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Jun 9th 2005
From The Economist print edition |
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Lasers: Few innovations of the past century have changed as
many aspects of everyday life as the laser. A laboratory
curiosity at first, there now seems no limit to its
usefulness.
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FOR
several years after the invention of the laser, Charles
Townes' colleagues teased him about its seeming irrelevance
to the real world. They told him bluntly that it was “a
solution looking for a problem”. But Dr. Townes, who later
received a Nobel prize for his work in developing the
technology, has had the last laugh. Over the past four
decades, he has watched the laser emerge from the obscurity
of the laboratory to become a ubiquitous technology that is
used in an amazing diversity of applications. Lasers can now
be found in everything from DVD players to weapons systems,
telephone networks to operating theatres.
Lasers pack a powerful punch, and have had a huge impact on
society. Today, they guide missiles, point mining equipment
in the right direction and enable astronomers to take
clearer pictures of the heavens. They also perform myriad
mundane tasks, such as powering office printers, removing
unwanted body hair and carrying voices from one telephone to
another. Lasers are synonymous with precision, from
industrial cutting machinery to sights for sniper rifles.
Yet the creators of the laser were not so sure what they
were aiming at when they began working on the technology
back in the 1950s.
The grandfather of the laser was Albert Einstein. In 1917,
the great physicist postulated that atoms could be persuaded
to emit tiny packets of energy, called photons, in an
organized manner though a process of “stimulated emission”.
But how could this be achieved in practice? The answer came
in a flash of inspiration on a park bench, or a
middle-of-the-night “Eureka!” moment—depending on whose
story you believe.
A tale of two epiphanies
As Dr. Townes tells it, he had his epiphany shortly after
dawn on a spring morning in 1951, as he was sitting on a
bench in Franklin Park in Washington, DC. He quickly jotted
down his idea on a scrap of paper he found in his pocket.
“It is perhaps a hackneyed device among dramatists to have a
scientist scribble his thinking on the back of an envelope,
but that is what I did,” he later wrote.
In essence, lasers work by setting up a chain reaction, in
which photons of a particular wavelength prompt other atoms
to emit further, identical photons. For all of this to work,
a suitable material, called a “gain medium”—which can be a
solid, a liquid or a gas—is required. To get the process
started, the gain medium is “pumped” using a burst of light
or an electrical discharge. This excites the atoms in the
medium, some of which then emit photons of a specific
wavelength. When a photon encounters an excited atom, it may
then cause it to emit an identical photon—a process called
“stimulated emission”. This causes a cascade effect, as each
photon stimulates the emission of additional, identical
photons. The gain medium acts, in short, as an amplifier of
light: hence the name laser, an abbreviation of “light
amplification by stimulated emission of radiation”.
Laser light has several unusual properties. It is
monochromatic, since it is made up of identical photons of a
specific wavelength, and the wavelength of light is what
determines its color. Next, laser light is coherent, which
means the peaks and troughs of the light waves are aligned.
The light is also emitted in a tight, concentrated beam.
Dr. Townes, who was the head of the Columbia University
Radiation Laboratory in New York, applied his idea to
generate an invisible beam of microwave radiation, rather
than visible light, using ammonia as the gain medium. His
team called their device a maser, which is short for
“microwave amplification by stimulated emission of
radiation”. Their first maser was not capable of continuous
output, but two Soviet scientists, Nikolai Basov and
Aleksan. Dr. Prokhorov, came up with a solution to this
problem. They subsequently shared the Nobel prize with Dr.
Townes in 1964.
Following the invention of the maser in 1953, the big
question was whether the same technique could be applied to
visible light. Dr. Townes and his brother-in-law Arthur
Schawlow, who worked at the legendary Bell Laboratories,
laid out much of the theoretical basis for such an “optical
maser” in a paper that appeared in the scientific journal
Physical Review in 1958. In particular, they had the idea of
putting mirrors at each end of the gain medium to cause
photons to bounce back and forth. But by another man's
account, the crucial insight that made lasers possible came
in the middle of a mild November night in 1957. Gordon
Gould, a doctoral student at Columbia under Dr. Townes, sat
up in bed when the idea hit him. He says that the flurry of
inspiration lasted several days, and he filled over six
pages of a notebook with the details of how the
light-emitting device would function. In his notes, he was
the first to refer to this device as a “laser”; Dr. Townes
still preferred the term “optical maser”. Dr. Gould rushed
off and had the pages notarized at a local sweet shop. But
he did not apply for a patent: he wrongly thought he would
have to build a working prototype of his design first.
