IEC 60793-1-49:2018

IEC 60793-1-49 ®
Edition 3.0 2018-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Optical fibres –
Part 1-49: Measurement methods and test procedures – Differential mode delay

Fibres optiques –
Partie 1-49: Méthodes de mesure et procédures d'essai – Retard différentiel de
mode
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IEC 60793-1-49 ®
Edition 3.0 2018-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Optical fibres –
Part 1-49: Measurement methods and test procedures – Differential mode delay

Fibres optiques –
Partie 1-49: Méthodes de mesure et procédures d'essai – Retard différentiel de

mode
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.10 ISBN 978-2-8322-5954-2

– 2 – IEC 60793-1-49:2018 © IEC 2018
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Apparatus . 8
4.1 Overview. 8
4.2 Optical source . 9
4.3 Probe fibre . 10
4.4 Scanning stage . 10
4.5 Probe to test sample coupling . 10
4.6 Cladding mode stripper . 10
4.7 Detection system . 10
4.8 Sampler and digitizer . 11
4.9 Computational equipment . 11
4.10 System performance . 11
4.10.1 General . 11
4.10.2 Pulse temporal stability . 12
4.10.3 System stability frequency limit (SSFL) . 12
5 Sampling and specimens . 13
5.1 Test sample . 13
5.2 Specimen end-faces . 13
5.3 Specimen length . 13
5.4 Specimen deployment . 13
5.5 Specimen positioning . 13
6 Procedure . 13
6.1 Fibre coupling and system setup . 13
6.2 Determination of centre . 14
6.3 Measurement of the test sample . 14
6.3.1 Selection of radii and quadrant . 14
6.3.2 Collection of scan data . 14
6.4 Determination of ∆T and ∆T . 14
PULSE REF
6.5 Reference test method . 14
7 Calculations and interpretation of results . 15
7.1 General . 15
7.2 Differential mode delay (DMD) . 15
7.2.1 General . 15
7.2.2 Deconvolution . 15
7.2.3 Pulse folding . 15
7.2.4 Determination of DMD . 16
7.3 Minimum calculated effective modal bandwidth . 17
7.3.1 General . 17
7.3.2 Time domain pulse computation . 17
7.3.3 Calculate the transfer function . 18
7.3.4 Compute the power spectrum . 18
7.3.5 Compute EMB and minEMB . 18
c c
7.4 Length normalization . 18

8 Documentation . 18
8.1 Information to be reported . 18
8.2 Information available upon request . 19
9 Specification information . 19
Annex A (normative) Source spectral width limitation . 20
A.1 Limiting the effect of chromatic dispersion (CD) on the value of DMD . 20
A.1.1 General . 20
A.1.2 Limit CD contribution to DMD to be measured . 20
A.1.3 Limit CD contribution to reference width . 20
A.1.4 Adjust ∆T to account for CD contribution . 21
REF
A.1.5 High-performance DMD fibres and spectral requirements . 21
A.2 Chromatic dispersion in multimode fibres . 22
Annex B (informative) Determination of fibre optical centre . 23
B.1 General . 23
B.2 Method . 23
Annex C (normative) Detection system modal measurement . 26
C.1 General . 26
C.2 Determination of coupling function . 26
C.2.1 Overview . 26
C.2.2 Fibre sample and coupling . 26
C.2.3 Detector response . 26
C.2.4 Reference response . 27
C.2.5 Coupling function determination . 28
Annex D (informative) Discussion of measurement details . 29
D.1 DMD . 29
D.2 EMB calculation . 30
c
Annex E (informative) Determining DMD weights for EMB calculation . 33
c
E.1 Selecting a set of weightings . 33
E.2 Procedure for generating DMD weightings given encircled flux data . 33
Annex F (informative) EMB calculation information . 35
c
F.1 Default DMD weightings for transmitters conforming to IEC 60793-2-10 . 35
F.2 Example method to determine if an adjusted bandwidth (BW) metric is
adequate. 36
Bibliography . 38
Figure 1 – Example apparatus . 9
Figure B.1 – Typical area data from centring waveforms . 24
Figure D.1 – Idealized DMD data . 29
Table A.1 – Worst-case chromatic dispersion. 22
Table C.1 – Theoretical normalized coupling efficiency . 27
Table F.1 – DMD weightings . 35
Table F.2 – DMD weightings . 36

