Performance and compatibility evaluation of uniform, apodized, and titled Fiber Bragg Grating profiles with new HMT-enhanced SnO2 coating for optical-based humidity sensing improvement

Optical-based sensors have been widely used for various applications, including biosensing, civil structural health monitoring, and humidity sensing. Fiber Bragg Grating (FBG) is one of the optical-based sensing approaches that are commonly used in these applications. The effect of hygroscopic coating on different grating profiles, including uniform, apodized, and tilted FBG, was investigated by connecting the FBG to a light source and optical spectrum analyzer (OSA). The reflected wavelength of the FBG, captured by the OSA at increasing and decreasing relative humidity ranging from 40 to 80% RH were recorded. Tin dioxide (SnO2) is one of the metal oxides with a hygroscopic nature, which is suitable to act as a coating material for FBG. For improved sensitivity, the HMT-enhanced technique was applied in this study to modify the surface morphology of SnO2, increasing the porosity of the nanostructure for water adsorption and desorption. The result showed that the sensitivity and linearity of the FBG-based humidity sensor can be enhanced via HMT-enhanced SnO2 nanostructure coating onto uniform FBG. A sensitivity of 1.30 pm/%RH and 1.52 pm/%RH was reported during incremental and decremental RH, respectively, and a fitting coefficient of >0.97 was recorded. This approach demonstrated the feasibility of hygroscopic coating on FBG to enable broad applications across various humidity sensing industries such as agriculture, pharmaceutical, and semiconductors.

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