Objective   Doppler lidar is one of the most powerful tools for the remote sensing of the three-dimensional wind field in the atmosphere at present. It is widely used in wind power generation, weather forecast, aviation safety, atmospheric science research and other fields. The traditional coherent detection or direct detection Doppler lidar requires a single-longitudinal-mode laser source with narrow linewidth, resulting in the shortcomings of the existing Doppler lidar system such as high cost, poor environmental adaptability, low laser energy utilization, which seriously restricts their industrialization and the airborne and spaceborne applications. Therefore, it is of great significance and scientific value to explore and study the technology of multi-longitudinal-mode (MLM) Doppler lidar using MLM laser as the emission source. For this purpose, the MLM Mie Doppler lidar technology based on dual Fabry-Perot interferometer (FPI) is proposed and studied.

  Methods   The detection principle of MLM Mie Doppler lidar based on dual FPI is analyzed (Fig.1). The theoretical formulas of radial wind speed and backscatter ratio measurement errors are derived, and the matching relationship between the longitudinal mode interval of the MLM laser source and the free spectral spacing of the dual FPI is analyzed, as well as the wind speed measurement error caused by the mismatch between the two. The lidar system structure (Fig.7) and parameters (Tab.1) are designed, and the detection performance of the designed lidar system is simulated using the 1976 USA atmospheric model and simulated cumulus clouds.

  Results and Discussions   The frequency matching condition between the longitudinal mode interval of the MLM laser source and the free spectral spacing of the dual FPI is that the former is an integral multiple of the latter. When the frequency matching condition is satisfied, the MLM wind measurement is equivalent to the superposition of each single-longitudinal-mode (SLM) wind measurement. In the low wind speed region, the percentage of the wind speed measurement error E_V caused by the frequency matching error increases rapidly with the increase of the matching error; When the frequency matching error remains unchanged, E_V decreases slowly with the increase of wind speed; When the frequency matching error is less than 10 MHz, E_V will be less than 5% (Fig.6). The simulation results of lidar detection performance show that, in the range of 0-10 km altitude and 0-50 m/s radial wind speed, when the range resolution is 30 m, the time resolution is 30 s and the zenith angle of laser emission is 30°, the radial wind speed measurement accuracy of the lidar system is better than 1.50 m/s and 1.02 m/s in daytime and nighttime respectively; Under cloudless conditions, the relative measurement accuracy of the backscatter ratio in daytime and nighttime is better than 6.57% and 4.53%, respectively (Fig.9).

  Conclusions   A Mie Doppler lidar technology based on multimode laser and dual FPI is proposed and studied. This technology requires that the longitudinal mode interval of the laser source should match the free spectral spacing of the dual FPI, and the center frequency of each longitudinal mode should be locked near the intersection of the dual FPI periodic spectrum curves. When the frequency matching condition is satisfied, the MLM wind measurement is equivalent to the superposition of each SLM wind measurement. The frequency matching error will increase the wind speed measurement error, but as long as the frequency matching error is controlled below 10 MHz, the impact of the matching error on the wind speed measurement accuracy is less than 5%, which can be easily achieved through system calibration. The simulation results show that the Doppler lidar system based on this technology has high detection accuracy of wind speed and backscatter ratio in all weather. These conclusions fully demonstrate the feasibility of this technology.