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

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extremely deep magazine becomes possible. These attributes combine to make solid­
state lasers very attractive for both civilian and military applications.
ROD AND SLAB LASERS
The earliest solid-state laser mediums took the shape of a cylindrical rod and were
optically pumped with flashlamps. Higher power solid-state lasers were later made
using larger rectangular slabs since the amount of laser power which can be extracted
increases with the lasing medium's volume. Additionally, rectangular geometries
allowed more efficient optical pumping and cooling. Newer solid-state lasers were
pumped with semiconductor diodes which emitted light more efficiently than flashlamps
and at wavelengths which were more useable by the gain medium. Like chemical
lasers, they are considerably less than 100 percent efficient overall and generate heat.
However, solid-state lasers accumulate heat in the glass lasing medium where the
resulting thermal stresses can distort the lasing beam enough to render it useless or
even crack the glass. Since the glass-based medium is a poor thermal conductor, it is
difficult to remove heat. Thus, thermal management of the gain medium imposes
fundamental limits to increasing the average power in a single slab laser. Much of the
recent research in slab lasers has focused on developing innovative ways to remove this
heat or to combine beams from many smaller slabs.
In order to combine laser beams in a fashion which retains good overall beam quality,
the wavelength, phase, and polarization of the individual beams must be tightly
controlled. This process is referred to as "coherent combination" or "phasing" and
results in the maximum irradiance (watts/cm 2) by minimizing the focused spot area
over which the total laser power is spread. If an application (such as welding or
machining) depends more on total power than on minimizing spot size, then coherent
combination is not needed and the laser beams can be separately focused to the same
spot with no regard to differences in wavelength, phase, or polarization. This technique
is called "incoherent combination."
Major progress has been made in slab laser development during the last few years.
Northrop-Grumman, under a DoD-funded program, recently produced 100 kW with
good beam quality. This laboratory device, called the Joint High-Power Solid-State
Laser (JHPSSL), uses a single Nd :YAG master-oscillator that feeds parallel, well-phased
Nd :YAG power amplifiers. Brute-force power scaling, well beyond 100 kW, seems
relatively straight-forward using approaches available today. Unfortunately, as more
lasers are combined the overall beam quality will deteriorate while the parts-count,
complexity, and fragility increases. This makes either militarization at the 100 kW level
or laboratory scaling to/beyond the MW level unlikely in the near-term with this
approach. However, given the recent rate of progress and the relative immaturity of
the technology, it would be reasonable to expect further major technological
breakthroughs in thermal management and phasing, cheaper/more powerful pump
diodes (the major cost today), and far more rugged engineering. These improvements
could be expected to yield affordable, rugged, militarily useful MW-class slab laser
systems with reasonable beam quality within the next 20 years. Much of the relevant
research is occurring outside of the United States.
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