AAWSAP DIRD, State of the Art and Evolution of High-Energy Lasers, March 2010

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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 µm, 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.
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