Prometheus stole fire from the
gods for the benefit of mankind. Zeus, the supreme
god on the Olympian Mountain, punished Prometheus
by having him bound to a rock. His liver was devoured
by an eagle everyday and his ordeal continued until
he was rescued by the hero Hercules. |
|
This extract from Greek mythology symbolizes the struggle
of mankind in striving for a brighter future and emerging
from barbarism. In the sky, the Sun gives mankind light and
warmth; it is the source of all life. On the Earth man has
conquered fire for light and warmth, leading to the advancement
of civilization. The story of Prometheus also highlights the
hazards of fire.
There are several milestones in the development
of civilization: the discovery and applications of fire; the
invention of Edison's incandescent lamp and moving pictures;
and the popularization of television. Light expels darkness
and develops civilization, but the flashes of atomic and hydrogen
bombs threaten the very existence of human beings.
Today, man creates a light source which emits
the brightest light. This light source is laser. The first
laser was a ruby laser built by Theodore H Maiman, a young
physicist at Hughes Research Laboratories in Malibu, California
(Fig.1). In mid 1960 he proudly demonstrated the world's first
laser: a rod of synthetic ruby with reflecting coatings on
both ends, surrounded by a helical flashlamp. When the lamp
was pulsed, a pulse of red light emerged from one partially
transparent coated end of the rod. Maiman had studied the
energy levels of ruby extensively but his road to the invention
was not smooth. He disagreed with main-stream opinions that
the best materials for lasers were gases, persisted when Hughes
management told him to stop, and did not lose his heart when
his report on ruby laser was rejected by the prestigious journal
Physical Review Letters. Finally he succeeded in inventing
the ruby laser and with its fine red beam the laser era was
born.
 |
(Figure 1 The
first laser was born in 1960. American physicist T H
Maiman extracted very bright and highly directional
light of extremely pure color from a rod of synthetic
ruby. Laser is the product of quantum theory and has
found extensive applications once it came out.)
|
(Laser Pointer - This photo was provided
by Physics World)
Laser is a powerful tool of mankind. A variety of lasers
are working for various applications. Lasers cut, weld, drill,
heat-treat workpieces (Fig.2), modify metal surfaces by alloying
and cladding, and mark serial numbers on parts in factories;
perform surgery and refix detached retinas of the human eye
in hospitals; measure distance by interference method; align
tunnels, drainage tile, ditches, building foundations, or
industrial equipment; read barcodes in supermarket checkouts;
create three dimension images by holography; produce spectra
in laboratories; and guide bombs in battle fields.
(Figure 2 Laser-cut saw-blade body)
One laser has entered every household. Its
light reads the digital codes on a thin aluminum film in a
plastic disc and converts them into beautiful music, or colorful
picture, or great works. Its light also transmits family conversation
or TV programs across thousands of miles of oceans, and prints
page after page of manuscripts or drawings. This is the semiconductor
laser (Fig.3) and is just several millimeters in size. It
is working hard in data processing, storage and retrieval,
and in communication. There has been a happy marriage of xerography,
computer word processing, and laser. Their child is laser
printing. The now-ubiquitous semiconductor lasers create a
miracle in the present information revolution and they tell
the biggest success story of lasers.
(Figure 3 Semiconductor lasers were invented in 1962. Researchers
of General Electricity, IBM and MIT in USA discovered that
the GaAs diode could transform electric energy into light.
Tens billion of laser diodes have been produced for telecommunication
and CD every year until now.)
Laser light is very different from natural light or light
from ordinary lamps. Laser light is monochromatic, directional
and coherent, i.e. highly "ordered".
Monochromatic
No matter how the size or the intensity of lasers may differ,
laser light has pure color, which is in contrast to sunshine
and lamplight. A white light is made up of many different
colors: Newton's prism, as well as the water drops in the
air after a shower, spread these colors into a rainbow. By
comparison, a semiconductor laser only emits one near infrared
light; an argon ion laser can emit green, blue or purple;
and a carbon dioxide laser emits invisible infrared light.
Directional
Lamplight propagates outwards in all directions, similar to
the explosion of a stick of dynamite. A laser emits light
along only one direction, like a bullet shot from a gun barrel.
A laser can thus concentrate all its power in one point to
generate extremely high intensities. When astronauts looked
back at the dark side of the Earth from the Moon, lights of
big cities could not be seen. However, the lasers aimed at
the Moon from the Earth gave distinct light spots. A helium-neon
laser of milliwatts produces a light spot brighter than the
sun.
Coherent
Laser light is "coherent". It is appropriate to
use laser light to perform interference experiments. In Young's
double-slit experiment using a laser, the interference fringes
are distinct even if a piece of glass is positioned over one
of the slits to increase the optical path difference. The
whole pattern will disappear if ordinary light is used. Light
is emitted in the form of individual finite wave trains which
are uncorrelated. The wave trains in ordinary light are so
short that the composite light is comparable to the crowd
in a market. The wave trains in laser light are much longer
than those in lamplight so that the light behaves like soldiers
marching in formation. Coherence is essential to holography.
