Transcript Introduction to Optical Networking: From Wavelength

Introduction to Optical Networking:
From Wavelength Division Multiplexing
to Passive Optical Networking
Dr. Manyalibo J. Matthews
Optical Data Networking Research
Bell Laboratories, Lucent Technologies
Murray Hill, NJ 07974 USA
University of Tokyo Visit – March 22, 2004
Evolution of Lucent and Matthews/Harris Lab:
T.Harris
A.Harris
1997
AT&T
M.Matthews
2000
Lucent ‘Uber Alles’
1996
Lucent ‘A la Carte’
2001
spectroscopy,NSOM,Confocal…device physics… network subsystems!
Akiyama
Quantum
Wire Lasers
Tunable
Lasers
Semiconductor Laser
Device Physics
Matthews
Telecom
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 Networks
Core/Backbone/LongHaul
Optical
Line System
Mesh
Backbone
Network
OLS 40/80G
OLS 400G
800G/1.6T
Optical
Cross
Connect
Regional
Point
of
Presence
CO-1
Metro
Edge
Switch
Access
C/DWDM
Metro
Edge
Switch
CO-n
C/DWDM
Metro
Edge
Switch
Node
Local
Service
Node
Metro
DMX
Metro
DMX
C/DWDM
EPON
node
Regional/Metro
PON
Access/Enterprise
DSL,
FTTH
Wavelength Division Multiplexed (WDM)
Long-Haul Optical Fiber Transmission System
Transmitter l1
Transmitter
l2
l3
Receiver
M
U
X
Transmitter
WDM “Routers”
Optical Amplifier
D
E
M
U
X
Receiver
Receiver
Erbium/Raman Optical Amplifier
Categorizing Optical Networks
Who Uses
it?
Span
(km)
Bit Rate
(bps)
Multiplexing
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/T
DM
SMF/
LWPF
DFB
APD/ PIN
Access/
LocalLoop
Small
Business,
Consumer
~10
~109
(56kbps
- 1Gbps)
TDM/
SCM/
SMF/
MMF
DFB/ FP
PIN
DWDM:
CWDM:
TDM:
SCM:
SMF:
MMF:
LWPF:
DCF:
EML:
DFB:
FP:
APD:
PIN:
Dense Wavelength Division Multiplexing (<1nm spacing)
Coarse Wavelength Division Multiplexing (20nm spacing)
Time Division Multiplexing (e.g. car traffic)
Sub-Carrier Multiplexing (e.g. Radio/TV channels)
Single-Mode Fiber (core~9mm)
Multi-Mode Fiber (core~50mm)
Low-Water-Peak Fiber
Dispersion Compensating Fiber
Externally modulated (DFB) laser
Distributed Feedback Laser
Fabry-Perot Laser
Avalanche Photodiode
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.
launch
receive
Chromatic Dispersion is caused mainly by the

t
t
wavelength dependence of the index of
refraction (dominant in SM fibers)
Modal Dispersion arises from the differences in

