High-sensitivity distributed relative salinity sensor based on frequency-scanning φ-OTDR

In this paper, a high-sensitivity distributed optical fiber salinity sensor based on frequency-scanning phase-sensitive optical time-domain reflectometry ( φ -OTDR) and polyimide-coated single-mode fiber is proposed. Distributed salinity sensing over an 1100 m polyimide-coated fiber with a 1 m spatial resolution was demonstrated, and a sensitivity of 782.4 MHz/(mol/L) was achieved with the salinity changing from 0 mol/L to 1.61 mol/L. Then the measurement accuracies of frequency shift and salinity were evaluated theoretically and experimentally. Both theoretical and experimental results show that the measurement accuracy deteriorates as the pulse width decreases, resulting in a trade-off between the spatial resolution and measurement accuracy. The measurement uncertainty of salinity is 0.022 mol/L in the case of 30 cm spatial resolution. And when the spatial resolution is set to be 2 m, the measurement uncertainty of salinity decreases to 0.005 mol/L. The response time of the fiber to external salinity change has also been investigated, and it takes about 8 minutes for the fiber to reach a stable state. The proposed salinity sensor exhibits high sensitivity and long measurement range, which may be used for distributed marine environmental monitoring.