Fiber-optic trace gas sensing based on graphite excited photoacoustic wave

A fiber-optic trace gas sensing system based on photoacoustic (PA) effect of solids is reported. The modulated laser light absorbed by the gas is incident on a graphite sheet. The graphite sheet and cantilever are enclosed together to form an optical power detector based on PA effect of solids. This scheme is different from the traditional PA trace gas detection, achieving the separation of the gas chamber and the PA detector, thereby isolating the interference of airflow and environmental noise. In addition, due to the large size of the graphite sheet, the laser incident on the graphite sheet does not need to be converged through a lens, which makes it much easier to adjust the optical path. The PA pressure wave generated by the graphite sheet leads to the vibration of the optical interferometric cantilever. A fiber-optic white-light interferometer is used for high-resolution cantilever readout to detect the PA signal generated by solid excitations. The trace acetylene gas is used to verify the system performance. The experimental results indicate that when the averaging time is 100 s, the minimum detection limit (MDL) is 24 ppb. The normalized noise equivalent absorption (NNEA) coefficient of the designed PA system is 5.4×10−9 cm−1·W·Hz−1/2. This gas sensing system has the characteristics of intrinsic safety and high sensitivity, which can realize remote measurement of corrosive, flammable and explosive gases.