Agarose-coated-FBG-based humidity sensing system based on the Vernier effect in an optoelectronic oscillator

In this paper, an agarose-coated fiber Bragg grating (AC-FBG)-based humidity sensing system with high sensitivity, leveraging the Vernier effect in an optoelectronic oscillator (OEO), is proposed and experimentally demonstrated. Changes in ambient humidity induce effective expansion and contraction of film morphology on the AC-FBG, leading to shifts of the Bragg wavelength. The Bragg wavelength variations of the AC-FBG can be converted to the changes of the microwave signal through the OEO system, effectively improving the resolution of sensing demodulation. The sensitivity of the proposed humidity sensing system is further enhanced by employing the Vernier effect in the OEO through the cascading of two FBGs, where tunable sensitivity can be achieved by adjusting the spacing between two FBGs. The proof-of-principle experimental results show that the sensitivity of the proposed AC-FBG-based humidity sensing system based on OEO without the Vernier effect is about 7.31 kHz/%RH, while the sensitivity based on the OEO with the Vernier effect is improved to 1.032 MHz/%RH, representing approximately 141.2 times higher than the system without the Vernier effect. The proposed scheme offers the advantages of simplicity, compact configuration, high resolution, and tunable sensitivity.