Introduction to DWDM Technology

James Tai

•General Background

•Why DWDM?

•Fundamentals of DWDM Technology

•Future Trend

Outline

General Background

Year

Optical TransmissionDoubling every 9 Months

Data StorageDoubling every 12 Months

Silicon Processing (Moore’s Law)Doubling every 18 Months

•Bandwidth Explosion

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

Carry multiple protocols (protocol independent)

Carry synchronous TDM hierarchy

No O-to-E conversion before signals being multiplexed/demuxed

Optical-to-electrical (O-to-E) conversion before signals being multiplexed/demuxed

Optically multiplex individual wavelengths over a single fiber

Electronically multiplex signals to a single higher bit rate at a single wavelength for transmission

WDM (DWDM)SONET TDM

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

• Fiber cable: core /cladding layer diameter

Multi-mode fiber (MMF): 50/125 or 62.5/125 µm

Single-mode fiber (SMF): 9/125 µm

SMF core

MMF core

Cladding layerLight path

Optical Fibers

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

“S”

ban

d

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

Output

Concept of Dispersion

Horse Race

Input

Dispersion

17 ps/(Km*nm) -100 ps/(Km*nm)

Optical Channels (Optical Frequency, nm)

Pow

er

Optical Fiber

Flat InputTilt Output (After SRS)

Stimulated Raman Scattering

Four-Wave Mixing

Optical Carriers (@ 50/100GHz Spacings)

f1 f2 f3 f4 Frequency

Four Wave Mixing creates

cross-talk for channel f1

Channels f1, f2, f3 interact to

create a intermodulation product

(sideband) at (f1+f2+f3)

• Fiber cable attenuation: Depend on core size and operating wavelength

MMF SMF

Core Diamater(µm)

Wavelength(nm)

Loss (dB/Km)

Dispersionps/(nm x Km)

50 62.5 9

1310 1550

0.35 0.22

171

2.7 3.2

850 1300 850 1300

0.8 0.9

BW (MHz) xLength (Km) 400 1000 200 500

Optical Fibers

New Fiber: 10GbpsX40m

•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

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)

BW

Noise

Linearity

Environmental Influence

LED FP DFB

WideNarrow

Application Digital,<1 Gbps

Digital &Analog

LowHigh

Poor

Unstable

Good

~

~

~

~

DWDM Digital (10Gb/s) &

Analog

Stable

• L-I Response of Light Source:

(a) LED (b) Laser Diode

Opt

ical

Pow

er (

mW

)

Bias (mA)

Distorted Signals

AC Signal

Modulated Optical Signals

AC Signal

Optical Light Sources and Detectors

Comparison of Key Performance Features for VCSEL, DFB, and FP lasers

VCSEL DFB Fabry Perot Emission Type Surface Edge Edge Emission Pattern Circular Elliptical Elliptical Divergence Angle ~ 10 degree ~ 30 degree ~ 30 degree Spectral Width 0.1 nm 0.1 nm 2 ~ 5 nm Peak Modulation Speed 20 Gb/s ~ 10 Gb/s ~ 10 Gb/s Threshold Current 1 ~ 5 mA 10 ~ 15 mA 2 ~ 5 mA Fiber Coupling Efficiency 80% 10% 10% Coupling Optics Not required Aspheric lens Aspheric lens Wavelength Drift ~0.1 nm/deg C ~0.1 nm/deg C ~0.5 nm/deg C Link Distance for 10 GbE Transponder

VSR (850 nm, 300m of new MM fiber)

IR (1310 nm, 2~12Km)

IR (Direct Modulation)

IR

Power Consumption for 10 GbE Transponder

3W ~ 4W 7W ~ 10W 7W ~ 10W

Rel. Price of packaged Laser @ 1Gb/s

1X 25X 4.5X

(Source: 2001, Mar. issue of Fiber Optic Product News)

Direct Modulation v.s. External Modulation

•Direct Modulation: Chirp can become a limiting factor at high bit rates (> 10 Gb/s)

•External Modulation: help to limit chirp

DFBOptical Output (SMF)RF Input

RF InputVRF

Bias ControlVBIAS

3 dB -Coupler

Phase Modulator

PM fiberin

SMF fiberoutDFB

λ @ ITU -grid

ITU Defined Wavelengths (100GHz = 0.8 nm)

C h a n n e l N u m b e r

W a v e l e n g t h ( n m )

F r e q u e n c y ( G H z )

C h a n n e l N u m b e r

W a v e l e n g t h ( n m )

F r e q u e n c y ( G H z )

