EN 60793-1-49:2006

SLOVENSKI STANDARD
01-december-2007
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Optical fibres -- Part 1-49: Measurement methods and test procedures - Differential
mode delay (IEC 60793-1-49:2006)
Optische Schnittstellen von Lichtwellenleiter-Steckverbindern -- Teil 2-1: Optische
Schnittstelle von nicht abgeschrägten Einmodenfasern mit physikalischem Kontakt (IEC
60793-1-49:2006)
Interfaces optiques de connecteurs pour fibres optiques -- Partie 2-1: Interfaces optiques
pour fibres unimodales en contact physique sans angles (IEC 60793-1-49:2006)
Ta slovenski standard je istoveten z: EN 60793-1-49:2006
ICS:
33.180.10 2SWLþQDYODNQDLQNDEOL Fibres and cables
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60793-1-49
NORME EUROPÉENNE
July 2006
EUROPÄISCHE NORM
ICS 33.180.10 Supersedes EN 60793-1-49:2003

English version
Optical fibres
Part 1-49: Measurement methods and test procedures -
Differential mode delay
(IEC 60793-1-49:2006)
Fibres optiques  Lichtwellenleiter
Partie 1-49: Méthodes de mesure et Teil 1-49: Messmethoden und
procédures d'essai - Prüfverfahren -
Retard différentiel de mode Gruppenlaufzeitdifferenz
(CEI 60793-1-49:2006) (IEC 60793-1-49:2006)

This European Standard was approved by CENELEC on 2006-07-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2006 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60793-1-49:2006 E
Foreword
The text of document 86A/1061/FDIS, future edition 2 of IEC 60793-1-49, prepared by SC 86A, Fibres
and cables, of IEC TC 86, Fibre optics, was submitted to the IEC-CENELEC parallel vote and was
approved by CENELEC as EN 60793-1-49 on 2006-07-01.
This European Standard supersedes EN 60793-1-49:2003.
It adds minimum calculated effective modal bandwidth (EMBc) to the test procedures, supporting
EN 60793-2-10.
This standard is to be read in conjunction with EN 60793-1-1 and EN 60793-2-10.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2007-04-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2009-07-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60793-1-49:2006 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60825-1 NOTE  Harmonized as EN 60825-1:1994 (not modified).
IEC 60825-2 NOTE  Harmonized as EN 60825-2:2004 (not modified).

__________
- 3 - EN 60793-1-49:2006
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

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.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

1) 2)
IEC 60793-1-1 - Optical fibres EN 60793-1-1 2003
Part 1-1: Measurement methods and test
procedures - General and guidance

1) 2)
IEC 60793-1-22 - Optical fibres EN 60793-1-22 2002
Part 1-22: Measurement methods and test
procedures - Length measurement

1) 2)
IEC 60793-1-41 - Optical fibres EN 60793-1-41 2003
Part 1-41: Measurement methods and test
procedures - Bandwidth
1) 2)
IEC 60793-1-42 - Optical fibres EN 60793-1-42 2002
Part 1-42: Measurement methods and test
procedures - Chromatic dispersion

1) 2)
IEC 60793-1-45 - Optical fibres EN 60793-1-45 2003
(mod)  Part 1-45: Measurement methods and test + corr. April 2004
procedures - Mode field diameter

1) 2)
IEC 60793-2-10 - Optical fibres EN 60793-2-10 2004
Part 2-10: Product specifications - Sectional
specification for category A1 multimode fibres

1) 2)
IEC 61280-1-4 - Fibre optic communication subsystem test EN 61280-1-4 2003
procedures
Part 1-4: General communication
subsystems - Collection and reduction of two-
dimensional nearfield data for multimode fibre
laser transmitters
1)
Undated reference.
2)
Valid edition at date of issue.

