Embedded T-type multi-pass cell based fiber-optic photoacoustic gas sensor

An innovative technology that introduces a multi-pass cell (MPC) and a fiber-optic cantilever into an embedded T-type photoacoustic cell (T-PAC) is reported, in which the light beam is reflected back and forth between miniaturized annular reflectors for a total of 26 times. The enhancement factor of effective excitation power reaches about 20. The photoacoustic frequency responses of three different embedded and non-embedded T-PACs are simulated and experimentally tested. Based on the structural particularity of T-PAC, the antinode of the acoustic wave is located at the end of the resonator , which conflicts with the assembly of the MPC. Fortunately, an acoustic hole on an annular reflector with an outer diameter and an inner diameter of 6 mm and 2 mm is used for acoustic wave detection, which breaks through this technical barrier. The change of light spot size with the reflection base length of MPC under different curvature radius parameters is analyzed, which laid a theoretical foundation for realizing multi-reflection on the miniaturized annular reflectors. This proposed novel fiber-optic photoacoustic sensor based on an acoustic resonator of 120 mm and an embedded T-type multi-pass photoacoustic cell (ETM-PAC) achieves a detection limit of 0.4 ppb methane with an averaging time of 400 s, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 1.5×10 −10 Wcm −1 Hz −1/2 . Moreover, methane content in the atmosphere is tested at Dalian University of Technology. • An innovative technology that introduces a multi-pass cell (MPC) and a fiber-optic cantilever into an embedded T-type photoacoustic cell (T-PAC) is reported. • The photoacoustic frequency responses of three different embedded and non-embedded T-PACs are simulated and experimentally tested. • An acoustic hole on an annular reflector with an outer diameter and an inner diameter of 6 mm and 2 mm is used for acoustic wave detection. • This proposed novel fiber-optic photoacoustic sensor based on an embedded T-type multi-pass photoacoustic cell (ETM-PAC) achieves a detection limit of 0.4 ppb methane.