otc000003 wdm principle issue1

WDM Principle

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WDM Principle

1 WDM Overview......................................................................................Page4

2 WDM Transmission Media....................................................................Page17

3 WDM Key Technologies......................................................................Page 25

4 Technology Specifications for WDM System........................................Page 49

WDM Principle


Page2Copyright © 2008 Huawei Technologies Co., Ltd. All rights reserved.

Foreword

With the development of telecommunication, the requirements

of the transmission capacity and service categories are

becoming bigger and bigger, under this background, WDM

technology emerged.

About this course:

This course mainly introduces the basic knowledge of WDM technologies,

expounds key technologies and optical transmission specification of WDM.

Through this course, you will have a relatively complete understanding of

the WDM knowledge and the development orientation of optical

transmission networks.

WDM Principle



Objectives

Upon completion of this course, you will be able to:

Describe the concepts, transmission modes and structure of WDM;

Classify the different types and characteristics of the fiber;

Outline the key technologies of WDM system;

List the technical specifications for WDM system.

Reference:

OTC000003 WDM principle

ITU-T G.694.1 and G.694.2 (about the wavelength distribution)

ITU-T G.671 (about the optical passive components)

ITU-T G.652 , G.653 and G.655 (about the fiber)

WDM Principle



Contents

1. WDM Overview

2. Transmission Media

3. Key Technologies

4. Technical Specifications

Objectives for this chapter:

To explain the basic concepts of WDM;

To list the components of WDM and their functions;

To outline the WDM background and technical features;

To make comparison between CWDM and DWDM.

WDM Principle



Solution of capacity expansion

SDM

Add fiber &

equipment

Time & cost

TDM

STM-16→ STM-64

Cost & Complication

WDM

Economical &

Mature & Quick

How to increase network capacity？

SDM increases the transmission capacity linearly by adding the number of

optical fibers, and the transmission equipment will be increased linearly, too.

TDM keeps the same transmission medium but increases the bit rate. The

equipment is getting more and more complicated and expensive. Additionally,

the maximum transported capability over a fiber pair is in the range of a few

10Gbps.

The way to scale to higher transported capacity is WDM. This technology keeps

the same fiber, the same bit rate, but uses multiple colours to multiply

transported capacity.

WDM is widely used in the national and metro backbone transmission systems.

WDM Principle



What's WDM？

Free Way

Gas Station

Patrol Car

Legend:

Freeway: Fiber

Patrol Car: Supervisory Signal

Gas Station: Optical Relay

Gray Car: Client Service

Colored Car: Service in different channels (wavelength)

Driveway: Optical Wavelength

Wave Division Multiplexing is a technology that utilizes the properties of

refracted light to both combine and separate optical signals based on their

wavelengths within the optical spectrum.

WDM Principle



WDM Concept

λ1

λ2┋

λ1 λ2 λn

┉λ

λn

SDH signal

IP package

ATM cells

Different signals with specific wavelength are multiplexed into a

fiber for transmission.

The Greek letter lambda ( λ ) , is often used to designate individual wavelengths.

Key word in the content is specific wavelength. How specific ? Please refer to

ITU-T series recommendations in chapter 4.

WDM allows for a more efficient use of existing fiber by providing multiple

optical paths along a single (pair of) fiber (s).

WDM allows for a greater range of protocol transmission better suited than

legacy network for data centric applications. (E.g.. GE, ESCON, Fiber Channel,

D1 video)

WDM Principle



The overall structure of the WDM system of N-path wavelength:

Optical Transponder Unit (OTU)

Optical Multiplexer Unit / Optical De-multiplexer Unit (OMU/ODU)

Optical Amplifier (OA)

Supervisory Channel (OSC/ESC)

System Structure

OTU

OTU

OTU

OM/OA

OA/OD

OTU

OTU

OTU

OSC OSCOSC

OLA

OTU: Access the client service and convent the wavelength complied with ITU

standards.

OMU: Multiplex several services with different wavelength into one main path

signal.

ODU: Demultiplex one main path signal into several individual signals.

OA: Amplifies the optical signal.