– 4 – IEC 60793-1-49:2018 © IEC 2018
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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60793-1-49 has been prepared by subcommittee 86A: Fibres and
cables, of IEC technical committee 86: Fibre optics.
This third edition cancels and replaces the second edition published in 2006. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) better alignment with original intent by filling some omissions and therefore improving
measurement rigor;
b) the measurement of fibres with smaller differential mode delay (and higher modal
bandwidth) such as type A1a.3 fibres of IEC 60793-2-10 [1] that are used in constructing
OM4 performance category cables; new requirements on specifying detector amplitude
and temporal response, specimen deployment conditions, four-quadrant scanning, and
uniformity of radial locations for calculating bandwidth.
The text of this International Standard is based on the following documents:
CDV Report on voting
86A/1812/CDV 86A/1860/RVC
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
This International Standard is to be used in conjunction with IEC 60793-1-1:2017.
A list of all parts in the IEC 60793 series, published under the general title Optical fibres, 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 "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
___________
Numbers in square brackets refer to the Bibliography.

– 6 – IEC 60793-1-49:2018 © IEC 2018
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 document 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 range of launch conditions, for example,
those produced by standardized laser transmitters.
With this method, the output from a probe fibre that is single-moded at the test wavelength
excites the multimode fibre under test. The probe spot is scanned across the end-face of the
fibre under test at specified radial positions, and a set of response pulses are acquired at
these positions.
Three specifiable parameters can be derived from the collected set of data.
• The first parameter, differential modal delay (DMD), is the difference in optical pulse delay
time between the fastest and slowest mode groups of the fibre under test. DMD
specifications place limits on modal delay over a specified range of probe fibre radial
offset positions. DMD specifications are determined by modeling and experimentation to
correspond to a minimum effective modal bandwidth (EMB) for the expected range of
transmitters used in a link at a given performance level.
• The second specifiable parameter is derived by combining the pulses using sets of
specific radial weights to determine an approximation of a set of pulses from typical
transmitters. Using Fourier transforms, the calculated effective modal bandwidth (EMB ) is
c
values (minEMB ) is the
determined for each weight set. The minimum of these EMB
c c
specifiable parameter.
• The third specifiable parameter, the computed overfilled launch bandwidth, OMB , is
c
determined in a manner similar to EMB , but by applying just one weight set to the set of
c
pulses; this weight set corresponds to the overfilling condition, where all mode groups are
equally excited.
The test's intent is to quantify the effects of interactions of the fibre modal structure and the
source modal characteristics excluding the source's spectral interaction with fibre chromatic
dispersion. Adding the effects of fibre chromatic dispersion and the source spectral
characteristics will reduce the overall transmission bandwidth, but this is a separate
calculation in most transmission models. In this test, the contribution of chromatic dispersion
is controlled by limiting the spectral width of usable test sources. Practical test sources will
have non-zero spectral width and will thus slightly distort the DMD, minEMB and OMB
c c
values. These chromatic dispersion effects are considered in Annex A.
NOTE Comparison between IEC 60793-1-49 and ITU recommendations: ITU-T Recommendation G.650.1 [2]
contains no information on how to measure the DMD of a graded-index multimode fibre.
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 60793-1-1:2017, Optical fibres – Part 1-1: 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 60793-1-45, Optical fibres – Part 1-45: Measurement methods and test procedures –
Mode field diameter
IEC 60825-1, Safety of laser products – Part 1: Equipment classification and requirements
IEC 60825-2, Safety of laser products – Part 2: Safety of optical fibre communication systems
(OFCS)
IEC 61280-1-4, Fibre optic communication subsystem test procedures – Part 1-4: General
communication subsystems – Light source encircled flux measurement method
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
maximum DMD
maximum DMD occurring between the outer (R ) and inner (R ) limits of radial offset
OUTER INNER
position over which the probe spot is scanned for one or more sets of R and R
OUTER INNER
3.2
minimum EMB
c
minEMB
c
minimum EMB among the EMB values calculated from a sequence of DMD weightings
c c
Note 1 to entry: The user of this document may also specify the calculated overfilled modal bandwidth (OMB ).
c
3.3
differential mode delay
DMD
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
Note 1 to entry: This note applies to the French language only.