t
Types of Dispersion
t
group velocity between the “modes” travelling
down the fiber (dominant in MM fibers)
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.
Cross-Phase Modulation:
Same as SPM, except involving more than
one WDM channel, causing cross-talk
between channels as well.
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.
Attenuation/Loss in Optical Fiber
3.0
First
Window
ATTENUATION (dB/km)
2.5
Second
Window
2.0
Third
Window
1.5
1.0
0.5
800
900
1000
1100
1200
1300
1400
1500
1600
1700
WAVELENGTH (nm)
1310nm
850nm
First window, second window,
third window correspond
(roughly) to first, second and
third generation optic
network technology
•
•
•
1550nm
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
Dispersion Characteristics*
DISPERSION COEFF, D (ps/km-nm)
Third
Window
Second
Window
3.0
0
-30
First
Window
-60
-90
-120
800
900
1000
1100
1200
1300
1400
1500
1600
1700
WAVELENGTH (nm)
850nm
•
•
* Modal dispersion not
included
•
1310nm
1550nm
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
Characterization of System Quality
Bit 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’
Distance (km)
Effect of Dispersion and Attenuation on Bit Rate
Attenuation limited
30
20
Dispersion limited
1310nm
850nm
10
1550nm
Coaxial
cable
1
Twisted Pair
0.1
1
Cat 3 Cat 5
limit limit
x
x
10
100
Bit rate (Mb/s)
Cat 7
limit
x
1000
10,000
• For short reaches (1-2 km), all optics are “Gigabit capable”
• For longer reaches (~10 km), only 1310/1550 nm optics are “Gigabit capable”
Technology Trends
850nm & 1310nm 
Preferred by high-volume,
moderate performance
data comm manufacturers
Reason? You need lots of them, they don’t need to go far,
and you’re not using enough fiber ($) to justify wavelength
division multiplexing (WDM), I.e. low-quality lasers are OK.
1310nm & 1550nm 
Preferred by high performance
but lower volume (today)
telecomm manufacturers
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
+
DFB
gain
gain
mirror
FP:
-
cleave
+
l
• Multi-longitudinal Mode
operation
mirror
Etched
grating
DFB:
-
AR coating
l
• Single-longitudinal Mode
operation
• Large spectral width
• Narrow spectral width
• high output power
• lower output power
• Cheap
• expensive
Fiber Bragg Grating External Cavity
Laser for Access/Metro Networks
Typical FBG-ECL:
Lensed
tip
gain
FBG
T=25C
HR
T=85C
AR
Optical Power (dBm)
0
Dl (3dB)
T=20C typ<0.5nm
dl/dT ~ 0.01nm/oC
-20
-40
-60
<1nm grating
Bell Labs FBG-ECL:
XB region
gain
FBG
-80
1309.0 1309.5 1310.0 1310.5 1311.0 1311.5 1312.0
Wavelength (nm)
T=25,
85C
?
HR
•
•
AR
1-2nm grating
SHOW PLOTS OF FBG-ECL DATA
SHOW PICTURE OF XPONENT’S EXTENDED REACH FP
(from Xponent Photonics, Inc.)
Fiber Bragg Grating External Cavity Laser
FBG-ECL
output
-20
Typical
FP output
• Narrow FBG bandwith limits
output Dl~1nm for extended
reach or WDM applications.
Power (dB)
-30
-40
• Simple design (AR-coated FP,
XBR, butt-coupled FBG)
-50
• Mode-hop free operation over 070C
-60
-70
1305
1310
1315
wavelength (nm)
1320
1325
Wavelength Stability of FBG-ECL
DFB drift ~ 0.1nm/oC
FP drift ~ 0.3nm/oC
1311.0
lave dependence 0.008nm/C
Wavelength(nm)
1310.9
1310.8
1310.7
1310.6
CW, ~40mA bias
1310.5
1310.4
1310.3
20
30
40
50
60
o
Temperature ( C)
70
80
Filter bandwidths of
WDM Mux/Demux
0.8nm (100GHz)
DWDM:
• High channel count, narrow channel spacing
• Temp-stablized DFBs required
• Temp-stablized AWGs required (typically)
1480nm
>100 channels (C+L+S)
1610nm
20nm
CWDM:
• Low channel count, large channel spacing
• Uncooled DFBs can be used
• Filters can be made athermal
1260nm
18 channels (O,E,S,C,L)
1610nm
3.2nm (400GHz)
xWDM?:
• Moderate channel count, moderate channel spacing
• FBG-ECL or Temp-stablized DFBs required
• Filters can be made athermal
• suitable for athermal WDM PON!
1480nm
32-64 channels (C+L+S)
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
50W line
47W chip resistor
CWDM System Improvement using
Electronic Dispersion Compensation
Example 2: Ethernet Passive Optical
Networks
Headend/CO
Outside Plant
PSTN
Internet
IP Video
Services
•
•
•
NO Active Elements in Outside Plant
Enable “triple-play” services
Simple & cheap
PON
Homes/Businesses
Choices of PONs
Architecture/Layout
Upstream Multiplexing
ONU
…
OLT
ONU
ONU
ONU
Linear Bus: lossy, fiber lean
TDM: simple, cheap
ONU
OLT
ONU
ONU
ONU
WDM:simple, expensive
Ring: lossy, protected
ONU
ONU
ONU
OLT
ONU
Simple or Cascaded Star: low loss
SCM: complex, expensive
OLT=Optical Line Termination (head-end)
