Research On V-shaped Off-axis Integrated Cavity Detection Technology And In-situ Measurement Of Greenhouse

Off-Axis Integrated Cavity Output Spectroscopy(OA-ICOS)is widely used to detect trace gases because of its high sensitivity,strong robustness,online detection,and simple structure.OA-ICOS detects though dense high-order transverse modes in the resonant cavity,without complicated and sensitive mode matching and no need to lock the cavity to the laser,it suppresses a large number of cavity mode noise,which is very suitable for field detection.However,the residual cavity mode noise is still the main factor limiting the sensitivity of OA-ICOS system.At the same time,the off-axis configuration also sacrifices the high transmittance of the coaxial resonant cavity,resulting in very weak light intensity transmitted from the cavity,which limits the further improvement of the detection sensitivity.In view of the above problems,this paper theoretically analyzed the cavity mode structure,detection principle and noise source of OA-ICOS.A number of OA-ICOS experimental measuring devices with different structures have been built,and in-depth studies have been carried out from the aspects of optical path simulation,cavity structure,signal analysis and optimization,and a variety of trace gases have been detected based on the measuring devices.The main achievements are as follows:1.A high sensitivity wavelength modulated reinjection OA-ICOS experimental setup was built at 2 μm band,and the high sensitivity absorption spectroscopy of CO2 gas was experimentally studied.Through the simulation of the optical reinjection optical path structure,the optimal parameters of the optical reinjection lens were determined for design and processing.The CO2 standard gas was used for experimental comparison and analysis,and the signal intensity of the device was increased by 6.3 times.Further combined with the wavelength modulation technique,the signal-to-noise ratio(SNR)of the output signal from the traditional OA-ICOS 179 to 1288,increased 7.2 times,Allan variance show that the detection limit of the system can reach 0.35ppm when the average time is 230 s.2.A novel V-shaped structure OA-ICOS experimental device was built,and the high sensitivity absorption spectroscopy of NH3 was studied at 2 μm band.The optical simulation software was used to simulate and optimize the experimental optical path,and the distribution law of light spots on the surface of the highly reflective mirrors in the V-shaped cavity was obtained.The analysis results showed that the design of the Vshaped cavity greatly increased the distribution area of light spots and presented a square distribution,greatly reduced the overlap of light spots on the surface and was conducive to the suppression of residual cavity mode noise.In addition,the V-shaped cavity design makes the device more compact.The concentration of NH3 in the room was measured by this device.After smoothing the spectrum with Savitzky-Golay algorithm,the SNR was increased from 23.96 to 46.06,and the detection limit was up to 16.93 ppb.3.A dual-path coupling V-OA-ICOS experimental device was set up to measure the stable isotope of water vapor at 3.66μm band.Based on the structural advantages of V-shaped cavity,combined with theoretical analysis,the dual-path coupling method is further proposed,that is,the light reflected back from the front cavity mirror is used again,and becomes a new independent optical path coupling into the cavity,which not only improves the signal output intensity of the device,but also increases the cavity mode density,and suppresses the residual cavity mode noise.The experimental results show that the dual-path coupling method can increase the output signal intensity by nearly one time and the SNR by 2.1 times.Combined with WMS-2f detection method to improve SNR,in the detection of water vapor stable isotopes HD16O and H216O,the SNR is 245 and 106.6 respectively.The water vapor isotope abundance δD was measured by this device,and the δD in indoor air was-95.7‰.The water vapor isotope in the actual outdoor atmosphere was measured continuously,the 8D value was between-220‰ and-120‰ and the variation trends of HD16O and H216O were analyzed through the experimental observation.