IEC TR 61282-5:2019

Fibre optic communication system design guidelines - Part 5: Accommodation and compensation of chromatic dispersion

IEC TR 61282-5:2019 which is a Technical Report, describes various techniques for accommodation and compensation of chromatic dispersion in fibre optic communication systems. These techniques include dispersion compensation with passive optical components, advanced dispersion management, and electronic accommodation of dispersion in the transmitters and receivers. This second edition cancels and replaces the first edition, published in 2002, and constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
a) extends the application space for dispersion compensation and accommodation to communication systems that employ non-zero dispersion-shifted fibres;
b) adds a discussion on the suitability of fibre types for long-haul transmission of wavelength-multiplexed signals;
c) updates the dispersion coefficient limits for dispersion-unshifted fibres;
d) adds information on the dispersion coefficients of dispersion-shifted fibres;
e) updates the naming of the fibre types to the revised naming conventions defined in IEC 60793-2-50:2018;
f) updates Table 2 to include the dispersion tolerance of phase-shift-keyed modulation formats used for the transmission of 40 Gbit/s and 100 Gbit/s signals;
g) adds information on dispersion management in terrestrial and submarine communication systems;
h) extends the description of passive dispersion compensators based on fibre Bragg gratings and etalons;
i) adds information on electronic dispersion accommodation in coherent communication systems (including transmitters and receivers);
j) updates the description of optical accommodation techniques to include soliton transmission and mid-span spectral inversion;
k) extends the list of system parameters for passive dispersion compensators to include wavelength-dependent loss, phase ripple, and latency;
l) updates the description of dispersion compensator applications in long-haul communication systems.
Keywords: chromatic dispersion

General Information

• Status: Published
• Publication Date: 16-Jul-2019
• ICS: 33.180.01 - Fibre optic systems in general
• Technical Committee: SC 86C - Fibre optic systems, sensing and active devices
• Drafting Committee: WG 1 - TC 86/SC 86C/WG 1
• Current Stage: PPUB - Publication issued
• Start Date: 17-Jul-2019
• Completion Date: 08-Aug-2019

Relations

• Revises: IEC TR 61282-5:2002 - Fibre optic communication system design guides - Part 5: Accommodation and compensation of dispersion
• Effective Date: 05-Sep-2023

Overview

IEC TR 61282-5:2019 - "Fibre optic communication system design guidelines - Part 5: Accommodation and compensation of chromatic dispersion" is a Technical Report from IEC (Edition 2.0, 2019). It surveys methods to manage chromatic dispersion in fibre‑optic systems, covering passive optical compensators, dispersion management strategies and electronic accommodation techniques in transmitters and receivers. This edition updates the 2002 report and extends guidance to modern systems such as WDM, coherent receivers and NZDSF (non‑zero dispersion‑shifted fibre) networks.

Key topics

• Dispersion mechanisms and impact - dependence on fibre type (dispersion‑unshifted, dispersion‑shifted, NZDSF), pulse broadening, pulse narrowing and dispersion‑limited distance.
• Passive compensation techniques - dispersion‑compensating fibre (DCF), chirped fibre Bragg gratings (FBG), etalon filters and dispersion compensation modules.
• Dispersion management - span‑by‑span and map‑based approaches for terrestrial and submarine links, and hybrid amplification scenarios.
• Electronic accommodation - transmitter pre‑distortion, receiver electrical compensation (ADC/DAC, MLSE) and coherent‑detection based CD compensation.
• Advanced optical techniques - soliton transmission, mid‑span spectral inversion and four‑wave mixing for spectral inversion.
• System and device parameters - chromatic dispersion, dispersion slope, operating wavelength range, insertion loss, wavelength‑dependent loss, phase ripple, latency, PMD, PDL and optical nonlinearity.
• Modulation and data‑rate relevance - updated dispersion tolerance information for phase‑shift‑keyed formats used at 40 Gbit/s and 100 Gbit/s and guidance for WDM transmission.

Note: IEC TR 61282-5 is informative (no normative references) and compiles state‑of‑the‑art techniques rather than mandatory requirements.

Applications and users

This report is practical for:

• System designers and network architects planning long‑haul, submarine or WDM networks who need to select fibre types and compensation strategies.
• Component manufacturers developing DCF, FBGs, DCMs and coherent receivers requiring informed parameter targets (e.g., dispersion slope, phase ripple).
• Test engineers and integrators evaluating end‑to‑end dispersion budgets and channel‑specific residual dispersion.
• Operators deploying 40/100 Gbit/s channels or migrating to coherent systems where electronic dispersion accommodation and multi‑band WDM are relevant.

