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Introduction to Optical Networking: From Wavelength Division
Multiplexing to Passive Optical Networking
Dr. Manyalibo J. MatthewsOptical Data Networking Research
Bell Laboratories, Lucent TechnologiesMurray Hill, NJ 07974 USA
University of Tokyo Visit – March 22, 2004
T.Harris A.Harris M.Matthews1997 2000
AT&T Lucent ‘Uber Alles’ Lucent ‘A la Carte’1996 2001
spectroscopy,NSOM,Confocal…device physics… network subsystems!
Evolution of Lucent and Matthews/Harris Lab:
Akiyama Matthews TunableLasers
TelecomLasersSemiconductor Laser
Device PhysicsQuantum
Wire Lasers
Outline• Introduction• Overview of Optical Networking
– Types of Networks– Fiber, Lasers, Receivers
• Coarse Wavelength Division Multiplexing
• Ethernet Passive Optical Networks• Conclusions & Future
Emergence of Optical NetworksO
ptic
alLi
ne S
yste
m
OLS 40/80GOLS 400G800G/1.6T
MeshBackboneNetwork Regional
Pointof
Presence
CO-1
CO-n
Core/Backbone/LongHaul
Regional/Metro
Access/Enterprise
EPONnode
MetroDMX
LocalServiceNodeMetro
EdgeSwitch
MetroEdge
Switch
OpticalCross
Connect
MetroDMX
Access
Node
Passive W
DM
Passive WDM
C/DWDM
C/DWDM
C/DWDM
MetroEdge
Switch
DSL,FTTH
PON
Wavelength Division Multiplexed (WDM)Long-Haul Optical Fiber Transmission System
Transmitter
Transmitter
Transmitter
Receiver
Receiver
Receiver
MUX
DEMUXOptical Amplifier
1
2
3
WDM “Routers” Erbium/Raman Optical Amplifier
Categorizing Optical NetworksWho Uses it?
Span (km)
Bit Rate(bps)
Multi-plexing
Fiber Laser Receiver
Core/LongHaul
Phone Company, Gov’t(s)
~103 ~1011
(100’s of Gbps)
DWDM/TDM
SMF/ DCF
EML/ DFB
APD
Metro/Regional
Phone Company, Big Business
~102 ~1010
(10’s of Gbps)
DWDM/CWDM/TDM
SMF/ LWPF
DFB APD/ PIN
Access/LocalLoop
Small Business, Consumer
~10 ~109
(56kbps- 1Gbps)
TDM/ SCM/
SMF/ MMF
DFB/ FP PIN
DWDM: Dense Wavelength Division Multiplexing (<1nm spacing)CWDM: Coarse Wavelength Division Multiplexing (20nm spacing)TDM: Time Division Multiplexing (e.g. car traffic)SCM: Sub-Carrier Multiplexing (e.g. Radio/TV channels)SMF: Single-Mode Fiber (core~9m)MMF: Multi-Mode Fiber (core~50m)LWPF: Low-Water-Peak FiberDCF: Dispersion Compensating FiberEML: Externally modulated (DFB) laserDFB: Distributed Feedback LaserFP: Fabry-Perot LaserAPD: Avalanche PhotodiodePIN: p-i-n Photodiode
Optical Fiber Attributes
Attenuation:Due to Rayleigh scattering and chemical absorptions, the light intensity along a fiber decreases with distance. This optical loss is a function of wavelength (see plot).
Dispersion: Different colors travel at different speeds down the optical fiber. This causes the light pulses to spread in time and limits data rates.
Types of DispersionChromatic Dispersion is caused mainly by thewavelength dependence of the index of refraction (dominant in SM fibers)Modal Dispersion arises from the differences in group velocity between the “modes” travelling down the fiber (dominant in MM fibers)
t
t t
t
launch receive
Non-Linear Effects in Fibers
Self-Phase Modulation: When the optical power of a pulse is very high, non-linear
polarization terms contribute and change the refractive index, causing pulse spreading and delay.
Four-wave Mixing: Non-linearity of fiber can cause ‘mixing’ of nearby wavelengths causing interference in WDM systems.
Stimulated Brillouin Scattering: Acoustic Phonons create sidebands that
can cause interference.
Cross-Phase Modulation: Same as SPM, except involving more than one WDM channel, causing cross-talk
between channels as well.
