Simultaneous detection of CO2/CH4 based on off-axis integrated cavity output spectroscopy and time-division-multiplexing-based wavelength modulation spectroscopy

A highly sensitive dual gases detection system was established for detecting the concentrations of carbon dioxide (CO2) and methane (CH4) simultaneously based on off-axis integrated cavity output spectroscopy (OAICOS) technology. Two Distributed Feedback (DFB) lasers with central wavelengths of 1580 nm and 1653 nm were used as the light sources of the system. A time-division-multiplexing-based wavelength modulation spectroscopy (WMS) method was used to scan the current of lasers for improving the signal-to-noise ratio of the system and getting the optimal modulation amplitudes of CO2 and CH4 simultaneously. In addition, the cavity mode noise was suppressed by radio frequency (RF) white noise loading and laser current to further improve the measurement accuracy. Allan deviation analysis indicates that measurement precision of 0.483 ppm for CO2 and 1.37 ppb for CH4 was achieved by averaging up to 993 s and 1093 s respectively. This compact and high-precision measurement system is suitable for measuring atmospheric greenhouse gases.