DWDM Principle Training Manual

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DWDM Principle Training Manual

ZTE CORPORATION


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COPYRIGHT

Copyright ZTE Corporation

All rights reserved.

All information contained herein are confidential information of ZTE and must be

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written consent of ZTE.

Author: Randy

Editor: Guo Yali

* * * *

ZTE UNIVERSITY

ZTE University, Dameisha, Yantian District, Shenzhen, P.R.China

Postcode: 518083

Tel: (+86755) 26778000

Fax: (+86755) 26778999

ZTE CORPORATION

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Postcode: 518057

Technical Support Websithttp://support.zte.com.cn

Client Support Hot line+8675526770800 800-830-1118

Fax: (+86755) 26770801

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Version: 1.0

S.N.:PXJCSW200512097

General


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Preface

Thanks for using DWDM Principle Training Manual. In order to use the Manual properly, please read the

Preface first.

1. Application

This Manual should not be used for the purpose of on-site installation or trouble shooting.

2. About This Manual

This manual is composed of one volumeand the table of contents of the volume is shown below:

Volume Course Code Course Name

I WM_001_E1 DWDM Principle

This manual is based on DWDM fundamentals. We will update this manual while the product is upgraded.

We apologize if there is any discrepancy between the manual and the products used in your company.

3. Conventions

Key pointsIndicates the key points in one section.

OOOO NoteA Note statement is used to alert the reader of installation, operation, or maintenance information that is

important.


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5555 Caution

Indicates a potentially hazardous situation which, if not avoided, could result in damages to the equipmentor personal injury. It may also be used to alert against unsafe practices.

&&&& Tips

Indicates a suggestion or hint to make things easier or more productive for the reader.

4. Manual Update history

Version Date Comments

1.0 Dec. 2005 New

5. From the Author

Thank you for using this manual and your continuous support. We would appreciate your comments and

suggestions on this Manual.

We can be reached at

Telephone+8675526778806

Fax+8675526778999

ZTE UNIVERSITY


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Introduction to ZTE Customer

DocumentationIn order to provide more professional and useful documentation and offer better services to you, ZTE

founded its Documentation R&D Department in 2005.

The Documentation R&D Department is responsible for writing, publishing and issuing various kinds

of customer documentation for ZTE Corporation, including product manuals, Maintenance Experience and

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01Guide to

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each manual, and how to use each manual.

02 Technical Manual

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the product, including:

1. Related fundamental knowledge2. Principle of hardware and software3. Structure of hardware and software4. Specifications of the overall system and each part/module5. Internal and external interfaces provided and signaling protocols used6. Service functions available7. Connection and networking scenarios and specific configurations

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swappable.

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04Installation

manual--Hardware

Introduces the hardware installation method and points for attention, including:

1. Preparation for installation project2. Hardware installation flow3. Installation of cabinets and shelves4. Connection of cables, including power cables, grounding wires, internal cables


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and external cables

5. Installation of alarm system and background system6. Installation of antenna and feeder system for radio equipment7. Check list and standards for judging whether the hardware installation is up to

standard.

8. How to insert and extract circuit boards, pre-power-on check, andpower-on/off procedures

05Installation

manualsoftware

Introduces how to install the software of the equipment and the points for attention,

including:

1. Connection mode between the foreground and background, the software to beinstalled, installation flow and the environment requirements for software

installation

2. Installation of the operating system3. Installation of software for various servers4. Installation of client software5. Installation of remote maintenance system

06Man-machine

interface manual

Introduces in detail all the man-machine interfaces of the operation and

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1. Structure and networking mode of the operation and maintenance system2. Startup, shutdown and man-machine interfaces of each server3. Startup, shutdown and man-machine interfaces of clients

07 Operation manual

This task-oriented manual describes in detail the purpose, pre-operation setup

(including hardware and software environment, e.g., cabling, equipment room and

grounding), operation steps and result verification for the following tasks:

1. How to commission an office/site2. Configuration of background operation and maintenance system3. Network (topology) configuration4. Hardware configuration (rack, shelf, board)5. Alarm system configuration6. Service interface configuration, user interface configuration

08 Command manual

Introduces in detail all the man-machine commands of the operation and

maintenance system, including the name, function, format, parameter description

and examples for each command.

09 Maintenance Serves as a reference manual for guiding equipment maintenance and


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manual troubleshooting, including:

1. Routine maintenance, including daily, weekly, monthly and quarterlymaintenance items

2. Notification messages and their handling3. Alarm messages and their handling4. Common problem handling5. Flow for handling serious accidents

10 User manual

The contents vary with the specific products. A typical comprehensive user manual

includes:

1. Structure and principle of the product2. Installation and debugging of hardware and software3. Power-on and power-off procedures, and how to operate and use the

equipment

4. How to maintain the equipment and handle common problems

The Documentation R&D Department offers you all-round services for accessing our documentation:

Our product manuals and maintenance experience monthly are available in hard copy, CD and

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You can contact us at any time through [email protected].

You can query and download the latest product manuals and Maintenance Experience from

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How to register to this website: Enter the website, click Register, select ZTE system

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your registered mailbox within two days after your registration. Starting from 2006, all trainees

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You can contribute to Maintenance Experience through [email protected]

The Documentation R&D Department has launched an action to hunt bugs in product

documentation. We welcome you to participate in this action, and there will be rewards for any good

suggestions.


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WM_000_E1 DWDM Basic Principles

Course Objectives:

z Grasping the significance and applicationenvironment of the DWDM networks

z Grasping the DWDM principles and the keytechnology for implementing the DWDM

References:

z Optical Wavelength Division MultiplexingSystem

z Modern Communication Base and Technology

z Principle and Test of DWDM TransmissionSystem


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Contents

1 DWDM Overview.......................................................................................................................................1

1.1 Background of DWDM Technology .................................................................................................1

1.1.1 Development of Multiplexing Technology in Optical Network .............................................1

1.1.2 PDH, SDH and DWDM .........................................................................................................2

1.2 DWDM Technology Overview..........................................................................................................5

1.2.1 Different between DWDM Technology and Other Multiplexing Technologies.....................5

1.2.2 Relationship between DWDM and SDH................................................................................7

1.2.3 Operation Wavelength Range...............................................................................................10

1.3 DWDM Features and Advantages...................................................................................................12

1.4 DWDM Development Trend...........................................................................................................13

2 Overview of Optical Fiber Communication ...........................................................................................15

2.1 Basic Knowledge of Optical Fibers.................................................................................................15

2.1.1 Brief Introduction to Optical Fibers .....................................................................................15

2.1.2 Usage Overview of Applicable Frequencies of Optical Fiber ..............................................18

2.1.3 Types and Features of Common SMFs.................................................................................19

2.2 Working Wavelength of DWDM System........................................................................................20

2.2.1 Introduction to Working Wavelength Area ...........................................................................20

2.2.2 Wavelength Allocation..........................................................................................................21

2.3 Fiber Transmission Features............................................................................................................25

2.3.1 Fiber Loss .............................................................................................................................25

2.3.2 Dispersion.............................................................................................................................26

2.3.3 Non-Linear Effect of Fiber ...................................................................................................29

2.4 New Optical Fiber Types.................................................................................................................33


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3 Key Technologies of DWDM System...................................................................................................... 35

3.1 Basic Structure of DWDM System ................................................................................................. 35

3.2 Light Source Technology ................................................................................................................ 36

3.3 Optical Wavelength Division Multiplexing and De-Multiplexing Technologies............................ 39

3.3.1 Overview.............................................................................................................................. 39

3.3.2 Introduction to OM .............................................................................................................. 39

3.3.3 Key Performance Indices ..................................................................................................... 42

3.4 OTU Technology............................................................................................................................. 44

3.4.1 Overview.............................................................................................................................. 44

3.4.2 Working Principle and Performance Indices........................................................................ 45

3.4.3 Classification and Applications of OTU .............................................................................. 48

3.5 Optical Amplifying Technology...................................................................................................... 49

3.5.1 EDFA Technology ................................................................................................................ 49

3.5.2 Raman Amplifying Technology ........................................................................................... 55

3.6 Supervision Technology.................................................................................................................. 56

3.6.1 Functions of Optical Supervision Channel (OSC) ............................................................... 57

3.6.2 Requirements for OSC ......................................................................................................... 57

3.6.3 Implementation of OSC ....................................................................................................... 58

4 Protection Principle of DWDM System ................................................................................................. 61

4.1 Brief Introduction to DWDM System Hierarchy............................................................................ 61

4.2 1+1 Protection................................................................................................................................. 62

4.2.1 Link 1+1 Protection ............................................................................................................. 62

4.2.2 Ring 1+1 Protection ............................................................................................................. 64

4.2.3 Features of 1+1 Protection ................................................................................................... 65

4.3 1: N Protection ................................................................................................................................ 65

4.3.1 Working Principle ................................................................................................................ 65


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4.3.2 Protection Implementation ...................................................................................................66

4.3.3 Features of 1:N Protection....................................................................................................66

4.4 Bidirectional Optical Channel Protection........................................................................................67

4.5 Bidirectional OMS Protection .........................................................................................................69


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1

1 DWDM Overview

Highlights:

z DWDM definitions and generation background.

z DWDM development trend.

