Simultaneous independent distributed strain and temperature measurements over 15 km using spontaneous Brillouin scattering

Long range simultaneous distributed strain and temperature sensors have many applications for measurements in the power and oil industries and also for structural monitoring. We present an efficient technique to measure both the intensity and frequency shift at every point along the sensing fibre with a low loss filtering device utilising two in-fibre Mach-Zehnder interferometers. From these two measurements, it is possible to compute accurately the strain and temperature profile. The first interferometer was used in a double-pass configuration and served to separate the Brillouin from the Rayleigh signal, the second allowed the frequency shift to be determined. It is possible to tune the Mach-Zehnder such that the launched source signal (and hence the Rayleigh) lies at the maximum or minimum of the transfer function. The Brillouin backscattered signals for the condition of maximum and minimum throughput at the signal wavelength are summed thus obtaining a Brillouin backscattered intensity measurement independent of frequency shift. The measurement obtained when the Rayleigh lies at a minimum of the transfer function provides information of both Brillouin intensity and frequency shift variations which then allows the frequency shift to be determined. The temperature and strain resolutions were estimated to be 4°C and 290με with a 10m spatial resolution over a sensing range of 15km.