The direction of propagation and phase of a light wave diffracted successively by two sound waves of the same frequency but different wavelengths can be controlled by varying the acoustic frequency and the relative phase of the sound waves. Because the Doppler frequency shifts produced by the two sound waves are of equal magnitude but opposite signs, they cancel each other, with the consequence that the diffracted-diffracted wave is coherent with the incident (or undiffracted) light. If a wavefront distorted by atmospheric turbulence is divided into subwavefronts, each subwavefront being planar, it can be corrected in real time if it is allowed to pass through an array of Bragg cells. Each cell carries two pairs of orthogonal sound waves to correct for tilts and phase errors of a corresponding subwavefront.
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