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Optoelectronics

Aplication notes :
Optoelectronics

Optoelectronics
is the study and application of electronic devices that interact with
light, and thus is usually considered a sub-field of photonics. In
this context,
light
often includes invisible forms of radiation such as
gamma
rays
,
X-rays,
ultraviolet
and
infrared.
Optoelectronic devices are electrical-to-optical or
optical-to-electrical transducers, or instruments that use such
devices in their operation.
Electro-optics
is often erroneously used as a synonym, but is in fact a wider branch
of physics that deals with all interactions between light and
electric fields, whether or not they form part of an electronic
device.

Optoelectronics
is based on the quantum mechanical effects of light on semiconducting
materials, sometimes in the presence of electric fields.

  • Photoelectric
    or photovoltaic effect, used in:

    • photodiodes
      (including solar cells)

    • phototransistors

    • photomultipliers

    • integrated
      optical circuit (IOC) elements

  • Photoconductivity,
    used in:

    • photoresistors

    • photoconductive
      camera tubes

    • charge-coupled
      imaging devices

  • Stimulated
    emission, used in:

    • lasers

    • injection
      laser diodes

  • Lossev
    effect, or radiative recombination, used in:

    • light-emitting
      diodes or LED

  • Photoemissivity,
    used in

    • photoemissive
      camera tube

Important
applications of optoelectronics include:

  • Optocoupler

  • optical
    fiber communications

Light

Light,
or
visible
light
,
is electromagnetic radiation of a wavelength that is visible to the
human eye (about 400–700 nm). In a scientific context, the word
light
is sometimes used to refer to the entire
electromagnetic
spectrum
.
Light
is composed of an elementary particle called a photon.

Three
primary properties of light are:

  • Intensity,
    or brightness;

  • Frequency
    or wavelength and;

  • Polarization
    or direction of the wave oscillation.

Light
can exhibit properties of both waves and particles. This property is
referred to as wave-particle duality. The study of light, known as
optics, is an important research area in modern physics.

Speed
of light

The
speed of light in a vacuum is exactly 299,792,458 m/s (about
186,282.397
miles per second). The speed of light depends upon the medium in
which it is traveling, and the speed will be lower in a transparent
medium. Although commonly called the “velocity of light”,
technically the word
velocity
is a vector quantity, having both magnitude and direction.
Speed
refers only to the magnitude of the velocity vector. This fixed
definition of the speed of light is a result of the modern attempt,
in physics, to define the basic unit of length in terms of the speed
of light, rather than defining the speed of light in terms of a
length.

Different
physicists have attempted to measure the speed of light throughout
history.
Galileo
attempted to measure the speed of light in the seventeenth century. A
good early experiment to measure the speed of light was conducted by
Ole
Rømer
,
a Danish physicist, in 1676. Using a telescope, Ole observed the
motions of Jupiter and one of its moons, Io. Noting discrepancies in
the apparent period of Io’s orbit, Rømer calculated that light
takes about 18 minutes to traverse the diameter of Earth’s orbit.
Unfortunately, this was not a value that was known at that time. If
Ole had known the diameter of the earth’s orbit, he would have
calculated a speed of
227,000,000 m/s.

Another,
more accurate, measurement of the speed of light was performed in
Europe by
Hippolyte
Fizeau

in 1849. Fizeau directed a beam of light at a mirror several
kilometers away. A rotating cog wheel was placed in the path of the
light beam as it traveled from the source, to the mirror and then
returned to its origin. Fizeau found that at a certain rate of
rotation, the beam would pass through one gap in the wheel on the way
out and the next gap on the way back. Knowing the distance to the
mirror, the number of teeth on the wheel, and the rate of rotation,
Fizeau was able to calculate the speed of light as
313,000,000 m/s.

Léon
Foucault

used an experiment which used rotating mirrors to obtain a value of
298,000,000 m/s in 1862.
Albert
A. Michelson

conducted experiments on the speed of light from 1877 until his death
in 1931. He refined
Foucault’s
methods

in 1926 using improved rotating mirrors to measure the time it took
light to make a round trip from Mt. Wilson to Mt. San Antonio in
California. The precise measurements yielded a speed of
299,796,000 m/s.


Refraction

Light
in a vacuum propagates at a maximum finite speed, defined above, and
denoted by the symbol
c.
While passing through any other transparent medium, the speed of
light slows to some fraction of
c.
The reduction of the speed of light traveling in a transparent medium
is indicated by the refractive index,
n,
which is defined as:

where
v
denotes the speed that light travels in the transparent medium.

Note,
n
= 1 in a vacuum and
n
> 1 in a transparent medium.

When
a beam of light crosses the boundary between a vacuum and another
medium, or between two different mediums, the wavelength of the light
changes, but the frequency remains constant. If the beam of light is
not orthogonal to the boundary, the change in wavelength results in a
change in the direction of the beam. This change of direction is
known as refraction.

The
refraction quality of lenses is frequently used to manipulate light
in order to change the apparent size of images. Magnifying glasses,
spectacles, contact lenses, microscopes and refracting telescopes are
all examples of this manipulation.


Optics

The
study of light and the interaction of light and matter is termed
optics. The observation and study of optical phenomena such as
rainbows
and the
aurora
borealis

offer many clues as to the nature of light as well as much enjoyment.


There
are many sources of light. The most common light sources are thermal:
a body at a given temperature emits a characteristic spectrum of
black-body radiation. Examples include
sunlight
(the radiation emitted by the chromosphere of the Sun at around
6,000 K peaks in the visible region of the
electromagnetic
spectrum
),
incandescent light bulbs (which emit only around 10% of their energy
as visible light and the remainder as infrared), and glowing solid
particles in flames. The peak of the blackbody spectrum is in the
infrared for relatively cool objects like human beings. As the
temperature increases, the peak shifts to shorter wavelengths,
producing first a red glow, then a white one, and finally a blue
color as the peak moves out of the visible part of the spectrum and
into the ultraviolet. These colors can be seen when metal is heated
to “red hot” or “white hot”. The blue color is
most commonly seen in a gas flame or a welder’s torch.

Atoms
emit and absorb light at characteristic energies. This produces
emission
lines

in the spectrum of each atom. Emission can be spontaneous, as in
light-emitting diodes, gas discharge lamps (such as neon lamps and
neon signs, mercury-vapor lamps, etc.), and flames (light from the
hot gas itself—so, for example, sodium in a gas flame emits
characteristic yellow light). Emission can also be stimulated, as in
a laser or a microwave maser.

Acceleration
of a free charged particle, such as an electron, can produce visible
radiation:
cyclotron
radiation, synchrotron radiation, and bremsstrahlung radiation

are all examples of this. Particles moving through a medium faster
than the speed of light in that medium can produce visible
Cherenkov
radiation.

Certain
chemicals produce visible radiation by chemoluminescence. In living
things, this process is called bioluminescence. For example,
fireflies produce light by this means, and boats moving through water
can disturb plankton which produce a glowing wake.

Certain
substances produce light when they are illuminated by more energetic
radiation, a process known as fluorescence. This is used in
fluorescent lights. Some substances emit light slowly after
excitation by more energetic radiation. This is known as
phosphorescence.

Phosphorescent
materials can also be excited by bombarding them with subatomic
particles. Cathodoluminescence is one example of this. This mechanism
is used in cathode ray tube televisions.

Certain
other mechanisms can produce light:

  • scintillation

  • electroluminescence

  • sonoluminescence

  • triboluminescence

  • Cherenkov
    radiation

When
the concept of light is intended to include very-high-energy photons
(gamma rays), additional generation mechanisms include:

  • radioactive
    decay

  • particle–antiparticle
    annihilation


Optical
theory

The
Muslim scientist
Ibn
al-Haytham

(c. 965-1040), known as
Alhacen
in the West, in his
Book
of Optics
,
developed a broad theory that explained vision, using geometry and
anatomy, which stated that each point on an illuminated area or
object radiates light rays in every direction, but that only one ray
from each point, which strikes the eye perpendicularly, can be seen.
The other rays strike at different angles and are not seen. He
described
the
pinhole camera

and invented the
camera
obscura
,
which produces an inverted image, and used it as an example to
support his argument.This contradicted Ptolemy’s theory of vision
that objects are seen by rays of light emanating from the eyes.
Alhacen held light rays to be streams of minute particles that
travelled at a finite speed. He improved
Ptolemy’s
theory

of the refraction of light, and went on to discover the laws of
refraction.

He
also carried out the first experiments on the dispersion of light
into its constituent colors. His major work
Kitab
al-Manazir

was translated into Latin in the Middle Ages, as well his book
dealing with the colors of sunset. He dealt at length with the theory
of various physical phenomena like shadows, eclipses, the rainbow. He
also attempted to explain binocular vision, and gave a correct
explanation of the apparent increase in size of the sun and the moon
when near the horizon. Because of his extensive research on optics,
Al-Haytham is considered the father of modern optics.

Al-Haytham
also correctly argued that we see objects because the sun’s rays of
light, which he believed to be streams of tiny particles travelling
in straight lines, are reflected from objects into our eyes. He
understood that light must travel at a large but finite velocity, and
that refraction is caused by the velocity being different in
different substances. He also studied spherical and parabolic
mirrors, and understood how refraction by a lens will allow images to
be focused and magnification to take place. He understood
mathematically why a spherical mirror produces aberration.


Wave
theory

In
the 1660s,
Robert
Hooke

published a
wave
theory of light
.
Christiaan Huygens worked out his own wave theory of light in 1678,
and published it in his
Treatise
on light

in 1690. He proposed that light was emitted in all directions as a
series of waves in a medium called the
Luminiferous
ether
.
As waves are not affected by gravity, it was assumed that they slowed
down upon entering a denser medium.

Thomas
Young’s sketch of the two-slit experiment showing the diffraction of
light. Young’s experiments supported the theory that light consists
of waves.

