Atmospheric compensation with a speckle beacon in strong scintillation conditions: directed energy and laser communication applications
ABSTRACT Wavefront control experiments in strong scintillation conditions (scintillation index, approximately equal to 1) over a 2.33 km, near-horizontal, atmospheric propagation path are presented. The adaptive-optics system used comprises a tracking and a fast-beam-steering mirror as well as a 132-actuator, microelectromechanical-system, piston-type deformable mirror with a VLSI controller that implements stochastic parallel gradient descent control optimization of a system performance metric. The experiments demonstrate mitigation of atmospheric distortions with a speckle beacon typical for directed energy and free-space laser communication applications.
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ABSTRACT: Adaptive optics methods have long been used by researchers in the astronomy field to retrieve correct images of celestial bodies. The approach is to use a deformable mirror combined with Shack-Hartmann sensors to correct the slightly distorted image when it propagates through the earth's atmospheric boundary layer, which can be viewed as adding relatively weak distortion in the last stage of propagation. However, the same strategy can't be easily applied to correct images propagating along a horizontal deep turbulence path. In fact, when turbulence levels becomes very strong (C-n(2)>10(-13) m(-2/3)), limited improvements have been made in correcting the heavily distorted images. We propose a method that reconstructs the light field that reaches the camera, which then provides information for controlling a deformable mirror. An intelligent algorithm is applied that provides significant improvement in correcting images. In our work, the light field reconstruction has been achieved with a newly designed modified plenoptic camera. As a result, by actively intervening with the coherent illumination beam, or by giving it various specific pre-distortions, a better (less turbulence affected) image can be obtained. This strategy can also be expanded to much more general applications such as correcting laser propagation through random media and can also help to improve designs in free space optical communication systems.SPIE Optical Engineering+ Applications; 10/2014
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ABSTRACT: In a previous experiment (Tunick, 2008: Optics Express 16, 14645-14654), values for the refractive index structure constant and the Fried parameter were calculated from measurements of signal intensity and angle-of-arrival statistics based on idealized models. Calculated turbulence parameters were evaluated in comparison to scintillometer-based measurements for several cases. It was found that the idealized models alone were insufficient to accurately describe complex, non-uniform microclimate and turbulence conditions. In addition, the signal intensity and focal spot displacement measurements were quite sensitive to platform and light source jitter. In order to compensate for adverse effects such as platform vibrations, an alternative differential image motion method is explored for optical turbulence parameter characterization. Hence, further experimental research is conducted along a 2.33 km free-space laser path to capture differential image centroid data from which Fried parameter and refractive index structure constant information can be obtained. This research is intended to provide useful information for US Army laser communications, long-range imaging and energy-on-target.Proceedings of SPIE - The International Society for Optical Engineering 08/2009; DOI:10.1117/12.823969 · 0.20 Impact Factor