Carbon Black Absorption Enhanced Fiber-Optic Photoacoustic Gas Sensing System with Ultrahigh Sensitivity

A highly sensitive trace gas sensing system based on carbon black absorption enhanced photoacoustic (PA) spectroscopy (PAS) is reported. A carbon black sheet and a fiber-optic cantilever microphone (FOCM) are integrated to form a fiber-optic cantilever spectrophone (FOCS). The gas concentration is obtained by measuring the acoustic wave amplitude generated by the carbon black sheet, which absorbs the laser passing through the interest gas. Due to the higher laser absorption rate of carbon black than that of flake graphite, the excited solid-state PA pressure wave is enhanced. The ability of the FOCS to detect weak sound waves is related to the laser power and the absorption length. Therefore, an Erbium-doped optical fiber amplifier and a multipass cell are also used to increase laser absorption by the tested gas, which combines with the FOCM to achieve multimechanism enhancement of the system performance. Different from traditional PAS gas detection systems, this system is a noncontact measurement solution, which not only effectively avoids gas flow noise but also makes the sensing element immune to the damage of corrosive gases. The experimental results show that the sensitivity of the system is about four times higher than that of the system using flake graphite as the light-absorbing element. When the average time is 100 s, the minimum detection limit of acetylene is 0.31 ppb. The normalized noise equivalent absorption coefficient of the designed PA system is achieved to be 7.1 × 10^-10 cm^-1·W·Hz^-1/2.