The
wave theory predicted that light waves could interfere with each
other like sound waves (as noted around 1800 by Thomas Young), and
that light could be polarized. Young showed by means of a
diffraction
experiment

that light behaved as waves. He also proposed that different colors
were caused by different wavelengths of light, and explained color
vision in terms of three-colored receptors in the eye.

Another
supporter of the wave theory was Leonhard Euler. He argued in
Nova
theoria lucis et colorum

(1746) that diffraction could more easily be explained by a wave
theory.

Later,
Augustin-Jean
Fresnel

independently worked out his own wave theory of light, and presented
it to the Académie des Sciences in 1817.
Simeon
Denis Poisson

added to Fresnel’s mathematical work to produce a convincing argument
in favour of the wave theory, helping to overturn Newton’s
corpuscular theory.

The
weakness
of
the wave theory was that light waves, like sound waves, would need a
medium for transmission. A hypothetical substance called the
luminiferous aether was proposed, but its existence was cast into
strong doubt in the late nineteenth century by the Michelson-Morley
experiment.

Newton’s
corpuscular theory implied that light would travel faster in a denser
medium, while the wave theory of Huygens and others implied the
opposite. At that time, the speed of light could not be measured
accurately enough to decide which theory was correct. The first to
make a sufficiently accurate measurement was Léon Foucault, in
1850. His result supported the wave theory, and the classical
particle theory was finally abandoned.


Electromagnetic
theory

A
linearly-polarized light wave frozen in time and showing the two
oscillating components of light; an electric field and a magnetic
field perpendicular to each other and to the direction of motion (a
transverse wave).

In
1845,
Michael
Faraday

discovered that the angle of polarization of a beam of light as it
passed through a polarizing material could be altered by a magnetic
field, an effect now known as Faraday rotation. This was the first
evidence that light was related to electromagnetism. Faraday proposed
in 1847 that light was a high-frequency electromagnetic vibration,
which could propagate even in the absence of a medium such as the
ether.

Faraday’s
work inspired
James
Clerk Maxwell

to study electromagnetic radiation and light. Maxwell discovered that
self-propagating electromagnetic waves would travel through space at
a constant speed, which happened to be equal to the previously
measured speed of light. From this, Maxwell concluded that light was
a form of electromagnetic radiation: he first stated this result in
1862 in
On
Physical Lines of Force
.
In 1873, he published
A
Treatise on Electricity and Magnetism
,
which contained a full mathematical description of the behaviour of
electric and magnetic fields, still known as Maxwell’s equations.
Soon after, Heinrich Hertz confirmed Maxwell’s theory experimentally
by generating and detecting radio waves in the laboratory, and
demonstrating that these waves behaved exactly like visible light,
exhibiting properties such as reflection, refraction, diffraction,
and interference. Maxwell’s theory and Hertz’s experiments led
directly to the development of modern radio, radar, television,
electromagnetic imaging, and wireless communications.


Light-emitting
diode

A
light-emitting
diode

(
LED)
is a semiconductor diode that emits incoherent narrow-spectrum light
when electrically biased in the forward direction of the p-n
junction, as in the common LED circuit. This effect is a form of
electroluminescence.

An
LED is usually a small area source, often with extra optics added to
the chip that shapes its radiation pattern. LED’s are often used as
small indicator lights on electronic devices and increasingly in
higher power applications such as flashlights and area lighting. The
color of the emitted light depends on the composition and condition
of the semiconducting material used, and can be infrared, visible, or
near-ultraviolet. An LED can be used as a regular household light
source.


LED
technology


Like
a normal diode, an LED consists of a chip of semiconducting material
impregnated, or
doped,
with impurities to create a
p-n
junction
.
As in other diodes, current flows easily from the p-side, or anode,
to the n-side, or cathode, but not in the reverse direction.
Charge-carriers—electrons and holes—flow into the junction from
electrodes with different voltages. When an electron meets a hole, it
falls into a lower energy level, and releases energy in the form of a
photon.

The
wavelength
of the light emitted, and therefore its color, depends on the band
gap energy of the materials forming the
p-n
junction
.
In silicon or germanium diodes, the electrons and holes recombine by
a
non-radiative
transition

which produces no optical emission, because these are indirect band
gap materials. The materials used for an LED have a direct band gap
with energies corresponding to near-infrared, visible or
near-ultraviolet light.

LED
development began with infrared and red devices made with
gallium
arsenide
.
Advances in materials science have made possible the production of
devices with ever-shorter wavelengths, producing light in a variety
of colors.

LEDs
are usually built on an n-type substrate, with an electrode attached
to the p-type layer deposited on its surface. P-type substrates,
while less common, occur as well. Many commercial LEDs, especially
GaN/InGaN, also use
sapphire
substrate
.
Substrates that are transparent to the emitted wavelength, and backed
by a reflective layer, increase the LED efficiency. The refractive
index of the package material should match the index of the
semiconductor, otherwise the produced light gets partially reflected
back into the semiconductor, where it may be absorbed and turned into
additional heat, thus lowering the efficiency. This type of
reflection also occurs at the surface of the package if the LED is
coupled to a medium with a different refractive index such as a glass
fiber or air. The refractive index of most LED semiconductors is
quite high, so in almost all cases the LED is coupled into a much
lower-index medium. The large index difference makes the reflection
quite substantial (per the
Fresnel
coefficients
),
and this is usually one of the dominant causes of LED inefficiency.
Often more than half of the emitted light is reflected back at the
LED-package and package-air interfaces. The reflection is most
commonly reduced by using a dome-shaped (half-sphere) package with
the diode in the center so that the outgoing light rays strike the
surface perpendicularly, at which angle the reflection is minimized.
An anti-reflection coating may be added as well. The package may be
cheap plastic, which may be colored, but this is only for cosmetic
reasons or to improve the contrast ratio; the color of the packaging
does not substantially affect the color of the light emitted. Other
strategies for reducing the impact of the interface reflections
include designing the LED to reabsorb and reemit the reflected light
(called
photon
recycling
)
and manipulating the microscopic structure of the surface to reduce
the reflectance, either by introducing random roughness or by
creating programmed
moth
eye

surface patterns.

Conventional
LEDs are made from a variety of inorganic semiconductor materials,
producing the following colors:

  • Aluminium
    gallium arsenide (AlGaAs) — red and infrared

  • Aluminium
    gallium phosphide (AlGaP) — green

  • Aluminium
    gallium indium phosphide (AlGaInP) — high-brightness orange-red,
    orange, yellow, and green

  • Gallium
    arsenide phosphide (GaAsP) — red, orange-red, orange, and yellow

  • Gallium
    phosphide (GaP) — red, yellow and green

  • Gallium
    nitride (GaN) — green, pure green (or emerald green), and blue
    also white (if it has an AlGaN Quantum Barrier)

  • Indium
    gallium nitride (InGaN) — 450nm – 470nm — near ultraviolet,
    bluish-green and blue

  • Silicon
    carbide (SiC) as substrate — blue

  • Silicon
    (Si) as substrate — blue (under development)

  • Sapphire
    (Al
    2O3)
    as substrate — blue

  • Zinc
    selenide (ZnSe) — blue

  • Diamond
    (C) — ultraviolet

  • Aluminium
    nitride (AlN), aluminium gallium nitride (AlGaN),
    aluminium
    gallium indium nitride

    (AlGaInN) — near to far ultraviolet (down to 210 nm)

With
this wide variety of colors, arrays of multicolor LEDs can be
designed to produce unconventional color patterns.


Ultraviolet
and blue LEDs

Ultraviolet
GaN LEDs.

Blue
LEDs are based on the wide band gap semiconductors
GaN
(gallium nitride)

and InGaN (
indium
gallium nitride
).
They can be added to existing red and green LEDs to produce the
impression of white light, though white LEDs today rarely use this
principle.

The
first blue LEDs were made in 1971 by Jacques Pankove (inventor of the
gallium nitride LED) at
RCA
Laboratories
.
However, these devices were too feeble to be of much practical use.
In the late 1980s, key breakthroughs in GaN epitaxial growth and
p-type doping by Akasaki and Amano (Nagoya, Japan) ushered in the
modern era of GaN-based optoelectronic devices. Building upon this
foundation, in 1993 high brightness blue LEDs were demonstrated
through the work of
Shuji
Nakamura

at Nichia Corporation.

By
the late 1990s, blue LEDs had become widely available. They have an
active region consisting of one or more InGaN quantum wells
sandwiched between thicker layers of GaN, called cladding layers. By
varying the relative InN-GaN fraction in the InGaN quantum wells, the
light emission can be varied from violet to amber. AlGaN aluminium
gallium nitride of varying AlN fraction can be used to manufacture
the cladding and quantum well layers for ultraviolet LEDs, but these
devices have not yet reached the level of efficiency and
technological maturity of the InGaN-GaN blue/green devices. If the
active quantum well layers are GaN, as opposed to alloyed InGaN or
AlGaN, the device will emit near-ultraviolet light with wavelengths
around 350–370 nm. Green LEDs manufactured from the InGaN-GaN
system are far
more
efficient and brighter

than green LEDs produced with non-nitride material systems.

With
aluminium containing nitrides, most often AlGaN and AlGaInN, even
shorter wavelengths are achievable. Ultraviolet LEDs are becoming
available on the market, in a range of wavelengths. Near-UV emitters
at wavelengths around 375–395 nm are already cheap, common to
encounter e.g., as black light lamp replacements for inspection of
anti-counterfeiting UV watermarks in some documents and paper
currencies. Shorter wavelength diodes, while substantially more
expensive, are commercially available for wavelengths down to 247 nm.

As
the photosensitivity of microorganisms approximately matches the
absorption spectrum of DNA, with peak at about 260 nm, UV LEDs
emitting at 250–270 nm are prospective for disinfecting devices.

Wavelengths
down to 210 nm were obtained in laboratories using aluminium nitride.

While
not actually an LED as such, an ordinary NPN bipolar transistor will
emit violet light if its emitter-base junction is subjected to
non-destructive reverse breakdown. This is easy to demonstrate by
filing the top off a metal-can transistor (BC107, 2N2222 or similar)
and biasing it well above emitter-base breakdown (≥ 20 V) via a
current limiting resistor.


