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Infrared gas sensors with multi-line filters

Selective and sensitive sensors can be made by tailoring infrared filters to fit the absorption pattern of particular gases. SINTEF is developing tunable multi-line filters using micro-optical and MEMS technology. The resulting sensors can be produced at low cost in high numbers.

Each gas has its own unique spectral absorbance function. For some gases, including carbon monoxide, nitrous oxide, and methane, the absorbance function is in the form of quasi-periodic lines.

A slab of a suitable infrared optical material has a periodical transmittance function, and this function can be fitted to a particular gas by choosing the appropriate thickness. One example is a 350 µm silicon wafer for carbon monoxide. A high-index material such as silicon is necessery to achieve sufficient optical throughput.

To make a robust infrared gas measurement one must switch between a matching (correlation) and a non-matching (anti-correlation) filter, which in practice comes to modulating the optical thickness of the slab. We have devised two methods for accomplishing this modulation. One is based on including a variable airgap in the slab. The other uses a slab with a mosaic pattern of two thicknesses.

The variable airgap is currently being developed as a microelectromechanical system (MEMS). Carbon monoxide measurements using a manually assembled device have verified the sensing principle.

The mosaic filter has been successfully fabricated and tested. A gas sensor based on this filter requires a scanning device and/or dual detectors.


Measuring CO concentration with correlation and anti-correlation filters. The difference between measurements through silicon slabs/wafers (Fabry-Perot filters), having an optical pathlength difference of a quarter wavelength, will give an estimate for the CO concentration. The required thickness difference is about 350 nm, only 1/1000 of the total thickness of 350 μm.

Key Factors

Project duration

01/01/2008 - 01/12/2008