Optimization of a direct detection UV wind lidar architecture for 3D wind reconstruction at high altitude

Abstract. An architecture for a UV wind lidar dedicated to measuring vertical and lateral wind in front of an aircraft for gust load alleviation is presented. To optimize performance and robustness, it includes a fiber laser architecture and a Quadri Mach-Zehnder (QMZ) interferometer with a robust design to spectrally analyze the backscattered light. Different lidar parameters have been selected to minimize the standard deviation of wind speed measurement projected onto the laser axis, calculated through end-to-end simulations of the instrument. The optimization involves selecting an emission/reception telescope to maximize the amount of collected photons backscattered between 100 m and 300 m, a background filter to reduce noise from the scene, and photo-multiplier tubes (PMT) to minimize detection noise. Simulations were performed to evaluate lidar performance as a function of laser parameters. This study led to the selection of three laser architectures: a commercial solid-state laser, a design of a fiber laser, and a hybrid fiber laser resulting in standard deviations of 0.18 m/s, 0.17 m/s, and 0.09 m/s, respectively, at 10 km of altitude. To reconstruct the vertical and lateral wind on the flight path, the lidar is addressed to four different directions to measure four different projections of the wind. We calculate analytically (and validate through simulations) the addressing angle with respect to the flight direction that minimizes the root mean squared error (RMSE) between the reconstructed vertical and lateral wind components and the actual ones, assuming turbulence that follows the Von Karman turbulence model. We found that the optimum angle for an estimation at 100 m is about 50°, resulting in an improvement of about 50 % compared to an angle of 15°–20° typically used in current studies.