Since the early 1980s when the Airborne Laser Laboratory (ALL) first demonstrated that a high-energy laser flown in a military aircraft was capable of performing both offensive and defensive missions, the concept of a speed-of-light weapon system has been in the mind of future weapons planners. The ALL used a powerful CO_2 laser which lased at 10.6 μm. The maximum intensity at the target for a diffraction-limited beam of power P_0 and aperture A is related to the inverse of wavelength λ times distance to the target R squared: I_o = P_oA/λ~2R~2. Thus, the natural aspiration was to develop high-energy lasers that lase at shorter wavelengths to increase the lethal range of the laser for the same power and aperture size. On the other hand, the reduction in diffraction-limited intensity due to aberrations, measured in rms optical path difference OPD_(rms) imprinted on the beams wavefront by turbulent density fluctuations in the air flowing over the exit pupil, is governed by the large-aperture approximation I/I_o ≌ e-(2πOPD_(rms)/λ)~2.
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