IEC 60793-1-40:2024

IEC 60793-1-40 ®
Edition 3.0 2024-11
REDLINE VERSION
INTERNATIONAL
STANDARD
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Optical fibres –
Part 1-40: Attenuation measurement methods

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IEC 60793-1-40 ®
Edition 3.0 2024-11
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Optical fibres –
Part 1-40: Attenuation measurement methods
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.10 ISBN 978-2-8327-0067-9

– 2 – IEC 60793-1-40:2024 RLV © IEC 2024
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 9
4 Calibration requirements . 9
5 Reference test method . 9
6 Apparatus . 9
7 Sampling and specimens Sample preparation . 9
7.1 SpecimenSample length . 9
7.2 SpecimenSample end face . 9
8 Procedure . 9
9 Calculations . 9
9.1 Methods A and B . 9
9.2 Method C . 10
9.3 Method D . 10
10 Results . 10
10.1 Information available with each measurement . 10
10.2 Information available upon request . 10
10.3 Method-specific additional information . 10
11 Specification information . 10
Annex A (normative) Requirements specific to method A – Cut-back . 11
A.1 General . 11
A.2 Apparatus . 11
A.2.1 General apparatus for all fibres. 11
A.2.2 Launch apparatus for all single-mode fibres . 13
A.2.3 Launch apparatus for A1 multimode fibres . 14
A.2.4 Launch apparatus for A2 to A4 multimode fibres . 16
A.2.5 Calibration requirements . 17
A.3 Procedure . 18
A.4 Calculations . 18
Annex B (normative) Requirements specific to method B – Insertion loss . 19
B.1 General . 19
B.2 Apparatus . 19
B.2.1 General set-ups . 19
B.2.2 Apparatus common to method A (cut-back). 19
B.2.3 Additional apparatus specific to method B (insertion-loss) . 19
B.2.4 Calibration requirements . 19
B.3 Procedure . 19
B.4 Calculations . 20
Annex C (normative) Requirements specific to method C – Backscattering . 21
C.1 General . 21
C.2 Apparatus . 21
C.2.1 General . 21

C.2.2 Optical transmitter . 21
C.2.3 Launch conditions . 22
C.2.4 Optical splitter . 22
C.2.5 Optical receiver . 22
C.2.6 Pulse duration and repetition rate . 22
C.2.7 Signal processor . 22
C.2.8 Display . 22
C.2.9 Data interface (optional) . 23
C.2.10 Reflection controller (optional) . 23
C.2.11 Splices and connectors . 23
C.3 Sampling and specimens . 23
C.4 Procedure . 23
C.4.1 General measurement steps . 23
C.4.2 Further steps for measuring attenuation. 24
C.4.3 Further steps for measuring point discontinuities . 25
C.4.4 Calibration . 27
C.5 Calculations . 27
C.6 Results . 28
Annex D (normative) Requirements specific to method D – Spectral attenuation
modelling . 29
D.1 General . 29
D.2 Apparatus . 29
D.3 Sampling and specimens . 29
D.4 Procedure . 29
D.5 Calculations . 30
D.6 Results . 31
Annex E (informative) Examples of short cable test results on A1 multimode fibres . 32
Bibliography . 34

Figure A.1 – Arrangement of equipment for loss measurement at a specified
wavelength . 11
Figure A.2 – Arrangement of equipment used to obtain loss spectrum . 12
Figure A.3 – General launch arrangement . 12
Figure A.4 – Limited phase space launch optics . 15
Figure A.5 – Two examples of optical fibre scramblers . 16
Figure A.6 – Lens system . 16
Figure A.7 – Launch fibre . 17
Figure A.8 – Mode scrambler (for A.4 fibre) . 17
Figure A.9 – A wide-spectrum source (line "b") could lead to attenuation measurement
errors due to sharp variations on spectral attenuation of polymer-core fibres (line "a") . 18
Figure B.1 – Calibration of insertion loss measurement set . 20
Figure B.2 – Measurement of insertion loss . 20
Figure C.1 – Block diagram of an OTDR . 21
Figure C.2 – Schematic OTDR trace for a "uniform" specimen preceded by a dead-
zone fibre . 24
Figure C.3 – Schematic OTDR trace for a "uniform" specimen not preceded by a dead-
zone fibre . 24

