We demonstrate a metastable helium Faraday optical filter operating on the 23S1 − 23P1 and 23S1 − 23P2 transition at 1083 nm by using a 3 cm long helium cell. The influence of the magnetic field and gas pressure of the helium cell on the filter characteristics is experimental studied. When the magnetic field is 230 Gs and the gas pressure of helium cell is about 110 Pa, the peak transmission corresponding to the two energy level transitions is about 32% and 57%, respectively. The equivalent noise bandwidth (ENBW) under this working condition is about 1.9 GHz. The metastable helium Faraday filter can be used to improve the optical inefficiency of a helium resonance fluorescence lidar to achieve the metastable helium density detection at 200–1000 km thermosphere.

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An ultranarrow-bandwidth-optical-receiver-based ultraviolet trifrequency Rayleigh Doppler wind lidar (DWL) technology is proposed that is able to simultaneously detect stratospheric wind with high precision during the daytime. The lidar system is designed, and the principle of wind measurement is analyzed. An ultranarrow-bandwidth element used for suppressing strong background light is designed as an important part of the ultranarrow-bandwidth optical receiver. A three-channel Fabry–Perot interferometer (FPI) is capable of measuring wind speed. A non-polarized beam splitter cube optically contacted on the three-channel FPI can offer a stable splitting ratio. The parameters of the three-channel FPI are optimized. The structure and parameters of the ultranarrow-bandwidth element are designed, and the transmission curve is measured. The transmission curve and stability of the three-channel FPI are validated. The background photon number is collected with the ultranarrow-bandwidth element and with an interference filter (IF) alternately from 08:00 to 18:00. Based on the selected system parameters and measured background photon number, the detection performance of the proposed lidar is simulated. Simulation results show that with 200 m range resolution from 15 to 25 km, 500 m range resolution from 25 to 40 km, and 30 min total accumulation time for paired line-of-sight (LOS) measurement, within ± 100 m / s LOS wind speed range, the daytime LOS wind speed error is below 4.77 m/s from 15 to 40 km altitude. Compared with the traditional IF-based dual-FPI Rayleigh Doppler lidar, the wind speed accuracies are improved by 1.29–16.29 times and the detection altitudes are improved from 23.55 to 40 km with the same wind-detecting precision.

The flood discharge atomization of high dams involves a complex coupled flow of water and air. Small-scale model tests are typically used to predict the atomization of flood discharge. However, the accuracy of the prediction results often suffers because of the scale effect between the model and the prototype. Considering that the numerical simulation method has the advantage of not being restricted by similarity scales, this paper studies the influence of the scale effect on the atomization of flood discharge based on the principle of water‒air two-phase flow. Taking the Shuibuya Hydropower Station as the research object, the distribution of the flood discharge atomized rainfall and the atomized wind speed are studied when the boundary conditions, ambient atmospheric pressure, and geometric dimensions meet similar requirements. The research results show that under the same boundary conditions, the geometric scale is the most important factor affecting flood discharge atomization. The smaller the geometric scale, the smaller the atomization wind speed and rainfall intensity obtained by the model, which means that smaller monitoring errors lead to larger prediction deviations. When the calculation model satisfies similar atmospheric pressure conditions, the atomization wind speed and rainfall obtained by the models with different geometric scales satisfy the standard exponential function relationship. By comparing with the atomized rainfall and wind speed data observed by the Shuibuya prototype, it is found that the prediction accuracy of the prototype can be greatly improved when the model satisfies a similar atmospheric pressure.

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