IEC 61300-3-43:2009

Fibre optic interconnecting devices and passive components - Basic test and measurement procedures - Part 3-43: Examinations and measurements - Mode transfer function measurement for fibre optic sources

IEC 61300-3-43:2009 describes the method for measuring the mode transfer function (MTF) to be used in characterising the launch conditions for measurements of attenuation and or return loss of multimode passive components. The MTF may be measured at the operational wavelengths. This standard cancels and replaces IEC/PAS 61300-3-43, published in 2006. This first edition constitutes a technical revision.

Dispositifs d'interconnexion et composants passifs fibroniques - Procédures fondamentales d'essais et de mesures - Partie 3-43: Examens et mesures - Mesures de la fonction de transfert modal pour sources fibroniques

IEC 61300-3-43: 2009 décrit la méthode de mesure de la fonction de transfert modal (MTF, Mode Transfer Function) à utiliser lors de la caractérisation des conditions d’injection pour les mesures de l’affaiblissement et/ou de l’affaiblissement de réflexion des composants passifs multimodaux. La MTF peut être mesurée au niveau des longueurs d’onde opérationnelles. La présente norme annule et remplace l’IEC/PAS 61300-3-43, publiée en 2006. Cette première édition constitue une révision technique.

General Information

• Status: Published
• Publication Date: 21-Jan-2009
• ICS: 33.180.20 - Fibre optic interconnecting devices
• Technical Committee: SC 86B - Fibre optic interconnecting devices and passive components
• Drafting Committee: WG 4 - TC 86/SC 86B/WG 4
• Current Stage: PPUB - Publication issued
• Start Date: 22-Jan-2009
• Completion Date: 31-Jan-2009

Relations

• Replaces: IEC PAS 61300-3-43:2006 - Fibre optic interconnecting devices and passive components - Basic test and measurement procedures - Part 3-43: Examinations and measurements - Mode transfer function measurement for fibre optic sources
• Effective Date: 05-Sep-2023

Overview

IEC 61300-3-43:2009 specifies a standardized method to measure the mode transfer function (MTF) of multimode fibre optic sources. The MTF measurement characterizes the launched modal distribution (LMD) - the mode content injected into multimode fibre - and is used to define launch conditions for accurate attenuation and return loss testing of multimode passive components. This first edition (2009) replaces IEC/PAS 61300-3-43 (2006) and is applicable at the operational wavelengths of the system under test.

Key topics and technical requirements

• MTF definition and theory: MTF is derived from the fibre near-field intensity distribution. The standard describes the mathematical relation between the near-field intensity I(r) and the MTF, and how to express results as a normalised mode number (m/M).
• Mode power distribution (MPD): MPD(m) = MTF(m) × m - used to show relative power across mode groups and identify dominant mode ranges.
• Measurement constraints:
  • Modes within a mode group must carry the same power and have random relative phases (mode-continuum approximation).
  • Linewidth requirements to avoid speckle: for a 50 µm core, Δλ > ~0.5 nm at 850 nm and Δλ > ~1.0 nm at 1 300 nm (typical).
  • If line-width is insufficient, temporal averaging (e.g., gentle fibre agitation) can be used; verify azimuthal symmetry by checking MTFs at 45° intervals.
  • The MPD peak should occur at a normalised mode number < 0.8 for valid results.
• Apparatus and procedure: The method uses a near-field imaging system (video microscope) - imaging and condensing lenses, beamsplitter, LED/source, fibre holder with XYZ manipulator, camera, video digitiser, and computer. Procedures cover mounting, optimisation, data acquisition, background subtraction, centroid location, differentiation of intensity profiles, and MTF/MPD computation.
• Alternative method: When fibre profile factor α is unknown, an alternative approach compares the derivative of the test intensity profile to that of a fully-filled reference profile.

Applications and users

• Who uses it: test laboratories, fibre and connector manufacturers, component qualification teams, standards engineers, and network equipment testers.
• Why it’s useful:
  • Establish repeatable launch conditions for attenuation and return-loss tests of multimode passive components (connectors, patch cords, lenses).
  • Diagnose and characterise light source behaviour and modal filling for system performance and interoperability.
  • Support quality control, R&D, and compliance testing for multimode fibre systems.

Related standards

• IEC 61300-1 - Basic test and measurement procedures: general and guidance
• IEC 61300-3-4 - Attenuation measurement procedures
• IEC 60793-1-20 - Optical fibres: measurement methods and test procedures (fibre geometry)

Keywords: IEC 61300-3-43, mode transfer function, MTF measurement, multimode fibre, launched modal distribution, MPD, near-field intensity, attenuation testing, return loss, fibre optic sources.