– 4 – IEC 60793-1-40:2024 RLV © IEC 2024
Figure C.4 – Schematic OTDR trace showing apparent loss due to point
discontinuities, one reflective and one non-reflective . 26
Figure C.5 – Schematic of an expanded OTDR trace showing two point discontinuities,
one with apparent gain, and another with no apparent loss or gain . 27
Figure E.1 – Example of attenuation coefficient tests on A1a.1 A1-OM2 fibre . 32
Figure E.2 – Example of attenuation coefficient tests on A1a.3 A1-OM4 fibre . 32
Figure E.3 – Example of attenuation coefficient tests on A1b A1-OM1 fibre . 33

Table A.1 – Size examples . 15
Table A.2 – Launch conditions for A2 to A4 fibres . 16

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-40: Attenuation measurement methods

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition IEC 60793-1-40:2019. A vertical bar appears in the margin
wherever a change has been made. Additions are in green text, deletions are in
strikethrough red text.
– 6 – IEC 60793-1-40:2024 RLV © IEC 2024
IEC 60793-1-40 has been prepared by subcommittee 86A: Fibres and cables, of IEC technical
committee 86: Fibre optics. It is an International Standard.
This third edition cancels and replaces the second edition published in 2019. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) modifying the definition of attenuation to be compatible with the definition in electropedia.org
The text of this International Standard is based on the following documents:
Draft Report on voting
86A/2355/CDV 86A/2446/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 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 webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

OPTICAL FIBRES –
Part 1-40: Attenuation measurement methods

1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the attenuation of
optical fibre, thereby assisting in the inspection of fibres and cables for commercial purposes.
Four methods are described for measuring attenuation, one being that for modelling spectral
attenuation:
– method A: cut-back;
– method B: insertion loss;
– method C: backscattering;
– method D: modelling spectral attenuation.
Methods A to C apply to the measurement of attenuation for all categories of the following fibres:
• class A multimode fibres;
• class B single-mode fibres.
Method C, backscattering, also covers the location, losses and characterization of point
discontinuities.
Method D is applicable only to class B fibres.
Information common to all four methods appears in Clause 1 to Clause 11, and information
pertaining to each individual method appears in Annex A, Annex B, Annex C, and Annex D,
respectively.
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, 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 60793-1-43, Optical fibres – Part 1-43: Measurement methods and test procedures –
Numerical aperture measurement
IEC 61746-1, Calibration of optical time-domain reflectometers (OTDR) – Part 1: OTDR for
single mode fibres
IEC 61746-2, Calibration of optical time-domain reflectometers (OTDR) – Part 2: OTDR for
multimode fibres
– 8 – IEC 60793-1-40:2024 RLV © IEC 2024
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60793-1-1 and the
following 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
3.1.1
attenuation
attenuation of optical power reduction along a fibre at wavelength λ between two cross-sections,
1 and 2, separated by a distance and defined as
P λ
( )
A λ = 10log
( ) (1)
P λ
( )
where
A(λ) is the attenuation, in dB, at wavelength λ;
P (λ) is the optical power traversing the first cross-section;
P (λ) is the optical power traversing the second cross-section.
Note 1 to entry: Attenuation is a measure of the decreasing optical power in a fibre at a given wavelength. It depends
on the nature and length of the fibre and is also affected by measurement conditions.
3.1.2
attenuation coefficient
attenuation per unit length for a uniform fibre under steady-state conditions
Note 1 to entry: It is possible to define the attenuation per unit length or the attenuation coefficient as follows:
A()λ
αλ() = (2)
L
which is independent of the chosen length of the fibre,
where
α(λ) is the attenuation coefficient;
A(λ) is the attenuation at wavelength λ;
L is the length, in kilometres.
Note 2 to entry: Uncontrolled launching conditions normally excite higher order lossy modes that produce transient
losses and result in attenuation that is not proportional to the length of the fibre. A controlled, steady-state launching
condition yields attenuation that is proportional to the fibre's length. Under steady-state conditions, an attenuation
coefficient of a fibre can be determined and the attenuation of concatenated fibres added linearly.
3.1.3
spectral attenuation modelling
technique that predicts the attenuation coefficients across a spectrum of wavelengths from a
small number (three to five) of discrete values measured directly at different wavelengths