White
LEDs

A
combination of red, green and blue LEDs can produce the impression of
white light, though white LEDs today rarely use this principle. Most
“white” LEDs in production today are modified blue LEDs:
GaN-based, InGaN-active-layer LEDs emit blue light of wavelengths
between 450 nm and 470 nm. This InGaN-GaN structure is
covered with a yellowish
phosphor
coating

usually made of cerium-doped yttrium aluminum garnet (Ce
3+:YAG)
crystals which have been powdered and bound in a type of viscous
adhesive. The LED chip emits blue light, part of which is efficiently
converted to a broad spectrum centered at about 580 nm (yellow)
by the Ce
3+:YAG.
Since yellow light stimulates the red and green receptors of the eye,
the resulting mix of blue and yellow light gives the appearance of
white, the resulting shade often called “lunar white”. This
approach was developed by Nichia and has been used since 1996 for the
manufacture of white LEDs.

The
pale yellow emission of the Ce
3+:YAG
can be tuned by substituting the cerium with other rare earth
elements such as terbium and gadolinium and can even be further
adjusted by substituting some or all of the aluminum in the YAG with
gallium. Due to the spectral characteristics of the diode, the red
and green colors of objects in its blue yellow light are not as vivid
as in broad-spectrum light. Manufacturing variations and varying
thicknesses in the phosphor make the LEDs produce light with
different color temperatures, from warm yellowish to cold bluish; the
LEDs have to be sorted during manufacture by their actual
characteristics.
Philips
Lumileds

patented conformal coating process addresses the issue of varying
phosphor thickness, giving the white LEDs a more consistent spectrum
of white light.

Spectrum
of a “white” LED clearly showing blue light which is
directly emitted by the GaN-based LED (peak at about 465 nanometers)
and the more broadband stokes shifted light emitted by the Ce
3+:YAG
phosphor which extends from around 500 to 700 nanometers.

White
LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs
with a mixture of high efficiency europium-based red and blue
emitting phosphors plus green emitting copper and aluminum doped zinc
sulfide (ZnS:Cu, Al). This is a method analogous to the way
fluorescent
lamps

work. However the ultraviolet light causes photodegradation to the
epoxy resin and many other materials used in LED packaging, causing
manufacturing challenges and shorter lifetimes. This method is less
efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift
is larger and more energy is therefore converted to heat, but yields
light with better spectral characteristics, which render color
better. Due to the higher radiative output of the ultraviolet LEDs
than of the blue ones, both approaches offer comparable brightness.

The
newest method used to produce white light LEDs uses no phosphors at
all and is based on homoepitaxially grown zinc selenide (ZnSe) on a
ZnSe substrate which simultaneously emits blue light from its active
region and yellow light from the substrate.

A
new technique developed by Michael Bowers, a graduate student at
Vanderbilt University in Nashville, involves coating a blue LED with
quantum dots that glow white in response to the blue light from the
LED. This technique produces a warm, yellowish-white light similar to
that produced by incandescent bulbs.


Organic
light-emitting diodes (OLEDs)

Combined
spectral curves for blue, yellow-green, and high brightness red
solid-state semiconductor LEDs. FWHM spectral bandwidth is
approximately 24–27 nanometres for all three colors.

If
the emitting layer material of an LED is an organic compound, it is
known as an
Organic
Light Emitting Diode (OLED). To function as a semiconductor, the
organic emitting material must have conjugated pi bonds. The emitting
material can be a small organic molecule in a crystalline phase, or a
polymer. Polymer materials can be flexible; such LEDs are known as
PLEDs or FLEDs.

Compared
with regular LEDs, OLEDs are lighter, and polymer LEDs can have the
added benefit of being flexible. Some possible future applications of
OLEDs could be:

  • Inexpensive,
    flexible displays

  • Light
    sources

  • Wall
    decorations

  • Luminous
    cloth

OLEDs
have been used to produce visual displays for portable electronic
devices such as cellphones, digital cameras, and MP3 players. Larger
displays have been demonstrated, but their life expectancy is still
far too short (<1,000 hours) to be practical.


Operational
parameters and efficiency

Most
typical LEDs are designed to operate with no more than
30–60
milliwatts

of electrical power. Around 1999, Philips Lumileds introduced power
LEDs capable of continuous use at one watt. These LEDs used much
larger semiconductor die sizes to handle the large power inputs.
Also, the semiconductor dies were mounted onto metal slugs to allow
for heat removal from the LED die.

One
of the key advantages of LED-based lighting is its high efficiency,
as measured by its light output per unit power input. White LEDs
quickly matched and overtook the efficiency of standard incandescent
lighting systems. In 2002, Lumileds made five-watt LEDs available
with a luminous efficacy of 18–22 lumens per watt. For comparison,
a conventional 60–100 watt incandescent lightbulb produces around
15 lumens/watt, and standard fluorescent lights produce up to 100
lumens/watt. (The luminous efficacy article discusses these
comparisons in more detail.)

In
September 2003, a new type of blue LED was demonstrated by the
company
Cree
,
Inc. to provide 24 mW at 20 mA. This produced a commercially packaged
white light giving 65 lumens per watt at 20 mA, becoming the
brightest white LED commercially available at the time, and more than
four times as efficient as standard incandescents. In 2006 they
demonstrated a prototype with a record white LED luminous efficacy of
131 lm/W at 20 mA. Also, Seoul Semiconductor has plans for 135 lm/W
by 2007 and 145 lm/W by 2008, which would be approaching an order of
magnitude improvement over standard incandescents and better even
than standard fluorescents. Nichia Corp. has developed a white light
LED with luminous efficacy of 150 lm/W at a forward current of 20 mA.

It
should be noted that high-power (≥ 1 Watt) LEDs are necessary for
practical general lighting applications. Typical operating currents
for these devices begin at 350 mA. The highest efficiency high-power
white LED is claimed by Philips Lumileds Lighting Co. with a luminous
efficacy of 115 lm/W (350 mA).

Today,
OLEDs operate at substantially lower efficiency than inorganic
(crystalline) LEDs. The best luminous efficacy of an OLED so far is
about 10% of the theoretical maximum of 683, or about 68 lm/W. These
claim to be much cheaper to fabricate than inorganic LEDs, and large
arrays of them can be deposited on a screen using simple printing
methods to create a
color
graphical display
.


Considerations
in use

Unlike
incandescent light bulbs, which light up regardless of the electrical
polarity, LEDs will only light with correct electrical polarity. When
the voltage across the
p-n
junction

is in the correct direction, a significant current flows and the
device is said to be
forward-biased.
If the voltage is of the wrong polarity, the device is said to be
reverse
biased
,
very little current flows, and no light is emitted. Some LEDs can be
operated on an
alternating
current

voltage, but they will only light with positive voltage, causing the
LED to turn on and off at the frequency of the AC supply.

While
the only 100% accurate way to determine the polarity of an LED is to
examine its datasheet, these methods are usually reliable:

sign:

+

terminal:

anode
(A)

cathode
(K)

leads:

long

short

exterior:

round

flat

interior:

small

large

wiring:

red

black

Less
reliable methods of determining polarity are:

sign:

+

marking:

none

stripe

pin:

1

2

PCB:

round

square

While
it is not an officially reliable method, it is almost univers
ally
true that the cup that holds the LED die corresponds to the cathode.
It is strongly recommended to apply a safe voltage and observe the
illumination as a test regardless of what method is used to determine
the polarity.

Because
the voltage versus current characteristics of an LED are much like
any diode (that is, current approximately an exponential function of
voltage), a small voltage change results in a huge change in current.
Added to deviations in the process this means that a voltage source
may barely make one LED light while taking another of the same type
beyond its maximum ratings and potentially destroying it.

Since
the voltage is logarithmically related to the current it can be
considered to remain largely constant over the LEDs operating range.
Thus the power can be considered to be almost proportional to the
current. In order to keep power nearly constant with variations in
supply and LED characteristics, the power supply should be a “current
source”, that is, it should supply an almost constant current.
If high efficiency is not required (e.g., in most indicator
applications), an approximation to a current source made by
connecting the LED in series with a current limiting resistor to a
constant voltage source is generally used.

Most
LEDs have low reverse breakdown voltage ratings, so they will also be
damaged by an applied reverse voltage of more than a few volts. Since
some manufacturers don’t follow the indicator standards above, if
possible the data sheet should be consulted before hooking up an LED,
or the LED may be tested in series with a resistor on a sufficiently
low voltage supply to avoid the reverse breakdown. If it is desired
to drive an LED directly from an AC supply of more than the reverse
breakdown voltage then it may be protected by placing a diode (or
another LED) in inverse parallel.

LEDs
can be purchased with built in series resistors. These can save PCB
space and are especially useful when building prototypes or
populating a PCB in a way other than its designers intended. However
the resistor value is set at the time of manufacture, removing one of
the key methods of setting the LED’s intensity. To increase
efficiency (or to allow intensity control without the complexity of a
DAC), the power may be applied periodically or intermittently; so
long as the flicker rate is greater than the human flicker fusion
threshold, the LED will appear to be continuously lit.

Provided
there is sufficient voltage available, multiple LEDs can be connected
in series with a single current limiting resistor. Parallel operation
is generally problematic. The LEDs have to be of the same type in
order to have a similar forward voltage. Even then, variations in the
manufacturing process can make the odds of satisfactory operation
low.

Bicolor
LED units contain two diodes, one in each direction (that is, two
diodes in
inverse
parallel
)
and each a different color (typically red and green), allowing
two-color operation or a range of apparent colors to be created by
altering the percentage of time the voltage is in each polarity.
Other LED units contain two or more diodes (of different colors)
arranged in either a
common
anode

or
common
cathode

configuration. These can be driven to different colors without
reversing the polarity, however, more than two electrodes (leads) are
required.

