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Signal processing for a single detector MOEMS based NIR micro spectrometer

: Heberer, A.; Grüger, H.; Zimmer, F.; Schenk, H.; Kenda, A.; Frank, A.; Scherf, W.

Chatard, J.-P. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Detectors and Associated Signal Processing II
Bellingham/Wash.: SPIE, 2005 (SPIE Proceedings Series 5964)
ISBN: 0-8194-5982-8
Conference on Optical Systems Design <2005, Jena, Germany>
Conference Paper
Fraunhofer IPMS ()

The examination of spectra in the NIR range is necessary for applications like process control, element analysis or medical systems. Typically integrated NIR spectrometers are based on optical setups with diffraction grating and detector arrays. The main disadvantage is price and availability of NIR array InGaAs-based-detectors.
The implementation of a scanning grating chip realized in a MOEMS technology which integrates the diffractive element makes it possible to detect spectra with single detecctors time resolved. Either simple InGaAs photodiodes or cooled detectors may be used.
The set up is a shrinked Czerny-Turner spectrometer. The light is coupled in by an optical fibre. After focussing the light passes the scanning grating moving at 150-500 Hz in a sinusoidal way. There it is split off in the different wavelength, the monochrome intensity is caught by a second mirror and led to the detector.
The detector signal is amplified by a transimpedance stage and converted to digital with 12 bit resolution. The main part of the signal processing is done by a digital signal processor, which is used to unfold the sinusoidal position and calculate the final spectra. The data rate can be up to 3 MHz, then a spectrum is acquired every 2ms by using a 500Hz Mirror.
Using the DSP, the spectrometer can operate autarkic without any PC. Then the spectrum is display on a 160 x 80 pixel graphic LCD. A keypad is used to control the functions. For communication a USB port is included, additional interfaces can be realized by a 16-pin expansion port, which is freely programmable, by the system firmware.