3.1.4
point discontinuity
temporary or permanent local deviation of the continuous optical time-domain reflectometer
(OTDR) signal in the upward or downward direction
Note 1 to entry: The nature of the deviation can vary with test conditions (e.g. pulse duration, wavelength, and
direction of the OTDR signal). Although a point discontinuity can have a length greater than the corresponding
displayed pulse duration (including transmitter and receiver effects), the length is usually about equal to the pulse
duration. For a correct interpretation, the guidelines in IEC 60793-1-22 should be followed for measuring length.
3.2 Abbreviated terms
FWHM full width at half maximum
LPS limited phase space
OTDR optical time-domain reflectometer
RMSW root-mean-squared width
RTM reference test method
4 Calibration requirements
See Annex A, Annex B, and Annex C for methods A, B, and C, respectively.
5 Reference test method
Method A, cut-back, is the reference test method (RTM), which shall be the one used to settle
disputes.
6 Apparatus
Annex A, Annex B, Annex C, and Annex D include layout drawings and other equipment
requirements for each of the methods, respectively.
7 Sampling and specimens Sample preparation
7.1 Specimen Sample length
The specimen sample shall be a known length of fibre on a reel, or within a cable, as specified
in the relevant specification.
7.2 Specimen Sample end face
Prepare a flat end face, orthogonal to the fibre axis, at the input and output ends of each
specimen sample.
8 Procedure
See Annex A, Annex B, Annex C, and Annex D for methods A, B, C and D, respectively.
9 Calculations
9.1 Methods A and B
Methods A and B, cut-back and insertion loss use Formula (1) and Formula (2) respectively,
which appear in 3.1.1 and 3.1.2.

– 10 – IEC 60793-1-40:2024 RLV © IEC 2024
9.2 Method C
See Annex C.
9.3 Method D
See Annex D.
10 Results
10.1 Information available with each measurement
Report the following information with each measurement:
– date and title of measurement;
– identification of specimen;
– optical source wavelength;
– specimen length;
– spectral attenuation, in dB, or attenuation coefficient, in dB/km, versus wavelength or at
specific wavelength(s), as required by the relevant specification.
10.2 Information available upon request
The following information shall be available upon request:
– measurement method used: A, B, C, or D;
– type of optical source used: centroidal wavelength(s) and spectral width(s);
– launching technique and conditions used;
– indication if a dead-zone fibre was used (for method C only);
– description of all key equipment;
– for type B fibres – dimensions and number of turns of the mode filter or mode scrambler;
– pulse duration(s), scale range(s), and signal-averaging details;
– details of computation technique (calculation method);
– any deviations to the procedure that were made;
– date of latest calibration of measurement equipment.
10.3 Method-specific additional information
For methods C and D, see the additional requirements in Clause C.6 and Clause D.6,
respectively. This particularly applies when using method C for measuring point discontinuities.
11 Specification information
The relevant specification shall specify the following information:
– type of fibre (or cable) to be measured;
– failure or acceptance criteria at the wavelength or wavelength range;
– any deviations to the procedure that apply;
– information to be reported.
Annex A
(normative)
Requirements specific to method A – Cut-back
A.1 General
The cut-back technique is the only method directly derived from the definition of fibre
attenuation, in which the power levels, P (λ) and P (λ), are measured at two points of the fibre
1 2
without change of input conditions. P (λ) is the power emerging from the end of the fibre, and
P (λ) is the power emerging from a point near the input after cutting the fibre. This explains its
wide acceptance as the reference test method for attenuation.
This measurement principle does not permit information to be obtained on the attenuation
behaviour over the length of the fibre, nor is it easy to measure the change of attenuation under
changing conditions. In some situations, its destructive nature is a disadvantage.
A.2 Apparatus
A.2.1 General apparatus for all fibres
A.2.1.1 General
See Figure A.1 and Figure A.2 for diagrams of suitable test set-ups.