OLA: Optical Line Amplifier

OSC: Optical Supervisory Channel

ESC: Electrical Supervisory Channel

WDM Principle



Transmission Modes

Single fiber unidirectional transmission

M40

M40

MUX DMUX

OTU

OTU

Unidirectional WDM system adopts two optical fibers. One only implements the

transmission of signals in one direction while the other implements the

transmission of the signals in the opposite direction.

This tansmission mode is widely used in the worldwide.

WDM Principle



M40

M40

MUX/DMUX DMUX/MUX

Transmission Modes

Single fiber bidirectional transmission

OTU

OTU

Bi-directional wave WDM system utilizes only one optical fiber. The single fiber

transmits optical signals in both directions simultaneously, and the signals in the

different directions should be assigned on different wavelengths.

Note:

To MUX/DEMUX the signals in one fiber, circulator is recommended.

This mode is usually used in the CWDM system to reduce the cost.

WDM Principle



Application Modes

Open System

M40

M40

MUX DMUX

OTU

OTU

Client Client

Open system has no special requirements for multiplex terminal optical

interfaces, the only requirement is that these interfaces meet the optical

interface standards defined in ITU-T.

WDM Principle



Application Modes

Integrated System

M40

M40

MUX DMUX

Client Client

Integrated system does not adopt the wavelength conversion technology,

instead, it requires that the wavelength of the optical signals at the multiplex

terminal conforms to the specifications for the WDM system.

The optical interface in the client equipment that could provide standard

wavelength is called colored interface. Huawei series OSN products could

support this function.

Thought:

Can some channels use OTU and some channels use colored interface?

WDM Principle



Advantages of WDM

Ultra high capacity

Data transparency transmission

Long haul transmission

Compatible with existing optical fibers

High performance-to-cost ratio

High networking flexibility, economy and reliability

Smooth expansion

Up to know the capacity is 1920Gbps at most.

Data Transparency Transmission:

WDM doesn’t change the structure or any byte in the frame for the client

signal.

Long Haul transmission: 5000km without REG / 230km long hop.

Smooth expansion: modularization and no affect the existing services.

WDM Principle



CWDM Vs DWDM

CWDM:

Coarse Wavelength Division Multiplex

DWDM:

Dense Wavelength Division Multiplex

Extended C band 192chs, 25GHz spacing

196.05THz 192.125THz

C band 160chs

192.05THz

Extended 32chs

191.275THz

ITU-T G.694.1

CWDM greatly reduces the system cost while providing certain amount of

wavelengths and transmission distance within 100 km.

Fewer channels=cheaper hardware

No amplification=a lower-cost system and distance-limited system

Comparison between CWDM and DWDM

192 x 10Gb/s = 1920G16 x 2.5Gb/s = 40GCapacity

5000km100kmApplication

100%70%Cost

Cooled LaserUn-cooled LaserLaser

C-band: 1529nm~1561nmL-band: 1570nm~1603nm

1311~1611nmBand

100GHz/50GHz/25GHz20nmChannel SpacingDWDMCWDMTypes

WDM Principle



Questions

What are WDM, DWDM and CWDM?

Difference between the two transmission modes

Difference between the two application modes

List the structure of the WDM system.

Fill in the blanks:

1.WDM System includes:________, _________, _________ and

__________;

2.CWDM system could use optical amplifiers (True or False) __________;

3.ESC means____________________________________. Need additional

wavelength to transmit in the fiber (True or False) _________.

4.Single fiber bidirectional transmission (can or can not )_________ use the

same wavelength for transmitting and receiving.

WDM Principle



Basic concepts and features of WDM, DWDM and CWDM;

WDM system structure ;

Transmission and application Modes of WDM system;

Summary

OTU, MUX/DeMUX, OA,OSC

False

Electrical Supervisory Channel,False

Can not

WDM Principle



Contents

1. WDM Overview


3. Key Technologies


Objectives for this chapter:

List the characteristics of the fiber;

Classify different types of the fiber;

Outline the methods to against the factors.

WDM Principle



Structure of Optical Fiber

Consists of a cylindrical glass core, a glass cladding and a plastic

wear-resisting coating.

θ

n2

n1

Refraction

Reflection

Cladding

Core

Coating

An optical fiber consists of two different types of solid glass —the core and

cladding—that are mixed with specific elements to adjust their refractive

indices. The difference between the refractive indices of the two materials

causes most of the transmitted light to bounce off the cladding and stay within

the core. The critical angle requirement is met by controlling the angle at which

the light is injected into the fiber. Two or more layers of protective coating

around the cladding ensure that the glass can be handled without damage.