1.1 Background of DWDM Technology

Prior to the introduction to DWDM technology, we should know about the

development of the optical network. This section will introduce the background of the

DWDM technology in terms of multiplexing technology and transmission technology.

1.1.1 Development of Multiplexing Technology in Optical Network

The communication network covers diversified transmission media, such as twisted

pair, coaxial cable, optical fiber and wireless transmission. Among them, the optical

fiber transmission features large transmission capacity, good quality, small attenuation,strict security and large trunk distance.

Since the broadband high-speed service ceaselessly develops in the information age,

the optical transmission system is not only expected to have larger capacity and longer

distance, but also expected to be interactive, fast and convenient. Therefore, the

multiplexing technology is introduced in the optical transmission system. The

multiplexing technology means to use the broadband and large-capacity features of the

optical fiber to simultaneously transmit multiple channels of signals on one optical

fiber or cable. In the multi-channel signal transmission system, the multiplexing modeof signals greatly affects system performance and cost.

The multiplexing technology of optical transmission network goes through three

development phases: Space Division Multiplexing (SDM), Time Division Multiplexing

(TDM) and Wavelength Division Multiplexing (WDM).

With simple design and practical feature, the SDM technology requires that the

quantity of fiber transmission cores must be configured in accordance with quantity

signal multiplexing channels, which means poor investment profit. The TDM


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WM_000_ E1 DWDM Basic Principles

2

technology is widely applied. It is the base of PDH, SDH, ATM and IP. But its

disadvantage is low line utilization ratio. The WDM technology supports multiple

wavelengths (channels) to be carried on one fiber. So it is the major measure for

expanding the current fiber communication network and is mostly used in trunk

network.

1.1.2 PDH, SDH and DWDM

The traditional fiber transmission technologies, such as Plesiochronous Digital

Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH), use

"one-wavelength-in-one-fiber" mode. They are so restricted by the characteristics of

their own equipment that neither the transmission capacity nor expansion mode can

meet the requirements of the communication network developing at a high speed, while

leaving the massive bandwidth resources of fibers far from being fully exploited.

The new Dense Wavelength Division Multiplexing (DWDM) becomes the most

effective and practical means for the fiber expansion. With its unique technical

advantages, the DWDM technology becomes a simple and economical means to

expand the fiber transmission capacity in a rapid and effective manner. It can fully

meet the current need of the network broadband service development and lays a solid

foundation for the development of the future fully-optical transmission network.

The development processes of PDH, SDH and DWDM are briefed below, as well as

interface specifications of each technology.

1. PDH

The early optical transmission system uses PDH, which introduces in Pulse

Coding Modulation (PCM) digital transmission technology based on the former

analog telephone network. It multiplexes signals of low rate level into

high-speed signals by means of bit filling and digit interleaving.

The primary signals of the PDH system are in synchronous TDM mode, and the

multiplexing of other high order groups are in plesiochronous (or called

asynchronous) TDM mode.

The PDH system covers three regional rate level standards in Europe, North

America and Japan, as shown in Table 1.1-1.


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Chapter 1 DWDM Overview

3

Table 1.1-1 PDH Code Rate

Country/RegionPrimary Group

(Primary)

Secondary

GroupTertiary Group Quartus Group

Europe and

China

2.048Mbit/s

30 channels

8.448Mbit/s

120 channels (30

4)

34.368Mbit/s

480 channels

(120 4)

139.264Mbit/s

1920 channels

(480 4)

North America1.544Mbit/s

24 channels

6.312Mbit/s

96 channels (24

4)

44.736Mbit/s

672 channels (96

7)

274.176Mbit/s

4032 channels

(672 6)

Japan

1.544Mbit/s

24 channels

6.312Mbit/s

96 channels (24 4)

32.064Mbit/s

480 channels (96 5)

97.728Mbit/s

1440 channels(480 3)

From early 1970's to 1980's, the PDH system and devices are popularly used in

the digital network. However, along with the developing fiber communication

technology and user's increasing demands for communication services, the

PDH disadvantages are more and more remarkable.

1) The compatibility between three rate standards is not available, which obstructs

development of international interconnection.

2) There is no worldwide standard optical interface specification. Private optical

interfaces developed by different manufacturers are not compatible with each

other, which limits networking flexibility and increases network complexity and

operation costs.

3) PDH is a kind of multiplexing structure based on point-to-point transmission. It

only supports point-to-point transmission, but cannot accommodate complicated

networking.

4) The operation, management and maintenance must depend upon manual digital

signal cross-connection and service-suspension test, which cannot meet

monitoring and NM requirements of modern communication network.

5) Along with rate increase, it is more and more difficult to implement

multiplexing of high-order groups through PDH technology, and requirements of

fiber digital communication for large-capacity and super-high speed

transmission cannot be met.


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2. SDH

In mid-1980's, the Bell Communication Research Institute in America putforward the concept of Synchronous Optical Network (SONET). In 1988, the

CCITT (former ITU-T) accepted the SONET concept, formed the worldwide

unified technology standard for transmission network, and rename it as SDH.

The SDH signals use synchronous multiplexing mode and flexible multiplexing

and mapping structure. Code streams of different levels are arranged regularly in

the payload of the frame structure. The payload is synchronous with the network,

so software can be used to directly de-multiplex a high-speed signal into

low-speed tributary signals at a time, called one-step de-multiplexing.

The rate specifications of the SDH system are shown in Table 1.1-2.

Table 1.1-2 SDH Signal Levels

SDH Level (ITU-T) OC Level (SONET) Line Rate (Mbit/s)

STM-1 OC-3 155.520

STM-4 OC-12 622.080

STM-16 OC-48 2488.320

STM-64 OC-192 9953.280

The SDH standardizes the features of the digital signals, such as frame structure,

multiplexing mode, transmission rate level and interface code pattern. It

provides a frame that is supported globally, on which a world-class telecom

transmission network has been developed, featuring flexibility, reliability and

convenient management. This kind of transmission network is easy to expand

and is applicable to the development of new telecom services. In addition, it

makes possible the interworking between the devices of different manufacturers.

But, after the transmission rate is higher than 10 Gbit/s, the system dispersion

and other negative influences will increase difficulty of long-distance

transmission. Furthermore, the SDH system is the TDM system based on the

single wavelength. The single-wavelength transmission cannot fully utilize the

broad bandwidth of fiber. Therefore, the WDM technology is introduced in the

backbone network, greatly enlarging the transmission capacity of fiber.


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3. DWDM

DWDM is one of WDM technologies. Due to small interval (1 nm ~ 10 nmorder) between adjacent wavelengths, it is called DWDM. At present, the

practical DWDM system works in 1550 nm window for the purpose of using the

gain spectrum feature of the EDFA to directly amplify the composite optical

wavelength signals. To meet the horizontal compatibility between systems, the

central wavelength of the optical channel must accord with G.692 standard.

In the DWDM system, each optical channel can bear different customer signals,

such as SDH signal, PDH optical signal and ATM signal.

