High Sensitivity Fiber Bragg Grating Humidity Sensors Made With Through-the-Coating Femtosecond Laser Writing and Polyimide Coating Thickening

• Published in: IEEE sensors letters Vol. 8; no. 6; pp. 1 - 4
• Main Authors: Mihailov, Stephen J., Ding, Huimin, Szabo, Katherine, Hnatovsky, Cyril, Walker, Robert B., Lu, Ping, De Silva, Manny
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01-06-2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Bragg gratings; C band; Coatings; Femtosecond pulses; fiber bragg gratings (FBG); Fiber gratings; Germanium; Humidity; Infrared lasers; Optical fiber polarization; optical fiber sensor; Optical fiber sensors; Optical fibers; photonic sensors; Pulse duration; Relative humidity; relative humidity (RH) sensor; Sensitivity; Sensor integration; Sensors; Silicon dioxide
• ISSN: 2475-1472; 2475-1472
• DOI: 10.1109/LSENS.2024.3398432

Durable fiber Bragg grating (FBG) relative humidity (RH) sensors are fabricated in commercially available polyimide (PI) coated 50-µm diameter Ge-doped silica optical fibers using an infrared femtosecond pulse duration laser and a phase mask. FBGs are inscribed directly through the 10-µm thick PI coating, which is subsequently thickened with PI using a customized fiber recoating system. Devices with varying PI thicknesses are evaluated for their response to RH. For telecom C-band wavelengths, FBG sensor sensitivities vary from 2.4 to 19 pm/%RH for 10 to 60-µm thick PI coatings, respectively. The 19 pm/%RH response represents the highest sensitivity reported for a PI coated silica FBG RH sensor. RH sensors with thinner coatings respond more rapidly to small changes in humidity and show smaller hysteresis when compared with devices with thicker coatings.