Due to its high sensitivity and fast measurement, the phase-sensitive optical time-domain reflectometer (φ-OTDR) is a good candidate for distributed dynamic strain sensing and has been widely used in intrusion monitoring, geophysical exploration, and other fields. In the case of frequency scan-based φ-OTDR, the phase change is expressed as a shift in the intensity distribution. The correlation between the reference and measured spectra is used for relative strain demodulation, which imposes a limitation on the absolute strain demodulation for continuous measurements. The Brillouin optical time domain analysis (BOTDA) fortunately enables the demodulation of the absolute strain in just one measurement. A combination of φ-OTDR and BOTDA is proposed and demonstrated in this work by using the same set of frequency-scanned optical pulses, and also introducing a frequency-agile technique for fast measurements. Measurements of 9.9 Hz vibrations were made at two different absolute strains (296.7 με and 554.8 με) with a strain range of 500 nε, made possible by integrating Rayleigh and Brillouin information. The submicron strain vibrations were demonstrated by φ-OTDR signals with a high sensitivity of 6.8 nε, while the absolute strains were measured by BOTDA signals with an accuracy of 5.4 με. Thus, the proposed sensor allows dynamic absolute strain measurements with high sensitivity, thus opening the door to new possibilities yet to be explored.
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