Practical benefits include guidance on selecting compensation locations (pre/post amplifiers, in‑line), expected trade‑offs (loss, latency, nonlinearity) and correlating fibre types with WDM‑system suitability.

Related standards

• IEC 60793-2-50:2018 (revised fibre type naming used in this TR)
• Other parts of the IEC 61282 series (fibre optic communication system design guidelines)

Keywords: chromatic dispersion, dispersion compensation, DCF, fibre Bragg grating, coherent communication, WDM, dispersion management.

Standards Content (Sample)

IEC TR 61282-5 ®
Edition 2.0 2019-07
TECHNICAL
REPORT
Fibre optic communication system design guidelines –
Part 5: Accommodation and compensation of chromatic dispersion

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

IEC Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform Electropedia - www.electropedia.org
The advanced search enables to find IEC publications by a The world's leading online dictionary on electrotechnology,
variety of criteria (reference number, text, technical containing more than 22 000 terminological entries in English
committee,…). It also gives information on projects, replaced and French, with equivalent terms in 16 additional languages.
and withdrawn publications. Also known as the International Electrotechnical Vocabulary

(IEV) online.
IEC Just Published - webstore.iec.ch/justpublished
Stay up to date on all new IEC publications. Just Published IEC Glossary - std.iec.ch/glossary
details all new publications released. Available online and once 67 000 electrotechnical terminology entries in English and
a month by email. French extracted from the Terms and Definitions clause of IEC
publications issued since 2002. Some entries have been
IEC Customer Service Centre - webstore.iec.ch/csc collected from earlier publications of IEC TC 37, 77, 86 and
If you wish to give us your feedback on this publication or need CISPR.

further assistance, please contact the Customer Service

Centre: [e-mail].
IEC TR 61282-5 ®
Edition 2.0 2019-07
TECHNICAL
REPORT
Fibre optic communication system design guidelines –

Part 5: Accommodation and compensation of chromatic dispersion

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.01 ISBN 978-2-8322-7131-5

– 2 – IEC TR 61282-5:2019 © IEC 2019
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and abbreviated terms . 6
3.1 Terms and definitions . 6
3.2 Abbreviated terms . 6
4 Background . 7
5 Impact of chromatic dispersion . 8
5.1 Dependence on fibre type . 8
5.2 Dispersion-unshifted fibres . 8
5.3 Dispersion-shifted fibres . 10
5.4 Pulse broadening . 11
5.5 Pulse narrowing and signal peaking . 13
5.6 Dispersion-limited transmission distance . 14
6 Compensation and accommodation of dispersion . 16
6.1 Passive dispersion compensation along the optical path . 16
6.1.1 General . 16
6.1.2 Dispersion compensating fibre . 16
6.1.3 Chirped fibre Bragg grating . 17
6.1.4 Etalon filter . 18
6.2 Dispersion management . 18
6.3 Accommodation of dispersion . 20
6.4 Pre-distortion of the transmitted signal . 20
6.5 Electrical accommodation in the receiver . 21
6.6 Dispersion-assisted transmission . 22
6.7 Mid-span spectral inversion . 23
7 Passive dispersion compensator parameters . 24
7.1 Compensated fibre length . 24
7.2 Operating wavelength range . 24
7.3 Chromatic dispersion . 24
7.4 Dispersion slope . 25
7.5 Insertion loss . 25
7.6 Wavelength-dependent loss . 25
7.7 Phase ripple . 26
7.8 Reflectance . 26
7.9 Polarization-mode dispersion . 26
7.10 Polarization-dependent loss . 27
7.11 Optical nonlinearity . 27
7.12 Latency . 27
8 Passive dispersion compensator applications . 28
8.1 Unamplified fibre spans . 28
8.2 Fibre links with in-line optical amplifiers . 28
8.3 Multi-channel WDM transmission systems . 29
8.4 Hybrid transmission systems . 30
8.5 Multi-band WDM transmission systems . 30

