Distributed fibre sensing using microwave heterodyne detection of spontaneous Brillouin backscatter
Brillouin scattering has been used for many years in optical fibre temperature and strain sensing applications, due to the dependence of both the power and the frequency shift of the scattered radiation on these two quantities. Simultaneous temperature and strain sensing is possible if both the power and frequency shift are measured. Much work has been published which presents either power or frequency shift measurements alone. Simultaneous measurement, however, has been less well documented, with all recent cases using direct detection of the backscattered power. As the input pulse power is increased, nonlinear processes ensue, compromising the accuracy of the sensor. These processes were investigated both experimentally and theoretically for a range of input pulse durations, focussing on their effect on Brillouin Power and frequency shift measurement.