Introduction to DWDM Technology James Tai
Outline • General Background • Why DWDM? • Fundamentals of DWDM Technology • Future Trend
General Background • Bandwidth Explosion Optical Transmission Doubling every 9 Months
Data Storage Doubling every 12 Months Silicon Processing (Moore’s Law) Doubling every 18 Months Year
Why DWDM? • High Bandwidth Demand: - Bandwidth are doubling every 3 months - Internet traffic increases thousand-fold every 3 years
How to increase Bandwidth • SONET& TDM: Increase the bit rate by using high speed electronics OC-12 OC-48 OC-192 OC-768 622 Mb/s 2.5 Gb/s 10 Gb/s 40 Gb/s Note: For signal rate <10 Gbps, the cost per bit will drop approximately 40% when the bit rate increases fourfold. • FDM: Increase the radio frequency channel (BW) • WDM: Increase the capacity of a single fiber by using a technology of combining and separating optical signals of different wavelengths sent along an optical fiber - e.g. Multiplex multiple TDM signals of OC-N over a single fiber (multiple OC-N signals each over a single fiber X N fibers) = NX(OC-N) signal over a single fiber
What is WDM ?
Evolution of WDM
TDM vs WDM SONET TDM
WDM (DWDM)
Optically multiplex individual Electronically multiplex signals to a single higher bit wavelengths over a single rate at a single wavelength fiber for transmission Optical-to-electrical (O-to-E) No O-to-E conversion conversion before signals before signals being being multiplexed/demuxed multiplexed/demuxed Carry synchronous TDM hierarchy
Carry multiple protocols (protocol independent)
WDM v.s. DWDM • DWDM spaces the wavelengths more closely than does WDM, and therefore has a greater overall capacity • State-of-the-art technology: 273 wavelengths, 40Gbps/wavelength 10.9Tbps over single fiber (NEC, Mar2001) this capacity means (1) 1,560M of DS0 or (2) 167M of MPEG-2 or (3) 2.5K of CD-ROM (500MB/CD-ROM)
Fundamentals of DWDM Technology 1. Optical Fiber 2. Optical Light Source and Detector 3. Optical Amplifier 4. DWDM Multiplexer and Demultiplexer 5. Optical Switch (Optical Cross-Connect) 6. Optical Add/Drop Multiplexer 7. Wavelength Router 8. Optical DWDM Transponder
Optical Fibers • Fiber cable: core /cladding layer diameter Multi-mode fiber (MMF): 50/125 or 62.5/125 µm Single-mode fiber (SMF): 9/125 µm
MMF core SMF core
Cladding layer Light path
“S” band
Fiber Attenuation & DWDM Operating Bandwidth
Note:DWDM BW (1) S-band: 1485~ 1520 nm (2) C-band:1530 ~ 1562 nm (3) L-band: 1570 ~1610 nm
Transmission Problems in Optical Fibers • Linear Effects: can be compensated (1) Attenuation (2) Dispersion • Non-Linear Effects: will accumulate (not so critical in short-haul network) (1) Polarization Mode Dispersion (not a problem at speeds < OC-192) (2) Stimulated Brillouin Scattering (3) Stimulated Raman Scattering (4) Self-Phase Modulation (5) Four-Wave Mixing (the most critical effect; will limit the channel capacity of DWDM system)
Dispersion Concept of Dispersion
Horse Race Input Output
Dispersion
17 ps/(Km*nm)
-100 ps/(Km*nm)
Stimulated Raman Scattering Tilt Output (After SRS)
Power
Flat Input
Optical Fiber
Optical Channels (Optical Frequency, nm)
Four-Wave Mixing
Channels f 1, f 2, f 3 interact to
Four Wave Mixing creates
create a intermodulation product
cross-talk for channel f 1
