1. DWDM Transmission Technology and Photonic Layer Network
Chao-Xiang Shi
Sprint
Transmission Network Development Group
Advanced Technology Laboratories
1 Adrian , Burlingame
CA 94010 1

2. Outline 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

3. Continue DWDM technology in Submarine network
- 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

4. Continue for all-optical networks - Key issue in Metro WDM network and
- 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

5. Today Technology Tomorrow Technology After…
DWDM Capacity and transmission:
Technology review
Today Technology
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
nm window (used to call L-band)
100 ~ 200 channels of 2.5 Gb/s
64 ~ 100 channels of 10 Gb/s
After…
nm window by Raman amplification

6. 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

7. 3 span: span distance 90 km, total 270 km
Span design in DWDM transmission
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

8. DFB laser with External modulation (for backbone long distance)
Optical transmitter in DWDM system:
DFB laser with external modulator
DFB laser with External modulation (for backbone long distance)
Wavelength stable, narrow band DFB laser
- DFB laser spectrum width : ~ 20 mHz
- wavelength stability: +/ 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)
- spectrum broaden
- wavelength stability

9. AWG (array waveguide grating)
Wavelength multiplex/demultiplex technology
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

10. Two-stage Optical fiber amplifier
DCF
optical
filter
OSC
980 nm
pump
EDFA1
1480 nm
EDFA2
WDM
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

11. Optical amplification, bandwidth , and capacity
Fiber loss
0.4 db
0.25 db
1310 nm
1550 nm
Wavelength (l)
C band: ~ 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: ~ 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: ~ 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

12. Fiber nonlinearity: SPM, XPM, SBS, and FWM
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.

13. Polarization mode dispersion limitation for beyond 10 Gbit/s
Y-polarization
Y-polarization
X-polarization t
X-polarization X
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.

14. PMD compensation technology
Y-polarization
X-polarization
Long distance
SM fiber
Polarization
controller (PC) X
PM fiber
Receiver
Transmitter
feedback
control signal
Electronic
process
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.

15. Capacity and transmission distance
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
Gbit/s WDM technologies
Gbit/s WDM technologies

16. …. …. Uniquely designed LCF fiber and non-zero
dispersion shift fiber (NZ-DSF)
EDFA
LCF fiber
NZ-DSF fiber
EDFA …. ….
25 km
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

17. ..…. …. Chromatic dispersion compensation in Submarine transmission
EDFA
LCF fiber
NZ-DSF fiber
EDFA
EDFA
Standard SMF fiber
EDFA
..…. ….
25 km
25 km
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

18. how is PMD impact for ultra- long distance such as
PMD concern in submarine
transmission
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.

19. One stage Er. Doped fiber amplifier
Opt.
isolator
ASE
filter
Gain equalization
filter
Er. fiber
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

20. more than 100 span and fiber amplifiers at 10 Gbit/s, but
Comparison of WDM transmission between
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.

21. Next Generation Networkl R
IP/WDM R R R l l R R R
Router
Non-IP Data Source
ATM Switch R
IP/SONET
SONET DCS or ADM R
Optical XC or ADM
IP/ATM l
Optical line System

22. All Optical Network: WDM Long Haul, Metro Backbone, and Local Collecting Ring
Central
Node 1 2 6
WDM Metro
Backbone ring
Hub 3
Hub 5
WDM local
collecting ring 4
WDM local
collecting ring

23. The ring size of metro backbone WDM network is
Description of Metro WDM Ring
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 .

