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OCR for page 17
Lasers in
Modern industries
Anthony ]. DeMaria
The development of the ammonia beam maser in 1954 ushered
in a new breed of active devices that electronic engineers could
relate to and use (Gordon et al., 1954~. The ammonia beam
maser was the first device to use stimulated emission from
inverted-population states of quantum mechanical resonances to
provide gain for an electromagnetic oscillator. The operation of
this quantum mechanical device initiated the field of quantum
electronics. In 1984 the field of quantum electronics was 30
years old.
In 1958 Arthur L. Schawlow and Charles H. Townes pub-
lished a classic paper suggesting the use of the maser principle
(with appropriate modification) for the generation of coherent
infrared, visible, or ultraviolet radiation (Schawlow and Townes,
19581. The operation of the first ruby laser by Theodore H.
Maiman in the latter part of 1960 made available for the first
time a visible light beam that had characteristics previously
associated only with radio frequency and microwave radiation
(Madman, 1960~. The acronym laser was formed from light
amplification by stimulated emission of radiation. The year 1985
was the 25th anniversary of the laser.
Laser action has now been observed in solids (crystalline and
noncrystalline insulators and semiconductors), liquids, gases,
and plasmas yielding thousands of discrete wavelengths vary-
ing from the vacuum ultraviolet to the millimeter wavelength
HISTORICAL
BACKGROUND
17
OCR for page 18
ANTHONY ]. DEMAR~A
portion of the electromagnetic spectrum. Dye, color centers,
and lead salt lasers now provide tunability over the visible,
near-infrared, and infrared spectrum. At present, the abilities
of electronic and laser devices overlap for generating radia-
tion in millimeter and submillimeter wavelengths. Scientists are
still working toward the generation of coherent radiation at
ever-higher frequencies extending to soft and hard x-ray radi-
ation.
Few developments in science have excited the imagination of
scientists and engineers as has the laser. The laser made it
possible to transport into the optical region all the basic tech-
niques developed for application in the radio and microwave
regions, such as harmonic generation; parametric amplification;
amplitude, frequency, and phase modulation; homodyne and
heterodyne detection; and chirping and pulse compression. In
the 25 years since the laser was first realized in the form of
pulsed coherent emission from a single ruby crystal, the field has
grown at a rate rarely experienced in science. The availability of
these intense, coherent optical radiation sources has made it
possible for scientists to experiment with optically generated
plasmas; optical harmonic generation; stimulated scattering
effects; photon echoes; self-induced optical transparency; opti-
cal pulses; optical pulse compression; holography; optical
shocks; self-trapping of optical radiation; optical parametric
amplification; optical ranging to the moon; extremely high
resolution spectroscopy; refined measurements of many basic
physical properties (length, the speed of light, and so forth); and
ultrafast relaxation processes in atoms and molecules.
A MULTIDISCIPLINARY FIELD
Today, the field of laser devices encompasses numerous disci-
plines. They include solid-state, molecular, and atomic physics;
spectroscopy; optics; acoustics; electronics; semiconductor tech-
nology; plasma physics; vacuum technology; organic and inor-
ganic chemistry; molecular and atomic kinetics; thin-film tech-
nology; glassworking technology; and crystallography. More
recently, the field has come to encompass electron-beams, x-
rays, fluid dynamics, aerodynamics, and combustion physics. In
sum, even without considering applications? the field has grown
so fast and proliferated so broadly that scientists are virtually
required to specialize within it. As a result, probably no individ-
ual today would claim authoritative knowledge over the whole
field of laser devices, or even be knowledgeable about most of
the significant literature.
OCR for page 19
LASERS IN MODERN INDUSTRIES 19
THE BIRTH OF THE TECHNOlOGY
During the first 15 years after development of the maser, from
approximately 1954 to 1969, the field of quantum electronics
was in the technology birth phase. After the operation of the
ruby laser in 1960, emphasis shifted from maser to laser devices.
This phase was characterized by numerous scientific discoveries
and inventions as well as by widely believed visions and predic-
tions of numerous medical, industrial, commercial, scientific,
and military applications. During this phase, many laser devices
were discovered from a large variety of gases and liquids, as well
as from both amorphous and crystalline dielectrics and semicon-
ductor solid-state materials.
Few business opportunities existed during this phase, except
to sell components, materials, and devices to researchers con-
cerned with developing the technology base of lasers. Some
opportunities were available to sell newly discovered laser
sources to researchers interested in probing the linear and
nonlinear electromagnetic behavior of atoms and molecules in
liquids, solids, and gases. Large corporations were funding
in-house research efforts in the technology, as well as capturing
significant government research and development contracts.
These contracts were directed toward determining the feasibility
of numerous military applications during this early develop-
ment cycle of the technology.
