without refueling the laser. The slewing from one target to another can be done in a relatively short time.
There are other limits to the useful range of high-energy lasers for boost-phase engagements. Even at the speed of light and the relatively thin atmosphere at the operating altitude of the ABL, atmospheric effects degrade the coherence of the beam unless variations in the structure of the atmospheric path from the laser to the target are compensated for in real time. This requires leading the target, and, even at the speed of light, the round trip of the calibration beam limits the effective range.
When a laser such as the ABL (now renamed the airborne laser test bed (ALTB)) is based on an aircraft, it is propagated through the atmosphere over a substantial distance, albeit at lower densities than propagation on the ground. Thus the properties of the atmosphere vis-à-vis the laser wavelength are very important. These properties include the absorption, scattering, and turbulence. Those wavelengths that are strongly absorbed by the constituents of the atmosphere are inappropriate for long-distance propagation. Notably, these are the wavelengths absorbed by atmospheric water vapor, carbon dioxide, carbon monoxide, oxides of nitrogen, and so forth, and they are ruled out. Fortunately, there are several windows in the atmospheric transmission where absorption is small enough to allow radiation to be propagated over hundreds of kilometers. The early lasers, starting with the carbon dioxide, hydrogen fluoride/deuterium fluoride (HF/DF), and COIL lasers, were chosen because their wavelengths were in such low-loss atmospheric windows.14
While the exact design power cannot be provided in this unclassified report, MDA describes the ABL as a multimegawatt laser and the ALTB as a megawatt-class laser. By the early part of 2010, the ALTB HEL had achieved about 80 percent of the Tail 1 design power. This performance can be improved, but substantial effort would be required.15 This does not appear to be a high priority at this time for MDA, and correctly so.
Inhomogeneities in the density of the atmosphere, called “turbulence” for short, result in inhomogeneities in the index of refraction. In turn, these variations perturb the propagation of a laser beam. A beam that would have been focused in a uniform atmosphere will no longer be focused. This is a problem well known to astronomers who use ground-based telescopes. The best solution for astronomers is to avoid the atmosphere and have the telescope in space—as, for instance, the Hubble Space Telescope—eliminating the problem. In the same way a laser weapon would be space based. For operation in the atmosphere, one would try to minimize the distance the beam travels through the atmosphere. Thus, a high flying aircraft is a much better platform than a device on the ground. There is a
14Additional information on the ABL/ALTB, including wavelength selection, is provided in the classified annex (see Appendix I).
15Missile Defense Agency. 2010. “ALTB Questions in Preparation for March 16-18 Presentations to the National Academy of Sciences,” March 17.