Due to unique advantages of fiber communication and its networking

technologies for accommodating multi-service and broadband requirements,

high-speed SDH system, N 2.5Gbit/s DWDM system and N 10Gbit/s

DWDM system become majority and backbone of the core network.

1.2 DWDM Technology Overview

The DWDM technology is this kind of fiber communication technology: Transmitting

multiple optical carriers with information (analog or digital) on one fiber and

expanding system only through wavelength (channel) increase. It can combine

(multiplex) optical signals with different wavelengths and then transmit them. After the

transmission, it can separate (de-multiplex) the combined optical signals and then send

them to different communication terminals. In other words, it can provide multiple

virtual fiber channels on one physical fiber.

1.2.1 Difference between DWDM Technology and Other MultiplexingTechnologies

This section compares the multiplexing technologies often used in the fiber

communication system.

1. TDM

TDM means that different channels of signals use different time intervals

(timeslot) for signal transmission on the same fiber.

The TDM has fixed timeslot allocation, which facilitate adjustment and control

and is applicable to the digital information transmission.


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Its disadvantage is low line utilization, because when a signal source has no data

for transmission, the corresponding channel will be idle but the other busy

channels cannot occupy this idle channel. In addition, restricted by modulation

capacity of high-speed electronic components and laser, it is difficult to

implement the system with a capacity over 40 Gbit/s.

The TDM technology is widely applied, such as PDH, SDH, ATM and IP.

2. SDM

SDM means the technology that divide space into different channels to

implement wavelength multiplexing. For example, you can add the core quantity

in the cable or use more fibers to form different channels.

The SDH performs optical intensity modulation to each channel of baseband

signals respectively. Each channel of signals are transmitted by one fiber,

different channels will not affect each other, leading to best transmission

performance.

The SDM technology has simple design and practical feature, but it requires that

the fiber transmission core quantity must be configured in accordance with the

signal multiplexing channel quantity, leading to poor investment profit.

3. SCM

Microwave Sub-Carrier Multiplexing (SCM) technology means to modulate

multiple baseband signals into the microwave carriers with different frequency

for the sake of electrical Frequency Division Multiplexing (FDM), and then use

this bit stream to modulate a single optical carrier into the fiber. At the receiving

end, pick the electrical FDM line signals through the photoelectrical detector

and then use microwave technology to de-multiplex the different microwave

carriers, to restore the former baseband signals. This technology is mostly used

in the CATV multi-band transmission system of access network.

4. WDM

WDM means to bear multiple wavelength (channel) systems on one fiber and

convert one fiber into multiple "virtual" fibers, each of which independently

works on different wavelengths. Boasting economical efficiency and

practicability, the WDM system technology is the major wavelength


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multiplexing technology commonly used in the current fiber communication

network.

The WDM is divided into three multiplexing modes: 1310 nm/1550

nm-wavelength multiplexing, Coarse Wavelength Division Multiplexing

(CWDM) and DWDM.

1) 1310 nm/1550 nm-wavelength multiplexing

In early 1970's, this multiplexing technology only uses two wavelengths: one in

1310 nm window and the other in 1550 nm window. It implements single-fiber

dual-window transmission through the WDM technology. It is the initial

wavelength division multiplexing case.

2) DWDM

In simple words, the DWDM technology refers to the WDM technology with

small interval between adjacent wavelengths, with working wavelength in the

1550 nm window. It can bear 8 ~ 160 wavelengths on one fiber, mostly used in

long-distance transmission system.

For the details, please refer to other chapters in this manual.

3) CWDM

The CWDM technology refers to the WDM technology with large interval

(usually greater than 20 nm) between adjacent wavelengths. Usually, its

wavelength quantity is 4 or 8 (16 at most). It uses 1200 nm ~ 1700 nm windows.

It adopts non-cooling laser and passive amplifier component, leading to a lower

cost than that of DWDM. Its disadvantages are small capacity and short

transmission distance. Therefore, the CWDM technology applies to the

communication cases with short distance, broad bandwidth and dense access

points, for example, the network communication inside the building or between

buildings.

1.2.2 Relationship between DWDM and SDH

1. Relationship between DWDM and SDH on the transmission layer of optical

network


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Both DWDM system and SDH system belong to the transmission network layer.

They are the transmission means established on the fiber transport media. Their

relationship in the transport network is shown in Fig. 1.2-1.

Circuit layer(for example,ATM and IP)

SDH channellayer

DWDM opticalchannel layer OADM

OTM

ADM

DXC

DWDM system

SDH system

Fig. 1.2-1 Relationship between DWDM and SDH in Transport Network

The SDH system implements multiplexing, cross-connection and networking on

the electrical channel layer. The WDM system implements multiplexing,

cross-connection and networking on the optical domain.

2. Multiplexing modes of DWDM and SDH for carrier signals

The SDH is the TDM system based on a single wavelength (one fiber

transmitting one wavelength channel). When the transmission rate exceeds

10 Gbit/s, the system dispersion and other negative influences will increase the

difficulty of long-distance transmission.

The DWDM technology simultaneously transmits multiple optical carrier

signals of different wavelengths in the same fiber, fully utilizing the bandwidth

resources of fiber and increasing system transmission capacity.

3. DWDM capable of transmitting different types of signals

At present, most customer-layer signals in the DWDM system are SDH signals.

But the wavelengths used in the DWDM system are mutually separated and


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irrelated with the service signal formats, so each wavelength can transmit the

optical signals with totally different features, for the sake of hybrid transmission

of multiple kinds of signals.

The relationship between DWDM system and some common services is shown

in Fig. 1.2-2.

IP

ATM

SDH

SDH ATM Ethernet Other

DWDM

Fiber physical layer

Open optical interface

Fig. 1.2-2 Relationship between DWDM and Other Services

4. Optical interface standards of DWDM and SDH signals

The optical interfaces of the SDH device should accord with the ITU-T G.957

recommendation, which does not stipulate the working central wavelength.

The optical interfaces in the DWDM system must accord with the ITU-T G.692

recommendation, which specifies the reference frequency of each optical

channel, channel interval, nominal central frequency (central wavelength),

central frequency offset and other parameters.

Therefore, the DWDM system can be either open DWDM system or integrated

DWDM system.

z Open system: The transmitting side of the WDM system provides the optical

wavelength converter (OTU) to converts the customer signals with non-standard

wavelength into the standard wavelength compliant with G.692 standard. "Open"

means that the DWDM system has no special requirement for the working

wavelength of the input signals, for example, the signals accessed through

"Open interface" shown in Fig. 1.2-2.


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z Integrated system: All the customer signals accessing the DWDM system must

accord with G.692 standard, for example, the signals accessed through

"Non-open interface" shown in Fig. 1.2-2.

5. Integrated application of DWDM and SDH

The transmission capacity of the fiber network can be effectively improved

through integrated application of DWDM and SDH.

1.2.3 Operation Wavelength Range

The quartz fiber has three low-loss windows: 860 nm, 1310 nm and 1550 nm, as shown

in Fig. 1.2-3.

0

0.5

1.0

1.5

2.0

2.5

3.0

800 1000 1200 1400 1600

Wavelength (nm)

Loss (dB/km)

~140THz

~50THz

OH- absorption peak

OH- absorptionpeak

OH- absorptionpeak

O ES C L

O: Original Band E: Extend Band S: Short Band C: Conventional Band L: Long Band

Fig. 1.2-3 Low-Loss Windows in Fiber Communication

1. 860 nm window

Its wavelength range is 600 nm ~ 900 nm. It is used in multi-mode fiber, with

large transmission loss (2 dB/km averagely). It is applicable to the short-distance

access network, such as Fiber Channel (FC) service.

2. 1310 nm window

The lower limit of the available wavelength here depends on the fiber cut-off

wavelength and fiber attenuation coefficient, and the upper limit depends on OH


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root absorption peak at 1385 nm. The working wavelength range is 1260 nm ~

1360 nm. The average loss is 0.3 dB/km ~ 0.4 dB/km.

