OPTICAL FIBERSOPTICAL FIBERS

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What are Fiber OpticsWhat are Fiber OpticsLong thin strands of very pure glass about the size of Long thin strands of very pure glass about the size of human hairhuman hair

Arranged in bundles called optical cablesArranged in bundles called optical cables

Used to transmit light signals over long distancesUsed to transmit light signals over long distances

Hundreds of thousands arranged in bundles to form Hundreds of thousands arranged in bundles to form optical cablesoptical cables

What is an Optical Fiber?What is an Optical Fiber?

An optical fiber is a waveguide for light

consists of :

core inner part where wave propagates

cladding outer part used to keep wave in core

buffer protective coating

jacket outer protective shield

Passage of light from a material with a high index of refraction(n1) to a material with a lower index of refraction(n2)

At the critical angle light will not go into n2 but instead travel along the surface between the two media

What are Optical FibresWhat are Optical Fibres ? ?   

• Optical Fibres are fibres of glass, usually about Optical Fibres are fibres of glass, usually about 120 micrometres in diameter, which are used 120 micrometres in diameter, which are used to carry signals in the form of pulses of light to carry signals in the form of pulses of light over distances up to 50 km without the need over distances up to 50 km without the need for repeaters. These signals may be coded for repeaters. These signals may be coded voice communications or computer datavoice communications or computer data

• The optical fiber can be used as a medium The optical fiber can be used as a medium for for telecommunicationtelecommunication and and networkingnetworking because it is flexible and can be bundled as because it is flexible and can be bundled as cables. cables.

• The light transmitted through the fiber is The light transmitted through the fiber is confined due to confined due to total internal reflectiontotal internal reflection within the material. within the material.

• In telecommunications applications, the In telecommunications applications, the light used is typically light used is typically infraredinfrared light light

• Fibers are generally used in pairs, with one Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each fiber of the pair carrying a signal in each direction direction

• Fibers, like Fibers, like waveguideswaveguides, can have various , can have various transmission modestransmission modes. The fibers used for . The fibers used for long-distance communication are known as long-distance communication are known as single modesingle mode fibers, as they have only one fibers, as they have only one strong propagation mode. strong propagation mode.

• Multi-modeMulti-mode fibers, where light transmitted fibers, where light transmitted in the different modes arrives at different in the different modes arrives at different times, resulting in times, resulting in dispersiondispersion of the of the transmitted signal. transmitted signal.

• single mode equipment is generally more single mode equipment is generally more expensive than multi-mode equipment. expensive than multi-mode equipment.

• single-mode optical fiber, data rates of up to single-mode optical fiber, data rates of up to 40 Gbit/s are possible in real-world use on a 40 Gbit/s are possible in real-world use on a single wavelength. single wavelength. Wavelength division multiplexingWavelength division multiplexing can then can then be used to allow many wavelengths to be be used to allow many wavelengths to be used at once on a single fiberused at once on a single fiber

Types of optical fibersTypes of optical fibers• Single mode Single mode

– only one signal can be transmittedonly one signal can be transmitted– use of single frequencyuse of single frequency

• Multi modeMulti mode– Several signals can be transmittedSeveral signals can be transmitted– Several frequencies used to modulate Several frequencies used to modulate

the signalthe signal

Types of FibresTypes of Fibres

nc

nc

nf

nc

nc

nf

nc

nc

GR

IN

Multi-mode step index

Single-mode step index

multi-mode graded index

Typical core and Typical core and cladding diameterscladding diameters

TypeType Core (Core (mmm)m) Cladding (Cladding (mmm) m)

Single mode Single mode 8 8 125 125

MultimodeMultimode 50 50 125 125

62.562.5 125 125

100100 140 140

Launching the LightLaunching the Light

Factors that effect the Factors that effect the Launching of LightLaunching of Light

•Intensity•Area•Acceptance Angle•Fresnell Loss

Signal ProductionSignal Production

• Convert electrical Convert electrical input to modulated input to modulated lightlight

2 Basic Schemes

On/Off Linear Variation

2 Common Devices used

Light Emitting Diode (LED)Laser Diode (LD)

Through the WireThrough the Wire

Light Propagates through the wire due Light Propagates through the wire due to total internal reflectionto total internal reflection

Fibre can be bent!!Fibre can be bent!!

Illustration of total internal Illustration of total internal reflectionreflection

Total internal reflectionTotal internal reflectionTrapping light in the fiberTrapping light in the fiber

Total Internal ReflectionTotal Internal Reflection

n i sin c n t 1( )n i sin c n t

sin c

n t

n i

n t

n i

Types of fiber endsTypes of fiber ends

beam patterns can be:

spherical

cylindrical

Fibers carry modes of lightFibers carry modes of light

a mode is :

• a solution to the wave equation

• a given path/distribution of light

higher # modes gives more light, which is not always desirable

2V~modesofnumber

Controlling the # of ModesControlling the # of Modes• From the V parameter, we see that we can From the V parameter, we see that we can

reduce the number of modes in a fiber by reduce the number of modes in a fiber by reducing:reducing:

(1) (1) NANA(2) (2) diameterdiameter (wrt (wrt ))

• This is exactly the case in single mode This is exactly the case in single mode fibers.fibers.

The V ParameterThe V Parameter

NAa

o 2V

a = fiber radiuso = incident wavelength

•known as the “V-parameter” or the fiber parameter

•an important parameter that governs the number of modes

•parameters that relates yucky EM wave solutions for both core and cladding

• The classical understanding of fiber optics The classical understanding of fiber optics comes once again from out longtime comes once again from out longtime

friend, friend, Snell’s Law!Snell’s Law!

