Highly sensitive near-infrared gas sensor system using a novel H-type resonance-enhanced multi-pass photoacoustic cell

A dual-gas sensor system was developed using a H-type resonance-enhanced multi-pass photoacoustic (PA) cell. The sensitivity was improved by the double enhancement of optical path length extension and acoustic resonance. Two silver-coated concave mirrors with a reflectivity of > 95% were fabricated to form a multi-pass PA cell. The laser beam was reflected 24 times to obtain an optical path length of ∼ 3.6 m. The sensor performance was evaluated using a 1572.34 nm laser for CO2 measurement and a 1653.73 nm laser for CH4 measurement. The PA signal amplitude is enhanced by a factor of >10. The limits of detection of CO2 and CH4 are respectively 20.83 parts per million by volume (ppmv) and 0.43 ppmv. The normalized noise equivalent absorption (NNEA) coefficients are 3.02×10-9 cm-1‧W‧Hz-1/2 for CO2 and 9.75×10-9 cm-1‧W‧Hz-1/2 for CH4. Online in-situ gas analysis in a coal mine verifies the practicability of the sensor system.