NORME CEI
INTERNATIONALE
IEC
60793-1-49
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2006-06
Fibres optiques –
Partie 1-49:
Méthodes de mesure et procédures d'essai –
Retard différentiel de mode
Optical fibres –
Part 1-49:
Measurement methods and test procedures –
Differential mode delay
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60793-1-49  IEC:2006 – 3 –
CONTENTS
FOREWORD.5
1 Scope.9
2 Normative references .9
3 Terms and definitions .11
4 Apparatus.13
4.1 Optical source .13
4.2 Stability .13
4.3 Launch system .13
4.4 Detection system.15
4.5 Computational equipment.17
5 Sampling and specimens.17
5.1 Test sample .17
5.2 Specimen endfaces .17
5.3 Specimen length.17
5.4 Specimen packaging .17
5.5 Specimen positioning .17
6 Procedure .17
6.1 Adjust and measure system response .17
6.2 Adjust detection system.19
6.3 Measure the test sample .19
7 Calculations and interpretation of results.21
7.1 Differential mode delay (DMD) .21
7.2 Minimum calculated effective modal bandwidth .21
7.3 Length normalization .25
8 Documentation .25
8.1 Report the following information for each test:.25
8.2 The following information shall be available upon request: .25
9 Specification information .25

Annex A (normative) Source spectral width limitation.29
Annex B (informative) Discussion of measurement details .35
Annex C (informative) Determining DMD weights for EMBc calculation .43
Annex D (informative)  EMBc calculation information .49
Annex E (informative)  Comparison between this standard and ITU recommendations .55

Bibliography.57

Figure B.1 – Idealized DMD data .35

Table A.1 –Highest expected dispersion for any of the commercially available
Category A1 fibres .33
Table D.1 – DMD weightings – Example set 1.49
Table D.2 – DMD weightings – Example set 2.51

60793-1-49  IEC:2006 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-49: Measurement methods and test procedures –
Differential mode delay
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 60793-1-49 has been prepared by subcommittee 86A Fibres and
cables, of IEC technical committee 86: Fibre optics.
This second edition cancels and replaces the first edition published in 2003, of which it
constitutes a technical revision. This edition adds minimum calculated effective modal
bandwidth (EMBc) to the test procedures, supporting IEC 60793-2-10.
The text of this standard is based on the following documents:
FDIS Report on voting
86A/1061/FDIS 86A/1077/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.

60793-1-49  IEC:2006 – 7 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
This standard is to be read in conjunction with IEC 60793-1-1 and IEC 60793-2-10.
IEC 60793-1-4X consists of the following parts, under the general title Optical fibres:
Part 1-40: Measurement methods and test procedures − Attenuation
Part 1-41: Measurement methods and test procedures − Bandwidth
Part 1-42: Measurement methods and test procedures − Chromatic dispersion
Part 1-43: Measurement methods and test procedures − Numerical aperture
Part 1-44: Measurement methods and test procedures − Cut-off wavelength
Part 1-45: Measurement methods and test procedures − Mode field diameter
Part 1-46: Measurement methods and test procedures − Monitoring of changes in optical
transmittance
Part 1-47: Measurement methods and test procedures − Macrobending loss
Part 1-48: Measurement methods and test procedures − Polarization mode dispersion
Part 1-49: Measurement methods and test procedures − Differential mode delay
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.
60793-1-49  IEC:2006 – 9 –
OPTICAL FIBRES –
Part 1-49: Measurement methods and test procedures –
Differential mode delay
1 Scope
This part of IEC 60793 applies only to multimode, graded-index glass-core (category A1)
fibres. The test method is commonly used in production and research facilities, but is not
easily accomplished in the field.
This standard describes a method for characterizing the modal structure of a graded-index
multimode fibre. This information is useful for assessing the bandwidth performance of a fibre
especially when the fibre is intended to support a variety of launch conditions such as those
produced by standardized laser transmitters.
With this method, the output from a fibre that is single-mode at the test wavelength excites
the multimode fibre under test. The probe spot is scanned across the endface of the fibre
under test, and the optical pulse delay is determined at specified offset positions.
Two results can be produced from the same data. First, the difference in optical pulse delay
time between the fastest and slowest mode groups of the fibre under test can be determined.
The user specifies the upper and lower limits of radial offset positions over which the probe
fibre is scanned in order to specify desired limits of modal structure. The DMD data is then
compared to DMD specifications that have been determined by modeling and experimentation
to correspond to a minimum EMB for a range of transmitters. Second, the optical pulse
shapes can be combined using specific weights to determine a calculated effective modal
bandwidth (EMBc), and by calculating a sequence of EMBc values with different sets of
weights, a minimum EMBc can be calculated, corresponding to a range of transmitters.
The test quantifies the effects of interactions of the fibre modal structure and the source
modal characteristics excluding the source spectral interactions with fibre chromatic
dispersion. Adding the effects of chromatic dispersion and source spectral width will reduce
the overall transmission bandwidth, but this is a separate calculation in most transmission
models. In this test, the effects of non-zero spectral width are minimized but any residual
effects will tend to increase the DMD value and decrease the EMBc value.
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.