– 8 – IEC 60793-1-49:2018 © IEC 2018
3.4
effective modal bandwidth
EMB
bandwidth associated with the transfer function, H(f), of a particular laser/fibre combination
Note 1 to entry: This note applies to the French language only.
3.5
calculated overfilled modal bandwidth
OMB
c
bandwidth associated with the transfer function, H(f), when the fibre is overfilled
3.6
quadrant
radial section at one of four possible azimuthal angles over which a radial set of pulse data
can be collected
Note 1 to entry: For example, a radial section may be taken from one of the sets x-positive, x-negative, y-positive
or y-negative.
3.7
mode field diameter
MFD
diameter of the mode emanating from the end-face of a single-mode fibre, as determined by
IEC 60793-1-45
Note 1 to entry: This note applies to the French language only.
3.8
reference test method
RTM
test method in which a given characteristic of a specified class of optical fibres or optical
cables (and associated components) is measured strictly according to the definition of this
characteristic, and which gives results that are accurate, reproducible and relatable to
practical use
Note 1 to entry: This note applies to the French language only.
3.9
full width quarter maximum
FWQM
full width at 25 % of maximum amplitude of an optical pulse
Note 1 to entry: This note applies to the French language only.

4 Apparatus
4.1 Overview
The apparatus shall provide a means to inject and detect short-duration pulses of light of a
small spot size launched into known locations of the core of the multimode fibre to be
measured. An example is diagrammed in Figure 1.

Sampler
digitizer
Optical
source
Processor
input trigger
Launch
fibre
Fibre
under
test
Detection
system
IEC
Figure 1 – Example apparatus
4.2 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 (FWQM), 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, so the
maximum allowable FWQM of the probe pulse is ~110 ps. Testing to the same DMD limit in a
10 000 m length of fibre requires measuring a DMD of 2 000 ps, therefore a pulse as wide as
~2 200 ps may be used. Detailed limits are given in 6.1, and may depend on the source
spectral width.
Source spectral width shall meet the requirements of Annex A. Chromatic dispersion induced
broadening resulting from source spectral width is limited through the methodologies
described 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. This requirement is challenging when measuring the highest
performance fibres (whose DMD can be as low as 0,1 ps/m). In these circumstances, the
pulse source's spectrum may be transform limited, in which case, no improvement can be
made.
The source centre wavelength requirement is given by the product specification documents,
which may require measurements at more than one wavelength. Each wavelength is
considered in this document as a single measurement (if no default wavelength specification
is defined for the product to be measured, the default of 850 nm ± 10 nm shall be used).
A mode-locked titanium-sapphire laser, pulsed semiconductor laser or mode-locked fibre laser
are examples of sources suitable for this application.
Reference shall be made to IEC 60825-1 and to IEC 60825-2 for an explanation of the safe
usage of these sources.
– 10 – IEC 60793-1-49:2018 © IEC 2018
4.3 Probe fibre
The optical source shall be coupled to a fibre which is single-moded at the wavelength of
measurement. It is required that this fibre be nominally of step-index design, and so shall
have a mode field diameter (MFD) satisfying the following equation:
(1)
MFD= (8,7λ− 2,39)± 0,5μm
where
λ is the source wavelength in µm;
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-moded by limiting the ability for higher-order
modes to propagate. Winding the probe fibre around a mandrel of a given diameter is an
example mode control device; a common example is three turns around a 25 mm diameter
mandrel.
4.4 Scanning stage
Either the probe fibre or the test sample shall be mounted to a scanning stage capable of
scanning the test sample relative to the probe fibre over the entire diameter of the test
sample's core in both x- and y-direction. The scanning stage x- and y-actuators shall be
capable of positioning the probe fibre to within 0,5 µm of the desired position. Often, the
scanning stage is used to adjust the gap between the probe fibre and the test sample's
end-face or, when an optical system is employed, to focus the probe spot image onto the
sample end-face.
The probe fibre output beam's angle of propagation shall be aligned with the test sample's
axis of propagation to within 1°.
The apparatus shall employ algorithms to reproducibly centre (with respect to the test
sample's core) the output spot of the probe fibre to within ±1,0 µm. Refer to Annex B for a
discussion of end-face centring.
4.5 Probe to test sample coupling
If directly coupled to the test sample, the gap between the output end of the probe fibre and
the end-face of the test sample shall be no more than 10 µm. Alternatively, a free-space
optics system of lenses or mirrors may be used to image the output spot of the probe fibre
onto the end-face of the test sample. When optics are employed, it is required that at each
radial scan position of the measurement, 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.
4.6 Cladding mode stripper
A cladding mode stripper provides 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.
4.7 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.