(sideband) at (f1+f2+f3)
f1
f2
f3
f4
Frequency
Optical Carriers (@ 50/100GHz Spacings)
Optical Fibers • Fiber cable attenuation: Depend on core size and operating wavelength SMF
MMF Core Diamater (µm)
50
62.5
9
Wavelength (nm)
850
1300
850
1300
Loss (dB/Km)
2.7
0.8
3.2
0.9
BW (MHz) x Length (Km)
400
1000
200
500
Dispersion ps/(nm x Km) New Fiber: 10GbpsX40m
1310
1550
0.35
0.22
1
17
Optical Fibers • Limitation on System Performance Using MMF (1) Insufficient bandwidth and transmission distance (2) Higher loss than SMF’s (3) Interference induced modal noise → SNR degradation
Optical Fibers • Three MajorTypes of Single Mode Fiber (SMF): (1) Non-dispersion-shifted fiber (NDSF), G.652 (standard SMF) (a) >95% of deployed plant; has serious fiber dispersion problem (b) suitable for TDM use in single channel 1310 nm or DWDM use in 1550 nm window (with dispersion compensators) (2) Dispersion-shifted fiber (DSF), G.653 (a) exhibits serious fiber nonlinearity problem, i.e. FWM (b) Suitable for TDM use in the 1550 nm window, but not suitable for DWDM (3) Non-zero dispersion-shifted fiber (NZ-DSF), G.655 (meet the needs of DWDM applications) As bit rates approach to 10 Gb/s and beyond, the interdependence between system and fiber design will be very important for system planning
Chromatic Dispersion
Optical Light Sources and Detectors • Light Source: (a) Light Emitting Diode (LED) (b) Laser Diode (LD): VCSEL, Fabry-Perot (FP) Laser, Distributed Feedback Laser (DFB)
LED
FP
DFB
BW
Narrow
~
Wide
Noise
High
~
Low
Linearity
Poor
~
Good
Environmental Influence
Unstable
~
Stable
Application
Digital, <1 Gbps
Digital & Analog
DWDM Digital (10Gb/s) & Analog
Optical Light Sources and Detectors • L-I Response of Light Source: (b) Laser Diode
Optical Power (mW)
(a) LED
Modulated Optical Signals Distorted Signals
Bias (mA) AC Signal
AC Signal
Comparison of Key Performance Features for VCSEL, DFB, and FP lasers (Source: 2001, Mar. issue of Fiber Optic Product News) VCSEL Emission Type Surface Emission Pattern Circular Divergence Angle ~ 10 degree Spectral Width 0.1 nm Peak Modulation Speed 20 Gb/s Threshold Current 1 ~ 5 mA Fiber Coupling Efficiency 80% Coupling Optics Not required Wavelength Drift ~0.1 nm/deg C Link Distance for 10 GbE VSR (850 nm, 300m of Transponder new MM fiber) IR (1310 nm, 2~12Km) Power Consumption for 10 3W ~ 4W GbE Transponder Rel. Price of packaged 1X Laser @ 1Gb/s
DFB Edge Elliptical ~ 30 degree 0.1 nm ~ 10 Gb/s 10 ~ 15 mA 10% Aspheric lens ~0.1 nm/deg C IR (Direct Modulation)
Fabry Perot Edge Elliptical ~ 30 degree 2 ~ 5 nm ~ 10 Gb/s 2 ~ 5 mA 10% Aspheric lens ~0.5 nm/deg C IR
7W ~ 10W
7W ~ 10W
25X
4.5X
Direct Modulation v.s. External Modulation • Direct Modulation: Chirp can become a limiting factor at high bit rates (> 10 Gb/s)
RF Input
DFB
Optical Output (SMF)
• External Modulation: help to limit chirp Phase Modulator RF Input Bias Control VRF VBIAS
DFB λ @ ITU -grid
PM fiber in
3 dB Coupler
SMF fiber out
ITU Defined Wavelengths (100GHz = 0.8 nm) Channel N u m b er
W a v e len g th (n m )
Frequency (G H z )
Channel N u m b er
W a v e len g th (n m )
Frequency (G H z )
15
1 5 6 5 .4 9 6 1
1 9 1 ,5 0 0
44
1 5 4 2 .1 4 2 5
1 9 4 ,4 0 0
16
1 5 6 4 .6 7 9 0
1 9 1 ,6 0 0
45
1 5 4 1 .3 4 9 6
1 9 4 ,5 0 0
17
1 5 6 3 .8 6 2 8
1 9 1 ,7 0 0
