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TitleLasers: The Technology and Uses of Crafted Light
LanguageEnglish
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Page 2

L A S E R S
N E W E D I T I O N

Page 88

population inversion among the CO2 molecules. (The CO2 molecules
play essentially the same role in the gas mixture that the chromium
atoms played in Maiman’s laser.) Just as with the ruby laser, one end of
the tube holding the gas mixture is highly reflective, but because the
waves produced are not in the visible part of the spectrum—CO2 lasers
produce invisible infrared electromagnetic waves—the mirror is made of
different materials. Sometimes copper-coated glass is used; sometimes
the metal molybdenum is used as a mirror, and other materials are used
as well. The opposite end of the tube, the end through which the laser
shines, cannot be made of glass because with respect to infrared waves,
glass is opaque. One material that is sometimes used to create a partially
reflective material is zinc selenide. Under certain conditions, sodium
chloride (table salt) is sometimes used as well.

The electromagnetic waves produced in the excited gas oscillate back
and forth in the tube and are amplified via the process of stimulated emis-
sion. Carbon dioxide lasers are easily made more powerful by lengthen-
ing the tube in which they are produced, and compared to many other
lasers they are remarkably efficient: A high-power CO2 laser will convert
approximately 20 percent of all the electrical energy that enters the tube
into laser energy. This means, however, that 80 percent of the electrical
energy that enters the tube must be removed to prevent the equipment
from overheating. This is part of the reason that the principal constitu-
ent of the mixture is helium. It improves the ability of the gas to conduct
heat, and this makes it easier to remove heat from the tube. The laser
operates efficiently only if the gas mixture is kept cool.

The Nd:YAG laser is very similar to the ruby laser. It uses a
crystal—called the yttrium-aluminum-garnet crystal (hence YAG)—
through which neodymium is diffused. (The neodymium plays the same
role as the chromium in the ruby laser.) Visible light usually is directed
into the crystal by some sort of flash lamp, and it is this light that creates
the population inversion. This is the most common method of initiat-
ing a population inversion, but increasingly, Nd:YAG lasers are pumped
with another laser. Laser excitation of the lasing medium increases the
efficiency of the Nd:YAG laser. Because the electromagnetic radiation
produced by the Nd:YAG is almost in the visible range—the wavelength of
the light emitted by this laser is a little longer than that of red light—the
beam behaves in a way that is very similar to light waves. In particular, they
travel through glass without difficulty, something that the beam emitted
by the CO2 laser cannot do. This proves to be an important advantage.

Carbon dioxide lasers have several advantages over the Nd:YAG
lasers. It is a relatively simple matter to make a powerful CO2 laser:

Lasers in Industry 73

Page 89

Provide sufficient electrical power to a sufficiently long tube. The lon-
ger the tube, the more powerful the laser. Further, CO2 lasers can be
operated in CW (continuous wave) mode or pulsed, in part, because they
are relatively easy to cool, and they are efficient. True, an efficiency
of 20 percent means that 80 percent of the input energy is wasted,
but 20 percent is very good for a laser. By contrast, an ordinary, opti-
cally pumped Nd:YAG laser may well have an efficiency of less than 1
percent. Much of the energy that enters this laser’s crystal is converted
to heat and must be removed in order to avoid damaging the crystal.
But even with a cooling system, the Nd:YAG crystal is difficult to
keep cool because producing the laser beam rapidly generates heat
inside the crystal, and heat moves toward the surface, where it can be
removed, by conduction. The difference in the rates of heat generation
and heat conduction explains why it is easy to overheat the crystal. For
this reason, Nd:YAG lasers are always operated in brief pulses. Finally,
CO2 lasers are robust. A well-made tube of gas is hard to break and
relatively inexpensive to replace. The crystal that makes up the heart of
the Nd:YAG laser is, by contrast, both more fragile and more expen-
sive to replace.

