High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering

The phase-sensitive optical time-domain reflectometry (φ-OTDR) is a good candidate for distributed dynamic strain sensing, due to its high sensitivity and fast measurement, which has already been widely used in intrusion monitoring, geophysical exploration, etc. For the frequency scanning based φ-OTDR, the phase change manifests itself as a shift of the intensity distribution. The correlation between the reference and measured spectra is employed for relative strain demodulation, which has imposed the continuous measurement for the absolute strain demodulation. Fortunately, the Brillouin optical time domain analysis (BOTDA) allows for the absolute strain demodulation with only one measurement. In this work, the combination of the φ-OTDR and BOTDA has been proposed and demonstrated by using the same set of frequency-scanning optical pulses, and the frequency-agile technique is also introduced for fast measurements. A 9.9 Hz vibration with a strain range of 500 nε has been measured under two different absolute strains (296.7με and 554.8 με) by integrating the Rayleigh and Brillouin information. The sub-micro strain vibration is demonstrated by the φ-OTDR signal with a high sensitivity of 6.8 nε, while the absolute strain is measured by the BOTDA signal with an accuracy of 5.4 με. The proposed sensor allows for dynamic absolute strain measurements with a high sensitivity, thus opening a door for new possibilities which are yet to be explored.