Aplication notes :
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
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
is based on the quantum mechanical effects of light on semiconducting
materials, sometimes in the presence of electric fields.
or photovoltaic effect, used in:
(including solar cells)
optical circuit (IOC) elements
emission, used in:
effect, or radiative recombination, used in:
diodes or LED
applications of optoelectronics include:
is electromagnetic radiation of a wavelength that is visible to the
human eye (about 400–700 nm). In a scientific context, the word
is sometimes used to refer to the entire electromagnetic
is composed of an elementary particle called a photon.
primary properties of light are:
or wavelength and;
or direction of the wave oscillation.
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 in a vacuum is exactly 299,792,458 m/s (about
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
physicists have attempted to measure the speed of light throughout
attempted to measure the speed of light in the seventeenth century. A
good early experiment to measure the speed of light was conducted by
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.
more accurate, measurement of the speed of light was performed in
Europe by Hippolyte
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.
used an experiment which used rotating mirrors to obtain a value of
298,000,000 m/s in 1862. Albert
conducted experiments on the speed of light from 1877 until his death
in 1931. He refined Foucault’s
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
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:
denotes the speed that light travels in the transparent medium.
= 1 in a vacuum and n
> 1 in a transparent medium.
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.
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.
study of light and the interaction of light and matter is termed
optics. The observation and study of optical phenomena such as
and the aurora
offer many clues as to the nature of light as well as much enjoyment.
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
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.
emit and absorb light at characteristic energies. This produces
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.
of a free charged particle, such as an electron, can produce visible
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
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.
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
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.
other mechanisms can produce light:
the concept of light is intended to include very-high-energy photons
(gamma rays), additional generation mechanisms include:
Muslim scientist Ibn
(c. 965-1040), known as Alhacen
in the West, in his Book
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
and invented the camera
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
of the refraction of light, and went on to discover the laws of
also carried out the first experiments on the dispersion of light
into its constituent colors. His major work Kitab
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.
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.
the 1660s, Robert
published a wave
theory of light.
Christiaan Huygens worked out his own wave theory of light in 1678,
and published it in his Treatise
in 1690. He proposed that light was emitted in all directions as a
series of waves in a medium called the Luminiferous
As waves are not affected by gravity, it was assumed that they slowed
down upon entering a denser medium.
Young’s sketch of the two-slit experiment showing the diffraction of
light. Young’s experiments supported the theory that light consists
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
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.
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
independently worked out his own wave theory of light, and presented
it to the Académie des Sciences in 1817. Simeon
added to Fresnel’s mathematical work to produce a convincing argument
in favour of the wave theory, helping to overturn Newton’s
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
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.
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
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
work inspired James
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.
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
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
a normal diode, an LED consists of a chip of semiconducting material
impregnated, or doped,
with impurities to create a p-n
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
of the light emitted, and therefore its color, depends on the band
gap energy of the materials forming the p-n
In silicon or germanium diodes, the electrons and holes recombine by
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
development began with infrared and red devices made with gallium
Advances in materials science have made possible the production of
devices with ever-shorter wavelengths, producing light in a variety
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
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
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
and manipulating the microscopic structure of the surface to reduce
the reflectance, either by introducing random roughness or by
creating programmed moth
LEDs are made from a variety of inorganic semiconductor materials,
producing the following colors:
gallium arsenide (AlGaAs) — red and infrared
gallium phosphide (AlGaP) — green
gallium indium phosphide (AlGaInP) — high-brightness orange-red,
orange, yellow, and green
arsenide phosphide (GaAsP) — red, orange-red, orange, and yellow
phosphide (GaP) — red, yellow and green
nitride (GaN) — green, pure green (or emerald green), and blue
also white (if it has an AlGaN Quantum Barrier)
gallium nitride (InGaN) — 450nm – 470nm — near ultraviolet,
bluish-green and blue
carbide (SiC) as substrate — blue
(Si) as substrate — blue (under development)
as substrate — blue
selenide (ZnSe) — blue
(C) — ultraviolet
nitride (AlN), aluminium gallium nitride (AlGaN), aluminium
gallium indium nitride
(AlGaInN) — near to far ultraviolet (down to 210 nm)
this wide variety of colors, arrays of multicolor LEDs can be
designed to produce unconventional color patterns.
LEDs are based on the wide band gap semiconductors GaN
and InGaN (indium
They can be added to existing red and green LEDs to produce the
impression of white light, though white LEDs today rarely use this
first blue LEDs were made in 1971 by Jacques Pankove (inventor of the
gallium nitride LED) at RCA
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
at Nichia Corporation.
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.
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.
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.
down to 210 nm were obtained in laboratories using aluminium nitride.
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.
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
usually made of cerium-doped yttrium aluminum garnet (Ce3+: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 Ce3+: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.
pale yellow emission of the Ce3+: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
patented conformal coating process addresses the issue of varying
phosphor thickness, giving the white LEDs a more consistent spectrum
of white light.