ENGINEERING DEVELOPMENT PHASE
For approximately the next 15 years, laser technology entered
the engineering development phase. This phase was character-
ized by a noticeable decrease in scientific breakthroughs and a
perceived impatience with the rate of technological progress
toward applications that addressed large markets. This was the
period when the statement "the laser is a solution in search of a
problem" was often heard. During this phase, many companies
with marginal interest in laser technology dropped out of the
field. In the same era, entrepreneurs invested considerable
effort in searching for markets with large growth potential. In
both of these early periods, the military market was larger than
the commercial and industrial markets.
MA NUFA CTURING TECHNOl OG Y PHASE
Laser technology has now definitively entered the manufactur-
ing technology phase. Sizable markets have been identified. A
-
OCR for page 20
20 ANTHONY J. DEMARIA
RECENT LASER
MARKETS
strong system and subsystem development effort is in place in
which laser devices either significantly lower costs or raise
performance leverage over older, more mature technologies.
Consequently, product developments have intensified, and
many new companies are being created. In addition, large,
well-established corporations promote and sell products aimed
at markets that laser technology can address uniquely: telecom-
munications, data processing and storage, entertainment, print-
ing, material working, and medical applications. In contrast to
earlier phases, the commercial and industrial markets are now
larger than the military market.
Consumers are also beginning to experience laser technology
directly through video and audio discs, laser printers for small
computers, bar code readers at checkout counters, fiber-optic
telecommunications, and various medical treatments. There is
evidence of consolidation among numerous small companies
oriented toward markets that use laser technology.
NEXT: MATURE TECHNOLOGY PHASE
In the future, laser technology will enter the mature technology,
or commodity product, phase, which will be characterized by
cost- and volume-driven markets (i.e., economies of scale),
requiring capital-intensive manufacturing plants. The laser di-
ode is the first laser device to achieve the mature technology
phase. As production volumes and techniques have approached
those in the high-volume manufacture of integrated circuits,
unit prices of laser diodes have dropped accordingly. Eventu-
ally, a few large companies will address the most important laser
markets, and chances are good that the surviving manufacturers
will not be those known today.
The dollar value of the 1984 worldwide laser market in the
commercial sector and the government and military sector was
approximately $2.855 billion and $1.305 billion, respectively,
for a total of approximately $4.16 billion (Spectra-Physics Corp.,
1983, 1984; DeMaria, 1985~. Figure 1 compares the 1983 and
1984 market dollar values. Of the commercial market, approx-
imately $2.502 billion was reported to be in systems and add-
ons, whereas laser devices themselves amounted to approxi-
mately $353 million of the 1984 world commercial market. As
reported in Lasers and Applications (1987), "Commercial sales of
individual lasers reached $509 million in 1986, up nearly 14%
OCR for page 21
LASERS IN MODERN INDUSTRIES 21
COMMERCIAL
$22,855 M
$1 ,985 M
SYSTEMS &
ADD-ON
$2,502 M
$1,705 M
LASER DEVICES
$353 M
$280 M
GOV. & MILITARY
$1,305 M
$1,230 M
. .. , , ............... ....... ... .....
................. .. .. ~
~'<~5~
· 1
//
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::--
............................. . .......
TOTAL MARKET
1984 -- $4,160 M
~3 1983--$3,215 M
I 1 1 1 1 1 1
0 400 800
1,200 1,600 2,000 2,400 2,800
DOLLARS x MILLIONS
FIGURE l Laser industry world markets. SOURCE: Spectra-Physics
Corp. (1983, 1984~.
Over 1985. This year's sales should increase about 10% to $559
million."
From this statement we interpolate that individual laser sales
in 1985 totaled approximately $447 million, up approximately
27 percent from the 1984 sales of $353 million. These estimates
would lead an entrepreneur rightly to conclude that business
opportunities are more plentiful with the inclusion of systems
and add-one in a line of laser devices.
It is important to note in Figure 1 that the military market was
smaller than the commercial market in 1983 and 1984. This
trend is expected to continue. A 1983 forecast predicted that the
worldwide laser market would grow 23 percent annually (Spec-
tra-Physics Corp., 1983, 1984), and that well over 75 laser
companies contributed significantly to these world markets.
According to Lasers and Applications (1987), "Overall, the laser
industry continues to grow at double-digit percentage rates.
However, the double digits are now in the low teens, not the low
twenties as was the case in the early 1980's."
In view of the slow industrial growth in 1986 and 1987,
growth of the laser industry in the low teens percentage rate is
very respectable. This remarkably high growth rate is compara-
ble to that experienced by the microelectronic and information
processing markets.
