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We present a microspectrometer based on a tunable interference filter for infrared or visible light that scans the desired part of the spectrum within milliseconds. A single pixel detector measures serially the intensity at selected wavelengths. This concept avoids expensive linear detectors as used for grating spectrometers. The tunable filter is fabricated by a new porous silicon technology using only two photolithography steps. A Bragg mirror or a Fabry-Perot bandpass filter for transmission wavelengths between 400 nm and 8 mum at normal incidence is created by modulations of the refractive index in the filter plate. Two thermal bimorph micro-actuators tilt the plate by up to 90degrees, changing the incidence angle of the beam to be analyzed. This tunes the wavelength transmitted to the detector by a factor of 1.16. The filter area can be chosen between 0.27 x 0.70 mm(2) and 2.50 x 3.00 mm(2), the filter thickness is typically 30 mum. The spectral resolution of Deltalambda/lambda = 1/25 is sufficient for most sensor applications, e.g., measurement Of CO2 and CO in combustion processes by their IR absorption bands as will be presented.
Hans Peter Herzig, Grégoire Maxime Smolik
Hatice Altug, Andreas Tittl, Aleksandrs Leitis