Humidity and Temperature Sensor Based on LPFG and Coupler in Hole-Assisted Dual-Core Fiber

A relative humidity (RH) and temperature dual parameter sensor based on a long-period fiber grating (LPFG) in a hole-assisted dual-core fiber (HADCF) is demonstrated. LPFG in the center core is written by a high-frequency-CO₂-laser and is used to measure the humidity. To improve the humidity sensitivity, LPFG is coated with polyvinyl alcohol (PVA)/polyethylene glycol (PEG) film. While the temperature is measured by the coupling between the center core and the suspension core, and the air hole is filled with refractive index matching liquid to increase the temperature sensitivity. LPFG and coupler are integrated in parallel in HADCF with the length of 3.3 cm. The humidity and the temperature sensitivities are 0.674 nm/%RH of 0.841 nm/°C for the LPFG and the coupler, respectively. The simultaneous measurement of temperature and humidity is achieved by establishing a two-parameter matrix. The sensor has the advantages of compact size and high sensitivity, suitable for environmental monitoring and industrial control fields.