Nearly 30 years afters its birth, the laser still seems exotic. Its most
dramatic uses remain in the laboratory or, in the case of 鈥淪tar Wars鈥� laser
weapons, in the realm of science fiction. Yet like satellites, lasers have
become part of our lives
LASERS GIVE out a special kind of light called coherent light. You can think
of coherent light as an army of soldiers, marching in step, in the same
direction, and wearing the same uniform. Ordinary light, though, is
incoherent. Like a tightly controlled army, laser light can do things that
less-organised light 鈥� or people 鈥� can鈥檛 do as well. Physicists have been
trying to control and manipulate laser light more effectively for almost 30
years.
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Light can be thought of as waves, and the light is said to be coherent when
the peaks and valleys in its waveform align with each other to move in phase.
The phase aspect is also rather like the soldiers marching together and
keeping in step. Two waves are in phase when their peaks and troughs coincide.
Light waves that are truly identical could stay that way forever, but even a
laser cannot emit light waves that are exactly the same. Because waves of
laser light are not exactly the same, they gradually drift out of phase as
they travel. The distance over which they remain coherent is called the
coherence length, and it varies considerably. For some lasers it is less than
a millimetre; for others it can be hundreds of metres.
Colour of light depends on its wavelength. Laser light is often said to be
鈥渕onochromatic鈥� as it is a single colour, although the wavelengths do differ
slightly. The range of wavelengths depends on the internal optics and the
nature of the laser material, but for many types of laser material the range
can be very narrow compared with that of the ordinary white light emitted by a
filament lamp.
The Sun emits radiation strongly at wavelengths from about 0.2 micrometres in
the ultraviolet to 2 micrometres in the infrared, with the most intense
emission in the visible region. The atmosphere of the Earth, particularly the
ozone layer, filters some of it (see Inside Science number 9) but this is
still a wide bandwidth. Many lasers operate on a fixed and narrow spread of
wavelengths, depending on the nature of the laser material. The red helium-
neon laser beam that reads bar codes at the till of some larger supermarkets
is visible, but its power is considerably less than a thousandth of a watt, so
it can鈥檛 do you any harm. Lasers can, however, be much more powerful. The US
armed forces are testing a chemical laser called the Mid Infrared Advanced
Chemical Laser (MIRACL) to shoot down missiles. MIRACL generates more than two
million watts for periods of a few seconds. Lasers can be even more powerful
if they emit their beams in short pulses. The Nova laser at the Lawrence
Livermore National Laboratory, in California, produces about 100 trillion
watts of power (100 000 000 000 000 W) in a pulse lasting under a billionth
(0.000 000 001) of a second. During that brief pulse, Nova鈥檚 power matches
that of 100 000 nuclear reactors. The giant laser is used for research on
nuclear fusion and weapons.
The basic idea of a laser is immortalised in its name, an acronym for Light
Amplification by the Stimulated Emission of Radiation. The radiation is
normally visible, ultraviolet, or infrared light. A process known as
stimulated emission triggers atoms or molecules to release some of their
energy as light; all laser light is produced in this way.
There is an important difference between light that is produced by stimulated
emission and ordinary light. Left alone, atoms or molecules in the higher of
two energy states can spontaneously release some excess energy as light by
decaying to the level of lower energy. Light of an appropriate wavelength can
also stimulate or trigger the atoms or molecules to undergo the transition
between higher and lower energy states, thus releasing energy in the form of
stimulated emission.
However, the atoms or molecules in the lower energy level can also absorb
light of the same wavelength and be promoted to the higher level.
Which of the two processes (stimulated emission or absorption loss) dominates
depends on whether there are more atoms in the lower level capable of
undergoing absorption or more in the upper level capable of stimulated
emission.
How lasers work
Stimulated emission
LEFT to themselves, atoms will always arrange themselves among their energy
states in such a way that there are more in the lower energy states than in
the higher states. So, in ordinary circumstances, absorption always wins.
Stimulated emission can however win over absorption if, for one pair of energy
states, there are more atoms in the higher state than in the lower one. Such a
condition is known as a population inversion.