Figure 2 compares the growth of laser markets in 1984 and
1983. Note that the total market in 1984 grew 29 percent over
the 1983 sales. Military sales experienced only a 6 percent
1 1
OCR for page 22
22 ANTHONY J. DEMAR~A
COMMERCIAL 44%
SYSTEMS & ADD-ON 47%
LASER DEVICES 26%
GOVERNMENT & MILITARY 6%
TOTAL MARKET 29%
0 10 20 30
% GROWTH
FIGURE 2 Laser industry growth, 1983 - 1984.
//////////////////////////,/////////A
//////////, ~ ,////,
40 50
increase in this period, and the commercial market grew by a
phenomenally large 44 percent.
COMMERCIAL LASER INDUSTRY
A more detailed look at the 1983 and 1984 commercial laser
world market reveals that approximately 63 and 50 percent,
respectively, of these 2 years' commercial markets ($1.25 billion
and $1.44 billion, respectively) was attributed to printing and
graphics associated with information and data processing (see
Figure 3~. Since Spectra-Physics does not have a product line in
the entertainment or computer fields, data on semiconductor
lasers and subsystems associated with the video and audio discs
and data storage markets were not included in its annual reports
(Spectra-Physics C:orp., 1983, 1984) and thus are not included in
the data shown in Figure 3. The market for audio and video disc
entertainment is bringing laser technology directly into the
home. Fortune forecast that music lovers in the United States
would buy 15 million discs in 1985 versus 5.8 million in 1984
(Fortune, 1985~. Fortune also forecast that sales of players and
discs would reach $1.3 billion worldwide in 1985.
The second largest segment of the world's commercial laser
market consists of laser material working, which accounted for
approximately 11 percent in 1983 ($210 million) and 10 percent
in 1984 ($285 million) of the total market. The third largest
market segment was communication, which accounted for ap-
proximately 8 percent in 1983 and 1984 ($150 million and $225
million, respectively). The medical market is the fourth largest
segment, with $ 105 million in 1983 and $ 150 million in 1984,
OCR for page 23
LASERS IN MODERN INDUSTRIES 23
capturing just over 5 percent in each of these 2 years. The laser
market in metrology, industrial inspection, and science was just
under 5 percent in 1983 and 1984 ($90 million and $ 135
million, respectively) of the total market, and it ranks fifth in size
after the medical market. The data capture sector of the market
(bar code readers, for example) is the sixth largest in size, with
1983 and 1984 sales of $70 million and $85 million, respectively,
and market percentages of just under 4 percent in 1983 and just
under 3 percent in 1984. The 1983 numbers include a $35
million miscellaneous category that is not shown as a bar plot in
Figure 3 but is included in the $1.986 million total (Spectra-
Physics Corp., 1983, 1984~.
The expanding applications of lasers in the medical field are
a source of great satisfaction to laser researchers. One of the
earliest medical applications of lasers was in retina operations.
Since then, much progress has been made. Lasers are now being
used or investigated for use in cataract surgery, treating bleed-
ing ulcers, opening blocked windpipes, reconnecting severed
nerves, removing tumors, and cleaning the plaque that clogs
blood vessels. Lasers are also starting to play a role in dermatol-
ogy, plastic surgery, gynecology, and podiatry (see Rodney C.
Perkins in this volume). It is no wonder that the market for laser
MATERIALS WORKING
$285 M
$210 M
COMMUNICATION
$225 M
$150 M
MEDICAL
$150 M
$105 M
METROLOGY & INSPECTION
$135 M
$90 m
DATA CAPTURE
$1 1 0 M
$75 M
ALIGNMENT
$85 M
$70 M
PRINTING & GRAPHICS
$1 ,440 M
$1,250 M
. _,,, ' ' ' 'A
, '- - - - - - - - - - - - - ' - - - - -]
/////
///
.. - 2 ]
.............. 3
it
...... ,
war
TOTAL
1984 -- $2,855 M
1983 -- $1,985 M
1 1 1 1 1 1 1
0 200 400 600 800 1,000 1,200 1,400
DOLLARS x MILLIONS
FIGURE 3 Commercial laser industry world market. SOURCE: Spec-
tra-Physics Corp. (1983, 1984).
11
Tl
OCR for page 24
24 ANTHONY J. DEMARIA
MATERIALS WORKING 36%
COMMUNICATION 50%
MEDICAL 43%
METROLOGY & INSPECTION 50%
DATA CAPTURE 47%
ALIGNMENT 21%
PRINTING & GRAPHICS 15%
COMMERCIAL MARKET 44%
systems for medical applications is expected to double in each of
the next several years. It is an embryonic but fast-growing
market.
Figure 4 compares 1984 sales growth with 1983 sales for the
market segments identified in Figure 3. The communications
market and the metrology and industrial inspection market had
an astonishing 50 percent growth in this 2-year period. The data
capture and medical sections of the market had outstanding
increases of 47 and 43 percent, respectively. The growth of 36
percent achieved by the materials working market and 21
percent by the alignment market would be the envy of most
high-technology industries. The printing and graphics segment
of the laser market had the smallest growth 15 percent—of the
identified market.
