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