Nd:YAG lasers, however, have their own advantages over the CO2
variety. The most important is that the wavelength of light emitted by the
Nd:YAG laser is easily transmitted along an optical fiber. (Recall that the

74 Lasers

Two images of the same laser-drilled hole. At a few hundred nanometers across, this
type of precision work can be accomplished only with a laser. (NASA)

Page 176

Raman,
Chandrasekhara
Venkata 141

Raman spectrometer
141, 141–142

razor blades, cutting
through 70

reference beam 128–
129, 130–131, 134,
134–135

reflection, in SLR
systems 117–118

reflection hologram
131–132

refraction, index of,
in fiber-optic cable
93–94, 94

refractive error,
correcting 64–67,
66, 67

resonance, in ruby
laser 36–37

resultant wave 22,
124–125, 125

retina
bleeding of 62
laser surgery on 56,
56–57

Roemer, Ole 3–4
ruby, chromium atoms
in 32–33

ruby laser 30–41, 34,
38
color of 33
cooling 39
efficiency of 39–41
laser beam from
34–37

light source of 33,
38

oscillation in 35–37
power source for
37–39

resonance in 36–37

ruby geometry in
34–36

in tattoo removal 68

S
satellite laser ranging
(SLR) 116, 116–117

Schawlow, Arthur
48–49

sculpture, casts of 130
selenium 87
semiconductor lasers
100–101

shortwave
communications
83–84

Silver, Bernard 99
SLR (satellite laser
ranging) 116, 116–117

sodium, in LGS
systems 121

solar system, atom
compared to 12–14

sound waves,
interference in 10

spatial light modulator
134

spectrometers 137,
140–144, 141

speed of light (c)
in astronomy, delay
from 3

estimation of 3–4
frequency in 6
in laser measurement
109–110

measuring 1–2
in vacuum 2–3
wavelength in 6

sphere, wave front
through 24–25

spontaneous emission
14–15

steam engine, energy
conversion in 26

steel, cutting, vapor
from 71

stimulated emission
15, 22–23
in ammonia gas 44
in chromium atoms
33

Einstein’s description
of 42

existence of
predicted 16

light leaks from 39
likelihood of 31
in plasma 52
population inversion
for 31–32, 43

“Stimulated Optical
Radiation in Ruby”
(Maiman) 39, 41

sunburn 8
sunlight

colors in 8, 19
incoherence of 15
information
transmitted by 86,
86–88

wavelengths in 8
surgery, lasers used in
57–61

surveying
lasers in 111–114,
113

measurement in
108–109, 109

T
Tacoma Narrows
Bridge 36, 36

Tainter, Charles
Sumner 86

tattoo 68, 68
tattoo removal 68–69

Index 161

Page 177

162 Lasers

tectonic plates,
measuring movement
of 114–118

telescope, in orbit
118–119

thermal energy, v.
mechanical energy
26

tissue
absorptive property
of 57–59

conduction of heat
through 60–61

Townes, Charles Hard
43, 43, 48–49

transmission hologram
131

TRG 56
tumor, laser used on
57–59

turbulence,
atmospheric
118–122

U
ulcers,
photocoagulation
of 62

ultraviolet laser 21,
61

ultraviolet rays 8
units of measurement
xiv

universal product
codes (UPCs) 96,
97–101, 98

U.S. Postal Service,
lasers used in 96–97

V
vapor

from cutting laser
71, 77

from drilling lasers 77
violet lasers 61
vision correction 64–
67, 66, 67

W
wasted energy

from laser 28
in ruby laser 38–39

water vapor, carbon
dioxide laser and 75

watts 26–27
waveform, model of
23

wave front 23
expanding 23–25, 24
in holography 129
intensity of 23–25
for laser beam 25
of lasers 126–127

wavelength (ν) 5–6, 6
in CD 103
coexisting 9–10
v. color 19–20
in DVD 105
in holography 126,
133

for laser applications
53–55

of laser light 19,
53–55

in lidar 114
of medical lasers
59–60

of Nd:YAG laser 75
in optical
microscopes 138

in photography 126
in PPM scheme
90–91

of resultant wave
125

in speed of light 6
in sunlight 8
in xenon flash lamp
38

waves
characteristics of
4–7, 6

combining 9, 10, 22,
124–126, 125

phases of 10–11
speed of 6
three-dimensional
movement of 24

two-dimensional
movement of 23

wave theory of light
4–11

welding lasers 80–82,
81

Woodland, Joseph 99
words, in PPM
scheme 89, 90

work, concept of
26–27

X
xenon flash lamp, in
ruby laser 33, 38

X-ray laser, heat from
52

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