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

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.