IEC 61978-1:2024

IEC 61978-1 ®
Edition 4.0 2024-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic interconnecting devices and passive components – Fibre optic
passive chromatic dispersion compensators –
Part 1: Generic specification
Dispositifs d'interconnexion et composants passifs fibroniques –
Compensateurs de dispersion chromatique passifs fibroniques –
Partie 1 : Spécification générique
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IEC 61978-1 ®
Edition 4.0 2024-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic interconnecting devices and passive components – Fibre optic

passive chromatic dispersion compensators –

Part 1: Generic specification
Dispositifs d'interconnexion et composants passifs fibroniques –

Compensateurs de dispersion chromatique passifs fibroniques –

Partie 1 : Spécification générique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.01  ISBN 978-2-8322-8702-6

– 2 – IEC 61978-1:2024 © IEC 2024
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
3.1 Component terms. 7
3.2 Performance terms . 8
4 Requirements . 10
4.1 Classification . 10
4.1.1 General . 10
4.1.2 Technology . 11
4.1.3 Types . 11
4.1.4 Wavelength band . 11
4.1.5 Application of PDCSs and their suitable technologies . 11
4.1.6 Interface style . 12
4.2 Documentation . 12
4.2.1 Symbols . 12
4.2.2 Drawings . 12
4.2.3 Tests and measurements . 12
4.2.4 Test report . 13
4.2.5 Instructions for use . 13
4.3 Standardisation system . 13
4.3.1 Interface standards . 13
4.3.2 Performance standards . 13
4.3.3 Reliability standards . 13
4.4 Design and construction . 14
4.4.1 Materials . 14
4.4.2 Workmanship . 14
4.5 Quality . 14
4.6 Performance requirements . 14
4.7 Identification and marking . 14
4.7.1 General . 14
4.7.2 Component marking . 14
4.7.3 Package marking . 15
4.8 Packaging . 15
4.9 Storage conditions . 15
4.10 Safety . 15
Annex A (informative) Example of dispersion compensating fibre (DCF) technologies . 16
Annex B (informative) Example of fibre Bragg grating (FBG) technologies . 18
Annex C (informative) Example of virtually imaged phased array (VIPA) technologies . 20
Annex D (informative) Example of GT etalon technologies . 22
Annex E (informative) Technology dependent characteristics of PCDCs . 23
Annex F (informative) Example of interface style . 24
Bibliography . 25

Figure A.1 – Chromatic dispersion in a standard single-mode optical fibre (SMF) . 16

Figure A.2 – Calculated contour for different dispersion at the wavelength of 1,55 µm
[CD(λ:1,55 µm)] for a step index core fibre . 17
Figure A.3 – Examples of refractive index profile used in DCF . 17
Figure B.1 – Illustration of the use of a chirped fibre Bragg grating for chromatic
dispersion compensation . 18
Figure B.2 – Expanded view over 10 nm of the insertion loss (attenuation) spectrum of
a multi-channel FBG . 19
Figure C.1 – Structure of virtually imaged phased array (VIPA) . 20
Figure C.2 – Detailed light path and mechanism of generating chromatic dispersion . 21
Figure D.1 – Gires-Tournois etalon . 22
Figure F.1 – Examples of interface style for fibre optic PCDCs . 24

Table 1 – Example of a typical fibre optic PDCS classification . 11
Table 2 – Application, channel numbers, passband and technologies of PDCSs . 12
Table E.1 – Summary of technology dependent characteristics of PCDCs . 23

– 4 – IEC 61978-1:2024 © IEC 2024
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC INTERCONNECTING DEVICES
AND PASSIVE COMPONENTS – FIBRE OPTIC PASSIVE
CHROMATIC DISPERSION COMPENSATORS –

Part 1: Generic specification
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
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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
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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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
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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
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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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
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the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61978-1 has been prepared by subcommittee 86B: Fibre optic interconnecting devices and
passive components, of IEC technical committee 86: Fibre optics. It is an International Standard.
This fourth edition cancels and replaces the third edition published in 2014. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) harmonization of terms and definitions with IEC TS 62627-09;
b) change of Clause 4 regarding requirements.