Standards Content (Sample)

IEC 61300-3-43 ®
Edition 1.0 2009-01
INTERNATIONAL
STANDARD
Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures –
Part 3-43: Examinations and measurements – Mode transfer function
measurement for fibre optic sources

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
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IEC 61300-3-43 ®
Edition 1.0 2009-01
INTERNATIONAL
STANDARD
Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures –
Part 3-43: Examinations and measurements – Mode transfer function
measurement for fibre optic sources

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
R
ICS 33.180.20 ISBN 978-2-88910-501-4
– 2 – 61300-3-43 © IEC:2009(E)
CONTENTS
FOREWORD.3
1 Scope.5
2 Normative references .5
3 General description .5
4 Theory.5
4.1 Alternative method .7
4.2 Mode power distribution .7
4.3 Constraints.8
5 Apparatus.9
5.1 General .9
5.2 Test sample .9
5.3 Sample positioning device.9
5.4 Optical system.10
5.5 Camera .10
5.6 Video digitiser .10
5.7 Calibration.10
6 Procedure .11
6.1 Mounting and aligning the sample .11
6.2 Optimisation .11
6.3 Acquiring the data .11
7 Calculations .11
7.1 Background level subtraction.11
7.2 Location of centroid of intensity profile .12
7.3 Differentiating the intensity profile .12
7.4 Computing the MTF.13
8 Results .14
Annex A (informative) .16
Bibliography.18

Figure 1 – Example of normalised MTF.7
Figure 2 – Example of normalised MPD .8
Figure 3 – Schematic of measurement apparatus.9
Figure 4 – Location of fibre centre using symmetry computation .13
Figure A.1 – Sensitivity of MTF and MPD to core diameter.16
Figure A.2 – Sensitivity of MTF and MPD to profile factor .17

61300-3-43 © IEC:2009(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC INTERCONNECTING DEVICES
AND PASSIVE COMPONENTS –
BASIC TEST AND MEASUREMENT PROCEDURES –

Part 3-43: Examinations and measurements –
Mode transfer function measurement for fibre optic sources

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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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.
International Standard IEC 61300-3-43 has been prepared by subcommittee 86B: Fibre optic
interconnecting devices and passive components, of IEC technical committee 86: Fibre optics.
This standard cancels and replaces IEC/PAS 61300-3-43, published in 2006. This first edition
constitutes a technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
86B/2780/FDIS 86B/2810/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

– 4 – 61300-3-43 © IEC:2009(E)
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this standard may be issued at a later date.

61300-3-43 © IEC:2009(E) – 5 –
FIBRE OPTIC INTERCONNECTING DEVICES
AND PASSIVE COMPONENTS –
BASIC TEST AND MEASUREMENT PROCEDURES –

Part 3-43: Examinations and measurements –
Mode transfer function measurement for fibre optic sources

1 Scope
This part of IEC 61300 describes the method for measuring the mode transfer function (MTF)
to be used in characterising the launch conditions for measurements of attenuation and or
return loss of multimode passive components. The MTF may be measured at the operational
wavelengths.
2 Normative references
The following referenced documents are indispensable for the application 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 61300-1, Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures – Part 1: General and guidance
IEC 61300-3-4, Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures – Part 3-4: Examination and measurements – Attenuation
IEC 60793-1-20, Optical fibres – Part 1-20: Measurement methods and test procedures –
Fibre geometry
3 General description
The modal distribution launched into multimode fibre can vary widely with different light
sources. This variation in launched modal distribution can result in significant differences in
measured attenuation in the same component. The MTF test method gives information about
the launched modal distribution (LMD) condition in a measured component. The MTF test
method is based on a measurement of the near-field intensity distribution in the fibre [1], [2] .
4 Theory
For a fibre with a power-law index profile n(r), given by,
0,5
α
⎡ ⎤
⎛ r⎞ ⎛ r⎞
n( r ) = n⎢1− 2Δ ⎥ ≤ 1 (1)
⎜ ⎟ ⎜ ⎟
a a
⎢ ⎝ ⎠ ⎥ ⎝ ⎠
⎣ ⎦
where
a is the fibre core radius;
α is the profile factor (α = 2 for a parabolic profile);
___________
Figures in square brackets refer to the Bibliography.

