| Due to its high spectral resolution,high signal-to-noise ratio(SNR),and easy integration,laser heterodyne spectroscopy detection technology has attracted extensive attention in recent years in the fields of greenhouse gases measurements and optoelectronic engineering.With the improvement of system performance,the measurement wavelength range also has a trend from long-wave infrared to short-wave infrared.However,the measurement of the atmospheric molecular profile,column concentration,and laser atmospheric propagation requires a high spectral resolution of the instrument.The SNR,spectral resolution,and time resolution of the laser heterodyne radiometers(LHRs)previously established in the laboratory are insufficient,and the measurement accuracy needs to be further improved.In addition,the measurement methods of system performance are not perfect,system integration and stability are not satisfactory.As a result,the parameters such as linear function and minimum detectable energy of the instrument are difficult to be directly measured,and the field data acquisition ability of the instrument is weak.For the above problems,this paper conducts in-depth research on the integration establishment,performance improvement,signal processing and optimization of the laser heterodyne spectral detection system.Based on the high-resolution spectral data obtained by the measurement system,the greenhouse gases column concentration and height profiles were inverted.The main results are as follows:1.A 1.316 μm Distributed Feedback(DFB)semiconductor laser was used as the local oscillator to establish a demodulation LHR.The optical fiber structure and balanced detection method were used to improve the optical transmission efficiency of the system and suppress the system noise.Compared with the previous versions of the laboratory,the performance of the system has been greatly improved.The spectral resolution and SNR are 0.009 cm-1 and 120,respectively.The absorption spectrum of H2O in Hefei was measured,and the inversion of the atmospheric transmittance,H2O profile,and column concentration were carried out.The inversion results of column concentration were compared with the Fourier transform spectrometer EM27/SUN.By comparison,the results are consistent,the maximum relative deviation is 13.5%.2.By analyzing the relationship between the integration time of the lock-in amplifier and the response speed of the system,a near-infrared non-demodulation optical fiber LHR was established.The system response speed and SNR have been effectively improved,the spectral resolution and SNR have reached 0.006 cm-1 and 300,respectively.The SNR and optimal average times of the system were measured.When the demodulation and non-demodulation acquisition times are 6 min and 2 min,respectively,the SNR is 58 and 103.In the case of an average of 9000 times,the noise ratio can reach the highest.Using this system,the absorption spectrum of CH4 in the atmosphere was measured,and the transmittance of the atmosphere,the profile,and column concentration of CH4 were inverted.In addition,the heterodyne optical path of the system was modularized and the overall system was preliminarily integrated.3.Based on the original work in the laboratory,a free-space optical path LHR was established with the distributed feedback inter-band cascade lasers(DFB-ICL)in the mid-infrared band as the local oscillator.The absorption spectra of atmospheric H2O,HDO,CO2,and O3 in Hefei and Golmud were measured,respectively,and the SNR and spectral resolution met the inversion requirements of molecular concentration profiles.The profiles of H2O and O3,the column concentrations of H2O,CO2,and O3,and the abundance of HDO were inverted.Through the research on laser heterodyne spectroscopy detection technology and greenhouse gas measurement,the spectral resolution,SNR,measurement speed,and integration of the system were greatly improved.The non-demodulation signal processing method provides a reference for the establishment of a fast response miniaturized LHR. |
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