N1 and N2, which one is larger ?

WDM Principle



Attenuation

900 1300 1400 1500 1600 1700

nm

dB/km

2

3

1

4

5

1200

Multi-m

ode

（850~900nm

）

Oband

E S C L U

OH-

Band Wavelength Bandwidth (nm)

Original 1260~1360 100

Extended 1360~1460 100

Short 1460~1525 65

Conventional 1525~1565 40

Long 1565~1625 60

Ultra long 1625~1675 50

Combining the above losses, the attenuation constant of single mode fiber at

1310nm and 1550nm wavelength areas is 0.3~0.4dB/km (1310nm) and

0.17~0.25dB/km (1550nm), respectively. As defined in ITU-T Recommendation

G.652, the attenuation constant at 1310nm and 1550nm should be less than

0.5dB/km and 0.4dB/km, respectively.

WDM Principle



Dispersion

Chromatic dispersion:

Time

Power

Optical pulses

TransmittingL1 (km)

TransmittingL2 (km)

Dispersion in fiber refers to a physical phenomenon of signal distortion caused

when various modes carrying signal energy or different frequencies of the signal

have different group velocity and disperse from each other during propagation.

WDM Principle



Dispersion coefficient

G.655

1550nm1310nm

17ps/nm.km

λ

Dispersion

G.652:widely used, need dispersion compensation for high rate transmission

G.653: Zero dispersion at 1550nm window.

G.655: Little dispersionto avoid FWM.

G.652 fiber is currently a single mode fiber for widely use, called 1310nm

property optimal single mode fiber and also called dispersion unshifted fiber.

G.653 fiber is called dispersion shifted fiber or 1550nm property optimal fiber.

By designing the refractive index cross section, the zero dispersion point of this

kind of fiber is shifted to the 1550nm window to match the minimum

attenuation window. This makes it possible to implement ultrahigh speed and

ultra long distance optical transmission.

G.655 fiber, a nonzero dispersion shifted single mode optical fiber, is similar to

G.653 fiber and preserves certain dispersion near 1550nm to avoid four-wave

mixing phenomenon in DWDM transmission. It is suitable for DWDM system

applications.

WDM Principle



Dispersion Compensation

The pulse will be broadened because of

Positive dispersion coefficient at 1550nm window

DCF has negative dispersion coefficient and can counteract

positive dispersion in transmission.

Dispersion Coefficient G.652

Normal DCF

DSCF: Dispersion Slope Compensation Fiber

wavelength

DCF is one special kind of optical fiber, with the negative dispersion at 1550nm

window.

The dispersion coefficient is -90~-120ps/nm.km

DCF can counter act positive dispersion while bring new insertion loss and

increasing of PMD.

WDM Principle



Questions

What’s difference between the refractive index of the cladding

and core?

What are the features of G.652, G.653 and G.655 fibers?

How to compensate the chromatic dispersion?

Fill in the blanks:

1. The attenuation coefficient of G.652 fiber is __________; approximately

________ for engineering planning;

2. The dispersion coefficient of G.655 at 1550nm window

is_______________;

3. The dispersion coefficient of G.652 at 1310nm window is__________;

at 1550nm window is___________;

WDM Principle



Structure of optical fiber

Types of optical fiber

Characteristics of optical fiber

Summary

Coating, Cladding, Core

G.652, G.653, G.655

Attenuation, Dispersion, Nonlinear effect

WDM Principle



Contents

1. WDM Overview


3. Key Technologies


WDM Principle



WDM System Key Technologies

Optical Source

Optical Amplifier Supervisory Technologies

Key Tech. in WDM

Optical Multiplexer and Demultiplexer

WDM Principle



Requirements of Optical Source

1 Larger dispersion tolerance value

2 Standard and stable wavelength

WDM Principle



Direct modulator

LD

Modulation current

Output laser is controlled by input current. The variation of the modulation

current causes the variation of output wavelength.