– 6 – 61300-3-43 © IEC:2009(E)
Δ is the relative index difference, given by
2 2
n − n
1 2
Δ = (2)
2n
where
n is the index at fibre centre;
n is the cladding index.
The near-field intensity profile in the fibre I(r) may be determined from an integration of the
mode transfer function MTF(δ) in the fibre, as follows (ignoring constants):
Δ
I( r ) = MTF(δ ) × dδ (3)
∫
α
r
Δ()
a
where
δ is the normalised propagation constant;
r/a is the normalised radial position.
Differentiating both sides gives the MTF as follows (ignoring constants):
⎡ ⎤
dI( r ) 1
MTF(δ ) = × (4)
⎢ ⎥
α −1 α
dr
⎣ r ⎦ r
δ =Δ()
a
The MTF is usually plotted as in terms of the principal mode number m divided by the
maximum principal mode number M, where
(2+α ) (2+α )
2α 2
δ
m ⎡ ⎤ ⎡ r⎤
= = (5)
⎢ ⎥ ⎢ ⎥
M Δ a
⎣ ⎦ ⎣ ⎦
The term (m/M) is usually referred to as the relative mode number, or the normalised mode
number.
The maximum principle mode number M, is given by
α n 2πa
⎛ ⎞
M = Δ (6)
⎜ ⎟
α + 2 λ
⎝ ⎠
A typical normalised MTF plot is shown in Figure 1, where it can be seen, in this example,
that normalised mode numbers up to about 0,6 are equally filled and higher order modes are
progressively less well-filled.

61300-3-43 © IEC:2009(E) – 7 –

1,0
0,75
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalised mode number
IEC  2371/08
Figure 1 – Example of normalised MTF
4.1 Alternative method
If the profile factor, α, in Equation (4) is not known, then an alternative expression for MTF
can be used.
It is known[3] that in a fully-filled fibre (i.e. MTF=1 for all mode numbers) the near-field
intensity profile, I , is approximately the same shape as the square of the refractive index
o
2 α-1
profile, n(r) . Furthermore, the term r Equation (4) is equal (ignoring constants) to the
differential of n(r) and so Equation(4) can be rewritten as:
⎡ dI( r ) 1 ⎤
MTF(δ ) = × (7)
⎢ ⎥
dr dI ( r ) dr
o r
⎣ ⎦
δ =Δ()
a
where a value of α=2 has been assumed in order to compute values for the normalised mode
number.
Thus the MTF is equal to the ratio of the derivative of the intensity profile under test to the
derivative of the intensity profile of the same fibre under fully-filled conditions.
4.2 Mode power distribution
For graded index multimode fibre the number of discrete modes in a particular mode group is
proportional to the principal mode number. Thus higher-order mode groups contain more
modes and therefore will carry more light if all the modes are equally excited. This can be
represented by the mode power distribution (MPD), defined as:
MPD( m) = MTF( m) × m (8)
Because of this relationship of modes within mode groups, the MPD transform effectively
displays the relative power in the mode groups.
Figure 2, where it can be seen, in this case,
An example of a normalised MPD is shown in
that the peak power level occurs around 0,65 normalised mode number.
Normalised MTF
– 8 – 61300-3-43 © IEC:2009(E)

1,0
0,75
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalised mode number
IEC  2372/08
Figure 2 – Example of normalised MPD
4.3 Constraints
The MTF measurement method described herein is only valid under certain conditions, as
follows:
• modes within a mode group carry the same power;
• there are random phases between the propagating modes.
It has been found[4] that both these conditions can be simultaneously met if the line-width Δλ
of the source is sufficiently broad, leading to the so-called "mode-continuum approximation",
given by:
Δλ 2Δ
(10)
≥
λ a × k × N
where
λ is the optical wavelength;
k = 2π/λ;
N is the group index, given by
dn
N = n − λ × (11)
dλ
Typically, for a 50 μm core diameter fibre, with 0,21 numerical aperture, then Δλ > 0,5 nm at
850 nm and Δλ > 1,0 nm at 1 300 nm satisfy this condition.
If the source line-width does not me
...

IEC 61300-3-43 ®
Edition 1.0 2009-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures –
Part 3-43: Examinations and measurements – Mode transfer function
measurement for fibre optic sources

Dispositifs d’interconnexion et composants passifs fibroniques – Procédures
fondamentales d’essais et de mesures –
Partie 3-43: Examens et mesures – Mesure de la fonction de transfert modal
pour sources fibroniques
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IEC 61300-3-43 ®
Edition 1.0 2009-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic interconnecting devices and passive components – Basic test and

measurement procedures –
Part 3-43: Examinations and measurements – Mode transfer function

measurement for fibre optic sources

Dispositifs d’interconnexion et composants passifs fibroniques – Procédures

fondamentales d’essais et de mesures –

Partie 3-43: Examens et mesures – Mesure de la fonction de transfert modal

pour sources fibroniques
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.20 ISBN 978-2-8322-9359-1

– 2 – IEC 61300-3-43:2009  IEC 2009
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 General description . 5
4 Theory . 5
4.1 Alternative method . 7
4.2 Mode power distribution . 7
4.3 Constraints . 8
5 Apparatus . 9
5.1 General . 9
5.2 Test sample . 9
5.3 Sample positioning device . 9
5.4 Optical system. 10
5.5 Camera . 10
5.6 Video digitiser . 10
5.7 Calibration . 10
6 Procedure . 11
6.1 Mounting and aligning the sample . 11
6.2 Optimisation . 11
6.3 Acquiring the data . 11
7 Calculations . 11
7.1 Background level subtraction . 11
7.2 Location of centroid of intensity profile . 12
7.3 Differentiating the intensity profile . 12
7.4 Computing the MTF . 14
8 Results . 15
Annex A (informative) . 16
Bibliography . 18