9 System parameters for passive dispersion compensators . 30
Bibliography . 32

Figure 1 – Range of the dispersion coefficient for B-652.D fibres . 9
Figure 2 – Distortions in a 10 Gbit/s NRZ signal at various amounts of CD . 14
Figure 3 – Summing the dispersions of a B-652 fibre and a DCF over the C-band . 17
Figure 4 – Reflectivity and time delay of an FBG-based PDC . 18
Figure 5 – Periodic dispersion map with span-by-span compensation . 19
Figure 6 – Transmitter for generating pre-compensated optical signals . 21
Figure 7 – Coherent optical receiver with electrical CD post-compensation . 22
Figure 8 – Spectral inversion of a modulated signal via four-wave mixing . 23
Figure 9 – Passive dispersion compensators placed at the receiver . 28
Figure 10– PDCs placed before optical booster amplifiers at the transmitter . 28
Figure 11 – PDCs placed after pre-amplifiers at the receiver. 28
Figure 12 – Optically amplified link with in-line PDCs . 29
Figure 13 – Optically amplified WDM communication link with in-line PDCs . 29
Figure 14 – WDM link with individual compensation of residual dispersion . 30
Figure 15 – Two-band WDM link with OA and PDC in the C-band . 30

Table 1 – Single-mode fibre types and range of dispersion coefficients at 1 550 nm . 11
Table 2 – Dispersion-limited transmission distances over B-652 fibre at 1 550 nm . 15
Table 3 – Primary system parameters for DCF-based PDCs . 31
Table 4 – Primary system parameters for FBG-based PDCs . 31

– 4 – IEC TR 61282-5:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SYSTEM DESIGN GUIDELINES –

Part 5: Accommodation and compensation of chromatic dispersion

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 61282-5, which is a Technical Report, has been prepared by subcommittee 86C: Fibre
optic systems and active devices, of IEC technical committee 86: Fibre optics.
This second edition cancels and replaces the first edition, published in 2002, and constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) extends the application space for dispersion compensation and accommodation to
communication systems that employ non-zero dispersion-shifted fibres;
b) adds a discussion on the suitability of fibre types for long-haul transmission of wavelength-
multiplexed signals;
c) updates the dispersion coefficient limits for dispersion-unshifted fibres;

d) adds information on the dispersion coefficients of dispersion-shifted fibres;
e) updates the naming of the fibre types to the revised naming conventions defined in
IEC 60793-2-50:2018;
f) updates Table 2 to include the dispersion tolerance of phase-shift-keyed modulation formats
used for the transmission of 40 Gbit/s and 100 Gbit/s signals;
g) adds information on dispersion management in terrestrial and submarine communication
systems;
h) extends the description of passive dispersion compensators based on fibre Bragg gratings
and etalons;
i) adds information on electronic dispersion accommodation in coherent communication
systems (including transmitters and receivers);
j) updates the description of optical accommodation techniques to include soliton transmission
and mid-span spectral inversion;
k) extends the list of system parameters for passive dispersion compensators to include
wavelength-dependent loss, phase ripple, and latency;
l) updates the description of dispersion compensator applications in long-haul communication
systems.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
86C/1573/DTR 86C/1581/RVDTR
Full information on the voting for the approval of this Technical Report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61282 series, published under the general title Fibre optic
communication system design guidelines, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

– 6 – IEC TR 61282-5:2019 © IEC 2019
FIBRE OPTIC COMMUNICATION SYSTEM DESIGN GUIDELINES –

Part 5: Accommodation and compensation of chromatic dispersion

1 Scope
This part of IEC 61282, which is a Technical Report, describes various techniques for
accommodation and compensation of chromatic dispersion in fibre optic communication
systems. These techniques include dispersion compensation with passive optical components,
advanced dispersion management, and electronic accommodation of dispersion in the
transmitters and receivers.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.2 Abbreviated terms
ADC analogue-to-digital converter
BER bit-error ratio
CD chromatic dispersion
CW continuous wave
DAC digital-to-analogue converter
DCF dispersion-compensating fibre
DCM dispersion compensation module
DGD differential group delay
DPSK differential phase-shift keying
DQPSK differential quaternary phase-shift keying
DSF dispersion-shifted fibre
DWDM dense wavelength-division multiplexing
FBG fibre Bragg grating
FWM four-wave mixing
I in-phase component
IL insertion loss
ITU International Telecommunication Union

