| Fast and accurate on-line monitoring of toxic,harmful and explosive gases is an important part of ensuring industrial and agricultural production and residents' safety.It is also an important guarantee for continuously promoting"Made in China 2025".The content of the gas component to be tested may be on the order of ppm or less,and the measurement condition often involves complex environments such as high temperature and high pressure and diversified components.For example,the "Tian Gong" space station,the "Bei Dou" navigation system and the "ChangE" series of satellites under construction in China are represented by the aerospace,lunar exploration project and domestic aircraft carrier,and the "Jiao Long" manned submersible.Trace gas should be strictly monitored inside and outside the aircraft and the carrier to ensure the safety of personnel and the normal operation of the equipment.The 19th National Congress pointed out that the concept of green development and the construction of ecological civilization are the millennium of sustainable development of the Chinese nation.The residues of combustion emissions in the process of thermal power generation,industrial production,and automobile exhaust emissions are strictly monitored.At the same time,energy is the engine of continuous development and progress of human civilization,and is a major event concerning national economy and people's livelihood and national security.Safe mining has always been highly valued by the energy industry.Real-time online monitoring questions,methane,acetylene and other olefins combustible gas is critical in the mining and transport of coal,oil and natural gas.With the rapid development of China in the UHV field,the relevant insulation gas monitoring in high-voltage equipment is an important guarantee for the safe transmission of power plants and the safe use of electricity by residents.With the frequent food and drug safety incidents in recent years,the safety of food and medicine has been It has attracted the attention of relevant government departments.The most extensive of the series of sampling tests for foods and drugs is the barrier properties of packaging materials and packaging residue detection,which requires high measurement sensitivity and response speed of gas sensing systems to ensure Productivity.The advantages of intrinsic safety,anti-electromagnetic interference,high temperature and high pressure resistance,easy remote transmission and reuse are increasingly valued by researchers.Not only that,but also gradually applied in actual industrial and agricultural production.The basic principle of the fiber-optic gas sensor is to detect the absorption of light power at a specific wavelength.At low concentrations,this change in optical power is very weak.Therefore,finding a highly sensitive detection method has always been the most important research content of fiber-optic gas sensing.Differential absorption is one of the most commonly used methods for detecting background noise.However,in addition to gas absorption,there are other interference factors that cause two differential outputs.Therefore,the sensitivity and reliability of the differential method are limited by external environmental fluctuations.The harmonic method adds high-frequency modulation to the light source,and uses the lock-in amplifier to detect the harmonic signal generated by the absorption,which can greatly improve the system signal-to-noise ratio.However,the residual amplitude modulation interference due to the intensity modulation is the final signal extraction band.Come to influence.White cell and other reciprocating designs can effectively increase the gas absorption path,and the harmonic detection system further enhances the measurement sensitivity,but its large size and vulnerability to environmental influences limit its application in some specific occasions.In addition,each doubling of the absorption path of the multiple rounds of absorption pool will greatly increase the design difficulty and processing cost.Laser cavity ring-down spectroscopy can effectively enhance the absorption of gas,showing a very good prospect,but still can not get rid of the influence of strong background light on detector saturation.The photoacoustic spectroscopy gas sensing technology analyzes the gas concentration by detecting the acoustic wave signal excited by the laser absorption,and gets rid of the interference of the background light and the background absorption.However,the low excitation light power has always been a bottleneck restricting the improvement of the photoacoustic signal.In addition,since the above methods are all based on spectral line absorption amplitude detection,they are inevitably affected by the absorption profile.In the measurement environment where the pressure changes and the background gas is complex and variable,the deviation and broadening of the absorption profile are inevitable,which will directly lead to the failure of the originally calibrated gas sensing system.This paper mainly focuses on the key technologies involved in the actual engineering application of the above gas sensing system,and proposes a series of solutions to eliminate various background interferences,focusing on improving the detection limit and background interference resistance of gas sensing systems.The main contents include:1.The advantages and disadvantages of the detection principle based on traditional gas detection methods such as catalytic combustion method and electrochemical method are introduced.The direct absorption spectrum technique and photoacoustic spectroscopy technology are introduced in detail.2.The spectral absorption theory of molecules is introduced in detail,including Beer-Lambert's law,absorption line strength theory,and absorption line linear function theory.3.The basic flow for constructing a direct absorption spectroscopy gas sensing system is studied in detail from the aspects of device selection and testing,absorption line selection,etc.The system structure of scanning absorption spectroscopy and wavelength modulation spectroscopy is introduced.Working mode and absorption signal demodulation principle,the subtraction method,division method,BRD,phase-locked amplifier signal demodulation circuit used in the above method are designed and the working mechanism of concentration demodulation is explained.As an extension,a study is carried out.A phase-locking-based phase detection module accurately measures the phase of the frequency to be measured from a complex weak signal whose amplitude is time-varying.4.The