In the automated offices of tomorrow, it has been widely
forecast that video screens will replace ink and paper. Today,
however, office automation is producing more rather than less
paper. The printers that help computers create much of this
paper have become a $2.4 billion industry, with the promise that
printer sales will more than double before the decade ends (The
Wall Street journal, 19841. Semiconductor laser printer technol-
ogy is expected to become the major competitor against ink jet
printers in the future computer printer market. One or both of
i////////////
///////////////////////~
////////////////////~
///////////////////~//~
/////////////////////~
///////
a////
////////////////////~
1 1 1 1 1 1
0 10 20 30 40 50 60
% GROWTH
FIGURE 4 Commercial laser industry growth, 1983-1984.
OCR for page 25
LASERS IN MODERN INDUSTRIES 25
5,000
4,000
cn
CC
o
C]
O
In
By
o
~ 2,000
x
cn
J
In
3,000
1,000
o
U.S.
WORLD
1982 1984 1986 1988 1990
FIGURE 5 Fiber optics market: fibers, cables, transceivers, components.
SOURCE: Business Week (19841.
these technologies is expected to displace the typewriter tech-
nology used today.
Much of the laser communications market that is, the fiber
optics telecommunications market—is held by large, vertically
integrated corporations such as AT&T, ITT, and Nippon
Telegraph & Telephone and is thus not available to other
manufacturers. Undoubtedly, this accounts for the relatively
small fraction of the total world market of the commercial laser
industry attributable to the laser communications market, as
shown in Figure 3. By 1990 the fiber optics world market is
expected to exceed $4.5 billion per year, whereas the U.S.
market will be approximately $2 billion per year (Figure 5~.
Business Week has predicted that the end of conventional copper
wire in the telecommunications industry could come as early as
the turn of the century; moreover, semiconductor lasers cou-
pled with fiber optics technology will make ground and under-
sea cable communication so inexpensive that few commercial
communication satellites will be launched in the l990s (Business
Week, 1984~. There were 250,000 miles of optical telecommu-
nication fibers installed in the United States in 1983. Northern
OCR for page 26
26 ANTHONY J. DEMAR~A
11
V DEO SEM -
D~SCS ~ LASER
~ lASER
\ PRINTING
AUDIO ~ ~ DATA
~FIBER-
_ 1 OPTICS
| TELECOMMU-
~CATIONSJ
-
\ ~ -
BAR
CODE
READING
/ \
FIGURE 6 Applications of semiconductor lasers.
_~
Business Information, Inc., forecast that approximately 1.3
million miles of telecommunication fiber would be installed in
the United States in 1986. This installation is expected to
increase to 4.5 million miles in 1990 (Business Week, 19841. The
development of laser communications technology is discussed in
detail by C. K. N. Patel later in this volume.
Over the last 20 years, three areas of laser technology have
received continuous and extensive research and development
support: laser weapons, controlled fusion, and semiconductor
laser development. By most estimates, the practical realization of
the first two is still believed to be 20 years away. Because of their
importance to national security and economic well-being, they
have received extensive government research and development
support in many countries. The semiconductor laser, on the
other hand, has been developed primarily with industrial re-
search and development funds. This development has spawned
many new sectors of major industries, such as telecommunica-
tions, printing, video and audio discs, data recording, and bar
code reading (Figure 61. In another example of the large
markets generated by semiconductor lasers, Frost & Sullivan,
Inc. (1986), has projected tremendous market growth over the
next few years for optical disc system manufacturers and retail-
OCR for page 27
LASERS IN MODERN INDUSTRIES 27
ers for personal computers. They have forecast a market of $2.5
billion in 1990 for optical disc systems.
Optical discs can offer much greater information density than
current magnetic storage devices at a lower cost per byte.
Consequently, they are expected to generate the next revolution
in mass storage. The technologies for write-once and read-only
optical discs have been well established for some time. They use
a modulated laser beam to permanently engrave a submicron-
size bubble or pit in the active layer of a medium. Intensive
research has been focused on erasable optical discs using a
semiconductor laser that causes either a phase change or
magneto-optical change in a medium. Compact disc read-only
memories store 550 megabytes of data that cannot be altered or
erased and were the first to reach the market. Write-once
discs are relatively new to the market and typically hold 1-2
gigabytes and let users store and update information without
eliminating data already stored on the disc. Erasable discs are
not expected to appear until 1988 and will permit continual
reuse.
