Lecture:3 Lightwave/Optical Systems
Ajmal Muhammad, Robert Forchheimer
Information Coding Group
ISY Department
Outline

Optical Networks



Optical Access Networks
Optical Amplifiers


Doped fibers, semiconductor optical amplifiers (SOAs)
Modulation


Core, metro, and access networks
Direct intensity, external modulation
Demodulation
Telecom Network Hierarchy
Long haul
- 100s-1000s km
- Mesh
Metro (interoffice)
- 10s of km
- Rings
Access
- a few km
- Hubbed rings
✕
The “Last” Mile
“First”
Part of core Network – Submarine Optical Cables
The longest submarine cable is the Southeast Asia—Middle East—Western Europe
(SEA-ME-WE 3) system stretching 39,000 km from Norden, Germany, to Keoje, South Korea
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
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 today’s 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
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
Optical Fiber Based Access
Networks
Power in the field required
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
Optical Line Terminal (OLT)
Optical Network Unit (ONT)
ONT for FTTH (Home)
ONT for FTTH outdoor unit
1G PON - Ethernet PON(EPON)
Broadcasting
1 Gb/s
1490-nm wavelength
Shared medium network for downstream traffic
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
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
Service Network Interface (SNI)
Media Access Control schedules
the right to use physical medium
ONU Structure
User to Network Interface (UNI)
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(@1310 nm) power budget = 30 dB
Downstream(@1490 nm) power budget= 22 dB
Second Generation PON:: LineRate 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 1.244Gb/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
expensive ONU devices
(symmetric
approach)
require
more
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
Hybrid (TDM/WDM) PON
Pareto principle
80% of the traffic is generated by only 20 % of the users
Utilize network resources (wavelengths) efficiently
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 <10-9)
Different functions of an optical amplifier
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
Optical Amplifiers :: Types
Rare-earth doped fiber amplifiers:
Doped (EDFA) – 1,500 – 1,600 nm band
Praseodymium Doped (PDFA) – 1,300 nm band
Erbium
Raman amplifiers – 1,280 – 1,650 nm band
Semiconductor Optical Amplifiers (SOAs) – 400 – 2,000 nm
band
Erbium Doped Fiber :: Amplification
Process
Erbium Doped Fiber :: Operation
Absorption and gain spectra for 1480 nm pump
Raman Amplifier
Raman Amplifier :: Operation
Semiconductor Optical Amplifier
SOA :: Amplification Process
SOA :: Design
Optical Amplifiers : Comparison
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 < 30 km, no EDFA
1310 nm
External Modulation
1550 nm transmitters

Expensive but can cover distance up to 120 km by using EDFA
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
Photodetector :: Types
The most commonly used photodetectors in optical
communications are:
Positive-Intrinsic-Negative (PIN)



No internal gain
Low bias voltage [10-50 V @ Lambda=850 nm, 5-15 V @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 @ Lambda=850 nm, 20-30 V @Lambda=
1300-1550 nm]
Costly
Photodiode (PIN) :: Structure
• No carrier in the I region
• No current flow
• Reverse-biased
• Photons generated electron-hole
• Current flow through the diode