32983103-Introduction-to-Fiber-Optics.pdf

R Telecommunications and Energy Networks Division

I N T R

O D U C T I O N

T O

F I B E

R

O P

T I C S

Daniel Daems - September 1997

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Telecommunications and Energy Networks Division

Introduction to Fiber Optics

CONTENTS WHAT IS FIBER OPTIC COMMUNICATION COMMUNICATION?........... ?....................... ....................... ...................... ............. .. 3 HISTORY HISTORY OF FIBER OPTICS ...................... ................................. ....................... ....................... ...................... ............... .... 4 BASICS OF OPTICAL OPTICAL FIBER TRANSMISSION.................... TRANSMISSION............................... ...................... ............. .. 7 Index of Refraction.......................................... Refraction................................................................. .............................................. ................................. .......... 7 Total internal reflection: Snell’s Law............................................................... Law..................................................................... ...... 8 Decibel (dB) Definition....................... Definition .............................................. .............................................. ........................................... .................... 10 TYPES OF OPTICAL OPTICAL FIBERS ...................... ................................. ....................... ....................... ...................... ............. .. 11 General ............................................ ................................................................... .............................................. .............................................. ......................... 11 Multimode, Step Index ........................................... .................................................................. .............................................. ......................... 12 Multimode Graded Index Fiber .............................................. ..................................................................... ............................... ........ 13 Single–Mode Fiber ............................................. ..................................................................... ............................................... ........................... .... 14 Production of Optical Fibers ......................................................... ................................................................................ ......................... 16 CHARACTERIS CHARACTERISTICS TICS OF OPTICAL OPTICAL FIBERS........... FIBERS ....................... ....................... ...................... ............. .. 18 FIBER OPTIC CABLES CABLES ....................... .................................. ...................... ....................... ....................... ...................... ........... 19 Why are ar e cables needed? ..................................................... ............................................................................ ................................... ............ 19 Fiber Optic Cable Constructions ............................................ ................................................................... ............................... ........ 20 FIBER OPTIC COMMUNICAT COMMUNICATION............ ION........................ ....................... ...................... ...................... ................. ...... 25 Light Sources................................... Sources.......................................................... .............................................. .............................................. ......................... 25 Detectors............................................................. Detectors...................................... ............................................... ............................................... ........................... .... 30 TDM: Time Division Multiplexing ............................................ ................................................................... ........................... .... 31 Comparison With Copper Networks ............................................. .................................................................... ......................... 32

References: Optical Fiber Systems Technology, Design and Applications Charles K. Kao McGraw-Hill Book Company

Technicians Guide to Fiber Optics Donald J. Sterling, Jr. Delmar Publishers Inc.


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WHAT IS FIBER OPTIC COMMUNICATION? THE TRANSPORTATION OF OPTICAL SIGNALS THROUGH AN OPTICAL FIBER

SEQUENCES OF PULSES (BITS)

CODER INFORMATION

PULSES OF LIGHT FIBER

LIGHT SOURCE

DETECTOR

DECODER

OUTPUT PULSES OF CURRENT

Principles of voice transmis sion • •

• • • • •

•

A voice will create an analog modulated electrical signal in the phone. A coder or analog to digital converter will transform the analog electrical signal into a digital electrical signal. Digital electrical signal consists of electrical pulses. These pulses will drive a light source. Optical pulses are then sent through an optical fiber to a detector. The detector will transform the optical pulses back into electrical pulses. A digital-to-analog converter will regenerate an analog electrical signal from the incoming digital signals. This analog electrical signal is transformed tr ansformed back into an analog acoustical signal in the telephone set. Daniel Daems - September 1997

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HISTORY OF FIBER OPTICS F i r s t E x p e r i m e n t s W i t h L i g h t T r an an s m i s s i o n

Sunlight

Water

Water jet

•

•

•

•

In 1890, John Tyndall demonstrated that light could be bent around a corner when it travels through a jet of pouring water. He claimed that light was guided through the water beam by total internal reflection of the light. Light travelling in a medium with a high refractive index (water) surrounded by a medium with a lower refractive index will be reflected at the contact surface of the two media. This principle of total internal reflection is still used in optical fiber transmission.


