Advanced Technology Laboratories
DWDM Transmission Technology and Photonic
Layer Network
Chao-Xiang Shi
Sprint
Transmission Network Development Group
Advanced Technology Laboratories
1 Adrian , Burlingame
CA 94010
Transmission Network Development
Outline

Advanced Technology Laboratories
DWDM Technology in terrestrial network
- DWDM capacity and transmission distance: technology review
- DWDM transmission system
- Span design in DWDM transmission
- Optical transmitter in DWDM system: DFB laser with
external modulator
- Wavelength multiplex/de-multiplex technology in DWDM:
AWG, Dielectric filter, and Fiber grating type
- Two-stage optical fiber amplifier
- Optical amplification, bandwidth , and capacity
- Optical fiber nonlinearity: SPM, XPM, SBS, and FWM
- Polarization mode dispersion (PMD) limitation for 10 Gbit/s
and beyond
Transmission Network Development
Continue
Advanced Technology Laboratories
- PMD compensation technology
•DWDM technology in Submarine network
- capacity and transmission distance : technology review
- uniquely designed LCF fiber and non-zero dispersion shift fiber
- chromatic dispersion compensation in Submarine transmission
- PMD concern in submarine transmission
- one stage Er. Doped fiber amplifier
- comparison of WDM transmission between terrestrial and
submarine network
• Photonic layer network
- Optical network architecture
- Protection and restoration mechanism for IP/ATM directly over
WDM optical network
Transmission Network Development
Continue
Advanced Technology Laboratories
- Issues of protocols and interfaces requirements
for all-optical networks
- Key issue in Metro WDM network and
possible solutions
- Application of Metro WDM equipment in transparent
transport network: Experimental Verification
• Emerging Technology of Optical Network
- Optical CDM (CDMA)
- Optical Packet Switching Network
Transmission Network Development
DWDM Capacity and transmission:
Technology review
Advanced Technology Laboratories
Today Technology
1530 - 1560 nm window (used to call C-band)
80 ~ 100 channels of 2.5 Gb/s (50 GHz spacing)
32 ~ 40 channels of 10 Gb/s (100 GHz spacing)
70 ~ 90 km span length
4 in-line optical amplifiers and 5 spans
total 400 km transmission for 10 Gbit/s
total 600 km transmission for 2.5 Gbit/s
Tomorrow Technology
1530 - 1600 nm window (used to call L-band)
100 ~ 200 channels of 2.5 Gb/s
64 ~ 100 channels of 10 Gb/s
After…
1480 - 1530 nm window by Raman amplification
Transmission Network Development
Advanced Technology Laboratories
DWDM transmission system
OC-48/
OC-192
70-90km
Tx
OC-48/
OC-192
Uni-directional
transmission
OSC
1510 nm or 1480 nm
OC-48/
OC-192
OC-48/
OC-192
70-90km
Bi-directional
transmission
1510 nm
or 1480 nm
Transmission Network Development
Span design in DWDM transmission
Advanced Technology Laboratories
OC192 (10 Git/s) +6~8 dBm/ch
• 3 span: span distance 90 km, total 270 km
• 4 span: span distance 80 km, total 320 km
• 5 span: span distance 70 km, total 350 km
OC 48 (2.5 Gbit/s)
• 3 span: span distance 120 km, total 360 km
• 5 span: span distance 100 km, total 500 km
• 8 span: span distance 80 km, total 640 km
Transmission Network Development
Optical transmitter in DWDM system:
DFB laser with external modulator
Advanced Technology Laboratories
DFB laser with External modulation (for backbone long distance)
• Wavelength stable, narrow band DFB laser
- DFB laser spectrum width : ~ 20 mHz
- wavelength stability: +/- 0.01 nm
•DFB laser integrated with EA modulator
- Low chirping effect
- polarization stability
- low driving power required
•DFB laser with external LN modulator
- polarization problem
- high driving power required
- chirping problem
DFB laser with Direct modulation (for local area short distance)
- chirping problem
- spectrum broaden
- wavelength stability
