Real-time detection of multiple trace gases using fiber Bragg gratings combined with gas ionization

Trace levels of water vapor (H₂O), nitrogen (N₂), and methane (CH₄) in high-purity helium pose challenges for gas monitoring. In this work, we report a real-time, multi-species sensing approach that combines glow-discharge gas ionization with wavelength-selective fiber Bragg gratings (FBGs). Dedicated FBG channels were engineered with reflection peaks near ∼308 nm, 357.7 nm, and 431.4 nm to match the characteristic ionization emission of the -OH group (water), N₂, and C-H bonds (methane), respectively, producing distinct optical signals for each species while retaining the intrinsic sensitivity and electromagnetic-immunity of FBG devices. The system was evaluated using three helium mixtures with known impurity concentrations (2.99-20 ppm). For all three gases, the FBG outputs exhibited strong linearity versus concentration ( R² > 0.97). Limits of detection were 47 ppb (H₂O), 103 ppb (N₂), and 39 ppb (CH₄). These results demonstrate that ionization-assisted FBG spectroscopy is a highly linear, sensitive, and reproducible method for trace-impurity monitoring in helium.