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

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completely in the vacuum of space, there is obviously no need for target loop adaptive
optics.
The measure of a beam control system's performance is its ability to maximize the HEL
beam's average irradiance (watts/cm 2) in the focused spot on the aim point and
maintain it there while sufficient fluence (watts/cm 2 x time or joules/cm 2) is
accumulated. The total power in this focused spot is typically about one-half of the
laser's output power further reduced by losses in the beam control system's optical
train and the atmosphere.
The laser beam's spot size on the target has many contributors. Optical diffraction
establishes the spot's minimum area at approximately (RA/D)2 where R is the range to
the target, 'A, is the laser beam's wavelength and D is the diameter of the pointing
telescope. Improvement can be only achieved by reducing the range to the target,
using a laser with shorter wavelength or increasing the pointing telescope's size. This
ideal (diffraction limited) spot area, is unachievable in a real system. Additional
contributors to the actual spot area include a less-than-perfect laser beam (beam
quality greater than one), mechanical jitter of the beam control system from the
tracker, the alignment systems, or base motion disturbance and atmospheric distortions
from turbulence or thermal blooming. Each of these contributors to the laser beam
spot's area serve to reduce the irradiance by spreading the laser power over a larger
area.
Figure 10 is a cartoon of the gimbaled portion of a beam control system and is used to
illustrate basic servo control functions. A tracking telescope and optical sensor are
mounted on the elevation over azimuth gimbal. Their purpose is to generate an
electronic image of the target and send it to the tracker. This tracker is a special­
purpose computer which then processes the target image, identifies the desired aim
point and measures the angle between it and the optical boresight of the telescope. Its
output is a command to the gimbals to rotate until the optical axis of the tracking
telescope is following the target and pointing at the aim point. If the gimbals are
mounted on a moving or vibrating platform, these disturbances introduce additional
tracking errors. Unlike target motion, this base motion disturbance can be directly
measured using gyros and accelerometers which are attached to the telescope. This
package, called an Inertial Reference Unit (IRU), provides an additional command to
the gimbals which stabilizes the telescope and improves the tracker's ability to measure
target motion errors. The HEL, shown simply in this cartoon as a box, provides the
weapon beam to a pointing telescope which is then mechanically boresighted (optical
axis made parallel) to the tracking telescope. Finally, the range to the target must be
measured so that a parallax correction can be applied to the pointing telescope. This
slightly tilts its optical axis to intersect the tracking telescope's optical axis at the range
of the target and thus place the HEL beam on the aim point.
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