Photoacoustic methane detection inside a MEMS microphone

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 N-2. A limit of detection (3 sigma-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 center dot 10(-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.

Automated summary

An innovative laser based photoacoustic gas sensing concept with intrinsic miniaturization potential was developed and investigated for methane trace gas detection. The concept uses an interband cascade laser to target a specific methane absorption line feature. A MEMS microphone is implemented as an intrinsically miniaturized PA cell, serving as the gas sensing volume, acoustic resonator, and sound transducer. The sensor shows sensitive methane detection and good long-term stability, with a noise equivalent concentration of 14 ppb.