Physics and
history
The range of sizes in which lasers exist is immense, extending from
microscopic diode lasers (top) to football field sized neodymium
glass lasers (bottom) used for inertial confinement fusion.The first
working laser was made by Theodore H. Maiman in 1960 at Hughes Research
Laboratories in Malibu, California, beating several research teams
including those of Charles Townes at Columbia University, and Arthur
L. Schawlow at Bell laboratories. Maiman used a solid-state flashlamp-pumped
synthetic ruby crystal to produce red laser light at 694-nanometres
wavelength. In the same year the Iranian physicist Ali Javan invented
the gas laser. He later received the Albert Einstein Award.
The basic physics of lasers centres around the idea
of producing a population inversion in a laser medium by "pumping"
the medium; i.e., by supplying energy in the form of light or electricity,
for example. The medium may then amplify light by the process of
stimulated emission. If the light is circulating through the medium
by means of a cavity resonator, and the gain (amplification) in
the medium is stronger than the resonator losses, the power of the
circulating light can rise exponentially. Eventually it will get
so strong that the gain is saturated (reduced). In continuous operation,
the intracavity laser power finds an equilibrium value which is
saturating the gain exactly to the level of the cavity losses. If
the pump power is chosen too small (below the "laser threshold"),
the gain is not sufficient to overcome the resonator losses, and
the laser will emit only very small light powers.
Population inversion is also the concept behind the
maser. The first maser was built by Charles H. Townes and graduate
students J. P. Gordon, and H. J. Zeiger in 1953. Townes later worked
with Arthur L. Schawlow to describe the theory of the laser, or
optical maser as it was known. The word laser was coined in 1957
by Gordon Gould. Gordon also coined the words iraser, intending
"aser" as the suffix and the spectra of light emitted
at as the prefix (examples: X-ray laser = xaser, UltraViolet laser
= uvaser) but these terms never became popular. Gordon was also
credited with lucrative patent rights for a gas-discharge laser
in 1987, following a protracted 30 year legal battle.
The first maser, developed by Townes, was incapable
of continuous output. Nikolai Basov and Alexander Prokhorov of the
USSR worked independently on the quantum oscillator and solved the
problem of continuous output systems by using more than two energy
levels. These systems could release stimulated emission without
falling to the ground state, thus maintaining a population inversion.
In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov shared
a Nobel Prize in Physics "for fundamental work in the field
of quantum electronics, which has led to the construction of oscillators
and amplifiers based on the maser-laser principle."
A further note on the terminology is necessary. As
laser stands for light amplification by stimulated emission of radiation,
it should be understood that the word light is here meant in the
expansive sense, as photons of any energy; not as simply photons
in the visible spectrum. Hence there are X-ray lasers, IR-lasers,
UV-lasers, microwave lasers, radio lasers, etc. However, microwave
lasers and radio lasers are usually called masers in the modern
terminology. In addition, some argue that all lasers can be thought
of as masers (see the article on maser for details).
Additionally, laser light can be highly intense—able
to cut steel and other metals. While the beam emitted by a laser
often has a very small divergence (highly collimated), a perfectly
collimated beam cannot be created, due to the effect of diffraction.
Nonetheless, a laser beam will spread much less than a beam of light
generated by other means. A beam generated by a small laboratory
laser such as a helium-neon (HeNe) laser spreads to approximately
1 mile (1.6 kilometres) in diameter if shone from the Earth's surface
to the Moon. Some lasers, especially semiconductor lasers due to
their small size, produce very divergent beams. However, such a
divergent beam can be transformed into a collimated beam by means
of a lens. In contrast, the light from non-laser light sources cannot
be collimated by optics as well or much. Using a waveguide such
as an optical fibre though, diffraction laws governing divergence
no longer apply. Other interesting effects happen in nonlinear optics.
Graph showing the history of maximum laser pulse
intensity throughout the past 40 years.An unforeseen discovery in
1992, lasing without maintaining the medium excited into a population
inversion, was discovered in sodium gas in and again in 1995 each
in sodium and rubidium gas by various international teams. Normally,
electrons in the ground state absorb the pumping and emitted radiation,
thwarting the laser gain by heating up the medium. So media with
electron levels and transitions amenable to the driving current
are desired, and generally those which involve three or four energy
levels rather than two make better lasers because the electrons
are kept above the ground state, excited, and optically-transparent
so as not to heat up, but such media are prone to noisy beams. By
using an external maser to induce "optical transparency"
in the media by introducing and destructively interfering the ground
electron transitions between two paths, the likelihood for the ground
electrons to absorb any energy has been cancelled. Though there
were initial hopes that this discovery would allow an increase in
efficiency (higher than the .01 to .3 for typical media and wavelengths),
the idea never panned out commercially or otherwise and remains
little more than a backwater in laser research.
In 1985 at the University of Rochester's Laboratory
for Laser Energetics a breakthrough in creating ultrashort-pulse,
very high-intensity (terawatts) laser pulses became
available using a technique called chirped pulse amplification,
or CPA, discovered by Gérard Mourou. Later, in
1994, it was discovered by Mourou and his team at University
of Michigan that the balance between the self-focusing
refraction (see Kerr effect) and self-attenuating diffraction
by ionization and rarefaction of a laser beam of terawatt
intensities in the atmosphere creates "filaments"
which act as waveguides for the beam thus preventing
divergence. If a light filament drops below the intensity
needed for this dynamic balance, called modulation instability,
it can merge with another filament and continue propagating
without broadening as with all earlier means of sending
light. The filaments, having made a plasma, though turn
the narrowband laser pulse into a broadband pulse having
a wholly new set of applications
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