JED - December 2015 - 82

EW 101

Directed Energy Weapons
- Part 3
Military Lasers, continued
By Dave Adamy
THIN DISK LASERS

G
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US IC
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FO OPT

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LA PUT
IN

OUTPUT
COUPLER

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MIR USING
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Free Electron Lasers (FEL) are expected to play a significant
role in ship protection against anti-ship cruise missiles (ASCMs), ballistic missiles, and other airborne and surface threats
when the supporting technologies are sufficiently advanced
(probably in the mid 2020s). These will be multi-megawatt lasers capable of defeating hardened targets at significant ranges.
An interesting feature of FELs is that they can be tuned
over a range of wavelengths. This is important in the ship
protection mission because of the nature of the thick, humid marine atmosphere at sea level. There are propagation
"windows" - wavelengths at which there is low propagation
loss, but these windows move in wavelength as environmental
conditions change.
Megawatt class FELs are, and are expected to remain, very
large - requiring multiple ship compartments until new hull
designs are available. Additional challenges will be shielding
to protect personnel and critical electronics from radiation
produced by escaping electrons.
As shown in Figure 2, the FEL generates a high power coherent laser beam in the X-ray wavelength range by passing a
stream of electrons through a sinusoidally varying magnetic
field at very high speed (near the speed of light). This magnetic field is produced in an undulator (also called a "wiggler"),
which has a series of powerful magnets with alternating magnetic fields
in a vacuum. As the electron stream
is "wiggled," it produces high-energy
beams of laser photons. The photons
are all at the same wavelength, but
are non-coherent. As the process continues, the electrons gather into very
short pulses with a period of one optical wavelength along the axis of the
electron stream. Within these pulses,
the laser beam becomes coherent.
LASER BEAM
FELs are currently very large. They
can be miles long and cost about $1 billion to build. They can also cost $100
million a year to operate. However
there is a current (very early) development in a cooperative effort between
the Massachusetts Institute of Technology and Arizona State University to
build an FEL that is only 3 feet long and
orders of magnitude less expensive.
Figure 1: A thin disk laser requires complex optics to produce a high quality beam.

COOLING

The Journal of Electronic Defense | December 2015

82

FREE ELECTRON LASERS

Thin disk lasers are solid-state lasers which use thin disks
of glass-like material about the size of a dime. The material is
yttrium aluminum garnet (YAG). It is doped with rare earth
elements, such as neodymium (Nd), erbium (Er), thulium (Tm)
and holmium (Ho), which cause the laser to emit at different
wavelengths between 1 and 2 microns. These wavelengths fall
into transmission "windows" in the atmosphere at which long
range laser propagation is possible.
As shown in Figure 1, a laser source is aimed at the disk,
and focusing mirrors are used to amplify the laser power.
The disk is very thin, with a high surface-area-to-volume
ratio, allowing heat to be efficiently removed. Multiple thin
disk lasers are combined into a single resonator to produce
a few kilowatts of beam power. A big advantage of thin
disk lasers is that they use laser sources with significantly
lower power than required for the fiber lasers described
last month.
Thin disk lasers are widely used in industry, but the technique shows great promise for the development of devices
emitting up to tens of megawatts. They require many optical
components, increasing their complexity, but their high power
potential and relatively small size make them potentially very
useful in future high energy lasers.

Figure 1: A thin disk laser requires complex optics to produce a high quality beam.



JED - December 2015

Table of Contents for the Digital Edition of JED - December 2015

The View from Here
Conferences Calendar
Courses Calendar
From the President
Letters
The Monitor
Washington Report
World Report
2015 AOC Award Winners
SIGINT on the Border
Jamming Down Under
Commercial Airliners: Still Vulnerable to MANPADS
New Products
Threat Monitor
EW 101
Report from the 2015 AOC Pacific Conference
Report from the 2015 AOC CEMA Conference
AOC News
Index of Advertisers
JED Quick Look
JED - December 2015 - cover1
JED - December 2015 - cover2
JED - December 2015 - igc
JED - December 2015 - OGC
JED - December 2015 - 3
JED - December 2015 - 4
JED - December 2015 - 5
JED - December 2015 - The View from Here
JED - December 2015 - 7
JED - December 2015 - Conferences Calendar
JED - December 2015 - 9
JED - December 2015 - Courses Calendar
JED - December 2015 - 11
JED - December 2015 - From the President
JED - December 2015 - 13
JED - December 2015 - Letters
JED - December 2015 - The Monitor
JED - December 2015 - 16
JED - December 2015 - 17
JED - December 2015 - 18
JED - December 2015 - 19
JED - December 2015 - 20
JED - December 2015 - 21
JED - December 2015 - 22
JED - December 2015 - 23
JED - December 2015 - 24
JED - December 2015 - 25
JED - December 2015 - 26
JED - December 2015 - 27
JED - December 2015 - 28
JED - December 2015 - 29
JED - December 2015 - Washington Report
JED - December 2015 - 31
JED - December 2015 - World Report
JED - December 2015 - 33
JED - December 2015 - 2015 AOC Award Winners
JED - December 2015 - 35
JED - December 2015 - 36
JED - December 2015 - 37
JED - December 2015 - 38
JED - December 2015 - 39
JED - December 2015 - 40
JED - December 2015 - 41
JED - December 2015 - 42
JED - December 2015 - 43
JED - December 2015 - SIGINT on the Border
JED - December 2015 - 45
JED - December 2015 - 46
JED - December 2015 - 47
JED - December 2015 - 48
JED - December 2015 - 49
JED - December 2015 - 50
JED - December 2015 - 51
JED - December 2015 - 52
JED - December 2015 - 53
JED - December 2015 - 54
JED - December 2015 - 55
JED - December 2015 - 56
JED - December 2015 - 57
JED - December 2015 - Jamming Down Under
JED - December 2015 - 59
JED - December 2015 - 60
JED - December 2015 - 61
JED - December 2015 - 62
JED - December 2015 - 63
JED - December 2015 - 64
JED - December 2015 - 65
JED - December 2015 - 66
JED - December 2015 - 67
JED - December 2015 - Commercial Airliners: Still Vulnerable to MANPADS
JED - December 2015 - 69
JED - December 2015 - 70
JED - December 2015 - 71
JED - December 2015 - 72
JED - December 2015 - 73
JED - December 2015 - 74
JED - December 2015 - 75
JED - December 2015 - New Products
JED - December 2015 - 77
JED - December 2015 - Threat Monitor
JED - December 2015 - 79
JED - December 2015 - 80
JED - December 2015 - 81
JED - December 2015 - EW 101
JED - December 2015 - 83
JED - December 2015 - 84
JED - December 2015 - 85
JED - December 2015 - Report from the 2015 AOC Pacific Conference
JED - December 2015 - 87
JED - December 2015 - 88
JED - December 2015 - 89
JED - December 2015 - Report from the 2015 AOC CEMA Conference
JED - December 2015 - 91
JED - December 2015 - 92
JED - December 2015 - AOC News
JED - December 2015 - 94
JED - December 2015 - 95
JED - December 2015 - 96
JED - December 2015 - Index of Advertisers
JED - December 2015 - JED Quick Look
JED - December 2015 - cover3
JED - December 2015 - cover4
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