The success of the semiconductor diode laser in bringing
about many new sectors of major industries is probably attrib-
utable to its compatibility with semiconductor integrated cir-
cuits. Its small size, low manufacturing cost, low voltage and
power requirements, and high efficiency make it compatible
with modern electronic technology. The semiconductor laser is
the first truly mass-produced laser. It has been reported that
Mitsubishi Electronic Corporation produced 400,000 semicon-
ductor lasers monthly in 1985. It is expected that 10 million
diode lasers will be sold in 1987, with the vast majority going into
audio disc players and low-cost printers. Laser diode production
has become a commodity process with commodity pricing strat-
egies.
PHOTONICS VERSUS
ELECTRONICS
In the early days of electronics, vacuum tubes played an impor-
tant role in developing the industrial base of the radio fre-
quency, microwave, and millimeter wave portion of the electro-
magnetic spectrum. Similarly, gas lasers and optically pumped
solid-state lasers have been important in developing the indus-
trial base of lightwave technology in the new field of quantum
electronics. There is also a clear analogy between the roles
played by the semiconductor diode laser and the transistor in
establishing the industrial base of their respective spectral re-
OCR for page 34
34 ANTHONY ]. DEMAR~A
Butt weld Tee weld with filler Butt weld
~ - 3.2mm
25.4 mm 1 1
:!5 4 mm
4.8 mm
Power: 12 kW Power: 13 kW Power: 8 kW
Speed: 12.7 mm/see Speed: 12.7 mm/see Speed: 21.2 mm/see
No filler Filler: 0.89 mm wire, 127 mm/see No filler
Matenal: Ship steel Matenal: A-36 steel Material: Low carbon steel
FIGURE 9 Typical weld configurations performed with a CO2 laser
under the conditions indicated.
interaction time of the radiation with a material required to
accomplish various important material-processing tasks (Banes
and Webb, 19821. For a laser beam moving continuously across
a material, the interaction time can be defined as the time
required for the incident laser spot to move one diameter
relative to the surface of the workpiece. For a material process
requiring short pulses of laser radiation, the interaction time is
the duration of the pulse, since the material can be assumed to
be stationary during the short irradiation process.
Figure 9 shows three typical laser welds performed with
carbon dioxide (CO2) lasers (Duhamel and Banas, 1983~. A
three-stage, gas-recirculating, closed-cycle, electric-discharge
CO2 laser can yield 9-12 kW of continuous output power. The
use of fast gas-flowing techniques to achieve several tens of
kilowatts of continuous power from electrically excited CO2
lasers (DeMaria, 1973) has been responsible for placing CO2
lasers in a dominant position for large material-working appli-
cations. In the next decade, laser material processing in manu-
facturing is expected to increase dramatically. The Nd3+:YAG,
ruby, Nd3+:glass, and CO2 lasers are expected to be the most
widely used in these applications.
LASER RADAR
1 1
Laser radar technology is an obvious progression of radar
technology from the radio frequency, microwave, and milli-
OCR for page 35
LASERS IN MODERN INDUSTRIES 35
meter wave region of the spectrum into the optical region (i.e.,
infrared, visible, near ultraviolet). Laser radar technology has
both advantages and disadvantages when compared with con-
ventional radar technology. Consequently, laser radar systems
will complement and not compete with conventional, lower
frequency radar systems. Laser radar systems will be used
predominantly in those applications that cannot be addressed by
conventional radar systems.
Range finders are the most basic radar systems of either the
microwave or the laser variety. They measure the range to a target
by measuring the time of flight of a transmitted and an echo pulse
of electromagnetic radiation. The speed of the target can be
obtained by measuring the change in range as a function of time.
Range finders can also provide information about the azimuth to a
target. Radars of this kind are known as incoherent radar systems.
Coherent radar systems are more complex and have the
ability to measure the velocity of the targets by means of the
Doppler effect. This type of radar was originally used exten-
sively in the early development of radar technology to detect
moving targets against stationary background clutter. Coherent
radar systems measure the Doppler shift of the echo radiation
by comparing the frequency of the received echo signal with the
frequency of the transmitted radiation. This comparison is
accomplished by heterodyning, or mixing, the returned signal
with the signal of the system's frequency reference (called the
local oscillator) on a detector. By maintaining the frequency of
the transmitter signal either above or below the local oscillator
signal by a fixed value determined by electronic control circuits,
and superimposing on the detector the local oscillator signal
with the return signal from a stationary target, an interference
pattern that modulates the amplitude of the detected laser
radiation is generated on the detector. This "beat" signal is equal
to the difference between the frequencies of the transmitted and
local oscillator signals. Since this beat signal is arranged to occur
within the radio frequency range (tens to hundreds of mega-
hertz), electronic amplifiers tuned to this frequency can process
the signal electronically and obtain the same signal-to-noise
benefits well known in conventional heterodyne radio receivers.
Measurement of the deviation of this known beat signal by the
Doppler effect caused by the moving target provides a measure-
ment of the speed of the target.