Transmission Network Development
Wavelength multiplex/demultiplex technology
Advanced Technology Laboratories
in DWDM: AWG, Dielectric filter, Fiber grating
l
WDM Mux/Demux
• AWG (array waveguide grating)
- Insertion loss : 6 ~ 8 dB (insertion loss is almost
- channel crosstalk ~ 25 db
- application for higher channel number
• Dielectric filter WDM Mux/Demux
-insertion loss: increases when channel number increases
-channel crosstalk: 25 ~ 30 dB
-application for lower channel number WDM Mux/Demux
• Fiber Bragg grating
- need optical circulator
- cascade multipile grating to form a WDM Mux/Demux
Transmission Network Development
Two-stage Optical fiber amplifier
WDM
EDFA1
DCF
Advanced Technology Laboratories
EDFA2
WDM
optical
filter
OSC
980 nm
pump
1480 nm
pump
• 980 nm low noise pump laser for first stage EDFA
• 1480 nm high power pump laser for second EDFA
• DCF (dispersion compensation fiber) is required for 10 Gbit/s
• Attenuater is needed for 2.5 Gbit/s
• Optical isolator is used to reduce back ASE noise impact
• Optical filter is used for gain equalization
• Total gain of fiber amplifier is from 25 dB to 30 dB
• N.F. (noise figure): 5 ~ 7dB
• Output power : +17 ~ +23 dBm
•Flatten gain : +/- 1 dB with 30 nm ~ 40 nm over Er. gain range
•Dynamic input range: 15 dB
Transmission Network Development
Fiber loss
Optical amplification, bandwidth , Advanced Technology Laboratories
and capacity
0.4 db
0.25 db
1310 nm
1550 nm
Wavelength (l)
C band:
1530 ~ 1560 nm (100 Ghz channel space for 10 Gbit/s, total 40 channels,
50 Ghz channel space for 2.5 Gbit/s, total 96 channels )
L band:
1560 ~ 1600 nm (40 channel available for 10 Gbit/s, i.e. 40 gbit/s, , and
100 channels available for 2.5 gbit/s)
S band:
1480 ~ 1520 nm (40 channel available for 10 Gbit/s, i.e. 40 gbit/s, , and
100 channels available for 2.5 gbit/s)
•Total 1.2 Tbit/s capacity
• S Band: Raman amplification
• L Band: EDFFA, Ti-EDFA
• C Band: EDFA
Transmission Network Development
Fiber nonlinearity: SPM, XPM,
SBS, and FWM
Advanced Technology Laboratories
SPM: Self-phase modulation
- Create positive chirping, which cause pulse distortion due to fiber dispersion
- Result in the optical spectrum broaden which limits the channel space
XPM: Cross phase modulation
- Phase modulation between two channels due to fiber Kerr effect
- Convert phase noise (due to ASE) to intensity noise via fiber dispersion
- Limit channel space (for 10 Gbit/s channel space is 100 Ghz , 0.8nm)
SBS: Stimulated Brillouin Scattering
- Creating a new wave in backward direction through interaction between light
wave and acoustic wave
- SBS threshold can be reduced by decreasing the power level and increasing
optical spectrum.
- For 10 Gbit/s, FM modulation (~100 Mhz) of DFB laser can reduce the SBS
threshold from +5 dBm to +10 dBm.
FWM: Four wave mixing
- Optical parametric process through 3 or 4 light wave.
- Cause nonlinear channel crosstalk when transmission near zero dispersion
wavelength (a critical problem for dispersion-shift fiber)
- Standard SMF-28 is good to suppress FWM, but has too much chromatic dispersion
- True wave fiber has larger enough dispersion to suppress FWM, and small
enough chromatic dispersion, but still has dispersion slope problem.
Transmission Network Development
Polarization mode dispersion limitation
for beyond 10 Gbit/s
Y
Advanced Technology Laboratories
Y-polarization
Y-polarization
X-polarization
X
X-polarization
t
t ~ c, (nx-ny) and L
• PMD is caused by differential group delay (DGD) between two
- polarization modes
• PMD is a statistic process satisfying Maxwellian distribution
• PMD becomes serious issue for 10 Gbit/s and beyond
• PMD design
- Instantaneous PMD should be smaller than 25% pulse width
- Assuming fiber PMD is 0.3 ps/km^1/2, 400 km fiber gives mean PMD 6 ps.
