Basics of Optical Fiber

7/30/2019 Basics of Optical Fiber

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What are Fiber Optics?

You hear about fiber-optic cables whenever people talk about thetelephone system, thecable

TV systemor the Internet. Fiber-optic lines are strands of OpticallyPureGlass as thin as ahuman hair that carry digital information over long distances. They are also used in medicalimaging and mechanical engineering inspection.

Parts of a single optical fiberIn this article, we will show youhow these tiny strands of glasstransmit light and the fascinatingway that these strands aremade.

Fiber optics (optical fibers) are

long, thin strands of very pureglass about the diameter of ahuman hair. They are arrangedin bundles called optical cablesand used to transmit lightsignals over long distances.

If you look closely at a singleoptical fiber, you will see that ithas the following parts:

Core - Thin glass center of the fiber where the light travels

Cladding - Outer optical material surrounding the core that reflects the light back intothe core

Buffer coating - Plastic coating that protects the fiber from damage and moistureHundreds or thousands of these optical fibers are arranged in bundles in optical cables. Thebundles are protected by the cable's outer covering, called a jacket.Optical fibers come in two types:

Single-mode fibersMulti-mode fibers

SeeTpub.com: Mode Theoryfor a good explanation.Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) andtransmit infraredlaserlight (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers have

larger cores (about 2.5 x 10

-3

inches or 62.5 microns in diameter) and transmit infrared light(wavelength = 850 to 1,300 nm) fromlight-emitting diodes(LEDs).

Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.

How Does an Optical Fiber Transmit Light?Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straightdown the hallway -- light travels in straight lines, so it is no problem. What if the hallway has a bend in it?You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway isvery winding with multiple bends? You might line the walls with mirrors and angle the beam so that itbounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.
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Diagram of total internal reflection in an optical fiber The light in a fiber-optic cable travels throughthe core (hallway) by constantly bouncing from

the Cladding (Mirror-Lined walls), a principlecalled total internal reflection. Because thecladding does not absorb any light from the

core, the light wave can travel great distances.However, some of the light signal degradeswithin the fiber, mostly due to impurities in theglass. The extent that the signal degradesdepends on the Purity of the glass and theWavelength of the transmitted light (for example,850 nm = 60 to 75 percent/km; 1,300 nm = 50 to

60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much lesssignal degradation -- less than 10 percent/km at 1,550 nm.

A Fiber-Optic Relay System

To understand how optical fibers are used in communications systems, let's look at an example from aWorld War II movie or documentary where two naval ships in a fleet need to communicate with eachother while maintaining radio silence or on stormy seas. One ship pulls up alongside the other. Thecaptain of one ship sends a message to a sailor on deck. The sailor translates the message into Morsecode (dots and dashes) and uses a signal light (floodlight with a venetian blind type shutter on it) to sendthe message to the other ship. A sailor on the deck of the other ship sees the Morse code message,

decodes it into English and sends the message up to the captain.

Now, imagine doing this when the ships are on either side of the ocean separated by thousands of milesand you have a fiber-optic communication system in place between the two ships. Fiber-optic relaysystems consist of the following:

Transmitter - Produces and encodes the light signalsOptical fiber - Conducts the light signals over a distanceOptical regenerator - May be necessary to boost the light signal (for long distances)Optical receiver - Receives and decodes the light signals

TransmitterThe transmitter is like the sailor on the deck of the sending ship. It Codes the received Electrical Signal

and directs the Optical device to turn the Light "on" and "off" in the correct sequence, therebygenerating a light signal.

The transmitter is physically close to the optical fiber and may even have a lens to focus the light into thefiber. Lasers have more power than LEDs, but vary more with changes in temperature and are moreexpensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm(infrared, non-visible portions of the spectrum).

Optical RegeneratorAs mentioned above, some signal loss occurs when the light is transmitted through the fiber, especiallyover long distances (more than a half mile, or about 1 km) such as with undersea cables. Therefore, one

or more optical regenerators is spliced along the cable to boost the degraded light signals.

An optical regenerator consists of Optical Fibers with a Special Coating (Doping). The doped portion is"Pumped" with a laser. When the degraded signal comes into the doped coating, the Energy from the

Laserallows the Doped Molecules to become LASER themselves. The doped molecules then emit anew, stronger light signal with the same characteristics as the incoming weak light signal. Basically, theregenerator is a laser amplifier for the incoming signal.

