Research And Application Of Broadband Quartz Tuning Fork Light-induced Thermoelastic Spectroscopy

With the continuous development of social economy,the impact of human activities on the global environment is becoming more serious.Among them,the environmental problems of gaseous substances have attracted much attention,so it is very important to trace the source of gas pollution and monitor its emission in real time.In addition,gas detection is also widely used in medical non-invasive diagnosis,industrial process control,aerospace and other fields.Therefore,the development of high-precision,high-sensitivity and high-selectivity gas sensors has great research significance and application value.Among gas sensing technologies,direct absorption spectroscopy has been widely used in different fields due to its advantages of high sensitivity and good selectivity.However,the commercial photodetectors used in traditional direct absorption spectroscopy have the limitations of narrow spectral response range and low power tolerance,which leads to the limited development of this technology.Light-induced thermoelasticity spectroscopy based on quartz tuning fork（QTF-based LITES）is based on direct absorption spectroscopy.Its essence is to use quartz tuning fork thermalelastic detector instead of commercial photodetectors to detect laser radiation in different wavebands.This technique can make up for the limitation of traditional direct absorption spectroscopy and expand its application.At present,the research of QTF-based LITES technology is mainly focused on the performance improvement and application of commercial standard quartz tuning fork thermalelastic detector,and there is no research group to systematically study the broadband response ability of this technology,as well as the detection accuracy and long-term stability in practical applications.Therefore,in this thesis,there are four research aspects:（1）Optimization scheme of a new quartz tuning fork thermalelastic detector was proposed to further improve the response ability of QTF-based LITES;（2）The broadband response capability of QTF-based LITES was characterized by theoretical simulation and experimental analysis;（3）The heterodyne QTF-based LITES was proposed to improve the detection accuracy of QTF-based LITES in practical application;（4）A QTF-based LITES atmospheric methane（CH₄）sensor was developed to verify the long-term stability of QTF-based LITES.The main research content and innovation of this thesis are as follows:1.Aiming at the limitations of current commercial quartz tuning fork thermoelastic detector in QTF-based LITES technology,optimization schemes of a new quartz tuning fork thermoelastic detector were proposed:（1）Optimizing vibration prongs and electrode coupling structures to improve charge collection capabilities;（2）Optimizing the electrode coating structure to enhance the response to laser radiation;（3）While maintaining a high-quality factor Q,reducing the resonance frequency f₀ to increase the energy accumulation time.Based on the above three improvement schemes,a T-shaped quartz tuning fork（T-QTF）thermoelastic detector was fabricated,with a resonance frequency of 9783.9 Hz and a quality factor of～11500 at atmospheric pressure.The results showed that the T-QTF thermoelastic detector has better performance than other current quartz tuning fork thermoelastic detectors.2.The optimal laser radiation position of the entire three-dimensional surface of the T-QTF thermoelastic detector was studied.By scanning the QTF-based LITES signal spectrum of the laser at different positions on the two-dimensional surface of the T-QTF thermoelastic detector,and combining the strain amplitude of the entire three-dimensional surface,the optimal radiation position of the laser on the T-QTF thermoelastic detector was determined to be the connection between the root of the vibrating prongs and the bottom bracket.3.The broad spectral response capability and noise of QTF-based LITES were studied,and the calculation formula of the voltage spectral responsivity and noise equivalent power of quartz tuning fork thermoelastic detector were derived based on Lambert-Beer’s law,modulation spectrum technology and harmonic demodulation principle.The temperature distribution and radiation intensity distribution of the laser radiation with different wavelengths on the surface of the quartz tuning fork thermoelastic detector were simulated using the COMSOL Multiphysics finite element analysis software,the results showed that the quartz tuning fork thermoelastic detector has wavelength-independent light-induced thermoelastic conversion capability in 1-11μm infrared spectral range.In addition,a QTF-based LITES gas sensing system based on five excitation light sources with different wavelengths was built.The voltage spectral responsivity and noise equivalent power of T-QTF thermoelastic detectors were calculated by analyzing the QTF-based LITES signals of target gases in different wavebands.Compared with the current commercial photodetectors,the results showed that the QTF-based LITES technology has a broad spectrum and high flat response capability.4.Aiming at the problem of the sensitivity drop caused by the shift of the resonance characteristics of the quartz tuning fork thermoelastic detector in the QTF-based LITES technology,a heterodyne QTF-based LITES self-calibration method was proposed.This method realized the simultaneous detection of the target gas concentration,the resonant frequency and the quality factor of the quartz tuning fork thermoelastic detector by detecting the response of the quartz tuning fork thermoelastic detector to the photothermal pulse radiation,and then realized the high-precision and long-term continuous monitoring of the target gas by the QTF-based LITES gas sensor.5.A compact and portable atmospheric CH₄ sensor was constructed based on QTF-based LITES technology.To adapt to field test conditions,a 1.65μm fiber-coupled distributed feedback（DFB）laser,a mini-multi-pass cell with an optical path length of 4.2m,and a small quartz tuning fork thermoelastic detector module were integrated into a 78mm×40 mm×40 mm gas detection module to realize the detection of atmospheric CH₄.With an integration time of 300 ms,the detection limit of the sensor is 52 ppb.And The continuous monitoring of atmospheric CH₄ was achieved,which verified the long-term stability of the QTF-based LITES atmospheric CH₄ sensor.