Laser wavefront propagation through turbulent separated shear layers: laboratory experiments, computations, and physical modeling

Laboratory experiments, computations, and physical modeling of laser wavefronts propagating through variable-refractive-index separated shear layers at large Reynolds numbers are conducted in order to examine the relation between the flow behavior and the laser wavefront behavior for airborne laser communications. The new element of this work is the focus on the dependence on scale of the optical behavior as well as of the flow behavior, using multiresolution analysis of the measured and computed data. The experiments are conducted using the UC Irvine variable-pressure turbulent flow facility. Direct non-intrusive imaging of the refractive index field is accomplished with laser-induced fluorescence and a high-resolution digital camera that resolves three decades of scales. Simultaneously, direct imaging of the propagated laser wavefront phase profile is conducted using a Shack-Hartmann array sensor that also has a resolution of three decades of scales. The computational component consists of near-field wavefront propagation through the measured refractive index field, validated by the direct wavefront measurements. We have conducted multiresolution analysis of the flow data and optical data, by a posteriori reducing the resolution of the refractive-index field and phase field. We present evidence of strong scale dependence at large scales, i.e. in the energy-containing range of scales. Physical modeling of this behavior is developed based on the structure of the coarse-grained refractive turbulent interfaces. This approach is useful in order to relate the root-mean-squared optical path difference and Strehl ratio, at variable resolutions, to the refractive-index variations along the laser wavefront propagation path. This facilitates the identification of the dominant refractive interfaces and serves as a guide to developing aero-optical optimization methods for airborne laser communication applications.