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.
A silicon cantilever-enhanced photoacoustic spectroscopy (PAS)based trace gas analyzer with low gas consumption is presented. A silicon cantilever-based fiber-optic Fabry−Perot (F−P) interferometric acoustic sensor with a compact structure and high sensitivity is designed for photoacoustic signal detection. The non-resonant photoacoustic cell (PAC) is a cylindrical copper tube with a volume of 0.56 mL. A near-infrared laser with a center wavelength of 1532.83 nm amplified using an erbium-doped fiber application amplifier is used as the excitation light. The wavelength modulation spectroscopy (WMS) technique is employed in the present work for second-harmonic photoacoustic signal detection. The experimental results show that the minimum detection limit of C 2 H 2 is 199.8 parts per trillion (ppt) with an average time of 60 s. The normalized noise equivalent absorption coefficient is calculated as 1.72 × 10 −9 cm −1 W/Hz 1/2 . Furthermore, the proposed silicon cantilever-enhanced non-resonant PAS-based gas analyzer can not only analyze the gas concentration in a closed small-capacity PAC with low gas consumption but also detect target gas leakage in real time at a long distance.
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