138 Simple Distributed Temperature Sensor based on Brillouin Gain Spectrum Analysis Marc NikIès, Luc Thévenaz, Philippe A. Robert Swiss Federal Institute of Technology, Metrology Laboratory CH-1015 Lausanne, Switzerland Brillouin gain spectrum measurement along an optical fiber has recently gained a lot of interests owing to its potentiality for strain monitoring in installed cables [1] and distributed temperature sensing [21. This measurement requires two lightwaves propagating in opposite directions through the fiber, since Stimulated Brillouin Scattering (SBS) amplification is possible only in the backward direction. One lightwave pumps the medium and the other acts as a probe signal, experiencing amplification when it lies within the Brillouin gain spectral range. The Brillouin Gain Spectrum (BGS) is downshifted with respect to the pump light frequency by the Brillouin frequency shift vB which takes values from 11.5 to 13 GHz depending on the fiber refractive index profile at a pump wavelength near 1300 nm. This frequency shift is given by VB = 2flVaI2, where 14 is the acoustic velocity within the fiber, n is the refractive index and . the wavelength of the incident lightwave. The temperature dependence of VB makes SBS suitable for temperature sensing. Therefore the issue of the sensing technique based on BGS analysis is to measure the BGS at different locations along the sensing fiber, because it gives access to VB and consequently to the temperature. Distributed measurements can be performed using a modified OTDR technique. Most of the methods reported so far use either two distinct laser sources [1,2], causing problems when the fiber ends are remote, or employs a single laser source at the expense of the setup complexity [31. Cleaved- Fig.1: Experimental setup used for distributed temperature measurement. A novel configuration is presented here, based on the pump and probe technique, that has the following advantages: 1) few optical elements are required, 2) a single laser source is used, 3) a 0-8 1 94-1699-1/94/$6.OO Pump Laser EOM end I Sensing Fiber Trig Downloaded from SPIE Digital Library on 01 Feb 2011 to 128.178.81.84. Terms of Use: http://spiedl.org/terms