Simultaneous Detection of Dual-Component Gases Using a Single Multipass Photoacoustic Cell with Cross-Patterned Spots

An innovative photoacoustic (PA) sensor based on a single multipass photoacoustic cell with cross-patterned spots has been developed to achieve simultaneous detection of dual-component gases. The multipass cell (MPC) integrates two fiber collimators, each operating at a distinct wavelength, to couple the respective excitation beams into the cell concurrently, inducing multiple reflections and forming two perpendicularly oriented sets of elliptically distributed spots. This dual-coupling design allows each collimator to be independently optimized for its target gas, thus fully utilizing available laser power even when absorption lines are far apart. The capability of the sensor was validated by employing two lasers at 1651 and 2327 nm to selectively excite methane (CH₄) and carbon monoxide (CO), respectively. The concentration of water vapor (H₂O) in the gas sample was maintained at about 18,000 ppm to avoid the influence from de-excitation rate change. In addition, the cross-interference between CH₄ and CO was reduced to almost zero by optimizing the modulation currents. The developed system, confirming the effectiveness of the sensor in resolving and analyzing dual-gas mixtures, achieves minimum detection limits (MDLs) of 25 ppb for CH₄ and 240 ppb for CO at 1 s averaging time, with corresponding normalized noise equivalent absorption (NNEA) coefficients of 2.8 × 10^-10 and 5.2 × 10^-10 cm^-1 W Hz^-1/2, respectively. The response time of the system is measured as 10 s using 10 ppm of CH₄ and 100 ppm of CO, which evaluates the rapid-response capability of the developed system.