Hot spot detection over 100 km with 2 meter resolution in a Raman- assisted Brillouin distributed sensor Xabier Angulo-Vinuesa 1 , Sonia Martin-Lopez 1 , Javier Nuño 1 , Pedro Corredera 1 , Juan Diego Ania-Castañon 1 , Luc Thévenaz 2 , Miguel Gonzalez-Herraez 3 1 Instituto de Óptica, CSIC, C/ Serrano 121, 28006, Madrid, Spain. 2 Inst. of Electrical Engineering, EPFL, STI-GR-SCI Station 11, CH-1015, Lausanne, Switzerland. 3 Dept. de Electrónica, Universidad de Alcalá, C. Universitario, 28871, Alcalá de Henares, Spain. ABSTRACT We have developed a long-range Brillouin distributed sensor featuring 100 km measuring distance with 2 meter resolution. To our knowledge, this is the first time that a high-resolution setup reaches the barrier of 100 km measurement range. The key improvements with respect to previous configurations are explained. Keywords: Brillouin scattering, distributed optic fiber sensor, Raman scattering, temperature sensor. 1. INTRODUCTION Brillouin Optical Time-Domain Analysis (BOTDA) allows distributed detection of temperature and strain along the fiber. The BOTDA was first proposed by Horiguchi and Tateda [1], [2] in the late 80s, and since then has evolved into a consolidated fiber sensing technology. The underlying physical phenomenon in this technology is a nonlinear optical effect called Stimulated Brillouin Scattering (SBS) [3] . This effect is an acousto-optic process which manifests as a narrowband amplification of a counter-propagating probe beam when an intense coherent pump light beam is introduced through one end of a single-mode fiber. If the source light is centered at f 0 frequency, the amplification of the counter- propagating beam occurs at a narrow spectral range around f 0 -v B . v B is known as the Brillouin shift of the fiber. In BOTDA, localization is achieved by pulsing the pump wave and analyzing the detected probe wave variations as a function of the time-of-flight of the pump pulse in the fiber. The resolution of the system is thus limited by the pump pulse width. The measurement process relies in mapping the variations of v B along the fiber, which depends on the fiber properties as:         a B nv v 2 (1) where n is the refractive index of the fiber, v a is the acoustic velocity along the fiber and  is the source wavelength. Thus, BOTDA systems can in principle measure all the physical parameters that modify the refractive index or the acoustic velocity of the fiber, being strain and temperature the most effective ones. Two of the most restrictive features of these systems are the measurement range and the spatial resolution. Generally, the measuring distance is limited to 20- 30 km, with 1-2 meter resolution [4] . The measurement range is limited by the fiber attenuation, which is approximately 0.2 dB/km in conventional fibers operating at 1550 nm. This attenuation causes a decay of both pump and signal powers as the distance grows, as well as an increase in the measurement uncertainty since the pump power is reduced with distance. At the same time, the resolution is set by the length of the pulses used to produce the Brillouin effect. Obviously, the objective is to acquire the highest possible resolution, but this requires the use of short optical pulses, which reduces both the effective distance for amplification and the amplification factor, while it increases the measurement uncertainty due to the associated spectral broadening of the interaction. Thus, generally, increasing the resolution strongly limits the measurement range. One may think that this problem can be circumvented by rising the pump power. However this approach is limited because pump depletion and other competing nonlinear effects become increasingly important. It is necessary then to find a proper balance between the length of the pulses and the required system specifications; short pulses for higher resolution or longer pulses for increased range. Several studies have proved that the fiber losses can be successfully compensated using distributed Raman amplification [5],[6],[7] , since it maintains the pump power to a level that guarantees a sufficient gain all over the fiber. This allows to have long-range systems with high resolution. In this work we present a first-order Raman assisted BOTDA sensor that features 100 km dynamic range 21st International Conference on Optical Fiber Sensors, edited by Wojtek J. Bock, Jacques Albert, Xiaoyi Bao, Proc. of SPIE Vol. 7753, 775309 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.885041 Proc. of SPIE Vol. 7753 775309-1 Downloaded from SPIE Digital Library on 23 May 2011 to 128.178.23.124. Terms of Use: http://spiedl.org/terms