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Characterization and Reduction of Background Signals from Etalon Effects in Wavelength Modulation Spectroscopy
Umeå University, Faculty of Science and Technology, Department of Physics.
2014 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Wavelength Modulation Absorption Spectroscopy (WMAS) is one of the modulation techniques which are used to detect the concentration of the molecules in gas phase. The technique has been of interest for long time due to the cheap equipment and the increase of sensitivity of detection in comparison with Direct Laser Absorption Spectroscopy (DLAS). Although the WMAS demonstrates higher signal to noise ratio than DLAS, the detectability of the technique is often limited by background signals originating from etalon. In this thesis, the set up for both DLAS and WMAS is developed for detection of CO molecules in 600 Torr and transition wavelength 6368.0861 cm-1. In order to characterize the background signals, an etalon with a length of 25 mm, which corresponds to free spectral range of 4 GHz is used. The theoretical description of the technique which is given in terms of Fourier coefficients of modulated absorption lineshape is presented and more over it is illustrated that the amplitude of both analytical signal and background signal has strong dependence on the modulation amplitude ( va ). In order to find the maximum signal to noise ratio (SNR), both the background signals and the analytical signals were scanned and plotted in terms of modulation amplitude. Also the optimum modulation amplitude is chosen to be va opt=3.84 GHz. Therefore it is concluded that for the set up with wavelength modulated distributed feedback (DFB) diode laser, in transition wavelength (6368.0861 cm-1) and in combination with etalon reference (4 GHz FSR), the optimum modulation amplitude which makes the setup etalon free is vFSR= 0.96 (for FSR normalized modulation amplitude) and  va D= 1.9 (for transition width normalized modulation amplitude).

Place, publisher, year, edition, pages
2014.
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-106677OAI: oai:DiVA.org:umu-106677DiVA: diva2:843887
Subject / course
Fysik D - examensarbete
Educational program
Master's Programme in Physics
Available from: 2015-10-29 Created: 2015-07-31 Last updated: 2015-10-29Bibliographically approved

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