AAWSAP DIRD, Laser Lightcraft Nanosatellites, November 2010

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achieved by laser weapons with 25 kW or 50 kW beam power, provided the energy is
transmitted with good beam quality. Laser propulsion requires megawatt-class lasers
which are only developed in directed energy weapons (DEW) programs. However,
BSSSL beam power can be scaled up further by improving presently known gain media
and doping combinations, inventing new gain media and doping combinations, by
combining the beams of several lower-power devices, or a combination of all these until
the ultimate optical/thermal/mechanical limit of slab gain materials is reached. BSSSL
beam output power is projected to reach the multi-megawatt level within three to five
years from the time of this writing (P. Zarubin, J. Albertine, and V. Hasson, private
communications, 2010).
Figure 20 shows DARPA's High Energy Liquid Laser Area Defense System (HELLADS) as
an example of a liquid-cooled bulk slab solid-state (ceramic) laser. HELLADS is
designed to be light and compact enough to fit on a jet fighter or drone aircraft, and yet
powerful enough to fire a 150 kW beam of energy. HELLADS makes use of a unique
cooling technique to save weight and size. The high-power laser uses a liquid that has
the same index of refraction as the mirrors inside the laser. That way, the laser can
fire away, even while it's being cooled. The HELLADS program will deliver a 150 kW
laser weapon at 2 m3 of system volume and 600 kg of system mass (not including the
prime power and cooling systems) to achieve the low specific mass (5 kg/kW) and
compact size need to be mounted on small tactical airborne platforms like the C-130
transport, jet fighters, or Predator-class UAVs. The device will be built by General
Atomics and the tracking system will be built by Lockheed-Martin.
A bulk solid-state laser is based on a bulk piece of doped crystal, disordered or
amorphous material (such as glass), glass ceramic (which is a combination of
crystalline-ordered structure and glassy phases disordered structure), or mixed crystals
as the laser gain medium. In most cases, the gain medium is doped either with rare­
earth ions or transition metal ions. Typically, these ions replace a small percentage of
other ions of similar size in the host medium. The laser-active ions have suitable
optical transitions for pumping and laser emission at wavelengths where the host
medium is transparent. A bulk laser resonator is often formed with laser mirrors placed
around the gain medium. However, there are also laser gain media with a highly
reflective dielectric mirror coating on one side, which serves as a resonator end mirror.
Also, there are monolithic solid-state lasers where the beam path is entirely inside the
gain medium. See Reference 30 for additional technical details.
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