What is photoacoustic spectroscopy (PAS)?

Identified in the 19th century, the photoacoustic effect allows the conversion of light into sound and has been employed in devices such as non-dispersive infrared absorption spectroscopy (NDIR), which was the first commercially available device to detect gases using this effect. Modulating the light intensity produces a periodic pressure variation, which is measured with a sound-detecting device like a microphone. Criteria to compare different implementations of the photoacoustic effect include selectivity, sensitivity and size.

With today’s efforts for increasingly efficient buildings, inherently poor airflow occurs. This lack of air flow can result in depletion of oxygen levels and a build-up of carbon dioxide (CO₂) concentrations. However, the detection of harmful levels can prevent health problems. Based on photoacoustic spectroscopy (PAS), Infineon created its XENSIV PAS environmental sensor for sensing CO₂. The miniaturized sensor overcomes existing size, cost and performance challenges.

Using a highly sensitive MEMS microphone as the detector and an integrated printed circuit board (PCB) design reduced space requirements by more than 75% compared with commercially available real CO₂ sensors. Integration on the PCB (in a 14 x 13.8 x 7.5 mm² module) includes the PAS transducer and detector, an infrared source and optical filter, a microcontroller for signal and algorithm processing and a MOSFET chip to drive the infrared source.

The IM69D130 MEMS microphone in the sensor detects the pressure changes generated by the CO₂ molecules with a signal-to-noise ratio (SNR) of 69 dB(A).

In addition to smart home and building automation applications such as demand-controlled ventilation systems, air purifiers and thermostats, the small size and low cost of the PAS sensor makes it applicable for in-cabin air quality monitoring in vehicles.