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.