| Quartz tuning fork（QTF）was first employed in photoacoustic spectroscopy in2002.Because of its unique advantages,such as small size,high quality factor（Q-factor）and low cost（<1 RMB）,it has been widely used in laser absorption spectroscopy（LAS）in recent years.QTF can be used not only as an acoustic transducer in quartz enhanced photoacoustic spectroscopy（QEPAS）,but also in quartz enhanced photothermal spectroscopy（QEPTS）based on thermo-elastic effect.This thesis focuses on the research of gas sensing technique based on QTF,sush as QEPAS and QEPTS.The work carried out in this thesis are summarized as follows:Firstly,the classification of gas absorption spectroscopy is introduced,especially the photoacoustic/photothermal spectroscopy focused on in this thesis.The research status and development direction of QEPAS/QEPTS are described in detail.Then,the technical basis of QEPAS/QEPTS is studied,including the theoretical model of QTF and the measurement method of QTFs’characteristic parameters,the signal generation and enhancement method,the response time and background noise of the corresponding sensor system,and the performance evaluation method.The numerical analysis methods used in QEPAS/QEPTS are introduced,and then three numerical models（the vibration mode analysis model of QTF,the numerical optimization model of photoacoustic spectrophone and the numerical optimization model of photothermal excitation parameters for QTF）are proposed for the numerical simulation analysis of QEPAS/QEPTS.Then,the QTF-embedded off-beam QEPAS was proposed.The background and significance are provided.The paramaters of the designed spectrophone is optimized by numerical simulation and experiment.It is confirmed that the QTF-embedded off-beam configuration has a simple assembly and collimation process（the excitation beam does not need to pass through the gap of QTF’s prongs with an off-beam configuration）,a high detection sensitivity（the signal-to-noise ratio gain of the dual-tube QTF-embedded off-beam configuration can reach～40,which is～1.3 times better than the realized signal-to-noise ratio gain of the common on-beam configuration with two resonant tubes）and a high acoustic coupling strength（Q-factor of the dual-tube configuration can be reduced from>10000 to～2500,which is equivalent to the acoustic coupling strength of the common on-beam configuration）.The detection performance of the QTF-embedded off-beam QEPAS is quantitatively evaluated through water vapor detection.The 1σdetection limit（σis the standard deviation）is 0.159 ppmv,and the corresponding normalized noise equivalent absorption coefficient（NNEA）is 6.59×10-9 cm-1·W·Hz-1/2,which proves the QTF-embedded off-beam QEPAS has a high detection sensitivity.Then,an all optical fiber QEPTS was proposed.The research background and significance of all optical fiber QEPTS are introduced.The optical excitation parameters were optimized in detail through simulation and experiment.The characteristics of all optical fiber QEPTS are studied in detail.Experimental results reveal that the main noise source of all optical fiber QEPTS is mode interference noise,which is different from the common free space QEPTS.Through methane detection experiments,the detection performance of all optical fiber QEPTS was evaluated.Thanks to the combination with optical fiber sensing technique,the corresponding sensor system is more compact and easier to been integrated into photonic circuits,which is helpful to realize long-distance and multi-point sensing application.The realized 1σdetection limit and NNEA are 48.8 ppmv and 9.66?10-9cm-1·W·Hz-1/2,respectively.Finally,four sensor systems for different application scenarios based on QEPAS/QEPTS are developed:（1）A high sensitivity methane detection system based on dual-tube-enhanced QTF-embedded off-beam QEPAS was developed.The light source is a distributed feedback（DFB）semiconductor tunable laser with a central emission wavenumber of 6046.9 cm-1.A photoacoustic detection module with a compact volume of 3?2?1 cm3 and a total weight of 9.7 g was designed and realized by three-dimensional（3D）laser printing technique.Wavelength modulation technique-based second harmonic（2f）detection technique was employed in the developed methane sensing system.The modulation depth and optical structure of the sensor system are optimized in detail.With a lock-in integration time of 0.3 s,the realized 1σdetection limit and NNEA are 8.62 ppmv and 1.80?10-8 cm-1·W·Hz-1/2,respectively.