The text of this International Standard is based on the following documents:
Draft Report on voting
86B/4866/FDIS 86B/4901/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61978 series, published under the general title Fibre optic
interconnecting devices and passive components – Fibre optic passive chromatic dispersion
compensators, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

– 6 – IEC 61978-1:2024 © IEC 2024
FIBRE OPTIC INTERCONNECTING DEVICES
AND PASSIVE COMPONENTS – FIBRE OPTIC PASSIVE
CHROMATIC DISPERSION COMPENSATORS –

Part 1: Generic specification
1 Scope
This part of IEC 61978 applies to fibre optic passive chromatic dispersion compensators, all
exhibiting the following features:
– they are optically passive;
– they have an optical input and an optical output for transmitting optical power;
– the ports are optical fibres or optical fibre connectors;
– they are wavelength sensitive;
– they can be polarization sensitive.
This document establishes uniform requirements for the passive chromatic dispersion
compensator.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60027 (all parts), Letter symbols to be used in electrical technology
IEC 60050-731, International Electrotechnical Vocabulary – Chapter 731: Optical fibre
communication
IEC 60617 (all parts), Graphical symbols for diagrams
IEC 61300 (all parts), Fibre optic interconnecting devices and passive components – Basic test
and measurement procedures
IEC 61753 (all parts), Fibre optic interconnecting devices and passive components performance
standard
IEC TR 61930, Fibre optic graphical symbology
IEC 62005 (all parts), Reliability of fibre optic interconnecting devices and passive components
IEC TS 62627-09, Fibre optic interconnecting devices and passive components – Vocabulary
for passive optical devices
ISO 129-1, Technical product documentation (TPD) – Presentation of dimensions and
tolerances – Part 1: General principles

ISO 1101, Geometrical product specifications (GPS) – Geometrical tolerancing – Tolerances of
form, orientation, location and run-out
ISO 8601-1, Date and time – Representations for information interchange – Part 1: Basic rules
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-731 and
IEC TS 62627-09 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1 Component terms
3.1.1
passive chromatic dispersion compensator
PCDC
two-port in-line passive device used to perform chromatic dispersion compensation
Note 1 to entry: PCDCs are commonly used to compensate the chromatic dispersion of an optical path by adding
the opposite sign chromatic dispersion.
Note 2 to entry: The typical optical paths comprise single-mode fibre, dispersion shifted fibre and/or non-zero
dispersion shifted fibre. PCDCs have either negative or positive chromatic dispersion values depending on the
chromatic dispersion sign of the optical path.
[SOURCE: IEC TS 62627-09:2016, 3.3.17]
3.1.2
dispersion compensating fibre
DCF
speciality fibre to compensate for the chromatic dispersion of an optical path
3.1.3
passive DCF based dispersion compensator
PCDC which constitutes DCF; realised by having chromatic dispersion characteristics of
opposite sign to that of the optical path which are controlled the refractive index profile of the
fibre
3.1.4
fibre Bragg grating
FBG
fibre type optical device which has periodically modulated refractive index profile in the core
along the fibre axis
3.1.5
passive FBG based dispersion compensator
PCDC which constitutes an FBG; PCDC is realised by a chirped FBG which has gradual change
in either modulation period or refractive index, or both, along the fibre axis
3.1.6
virtually imaged phased array
VIPA
optical device having a glass plate with a highly reflective mirror

– 8 – IEC 61978-1:2024 © IEC 2024
Note 1 to entry: A VIPA has the same functions as a grating.
3.1.7
passive VIPA based dispersion compensator
PCDC consisting of a VIPA, focusing lens and 3-dimensional mirror
Note 1 to entry: PCDC produces both positive and negative chromatic dispersion by the movement of the
3-dimensional mirror to compensate for the chromatic dispersion of an optical path.
3.1.8
etalon
optical cavity which consists of a pair of parallel reflective mirrors
3.1.9
Gires-Tournois etalon
GT etalon
etalon having a highly reflective mirror and a half mirror
Note 1 to entry: The GT etalon is sometimes called a GT interferometer.
3.1.10
passive GT etalon based dispersion compensator
PCDC which comprises a GT etalon
3.2 Performance terms
3.2.1
chromatic dispersion compensation
process by which a specific amount of chromatic dispersion is removed in order to mitigate the
system impairment caused by unwanted dispersion
3.2.2
group delay
time by which a pulse is delayed by an optical device
Note 1 to entry: The group delay generally varies with the operating wavelength.
3.2.3
chromatic dispersion
derivative of group delay with respect to wavelength or frequency
Note 1 to entry: A typical unit is ps/nm or ps/GHz. The chromatic dispersion generally varies with the operating
wavelength.
Note 2 to entry: The unit of ps/GHz are not commonly used; however, it is suitable for the evaluation of transmission
system influence.
3.2.4
dispersion slope
derivative of chromatic dispersion with respect to wavelength or frequency
2 2 2
Note 1 to entry: A typical unit is ps/nm or ps/GHz .The unit of ps/GHz is not commonly used; however, it is suitable
for the evaluation of transmission system influence.
Note 2 to entry: The dispersion slope generally varies with the operating wavelength.
3.2.5
operating wavelength
nominal wavelength λ at which a passive device operates with the specified performance
Note 1 to entry: Operating wavelength includes the wavelength to be nominally transmitted, attenuated and isolated.

