The Basics of Fiber The Basics of Fiber OpticsOptics

Ch 2Ch 2

Fiber Optics Technician’s Fiber Optics Technician’s Manual, 3Manual, 3rdrd. Ed. Ed

Jim HayesJim Hayes

Optical FiberOptical Fiber

Fiber v. CopperFiber v. Copper

Optical fiber transmits light pulsesOptical fiber transmits light pulses• Can be used for analog or digital Can be used for analog or digital

transmissiontransmission• Voice, computer data, video, etc.Voice, computer data, video, etc.

Copper wires (or other metals) can Copper wires (or other metals) can carry the same types of signals with carry the same types of signals with electrical pulseselectrical pulses

Advantages of FiberAdvantages of Fiber

Fiber has these advantages Fiber has these advantages compared with metal wirescompared with metal wires• Bandwidth – more data per secondBandwidth – more data per second• Longer distanceLonger distance• FasterFaster• Special applications like medical Special applications like medical

imaging and quantum key distribution imaging and quantum key distribution are only possible with fiber because are only possible with fiber because they use light directlythey use light directly

Elements of a Fiber Data LinkElements of a Fiber Data Link

Transmitter emits light pulses (LED Transmitter emits light pulses (LED or Laser)or Laser)

Connectors and Cables passively Connectors and Cables passively carry the pulsescarry the pulses

Receiver detects the light pulsesReceiver detects the light pulses

Transmitter ReceiverCable

RepeatersRepeaters

For long links, repeaters are needed For long links, repeaters are needed to compensate for signal lossto compensate for signal loss

FiberRepeaterRepeater Repeater

Fiber FiberFiber

Optical FiberOptical Fiber CoreCore

• Glass or plastic with a higher Glass or plastic with a higher index of refraction than the index of refraction than the claddingcladding

• Carries the signalCarries the signal

CladdingCladding• Glass or plastic with a lower Glass or plastic with a lower

index of refraction than the coreindex of refraction than the core

BufferBuffer• Protects the fiber from damage Protects the fiber from damage

and moistureand moisture

JacketJacket• Holds one or more fibers in a Holds one or more fibers in a

cablecable

Singlemode FiberSinglemode Fiber Singlemode fiber has a core diameter Singlemode fiber has a core diameter

of 8 to 9 microns, which only allows of 8 to 9 microns, which only allows one light path or one light path or modemode• Images from arcelect.com (Link Ch 2a)Images from arcelect.com (Link Ch 2a)

Index of refraction

Multimode Step-Index FiberMultimode Step-Index Fiber Multimode fiber has a core diameter Multimode fiber has a core diameter

of 50 or 62.5 microns (sometimes of 50 or 62.5 microns (sometimes even larger)even larger)• Allows several light paths or Allows several light paths or modesmodes• This causes This causes modal dispersionmodal dispersion – some modes – some modes

take longer to pass through the fiber than others take longer to pass through the fiber than others because they travel a longer distancebecause they travel a longer distance

• See animation at link Ch 2fSee animation at link Ch 2fIndex of refraction

Multimode Graded-Index FiberMultimode Graded-Index Fiber The index of refraction gradually The index of refraction gradually

changes across the corechanges across the core• Modes that travel further also move fasterModes that travel further also move faster• This reduces This reduces modal dispersion modal dispersion so the so the

bandwidth is greatly increasedbandwidth is greatly increased

Index of refraction

Step-index and Graded-indexStep-index and Graded-index

Step index multimode was developed Step index multimode was developed first, but rare today because it has a first, but rare today because it has a low bandwidth (50 MHz-km)low bandwidth (50 MHz-km)

It has been replaced by graded-index It has been replaced by graded-index multimode with a bandwidth up to 2 multimode with a bandwidth up to 2 GHz-kmGHz-km

Plastic Optical FiberPlastic Optical Fiber

Large core (1 mm) step-index Large core (1 mm) step-index multimode fibermultimode fiber

Easy to cut and work with, but high Easy to cut and work with, but high attenuation (1 dB / meter) makes it attenuation (1 dB / meter) makes it useless for long distancesuseless for long distances

Sources and WavelengthsSources and Wavelengths

Multimode fiber is used withMultimode fiber is used with• LED sources at wavelengths of 850 and LED sources at wavelengths of 850 and

1300 nm for slower local area networks1300 nm for slower local area networks• Lasers at 850 and 1310 nm for networks Lasers at 850 and 1310 nm for networks

running at gigabits per second or morerunning at gigabits per second or more

Sources and WavelengthsSources and Wavelengths

Singlemode fiber is used withSinglemode fiber is used with• Laser sources at 1300 and 1550 nmLaser sources at 1300 and 1550 nm• Bandwidth is extremely high, around Bandwidth is extremely high, around