background interference in the direct absorption spectroscopy gas sensing system is studied,and the corresponding solutions are proposed.Taking water vapor detection as an example,the main sources of interference factors are analyzed from the aspects of additional absorption background interference and non-absorptive background interference.The additional absorption background interference originates from the background gas absorption introduced in the optical device packaging process,and the photodetection is studied.Pairing,long-pathlength absorption cell dilution,and waterless optics reduce interference from additional background gas absorption,reducing the additional background absorption of the water vapor sensing system from 727.7 ppm to 1.7 ppm;The source of non-absorptive power fluctuations is studied.The different performance effects of subtraction,division and BRD demodulation circuits in the face of non-absorptive power fluctuations are studied.A single optical path absorption peak demodulation algorithm is studied to suppress non-absorbent power.The interference of fluctuations on the measured signal.5.The photo-acoustic spectroscopy gas detection technology is introduced.According to the different methods of photoacoustic signal detection,the vocal cavity photoacoustic spectroscopy,Fabry-chamber photoacoustic spectroscopy and quartz tuning fork enhanced photoacoustic spectroscopy gas sensing system are introduced.The quartz tuning fork enhanced photoacoustic spectroscopy gas sensing system is mainly studied,which optimizes the optimal laser scanning frequency and wavelength modulation coefficient in photoacoustic spectroscopy applications.The center frequency and sound detection efficiency of quartz tuning fork are studied in a long-term stability experiment;several photoacoustic resonance structures were compared by experiments,and finally the coaxial double-tube resonance structure was able to achieve the best photoacoustic enhancement effect in standard quartz tuning fork photoacoustic detection.In order to improve the measurement sensitivity of photoacoustic spectroscopy gas sensing system,a second harmonic-based wavelength calibration technique is studied,which enables us to lower the filter bandwidth and apply the averaging algorithm.The signal-to-noise ratio is increased by more than one order of magnitude;a round-trip photoacoustic enhancement structure is designed to nearly double the photoacoustic signal without introducing unwanted noise;a fiber laser intra-cavity photoacoustic spectroscopy is proposed,using the characteristics of high power and light transmission in the fiber laser cavity,greatly enhances the photoacoustic signal,and realizes 6 watt photoacoustic excitation power through active Q-switching technology,and realizes ppt level detection limit of acetylene.6.Combining the theory of interference with the technique of absorption spectroscopy,a Fourier-domain optical coherent absorption spectroscopy technique is proposed,which can realize the absorption integration detection and distributed gas detection,and establish the corresponding theoretical model.The feasibility of this theory for gas detection is verified by experiments.The innovations of this paper are:1.A single optical path absorption peak demodulation algorithm is proposed.The algorithm can flatten the scan baseline with absorption peak information and obtain the absorption peak signal for gas concentration demodulation without the reference optical path and reference signal.The optical power normalization coefficient is integrated in the algorithm.In theory,the interference of the non-absorbent power fluctuation on the measured absorption peak signal can be completely eliminated,and verified in the experiment,and good results are obtained.2.The influence of laser scanning frequency on the photoacoustic signal of the photoacoustic spectroscopy gas sensing system is studied.The photoacoustic signal is generated by the absorption of photon energy by the substance,and then converted into a pressure wave,that is,the form of sound wave.Photodetectors have different high-speed detection of light.The generation and detection of photoacoustic signals involve the conversion,accumulation and propagation of energy.Therefore,the scanning frequency cannot be too fast.This paper optimizes the scanning frequency in photodetection gas detection applications to obtain The largest photoacoustic signal;the wavelength modulation coefficient m is optimized by experiments,and the conclusion that the optimal modulation factor is not necessarily 2.2 in practical applications is obtained and explained;for the quartz tuning fork enhanced photoacoustic spectroscopy gas detection system The bare quartz tuning fork of the photoacoustic detection element was subjected to a long-term stability test for 8 months.The experiment involved its center frequency and sound detection efficiency.It was found that the exposed tuning fork was exposed to the air environment for a long time,and its center frequency was shifted.The sound detection efficiency will also decrease.3.In order to improve the measurement sensitivity of the photoacoustic spectroscopy gas detection system,a wavelength calibration technique based on the second harmonic peak is proposed.This technology can be combined with the constant wavelength-wavelength modulation technology to avoid the light source in the photoacoustic spectrum gas detection system.The limitation of scanning frequency makes it possible to compress the bandwidth of the lock-in amplifier and average the measurement results several times,and greatly improve the signal-to-noise ratio of the detection system.The experimental results show that the signal-to-noise ratio is increased by more than one order of magnitude;a miniaturized round trip is designed.The photoacoustic resonance enhances the optical path,and the photoacoustic signal is nearly doubled without introducing excessive noise.4.The Fourier domain optical coherent absorption spectroscopy technique is proposed,which combines the interference theory with the absorption spectroscopy.The technique has the characteristics of absorption line line type internal absorption area detection,and the absorption line type broadening contrast amplitude caused by pressure and background gas.The influence of the value detection method has an inhibitory effect.In addition,distributed gas detection can be realized by performing Fourier transform on the interference signal measured by the method. |
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