The laser guidebook

485 Pages·1999·3.454 MB·English

by  Jeff Hecht

Source: The Laser Guidebook
Chapter
1
Introduction
The word laser is an acronym for “light amplification by the stimulated
emission of radiation,” a phrase which covers most, though not all, of
the key physical processes inside a laser. Unfortunately, that concise
definition may not be very enlightening to the nonspecialist who wants
to use a laser, and cares less about its internal physics than its external
characteristics. The laser user is in a position analogous to the
electronic circuit designer. A general knowledge of laser physics is as
helpful to the laser user as a general understanding of semiconductor
physics is to the circuit designer. However, their jobs require them to
understand the operating characteristics of complete devices, not to
assemble lasers or fabricate integrated circuits.
This book is written and organized with the needs of the laser user in
mind. The first few chapters describe the basic ideas behind lasers, how
they work, and the characteristics that are most important from a
user’s standpoint. They give an overview of the field, but cannot replace
a good textbook or tutorial introduction to lasers. The following
chapters focus on individual laser types, first outlining operating
principles, then describing specific characteristics such as output
power, wavelength, and input power requirements. The Appendix
tabulates the most important characteristics of major lasers. The
chapters on individual types of lasers follow the same basic outline and
are structured both to be read as chapters and to be used for looking up
specific data. Inevitably, such a structure brings with it some
redundancy; wavelength and power range, for example, may be
mentioned a few times in the same chapter. However, what may seem
repetitive when reading the chapter also makes the information easier
to find when searching out specific data in the book.
As a guide to practical laser technology, this book concentrates on
lasers which are available commercially or are otherwise important to
1
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Introduction
2 Chapter One
current and potential laser users. Details on performance are given for
lasers where the technology is reasonably well established, such as
helium-neon; newer types such as free-electron lasers are described in
more general terms. Development of high-energy laser weapons is
covered only briefly because the technology is classified and of limited
interest. In a few cases—notably chemical, free-electron, and x-ray
lasers—far more effort has gone into military weapons programs than
into development of lower-power lasers for civilian use.
An annotated bibliography and reference list follows each chapter.
The listing includes selected review articles as well as references cited
in the text so that readers can search out more details if they desire.
Some references may appear dated, but some fields of laser technology
are not as fast-moving as they may appear from a distance. For
example, the fundamentals of rare gas ion lasers have changed little in
the past few years, although commercial models have pushed further
into the ultraviolet. In addition, few good review articles on more
mature lasers—in particular, many gas types—have come to my
attention in recent years.
No book can hope to keep up with the continual changes in the product
lines of laser manufacturers. Specific models are mentioned in a few
places, but only as examples of the characteristics to expect in commercial
products. Companies also come and go in the field. The best way to follow
the laser industry is by reading one or more of its monthly trade
magazines: Lasers & Optronics, Laser Focus World, and Photonics
Spectra. (To make my biases perfectly clear, I will identify myself as a
cofounder and contributing editor of Lasers & Optronics. The other
magazines provide the sort of competition that keeps all three publications
working hard to serve their readers.) Many other magazines and scholarly
journals publish useful material on lasers and related technology.
All three major trade magazines publish industry directories, and
those listings are the best way to locate manufacturers of specific
lasers. They also tabulate specifications for selected lasers and
advertisements from manufacturers. I find the Lasers & Optronics
Buying Guide the easiest to use, but that may be because I helped
organize it. The bibliography gives information on all three directories.
Definition and Description of a Laser
From a practical standpoint, a laser can be considered as a source of a
narrow beam of monochromatic, coherent light in the visible, infrared,
or ultraviolet parts of the spectrum. The power in a continuous beam
can range from a fraction of a milliwatt to around 25 kilowatts (kW) in
commercial lasers, and up to more than a megawatt in special military
lasers. Pulsed lasers can deliver much higher peak powers during a
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Introduction
Introduction 3
pulse, although the power averaged over intervals while the laser is off
and on is comparable to that of continuous lasers.
