Download How laser works

LASER
"Laser" is an acronym for light amplification by stimulated emission of radiation. If
the electrons in special glasses, crystals, or gases are energized, they will emit light
photons in response to a weak laser pulse. The photons will all be at the same
wavelength and will also be "coherent," meaning the light wave's crests and troughs
are all in lockstep. In contrast, ordinary visible light comes in multiple wavelengths
and is not coherent.
How laser works
A laser can be as small as a microscopic computer chip or as immense as the
National Ignition Facility (NIF), which is three football fields wide. Clearly size has
nothing to do with what makes a laser. In NIF, as in most large lasers, intense flashes
of white light from giant flashlamps "pump" the atomic electrons in large slabs of
laser glass to a higher or more "excited" energy state. But this excited state is
transitory, lasting only about one-millionth of a second. A small pulse of laser light
"tuned" to the excited electrons' energy is directed through the glass slabs. This
laser pulse stimulates the electrons to drop to their lower energy states, or "ground"
states, and emit a laser photon of exactly the same wavelength.
A system of mirrors at both ends of the laser glass amplifier causes the photons to
travel back and forth through the glass, stimulating more electrons to drop to their
lower energy states and emit laser photons. This process results in "amplification,"
or the production of huge numbers of photons of the same wavelength and
direction. In NIF, amplification produces trillions upon trillions of photons—
1023 photons to be precise. As the photons encounter each other inside the laser
amplifier, they vibrate coherently with the same frequency and direction. In
essence, the photons "clone" themselves, creating a perfect copy of the first photon.
Coherence causes the light to shine in a beam that is extremely bright and straight,
familiar to anyone who has used a laser pointer.
The initial weak pulse has by now been amplified and increased enormously in
energy. In small lasers, a partially reflecting mirror at one end of the lasing medium
is designed to allow some of the amplified light—now laser light—to pass through
(see "The First Ruby Laser" below). In NIF, a special optical switch traps the lowenergy laser pulse in the main amplifier for four passes back and forth through the
laser glass slabs. Only then are the laser beams switched out of the main amplifier to
continue their journey to the target chamber. This system of glass, mirrors, and
other special optics is known as a laser amplifier cavity. Precise design of the
amplifier components allows scientists to control how much energy the laser beam
will produce. In NIF, a series of amplifiers increases the energy of the initial weak
pulse of light by more than a quadrillion times to create NIF's 192 highly energetic,
tightly focused laser beams.
History
The laser would not have been possible without an understanding that light is a
form of electromagnetic radiation. Max Planck received the Nobel Prize in physics in
1918 for his discovery of elementary energy quanta. Planck was working in
thermodynamics, trying to explain why “blackbody” radiation, something that
absorbs all wavelengths of light, didn’t radiate all frequencies of light equally when
heated.
In his most important work, published in 1900, Planck deduced the relationship
between energy and the frequency of radiation, essentially saying that energy could
be emitted or absorbed only in discrete chunks – which he called quanta – even if
the chunks were very small and name them photons.
In 1917, Einstein proposed the process that makes lasers possible, called stimulated
emission. He theorized that, besides absorbing and emitting light spontaneously,
electrons could be stimulated to emit light of a particular wavelength (for more on
the pioneers of the laser, see “On the Shoulders of Giants” by Lynn Savage, page 70).
But it would take nearly 40 years before scientists would be able to amplify those
emissions, proving Einstein correct and putting lasers on the path to becoming the
powerful and ubiquitous tools they are today.
On May 16, 1960, Theodore H. Maiman operated the first functioning
laser,[16][17] at Hughes Research Laboratories, Malibu, California, ahead of several
research teams, including those of Townes, at Columbia University, Arthur
Schawlow, at Bell Labs,[18] and Gould, at the TRG (Technical Research Group)
company. Maiman's functional laser used a solid-state flashlamp-pumped
synthetic ruby crystal to produce red laser light, at 694 nanometres wavelength;
however, the device only was capable of pulsed operation, because of its three-level
pumping design scheme. Later in 1960, the Iranianphysicist Ali Javan, and William
R.
Bennett,
and Donald
Herriott,
constructed
the
first gas
laser,
using helium and neon that was capable of continuous operation in the infrared
(U.S. Patent 3,149,290); later, Javan received the Albert Einstein Award in 1993.
Basov and Javan proposed the semiconductorlaser diode concept. In 1962, Robert N.
Hall demonstrated the first laser diode device, made of gallium arsenide and emitted
at 850 nm the near-infrared band of the spectrum. Later, in 1962, Nick Holonyak,
Jr. demonstrated the first semiconductor laser with a visible emission. This first
semiconductor laser could only be used in pulsed-beam operation, and when cooled
to liquid nitrogentemperatures (77 K). In 1970, Zhores Alferov, in the USSR, and
Izuo Hayashi and Morton Panish of Bell Telephone Laboratories also independently
developed room-temperature, continual-operation diode lasers, using
the heterojunction structure.
First ruby laser
The ruby laser was the first laser invented in 1960. Ruby is an aluminum oxide
crystal in which some of the aluminum atoms have been replaced with chromium
atoms. Chromium gives ruby its characteristic red color and is responsible for the
lasing behavior of the crystal. Chromium atoms absorb green and blue light and emit
or reflect only red light.
For a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror
is placed on one end and a partially reflecting mirror on the other. A high-intensity
lamp is spiraled around the ruby cylinder to provide a flash of white light that
triggers the laser action. The green and blue wavelengths in the flash excite
electrons in the chromium atoms to a higher energy level. Upon returning to their
normal state, the electrons emit their characteristic ruby-red light. The mirrors
reflect some of this light back and forth inside the ruby crystal, stimulating other
excited chromium atoms to produce more red light, until the light pulse builds up to
high power and drains the energy stored in the crystal.