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HISTORY OF FIBER OPTICS (CONT.) Bell's Photophon e

RECEIVER

TRANSMITTER 200 meters

An Early Free-Space Optical System

•

•

• •

•

In 1880, Alexander Graham Bell patented a telephone which used modulated light to carry speech. A series of lenses and mirrors brought sunlight onto a flat mirror attached to a mouthpiece. The voice vibrated the mirror, thereby modulating the incident light. The receiver was a selenium detector whose electrical resistance varied with the intensity of the modulated light. Bell managed to transmit voices over a distance of 200 meters.


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HISTORY OF FIBER OPTICS (CONT.) 1927

Baird and Hanell use optical fiber bundles to transmit television screen information to another location.

1960

The first laser was built.

1966

A technical study by C. Kao and C. Hockam proposed that optical fibers could be competitive to copper wires when losses could be reduced to 20 dB/km.

1970

Production of the first optical fibers with losses below 20 dB/km (Corning).

1970

First semiconductor laser.

1973

Optical fibers with losses of 4 dB/km (@ 850 nm).

1974

Optical fibers with losses of 3 dB/km (@ 850 nm).

1976

Fusion splicing machine to interconnect fibers.

1976

Low loss fibers (0.5 dB/km @ 1300 nm).

1977

First commercial transmission systems based on optical fibers. f ibers.

1979

Optical loss of fibers 0.2 dB/km @ 1550 nm

1983

Single–mode optical fibers commercially available, but too difficult to use.

1985

Telecom industry standardises on single–mode fiber.

1986

FOSC 100D (mainly for trunk and junction j unction networks).

1989

FOSC 100B (smaller size).

1993

FIST (for distribution networks).


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BASICS OF OPTICAL FIBER TRANSMISSION INDEX OF REFRACTION

•

n=1

n = 1.5

Velocity = c

Velocity v < c

The refractive index, symbolised by n, of a material is the ratio of the velocity of light c in free space (vacuum) to its velocity v in a specific material:

n= c/v •

The velocity of light in free space is about 3.108 meter/second.

•

Light in a medium with a higher index of refraction (like glass or water) will travel slower than light in free space.

•

Typical values for refractive index are: water 1. 3 glass 1.5 air 1.0003


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BASICS OF OPTICAL FIBER TRANSMISSION (CONT.) TOTAL INTERNAL REFLECTION: SNELL’S LAW •

•

•

Light passing from a material of higher refractive index to one of a lower one refracts away from the normal. A small portion of the light is reflected refl ected on the interface of the 2 media with different refractive index. When the angle Ø1 of the incident light is increased, the angle Ø2 of the refracted light will increase too.

n2

2

n1

1

n1 sin Ø1 = n2 sin Ø2

•

•

When the angle Ø1 of the incident light is chosen in such a way that the angle Ø2 of reflection becomes 90°, the angle Ø1 is called the critical angle Øc. If the angle Ø1 of the incident light increases beyond the critical angle all light is totally reflected back at the interface.

n2

1

1

n1

Ø1 > Øc = 1/sin (n2 /n1)


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BASICS OF OPTICAL FIBER TRANSMISSION (CONT.)

Acceptance cone c

NA

n1 = 1.48

Core

n2 = 1.46

Cladding Coating

Numerical Aperture: Aperture: NA = sin ØNA = n1. cos Øc where the critical angle: Øc = Bgsin (n2 /n1) •

• •

•

Only light that enters the fiber core with an angle smaller than the angle ØNA will be propagated in the core. These angles form a cone, called the acceptance cone. The numerical aperture NA is another measure for the angle of the acceptance cone. Light which enters the core with an angle above the angle ØNA will escape from the core and will be lost in the cladding of the fiber.