60793-1-49  IEC:2006 – 11 –
IEC 60793-1-1: Optical fibres − Part 1: Measurement methods and test procedures –- General
and guidance
IEC 60793-1-22: Optical fibres − Part 1-22: Measurement methods and test procedures –
Length measurement
IEC 60793-1-41: Optical fibres – Part 1-41: Measurement methods and test procedures –
Bandwidth.
IEC 60793-1-42: Optical fibres − Part 1-42: Measurement methods and test procedures −
Chromatic dispersion
IEC 60793-1-45: Optical fibres − Part 1-45: Measurement methods and test procedures -
Mode field diameter
IEC 60793-2-10: Optical fibres – Part 2-10: Product specifications – Sectional specification for
category A1 multimode fibres
IEC 61280-1-4: Fibre optic communication subsystem test procedures – Part 1-4: General
communication subsystems – Collection and reduction of two-dimensional nearfield data for
multimode fibre laser transmitters
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE The user of this standard specifies either the maximum DMD for the outer (R ) and inner (R )
OUTER INNER
limits of radial offset position over which the probe spot is scanned, or the minimum EMBc among the EMBc values
calculated from a sequence of DMD weightings.
3.1
differential mode delay
DMD
the estimated difference in optical pulse delay time between the fastest and slowest modes
excited for all radial offset positions between and including R and R
INNER OUTER
3.2
effective modal bandwith
bandwidth associated with the transfer function, H(f), of a particular laser/fibre combination
3.3
inner limit
R
INNER
outer limit
R
OUTER
limits of radial offset positions on the endface of the fibre under test over which the probe spot
is scanned
60793-1-49  IEC:2006 – 13 –
4 Apparatus
4.1 Optical source
Use an optical source that introduces short duration, narrow spectral width pulses into the
probe fibre.
The temporal duration of the optical pulse shall be short enough to measure the intended
differential delay time. The maximum duration allowed for the optical pulse, characterized as
full width at 25 % of maximum amplitude, will depend both on the value of DMD to be
determined and the sample length. For example, if the desired length-normalized DMD limit is
0,20 ps/m over a sample of length 500 m, the DMD to be measured is 100 ps, and a pulse of
duration less than ~110 ps is needed. Testing to the same DMD limit in a 10 000 m length of
fibre requires measuring a DMD of 2 000 ps, and a pulse a wide as ~2 200 ps may be used.
Detailed limits are given in 6.1, and may depend on the source spectral width.
Chromatic dispersion induced broadening resulting from source spectral width shall be within
the limits indicated in Annex A. The requirement on spectral width may be met either by using
a spectrally narrow source, or alternatively by the use of appropriate optical filtering at either
the source or detection end.
The centre wavelength shall be within ±10 nm of the nominal specified wavelength.
A mode locked titanium-sapphire laser is an example of a source usable for this application.
4.2 Stability
Devices shall be available to position the input and output ends of the test specimen with
sufficient stability and reproducibility to meet the conditions of 4.3 and 4.4.
4.3 Launch system
The probe fibre between the light source and test sample shall propagate only a single mode
at the measurement wavelength. The mode field diameter of the probe fibre at λ shall be
(8,7λ – 2,39) ± 0,5 µm, where λ is the measurement wavelength in micrometers, and the
mode field diameter is determined using IEC 60793-1-45. This equation produces a mode
field diameter of 5 µm at 850 nm and 9 µm at 1 310 nm, which corresponds to commercially
available single-mode fibres.
Ensure that the output of the probe fibre is single-mode. One method to do this is to strip
higher order modes by wrapping the probe fibre three turns around a 25-mm diameter
mandrel.
The output spot of the probe fibre shall be scanned across the endface of the test sample with
a positional accuracy less than or equal to ±0,5 µm.
The output beam from the probe fibre shall be perpendicular to the endface of the test sample
to within an angular tolerance of less than or equal to 1,0 degree.
The launch system shall be capable of reproducibly centring the output spot of the probe fibre
to within ±1,0 µm.
60793-1-49  IEC:2006 – 15 –
If directly coupled to the test sample, the gap between the output end of the probe fibre and
the endface of the test sample shall be no more than 10 µm.
A free space optics system of lenses or mirrors may be used to image the output spot of the
probe fibre onto the endface of the test sample. When using this type of launch system, care
should be taken to ensure that substantially the same modes are excited in the test fibre as
would be if the beam were coupled directly from the output of the single-mode probe fibre. For
example, a system of lenses or mirrors may be used to image the output of a single-mode
fibre onto the end face of the test sample.
Provide means to remove cladding light from the test sample. Often the fibre coating is
sufficient to perform this function. Otherwise, use cladding mode strippers near both ends of
the test sample. If the fibre is retained on the cladding mode stripper(s) with small weights,
care shall be taken to avoid microbending at these sites.
4.4 Detection system
Use an optical detection apparatus suitable for the test wavelength. The detection apparatus
shall couple all of the guided modes from the test sample onto the detector's active area, such
that the detection sensitivity is not significantly mode dependent. The detector, along with any
signal preamplifier, shall respond linearly (within ±5 %) over the range of power detected.
The temporal response of the detector system, including any optional optical attenuator, shall
not be significantly mode dependent.
A specific test for mode dependence is given in 6.1. Alternatively, the detector’s temporal
response may be a function of offset as long as it is stable over the course of the
measurement (i.e. ΔT (r) shall fulfil the ±5 % requirement of 6.1).
PULSE
Ringing of the detector system shall be limited such that maximum overshoot or undershoot
shall be less than 5 % of the peak amplitude of the detected optical signal as measured on
the reference.
The waveform of the detected optical signal shall be recorded and displayed on a suitable
instrument, such as a high-speed sampling oscilloscope with calibrated time sweep. The
recording system should be capable of averaging the detected waveform for multiple optical
pulses.
Use a delay device, such as a digital delay generator, to provide a means of triggering the
detection electronics at the correct time. The delay device may trigger the optical source, or
be triggered by it. The delay device may be internal or external to the recording instrument.
The combined effect of timing jitter and noise in the detection system shall be small enough
that the difference between successive measurements of optical delay times for any fixed
launch used in the measurement shall be less than 5 % of the measured value of DMD.
Averaging the detected waveform for multiple optical pulses may be used to reduce the
effects of timing jitter and noise. If averaging is used, each waveform shall be recorded using
at least the number of averages used when determining ΔT in 6.1 The system shall
PULSE
maintain this level of stability over the course of the measurement.