Types of lasers
Gas lasers
Following the invention of the HeNe gas laser, many other gas discharges have been
found to amplify light coherently. Gas lasers using many different gases have been
built and used for many purposes. The helium-neon laser (HeNe) is able to operate
at a number of different wavelengths, however the vast majority are engineered to
lase at 633 nm; these relatively low cost but highly coherent lasers are extremely
common in optical research and educational laboratories.
Chemical lasers
Chemical lasers are powered by a chemical reaction permitting a large amount of
energy to be released quickly. Such very high power lasers are especially of interest
to the military, however continuous wave chemical lasers at very high power levels,
fed by streams of gasses, have been developed and have some industrial
applications. As examples, in the Hydrogen fluoride laser(2700–2900 nm) and
the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen
or deuterium gas with combustion products of ethylene in nitrogen trifluoride.
Excimer lasers
Excimer lasers are a special sort of gas laser powered by an electric discharge in
which the lasing medium is an excimer, or more precisely an exciplex in existing
designs. These are molecules which can only exist with one atom in an excited
electronic state.
Solid-state lasers
Solid-state lasers use a crystalline or glass rod which is "doped" with ions that
provide the required energy states.
Fiber lasers
Solid-state lasers or laser amplifiers where the light is guided due to the total
internal reflection in a single mode optical fiber are instead called fiber lasers.
Guiding of light allows extremely long gain regions providing good cooling
conditions; fibers have high surface area to volume ratio which allows efficient
cooling. In addition, the fiber's waveguiding properties tend to reduce thermal
distortion of the beam. Erbium and ytterbium ions are common active species in
such lasers.
Photonic crystal lasers
Photonic crystal lasers are lasers based on nano-structures that provide the mode
confinement and the density of optical states (DOS) structure required for the
feedback to take place. They are typical micrometre] and tunable on the bands of the
photonic crystals.
Dye lasers
Dye lasers use an organic dye as the gain medium. The wide gain spectrum of
available dyes, or mixtures of dyes, allows these lasers to be highly tunable, or to
produce very short-duration pulses (on the order of a few femtoseconds). Although
these tunable lasers are mainly known in their liquid form, researchers have also
demonstrated narrow-linewidth tunable emission in dispersive oscillator
configurations incorporating solid-state dye gain media.[26] In their most prevalent
form these solid state dye lasers use dye-doped polymers as laser media.
Free-electron lasers
Free-electron lasers, or FELs, generate coherent, high power radiation, that is widely
tunable, currently ranging in wavelength from microwaves, throughterahertz
radiation and infrared, to the visible spectrum, to soft X-rays. They have the widest
frequency range of any laser type. While FEL beams share the same optical traits as
other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas,
liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs
use a relativistic electron beam as the lasing medium, hence the term free electron
lasers.
Uses of lasers
MedicalLasers:
Medical lasers can be used as a scalpel. Since the laser can be controlled and can
have such a small contact area it is ideal for fine cutting and depth control. Medical
lasers can also be used to reattach retinas and can be used in conjunction with fiber
optics to place the laser beem where it needs to be. Medical lasers can also be used
to stitch up incisions after surgery, by fusing together skin. (LFI)
Entertainment:
Laser shows are quite popular and the special effects are amazing. These use lasers
that are in the visible spectrum along with vibrating mirrors to paint images in the
air. Here is an example of a dance with lasers in the background.
You might noticed the fog in the background, that is what allows the laser light to
reflect and you to view it. Another example of laser entertainment is the use of laser
signs at trade shows. Here is an example of a laser Microsoft sign.
Computer and Music
One popular use of lasers is the reading of CD. CD's function by having a reflective
aluminum layer that has very small pits put in the aluminum. The pits and the lack
of are translated into binary by the computer and then are used for
information. Another use of lasers is in the use of fiber optics. Since lasers travel
very fast they make an ideal way to communicate. The laser is shot down a
fiberglass tube to a receiver. These wires can be very long with no loss of signal
quality. Also modern multiplexing of the line lets two lasers of different frequencies
share the same line. (Serway)
Metal working:
Lasers very accurate point and solid state construction make it ideal or industrial
production. Lasers allow better cuts on metals and the welding of dissimilar metals
with out the use of a flux. Also lasers can be mounted on robotic arms and used in
factors. This is safer then oxygen and acetylene, or arc welding. (Impulse)
As weapons
Lasers of all but the lowest powers can potentially be used as incapacitating
weapons, through their ability to produce temporary or permanent vision loss in
varying degrees when aimed at the eyes. The degree, character, and duration of
vision impairment caused by eye exposure to laser light varies with the power of the
laser, the wavelength(s), the collimation of the beam, the exact orientation of the
beam, and the duration of exposure. Lasers of even a fraction of a watt in power can
produce immediate, permanent vision loss under certain conditions, making such
lasers potential non-lethal but incapacitating weapons. The extreme handicap that
laser-induced blindness represents makes the use of lasers even as non-lethal
weapons morally controversial, and weapons designed to cause blindness have been
banned by the Protocol on Blinding Laser Weapons. Incidents of pilots being
exposed to lasers while flying have prompted aviation authorities to implement
special procedures to deal with such hazards.[44]
Laser weapons capable of directly damaging or destroying a target in combat are
still in the experimental stage. The general idea of laser-beam weaponry is to hit a
target with a train of brief pulses of light. The rapid evaporation and expansion of
the surface causes shockwaves that damage the target.[citation needed] The power
needed to project a high-powered laser beam of this kind is beyond the limit of
current mobile power technology, thus favoring chemically powered gas dynamic
lasers. Example experimental systems includeMIRACL and the Tactical High Energy
Laser.