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CONTINUOUS-WAVE COMB FOURIER TRANSFORM SPECTROSCOPY

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Author(s)
Kraetschmer, Thilo
Advisor(s)
Sanders, Scott T.
Date
Jun 24, 2009
Abstract
Due to its versatility, Fourier-transform spectroscopy (FTS) has found wide-spread application in research and development, monitoring of industrial processes, forensics, etc1; its uses continue to expand2. Despite decades of development and improvement of Fouriertransform (FT) spectrometers many of their attributes still show the potential for significant improvement. Two attributes that are of major significance for the performance of FT spectrometers are the increase of spectral radiance of the light sources employed in conventional spectrometers and the increase of measurement speed. Modern superluminescent light sources offer superior levels of spectral radiance when compared to traditionally employed incandescent light sources. Due to this higher brightness, measurements with a higher signal-to-noise ration can be made. Also, the higher light throughput enables measurements at a higher repetition rate, so that the rate is ultimately limited by the dynamics of the (usually mechanical) scanning process employed in FT spectrometers. Repetition rates of up to 2 kHz can be achieved with rotating interferometers3, ii and non-mechanical interferometers exhibiting even higher repetition rates are under development4. An entirely different approach to high-throughput high-speed FTS was pursued by van der Weide et al.5-7. It is based on heterodyne frequency-comb spectroscopy and employs two frequency combs with slightly different mode spacing. This approach, termed frequencycomb Fourier-transform spectroscopy (c-FTS), relies on expensive mode-locked lasers but enables high-throughput FTS with demonstrated repetition rates of ~ 1 kHz. However, even in this case one has to rely on moving cavity mirrors5. This thesis presents a new approach to c-FTS that employs continuous-wave light sources (CW c-FTS). It combines the benefits of conventional FTS (broad spectral coverage, high throughput) with the following advantages: no moving parts, high repetition rate, and good signal-to-noise ratio (SNR) due to high spectral radiance sources. By engineering stable, broadband combs, CW c-FTS could result in a universal and simple approach for spectroscopy at almost arbitrary measurement speeds and spectral resolutions limited only by Fourier principles.
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http://digital.library.wisc.edu/1793/35129 
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