3.2.6
operating wavelength range
specified range of wavelengths including all operating wavelengths
Note 1 to entry: Operating wavelength range shall include all passbands when two or more the passbands are exist.
3.2.7
figure of merit
FoM
ratio of the dispersion to the insertion loss of a PCDC at a particular operating wavelength
3.2.8
passband
wavelength range within which a passive optical device is required to operate with optical
attenuation less than or equal to a specified optical attenuation value
Note 1 to entry: There can be one or more passbands for a PCDC.
3.2.9
passband ripple
maximum peak-to-peak variation of insertion loss in the passband
Note 1 to entry: The passband ripple of a PCDC is defined as the maximum passband ripple for all passbands.
3.2.10
group delay ripple
GDR
maximum peak-to-peak variation of the group delay approximated by a desired function of
wavelength (or frequency), typically a linear fit, within a channel wavelength (or frequency)
range
3.2.11
phase ripple
maximum peak-to-peak variation in measured phase spectrum when compared to a quadratic
fit within a channel wavelength (or frequency) range
Note 1 to entry: Phase ripple (unit: radian) is calculated as the product of a peak-to-peak group delay ripple (unit: s)
and a period of group delay ripple (unit: Hz). Refer to IEC 61300-3-38.
3.2.12
insertion loss
reduction in optical power between an input and output port of a passive device
Note 1 to entry: expressed in decibels (dB).
Note 2 to entry: insertion loss is expressed as follows:
P
a
a=−10log
P
where
P is the optical power launched into the input port;
P is the optical power received from the output port.
a
3.2.13
return loss
fraction of input power that is returned from a port of a passive device expressed in decibels

– 10 – IEC 61978-1:2024 © IEC 2024
Note 1 to entry: The return loss is defined as follows:
P
r
RL=−10log
P
where
P is the optical power launched into a port;
P is the optical power received back from the same port.
r
3.2.14
reflectance
ratio of the optical power returning back from a port to input power expressed in %
3.2.15
polarization dependent loss
PDL
maximum variation of insertion loss (attenuation) due to a variation of the state of polarization
(SOP) over all the SOPs
3.2.16
wavelength dependent loss
WDL
maximum variation of the insertion loss (attenuation) over operating wavelength range
3.2.17
polarization mode dispersion
PMD
average delay of the travelling time between the two principal states of polarization (PSP), when
an optical signal passes through a passive optical device
4 Requirements
4.1 Classification
4.1.1 General
Fibre optic passive chromatic dispersion compensators (PCDCs) are classified either wholly or
partially within the following categories:
– technology;
– type;
– wavelength band;
– categories of transmission fibre;
– interface style.
An example of a typical fibre optic PDCS classification is given in Table 1.

Table 1 – Example of a typical fibre optic PDCS classification
Items Classification
Technology DCF
Type Wavelength dispersion compensating
Wavelength band C-band
Category of transmission fibre B-652
Interface style Configuration D
Fibre category: IEC 60793-2-50, B-652
IEC 61754-4 (SC connector)
4.1.2 Technology
PCDCs typically use the following technologies:
– dispersion compensating fibre (DCF);
– fibre Bragg grating (FBG);
– Virtual Image Phased Array (VIPA);
– GT etalon.
Each technology of PCDCs is described in Annex A to Annex D.
4.1.3 Types
– Wavelength dispersion compensation;
– Wavelength dispersion slope compensation.
4.1.4 Wavelength band
– O-band;
– S-band;
– C-band;
– L-band;
– C-band and L-band;
– other wavelength band or combination of wavelength bands above.
4.1.5 Application of PDCSs and their suitable technologies
The application of PCDCs and the suitable technologies are summarized in Table 2.
Technology dependent characteristics of PCDCs are summarized in Annex E.

