The Effect of Hygroscopic Coating in FBG’s Sensitivity and Linearity Improvement

Fiber Bragg grating (FBG) sensors are optical fiber sensors that utilize Bragg gratings imprinted in the core of the fiber to detect changes in temperature, strain, vibration, and other parameters. This paper explores coating an FBG with zinc oxide nanostructures synthesized using a hydrothermal method with hexamethylenetetramine (HMT) to enhance its humidity sensing performance. The ZnO-HMT coating increases the hygroscopicity and humidity sensitivity of the FBG sensor. Experimental results demonstrate the coated FBG has a humidity sensitivity of 0.002nm/%RH compared to 0.00009nm/%RH for an uncoated FBG across a 40-80%RH range. The ZnO-HMT coated FBG also has a 92% linearity. Scanning electron microscopy reveals the ZnO-HMT coating has a porous, island-like nanostructure on the FBG which aids water vapor adsorption. The coating causes an initial conditioning effect, requiring pre-exposure to stabilize the sensitivity over humidity cycles. Overall, the ZnO-HMT coated FBG shows significantly improved sensitivity and reliable response after conditioning, confirming that hygroscopic coatings can enhance FBG performance for humidity sensing.