A near-infrared C2H2/CH4 dual-gas sensor system combining off-axis integrated-cavity output spectroscopy and frequency-division-multiplexing-based wavelength modulation spectroscopy

Abstract

By combining frequency division multiplexing assisted wavelength modulation spectroscopy (FDM-WMS) and off-axis integrated-cavity output spectroscopy (OA-ICOS), a near-infrared (near-IR) dual-gas sensor system was demonstrated for simultaneous chemical gas-phase detection of acetylene (C₂H₂) and methane (CH₄). Two distributed feedback (DFB) lasers modulated at the frequency of 3 kHz and 4 kHz with an emitting wavelength of 1532 and 1653 nm were used to target two absorption lines, C₂H₂ at 6523.88 cm⁻¹ and CH₄ at 6046.95 cm⁻¹, respectively. A 6 cm-long cavity was fabricated, which reveals an effective path length of 9.28 m (@1532 nm, C₂H₂) and 8.56 m (@1653 nm, CH₄), respectively. Performances of the dual-gas sensor system were experimentally evaluated using C₂H₂ and CH₄ samples generated by an Environics gas mixing system. An Allan deviation of 700 parts-per-billion in volume (ppbv) for C₂H₂ with an averaging time of 200 s and 850 ppbv for CH₄ with an averaging time of 150 s was achieved for these two gas species. Dynamic measurements of a C₂H₂/CH₄ : N₂ mixture were performed for monitoring both C₂H₂ and CH₄ simultaneously. This dual-gas sensor has the merits of reduced size and cost compared to two separate OA-ICOS sensors and reveals the minimum detectable column density (DCD) compared to other reported C₂H₂ and CH₄ sensor systems.