Edge technique for direct detection of strain and temperature based on optical time domain reflectometry

A hybrid technique for real-time direct detection of strain and temperature along a single-mode fiber is proposed. The temperature is directly detected from the Raman backscattering in the time domain. To retrieve the strain profile from the Brillouin backscattering, an edge technique is introduced and a response function of the Fabry-Perot interferometer for the Brillouin backscattering is defined for the first time to our knowledge. The outgoing laser and the Brillouin backscattering are measured on different interference orders through different channels of the Fabry-Perot interferometer. A low-resolution reference channel and a high-resolution Brillouin channel are designed to keep both a high measurement sensitivity and a wide dynamic range. The measurement is based on detecting the bandwidth changes and the frequency shifts of the Brillouin backscattering; thus the resulting measurement is insensitive to the power fluctuation of the backscattering and the laser frequency jitter or drift. Neither time-consuming frequency scanning nor heavy data processing is needed, which makes real-time detection possible. The dynamic range of the edge technique can be increased substantially by using a piezoelectric tunable and capacitive-servo-stabilized Fabry-Perot interferometer. We highlight the potential of this technique by numerical simulations. Given that the uncertainty of the temperature measurement is 0.5 degrees C and that the spatial and temporal resolutions are 10 cm and 1 s, the strain uncertainty is less than 20 microepsilon within a 2 km distance when the strain is below 0.4%, and it is not more than 110 microepsilon within a 4 km distance when the strain is below 0.6%.