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Over the course of a patent war spanning the next 30 years,
Dr. Gould told countless courtrooms that Dr. Townes had
appropriated his ideas. He also argued that he had always
seen the laser's potential in real-world applications—which
were poorly outlined in the laser patent granted to Dr.
Townes and Dr. Schawlow. In the 1970s and 1980s, Dr. Gould
won several important legal battles, and was awarded a
number of patents relating to lasers. Neither Dr. Townes nor
Dr. Gould constructed the first working laser, however. That
happened in 1960, when Theodore Maiman, a physicist at the
Hughes Research Laboratories in Malibu, California, finally
hit upon the correct configuration of mirrors, gain medium
and pumping mechanism to make a working laser—beating Dr.
Townes's own team at Columbia, and Dr. Schawlow's at Bell
Labs, in the process.
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“It is not surprising that the pioneers of the laser could
not predict what it would be used for, since its
applications have been so many and varied.” |
Beaming with success
It is not surprising that Dr. Townes and Dr. Schawlow could
not predict what the laser would be used for, since its
applications have been so many and varied. “Asking how
lasers have influenced modern life is like asking how
electricity has influenced modern life,” says Lou
Bloomfield, a physicist at the University of Virginia in
Charlottesville. To start with, it seemed that lasers would
be most useful in industry and in scientific research: the
first examples had been devised for use in spectroscopy, to
probe the properties of matter. But the advent of small,
cheap, low-power semiconductor lasers meant they could be
incorporated into mass-produced consumer products. Today,
such lasers are the most numerous: they can be found at the
heart of hundreds of millions of CD and DVD players, PCs and
games consoles.
In an optical drive, a laser beam is focused on to the
microscopic bumps and pits on the disc's surface. The bumps
and pits are formed by injection-molding the plastic disc;
its surface is then covered with a very thin reflective
coating, and the whole thing is encased in a transparent
protective layer. As the disc spins, the laser's light is
reflected off the bumps and pits, each of which reflects the
light differently. These differences are detected using an
optical sensor, which produces a stream of digital
information corresponding to the sequence of bumps and pits.
CD and DVD players work in essentially the same way, but
DVDs can contain more information, since they are read using
laser light of a shorter wavelength (650 nanometers, as
opposed to 780 nanometers in the case of CDs). This shorter
wavelength allows smaller bumps and pits to be
distinguished, which means more of them, and hence more
data, can fit on the disc. The next generation of optical
drives will be based on blue lasers (with a wavelength of
405 nanometers), providing enough storage capacity for
high-definition movies.
Compared with videocassette players, which rely on magnetic
tapes and contain lots of complicated and expensive moving
parts, laser-based gadgets use cheaper off-the-shelf
components, explains Paul Jackson of Forrester, a
consultancy. As a result, the prices of DVD players fell far
more quickly than those of VCRs. The DVD player became one
of the fastest-adopted products in the history of
technology: the first players appeared in 1997, but around
half the households in the developed world now have one, and
the cheapest models cost a mere $40.
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Seeing well and looking good
As well as bringing crisper images to television screens,
laser technology has also made it easier for people to see
them without the help of glasses, thanks to the development
of laser eye surgery. As early as the 1960s, the possibility
of using the technology in medicine had occurred to
researchers such as Leon Goldman, often called the father of
medical lasers. He had been asked to evaluate the safety of
industrial lasers, and his investigations into the
biological effects of lasers led him to consider their use
in surgery.
In 1965 doctors first used an argon laser to repair a
detached retina, a sight-threatening condition in which the
light-sensitive layer of the inner eye becomes separated
from its supportive tissue. Focusing a laser on to the
retina and making tiny burns produces scar tissue that
“welds” the retina back into place. To the laser's
inventors, this application of the technology was a
completely unexpected development. “I had never heard of a
detached retina,” says Dr. Townes.
More recently, an increasing number of people have put aside
their glasses and contact lenses to take advantage of a
speedy procedure known as “laser-assisted in situ
keratomileusis”, better known as LASIK. Around 2m people in
America alone will undergo the operation this year to
correct near- or far-sightedness. During LASIK surgery, an
instrument cuts a thin flap in the outer portion of the eye,
called the cornea. This flap is then peeled back, allowing
laser pulses to vaporise and remove a microscopic portion of
the underlying tissue, thus reshaping the cornea. Since it
is the shape of the cornea that determines how near- or
far-sighted someone is, this reshaping has the effect of
correcting the patient's vision and reducing the need for
glasses or contact lenses.