（2）A dual-gas（methane/acetylene）detection system based on dual-channel QTF-embedded off-beam QEPAS and time division multiplexing（TDM）technique was developed.The selected absorption lines of methane and acetylene are6046.9 cm-1 and 6521.2 cm-1,respectively.The output light of two DFB lasers is respectively guided through two independent detection channels of the photoacoustic spectrophone through two fiber-coupled collimators.By using TDM technique,only a QTF and a lock-in amplifier are employed to realize dual-gas detection application.The construction and optimization process of the sensor system are provided.In order to meet the needs of dual-channel detection,the photoacoustic detection module is designed and realized by using 3D laser printing technique.With a lock-in integration time of 1 s,the realized 1σdetection limits of methane and acetylene are 7.63 ppmv and 17.47 ppmv,respectively,and the corresponding NNEAs are 7.24?10-8cm-1·W·Hz-1/2 and 3.73?10-8 cm-1·W·Hz-1/2,respectively.（3）Long-distance QEPTS methane detection system based on optical fiber-coupled sensing probe was developed.The optical fiber-coupled sensing probe with fiber in and fiber out configuration can be used as a gas absorption cell in QEPTS to realize long-distance gas sensing application.In order to realize a real-time in-situ monitoring,the wavelength locked detection scheme is employed to reduce the sensor response time,and a field gas leakage experiment is carried out in Jilin University campus,which proves the long-distance detection ability of the sensor system.And a response time of<12 s was realized.The lock-in integration time is 0.3 s in the proposed sensor system.The realized 1σdetection limit is～11 ppmv for methane,and the corresponding NNEA is 6.03?10-9 cm-1·W·Hz-1/2.（4）A QEPTS methane detection system based on a dual optical path multi-pass cell（MPC）was developed.Combined with the dual optical path MPC,the detection performance of the QEPTS sensor system under two different optical path lengths is analyzed.The integration time of the lock-in amplifier is set to 30 ms.when the optical path is increased from 6 m to 20m,the 1σdetection limit of the system is reduced from 7.19 ppmv to 2.59 ppmv,and the corresponding NNEAs are 3.68?10-9 cm-1·W·Hz-1/2 and 8.06?10-10cm-1·W·Hz-1/2,respectively.It is confirmed that the minimum detection limit of the sensor system will be reduced with the increase of optical path length within a certain range,but it is not a simple linear relationship.With the change of optical path,the factors that affect the system performance may include optical transmission loss,gas absorbance and background noise,etc.Therefore,in order to improve the detection sensitivity of free space QEPTS-based sensor system,the optical path length must be optimized according to the specific situation.The innovations of this thesis are summaried as follows:（1）In order to achieve a high acoustic coupling strength and detection sensitivity and to simplify the assembly and alignment process in QEPAS,a novel QTF-embedded off-beam QEPAS was proposed.The structure parameters of QTF-embedded off-beam QEPAS spectrophone were optimized by simulation and experiment,and the detection performance of the corresponding sensor system is quantitatively evaluated by gas detection experiment.（2）Aiming at the problems of large volume and poor compactness of traditional QEPTS-based gas sensor system,an all fiber QEPTS is proposed.The excitation parameters of laser beam in all fiber QEPTS are optimized by simulation and experiment.A good detection sensitivity is rezlized in all fiber QEPTS,and the methods to further improve the detection sensitivity are discussed.（3）In order to solve the problem that the traditional QEPTS-based gas sensor system is not convenient for remote gas monitoring,a fiber coupling probe is introduced into the QEPTS,and a remote real-time methane monitoring system is developed by locking the central wavelength of the laser at the target gas absorption line.Through the field gas leakage monitoring experiment,it is proved that the developed sensor system has the ability of remote monitoring and short response time（<12 s）. |
