Long-range distributed temperature and strain optical fibre sensor based on the coherent detection of spontaneous Brillouin scattering with in-line Raman amplification

We report an extended range distributed temperature and strain optical fibre sensor based on the coherent detection of spontaneous Brillouin scattering combined with Raman amplification. The Raman amplification was achieved within the sensing fibre using either co- or counter-propagating Raman pump configuration with respect to the probe pulse and experiments were conducted to investigate the optimum pump and probe power combination. Using Brillouin frequency shift measurements with co- propagating Raman pump configuration, a temperature resolution of 1.7 degrees C with a 20 m spatial resolution at 100 km was achieved. With the counter-propagating pump configuration, a temperature resolution of 5 degrees C with a 50 m spatial resolution at 150 km was achieved. Measuring both the power and frequency of the Brillouin signal, a simultaneous temperature and strain measurement was performed over 50 km using co- propagating Raman pump. Temperature and strain resolutions of 3.5 degrees C and 85 mu epsilon with 5 m spatial resolution were achieved.