LEDs
are usually constantly illuminated when a current passes through
them, but flashing LEDs are also available. Flashing LEDs resemble
standard LEDs but they contain an integrated multivibrator circuit
inside which causes the LED to flash with a typical period of one
second. This type of LED comes most commonly as red, yellow, or
green. Most flashing LEDs emit light of a single wavelength, but
multicolored flashing LEDs are available too.

Generally,
for newer common standard LEDs in 3 mm or 5 mm packages,
the following forward DC potential differences are typically
measured. The forward potential difference depending on the LED’s
chemistry, temperature, and on the current (values here are for
approx. 20 milliamperes, a commonly found maximum value).

Color

Potential
Difference

Infrared

1.6
V

Red

1.8
V to 2.1 V

Orange

2.2
V

Yellow

2.4
V

Green

2.6
V

Blue

3.0
V to 3.5 V

White

3.0
V to 3.5 V

Ultraviolet

3.5
V

Many
LEDs are rated at
5
V maximum

reverse voltage.

LEDs
also behave as photocells, and will generate a current depending on
the ambient light. They are not efficient as photocells, and will
only produce a few microamps, but will put out a surprising voltage
level, as much as 2 or 3 volts. This is enough to operate an
amplifier or CMOS logic gate. This effect can be used to make an
inexpensive light sensor, for example to decide when to turn on an
LED illuminator.


Advantages
of using LEDs

  • LEDs
    produce more light per watt than do incandescent bulbs; this is
    useful in battery powered or energy-saving devices.

  • LEDs
    can emit light of an intended color without the use of color filters
    that traditional lighting methods require. This is more efficient
    and can lower initial costs.

  • The
    solid package of an LED can be designed to focus its light.
    Incandescent and fluorescent sources often require an external
    reflector to collect light and direct it in a usable manner.

  • When
    used in applications where dimming is required, LEDs do not change
    their color tint as the current passing through them is lowered,
    unlike incandescent lamps, which turn yellow.

  • LEDs
    are ideal for use in applications that are subject to frequent
    on-off cycling, unlike fluorescent lamps that burn out more quickly
    when cycled frequently, or HID lamps that require a long time before
    restarting.

  • LEDs,
    being solid state components, are difficult to damage with external
    shock. Fluorescent and incandescent bulbs are easily broken if
    dropped on the ground.

  • LEDs
    can have a relatively long useful life. One report estimates 35,000
    to 50,000 hours of useful life, though time to complete failure may
    be longer.

    Fluorescent
    tubes typically are rated at about
    30,000
    hours
    ,
    and incandescent light bulbs at
    1,000–2,000
    hours
    .[citation
    needed
    ]

  • LEDs
    mostly fail by dimming over time, rather than the abrupt burn-out of
    incandescent bulbs.

  • LEDs
    light up very quickly. A typical red indicator LED will achieve full
    brightness in microseconds; LEDs used in communications devices can
    have even faster response times.

  • LEDs
    can be very small and are easily populated onto printed circuit
    boards.

  • LEDs
    do not contain
    mercury,
    while compact fluorescent lamps do.

LEDs
are produced in an array of shapes and sizes. The 5 mm
cylindrical package (red, fifth from the left) is the most common,
estimated at 80% of world production. The color of the plastic lens
is often the same as the actual color of light emitted, but not
always. For instance, purple plastic is often used for infrared LEDs,
and most blue devices have clear housings. There are also LEDs in
extremely
tiny packages
,
such as those found on blinkies (not shown).


Disadvantages
of using LEDs

  • LEDs
    are currently more expensive, price per lumen, on an initial capital
    cost basis, than more conventional lighting technologies. The
    additional expense partially stems from the relatively low lumen
    output and the drive circuitry and power supplies needed. However,
    when considering the total cost of ownership (including energy and
    maintenance costs), LEDs far surpass incandescent or halogen sources
    and begin to threaten compact fluorescent lamps. In December 2007,
    scientist at
    Glasgow
    University

    claimed to have found a way to make Light Emitting Diodes brighter
    and use less power than energy efficient light bulbs currently on
    the market by imprinting the holes into billions of LEDs in a new
    and cost effective method using a process known as nanoimprint
    lithography.

  • LED
    performance largely depends on the ambient temperature of the
    operating environment. Driving an LED hard in high ambient
    temperatures may result in overheating of the LED package,
    eventually leading to device failure. Adequate heat-sinking is
    required to maintain long life. This is especially important when
    considering automotive, medical, and military applications where the
    device must operate over a large range of temperatures, and is
    required to have a low failure rate.

  • LEDs
    must be supplied with the correct current. This can involve shunt
    resistors or regulated power supplies.

  • The
    spectrum of some white LEDs differs significantly from a black body
    radiator, such as the sun or an incandescent light. The spike at
    460 nm and dip at 500 nm can cause the color of objects to
    be perceived differently under LED illumination than sunlight or
    incandescent sources, due to metamerism. However, it should be noted
    that color rendering properties of common fluorescent lamps are
    often inferior to what is now available in state-of-art white LEDs.

  • LEDs
    do not approximate a “point source” of light, so cannot be
    used in applications that need a sharply directive and collimated
    beam. LEDs are not capable of providing directivity below a few
    degrees. In such cases LASERs (or amplified spontaneous emission
    devices) may be a better option.

  • There
    is increasing concern that blue LEDs and white LEDs are now capable
    of exceeding safe limits of the so-called blue-light hazard as
    defined in eye safety specifications such as ANSI/IESNA RP-27.1-05:
    Recommended Practice for Photobiological Safety for Lamp and Lamp
    Systems.


Types

There
are 3 main types of LEDs: miniature LEDs, alphanumeric LEDs, and
lighting LEDs.


Miniature
LEDs

These
are mostly single die LEDs used as indicators, and come in various
size packages:

  • surface
    mount

  • 2mm

  • 3mm

  • 5mm

  • Other
    sizes are also available, but less common.

There
are 3 main categories of miniature single die LEDs:

  • Low
    current – typically rated for 2mA at around 2v (apx 4mW
    consumption).

  • Standard
    – 20mA LEDs at around 2v (apx 40mW) for red, orange, yellow &
    green, and 20mA at 4-5v (apx 0.1W) for blue, violet and white.

  • Ultra
    high output – 20mA at apx 2v or 4-5v, designed for viewing in direct
    sunlight. These have enough light output to light a very small
    object, or may be used in low output torches.


Multicolour
LEDs

Bicolour
LEDs

contain 2 dice of different colours connected back to back, and can
produce any of 3 colours. Current flow in one direction produces one
colour, current in the other direction produces the other colour, and
bidirectional current produces both colours mixed together.

Tricolour
LEDs

contain 3 dice. Often these are RGB dice, but not always. These LEDs
can be driven to produce a wider range of colours, and in the case of
RBG LEDS nearly the whole visible spectrum of colours.


5v
& 12v LEDs

These
are miniature LEDs incorporating a series resistor, and may be
connected directly to 5v or 12v.


Flashing
LEDs

These
miniature LEDs flash when connected to 5v or 12v. Used as attention
seeking indicators where it is desired to avoid the complexity of
external electronics.


Alphanumeric
LEDs

LED
displays are available in
7
segment

and starburst format.
7
segment displays

handle all numbers and a limited set of letters. Starburst displays
can display all letters.

7
segment LED displays were in widespread use in the 1970s and 1980s,
but increasing use of LCD displays, with their lower power
consumption and greater display flexibility, has reduced the
popularity of numeric and alphanumeric LED displays.


Lighting
LEDs

LED
lamps (also called LED bars or Illuminators) are usually clusters of
LEDs in a suitable housing. They come in different shapes, among them
the light bulb shape with a large E27 Edison screw and MR16 shape
with a bi-pin base. Other models might have a small Edison E14
fitting, GU5.3 (Bipin cap) or GU10 (bayonet socket). This includes
low voltage (typically 12 V halogen-like) varieties and replacements
for regular AC mains (120-240 V AC) lighting. Currently the latter
are less widely available but this is changing rapidly.


LED
applications

  1. LED
    panel light source used in an experiment on plant growth. The
    findings of such experiments may be used to grow food in space on
    long duration missions.

  2. Light
    sources for machine vision systems.

  3. An
    LED destination display on a bus. Note how the camera has had
    difficulty catching all the LEDs.

  4. Old
    calculator LED display.

  5. Flashlights
    and lanterns that utilise white LEDs are becoming increasingly
    popular due to their durability and longer battery life.

  6. Single
    high-brightness LED with a glass lens creates a bright carrier beam
    that can stream DVD-quality video over considerable distances. The
    device, RONJA, can be built very simply by enthusiasts.

  7. LED
    lights on an Audi S6


List
of LED applications

Some
of these applications are further elaborated upon in the following
text.

  • Streetlights

  • Large
    scale video displays

  • Architectural
    lighting

  • Status
    indicators on all sorts of equipment

  • Traffic
    lights and signals

  • Light
    source for machine vision systems, requiring bright, focused,
    homogeneous and possibly strobed illumination.

  • Exit
    signs

  • Motorcycle
    and Bicycle lights

  • Toys
    and recreational sporting goods, such as the Flashflight

  • Railroad
    crossing

    signals

  • Continuity
    indicators

  • Flashlights,
    including some mechanically powered models.

  • Emergency
    vehicle lighting

  • Elevator
    Push Button Lighting

  • Thin,
    lightweight message displays at airports and railway stations and as
    destination displays for trains, buses, trams and ferries.

  • Red
    or yellow LEDs are used in indicator and alphanumeric displays in
    environments where night vision must be retained: aircraft cockpits,
    submarine and ship bridges, astronomy observatories, and in the
    field, e.g. night time animal watching and military field use.

  • Red,
    yellow, green, and blue LEDs can be used for model railroading
    applications

  • Remote
    controls
    ,
    such as for TVs and VCRs, often use infrared LEDs.

  • The
    Nintendo
    Wii’s
    sensor

    bar uses infrared LEDs.