LO local oscillator
MLSE maximum-likelihood sequence estimation
NRZ non-return-to-zero
NZDSF non-zero dispersion-shifted fibre
OA optical amplifier
OOK on-off keying
OSNR optical signal-to-noise ratio
PAM pulse-amplitude modulation
PBS polarization splitter
PCD pre-compensated dispersion
PD photo-detector
PDC passive dispersion compensator
PDL polarization-dependent loss
PMD polarization-mode dispersion
PSK phase-shift keying
Q quadrature-phase component
QAM quadrature amplitude modulation
QPSK quaternary phase-shift keying
RD residual dispersion
RDPS residual dispersion per span
RMS root-mean-square
Rx optical receiver
RZ return-to-zero
SPM self-phase modulation
TIA transimpedance amplifier
Tx optical transmitter
WDL wavelength-dependent loss
WDM wavelength-division multiplexing
XPM cross-phase modulation
XPolM cross-polarization modulation
XI
in-phase component of X-polarized signal
XQ quadrature-phase component of X-polarized signal
YI in-phase component of Y-polarized signal
YQ quadrature-phase component of Y-polarized signal
4 Background
Optical communication fibres often exhibit a considerable amount of chromatic dispersion (CD).
This means that optical signals at different wavelengths propagate at different speeds through
the fibre and, hence, arrive at different times at the receiver. In some communication links, the
fibre dispersion can be large enough to also introduce significant differential time delays
between the various frequency components forming a single modulated optical signal. These
time delays may cause severe waveform distortions in the transmitted optical signal. Chromatic
dispersion accumulates linearly with fibre length and, hence, can severely limit the maximal
distance over which an optical signal may be transmitted without intermediate electrical
regeneration.
– 8 – IEC TR 61282-5:2019 © IEC 2019
To overcome these distance limitations, special fibres have been developed that exhibit
relatively small or even negligible dispersion in the wavelength range of interest. It was found,
however, that fibres with vanishing dispersion are not well suited for long-haul communication
systems employing dense wavelength-division multiplexing (DWDM) because of signal
distortions due to nonlinear optical interactions between the various multiplexed signals, such
as cross-phase modulation (XPM) and four-wave mixing (FWM). In fibres with relatively large
CD, the nonlinear signal distortions accumulate much more slowly than in fibres with only small
or even vanishing CD. The reason is that dispersion introduces differential time delays between
the various multiplexed signals as they travel through the fibre, which have the effect that they
de-phase the nonlinear interactions between the signals. For this reason, DWDM
communication systems usually employ fibres that have non-vanishing dispersion in the
wavelength range of interest.
If not properly compensated or otherwise accommodated, the accumulated dispersion at the
end of the fibre link may cause severe signal distortions in the transmitted signals, especially
in long-haul communication systems and for signals that are modulated at symbol rates of
10 GBd or higher. Without dispersion compensation, the maximal transmission distances
decrease rapidly with increasing modulation rate of the transmitted signals.
Techniques for reducing the waveform distortions caused by accumulated CD include the
insertion of passive optical elements with opposite dispersion along the fibre link (optical
dispersion compensation), dispersion-assisted transmission of optical signals (soliton pulses),
and electrical accommodation of CD-induced waveform distortions in the optical transmitters
and receivers (pre- and post-compensation). Optical compensation techniques are primarily
applied in medium- to long-haul DWDM transmission systems using direct-detection (i.e. non-
coherent) receivers, whereas electrical accommodation techniques are widely employed in
transmission systems using coherent receivers and complex vector modulation.
Depending on the fibre type, short-reach communication systems, in particular those operating
in the 1 300-nm wavelength range, may not require dispersion mitigation, because of their short
length (typically less than 10 km) and small dispersion coefficient.
5 Impact of chromatic dispersion
5.1 Dependence on fibre type
Chromatic dispersion in optical communication fibres is usually characterized by a length-
independent dispersion coefficient D(λ), expressed in units of ps/(nm·km) or ps/nm-km. The
total amount of dispersion in a fibre of length L is given by D(λ)×L and, hence, increases linearly
with fibre length. The magnitude and sign of the dispersion coefficient generally vary with optical
wavelength λ and can differ substantially from fibre type to fibre type.
The various fibre types used in single-mode optical communication links are categorized in
IEC 60793-2-50 according to their design and dispersion characteristics. They include
dispersion-unshifted fibres as well as various types of dispersion-shifted fibres.
IEC 60793-2-50 also specifies acceptable ranges for the dispersion coefficients D(λ) of these
fibres, which mirror those specified in ITU-T Recommendations G.652 through G.657 for single-
mode fibres and cables [1] to [6].
The amount of distortion caused by chromatic dispersion in a transmitted optical signal thus
depends on the fibre type, the length of the fibre, and the wavelength of the signal. The
magnitude and wavelength dependence of D(λ) for the various fibre types and their impact on
signal transmission is discussed in 5.2 and 5.3.
5.2 Dispersion-unshifted fibres
The first generation of single-mode fibres used in optical communication systems were
dispersion-unshifted fibres, which are defined in IEC 60793-2-50 as category B-652 fibres
(formerly known as category B1 fibres). Although originally intended for signal transmission

