Measurement Technology and Analytics

Tunable band-pass filters for infrared light

© Fraunhofer ENAS
Photograph of FPIs with distributed Bragg reflectors.
© Fraunhofer ENAS
Photograph of FPI with subwavelength grating reflectors.

Fraunhofer ENAS develops together with its partners Technische Universität Chemnitz / Center for Microtechnologies and InfraTec GmbH electrically tunable band-pass filters for infrared light. The development is part of several funded projects (MIDAS1, SIRKO2, NanoRef3, Nano3pt4) as well as contracts with InfraTec GmbH. Some types of the infrared filters are commercially available already and are fabricated successfully in a 6-inch MEMS wafer-level process, with up to 240 chips per wafer compound. New types demonstrate advanced technologies, e.g. reflectors made of subwavelength gratings, fabricated with nano-imprint lithography, to enhance spectral range, resolution, mechanical stability and process technology.

The target application in most of the projects is spectral gas analysis in medical and technical areas. In medical diagnostics especially the monitoring of carbon dioxide and anesthetics, such as halogenated ethers and nitrous oxide, in respiratory air are of strong interest. In general, the analysis is done by measuring the infrared absorption spectrum of the gases. Many medical and technical gases have strong absorption bands in the infrared spectral range especially between 3 µm and 12 µm wavelength. In a common spectrometer set-up, the band-pass filters are used as wavelength selecting element in front of a broadband infrared detector. The gas is placed in a sample cell between an infrared emitter and the spectrometer set-up. The absorption spectrum of the gas is measured in transmittance by stepwise tuning the infrared filter through the relevant spectral range.

InfraTec GmbH integrates the components of the spectrometer set-up in a tiny TO-8 package. They are commercially available for different spectral ranges, as well as in single-band and dual-band configurations. A dual-band configuration uses two passbands of the infrared filter and allows for the simultaneous measurement in these two spectral ranges. This is achieved by adding a dichroic beam splitter and a second broadband infrared detector to the spectrometer set-up. The dual-band type still fits in a TO-8 package, with only a small increase in the sensor height. The MEMS micro-spectrometers have many advantages, e.g. small size, small prize, small weight and mechanical robustness and are therefore a very promising enhancement of existing spectrometer solutions.

Technically, the tunable infrared filters are tiny Fabry-Pérot interferometers (FPI). They consist of two mirrors that build an optical resonator in between. To obtain an excellent spectral performance the surfaces of the mirrors have to be smooth and highly reflective and the mirrors have to be aligned parallel to each other very precisely. The transmittance spectrum of the FPI shows a passband at each interference order and strong blocking of the radiation in between. The passbands can be shifted by varying the distance between the mirrors. The spectral range between two adjacent passbands is call the free spectral range of the FPI and is the maximum tuning range of the infrared filter.

The filters are fabricated in a sophisticated MEMS wafer-level process. They consist of two silicon wafers, which are bonded together by either silicon fusion bonding or an intermediate SU-8 polymer layer. The optical resonator is built between the bonded wafers. The mirrors are deposited before bonding on the inner sides of the wafers in very precisely wet-etched cavities. On the other side of the wafers, an anti-reflective coating is deposited. At least one wafer of a compound consists of moveable mirror carriers that are actuated electrostatically to vary the distance between the mirrors.

Different types of the infrared filters cover the spectral range between 3 µm and 11 µm. They have an optical aperture of up to 2 mm x 2 mm. The peak transmittance is up to 90 % and the full width at half maximum can be as small as 20 nm. The tuning voltage depends on the spectral range, but is below 50 V for many types. The chips have a size of 7 mm x 7 mm x 0.6 mm.