46
1 5 4 0 .5 5 7 6
1 9 4 ,6 0 0
18
1 5 6 3 .0 4 7 5
1 9 1 ,8 0 0
47
1 5 3 9 .7 6 6 3
1 9 4 ,7 0 0
19
1 5 6 2 .2 3 2 9
1 9 1 ,9 0 0
48
1 5 3 8 .9 7 5 9
1 9 4 ,8 0 0
20
1 5 6 1 .4 1 9 3
1 9 2 ,0 0 0
49
1 5 3 8 .1 8 6 3
1 9 4 ,9 0 0
21
1 5 6 0 .6 0 6 5
1 9 2 ,1 0 0
50
1 5 3 7 .3 9 7 4
1 9 5 ,0 0 0
22
1 5 5 9 .7 9 4 5
1 9 2 ,2 0 0
51
1 5 3 6 .6 0 9 4
1 9 5 ,1 0 0
23
1 5 5 8 .9 8 3 4
1 9 2 ,3 0 0
52
1 5 3 5 .8 2 2 2
1 9 5 ,2 0 0
24
1 5 5 8 .1 7 3 1
1 9 2 ,4 0 0
53
1 5 3 5 .0 3 5 8
1 9 5 ,3 0 0
25
1 5 5 7 .3 6 3 6
1 9 2 ,5 0 0
54
1 5 3 4 .2 5 0 3
1 9 5 ,4 0 0
26
1 5 5 6 .5 5 5 0
1 9 2 ,6 0 0
55
1 5 3 3 .4 6 5 5
1 9 5 ,5 0 0
27
1 5 5 5 .7 4 7 3
1 9 2 ,7 0 0
56
1 5 3 2 .6 8 1 5
1 9 5 ,6 0 0
28
1 5 5 4 .9 4 0 4
1 9 2 ,8 0 0
57
1 5 3 1 .8 9 8 3
1 9 5 ,7 0 0
29
1 5 5 4 .1 3 4 3
1 9 2 ,9 0 0
58
1 5 3 1 .1 1 5 9
1 9 5 ,8 0 0
30
1 5 5 3 .3 2 9 0
1 9 3 ,0 0 0
59
1 5 3 0 .3 3 4 4
1 9 5 ,9 0 0
31
1 5 5 2 .5 2 4 6
1 9 3 ,1 0 0
60
1 5 2 9 .5 5 3 6
1 9 6 ,0 0 0
32
1 5 5 1 .7 2 1 0
1 9 3 ,2 0 0
61
1 5 2 8 .7 7 3 6
1 9 6 ,1 0 0
33
1 5 5 0 .9 1 8 3
1 9 3 ,3 0 0
62
1 5 2 7 .9 9 4 4
1 9 6 ,2 0 0
34
1 5 5 0 .1 1 6 3
1 9 3 ,4 0 0
63
1 5 2 7 .2 1 6 0
1 9 6 ,3 0 0
35
1 5 4 9 .3 1 5 3
1 9 3 ,5 0 0
64
1 5 2 6 .4 3 8 4
1 9 6 ,4 0 0
36
1 5 4 8 .5 1 5 0
1 9 3 ,6 0 0
65
1 5 2 5 .6 6 1 6
1 9 6 ,5 0 0
37
1 5 4 7 .7 1 5 5
1 9 3 ,7 0 0
66
1 5 2 4 .8 8 5 6
1 9 6 ,6 0 0
38
1 5 4 6 .9 1 6 9
1 9 3 ,8 0 0
67
1 5 2 4 .1 1 0 3
1 9 6 ,7 0 0
39
1 5 4 6 .1 1 9 1
1 9 3 ,9 0 0
68
1 5 2 3 .3 3 5 9
1 9 6 ,8 0 0
40
1 5 4 5 .3 2 2 2
1 9 4 ,0 0 0
69
1 5 2 2 .5 6 2 2
1 9 6 ,9 0 0
41
1 5 4 4 .5 2 6 0
1 9 4 ,1 0 0
70
1 5 2 1 .7 8 9 3
1 9 7 ,0 0 0
42
1 5 4 3 .7 3 0 7
1 9 4 ,2 0 0
71
1 5 2 1 .0 2 0 0
1 9 7 ,1 0 0
43
1 5 4 2 .9 3 6 2
1 9 4 ,3 0 0
72
1 5 2 0 .2 5 0 0
1 9 7 ,2 0 0
ITU-Grid (ITU-G.692) Wavelengths • Optical channel numbers can be increased by spacing the wavelengths more closely, at 50 GHz, to double the number of channels. However, spacing at 50 GHz limits the maximum data rate per λ to 10 Gb/s • The closer the wavelength spacings, the more optical channel crosstalk results • Nonlinear interactions among different DWDM channels creates intermodulation products (FWM) that can induce interchannel interference, resulting in crosstalk and SNR degradation. The closer the spacings, the more FWM interference results
Optical Transceiver Evolution (using SMF)
Optical Line Rate
100
10
Short Reach
Intermediate Reach
Short Reach Long Reach 1300 nm 1550 nm
VCSEL
1
0.1
0.01 1
10 Distance (Km)
100
Spectral Response for Photodiode
Si
Ge
InGaAs
Optical Receiver Design Issue • PIN Photodiode:
CONTACT METALIZATION
p
Wd
DEPLETION LAYER
i
SUBSTRATE n
p-InGaAs or p-InP
electron diffusion
n-InGaAs
carrier drift
n-InP
hole diffusion
V
RL Photodiode
Tuning + Matching Circuit
To 50 Ohm Load
hν
- Two Important Design Issues for “impedance matched receiver”: (1) Low Noise (2) Wide Bandwidth
Photodiode
PIN Photodiode Photon-Electron Conversion Receiver Sensitivity Cost Reliability Temperature Sensitivity
1:1
Avalanche Photodiode (APD) 1:N (N=10)
Medium Low High Low
High High Moderate High
Optical Amplifiers - DWDM Enabler (1) Conventional Design Tx
Repeater
Rx
3R Functions: - Retiming - Reshaping - Retransmission
(2) New Design (can save 60 to 80% regenerator costs) Tx