  • In
    optical fiber and
    Free
    Space Optics communications
    .

  • In
    dot matrix arrangements for displaying messages.

  • Glowlights,
    as a more expensive but longer lasting and reusable alternative to
    Glowsticks.

  • Grow
    lights composed of LEDs are more efficient, both because LEDs
    produce more lumens per watt than other alternatives, and also
    because they can be tuned to the specific wavelengths plants can
    make the most use of.
    [citation
    needed
    ]

  • Movement
    sensors, for example in optical computer mice

  • Because
    of their long life and fast switching times, LEDs have been used for
    automotive high-mounted brake lights and truck and bus brake lights
    and turn signals for some time, but many high-end vehicles are now
    starting to use LEDs for their entire rear light clusters. Besides
    the gain in
    reliability,
    this has styling advantages because LEDs are capable of forming much
    thinner lights than incandescent lamps with parabolic reflectors.
    The significant improvement in the time taken to light up (perhaps
    0.5s faster than an incandescent bulb) improves safety by giving
    drivers more time to react. It has been reported that at normal
    highway speeds this equals one car length increased reaction time
    for the car behind. White LED headlamps are beginning to make an
    appearance.

  • Backlighting
    for
    LCD
    televisions and displays. The availability of LEDs in specific
    colors (RGB) enables a full-spectrum light source which expands the
    color gamut by as much as 45%.

  • New
    stage lighting equipment is being developed with LED sources in
    primary red-green-blue arrangements.

  • Lumalive,
    a photonic textile

  • LED-based
    Christmas lights have been available since 2002, but are only now
    beginning to gain in popularity and acceptance due to their higher
    initial purchase cost when compared to similar incandescent-based
    Christmas lights. For example, as of 2006, a set of 50 incandescent
    lights might cost US$2, while a similar set of 50 LED lights might
    cost US$10. The purchase cost can be even higher for single-color
    sets of LED lights with rare or recently-introduced colors, such as
    purple, pink or white. Regardless of the higher initial purchase
    price, the total cost of ownership for LED Christmas lights would
    eventually be lower than the TCO for similar incandescent Christmas
    lights
    [citation
    needed
    ]
    since an LED requires much less power to output the same amount of
    light as a similar incandescent bulb. More to the point, LEDs have
    practically unlimited life and are hard-wired rather than using
    unreliable sockets as do replaceable bulbs. So a set of LED lights
    can be expected to outlive many incandescent sets, and without any
    maintenance.

  • LED
    phototherapy for acne using blue or red LEDs has been proven to
    significantly reduce acne over a 3 month period.
    [citation
    needed
    ]

  • As
    a medium quality
    voltage
    reference

    in electronic circuits. The forward voltage drop (e.g., about 1.7 V
    for a normal red LED) can be used instead of a Zener diode in
    low-voltage regulators. Although LED forward voltage is much more
    current-dependent than a good Zener, Zener diodes are not available
    below voltages of about 3 V.

  • Some
    flatbed scanners use an array of red, green, and blue LEDs rather
    than the typical cold-cathode fluorescent lamp as the light source.
    Having independent control of three illuminant colors allows the
    scanner to calibrate itself for more accurate color balance, and
    there is no need for warm-up.

  • Computers,
    for hard drive activity and power on. Some custom computers feature
    LED accent lighting to draw attention to a given component. Many
    computer manufactuers use LEDs to tell the user its current state.
    One example would be the Mac, which tells its user when it is asleep
    by fading the LED activity lights in and out, in and out.

  • Light
    bulbs

  • Lanterns


Optoisolators
and optocouplers

Optocoupler
schematic showing LED and phototransistorAn LED may be combined with
a photodiode or phototransistor in a single electronic device to
provide a signal path with electrical isolation between two circuits.
An optoisolator will have typical breakdown voltages between the
input and output circuits of typically 500 to 3000 volts. This is
especially useful in medical equipment where the signals from a low
voltage sensor circuit (usually battery powered) in contact with a
living organism must be electrically isolated from any possible
electrical failure in a recording or montoring device operating at
potentially dangerous voltages. An optoisolator also allows
information to be transferred between circuits not sharing a common
ground potential. An optocoupler may not have such high breakdown
voltages and may even share a ground between input and output, but
both types are useful in preventing electrical noise, particularly
common mode electrical noise, on a sensor circuit from being
transferred to the receiving circuit (where it may adversly affect
the operation or durability of various components) and/or
transferring a noisy signal. Optoisolators are also used in the
feedback circuit of a DC to DC converter, allowing power to be
transferred while retaining electrical isolation between the input
and output.


Indicator
LEDs

Miniature
indicator LEDs are normally driven from low voltage DC via a current
limiting resistor. Currents of 2mA, 10mA and 20mA are common. Some
low current indicators are only rated to 2mA, and should not be
driven at higher current.

Sub-mA
indicators may be made by driving ultrabright LEDs at very low
current. Efficacy tends to reduce at low currents, but indicators
running on 100uA are still practical. The cost of ultrabrights is
higher than 2mA indicator LEDs.

LEDs
have a low max repeat reverse voltage rating, ranging from apx 2v to
5v, and this can be a problem in some apps. Back to back LEDs are
immune to this problem. These are available in single colour as well
as bicolor types. There are various strategies for reverse voltage
handling.

In
niche applications such as IR therapy, LEDs are often driven at far
above rated current. This causes high failure rate and occasional LED
explosions. Thus many parallel strings are used, and a safety screen
and ongoing maintenance are required.


Alphanumeric
LEDs

These
use the same drive strategy as indicator LEDs, the only difference
being the larger number of channels, each with its own resistor. 7
segment and starburst LED arrays are available in both common anode
or common cathode forms.


Lighting
LEDs on mains

A
CR dropper followed by full wave rectification is the usual ballast
with series-parallel LED clusters.

A
single series string minimises dropper losses, while parallelled
strings increase reliability. In practice usually 3 strings or more
are used.

Operation
on square wave and modified sine wave (MSW) sources, such as many
invertors, causes heavily increased resistor dissipation in CR
droppers, and LED ballasts designed for sine wave use tend to burn on
non-sine waveforms. The non-sine waveform also causes high peak LED
currents, heavily shortening LED life. An inductor & rectifier
makes a more suitable ballast for such use, and other options are
also possible.


Lighting
LEDs on low voltage

LEDs
are normally operated in parallel strings of series LEDs, with the
total LED voltage typically adding up to around 2/3 of the supply
voltage, and resistor current control for each string.

LED
current is proportional to power supply (PSU) voltage minus total LED
string voltage. Where battery sources are used, the PSU voltage can
vary widely, causing large changes in LED current and light output.
For such applications, a
constant
current

regulator is preferred to resistor control. Low drop-out (LDO)
constant current regs also allow the total LED string voltage to be a
higher percentage of PSU voltage, resulting in improved efficiency
and reduced power use.

Torches
run 1 or more lighting LEDs on a low voltage battery. These usually
use a resistor ballast.

In
disposable coin cell powered keyring type LED lights, the resistance
of the cell itself is usually the only current limiting device. The
cell should not therefore be replaced with a lower resistance type,
such as one using a different battery chemistry.

Finally,
an LED can be run from a single cell by use of a constant current
switched mode invertor. The extra expense makes this option
unpopular.


LED
panels

The
1,500 foot long LED display on the
Fremont
Street Experience

is currently the largest in the world.

There
are two types of LED panels: conventional, using discrete LEDs, and
surface
mounted device (SMD)

panels. Most outdoor screens and some indoor screens are built around
discrete LEDs, also known as individually mounted LEDs. A cluster of
red, green, and blue diodes is driven together to form a full-color
pixel, usually square in shape. These pixels are spaced evenly apart
and are measured from center to center for absolute pixel resolution.
The largest LED display in the world is over 1,500 foot
(457.2 m) long and is located in Las Vegas, Nevada covering the
Fremont Street Experience.

Most
indoor screens on the market are built using SMD technology—a trend
that is now extending to the outdoor market. An SMD pixel consists of
red, green, and blue diodes mounted on a chipset, which is then
mounted on the driver PC board. The individual diodes are smaller
than a pinhead and are set very close together. The difference is
that the maximum viewing distance is reduced by 25% from the discrete
diode screen with the same resolution.

LED
panels allow for smaller sets of interchangeable LEDs to be one large
display.

Indoor
use generally requires a screen that is based on SMD technology and
has a minimum brightness of 600 candelas per square meter
(unofficially called nits). This will usually be more than sufficient
for corporate and retail applications, but under high
ambient-brightness conditions, higher brightness may be required for
visibility. Fashion and auto shows are two examples of
high-brightness stage lighting that may require higher LED
brightness. Conversely, when a screen may appear in a shot on a
television show, the requirement will often be for lower brightness
levels with lower color temperatures (common displays have a white
point of 6500 to 9000 K, which is much bluer than the common
lighting on a television production set).

A
large LED screen in
Razorback
Stadium

For
outdoor use, at least 2,000 nits are required for most situations,
whereas higher brightness types of up to 5,000 nits cope even better
with direct sunlight on the screen. (The brightness of LED panels can
be reduced from the designed maximum, if required.)

Suitable
locations for large display panels are identified by factors such as
line of sight, local authority planning requirements (if the
installation is to become semi-permanent), vehicular access (trucks
carrying the screen, truck-mounted screens, or cranes), cable runs
for power and video (accounting for both distance and health and
safety requirements), power, suitability of the ground for the
location of the screen (if there are no pipes, shallow drains, caves,
or tunnels that may not be able to support heavy loads), and overhead
obstructions.


Early
LED flat panel TV history

The
first recorded flat panel LED television screen prototype to be
developed was by James P. Mitchell in 1977. The modular, scalable
display was enabled by MV50 LEDs and newly available TTL (transistor
transistor logic) memory addressing circuit technology. The prototype
and paper were displayed at an Engineering Exposition in Anaheim May
1978, and organized by the Science Service in Washington D.C. The LED
TV display received special recognition from
NASA,
General Motors Corporation and area universities including The
University of California Irvine, Robert M. Saunders Prof. of
Engineering and IEEE President 1977. Additionally, technology
business representatives from the U.S. and overseas witnessed
operation of the monochromatic LED television display. The prototype
remains operational. An LCD (liquid crystal display) matrix design
was also presented in the accompanying scientific paper as a future
television display method using a similar array scanning design.

