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Laser
I INTRODUCTION Laser, acronym for light amplification by stimulated emission of radiation. Lasers are devices that amplify light and produce coherent light beams, ranging from infrared to ultraviolet. A light beam is coherent when its waves, or photons, propagate in step, or in phase, with one another . Laser light, therefore, can be made extremely intense, highly directional, and very pure in colour (frequency). Laser devices now extend into the X-ray frequency range. Masers are similar devices for microwaves.
II
PRINCIPLES OF OPERATION
Lasers harness atoms to store and emit light in a
coherent fashion. The electrons in the atoms of a laser medium are first pumped,
or energized, to an excited state by an energy source. They are then
“stimulated” by external photons to emit the stored energy in the form of
photons, a process known as stimulated emission. The photons emitted have a
frequency characteristic of the atoms and travel in step with the stimulating
photons. These photons in turn impinge on other excited atoms to release more
photons. Light amplification is achieved as the photons move back and forth
between two parallel mirrors, triggering further stimulated emissions. At the
same time the intense, directional, and monochromatic laser light “leaks”
through one of the mirrors, which is only partially silvered.
Stimulated emission, the underlying process for
laser action, was first described theoretically by Albert Einstein in 1917. The
working principles of lasers were outlined by the American physicists Arthur
Schawlow and Charles Hard Townes in their 1958 patent application. The patent
was granted, but was later challenged by the American physicist and engineer
Gordon Gould. In 1960 the American physicist Theodore Maiman observed the first
laser action in solid ruby. A year later a helium-neon gas laser was built by
the Iranian-born American physicist Ali Javan. Then in 1966 a liquid laser was
constructed by the American physicist Peter Sorokin. The United States Patent
Office court in 1977 affirmed one of Gould's claims over the working principles
of the laser.
III
TYPES OF LASERS
According to the laser medium used, lasers are
generally classified as solid state, gas, semiconductor, or liquid.
A Solid-State Lasers The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rod are fashioned into two parallel surfaces coated with a highly reflecting non-metallic film. Solid-state lasers offer the highest power output. They are usually operated in a pulsed manner to generate a burst of light over a short time. Bursts as short as 12 × 10-15 sec have been achieved, which are useful in studying physical phenomena of very brief duration. Pumping is achieved with light from xenon flash tubes, arc lamps, or metal-vapour lamps. The frequency range has been expanded from infrared (IR) to ultraviolet (UV) by multiplying the original laser frequency with crystal-like potassium dihydrogen phosphate, which are even shorter, and X-ray wavelengths, which are even shorter, have been achieved by aiming laser beams at yttrium targets.
B
Gas Lasers
The laser medium of a gas laser can be a pure
gas, a mixture of gases, or even metal vapour, and is usually contained in a
cylindrical glass or quartz tube. Two mirrors are located outside the ends of
the tube to form the laser cavity. Gas lasers are pumped by ultraviolet light,
electron beams, electric current, or chemical reactions. The helium-neon laser
is known for its high frequency stability, colour purity, and minimal beam
spread. Carbon dioxide lasers are very efficient, and consequently they are the
most powerful continuous wave (CW) lasers.
C Semiconductor Lasers The most compact of lasers, the semiconductor laser usually consists of a junction between layers of semiconductors with different electrical conducting properties. The laser cavity is confined to the junction region by means of two reflective boundaries. Gallium arsenide is the semiconductor most commonly used. Semiconductor lasers are pumped by the direct application of electrical current across the junction, and they can be operated in the CW mode with better than 50 per cent efficiency. A method that permits even more efficient use of energy has been devised. It involves mounting tiny lasers vertically in such circuits, to a density of more than a million per square centimetre. Common uses for semiconductor lasers include CD players and laser printers.
D Liquid Lasers The most common liquid laser media are inorganic dyes contained in glass vessels. They are pumped by intense flash lamps in a pulse mode or by a gas laser in the CW mode. The frequency of a tunable dye laser can be adjusted with the help of a prism inside the laser cavity.
E Free-Electron Lasers Lasers using beams of electrons unattached to atoms and spiralling around magnetic field lines to produce laser radiation were first developed in 1977 and are now becoming important research instruments. They are tunable, as are dye lasers, and in theory a small number could cover the entire spectrum from infrared to X-rays. Free-electron lasers should also become capable of generating very high-power radiation, which is currently too expensive to produce.
IV
LASER APPLICATIONS
The use of lasers is restricted only by
imagination. Lasers have become valuable tools in industry, scientific research,
communication, medicine, military technology, and the arts.
A
Industry
Powerful laser beams can be focused on a small
spot with enormous power density. Consequently, the focused beams can readily
heat, melt, or vaporize material in a precise manner. Lasers have been used, for
example, to drill holes in diamonds, to shape machine tools, to trim
microelectronic components, to heat-treat semiconductor chips, to cut fashion
patterns, to synthesize new material, and to attempt to induce controlled
nuclear fusion . The powerful short pulse produced by a laser also makes
possible high-speed photography with an exposure time of several trillionths of
a second. Highly directional laser beams are used for alignment in road and
building construction.
Lasers are used for monitoring crustal movements and for geodetic surveys. They are also the most effective detectors of certain types of air pollution. In addition, lasers have been used for precise determination of the Earth-Moon distance and in tests of relativity. Very fast laser-activated switches are being developed for use in particle accelerators, and techniques have been found for using laser beams to trap small numbers of atoms in a vacuum for extremely precise studies of their spectra.
B
Scientific Research
Because laser light is highly directional and
monochromatic, extremely small amounts of light scattering or small frequency
shifts caused by matter can easily be detected. By measuring such changes,
scientists have successfully studied molecular structures. With lasers, the
speed of light has been determined to an unprecedented accuracy, chemical
reactions can be selectively induced, and the existence of trace substances in
samples can be detected.
C Communication Laser light can travel a large distance in outer space with little reduction in signal strength. Lasers are therefore ideal for space communications. Because of its high frequency, laser light can carry, for example, 1,000 times as many television channels as are now carried by microwaves. Low-loss optical fibres have been developed to transmit laser light for earthbound communication in telephone and computer systems . Laser techniques have also been used for high-density information recording. For instance, laser light simplifies the recording of a hologram, from which a three-dimensional image can be reconstructed with a laser beam
D Medicine Intense, narrow beams of laser light can cut and cauterize certain tissues in a small fraction of a second without damaging the surrounding healthy tissues. They have been used to “weld” the retina, bore holes in the skull, vaporize lesions, and cauterize blood vessels. Laser techniques have also been developed for lab tests of small biological samples.
E Military Technology Laser guidance systems for missiles, aircraft, and satellites are commonplace. The use of laser beams against hostile ballistic missiles has been proposed, as in the defence system urged by US President Ronald Reagan in 1983 . The ability of tunable dye lasers to excite selectively an atom or molecule may open up more efficient ways to separate isotopes for construction of nuclear weapons.
V
LASER SAFETY
Because the eye focuses laser light as it does
other light, the chief danger in working with lasers is eye damage. Therefore,
laser light should not be viewed, whether it is direct or reflected. Lasers
should be used only by trained personnel wearing protective goggles.