Cover Story：Wang B Z, Ba D X, Chu Q, Qiu L Q, Zhou D W et al. High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering. Opto-Electron Adv 3, 200013 (2020). 

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, and has already been widely used in intrusion monitoring, and 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 limit for the absolute strain demodulation. Recently, the research group of Professor Yongkang Dong from Harbin Institute of Technology has proposed and demonstrated the combination of the φ-OTDR and Brillouin optical time-domain analysis (BOTDA) by using the same set of frequency-scanning optical pulses, and moreover the frequency-agile technique is introduced for fast measurements. A 9.9 Hz vibration with a strain range of 500 nε has been measured under two different absolute strains (296.7 and 554.8 με), enabled 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.