Optical Communication Systems

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Optical Communication Systems

• NITIN KUMAR
• Asst Professor
• Electronics comm. Engineering Deptt
• SDEC, GHAZIABAD

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OVERVIEW

• Information Systems Evolution What is it ?
• Why there is Demand of Large bandwidth ?
• Why Optical Fiber Technology ?
• Optical Transmission fundamentals.
• How to Explode the optical fiber bandwidth ?
• Data rate requirements for high speed networks.
• Optical Fiber Solutions for todays Systems
  Networks.

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An Information Model

• Definition
• Delivering information to an authorized user
  when it is needed, wherever it is needed i.e,
  regardless of the physical location of the user
  or of the information, and whatever form it is
  needed in a secure way.

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Information Systems Evolution

• Compared to legacy systems todays Systems are
• - Data oriented, large, and complex
• - On-line, interactive with strong emphasis on
  user interface e.g. Graphical User Interface
• - Global, distributed and extensive in their
  reach
• - More volatile and subjective to constant
  change
• Todays systems often require reuse of components
  of existing systems and building new systems to
  deal with changes

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Needs For Todays Optical Systems

• ? Increase capacity of transmission (bit/sec).
• ? Minimize insertion loss (dB).
• ? Minimize polarization dependent loss (PDL).
• ? Minimize temperature dependence of the optical
  performance (a thermal solutions).
• ? Minimize component packaging size
  (integrability).
• ? Modularity of components is an advantage
  (versatility)

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Trends

• Internet A Deriving force
• SOME ACTUAL FACTS
• 12 Million email messages in next minute
• 0.5 Million voice mail messages in next minute
• 3.7 Million people log on the net today
• Next 100 days, Internet traffic doubles
• 100 Million additional internet users every year
• Data based on the survey at Bell Laboratories,
  USA in Nov., 2000.
• DEMAND FOR MORE BANDWIDTH
• ONLY SOLUTION IS
• OPTICAL COMMUNICATION

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The Race for Bandwidth
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Exploding Demands for Bandwidth
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Optical Fiber Bandwidth as a function of time40
X OC 92 denotes 40 wavelength channelsOC-48
2.5Gb/s, OC-19210Gb/s, OC-76840Gb/s
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Trunk transmission capacity
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Do We Need Terabits ?

• Information Systems
• Computing Shift
• The Internet
• Ligthwave Capacity Trends
• Global Networking

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Facts Regarding Optical Transmission
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Capacity Growth of Optical Fiber Each Year

• Year Capacity (Gb/s)
• 1980                0.1
• 1985                1 
• 1990 3
• 1995                5
• 2000                100 (40 practically shown)
• 2005                1,000 (If limitations due
  to Dispersion Nonlinearities are overcome)

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The optical world is approaching towards

• 1. 50 THz Transmission Window
• 1000 Channel WDM
• 100 Gb/s TDM
• 1000 km Repeater less transmission 
• If Nonlinearities can be controlled,
  transmission window will be 300THz

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Optical Fiber Applications
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Fiber to the Home
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OFC Backbone Capacity
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Bandwidth-What is it ?

• Bandwidth is the a measure of information
  carrying capacity of a medium.
• To the digital word, it is translated into a
  maximum bit rate at which signals can be sent
  without significant signal degradation
• Fiber bandwidth is typically quoted in frequency
  and normalized to fiber length (MHz-Km)
• - As length increases bandwidth decreases
• A fiber bandwidth is determined by its pulse
  spreading properties

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Bandwidth-What is it ?

• The difference between the highest and lowest
  frequencies of a band that can be passed by a
  transmission medium without undue distortion.
• A term used to indicate the amount of
  transmission or processing capacity possessed by
  a system or specific location in a system
  (Usually a network system)

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Copper Versus Fiber Repeaters
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Eliminate the dangers found in areas of high
lightning-strike
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Fiber links offer over 1,000 times as much
bandwidth and distances over 100 times
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Electromagnetic Spectrum
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Introduction to Optical COmmunication

• The first practical scheme of optical
  communication, was invented by Alexander Grahm
  Bell, in 1880, the Photophone.
• Photophone Device in which speech can be
  transmitted on a beam of light, using mirrors
  selenium detectors.
• Present optical communication systems use Laser
  Optical Fiber technologies.
• Optical frequency is typically 1014 Hz, which can
  support wideband modulation. Compared to
  microwave frequencies 109 Hz, the optical career
  can offer 105 times more bandwidth.