Optical ReceiverThe optical receiver is like the sailor on the deck of the receiving ship. It takes the Incoming Digital Light

signals, decodes them and sends the Electrical Signal to the other user's computer, TV ortelephone(receiving ship's captain). The receiver uses a Photocell orPhotodiode to detect the light.
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Advantages of Fiber Optics

Why are fiber-optic systems revolutionizing telecommunications? Compared to conventionalmetal wire (copper wire), optical fibers are:

Less expensive - Several miles of optical cable can be made cheaper than equivalentlengths of copper wire. This saves your provider (cable TV, Internet) and you money.

Thinner - Optical fibers can be drawn to smaller diameters than copper wire.Higher carrying capacity - Because optical fibers are thinner than copper wires, morefibers can be bundled into a given-diameter cable than copper wires. This allows morephone lines to go over the same cable or more channels to come through the cable intoyour cable TV box.Less signal degradation - The loss of signal in optical fiber is less than in copper wire.Light signals - Unlike electrical signals in copper wires, light signals from one fiber donot interfere with those of other fibers in the same cable. This means clearer phoneconversations or TV reception.Low power - Because signals in optical fibers degrade less, lower-power transmitterscan be used instead of the high-voltage electrical transmitters needed for copper wires.Again, this saves your provider and you money.Digital signals - Optical fibers are ideally suited for carrying digital information, which isespecially useful in computer networks.Non-flammable - Because no electricity is passed through optical fibers, there is no firehazard.Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber-optic cables take up less space in the ground.Flexible - Because fiber optics are so flexible and can transmit and receive light, theyare used in many flexibledigital camerasfor the following purposes:

Medical imaging - in bronchoscopes, endoscopes, laparoscopesMechanical imaging - inspecting mechanical welds in pipes and engines (inairplanes,rockets,space shuttles,cars)

Plumbing - to inspectsewer linesBecause of these advantages, you see fiber optics in many industries, most notablytelecommunications and computer networks. For example, if you telephone Europe from theUnited States (or vice versa) and the signal is bounced off a communicationssatellite, you oftenhear an echo on the line. But with transatlantic fiber-optic cables, you have a direct connectionwith no echoes.

How Are Optical Fibers Made?

Now that we know how fiber-optic systems work and why they are useful -- how do they makethem? Optical fibers are made of extremely pure optical glass. We think of a glass window astransparent, but the thicker the glass gets, the less transparent it becomes due to impurities in

the glass. However, the glass in an optical fiber has far fewer impurities than window-paneglass. One company's description of the quality of glass is as follows: If you were on top of anocean that is miles of solid core optical fiber glass, you could see the bottom clearly.

Making optical fibers requires the following steps:

1. Making a Preform Flass Cylinder2. Drawing the fibers from the preform3. Testing the fibers

Making the Preform Blank

The glass for the preform is made by a process called modified chemical vapor deposition

(MCVD).
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In MCVD, oxygen is bubbledthrough solutions of silicon chloride(SiCl4), germanium chloride (GeCl4)and/or other chemicals. The precisemixture governs the various physicaland optical properties (index ofrefraction, coefficient of expansion,melting point, etc.). The gas vaporsare then conducted to the inside of asynthetic silica or quartz tube(cladding) in a special lathe. As thelathe turns, a torch is moved up anddown the outside of the tube. Theextreme heat from the torch causestwo things to happen:

The silicon and germanium react with oxygen, formingsilicon dioxide (SiO2) and germanium dioxide (GeO2).

The silicon dioxide and germanium dioxide deposit onthe inside of the tube and fuse together to form Pure Glass.The lathe turns continuously to make an even coating andconsistent blank. The purity of the glass is maintained by usingcorrosion-resistant plastic in the gas delivery system (valveblocks, pipes, seals) and by precisely controlling the flow andcomposition of the mixture. The process of making the PreformBlank is highly automated and takes several hours. After thepreform blank cools, it is tested for quality control (index of

refraction).

Drawing Fibers from the Preform BlankOnce the preform blank has been tested, it gets loaded intoa fiber drawing tower.