60793-1-49  IEC:2006 – 17 –
4.5 Computational equipment
This test method generally requires a computer to store the intermediate data and calculate
the test results.
5 Sampling and specimens
5.1 Test sample
The test sample shall be graded-index glass-core (category A1) multimode fibre.
5.2 Specimen endfaces
Prepare flat endfaces at the input and output ends of the specimen.
5.3 Specimen length
The length of the fibre shall be measured using a suitably accurate method such as that of
IEC 60793-1-22.
5.4 Specimen packaging
Support the test fibre in a manner that relieves tension and minimizes microbending.
5.5 Specimen positioning
Position the input end of the test sample such that it is aligned to the output end of the launch
system as described in 4.3.
Position the output end of the test sample such that it is aligned with the detection system, as
described in 4.4.
6 Procedure
6.1 Adjust and measure system response
Couple the output of the probe fibre into the detection apparatus. This may be accomplished
by mounting the probe fibre in the detection apparatus, or by using a short (<10 m) length of
fibre mounted between the launch system and the detection system, or by directly coupling
the probe output to the detector via a system of lenses and mirrors. If using a short fibre, it
shall be of the same type fibre as the test fibre.
Adjust the amplitude of the optical pulse to match the smallest peak amplitude expected from
the test fibre during the measurement. The smallest peak amplitude from the test fibre will
usually occur for the largest radial offset.
Adjust the time scale of the detection system to match the time scale used in acquiring data
from the test sample to ensure that the entire pulse is captured (see 6.2).
Measure the waveform of the optical pulse, and determine its temporal width at 25 % of the
peak amplitude. This value will be used to calculate the test results, and will be called
ΔT . Linear interpolation may be used between successive time points to calculate
PULSE
ΔT for improved accuracy.
PULSE
60793-1-49  IEC:2006 – 19 –
– Repeated measurements of ΔT shall differ by no more than 5 % of the value of DMD
PULSE
being measured
– If using either a short length of fibre, or a system of lenses and mirrors, the values of
ΔT shall differ by no more than 5 % from the values obtained by coupling the probe
PULSE
fibre directly into the detection apparatus.
– To test and verify that the detector apparatus is not significantly mode dependent, prepare
a special short-length test sample of the same type as the fibre to be tested. Measure the
value of ΔT for each radial offset to be used in the measurement. This value shall
PULSE
meet the requirement of 6.1.
Use Annex A to calculate a value of ΔT appropriate for the values of ΔT , source
REF PULSE
spectral width, and fibre chromatic dispersion.
6.2 Adjust detection system
Launch light from the probe fibre into the test fibre. Adjust the time scale and trigger delay of
the detection system such that one entire optical pulse is displayed for all relevant offsets of
the probe spot, including all leading and trailing edges having amplitude greater than or equal
to 1 % of the peak amplitude. All data from the test fibre shall be obtained without further
adjustment of the delay and time scale.
Find the centre of the core of the test fibre. One method for finding the centre is to scan the
position of the probe spot across the face of the test fibre. Find both edges of the core of the
fibre along some arbitrary “x” axis, with the edge defined as the position for which the total
received power reaches a threshold of about 15 % of maximum. Centre the probe spot along
the “x” axis. Now scan the probe spot along the orthogonal “y” axis, finding the fibre core
edges and centering along the “y” axis. Iterate, as necessary, to achieve the required
positional tolerance. When the probe spot is centred, the DMD will be symmetric between
positive and negative offsets along the “x” or “y” axes. IEC 61280-1-4 also describes another
method for finding the optical centre of the fibre (see 5.4 of IEC 61280-1-4).
6.3 Measure the test sample
Measure the response of the test sample, U(r,t), for radial offsets, r, of the probe spot. For
measurement of DMD, r ranges from R ≤ r ≤ R at intervals of ≤2 µm. R and
INNER OUTER INNER
R shall be provided in the specification (see item 3 in clause 9). Depending on the
OUTER
values specified for R and R , intervals less than 2 µm may be required.
INNER OUTER
Example: If the specification calls for R = 0 and R = 17 µm, the fewest number
INNER OUTER
of radial offsets will be ten. Either (0, 2, …, 16, 17) µm or (0, 1, …, 15, 17) µm
would meet the minimum requirement. Alternatively, one could use 18 offsets at
(0, 1, 2, …, 16, 17) µm.
For EMBc measurements, scan from the optical centre to within 1 µm of the nominal core
radius. Additional radial offsets may be used. For 50 µm core diameter A1a.2 multimode fibre
EMBc measurements, measure U(r,t) over the range 0 ≤ r ≤ 24 µm at intervals of ≤2 µm.
At each radial offset, measure the waveform of the optical pulse, and determine the temporal
position of the leading and trailing edges at 25 % of the maximum amplitude of the resulting
waveform (see Annex B). Linear interpolation may be used between successive time points to
estimate the leading and trailing edge times for improved accuracy. Record the leading and
trailing edge times for each radial offset position.