Typical values Step index multimode: Single–mode:

NA = 0.2 NA = 0.11


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BASICS OF OPTICAL FIBER TRANSMISSION (CONT.) DECIBEL (DB) DEFINITION

dB (loss) = -10 log10 (POUT /PIN) where: POUT PIN

= Power out (in Watt) = Power in (in Watt)

Example If the power out is only half of the power in: dB loss = -10 log10 (1/2) = 3 dB Note that the loss expressed in dB gives only a relative expression of the power loss. The absolute power loss is not known (the wattage value lost is unknown).

•

The optical loss through a fiber is is expressed in dB/km.

•

Optical fibers manufactured today will have a loss of 0.3 dB/km for light with a wavelength of 1550 nm.

•

This means that for a 1 km length of optical fiber the outgoing optical power is 93% of the ingoing optical power.


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TYPES OF OPTICAL FIBERS GENERAL M ul ti mode, mode, Step tep I ndex. ndex. •

•

•

•

•

These fibers are hardly used in telecommunication networks today because of their limited bandwidth. This is expressed by the bandwidth-length figure (100 MHz.km for f or step index multimode fiber). The bandwidth expresses the maximum amount of information per second that can be sent through a fiber. In this case you can send: 100 MHz through 1 km of fiber 10 MHz through 10 km of fiber 5 MHz through 20 km of fiber 500 MHz over 200 meters of fiber Multimode step index fiber is still used in very specific applications, e.g. in the nuclear industry using radiation resistant fibers.

M ul ti mode mode, Graded Graded I ndex. ndex. •

•

•

These fibers are more difficult to make because of the complex refractive index profile in the core. Graded index fibers will have a larger l arger bandwidth compared to the step index multimode fibers (up to 1000 MHz.km). These graded index multimode fibers are mainly used in Lans (Local Area Networks).

Sin gle gl e–M ode, ode, Step Step I n dex dex The production of these fibers requires high precision due to the small tolerances on the dimensions. About 85% of world-wide production is single–mode fiber. This fiber type guarantees the largest bandwidth (100,000 Mhz.km), with electronics as the limiting factor for the bandwidth.

Coating Cladding Core

These fiber types will be discussed in more detail in the following sections. Daniel Daems - September 1997

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TYPES OF OPTICAL FIBERS (CONT.) MULTIMODE, STEP INDEX Index profile

Higher order mode Fundamental mode

125 µm 50 µm Core Cladding •

A mode can be considered as the path that a light ray follows when travelling through a fiber. Each mode carries a quantity of optical energy.

•

In a multimode step index fiber with a 50 µm core over 1000 modes can exist.

•

The path length for each mode will be different and therefore light rays will take different times to travel the length of the fiber.

•

This spreading of light energy is called “modal dispersion” and causes the spreading of the light pulse when travelling through the fiber.

•

Eventually, 2 pulses will merge together and cannot then be distinguished from each other.

Pulses Pulses merged

Optical fiber


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TYPES OF OPTICAL FIBERS (CONT.) MULTIMODE GRADED INDEX FIBER Index profile Cladding Core 125 µm 50 µm

•

By using a parabolic refractive index profile in the core the refractive index in the central axis of the core will be higher than that at the outside of the core.

•

Due to this profile, a light ray diverging from the center of the core will be gradually bent until it rejoins the center of the core. Light thus travels through the fiber in a sinusoidal pattern.

•

•

Light travels faster in a lower index of refraction. Hence those rays that follow the longest path by travelling near the outside of the core will have a faster velocity in this region. In contrast, light travelling near the center of the core has a lower velocity. As a result, all rays tend to reach the end of the t he fiber at the same time. This graded-index fiber reduces modal dispersion to less than 1 ns/km. In addition, pulse spreading will be lower and therefore a higher bandwidth can be achieved.