– 12 – IEC 61978-1:2024 © IEC 2024
Table 2 – Application, channel numbers, passband and technologies of PDCSs
Applications Channel number Passbands Technologies
TDM (time division multiplexing) Single channel Narrow Dispersion compensating fibre
(DCF)
Fibre Bragg grating (FBG)
GT etalon
WDM (wavelength division multiplexing) Single channel Narrow FBG
a)
Narrow FBG
Multi-channel
GT etalon
Virtually imaged phased array
(VIPA)
Wide DCF
a)
Multi-channel PCDCs can be used for a single channel use.

4.1.6 Interface style
PCDC style shall be defined based on the following elements:
– the input and output port configuration;
– the connector set type(s), if any.
NOTE Examples of interface style are provided in Annex F.
4.2 Documentation
4.2.1 Symbols
Graphical and letter symbols shall, whenever possible, be taken from IEC 60027 series,
IEC 60617 and IEC TR 61930.
4.2.2 Drawings
4.2.2.1 General
The drawings and dimensions given in the relevant specifications shall not restrict detail
construction nor be used as manufacturing drawings.
4.2.2.2 Projection system
Either first angle or third angle projection shall be used for the drawings in documents covered
by this document. All drawings within a document shall use the same projection system and the
drawings shall state which system is used.
4.2.2.3 Dimensional system
All dimensions shall be given in accordance with ISO 129-1 for general information of
dimensions and tolerances, ISO 286-1 for tolerances of form, orientation location and run out
for information, and ISO 1101 for information interchange. The metric system shall be used in
all specifications. Dimensions shall not contain more than five significant digits. When units are
converted, a note shall be added in each relevant specification.
4.2.3 Tests and measurements
4.2.3.1 Tests and measurements procedures
The tests and measurements procedures for optical, mechanical, climatic, and environmental
characteristics of fibre optic PCDCs to be used shall be defined and selected preferentially from

IEC 61300 series. The size measurement method to be used shall be specified in the relevant
IEC 61753 series performance standard or IEC 62005 series reliability standard, for
dimensions which are specified within a total tolerance zone of 0,01 mm or less.
4.2.3.2 Reference components
Reference components for measurement purposes, if required, shall be specified in the relevant
IEC 61300 basic test and procedure standard.
4.2.4 Test report
The test reports shall be prepared for each test conducted as required by a relevant IEC 61753
series performance standard or IEC 62005 series reliability standard. The reports shall be
included in the qualification test report and in the periodic inspection report.
Test reports shall contain the following information as a minimum:
– title and date of test;
– test equipment used;
– all applicable test details;
– all measurement values and observations.
4.2.5 Instructions for use
Instructions for use, when required, shall be given by the manufacturer.
4.3 Standardisation system
4.3.1 Interface standards
Refer to the proper optical connector interface of the IEC 61754 series when an optical
connector is used.
4.3.2 Performance standards
Performance standards (IEC 61753 series) contain a series of tests and measurements (which
can be grouped into a specified schedule depending on the requirements of that standards)
with clearly defined conditions, severities, and pass/fail criteria. The tests are intended to be
run on a one-off basis to prove the ability of any product to satisfy the performance standards
requirement. Each performance standard has a different set of tests severities, and groupings,
representing the requirements of a market sector, user group or system location.
A product that has been shown to meet all the requirements of a performance standard can be
declared as complying with a performance standard but should then be controlled by a quality
assurance and quality conformance programme.
4.3.3 Reliability standards
Reliability standards are intended to ensure that a component can meet performance
specifications under stated conditions for a stated time period.

