Characterizing atmospheric turbulence and its impact on electro-optical systems

Electro-optical (EO) systems, whether used for astronomical observations, remote-sensing and surveillance from space, tracking and high-resolution imaging of satellites, delivery of directed energy to space-based platforms, or horizontal-path imaging and laser communications, are always affected by atmospheric turbulence and in the majority of cases this turbulence imposes a fundamental limitation to their performance. In order to be able to compensate these effects one first needs to characterize turbulence: its strength and properties. Most EO systems are operated in the lower atmospheric boundary layer which is highly affected by atmospheric turbulence. It is absolutely necessary to describe these optical disturbances in order to predict the performance of the systems. Atmosphere is a complex system, so simplifications and approximations have to be made. We rely on statistical descriptions of turbulence to model its effect on EO systems. Kolmogorov's theory is the fundamental approach for the description of optical turbulence but it is not always valid, especially close to the ground where turbulence might not be fully developed and the short outer scale might seriously limit the range of spatial frequencies where Kolmogorov theory is valid, i.e. the inertial range. To investigate the statistical behaviour of turbulence effects, Fraunhofer IOSB performed several experiments with a new technique based on differential motion measurements from an array of light-emitting diodes. The technique allows for measurement of not only the path-averaged turbulence strength but also of the outer scale, anisotropy coefficient, possibly non-Kolmogorov slope and transverse wind speed. The extracted values of these characteristics serve as input for simulations of laser beam propagation through realistic atmospheric turbulence.