Measurement with 2 m resolution using a Raman-assisted BOTDA sensor featuring 75 km dynamic range S. Martin-Lopez *a , F. Rodriguez-Barrios a , A. Carrasco-Sanz b , P. Corredera a , J. D. Ania-Castañon c , L. Thevenaz d , M. Gonzalez-Herraez e a Dept. de Metrología, Inst. de Física Aplicada, CSIC, Serrano 144, 28006, Madrid (Spain); b Dept. de Óptica, Fac. de Ciencias, UGR, C. Universitario Fuentenueva, 18071, Granada (Spain); c Dept. Imágenes, Visión y Óptica Física, Inst. de Óptica, CSIC, Serrano 121, 28006, Madrid (Spain); d Inst. of Electrical Engineering, EPFL. STI-GR-SCI Station 11, CH-1015, Lausanne (Switzerland); e Dept. de Electrónica, Universidad de Alcala, C. Universitario, 28871, Madrid (Spain) ABSTRACT We have used distributed Raman amplification to extend the measurement distance of a Brillouin Optical Time-Domain Analysis (BOTDA) sensor. We successfully demonstrate a dynamic range of 75 km with 2 meter spatial resolution. Keywords: Stimulated Brillouin scattering, Raman scattering, BOTDA, Raman amplification. 1. INTRODUCTION Distributed optical fiber Brillouin sensors are attractive solutions for the monitoring of temperature and strain in large structures [1,2]. The operating range of these sensors is typically of the order of 20-30 km (for 1-2 meter resolution), which limits their use in certain applications. This limitation is basically due to fiber attenuation, which reaches a minimum of about 0.2 dB/km in modern optical fibers at a wavelength of 1.5 μm. To achieve longer measurement ranges, previous works [3,4] have used distributed Raman amplification in the sensing fiber. However, relatively low resolution values (20-50 meters) were reported in those works. In many applications it is desirable to keep the resolution in the meter range while extending the working distance. In this work, we have studied different implementations for Raman amplification to extend the measurement distance of a Brillouin Optical Time-Domain Analyzer (BOTDA). We successfully demonstrate a dynamic range of 75 km while keeping the spatial resolution of the sensor at 2 meters. We demonstrate the measurement of a 2m hot-spot close to the end of the fiber. 2. EXPERIMENTAL SETUP BOTDA systems use two distinct counter-propagating single-frequency light waves in the fiber. One of these light waves (centered at f 0 ) is pulsed and is meant to generate some amplification on the probe; it is designated as the pump wave. The other one (centered around f 0 -υ B ) is a continuous wave, called the probe wave, which will be locally amplified by the pump pulse through stimulated Brillouin scattering (SBS). As the pump pulse travels down the fiber, it will induce a different amplification on the probe depending on the local value of the Brillouin shift. The local amplification of the probe wave will yield a time-dependent variation of the detected probe signal at the pump end. The amplification at each point will be maximized when the pump-probe frequency separation is exactly the Brillouin shift at that point. By scanning for this maximum at all positions, one can obtain a map of the Brillouin shift of the fiber across its whole length [5]. To improve the dynamic range of the BOTDA we propose to introduce Raman pumping from one or both sides of the sensing fiber. The objective of this study is to determine which pumping configuration provides the largest increase in the dynamic range and enables the longest measurement distance: co-, counter- or bi-directional propagation of the Raman pump with respect to the Brillouin pump wave. In Fig. 1 we show a schematic diagram of the experimental setup used for this purpose. The setup includes a conventional BOTDA working at 1550 nm. In this experiment, the Brillouin pump and probe waves are obtained from * soniaml@cetef.csic.es ; phone: +34 915 618 806: fax: +34 915 642 122 Fourth European Workshop on Optical Fibre Sensors, edited by José Luís Santos, Brian Culshaw, José Miguel López-Higuera, William N. MacPherson, Proc. of SPIE Vol. 7653, 76532T © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.866442 Proc. of SPIE Vol. 7653 76532T-1