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Microspectrometers: small but wow!

Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. : 28 May, 2006  (New Product)
With a microspectrometer containing just a single photo-diode, research scientists can measure the colors of light, visible and invisible in the UV and infrared range. The one kilo spectrometer offers an interesting alternative to measurements with gallium-arsenide detectors.
What is the composition of the Sun's atmosphere or rock on Mars? What exhaust emissions does an internal combustion engine produce? Is it polystyrene or polypropylene on the conveyor belt in the refuse sorting plant? Spectrometers provide the answer. Solids, liquids and gases can be identified and analyzed with the spectral fingerprint. The key element in most spectrometers is a diffraction grating. Unlike prisms which diffract short-wave violet fractions of light more strongly than the long-wave red fractions, diffraction gratings provide an evenly fanned-out spectrum. Most diffraction grating spectrometers guide the spectrum produced directly onto a series of light-sensitive elements which measure the intensity of the signal for each color. From the light absorption of the material being analyzed, conclusions can be drawn about the chemical composition. Low-cost silicon diodes already exist for the visible light range. For measurements in the near-infrared or infrared spectrum, however, arrays are required which are made of several hundred gallium-arsenide detectors, and they are expensive.

Scientists at the Fraunhofer Institute for Photonic Microsystems IPMS in Dresden working in cooperation with an industrial partner, CTR AG in Villach, have developed a new solution. The new microspectrometer needs only a single detector element. The key feature of the system is an oscillating mirror, which the engineers in Dresden originally devised as a microscanner for barcode readers. To produce the microspectrometer, a high-precision, reflecting diffraction grating was etched into the smooth scanner mirror. From the position of the mirror, which constantly oscillates, research scientists can calculate the associated wavelength. In this way, the entire spectrum can be scanned with just one detector.

The prototype of the new spectrometer measures just 165 x 110 x 80 millimeters, weighs one kilo and contains a device which processes the detector data directly in the unit. Depending on the diode used, the detection range extends from ultraviolet at 200 nanometers to the near-infrared at 2,500 nanometers. Measurements are already possible using the light from a 20-watt halogen lamp, with the fastest oscillating grating scanning a 500-nanometer spectrum every millisecond. The optical resolution lies in the region of 5 to 10 nanometers and is thus higher than that of many conventional spectrometers.
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