– 14 – IEC 61978-1:2024 © IEC 2024
4.4 Design and construction
4.4.1 Materials
4.4.1.1 General
All housing materials used in the construction shall be manufactured with materials which meet
the requirements of the relevant IEC 61753 performance standard or IEC 62005 reliability
standard.
4.4.1.2 Non-flammable materials
When non-flammable materials are required, the requirements shall be specified, and reference
shall be made to IEC 60695-11-5. If an alternate standard is used for non-flammable materials,
it shall be declared.
4.4.2 Workmanship
Components and associated hardware shall be manufactured to a uniform quality and shall be
free of sharp edges, burrs, or other defects that would affect life, service ability or appearance.
Particular attention shall be given to neatness and thoroughness of marking, plating, soldering,
bonding, etc.
4.5 Quality
Fibre optic PCDCs shall be controlled by the quality assessment procedures and declared.
4.6 Performance requirements
Fibre optic PCDCs shall meet the performance requirements specified in the relevant IEC 61753
performance standard or IEC 62005 reliability standard.
4.7 Identification and marking
4.7.1 General
Components, associated hardware and shipping packages shall be permanently and legibly
identified and marked when required by the relevant IEC 61753 performance standard or
IEC 62005 reliability standard.
4.7.2 Component marking
Component marking, if required, should be specified in the relevant IEC 61753 performance
standard or IEC 62005 reliability standard. The preferred order of marking is:
a) port identification;
b) manufacturer's part number (including serial number, if applicable);
c) manufacturer's identification mark or logo.
If space does not allow for all the required marking on the component, each unit shall be
individually packaged with a data sheet containing all of the required information which is not
marked.
4.7.3 Package marking
Several devices may be packaged together for shipment.
Package marking, if required, shall be specified in the relevant IEC 61753 performance
standard or IEC 62005 reliability standard. The preferred order of marking is:
a) manufacturer's identification mark or logo;
b) manufacturer's part number.
When applicable, individual unit packages (within the sealed package) should be marked with
the reference number of the certified record of released lots, the manufacturer's factory identity
code and the component identification.
4.8 Packaging
Packaging shall be securely without any damage to passive optical components during
transportation and storage.
Packages shall include instructions for use when required by the relevant IEC 61753
performance standard or IEC 62005 reliability standard (see 4.2.5).
4.9 Storage conditions
Where short-term degradable materials, such as adhesives, are supplied with the package, the
manufacturer shall mark these with the expiry date according to ISO 8601-1 together with any
requirements or precautions concerning safety hazards or environmental conditions for storage.
4.10 Safety
Fibre optic PCDCs, when used on an optical fibre transmission system and/or equipment, can
emit potentially hazardous radiation from an uncapped or unterminated output port or fibre end.
The fibre optic PCDC manufacturers shall provide sufficient information to alert system
designers and users about the potential hazard and shall indicate the required precautions and
working practices.
In addition, each relevant IEC 61753 performance standard or IEC 62005 reliability standard
shall include the following:
WARNING – Care should be taken when handling small diameter fibre to prevent
puncturing the skin, especially in the eye area. Direct viewing of the end of an optical
fibre or an optical fibre connector when it is propagating energy is not recommended,
unless prior assurance has been obtained as to the safety energy output level.

– 16 – IEC 61978-1:2024 © IEC 2024
Annex A
(informative)
Example of dispersion compensating fibre (DCF) technologies
Chromatic dispersion in optical fibre is expressed as the sum of material dispersion caused by
wavelength dependence of the refractive index of the glass materials and waveguide dispersion
caused by index profile of optical fibre (Figure A.1). Silica glass optical fibre material dispersion
does not vary greatly. Waveguide dispersion can be controlled by changing the index profile of
the optical fibre. DCFs are designed to control waveguide dispersion to achieve the desired
dispersion characteristics.
Figure A.1 – Chromatic dispersion in a standard single-mode optical fibre (SMF)
Figure A.2 shows the contour for different dispersions at a wavelength of 1,55 µm as a function
of the refractive index contrast, ∆, from a pure silica cladding index value level and core
diameter in a step-index profile with the germanium-doped silica core. From this figure, a DCF
with a large negative chromatic dispersion can be obtained by increasing ∆ and decreasing the
diameter of the core.
NOTE ∆ is the relative index contrast. Refer IEC IEV 731-02-20.

