Department of Defense Directed Energy Weapons: Background and Issues for Congress
Congressional Research Service 21
weapon. Absorption by water vapor is a particular consideration for shipboard
lasers because marine environments feature substantial amounts of water vapor in
the air. There are certain wavelengths of light (i.e., “sweet spots” in the
electromagnetic spectrum) where atmospheric absorption by water vapor is
markedly reduced. Lasers can be designed to emit light at or near those sweet
spots, so as to maximize their potential effectiveness. Absorption generally grows
with distance to target, making it in general less of a potential problem for short-
range operations than for longer-range operations. Adaptive optics, which make
rapid, fine adjustments to a laser beam on a continuous basis in response to
observed turbulence, can counteract the effects of atmospheric turbulence. Even
so, lasers might not work well, or at all, in rain or fog, preventing lasers from
being an all-weather solution.
• Thermal blooming. A laser that continues firing in the same exact direction for a
certain amount of time can heat up the air it is passing through, which in turn can
defocus the laser beam, reducing its ability to disable the intended target. This
effect, called thermal blooming, can make lasers less effective for countering
targets that are coming straight at them, on a constant bearing (i.e., “down-the-
throat” shots). Most tests of laser systems have been against crossing targets
rather than “down-the-throat” shots. In general, thermal blooming becomes more
of a concern as the power of the laser beam increases.
• Saturation attacks. Since a HEL weapon can attack only one target at a time,
requires several seconds to disable the target, and requires several more to be
redirected to the next one, a HEL weapon can disable only so many targets within
a given period of time. This places an upper limit on the ability of an individual
laser to deal with saturation attacks—attacks by multiple weapons that approach
the platform simultaneously or within a few seconds of one another. This
limitation can be mitigated by installing more than one laser on the platform, up
to space and energy availability.
• Hardened targets and countermeasures. Less powerful lasers—that is, lasers
with beam powers measured in kilowatts (kW) rather than megawatts (MW)—
can be less effective against targets that incorporate shielding, ablative material,
or highly reflective surfaces, or that tumble or rotate rapidly (so that the laser
spot does not remain continuously on a single location on the target’s surface).
Smoke or other obscurants can reduce the susceptibility of a target platform to
laser attack. Such measures, however, can increase the cost and/or weight of the
target platform.
Potential Advantages of HPM Weapons
In addition to deep magazines, low costs per shot, fast engagement times, and graduated
responses, potential advantages of HPM weapons include the following:
• Temporary or system-specific effects. HPM weapons can generate waves at
different frequencies and power levels to temporarily or permanently disrupt
targeted electronic systems while leaving others unaffected.
• Broad effects. HPM weapons can destroy a wide array of unshielded electronic
systems, including both military and commercial systems. In addition, they are
capable of disabling any unshielded electronic system within their