An additional advantage of the coherent radar system is that
one can increase the power of the local oscillator signal on the
detector to achieve the theoretical detector performance, which
is the quantum noise-limited sensitivity. A laser radar, using CO2
lasers, typically operates in the 1 0.6-,um wavelength region
1~1
OCR for page 36
36 ANTHONY J. DEMARIA
(Silverman, 19821. At this wavelength, HgCdTe detectors at
present provide the optimum sensitivity. A figure of merit for
detectors is usually given in terms of noise-equivalent power, or
NEP. Heterodyne NEPs of 2 x 10-~9, 5 x 10-~8, and 2 x 10-~7
WHz have been measured with HgCdTe detectors operating at
1 GHz at 77 K, i95 K, and 300 K, respectively.
Table 1 compares some of the relevant parameters of x-band
and CO2 laser radars. The laser radar operates at a frequency
3,000 times higher (or at a wavelength 3,000 times shorter) than
an x-band radar. The large difference in wavelengths between
the CO2 laser radar and the x-band radar results in large
differences in reflection characteristics of targets for the two
technologies. Variation in target surface dimensions (i.e., sur-
face roughness) typically are greater than the wavelengths of
CO2 laser radars, but less than the wavelengths of x-band radars.
Since man-made targets usually have smoother surfaces than
natural targets, even small man-made targets such as wires have
a larger cross-section than natural targets for laser radars. Figure
10 shows the ratio of detector signal current to noise current (is/in)
as a function of range for various natural and man-made targets
irradiated with a pulsed, coherent CO2 laser radar system having
400 mW of average power and an HgCdTe detector.
TABLE ~ Comparison of Basic Radar Parameters
CO2 Laser
Radar Characteristic Radar x-Band Radar
Frequency, Hz 3 x 10'3 10~°
Wavelength, cm 10-3 3
Beamwidth, AID, radians 10-3/dia. 3/dia.
Doppler sensitivity, Sv/A, Hz 2,000 x velocity 2/3 x velocity
Photon energy, joules 2 x 10-20 6.6 x 10-24
Note: D= diameter, in cm; v = velocity, in cm/s.
Since the beam divergence varies directly with wavelength and
indirectly with transmitting aperture, CO2 laser radars have
3,000 times smaller beam divergence than x-band radars with
the same aperture. Since the Doppler shift varies inversely with
wavelength, the CO2 laser radar has three orders of magnitude
higher Doppler sensitivity than an x-band radar (see Figure 1 1~.
Figure 11 shows, for instance, that for a radar frequency of 30
THz (CO2 laser frequency), a target moving at 0.5 km/in (about
1/10 the speed of a person walking) yields a Doppler signal of
100 kHz, whereas a 30-GHz microwave radar would yield a
Doppler signal of 0.1 kHz.
OCR for page 37
LASERS IN MODERN INDUSTRIES 37
100
_ -
—<,, 10
1
~ ~ GRASS
WIRE \
WD-1 NORMAL°\
O VHF ANTENNA
\POLE O O TV ANTENNA
~ ° CABLE
\ No
WIRE WD-1 160° \
HOUSE 0
\ TREES
SNOW ~
\
1 00 1 ,000 1 0,000
RANGE, m
FIGURE 10 Signal-to-noise ratio of a CO2 laser radar (400 mW average
power, 75 W peak power) for various natural and man-made targets.
1
10-1
10-2
co
ID
~ 10
~7
10
10-5
10-6
10-7
300 MHz /
/ /~-0,
//
.
~/~
/ CO2; f ~ 3 x 1 o1 3~
2/V/~//
-2 1o~1 1 10 1o2 103 10
2 x velocity -- km/hr
~ RF
Microwaves
Millimeter waves
1
Infrared
Tvisible
luv
FIGURE l l Comparison of Doppler sensitivity: Doppler frequency
shift as a function of radar wavelength and target velocity.
OCR for page 38
38 ANTHONY J. DEMARIA
11
Since the photon energy of the CO2 laser radar is 3,000 times
higher than that of the x-band radar, the laser radar beam has
3,000 times fewer photons per unit of energy than the x-band
radar. If one photon in unit time is the minimum detectable
signal, then the operation of a CO2 laser radar is limited to a
smaller field of view than the x-band radar for the same
transmitted power. Consequently, the laser radar is not suited to
wide-area search applications, but is well suited to applications
requiring ultrahigh sensitivity in range, azimuth, Doppler shift,
image resolution, and small field of view.
It is important to point out that laser radar suffers from
poorer propagation characteristics through the atmosphere
than conventional microwave radar because of higher back-
scatter from rain, snow, haze, and fog and because of higher
absorption by water in the atmosphere. Consequently, in the
earth's atmosphere, laser radars have a shorter range than
microwave radars. Fortunately, the operating wavelengths of
CO2 lasers falls within one of the best atmospheric windows
when compared with other laser wavelengths. Consequently, for
applications in the atmosphere, the relatively long wavelength of
10.6 ,um for CO2 lasers over other lasers, such as Nd3+:YAG,
ruby, and semiconductor lasers, makes the CO2 laser radar the
system of choice for most applications.