The coherent length of a ruby laser can reach 10 m and that
of a He-Cd laser can be stretched to about 100 m.
A Laser contains 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 (Fig.4).
Hundreds of materials in gas, liquid or solid state can provide
laser emission but only a couple of dozen types have proved
to be practical in the laser industry. According to the medium
lasers are divided into four groups: solid-state (e.g. ruby,
Nd:YAG or Nd:glass lasers), semiconductor (GaAs diode laser),
gas (He-Ne, Ar, CO, and  lasers)
and dye lasers. Lasers are also classified according to their
relative hazards to eye and skin into four classes so as to
specify appropriate controls.
(Figure 4 A simplified view of ruby laser,
showing the basic components that make a laser.)
All laser action begins with the input of energy to a laser
medium, called pumping. Atoms or molecules of the laser medium
absorb the input energy and are excited. Excitation is the
process of raising atoms or molecules from a lower energy
level to a higher one. It could be fulfilled by passing electric
current through the laser medium, or shining strong light
on to it, or causing chemical reactions, etc. Nature keeps
everything in the lowest possible state so that ordinarily,
the population of atoms or molecules in the lower energy level
is always larger than that in the higher level. Energy input
makes it possible that the population in the higher level
is larger than that in the lower level. This unusual situation
is given a special name: population
inversion, which is essential for laser action.
An analogue is the wealth distribution in different classes
of the population of a society. Population inversion is similar
to an inversed pyramid of wealth distribution.
Just like a bubble will inevitably burst and Newton's apple
would fall to the ground or onto Newton's head, the atoms
or molecules in the higher energy level will drop to a lower
energy level in a process called transition. In free falling,
an apple's potential energy is converted into kinetic energy.
Similarly, part of the energy of atoms or molecules may transform
to light in the transition from a higher energy level to lower
energy one. Albert Einstein, the greatest physicist of all
time, predicted that besides the usual spontaneous emission
of light, light with energy corresponding to that of an energy-level
transition could induce or stimulate atoms or molecules in
the upper level to drop to the lower level, with emission
of light. The stimulated and stimulating lights are coherent,
travel in the same direction and have the same polarization.
This is stimulated emission. Laser emission concerns stimulated
emission only and all atoms in laser radiate in step with
each other.
Laser is the acronym of "Light Amplification by Stimulated
Emission of Radiation", but its result is only a monochromatic
(one color) and coherent light bulb instead of a high-intensity
laser beam. Solar radiation excites carbon dioxide in the
tenuous upper layers of the Martian atmosphere. The
laser transition occurs but only extremely weak light is observed
since the laser emission is dissipated randomly into space.
The essence of a laser is light oscillation: the stimulated
emission reflects back and forth between a pair of mirrors
on either end of a resonance cavity so as to produce further
stimulated emission and a laser beam. The resonator determines
the established standing waves and part of the oscillating
stimulate emission is extracted as output laser beam. Laser
beam spread little as it travels (Fig.5).
 |
(Figure
5 Laser beam from an observatory. Accurate laser measurement
of distance on satellites is an important technique
for earth dynamics, earth physics, geodesy and earthquake
forecasting.)
|
Lasers transform various forms of energy into
light. The overall efficiency is low by electrical standards,
although comparable with that of other light source. Carbon
dioxide lasers have the highest efficiency of about 30 per
cent and the efficiency of semiconductor lasers is 20 per
cent but Ar ion lasers only convert 0.02 per cent of input
electric energy into laser beam. For comparison, only a few
percent of the electrical energy used by an ordinary incandescent
lamp is emitted as visible light.
A small laser is as large as a grain of salt
and outputs a fraction of a milliwatt in a continuous beam.
The power in a continuous beam can reach tens of kilowatts
(kW) in commercial lasers, and up to a megawatt in special
military lasers. Pulsed lasers can deliver much higher peak
powers, but only during a short pulse. The largest laser,
the Nova laser, has a peak power of 
W, occupies the whole building and is used for inducing nuclear
fusion.
All kinds of new lasers have been and are being invented.
Unexpected applications of this powerful tool result in new
technology and great achievements in industries and scientific
research (fig.6). Military high-energy laser weapon programs
spend hundreds of million US dollars and would like to use
a laser weapon to shoot down an unfriendly missile. Laser
beams are narrow and straight but their applications orient
to all directions. This great laser explosion needs not only
knowledge but also courage of young people. Lasers illuminate
our future.
(Figure 6 Professor Steven Di-Wen Chu
of Stanford University, California, USA became one Nobel Laureate
in Physics in 1997 for development of methods to cool and
trap atoms with laser light.)
|
