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An optical wavefront propagating through the atmosphere will be perturbed by local variations in the refractive index of the atmospheric gases. When accumulated over long optical path distances they will impart a spatial and temporally random distortion to the wavefront. These distortions have a characteristic spatially coherence length r0, and an atmospheric decorrelation time τo. In directed energy applications, atmospheric distortions can reduce the peak target energy densities of large diameter laser beams by orders of magnitude. The problem is not solved through the use of larger apertures; once the aperture size increases beyond one or two τ0 the far-field spot remains constant in size. Hence, for large aperture systems, the overall performance is set by the spatial coherence of the atmosphere and not by the systems' exit pupil. An adaptive optics (AO) system can compensate for the degrading effects of the atmosphere and significantly restore diffraction limited performance. A segmented or deformable mirror with appropriate control signals can "pre-distort" the outgoing beam to cancel the atmospheric aberration. To be effective, the system must measure and correct wavefront variations over spatial resolution elements on the order of one ro and generate the compensation in a time less than τo. ITC is currently producing its third generation of segmented mirrors for atmospheric compensation. Advanced systems for short wavelength operation with five hundred segments have been tested and proven as part of integrated adaptive optical systems, see Figure 1. These systems have excellent optical figure, wavefront fitting, and dynamic performance as well as low cost. The segmented mirrors' inherent modularity has eliminated the classic optical fabrication problems associated with large optics. Using TTC's third generation technology it is now practical to produce segmented wavefront correctors with more than 10,000 segments for compensation of short wavelength systems with apertures of 4 m and beyond.
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