Development of a lidar technique for profiling optical turbulence

Many techniques have been proposed for active optical remote sensing of the strength of atmospheric refractive turbulence. The early techniques, based on degradation of laser beams by turbulence, were susceptible to artifacts. In 1999, we began investigating a new idea, based on differential image motion (DIM), which is inherently immune to artifacts. The new lidar technique can be seen as a combination of two astronomical instruments: a laser guide star transmitter/receiver and a DIM monitor. The technique was successfully demonstrated on a horizontal path, with a hard-target analog of a lidar, and then a true lidar was developed. Several investigations were carried out first, including an analysis to predict the system's performance; new hard-target field measurements in the vertical direction; development of a robust inversion technique; and wave optics simulations. A brassboard lidar was then constructed and operated in the field, along with instruments to acquire truth data. The tests revealed many problems and pitfalls that were all solvable with engineering changes, and the results served to verify the new lidar technique for profiling turbulence. The results also enabled accurate performance predictions for future versions of the lidar. A transportable turbulence lidar system is currently being developed to support field tests of high-energy lasers.