| In recent years,gas sensors have been widely used in civilian and military fields such as petrochemical industry,atmospheric environmental monitoring,deep sea exploration,aerospace,biomedicine,and food safety.With the increase of application scenarios and market demands,the requirements for detection accuracy and types are also increasing.In traditional industries such as petroleum exploration,oil-gas field development,and coal mining,gas sensors are used to ensure safety,monitor and measure gas produced in the production process.The " 14th Five-Year Plan" points out that it is necessary to achieve new progress in the construction of ecological civilization,continue to reduce the discharge of major pollutants,improve the ecological environment,make the ecological security barrier stronger,and significantly improve the urban and rural human settlement environment.This requires a highly sensitive gas sensor for real-time monitoring of toxic and harmful gases in the atmospheric environment,which is used to measure the distribution and content of different kinds of gases.With the development of the industrial automation and artificial intelligence industry,Jiaolong sea exploration,Mozi communication,Tiangong travel,Shenzhou flying,Wukong exploration,Chang'e moon exploration and other deep-sea,aerospace projects are carried out and implemented.In order to ensure the safety of personnel and the normal operation of equipment,there is an urgent need for technical improvements in the detection of gas concentration and composition.In addition,in order to ensure the safety of medicines and foods,it is necessary to test the gas barrier performance of the packaging materials and the residual content of the gas in the packaging.And it is usually required to be in ppm level or even lower,which requires the gas sensor to be used with high detection accuracy.Therefore,the development of high-precision gas sensors has always been one of the important research contents in the scientific field.The gas sensor based on quartz tuning fork not only has the advantages of low detection limit,corrosion resistance,high sensitivity,low loss,anti-electromagnetic interference,intrinsic safety and suitable for remote online measurement,but also has full wavelength response,positive correlation between signal and incident laser intensity,immune to external uncorrelated noise,narrow response bandwidth,high Q value,low price,compact structure and other advantages,and it has been verified by researchers that it can be used for the detection of dozens of gases in recent years,so it has great potential in the development of high-performance gas sensors.This thesis has carried out a series of researches on the performance improvement and key problem solving of gas sensors based on quartz tuning fork,and provided technical support for promoting the commercialization of gas sensors based on quartz tuning fork.The main contents of this thesis are as follows:(1)The detection principles,advantages and disadvantages of several common gas sensors are briefly introduced.The current development status and three technical branches of photoacoustic spectroscopy gas detection technology are introduced in detail.The research status of quartz tuning fork gas detection technology in China and other countries is mainly introduced.(2)The basic theory of infrared spectroscopy gas detection technology and harmonic detection theory,as well as the detection process of QEPAS signal are described,and the quartz tuning fork is introduced,including the vibration mode of quartz tuning fork,the calculation of energy accumulation time and resonance frequency of the quartz tuning fork.(3)The structure and performance of the QEPAS gas detection system are optimized and improved.A wavelength calibration technique based on the second harmonic is proposed,which effectively avoids the laser output wavelength drift caused by the ambient temperature change in the constant driving mode,and the signal can be averaged multiple times in a short time.Compared with the traditional wavelength modulation spectroscopy technology,the detection accuracy is increased by 12.87 times.A laser-receiving quartz tuning fork photoacoustic detection module is designed and optimized,which can prevent gas flow interference and is easy to be aligned.The length and inner diameter of the resonance tube used in the quartz tuning fork photoacoustic detection module and the distance between the resonance tube and the top of the quartz tuning fork are optimized.In 6 months,the resonance frequency and acoustoelectric conversion efficiency of quartz tuning fork are tested.A four off-axis resonance tube quartz tuning fork photoacoustic detection module is invented.Compared with the bare quartz tuning fork,the signal is increased by 25.84 times.The preamplifier circuit of quartz tuning fork is designed.The principle of the lock-in amplifier is introduced,and a dual path lock-in differential method to eliminate the residual amplitude modulation of the second harmonic signal is proposed.The asymmetry factor of the second harmonic signal in the photoacoustic spectroscopy gas detection system is greatly reduced.(4)The Q-switched fiber laser is introduced,and the relationship between the excitation laser source power and photoacoustic signal in QEPAS gas detection system is simulated.An acousto-optic Q-switched fiber laser-based intra-cavity QEPAS gas detection system and a scanned-wavelength intra-cavity QEPAS gas detection system with injection seeding technique are proposed.The parameters affecting the photoacoustic signal of the two systems are optimized,such as the center wavelength of the fiber grating,the pump power of the pump source,the modulation duty cycle of the modulation signal in the first system and the scanning time of the seed laser source,the output coupling ratio of the fiber coupler,the pump power of the pump source in the second system.The performance of the two intra-cavity systems is evaluated and tested,and their advantages and disadvantages are compared.