Etalon immune distances for reduction of etalon background signals in frequency modulation spectrometry
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Frequency modulation spectrometry (FMS) is a sensitive laser based spectroscopic technique often used for investigation of molecules and atoms in gas phase. There is an ambition to reduce background signals and noise in spectroscopic systems and thereby increasing sensitivity in detection of gases which would extend the applicability of these types of techniques. This ambition was the starting point for this thesis.
FMS is based on modulation of monochromatic light. At not too large modulation indices, the modulation creates a frequency modulated (FM) triplet with one carrier wave at the laser frequency and two equally frequency detuned sidebands that are exactly out of phase. This property of the FM light can be used for reduction of background signals from so called etalons.
Etalons are background signals originating from multiple reflections between surfaces in the experimental setup. Since these signals can give rise to drifts and noise in detection systems it is of great importance to reduce this background. This thesis studies a method to reduce etalons called etalon immune distances (EID). The basic idea of the technique is to use the FM triplet that contains two sidebands that are completely out of phase. By matching the modulation frequency to the free spectral range (FSR) of the surfaces it is possible, in theory, to completely eliminate etalon background signals. Thus by placing the optical components at specific physical distances, the etalon signals can be reduced. The method of EID is examined in order to gain understanding of the behavior of etalon signals when FM light is used. The purpose of the thesis is to examine to what extent the method can be used for reduction of etalon background signals when FMS is used.
The experiential work shows that EID is in fact a good method for reduction of these background signals. By placing the surfaces at EID it is possible to reduce the etalons significantly. However, the results also show that the method is very sensitive to deviations from perfect placing of the surfaces. For the modulation frequency of 396.2 MHz that was used in the experiment, a displacement of 2.7 mm from EID will give rise to 5% of the maximum measured etalon fringe amplitude in absorption phase. For dispersion phase the corresponding value is 25.3 mm. This indicates that detection in dispersion phase is much less sensitive to displacements of the surfaces than it is in absorption phase. Results thus show that it is important to be aware that the placing of the optical components on the optical table is of great importance in reduction of the etalon background signals.
Place, publisher, year, edition, pages
etalon, frequency modulation spectrometry
Atom and Molecular Physics and Optics
IdentifiersURN: urn:nbn:se:umu:diva-84944OAI: oai:DiVA.org:umu-84944DiVA: diva2:690258
Subject / course
Examensarbete i teknisk fysik
Master of Science Programme in Engineering Physics
2014-01-17, Universitetsklubben, Umeå, 11:00 (English)
Ehlers, Patrick, Doctoral student
Axner, Ove, Professor