Figure 1 – Example of normalised MTF . 7
Figure 2 – Example of normalised MPD . 8
Figure 3 – Schematic of measurement apparatus . 9
Figure 4 – Location of fibre centre using symmetry computation . 13
Figure A.1 – Sensitivity of MTF and MPD to core diameter. 16
Figure A.2 – Sensitivity of MTF and MPD to profile factor . 17

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC INTERCONNECTING DEVICES
AND PASSIVE COMPONENTS –
BASIC TEST AND MEASUREMENT PROCEDURES –

Part 3-43: Examinations and measurements –
Mode transfer function measurement for fibre optic sources

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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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.
International Standard IEC 61300-3-43 has been prepared by subcommittee 86B: Fibre optic
interconnecting devices and passive components, of IEC technical committee 86: Fibre optics.
This standard cancels and replaces IEC/PAS 61300-3-43, published in 2006. This first edition
constitutes a technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
86B/2780/FDIS 86B/2810/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

– 4 – IEC 61300-3-43:2009  IEC 2009
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
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.
FIBRE OPTIC INTERCONNECTING DEVICES
AND PASSIVE COMPONENTS –
BASIC TEST AND MEASUREMENT PROCEDURES –

Part 3-43: Examinations and measurements –
Mode transfer function measurement for fibre optic sources

1 Scope
This part of IEC 61300 describes the method for measuring the mode transfer function (MTF)
to be used in characterising the launch conditions for measurements of attenuation and or
return loss of multimode passive components. The MTF may be measured at the operational
wavelengths.
2 Normative references
The following referenced documents are indispensable for the application 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 61300-1, Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures – Part 1: General and guidance
IEC 61300-3-4, Fibre optic interconnecting devices and passive components – Basic test and
measurement procedures – Part 3-4: Examination and measurements – Attenuation
IEC 60793-1-20, Optical fibres – Part 1-20: Measurement methods and test procedures –
Fibre geometry
3 General description
The modal distribution launched into multimode fibre can vary widely with different light
sources. This variation in launched modal distribution can result in significant differences in
measured attenuation in the same component. The MTF test method gives information about
the launched modal distribution (LMD) condition in a measured component. The MTF test
method is based on a measurement of the near-field intensity distribution in the fibre [1], [2] .
4 Theory
For a fibre with a power-law index profile n(r), given by,
0,5
α
 
 r  r
n( r )= n1− 2∆  ≤ 1 (1)
   
a a
     
 
where
a is the fibre core radius;
α is the profile factor (α = 2 for a parabolic profile);
___________
Figures in square brackets refer to the Bibliography.

– 6 – IEC 61300-3-43:2009  IEC 2009
∆ is the relative index difference, given by
2 2
n − n
1 2
∆= (2)
2n
where
n is the index at fibre centre;
n is the cladding index.
The near-field intensity profile in the fibre I(r) may be determined from an integration of the
mode transfer function MTF(δ) in the fibre, as follows (ignoring constants):
∆
I( r )= MTF(δ )× dδ (3)
∫
α
r
∆( )
a
where
δ is the normalised propagation constant;
r/a is the normalised radial position.
Differentiating both sides gives the MTF as follows (ignoring constants):
 
dI( r ) 1
MTF(δ )= × (4)
 
α−1 α
dr
 r  r
δ=∆( )
a
The MTF is usually plotted as in terms of the principal mode number m divided by the
maximum principal mode number M, where
(2+α ) (2+α )
2α 2
m δ r
   
= = (5)
   
M ∆ a
   
The term (m/M) is usually referred to as the relative mode number, or the normalised mode
number.
The maximum principle mode number M, is given by
α n 2πa
 
M= ∆ (6)
 
α+ 2 λ
 
A typical normalised MTF plot is shown in Figure 1, where it can be seen, in this example,
that normalised mode numbers up to about 0,6 are equally filled and higher order modes are
progressively less well-filled.