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 Ce3+:YAG
phosphor which extends from around 500 to 700 nanometers.
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
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.
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.
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.
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.
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.
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:
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.
typical LEDs are designed to operate with no more than 30–60
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.
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.)
September 2003, a new type of blue LED was demonstrated by the
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.
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).
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
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
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
very little current flows, and no light is emitted. Some LEDs can be
operated on an alternating
voltage, but they will only light with positive voltage, causing the
LED to turn on and off at the frequency of the AC supply.
the only 100% accurate way to determine the polarity of an LED is to
examine its datasheet, these methods are usually reliable:
reliable methods of determining polarity are:
it is not an officially reliable method, it is almost universally
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 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.
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.
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.
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.
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
LED units contain two diodes, one in each direction (that is, two
diodes in inverse
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
configuration. These can be driven to different colors without
reversing the polarity, however, more than two electrodes (leads) are
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.
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).
LEDs are rated at 5
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
produce more light per watt than do incandescent bulbs; this is
useful in battery powered or energy-saving devices.
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.
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.
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.
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
being solid state components, are difficult to damage with external
shock. Fluorescent and incandescent bulbs are easily broken if
dropped on the ground.
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
tubes typically are rated at about 30,000
and incandescent light bulbs at 1,000–2,000
mostly fail by dimming over time, rather than the abrupt burn-out of
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.
can be very small and are easily populated onto printed circuit
do not contain mercury,
while compact fluorescent lamps do.
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
such as those found on blinkies (not shown).
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
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
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.
must be supplied with the correct current. This can involve shunt
resistors or regulated power supplies.
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.
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.
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
are 3 main types of LEDs: miniature LEDs, alphanumeric LEDs, and
are mostly single die LEDs used as indicators, and come in various
sizes are also available, but less common.
are 3 main categories of miniature single die LEDs:
current – typically rated for 2mA at around 2v (apx 4mW
– 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.
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.
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.
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.
are miniature LEDs incorporating a series resistor, and may be
connected directly to 5v or 12v.
miniature LEDs flash when connected to 5v or 12v. Used as attention
seeking indicators where it is desired to avoid the complexity of
displays are available in 7
and starburst format. 7
handle all numbers and a limited set of letters. Starburst displays
can display all letters.
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.
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.
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.
sources for machine vision systems.
LED destination display on a bus. Note how the camera has had
difficulty catching all the LEDs.
calculator LED display.
and lanterns that utilise white LEDs are becoming increasingly
popular due to their durability and longer battery life.
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.
lights on an Audi S6
of these applications are further elaborated upon in the following
scale video displays
indicators on all sorts of equipment
lights and signals
source for machine vision systems, requiring bright, focused,
homogeneous and possibly strobed illumination.
and Bicycle lights
and recreational sporting goods, such as the Flashflight
including some mechanically powered models.
Push Button Lighting
lightweight message displays at airports and railway stations and as
destination displays for trains, buses, trams and ferries.
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.
yellow, green, and blue LEDs can be used for model railroading
such as for TVs and VCRs, often use infrared LEDs.
bar uses infrared LEDs.
optical fiber and Free
Space Optics communications.
dot matrix arrangements for displaying messages.
as a more expensive but longer lasting and reusable alternative to
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
sensors, for example in optical computer mice
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
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%.
stage lighting equipment is being developed with LED sources in
primary red-green-blue arrangements.
a photonic textile
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
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
phototherapy for acne using blue or red LEDs has been proven to
significantly reduce acne over a 3 month period.[citation
a medium quality voltage
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.
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.
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.
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
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.
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.
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
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.
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.
CR dropper followed by full wave rectification is the usual ballast
with series-parallel LED clusters.
single series string minimises dropper losses, while parallelled
strings increase reliability. In practice usually 3 strings or more
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
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.
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
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.
run 1 or more lighting LEDs on a low voltage battery. These usually
use a resistor ballast.
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.
an LED can be run from a single cell by use of a constant current
switched mode invertor. The extra expense makes this option
1,500 foot long LED display on the Fremont
is currently the largest in the world.
are two types of LED panels: conventional, using discrete LEDs, and
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.
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.
panels allow for smaller sets of interchangeable LEDs to be one large
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).
large LED screen in Razorback
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.)
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
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.
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.
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.
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.
with a LASER(Light
amplified by stimulated emmision of radiation)
is an electronic-optical device that produces coherent
The term “laser” is an acronym
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
laser consists of a gain
inside an optical
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.
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.
first working laser was demonstrated in May 1960 by Theodore
at Hughes Research Laboratories. Recently, lasers have become a
multi-billion dollar industry. The most widespread use of lasers is
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
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.
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.
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.
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).
also: Laser science and Laser construction
laser is composed of an active
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
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
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
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.