Heating can increase the average energy of atoms or molecules, but heating
alone cannot create a population inversion. Heat moves all the atoms a little
higher on the energy level ladder. It does not increase the ratio of atoms in
a higher state. Instead, the energy must selectively excite them to certain
high-energy states, where they keep their excess energy for an unusually long
time. The atoms in lasers can be excited by light from other sources, by
electrical currents passing through gases or semiconductors, by chemical
reactions, or in other ways.
In a ruby laser, chromium atoms in a crystal of aluminium oxide absorb energy
from a bright flash. The flash is so bright that most of the chromium atoms
absorb extra energy. The atoms release some energy, but then become trapped in
a long-lived intermediate state known as the metastable state. A few atoms in
the metastable state release their extra energy as red light, and that light
stimulates emission from other chromium atoms, building up a pulse of red
light.
Ruby is not the best material with which to create a population inversion
because the lower level is the ground state, normally occupied by most
chromium ions. It takes more energy to create a population inversion if the
lower energy level is the ground state than if an intermediate energy is used.
More efficient, solid-state lasers use four energy levels. In these lasers, a
flash of light excites ions from their ground state to a short-lived highly
excited level, as with a ruby laser. The atom then drops in energy to the
metastable upper laser level, where it waits to be stimulated to the higher
level. Unlike the three-level laser, the atom does not drop back to its ground
state. Instead, it drops to an intermediate state, with more energy than the
ground level. Normally, a few atoms are in this intermediate level, so it
takes fewer atoms in the upper laser level to produce a population inversion
than for a three-level laser. For this reason, four-level lasers need less
energy than three-level lasers to start operating.
Stimulated emission produces light that travels in the same direction as the
light that triggers it. In most laser materials, stimulated emission is a weak
effect that can build up to high powers only after the light has travelled a
long distance through the laser, interacting with many atoms. The easiest way
to make the light go a long distance through the laser material is to put
mirrors on the ends of a cylinder or rod of laser material. As light bounces
back and forth between the mirrors, it increases in power. However, the
mirrors can reflect the laser light many times only if it is travelling
directly between them. Thus the laser beam is strongest along the line between
the two mirrors, and spreads out gradually beyond them, depending on how the
mirrors are arranged.
Lasers are not very efficient. The best lasers convert about 30 per cent of
the electrical energy that powers them into light energy, but many convert
much less than 1 per cent. The rest is lost as heat. The losses occur in
converting input power into a form suitable to drive the laser, in
transferring input power to the laser, in exciting atoms and molecules above
the upper laser level, and in failing to extract all the energy from the
excited laser material.
Lasers come in many different types, from tiny semiconductor lasers no bigger
than a grain of salt, to monsters the size of buildings which scientists
develop to research into high-power laser effects and laser weapons.
For many years, the most common lasers were helium-neon gas lasers. These
lasers produce light by passing an electric discharge through a tube
containing helium and neon gases. The discharge generates a steady red beam
when helium atoms excited in the discharge transfer their energy to the neon
atoms where they collide. Another common gas laser is the carbon dioxide
laser. These lasers are also powered by an electric discharge but this time
the discharge passes through a mixture of carbon dioxide, helium, and
nitrogen. This process produces up to 20 000 watts of steady infrared light of
wavelengths near 10 micrometres.
Today, the most widely used lasers are semiconductor lasers, which are small
chips of semiconducting material about the size of a grain of salt. The way in
which they work is similar to that of light-emitting diodes (LEDs), which are
also made from semiconductors. An electric current passing through the chip
causes current carriers to accumulate at the junction of two parts of the
crystal with slightly different composition. Whereas LEDs emit light
spontaneously, semiconductor lasers produce stimulated emission, with power
from a thousandth of a watt to a few watts.
Ruby lasers were the first in a family of crystalline solid-state lasers which
are still in use today. In the type most widely used, atoms of neodymium dope
a crystal of yttrium aluminium garnet. Neodymium lasers are four-level lasers
which emit pulsed beams when powered by a bright pulse of light from a
flashlamp, or steady beams when powered by a bright arc lamp. Semiconductor
lasers can also act as a power source for neodymium lasers, emitting either
pulsed or steady light.
For research purposes, many scientists use dye molecules dissolved in an
organic solvent, such as enthanol or sometimes water as the lasing material,
rather than crystals. The dyes have such complex energy level structures that
they can emit light at many wavelengths or at one selected band if the mirrors
and lenses are adjusted or 鈥渢uned鈥� to respond to a small wavelength range
only.
