Lecture:3 Lightwave/Optical Systems

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Lecture3 Lightwave/Optical Systems
Ajmal Muhammad, Robert Forchheimer Information
Coding Group ISY Department
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Outline

• Optical Networks
• Core, metro, and access networks
• Optical Access Networks
• Optical Amplifiers
• Doped fibers, semiconductor optical amplifiers
  (SOAs)
• Modulation
• Direct intensity, external modulation
• Demodulation

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Telecom Network Hierarchy
Long haul - 100s-1000s km - Mesh
Metro (interoffice) - 10s of km - Rings
Access - a few km - Hubbed rings
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The Last Mile First
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Part of core Network Submarine Optical Cables
The longest submarine cable is the Southeast
AsiaMiddle EastWestern Europe (SEA-ME-WE 3)
system stretching 39,000 km from Norden, Germany,
to Keoje, South Korea
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Metropolitan-Area Networks (MANs)

• MAN is connected to a WAN at egress nodes (EN)
• MAN is connected to LANs at access nodes (AN).
  ADM stands for add-drop multiplexer
• Several MANs can be interconnected with a ring to
  form a regional network
• Regional rings provide protection against failures

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The First Mile Access Networks

• Telephone companies xDSL (Digital Subscriber
  Line)
• - DSL data rate 128kb/s - 1.5Mb/s
• - Maximum subscriber distance from central office
  5.5 km
• Other flavors ADSL (asymmetric DSL) 12Mb/s,
  VDSL (very-high-bit-rate) 50Mb/s-0.5 km,
  HDSL(high-bit-rate DSL)
• Cable TV companies CM (Cable Modem)
• Dedicated radio channel for data
• Problems with todays access technologies (xDSL,
  CM)
• - Originally designed and built for voice and TV,
  respectively
• - Retrofitting for data not working well
• - Limitations in Reach, Bandwidth, Scalability,
  Flexibility, Cost

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Fiber Access Network

• Fiber-to-the-x (FTTx) where x H,B,C,P,BS,AP,
• Platform for triple play service, i.e., voice,
  data and video
• Long reach 0-20 km
• Fiber plant has long life span (20 years)
• Able to scale and incorporate new technologies
  without digging new trenches
• Leverage long reach to facilitate broadband
  wireless access over shorter distance

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Optical Fiber Based Access Networks
Power in the field required
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Passive Optical Network (PON)
Passive Splitter
- Point-to-multipoint topology - Low cost
implementation - Relative ease of deployment -
Future-proof
OLT Optical line terminal ONU Optical network
unit
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Optical Line Terminal (OLT)
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Optical Network Unit (ONT)
ONT for FTTH (Home)
ONT for FTTH outdoor unit
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1G PON - Ethernet PON(EPON)
Broadcasting
1 Gb/s 1490-nm wavelength
Shared medium network for downstream traffic
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1G PON - Ethernet PON(EPON)
Time Division Multiplexing
1 Gb/s 1310-nm wavelength Low cost FP lasers
Point-to-point network for upstream traffic
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OLT Structure
Physical Media Dependent defines the optical
transceiver the wavelength demulplexer
Service adaptation provides the translation
between the signal format required for client
equipment connection and the PON signal format
Media Access Control schedules the right to use
physical medium
Service Network Interface (SNI)
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ONU Structure
User to Network Interface (UNI)
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Typical PON Configuration

• Wavelength
• Dual fiber 1310 nm
• Single fiber upstream (downstream) on 1310 (1490)
  nm
• Transceiver
• ONU Fabry-Perot (upstream), PIN (downstream)
• ONT APD(upstream), DFB(downstream)
• Transceiver Assumptions
• Upstream(_at_1310 nm) power budget 30 dB
• Downstream(_at_1490 nm) power budget 22 dB

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Second Generation PON Line-Rate Upgrade