This window is applied to intra-office, short-distance and long-distance

communication of STM-N signal (N = 1, 4 or 16). The light source can be

multi-longitudinal mode laser (MLM) and LED. Since the broadband optical

amplifier working in 1310 nm window is not available at present, this window is

not suitable for the DWDM system.

3. 1550 nm window

The lower limit of the available wavelength here depends on OH root absorption

peak at 1385 nm, and the upper limit depends on infrared absorption loss and

bending loss. . The working wavelength range is 1460 nm ~ 1625 nm. The

average loss is 0.19 dB/km ~ 0.25 dB/km.

The loss in the 1550 nm window is the lowest, so it can be applied to

short-distance and long-distance communication of SDH signals. In addition, the

EDFA often used currently has sound gain flatness in this window, so this

window is applicable to the DWDM system as well.

The working wavelength in the 1550 nm window is divided into three parts (S

band, C band and L band), with the wavelength range shown in Fig. 1.2-4.

Fig. 1.2-4 Division of Working Wavelength in 1550 Window

1) S band (1460 nm ~ 1530 nm): Since the working wavelength range of EDFA is

in C band or L band, S band is not used in the DWDM system at present.

2) C band (1530 nm ~ 1565 nm): It is often used as the working wavelength area of

the DWDM system under 40 waves (with band interval as 100 GHz), DWDM

system under 80 waves (with band interval as 50 GHz) and SDH system.

3) L band (1565 nm ~ 1625 nm): Working wavelength area of the DWDM system

above 80 waves. In this case, the band interval is 50 GHz.


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1.3 DWDM Features and Advantages

1. Fully utilizing fiber bandwidth resources and featuring large transmission

capacity

The DWDM technology makes full use of the huge bandwidth (about 25 THz)

resource of fibers to expand the transmission capacity of the system.

2. Super-long transmission distance

Through EDFA and other super-long distance transmission technologies, the

channel signals in the DWDM system are amplified at the same time, for the

sake of long-distance transmission of the system.

3. Abundant service access types

The wavelengths in the DWDM system are separated to each other, capable of

transmitting different services in transparent way, such as SDH, GbE and ATM

signals, for the sake of hybrid transmission of multiple kinds of signals.

4. Saving fiber resource

The DWDM system multiplexes multiple single-channel wavelengths for

transmission in one fiber, greatly saving fiber resource and reducing line

construction cost.

5. Smooth upgrading and expansion

Since the DWDM system transmits the data in each wavelength channel in

transparent way and does no process the channel data, only more multiplexing

wavelength channels should be added for expansion, which is convenient and

practical.

6. Fully utilizing well-developed TDM technology

At present, the optical transmission technologies in TDM mode, such as SDH,

have been well developed. Through the WDM technology, the transmission

capacity can be enlarged by several times or even dozens of times, with

expansion cost lower than that in the TDM mode.

7. Forming full optical network

Full optical network is the development trend of the optical transport network.

In such network, the WDM system is connected with Optical Add/Drop

Multiplexer (OADM) and Optical Cross-connection (OXC) device, to directly


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13

performing optical channel adding/dropping and cross-connection to the services

on the optical wavelength signals, and thus forming full optical network with

high flexibility, reliability, survivability and economical efficiency to meet the

requirements of future information society for the broadband transport network.

1.4 DWDM Development Trend

1. Higher channel rate

The channel rate of the DWDM system has developed to 10 Gbit/s from 2.5

Gbit/s, and the system at 40 Gbit/s rate is in experimentation and its technology

becomes more and more mature.

2. Greater wavelength multiplexing quantity

The DWDM system at early phase usually works at 8/16/32 wavelengths, with

channel interval as 100 GHz and working wavelength in C band. Along with

ever development of the technology, the working wavelength of DWDM system

can cover C and L bands, with interval as 50 GHz. For example, ZTE's ZXWM

M900 device can provide the multiplexing of up to 160 waves.

3. Super-long fully-optical transmission distance

The initial cost and operation cost for network construction can be reduced

through extending full optical transmission and reducing electrical regeneration

points.

The traditional DWDM system uses EDFA to extend the current-free delay

transmission distance. At present, this distance can be extended from 600 km to

above 2000 km, through distributed Raman amplifier and super-powerful

Forward Error Correction (FEC) technology, dispersion management technology

and optical balancing technology and effective modulation formats.

4. Evolving from point-to-point WDM to full optical network

The ordinary point-to-point DWDM system consists of Optical Terminal

Multiplexer (OTM). Although it has a huge transmission capacity, it only

provides primitive transmission bandwidth and features inflexible networking

capability. Along with ceaseless development of electrical cross-connection

system, the node capacity ever increases, and the point-to-point networking

obviously cannot keep up with the growth of network transmission link capacity.


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The further expansion opportunity depends on the optical nodes, that is, OADM

and OXC.

Through OADM, we can construct chain and ring optical networks. The OADM

controls the optical signals of different wavelength channels to be sent to the

proper locations, for the sake of protection and restoration of optical services.

OXC is the route switch of next generation optical communication. In the full

optical network, it provides these functions: Providing connection function

based on wavelengths, providing wavelength add/drop function of optical

channels, leading the wavelength channels for the sake of best utilization of

fiber infrastructure, and implementing protection and restoration on wavelength,wavelength and fiber levels. The OXC is set at the important tandem point of the

network, converging input of different wavelengths from different directions and

then output signals with proper wavelengths. Through OADM and OXC, we can

construct more complicated ring network. In the next generation IP Over

DWDM telecom/network architecture, the OXC may replace the existing

electrical switching/route by optical signals.

5. Development of IP over DWDM technology

The bandwidth of the Internet backbone network increases quickly, so if the

DWDM technology is not used, Internet data streams alone will occupy the

capacity of the whole single-wavelength fiber system (at present, the maximum

transmission rate of the single-wavelength fiber system for commercial use is 10

Gbit/s). Therefore, the IP over DWDM will be the major technology of the

network communication in the future.


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2 Overview of Optical FiberCommunication

Highlights:

z Basic knowledge on optical fiber communication.

z Types and applications of common optical fibers.

z Transmission features of optical fibers.

2.1 Basic Knowledge of Optical Fibers

2.1.1 Brief Introduction to Optical Fibers

1. Structure of optical fibers

Optical fiber is a kind of cylinder glass fiber with sound light conducting

performance and small diameter, consisting of fiber core, cladding and coating

layer, as shown in Fig. 2.1-1.

n1n2

Fiber coreCladding

Coating layer

n1: Refractive index of fiber core n2: Refractive index of cladding

Fig. 2.1-1 Structure of the Optical Fiber

1) Fiber core

It is made of SiO2 (quartz). It also comprises few doped chemical, such as GeO2,

to improve refractive index (n1) of the fiber core. The diameter of the fiber core

usually ranges 5 m ~ 50 m.


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2) Cladding

Usually, it is made of SiO2, with outer diameter as 125 m. The refractive index(n2) of cladding is less than that (n1) of fiber core.

3) Coating layer

It is made of high molecular materials, such as epoxide resin and silicone rubber,

with outer diameter as about 250 m. Through adding coating, we can improve

flexibility, mechanical strength and aging-resistance features of the optical fiber.

2. Fiber categories

1) By distribution shape of refractive index

When the light is transmitted in the fiber, each light shoots into the juncture

between fiber core and cladding in proper angle. Since the refractive index (n1)

of the fiber core is greater than that (n2) of the cladding, when the shooting-in

angle of the light meets the full reflection condition, the light can be repeatedly

reflected on the juncture and move forwards in "zigzag" way, and thus

restricting the light inside the fiber core and forming transmission wave.

Depending on the refractive index radio distribution on the fiber cross section,

the fiber is divided into step-type fiber and graded fiber. The relationship

between refractive index and fiber structure as well as the transmission of light

in the fiber are shown in Fig. 2.1-2.

CladdingLightFiber core

n2

n1

CladdingLightFiber core

n2

n1

a: Step-type fiber b: Graded fiber

Fig. 2.1-2 Comparison between Step-Type Fiber and Graded Fiber


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Chapter 2 Overview of Optical Fiber Communication

17

2) By fiber material

Besides quartz fiber, the fibers can be divided into quartz fiber, glass fiber withmany compositions, quartz-plastic fiber with quartz core and plastic cladding,

and full-plastic fiber with plastic core and plastic cladding, by material.