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TYPES OF OPTICAL FIBERS (CONT.) SINGLE –MODE FIBER Index profile

125 µm

10 µm

Core

Cladding •

• •

•

•

Modal dispersion can be eliminated by reducing the core’s diameter until only one mode is propagated through the fiber. Single–mode fibers have a very small core diameter ranging from 7 to 10µm. Higher order modes will immediately disappear into the cladding. Because there is no modal dispersion, the available bandwidth is increased to 100 GHz.km. Single–mode transmission only occurs when the transmitted light has a wavelength above a certain limit, called the cut-off wavelength. Below the cut-off wavelength the fiber will act as a multimode fiber. Typically the cut-off wavelength is around 1200 nm. Not all single-mode fibers use a step index profile. More complex designs like depressed cladding are used to optimize the transmission at a certain wavelength. Index

Index

Radius

Step Index

Radius

Depressed Cladding


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TYPES OF OPTICAL FIBERS (CONT.) Fiber types are designated according to the type t ype of protective coating:

Primary-coated fibers (diameter 250 µm).

Secondary-coated fibers are primary-coated fiber with an extra coating (diameter 900 µm).

Ribbonized fibers (or ribbon) are primary-coated fibers placed next to each other in one plane. An acrylate coating or polyester tape keeps the fibers together. Typically 4-, 8- and 12-fiber ribbons are used; but in the past 2-, 6- and 10-fiber ribbons existed too. Today, some manufacturers are planning for 16- and 24-fiber ribbons.

Advantage of ribbons: less time is required to splice a large number of fibers. • Disadvantages: splice loss is higher than for single-fiber splices due to the lack of individual • fiber alignment. Today’s ribbon fusion splicing machines only use V-groove alignment. fi bers need to be separated from the • At the equipment side of the network the fibers ribbon, by ribbon fan-out.


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TYPES OF OPTICAL FIBERS (CONT.) PRODUCTION OF OPTICAL FIBERS Several techniques are used to make optical fibers. Most of them first make a preform made out of pure silica glass. • This pure glass can only by made by gaseous chemical reactions. • This is the only way to reduce the number of impurities to 10 - 50 parts per billion. Impurities like iron and copper atoms will cause attenuation of light inside the fiber. Later, optical fiber will be made out of the preform. • The following techniques exist to make preforms: OVD: Outside Va Vapour De Depositio tion (C (Corning) • VAD: Vapour Ax Axial De Deposition (Japan) • MCVD MCVD:: Modi Mo difie fied dC Che hemi mica call V Vap apou ourr Dep Depos ositi ition on (AT&T (AT&T)) • PCVD PCVD:: Plasm Plasma a Chem Chemic ical al Va Vapo pour ur Depo Deposi sitio tion n (Phi (Philip lips) s) • Most production techniques are based on the chemical reaction of gasses (SiCl4, O2 and CH4) at temperatures of between 1000 and 1500°C. Glass is deposited onto an existing glass tube or rod. • The refractive index can be increased by adding germanium (as ( as GeCl 4) to the • gasses. fused quartz tube O2 O2 Cl2 exhaust dust

flow meters, mass flow controllers and manifold SiCl4 GeCl 4 bubblers

BCl3

multi-burner torch

deposited core glass layer

O2 H2 translation

• •

The end product is a preform with a diameter of 5 cm and a length of 30 cm. From this the fiber will be made in subsequent production steps.


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TYPES OF OPTICAL FIBERS (CONT.) PRODUCTION OF OPTICAL FIBERS (CONT.) •

• •

The preform contains the core and cladding in the correct dimensional proportions. By heating the preform a fiber can be drawn from it. The fiber is immediately protected with a UV curable polymer coating to prevent mechanical degradation of the glass surface.

downfeed mechanism 18/09/97

furnace preform

waveguide

winding drum

diameter monitor

coater drawing tractors

tensile strength monitor

Waveguide Draw Schematic


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CHARACTERISTICS OF OPTICAL FIBERS Attenuation Attenuation is the loss of optical power as light travels through a fiber. It is caused by 3 factors: Raleigh scattering, • absorption, and • bending losses. •