Figure 12 shows general areas of applications of various radar
technologies. Ladar is a commonly used acronym for "laser
radar," and was formed from laser detection and ranging
following the example of the word radar, which was originally an
acronym formed from radio detection and ranging. (Lidar, light
detection and ranging, is also used.)
Figure 13 compares a telescopic photograph of a control
tower at a range of 1.2 km with an image taken by a 15-year-old
coherent CO2 laser radar. The radar system used a binary (black
and white) gray scale and was not intended to produce a
photographic-quality image. The output power of the early
radar used to produce this image was 0.25 W at a pulse repetition
rate of 30,000 pulses/s. The trees in the far background of the
scene do not show up in the laser radar image because of a
range-gating technique used by the radar system. The evergreen
trees at the bottom of the photograph do show up in the radar
image because their return signals fell within the time window of
the time-gated receiver. Since glass is absorbing at 10.6 ,um, the
windows appear black in the radar image. The antennas on top
of the control tower are difficult to see against the sky in the
photograph but are easily visible in the laser radar image. The
ability of CO2 radars to detect small obstacles such as wires,
OCR for page 39
1o2
1o1
E
,o
-
8
~ -1
~ 10
-
o
3 rsynt~ Ire Microwave
S~irface/airborne/space surface radar
~ \ Micrwave _~
J
/ Tactical lADAR
As/ / Surfacelairborne
= ~ 1
10-2
10-3
- 4
10
Strategic LADAR
Surface/space
~1 , ~ 11 1 1
10 4 10 3 10 2 10 1 0 1 10
Field of view, sterad
LASERS IN MODERN INDUSTRIES 39
~ 1o2
. 104
~ 106 <~)
._
n
0
8
010 Z
FIGURE 12 Active sensors capabilities: general areas of applications of various
radar technologies from an angular resolution and field-of-view perspective.
FIGURE 13 Comparison of a photographic image (taken through a
telescope) with a black-and-white binary scale CO2 coherent laser radar
composite image. The trees in the far background were not recorded in
the radar image because of range-gating techniques used in the radar
system. Range: 1,200 m; average power: i/ W; pulse rate: 30,000/s.
OCR for page 40
40 ANTHONY ]. DEMARIA
poles, and antennas makes them ideally suited for obstacle and
terrain avoidance applications in helicopters. They are also
compatible with the 8- to 12-,um passive night-viewing avionic
systems now in use.
One of the most exciting potential applications of radar is in
the measurement of wind velocities in the upper atmosphere by
measuring, from the space shuttle, the Doppler shift from
backscatter off naturally occurring aerosols in the upper atmo-
sphere. The operation of such a system is expected to improve
greatly the accuracy of weather forecasting.
~~ ,
ElECTRlC CABlE INSPECTION
When it was established that polyethylene (PE) and cross-linked
PE (XLPE) material had intrinsically high dielectric strengths,
on the order of 800 kV/mm, the electric power industry ex-
pected 40-year lifetimes for underground electric power cables
in distribution systems using such materials. Consequently, in
the 1960s, the electric power industry began to make extensive
use of underground cables using PE and XLPE as the dielectric
between the inner and outer conductors. The expected lifetime
was not achieved even at average stress levels of 2-4 kV/mm,
even though such stress levels provided 200-400 times smaller
voltage gradients than the intrinsic dielectric strength of the
material. The failure rate for cables put into service since the
1960s reached a level that disturbed the electric power industry.
It led the Electric Power Research Institute, the U.S. Depart-
ment of Energy, and cable manufacturers to launch a research
and development program in the 1970s to solve the problem of
the premature failures of PE and XLPE cables.
The cables are produced in a continuous operation. A central
conductor of stranded copper wire passes through an extruder
that coats it with a smooth, thin semiconducting shield consisting
of PE filled with carbon black. Over this opaque semiconducting
surface, the white PE insulation is extruded and then cross-
linked with heat, ultraviolet radiation, or electron bombard-
ment. A second semiconducting shield of PE and carbon black is
then extruded over the insulation, followed by a mesh of
stranded copper wires and finally a protective plastic coating.
