High sensitivity fiber Bragg grating humidity sensor based on Agar/PVA composite film

• Fiber Bragg grating humidity sensor using Agar/PVA as composite film is proposed firstly. • The sensor achieves high sensitivity with an average of 21.6pm/%RH, and its response time is 18s. • Sensitivity comparison of sensors made from 80 μm remaining-cladding polished SMF and 80 μm thin-clad SMF. • The sensor have good repeatability and exhibit excellent stability over measurements of up to 60 days. A fiber Bragg grating (FBG) humidity sensor was developed using an Agar/polyvinyl alcohol (PVA) composite film (4:1 ratio). Initial sensitivity reached 10.8 pm/%RH (25 %–85 %RH) with a 24 s response time. Sensitivity enhancement was achieved by polishing hydrogen-loaded single-mode fiber (SMF) claddings to 105.4 µm and 90.1 µm before FBG inscription and film coating, yielding 13.7 pm/%RH and 17.1 pm/%RH, respectively. Further optimization involved polishing the SMF claddings to 80 μm and utilizing a hydrogen-loaded thin-clad SMF with an 80 μm cladding. Comparative testing showed average sensitivities of 21.6 pm/%RH (polished SMF) and 20.7 pm/%RH (thin-clad SMF), demonstrating superior sensitivity for the polished SMF. The optimized sensor (80 µm polished SMF) achieved an 18 s response time, excellent repeatability, and sustained stability throughout up to two months. Temperature cross-sensitivity was measured at 0.41 %RH/°C, mitigated via cascaded FBG temperature compensation.