If we use safety number 4 for Maxwellian distribution, the instantaneous
PMD is 24 ps. Which means 0.3 ps/km^1/2 PMD gives 400 km distance
limitation for 10 Gbit/s.
Transmission Network Development
PMD compensation technology
Y
X
Advanced Technology Laboratories
Y-polarization
X-polarization
Long distance
SM fiber
Polarization
controller (PC)
PM fiber
Transmitter
feedback
control signal
Electronic
process
Receiver
• PM fiber: with high PMD due to strong fiber birefringence
• PMD induced by long distance single mode fiber can be canceled by
using a short PM fiber with a greater PMD
• Feedback control signal to adjust input polarization of PM
fiber, so that the fast polarization axis of single mode fiber matches
to the slow axis of PM fiber and vice versa.
Transmission Network Development
Capacity and transmission distanceAdvanced Technology Laboratories
Current Transmission Technology
•1530 ~1560 nm window of EDFA
-
10 Gbit/s X 16 ch transmission (channel space 0.6 nm)
45 ~ 50 km span length
~ 150 in-line optical amplifiers
total 7500 km transmission without electronic regenerter
for 10 Gbit/s
Future Transmission Technology
- 10 Gbit/s x N (N=32~50) transmission
- 20 Gbit/s WDM technologies
- 40 Gbit/s WDM technologies
Transmission Network Development
Uniquely designed LCF fiber and non-zero
Advanced Technology Laboratories
dispersion shift fiber (NZ-DSF)
EDFA
LCF fiber
NZ-DSF fiber
….
25 km
EDFA
….
25 km
• LCF (Large core fiber)
- chromatic fiber dispersion -2 ps/km.nm
- large effective area 75 ~ 80 um^2
- bigger dispersion slope
- suppression of nonlinear effect
- used in first half span distance for higher channel power
• NZ-DSF fiber
- chromatic fiber dispersion -2 ps/km.nm
- smaller dispersion slope
- used in second half span for smaller power
- to reduce accumulation of chromatic dispersion
Transmission Network Development
Chromatic dispersion compensation
Advanced Technology Laboratories
in Submarine transmission
EDFA
LCF fiber
25 km
NZ-DSF fiber EDFA
..….
25 km
EDFA
Standard SMF fiber
EDFA
….
50 km
10 span 500 km
• Standard single mode fiber (SMF) is used for chromatic dispersion
compensation
• Dispersion compensation is performed at every 10 span (500 km)
• In order to resolve dispersion slope problem, pre-dispersion and
post-dispersion compensation are needed at transmitter and
receiver ends
Transmission Network Development
PMD concern in submarine
transmission
Advanced Technology Laboratories
• how is PMD impact for ultra- long distance such as
Submarine transmission (7500 km)?
- PMD is accumulated through the long distance
transmission by both fiber cable and every optical
component.
- define a low PMD fiber (PMD as low as
0.008 ps/km^1/2). Over 7500 km, mean fiber
PMD =6.9 ps .
- define each optical component with a small
PMD, e.g, EDFA with 0.1 ps, WDM with 0.1ps.
Transmission Network Development
One stage Er. Doped fiber
amplifier
Er. fiber
Advanced Technology Laboratories
Opt.
isolator
ASE
filter
Gain equalization
filter
980 nm pump
laser module
• 980 nm low noise pump laser module for first stage EDFA
•
•
•
•
•
•
Optical isolator is used to reduce back ASE impact
Optical filter is used for gain equalization
ASE filter (FBG) is used to get off ASE and its accumulation
Total gain of fiber amplifier is from 10 dB to 12 dB
small N.F. (noise figure): ~4 dB
Output power : ~ +11 dBm
Transmission Network Development
Comparison of WDM transmission between
Advanced Technology Laboratories
terrestrial and submarine network
• Why submarine network can transmit over 7500 km with
more than 100 span and fiber amplifiers at 10 Gbit/s, but
terrestrial network can only handle 5 span over 400 km?