800 900 1000 1100 1200 1300 1400 1500 1600 1700
0.5
1.0
1.5
2.0
2.5
3.0First
Window SecondWindow
ThirdWindow
ATTE
NUAT
ION
(dB/
km)
WAVELENGTH (nm)1310nm 1550nm
Attenuation/Loss in Optical Fiber
• First Window @ 850nm– High loss; First-gen. semiconductor diodes (GaAs)
• Second Window @ 1310nm – Lower Loss; good dispersion; second gen. InGaAsP
• Third Window @ 1550nm– Lowest Loss; Erbium Amplification possible
850nm
First window, second window, third window correspond (roughly) to first, second and third generation optic network technology
Dispersion Characteristics*
1310nm 1550nm850nm
800 900 1000 1100 1200 1300 1400 1500 1600 1700
-120
-90
-60
-30
0
3.0
FirstWindow
SecondWindow
ThirdWindow
DISP
ERSI
ON C
OEFF
, D (
ps/k
m-n
m)
WAVELENGTH (nm)
• Standard SMF has zero dispersion at 1310nm– Low Dispersion => Pulses don’t spread in time
• Dispersion compensation needed at 1550nm– Limits data transmission rate due to ISI (inter-
symbol interference)• Dispersion not so important at 850nm
– Loss usually dominates
* Modal dispersion not included
Characterization of System QualityBit Error Rate:input known pattern of ‘1’s and ‘0’s and see how many
are correctly recongnized at output.Eye Diagram: Measure ‘openness’ of transmitted 1/0 pattern using
scope triggered on each bit.
‘Eye opening’
Effect of Dispersion and Attenuation on Bit Rate
30
10
1
Bit rate (Mb/s)
Dist
ance
(km
)
0.1 10 100 1000 10,0001
1550nm
1310nm850nm
Dispersion limitedAttenuation limited
single-mode fiber
multi-m
ode fiberCoaxialcable
• For short reaches (1-2 km), all optics are “Gigabit capable”• For longer reaches (~10 km), only 1310/1550 nm optics are “Gigabit capable”
20
x x
Cat 3 limit
Cat 7 limit
Cat 5 limit
x
Twisted Pair
Technology Trends850nm & 1310nm Preferred by high-volume,
moderate performancedata comm manufacturers
1310nm & 1550nm Preferred by high performancebut lower volume (today)telecomm manufacturers
Reason? You need lots of them, they don’t need to go far, and you’re not using enough fiber ($) to justify wavelengthdivision multiplexing (WDM), I.e. low-quality lasers are OK.
Reason? You don’t need lots, but they have to be good enough to transmit over long distances… cost of fiber (and TDM) justifies WDM… 1550nm is better for WDM
DFB vs. FP laser
Simple FP
mirror
gain
cleave
+
- mirror
gain
AR coating
+
-Etchedgrating
DFB
FP: • Multi-longitudinal Mode operation• Large spectral width • high output power• Cheap
DFB: • Single-longitudinal Mode operation• Narrow spectral width• lower output power• expensive
Fiber Bragg Grating External Cavity Laser for Access/Metro Networks
• SHOW PLOTS OF FBG-ECL DATA• SHOW PICTURE OF XPONENT’S EXTENDED REACH FP
Typical FBG-ECL:
Bell Labs FBG-ECL:
HR AR
gainFBGLensed
tipT=25C
T=85C
HR AR
gainFBG
XB region T=25, 85C
1-2nm grating
<1nm grating
1309.0 1309.5 1310.0 1310.5 1311.0 1311.5 1312.0-80
-60
-40
-20
0
Wavelength (nm)
T=20C
Opt
ical
Pow
er (d
Bm
)
(3dB) typ<0.5nmddnm/oC
?
(from Xponent Photonics, Inc.)
Fiber Bragg Grating External Cavity Laser
FBG-ECLoutput
TypicalFP output
1305 1310 1315 1320 1325-70
-60
-50
-40
-30
-20
Pow
er (d
B)
wavelength (nm)
• Narrow FBG bandwith limitsoutput ~1nm for extended reach or WDM applications.
• Simple design (AR-coated FP, XBR, butt-coupled FBG)
• Mode-hop free operation over 0-70C
20 30 40 50 60 70 801310.3
1310.4
1310.5
1310.6
1310.7
1310.8
1310.9
1311.0
ave
dependence 0.008nm/C
Wav
elen
gth(
nm)
Temperature (oC)
Wavelength Stability of FBG-ECL
CW, ~40mA bias
DFB drift ~ 0.1nm/oCFP drift ~ 0.3nm/oC
Filter bandwidths of WDM Mux/Demux
0.8nm (100GHz)
>100 channels (C+L+S)
20nm
18 channels (O,E,S,C,L)
3.2nm (400GHz)
32-64 channels (C+L+S)
DWDM:• High channel count, narrow channel spacing• Temp-stablized DFBs required• Temp-stablized AWGs required (typically)
CWDM:• Low channel count, large channel spacing• Uncooled DFBs can be used• Filters can be made athermal
xWDM?:• Moderate channel count, moderate channel spacing• FBG-ECL or Temp-stablized DFBs required• Filters can be made athermal• suitable for athermal WDM PON!
