Distributed optical fiber humidity sensing system based on phase-sensitive optical time-domain reflectometer

Humidity monitoring is crucial for the safe storage of chemicals and medicines, the storage of concrete sealing and curing agents, and the long-term storage of seeds and grains. Most existing and humidity sensors are classified as point sensors.1 Although these sensors are widely used, they exhibit limitations in the continuous monitoring of large areas. This paper proposes an all-fiber sensing system based on Phase-Sensitive Optical Time-domain Reflectometer(Φ-OTDR) and introduces three sections of single-mode fiber coated with polyimide lithium formaldehyde as the sensing layer. This configuration aims to develop a humidity-sensitive sensor capable of measuring relative humidity and verifying its consistency and repeatability. Experimental results demonstrate that, with a spatial resolution set at 5m and an ambient temperature of 23°C, the relative humidity measurement sensitivity of a 7m-long humidity-sensitive optical fiber is 22.78rad/%RH*m (covering a relative humidity range of 30%RH to 80%RH). Compared to a 13m-long humidity-sensitive optical fiber, the sensitivity deviation is within 3%. With the system parameters held constant, the humidity of the environment consistently ranged from 35%RH to75%RH, and the phase difference change was found to be positively correlated with the humidity change. The method proposed in this article offers broad application prospects in food and drug preservation, weapons and ammunition storage, and other fields that require large-scale humidity monitoring.