Amplifiers and signal conditioning electronics may be employed, but typically biased PIN
photodetectors or avalanche-mode photodiodes are employed which are coupled directly to
the electronics of the sampling system. The detector, in combination with any amplifiers and
other electronics, shall have a combined amplitude nonlinearity no greater than 5 % over the
expected range of signals. Ringing of the detector system shall be limited such that maximum
overshoot or undershoot be less than 5 % of the peak amplitude of the detected optical signal
as measured on the reference pulse.
These detection systems may exhibit modally dependent amplitude responses. The
determination of DMD depends little on this modal response error since each pulse's relative
amplitude is used to determine the location of its leading and training edges. However, the
determination of EMB and OMB (defined in IEC 60793-1-41) rely on the pulse amplitudes in
c c
relation to all the pulses in the data set, so a modally dependent detector can distort these
measurements. Annex C describes a method for qualifying modally dependent detectors by
scanning the detector's spatial uniformity and computing a coupling function, C(r). It is
required that the detector's coupling function satisfy 0,9 ≤ C(r) ≤ 1,1 over the range of radii to
be measured.
4.8 Sampler and digitizer
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 an integral part of the recording instrument or it may
be an external device.
When averaging is employed to improve the signal-to-noise ratio (SNR) of the measurement,
pulse-to-trigger jitter or wander statistics may affect the measurement in various ways. For
example, if significant high-frequency jitter is present, averaging several pulses will effectively
widen the probe pulse, and the jitter statistics may be dependent on the amount of delay
employed; the averaging scheme shall be consistent when measuring the reference pulse and
the scanned pulses so that the effective reference pulse remains constant. Some delay
systems have difficult jitter statistics and attention shall be paid to these effects to ensure
good measurements.
4.9 Computational equipment
This test method generally requires a computer to automate the procedure, including the
control of the scanning stage and waveform acquisition, storage of the intermediate data and
calculation of the results.
4.10 System performance
4.10.1 General
The stability of the system in both the temporal and frequency domains is critical to ensure
valid, repeatable measurements. Subclause 4.10 defines a characterization process that shall
be performed when a measurement system is commissioned, serviced and checked at regular
intervals to ensure the measurement system is performing as expected. Both tests begin by
coupling the output of the probe fibre into the detection apparatus and adjusting the signal
level and time base. Each shares the following common steps:
• Couple the pulse directly into the detection apparatus using one of these three methods:
– the probe fibre can be coupled directly to the detection apparatus.

