Speckle Analysis Method for Distributed Detection of Temperature Gradients With $Φ $ OTDR

A method to extend the operation of traditional single-frequency phase-sensitive optical time-domain reflectometry ( phi-OTDR) to the monitoring of distributed temperature gradients along an optical fiber is proposed and experimentally validated. The measurement principle is derived from the perturbation response of a single-wavelength phi-OTDR signal, which is analyzed as a unidimensional speckle pattern. The method could be implemented in parallel to standard phi-OTDR systems used for distributed vibration sensing with a close to zero cost and without affecting its operation, as it only requires a low computational cost post-processing of the traces which are already acquired. Frequency scanning of the laser, heterodyning or additional hardware are not required. The distributed detection of a temperature gradient of 2.5 °C over 10 min is demonstrated.