We experimentally examine the validity of previous assumptions that the Brillouin coefficients describing the variation of the power of the spontaneous Brillouin and its frequency shift with respect to either strain or temperature are independent of the other parameter. Keywords: Optical fibre sensors, Brillouin scattering, distributed temperature sensors
INTRODUCTION AND THEORYResults of earlier attempts towards the characterization of the Brillouin frequency [1][2] and its power to temperature and strain [3][4] influences, validated it as a strong contender for optical fiber distributed temperature and strain sensing. Since then there have been numerous contributions to the field of distributed sensing exploiting the dependence of the Brillouin scattering to temperature and strain [5][6][7][8][9][10] . The layout of sensing rigs for Brillouin scattering based distributed sensing to date, have comprised separate sections of sensing fiber subjected to either temperature or strain influences. Interpretation of such experiments have been based on the premise that the Brillouin coefficients used to describe the influence of strain or temperature remain valid under circumstances where a sensing fiber is subjected to the combined influence of both temperature and strain. This paper for the first time experimentally investigates this premise.
1.1The Brillouin coefficients The change in Brillouin frequency and power due to strain and temperature has been represented by the matrix equation Equation 2 highlights that to estimate strain or temperature would require the usage of the Brillouin coefficients which so far have been measured by varying only one parameter, for example strain, whilst keeping the other constant i.e. temperature or vice versa, and the assumption has been that these coefficients are independent of the other parameter. This paper is the first attempt to experimentally investigate this.
1.2Brillouin frequency measurements Brillouin frequency measurements were made with the well established Brillouin optical correlation domain analysis (BOCDA) technique [ 8 ] . In BOCDA a frequency swept laser output is split into two. One of which is frequency shifted by an amount close to the expected Brillouin frequency shift, while the other beam remains un-shifted. Both beams are then launched into the opposite ends of the sensing fibre. A strong Brillouin interaction results in a localized region of the fibre corresponding to zero delay between the counter-propagating beams of light. The peak Brillouin frequency corresponding to this localised region is obtained by scanning the frequency of the frequency shifter. Delaying the phase of one beam relative to the other allows the position of the localised Brillouin interaction region to be swept along the