100 THz-km100 THz-km

Fiber Optic SpecificationsFiber Optic Specifications

AttenuationAttenuation• Loss of signal, measured in dBLoss of signal, measured in dB

DispersionDispersion• Blurring of a signal, affects bandwidthBlurring of a signal, affects bandwidth

BandwidthBandwidth• The number of bits per second that can be The number of bits per second that can be

sent through a data linksent through a data link Numerical ApertureNumerical Aperture

• Measures the largest angle of light that can Measures the largest angle of light that can be accepted into the corebe accepted into the core

Attenuation and DispersionAttenuation and Dispersion

See See animation animation at link Ch at link Ch 2e2e

Measuring BandwidthMeasuring Bandwidth

The The bandwidth-distance productbandwidth-distance product in units in units of of MHzMHz×km shows how fast data can be ×km shows how fast data can be sent through a cablesent through a cable

A common multimode fiber with A common multimode fiber with bandwidth-distance product of 500 bandwidth-distance product of 500 MHz×km could carryMHz×km could carry• A 500 MHz signal for 1 km, orA 500 MHz signal for 1 km, or• A 1000 MHz signal for 0.5 kmA 1000 MHz signal for 0.5 km

From WikipediaFrom Wikipedia

Numerical ApertureNumerical Aperture If the core and cladding have almost the same If the core and cladding have almost the same

index of refraction, the index of refraction, the numerical aperture numerical aperture will will be smallbe small

This means that light must be shooting right This means that light must be shooting right down the center of the fiber to stay in the coredown the center of the fiber to stay in the core

See Link Ch 4dSee Link Ch 4d

Fiber Types and SpecificationsFiber Types and Specifications

From Lennie Lightwave (www.jimhayes.com/lennielw/fiber.html)

Popular Fiber TypesPopular Fiber Types

At first there were only At first there were only two common types of fibertwo common types of fiber• 62.5 micron multimode, intended for 62.5 micron multimode, intended for

LEDs and 100 Mbps networksLEDs and 100 Mbps networks There is a large installed base of 62.5 micron There is a large installed base of 62.5 micron

fiberfiber

• 8 micron single-mode for long distances 8 micron single-mode for long distances or high bandwidths, requiring laser or high bandwidths, requiring laser sourcessources

Corning’s SMF-28 fiber is the largest base of Corning’s SMF-28 fiber is the largest base of installed fiber in the world (links Ch 2j, 2k)installed fiber in the world (links Ch 2j, 2k)

Gigabit EthernetGigabit Ethernet

62.5 micron multimode fiber did not 62.5 micron multimode fiber did not have enough bandwidth for Gigabit have enough bandwidth for Gigabit Ethernet (1000 Mbps)Ethernet (1000 Mbps)

LEDs cannot be used as sources for LEDs cannot be used as sources for Gigabit Ethernet – they are too slowGigabit Ethernet – they are too slow

So Gigabit Ethernet used a new, So Gigabit Ethernet used a new, inexpensive source:inexpensive source:•Vertical Cavity Surface Emitting Laser

(VCSEL)

Multimode Fiber Designed for VCSELs

First came laser-rated 50 micron First came laser-rated 50 micron multimodemultimode• Bandwidth 500 MHz-km at 850 nmBandwidth 500 MHz-km at 850 nm

Then came laser-optimized 50 Then came laser-optimized 50 micron multimodemicron multimode• Bandwidth 2000 MHz-km at 850 nmBandwidth 2000 MHz-km at 850 nm• Distinctive aqua-colored jacket Distinctive aqua-colored jacket

See links Ch 2g, 2h, 2iSee links Ch 2g, 2h, 2i

Don’t Mix Fiber TypesDon’t Mix Fiber Types

You can’t mix singlemode and You can’t mix singlemode and multimode fiber – you lose 20 dB at multimode fiber – you lose 20 dB at the junction (99% of the light!)the junction (99% of the light!)

Mixing 50 micron and 62.5 micron Mixing 50 micron and 62.5 micron multimode is not as bad, but you lose multimode is not as bad, but you lose 3 dB (half the power) which is usually 3 dB (half the power) which is usually unacceptableunacceptable

Flash CardsFlash Cards

To memorize this stuff, I use online To memorize this stuff, I use online flash cardsflash cards• Go to samsclass.infoGo to samsclass.info• Click on CNIT 211Click on CNIT 211• Click on FlashcardsClick on Flashcards• Choose Ch 2a: Fiber TypesChoose Ch 2a: Fiber Types

Fiber ManufactureFiber Manufacture

Three MethodsThree Methods

Modified Chemical Vapor Deposition Modified Chemical Vapor Deposition (MCVD)(MCVD)