1,0
0,75
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalised mode number
IEC  2371/08
Figure 1 – Example of normalised MTF
4.1 Alternative method
If the profile factor, α, in Equation (4) is not known, then an alternative expression for MTF
can be used.
It is known[ 3] that in a fully-filled fibre (i.e. MTF=1 for all mode numbers) the near-field
intensity profile, I , is approximately the same shape as the square of the refractive index
o
2 α-1
profile, n(r) . Furthermore, the term r Equation (4) is equal (ignoring constants) to the
differential of n(r) and so Equation(4) can be rewritten as:
 
dI( r ) 1
MTF(δ )= × (7)
 
dr dI ( r ) dr
o
  r
( )
δ=∆
a
where a value of α=2 has been assumed in order to compute values for the normalised mode
number.
Thus the MTF is equal to the ratio of the derivative of the intensity profile under test to the
derivative of the intensity profile of the same fibre under fully-filled conditions.
4.2 Mode power distribution
For graded index multimode fibre the number of discrete modes in a particular mode group is
proportional to the principal mode number. Thus higher-order mode groups contain more
modes and therefore will carry more light if all the modes are equally excited. This can be
represented by the mode power distribution (MPD), defined as:
MPD( m)= MTF( m)× m (8)
Because of this relationship of modes within mode groups, the MPD transform effectively
displays the relative power in the mode groups.
An example of a normalised MPD is shown in Figure 2, where it can be seen, in this case,
that the peak power level occurs around 0,65 normalised mode number.
Normalised MTF
– 8 – IEC 61300-3-43:2009  IEC 2009

1,0
0,75
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalised mode number
IEC  2372/08
Figure 2 – Example of normalised MPD
4.3 Constraints
The MTF measurement method described herein is only valid under certain conditions, as
follows:
• modes within a mode group carry the same power;
• there are random phases between the propagating modes.
It has been found[4] that both these conditions can be simultaneously met if the line-width ∆λ
of the source is sufficiently broad, leading to the so-called "mode-continuum approximation",
given by:
∆λ 2∆
(10)
≥
λ a× k × N
where
λ is the optical wavelength;
k = 2π/λ;
N is the group index, given by
dn
N= n −λ× (11)
dλ
Typically, for a 50 µm core diameter fibre, with 0,21 numerical aperture, then ∆λ > 0,5 nm at
850 nm and ∆λ > 1,0 nm at 1 300 nm satisfy this condition.
If the source line-width does not meet this criterion then interference between propagating
modes may take place, resulting in "speckle" in the near-field image. The method can,
however, still be applied to such sources by gently shaking, or somehow agitating, the fibre
under test so as to cause a temporal averaging of the speckle pattern. In this case, it is
important to ensure the near-field is azimuthally symmetric. This can be achieved by checking
that the MTFs measured at 45° intervals around the fibre coincide with each other[5].
• The peak of the MPD occurs at a normalised mode number of <0,8.
Normalised MPD
It is known that deviation of the measured near-field intensity profile I(r) from the power law
profile in Equation (1), for fibres that are well-filled, may occur towards the core/cladding
boundary. It is recommended that, in this case, the alternative method for the determination of
MTF described in 4.1 is employed.
5 Apparatus
5.1 General
The apparatus is essentially a video microscope where a near-field image of the end of the
fibre under test is formed on the surface of a camera by an optical system. The camera image
is then digitised by a video digitiser and transferred to a computer for analysis and data
presentation.
A schematic of a typical measurement configuration is shown in Figure 3.

Condensing
Imaging
lens
lens
Beamsplitter
LED
source
Fibre holder and
XYZ manipulator
Optional neutral
density filter
Camera
Computer
IEC  2373/08
Figure 3 – Schematic of measurement apparatus
5.2 Test sample
The test sample consists of a multimode patch cord attached to a light source. It should be
recognised that the mode distribution at the output of the patch cord is a product of both the
launch conditions of the source and of the patch cord itself. The resultant MTF is therefore not
a parameter of either the light source or the patch cord individually but rather of the
combination, including the particular conditions under which the patch cord is disposed, such
as bend radius.
5.3 Sample positioning device
A positioning device is required to ensure that the end of the patch cord under test is located
on the optical axis of the instrument and also in the correct axial position to give a well-
focussed image on the camera. For this purpose, an XYZ manipulation stage may be used or,
preferably, a suitable connector receptacle mounted axially with the optics. An example is a
standard 2,5 mm ferrule receptacle which is able to accommodate several connector types,

