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Chapter 6:
Systems —€” Putting It All Together
Adaptive optics are useful for compensating a wide variety of aberrations, not just those imposed by the atmosphere. Early investigation of adaptive optics included compensation for the deleterious effects of high energy lasers, mostly thermal distortions in the laser gain medium and in the optics. High energy lasers were developed so that their energy, when focused down to a small spot on an enemy missile, would melt through it and destroy it, but problems arose when the energy was being propagated out through an optical system. If the power was enough to melt enemy missiles, it was enough to melt and destroy the optics that direct it, so the field of high energy laser beam control evolved to include high reflectivity optical coatings and liquid-cooled optics. Since it was impractical for a missile to have a high reflectivity coating and really a mess to require a heavy water-cooled surface, the advantage goes to the high energy beam control system. When it's laser versus missile, it becomes laser: game, set, match. Thermal distortions in optics come in two flavors. The first arises when a flat plate, the mirror, is constrained around its edges and is not free to expand inplane when heated. As the mirror absorbs energy, even one with a good coating bows and its surface assumes a parabolic shape, adding defocus into the beam. The bowing distortion is proportional to the total power incident and is called power-induced distortion. When the mirror is hit with a nonuniform intensity pattern, like from just about every high power beam in existence, the expansion goes in an out-of-plane direction; hot intensity spots distort or bubble up more than cold spots. The intensity mapping distortion, or flux-induced distortion, is proportional to the incident intensity at each point and the absorption of the laser energy at each point.
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