Outside Vapor Deposition (OVD)Outside Vapor Deposition (OVD) Vapor Axial Deposition (VAD)Vapor Axial Deposition (VAD)

Modified Chemical Vapor Modified Chemical Vapor Deposition (MCVD)Deposition (MCVD)

A hollow, rotating glass A hollow, rotating glass tube is heated with a tube is heated with a torchtorch

Chemicals inside the tube Chemicals inside the tube precipitate to form precipitate to form sootsoot

Rod is collapsed to crate a Rod is collapsed to crate a preformpreform

Preform is stretched in a Preform is stretched in a drawing tower drawing tower to form a to form a single fiber up to 10 km single fiber up to 10 km longlong• Image from thefoa.orgImage from thefoa.org

Modified Chemical Vapor Modified Chemical Vapor Deposition (MCVD)Deposition (MCVD)

Outside Vapor Deposition (OVD)Outside Vapor Deposition (OVD) A mandrel is coated with a porous A mandrel is coated with a porous

preform in a furnacepreform in a furnace Then the mandrel is removed and Then the mandrel is removed and

the preform is collapsed in a process the preform is collapsed in a process called called sinteringsintering• Image from csrg.ch.pw.edu.plImage from csrg.ch.pw.edu.pl

Vapor Axial Deposition (VAD)Vapor Axial Deposition (VAD) Preform is Preform is

fabricated fabricated continuouslycontinuously

When the preform When the preform is long enough, it is long enough, it goes directly to goes directly to the drawing towerthe drawing tower• Image from Image from

csrg.ch.pw.edu.plcsrg.ch.pw.edu.pl

DrawingDrawing

The fiber is drawn from The fiber is drawn from the preform and then the preform and then coated with a protective coated with a protective coatingcoating

Index of RefractionIndex of Refraction

When light enters a dense medium like When light enters a dense medium like glass or water, it slows downglass or water, it slows down

The index of refraction (n) is the ratio of The index of refraction (n) is the ratio of the speed of light in vacuum to the speed the speed of light in vacuum to the speed of light in the mediumof light in the medium

Water has n = 1.3 Water has n = 1.3 • Light takes 30% longer to travel through itLight takes 30% longer to travel through it

Fiber optic glass has n = 1.5Fiber optic glass has n = 1.5• Light takes 50% longer to travel through itLight takes 50% longer to travel through it

Fiber ApplicationsFiber Applications

Step-index MultimodeStep-index Multimode

Large core size, so source power can Large core size, so source power can be efficiently coupled to the fiberbe efficiently coupled to the fiber

High attenuation (4-6 dB / km)High attenuation (4-6 dB / km) Low bandwidth (50 MHz-km)Low bandwidth (50 MHz-km) Used in short, low-speed datalinksUsed in short, low-speed datalinks Also useful in high-radiation Also useful in high-radiation

environments, because it can be environments, because it can be made with pure silica coremade with pure silica core

Graded-index MultimodeGraded-index Multimode

Useful for “premises networks” like Useful for “premises networks” like LANs, security systems, etc.LANs, security systems, etc.

62.5/125 micron has been most 62.5/125 micron has been most widely usedwidely used• Works well with LEDs, but cannot be Works well with LEDs, but cannot be

used for Gigabit Ethernetused for Gigabit Ethernet 50/125 micron fiber and VSELS are 50/125 micron fiber and VSELS are

used for faster networksused for faster networks

Singlemode FIberSinglemode FIber

Best for high speeds and long Best for high speeds and long distancesdistances

Used by telephone companies and Used by telephone companies and CATVCATV

Fiber PerformanceFiber Performance

AttenuationAttenuation Modern fiber material is very pure, but there is Modern fiber material is very pure, but there is

still some attenuationstill some attenuation The wavelengths used are chosen to avoid The wavelengths used are chosen to avoid

absorption bandsabsorption bands• 850 nm, 1300 nm, and 1550 nm850 nm, 1300 nm, and 1550 nm• Plastic fiber uses 660 nm LEDsPlastic fiber uses 660 nm LEDs

Image from iec.org (Link Ch 2n)Image from iec.org (Link Ch 2n)

Three Types of DispersionThree Types of Dispersion

Dispersion is the spreading out of a Dispersion is the spreading out of a light pulse as it travels through the light pulse as it travels through the fiberfiber

Three types:Three types:• Modal DispersionModal Dispersion• Chromatic DispersionChromatic Dispersion• Polarization Mode Dispersion (PMD)Polarization Mode Dispersion (PMD)

Modal DispersionModal Dispersion

Modal DispersionModal Dispersion• Spreading of a pulse because different Spreading of a pulse because different

modes (paths) through the fiber take modes (paths) through the fiber take different timesdifferent times