– 10 – IEC 61300-3-43:2009  IEC 2009
such as FC, ST and SC. In this case, the XY positioning of the patch cord is well-defined and
only a focussing adjustment is required.
5.4 Optical system
The optical system comprises magnifying optics to produce an image of the fibre end on the
camera. To optimise measurement resolution, it is recommended that the optical
magnification shall be chosen so that the image of the fibre core fills a reasonable proportion
of the camera. Typically, this might be between 20 % and 50 % of the vertical extent of the
camera.
The numerical aperture of the imaging system shall be greater than the numerical aperture of
the fibre under test.
A means of illuminating the end face of the fibre in reflection may also be provided, such as a
beam splitter and an LED source positioned between the focussing lens and the camera.
Neutral density (ND) filters may also be provided to control the amount of light reaching the
camera.
5.5 Camera
A high quality camera shall be used that has demonstrable geometrical uniformity and
intensity linearity. The pixel size of the camera, picsize, shall be sufficiently small compared
with the magnified near-field image as to be less than the system diffraction limits by a factor
of 2, given by
0,61Magλ
Picsize< (12)
2NA
where
Mag is the system magnification;
NA is the numerical aperture of the fibre.
For example, if Mag = 20, NA = 0,21, λ = 850 nm then picsize < 24 µm. It is recommended,
however, that the camera pixel size is much smaller than this. In this example, the
corresponding pixel size at the fibre would be equal to picsize divided by Mag, which is equal
to 1,2 µm.
5.6 Video digitiser
The video digitiser, which is connected to the camera, provides the computer with a digitised
image of the fibre end. A typical video digitiser will provide an 8 bit image, although a digitiser
providing more bits, for example 12, may be used for increased resolution.
5.7 Calibration
The calibration factor is expressed in units of µm/pixel. It is required in 7.4 to convert the
processed data between pixel space and µm units.
The optical system may be calibrated by measuring an artefact of known dimension, such as
a microscope graticule or an optical fibre of known cladding diameter. The calibration artefact
is positioned in the object plane of the system and focussed onto the camera. In the case of a
graticule, illumination may be by transmitted or reflected light. In the case of an optical fibre,
reflected light must be used. This is typically achieved by the use of a light source and beam
splitter positioned in the optical system between the focussing lens and the camera.

NOTE The wavelength of the illumination source should be within 30 nm of the nominal wavelength of the source
under test so as to minimise chromatic effects on the system magnification.
Measure the size of the calibration artefact in pixels, n . If the size of the artefact in µm is
pix
n , then the calibration factor, calfactor, is given by
cal
n
cal
Calfactor= (13)
n
pix
The system magnification, Mag, which is required in 5.5 may be calculated from the
calibration factor as follows:
picsize
Mag= (14)
calfactor
NOTE In the case where the camera pixels are non-square, then the calibration factor must be determined along
the particular axis of the camera that is used for subsequent MTF measurements, (see Clause 7).
6 Procedure
6.1 Mounting and aligning the sample
Mount the fibre to be measured in the sample positioning device in the object plane of the
optical system and switch on the end illumination source. Align the lateral position of the fibre
end, if necessary, and adjust the focus position of the fibre to give a well-focussed near-field
image on the camera. Switch off the end illumination and switch on the source under test,
which, if necessary, should be allowed to stabilise.
6.2 Optimisation
In order to utilise the full analogue-to-digital converter (ADC) range of the video digitiser
effectively adjust the intensity of the image so that it fills typically about 90 % of the ADC
range. This may be achieved by any or a combination of the following means:
• adjusting the intensity of the light source;
• the use of neutral density (ND) filters in front of the camera;
• adjusting the gain and/or electronic shuttering of the camera.
6.3 Acquiring the data
A digitised image of the fibre end is then transferred by the controlling computer for analysis.
Typically the image is then converted to a two-dimensional array of ADC values for
subsequent processing. In order to improve signal-to-noise ratio, several images or frames,
can be serially acquired and their ADC values averaged on a pixel-by-pixel basis. A typical
number of frames is ten to twenty, although, in the case of a coherent source where agitation
must be used to break up the speckle pattern, several hundred frames is typical.
If the alternative method (4.1) is being used then it is necessary to disconnect the source
under test from the patchcord and replace this with a source which overfills the patchcord. A
second digitised image is then obtained in the same manner as above.
7 Calculations
7.1 Background level subtraction
It is important that the background level, or dark level, of the camera is uniform to avoid
unwanted noise caused by the differential in Equation (4). The background uniformity may be