The range of laser devices is broad. The laser medium, or material
emitting the laser beam, can be a gas, liquid, glass, crystalline solid, or
semiconductor crystal and can range in size from a grain of salt to
filling the inside of a moderate-sized building. Not every laser produces
a narrow beam of monochromatic, coherent light. Semiconductor diode
lasers, for example, produce beams that spread out over an angle of 20
to 40°, hardly a pencil-thin beam. Liquid dye lasers emit at a broad or
narrow range of wavelengths, depending on the optics used with them.
Other types emit at a number of spectral lines, producing light that is
neither truly monochromatic nor coherent.
Practically speaking, lasers contain three key elements. One is the
laser medium itself, which generates the laser light. A second is the
power supply, which delivers energy to the laser medium in the form
needed to excite it to emit light. The third is the optical cavity or
resonator, which concentrates the light to stimulate the emission of
laser radiation. All three elements can take various forms, and
although they are not always immediately evident in all types of lasers,
their functions are essential. Figure 1.1 shows these elements in a ruby
and a helium-neon laser; the internal workings of lasers are described
in more detail in Chap. 3.
There are several general characteristics which are common to most
lasers which new users may not expect. Like most other light sources,
lasers are inefficient in converting input energy into light. Efficiencies
range from under 0.01 to over 30 percent, but few types are much above
1 percent efficient. These low efficiencies can lead to special cooling
requirements and duty-cycle limitations, particularly for highpower
lasers. In some cases, special equipment may be needed to produce the
right conditions for laser operation, such as cryogenic temperatures for
the lead salt semiconductor lasers described in Chap. 21. Operating
characteristics of individual lasers depend strongly on structural
components such as cavity optics, and in many cases a wide range is
possible. Packaging can also have a strong impact on laser
characteristics and the use of lasers for certain applications. Thus wide
ranges of possible characteristics are specified in many chapters,
although single devices will have much more limited ranges of operation.
Differences from Other Light Sources
The basic differences between lasers and other light sources are the
characteristics often used to describe a laser: the output beam is
narrow, the light is monochromatic, and the emission is coherent. Each
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Introduction
4 Chapter One
Figure 1.1 Simplified views of two common lasers, (a) ruby and (b)
helium-neon, showing the basic components that make a laser.
of these features is important for certain applications and deserves
more explanation.
Most gas or solid-state lasers emit beams with a divergence angle of
about a milliradian, meaning that they spread to about one meter in
diameter after traveling a kilometer. (Semiconductor lasers have much
larger beam divergence, but suitable optics can reshape the beam to
make it much narrower.) The actual beam divergence depends on the
type of laser and the optics used with it. The fact that laser light is
contained in a beam serves to concentrate the output power onto a
small area. Thus a modest laser power can produce a high intensity
inside the small area of the laser beam; the intensity of light in a 1-
milliwatt (mW) helium-neon laser beam is comparable to that of
sunlight on a clear day, for example. The beams from high-power
lasers, delivering tens of watts or more of continuous power or higher
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Introduction
Introduction 5
peak powers in pulses, can be concentrated to high enough intensities
that they can weld, drill, or cut many materials.
The laser beam’s concentrated light delivers energy only where it is
focused. For example, a tightly focused laser beam can write a spot on a
light-sensitive material without exposing the adjacent area, allowing
high-resolution printing. Similarly, the beam from a surgical laser can
be focused onto a tiny spot for microsurgery, without heating or
damaging surrounding tissue. Lenses can focus the parallel rays in a
laser beam to a much smaller spot than they can the diverging rays
from a point source, a factor which helps compensate for the limited
light-production efficiency of lasers.
Most lasers deliver a beam that contains only a narrow range of
wavelengths, and thus the beam can be considered monochromatic for
all practical purposes. Conventional light sources, in contrast, emit
light over much of the visible and infrared spectrum. For most
applications, the range of wavelengths emitted by lasers is narrow
enough to make life easier for designers by avoiding the need for
achromatic optics and simplifying the task of understanding the
interaction between laser beam and target. However, for some
applications in spectroscopy and communications, that range of
wavelengths is not narrow enough, and special line-narrowing options
may be required.
