Advanced NO Sensors on Notched Long-Period Fiber Gratings Covered by Mesoporous WO3

This paper presents a notched long-period fiber grating (NLPFG) for a gas sensor that was produced by dry etching and includes a refractive index sensitive layer of tungsten trioxide (WO3) thin film that was fabricated using the sol-gel process. This development expands the potential application of NLPFG technologies in reliable sensors offering fast detection at room temperature. We propose that spectral changes in the coupling of the fundamental guided mode to the cladding modes caused by nitrogen oxide (NO) gas were captured by the WO3 thin film, causing refractive index changes. A maximum transmission loss of 3.88 dB was obtained with a fiber grating depth of $16.1~\mu \text{m}$ and a diameter of $50~\mu \text{m}$ . The presented gas sensors can be regenerated by being heated to 200 °C and purging with nitrogen gas, but the results also demonstrated the repeatability of the gas sensors.