7500 km vs/ 400 km is a big difference!
- Submarine transmission network is a pre-defined
system, which is more like a well controlled
experimental system in Lab.
- In terrestrial network, the characteristic of fiber in
underground is unknown. The system designer should
build equipment to cover a lot of statistic cases.
Transmission Network Development
Next Generation Network
Advanced Technology Laboratories
R
l
R
IP/WDM
R
R
R
l
l
R
R
R
Router
Non-IP Data Source
R
ATM Switch
IP/SONET
SONET DCS or ADM
R
IP/ATM
Optical XC or ADM
l
Optical line System
Transmission Network Development
All Optical Network: WDM LongAdvanced
Haul, Technology
Metro Backbone,
and
Laboratories
Local Collecting Ring
WDM Long Haul
Central
Node
Hub
1
2
6
WDM Metro
Backbone ring
3
5
Hub
4
WDM local
collecting ring
WDM local
collecting ring
Transmission Network Development
Description of Metro WDM Ring
Advanced Technology Laboratories
The ring size of metro backbone WDM network is
defined to be from 100 km to 200 km, and WDM local
collecting ring is defined from 20 km to 50 km.
In order to have a transparent (protocol independent)
transport optical network also for the low cost reason, no
electronic regenerators should be allowed in Metro WDM
rings.
Optical amplifiers might be needed in WDM metro backbone
ring network, but not in WDM local collecting ring.
Metro WDM ring should be self-healing optical ring.
network protection and restoration should be at
photonic layer .
Transmission Network Development
Optical Protection Efficiency
Advanced Technology Laboratories
1+1 OSNCP (Path Switch) vs. OSPRING (Optical Line)
l5
l5
l5
OSPRING
1+1 OSNCP
l5
l5
l5
Interconnections between routers requires 4
protection wavelengths with path switch
l5
l5
Same interconnections between routers requires
1 protection wavelength with OSPRING
Transmission Network Development
2-Fiber OMS/SPRING
(conventional switching)
Advanced Technology Laboratories
No Wavelength Conversion
Required
Working Protection
lN/2 - lN
li - lN/2
(lk)
(li)
(lk)
(li)
fiber 1
fiber 2
B
fiber
cut
AC
Protection Working
lN/2 - lN
li - lN/2
fiber 1
fiber 2
A
Ring Switch
CA
C
AC
CA
D
Transmission Network Development
2-Fiber OMS/SPRING
(w/G.841 undersea protocol)
Advanced Technology Laboratories
No Wavelength Conversion
Required
Working Protection
lN/2 - lN
li - lN/2
(lk)
(li)
(lk)
(li)
fiber 1
fiber 2
B
fiber
cut
AC
Protection Working
lN/2 - lN
li - lN/2
fiber 1
fiber 2
A
Ring Switch
CA
C
AC
CA
D
Transmission Network Development
Advanced Technology Laboratories
Optical Network Evolution Issues







How to transport large pipes (OC-48c & above) reliably? Should OC-192
be deployed in an existing OC-48 based network?
Should SONET be bypassed for ATM, FR, and IP transport over
wavelengths?
No standards on optical data interface, multi-vendor interoperability
What survivability architecture best balances performance, cost, and
flexibility?
Is synchronization required for optical network?
Mechanisms for providing OCH trail trace, mechanisms to discover fiber
topology, performance monitor and management across administrative
boundaries.
Meeting latency requirements in detecting, reporting, localizing, and
reacting to faults (e.g. protection switching).
Transmission Network Development
Advanced Technology Laboratories
Survivability Alternative Tradeoffs
Every survivability mechanism makes tradeoffs:
Speed vs. Facility Cost (Overbuild) is most fundamental
Centralized
Mesh
Good
Maximum Outage
Service
Layer Mesh
Distributed
Mesh
MS/
SPRING
SNCP
MSP
Good
Facility Cost [Restoration Overbuild]
Transmission Network Development
Metro WDM Network’s Key Issue: Limited
Advanced Technology Laboratories
Number of OADM Nodes and Small Ring Size
OADM
OADM
OADM
Central
Node
OADM
OADM
OADM
Transmission Network Development
Metro WDM Network’s solution: Boost and
PreAdvanced Technology Laboratories
Amplifiers
l1
Central
Node
l4
OADM
OADM
OADM
l1
:
l3
l2
Att.