This variation, called modulation chirp, is actually a kind of wavelength

(frequency) jitter inevitable for direct modulation of the sources. The chirp

broadens the bandwidth of the emitting spectrum of the laser, deteriorates its

spectrum characteristics and limits the transmission rate and distance of the

system.

Transmission rate is limited to 2.5Gbit/s, and transmission distance is less than

100km.

Similar Specification –This kind of modulator is Widely used in CWDM system.

WDM Principle



Electro-Absorption (EA) external modulator

LD EADC current drive ITU λ

Modulation current

EA modulator adopts different structure, use stable DC current to let LD output

a standard wavelength (complied with ITU-T). EA module act as a door that

open only happens to the current change. In this way, the information is

modulated into the wavelength.

Less chirp = Support long haul transmission (2.5Gb/s > 600km)

High Dispersion tolerance (2.5Gb/s: 7200~12800ps/nm)

Most widely used in DWDM

WDM Principle



DC current drive

ITU λ

Modulation current

LD

Mach-Zehnder (M-Z) external modulator

This modulator separates the light input into two equal signals which enter the two optical branches of the modulator respectively. These two optical branches employ an electro-optical material whose refractive index changes with the magnitude of the external electrical signal applied to it. Changes of the refractive index of the optical branches will result in the change variation of the signal phases. Hence, when the signals from the two branches recombine at the output end, the combined optical signal is an interference signal with varying intensity. With this method, the frequency chirp of the separated external modulated laser can be equal to zero.

Long dispersion limited distance

High cost with good performance

Negligible chirp

Not widely used.

WDM Principle



Comparison of Modulators

better

expensive

7200~12800

EA Modulator

bestgoodWavelength Stability

very expensivemoderateCost

>128001200~4000

Max. dispersion

toleration (ps/nm)

M-Z ModulatorDirect ModulatorTypes

As a maturing technology, direct modulator and indirector modulator are

widely used in WDM system.

WDM Principle



Optical Amplifiers

EDFA

RFA Raman Fiber Amplifier

Erbium Doped Fiber Amplifier

OA

The EDFA amplifier is widely used in WDM system.

WDM Principle



Stimulated radiationStimulated radiation

Er3+ energy level diagram

Erbium Doped Fiber Amplifier

E2 meta-stable state

E3 excited state

E1 ground state

1550nmsignal light

1550nmsignal light

980nmpump light

DecayDecay

Principle:

The outer electrons of Er ions have 3 energy levels, where E1 is the basic

state energy level, E2 is the metastable state energy level and E3 is the

high energy level.

When high-energy pump lasers are used to excite the EDF, lots of bound

electrons of the erbium ions are excited from E1 to E3 level, then soon

dropped to the E2 level via a non-radiation decay process (i.e. no photon

but heat is released).

When a signal with the wavelength of 1550nm passes through this

erbium-doped fiber, particles in the metastable state are transited to the

basic state via stimulated radiation and generate photons identical to

those in the incident signal light.

WDM Principle



Structure of EDFA

Coupler

EDF

ISO

Pumping laser

ISO

PD

TAP

Signal input

TAP

Signal Output

PD

ISO: Isolator

PD: Photon Detector

TAP is used to spilt out a little part of energy and send it to the PD to detection.

ISO is used to make sure the signal transmit in one direction.

Pump laser has two type: with 980nm and with 1480nm.

If we want to get a high gain, we could cascade EDF and pumping laser

WDM Principle



Features of EDFA

Consistent with the low

attenuation window

High energy conversion efficiency

High gain with little cross-talk

Good gain stability

…

Fixed gain range

Gain un-flatness

Optical surge problem

…Advantages Disadvantages

Advantage:

Fortunately, 1550nm is in the low attenuation window, the emergence of

EDFA greatly activate the development of WDM.

Disadvantage:

Gain un-flatness

WDM Principle



Automatic Gain Control

Pin Pout

Gainλ1~ λn

λ1~ λn

Gain no change!

EDFA

PINpump

PINDSP

splitter splitter

EDFInput Power: Pin Output Power: Pout

Gain = Pout / Pin is invariablecoupler

If we cannot control the gain, optical surge generates.

With AGC function:

When add wavelengths from 1 to 40, the gain will be not changed.

When drop wavelengths from 40 to 1, the gain will be not changed also

Key Component is the DSP that makes the nonlinear calculation.