The research and development programs indicated that the
aging of the insulation generates branched channels caused by
dielectric breakdown, which in turn causes an electrical short
circuit between the outer ground conductor and the inner
conductor (see Figure 141. The branched channel structure, or
"trees," of dielectric breakdown in the insulation is believed to be
OCR for page 41
LASERS IN MODERN INDUSTRIES 41
Nn',tral Arraign -
Microvoids
Void
Contaminant
,
~°~`
/N
Voids and j' EN
at interface a\ OWN
Conductor in contact
with insulation
A/ ~ Undisoersed antioxidants
kY~
,_ ~
contaminants ~
Insulation and \~/
shield eccentricity
Electrical tree
Neutral wire embedded
in insulation shield
Strand shield fall-in
~ Bow be Bee
3
/~
nits
~ ' Electrochemical trees
· Loss of shield conductivity
· Damage during installation
· Loose fitting insulation shield
FIGURE 14 Common causes of electric power cable failures.
caused by surface imperfections at interfaces within the cable
and by irregularities in the insulating materials. These irregu-
larities are caused by gas- or vapor-filled voids, contaminating
particles, inhomogeneous variation of density in the materials,
and other defects. Unfortunately, visual inspection is not possi-
ble during the manufacturing process, where these defects arise,
because PE is normally a milky white, opaque material except
when immersed in hot oil.
Corona testing is a nondestructive inspection technique that
can detect 50-,um-diameter voids in 500-ft lengths of cable. The
disadvantage of the technique is that it cannot detect contami-
nants and flaws, voids filled with liquids or vapors, and micro-
voids 1-10 ,um in diameter. The technique also does not provide
on-line inspection during the manufacturing process, nor can it
locate the position of the defect.
The inspection procedure now commonly used is to cut out a
2-in. piece of manufactured cable every 10,000 ft. slice it into
0.5-in. portions, slice these portions into 0.020-in. wafers, make
these wafers transparent by immersing them in hot oil, and then
inspect the wafers visually under a 15-power microscope. The
obvious disadvantages of this technique are that it is neither an
. . . . .
On- 1ne, rea -time inspection process nor a none .estructlve pro-
cedure.
One promising approach to an on-line, real-time technique
tor nondestructive inspection of cable is based on the fact that
PE and XLPE are almost transparent in the far infrared.
OCR for page 42
42 ANTHONY J. DEMAR~A
~ .~
, 250 Am void
Microvoid background
~ AN devoid
Add-- ~
-
_~ _~ _
-~ Detector noise
FIGURE 15 Signals of a far-infrared laser inspection system from voids
in cross-linked polyethylene insulation used in electric power distribu-
tion cable (25 KV moving cable'.
Investigation of lasers that emit radiation in the far infrared
(Chang et al., 1970) led to a nondestructive technique for
continuously monitoring the quality of electric power cables in
real time (Cantor et al., 1981~. A monitoring system based on
this technique scans around the cable before the outer ground
conductor and its protective coating are extruded onto the cable.
The laser radiation scattered by voids, contaminants, or other
defects in the dielectric is collected and detected, and its mag-
nitude is digitally recorded. The speed of the cable through the
system is monitored to maintain a complete record of signal
amplitude caused by the scattered radiation as a function of
cable position. Figure 15 shows typical signals obtained at a
118-,um laser wavelength and 0.1-W laser power with a germa-
nium-doped silicon detector cooled with liquid- helium. Figure
16 shows an experimental arrangement of such a system. It is
important to note that because of the long laser wavelength
(submillimeter wavelengths), the mirrors are fabricated from
finely machined aluminum and do not require extensive polish-
~ng.
at,
It is too early to determine the practical effect of laser
inspection in the manufacture of electrical power cable, but it is
already apparent that the technology will provide useful re-
search instrumentation for the industry.
OCR for page 43
LASERS IN MODERN INDUSTRIES 43
I r ;
_
_,..._
._
.~
FIGURE 16 Optics used in a far-infrared laser inspection system for
electric power cable insulation.
Laser technology is young and robust, with a highly promising
and exciting future. It is now spawning new products and
opening major new segments of basic industries that will ensure
its growth well into the next century. The fields of fiber-optic
telecommunications, optical audio and video discs, optical data
storage, optoelectronics, lasers for material working (cutting,
welding, heat treating, hole drilling, and scribing), laser appli-
cations in medicine, laser instrumentation, and military applica-
CONCLUS~ONS
OCR for page 44
44 ANTHONY J. DEMA~A
REFERENCES
tions are still in their infancy; thus, considerable growth is yet to
come.
The most serious challenge in laser technology is the continu-
ing shortage of photonic engineers required to develop the
numerous new and rapidly evolving products the technology is
generating, to continually advance the state of the art required
to meet new product needs, and to work at the interface between
electronics and photonics technologies. An engineer in this field
needs a background in optics and electronics and in quantum
electronics. Most engineers working ire the field today are either
physicists who have learned some electronics or electronic engi-
neers who have learned some optics. The offering of a formal
undergraduate engineering curriculum in photonic engineering
would be a big boost to this important emerging field of
technology.
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Representative terms from entire chapter:
modern industries