around 1 310 nm wavelength, B-652 fibres are also frequently used in the 1 550-nm range,
where the optical attenuation coefficient is significantly smaller than at 1 310 nm. The
dispersion coefficient of these fibres vanishes at some wavelength around 1 310 nm, called the
zero-dispersion wavelength λ , but becomes fairly large at wavelengths around 1 550 nm. As
for most fibre types, the zero-dispersion wavelength and the wavelength dependence of the
dispersion coefficient D(λ) may differ from fibre to fibre because of variations in the fibre design
and the manufacturing process.
IEC 60793-2-50 specifies the acceptable variations in the zero-dispersion wavelength λ and
the slope of the dispersion coefficient, thus setting boundaries for the dispersion coefficient
λ) as a function of wavelength [7] [8]. This is shown in Figure 1 for the example of sub-
D(
category B-652.D fibres. The solid curve displays the maximal values allowed for D(λ), while
the dashed curve shows the corresponding minimal values. At 1 310 nm wavelength, the
dispersion coefficient is bound between –1,3 ps/nm-km and +0,9 ps/nm-km, whereas it
increases to at least 13,3 ps/nm-km but no more than 18,6 ps/nm-km at 1 550 nm. Other types
of dispersion-unshifted fibres may have slightly different limits for D(λ).

Figure 1 – Range of the dispersion coefficient for B-652.D fibres
Short-reach communication systems (with less than 40 km transmission distance) using
category B-652 fibres and signal sources around 1 300 nm wavelength may not be impacted by
CD, whereas medium- and long-haul communication systems operating in the C-band
(1 530 nm to 1 565 nm) or in the L-band (1 565 nm to 1 625 nm) can be severely affected by
CD because of the higher dispersion coefficient and longer length.
At 1 550 nm wavelength, a dispersion coefficient of around 17 ps/nm-km may be considered
typical for dispersion-unshifted fibres. Thus, in a 100-km long fibre link, the accumulated
dispersion is about 1 700 ps/nm. Insertion of a properly selected optical dispersion
compensator can decrease this value to about 100 ps/nm or less. However, in DWDM
applications, the slope of the dispersion coefficient also becomes important. Around 1 550 nm,
the dispersion-slope coefficient is about 0,057 ps/nm -km, which means that the accumulated

– 10 – IEC TR 61282-5:2019 © IEC 2019
dispersion in a 100-km long link typically increases by about 200 ps/nm between 1 530 nm and
1 565 nm wavelength.
5.3 Dispersion-shifted fibres
Fibre dispersion is the sum of material and waveguide dispersion. It is therefore possible to
move the zero-dispersion wavelength λ of a fibre to a different value by changing the
waveguide dispersion in the light-guiding fibre core. Shifting λ to longer wavelengths typically
reduces the dispersion coefficient in the 1 550-nm range. These types of fibres are known as
dispersion-shifted fibres (DSFs). In category B-653 fibres (formerly category B2 fibres), the
zero-dispersion wavelength is shifted to around 1 550 nm, as specified in IEC 60793-2-50.
Consequently, the magnitude of the dispersion coefficient in B-653 fibres is very small across
the C-band.
Transmission of DWDM signals over zero- or low-dispersion fibres can be severely impaired by
nonlinear optical interactions between the various optical channels which occur along the fibre
link, as described in IEC TR 61282-4 [9]. These non-linear interactions manifest themselves in
cross-phase modulation (XPM), cross-polarization modulation (XPolM), and four-wave mixing
(FWM). It was found that signal distortions caused by XPM, XPolM and FWM accumulate much
more slowly in fibres with large dispersion coefficients than in those with small or nearly
vanishing dispersion coefficients [10]. Therefore, B-653 fibres are not suitable for long-haul
transmission of DWDM signals in the C-band, although they may be used for DWDM
transmission in the L-band, where the dispersion coefficient is significantly larger. For this
reason, B-653 fibres are no longer deployed in long-haul optical communication systems.
Newer generations of dispersion-shifted fibres are designed to have relatively small but non-
vanishing dispersion coefficients within the C-band, so as to allow DWDM transmission over
long fibre links. In these non-zero dispersion-shifted fibres (NZDSF), defined as category
B-655 fibres in IEC 60793-2-50 (formerly category B4), λ is shifted either to a wavelength
below 1 530 nm, so that D(λ) is greater than zero in the entire C-band, or to one above
1 565 nm, so that D(λ) is lower than zero in the C-band. Since fibres with λ > 1 565 nm do not
support DWDM transmission in the L-band (1 565 nm to 1 625 nm), newer generations of
NZDSFs, like sub-category B-655.D fibres, specify λ to be below 1 530 nm, so that D(λ) is
greater than zero in the C- and L-bands. In category B-656 fibres (formerly category B5), which
were designed for wideband optical transport networks, λ is shifted to below 1 460 nm, so that
D(λ) is greater than zero over the extended wavelength range from 1 460 nm to 1 625 nm,
covering the S-, C- and L-bands. Consequently, category B-656 fibres exhibit significantly larger
dispersion at 1 550 nm than some of the earlier generations of NZDSF.

