High-sensitivity photoacoustic gas detector by employing multi-pass cell and fiber-optic microphone.

A high-sensitivity photoacoustic (PA) spectroscopy (PAS) system is proposed for dual enhancement from both PA signal excitation and detection by employing a miniaturized Herriott cell and a fiber-optic microphone (FOM). The length of the optical absorption path of the PA cell is optimized to ∼374 mm with 17 reflections. The volume of the PA cell is only 622 µL. The FOM is a low-finesse fiber-optic Fabry-Pérot (FP) interferometer. The two reflectors of the FP cavity are formed by a fiber endface and a circular titanium diaphragm with a radius of 4.5 mm and a thickness of 3 µm. A fast demodulated white-light interferometer (WLI) is utilized to measure the absolute FP cavity length. The acoustic responsivity of the FOM reaches 126.6 nm/Pa. Several representative PA signals of trace acetylene (C2H2) are detected to evaluate the performance of the trace gas detector in the near-infrared region. Experimental results show that the minimum detectable pressure (MDP) of the FOM is 3.8 µPa/Hz1/2 at 110 Hz. The noise equivalent minimum detection concentration is measured to be 8.4 ppb with an integration time of 100 s. The normalized noise equivalent absorption (NNEA) coefficient is calculated as 1.4×10-9 cm-1·W·Hz-1/2.