Such fibers have greater loss than the quartz fiber, so they are usually used by

the short-distance systems inside buildings or rooms.

3) By transmission mode

Light is a kind of electromagnetic wave. Therefore, the light transmitted in the

fiber should not only meet full-reflection condition between fiber core and

cladding, but also meet the related conditions for electromagnetic wave in the

transmission process.

For the specified fiber structure, only a series specified electromagnetic wave

can be effectively transmitted in the fiber. Such specified electromagnetic wave

is called optical fiber mode. In the fiber, the conductible mode quantity depends

on structure and refractive index radial distribution of the fiber.

If the fiber supports only one conduction mode (base mode), this fiber is called

Single-Mode Fiber (SMF) and its core transmits only one light. If the fiber

supports multiple conduction modes, this fiber is called Multi-Mode Fiber

(MMF), and each light in its core is in a transmission mode. Fig. 2.1-2 shows

two typical multi-mode fibers.

The differences between SMF and MMF are listed in Table 2.1-1.


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Table 2.1-1 Differences Between SMF and MMF

SMF MMF

Transmission

mode

Only supporting transmission in base

modeSupporting multiple conduction modes

Fiber core Smaller (about 5 m ~ 10 m) Greater (about 50 m)

Dispersion

influence

Caused by transmission rates of

different frequency components in the

optical signal; increasing along with

increased optical signal spectrum width

Large mode dispersion due to different

transmission rates of different modes,

directly affecting transmission

bandwidth and transmission distance

Type

Ordinary SMF, Dispersion Shifted

Fiber (DSF) and DispersionCompensation Fiber (DCF)

Ordinary MMF

Working

window1310 nm and 1550 nm 850 nm and 1310 nm

ApplicationsLong-distance fiber communication

system with large capacity

Short-distance fiber communication

system at low rate

2.1.2 Usage Overview of Applicable Frequencies of Optical Fiber

Due to improving fiber manufacture technique, the fiber transmission loss is lower and

lower. At present, there are five low-loss windows, as shown in Fig. 2.1-3.

0

0.5

1.0

1.5

2.0

2.5

3.0

800 1000 1200 1400 1600Wavelength (nm)

Loss (dB/km)

~140THz

~50THz

OH- absorption peak

OH- absorptionpeak

OH- absorptionpeak

O ES C L

III III IVV

O: Original Band E: Extend Band S: Short Band C: Conventional Band L: Long Band

Fig. 2.1-3 Division of Low-Loss Windows


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The optical signal mark, wavelength range, applied fiber types and application of each

window are described in Table 2.1-2.

Table 2.1-2 Feature Comparison between Low-Loss Windows

Window I II III IV V

Mark (nm) 8501310 (O

band)1550 (C band) 1600 (L band)

1360 ~ 1530

(E + S bands)

Wavelength

range (nm)600~900 1260~1360 1530~1565 1565~1625 1360~1530

Fiber type MMFMMF/G.652/

G.653

G.652/G.653/G

.655

G.652/G.653/G

.655

Full-wave

fiber

Applications

Short

distance and

low rate

Short

distance and

low rate

Long distance and high rate

2.1.3 Types and Features of Common SMFs

This section briefly introduces features and functions of three SMFs, G.652, G.653 and

G.655. The fiber types applied in the DWDM system are also involved.

1. G.652 (ordinary SMF)

It is also called dispersion non-shifted SMF, used in 1310 nm and 1550 nm

windows. In the 1310 nm window, it has dispersion close to zero. But in the

1550 nm window, its loss is the smallest, with dispersion of 17 ps/km nm.

When it is used in the 1310 nm window, it is only applicable to the SDH system;

when it is used in the 1550 nm window, it is applicable to both SDH system and

DWDM system, requiring dispersion compensation when the single channel rate

is over 2.5 Gbit/s.

2. G.653 (dispersion shifted SMF)

It has the smallest loss and the smallest dispersion in the 1550 nm. Therefore, it

usually works in the 1550 nm window.

It is applicable to the high-rate and long-distance single-wavelength

communication system. When the DWDM technology is used, serious

non-linear Four Wave Mixing (FWM) problem will occur in zero-dispersion

wavelength area, resulting in optical signal attenuation in multiplexing channels

and channel crosstalk.


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3. G.655 (non-zero dispersion shifted SMF)

In the 1550 nm window, the absolute value of its dispersion is not zero andwithin a certain range (ensuring smallest loss and small dispersion in this

window).

It is applicable to the high-rate and long-distance optical communication system.

In addition, non-zero dispersion suppresses the influence of non-linear FWM

over DWDM system. Therefore, this kind of fiber is usually used in the DWDM

system.

2.2 Working Wavelength of DWDM System

2.2.1 Introduction to Working Wavelength Area

Based on multiplexing channel quantity and frequency interval of the DWDM system,

the working wavelengths of the systems below 40 wavelengths, 80-wavelength system

and 160-wavelength system are introduce below.

1. 8/16/32/40-wavelength system

Working wavelength range: C band (1530 nm ~ 1565 nm)

Frequency range: 192.1 THz ~ 196.0 THz

Channel interval: 100 GHz

Central frequency offset: 20 GHz (at rate lower than 2.5 Gbit/s); 12.5 GHz (at

rate 10 Gbit/s)

2. 80-wavelength system

Working wavelength range: C band (1530 nm ~ 1565 nm)

Frequency range: C band (192.1 THz ~ 196.0 THz)


Central frequency offset: 5 GHz

3. 160-wavelength system

Working wavelength range: C band (1530 nm ~ 1565 nm) + L band (1565 nm ~

1625 nm)


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Frequency range: C band (192.1 THz ~ 196.0 THz) + L band (190.90 THz ~

186.95 THz)


Central frequency offset: 5 GHz

2.2.2 Wavelength Allocation

The working wavelength of the DWDM system complies with the specific central

wavelength and central frequency values in the multi-channel system, as stipulated by

the ITU-T Recommendation G.692.

1. The wavelength allocation for C band 40-wavelength system with wavelength

interval of 100 GHz is shown in Table 2.2-1.

Table 2.2-1 Wavelength Allocation of 40CH/100 GHz Interval on C Band

No. Central Frequency (THz) Wavelength (nm)

1 192.1 1560.61

2 192.2 1559.79

3 192.3 1558.98

4 192.4 1558.175 192.5 1557.36

6 192.6 1556.55

7 192.7 1555.75

8 192.8 1554.94

9 192.9 1554.13

10 193.0 1553.33

11 193.1 1552.52

12 193.2 1551.72

13 193.3 1550.92

14 193.4 1550.12

15 193.5 1549.32

16 193.6 1548.51

17 193.7 1547.72

18 193.8 1546.92

19 193.9 1546.12

20 194.0 1545.32

21 194.1 1544.53

22 194.2 1543.73

23 194.3 1542.94


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No. Central Frequency (THz) Wavelength (nm)

24 194.4 1542.14

25 194.5 1541.35

26 194.6 1540.56

27 194.7 1539.77

28 194.8 1538.98

29 194.9 1538.19

30 195.0 1537.40

31 195.1 1536.61

32 195.2 1535.82

33 195.3 1535.04

34 195.4 1534.2535 195.5 1533.47

36 195.6 1532.68

37 195.7 1531.90

38 195.8 1531.12

39 195.9 1530.33

40 196.0 1529.55

2. The wavelength allocation for C/C+ band 80-wavelength system with

wavelength interval of 50 GHz is shown in Table 2.2-2.