The
early display prototype was red monochromatic. Low-cost efficient
blue LEDs did not emerge until the early 1990s, completing the
desired RGB color triad. High-brightness colors gradually emerged in
the 1990s enabling new designs for outdoor signage and huge video
displays for billboards and stadiums.


Multi-touch
sensing

Since
LEDs share some basic physical properties with photodiodes, which
also use
p-n
junctions with band gap energies in the visible light wavelengths,
they can also be used for photo detection. These properties have been
known for some time, but more recently so-called bidirectional LED
matrices have been proposed as a method of touch-sensing. In 2003,
Dietz, Yerazunis, and Leigh published a paper describing the use of
LEDs as cheap sensor devices.

In
this usage, various LEDs in the matrix are quickly switched on and
off. LEDs that are on shine light onto a user’s fingers or a stylus.
LEDs that are off function as photodiodes to detect reflected light
from the fingers or stylus. The voltage thus induced in the
reverse-biased LEDs can then be read by a microprocessor, which
interprets the voltage peaks and then also uses them elsewhere.


Laser

Experiment
with a LASER(
Light
amplified by stimulated emmision of radiation
)
(US Military)

A
laser
is an electronic-optical device that produces
coherent
radiation
.
The term “laser” is an
acronym
for
Light
Amplified
by
Stimulated
Emission
of
Radiation.
A
typical laser emits light in a narrow, low-divergence beam and with a
well-defined wavelength (i.e., monochromatic, corresponding to a
particular colour if the laser is operating in the visible spectrum).
This is in contrast to a light source such as the incandescent light
bulb, which emits into a large solid angle and over a wide spectrum
of wavelength.

A
laser consists of a
gain
medium

inside an
optical
cavity
,
with a means to supply energy to the gain medium. The gain medium is
a material (gas, liquid, solid or free electrons) with appropriate
optical properties. In its simplest form, a cavity consists of two
mirrors arranged such that light bounces back and forth, each time
passing through the gain medium. Typically, one of the two mirrors,
the output coupler, is partially transparent. The output laser beam
is emitted through this mirror.

Light
of a specific wavelength that passes through the gain medium is
amplified (increases in power); the surrounding mirrors ensure that
most of the light makes many passes through the gain medium. Part of
the light that is between the mirrors (i.e., is in the cavity) passes
through the partially transparent mirror and appears as a beam of
light. The process of supplying the energy required for the
amplification is called pumping and the energy is typically supplied
as an electrical current or as light at a different wavelength. In
the latter case, the light source can be a flash lamp or another
laser. Most practical lasers contain additional elements that affect
properties such as the wavelength of the emitted light and the shape
of the beam.

The
first working laser was demonstrated in May 1960 by
Theodore
Maiman

at Hughes Research Laboratories. Recently, lasers have become a
multi-billion dollar industry. The most widespread use of lasers is
in
optical
storage devices

such as compact disc and DVD players, in which the laser (a few
millimeters in size) scans the surface of the disc. Other common
applications of lasers are bar code readers and laser pointers. In
industry, lasers are used for
cutting
steel

and
other metals and for inscribing patterns (such as the letters on
computer keyboards). Lasers are also commonly used in various fields
in science, especially spectroscopy, typically because of their
well-defined wavelength or short pulse duration in the case of pulsed
lasers. Lasers are also used for military and medical applications.

A
helium-neon laser demonstration at the Kastler-Brossel Laboratory at
Univ. Paris 6. The glowing ray in the middle is an electric discharge
producing light in much the same way as a neon light. It is the gain
medium through which the laser passes,
not
the laser beam itself, which is visible there. The laser beam crosses
the air and marks a red point on the screen to the right.

Spectrum
of a helium neon laser showing the very high spectral purity
intrinsic to nearly all lasers. Compare with the relatively broad
spectral emittance of a light emitting diode.

To
edward understand the fundamentals of how lasers work and what makes
their emissions so special requires a knowledge of the interaction of
electromagnetic radiation and matter (
see
the “introduction to quantum mechanics” article
).

See
also: Laser science and Laser construction

A
laser is composed of an
active
laser medium
,
or
gain
medium
,
and a resonant optical cavity. The gain medium transfers external
energy into the laser beam. It is a material of controlled purity,
size, concentration, and shape, which amplifies the beam by the
process of stimulated emission. The gain medium is energized, or
pumped,
by an external energy source. Examples of pump sources include
electricity and light, for example from a flash lamp or from another
laser. The pump energy is absorbed by the laser medium, placing some
of its particles into high-energy (“excited”)
quantum
states
.
Particles can interact with light both by absorbing photons or by
emitting photons. Emission can be spontaneous or stimulated. In the
latter case, the photon is emitted in the same direction as the light
that is passing by. When the number of particles in one excited state
exceeds the number of particles in some lower-energy state,
population inversion is achieved and the amount of stimulated
emission due to light that passes through is larger than the amount
of absorption. Hence, the light is amplified. Strictly speaking,
these are the essential ingredients of a laser. However, usually the
term
laser
is used for devices where the light that is amplified is produced as
spontaneous emission from the same gain medium as where the
amplification takes place. Devices where light from an external
source is amplified are normally called optical amplifiers.

by
stimulated emission is very similar to the input signal in terms of
wavelength, phase, and polarization. This gives laser light its
characteristic coherence, and allows it to maintain the uniform
polarization and often monochromaticity established by the optical
cavity design.

The
optical cavity, a type of
cavity
resonator
,
contains a coherent beam of light between reflective surfaces so that
the light passes through the gain medium more than once before it is
emitted from the output aperture or lost to diffraction or
absorption. As light circulates through the cavity, passing through
the gain medium, if the gain (amplification) in the medium is
stronger than the resonator losses, the power of the circulating
light can rise exponentially. But each stimulated emission event
returns a particle from its excited state to the ground state,
reducing the capacity of the gain medium for further amplification.
When this effect becomes strong, the gain is said to be
saturated.
The balance of pump power against gain saturation and cavity losses
produces an equilibrium value of the laser power inside the cavity;
this equilibrium determines the operating point of the laser. If the
chosen pump power is too small, the gain is not sufficient to
overcome the resonator losses, and the laser will emit only very
small light powers. The minimum pump power needed to begin laser
action is called the
lasing
threshold
.
The gain medium will amplify any photons passing through it,
regardless of direction; but only the photons aligned with the cavity
manage to pass more than once through the medium and so have
significant amplification.

The
beam in the cavity and the output beam of the laser, if they occur in
free space rather than waveguides (as in an optical fiber laser),
are, at best, low order Gaussian beams. However this is rarely the
case with powerful lasers. If the beam is not a low-order Gaussian
shape, the transverse modes of the beam can be described as a
superposition of Hermite-Gaussian or Laguerre-Gaussian beams (for
stable-cavity lasers). Unstable laser resonators on the other hand,
have been shown to produce fractal shaped beams. The beam may be
highly
collimated,
that is being parallel without diverging. However, a perfectly
collimated beam cannot be created, due to diffraction. The beam
remains collimated over a distance which varies with the square of
the beam diameter, and eventually diverges at an angle which varies
inversely with the beam diameter. Thus, a beam generated by a small
laboratory laser such as a
helium-neon
laser

spreads to about 1.6 kilometers (1 mile) diameter if shone from the
Earth
to the Moon
.
By comparison, the output of a typical semiconductor laser, due to
its small diameter, diverges almost as soon as it leaves the
aperture, at an angle of anything up to 50°. 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.

The
output of a laser may be a continuous constant-amplitude output
(known as
CW
or
continuous
wave
);
or pulsed, by using the techniques of Q-switching, modelocking, or
gain-switching. In pulsed operation, much higher peak powers can be
achieved.

Some
types of lasers, such as
dye
lasers

and
vibronic
solid-state lasers

can produce light over a broad range of wavelengths; this property
makes them suitable for generating extremely short pulses of light,
on the order of a few femtoseconds (10
-15
s).

Although
the laser phenomenon was discovered with the help of quantum physics,
it is not essentially more quantum mechanical than other light
sources. The operation of a free electron laser can be explained
without reference to
quantum
mechanics
.

It
is understood that the word
light
in the acronym
Light
Amplification by Stimulated Emission of Radiation

is typically used in the expansive sense, as photons of
any
energy; it is not limited to photons in the visible spectrum. Hence
there are
infrared
lasers
,
ultraviolet
lasers
,
X-ray
lasers
,
etc. For example, a source of atoms in a coherent state can be called
an atom laser.

Because
the microwave equivalent of the laser, the
maser,
was developed first, devices that emit microwave and radio
frequencies are usually called
masers.
In early literature, particularly from researchers at
Bell
Telephone Laboratories
,
the laser was often called the
optical
maser
.
This usage has since become uncommon, and as of 1998 even Bell Labs
uses the term
laser.


History

Foundations

In
1917,
Albert
Einstein

in his paper
Zur
Quantentheorie der Strahlung (On the Quantum Theory of Radiation)
,
laid the foundation for the invention of the laser and its
predecessor, the maser, in a ground-breaking rederivation of Max
Planck’s law of radiation based on the concepts of probability
coefficients (later to be termed ‘Einstein coefficients’) for the
absorption, spontaneous, and stimulated emission.

In
1928
,
Rudolph W. Landenburg

confirmed the existence of stimulated emission and negative
absorption.

In
1939,
Valentin
A. Fabrikant

(USSR) predicted the use of stimulated emission to amplify “short”
waves.

In
1947,
Willis
E. Lamb and R. C. Retherford

found apparent stimulated emission in hydrogen spectra and made the
first demonstration of stimulated emission.