ONU=Optical Network Unit (user-end)
EPON Access Platform
“premium access”
Management
Business
Data
optical
splitter
DFB
32 subscribers
Per EPON
.
.
.
EPON
BigIron 4000
NETWO RKS
1
2
3
4
Link
5
6
Link
7
8
Link
B 8G
8-port
Gigabit
Link
A ctivity
A ctivity
A ctivity
A ctivity
Base-FX
100
B24FX
Metro
Network
FOUNDRY
10G Ethernet
Or up to 6 1GbE
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
12 EPONS
Metro Edge
optical
splitter
Broadcast Video
VOD
Panther EPON OLT Chassis
Voice/IP
Services
1232  384 subscribers
Dynamic bandwidth
Guaranteed QOS
Note on Lasers:
-Use DFB at headend (shared)
-Use FP at Homes (not shared)
Residence
Lucent
EPON ONU
+ Gateway Video/IP Television
Voice/IP POTS service
High-speed data
FP
ONU Design
PON
1.25G BM
BiDi Xcvr
SERDES
(w/CDR)
GigE uplink
watchdog1
“CHILD”
BOARD
watchdog0
FPGA w/
Embedded
mProcessor
discovery
Periodic
Report
generator
Packet
memory
GMII
TX
Packet
Memory
Serial
Port
10/100bT
diagnostic
port
Mux
Demux
Memory
manager
Queue
manager
RX
Flash (CPU)
memory
CPU
TX
EPON MAC
EPON core
Report
Generator
“PARENT”
BOARD
FPGA
EPON driver
Timesta
CRC
mp
LLID
RX
Control
Parser
SERDE
S
&
Optics
ONU
PON
OLT Design
watchdog1
GigE uplink
SERDES
(w/CDR)
1.25G BM
BiDi Xcvr
watchdog0
discovery
Keepalive
scheduler
EPON driver MPCP driver
FPGA w/
Embedded
mProcessor
EPON core MPCP core
Grant
List
RX
GMII
TX
Packet
Memory
RTT table
Memory
manager
Queue
manager
RTT Processor
Report processor
Report
table
10/100bT
diagnostic
port
Flash (CPU)
memory
Serial
Port
Mux
Gate
Generator
TX
EPON MAC
Demux
Packet
memory
Timesta
CRC
mp
LLID
RX
SERDE
S
&
Optics
Control
Parser
FPGA
CPU
EPON downstream/upstream
traffic
Control “Gates”
1
Edge Router
•
3
2
OLT
Downstream: continuous, MAC addressed
– Uses Ethernet Framing and Line Coding
– Packets selected by MAC address
– QOS / Multicast support provided by Edge Router
Edge Router
•
2
O
N
U
OLT
1
2
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
3 3
2
1
O
N
U
O
N
U
2
2
3
Control “Reports”
O
N
U
O
N
U
1
O
N
U
2
2
3 3
ONU: Optical Network Unit
OLT: Optical Line Termination
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
Data
Tx FIFO
Encoder
Serializer
Clock
Prebias
Optical
output
• Driving LD below
Threshold causes
Jitter
• Off-state ~ -40dBm
“1”
“0”
“off”
Ith
current
Modulation
current
Transmitter
Physical
Media
BURST-MODE RECEIVERS
Data
Rx FIFO
Decoder
Deserializer
Clock
Reset
•
•
PROBLEM OF FAST CDR LOCKING
GAIN LEVELING & DYNAMIC
RANGE OF OPTICAL RECEIVER
CDR
Limiting
Amp
Receiver
IMPACT ON EFFICIENCY
Upstream Bursts
Cascaded PON
ONU 2
ONU 1
OLT
1:4
1:8
ONU 1
ONU 2
..
.
guardband
Throughput Efficiency
1.05
Burst-mode transceivers
Utilisation
1
0.95
0.9
0.85
Our current situation
0.8
0.75
Standa
rd GE
transc
eivers
0.7
0
1000
2000
3000
Laser AGC CDR
on settle lock
D
M
A
C
S
M
A
C
V
L
A
N
H
L
E
N
O P C
T L
T
S
I F R H
O E
T
I
D F O K
S N
L
P
ST T SM
AGC+CDR+LASER ON/OFF (ns)
Ethernet
Byte ONU1 payload Laser
sync (Ethernet Frames) off
IP
H F WC
U
D S D S A
L L S H
R
I P P E C
E A Z K
G
P T T Q K
N GS E SM
64 Bytes
TCP
Data
C
R
C
~1460 Bytes
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:
loss  10log N  L  other.losses  22dB
(For N=32, L=20km; typically ~ 24-26dB w/ connectors, splices, etc.)
• Dispersion penalty for MLMs (Agrawal 1988)
 ISI  14( BDL )2  2.8dB
(for worst case, D=6ps/nmkm, L=20km, B=1.25Gbps, =3nm
• Typical p-i-n receivers w/ ~150nA current noise, 1.25Gbps, R~1 
• -27dBm (about 1mW)
• Typical 1310nm FP lasers 0dBm output power (about 1mW)
MODE PARTITION NOISE EFFECT
D (ps/nm.km)
• 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, dl/dT ~
0.3nm/oC, and D(l)~S0l, 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!
l0
l (nm)
Bit Rate and Reach Limits due to MPN
Power penalty due to MPN given by
20
(Ogawa 1985):
18
Reach (km)
2

mpn  5 log1  Q2 mpn
k=0.5
k=0.7
k=0.9
16
14
12
Q~6.7 (BER 10-11)
2dB penalty
10
8
6
4
L  DB 
1
2
0.5
1.0
1.5
2.0
Bit Rate (Gbps)
•
•
•


  BDL


k

ln
 k 

2
mpn


0
0.0
 mpn
k
 2

1 e
2
2.5
3.0
Where k is the MPN coeficient,
dependent on mode power correlations.
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.
REDUCING MPN
• Dispersion Compensation at OLT
– Additional Loss, some cost
– One-size won’t fit all, SMF l0 ~ 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)
Arrayed
waveguides
Star coupler
Output
waveguides
Input
waveguides
P-doped v-SiO2 core
TM, y
B,P-doped v-SiO2
} core layer
TE, x
Thermal v-SiO2
(100) Si
Types of Lasers & Receivers used for
Telecommunications