Table 2.2-2 Wavelength Allocation of 80CH/50 GHz Interval on C/C+ Band

Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

1 196.05 1529.16 41 194.05 1544.92

2 196.00 1529.55 42 194.00 1545.32

3 195.95 1529.94 43 193.95 1545.72

4 195.90 1530.33 44 193.90 1546.12

5 195.85 1530.72 45 193.85 1546.52

6 195.80 1531.12 46 193.80 1546.92

7 195.75 1531.51 47 193.75 1547.32

8 195.70 1531.90 48 193.70 1547.72

9 195.65 1532.29 49 193.65 1548.11

10 195.60 1532.68 50 193.60 1548.51

11 195.55 1533.07 51 193.55 1548.91

12 195.50 1533.47 52 193.50 1549.32

13 195.45 1533.86 53 193.45 1549.72


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Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

14 195.40 1534.25 54 193.40 1550.12

15 195.35 1534.64 55 193.35 1550.52

16 195.30 1535.04 56 193.30 1550.92

17 195.25 1535.43 57 193.25 1551.32

18 195.20 1535.82 58 193.20 1551.72

19 195.15 1536.22 59 193.15 1552.12

20 195.10 1536.61 60 193.10 1552.52

21 195.05 1537.00 61 193.05 1552.93

22 195.00 1537.40 62 193.00 1553.33

23 194.95 1537.79 63 192.95 1553.73

24 194.90 1538.19 64 192.90 1554.13

25 194.85 1538.58 65 192.85 1554.54

26 194.80 1538.98 66 192.80 1554.94

27 194.75 1539.37 67 192.75 1555.34

28 194.70 1539.77 68 192.70 1555.75

29 194.65 1540.16 69 192.65 1556.15

30 194.60 1540.56 70 192.60 1556.55

31 194.55 1540.95 71 192.55 1556.96

32 194.50 1541.35 72 192.50 1557.36

33 194.45 1541.75 73 192.45 1557.77

34 194.40 1542.14 74 192.40 1558.17

35 194.35 1542.54 75 192.35 1558.58

36 194.30 1542.94 76 192.30 1558.98

37 194.25 1543.33 77 192.25 1559.39

38 194.20 1543.73 78 192.20 1559.79

39 194.15 1544.13 79 192.15 1560.20

40 194.10 1544.53 80 192.10 1560.61


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3. The wavelength allocation for L/L+ band 80-wavelength system with

wavelength interval of 50 GHz is shown in Table 2.2-3.

Table 2.2-3 Wavelength Distribution of 80CH/50 GHz Interval on L/L+ Band

Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

1 190.90 1570.42 41 188.90 1587.04

2 190.85 1570.83 42 188.85 1587.46

3 190.80 1571.24 43 188.80 1587.88

4 190.75 1571.65 44 188.75 1588.30

5 190.70 1572.06 45 188.70 1588.73

6 190.65 1572.48 46 188.65 1589.15

7 190.60 1572.89 47 188.60 1589.57

8 190.55 1573.30 48 188.55 1589.99

9 190.50 1573.71 49 188.50 1590.41

10 190.45 1574.13 50 188.45 1590.83

11 190.40 1574.54 51 188.40 1591.26

12 190.35 1574.95 52 188.35 1591.68

13 190.30 1575.37 53 188.30 1592.10

14 190.25 1575.78 54 188.25 1592.52

15 190.20 1576.20 55 188.20 1592.95

16 190.15 1576.61 56 188.15 1593.37

17 190.10 1577.03 57 188.10 1593.79

18 190.05 1577.44 58 188.05 1594.22

19 190.00 1577.86 59 188.00 1594.64

20 189.95 1578.27 60 187.95 1595.06

21 189.90 1578.69 61 187.90 1595.49

22 189.85 1579.10 62 187.85 1595.91

23 189.80 1579.52 63 187.80 1596.34

24 189.75 1579.93 64 187.75 1596.76

25 189.70 1580.35 65 187.70 1597.19

26 189.65 1580.77 66 187.65 1597.62

27 189.60 1581.18 67 187.60 1598.04

28 189.55 1581.60 68 187.55 1598.47

29 189.50 1582.02 69 187.50 1598.89

30 189.45 1582.44 70 187.45 1599.32

31 189.40 1582.85 71 187.40 1599.75


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Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

Wavelength

No.

Nominal

Central

Frequency

THz

Nominal

Central

Wavelength

nm

32 189.35 1583.27 72 187.35 1600.17

33 189.30 1583.69 73 187.30 1600.60

34 189.25 1584.11 74 187.25 1601.03

35 189.20 1584.53 75 187.20 1601.46

36 189.15 1584.95 76 187.15 1601.88

37 189.10 1585.36 77 187.10 1602.31

38 189.05 1585.78 78 187.05 1602.74

39 189.00 1586.20 79 187.00 1602.17

40 188.95 1586.62 80 186.95 1603.57

2.3 Fiber Transmission Features

2.3.1 Fiber Loss

Power transmission loss is a basic and important parameter of the fiber. Due to

existence of fiber loss, the optical power transmitted in the fiber will attenuate by index

with the increase of transmission distance.

1. Generation of fiber loss and low-loss window

The fiber loss covers two aspects:

1) Loss coming from fiber, including inherent absorption loss of fiber materials,

absorption loss of material impurity (especially the loss caused by the remained

OH component in the fiber), Raileigh dispersion loss, and dispersion loss due to

incomplete fiber structure.

2) The fiber additional loss caused by optical cable layout, fiber connection and

system coupling/connection in all kinds of environment, because the fibers are

bundled into cable. This aspect involves bending loss and minor bending loss of

optical fiber/cable, connection loss in the fiber line, and coupling loss between

optical components.

The fiber attenuation spectrum is shown in Fig. 2.1-3. The average loss in

Window I is 2 dB/km, the one in Window II is 0.3 dB/km ~ 0.4 dB/km, and the

one in Window III is 0.19 dB/km ~ 0.25 dB/km. The 1380 nm point in Window

V exist OH absorption peak.


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2. The line loss values of the common fibers are shown in Table 2.3-1.

Table 2.3-1 SMF Loss

Fiber Type G.652 G.653 G.655

Typical loss value (1310 nm) 0.3 dB/km ~ 0.4 dB/km - -

Typical loss value (1550 nm) 0.15 dB/km ~ 0.25

dB/km

0.19 dB/km ~

0.25dB/km

0.19 dB/km ~

0.25 dB/km

Working window 1310 nm and 1550 nm 1550 nm 1550 nm

2. Relationship between fiber loss and OSNR

OSNR means the ratio between optical signal power and noise power. It is very

important for estimating and measuring bit error performance, engineering

design and maintenance.

Take the OSNR at the receiving end of the DWDM system as an example, the

calculation formula is:

OSNR = Pout 10log

M - L + 58 - NF - 10log

N

Here, Pout: In-fiber optical power (dBm).

M: Number of multiplexing channels of the WDM system

L: Loss between any two optical amplifiers, that is, sectional loss (dB)

NF: Noise index of the EDFA.

N: Number of the EDFAs between optical multiplexer and optical

de-multiplexer of the WDM system.

The formula shows this: When the other parameters keep unchanged, greater

line loss leads to lower OSNR, which means decreased transmission quality of

the optical line.

In the initial design of the DWDM, except loss limit and dispersion limit, the

OSNR at the receiving end, Q value and BER should also be considered. The

design is qualified only when these three factors are qualified.

2.3.2 Dispersion

After the optical pulse signals entering the fiber through input end are transmitted for a

long distance, the pulse wave shape spreads by time at the fiber output end, this


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phenomenon is called dispersion. We take the dispersion in the SMF as an example, as

shown in Fig. 2.3-1.

Time

Opticalpower

Incoming opticalpulse waveform

SMF

Time

Opticalpower

Outgoing opticalpulse waveform

Fig. 2.3-1 Dispersion in Fiber

Dispersion will cause inter-symbol interference, affects correct judgment of optical

pulse signal at the receiving end, deteriorates BER performance and even affects

information transmission.

The dispersion in the SMF is caused by different transmission rates of different

frequency components in the optical signal, and is called chroma dispersion. In the area

with negligible chroma dispersion, the polarization mode dispersion is the major part of

SMF dispersion.

The phenomena and causes of chroma dispersion and polarization mode dispersion will

be introduced below, as well as their influences to the DWDM system.

2.3.2.1 Chroma Dispersion

1. Brief introduction to chroma dispersion

Chroma dispersion is divided into material dispersion and waveguide dispersion.