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
optical cavity, a type of cavity
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
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
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
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
spreads to about 1.6 kilometers (1 mile) diameter if shone from the
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.
output of a laser may be a continuous constant-amplitude output
(known as CW
or pulsed, by using the techniques of Q-switching, modelocking, or
gain-switching. In pulsed operation, much higher peak powers can be
types of lasers, such as dye
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
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
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
etc. For example, a source of atoms in a coherent state can be called
an atom laser.
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
the laser was often called the optical
This usage has since become uncommon, and as of 1998 even Bell Labs
uses the term laser.
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.
Rudolph W. Landenburg
confirmed the existence of stimulated emission and negative
(USSR) predicted the use of stimulated emission to amplify “short”
E. Lamb and R. C. Retherford
found apparent stimulated emission in hydrogen spectra and made the
first demonstration of stimulated emission.
(Nobel Prize for Physics 1966) proposed the method of optical
pumping, which was experimentally confirmed by Brossel, Kastler and
Winter two years later.
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
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
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.
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
1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell
Labs, began a serious study of the infrared laser. As ideas were
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).
first page of Gordon Gould’s laser notebook in which he coined the
acronym LASER and described the essential elements for constructing
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
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.
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.
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.
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
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
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,
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
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.
concept of the semiconductor laser diode was proposed by Basov and
Javan. The first laser
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
temperatures (77 K).
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
showing the history of maximum laser pulse intensity throughout the
past 40 years.
the early period of laser history, laser research has produced a
variety of improved and specialized laser types, optimized for
different performance goals, including:
average output power
peak output power
output pulse duration
this research continues to this day.
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.
1985 at the University of Rochester’s Laboratory for Laser Energetics
a breakthrough in creating ultrashort-pulse, very high-intensity
laser pulses became available using a technique called chirped pulse
amplification, or CPA, discovered by Gérard Mourou. These high
intensity pulses can produce filament
in the atmosphere.
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.
(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.
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.
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
as the average power of the laser (were it running in CW mode) is
packed into a shorter time frame.
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
(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
modelocked laser is a most versatile tool for researching processes
happening at extremely fast time scales (femtosecond
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.
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.
lasers using many gases have been built and used for many purposes.
They are one of the oldest types of laser.
(HeNe) emits at a variety of wavelengths and units operating at 633
nm are very common in education because of its low cost.
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 CO2
laser is over 10%.
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.
discharge in gas at atmospheric
pressure (TEA) laser is an inexpensive gas laser producing UV Light
at 337.1 nm.
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
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.
lasers are powered by a chemical reaction involving an excited
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 F2
(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]).
50 W FASOR, based on a Nd:YAG laser, used at the Starfire Optical
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.
is a common dopant in various solid state laser crystals, including
yttrium orthovanadate (Nd:YVO4),
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.
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.
sapphire (Ti:sapphire) produces a highly tunable infrared laser,
commonly used for spectroscopy as well as the most common ultrashort
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 (dn/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.
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 106
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.
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.
The efficient use of pump in fiber laser can be achieved at the
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
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.
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
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
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.
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.
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
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
September 2007, the BBC News reported that there was speculation
about the possibility of using positronium annihilation to drive a
very powerful gamma
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.
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.
lasers were invented in 1960, they were called “a solution
looking for a problem”.[citation
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.
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
player was the first laser-equipped device to become truly common in
consumers’ homes, beginning in 1982, followed shortly by laser
of the other applications include:
Bleedless surgery, laser healing, survical treatment, kidney stone
treatment, eye treatment, dentistry
Cutting, welding, material heat treatment, marking parts
Marking targets, guiding munitions, missile defence, electro-optical
countermeasures (EOCM), RADAR alternative
Spectroscopy, laser ablation, Laser annealing, laser scattering,
laser interferometry, LIDAR
development/Commercial: Laser Printers, CDs, Barcode scanners, laser
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
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
(constant output) lasers, as are used in communication, or cutting.
Output power is always less than the input power needed to generate
peak power required for some uses:
mW – CD-ROM drive
mW – DVD player
mW – CD-R drive
mW – output power of Sony SLD253VL red laser diode, used in consumer
48-52 speed CD-R burner.
mW – output power of Sony SLD1332V red laser diode, used in consumer
W – green laser in current Holographic
Versatile Disc prototype development.
to 3000 W (peak output 1.5 kW) – typical sealed CO2
lasers used in industrial laser cutting.
kW – Output power expected to be achieved by “a single 1 cm
diode laser bar”
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.
petawatts (PW) – world’s most powerful laser (claimed on 23 May 1996
by Lawrence Livermore Laboratory).
: My teacher Aplication notes, LED.
this for such Information above, producer of LED :
LED are still popular.
complete data sheet in PDF versions.
useful. Regards : Yonni. M