Lasers have many other surgical uses, too. They have been
adapted to zap away spider veins, warts and wrinkles. “The
impact of lasers and laser-like technology in dermatology
over the last 20 years has been nothing short of
miraculous,” says David Goldberg, who is the director of
laser research at the Mount Sinai School of Medicine in New
York. A tiny beam can be used to heat up a wrinkle, for
example, and collagen proteins, which have a firming effect,
are then produced as part of the skin's healing mechanism.
As people become older, richer and vainer across the
developed world, researchers are devoting a lot of attention
to improving such cosmetic procedures.
Lasers also have less frivolous medical uses, of course, not
just improving people's appearances, but also saving lives.
They can be used to zap tumors, remove polyps and stem
bleeding. Lasers are more precise than conventional surgical
instruments, doing less damage and allowing for faster
healing and recovery. By directing a beam along a flexible
optical fiber, they can even be used inside the body.
Laser pulses traveling along optical fibers can heal, but
are better known for carrying information. In 1988 the first
transatlantic fiber-optic cable went into service, and such
cables have since redefined the economics of
telecommunications. “If you look at a world map now, it's
just a spider web of these undersea cables,” says Paul
Shumate, executive director of the Laser and Electro-Optics
Society at the Institute of Electrical and Electronics
Engineers (IEEE). “In 1975 there wasn't a fiber in the
telephone network. Now, 30 years later, there's hardly any
copper except at the last connection from the curb.” Stephen
Anderson, editor-in-chief of Laser Focus World, an industry
journal, points out that people have forgotten the
challenges of making transatlantic calls. As recently as 40
years ago, such calls required a lot of organization.
“Bookings were made two days in advance, and you were lucky
if you could understand one another,” he says. During the
1990s, enthusiasm for fiber networks led to an enormous
overbuild, huge overcapacity, and plunging prices. The
result was meltdown for the telecoms industry, but far
cheaper and easier communications for its customers.
Using several lasers, each with a different wavelength or
color, it is possible to send multiple streams of data down
a single optical fiber—a trick called “wavelength division
multiplexing”. Improving the precision with which particular
colors of laser light can be generated—in particular,
through careful temperature control—therefore has the effect
of increasing the capacity of a fiber. At the beginning of
the 1980s, fibers carried a single stream of data at 45
megabits per second, says Dr. Shumate. The capacity of each
stream has since increased to 10 gigabits per second, and
researchers have achieved speeds of 40 gigabits per second
in laboratory conditions. Sending dozens of such streams,
using many slightly different wavelengths, down a single
fiber increases its capacity to several terabits (millions
of megabits). Improvements in laser technology, in short,
have increased the capacity of networks without the need to
lay any new fiber.
New ways to use lasers continue to proliferate—and one of
them might even be coming to a supermarket near you.
Retailers already use laser scanners to read bar-codes. But
Sherwood Technology, a firm based in Widnes, Britain, has
devised a system called “DataLase” that uses lasers to write
prices, dates and other information on to fruits,
vegetables, pills and even confectionery. An edible,
light-sensitive coating is applied to the surface and then
drawn on using a low-power laser, which causes the coating
to change color. It is yet another example of the way that
new uses continue to be developed for lasers, decades after
their invention.
The lesson of the laser
And therein lies a lesson. The technology was originally
developed for use in spectroscopy, and its myriad other uses
were an unexpected bonus. “People just didn't imagine the
many applications which now seem so obvious,” says Dr.
Townes. “But that's a characteristic of most really brand
new ideas.” Dr. Schawlow has even suggested that trying to
anticipate the possible uses for the laser might have
hindered its development. “We had no application in mind,”
he said in 1981. “If we had, it might have hampered us and
not worked out as well.”
Some people worry that the research climate that produced
this fundamental technology—one that allowed bright minds to
pursue abstract ideas—has become regrettably rare. Dr.
Bloomfield, who himself once worked at Bell Labs, feels that
today's corporate research laboratories tend to have a
shortsighted view, focusing only on the next six months.
“More and more, everyone's waiting for someone else to do
the pioneering work and hoping to take advantage of other
people's foresight,” he says.
The laser is an object lesson in the value of blue-sky
research, and a reminder that even a seemingly obscure
technology can go on to have an extraordinary range of uses.
So has Dr. Townes ventured back to the Franklin Park bench
since the morning he came up with his breakthrough idea? No,
he admits, but perhaps it would lead to new insights. “I
probably ought to go back,” he says. |
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