– 12 – IEC 60793-1-49:2018 © IEC 2018
– the probe fibre's output can be coupled by using a short length of fibre (< 10 m of the
same type fibre as the test fibre) mounted between the launch system and the
detection system.
– the probe output can be coupled to the detector via a system of lenses and mirrors.
• Adjust the amplitude of the optical pulse to ensure good SNR without causing signal
compression in the receiver.
• Adjust the sampling window of the detection system to match the smallest time window
used to acquire data from the expected range of test samples. Ensure the entire pulse is
captured and the Nyquist limit is not violated.
These characterizations test the system stability to ensure the system's suitability for
measurement. Each characterization should be performed over a time interval, T, which
should be no shorter than the time required to perform a four-quadrant measurement at 1 µm
intervals at maximum averaging. Over T, several reference pulse waveforms are acquired,
and, computing intermediate results, the particular stability parameter is characterized. The
number of waveforms taken over the time interval, T, should be approximately the number of
waveforms in a four-quadrant scan of 1 µm intervals (i.e., 102 samples for a 50 µm fibre).
4.10.2 Pulse temporal stability
This test characterizes the system's temporal limits and stability.
At each time t, record the 25 % width (FWQM) using linear interpolation to improve precision.
Record FWQM as a function of time over T. Determine the temporal stability parameter,
∆FWQM :
stab
FWQM − FWQM
MAX MIN
∆FWQM 100×
stab
FWQM
(2)
where
is the average FWQM over the interval.
FWQM
∆FWQM shall be less than 5 % to satisfy the temporal stability requirement. If ∆FWQM
stab stab
lies outside this range, the system is disqualified.
4.10.3 System stability frequency limit (SSFL)
Define
( ())
FT R t
G ( f)=
ref
FT(R (t))
(3)
where
R is a reference pulse taken at the beginning of the characterization;
R is any subsequent reference pulse;
FT means Fourier transform.
At each t, record R and compute G and then, for that time t, record F (t) as the lowest
ref MAX
frequency where |G(f)| exceeds 1,0 ± 0,05. Over the complete interval, record the minimum of
(t) as the system's SSFL.
the set of F
MAX
=
Both R and R should be acquired with enough averaging to reduce the noise of the ratio to be
less than 1 % over the frequency range of interest.
If the calculated minEMB or OMB for a fibre/laser combination exceeds the SSFL, report the
c c
normalized bandwidth value as greater than SSFL multiplied by the length.
5 Sampling and specimens
5.1 Test sample
The test sample shall be graded-index glass-core (category A1) multimode fibre.
5.2 Specimen end-faces
Prepare flat end-faces at the input and output ends of the specimen. The quality of the input
end-face is critical; they shall have end angles no greater than 1,5°.
5.3 Specimen length
The length of the fibre shall be measured using IEC 60793-1-22. The length of the sample
shall be known to ±1 %. To resolve disputes, the reference test length shall be specified by
the product specification.
5.4 Specimen deployment
Support the test fibre in a manner that relieves tension and minimizes microbending, such as
on a measurement spool having a minimum radius of 150 mm that imparts less than 5 g of
fibre tension. Deployment shall not impart macrobends of radius less than 40 mm.
The thermal stability of the specimen shall meet the required measurement precision. This
requirement can be quite demanding for high performance fibres. The thermal coefficient of
optical transit time for these fibres is approximately 0,035 ps/m·K. If the sample undergoes a
3 K temperature change during the time of measurement, the error will be 0,1 ps/m, which
subsumes the entire specification for high-performance fibres.
5.5 Specimen positioning
Position the input end of the test sample such that it is aligned to the output end of the probe
fibre 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 6.2 (careful centring is part of the measurement procedure below).
6 Procedure
6.1 Fibre coupling and system setup
Launch the light from the probe fibre into the test fibre. Adjust the time scale and trigger delay
of the detection system such that, for all relevant radial offsets of the probe spot, the pulses
are completely contained inside the digitisation window ("contained" means that all leading
and trailing edges having amplitude greater than or equal to 1 % of the peak amplitude are
inside the window). All data from the test fibre shall be obtained without further adjustment of
the delay and time scale. The reference pulse acquisition may use a different amount of
delay, but shall use the same time scale.

– 14 – IEC 60793-1-49:2018 © IEC 2018
6.2 Determination of centre
Find the centre of the optical axis of the test fibre with respect to the probe fibre. Refer to
Annex B for suggested approaches to determine the fibre's centre.
6.3 Measurement of the test sample
6.3.1 Selection of radii and quadrant
The selection of radii and quadrant is determined by two considerations: measurement
efficiency and how the data set is to be used. When DMD masks are applied to the data, it is
required that the endpoint radii of all masks be included in the radial points scanned. In no
case, for either DMD or EMB , can the radial spacing be greater than 2 µm between adjacent
c
radii. The reference test method (see 6.5) requires the radial spacing be nominally 1 µm.
One or more quadrants may be scanned. The reference test method (see
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