Infrared Spectrometer Filter Is Electrically Tunable

Paul Mortensen

Portable infrared spectrometers enable in situ analysis of compounds in a variety of hazardous environments on Earth and in space. Such possibilities have led researchers at Ryukoku University in Otsu, Japan, to develop a small, robust infrared filter that can be tuned electrostatically to select specific wavelengths.

Most spectrometers use a Michelson interferometer with a moving mirror, a fixed mirror, a beamsplitter and a light source, but their design is bulky and overly sensitive to movement. The new filter is a Fabry-Perot interferometer, a much simpler design that could lead to size and weight reductions in such instruments.

To construct the filter, the researchers stacked two 400 x 25 x 25-µm silicon plates, chosen for their electrical conductivity, infrared transparency and high reflectance. They applied an adhesive containing glass spacers to two opposite edges of these cavity reflectors, creating a 7-µm spacing.

When they applied voltage, the positive and negative charges induced on the silicon plates attracted them to each other. With 88 V, for example, the separation between the plates decreased from 7 to 5 µm, and the transmission spectrum shifted to shorter wavelengths, with the first-order peak shifting from 14.3 to 9.5 µm.

By electrostatically deforming the reflectors of the filter, the researchers found that they could tune for infrared wavelengths and could modulate their intensities to select the signatures of specific compounds. They attained a modulation factor of more than 80 percent below 10 Hz.

Earth first

Although they believe that the Fabry-Perot filter could be used in space for astronomical spectrosco-py, an immediate application is on Earth. Mitsunori Saito, one of the researchers on the project, noted that a simple, compact spectrometer is needed for atmospheric and factory applications.

Using thinner silicon plates to lower the tuning voltage and coating the outer surfaces to enhance transmittance and modulation should improve the filter's performance. To further the work, the team is setting up a research and development project with a Japanese company and is applying for funding from Japan's Ministry of Economy, Trade and Industry, Saito said.