Raleigh scattering is the loss of optical energy due to imperfections in the fiber and from the basic structure of the fiber. Scattering decreases rapidly at longer wavelengths. Absorption : Higher wavelengths above 1700 nm are heavily absorbed by the glass molecules. Impurities like water- (OH ), iron- and copper-ions will absorb absorb optical energy in the regions of 1250 nm and 1390 nm. Bending: (micro and macro bends). Excessive fiber bending will cause the critical angle for total internal reflection to be exceeded. Light will escape from the core and will be lost inside the cladding. This phenomenon can be utilised to tap off light from a fiber without breaking the fiber (= bent fiber coupler). The spectral attenuation curve of an optical fiber is shown below: 2.5

Attenuation (dB (dB/km)

2.0 Absorption peak of water

1.5 1.0 .5

Wavelength (nm) 0 800

1st window

10 00

1 2 00

140 0

2nd window

160 0

3rd window


1800

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FIBER OPTIC CABLES WHY ARE CABLES NEEDED? To Protec Protect F iber iber s Du r in g I nstallation nstallation Cabling will protect the fragile optical fibers from mechanical or environmental influences. Today’s fiber optics cables are made in such a way that can be installed as if they were copper cables (pulling through ducts, direct buried). Only a few limitations need to be taken into account, i.e. maximum pulling force and bend diameter.

Protec Protection of F iber iber s Du r in g Cable Cable L if etime When properly specified, fiber optic cables can survive in any environmental condition that can be imagined. This requires the correct construction and material choice for all cable elements.

F iber iber I denti denti f ication ication After the cabling process the fibers can still be individually identified. This is achieved by using color codes or location l ocation in the cable. Fibers need to be bundled in such a way that they are easy to retrieve from the cable (for example when they need to be spliced to other cables).

M aintaini ng Optical Optical and M echani chani cal cal Prope Proper ties ties of F iber iber s The cabling process should be done in such a way that the fibers are not put under mechanical stress. The optical characteristics of the fiber should not be affected during the lifetime of the cable.


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FIBER OPTIC CABLES FIBER OPTIC CABLE CONSTRUCTIONS •

4 basic cable designs are used today: Loose tube ∗ Slotted core ∗ Central core ∗ Tight buffer ∗

•

These cable constructions use different cable jacket materials and types of mechanical protection, depending upon the environment or application. ∗

Cable jacket materials are polyethylene, polyurethane, PVC or Teflon.

∗

Mechanical protection can be steel sheaths or wires, glass or aramid yarns, etc... Some cables are completely metal free (i.e. all-dielectric) to prevent lightning strikes.

∗

The number of fibers per cable can range from 1 to 1000! There are even cables containing 5000 fibres! Sometimes copper pairs are included in the cable to allow communication when working on a cable or to allow the use of humidity sensors.

∗

Most cables are grease-filled to prevent the ingress of water when the cable jacket is damaged. Others are use materials which swell in the presence of water and hence prevent its ingress. Indoor cables are not generally grease-filled.


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Fiber Optic Cable Constructions (cont.) L oose oose-Tu -T u be Cable Cabl e

fiber

overcoated central member (steel/dielectric)

loose tube buffer (filled)

kevlar

interstitial filling

PE jacket

Loose-Buffer-Tube Loose-Buffer-Tube Cable Cross Section •

• •

•

• •

In a loose-tube cable design the fibers lie in a buffer tubes having diameters ranging from 1 to 3 mm. Typically one loose-tube contains 4 to 12 primary-coated fibers. The tube isolates the fiber from the rest of the cable and the mechanical forces acting on it. As the cable expands or shrinks with changes in temperature, the fibers are not affected. The diameter of the loose-tubes depends on material choice, the temperature range,minimum bend diameter, number of fibers, etc. The fiber in the tube is slightly longer than the tube itself. The overlength is about 0.1 to 0.3%.