WDM Principle



Raman Fiber Amplifier

Stimulated Raman Scattering

PumpGain

30nm

13THz

Pump3

70~100nm30nm

GainPump2Pump1

Principle:

Fiber has wide SRS gain spectrum and a wide gain peak around a frequency

13THz lower than that of the pumping light. If a weak signal and a strong

pumping light wave are transmitted through the fiber at the same time, and the

wavelength of the weak signal is set within the Raman gain bandwidth of the

strong pumping light, the weak signal can be amplified. Such SRS-based OA is

call Raman optical amplifier. Raman optical amplifier’s gain is the switch gain,

that is, the difference between the output power when the amplifier is on and

that when the amplifier is off.

WDM Principle



Features of Raman

Flexible gain wavelength

Simple structure

Nonlinear effect can be reduced;

Low noise

…

High pump power, low

efficiency and high cost;

Components & fiber

undertake the high power;

…Advantages Disadvantages

Advantage:

The gain wavelength is determined by the pumping light wavelength.

The gain medium is the transmission fiber itself, low noise.

As the amplification is distributed along the fiber with the comparatively

low signal power, it reduces the interference from non-linear effect,

especially FWM effect.

Disadvantage:

High power is harmful for body.

Be careful when put operation on Raman.

WDM Principle



Application of OA

Booster amplifier Line Amplifier Pre-amplifier

M40

OTU

OTU

M40

M40

OTU

OTU

M40

MUX

DMUX

OA OA OA

According to its application:

BA: Booster amplifier, mainly used in the transmit end. For the hardware

description, you will see OBU card.

LA: Line amplifier, mainly used in the amplifier station, could be

recognized as BA+PA. For the hardware description, you will see OAU

card.

PA: Pre-amplifier, mainly used in the receive end. For the hardware

description, you will see OPU card.

WDM Principle



Optical Multiplexer and Demultiplexer

Multiplexer

λ1

λ2

λn

λ1 λ2 λn

Demultiplexer

λ1λ2

λn

λ1 λ2 λn

TFF

AWG Arrayed Waveguide Grating

Thin Film Filter

For all the optical lights are bidirectional, the mechanisms of multiplexer and

demultiplexer are the same. Here in after we just discuss about the multiplexer,

if you reverse the direction, it could also be considered as a demultiplexer.

WDM Principle



λ 1- λ 4

λ 4

λ 2

λ 3

Self-focusing lens

λ 1 filter

λ 3 filter

Glass

λ 1

Thin Film Filter

Film Filter offers good stability and isolation between channels at moderate cost,

but with a high insertion loss.

So the number of dropping wavelength is limited.

WDM Principle



Arrayed Waveguide Grating

λ1,λ2… λn

Arrayed of waveguides 1…n

λ1

λnArrayed of fibers

The waveguides are connected to cavities at the input and output. When the

light enters the input cavity, it is diffracted and enters the waveguide array.

There the optical length difference of each waveguide introduces phase delays

in the output cavity, where an array of fibers is coupled. The process results in

different wavelengths having maximal interference at different locations, which

correspond to the output ports.

WDM Principle



Supervisory Technologies

OSC Optical Supervisory Channel Technology

ESC Electrical Supervisory Channel Technology

OSC is often used in the backbone wavelength system,and ESC is normally used

in metropolitan system.

WDM Principle



Optical Supervisory Channel

Requirements:

Operating wavelength should be different from the pumping wavelength of OA.

Operating wavelength should not take 1310nm window.

Available when OA fails;

Suitable for long distance transmission.

M40

M40

FIU

OTU1

OTU2

OTU3

OTU4

OTU1

OTU2OTU3

OTU4

FIU

OSC OSC

SCC

SCC

Pumping wavelength of OA: 980nm or 1480nm.

1310nm already defined by ITU-T for future use.

OA fails, all signal lost, requires the supervisory signal continue to transmit

alarms and other indications.

The receive sensitivity of the OSC unit is very good, up to -48dBm.

WDM Principle



Typical frame structure of OSC

Reserved

E2 byte

F3 byte

F2 byte

D1-D12 bytes

ALC byte

F1 byte

E1 byte

FA

TS3-TS13, TS15

OthersTS14

TS19TS2

TS18TS1

TS17TS0

TS31……TS16TS15TS14……TS3TS2TS1TS0

FA: Frame alignment.