Other lasers promise extremely high powers. The most powerful laser in the US
is MIRACL, at White Sands Missile Range in New Mexico. MIRACL generates 2
million watts at wavelengths between 3.6 and 4.0 micrometres, which is in the
infrared. The laser mixes hydrogen and fluorine to produce excited hydrogen
fluoride molecules.
Military researchers also are working on 鈥渇ree electron鈥� lasers, powered by a
beam of electrons travelling at speeds very close to the speed of light.
Because, in such a laser, the electrons are not bound to atoms, they can emit
light at a broad range of wavelengths, from the ultraviolet to the infrared.
The armed forces are interested in such lasers because they hope to convert
much of the energy in a beam of electrons into laser light in order to pierce
armour plating.
After physicists built the first lasers on Earth, astronomers discovered
stimulated emission in the sky. 鈥淐osmic masers鈥� are hot clouds of interstellar
gas which emit microwaves. Carbon dioxide molecules in the upper atmospheres
of Venus and Mars produce stimulated emission near the 10-micrometre
wavelengths of the carbon dioxide laser. Lacking mirrors, these natural lasers
do not generate tightly focused beams, but they do give valuable information
to astronomers.
The dual nature of light and the physics behind lasers
LIGHT has a dual nature 鈥� it sometimes acts like a wave and other times acts
like a quantum-mechanical particle. Its wave nature gives it a wavelength and
an oscillation frequency. Its particle side is manifested as the photon, which
carries a discrete amount of energy. Other electromagnetic radiation also acts
like waves and particles. The photon鈥檚 energy is proportional to its
frequency.
Both wave and particle viewpoints help in understanding lasers, but the
particle view 鈥� a photon carrying a unit or quantum of energy 鈥� tends to be
more important.
Quantum mechanics limits atoms and molecules to certain discrete states of
energy, the lowest of which is called the ground state. Atoms and molecules
make essentially instantaneous transitions between energy levels. To go to a
higher energy from a lower one, they must absorb a photon of energy. When
dropping from a higher energy level to a lower one, they release a photon. The
transition energy is often (but not always) absorbed or is released as a
photon.
Atoms and molecules each have characteristic sets of energy states, which form
a kind of ladder. Each atom or molecule has unique energy states, which are
like the rungs of the ladder.
The energy levels are not evenly spaced and their positions are determined by
the number of protons and electrons in the atom. The energy states of atoms
correspond to the different orbits their electrons can occupy around the
nucleus. Molecules have two other energy states that they can exist in. One
represents vibrational states of the molecule; the other represents rotational
states.
Lasers can operate on electronic, vibrational, or rotational transitions, or
on transitions in which a molecule changes two or more energy states at the
same time.
How scientists harnessed the power of the laser
ALBERT EINSTEIN took the first step to the eventual development of the laser
by suggesting in 1917 that atoms could be stimulated to emit light. He was
proved right a decade later but, until after the Second World War, physicists
thought that spontaneous emission of photons would always overwhelm stimulated
emission. In 1954, Charles H. Townes, James P. Gordon and Herbert Zeiger of
Columbia University in New York built the first 鈥渕aser鈥�, that produced a
microwave beam. The development of the maser spurred Townes and others to try
to extend the idea to visible and infrared light.
By 1957, Townes and Arther L. Schawlow, then at Bell Telephone Laboratories,
analysed how to make an 鈥渙ptical maser鈥�. Meanwhile, Gordon Gould, then a 37-
year-old graduate student at Columbia, filled his notebooks with similar ideas
from what he called a 鈥渓aser鈥�. Townes and Schawlow published their results in
the prestigious journal Physical Review (volume 112, p 1940, 1958). Gould
sought a patent. People still argue who had the laser idea first. Townes鈥檚
work earned him a share in the 1964 Nobel Prize for Physics.
The Townes-Schawlow paper stimulated many efforts to make lasers. Gould鈥檚
proposal reached the US Department of Defense, which paid for research but
denied Gould a security clearance as he and his first wife were involved in a
Marxist group in the 1940s. However, the winner of the great laser race was a
little-known young American physicist, Theodore Maiman, then at Hughes
Research Laboratories in Malibu, California. On 15 May 1960, he slipped a
small ruby rod, with silvered ends, into a spring-shaped flashlamp. When he
fired the flashlamp, the ruby rod emitted a bright pulse of deep red light 鈥�
the first laser beam. The device was small enough to fit in his hand.
