Technology Today issue 1 2008 - Raytheon

To maintain an advantage in today’s
battlespace, our forces require the
ability to engage targets at longer
ranges and see them with higher resolution.
Active electro-optical (EO) systems
address this need, providing important mission
capabilities such as ranging, tracking,
marking, designating, 3-D imaging (ladar),
chemical and biological agent detection,
and laser defense using high-energy lasers
(HELs). Compared to passive EO systems
(FLIR or camera) or active radio frequency
(RF) systems (radar), active EO systems allow
us to increase both range and resolution
and also to perform new types of missions.
A critical component of any active EO
system is the laser used to illuminate the
target. To offer our customers the highest
performance systems, we must utilize
advanced lasers meeting ever more
challenging requirements in the areas of
laser power, beam quality, efficiency, size
and weight.
Lasers are inherently inefficient, converting
only a portion of the electrical input power
into useful laser output power. The size,
weight and input power of a laser system
are largely driven by two factors: 1) the
average output power of the laser and 2)
its efficiency. In the design of our active EO
systems, we typically optimize the system
design to minimize the required average
power from the laser and then optimize the
laser design for high efficiency. All lasers
require a “gain medium,” a material that
can emit a laser beam, and a “pump
source,” a means of exciting atoms in the
gain medium so that they emit light.
Historically, most military laser systems have
used arclamps to excite the laser gain medium
with resulting efficiencies of just a few
percent. During the past decade, efficiencies
of 20 percent or more have been
achieved by diode-pumped solid-state lasers
due to development of high-power laser
diodes as a more efficient means of exiting
the gain medium, as well as advances in
the gain medium configurations utilized.
While laser diode pumping is a key factor in
enabling high efficiency, scaling laser-output
power while maintaining high beam quality
necessitates improvements in the geometry
of the gain medium itself. Due to the fact
that lasers are not 100 percent efficient,
significant amounts of waste heat are
generated in the gain medium during laser
operation. This waste heat can create
distortions in the gain medium, adversely
affecting important properties of the laser
beam. Raytheon has long been at the
forefront of laser technology development
for military applications, and is currently
focused on advanced laser architectures
that leverage the benefits of laser diode
pumping and address the shortcomings
of conventional laser gain medium
Feature
Next-Generation Lasers
for Advanced
Active EO Systems
architectures such
as the venerable cylindrical
rod and, more recently, bulk
slab geometries. The optimal gain
medium geometry for a given application
varies, depending on the average power
and laser waveform, but all of the
advanced gain medium geometries
employed by Raytheon seek to minimize
the amount of waste heat and remove the
waste heat from the gain medium in a
manner that minimizes adverse effects on
the quality of the laser beam. The goal of
minimizing adverse thermal gradients within
the gain medium has led Raytheon to
focus on three primary gain medium
geometries for advanced laser systems:
microchip lasers, fiber lasers and planar
waveguide lasers.
Microchip lasers are very simple, robust
devices for applications requiring up to
~1W of average laser power. They can be
operated in a pulsed mode with pulse energies
up to ~1mJ and pulse widths as short
as ~1 nanosecond, enabling peak powers
up to 1MW. Fiber lasers and planar waveguide
lasers both enable scaling of laser
average power up to the kW level by using
gain medium geometries with large surface-
Continued on page 10
RAYTHEON TECHNOLOGY TODAY 2008 ISSUE 1 9