Dual-gas quartz-enhanced photoacoustic spectroscopy sensor exploiting two fiber-combined interband cascade lasers

In this work, a novel indium fluoride glass 2-input-1-output fiber combiner was designed and fabricated to combine two Interband Cascade Laser (ICL) sources emitting in the mid-infrared wavelength range. To test the combiner performance, a dual-gas quartz-enhanced photoacoustic spectroscopy sensor was demonstrated for the detection of carbon dioxide (CO₂) and nitric oxide (NO), employing two fiber-coupled ICLs having central emission wavelengths of 4,234 nm and 5,263 nm, respectively. The laser beams were coupled via the fiber combiner and then focused into a commercial acoustic detection module equipped with an input fiber-port, thus resulting in a plug-and-play sensing system. Tens of ppm-level detection limits at 3σ are achieved for both pollutants with a lock-in integration time (τ) of 0.1 s. Finally, an Allan-Werle analysis demonstrated the stability of the sensor, allowing the achievement of detection limit of 13 ppm and 4 ppm at τ = 10 s for CO₂ and NO, respectively.