Sandwiched Long-Period Fiber Grating Fabricated by MEMS Process for CO₂ Gas Detection

This paper presents an optical fiber gas sensor based on sandwiched long-period fiber grating (SLPFG) that is fabricated via the microelectromechanical systems (MEMS) process and coated with amino silica adsorbent for carbon dioxide (CO₂) gas sensing. The amine-modified nanoporous silica foams were coated onto the SLPFG for CO₂ adsorption. To characterize the CO₂ adsorption of the SLPFG sensor, a gas sensing test was conducted with a mixed gas consisting of 15% CO₂ and 85% nitrogen at a flow rate of 0.2 L/min. The results showed that the spectra of the SLPFG were varied with the gas flow within 21 min. After that, the transmission spectra of the SLPFG held steady and exhibited no further change. This phenomenon was caused by the adsorption saturation of the amine-modified nanoporous silica foams which were coated onto the SLPFG. During the absorption process, the transmission was increasing by about 11.27 dB (from -23.11 to -11.84 dB), and the increasing rate of transmission was 0.4598 dB/min. Repeatable adsorption and desorption experiment results showed that the SLPFG CO₂ gas sensor exhibited good repeatability and a short response time. The recovery rate for each cycle was about 85%, and the required recovery time was short. Therefore, elaborated SLPFG gas sensor could potentially be used as a gas sensor for monitoring CO₂ adsorption in the context of various industrial, agricultural, and household applications.