• 10G-PON Suppose symmetric 10-Gb/s downstream and
  upstream, and asymmetric 10-Gb/s downstream and
  1-Gb/s upstream
• GPON Suppose asymmetric 2.488-Gb/s downstream
  and 1.244-Gb/s upstream
• XG-PON Suppose coexistence with GPON on the same
  fiber plant. Downstream 10-Gb/s and upstream
  2.5-Gb/s
• High upstream capability (symmetric approach)
  require more expensive ONU devices

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Candidate Technologies for the NG-PON
Wavelength division multiplexing (WDM) PON
State-of-the-art experimental WDM PON support
100Mb/s 2Gb/s symmetric communication per
wavelength channel with 32 ONUs
Wavelength-routed WDM PON
Migration requirements - Change the power
splitter with the AWG - Coexistence with previous
generations of deployed devices not possible
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Hybrid (TDM/WDM) PON
Pareto principle 80 of the traffic is generated
by only 20 of the users
Utilize network resources (wavelengths)
efficiently
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Optical Amplifiers

• Typical fiber loss around 1.5 um is 0.2 dB/km
• After traveling 100 km, signals are attenuated
  by 20dB
• Signals need to be amplified or signal-to-nose
  (SNR) of detected signals is too low and bit
  error rate (BER) becomes too high (typically want
  BER lt10-9)

Different functions of an optical amplifier
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Optical Amplifiers Characteristics

• An optical amplifier is characterized by
• Gain ratio of output power to input power (in
  dB)
• Gain efficiency gain as a function of input
  power (dB/mW)
• Gain bandwidth range of wavelengths over which
  the amplifier is effective
• Gain saturation maximum output power, beyond
  which no amplification is reached
• Noise undesired signal due to physical
  processing in amplifier

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Optical Amplifiers Types

• Rare-earth doped fiber amplifiers
• Erbium Doped (EDFA) 1,500 1,600 nm band
• Praseodymium Doped (PDFA) 1,300 nm band
• Raman amplifiers 1,280 1,650 nm band
• Semiconductor Optical Amplifiers (SOAs) 400
  2,000 nm band

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Erbium Doped Fiber Amplification Process
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Erbium Doped Fiber Operation
Absorption and gain spectra for 1480 nm pump
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Raman Amplifier
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Raman Amplifier Operation
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Semiconductor Optical Amplifier
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SOA Amplification Process
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SOA Design
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Optical Amplifiers Comparison
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Modulation

• The process transmitting information via light
  carrier (or any carrier signal)
• Direct Intensity (current) 1310 nm transmitters
• Inexpensive light emitting diode (LED)
• Laser diode (LD) suffer from chirp up to 1nm
  (wavelength variation due to variation in
  electron densities in the lasing area)
• Distance lt 30 km, no EDFA

1310 nm
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External Modulation

• 1550 nm transmitters
• Expensive but can cover distance up to 120 km by
  using EDFA

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Optical Receiver

• To extract the optical signal (low level) from
  various noise disturbances
• To reconstruct original information correctly
• Selection criteria
• Optical sensitivity for a given SNR and BER,
  operating wavelength
• Dynamic range, simplicity, stability

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Photodetector Types

• The most commonly used photodetectors in optical
  communications are
• Positive-Intrinsic-Negative (PIN)
• No internal gain
• Low bias voltage 10-50 V _at_ Lambda850 nm, 5-15 V
  _at_Lambda 1300-1550 nm
• Highly linear, low dark current
• Avalanche Photo-Detector (APD)
• Internal gain (increased sensitivity)
• Best for high speed and highly sensitive
  receivers
• Strong temperature dependence
• High bias voltage 250 V _at_ Lambda850 nm, 20-30 V
  _at_Lambda 1300-1550 nm
• Costly

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Photodiode (PIN) Structure

• No carrier in the I region
• No current flow

• Reverse-biased
• Photons generated electron-hole
• Current flow through the diode

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