Atmospheric temperature is an important meteorological parameter in atmospheric physics, weather forecasting, and environmental monitoring. Because of the advantages of high temporal and spatial resolution, atmospheric temperature lidar has become a research hotspot. Because of the influence of aerosols at the bottom of the atmosphere, pure rotational Raman and Rayleigh high optical spectral lidars have become important equipment for observing atmospheric boundary layer temperature profiles. Among them, the pure rotational Raman lidar with a medium optical spectral resolution has particularly developed rapidly. However, it still has problems such as daytime detection capability and system stability, which have become critical factors in limiting its application. Based on the current application status of these optical spectroscopic bulk optics in the pure rotational Raman lidar, particularly the application of F-P interferometer (FPI) in the temperature relative detection technique of pure rotational Raman lidar, a multichannel pure rotational Raman lidar spectroscopic system is proposed and designed by combining with the research progress of our research group in the field of absolute-probing temperature lidar.

Considering the influence of the atmospheric fluorescence signal, the pure rotational Raman signal of the anti-Stokes branch is selected in the pure rotational Raman fine spectral-line structure of nitrogen molecules in the atmosphere to retrieve the atmospheric temperature. The FPI is used as a frequency comb filter to configure the primary pure rotational Raman spectroscopic system to effectively filter the daytime solar background noise and the pure rotational Raman spectral signals of other gas components such as oxygen molecules, which are distributed between the pure rotational Raman spectral signals of nitrogen molecules. A secondary pure rotational Raman spectroscopy system is constructed using the advantages of a low-order diffraction grating for spatial spectroscopy combined with a precise and uniform fiber line array to effectively extract the pure rotational Raman signal of the even rotational quantum number of nitrogen molecules. Based on FPI and low-order diffraction gratings, a two-stage pure rotational Raman spectroscopy system is promoted to effectively filter the background noise between the pure rotational Raman spectral lines and achieve a suppression rate of approximately 60 dB for elastic scattering signal, thereby achieving absolute detection of atmospheric temperature profile based on the pure rotational Raman lidar.

First, the FPI application results in the relative detection temperature lidar and multichannel pure rotational Raman absolute detection temperature lidar technique are combined, and the direct retrieval model of the atmospheric temperature from the multichannel pure rotational Raman signals is analyzed based on the principle of least squares. A statistical error model for retrieving atmospheric temperature from multichannel pure rotation Raman signal is proposed using sensitivity and signal-to-noise ratio (SNR) theory. A two-stage parallel 8-channel pure rotational Raman spectroscopy with FPI and low-order diffraction grating as the core is designed, followed by the construction of a pure rotational Raman lidar for absolute temperature measurement throughout the day (Fig. 2). Second, based on the spectroscopic principle of multibeam interference of FPI, it finely matches the spectral structure of the pure rotational quantum number of nitrogen molecules and optimizes performance parameters such as the free spectral range, fineness, extreme transmittance (maximum and minimum transmittance), and full width of half maximum (FWHM) of FPI. Considering the film loss and mirror defects of FPI, the structural parameters (Table 2) and performance parameters (Fig. 5) of FPI are optimized to match the pure rotational quantum number Raman spectra of the anti-Stokes branch of the nitrogen molecule. Therefore, we can suppress the elastic scattering signal by approximately 60 dB using a two-stage spectral filter (Table 3), and effectively filtering out the solar background noise and neighboring spectra in these eight channels. Finally, assuming that the radiant energy density of the solar background light is approximately 0.5 W·m^-2·sr^-1·nm^-1, and the standard atmosphere model and statistical error analysis model are combined based on the SNR, the signal power and SNR of the rotating Raman lidar for absolute profiling atmospheric temperature throughout a day are simulated (Fig. 6) using the system parameters of the pure rotation Raman lidar (Table 1) and the multichannel pure rotation Raman spectroscopy parameters (Table 3), and then the retrieval temperature deviation of the system are analyzed (Fig. 7) to verify the effectiveness of 8-channel rotating Raman spectroscopy system.

In this study, we propose and design a two-stage parallel 8-Raman channel spectroscopic system with FPI and a low-order diffraction grating. The mirror spacing and FWHM of the frequency comb filter FPI are optimized to be 629.300 μm and 6 pm, respectively. The reflectivity and defect fineness are 0.9519 and 45.63, respectively, to efficiently filter out the solar background radiation noise and adjacent Raman spectral-line signals. The eight even rotational quantum numbers of nitrogen molecules (J=6-20) are chosen as the retrieval spectral signals for the absolute detection of atmospheric temperature profiles. The pure rotational Raman channel can suppress the elastic scattering signal by approximately 60 dB when the test results of the first-order diffraction grating by our research group are combined. The simulation analysis results show that, while the accumulation time is 17 min, the pure rotational Raman spectroscopic system can achieve the synchronous extraction of 8-channel pure rotational Raman signals of up to 1.8 km during the daytime with SNR better than 10. The detection height of the lidar for directly retrieving temperature is then increased from 0.4 to 0.8 km with a deviation less than 1 K. Considering the actual lidar system overlap factor, the temperature profile of a certain height in the troposphere can be obtained, allowing for uncorrected detection of atmospheric temperature throughout the day.