OADM
BoostAmp
l8
PreAmp
OADM
l8
OADM
l7
OADM
l6
OADM
l5
Transmission Network Development
Metro WDM Network’s solution: One LineAdvanced Technology Laboratories
amplifier
l1
l2
l3
OADM
OADM
l4
ATT
OADM
OADM
EDFA Input (before
Tx ATT)
l1
l2
Central l3
Office l4
..
l4l3
l2l1
l5 l7l8
ATT.
EDFA Input
(after ATT control)
l4 l3 l1 l5 l7l8
EDFA
l8
l8
l1l2
...
OADM
l8
OADM
OADM
OADM
l7
l6
l5
EDFA Output
Transmission Network Development
Advanced Technology
Laboratories
Metro WDM Network’s solution: Line-Amplifier
with Gain
Slope
l1
OADM
l3
l4
OADM
OADM
l2
OADM
EDFA Input
l1
l2
l3
l4
..
l4 l3
l2l1
l5 l7l8
Central
Office
Gain curve
EDFA
l8
l8 l
l4 l3
...
OADM
l8
OADM
l7
OADM
l6
OADM
l5
EDFA Output
Transmission Network Development
Metro WDM Network:
Experimental Set-up
Advanced Technology Laboratories
7dB
C
A
7dB
7dB
D
B
7dB
Transmission Network Development
Transparent WDM Network: SONET-Less
, Photonic
Advanced Technology Laboratories
Layer Restoration By Metro WDM Equipment
c
Metro WDM
Network 3
Hub
OADM
A
Hub
B
Metro WDM
Network 1
WDM Long Haul
Network
OADM
OADM
D
Hub
OADM
Metro WDM
Network 2
Transmission Network Development
Hybrid WDM Metro and Long Haul:
Experimental Set Up
Advanced Technology Laboratories
16 ch. Long Haul WDM
Transmission 500 km
Fiber
Transponder
l16
HP Digital
Scope
Error
output
Metro WDM
Network 1
B
lC
A lA lA D
lC
C
..
.
:
:
l1
l1
F
lF
40 ch. Long Haul WDM
Transmission 500km
l1’
l1’
LA4
:
lF
H
:
TA
l40’
lE G
E lE
LA3
LA2
LA1
Tektronix
ST2400
SONET testset
l16
Fiber
cut
RA
l40’
Transmission Network Development
Protection time when 16 channel long haul
Advanced Technology Laboratories
WDM fails
Error period
Error free
Error free
Transmission Network Development
Protection time when 40 channel long haul
Advanced Technology Laboratories
WDM fails
Error period
Error free
Error free
Transmission Network Development
BER results for working and
protection path
Advanced Technology Laboratories
working path
protection path
with other Metrio traffics (working path)
1.00E-04
1.00E-05
1.00E-06
BER
1.00E-07
1.00E-08
1.00E-09
1.00E-10
1.00E-11
1.00E-12
-34
-33.5
-33
-32.5
-32
-31.5
-31
-30.5
Receiving power (dBm)
Transmission Network Development
Emerging Technology: Optical CDM
(CDMA) using Fiber Brag Gratings
“1”
Optical
circuit
dd
l1
Input
...
l2
Advanced Technology Laboratories
dd
FBG n
ln
l1
dt
... FBG n
l2
ln
dt
“1”
“1 0 1 0 0 1 ”
“1 0 1 0 0 1 ”
Fiber
EDFA
Output
Dispersion
Compensation
Transmission Network Development
Optical Packet Switching Network
IP/ATM
Advanced Technology Laboratories
Node
IP/ATM
Network
100
Network
l100
Node
Node
1
5
IP/ATM
Network
l1
l5
Node
IP/ATM
2
Network
IP/ATM
Optical packet switching
ring network
Node
l2
Node
l4
4
IP/ATM
Network
3
Network
Transmission Network Development