A Narrow-band Filter For Helium Resonance Fluorescence Lidar Based On He 2~3S-2~3P Faraday Dispersion Effect

The thermosphere and exosphere are the key areas in the research on atmospherespace interaction,and the resonance fluorescence lidar is a significant means to detect the atmospheric dynamic parameters of the thermosphere and exosphere.There are a large number of helium atoms,ionized helium and metastable helium in the neutral atmosphere of the thermosphere and exosphere.Therefore,the metastable helium atoms in the thermosphere can be taken as tracers to develop helium resonance fluorescence lidar so as to realize the measurement of helium density in the height range from 200km to 1000km.This paper introduces the basic principles of the helium resonance fluorescence lidar,proposes a bistatic helium lidar system based on continuous wave laser,simulates and analyzes the detection performance of the system.Meanwhile,because of the need to extract weak laser signals from strong background light in lidar atmospheric detection,this paper proposes a new narrow-band Faraday Anomalous Dispersion Optical Filter(He 1083nm FADOF)based on He 23S-23P Faraday anomalous dispersion effect.The main content includes the following aspects:Firstly,this paper expounds the basic principles of the helium resonance fluorescence lidar,introduces the fine energy level structure of helium atoms and the radiation characteristics of energy level transitions,and derives the resonance fluorescence lidar equation.Meanwhile,this paper introduces the components of the bistatic helium lidar system,conducts a specific analysis on the detection performance of the system,and obtains the corresponding relationship between the number of photons received by a single pixel of the system and the detection height in winter and summer.Through simulation,the changes of the system's signal-to-noise ratio and relative error with the detection height is analyzed when the detection performance parameters of the detector are different.The result shows that when the metastable helium density is at its maximum(winter),the signal received by the system is stronger,and the signal-to-noise ratio is significantly higher than when the metastable helium density is small(summer).Therefore,the signal-to-noise ratio of the system using InGaAs CCD in winter is better.When the integration time is 2h,the range resolution is 50km,and the height range from 400km to 1000km,the signal-to-noise ratio ranges from 10 to 50.Secondly,the principle of saturation absorption is introduced.The saturated absorption spectrum of He 1083nm transition is measured,and three obvious saturated absorption peaks are obtained.The central wavelengths are 1082.9076nm,1083.0246 nm,1083.0292nm(cross absorption peak)and 1083.0338nm respectively,among which the saturated absorption peak at 1083.0246nm is the narrowest.Meanwhile,the saturated absorption spectrum when the pressure in the helium cell increases to 200Pa is also measured.The result shows that when the pressure in the helium cell increases,the transition intensity between the energy levels decreases significantly,and the line width of the spectrum increases slightly.Finally,this paper proposes an optical filter based on He 23S-23P Faraday anomalous dispersion effect(He 1083nm FADOF).Through the theoretical calculation and experimental measurement research,the filtering characteristics of He FADOF system under different magnetic field intensity,helium cell gas pressure and helium cell length are analyzed.The results show that the experimental measurement results are basically consistent with the theoretical calculation results.The experimental measurement shows that when the length of the helium cell is 3cm,the pressure of the helium cell is 110Pa,and the magnetic field intensity is 230Gs,the peak transmittance of the two transmission peaks corresponding to the 23S-23P1,2 transition reaches the maximum,which is about 32%and 57%,respectively.In addition,the results also show that as the magnetic field and helium cell pressure increase,the equivalent noise bandwidth(ENB W)of the FADOF system gradually increases.Through the study of the above content,this paper shows that the helium resonance fluorescence lidar,which uses metastable helium atoms in the thermosphere as a tracer,is a new type of resonance fluorescence lidar that can be applied to achieve the measurement of helium density in the neutral atmosphere of the thermosphere in the height range from 200km to 1000km.Meanwhile,the He 1083nm FADOF proposed in this paper is connected in series with the interference filter to form an ultra-narrowband filtering system,which can be applied to the helium lidar system to realize the extraction of weak laser signals.