Next-Generation Lasers for Advanced Active EO Systems - Raytheon

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Next-Generation Lasers for Advanced Active EO Systems - 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

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