The blank gets lowered into a graphite furnace (3,452 to3,992 degrees Fahrenheit or 1,900 to 2,200 degreesCelsius) and the tip gets melted until a molten glob fallsdown bygravity. As it drops, it cools and forms a thread.

The operator threads the strand through a series of coatingcups (Buffer Coatings) andultraviolet light curing ovens onto a

tractor-controlled spool. The tractormechanism slowly pulls the fiberfrom the heated preform blank andis precisely controlled by using alaser micrometer to measure thediameter of the fiber and feed theinformation back to the tractormechanism. Fibers are pulled fromthe blank at a rate of 33 to 66 ft/s(10 to 20 m/s) and the finishedproduct is wound onto the spool. It is

not uncommon for spools to contain more than 1.4 miles(2.2 km) of optical fiber.

Image courtesyFiberCore Ltd.MCVD process for making the Preform Blank

Photo courtesy Fibercore Ltd.Lathe used in preparing

the preform blank

O/Fdrawing tower from a preform blank
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Testing the Finished Optical Fiber

The finished optical fiber is tested for the following:

Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniformTensile strength - Must withstand 100,000 lb/in2 or more

Refractive index profile - Determine numerical aperture as well as screen for opticaldefectsAttenuation - Determine the extent that light signals of various wavelengths degradeover distanceInformation carrying capacity (bandwidth) - Number of signals that can be carried atone time (multi-mode fibers)Chromatic dispersion - Spread of various wavelengths of light through the core(important for bandwidth)Operating temperature/humidity rangeTemperature dependence of attenuationAbility to conduct light underwater - Important for undersea cables

Once the fibers have passed the quality control, they are soldto telephone companies, cable companies and networkproviders. Many companies are currently replacing their oldcopper-wire-based systems with new fiber-optic-basedsystems to improve speed, capacity and clarity.

Physics of Total Internal Reflection

When light passes from a Medium with oneindex of refraction(m1) to anotherMedium witha lower index of refraction (m2), it bends orrefracts away from an imaginary lineperpendicular to the surface (normal line). Asthe angle of the beam through m1 becomesgreater with respect to the normal line, therefracted light through m2 bends further away

from the line.

At one particular angle (critical angle), therefracted light will not go into m2, but insteadwill travel along the surface between the twomedia (sine [critical angle] = n2 / n1 where n1and n2 are the indices of refraction [n1 isgreater than n2]). If the beam through m1 is greater than the critical angle, then the refractedbeam will be reflected entirely back into m1 (total internal reflection), even though m2 may betransparent!

In physics, the critical angle is described with respect to the normal line. In fiber optics, thecritical angle is described with respect to the parallel axis running down the middle of the fiber.

Therefore, the fiber-Optic critical angle = (90* degrees - Physics critical angle).

Finished spool of optical fiber

Photo courtesy Corning

Total internal reflection in an optical fiber
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In an Optical Fiber, the light travels through the core (m1, Higher Index of Refraction) byconstantly reflecting from the cladding (m2, lower index of refraction) because the angle of thelight is always greater than the critical angle. Light reflects from the cladding no matter whatangle the fiber itself gets bent at, even if it's a full circle!

Because the cladding does not absorb any light from the core, the light wave can travel greatdistances. However, some of the light signal degrades within the fiber, mostly due to impuritiesin the glass. The extent that the signal degrades depends upon the purity of the glass and thewavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers showmuch less signal degradation -- less than 10 percent/km at 1,550 nm.

Polarization(alsopolarisation)

http://en.wikipedia.org/wiki/Polarization_(waves)

is a property of certain types of waves that describes the orientation of their oscillations.Electromagnetic waves, such as light, and gravitational waves exhibit polarization; acousticwaves (sound waves) in a gas or liquid do not have polarization because the direction ofvibration and direction of propagation are the same.

By convention, the polarization of light is described by specifying the orientation of the wave'selectric field at a point in space over one period of the oscillation. When light travels in freespace, in most cases it propagates as atransverse wavethe polarization is perpendicular tothe wave's direction of travel. In this case, the electric field may be oriented in a single direction(linear polarization), or it may rotate as the wave travels (circularorelliptical polarization). In thelatter cases, the oscillations can rotate either towards the right or towards the left in the directionof travel. Depending on which rotation is present in a given wave it is called the wave'schiralityor handedness. In general the polarization of an electromagnetic (EM) wave is a complex issue.

