Long-range temperature sensing based on forward Brillouin scattering in highly nonlinear fiber

•Long-range forward Brillouin scattering temperature sensor is proposed and performed.•The approach for quickly acquiring an optimal R0, m mode spectrum is presented.•Frequency shift of remote FBS sensing shows a linear correlation with temperature.•Impact of SMF length in the main sensing unit on the FBS gain spectrum is discussed.•Impact of SMF length in transmission channel on the FBS gain spectrum is discussed. We propose and experimentally implement a long-range forward Brillouin scattering (FBS) temperature sensor, in which a 25-m highly nonlinear fiber (HNLF) is included in a Sagnac ring and is employed as the main sensing unit, and a 40-km standard single-mode fiber (SMF) is used as the transmission channel to connect the main sensing unit to the data processing center. The approach for quickly acquiring an optimal R0, m mode spectrum is presented. After taking into account the signal-to-noise ratio, sensitivity, and Lorentzian spectral shape, the R0, 17 mode was selected to perform temperature sensing. The frequency shift of the long-range and local FBS sensing demonstrates a linear correlation with the temperature change from 30 ℃ to 60 ℃, with respective frequency shift-temperature coefficients of 73.1 kHz/℃ and 73.4 kHz/℃. Impact of the length of the SMF as the main sensing unit and the SMF as the transmission channel on the FBS gain spectrum has also been discussed, respectively. The proposed long-range FBS scheme is expected to find applications in the investigation of submarine oil and gas pipelines, as well as temperature gradients in underwater and mountainous environments.