(5)A quartz tuning fork photodetector based on photothermal effect is designed,the distance between the incident spot on the QTF arm and the top of the QTF,the distance between the fiber collimator and the QTF arm,and duty ratio of modulation signal are optimized to obtain the maximum signal.The experimental results show that the quartz tuning fork detector has the advantages of wide response wavelength range,high saturation power and high sensitivity.Based on the characteristics of the quartz tuning fork detector,the self-differential technique is proposed and applied to the scanning direct absorption spectroscopy gas detection system.The detection signal and reference signal are directly differentiated by quartz tuning fork.Compared with the traditional differential technique,the signal-to-noise ratio is increased by 5.1 times.In the gas detection system based on quartz tuning fork detector,long absorption optical path and high laser power are beneficial to improve the signal amplitude of the system.A long optical path and high power gas detection system based on fiber laser and quartz tuning fork detector is designed.Acetylene gas is detected by this system,and the minimum detection limit reaches 6.1 ppb.(6)A multi-component gas detection system based on quartz tuning fork frequency division multiplexing technique is designed.Three quartz tuning forks with different frequencies are used to simultaneously detect water vapor,methane and acetylene.The corresponding minimum detection limits are 1.3 ppm,79 ppm and 5 ppm,respectively.The QEPAS gas detector and multi-component photoacoustic spectroscopy gas detector are developed and tested in the field.Innovations of this thesis:(1)The QEPAS gas detection system is optimized from the system structure and signal processing.In terms of structure,a four-off-axis resonance tube quartz tuning fork photoacoustic detection module is designed.Three right-angle prisms allow laser to pass through the gas four times,and the photons are absorbed by the gas molecules to generate sound signals.Four off-axis resonance tubes are used to amplify the sound signals.In terms of signal processing,a dual path lock-in differential method to eliminate the residual amplitude modulation of the second harmonic signal is proposed.The phases of the two lock-in amplifier reference signals are adjusted separately,one of them obtains the second harmonic signal with residual amplitude modulation,and the other one only contains the residual amplitude modulation signal caused by the first harmonic signal and the third harmonic signal.By adjusting the amplitude of the two signals,the residual amplitude modulation terms of the two signals are equal,and then the two signals are subtracted to obtain the second harmonic signal without residual amplitude modulation.(2)In the photoacoustic spectroscopy gas detection system,the photoacoustic signal is proportional to the power of the excitation laser source.Based on this characteristic,intra-cavity photoacoustic spectroscopy gas detection systems are proposed.The quartz tuning fork photoacoustic detection module is placed in the ring cavity of the Q-switched fiber laser,and the high-power pulse in the cavity is used as the excitation laser source to detect the gas.Two types of intra-cavity photoacoustic spectroscopy gas detection systems are designed.In the first method,a fiber laser with fiber Bragg grating as wavelength selective device is used as excitation laser source,which can make full use of the power in the fiber laser cavity.The minimum detection limit of this system can reach ppt level in theory.In the second method,a fiber laser with injection seed laser as wavelength selective device is used as excitation laser source.This system needs a fiber coupler to connect the injection seed laser into the fiber laser cavity,which is equivalent to increasing the loss of the laser resonant cavity.Compared with the previous system,the intra-cavity power is decreased under the same conditions,but this method can get the whole gas absorption line and reduce the influence of ambient temperature on the system.(3)The self-differential characteristic of the quartz tuning fork detector is discovered and used in the scanning direct absorption spectroscopy gas detection system.The quartz tuning fork is directly used to differentiate the detection signal and the reference signal.Compared with the traditional differential technique,it effectively improves the detection accuracy.A high power and long optical path gas detection system based on quartz tuning fork detector is designed.In this system,the fiber laser is used as the excitation laser source.In the process of laser forming,the laser passes through the gas cell many times,which is equivalent to increasing the effective absorption optical path.In addition,the Q-switched fiber laser also has the characteristics of high power.In this system,high power and long effective absorption optical path are both beneficial to increase the signal.(4)A multi-component gas detection system based on quartz tuning fork frequency division multiplexing technique is designed.Three DFB lasers of different wavelengths with modulation frequencies of 15.346 kHz,15.987 kHz,and 16.373 kHz are combined by a fiber coupler,and then pass through three quartz tuning forks with resonance frequencies of 30.692 kHz,31.974 kHz,and 32.746 kHz.The experimental results show that there is no cross interference when the three gases are detected by quartz tuning forks with different resonance frequencies at the same time.The system has been used to achieve simultaneous measurement of water vapor,methane,and acetylene. |
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