For instance in a waveguide such as an optical fiber, or for radially polarized beams in freespace,[1] the description of the wave's polarization is more complicated, as the fields can havelongitudinal as well as transverse components. Such EM waves are eitherTM or hybrid modes.

Forlongitudinal wavessuch assound wavesinfluids, the direction of oscillation is by definitionalong the direction of travel, so there is no polarization.

Linear Circular Elliptical
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Polarization state : The shape traced out in a fixed plane by the electric vector as such aplane wave passes over it (a Lissajous figure) is a description of the polarization state. Thefollowing figures show some examples of the evolution of the electric field vector (black), withtime(the vertical axes), at a particular point in space, along with its xand ycomponents (red/leftand blue/right), and the path traced by the tip of the vector in the plane (purple): The sameevolution would occur when looking at the electric field at a particular time while evolving thepoint in space, along the direction opposite to propagation.

Wave PlateorRetarder

is anopticaldevice (simply abirefringentcrystalwith a carefully chosen orientation andthickness) that alters thepolarizationstate of alightwave travelling through it.

http://optics.byu.edu/animation/polarwav.mov

http://en.wikipedia.org/wiki/Wave_plate

A half-wave plate. Linearly polarized light entering awave plate can be resolved into two waves, parallel(shown as green) and perpendicular (blue) to theoptical axis of the wave plate. In the plate, theparallel wave propagates slightly slower than theperpendicular one. At the far side of the plate, theparallel wave is exactly half of a wavelength delayed relative to the perpendicular wave, and the resultingcombination (red) is orthogonally polarized compared to its entrance state.

A wave plate works by shifting thephasebetween two perpendicular polarization components ofthe light wave. A typical wave plate is simply a birefringent crystal with a carefully chosenorientation and thickness. The crystal is cut so that the extraordinary axis or "optic axis" isparallel to the surfaces of the plate. Light polarized along this axis travels through the crystal ata different speed than light with the perpendicular polarization, creating a phase difference.When the extraordinary index is smaller than the ordinary index, as incalcite, the extraordinaryaxis is called the "fast axis" and the perpendicular direction in the plane of the surfaces is calledthe "slow axis".

Depending on the thickness of the crystal, light with polarization components along both axeswill emerge in a different polarization state. The wave plate is characterized by the amount ofrelative phase, , that it imparts on the two components, which is related to the birefringence nand the thickness L of the crystal by the formula

where 0 is the vacuum wavelength of the light. For instance a quarter-wave plate creates aquarter-wavelength phase shift and can change linearly polarized light to circular and viceversa. This is done by adjusting the plane of the incident light so that it makes 45 angle with thefast axis. This gives ordinary and extraordinary waves with equal amplitude.

The other common type of wave plate is a half-wave plate, which retards one polarization byhalf a wavelength, or 180 degrees. This type of wave plate changes the polarization direction oflinear polarized light.
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Magneto-Optic or Faraday effect

Inphysics, the Faraday effect orFaraday rotation is aMagneto-opticalphenomenon, that is,an interaction between light and a magnetic field in a medium. The Faraday effect causes arotation of the plane of polarization which is linearly proportional to the component of themagnetic field in the direction of propagation.

The Faraday effect causes left and right circularlypolarized waves to propagate at slightly differentspeeds, a property known as circular birefringence.

The Faraday effect has a few applications in measuring

instruments. For instance, the Faraday effect has beenused to measure optical rotatory power and for remotesensing ofmagnetic fields.

Polarization mechanism due to the Faraday effect. The field linesare usually closed through apermanent magnetaround the rotator.

http://en.wikipedia.org/wiki/Faraday_rotator

The plane of linearly polarized light is rotated when amagnetic fieldis appliedparallelto thepropagationdirection. The empiricalangle of rotationis given by:

Where is the angle of rotation (inradians)

Bis the magnetic flux density in the direction of propagation (inteslas).dis the length of the path (in metres) where the light and magnetic field interact.