24. Optical Protection Efficiency
1+1 OSNCP (Path Switch) vs. OSPRING (Optical Line) l5
1+1 OSNCP l5
OSPRING
Interconnections between routers requires 4
protection wavelengths with path switch
Same interconnections between routers requires
1 protection wavelength with OSPRING

25. 2-Fiber OMS/SPRING (conventional switching)
No Wavelength Conversion
Required
Working
Protection
Protection
Working
li - lN/2
lN/2 - lN
li - lN/2
lN/2 - lN
(li)
(lk)
(li)
(lk)
fiber 1
fiber 2 B
fiber cut
fiber 2
fiber 1
AÔC A
Ring Switch
AÔC C
CÔA
CÔA D

26. 2-Fiber OMS/SPRING (w/G.841 undersea protocol)
No Wavelength Conversion
Required
Working
Protection
Protection
Working
li - lN/2
lN/2 - lN
li - lN/2
lN/2 - lN
(li)
(lk)
(li)
(lk)
fiber 1
fiber 2 B
fiber cut
fiber 1
fiber 2
AÔC A
Ring Switch
AÔC C
CÔA
CÔA D

27. 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).

28. Survivability Alternative Tradeoffs
Service Layer Mesh
Every survivability mechanism makes tradeoffs:
Speed vs. Facility Cost (Overbuild) is most fundamental
Physical Layer (SONET & Optical) Schemes
Centralized
Mesh
Maximum Outage
Distributed
Mesh
Good
MS/
SPRING
SNCP
MSP
Good
Facility Cost [Restoration Overbuild]

29. Metro WDM Network’s Key Issue: Limited Number of OADM Nodes and Small Ring Size
Central
Node
OADM
OADM
OADM

30. l3 l4 l1 l2 l1 Central Node : l8 l8 l6 l5 l7
Metro WDM Network’s solution: Boost and Pre-Amplifiers l3 l4 l1 l2
Att.
OADM
OADM
OADM
OADM
Boost-
Amp l1
Central
Node : l8
Pre-
Amp
OADM
OADM
OADM
OADM l8 l6 l5 l7

31. Metro WDM Network’s solution: One Line-amplifier
ATT.
ATT
OADM
OADM
OADM
OADM
EDFA Input
(after ATT control)
EDFA Input (before
Tx ATT) l1 l2 l4 l3
Central
Office l3 l2 l1 l4 l5 l7 l8 l3 l1 l5 l7 l8 l4 .
EDFA l8 l1 l2 l8
...
OADM
OADM
OADM
OADM
EDFA Output l8 l7 l6 l5

32. Metro WDM Network’s solution: Line-Amplifier with Gain Slope
OADM
OADM
OADM
OADM
EDFA Input l1 l4 l3 l2 l2 l1 l5 l7 l8 l3
Central
Office l4 .
Gain curve
EDFA l8 l l4 l3 l8
...
OADM
OADM
OADM
OADM
EDFA Output l8 l7 l6 l5

33. Metro WDM Network: Experimental Set-upB C D TX RX LR
Splitter lA lB
7dB SR
Client
Combiner
Switch
OADM
filter

34. Transparent WDM Network: SONET-Less , Photonic Layer Restoration By Metro WDM Equipment
Hub
OADM
OADM A
WDM Long Haul
Network D
Hub
Hub B
OADM
OADM
Metro WDM
Metro WDM
Network 1
Network 2

35. Hybrid WDM Metro and Long Haul: Experimental Set Up
16 ch. Long Haul WDM Transmission 500 km
Fiber
Transponder
Fiber cut
l16
l16
... : :
Metro WDM
Network 1
HP Digital
Scope l1 l1 B
40 ch. Long Haul WDM Transmission 500km F lC lF G A lA lA D E lE lE
LA2
LA3 lC lF
Error
output C H
l1’
l1’
LA1
LA4 : :
Tektronix ST2400 SONET testset TA RA
l40’
l40’

36. Protection time when 16 channel long haul WDM fails
Error period
Error free
Error free

37. Protection time when 40 channel long haul WDM fails
Error free
Error period

38. BER results for working and
protection path

39. Emerging Technology: Optical CDM (CDMA) using Fiber Brag Gratings
“1”
Optical circuit dd dd
...
FBG n
...
FBG n l1 l2 ln
Input l1 l2 ln dt dt
“1”
“ ”
“ ”
Output
Fiber
Dispersion Compensation
EDFA

40. Optical Packet Switching Network
Node 2 1
100 4 5 3
IP/ATM
Network
Optical packet switching
ring network ….
l100 l1 l2 l5 l4