Sub-ppb-level CH 4 detection by exploiting a low-noise differential photoacoustic resonator with a room-temperature interband cascade laser.

An ultra-highly sensitive and robust CH ₄ 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 CH ₄ 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 CH ₄ sensor.