Vis theVerdet constantfor the material. This empirical proportionality constant (in units ofradians per tesla per metre, rad/(Tm)) varies with wavelength and temperature and istabulated for various materials.

A positive Verdet constant corresponds to L-rotation (anticlockwise) when the direction ofpropagation is parallel to the magnetic field and to R-rotation (clockwise) when the direction ofpropagation is anti-parallel. Thus, if a ray of light is passed through a material and reflectedback through it, the rotation doubles. .. i.e. Faraday rotation is an example ofnon-reciprocaloptical propagation.

The geometry of non-reciprocal propagation may at first appear paradoxical. In an opticallyactive medium, the polarization direction twists in the same sense (e.g. like a right-handedscrew) during the forward and backward passes, whereas in a Faraday medium, because thelight reverses its propagation direction with respect to the magnetic field, the helicity of thepropagation also reverses. But because the propagation axis has also reversed, this reversal ofhelicity is just what is needed cause the back-reflected light to have different polarization fromthe incident light. If the Faraday medium is of such thickness as to cause a 45 degree rotationon the way in, the back-reflected beam will have polarization perpendicular to the incidentbeam, allowing it to be cleanly blocked. This allows Faraday Rotators to be used to constructdevices such asoptical isolatorsto prevent undesired back propagation of light from disruptingor damaging an optical system.
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An Optical Circulator is a specialfiber-opticcomponent that can be usedto separate optical signals that travel in opposite directions in an opticalfiber, analogous to the operation of an electronic circulator. An opticalcirculator is a three-port device designed such that lightentering any portexits from the next. This means that if light enters port 1 it is emitted fromport 2, but if some of the emitted light is reflected back to the circulator, itdoes not come out of port 1, but instead exits from port 3. Circulators canbe used to achieve bi-directional transmission over a single fiber.Because of its highisolationof the input and reflected optical powers and its lowinsertion loss,optical circulators are widely used in advancedcommunication systemsandfiber-optic sensorapplications.

http://en.wikipedia.org/wiki/Magneto-optic_effect

Electro-Optic Effect

The phenomenon that the refractive index of a material can be modifiedwith an electric field

The electro-optic effect (or electrooptic effect) is the modification of the refractive index of amedium, caused by an electric field. Only non-centrosymmetric materials (mostly crystals)exhibit the linearelectro-optic effect, also called thePockels effect, where the refractive indexchange is proportional to the electric field strength (see the article onPockels effect for moredetails). Materials exhibiting the Pockels effect are called electro-optic materials.

All Centro-Symmetric media exhibit only the Kerrelectro-optic effect, where the refractive indexchange is proportional to the squareof the electric field strength, and is typically much weakerthan for the linear effect.

The linear electro-optic effect is exploited in Pockels cells, which can be part of electro-optic

modulators, and forelectro-optic sampling.

Pockels Cells

A Pockels cell is a device consisting of anelectro-opticcrystal (with some electrodes attachedto it) through which a light beam can propagate.Only certain crystalline solids show the Pockelseffect, as it requires lack of inversion symmetry.such as lithium niobate or galliumarsenide and in other non-centro-symmetric media such as electric-field poled polymers orglasses.

The phase delay in the crystal (Pockels effect) can be modulated by applying a variableelectric voltage. The Pockels cell thus acts as a voltage-controlledwaveplate. Pockels cells arethe basic components ofelectro-optic modulators, used e.g. forQ switchinglasers.

Pockels cells can have two different geometries concerning the direction of the applied electricfield: Longitudinal deviceshave the electric field in the direction of the light beam, which passes

through holes in the electrodes. Large apertures can easily be realized, as the required drivevoltage is basically independent of the aperture. The electrodes can be metallic rings(Figure 1, left) or transparent layers on the end faces (right) with metallic contacts.

Figure 1: Pockels cells with longitudinal electric field.The electrodes are either rings on the end faces (left

side) or on the outer face (right side).
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Transverse devices have the electric field perpendicular to the light beam. The field isapplied through electrodes at the sides of the crystal. For small apertures, they can have

lower switching voltages.Figure 2: Pockels cells with transverseelectric field. On the left is a bulk modulatorand on the right a waveguide modulator.