Optical Amplifier 1R Function: -Retransmission or Reamplification
Rx
Optical Amplifiers
DWDM Bandwidth
Optical Amplifiers • Optical Fiber Amplifier - Pr-Doped Fiber Amplifier (PDFA; 1310nm region) - Th-Doped Fiber Amplifier (TDFA; S Band in 1500 nm region, 20 dB gain, 35 nm gain BW) - Er-Doped Fiber Amplifier (EDFA; C or L Band in1550nm region, 30~ 40 dB gain) • EDWA: Er-Doped Waveguide Amplifier (14dB gain) • Semiconductor Optical Amplifier (SOA) - can operate in 1310 nm or 1550 nm region, 30 nm gain BW - not suitable for DWDM transmission • Raman Amplifier - can provide gain from 1300 to 1550 nm or wider, 20 dB gain
Erbium-Doped Fiber Amplifier • Single Channel EDFA
• DWDM EDFA EDF, pre-amp stage
EDF, booster stage
Dispersion Gain Flattening Compensation Unit Filter
980-nm pumps
1480-nm pumps
EDFA Flattened Gain Response
Erbium-Doped Waveguide Amplifier
Gain @3~5dB/cm; Total length: 5~ 10 cm
Note: Pump Mux, Tap Coupler, and Mode Adapter can be integrated on to a single chip. (Drawback: absence of integrated isolators)
Performance Comparison among Optical Amplifiers
Optical Raman Amplifier
(A) Discrete Raman Amplifier (using specialty fiber)
(B) Distributed (Lumped) Raman Amplifier (using transmission fiber) - pump @ 1450 nm, - remote & back inject into 100Km fiber - distributed gain over 40 Km - pumping efficiency ~ 1/5* EDFA’s
Why use Raman Amplifier? • Improve system signal-to-noise ratio (SNR) • Permit higher-speed (40Gbps) transmission by reducing fiber nonlinearity • Extend repeater span • Raman gain from 1300 to 1500 nm or wider
DWDM Transmission Span • 80 Km for each span • DWDM terminal spacing ~ 400~600 Km (followed by a regenerator)
Cascaded Optical Amplifiers 400 ~ 600Km (link)
• Concerned Factors: (1) Fiber type (2) Transmission distance (3) Channel count and bit rate
(4) Amplifier spacing (5) Amplifier noise (6) Amplifier power
DWDM
DWDM
80 Km (span)
DWDM Multiplexer/Demultiplexer
Technologies include: • Thin film coating filters • Fiber Bragg gratings • Diffraction gratings
• Arrayed waveguide gratings • Fused biconic tapered devices • Inter-leaver devices
Device Aspects of WDM Filter - Figure of merit, -0.5 dB bandwidth/ -30 dB bandwidth - Low loss - Low Polarization sensitivity - Flat top Channel Spacing
- Steep roll-off - Stable & Manufacturable Crosstalk
Filter Bandwidth
DWDM Multiplexer/Demultiplexer Advantages Thin Film Coating Filters (1) Flexible in channel count and irregular wavelength plan (2) Totally passive/temperature stable (3) Good optical performance in isolation, insertion loss, PDL, and PMD (4) wideband application (up to 16 Chs) Fiber Bragg Gratings (1) Excellent filter shape (2) Good optical performance in isolation, insertion loss(when used as a notch filter) (3) Short development time (4) Fused coupler + FBG, achieve 50 GHz spacing Arrayed Waveguide Gratings (1) Cost is not proportional to channel Count (cost effective for DWDM ) (2) Short development time to dense channel spacings (5) Relative low insertion loss for high channel count (6) Compact size (7) Potential to integrate with other functions
Disadvantages (1) Takes longer time to develop and accumulate filters with dense channel spacing (2) Cost is proportional to channel count
(1) Not suitable for wideband applications (2) Need temperature stabilization (3) Cost is proportional to channel count
