Simultaneous distributed measurement of strain and temperature from noise-initiated Brillouin scattering in optical fibers

The simultaneous determination of strain and temperature distributions from the measurement of noise-initiated Brillouin scattering (NIBS) power and frequency shift in optical fibers is discussed. Equations governing the growth of the NIBS signal are derived and from these, we calculate the dependence of the Brillouin power on temperature and strain. We study the potential problem given by the need to normalize the nonlinear Brillouin signal and present a new technique that solves this problem by mathematically combining the values of the Stokes and anti-Stokes powers to produce a linear effective power. Experimental results are presented that support this theory and allow the verification of the coefficients governing the dependence of the Brillouin power and frequency shift on temperature and strain. The signal-to-noise ratio of the sensor is discussed, and it is found that the noise associated with the field statistics plays a limiting role in the sensor performance and that an optimum value for the Brillouin gain factor can be determined. A simultaneous distributed temperature and strain sensor is demonstrated; preliminary results show a strain resolution of 100-μm strain, a temperature resolution of 4°C, and a spatial resolution of 40 m, over a sensing length of 1200 m