1) Material dispersion: The quartz glass, fiber material, has different refractive

index for different optical wavelengths. While the light source has certain

spectrum width, and different wavelengths results in different group rates, so the

optical pulse spreading will occur.

2) Waveguide dispersion: For a transmission mode of the fiber, the pulse spreading

occurs due to different group rates in different optical wavelengths. This

dispersion is related to the waveguide effect of fiber structure, so it is also called

structure dispersion.

Material dispersion is greater than waveguide dispersion. According to the

dispersion calculation formula, the material dispersion at a specific wavelength


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may be zero, and this wavelength is called the zero dispersion wavelength of the

material. Luckily, this wavelength is in the low-loss window near 1310 nm. For

example, G.652 fiber is the zero dispersion fiber.

Although the optical components are heavily affected by the dispersion, there is

the tolerable maximum dispersion value (that is, dispersion tolerance). Normal

transmission can be ensured when the generated dispersion is within the

tolerance.

2. Influence of chroma dispersion

Chroma dispersion will result in pulse spreading and chirp effect.

1) Pulse spreading

Pulse spreading is the major influence of chroma dispersion to system

performance. When the transmission distance is longer than the fiber dispersion

length, the pulse spreading is too large. At this time, the system will have serious

inter-symbol interference and bit errors.

2) Chirp effect

Dispersion not only results in pulse spreading but also makes pulse generate

phase modulation. Such phase modulation makes different parts of the pulse

make different offset from the central frequency with different frequencies,

which is called chirp effect of pulse.

Due to chirp effect, the fiber is divided into normal dispersion fiber and

abnormal dispersion fiber. In the normal dispersion fiber, the high-frequency

component of the pulse is located at the rear edge of the pulse and the

low-frequency component is located at the front edge of the pulse. In the

abnormal dispersion fiber, the low-frequency component of the pulse is located

at the rear edge of the pulse and the high-frequency component is located at the

front edge of the pulse. In the transmission line, we can properly use these two

fibers to offset the chirp effect and remove the pulse dispersion spreading.

3. Removing influence of chroma dispersion to DWDM system

Since the DWDM system is mostly used in the 1550 nm window, if G.652 fiber

is used, it is required to use the DCF with negative wavelength dispersion to

compensate the dispersion and reduce the total dispersion value of the whole

transmission value.


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2.3.2.2 Polarization Mode Dispersion (PMD)

PMD is a kind of physical phenomenon existing in optical fiber and optical component

fields.

The baseband in the SMF has two polarization modes that are orthogonal. In the ideal

case, two polarization modes should have the same feature curve and transmission

characteristics. Due to geometrical and pressure asymmetry, two polarization modes

have different transmission rates, resulting in delay and PMD, as shown in Fig. 2.3-2.

Usually, the unit of PMD is ps/km1/2

.

Incoming light Outgoing light

Delay

Optical fiber

Fig. 2.3-2 PMD in SMF

In the digital transmission system, the PMD will result in pulse separation and pulse

spreading, degrade transmission signal and limit transmission rate of carriers.

Compared with other dispersions, the PMD can almost be omitted. But it cannot be

totally extinguished, but can be minimized through optical components. The narrower

the pulse in the super-speed system is, the greater the PMD influence is.

2.3.3 Non-Linear Effect of Fiber

In the common fiber communications system, the transmitting optical power is low and

the fiber has linear transmission feature. But, for the DWDM system, the fiber has

non-linear effect after the EDFA is used.

The non-linear effect of the fiber will result in serious crosstalk between

multi-wavelength channels of the DWDM system, which will lead to additional

attenuation of the fiber communication system as well as restriction of optical power,

EDFA amplifying performance and current-free regenerative relay distance.


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The non-linear effect involves Self-Phase Modulation (SPM), Cross-Phase Modulation

(XPM), Four Wave Mixing (FWM), Stimulated Raman Scattering (SRS) and

Stimulated Brillouin Scattering (SBC).

1. SPM

Due to dependency relationship between refractive index and light intensity,

refractive index changes during optical pulse continuance, with the pulse peak

phase delayed for both front and rear edges. With more transmission distance,

the phase shift is accumulated continuously and represents large phase

modulation upon certain distance. As a result, the spectrum spreading results in

pulse spreading, which is called SPM, as shown in Fig. 2.3-3.

Intensity

Pulse width beforetransmission

Pulse width aftertransmission

Optical spectrumbefore transmission

Optical spectrum aftertransmission

Intensity

Fig. 2.3-3 SPM

When the system works in the fiber working area (for example, the short

wavelength area of G.653 fiber or working area with negative dispersion of

G.655 fiber) with negative dispersion index, the SPM will result in smaller

dispersion limit distance. When the system works in the fiber working area (for

example, the long wavelength area of G.652/G.653 fiber or working area with

positive dispersion of G.655 fiber) with positive dispersion index, the SPM will

result in greater dispersion limit distance.


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The SPM effect occurs in certain distance from the transmitter end. In addition

the low-dispersion fiber can also reduce the influence of SPM to the system

performance.

2. XPM

When two or more optical waves with different frequencies are simultaneously

transmitted in the non-linear media, the amplitude modulation of each frequency

wave will result in the corresponding change of the fiber refractive index,

resulting in non-linear phase modulation of the optical wave with other

frequencies, which is called XPM.

XPM often occurs along with SPM. XPM will result in a series of non-linear

effects, such as signal interference between DWDM system channels and

non-linear dual-refraction of the fiber, leading to unstable polarization of the

fiber transmission. Meanwhile, the XPM will also affect wave shape and

spectrum of pulse.

Adding dispersion properly can reduce the XPM influence.

3. FWM

FWM refers to a physical process of energy exchange between multiple optical

carriers caused by non-linear effect of the fiber, when multiple frequencies of

optical carriers with high power are simultaneously transmitted in the fiber.

FWM results in optical signal energy attenuation in multiplexing channels and

channel crosstalk. As shown in Fig. 2.3-4, a new optical wave appears on

another wavelength, due to FWM.


New optical wave

Fig. 2.3-4 FWM

The generation of FWM is related to the fiber dispersion. For zero dispersion,

the mixing efficiency is the highest. Along with dispersion increase, the mixing


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efficiency reduces fast. The DWDM system uses G.655 fiber to slider over the

FWM effect in 1550 nm zero-dispersion wavelength area.

4. SRS

SRS belongs to the stimulated non-elastic scattering process caused by

non-linear effect. It comes of mutual action and energy exchange between

photon and optical phonon (molecular vibration status).

SRS effect results in attenuation of signals with short wavelength and

reinforcement of signals with long wavelength, as shown in Fig. 2.3-5.

Power

1 2 3

Power

1 2 3


... ...

Fig. 2.3-5 SRS

SRS effect is widely applied in the fiber communication, for example, making

distributed Raman amplifier based on Raman gain, to provide distributed

broadband amplifying for optical signals. The DRA board of ZTE DWDM

device implements the optical amplifying function through the SRS effect. On

the other hand, SRS introduces in negative influence to the communication

system. In the DWDM system, light in the short-wavelength channel will serve

as pump light to transfer energy to the long-wavelength channel, resulting in

Raman crosstalk between channels.

5. SBS

SBS belongs to the stimulated non-elastic scattering process caused bynon-linear effect. It comes of mutual action and energy exchange between

photon and acoustic phonon (crystal vibration status).

SRS effect can be used to make fiber Brillouin laser and amplifier. On the other

hand, SBS will result in unstable signal light source and crosstalk between

reverse transmission channels. However, along with increase of system

transmission rate, the SBS peak gain obviously reduces. So, SBS will not greatly

affect the high-speed fiber transmission system.


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2.4 New Optical Fiber Types

This section will briefly introduce features and applications of some new-type fibers.

1. G.654 optical fiber

The G.654 fiber works in the 1550 nm window, with average loss as 0.15 dB/km

~ 0.19 dB/km, which is less than those of the other types of fibers. The zero

dispersion point is still in the 1310 nm window.

It is applicable to the long/medium-distance optical transmission system.

2. Full-wave fiber

The full-wave fiber, water peak free fiber, eliminates OH- ions near the 1385 nm

wavelength and thus also eliminates the appended water peak attenuation caused

by the OH-ions. In this way, the fiber attenuation is only determined by the

internal scattering loss of the silicon glass.

