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The Defense Intelligence Agency produced this reference document, dated 31 March 2010, under its Advanced Aerospace Weapon System Applications (AAWSA) Program. It reviews the state of the art and likely evolution of high-energy laser weapons, including chemical, solid-state, fiber and free-electron lasers, as well as beam control and the history of DoD laser research. It concludes that electrically powered lasers could make megawatt-class weapons practical and that spacecraft laser weapons are conceivable within 20 years. It also recommends ways to protect spacecraft from laser damage.
UNCLASSIFIED/ ,'F811. 8FFll!l*L 1!1!11!! 8IU:!Y The lower atmosphere is a difficult medium through which to propagate a laser beam. The molecules which make up the atmosphere and the aerosols which are suspended in it can scatter and absorb light, reducing the number of photons which arrive at the target. These effects are collectively referred to as "atmospheric extinction." Atmospheric ducting can bend the laser beam (this causes mirages), and atmospheric turbulence or thermal blooming (explained later) can enlarge the spot. These effects combine to reduce the beam's irradiance (measured in watts/cm 2) on the target and its effectiveness. Some are almost independent of wavelength while others can vary dramatically with small changes in wavelength. As a result, the wavelength for a laser weapon which operates in the lower atmosphere must be carefully chosen. Scattering occurs when photons strike molecules or aerosols in the atmosphere and are reflected away from the target direction. This effect is commonly observed in haze or fog but always exists, even in the clearest of skies. For example, as much as 50 to 75 percent of a laser beam's energy can be lost over a 10 km lower atmosphere path on a fairly clear day. Within reasonable limits, this reduction in photons reaching the target can be accommodated by increasing the laser power or reducing the range to the target. The losses due to scattering vary quite slowly with changes in wavelength; longer (infrared) wavelengths are less affected than shorter (visible) as can be seen from the green curve in Figure 12. Absorption occurs when molecules or aerosols in the atmosphere absorb some of the photons. This phenomenon, caused by resonant interaction, is very strongly dependent on wavelength and can vary by orders of magnitude with wavelength changes of a fraction of a micron (red curve in Figure 12). Extinction (the sum of absorption and scattering) is shown by the yellow curve in Figure 12 and for a low-power laser, there is no apparent difference as both effects simply reduce the number of photons which strike the target. However, if enough energy is absorbed in the laser beam's path (when using a high-power laser), it heats slightly which causes a distortion and defocusing of the laser beam. This effect is called thermal blooming and can be a significant limit on the performance of a lower atmosphere based MW-class laser system. It was a key reason (beyond the desire to eliminate chemicals) why the Navy abandoned development of DF chemical laser weapons for self-defense where thermal blooming is at its worst. One can see from Figure 12 that wavelengths around 1 ~1m, 1.2 μm, 1.6 μm and 2.2 μm offer the lowest absorption windows. Unfortunately, no MW-class lasers existed at those wavelengths and no candidates were apparent. For this reason, the Navy began the development of FEL technology which can be tuned to any desired optical or infrared wavelength. 15 UNCLASSIFIED/ ,'F811. IIFFUiil,r.k I.Iii O•lk¥
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 31 pages are in the text index: search them above, or from the library's search.