Defense Intelligence Reference Document State Of The Art And Evolution Of High-Energy Laser Weapons

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FIBER LASERS
Fiber lasers are also solid-state lasers but are frequently considered separately due
their many differences in implementation. They are made of the same materials listed
in Figure 4 and lase at the same wavelengths. A fiber laser can be visualized as a rod
laser which has been stretched many meters with a resulting diameter about the size of
a human hair. The fundamental reason is to improve heat dissipation. The ratio of a
fiber's surface area to its volume is much greater than a rod or slab and the distance
which heat must travel to the edge of the lasing medium is reduced by orders of
magnitude. The lasing fiber is surrounded by a concentric, larger diameter undoped
fiber into which pump light is injected (usually at the ends) and then allowed to leak
into the lasing fiber along the entire length. Fiber lasers are manufactured or "pulled"
in much the same way as conventional optical communications fibers. Because the
fibers are so thin, the power which can be generated in any one lasing fiber before the
onset of damage is currently quite limited. New approaches to injecting the pump light
and controlling the laser beam as it is generated or amplified are occurring almost daily.
Fibers have been constructed using multiple cores, photonic crystals and air spaces in
order to increase the power of individual fiber lasers with good beam quality and in
ways to combine the beams.
The smallest diameter fiber lasers, which produce single mode beams with very good
wavelength control and stable polarization, have demonstrated a few hundred watts
and are currently thought to have a damage threshold upper bound of about 10 kW.
Small numbers of these beams have been coherently combined using techniques which
match wavelength, phase and polarization of each fiber laser in the cluster. The
bandwidths of these laser/amplifier chains must be extremely narrow ( < < 1 GHz) to
maintain polarization and to then be coherently phased to about 01. t. rms. It remains
to be seen how much total power can be generated with good beam quality and what
are the fundamental limits to coherent combining. Phasing research into mechanical
techniques which use deformable mirrors and passive approaches which use stimulated
Raman scattering are both being aggressively pursued. These combining techniques,
when applied to fiber lasers, are in their infancy and offer the potential for weapons
with fairly long tactical ranges.
Other single mode fiber lasers have been produced which generated as much as 5 kW.
Although their outputs are of good beam quality, they lase over a wavelength band
which is too broad to maintain polarization and cannot be coherently combined.
However, these types of beams can be combined incoherently by individually focusing
them onto the same spot (Figure 6), similar to incoherent combination of slab laser
beams or by physically splicing the output fibers of the individual lasers onto a larger
undoped fiber (Figure 7) which can accommodate the higher total power.
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