E1 E2 : Orderwire.

ALC: Automatic Level Control.

F1 F2 F3 : transparent serials data.

D1-D12: DCC bytes, data communication channel.

WDM Principle



Electrical Supervisory Channel

Features:

Simple structure & cost saving

Redundancy supported

Improve power budget

Reduce system complexity

M40

M40

OTU1

OTU2

OTU3

OTU4

OTU1

OTU2

OTU3

OTU4

SCC

SCC

The optical transponder unit (OTU) multiplexes the supervisory information into

the service channel for transmission.

The ESC reduces the investment of the OSC. It also deletes the insertion loss of

the FIU. This lowers the cost and the power budget of optical channels.

WDM Principle



Questions

What is the mechanism of electro-absorption modulation?

How many types of multiplexer are there used for WDM?

What is the difference between EDFA and Raman?

What are the working wavelength and bit rate of OSC signal?

Fill in the blanks:

1.EDFA means:______________________; its pumping wavelength

is___________; We can calculate noise figure by _________。

2.AWG means:______________________; TFF

means:________________________;

3.OSC signal’s frame structure is_____________, (can, can not) by

amplified by OA.

4.ESC support OLA station ?_______(True, False)

WDM Principle



Optical source

Optical amplifier

Optical multiplexer

Supervisory technologies

Summary

LD, EA, M-Z

EDFA, Raman

TFF, AWG

OSC, ESC

WDM Principle



Contents

1. WDM Overview


3. Key Technologies


WDM Principle



Related ITU-T recommendations

G.652 Characteristics of a single-mode optical fiber cable

G.655 Characteristics of a dispersion-shifted SMF

G.661/G.662/G.663 Relevant recommendations of OA

G.671 Characteristics of passive optical components

G.957 Optical interfaces relating to SDH system

G.691 Optical interfaces for single channel STM-64, STM-256 systems

and other SDH systems with OA

G.692 Optical interfaces for multi-channel systems with OA

G.709 Interfaces for the optical transport network (OTN)

G.975 Forward error correction for submarine systems (FEC)

ITU-G.692 – Optical Interfaces for Multi-Channel Systems with Optical

Amplifiers

This recommendation specifies multi-channel optical line system

interfaces for the purpose of providing future transverse compatibility

among such systems. The current recommendation defines interface

parameters for systems of four, eight, and sixteen channels operating at

bit rates of up to STM-16 on fibers, as described in Recommendations

G.652, G.653, and G.655 with nominal span lengths of 80 km, 120 km,

and 160 km and target distances between regenerators of up to 640 km.

A frequency grid anchored at 193.1 THz with inter-channel spacing at

integer multiples of 50 GHz and 100 GHz is specified as the basis for

selecting channel central frequencies.

WDM Principle



Transmission Channel Reference Points

The WDM system in the above figure has the following reference points:

S1…Sn: The reference points on the fiber at transmitter optical output

connector in channels 1…n;

RM1 RMn: The reference points on the fiber at OM/OA optical input

connector in channels 1…n;

MPI-S: A reference point on the optical fiber just behind the OM/OA

optical output connector;

S': A reference point on the optical fiber behind the optical output

connector of the optical line amplifier;

R': A reference point on the optical fiber in front of the optical input

connector of the optical line amplifier;

MPI-R: A reference point on the optical fiber in front of the OA/OD input

optical connector;

SD1…SDn: The reference points at the OA/OD optical output connector;

R1…Rn: The reference points at receiver optical transmitter input

connector.

WDM Principle



Distribution of Optical Wavelength Areas

Nominal central frequency refers to the central wavelength

corresponding to each channel in WDM systems. Channel frequency

allowed in G.692 is based on frequency and spacing series of

reference frequency 193.1THz and minimum spacing 100GHz , 50GHz

or 25GHz.

WDM Principle



Questions

Which are the ITU-T recommendations involved for WDM part?

What is the absolute reference frequency for WDM systems?

WDM Principle


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otc000003 wdm principle issue1

Documents

main path

optical transponder

arrayed waveguide

optical line

single mode

otu otu otu

1550nm window

supervisory