Cable reinforcement elements: The central strength members can be made of steel or fiberglass reinforced • epoxy rods. It will give mechanical protection to the fibers against push and pull forces during and after installation of the cable or during extreme temperature variations. In almost every cable aramid yarns or kevlar are used to give mechanical • protection against tensile stresses during and after installations. This type of cable is used in: USA, UK, Germany, Belgium, The Netherlands, etc.


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Fiber Optic Cable Constructions (cont.) Sl otted-Cor otted-Cor e Cable Cabl e tensile member (steel or dielectric)

extruded plastic fiber(s)/ twisted pair

heat-barrier polyethylene outer jacket

Slotted-Core Cable Design

•

• •

• •

•

A slotted-core cable consists of a grooved polymeric core extruded around a strength member. Each slot can contain several fibers (up to 18) or ribbons (up to 5 ribbons). The slots perform in the same way as a loose-tube (keeping fibers free from mechanical stress). Slotted-core cables can have 2 to 12 grooves per core. Slotted-core constructions for single fibers are no longer popular because of difficulties in cable preparation. However, slotted-core is gaining more interest for ribbon constructions. The slotted-core cable design gives excellent protection in direct-buried applications due to the crush resistance of the core.

Single fiber cables are used in: France, Sweden, Denmark, Italy, and New Zealand. Ribbon fiber cables are used in: Japan, Italy, and Sweden.


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Fiber Optic Cable Constructions (cont.) Centr Centr al Core Cable Cable stainless steel wires lightguide ribbon

inner sheath

intermediate sheath polyester tape

HDPE outer sheath fiber core and cladding coated fiber

•

Central core cable is used mainly in the USA and Finland.

•

The single fibers or ribbons are located in one big, strong tube.

•

The strength members are placed outside the centre.

•

AT&T's most popular ribbon cable hold 16 ribbons with 12 fibers/ribbon (= 192 fibers in total). The ribbons are stacked on each other inside the central tube.


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Fiber Optic Cable Constructions (cont.) Ti ght Buf fer fer

flame-retardent PVC jacket

kevlar

buffered fiber (500 - 1000 µm)

•

•

•

The primary-coated fiber (250 µm) is covered by an extra coating with a diameter of 500 to 1000 µm. This secondary-coated or tight coated fiber is then covered by kevlar yarns to give it mechanical protection during handling of the cable. The cable jacket is a flame retardant PVC jacket.

This cable construction is used for patchcords (= connectors on both sides) or pigtail cords (= connector on one side) in central offices (racks) or at customer terminations, or in other words: at the extremities of the network. This cable construction will also be used in cross connect points. A similar construction exists with a primary-coated fiber inside a small loose-tube (900µm). This construction is called semi-tight.


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FIBER OPTIC COMMUNICATION LIGHT SOURCES A light source converts the electrical signals into optical signals. Two types of semiconductor sources are used in optical-fiber transmission: LED (Light Emitting Diode) • LD (Laser Diode) •

A dvant dvantage ages s of L D s Compar Compar ed to LE L E D s • • • •

Higher bit rates High optical power output Higher coupling efficiency into fiber Small spectral width (results in less l ess chromatic dispersion)

D i sadvantage advantages s of L D s compar compare ed to L ED s • • • •

Threshold current (lasing starts above threshold current, typically 50 mA) Complicated electronics More expensive Shorter lifetime

Depending on the required transmission bandwidth, distance and reliability, reliabili ty, a choice is made between the use of LEDs or LDs. In general LDs are used in telecommunications while LEDs are more frequently used in LANs (Local Area Networks) for data transmission.