One of the laser beam’s most unique properties is its coherence, the
property that the light waves it contains are in phase with one another.
Strictly speaking, all light sources have a finite coherence length, or
distance over which the light they produce is in phase. However, for
conventional light sources that distance is essentially zero. For many
common lasers, it is a fraction of a meter or more, allowing their use for
applications requiring coherent light. The most important of these
applications is probably holography, although coherence is useful in
some types of spectroscopy, and there is growing interest in
communications using coherent light.
Some types of lasers have two other advantages over other light
sources: higher power and longer lifetime. For some high-power
semiconductor lasers, lifetime must be traded off against higher power,
but for most others the life-vs.-power trade-off is minimal. The
combination of high power and strong directionality makes certain
lasers the logical choice to deliver high light intensities to small areas.
For some applications, lasers offer longer lifetimes than do other light
sources of comparable brightness and cost. In addition, despite their
low efficiency, some lasers may be more efficient in converting energy
to light than other light sources.
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Introduction
6 Chapter One
The Laser Industry
Commercial Lasers. There is a big difference between the world of
laser research and the world of the commercial laser industry.
Unfortunately, many text and reference books fail to differentiate
between types of lasers that can be built in the laboratory and those
which are readily available commercially. That distinction is a crucial
one for laser users.
Laser emission has been obtained from hundreds of materials at
many thousands of emission lines in laboratories around the world.
Extensive tabulations of these laser lines are available (Weber, 1982),
and even today researchers are adding more lines to the list. However,
most of these laser lines are of purely academic interest. Many are
weak lines close to much stronger lines which dominate the emission in
practical lasers. Most of the lasers that have been demonstrated in the
laboratory have proved to be cumbersome to operate, low in power,
inefficient, and/or simply less practical to use than other types.
Only a couple of dozen types of lasers have proved to be commercially
viable on any significant scale; these are described in the rest of this
book and summarized in the Appendix. Some of these types, notably
the ruby and helium-neon lasers, have been around since the beginning
of the laser era. Others, such as vibronic solid-state, are promising
newcomers. The family of commercial lasers is expanding slowly, as
new types such as titanium-sapphire come on the market, but with the
economics of production a factor to be considered, the number of
commercially viable lasers will always be limited.
There are many possible reasons why certain lasers do not find their
way onto the market. Some require exotic operating conditions or laser
media, such as high temperatures or highly reactive metal vapors.
Some emit only feeble powers. Others have only limited applications,
particularly lasers emitting low powers in the far-infrared, or in parts
of the infrared where the atmosphere is opaque. And some simply
cannot compete with materials already on the market.
Market Size. Sales of lasers per se—including power supply and
optics—was about $750 million in the noncommunist world in 1990
(Lasers & Optronics, 1990). Total sales of systems containing lasers
was much higher, probably several billion dollars. The difference
reflects an important fact: for many applications, the cost of the laser
represents only a small part of the system cost.
Because some lasers are much more expensive than others, there are
large dichotomies between rankings of laser sales based on number and
those based on dollars. Over 25 million semiconductor diode lasers
were sold in 1990, representing 98 percent of the total number of lasers.
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Introduction
Introduction 7
However, diode lasers accounted for less than 30 percent of the dollar
volume of sales. Conversely, some 4000 carbon dioxide gas lasers
accounted for 21 percent of 1990 laser sales dollars (Lasers &
Optronics, 1990).
Types of Organizations. Many companies in the laser industry are
young, and some are not far removed from humble origins in a garage
or basement. Some large companies started their own laser divisions,
but most laser groups began as independent start-ups. The first wave of
laser start-ups were formed in the early to mid- 1960s, in the wake of
the demonstration of the first laser in 1960. That wave included the two
companies generally considered the largest laser manufacturers,
Spectra-Physics Inc. and Coherent Inc. Many more laser companies
have been formed since, with start-ups continuing to play an important
role in developing new technology through the 1980s.
