Design of long optical path and resonant sphere-tube coupled photoacoustic cell

Abstract

Photoacoustic (PA) spectroscopy technology is widely used in the detection of various trace gases because it has some advantages such as zero background detection, strong reliability and high sensitivity. The capability of gas detection is directly affected by the performance of the PA cell as an important part of the PA detection device. A long optical path resonant sphere-tube coupled PA cell (STCell) was designed, which based on the spherical PA cell. The inner wall of the sphere was coated with diffuse reflection material as an integrating sphere, and the modulated light was reflected multiple times in the sphere to increase the optical path. In order to further amplify the PA signal, an acoustic resonance tube was coupled with the sphere. One-dimensional longitudinal resonance frequency of the tube was used to modulate light source, the maximum PA signal was got at the end of the tube away from the sphere. When the modulated light passed through the sphere, the structure was similar to the T-type PA cell (TCell). A PA experiment was carried out to compare the performance between TCell and STCell, and the gas to be measured was NO₂. The PA signal amplitude of STCell was 10 times that of TCell when the concentration of NO₂ was 10 ppm, and the limit of detection (LOD) (3σ) was 2ppb, which was reduced by 13.5 times. In this paper, long optical path with acoustic resonance was combined, which has reference significance for the design of trace gas PA cells.