Design and Modelling of a Fourier Spectrometer Based on Sampling a Standing Wave
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DESIGN AND MODELLING OF A FOURIER SPECTROMETER BASED ON SAMPLING A STANDING WAVE D. Knippa, H. Stiebigb, H.-J. Büchnerc, G. Jägerc, M. Rosaa, R.A. Streeta a) Xerox Palo Alto Research Center, Palo Alto, CA 94304, USA b) Research Center Jülich, Institute of Photovoltaic, 52425 Jülich, Germany c) TU Ilmenau, Institute of Measurement and Sensor Technology, 98684 Ilmenau, Germany ABSTRACT The design of a novel spectrometer based on a thin film diode in combination with an electro statically tunable micro machined mirror will be presented. The semi transparent diode is introduced into a standing wave created in front of a reflector to sample the profile of the standing wave. Varying the position of the reflector results in a shift of the phase of the standing waves and thus in a change of the optical generation profile within the detector. The spectral information of the incoming light can be determined by the Fourier transformation of the transient response of the sensor. An analytical optical model will be presented which facilitates the evaluation of different detector concepts. The model will be applied to discuss different device designs regarding the resolution of the spectrometer, the spectral range and the linearity. INTRODUCTION Various miniature spectrometers for different applications have been developed in recent years [1-4]. Most of the progress in this area has been stimulated by MEMS technology. Especially the interest in optical MEMS has initiated a lot of new research activity in optical metrology. The activities are driven by improvements of size, cost and spectral resolution. Consequently, several spectrometer concepts have been transferred from the macro to the micro scale like Fabry-Perot spectrometers [1], grating spectrometers [2] and Fourier spectrometers [3]. More recently Kung and coworkers [4] have presented a new type of Fourier spectrometer. In this case a thin amorphous silicon photoconductor was applied to sample a standing wave created in front of a micro-machined mirror. The novel spectrometer reduces the number of components to a minimum, simplifies the setup to a linear arrangement of a detector and a mirror and facilitates the realization of 1D or 2D spectrometer arrays. The concept of using a standing wave in order to realize an interferometer is known for many years [5, 6], but the application of such a concept has been limited by technological reasons. Sampling a standing wave by a semiconductor device requires a highly transparent and very thin detector, which enables the creation of a standing wave and is sensitive to the wavelength of the incident light. Further, sufficient light has to be absorbed to generate an electrical signal as a consequence of the intensity modulation within the sensor. Nevertheless, Fourier spectrometers based on sampling a standing wave are of high interest, due to the simple setup and the ability to realize spectrometer arrays. However, the advantages require a careful processing and device design. The performance of the spectrometer is mainly determined
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