Table 1 – Single-mode fibre types and range of dispersion coefficients at 1 550 nm
D(λ) at 1 550 nm
IEC category
ps/nm-km
Fibre type
a b
Example Min. Max.
Old New
c
Dispersion-unshifted B1.1 B-652.B B-652.B -
Cut-off shifted B1.2 B-654 B-654.E 17 23
Dispersion-unshifted (reduced water peak) B1.3 B-652.D B-652.D 13,3 18,6
Bending-loss insensitive B6 B-657 B-657.A 13,3 18,6
Dispersion-shifted B2 B-653 B-653.B –2,3 +2,3
Non-zero dispersion-shifted B4 B-655 B-655.E 6,1 9,3
Wideband non-zero dispersion-shifted B5 B-656 B-656 3,6 9,3
NOTE The minimal and maximal dispersion coefficients are listed for illustrative purposes and apply only to the
specified sub-categories. The dispersion ranges for other fibre sub-categories and/or at other
wavelengths can be found in IEC 60793-2-50.
a
IEC 60793-2-50:2015 and older [7].
b
IEC 60793-2-50:2018 and newer [8].
c
Calculated from the minimal zero-dispersion wavelength and the maximal zero-dispersion slope
according to [1].
Table 1 lists examples of the various types of dispersion-unshifted and dispersion-shifted fibres
and their specifications for minimal and maximal dispersion coefficients at 1 550 nm. Thus,
even with non-zero dispersion-shifted fibres, the total accumulated chromatic dispersion at the
end of a long fibre link may become too large to allow error-free transmission in long-distance
communication links. In this case, it is necessary to either compensate the fibre CD by inserting
optical dispersion compensators or otherwise accommodate its effects.
5.4 Pulse broadening
Fibre dispersion generally leads to waveform distortions in the transmitted optical signal, which
include pulse broadening as well as signal peaking, as discussed in 5.5. The reason for these
waveform distortions is that the various frequency components of a modulated signal travel at
different speeds through the fibre and, hence, arrive at different times at the end of the fibre
link. The differential time delay ∆t between two signals at different wavelengths is proportional
to their wavelength difference ∆λ and the total dispersion in the fibre link, which is determined
by the dispersion coefficient D(λ) and fibre length L [11]:
ΔΔt λ Dλ××L λ (1)
( ) ( )
A positive dispersion coefficient means that longer wavelengths experience longer transit times
than shorter wavelengths. For negative dispersion coefficients, the order is reversed, so that
longer wavelengths experience shorter transit times than shorter wavelengths.
The optical spectrum of a modulated signal always has a finite spectral width ∆λ and, hence,
m
contains a multitude of wavelengths, which all experience different time delays when traveling
through a dispersive fibre. If these time delays become too large, they can severely distort the
waveform of the transmitted signal. The differential phase shifts ∆φ introduced by the CD-
induced time delays can be described in the wavelength domain by a simple multiplication of
the complex spectral components with the complex transfer function:
=
– 12 – IEC TR 61282-5:2019 © IEC 2019

π ×ΔΔt λ ××c λ π ×D λ ×L××c Δλ
( ) ( )
  