For a Pockels cell with longitudinalelectric field, the crystal length does not matter, since e.g. ashorter length also increases the electric field strength for a given voltage. Larger apertures arepossible without increasing the half-wave voltage.

Spun elliptically birefringent fibre has been fabricated by spinning the preform of a highly linearlybirefringent photonic crystal fibre (PCF) during the drawing process. The resulting Spun HighlyBirefringent (SHi-Bi) PCF offers sensitivity to magnetic fields for current measurements withgreatly reduced temperature dependence in comparison with conventional spun stressbirefringence fibres. The ellipticity of the birefringence has been measured and temperature

independence has been demonstrated.http://iopscience.iop.org/0957-0233/18/10/S04;jsessionid=C32C7BC99CB018F6822F36FB13BBF751.c2
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http://science.howstuffworks.com/laser5.htm

The Basics of an Atom

There are only about 100 different kinds of

atomsin the entire universe. Everything we seeis made up of these 100 atoms in an unlimitednumber of combinations. How these atoms arearranged and bonded together determineswhether the atoms make up a cup of water, apiece of metal, or the fizz that comes out of yoursoda can!

Atoms are constantly in motion. Theycontinuously vibrate, move and rotate. Even theatoms that make up the chairs that we sit in aremoving around. Solids are actually in motion!

Atoms can be in different states of excitation.In other words, they can have differentenergies. If we apply a lot of energy to an atom,it can leave what is called the ground-state energy level and go to an excited level. The levelof excitation depends on the amount of energy that is applied to the atom via heat, light, orelectricity.

Here is a classic interpretation of what the atom looks like:

This simple atom consists of a nucleus (containing the protons and neutrons) and an electroncloud. Its helpful to think of the electrons in this cloud circling the nucleus in many differentorbits.

Absorbing Energy

Consider the illustration from the previous page. Although more modern views of the atom donot depict discrete orbits for the electrons, it can be useful to think of these orbits as thedifferent energy levels of the atom. In other words, if we apply some heat to an atom, we mightexpect that some of the electrons in the lower-energy orbitals would transition to higher-energyorbitals farther away from the nucleus.

This is a highly simplified view of things, but itactually reflects the core idea of howatomsworkin terms of lasers.

Once an electron moves to a higher-energyorbit, it eventually wants to return to the groundstate. When it does, it releases its energy as aphoton -- a particle oflight. You see atomsreleasing energy as photons all the time. Forexample, when the heating element in atoasterturns bright red, the red color is caused byatoms, excited by heat, releasing red photons.When you see a picture on aTV screen, whatyou are seeing is phosphor atoms, excited by

high-speed electrons, emitting different colors of light. Anything that produces light --fluorescentlights,gas lanterns,incandescent bulbs-- does it through the action of electrons changing orbits

and releasing photons.

An atom, in the simplest model,consists of a nucleus and orbiting electrons.

Absorption of energy: An atom absorbs energy inthe form of heat, light, or electricity. Electronsmay move from a lower-energy orbit to a higher-energy orbit.
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The Laser/Atom ConnectionA laser is a device that controls the way that energized atoms release photons. "Laser" is an acronym forlightamplification by stimulated emission of radiation, which describes very succinctly how a laser works.

Although there are many types of lasers, all have certain essential features. In a laser, the lasing medium is pumpedto get theatomsinto an excited state. Typically, very intense flashes of l ight or electrical discharges pump the lasing

medium and create a large collection of excited-state atoms (atoms with higher-energy electrons). It is necessary tohave a large collection of atoms in the excited state for the laser to work efficiently. In general, the atoms are excitedto a level that is two or three levels above the ground state. This increases the degree ofpopulation inversion. Thepopulation inversion is the number of atoms in the excited state versus the number in ground state.

Once the lasing medium is pumped, it contains a collection of atoms with some electrons sitting in excited levels. Theexcited electrons have energies greater than the more relaxed electrons. Just as the electron absorbed some amountof energy to reach this excited level, it can also release this energy. As the figure below illustrates, the electron cansimply relax, and in turn rid itself of some energy. This emitted energy comes in the form ofphotons (light energy).The photon emitted has a very specificwavelength(color) that depends on the state of the electron's energy whenthe photon is released. Two identical atoms with electrons in identical states will release photons with identicalwavelengths.
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