1260nm 1610nm
1480nm 1610nm
1480nm 1610nm
Example 1: 10Gbps Coarse WDM -Used currently in Metro systems (rings, linear, mesh)-Spacing of CWDM ‘grid’ determined by DFB wavelength drift-Current systems limited to 2.5Gbps due to cheaper optics-Possible upgrade to 10Gbps?
CWDM Lasers 16 uncooled, directly modulated CWDM lasers (DMLs)
rated for 2.5 Gb/s direct modulation (cheap! - $350 a piece)
NRZ-modulation at 10 Gb/s (careful laser mounting; no device selection)
2.5-Gb/s DML 50line
chip resistor
CWDM System Improvement using Electronic Dispersion Compensation
Example 2: Ethernet Passive Optical Networks
• NO Active Elements in Outside Plant• Enable “triple-play” services• Simple & cheap
IP VideoServices
PSTN
InternetPON
Headend/COHomes/BusinessesOutside Plant
Choices of PONs
Architecture/Layout Upstream Multiplexing
OLT …
ONU
ONU
OLT
WDM:simple, expensive
TDM: simple, cheap
SCM: complex, expensive
Linear Bus: lossy, fiber lean
Ring: lossy, protected
OLTONU
Simple or Cascaded Star: low loss
ONUONUONU
ONUONUONU
ONUONUONU
OLT=Optical Line Termination (head-end)ONU=Optical Network Unit (user-end)
EPON Access Platform
Video/IP TelevisionVoice/IP POTS serviceHigh-speed data
Residence
Metro Edge
Voice/IPServices
Business
Broadcast Video VOD
Management
Metro Network
Data
10G EthernetOr up to 6 1GbE
EPON
opticalsplitter
opticalsplitter
32 subscribersPer EPON
Panther EPON OLT Chassis1232 384 subscribersDynamic bandwidthGuaranteed QOS
“premium access”
.
.
.
12 EPONS
Lucent EPON ONU + Gateway
Note on Lasers:-Use DFB at headend (shared)-Use FP at Homes (not shared)
DFB
FP
ONU Design
ReportGenerator
Packet Memory
TX
RX
ControlParser
Dem
ux
watchdog0
watchdog1
discoveryPeriodicReport
generatorEPON driver
EPON core
RX
TX
EPON MAC
Mux Timesta
mpCRC LLIDMemory
managerQueue
manager
GMII
SERDES&
Optics
CPUFPGA
Serial Port
GigE uplink
Packet memory
1.25G BM BiDi Xcvr
Flash (CPU)memory
10/100bTdiagnosticport
SERDES(w/CDR)
PON
FPGA w/EmbeddedProcessor
“CHILD” BOARD
“PARENT”BOARD
ONU
GrantList
GateGenerator
Packet Memory
RTT table
TX
RX
ControlParser
Dem
ux
watchdog0
watchdog1
discovery Keepalive scheduler
EPON driver MPCP driver
EPON core MPCP core
RX
TX
EPON MAC
Mux Timesta
mpCRC LLIDMemory
managerQueue
manager
RTT Processor
Report processor
GMII
SERDES&
Optics
Report table CPUFPGA
OLT Design
Serial Port
GigE uplink
Packet memory
1.25G BM BiDi Xcvr
Flash (CPU)memory
10/100bTdiagnosticport
SERDES(w/CDR)
PON
FPGA w/EmbeddedProcessor
• Downstream: continuous, MAC addressed– Uses Ethernet Framing and Line Coding– Packets selected by MAC address– QOS / Multicast support provided by Edge Router
• Upstream: Some form of TDMA– ONU sends Ethernet Frames in timeslots– Must avoid timeslot collisions– Must operate in burst-mode– BW allocation easily mapped to timeslots
EPON downstream/upstream traffic
1 2 3 2
1
2 2
3
1 2 3 21
2 2
3
1 2 3 2
1 2 3 21
23
2
12
2
OLT
OLT
3
3
3 3
ONU
ONUO
NU
ONU
ONUO
NU
Edge Router
ONU: Optical Network UnitOLT: Optical Line Termination
Edge Router
Control “Gates”
Control “Reports”
PON TDMA BURSTMODE OPTICS
• Because upstream transmissions must avoid collisions, each ONU must transmit only during allowed timeslot
• Transmitting “0”s during quiet time is not allowed!– Average “0” power ~ -10 to –5 dBm – Summing over 16 ONUs would result in a ~1dBm noise floor
• Distinct from “Bursty” nature of Ethernet TRAFFIC – Ethernet transmitters never stop transmitting (Idle characters)– CDR circuit at receiver stays locked even when no data is transmitted
• Besides PONs, other systems use burstmode– Wireless– Shared buses/backplanes– Optical burst switched (OBS) systems
BURSTMODE TRANSMITTERS
Tx FIFO Encoder Serializer TransmitterData
ClockPrebias
Physical Media
currentIth
Optical output
“0”
“1”
Modulationcurrent
“off”
• Driving LD belowThreshold causesJitter• Off-state ~ -40dBm
BURST-MODE RECEIVERS
• PROBLEM OF FAST CDR LOCKING• GAIN LEVELING & DYNAMIC
RANGE OF OPTICAL RECEIVER
Rx FIFO CDR LimitingAmp ReceiverData
Clock
DeserializerDecoder
Reset
IMPACT ON EFFICIENCY
~1460 Bytes64 Bytes
CRC
DMAC
SMAC
VLAN
HLEN
TOS
LEN
ID
OFF
ST
TTL
PROT
CHK
SM
SIP
DIP
ACK
HLEN
FLA
GS
WSZE
CHK
SM
URG
SPT
DPT
SEQ Data
1:4OLT
ONU 1
1:8ONU 2...