The last half of the 1980s saw many mergers and much
restructuring of the laser industry. Two of the largest independent
laser companies, Spectra-Physics and Lumonics Inc., were acquired
by much larger corporations, Ciba-Geigy and Sumitomo, respectively.
(However, in mid-1990, Ciba-Geigy sold Spectra-Physics to Pharos
AB, a Swedish company affiliated with the Nobel group, but
comparable in size to Spectra-Physics.) Some smaller companies also
were sold, reflecting a general consolidation of the industry, but that
was offset by formation of new companies and spin-offs of laser
divisions of some larger firms.
No single company offers every kind of laser, but many of the larger
firms do produce several different kinds. In contrast, most smaller
companies specialize in just one or a few types of lasers. While some
companies produce related products, others stick to lasers. Likewise,
many other companies concentrate on related products such as optics
or power supplies and stay away from producing lasers per se. Some
companies build systems around lasers they purchase from outside
vendors; others produce their own lasers for use in systems. The field is
a dynamic one, and readers who want to keep up with who’s making
what should follow the trade magazines which cover the field: Lasers &
Optronics, Laser Focus World, and Photonics Spectra.
Market Structure. There are few clear-and-fast patterns in the structure
of the laser market. The market has become an international one, with
major activity in North America, western Europe, and Japan. The two
largest laser makers, Spectra-Physics and Coherent Inc., for many years
concentrated on gas and dye lasers, but both now offer broader product
lines. Both are nominally based in the United States but have strong ties
to Europe; Spectra-Physics is a subsidiary of a Swedish company, while a
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Introduction
8 Chapter One
principal Coherent subsidiary is based in Germany. A third large laser
manufacturer, Lumonics Inc., is a Canadian subsidiary of a Japanese
company, and in turn has a major division based in the United States.
Japanese electronics giants dominate the world market for low-priced
semiconductor lasers, but they have some competition in other
semiconductor lasers, and so far have not become major suppliers of
high-power semiconductor laser arrays.
Lasers have long been among the military sensitive technologies
subject to export controls by the United States and many of its allies.
The severity of restrictions depends on the political climate, and eased
with the decreased international tensions of 1990. However, as of 1990,
there is little trade in laser or related technologies between the Soviet
Union, eastern Europe, and China on one hand and North America,
Japan, and western Europe on the other. In general, developing
countries must import laser equipment because they lack domestic
industries.
The biggest laser makers also offer optical components and
accessories for single-stop shopping, but many laser makers offer
accessories only if they can be used with their own products. There are
many small companies (and some large ones) which make laser optics,
often custom-made for specific applications. There are several
companies which function as catalog supply houses for optics and
accessories. All told, about a hundred companies make lasers, and
several hundred more provide laser accessories and offer services to the
laser industry. The cast of companies changes; for current information
consult an industry directory.
An Introduction to Laser Safety
Laser safety has been controversial since lasers began appearing in
laboratories. The two major concerns are exposure to the beam (which
presents much more danger to the eyes than to the rest of the body)
and high voltages within the laser and power supply. Many standards
have been developed covering either the performance of laser
equipment or the safe use oflasers; some developed by government
agencies have legal status, while others are recommendations by
voluntary organizations. Tabulations of these standards are available
(Weiner, 1990).
High-power laser beams can burn the skin, but the most important
hazards of laser beams are to the eyes, which are the part of the body
most sensitive to light. Like sunlight, laser light arrives in parallel
rays, which the eye focuses to a point on the retina, the layer of cells
that responds to light. Just as staring at the sun can damage vision,
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Introduction
Introduction 9
exposure to a laser beam of sufficient power can cause permanent eye
damage.