– 12 – IEC 61300-3-43:2009  IEC 2009
improved by acquiring image data with the light source turned off and then subtracting this on
a pixel-by-pixel basis from the measured fibre image.
7.2 Location of centroid of intensity profile
The centroid, or centre of gravity, of the near-field image is required so that an intensity
profile through the fibre centre can be extracted. To do this, only the vertical centroid is
required. A typical method is as follows:
a) locate the co-ordinates of the position of peak power in the image;
b) extract a 2-D matrix of pixels, I , from the acquired, background-subtracted image,
core
centred on the position of peak. The first index of I is the row index (y-dimension)
core
whose extent is rows. The second index of I is the column index (x-dimension) whose
core
extent is cols. I shall contain the entire core image although effort should be made to
core
limit the dark pixels since they contribute only noise to the following computations;
c) compute the sum of the intensity values along each row in I , sumrow(i), yielding a 1D
core
array of sums. This is called collapsing the 2D data onto the Y axis:
cols
Sumrow( i )= Icore( i, j ) (15)
∑
j=1
d) compute the sum of the elements of the array of sums, yielding a single scalar number,
sumofsums;
rows
Sumofsums= sumrow( i ) (16)
∑
i=1
e) compute the product of each element of the array of sums with its array co-ordinate and
sum these products to yield a single scalar number, sumproduct;
rows
Sumproduct= sumrow( i )× i (17)
∑
i=1
f) the centroid, in pixel units, is then given by the sum of the products divided by the sum of
the sums:
sumproduct
Centroid= (18)
sumofsums
g) the intensity profile, I(i), along the row that is nearest to the centroid is then extracted for
analysis. Note that, for cameras meeting the requirements of Equation (12), the error in
this approximation is negligible.
7.3 Differentiating the intensity profile
The next step is to differentiate the near-field intensity profile, as required by Equation (4).
Any suitable numerical method can be used but a recommended method is that of the
Savitsky-Golay filter[6]. This filter effectively fits a sliding polynomial across the data-set and
computes the differential from the fitted coefficients. One such polynomial is that of a
quadratic. A required parameter is the number of data-points over which the polynomial is
fitted, known as the fit-window. Typically, the wider the fit-window the greater the data
smoothing that occurs, similar to a low-pass filter. A trade-off exists, therefore between the
level of noise in the differentiated data and amount of detail that is lost by the smoothing
process.
The intensity profile that was extracted in 7.2 extends well beyond the extent of the fibre core.
However, the MTF is only defined between the fibre centre and the edge of the core so the

end points need to be defined. The fibre centre is located from the differentiated data as
follows:
a) locate the approximate centre of the fibre by computing the mean pixel position, Xc, of the
positions corresponding to the maximum and minimum values of the differentiated data-
set, Idiff(i);
b) compute the symmetry function, Sym(k), about this position, as follows:
k−1 Xc+nsym
Sym( k)= Idiff(i)× k− i + Idiff(i)× k− i (19)
∑ ∑
i=Xc−nsym i=k+1
where
nsym is the width of window for the symmetry computation, typically similar to the core
radius, in pixels;
k takes integer values from (Xc-nsym) to (Xc+nsym).
c) locate the pixel nearest to the minimum of Sym(k). This corresponds to the fibre centre.
An example of a computed symmetry function for a particular intensity profile is shown in
Figure 4, where the position of maximum symmetry, corresponding to the minimum of the
symmetry, zeropos, corresponding to the minimum of the symmetry function, is indicated.

Zeropos
250 300 350 400 450 500
Pixel number
Symmetry function
Intensity profile
IEC  2374/08
Figure 4 – Location of fibre centre using symmetry computation
Next, in order to compute the MTF, separate the differentiated data-set into two halves, left
and right, about the computed fibre centre and average these together on a pixel-by-pixel
basis.
For diagnostic purposes, the MTF may also be independently computed for both the left and
right halves of the differentiated data-set. Comparison of the resulting curves provides a
useful check of the requirement for azimuthal symmetry (4.3). Differences between the two
curves may indicate, for example, that part of the fibre end is scratched or contaminated.
Relative intensity
– 14 – IEC 61300-3-43:2009  IEC 2009
7.4 Computing the MTF
α-1
The final step is to divide the differential, dI(r)/dr, by the factor (r ), in pixel space, shown
in Equation (4) and reproduced below as a function of the principal mode number:
 dI( r ) 1 
(2+α)
MTF( m)= × (20)
 
α−1
r 2
 
dr
r
 
m=M
 
a
 
The MTF is then normalised and plotted as a function of normalised mode number, given by
Equation(5) as:
( 2+α )
m  r
= (21)
 
M a
 
where in Equation (21) the fibre core radius, a, is replaced by the number of pixels
corresponding to the fibre core radius, pixrad:
a
Pixrad= (22)
calfactor
where calfactor is the calibration factor of the optical system, described in 5.7, and expressed
in units of µm/pixel.
NOTE If the fibre core radius is unknown, then it may be determined according to the procedures given in
IEC 60793-1-20.
If the alternative method is being used (see 4.1) then the reference image obtained in 6.3 is
processed in the same way as described in 7.1 to 7.3. The MTF is computed, in pixel space,
according to Equation (7), which is reproduced below as a function of the principal mode
number:
 
dI( r ) 1
MTF( m)= × (23)
  2
dr dI ( r ) dr  r
 o 
m=M
 
a
 
The MTF is then normalised and plotted as a function of the normalised mode number, given
by:
m r
 
= (24)
 
M a
 
where in Equation (24) the fibre core radius, a, is replaced by the number of pixels
corresponding to the fibre core radius, pixrad, defined in Equation (22).
NOTE For display purposes, data points for a normalised mode number below 0,05 may be ignored in the
normalisation and values greater than 1 in this region may not be plotted. Additionally, negative values may be
omitted from the plot.
8 Results
The following information shall be provided with each measurement:
– date and title of measurement;
– identification of test method (this document);
– identification and description of specimen, including light source and patch cord;
– the test wavelength;
– the fit-window used in differentiating the profile intensity data, in µm;
– the number of frames averaged;
– the normalised mode transfer function (MTF);
– the normalised mode power distribution (MPD);
The following information may also be provided if required:
– the near-field image (bitmap).

