Photoacoustic methane detection inside a MEMS microphone

Thomas Strahl; Jonas Steinebrunner; Christian Weber; Jürgen Wöllenstein; Katrin Schmitt

doi:10.1016/j.pacs.2022.100428

Photoacoustic methane detection inside a MEMS microphone

Photoacoustics. 2022 Dec 1:29:100428. doi: 10.1016/j.pacs.2022.100428. eCollection 2023 Feb.

Authors

Thomas Strahl^{1

2}, Jonas Steinebrunner², Christian Weber^{1

2}, Jürgen Wöllenstein^{1

2}, Katrin Schmitt^{1

2}

Affiliations

¹ Laboratory for Gas Sensors, Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 102, Freiburg, 79110, Germany.
² Department of Gas and Process Technology, Fraunhofer Institute for Physical Measurement Techniques IPM, Georges-Köhler-Allee 301, Freiburg, 79110, Germany.

Abstract

An innovative laser based photoacoustic (PA) gas sensing concept with intrinsic miniaturization potential was developed and investigated for methane trace gas detection. An interband cascade laser (ICL) with an optical power of 8.5 mW targets a methane (CH₄) absorption line feature around 3057.7 cm^-1 (or 3270 nm). The ICL was focused into the sound port of a MEMS microphone, where the PA signal was generated and detected using a wavelength modulation concept (2f-WMS-PAS). The MEMS microphone was successfully implemented as an intrinsically miniaturized PA cell being gas sensing volume, acoustic resonator and sound transducer at once. Frequencies between 2 kHz and 100 kHz were investigated and used for methane detection. A sensitive and resonant methane detection at 41.8 kHz was investigated by concentration variations between 0 and 10 ppm CH₄ in N₂. A limit of detection ( $3 σ$ -LOD) of 329 ppb was estimated. The long term stability of this sensor was investigated by the measurement of methane in ambient air. A noise equivalent concentration (NEC) of 14 ppb (parts per billion) at an average time of 10 s was estimated. This value corresponds to a normalized noise equivalent absorption (NNEA) of $2 \cdot 1 0^{- 8}$ W cm^-1 Hz^-1/2. Using the MEMS microphone directly as PA cell offers the possibility for an extremely miniaturized, highly sensitive and very cost-efficient photoacoustic trace gas sensor.

Keywords: Gas sensing; Laser spectroscopy; MEMS microphone; Methane; Photoacoustic.