• Only happens in multimode fiberOnly happens in multimode fiber• Reduced, but not eliminated, with Reduced, but not eliminated, with

graded-index fibergraded-index fiber

Chromatic DispersionChromatic Dispersion

Different wavelengths travel at Different wavelengths travel at different speeds through the fiberdifferent speeds through the fiber

This spreads a pulse in an effect This spreads a pulse in an effect named named chromatic dispersionchromatic dispersion

Chromatic dispersion occurs in both Chromatic dispersion occurs in both singlemode and multimode fibersinglemode and multimode fiber• Larger effect with LEDs than with lasersLarger effect with LEDs than with lasers• A far smaller effect than modal A far smaller effect than modal

dispersiondispersion

Polarization Mode DispersionPolarization Mode Dispersion

Light with different polarization can Light with different polarization can travel at different speeds, if the fiber travel at different speeds, if the fiber is not perfectly symmetric at the is not perfectly symmetric at the atomic levelatomic level

This could come from imperfect This could come from imperfect circular geometry or stress on the circular geometry or stress on the cable, and there is no easy way to cable, and there is no easy way to correct itcorrect it

It can affect both singlemode and It can affect both singlemode and multimode fiber.multimode fiber.

Modal DistributionModal Distribution

In graded-index fiber, the off-axis In graded-index fiber, the off-axis modes go a longer distance than the modes go a longer distance than the axial mode, but they travel faster, axial mode, but they travel faster, compensating for dispersioncompensating for dispersion• But because the off-axis modes travel But because the off-axis modes travel

further, they suffer more attenuationfurther, they suffer more attenuation

Equilibrium Modal DistributionEquilibrium Modal Distribution

A long fiber that has lost the high-A long fiber that has lost the high-order modes is said to have an order modes is said to have an equilibrium modal distributionequilibrium modal distribution

For testing fibers, devices can be For testing fibers, devices can be used to condition the modal used to condition the modal distribution so measurements will be distribution so measurements will be accurateaccurate

Mode StripperMode Stripper

An index-matching substance is put An index-matching substance is put on the outside of the fiber to remove on the outside of the fiber to remove light travelling through the claddinglight travelling through the cladding• Figure from fiber-optics.info (Link Ch 2o)Figure from fiber-optics.info (Link Ch 2o)

Mode ScramblerMode Scrambler

Mode scramblers mix light to excite Mode scramblers mix light to excite every possible mode of transmission every possible mode of transmission within the fiberwithin the fiber• Used for accurate measurements of Used for accurate measurements of

attenuation attenuation • Figure from Figure from

fiber-optics.infofiber-optics.info (Link Ch 2o) (Link Ch 2o)

Mode FilterMode Filter

Wrapping the fiber aroundWrapping the fiber around a 12.5 mm mandrel a 12.5 mm mandrel• Exceeds the critical angle for total Exceeds the critical angle for total

internal reflection for very oblique internal reflection for very oblique modesmodes

• The high-order modes leak into the The high-order modes leak into the cladding and are lostcladding and are lost

• That creates an equilibrium modal That creates an equilibrium modal distributiondistribution

• Allows an accurate test with a short test Allows an accurate test with a short test cablecable

Figure from fiber-optics.info (Link Ch 2o)Figure from fiber-optics.info (Link Ch 2o)

Decibel UnitsDecibel Units

Optical Loss in dB (decibels)Optical Loss in dB (decibels)

If the data link is perfect, and loses no powerIf the data link is perfect, and loses no power• The loss is 0 dBThe loss is 0 dB

If the data link loses 50% of the powerIf the data link loses 50% of the power• The loss is 3 dB, or a change of – 3 dBThe loss is 3 dB, or a change of – 3 dB

If the data link loses 90% of the powerIf the data link loses 90% of the power• The loss is 10 dB, or a change of – 10 dBThe loss is 10 dB, or a change of – 10 dB

If the data link loses 99% of the powerIf the data link loses 99% of the power• The loss is 20 dB, or a change of – 20 dBThe loss is 20 dB, or a change of – 20 dB

dB = 10 log (Power Out / Power In)dB = 10 log (Power Out / Power In)

Data LinkPower In Power Out

Absolute Power in dBmAbsolute Power in dBm The power of a light is measured in The power of a light is measured in

milliwattsmilliwatts For convenience, we use the dBm units, For convenience, we use the dBm units,

wherewhere-20 dBm -20 dBm = 0.01 milliwatt= 0.01 milliwatt

-10 dBm -10 dBm = 0.1 milliwatt= 0.1 milliwatt

0 dBm 0 dBm = 1 milliwatt= 1 milliwatt

10 dBm 10 dBm = 10 milliwatts= 10 milliwatts

20 dBm 20 dBm = 100 milliwatts= 100 milliwatts