Figure A.1 – Arrangement of equipment for loss measurement at
a specified wavelength
– 12 – IEC 60793-1-40:2024 RLV © IEC 2024

Figure A.2 – Arrangement of equipment used to obtain loss spectrum
A.2.1.2 General launch arrangement
Figure A.3 shows the general launch arrangement used for all fibres. See A.2.2 to A.2.4 for
further details as they apply to specific categories of single-mode and multimode fibres.
A.2.1.3 Optical source
Use a suitable radiation source, such as a lamp, laser or light-emitting diode. The choice of
source depends upon the type of measurement. The source shall be stable in position, intensity
and wavelength over a time period sufficiently long to complete the measurement procedure.
Specify the spectral line width (between the 50 % optical intensity power points of the sources
used) such that the line width is narrow, for example <10 nm, compared with any features of
the fibre spectral attenuation. Align the fibre to the launch cone or connect it to a launch fibre.

Figure A.3 – General launch arrangement
A.2.1.4 Source wavelength
Measurements can be made at one or more wavelengths. Alternatively, a spectral response
can be obtained over a range of wavelengths.

A.2.1.5 Optical detection assembly
Means shall be provided to couple all power emitted from the specimen to the active region of
the detector. For example, an optical lens system, a butt spliced to a fibre pigtail, or a coupling
directly to the detector may can be used. If the detector is already pigtailed, the pigtail fibre
shall have sufficiently large core diameter and numerical aperture to capture all of the light
exiting the reference and specimen fibres.
Use an optical detector that is linear and stable over the range of intensities and measurement
times that are encountered in performing this measurement. A typical system might include a
photovoltaic mode photodiode amplified by a current input amplifier, with synchronous detection
by a lock-in amplifier.
A.2.1.6 Signal processing
It is customary to modulate the light source to improve the signal to noise ratio at the receiver.
If such a procedure is adopted, link the detector to a signal processing system synchronous
with the source modulation frequency. The detecting system should be substantially linear or
have been fully characterized with a response function.
A.2.1.7 Cladding mode stripper
Use suitable techniques to remove optical power propagating in the cladding where this would
significantly influence the received signal.
A.2.2 Launch apparatus for all single-mode fibres
A.2.2.1 General
An optical lens system or fibre pigtail may can be employed to excite the test fibre. The power
coupled into the fibre shall be stable for the duration of the measurement. See Figure A.1.
A.2.2.2 Fibre pigtail
If using a pigtail, it may can be necessary to use index-matching material between the source
pigtail and test fibre to eliminate interference effects.
A.2.2.3 Optical lens system
If using an optical lens system, provide a means of stably supporting the input end of the fibre,
such as a vacuum chuck. Mount this support on a positioning device so that the fibre end can
be repeatedly positioned in the input beam. A method of making the positioning of the fibre less
sensitive is to overfill the fibre end spatially and angularly.
A.2.2.4 High-order mode filter
Use a method to remove high-order propagating modes in the wavelength range of interest. An
example of such a high-order mode filter is a single loop of radius sufficiently small to shift the
cut-off wavelength below the minimum wavelength of interest. For bending loss insensitive
single-mode fibres, multiple loops with smaller radius or longer cut-back specimen length can
be applied. Care should be taken that the radius is not too small as to induce
wavelength-dependent oscillations. Increase of the cut-back specimen length should be
accounted for in the attenuation computation.

