coding [8] and distributed first- and second-order Raman amplification [9]–[11] have made it 0733-8724 © 2013 IEEE