| With the development of society and the accelerating process of industrialization,the emission of various toxic and harmful polluting gases has led to the continuous deterioration of the atmospheric environment on which human beings depend,and natural disasters such as the greenhouse effect,acid rain,and smog are emerging one after another.Accurate and effective monitoring of trace gases in the atmosphere and various polluting gases emitted by industries is an important measure to guide the harmonious coexistence of man and nature and green and sustainable development.Therefore,it is of great research significance and application value to develop a gas sensor with high recognition accuracy,short response time,high detection sensitivity,compact structure,low price,and real-time monitoring of target gas concentration.Among many gas sensing technologies,laser spectroscopy has been widely used in atmospheric environment,industrial process control,human breath composition analysis,energy exploration and other fields for research and development of various types of gas sensors.In recent years,thanks to the high-quality factor,anti-interference,miniaturization,low price,and no limitation of wavelength response bandwidth of quartz tuning fork,the development of gas sensors based on quartz tuning fork detectors and the optimization of related spectroscopic techniques have become a research hotspot in the field of laser spectroscopy.Based on the technical principles of absorption spectroscopy,photoacoustic spectroscopy and related technical principles of quartz tuning fork detectors,this paper has carried out the following research contents:1、A dual-spectroscopy detection technique based on quartz tuning fork detector is proposed.First,the quartz tuning fork was sealed in a home-designed and processed gas photoacoustic cell,and the photoacoustic spectral response characteristics of the tuning fork detector were studied.The external cavity quantum cascade laser（ECQCL）was used to conduct experimental analysis on the volatile organic compound（VOCs）ethanol volatilization gas.Then,two quartz tuning forks with both resonance frequencies of 32751 Hz were used as photoacoustic signal detector and direct absorption signal detector,respectively,to build a N2O dual-spectrum detection system.The system can detect the photoacoustic spectral signal and the direct absorption spectral signal respectively,and can also realize the simultaneous detection of the two spectral signals,and can realize a dual-spectral signal measurement in which the photoacoustic and photothermal superimpose each other by using the dual-spectral effect.The experimental results show that the signal-to-noise ratio of the dual spectral signal of N2O is increased by 4-7 times compared with the photoacoustic spectral signal of the system under the same gas concentration,which can effectively improve the detection sensitivity of the gas sensor.2、A multi-frequency modulation technique based on quartz tuning fork detector is proposed.Using the piezoelectric effect of the resonance characteristic of the quartz tuning fork,a multi-frequency modulation technique based on the fast Fourier transform（FFT）algorithm is established within the resonance frequency response bandwidth of the tuning fork.On the basis of this technology,the high-sensitivity simultaneous detection of CH4 gas in the near-infrared and mid-infrared bands was successfully achieved by using the full-band response characteristics of the quartz tuning fork.The detection limits of CH4 are 5ppm and 3.5ppm,respectively,and the corresponding normalized noise equivalent absorption coefficients（NNEA）are 1.33×10-9 cm-1W/Hz1/2、2.20×10-10 cm-1W/Hz1/2.In addition,a portable multi-component gas simultaneous detection system was built based on a single quartz tuning fork and three semiconductor lasers,which realized the simultaneous detection of H2O,CO2,and CH4.The results show that the detection limits of the three gases are 1.4 ppm,353 ppm and 3.1ppm,respectively,corresponding to the NNEA of 2.65×10-10 cm-1W/Hz1/2,8.09×10-10 cm-1W/Hz1/2,8.28×10-10 cm-1W/Hz1/2.Experiments show that the system sensitivity can be further improved by using the fiber power amplifier（EDFA）.And this spectroscopic technique has the significant advantages of easier optical alignment,lower cost,and more compact and portable than traditional direct absorption spectroscopic technique sensors based on multiple photodetectors.3、A 2f/1f-WMS technology based on quartz tuning fork is proposed.A novel laser wavelength modulation strategy combined with the frequency division multiplexing demodulation algorithm in the above research content realizes the simultaneous detection of the first harmonic（1f）and second harmonic（2f）signals in wavelength modulation spectroscopy（WMS）.Compared with the commonly used 1f or 2f detection methods,the results show that the proposed 2f/1f-WMS technology is effective for some typical environmental interference factors（such as light intensity jitter,gas disturbance,mechanical vibration）caused by the test of atmospheric water vapor.The strong change has good immunity,which proves that it has great application potential in external field,especially in harsh environment.4、A method for enhancing the photothermal detection performance of a quartz tuning fork is proposed.By coating a two-dimensional semiconductor heat-absorbing material—a super-thin iron-doped cobalt oxide coating（Co O-Fe）on the surface of the quartz tuning fork,the light and heat absorption efficiency of the surface of the quartz tuning fork is improved,thereby improving the detection sensitivity of the tuning fork.In the experiment,the comparison and simulation of traditional bare tuning fork and quartz tuning fork coated with endothermic material were carried out.The research results show that this method has a significant enhancement effect,which is manifested as the surface temperature of the quartz tuning fork coated with endothermic material increases,the deformation increases,and the resonance frequency signal amplitude of the tuning fork changes from the original 0.43 V to2.7 V.Quality factor increased from 4014 to 9417.Using this quartz tuning fork to detect the concentration of CH4,the detection limit is 880ppb under the integration time of 1s,and the corresponding normalized noise equivalent absorption coefficient is 2.2×10-10 cm-1W/Hz1/2.In summary,in view of the difficulties in the application of quartz tuning forks in the field of spectral detection and related technical requirements,this paper has carried out the improvement of the spectral signal-to-noise ratio of quartz tuning forks,the simultaneous detection of multiple gases based on a single quartz tuning fork,the new quartz tuning fork2f/1f-WMS,detection performance optimization of quartz tuning fork,etc.Related technologies can be applied to real-time online monitoring of gas molecules such as atmospheric trace gases,greenhouse gases,and volatile organic compounds. |
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