Photo acoustics

Photoacoustic trace gas analysis
© Fraunhofer IBP
Photoacoustic methods and detectors for trace gas analysis.

Gas analysis with photoacoustic methods

The applied research conducted by our photoacoustics experts focuses on the development of photoacoustic gas sensors and monitors. We also carry out important fundamental research in our laboratories to find ways of improving the capabilities of the photoacoustic detection method and to develop innovative photoacoustic sensors. The photoacoustic effect is based on the sensitive detection of acoustic waves. These are generated by the absorption of pulsed or modulated monochromatic light via transient local heating and expansion processes in a gas, liquid, or solid. This effect is due to the conversion of at least part of the absorbed light energy into kinetic energy of the gas molecules by energy exchange processes.

Light sources and photoacoustic measuring set-up

Due to the narrow width of their emission line, lasers are best suited as light sources for photoacoustic applications. Many practical applications, such as trace gas detection, require compact tunable laser sources. For this purpose, we use single-mode DFB (distributed feedback) near-infrared diode lasers with a high spectral resolution.

Detecting traces of gaseous nitrogen compounds

Depending on the laser source used, photoacoustics can be applied for high-resolution spectroscopy and, based on this, for the selective and sensitive detection of traces of  molecules. An example is the detailed investigation of the adsorption and desorption processes of polar molecules (e.g., ammonia on walls), which limit the detection sensitivity in this class of molecules. State-of-the-art laser techniques and optimized photoacoustic set-ups are used to analyze traces of gas in the ambient atmosphere, for example, to investigate harmful compounds in the nitrogen cycle. Other applications include the detection of dinitrogen monoxide molecules and polar nitrogen dioxide molecules with quantum cascade lasers, as well as the detection of formaldehyde in gas mixtures and ambient air.