Single-photon Distributed Free-space Spectroscopy

Spectroscopy can be traced back to the 17th century when Newton used a prism to disperse white light into colorful light bands.With the development of quantum mechanics and laser technology,precise spectroscopic analysis methods can be used to analyze the substances composition qualitatively and quantificationally,which are widely applied and play an important role in the fields of chemistry and biology.In terms of atmospheric gas remote sensing,spectral measurement methods mainly include passive methods,such as grating spectrometers,Fourier infrared spectrometers,etc.,and active methods,such as optical frequency comb remote sensing,and multi-wavelength differential absorption Lidars.However,these technologies for atmospheric gas measurement are limited to column-integrated spectral measurement.Challenges exist in precise spectral remote sensing in free space,which hinders the further development of various gas components and chemical processes in the atmosphere.In this paper,the importance of carbon neutrality and carbon peaking is introduced.Besides,the urgency of greenhouse gas emission monitoring and the development of localized observation equipment are pointed out.Furthermore,the currently methods used for atmospheric gas measurement including passive and active remote sensing technologies are compared from the applicable fields to limitations.The paper uses superconducting nanowire single-photon detection technology to reduce the high requirements for laser energy in traditional Lidar methods and further improve the signal-to-noise ratio.Besides,by using the comb-referenced locking technology,the emitted frequency of the tunable laser can be controlled accurately within a wide range.Hence,the distributed spectra in free-space can be realized,which holds much potential for carbon emission monitoring,leakage warning,and atmospheric chemistry researches.The main work of the thesis is as follows:1.The principles and design of the free-space distributed spectral remote sensing based on single-photon are described.First,the theoretical analysis of the molecular absorption spectroscopy is introduced.Second,the spectrum and the corresponding concentration inversion process are derived according to the formula based on Lidar principle.Then,the principle of frequency comb locking is introduced and the whole system is preliminary designed by combining superconducting nanowire single photon detector.2.According to the line selection criteria for gas spectrum measurement,the absorption lines(R16 line with a central wavelength of 1572.335 nm)that not only have suitable spectroscopy parameters but also have detection advantages for CO2 spectral remote sensing are analyzed by simulations,with the influence of HDO as an interference gas considered.Besides,by simulating the Lidar signal with different noise reduction methods,the spectrum and concentration distribution are obtained by inversion algorithm.The change trend of the simulated result is consistent with the preset ideal value.On the basis of theoretical simulation,the feasibility of the system for free-space distributed spectral remote sensing is analyzed.3.The range resolved spectral remote sensing experiment is carried out.The system parameters and corresponding experimental settings are introduced in detail.Besides,the control loop design based on frequency comb locking technology and the receiving system based on single-photon detection are also described.The experiments mainly include:(1)The atmospheric spectrum is remotely sensed and compared with different frequency sampling intervals based on frequency comb locking technology to verify the adaptability and controllability of the sampling mode;(2)The spectrum is measured and compared without and with the frequency locking to verify the stability and necessity of locking;(3)The mixture spectra of CO2 and HDO are remotely sensed in winter and summer,respectively,which verifies that HDO acting as an interference gas has a non-negligible effect on CO2.Besides,the spectrum of HDO is separated by the three-peak Voigt fitting method with its concentration obtained,which further weakens the influence on CO2;(4)A continuous observation experiment over 72 hours is carried out,which further proves the system ability for day and night observation.Meanwhile,the retrieved concentrations of CO2 and HDO are compared with the in-situ measurements,which shows good agreement.The experiment verifies the ability of the system to be used for distributed spectroscopy remote sensing.