– 16 – IEC 61300-3-43:2009  IEC 2009
Annex A
(informative)
Sensitivity of MTF and MPD to core parameters

The measurement of the modal distribution according to Equation (4) depends on a
knowledge of the fibre core radius and the index profile factor.
Examples of the effect on the MTF and MPD of entering different core diameters into Equation
(4) are shown in Figure A.1
1,0
50,5um
50,0um
0,75
51,0um
0,50
0,25
0,0
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
Normalized mode number
IEC  2375/08
1,0
50,5um
0,75 50,0um
51,0um
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalized mode number
IEC  2376/08
Figure A.1 – Sensitivity of MTF and MPD to core diameter
Examples of the effect on the MTF and MPD of entering different profile factors, α, into
Equation (4) are shown in Figure A.2
Normalized MTF
Normalized MPD
1,0
2,0
1,8
0,75
2,2
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalized mode number
IEC  2377/08
1,0
2,0
0,75
1,8
2,2
0,50
0,25
0,0
0,0 0,2 0,4 0,6 0,8 1,0
Normalized mode number
IEC  2378/08
Figure A.2 – Sensitivity of MTF and MPD to profile factor

Normalized MTF
Normalized MPD
– 18 – IEC 61300-3-43:2009  IEC 2009
Bibliography
[1] DAIDO Y. et al., Determination of modal power distribution in graded-index optical
waveguides from near-field patterns and its application to differential mode attenuation,
Appl. Opt., vol. 18, no. 13, pp. 2207-2213, 1979.
[2] LEMINGER OG. and GRAU GK., Near-field intensity and modal power distribution in
multimode graded-index fibres, Electron. Lett., vol. 16, no. 17, pp. 678-679, 1980.
[3] GLOGE D. and MARCATILI EAJ., Multimode theory of graded-core fibers, Bell Syst.
Tech. J., vol. 52, no. 9, pp. 1563-1579, 1973.
[4] MICKELSON AR. and ERIKSRUD M., Mode-continuum approximation in optical fibers,
Opt. Lett., vol. 7, no. 11, pp. 572-574, 1982.
[5] RITTICH D., Practicability of determining the modal power distribution by measured
near and far-fields, IEEE J. Lightwave Technol., vol. 3, no. 3, pp. 625-661, 1985.
[6] PRESS WH. et al., Numerical Recipes in C: The Art of Scientific Computing, Cambridge
University Press, ch.14.8.
___________
– 20 – IEC 61300-3-43:2009  IEC 2009
SOMMAIRE
AVANT-PROPOS . 21
1 Domaine d’application . 23
2 Références normatives . 23
3 Description générale . 23
4 Théorie . 24
4.1 Autre méthode . 25
4.2 Distribution de puissance modale . 26
4.3 Contraintes. 26
5 Appareillage . 27
5.1 Généralités. 27
5.2 Echantillon d’essai . 28
5.3 Dispositif de positionnement de l’échantillon . 28
5.4 Système optique. 29
5.5 Caméra . 29
5.6 Numériseur vidéo . 29
5.7 Etalonnage . 29
6 Procédure . 30
6.1 Montage et alignement de l’échantillon . 30
6.2 Optimisation . 30
6.3 Acquisition des données . 30
7 Calculs . 31
7.1 Soustraction du bruit de fond . 31
7.2 Emplacement du centroïde du profil d’intensité . 31
7.3 Différentiation du profil d’intensité . 32
7.4 Calcul de la MTF . 33
8 Résultats . 34
Annex A (informative) Sensibilité de la MTF et la MPD aux paramètres du cœur . 35
Bibliographie . 37

Figure 1 – Exemple de MTF normalisée . 25
Figure 2 – Exemple de MPD normalisée . 26
Figure 3 – Schéma de l’appareillage de mesure . 28
Figure 4 – Localisation du centre de la fibre à l’aide du calcul de symétrie. 33
Figure A.1 – Sensibilité de la MTF et la MPD au diamètre du cœur . 35
Figure A.2 – Sensibilité de la MTF et la MPD au facteur de profil . 36

COMMISSION ÉLECTROTECHNIQUE INTERNATIONALE
___________
DISPOSITIFS D’INTERCONNEXION
ET COMPOSANTS PASSIFS FIBRONIQUES –
PROCÉDURES FONDAMENTALES D’ESSAIS ET DE MESURES –

Partie 3-43: Examens et mesures –
Mesure de la fonction de transfert modal pour sources fibroniques

AVANT-PROPOS
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d
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