T-type differential Helmholtz photoacoustic sensor for ultra-sensitive gas detection with noise suppression

To achieve ultra-sensitivity gas detection, a photoacoustic (PA) gas sensor system was developed using a multi- pass T-type resonance-enhanced differential Helmholtz (MTRDH) PA cell. The MTRDH PA cell combines the advantages of both the T-type PA cell and the Helmholtz PA cell, increasing the intensity of PA signal, while greatly suppressing the background noise. The structure of the MTRDH PA cell was optimized by comprehensively considering the absolute sound pressure, resonant frequency and the quality factor. The performance of the sensor system was evaluated using a distributed feedback laser with a central wavelength of 1532.68 nm for acetylene (C₂H₂) detection. Two right-angle prisms were installed at both ends of the buffer cavity to form a multi-pass PA cell, achieving 4 reflections of incident light. Compared with the single-pass PA cell, the PA signal of the multi-pass PA cell increased by 3.49 times. The experimental results show that the sensor system has a good linear response to the C₂H₂ gas concentration, achieving a limit of detection of 105 ppb and a normalized noise equivalent absorption coefficient of 1.98 × 10⁻⁹ cm⁻¹ W Hz^−1/2.