Application Research Of Photoacoustic Spectroscopy Technology In Environmental Monitoring And Power System

Trace gas sensors based on photoacoustic spectroscopy（PAS）technology,with their high detection sensitivity,high gas selectivity,compact structure,small size,and low cost,are important in many application areas,such as atmospheric environment monitoring,industrial process control and non-invasive medical diagnosis.Techniques based on PAS for trace gas sensing,compared to other techniques,have these characteristics: zero background detection,independence of the excitation wavelength and photoacoustic signal amplitude proportional to the optical power.For the detection of atmospheric environmental pollution gases and the safety monitoring of electrical equipment in power systems,a variety of high-performance photoacoustic gas sensors are developed.By using the resonant photoacoustic cell and quartz tuning fork as the acoustic transducer,the following innovative researches have been obtained:1.Based on the theory understanding of acoustic resonator,a two-channel differential photoacoustic cell is designed.Due to the two photoacoustic resonant cavities with exactly the same shape and volume,the noise interference caused by the gas velocity,window absorption and external electromagnetic interference can be eliminated by using a differential circuit,which improves the detection signal-to-noise ratio of the photoacoustic sensor.Without increasing the gas flow rate noise,the photoacoustic sensor can be operated at a high gas flow rate,which can eliminate the possible saturation absorption effect under the high power excitation light source,thus reducing the viscous effects of gases（such as NH₃ and SO₂）,and improving the response time of the sensor system;2.Based on the differential photoacoustic cell,a variety of gas sensors have been developed by using the excitation wavelength from ultraviolet light sources to visible light,near-infrared and mid-infrared excitation light sources.The gas sensors can be used for long-term online monitoring of nitrogen oxide,nitric oxide and sulfur dioxide pollution gases,and can simultaneously detect gas molecules such as hydrogen sulfide,carbon monoxide and sulfur dioxide under SF₆ carrier gas,which meets the application needs of atmospheric pollutants and power systems;3.With the high-power excitation light,a simplified two-level model of the absorption process was used to describe the photoacoustic production due to photon absorption.The high gas flow can reduce the gas saturation absorption effect that was experimentally verified.Therefore,in the case where the photoacoustic signal is not saturated,an inexpensive high-power LD laser is used to detect NO₂.Moreover,a commercial fiber amplifier is used to pump the power of the near-infrared light source to the watt level,which makes up for the near-infrared weak absorption line strength.The obtained detection limit was comparable with that obtained with a mid-infrared light source;4.Based on the physical characteristics of SF₆ gas,a high-Q differential gas photoacoustic cell module is designed.The absorption spectrum of SF₆ molecules in a high concentration at 3.3-10 μm was also obtained by using a Fourier infrared spectrometer.Therefore,an excitation light source with a wavelength of ultraviolet or near-infrared is selected to avoid high background signals when using a mid-infrared light source.In addition,using the time division multiplexing method,a multicomponent SF₆ gas decomposition detection gas sensor has been designed,which can simultaneously detect hydrogen sulfide,carbon monoxide and sulfur dioxide gas at ppb-level,and meet the gas detection requirement of < 1 ppm in the power system;5.The photoacoustic detection modules with different structures based on the quartz-enhanced photoacoustic spectroscopy were studied.By using the electrical modulation elimination method,the high background noise amplitude generated by the high-power excitation light source can reduce to μV level.The V-T relaxation time of CO molecules under dry and wet nitrogen was also obtained.