How Fibers WorkHow Fibers Work

•Step index fibers: Total Internal Reflection

Optical Fiber BandwidthOptical Fiber Bandwidth Bandwidth LimitationBandwidth Limitation• Light entering at different angles reach the end of the cable at Light entering at different angles reach the end of the cable at

different times different times

• Smearing is produced: uncertainty of beginning and end of signalSmearing is produced: uncertainty of beginning and end of signal

• less smearing higher the bandwidth • smearing can be reduced by reducing the size of the fiber core

Areas of ApplicationAreas of Application

    • Telecommunication'sTelecommunication's

– Optical fibres are now the standard point to Optical fibres are now the standard point to point cable link between telephone point cable link between telephone substations. substations.

• Local Area Networks (LAN's)Local Area Networks (LAN's)– Multimode fibre is commonly used as the Multimode fibre is commonly used as the

"backbone" to carry signals between the "backbone" to carry signals between the hubs of LAN's from where copper coaxial hubs of LAN's from where copper coaxial cable takes the data to the desktop. Fibre cable takes the data to the desktop. Fibre links to the desktop, however, are also links to the desktop, however, are also common.common.

• Cable TVCable TV– As mentioned above domestic cable TV As mentioned above domestic cable TV

networks use optical fibre because of its very networks use optical fibre because of its very low power consumption.low power consumption.

• CCTVCCTV– Closed circuit television security systems use Closed circuit television security systems use

optical fibre because of its inherent security, optical fibre because of its inherent security, as well as the other advantages mentioned as well as the other advantages mentioned above.above.

• Optical Fibre SensorsOptical Fibre Sensors

• Long-haul trunksLong-haul trunks common in telephone networks common in telephone networks • Metropolitan trunksMetropolitan trunks to join phone exchanges in metro to join phone exchanges in metro

areasareas• Rural exchange trunksRural exchange trunks connect exchanges of different connect exchanges of different

phone companiesphone companies

• Subscriber loopsSubscriber loops

central exchange to subscribercentral exchange to subscriber• LANsLANs

Can support hundreds of stations Can support hundreds of stations on a campuson a campus

Other ApplicationsOther Applications

• EndoscopeEndoscope• X-ray ImagingX-ray Imaging• Night VisionNight Vision

Advantages of optical Advantages of optical FibresFibres

• Can carry much more informationCan carry much more information• Much higher data ratesMuch higher data rates• Much longer distances than co-axial Much longer distances than co-axial

cablescables• Immune to electromagnetic noiseImmune to electromagnetic noise• Light in weightLight in weight• Unaffected by atmospheric agentsUnaffected by atmospheric agents

Disadvantages of optical Disadvantages of optical FibresFibres

• expensiveexpensive• need to convert electrical signal need to convert electrical signal

into optical signal when into optical signal when transmitting and convert it back to transmitting and convert it back to electrical signal when receivingelectrical signal when receiving

The Optical Transmitter: The Optical Transmitter:

•

• The source of the optical signal can be either a light emitting diode, or a solid state laser diode.

• The transmitter converts an electrical analog or digital signal into a corresponding optical signal.

• The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers.

Optical ReceiversOptical Receivers

Converts modulated Converts modulated light from the cable light from the cable into the original signalinto the original signal

• Photodiode: Pin or Photodiode: Pin or Avalanche typeAvalanche type

• High gain internal High gain internal amplifiersamplifiers

• Large sensitive Large sensitive detecting area detecting area several microns thickseveral microns thick

The Optical Receiver:The Optical Receiver:

• The receiver converts the optical signal back into The receiver converts the optical signal back into a replica of the original electrical signal. The a replica of the original electrical signal. The detector of the optical signal is either a PIN-type detector of the optical signal is either a PIN-type

photodiode or avalanche-type photodiodephotodiode or avalanche-type photodiode..

Degradation of the SignalDegradation of the Signal Glass must be extremely pureGlass must be extremely pure Most general purpose optical fiberMost general purpose optical fiber

Signal losses per km traveledSignal losses per km traveled• 850nm = 60-75%850nm = 60-75%• 1300nm = 50-60%1300nm = 50-60%• 1550nm = 40%1550nm = 40% Excessive bendingExcessive bending

Signal RegenerationSignal Regeneration Optical regenerators spliced along the cable to Optical regenerators spliced along the cable to

boost weakened signalsboost weakened signals Optical RegeneratorOptical Regenerator• Optical fibers with specially doped coatingOptical fibers with specially doped coating• Doped portion is pumped with a laserDoped portion is pumped with a laser• When signals enters energy from the laser When signals enters energy from the laser

allows doped material to imitate lasersallows doped material to imitate lasers• Doped molecules now emit a stronger signal Doped molecules now emit a stronger signal

with the same initial characteristicswith the same initial characteristics

Optical Fiber - Transmission Optical Fiber - Transmission CharacteristicsCharacteristics

• Act as wave guide for 10Act as wave guide for 101414 to 10 to 101515 Hz Hz – Portions of infrared and visible spectrumPortions of infrared and visible spectrum

• Light Emitting Diode (LED)Light Emitting Diode (LED)– CheaperCheaper– Wider operating temp rangeWider operating temp range– Last longerLast longer

• Injection Laser Diode (ILD)Injection Laser Diode (ILD)– More efficientMore efficient– Greater data rateGreater data rate

• Wavelength Division MultiplexingWavelength Division Multiplexing - - Multiple beams of light at different frequencies can Multiple beams of light at different frequencies can

be transmitted simultaneouslybe transmitted simultaneously

Global crossing fibre networksGlobal crossing fibre networks

Atlantic crossing networksAtlantic crossing networks

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