H Δλexp jλΔΔφ=exp j exp j (2)
( ) { ( )}   
λλ

 

where
c is the speed of light in vacuum;
∆λ is the wavelength difference between each spectral component and the centre frequency of
the spectrum [11].
Thus, the wider the modulated optical spectrum is, the larger ∆φ can be. It is important to note
that ∆φ grows proportionally with ∆λ , which means that the dispersion-induced waveform
λ and thus with
distortions increase steeply with the width of the modulated optical spectrum ∆
m
the modulation rate of the optical signal.
For example, a binary non-return-to-zero on-off-keyed signal (NRZ-OOK) at bit-rate B carries
significant spectral content in a frequency interval of 2B. For B = 10 Gbit/s and λ = 1 550 nm,
this interval corresponds to a spectral width of ∆λ = 0,16 nm, whereas for B = 40 Gbit/s,
m
∆λ is four times larger, i.e. 0,64 nm. However, the CD-induced maximal differential phase
m
shifts in the optical spectrum increase with the square of B, as described by Equation (2). For
this reason, a binary 40 Gbit/s NRZ-OOK signal is 16 times more sensitive to CD than a binary
10 Gbit/s NRZ-OOK signal, and 256 times more sensitive than a binary 2,5 Gbit/s NRZ-OOK
signal.
In most instances, the CD-induced differential phase shifts ∆φ lead to pulse broadening in digital
communication systems and to modulation-frequency roll-off in analogue communication
systems. This can be better seen in the time domain, where the transfer function H(∆λ)
corresponds to a convolution of the time-varying optical signal amplitude A(t) with the complex
transfer function:

1 t

ht( ) exp−j (3)

2α
α


where
α = D(λ) x L x λ /(2 x π x c)
When applied to a single chirp-free optical pulse, this convolution simply spreads the optical
energy of the pulse in time and thus lengthens the pulse duration. However, the waveform
distortions of modulated optical signals tend to be more complicated, because the pulse
distortion of each individual symbol depends on the amplitudes and phases of the preceding
and succeeding symbols. As a result, the waveform distortions become pattern-dependent (as
in Figure 2) and may exhibit pulse broadening and pulse narrowing as well as signal peaking.
The larger the accumulated dispersion, the more symbols are involved in the convolution.
The dispersion-induced waveform distortions depend on the overall width of the optical
spectrum, which is determined by the information content of the digital or analogue information
to be transmitted as well as by the particular modulation format used to encode the optical
signal. For a given modulation format, the spectral width increases linearly with the modulation
frequency (in analogue systems) or the symbol rate (in digital systems). In general, binary
encoded signals have broader optical spectra than those using modulation formats of higher
cardinality, such as quaternary pulse-amplitude-modulated (PAM-4) or quaternary phase-shift
keyed (QPSK) signals. However, polarization-multiplexed signals have the same spectral width
than single-polarized signals of the same modulation format and symbol rate.
The optical spectrum may be further broadened by frequency chirping and nonlinear modulation
in the transmitter, as discussed in 5.5, or by the finite spectral width of the unmodulated light
source in the transmitter. However, the latter is usually negligible, because long-haul
=
= =
communication systems operating at bit rates of 10 Gbit/s or above generally employ single-
longitudinal-mode lasers with linewidths below 100 MHz.
5.5 Pulse narrowing and signal peaking
The dispersion-induced waveform distortions are affected by frequency chirping and nonlinear
modulator response in the optical transmitter. Chirping can occur, particularly in directly
modulated lasers, where the digital or analogue modulation signal is directly applied to the laser
drive current. This type of modulation may cause the laser wavelength to vary with time. In
digital transmission systems, for example, the laser wavelength may move towards shorter
wavelengths during the ramp-up of the pulse and back towards longer wavelengths during ramp-
down (positive chirp). This frequency chirping causes significant broadening of the modulated
spectrum as well as further broadening of the transmitted pulses at the end of a fibre with
positive dispersion, because the leading edges of the pulses have shorter wavelengths and
hence arrive sooner than the trailing edges, which have longer wavelengths.
Conversely, in the negative dispersion region (at wavelengths below λ ) positive chirping can
,
result in pulse narrowing. Therefore, significant pulse compression can occur after certain fibre
lengths, but then the pulses broaden again. Therefore, proper frequency chirping can extend
the dispersion-limited transmission distance [12]. A similar effect can be achieved with negative
frequency chirp when the fibre dispersion is positive. This technique is an example of
dispersion accommodation discussed in 6.4.
Chirping can be alleviated or completely avoided when using an external modulator in
conjunction with an unmodulated (continuous-wave) laser. A semiconductor electro-
absorption modulator normally exhibits significantly less chirp than a directly modulated
laser but introduces considerable insertion loss. High-speed transmission systems at bit
rates of 10 Gbit/s and beyond frequently use electro-optic modulators based on Mach-
Zehnder interferometers, which can be designed to introduce negligible frequency chirp or,
if so desired, a fixed amount of positive or negative chirp.
In general, the optical output amplitude of interferometric Mach-Zehnder modulators is not a
linear function of the modulating drive signal, particularly when the modulator is swept from the
completely-on state to the completely-off state. Such nonlinear modulation generates harmonic
frequency components in the modulated spectrum, which can cause significant signal peaking
and even pulse narrowing after experiencing fibre dispersion. Figure 2 displays simulations of
CD-induced waveform distortions in a 10 Gbit/s NRZ-OOK signal generated with a chirp-free
interferometric Mach-Zehnder modulator. Pulse narrowing can be observed in the isolated “1”
symbols when the signal has experienced moderate amounts of CD, i.e. 670 ps/nm and
1 340 ps/nm, corresponding to about 40 km and 80 km of dispersion-unshifted fibre, which is
caused by clock frequency components in the optical amplitude spectrum [13]. After larger
amounts of accumulated CD (i.e. above 2 000 ps/nm), the initial pulse narrowing of the isolated
“1” symbols has turned into pulse broadening, but the signal peaking in the broader pulses
(formed by several consecutive “1” symbols) has grown even larger.

