High sensitivity miniaturized multi-pass absorption enhanced differential Helmholtz photoacoustic gas sensor

A high sensitivity photoacoustic (PA) gas sensor based on multi -pass absorption differential Helmholtz cell with a chamber volume of only 37 mL was proposed. The reported cell has an absorption path length of 2.5 m, which can effectively enhance the optical excitation. The differential Helmholtz structure can heighten the photoacoustic signal and effectively suppress ambient noise. To optimize the differential Helmholtz resonant PA cell, the PA field distribution of the cell was calculated by thermoviscous acoustic theory using the finite element method. The detection performance of the sensor was evaluated using a distributed feedback laser operating at a center wavelength of 1650.44 nm and targeting CH4 gas. Compared with a single pass, the PA signals were enhanced by 11 times in multiple pass mode. The differential mode suppressed 30 % of background noise. The calculated normalized noise equivalent absorption (NNEA) coefficient was 3.45 x 10-10 cm-1WHz1/2. The experimental results showed that the minimum detection limit (MDL) of CH4 was 6.6 ppb with an averaging time of 100 s.