– 14 – IEC 60793-1-40:2024 RLV © IEC 2024
A.2.2.5 Cladding mode stripper
The cladding mode stripper ensures that no radiation modes, propagating in the cladding region,
will be detectable after a short distance along the fibre. The cladding mode stripper often
consists of a material having a refractive index equal to or greater than that of the fibre cladding.
This may can be an index-matching fluid applied directly to the uncoated fibre near its ends;
under some circumstances the fibre coating itself will perform this function.
A.2.3 Launch apparatus for A1 multimode fibres
A.2.3.1 General
The launching conditions are of paramount importance in meeting the objectives stated in
Clause 1. Launching conditions are established to avoid launching power into higher-order,
transient modes. By not launching power into these transient modes of the test fibre,
attenuations which add in an approximately linear fashion will be measured. Because these
power distributions are essentially unaltered by the fibre, they are called "steady-state
distributions".
There are two commonly used techniques to produce steady-state launch conditions for
attenuation measurements: mode filters and a geometrical optics launch. Proper care in the use
of each technique gives comparable results.
Care should be takenEnsure that mode distribution is related with specimen length. For short A1
multimode fibre cables (less than 1 km), it is possible that the mode distribution may will not
reach a steady state. This will induce an increase in attenuation values towards shorter fibre
lengths, where the magnitude of the length dependence depends on fibre type, launch condition,
etc. In these cases, attenuation values should be obtained from cables long enough to reach a
steady-state condition, or they can be taken from the original longer donor cable. As guidance
for sufficient cable lengths, see examples of cable test results on A1 multimode fibres in
Annex E.
See Figure A.3 for a generic example of the launching arrangement using a mode filter.
Examples of each mode filter appear below.
A.2.3.2 Examples of mode filters
A.2.3.2.1 Dummy-fibre mode filter
Select a fibre of a similar type to that of the test fibre. The fibre should be long enough (typically
equal to or greater than 1 km) so that the power distribution carried by the fibre, when the
launch source of A.2.1.2 is used, is a steady-state distribution.
A.2.3.2.2 Mandrel-wrapped mode filter
Another mode filter takes the form of a mandrel around which a few turns (typically three to five
turns) of the fibre under test are wound with low tension. Select the mandrel diameter to ensure
that the transient modes excited in the test fibre have been attenuated to steady-state. Use a
far-field measurement to compare the power distribution exiting a long length of test fibre
(greater than 1 km) that has been excited with a uniformly overfilling source, with the power
distribution exiting a short length of the fibre with the mandrel applied. Select the mandrel
diameter to produce a far-field distribution in the short length that approximates the long length
far-field power distribution.
The numerical aperture (as measured by IEC 60793-1-43) of the radiation pattern exiting the
short length shall be 94 % to 100 % of the numerical aperture of the long-length pattern.

The diameter of the mandrel may can differ from fibre to fibre depending on fibre and coating
type. Common prescriptions consist of diameters in the range of 15 mm to 40 mm, with five
turns of fibre within a 20 mm length of the mandrel. While mandrels of different size and
arrangement can be selected, Table A.1 illustrates common mandrel sizes for fibres of different
core diameters.
Table A.1 – Size examples
Core diameter Mandrel diameter
µm mm
50 25
62,5 20
100 25
A.2.3.3 Example of geometrical optics launch
A limited phase space (LPS) launch is defined as a geometrically produced launch that
uniformly fills 70 % of the test fibre's core diameter and 70 % of the test fibre's numerical
aperture. This is the maximum geometrically launched power distribution that does not launch
power into leaky, unbounded modes. For a 50/125 µm, 0,2 NA graded-index multimode fibre,
the LPS launch condition consists of a uniform 35 µm spot and 0,14 NA.
An example of the optics necessary to produce the LPS launch is given in Figure A.4. It is
important to ensure that the axis of the launch beam is coincident with the axis of the fibre so
that the spot and incident cone of light are centred on the core of the fibre. Also, set up the
optical system at the wavelengths of operation to ensure proper measurement. While mandrels
of different size and arrangement can be selected, common mandrel sizes for fibres of different
core diameters, are shown in Table A.1.

Figure A.4 – Limited phase space launch optics
A.2.3.4 Mode scrambler
An essentially uniform power distribution is launched prior to the mode filter. For a source such
as an LED or laser, which does not form a uniform power distribution, use a mode scrambler.
The mode scrambler shall comprise a suitable fibre arrangement (for example, a step-graded-
step index profile sequence).
A "mode scrambler" is a device which is positioned between the light source and test fibre to
control launching conditions. A particular mode scrambler design is not specified. It should be
emphasized that the performance of these scramblers depends upon the launch optics and fibre
sizes (core and NA) used in the actual construction.