– 14 – IEC TR 61282-5:2019 © IEC 2019

Figure 2 – Distortions in a 10 Gbit/s NRZ signal at various amounts of CD
Dispersion-induced signal peaking tends to be larger in on-off keyed signals than in phase-shift
keyed signals. In either case, it can cause additional waveform distortions through nonlinear
effects, such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave
mixing (FWM), which are difficult to mitigate. The nonlinear waveform distortions may be
minimized through careful management of the accumulated dispersion along the fibre link, as
discussed in 6.2.
5.6 Dispersion-limited transmission distance
Without proper compensation or accommodation, dispersion can severely limit the distance
over which an optical signal may be transmitted without intermediate opto-electronic
regeneration. Subclause 5.6 describes how the maximal transmission distance depends on the
total amount of accumulated dispersion in the fibre link as well as on the symbol rate and
modulation format of the transmitted optical signals.
For binary NRZ on-off-keyed signals, ITU-T Recommendation G.957 specifies that pulse
spreading due to chromatic dispersion should not exceed a fraction ε of the symbol period 1/B
of the digital modulation [14], where B denotes the modulation rate, so as to not exceed the
maximal allowable dispersion power penalty at a particular bit-error ratio (BER). For a 1 dB
–10
power penalty at a BER of 10 , the value of ε is about 0,3. It should be noted that ε depends
upon certain component parameters of the transmission equipment and, therefore, should be
individually determined for each system design.
With Δt = ε/ B, one can then calculate the dispersion-limited maximal transmission distance L
D
for binary NRZ signals from Equation (1). Neglecting the spectral width of the unmodulated light
source, L is approximately given by:
D
ε
L = (4)
D
BD××2Δλ
m
where
3Bλ
∆λ = is the root-mean-square spectral width of the modulated optical spectrum;
m
2πc
D is the dispersion coefficient of the fibre;
λ is the centre wavelength of the spectrum.
NOTE It is assumed that the transmission distance is not limited by fibre attenuation, optical amplifier noise, or
non-linear effects in the fibre link. The maximal transmission lengths are not specified in any ITU recommendation.
Table 2 provides examples of the maximal accumulated chromatic dispersion D × L that can be
tolerated in modulated optical signals of various bit-rates and modulation formats, as well as
the corresponding dispersion-limited transmission lengths over uncompensated category B-652
fibre, assuming D(λ) = 17 ps/nm-km at 1 550 nm. These data assume chirp-free modulation and
an ideal light source having negligible spectral width when unmodulated [15].
Table 2 – Dispersion-limited transmission distances over B-652 fibre at 1 550 nm
Bit rate and modulation format
40 Gbit/s 40 Gbit/s 40 Gbit/s 100 Gbit/s
Maximal dispersion 2,5 Gbit/s 10 Gbit/s
NRZ-DPSK RZ-DQPSK PM- PM-
and transmission NRZ-OOK NRZ-OOK
(D)QPSK (D)QPSK
distance
(OTU3/ (OTU3/
(OTU1/ (OTU2/
STM-256/ STM-256/ (OTU3/ (OTU4/
STM-16) STM-64)
40GbE) 40GbE) STM-256) 100GbE)
Maximal chromatic
a a
dispersion
...