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

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25 to 50 fibers have been combined into an even larger fiber with a resultant output
power of 50 kW and an effective beam quality of about 30. Highly reliable commercial
welders and cutters are made today with fiber lasers which combine beams by splicing
laser fibers together to increase output power. Some of these devices operate at a
reported wall plug to light-out efficiency > 30 percent but these numbers do not include
any power required for cooling. As a weapon, these off-the-shelf lasers could produce
useful beams for short distances.
A significant advantage of fiber lasers is their inherent ruggedness when compared to
slab or rod lasers. Most of the optical functions (such as wavelength and polarization
control, beam splitters and combiners) which are required to produce a high-power
beam with good beam quality can be integrated into the fibers while slab laser systems
require discrete components which must be kept clean and held in mechanical
alignment.
Given the immaturity and rate of advance of fiber lasers today, it is impossible to
predict where they will go. Current power scaling research has been centered on
Yb:YAG and Nd:YAG fibers at 1 µm with limited effort at 1.5 µm and 2 µm. Although
lagging the maturity of 1 µm fibers, they offer potential for increased eye safety.
If the maturation process continues as it has, fiber lasers will offer the best promise for
high power and good beam quality with inherent ruggedness and reliability. Militarized
devices at the 50 kW to 100 kW should be available within the next 15 years and it's
not difficult to imagine MW systems within 20 to 30 years.
ULTRA-SHORT PULSE LASERS
Ultra-Short Pulse (USP) lasers, sometimes called femtosecond (fs) lasers, produce
pulses of light shorter than 1 picosecond (10-12 second). Some USP lasers have
produced pulses less than 10 fs which is a length of 3 microns and equivalent to only a
few cycles of visible light (by comparison, a human hair is about 100 microns in
diameter). They were developed in the 1970s using long pulse lasers to illuminate
chemical dyes which then produced the short pulses; today USP lasers employ slabs
such as Titanium:Sapphire or a variety of fibers and table-top size devices are
commercially available. Although the average power of these devices is typically sub­
watt, the short pulses result in peak powers which can range from a terawatt (10 12
watts) to a petawatt (1015 watts). The extremely high power contained in very short
pulses has proven useful for precision machining, drilling tiny holes, and selective
material removal in a wide variety of materials. In addition to commercial machining
applications, medical uses such as eye surgery and cancer cell destruction have been
developed. When propagated in the atmosphere, these ultra short pulses do not obey
the usual laws of diffraction spreading and can (under certain circumstances) remain
tightly focused over long distances. This technology remains in its infancy and research
is ongoing to increase average power (generally with a higher pulse rate), to better
understand the physics of USP propagation, and to explore material interactions with
USP. Much of the USP research today is found in world-wide academic institutions.
Other than the potential for inflicting sensor damage, the military potential of these
devices is yet to be determined.
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