– 16 – IEC 60793-1-40:2024 RLV © IEC 2024
EXAMPLE The two designs given in Figure A.5 are for illustration purposes only.

a)
b)
Figure A.5 – Two examples of optical fibre scramblers
A.2.4 Launch apparatus for A2 to A4 multimode fibres
Some examples of generic launching arrangements for short-distance fibres are described in
Figure A.6, Figure A.7 and Figure A.8.
The reproducibility of the attenuation measurements of multimode fibres is critical. Therefore,
a well-defined launching set-up description is necessary. Such a set-up can be achieved by
using commercially available optical components and shall be capable of providing for spot
sizes and launch NAs as given in Table A.2.
Table A.2 – Launch conditions for A2 to A4 fibres
Fibre category
a
Attribute A3 fibre A4 fibre
A2.2 fibre
Glass core/plastic cladding Plastic core/plastic cladding
Glass core/glass cladding
Spot size = fibre core size = fibre core size = fibre core size with full
mode launch (or use mode
scrambler with equilibrium
mode launch)
b c
Numerical aperture = fibre max. NA, with full
= fibre max. Na = fibre max. NA
c
(NA)
mode launch
a
Category A2.1 fibre requires further study.
b
This launch condition can be produced by overfilling a mode filter made from 2 m of fibre identical to the fibre
under test, with appropriate cladding mode stripping and using the output from this mode filter to launch into
the fibre under test.
c
This launch condition can be produced in the same manner as described in footnote b. However, some types
of A3 and A4 fibre will not require cladding mode stripping for the mode filter.

Figure A.6 – Lens system
Figure A.7 – Launch fibre
Figure A.8 – Mode scrambler (for A.4 fibre)
A.2.5 Calibration requirements
A.2.5.1 General calibration requirements
Calibrate the optical source's centroidal wavelength to within ±10 nm.
A.2.5.2 Requirements for A4 fibres
For A4 fibres it is common to perform attenuation measurements at specific wavelengths using
an LED as optical source. Owing to characteristic strong sharp variations in attenuation over
the wavelength spectrum of some polymeric materials, additional optical characterization
measurements should be performed to take into account effects that could affect the
measurement when calibrating wide-spectrum sources used for attenuation measurement,
especially when the centroidal wavelength is significantly far from the intended wavelength
measurement. A full characterization will ensure repeatability of the measurements and avoid
the negative influence of the following effects:
– Distortion on the attenuation measurement
An optical source with wide spectrum, for example, an LED, will cause measurement errors
on the measurements, since parts of the optical spectrum lie in low-loss wavelengths and
other parts lie in higher-loss wavelengths. This is illustrated in Figure A.9 with the Gaussian
line "b" showing the spectral response for an LED source used to measure A4 fibres and
with the expected spectral attenuation indicated by the line "a". To take proper consideration
of the potentially high attenuation variations, the source shall be calibrated both in its
centroidal wavelength and spectral width and it should be checked that these two
characteristics match the expected wavelength attenuation of the fibre under test.
– Spectral filter effect
Light with a wide spectrum undergoes relatively little attenuation at some wavelengths while
other spectral parts suffer higher losses when propagating through A4a fibres. With longer
measured fibre lengths, the detected LED spectral maximum shifts towards the fibre
attenuation-minimum wavelength. This can be seen in Figure A.9, where the original
spectral source is illustrated with the line "b" (characterized through a 0 m fibre length) and
the same spectra detected after passing different lengths of an A4 fibre. As the
measurement-fibre length increases, a shift on the maximum of the detected Gaussian
signal occurs towards the wavelength of minimum attenuation of the fibre (lines "c" to "f" in
Figure A.9).
– 18 – IEC 60793-1-40:2024 RLV © IEC 2024

Figure A.9 – A wide-spectrum source (line "b") could lead to attenuation measurement
errors due to sharp variations on spectral attenuation of polymer-core fibres (line "a")
A.3 Procedure
A.3.1 Set the fibre under test in the measurement apparatus. Record the output power, P (λ).
A.3.2 Keeping the launching conditions fixed, cut the fibre to the
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