1 5 1 5 6 5 . 4 9 6 1 1 9 1 , 5 0 0 4 4 1 5 4 2 . 1 4 2 5 1 9 4 , 4 0 0

1 6 1 5 6 4 . 6 7 9 0 1 9 1 , 6 0 0 4 5 1 5 4 1 . 3 4 9 6 1 9 4 , 5 0 0

1 7 1 5 6 3 . 8 6 2 8 1 9 1 , 7 0 0 4 6 1 5 4 0 . 5 5 7 6 1 9 4 , 6 0 0

1 8 1 5 6 3 . 0 4 7 5 1 9 1 , 8 0 0 4 7 1 5 3 9 . 7 6 6 3 1 9 4 , 7 0 0

1 9 1 5 6 2 . 2 3 2 9 1 9 1 , 9 0 0 4 8 1 5 3 8 . 9 7 5 9 1 9 4 , 8 0 0

2 0 1 5 6 1 . 4 1 9 3 1 9 2 , 0 0 0 4 9 1 5 3 8 . 1 8 6 3 1 9 4 , 9 0 0

2 1 1 5 6 0 . 6 0 6 5 1 9 2 , 1 0 0 5 0 1 5 3 7 . 3 9 7 4 1 9 5 , 0 0 0

2 2 1 5 5 9 . 7 9 4 5 1 9 2 , 2 0 0 5 1 1 5 3 6 . 6 0 9 4 1 9 5 , 1 0 0

2 3 1 5 5 8 . 9 8 3 4 1 9 2 , 3 0 0 5 2 1 5 3 5 . 8 2 2 2 1 9 5 , 2 0 0

2 4 1 5 5 8 . 1 7 3 1 1 9 2 , 4 0 0 5 3 1 5 3 5 . 0 3 5 8 1 9 5 , 3 0 0

2 5 1 5 5 7 . 3 6 3 6 1 9 2 , 5 0 0 5 4 1 5 3 4 . 2 5 0 3 1 9 5 , 4 0 0

2 6 1 5 5 6 . 5 5 5 0 1 9 2 , 6 0 0 5 5 1 5 3 3 . 4 6 5 5 1 9 5 , 5 0 0

2 7 1 5 5 5 . 7 4 7 3 1 9 2 , 7 0 0 5 6 1 5 3 2 . 6 8 1 5 1 9 5 , 6 0 0

2 8 1 5 5 4 . 9 4 0 4 1 9 2 , 8 0 0 5 7 1 5 3 1 . 8 9 8 3 1 9 5 , 7 0 0

2 9 1 5 5 4 . 1 3 4 3 1 9 2 , 9 0 0 5 8 1 5 3 1 . 1 1 5 9 1 9 5 , 8 0 0

3 0 1 5 5 3 . 3 2 9 0 1 9 3 , 0 0 0 5 9 1 5 3 0 . 3 3 4 4 1 9 5 , 9 0 0

3 1 1 5 5 2 . 5 2 4 6 1 9 3 , 1 0 0 6 0 1 5 2 9 . 5 5 3 6 1 9 6 , 0 0 0

3 2 1 5 5 1 . 7 2 1 0 1 9 3 , 2 0 0 6 1 1 5 2 8 . 7 7 3 6 1 9 6 , 1 0 0

3 3 1 5 5 0 . 9 1 8 3 1 9 3 , 3 0 0 6 2 1 5 2 7 . 9 9 4 4 1 9 6 , 2 0 0

3 4 1 5 5 0 . 1 1 6 3 1 9 3 , 4 0 0 6 3 1 5 2 7 . 2 1 6 0 1 9 6 , 3 0 0

3 5 1 5 4 9 . 3 1 5 3 1 9 3 , 5 0 0 6 4 1 5 2 6 . 4 3 8 4 1 9 6 , 4 0 0

3 6 1 5 4 8 . 5 1 5 0 1 9 3 , 6 0 0 6 5 1 5 2 5 . 6 6 1 6 1 9 6 , 5 0 0

3 7 1 5 4 7 . 7 1 5 5 1 9 3 , 7 0 0 6 6 1 5 2 4 . 8 8 5 6 1 9 6 , 6 0 0

3 8 1 5 4 6 . 9 1 6 9 1 9 3 , 8 0 0 6 7 1 5 2 4 . 1 1 0 3 1 9 6 , 7 0 0

3 9 1 5 4 6 . 1 1 9 1 1 9 3 , 9 0 0 6 8 1 5 2 3 . 3 3 5 9 1 9 6 , 8 0 0

4 0 1 5 4 5 . 3 2 2 2 1 9 4 , 0 0 0 6 9 1 5 2 2 . 5 6 2 2 1 9 6 , 9 0 0

4 1 1 5 4 4 . 5 2 6 0 1 9 4 , 1 0 0 7 0 1 5 2 1 . 7 8 9 3 1 9 7 , 0 0 0

4 2 1 5 4 3 . 7 3 0 7 1 9 4 , 2 0 0 7 1 1 5 2 1 . 0 2 0 0 1 9 7 , 1 0 0

4 3 1 5 4 2 . 9 3 6 2 1 9 4 , 3 0 0 7 2 1 5 2 0 . 2 5 0 0 1 9 7 , 2 0 0

•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

ITU-Grid (ITU-G.692) Wavelengths

Op

tica

l Lin

e R

ate

Distance (Km)1 10 100

0.01

0.1

1

10

100

Short Reach Intermediate Reach

Short Reach1300 nm

Long Reach1550 nm

VCSEL

Optical Transceiver Evolution (using SMF)