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Basics of Fiber Optic Communication

• Fiber Optics is a revolutionary development that
  has changed the face of telecommunications around
  the world
• Transmission of data as a light pulses through
  optical fiber (first converting electronic binary
  signals to light and then finally converting back
  to electronic signals)
• Elements of Fiber Optics
• Transmission
• Light Source (such as Infrared LED converts
  pulses and sends into optical fiber)
• 850 nm, 1300 nm
• Low cost, easy to use
• Used for multi mode fiber
• Special edge emitting LEDs for single mode fiber

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Basics of Fiber Optic Communication (Contd..)

• Laser Source having properties
• Coherence
• Monochromaticity
• Directionality
• High Specific Intensity
• 850 nm, 1300 nm, 1550 nm
• Very high power output
• Very high speed operation
• Very expensive
• Need specialized power supply circuitry
• Reception
• Photo detector converts back to electrical pulses
• PIN DIODES
• 850, 1300, 1550 nm
• Low cost
• APDs (Avalanche Photodiodes)
• 850, 1300, 1500 nm
• High sensitivity, can operate at very low power
  levels
• expensive

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Basics of Fiber Optic Communication (Contd..)

• Propagation in Fiber
• Light propagates by mans of total internal
  reflection.
• Optical Fiber consists of two concentric layers
• Core inner layer
• Cladding outer layer
• Refractive index of core is greater than
  cladding, necessary for total internal reflection
• Light entering with acceptance angle propagates
  through fiber
• Strikes core cladding interface gt critical angle
  and gets reflected completely.
• Zig-zags down length of core through repeated
  reflections.
• Fairly lossless propagation through bends also.
• Optical fiber
• Multimode (Graded Index 50/125? 62.5/125 ? )
• Single mode (8.7 /125 ? )

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Basics of Fiber Optic Communication (Contd..)

• Major Advantages of FOC
• Large Bandwidth (Extremely high information
  carrying capacity)
• Carrier frequency Light 1014 Hz
• Makes possible widespread long distance
  communication of high bandwidth signals
• Color video
• High speed network
• High degree of Multiplexing, without much
  interference among them.
• Low Loss (Long repeaterless link length/repeater
  spacing)
• Loss as low as 0.1 dB/Km
• Repeater spacing of over 100 Km possible over
  land under sea.
• EMI immunity (Even in noisy or harsh
  environments-Lightning, factory floor, high
  voltage lines, broadcast towers)

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Basics of Fiber Optic Communication (Contd..)

• Major Advantages of FOC (Contd..)
• Compact and light weight
• Single fiber can easily replace 1000 pair copper
  cable of 10 cm dia.
• Security (impossible to tap)
• Safety (insulator no sparks ideal for
  hazardous environment)
• Can be used in
• Oil exploration
• Oil refineries
• Mines
• Explosives
• Petrochemical
• Other hazardous chemical

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Basics of Fiber Optic Communication (Contd..)

• Some practical disadvantages of FOC
• Fiber is expensive
• Connectors very expensive (due to degree of
  precision involved)
• Connector installation time consuming highly
  skilled operation
• Joining (splicing) of fibers requires expensive
  equipment skilled operators
• Connections joints are relatively lossy
• Difficult to tap in out (for bus architectures)
  need expensive couplers
• Relatively careful handling required

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Advances in Optical Communication

• First Generation Support
• Operating at 850 nm
• Bit Rates 50 -100 Mbps
• Repeater Spans 10 Kms
• Sources Detectors made of InGaAsP compound
  semiconductor
• Second Generation Support
• Operating at 1300 nm
• Bit Rates 1-2 Gbps
• Repeater Spans 40 -50 Kms
• Sources Detectors made of InGaAsP compound
  semiconductor
• Third Generation Support
• Operating at 1550 nm
• Bit Rates 2.4 Gbps
• Repeater Spans 100 Kms

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Advances in Optical Communication (Contd..)

• Present Standards Supported
• Various multiplexing techniques for enhanced
  capacity utilization, use of optical amplifiers
  Soliton based transmission systems developed.
• Speed Repeater spacing due to fiber optic
  systems, newer standards such as
• FDDI (Fiber Distributed Data Interface)
• DQDB (Dual Queue Distributed Bus)
• SONET (Synchronous Optical Network)
• SDH (Synchronous Digital Hierarchy)

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Advances in Optical Communication (Contd..)

• More Advanced Systems
• Era of high capacity Trans Atlantic
  Telecommunication (TAT) began as under
• TAT - 2 in 1959
• TAT 6 in 1976
• TAT 7 in 1983 (offered a capacity of about 4000
  analog circuits)
• Optical fiber based TAT 8 in 1989 (offered
  40,000 circuits, 64,000 Km long, 280 Mbps, 40 Km
  repeater distance )
• TAT - 12/13 with many new features is now
  operational
• Some other fiber systems include HAW 4
  (Hawaiian Cable 4), TPC 3(Trans Pacific Cable
  3)

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Advances in Optical Communication (Contd..)