In
1950,
Alfred
Kastler

(Nobel Prize for Physics 1966) proposed the method of optical
pumping, which was experimentally confirmed by Brossel, Kastler and
Winter two years later.
[8]


Maser

In
1953,
Charles
H. Townes

and graduate students James P. Gordon and Herbert J. Zeiger produced
the first microwave amplifier, a device operating on similar
principles to the laser, but amplifying microwave rather than
infrared or visible radiation. Townes’s maser was incapable of
continuous output. Nikolay Basov and Aleksandr Prokhorov of the
Soviet Union worked independently on the quantum oscillator and
solved the problem of continuous output systems by using more than
two energy levels and produced the first maser. These systems could
release
stimulated
emission

without falling to the ground state, thus maintaining a population
inversion. In 1955 Prokhorov and Basov suggested an optical pumping
of multilevel system as a method for obtaining the population
inversion, which later became one of the main methods of laser
pumping.

Townes
reports that he encountered opposition from a number of eminent
colleagues who thought the maser was theoretically impossible —
including Niels Bohr, John von Neumann, Isidor Rabi, Polykarp Kusch,
and Llewellyn H. Thomas.

Townes,
Basov, and Prokhorov shared the Nobel Prize in Physics in 1964 “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”.


Laser

In
1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell
Labs, began a serious study of the infrared laser. As ideas were
developed,
infrared
frequencies

were abandoned with focus on visible light instead. The concept was
originally known as an “optical maser”. Bell Labs filed a
patent application for their proposed optical maser a year later.
Schawlow and Townes sent a manuscript of their theoretical
calculations to Physical Review, which published their paper that
year (Volume 112, Issue 6).

The
first page of Gordon Gould’s laser notebook in which he coined the
acronym LASER and described the essential elements for constructing
one.

At
the same time Gordon Gould, a graduate student at Columbia
University, was working on a doctoral thesis on the energy levels of
excited thallium. Gould and Townes met and had conversations on the
general subject of
radiation
emission
.
Afterwards Gould made notes about his ideas for a “laser”
in November 1957, including suggesting using an open resonator, which
became an important ingredient of future lasers.

In
1958, Prokhorov independently proposed using an open resonator, the
first published appearance of this idea. Schawlow and Townes also
settled on an open resonator design, apparently unaware of both the
published work of Prokhorov and the unpublished work of Gould.

The
term “laser” was first introduced to the public in Gould’s
1959 conference paper “The LASER, Light Amplification by
Stimulated Emission of Radiation”.


Gould intended “-aser” to be a suffix, to be used with an
appropriate prefix for the spectra of light emitted by the device
(x-ray laser = xaser, ultraviolet laser = uvaser, etc.). None of the
other terms became popular, although “raser” was used for a
short time to describe radio-frequency emitting devices.

Gould’s
notes included possible applications for a laser, such as
spectrometry, interferometry, radar, and nuclear fusion. He continued
working on his idea and filed a patent application in April 1959. The
U.S.
Patent Office denied

his application and awarded a patent to Bell Labs in 1960. This
sparked a legal battle that ran 28 years, with scientific prestige
and much money at stake. Gould won his first minor patent in 1977,
but it was not until 1987 that he could claim his first significant
patent victory when a federal judge ordered the government to issue
patents to him for the optically pumped laser and the gas discharge
laser.

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 Townes at Columbia University, Arthur L.
Schawlow at Bell Labs,

and
Gould at a company called TRG (Technical Research Group). Maiman used
a solid-state flashlamp-pumped synthetic ruby crystal to produce red
laser light at 694 nanometres wavelength. Maiman’s laser, however,
was only capable of pulsed operation due to its three energy level
pumping scheme.

Later
in 1960 the
Iranian
physicist Ali Javan
,
working with William R. Bennett and Donald Herriot, made the first
gas laser using helium and neon. Javan later received the Albert
Einstein Award in 1993.

The
concept of the semiconductor laser diode was proposed by Basov and
Javan. The first
laser
diode

was demonstrated by Robert N. Hall in 1962. Hall’s device was made of
gallium arsenide and emitted at 850 nm in the near-infrared region of
the spectrum. The first semiconductor laser with visible emission was
demonstrated later the same year by Nick Holonyak, Jr. As with the
first gas lasers, these early semiconductor lasers could be used only
in pulsed operation, and indeed only when cooled to
liquid
nitrogen
temperatures (77 K).

In
1970, Zhores Alferov in the Soviet Union and Izuo Hayashi and Morton
Panish of Bell Telephone Laboratories independently developed laser
diodes continuously operating at room temperature, using the
heterojunction structure.


Recent
innovations

Graph
showing the history of maximum laser pulse intensity throughout the
past 40 years.

Since
the early period of laser history, laser research has produced a
variety of improved and specialized laser types, optimized for
different performance goals, including:

  • new
    wavelength bands

  • maximum
    average output power

  • maximum
    peak output power

  • minimum
    output pulse duration

  • maximum
    power efficiency

  • maximum
    charging

  • maximum
    firing

and
this research continues to this day.

Lasing
without maintaining the medium excited into a population inversion,
was discovered in 1992 in sodium gas and again in 1995 in rubidium
gas by various international teams. This was accomplished by using an
external maser to induce “optical transparency” in the
medium by introducing and destructively interfering the ground
electron transitions between two paths, so that the likelihood for
the ground electrons to absorb any energy has been cancelled.

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. These high
intensity pulses can produce
filament
propagation
in the atmosphere.


Continuous
wave and pulsed lasing

A
laser may either be built to emit a continuous beam or a train of
short pulses. This makes fundamental differences in construction,
usable laser media, and applications.


Continuous
wave operation

In
the
continuous
wave

(CW) mode of operation, the output of a laser is relatively
consistent with respect to time. The population inversion required
for lasing is continually maintained by a steady pump source.


Pulsed
operation

In
the pulsed mode of operation, the output of a laser varies with
respect to time, typically taking the form of alternating ‘on’ and
‘off’ periods. In many applications one aims to deposit as much
energy as possible at a given place in as short time as possible. In
laser ablation for example, a small volume of material at the surface
of a work piece might evaporate if it gets the energy required to
heat it up far enough in very short time. If, however, the same
energy is spread over a longer time, the heat may have time to
disperse into the bulk of the piece, and less material evaporates.
There are a number of methods to achieve this.


Q-switching

In
a Q-switched laser, the population inversion (usually produced in the
same way as CW operation) is allowed to build up by making the cavity
conditions (the ‘Q’) unfavorable for lasing. Then, when the pump
energy stored in the laser medium is at the desired level, the ‘Q’ is
adjusted (electro- or acousto-optically) to favorable conditions,
releasing the pulse. This results in
high
peak powers

as the average power of the laser (were it running in CW mode) is
packed into a shorter time frame.


Modelocking

A
modelocked laser emits extremely short pulses on the order of tens of
picoseconds down to less than 10 femtoseconds. These pulses are
typically separated by the time that a pulse takes to complete one
round trip in the resonator cavity. Due to the
Fourier
limit

(also known as energy-time uncertainty), a pulse of such short
temporal length has a spectrum which contains a wide range of
wavelengths. Because of this, the laser medium must have a broad
enough gain profile to amplify them all. An example of a suitable
material is titanium-doped, artificially grown sapphire
(Ti:sapphire).

The
modelocked laser is a most versatile tool for researching processes
happening at extremely fast time scales (
femtosecond
physics

and femtosecond chemistry, also called ultrafast science), for
maximizing the effect of nonlinearity in optical materials (e.g. in
second-harmonic generation, parametric
down-conversion,
optical parametric oscillators and the like), and in ablation
applications. Again, because of the short timescales involved, these
lasers can achieve extremely high powers.


Pulsed
pumping

Another
method of achieving pulsed laser operation is to pump the laser
material with a source that is itself pulsed, either through
electronic charging in the case of flashlamps, or another laser which
is already pulsed. Pulsed pumping was historically used with dye
lasers where the inverted population lifetime of a dye molecule was
so short that a high energy, fast pump was needed. The way to
overcome this problem was to charge up large capacitors which are
then switched to discharge through flashlamps, producing a broad
spectrum pump flash. Pulsed pumping is also required for lasers which
disrupt the gain medium so much during the laser process that lasing
has to cease for a short period. These lasers, such as the excimer
laser and the copper vapour laser, can never be operated in CW mode.



Gas
lasers

Gas
lasers using many gases have been built and used for many purposes.
They are one of the oldest types of laser.

The
helium-neon
laser

(HeNe) emits at a variety of wavelengths and units operating at 633
nm are very common in education because of its low cost.

Carbon
dioxide lasers can emit hundreds of kilowatts at 9.6 µm and
10.6 µm, and are often used in industry for cutting and
welding. The efficiency of a CO
2
laser is over 10%.

Argon-ion
lasers

emit light in the range 351-528.7 nm. Depending on the optics and the
laser tube a different number of lines is usable but the most
commonly used lines are 458 nm, 488 nm and 514.5 nm.

A
nitrogen
transverse
electrical
discharge in gas at
atmospheric
pressure (TEA) laser is an inexpensive gas laser producing UV Light
at 337.1 nm.

Metal
ion lasers are gas lasers that generate deep ultraviolet wavelengths.
Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two
examples. These lasers have particularly narrow oscillation
linewidths of less than 3 GHz (0.5 picometers), making them
candidates for use in fluorescence suppressed
Raman
spectroscopy
.


Chemical
lasers

Chemical
lasers are powered by a chemical reaction, and can achieve high
powers in continuous operation. For example, 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. They were
invented by George C. Pimentel.


Excimer
lasers

Excimer
lasers are powered by a chemical reaction involving an
excited
dimer
,
or
excimer,
which is a short-lived dimeric or heterodimeric molecule formed from
two species (atoms), at least one of which is in an excited
electronic state. They typically produce ultraviolet light, and are
used in semiconductor
photolithography
and in LASIK eye surgery. Commonly used excimer molecules include F
2
(fluorine, emitting at 157 nm), and noble gas compounds (ArF [193
nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).


