Design And Optimizaition Of Novel Quartz-enhanced Photoacoustic Spectrophone

Quartz enhanced photoacoustic spectroscopy?QEPAS?is developed by Prof.Frank Tittel's group at Rice university in 2002.The QEPAS employed a commercial available standard quartz tuning fork?QTF?,instead of microphone,to detect the weak sound waves induced by the photoacoustic effect.QEPAS is widely applied to the trace gas detection in the space science,industry,agriculture,and environmental protection with the advantages of high sensitivity,strong noise immunity,and compact size.This thesis is focused on the development of novel QEPAS techniques by designing and optimizing new kinds of QEPAS spectrophones.The coulping effect between the acoustic microresonators?AmRs?and QTF is improved to enhance the amplitue of the QEPAS signal.The background noise is suppressed to increase the QEPAS detection signal-to-noise ratio?SNR?.The thesis is organized as following:1.History and fundamentals of photoacoustic spectroscopy?PAS?are introduced.The light sources,PAS gas cells,selectivities and limitations of PAS are also discussed.2.The principle of QEPAS is introduced,starting from the properties of QTF.The geometric structure,vibration modes,resonance frequency,and quality factor Q of QTF are described in detail.The application of QTF in force sensing and QEPAS are discussed.Several kinds of QEPAS spectrophone configurations are introduced.3.The traditional QEPAS system based on standard QTF is optimized.The impacts of QTF geometry,laser focus position,and external contamination on QEPAS are investigated.Two kinds of QEPAS spectrophones,the double-pass QEPAS spectrophone and multi-QTF enhanced on-beam QEPAS spectrophone,are developed to enhance the QEPAS signal.4.Two methods,scattered light modulation cancellation method?SL-MOCAM?and electrical modulation cancellation method?E-MOCAM?,are developed to suppress the background noise in the QEPAS system which employs light sources with poor beam quality.5.The QEPAS spectrophones based on custom QTFs are developed.The overtone resonance of QTF was applied to the QEPAS.Innovations:1.Ten kinds of QTFs are compared in terms of the geometry,resonance frequency,quality factor?Q factor?and QEPAS signal to evaluate the performance of QTF.The laser focus position with respect to a QTF is optimized.The optimal laser focus position appears to be 0.6-0.7 mm away from QTF opening.The influence of external contamination on the resonant frequency and Q factor of QTFs are discussed.2.Two spectrophone configurations to enhance the QEPAS signal amplitude are introduced.Base on the traditional on-beam QEPAS spectrophone,a double-pass QEPAS spectrophone configuration is developed by positioning a concave mirror behind the acoustic detection module?ADM?.Benifiting from the multi-pass absorption,the QEPAS detection sensitivity is increased by³ times.A multi-QTF enhanced on-beam QEPAS spectrophone is developed by configuring an additional QTF to the traditional on-beam QEPAS spectrophone.Compared to the traditional on-beam QEPAS spectrophone based on a single QTF,the multi-QTF configuration effectively increases the detection sensitivity by 1.5 times.3.To supress the background noise caused by the poor beam quality in QEPAS,two noise supression methods are introduced:scattered light modulation cancellation method?SL-MOCAM?and electrical modulation cancellation method?E-MOCAM?.SL-MOCAM allows the balance sources to operate in scattered light mode,thus eliminating the optical components for beam combination in the MOCAM system.SL-MOCAM simplifies the optical system and reduces the background noise by more than 90%.The E-MOCAM employed electrical signal instead of the balance source to balance out the noise caused by excitation source.E-MOCAM significantly simplifies the system and supresses the background noise for more than 3orders of magnitude.4.QEPAS spectrophone based on custom QTF is developed.Benefiting from the big prong spacing of the custom QTF,a novel spectrophone configuration single-tube on-beam QEPAS?SO-QEPAS?is developed.The AmR made of a single stainless micro tube is configured between the pongs of the custom QTF.The SO-QEPAS configuration increased the acoustic coulping effect by orders of magnitude.In the case of detecting 5%dry CO₂ in N₂,the SO-QEPAS inceases the detection sensitivity by>100 times,compared to a bare custom QTF without AmR.The custom QTF can also be operated in the first overtone resonance modes.The first overtone resonance of the custom QTF shows a higher Q factor than its fundamental resonance mode when the geometry of QTF prong is carefully designed.The combination of custom QTF overtone resonance and the SO-QEPAS spectrophone configuration results in a shorter AmR length and a higher detection sensitivity.The overtone resonance enhanced SO-QEPAS spectrophone increases the detection sensitivity by 380 times,compared to a bare custom QTF operated in the fundamental resonance mode.