• Further achievements include
• Fiber losses 0.16 dB/Km (at 1550 nm)
• Laser with threshold currents of few
  milli-amperes and life time of over a million
  hours
• Repeater spans of more than 200 Kms.
• Transmission rates in excess of 2 Gbps
• Advent of EDOFA (Erbium-Doped optical fiber
  amplifier), using dispersion compensating Soliton
  transmission techniques or the use of dispersion
  compensating fibers (DCF) and the improvements
  made in the attenuation dispersion
  characteristics of the modern optical fiber have
  led to the demonstration of data transmission in
  experiments with repeaterless spans of over
  10,000 Km and bit rates in excess of 10 Gbps
• More complex coherent optical communication,
  wavelength routed, dense wavelength division
  multiplexing (DWDM) links are available.

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Advances in Optical Communication (Contd..)

• Coherent communication systems make use of
• Sources detectors made of quantum well
  structures with high directional properties.
• Single mode single polarization optical fiber
  having very low loss and very low dispersion.
• Has superior SNR capabilities, long repeater
  spans high bit rates.
• WDM (Wavelength Division Multiplexing)
• Provides an easy way to increase the utilization
  of the high channel channel capacity of the
  optical fiber.
• Integrated Optics
• Deals with the miniaturization integration on a
  single substrate optical components such as
• - electro optic modulator
• - polarization controller
• - splitters / combiners
• - directional couplers
• - lenses

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Advances in Optical Communication (Contd..)

• -Optical MEMs make use of silicon micro machining
  to realize micro-opto-mechanical elements
• Soliton Propagation in Optical Fibers
• Initially launched pulse may propagate with
  ultra-low dispersion over thousands of Kilometers
• Active devices within fibers EDFA (Erbium Doped
  Fiber Amplifiers) are now available.
• Photonic switching architectures (which use
  integrated optic switches) optical MEMs
  provides data rate transparent switching
  services to optical fiber based trunks

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Advances in Optical Communication (Contd..)
Features of Present Optical Communication
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Advances in Optical Communication (Contd..)
System Design Issues
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Information Transmission Sequence
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Optical Communication Systems
Attenuation
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Fiber Structure

• A Core Carries most of the light, surrounded by
• A Cladding, Which bends the light and confines it
  to the core, covered by
• A primary buffer coating which provides
  mechanical protection, covered by
• A secondary buffer coating, which protects
  primary coating and the underlying fiber.

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Fiber Structure Cont
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Types Of Optical Fibre
Light ray
n1 core
n2 cladding
Single-mode step-index fibre
no air
n1 core
n2 cladding
Multimode step-index fibre
no air
Variable n
Multimode graded-index fibre
Index porfile
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Multimode Step Index Fiber

• Core diameter range from 50-1000mm
• Light propagate in many different ray paths, or
  modes, hence the name multimode
• Index of refraction is same all across the core
  of the fiber
• Bandwidth range 20-30 MHz

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Multimode Graded Index Fiber

• The index of refraction across the core is
  gradually changed from a maximum at the center to
  a minimum near the edges, hence the name Graded
  Index
• Bandwidth ranges from 100MHz-Km to 1GHz-Km

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Pulse Spreading
T
Pulse from zero-order mode
T
Pulses from other modes
T
T
Pulse from highest-order mode
?T
Resulting pulse
time
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Calculation of Pulse Spread
y/2
y/2
?C
?C
x
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Modes of Vibration of a String

• Lowest order mode
• Second order mode
• Third order mode

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Single-Mode Graded Index Fiber

• The Core diameter is 8 to 9mm
• All the multiple-mode or multimode effects are
  eliminated
• However, pulse spreading remains
• Bandwidth range 100GHz-Km

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Typical Core and Cladding Diameters (mm)
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Acceptance Cone Numerical Aperture
n2 cladding
Acceptance Cone
qC
n1 core
n2 cladding
Acceptance angle, qc, is the maximum angle in
which external light rays may strike the
air/fibre interface and still propagate down the
fibre with lt10 dB loss.
Numerical aperture NA sin qc ?(n12 - n22)
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Multiple OFC
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Standard Optical Core Size

• The standard telecommunications core sizes in use
  today are
• 8.3 µm (single-mode),
• 50-62.5 µm (multimode)

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Thanks