60793-1-49  IEC:2006 – 21 –
7 Calculations and interpretation of results
The minimum effective modal bandwidth (EMB) of a fibre is the minimum bandwidth
corresponding to excitation from transmitters conforming to defined launch conditions. For
example, the minimum EMB specified in IEC 60793-2-10 is applicable to launch conditions
also specified in IEC 60793-2-10. The minimum EMB is determined by calculating either the
DMD or the minimum calculated EMB (EMBc). The purpose of either calculation is to ensure
that the EMB of the fibre will exceed the requirement for any mode power distribution
consistent with conforming transmitters. The conformance of the transmitters may be defined,
for example, by encircled flux requirements such as those found in IEC 60793-2-10 measured
by IEC 61280-1-4.
7.1 Differential mode delay (DMD)
Find T , the minimum of the leading edge times for excitation between R and
FAST INNER
R from among the output pulses recorded in 6.3.
OUTER
Find T , the maximum of the trailing edge times for excitation between R and
SLOW INNER
R from among the output pulses recorded in 6.3.
OUTER
Calculate DMD:
– Option 1 (See Annex B):
Using the value of ΔT from 6.1, DMD = (T – T ) – ΔT
REF SLOW FAST REF
The lower reporting limit for DMD using this equation is 0,9(ΔT ) because of the
REF
practical measurement problems discussed in Annex B. Consequently, if the value
calculated for DMD using the above equation is less than 0,9(ΔT ), report the result as
REF
being "less than 0,9(ΔT )".
REF
– Option 2
DMD can alternatively be calculated by deconvolving the reference pulse from the pulses
gathered exiting the test fibre. To utilize deconvolution, the algorithm shall not introduce
significant error for the pulse shapes encountered in the measurement, especially arising
from the choice of a high-frequency noise filter.
– Multiple DMD values
A fibre may be characterized by multiple DMD values, with each value evaluated for a
different range of R and R . In this case, all DMD values may be evaluated
INNER OUTER
from among the output pulses recorded in 6.3, provided that the radial offset requirements
of 6.3 are met for each of the ranges of R and R .
INNER OUTER
7.2 Minimum calculated effective modal bandwidth
The minimum EMBc is the minimum value of EMBc determined for a specific fibre using the
full set of weightings corresponding to a range of mode power distributions using the
calculations of 7.2.1 to 7.2.4.
The DMD weightings correspond to the range of mode power distributions consistent with the
launch condition specifications of the optical transmitters utilized in the application. They are
specified by the user’s detailed specification. The user may also specify an additional
multiplier that is used to align EMBc to the theoretical effective modal bandwidth required by
the application. A default set of weightings applicable to, for example, IEEE 802.3 10GBASE-
S and INCITS 364 10GFC is specified in IEC 60793-2-10 and is also included as an example
in Annex D of this document. Annex C provides a procedure for generating DMD weights from
encircled flux data.
60793-1-49  IEC:2006 – 23 –
The following calculations involve the use of weight functions that are derived from near-field
encircled flux data of laser sources that are characteristic of applications. For a given fibre,
the application of several weight functions will yield a number of EMBc values, the minimum
of which is the minimum EMBc for the fibre.
NOTE When DMD data are collected at offsets separated by 2 µm, the U(r,t) values at the intervening 1 µm
offsets may be interpolated for the purpose of these calculations.
7.2.1 Calculate the output pulse
Calculate a resultant output temporal response, P (t) utilizing the fibre output pulse
o
information and a weighting function.
P (t) = W(r)U(r,t) (1)
o ∑
r
where
U ( r ,t ) is the sample output pulse measured at each radial offset r as a function of time t.
