Sub-ppb-level CH₄ detection by exploiting a low-noise differential photoacoustic resonator with a room-temperature...

An ultra-highly sensitive and robust CH₄ sensor is reported based on a 3.3 µm interband cascade laser (ICL) and a low-noise differential photoacoustic (PAS) cell. The ICL emission wavelength targeted a fundamental absorption line of CH₄ at 2988.795 cm^-1 with an intensity of 1.08 × 10^-19 cm/molecule. The double-pass and differential design of the PAS cell effectively enhanced the PAS signal amplitude and decreased its background noise. The wavelength modulation depth, operating pressure and V-T relaxation promotion were optimized to maximize the sensor detection limit. With an integration time of 90 s, a detection limit of 0.6 ppb was achieved. No additional water or air laser cooling were required and thereby allowing the realization of a compact and robust CH₄ sensor.