Upstream BurstsCascaded PON
guardband
ONU 1ONU 2
Ethernet IP TCP
Laser on
AGCsettle
CDRlock
Bytesync
ONU1 payload(Ethernet Frames)
Laser off
Throughput Efficiency
0.70.75
0.80.85
0.90.95
11.05
0 1000 2000 3000
AGC+CDR+LASER ON/OFF (ns)
Util
isat
ion
Our current situation Standard GE transceivers
Burst-mode transceivers
Conclusions• Optical Networking getting closer and
closer to end user• For Metro, CWDM is lowest cost solution,
but must be improved to handle 10Gbps• PON systems could deploy ‘in mass’ over
next 1-2 years, with EPON one of the leading standards
• Lasers dominate cost, therefore useful to study physics of low-cost laser structures!
THANK YOU VERY MUCH!(Domo Arigato Gozaimashita!)
Spare Slides
SYSTEM PENALITIES in PONs• Attenuation in PONs dominated by power splitters:
• Dispersion penalty for MLMs (Agrawal 1988)
• Typical p-i-n receivers w/ ~150nA current noise, 1.25Gbps, R~1 • -27dBm (about 1W)• Typical 1310nm FP lasers 0dBm output power (about 1mW)
dBBDLISI 8.2)(14 2
(for worst case, D=6ps/nmkm, L=20km, B=1.25Gbps, =3nm
dBlossesotherLNloss 22.log10
(For N=32, L=20km; typically ~ 24-26dB w/ connectors, splices, etc.)
MODE PARTITION NOISE EFFECT
• Mode Partition Noise is due to fluctuations in individual Fabry Perot modes coupled with optical fiber dispersion.
• Due to uncontrolled temperature and wavelength drift in FP diodes, d/dT ~ 0.3nm/oC, and D()~S0, the magnitude of this penalty will change with time.
• Due to lack of screening of FP mode partition coefficient, k, the magnitude of this penalty will also depend on particular FP!D
(ps/
nm.k
m)
(nm)0
Bit Rate and Reach Limits due to MPN
• Reach dependent on “quality” of laser (k factor)• (another) Reason why asymmetry in PONs (e.g., 155/622Mbps) are favored… GigE?• Worst-case isn’t quite fair… statistical model shows most fiber-laser combinations, D<3ps/nmkm, k<0.5.
2ln1
mpnkkBDL
2
12
ekmpn
BDL
Power penalty due to MPN given by(Ogawa 1985):
221log5 mpnmpn Q
Where k is the MPN coeficient, dependent on mode power correlations. 0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
2
4
6
8
10
12
14
16
18
20
Q~6.7 (BER 10-11)2dB penalty
Rea
ch (k
m)
Bit Rate (Gbps)
k=0.5 k=0.7 k=0.9
REDUCING MPN
• Dispersion Compensation at OLT– Additional Loss, some cost– One-size won’t fit all, SMF 0 ~ 1300-1325nm
• High-pass filtering using SOA– Low frequency MPN components are partially removed
• Very low noise FP LD driver• Replace FP w/ narrow-line source
– DFB is current solution– 1310nm VCSEL (high-power)– Fiber Bragg Grating ECL also a possibility if cost/integration improves
Structure of WDM MUX/DEMUX (Arrayed Waveguide Grating)
(100) Si
B,P-doped v-SiO2
Thermal v-SiO2
P-doped v-SiO2 core
} core layer
TM, y
TE, x
Inputwaveguides
Outputwaveguides
Arrayedwaveguides Star coupler
Types of Lasers & Receivers used for Telecommunications