Each output pulse is raw (un-normalized in amplitude), after an appropriate
subtraction of baseline noise;
W( r ) is the DMD weighting function corresponding to the transmitter used in the
application (see Annex C for details on calculating W(r), and see Annex D for
example W(r) values corresponding to a particular launch specification)
7.2.2 Calculate the transfer function
Deconvolve the reference temporal response, R(t), from the resultant output response, P (t),
o
in a similar fashion to that done in bandwidth measurements described in IEC 60793-1-41.
This gives the fibre frequency response, H (f ) , also called the fibre transfer function.
Fib
H ( f ) = FT{ P (t )} / FT{R(t )} (2)
Fib O
where:
P (t) is the resultant output pulse from 7.2.1;
o
R (t) is the resultant reference pulse from 6.1;
FT is the Fourier transform function.
NOTE These calculations yield an array of complex numbers.
7.2.3 Calculated effective modal bandwidth (EMBc)
Calculate the –1,5 dB optical bandwidth. It is determined from the lowest frequency where the
magnitude of the transfer function is 1,5 dB down from the zero frequency value. The –1,5 dB
optical value is then extrapolated to –3 dB using Gaussian assumptions by multiplying it by
1,414.
NOTE The bandwidth can be determined by the traditional 3 dB definition (the first point at which the transfer
function, H (f) reaches 50 % or 3 dB). However, highly non-Gaussian responses can be generated using real fibre
Fib
and real sources. For these responses, the measured 3 dB value has been shown not to provide a good correlation
to system performance. The 1,5 dB metric addresses some of the limitations of a wavy transfer function and its
effect on the –3 dB value.
60793-1-49  IEC:2006 – 25 –
7.2.4 System stability frequency limit (SSFL)
FT()R ()t
A
Define G ()f = (3)
ref
FT()R ()t
B
where
R and R are two independent reference pulses;
A B
SSFL is the minimum frequency where |G(f)| exceeds 1,0 ± 0,05 (see 60793-1-41).
If the EMBc calculated for a fibre/laser combination exceeds the SSFL, report the normalized
bandwidth value as greater than SSFL × length.
7.3 Length normalization
It may be desirable to normalize the value of DMD or EMBc to a unit length, such as ps/m
or MHz⋅km. If normalization to a unit length is used, the length dependence formula shall be
reported.
8 Documentation
8.1 Report the following information for each test:
– test sample identification;
– test sample length;
– length normalization formula, if used;
– test date;
– source wavelength (nominal or actual);
– minimum and maximum radial offsets, R , R
INNER OUTER;
– test result: DMD(R , R ) and/or minimum EMBc.
INNER OUTER
8.2 The following information shall be available upon request:
– the measurement method used;
– description of the test equipment, including: source type and actual source centre
wavelength, maximum specified or actual spectral width (r.m.s.);
– for DMD measurement, documentation of method used to calculate ΔT . For minimum
REF
EMBc, the transfer function features that are used to determine bandwidth, and the set of
weightings used;
– detector type and operating conditions;
– mode field diameter of probe fibre at measurement wavelength (nominal or actual);
– method of stripping cladding light;
– date of latest calibration of test equipment.
9 Specification information
When specifying fibre performance using this test method, the following information shall be
specified:
– number and type of samples to be tested;
– test procedure (IEC 60793-1-49);
– test wavelength(s);
60793-1-49  IEC:2006 – 27 –
– DMD requirements: Required DMD value for a stated range of minimum and maximum
radial offsets, DMD(R , R ). Evaluation of several different DMD values for
INNER OUTER
different stated ranges in R and R may be required;
INNER OUTER
– for DMD measurements, reporting method option from 7.1;
– for EMBc requirements: Required minimum EMBc value;
– for EMBc requirements: Required set of weights per Annex C.