(1) Poor filter shape (2) High nonadjacent channel noise (3) Need temperature stabilization (4) High PDL and PMD
DWDM Multiplexer/Demultiplexer
Interleaver
Optical Switch • MEMS(micro-electromechnical system)-Based Photonic Switch:
Performance for 1X2/2X2 MEMS-Based Latching Optical Switch (using 2-D MEMS)
MEMS Crossconnects • 2-D Design MEMS Plan 4
Plan 3
Plan 3
Plan 2 2 Plan Plan 1
• 3-D Design
Plan 2 Plan 1
Plan 2 Plan 1
Optical Add/Drop Multiplexer • Current OADM (Add/Drop fixed wavelengths) • Emerging OADM (Add/Drop any selection of wavelengths)
Characteristics of Optical Add/Drop Multiplexer • Has one or more optical fiber inputs and corresponding outputs, with multiple wavelengths multiplexed on each fiber • Demultiplexes some or all of the wavelengths on the coming fiber and drops these wavelengths, one wavelength per fiber, to subscribers and directly or via electronic demultiplexing to lower data rates • Add signals from subscribers, one wavelength per fiber, multiplexes these on outgoing fiber
Optical Add/Drop Multiplexer
Optical Amplifier
DWDM
λ1 ~ λ8
DWDM
• Current Throughput: 8 ~ 16 X 2.5 Gb/s = 20 ~ 40 Gb/s
R R Fiber to Subscriber
T T
Electronic Add/Drop
R: Receiver T: Transmitter @ fixed λ
Fiber from Subscriber
Optical Add/Drop Multiplexer • Estimated Throughput in 2008: 128 X10Gb/s = 1.28 Tb/s
Optical Amplifier
DWDM
λ1 ~ λ128
DWDM
Optical Crossconnect (128X256)
R R Fiber to Subscriber
T T
Electronic Add/Drop
R: Receiver T: Tunable Transmitter
Fiber from Subscriber
Wavelength Router (Dynamic WDM Crossconnect)
(Source: 2001, Mar issue of Lightwave) Tunable laser inside (1) Tuning speed < 2 ns, (2) Tuning throughout the C-band <15ns (3) Synchronization time < 40 ns
Optical Transponder / Wavelength Adapter
DWDM
DWDM Optical Transponder λ @850/1310/1550nm)
Optical Transponder
(1) Embeded DWDM Operation
(2) Open DWDM Operation
Pre-
DWDM
DWDM
Operation of DWDM-Based System
Optical Bandwidth • Optical bandwidth can be increased by increased by improving DWDM system in three areas:
Current benchmark
Channel Spacing
Channel Bit Rate
Fiber Bandwidth
50 GHz
10 Gb/s @ 50GHz spacing C band
State-of-the-art technology 25 GHz
40Gb/s @ 100GHz spacing S and L band
Improvement gain
X2
X2
X3
Challenge
(1) Laser stabilization (2) Mux/DeMux tolerance (3) Filter technology (4) Fiber nonlinear effects
(1) PMD mitigation (2) Dispersion compensation (3) High speed SONET Mux/DeMux
(1) Optical Amplifier (2) Band Splitters & Combiners (3) Gain tilt due to stimulated Raman scattering
Current Networking Status
Migrating the SONET Ring to DWDM DWDM Terminal
Future Trend Elimi
natin
Current IP / ATM / SONET Layering
g Pro tocol layer s
IP / MPLS
Packet-over-SONET Layering
ATM
IP / MPLS
SONET / SDH
SONET / SDH
Optical Transport Time
Direct IP-over-DWDM Layering IP / MPLS
Key requirements in the MAN for DWDM systems
• Multiprotocol support • Scalability • Reliability and availability • Openness (interface, network management, stand fiber types, electromagnetic compatibility) • Ease of installation and management • Size and power consumption • Cost effectiveness
Metro DWDM Systems
Metropolitan Area Networks • Metro Core • Metro Access • Enterprise
Optical Networking Applications in MAN