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FIBER OPTIC COMMUNICATION (CONT.) Light Sources (cont.) L E D +

+ +

•

-

+

-

-

+

•

-

The recombination of electron-hole pairs in the depletion zone of heavily doped P-N junctions creates light. The light emitted has a wide spectral width (i.e. many wavelengths are emitted). A typical spectral width value is 100 nm. Power

100 nm

1200

1300

1400

Wavelength (nm) •

•

• •

•

The refractive index, together with the velocity of light, depends on the wavelength of the transmitted light. When light is used with many spectral components at different wavelengths, each component will travel at a different velocity through the fiber. This is called material dispersion or chromatic dispersion. The effect is very similar to modal dispersion. Optical pulses with many spectral components will spread when travelling though a fiber and this will limit the bandwidth of the transmission line. The only way to solve it is to use a light source with a very narrow spectrum: the laser diode.


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FIBER OPTIC COMMUNICATION (CONT.) LIGHT SOURCES (CONT.) L ase aser Di ode ode (L D): D) :

confining layers (double heterojunction)

emitting region active layer

Light Amplification by Stimulated Emission of Radiation

There are many types of LD’s, but all are based on the same physical phenomenon. Below a certain current, called the threshold current (typically 50 mA) the LD • will act as an LED. Once the threshold current is exceeded a laser effect will take place. Photons • passing atoms in an excited state will pick up the energy from this atom and will be amplified. More photons are created and these photons will in turn create other photons • when passing excited atoms. This avalanche effect will create a high power, monochromatic and coherent • light. The spectral width of the emitted light is around 4 nm (compared to 100 nm for • an LED).


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FIBER OPTIC COMMUNICATION (CONT.) LIGHT SOURCES (CONT.) Power

LED

4 nm

12 0 0

1300

LD

14 0 0

Wavelength (nm)

With this small spectral width the chromatic dispersion will be negligable. The optical pulses will not spread out and this will result in a huge potential bandwidth


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FIBER OPTIC COMMUNICATION (CONT.) LIGHT SOURCES (CONT.) Coupli Coupli ng Ef fi ciency ciency The following typical coupling losses are achieved for interconnections of sources with fibers: LD in single–mode fiber:

5 dB (32% into fiber)

LD in multimode fiber:

2.5 dB (56% into fiber)

LED in single–mode fiber:

30 dB (0.1% into fiber)

LED in multimode fiber:

12 dB (6% into fiber)

Despite the high coupling losses the optical power can still be very high inside the fiber. Never look directly into the source or into the fiber end of a cleaved fiber or connector.

Remember that you can not see the infrared light.


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FIBER OPTIC COMMUNICATION (CONT.) DETECTORS A detector converts optical energy into electrical energy. The most commonly used detector is the semiconductor photodiode. heavily-doped P-type and N-type • A lightly-doped intrinsic layer separates the heavily-doped materials = PIN diode. Light

P

I

N -

+

Current

V -

• •

• •

+

Light that falls in the depletion region will create an electron-hole pair. These carriers will start to move when an electric field is applied to the diode i.e. a current is generated. The intensity of the current is proportional to the amount of incident light. Avalanche photodiodes (APD) work on the same principle as the PIN diode, but due to a stronger electrical field the generated primary electron-hole carriers will generate secondary electron-hole carriers through collisions with neutral atoms. As a result a higher current is generated for the same amount of light.


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FIBER OPTIC COMMUNICATION (CONT.) TDM: TIME DIVISION MULTIPLEXING

A/D

A/D

A/D

Time Division Multiplexer

LD

Pulses shifted in time Fiber

Detector

Demultiplexer

D/A

D/A


D/A

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FIBER OPTIC COMMUNICATION (CONT.) COMPARISON WITH COPPER NETWORKS A dvantage dvantages s of F i ber ber • • • • • •

Lighter No electromagnetic effects No electrostatic interference (lightning) Fewer amplifiers required Large bandwidth (when using single–mode fiber) Difficult to tap-off light without being detected

D i sadvant advantage ages s of F i ber ber • • •

•

Relatively new technology (about 20 years of experience) High cost in local access network A cable accident can disconnect thousands of people because of the large bandwidth per fiber (about 10.000 voices are possible per fiber) Fiber handling can cause transmission errors in active fibers due to uncontrolled bends


32983103-Introduction-to-Fiber-Optics.pdf

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