Solid-state
lasers

A
50 W FASOR, based on a Nd:YAG laser, used at the Starfire Optical
Range

Solid
state laser materials are commonly made by doping a crystalline solid
host with ions that provide the required energy states. For example,
the first working laser was a ruby laser, made from ruby
(chromium-doped corundum). Formally, the class of solid-state lasers
includes also fiber laser, as the active medium (fiber) is in the
solid state. Practically, in the scientific literature,
solid-state
laser usually means a laser with bulk active medium; while wave-guide
lasers are caller fiber lasers.

Neodymium
is a common dopant in various solid state laser crystals, including
yttrium orthovanadate (Nd:YVO
4),
yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet
(Nd:YAG). All these lasers can produce high powers in the infrared
spectrum at 1064 nm. They are used for cutting, welding and marking
of metals and other materials, and also in spectroscopy and for
pumping dye lasers. These lasers are also commonly frequency doubled,
tripled or quadrupled to produce 532 nm (green, visible), 355 nm (UV)
and 266 nm (UV) light when those wavelengths are needed.

Ytterbium,
holmium, thulium, and erbium are other common dopants in solid state
lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW,
Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-1050 nm.
They are potentially very efficient and high powered due to a small
quantum defect. Extremely high powers in ultrashort pulses can be
achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and
form an efficient laser operating at infrared wavelengths strongly
absorbed by water-bearing tissues. The Ho-YAG is usually operated in
a pulsed mode, and passed through optical fiber surgical devices to
resurface joints, remove rot from teeth, vaporize cancers, and
pulverize kidney and gall stones.

Titanium-doped
sapphire (Ti:sapphire) produces a highly tunable infrared laser,
commonly used for spectroscopy as well as the most common ultrashort
pulse laser.

Thermal
limitations in solid-state lasers arise from unconverted pump power
that manifests itself as heat and phonon energy. This heat, when
coupled with a high thermo-optic coefficient (d
n/dT)
can give rise to thermal lensing as well as reduced quantum
efficiency. These types of issues can be overcome by another novel
diode-pumped solid state laser, the diode-pumped thin disk laser. The
thermal limitations in this laser type are mitigated by utilizing a
laser medium geometry in which the thickness is much smaller than the
diameter of the pump beam. This allows for a more even thermal
gradient in the material. Thin disk lasers have been shown to produce
up to kilowatt levels of power.


Fiber-hosted
lasers

Solid-state
lasers

where the light is guided due to the total internal reflection in a
wavequide are called fiber lasers because of huge ratio of the length
to the transversal size; this ratio may vary from 10
6
to 10
9;
visually, the active element of such a laser looks as a fiber.
Guiding of light allows extremely long gain regions providing good
cooling conditions; fibers have high surface area to volume ratio
allows efficient cooling. In addition, the fiber’s waveguiding
properties tend to reduce thermal distortion of the beam.

double-clad
fibers
.
Quite often, the fiber laser is designed as a double-clad fiber. This
type of fiber consists of a fiber core, an inner cladding and an
outer cladding. The index of the three concentric layers is chosen so
that the fiber core acts as a single-mode fiber for the laser
emission while the outer cladding acts as a highly multimode core for
the pump laser. This lets the pump propagate a large amount of power
into and through the active inner core region, while still having a
high numerical aperture (NA) to have easy launching conditions.

Fiber
disk lasers
.
The efficient use of pump in fiber laser can be achieved at the
transversal
delivery
of pump
;
however, several lasers should be formed into a stack. Such stack may
have shape of a disk, which is an alternative to the double-clad
fiber.

Maximal
length

of a fiber laser. Fiber lasers have a fundamental limit in that the
intensity of the light in the fiber cannot be so high that optical
nonlinearities induced by the local electric field strength can
become dominant and prevent laser operation and/or lead to the
material destruction of the fiber. This effect is called
photodarkening. In bulk laser materials, the cooling is not so
efficient, and it is difficult to separate the effects of
photodarkening from the thermal effects, but the experiments in
fibers the photodarkening can be attributed to the forming og
long-living color centers.


Semiconductor
lasers

Commercial
laser diodes emit at wavelengths from 375 nm to 1800 nm, and
wavelengths of over 3 µm have been demonstrated. Low power
laser diodes are used in
laser
printers

and CD/DVD players. More powerful laser diodes are frequently used to
optically pump other lasers with high efficiency. The highest power
industrial laser diodes, with power up to 10 kW, are used in industry
for cutting and welding. External-cavity semiconductor lasers have a
semiconductor active medium in a larger cavity. These devices can
generate high power outputs with good beam quality,
wavelength-tunable narrow-linewidth radiation, or ultrashort laser
pulses.

Vertical
cavity surface-emitting lasers

(VCSELs) are semiconductor lasers whose emission direction is
perpendicular to the surface of the wafer. VCSEL devices typically
have a more circular output beam than conventional laser diodes, and
potentially could be much cheaper to manufacture. As of 2005, only
850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to
be commercialized, and 1550 nm devices an area of research. VECSELs
are external-cavity VCSELs. Quantum cascade lasers are semiconductor
lasers that have an active transition between energy
sub-bands
of an electron in a structure containing several quantum wells.

The
development of a silicon laser is
important
in the field of optical computing, since it means that if silicon,
the chief ingredient of computer chips, were able to produce lasers,
it would allow the light to be manipulated like electrons are in
normal integrated circuits. Thus, photons would replace electrons in
the circuits, which dramatically increases the speed of the computer.
Unfortunately, silicon is a difficult lasing material to deal with,
since it has certain properties which block lasing. However, recently
teams have produced silicon lasers through methods such as
fabricating the lasing material from silicon and other semiconductor
materials, such as indium(III) phosphide or gallium(III) arsenide,
materials which allow coherent light to be produced from silicon.
These are called hybrid silicon laser. Another type is a Raman laser,
which takes advantage of Raman scattering to produce a laser from
materials such as silicon.


Dye
lasers

Dye
lasers use an organic dye as the gain medium. The wide gain spectrum
of available dyes allows these lasers to be highly tunable, or to
produce very short-duration pulses (on the order of a few
femtoseconds)


Free
electron lasers

Free
electron lasers, or FELs, generate coherent, high power radiation,
that is widely tunable, currently ranging in wavelength from
microwaves, through terahertz 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
.


Nuclear
reaction lasers

In
September 2007, the BBC News reported that there was speculation
about the possibility of using positronium annihilation to drive a
very powerful
gamma
ray laser
.
This laser is believed to be powerful enough to jump-start a nuclear
reaction, with a single gamma ray laser, rather than the hundreds of
conventional lasers involved in current experiments.


Uses

Lasers
range in size from microscopic diode lasers (top) with numerous
applications, to football field sized neodymium glass lasers (bottom)
used for inertial confinement fusion, nuclear weapons research and
other high energy density physics experiments.

When
lasers were invented in 1960, they were called “a solution
looking for a problem”.
[citation
needed
]
Since then, they have become ubiquitous, finding utility in thousands
of highly varied applications in every section of modern society,
including consumer electronics, information technology, science,
medicine, industry, law enforcement, entertainment, and the military.

The
first
application
of lasers visible in the daily lives of the general population was
the supermarket barcode scanner, introduced in 1974. The laserdisc
player, introduced in 1978, was the first successful consumer product
to include a laser, but the
compact
disc
player was the first laser-equipped device to become truly common in
consumers’ homes, beginning in 1982, followed shortly by laser
printers.

Some
of the other applications include:

  • Medicine:
    Bleedless surgery, laser healing, survical treatment, kidney stone
    treatment, eye treatment, dentistry

  • Industry:
    Cutting, welding, material heat treatment, marking parts

  • Defense:
    Marking targets, guiding munitions, missile defence, electro-optical
    countermeasures (EOCM), RADAR alternative

  • Research:
    Spectroscopy, laser ablation, Laser annealing, laser scattering,
    laser interferometry, LIDAR

  • Product
    development/Commercial: Laser Printers, CDs, Barcode scanners, laser
    pointers, Holograms)

In
2004, excluding diode lasers, approximately 131,000 lasers were sold
world-wide, with a value of US$2.19 billion.


In the same year, approximately 733 million diode lasers, valued at
$3.20
billion,
were
sold.


Example
uses by typical output power

Different
uses need lasers with different output powers. Many lasers are
designed for a higher peak output with an extremely short pulse, and
this requires different technology from a
continuous
wave

(constant output) lasers, as are used in communication, or cutting.
Output power is always less than the input power needed to generate
the beam.

The
peak power required for some uses:

  • 5
    mW – CD-ROM drive

  • 5-10
    mW – DVD player

  • 100
    mW – CD-R drive

  • 250
    mW – output power of Sony SLD253VL red laser diode, used in consumer
    48-52 speed CD-R burner.

  • 500
    mW – output power of Sony SLD1332V red laser diode, used in consumer
    DVD-R burner.

  • 1
    W – green laser in current
    Holographic
    Versatile Disc prototype development.

  • 100
    to 3000 W (peak output 1.5 kW) – typical sealed CO
    2
    lasers used in industrial laser
    cutting.

  • 1
    kW – Output power expected to be achieved by “a single 1 cm
    diode laser bar”

  • 700
    terawatts (TW) – The National Ignition Facility is working on a
    system that, when complete, will contain a 192-beam, 1.8-megajoule
    laser system adjoining a 10-meter-diameter target chamber. The
    system is expected to be completed in April of 2009.

  • 1.25
    petawatts (PW) – world’s most powerful laser (claimed on 23 May 1996
    by Lawrence Livermore Laboratory).

From
: My teacher Aplication notes, LED.

Click
this for such Information above, producer of LED :

http://www.maxim-ic.com/
LED are still popular.

http://pdfserv.maxim-ic.com/en/an/AN1883.pdf
complete data sheet in PDF versions.

Hopely
useful. Regards : Yonni. M

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