Si Ge InGaAs

Spectral Response for Photodiode

Optical Receiver Design Issue

p

n

i

p-InGaAs or

p-InP

n-InGaAs

n-InP

CONTACT METALIZATION

hν

RL

V

carrier drift

electrondiffusion

holediffusionSUBSTRATE

DEPLETIONLAYERWd

Tuning + Matching

CircuitPhoto-diode

To 50 Ohm Load

- Two Important Design Issues for “impedance matched receiver”:

(1) Low Noise

(2) Wide Bandwidth

•PIN Photodiode:

PIN Photodiode Avalanche Photodiode (APD)

Photon-Electron Conversion

1:1 1:N (N=10)

Receiver Sensitivity Medium High Cost Low High Reliability High Moderate Temperature Sensitivity Low High

Photodiode

Optical Amplifiers- DWDM Enabler -

Tx Repeater Rx

3R Functions: - Retiming- Reshaping- Retransmission

TxOptical

Amplifier Rx

(1) Conventional Design

(2) New Design (can save 60 to 80% regenerator costs)

1R Function: -Retransmission orReamplification

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

•Single Channel EDFA

•DWDM EDFA

Erbium-Doped Fiber Amplifier

Gain Flattening Filter

980-nm pumps 1480-nm pumps

EDF, pre-amp stage EDF, booster stage

DispersionCompensation Unit

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

(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

Optical Raman Amplifier

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

•80 Km for each span•DWDM terminal spacing ~ 400~600 Km (followed by a regenerator)

DWDM Transmission Span

•Concerned Factors:

(1) Fiber type

(2) Transmission distance

(3) Channel count and bit rate

DW

DM

DW

DM

80 Km (span)

Cascaded Optical Amplifiers

400 ~ 600Km (link)

(4) Amplifier spacing

(5) Amplifier noise

(6) Amplifier power

DWDM Multiplexer/Demultiplexer

Technologies include:

•Thin film coating filters

•Fiber Bragg gratings

•Diffraction gratings

•Arrayed waveguide gratings

•Fused biconic tapered devices

• Inter-leaver devices

Channel Spacing

Crosstalk

FilterBandwidth

Device Aspects of WDM Filter

- Figure of merit, -0.5 dB bandwidth/ -30 dB bandwidth

- Low loss

- Low Polarization sensitivity

- Flat top

- Steep roll-off

- Stable & Manufacturable

DWDM Multiplexer/Demultiplexer Advantages Disadvantages 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)

(1) Takes longer time to develop and accumulate filters with dense channel spacing

(2) Cost is proportional to channel count

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

(1) Not suitable for wideband applications

(2) Need temperature stabilization (3) Cost is proportional to channel

count

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

(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)

•2-D Design MEMS

•3-D Design

MEMS Crossconnects

Plan 2

Plan 1 Plan 1 Plan 1

Plan 2 Plan 2 Plan 2

Plan 3 Plan 3

Plan 4

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 electronicdemultiplexing to lower data rates

•Add signals from subscribers, one wavelength per fiber, multiplexes theseon outgoing fiber

Optical Add/Drop Multiplexer

• Current Throughput: 8 ~ 16 X 2.5 Gb/s = 20 ~ 40 Gb/s

R: ReceiverT: Transmitter @ fixed λ

DW

DMλ1 ~ λ8

Optical Amplifier

DW

DM

R R

Fiber to Subscriber

TT

Fiber from Subscriber

Electronic Add/Drop

Optical Add/Drop Multiplexer

•Estimated Throughput in 2008: 128 X10Gb/s = 1.28 Tb/s

DW

DMλ1 ~ λ128

Optical Amplifier

Optical Crossconnect

(128X256)

DW

DM

R R

Fiber to Subscriber

TT

Fiber from Subscriber

Electronic Add/Drop

R: ReceiverT: Tunable Transmitter

Wavelength Router (Dynamic WDM Crossconnect)

Tunable laser inside(1) Tuning speed < 2 ns, (2) Tuning throughout the C-band <15ns(3) Synchronization time < 40 ns

(Source: 2001, Mar issue of Lightwave)

Optical Transponder / Wavelength Adapter

λ @850/1310/1550nm)

DWDM Optical Transponder

DW

DM

(1) Embeded DWDM Operation (2) Open DWDM Operation

Optical Transponder

Operation of DWDM-Based System

DW

DM

DW

DM

Pre-

Optical Bandwidth

Channel Spacing Channel Bit Rate Fiber Bandwidth

Current benchmark 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

•Optical bandwidth can be increased by increased by improving DWDM system in three areas:

Current Networking Status

DWDM Terminal

Migrating the SONET Ring to DWDM

Optical Transport

SONET / SDH

ATM

IP / MPLS

Current IP / ATM / SONETLayering

SONET / SDH

IP / MPLS

IP / MPLS

Packet-over-SONETLayering

Direct IP-over-DWDMLayering

Time

Future Trend

Eliminating Protocol layers

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

Metropolitan Area Networks

•Metro Core

•Metro Access

•Enterprise

Metro DWDM Systems

Optical Networking Applications in MAN