Simultaneous methane and acetylene detection using frequency-division multiplexed laser absorption spectroscopy

A dual-gas optical sensor was developed using frequency-division multiplexed (FDM) scanned-wavelength modulation spectroscopy for simultaneous detection of methane (CH₄) and acetylene (C₂H₂). Two infrared lasers were combined to exploit C₂H₂ and CH₄ absorption lines centered at 6539.45 cm⁻¹ and 6057.09 cm⁻¹, respectively. Scanned-WMS with the second-harmonic detection (WMS-2f) and a compact astigmatic multipass gas cell (36-m path length in a 0.3 L volume) were employed to improve detection sensitivity. Simultaneous dual-gas detection was realized by modulating each laser at a different frequency (5 kHz for 1529 nm and 13 kHz for 1650 nm) and software lock-in amplifiers were used to obtain the corresponding WMS-2f signals from a single photodetector. The Allan deviation analysis was performed to assess the long-term stability and minimum detection limit (MDL) of the sensor system. The MDL of 0.92 ppm for CH₄ and 0.64 ppm for C₂H₂ was achieved at 0.02-s integration time, which can be further improved to 55 ppb for CH₄ and 10 ppb for C₂H₂ at an optimum integration time of 10-s and 15-s, respectively. Such a dual-gas sensor has a promising potential for detecting dissolved gases (CH₄ and C₂H₂) in transformer oil for fault diagnosis due to its high sensitivity, hardware simplicity, and fast temporal resolution.