Full-wave fiber is numbered as G.652 C&D in ITU-T Recommendations. It is

one kind of G.652 fiber. Its full name is wavelength-expanded dispersion

non-shifted single-mode fiber.

The attenuation of the full-wave fiber becomes even at the band of 1310 nm~

1600 nm. As internal OH-ions are already eliminated, no water peak attenuation

will occur even when the fiber is exposed to hydrogen gas and the long-term

attenuation is reliable.

Full-wave optical fiber can provide a complete transmission band from 1280 nm

to 1625 nm. The available wavelength range is about 1.5 times of the

wavelength range of ordinary fibers.

3. Real-wave fiber

Real-wave fiber is a kind of non-zero dispersion shifted single-mode fiber

(G.655 fiber) widely used at present. Its fiber characteristics are similar to those

of G.655 fiber. Its zero dispersion point is in short-wavelength area below 1530

nm. In 1549 nm ~ 1561 nm band, the dispersion index is 2.0 ps/nmkm ~ 3.0

ps/nmkm.

It has small dispersion slope and dispersion coefficient, capable of tolerating

higher non-linear effect. It is applicable to large-capacity optical transmission

system to reduce network construction cost.


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4. Fiber with large effective fiber core area

It also belongs to non-zero dispersion shifted single-mode fiber (G.655 fiber).Essentially, it improves non-linear resistance capability of the system.

The super-speed system performance is mostly limited by dispersion and

non-linear effect. Usually, dispersion can be distinguished through dispersion

compensation. But the non-linear effect cannot be distinguished only through

linear compensation. The effective area of the fiber determines the fiber

non-linear effect. Larger effective area means high affordable optical power, that

is, better resistance to non-linear effect.


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3 Key Technologies of DWDM System

Highlights:

z Basic structure of the DWDM system.

z Light source technology.

z Optical wavelength division multiplexing and de-multiplexing technologies.

z OTU technology.

z Optical amplifying technology.

z Supervision technology.

3.1 Basic Structure of DWDM System

The DWDM system multiplexes several or dozens of optical channel signals with

different nominal wavelengths to one fiber for transmission, with each optical channel

bearing one service signal.

The basic structure of a unidirectional DWDM system is shown in Fig. 3.1-1.

Receiver/transmitter ofoptical supervision channel

n

2

1

3

G.692

...

Opticaltransponder

OM OBA OLA OPA

Receiver of opticalsupervision channel

Transmitter of opticalsupervision channel

OD

n

3

2

1

...

Optical transmitter Optical receiver

Optical relay amplifier

TX1

TX2

TX3

TXn

RX1

RX2

RX3

RXn

Opticaltransponder

Opticaltransponder

Opticaltransponder

Opticaltransponder

Opticaltransponder

Opticaltransponder

Opticaltransponder

Fig. 3.1-1 Composition of DWDM System

1. Optical transmitter end


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TX1TXn, the optical transmitters of all the multiplexing channels,

respectively transmit the optical signals (1, 2 n, with the corresponding

frequencies as f1, f2fn) with different nominal wavelengths. Each optical

channel bears different service signals, such as standard SDH signal, ATM signal

and Ethernet signal. After that, the optical multiplexer combines these signals

into one beam of optical wave, which will be output by the OBA to the fiber for

transmission.

2. Optical receiver end

After the line fiber goes through amplifying of OPA, the optical channel signals

are de-multiplexed by the optical de-multiplexer and then respectively input tothe corresponding multiplexing channel optical receivers, RX1RXn.

3. Optical regenerating amplifier end

Located in the middle of the optical transmission section, it uses OLA to amplify

the optical signals.

4. Optical monitoring channel

In the DWDM system shown in Fig. 3.1-1, an independent wavelength (1510

nm) is used as the optical monitoring channel for transmitting optical monitoring

signals. The optical monitoring signals are used to bear NE management and

monitoring information of the DWDM system, for the sake of effectively

management of network management system over the DWDM system.

5. Network management system

This module is omitted in Fig. 3.1-1. The DWDM NMS should be capable of

managing optical amplifying units (such as OBA, OLA and OPA), wavelength

division multiplexer, Optical Transponder Unit (OTU) and channel performance

supervision on one platform. It can manage the device in terms of performance,

fault, configuration and security. The information in the NMS is borne by the

monitoring signals in the optical monitoring channel.

3.2 Light Source Technology

1. Type of optical sources

At present, the semi-conductor optical sources widely used now are Laser

Device (LD) and Light-Emitting Diode (LED).


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LD is coherence light source, with large in-fiber power, small spectral line width

and high modulation rate. It is applicable to the long-distance high-speed system.

The LED is non-coherence light source, with small in-fiber power, large spectral

line width and low modulation rate. It is applicable to short-distance low-speed

system.

The light source of the DWDM system adopts the semi-conductor laser.

2. Features of DWDM system light source

1) Providing standard and stable wavelength

The DWDM system has very strict requirements for the working wavelength of

each multiplexing channel. Wavelength drift will cause unstable and unreliable

operation of the system.

The common wavelength stabilization measures are temperature feedback

control method and wavelength feedback control method.

2) Providing rather large dispersion tolerance

Fiber transmission may be limited by system loss and dispersion. Along with

increased transmission rate, the dispersion influence is larger. Here, dispersion

limit can be solved through using optical fibers/cables with small dispersion

coefficient or semi-conductor laser with small spectral width. After the optical

cables are laid, minimizing spectral width of light source devices is an effective

measure for solving dispersion limit.

3. Modulation modes of DWDM system laser

At present, there are two methods of light source intensity modulation: Direct

modulation and indirect modulation (that is, external modulation).

1) Direct modulation

Direct modulation means directly controlling the working current of

semi-conductor laser through electrical pulse code stream, and thus making it

generate the optical pulse stream corresponding to the electrical signal pulse. For

example, when the electrical pulse signal is "1", the working current of the laser

is greater than its current threshold, therefore it generates an optical pulse. When

the electrical pulse signal is "0", the working current of the laser is smaller than

its current threshold, therefore it does not generate optical pulse.


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The direct modulation mode is simple, with low loss and low cost. But, the

super-speed change of working current of the laser will make modulation chirp

easily. And chirp will limit transmission rate and distance of the system.

The direct modulation mode is often used in the transmission system composed

of G.652 fiber, with transmission shorter than 100 km and rate lower than

2.5 Gbit/s.

2) Indirect modulation (external modulation)

The external modulation mode refers to indirectly control (modulate) the

continuous light generated by the laser which is in the continuous light emitting

status, and thus obtaining optical pulse stream.

Therefore, in external modulation case, the laser nit generates stable high-power

laser, and the external modulator will modulate it in low chirp, to obtain the

maximum dispersion value much greater than that in the case of direct

modulation.It is applicable to the long-distance transmission system at rate over

2.5 Gbit/s.

At present, the external modulators often used are electrical absorption

modulator (EA) and waveguide Mach-Zehnder (MZ) modulator.

z EA modulator

It uses absorber controlled by electrical pulse signal to absorb or not absorb the

optical wave transmitted by the continuous wave semi-conductor laser (CW),

and thus making optical pulse stream under indirect control of electrical pulse

signal stream.

The EA light source features small size, high integration, low driving power and

low power consumption. The maximum dispersion can reach 12 000 ps/nm.

z Waveguide M-Z modulator

At the input end, the CW is in continuous wave working status. The optical

wave emitted by it is divided by the optical de-multiplexer into two equal signal

channels, which will respectively enter two optical tributaries of the modulator.

Under control of electrical pulse stream, it performs phase modulation to the

optical signals. At the output end, two optical tributaries are combined by the

optical multiplexer. When the signal phases in two optical tributaries are reverse

to each other, the optical multiplexer has no optical signal output; when the


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Chapter 3 Key Technologies of DWDM System

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signal phases in two optical tributaries are the same, the optical multiplexer has

optical signal output. In this way, the optical pulse stream is controlled by the

electrical pulse stream.

The M-Z light source features high modula

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