The solution that solved several problems
IN THE 1960s, some scientists used to gibe that the laser was 鈥渁 solution
looking for a problem鈥�. The label stuck because lasers then had few practical
uses. That is no longer true, but few people know how many ways lasers affect
them.
The greatest use of lasers is in handling and processing information. Every
day, millions of lasers play music in Compact Disc audio players. The infrared
beam from a tiny semiconductor laser reads music encoded as dark spots on a
reflective disk. The beam can be focused to a spot just a micrometre across,
so a 12-centimetre disk can store 75 minutes of digitised music. The same
technology can retrieve 600 million bytes (eight-bit units) of computer data
recorded on a 12-centimetre disk.
Lasers are also at the heart of many computer printers. Laser printers are
similar to small photocopiers, but instead of copying a printed page, they
write information with a semiconductor laser beam. Although laser printers are
expensive, they are widely used because they are the fastest way to print a
page attractively.
Fibre-optic telephone links rely on semiconductor lasers to send signals
through many kilometres of optical fibre. One laser beam can carry hundreds or
thousands of telephone calls. Most fibre-optic phone lines run between cities,
and some run under the sea, including transatlantic telephone cables such as
TAT-8, which began operating at the end of 1988.
Measurement is another important application for lasers. 快猫短视频s use lasers
to make ultra-precise measurements of the size of mechanical components, by
comparing the distances that different beams travel. Engineers make use of the
red beams from helium-neon lasers to cast accurate straight lines in the
construction of tunnels and in irrigation. Timing the round trip of laser
pulses to a reflective satellite and back helps geologists to measure small
motions of the Earth鈥檚 crustal plates. In research laboratories, dye lasers
help scientists to examine structures of atoms and molecules, and to analyse
the composition of materials. Pioneering work in that field earned Arthur L.
Schawlow and Nicolaas Bloembergen of Harvard University in Cambridge,
Massachusetts, the Nobel Prize in Physics in 1981.
Pulses of visible laser light have helped forestall blindness in millions of
diabetics suffering from diabetic retinopathy, a disease in which leaky blood
vessels block vision. Surgeons use carbon-dioxide lasers to perform
specialised surgery, especially in sensitive areas likely to bleed, such as
the larynx and the female reproductive tract. Dye lasers can be used to bleach
away birthmarks and to shatter kidney stones that are hard to treat by other
methods.
Manufacturers use lasers to cut and drill plastic, wood, rubber, and paper.
They can also use lasers to drill holes in the diamond dies through which they
pull metal wires. Titanium, a metal valued by the aerospace industry because
of its strength and light weight, can be cut more easily by lasers than with
saws. Lasers write identification patterns on semiconductor cases and on some
car parts. Laser beams can repair defective integrated circuits. Engineers are
working on many new industrial uses of lasers, including writing patterns for
semiconductor manufacture and fabrication of thin films of new high-
temperature superconductors.
The armed forces were interested in laser technology long before the American
鈥淪tar Wars鈥� program started work on laser battle stations for missile defence.
In the 1970s, the US began using lasers on the battlefield to measure the
range to potential targets. Coded series of laser pulses could 鈥渕ark鈥� targets,
so optical sensors could direct 鈥渟mart鈥� bombs to home in on them. Lower-energy
semiconductor lasers fire coded pulses to keep score in war games. High-energy
lasers like the 100 trillion watt Nova laser heat and compress nuclear fuels
to stimulate the fusion reactions that occur in nuclear bombs, letting
military researchers test their ideas without exploding actual weapons.
Further reading
Burkig, Valerie, Photonics: the New Science of Light (Enslow Publishers,
Hillside, New Jersey 1986 鈥� juvenile). Hitz, C. Breck, Understanding Laser
Technology (PennWell Publishing Co, Tulsa, Oklahoma, 1985). Hecht, Jeff,
Understanding Lasers (2nd edition (IEEE Press, Piscataway, New Jersey, 1994).
Kock, Winston E., Lasers and Holography, 2nd ed. (Dover Publications, New
York, 1981).