Ultra-High Sensitive Multipass Absorption Enhanced Fiber-Optic Photoacoustic Gas Analyzer
Xinyu Zhao, Ke Chen, Min Guo, Chenxi Li, Chenyang Li, Guangyin Zhang, Zhenfeng Gong, Zhi Zhou, Wei Peng
Abstract
We proposed and designed a high-sensitivity fiber-optic photoacoustic gas analyzer based on resonant Herriott multipass cell (MPC) and fiber-optic cantilever acoustic sensor, which realized the double enhancement of the excitation and detection of the photoacoustic signal. The chamber volume of the miniature photoacoustic cell was 157 mL. The laser was reflected 20 times and reached an optical path length of 4 m in the PA cell, achieving the excitation enhancement of the photoacoustic signal. We conducted a comparative experiment between the Herriott-MPC and the single-pass cell, and the results showed that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2f$ </tex-math></inline-formula> signal of the photoacoustic system based on Herriott-MPC was enhanced by about 16 times. By using a near-infrared (NIR) laser with a center wavelength of 1651 nm, the performance of the PA gas analyzer for CH4 detection was evaluated. Experimental results show that the minimum detection limit (MDL) of the system is achieved to be 3.7 ppb. Furthermore, the calculated normalized noise equivalent absorption (NNEA) coefficient is equal to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.3\times 10^{-10}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-1}\cdot $ </tex-math></inline-formula> WHz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-1/2}$ </tex-math></inline-formula> . The continuous monitoring of CH4 in the environment for five consecutive days proved the feasibility and stability of the trace gas analyzer.