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) p ower and frequency shift in optical fibers is discussed, Equations gov erning the growth of the IVIES signal are derived and from these, we c alculate the dependence of the Brillouin power on temperature and stra in, We study the potential problem given by the need to normalize the nonlinear Brillouin signal and present a new technique that solves thi s problem by mathematically combining the values of the Stokes and ant i-Stokes powers to produce a linear effective power, Experimental resu lts are presented that support this theory and allow the verification of the coefficients governing the dependence of the Brillouin power an d 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 perform ance and that an optimum value for the Brillouin gain factor can be de termined, A simultaneous distributed temperature and strain sensor is demonstrated; preliminary results show a strain resolution of 100-mu m strain, a temperature resolution of 4 degrees C, and a spatial resolu tion of 40 m, over a sensing length of 1200 m.