Eye hazards have attracted considerable attention from standards
writers and regulators. They depend on parameters such as laser
wavelength, average power over a long interval, peak power in a pulse,
beam intensity, and distance from the laser. Wavelength is important
because only certain wavelengths—between about 0.4 and 1.5
micrometers (µm)—can penetrate the eye well enough to damage the
retina. Ultraviolet light can damage surface layers of the eye (and some
can penetrate to the retina, especially in people whose natural ocular
lens has been removed). Infrared light also can damage the surface of
the eye, although the damage threshold is higher than that for
ultraviolet light. Eye response also differs within the range that
penetrates the eyeball because the eye has a natural aversion response,
which makes it turn away from a bright visible light which is not
triggered by infrared wavelengths longer than 0.7 µm.
High-power laser pulses pose dangers different from those due to
lower-power continuous beams. A single high-power pulse lasting less
than a microsecond can cause permanent damage if it enters the eye. A
lower-power beam presents danger only for longer-term exposure.
Distance reduces laser power density, thus decreasing the potential for
hazards to the eye.
Many countries have safety standards which must be met by laser
products sold within their borders. The National Center for Devices
and Radiological Health, part of the U.S. Food and Drug
Administration, has established standards in the United States
(CDRH, 1985). Many other countries have individual standards based
largely on recommendations of the International Electrotechnical
Commission (IEC, 1984). Laser product standards include provisions
for warning labels indicating hazard class. (In the United States, the
hazards increase with class number, with Class IV covering the most
powerful lasers.) Depending on hazard classification, lasers sold in the
United States may require beam shutters to block the beam when not
in use, key interlocks, and other safety features.
The United States does not have federal standards for the safe use of
lasers, but several states have set their own standards, and many other
countries have standards for laser use. The IEC recommended standard
covers the safe use of lasers. The American National Standards Institute
(ANSI) has developed a voluntary standard for laser use. These
standards concentrate on avoiding eye exposure.
Systems incorporating lasers also must meet safety standards. In
many of these systems the beam is contained so that it cannot leave the
box or reach the user’s eye, thus avoiding the need for warning labels.
Laser printers and compact-disk players are the most common
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Introduction
10 Chapter One
examples. Some scan a low-power beam so rapidly that the beam does
not stay in one place long enough to pose a hazard; laser scanners at
supermarket checkout counters are an example. High-power lasers
used in manufacturing may operate behind beam-absorbing shields,
which separate the operator from the workpiece and laser beam. Such
shields also may be used in some laboratories.
People working where they cannot avoid exposure to laser beams
that pose eye hazards should wear safety glasses or goggles. Special
filters in the safety goggles block light at certain laser wavelengths,
while transmitting enough other light to enable the worker to see. The
proper goggles should be used for work with each type of laser; it does
no good to block the blue-green light from an argon laser when working
with a red helium-neon laser. Users can select general-purpose goggles
that fit over the eyes and spectacles, or can have prescription spectacles
made from laser-blocking filter glasses. Goggles should block all
possible paths for laser light to reach the eye, because hazardous pulses
can be reflected from any shiny surface and arrive from unexpected
angles.
High-voltage hazards have attracted less attention, but in practice
are far more deadly. There is no public record of fatal injury due to a
laser beam, but several people have been electrocuted by high voltages
in a laser or power supply. Except for semiconductor lasers, virtually all
lasers require high voltages and sometimes high currents to generate a
beam. The high voltage may be applied directly to the laser medium or
to a pump lamp, but it is present in the system. Capacitors and some
other components may retain dangerous voltages for a considerable
time after the laser is used, especially in pulsed lasers. Be wary of these
electrical hazards.
If you plan to work directly with lasers, you should read a book that
discusses laser safety in more detail. The most exhaustive survey is by
David Sliney and Myron Wolbarsht (1980). A briefer guide to laser
safety with more emphasis on practical measures has been written by
D.C.Winburn (1990). The Lawrence Livermore National Laboratory
has issued a booklet on safety eyewear (1987). Documents also are
available from the National Center for Devices and Radiological Health
(1390 Piccard Dr., Rockville, MD. 20850), and the American National
Standards Institute (1430 Broadway, New York, NY 10018).
Overview of Laser Applications
Laser applications are far too broad and diverse to cover in any detail
here. Individual chapters cover the most important applications of each
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