Figure A.2 – Calculated contour for different dispersion at the wavelength
of 1,55 µm [CD(λ:1,55 µm)] for a step index core fibre
Figure A.3 shows two examples of refractive index profiles of DCFs. The refractive index
contrast between the core and the cladding is larger and the core diameter is smaller than those
of standard single-mode fibres. These design differences result in larger waveguide dispersion.
As for the double-cladding type DCF, much larger waveguide dispersion can be obtained than
in the case of the matched cladding type DCF.
Double cladding type DCF can give negative dispersion slope in C-band and/or L-band.
Because of this, the positive dispersion slope of SMFs (IEC 60793-2-50, B1 fibres) can be
compensated by using this type of DCF. Dispersion slope compensation is important to achieve
a uniform dispersion value over the wavelength range of a WDM transmission system.

a) Matched cladding type b) Double cladding type
Figure A.3 – Examples of refractive index profile used in DCF

– 18 – IEC 61978-1:2024 © IEC 2024
Annex B
(informative)
Example of fibre Bragg grating (FBG) technologies
Fibre Bragg grating (FBG) is a fibre type optical device that has a modulated refractive index
profile in the core along the longitudinal axis. A FBG functions as a reflective filter where
reflection wavelength is defined in Formula (B.1). Generally, refractive index modulation is
generated by using UV radiation induced refractive index change.
λ= 2××Λ n
(B.1)
B eff
where
λ is the reflection wavelength (Bragg wavelength);
B
Λ is the refractive index modulation period;
n is the effective refractive index.
eff
The basic principle of dispersion compensation using a chirped FBG is shown in Figure B.1. In
chirped FBG, either the grating period or effective refractive index, or both, are gradually
changed, and reflection wavelength changes along the fibre axis. After travelling through the
transport fibre, the signal experiences a positive chromatic dispersion so that its shorter-
wavelength part arrives before its longer-wavelength part. The chirped FBG provides more
group delay for the shorter-wavelength part of the signal thus compensating for the effect of the
chromatic dispersion. The slope of the group delay spectrum corresponds to the dispersion the
FBG provides. To conveniently access the output signal, an optical circulator is used.

Figure B.1 – Illustration of the use of a chirped fibre Bragg grating
for chromatic dispersion compensation
The FBG can be made multi-channel, allowing for a simultaneous compensation of the
chromatic dispersion accumulated in all channels of a WDM system. The multi-channel
character of the FBG is typically obtained through a sampling approach, i.e. a spatial modulation
of its physical properties. As an example, Figure B.2 shows an expanded view over 10 nm of
the spectral characteristics of a multi-channels FBG tailored for compensating the chromatic
dispersion accumulated over 100 km of single-mode fibre, specified in IEC 60793-2-50,
category B1.
a) Insertion loss (attenuation) including the optical circulator

b) Group delay spectrum
Figure B.2 – Expanded view over 10 nm of the insertion loss (attenuation)
spectrum of a multi-channel FBG

– 20 – IEC 61978-1:2024 © IEC 2024
Annex C
(informative)
Example of virtually imaged phased array (VIPA) technologies
Figure C.1 shows the structure of a virtually imaged phased array (VIPA). The input light from
a single-mode fibre is line-focused into a glass plate. The glass plate is coated on both surfaces
and collimated light is emitted from the reverse side of the glass plate after multiple reflections
between the coated surfaces. The light from the glass plate is then focused onto a curved mirror.
The reflected light travels back to the glass plate and is finally coupled back into the fibre.

Figure C.1 – Structure of virtually imaged phased array (VIPA)
Figure C.2 shows the detailed light path. Each time that the light is reflected at the right-angle
surface of the glass plate, a small percentage of the power passes through the partially
reflecting coating. This creates multiple beams that diverge from the corresponding beam waist
in the virtual image. The interference of these diverging beams generates collimated light. This
collimated light travels at an angle from the optical axis which varies with the wavelength.
Chromatic dispersion, i.e. the wavelength dependence of distance travelled, is determined by
the wavelength dependence of the pointing angle of collimated light from the glass plate and
the surface profile of the reflection mirror. The convex portion of the mirror produces negative
chromatic dispersion, and the concave portion of the mirror produces positive chromatic
dispersion. Figure C.1 shows that the collimated light (grey line area on the surface of the 3-D
mirror) reflects along the concave mirror surface
...