60793-1-49  IEC:2006 – 29 –
Annex A
(normative)
Source spectral width limitation

A.1 Limiting the effect of chromatic dispersion on the value of DMD
The effect of errors introduced by chromatic dispersion on the value of DMD shall be less than
10 %. This requirement may be met either by using a source with a spectral width small
enough that chromatic dispersion can be ignored, or by accurately determining the spectral
shape of the source and calculating the appropriate value of ΔT .
REF
The chromatic dispersion D(λ) may be estimated using the data given in Clause A.2.
Alternatively, one may use D(λ) obtained using IEC 60793-1-42 for the particular type of fibre
being tested. The requirement on spectral width may be met either by using a spectrally
narrow source, or by using an optical filter at either the source or detection end.
Several examples of methods for meeting the requirement of this annex are now given.
A.1.1 Use a source with sufficiently narrow spectral width such that the value of
Δt = 4 ⋅ ln(2) ⋅ δλ ⋅ D()λ ⋅ L (A.1)
chrom
is less than 10 % of the DMD to be measured. This gives a constraint on r.m.s. spectral width
δλ,
0,1⋅ DMD DMD
min min
δλ ≤ = 0,030 ⋅ (A.2)
D(λ) ⋅ L
4 ln(2) ⋅ D(λ) ⋅ L
Here, DMD is the smallest value of DMD to be determined, D(λ) is the chromatic
min
dispersion, and L is the sample length. Under the typical assumption that the modal delays of
a fibre scale linearly with length, this constraint has no length dependence.
Use ΔT = ΔT in 6.1. and for calculating the value of DMD.
REF PULSE
Example: DMD values as small as 100 ps are to be tested on fibre lengths of 0,5 km at a
wavelength of 850 nm. From Table A.1 in A.2, the value of D(λ) at 850 nm is
107 ps/nm-km. Substituting this information in equation (A.2), the source r.m.s.
spectral width δλ should be ≤ (0,03 × 100 ps)/(107 ps/nm-km × 0,5 km) =
0,056 nm. The same source would work for a 10 km test length with DMD values
as low as 2 000 ps.
60793-1-49  IEC:2006 – 31 –
A.1.2 Use a source with sufficiently narrow spectral width that ignoring Δt in relation to
chrom
ΔT changes the value of ΔT by less than 10 %. This gives a constraint on r.m.s.
PULSE REF
spectral width δλ,
0,21 ⋅ ΔT ΔT
PULSE PULSE
δλ ≤ = 0,138 ⋅ (A.3)
D(λ) ⋅ L
4 ln(2) ⋅ D(λ) ⋅ L
Use ΔT = ΔT in 6.1, and for calculating the value of DMD.
REF PULSE
In this case, there is no explicit dependence of source spectral width δλ on the value of DMD
being measured. Instead, the minimum value of DMD that can be measured
...