IEC 60793-1-41:2010

IEC 60793-1-41 ®
Edition 3.0 2010-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical fibres –
Part 1-41: Measurement methods and test procedures – Bandwidth

Fibres optiques –
Partie 1-41: Méthodes de mesure et procédures d'essai – Largeur de bande

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IEC 60793-1-41 ®
Edition 3.0 2010-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical fibres –
Part 1-41: Measurement methods and test procedures – Bandwidth

Fibres optiques –
Partie 1-41: Méthodes de mesure et procédures d'essai – Largeur de bande

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
U
CODE PRIX
ICS 33.180.10 ISBN 978-2-88912-170-0
– 2 – 60793-1-41 © IEC:2010
CONTENTS
FOREWORD.4
1 Scope.6
2 Normative references .6
3 Terms and definitions .7
4 Apparatus.7
4.1 Radiation source .7
4.1.1 Method A – Time domain (pulse distortion) measurement.7
4.1.2 Method B – Frequency domain measurement .8
4.1.3 Method C – Overfilled launch modal bandwidth calculated from
differential mode delay (OMBc) .8
4.1.4 For methods A and B.8
4.2 Launch system .8
4.2.1 Overfilled launch (OFL) .8
4.2.2 Restricted mode launch (RML) .9
4.2.3 Differential mode delay (DMD) launch .10
4.3 Detection system.10
4.4 Recording system.10
4.5 Computational equipment.11
4.6 Overall system performance .11
5 Sampling and specimens.11
5.1 Test sample .11
5.2 Reference sample .11
5.3 End face preparation .11
5.4 Test sample packaging.12
5.5 Test sample positioning.12
6 Procedure .12
6.1 Method A – Time domain (pulse distortion) measurement.12
6.1.1 Output pulse measurement.12
6.1.2 Input pulse measurement method A-1: reference sample from test
sample .12
6.1.3 Input pulse measurement method A-2: periodic reference sample .12
6.2 Method B – Frequency domain measurement .13
6.2.1 Output frequency response.13
6.2.2 Method B-1: Reference length from test specimen.13
6.2.3 Method B-2: Reference length from similar fibre .13
6.3 Method C – Overfilled launch modal bandwidth calculated from differential
mode delay (OMBc).13
7 Calculations or interpretation of results .14
7.1 -3 dB frequency, f .14
3 dB
7.2 Calculations for optional reporting methods .15
8 Length normalization .15
9 Results .15
9.1 Information to be provided with each measurement .15
9.2 Information available upon request.15
10 Specification information .16
Annex A (normative) Intramodal dispersion factor and the normalized intermodal
dispersion limit.17

60793-1-41 © IEC:2010 – 3 –
Annex B (normative) Fibre transfer function, H(f), power spectrum, |H(f)|, and f .20
3 dB
Annex C (normative) Calculations for other reporting methods.22
Annex D (normative) Mode scrambler requirements for overfilled launching conditions
to multimode fibres .23
Bibliography.28

Figure 1 – Mandrel wrapped mode filter .10
Figure D.1 – Two examples of optical fibre scramblers .24

Table 1 – DMD weights for calculating overfilled modal bandwidth (OMBc) from DMD
data for 850 nm only .14
Table A.1 – Highest expected dispersion for commercially available A1 fibres .17

– 4 – 60793-1-41 © IEC:2010
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-41: Measurement methods and test procedures –
Bandwidth
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 60793-1-41 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 2003. This edition
constitutes a technical revision.
The main change with respect to the previous edition is the addition of a third method for
determining modal bandwidth based on DMD data and to improve measurement procedures
for A4 fibres.
This standard should be read in conjunction with IEC 60793-1-1 and IEC 60793-1-2, which
cover generic specifications.
60793-1-41 © IEC:2010 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
86A/1294/CDV 86A/1329/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.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60793-1-4x series, published under the general title Optical fibres
– measurement methods and test procedures, can be found on the IEC website
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – 60793-1-41 © IEC:2010
OPTICAL FIBRES –
Part 1-41: Measurement methods and test procedures –
Bandwidth
1 Scope
This part of IEC 60793 describes three methods for determining and measuring the modal
bandwidth of multimode optical fibres (see IEC 60793-2-10, IEC 60793-30 series and
IEC 60793-40 series). The baseband frequency response is directly measured in the
frequency domain by determining the fibre response to a sinusoidaly modulated light source.
The baseband response can also be measured by observing the broadening of a narrow pulse
of light. The calculated response is determined using differential mode delay (DMD) data. The
three methods are:
• Method A – Time domain (pulse distortion) measurement
• Method B – Frequency-domain measurement
• Method C – Overfilled launch modal bandwidth calculated from differential mode delay
(OMBc)
Methods A and B can be performed using one of two launches: an overfilled launch (OFL)
condition or a restricted mode launch (RML) condition. Method C is only defined for A1a.2
(and A1a.3 in preparation) multimode fibre and uses a weighted summation of DMD launch
responses with the weights corresponding to an overfilled launch condition. The relevant test
method and launch condition should be chosen according to the type of fibre.
NOTE 1 These test methods are commonly used in production and research facilities and are not easily
accomplished in the field.
NOTE 2 OFL has been used for the modal bandwidth value for LED-based applications for many years. However,
no single launch condition is representative of the laser (e.g. VCSEL) sources that are used for gigabit and higher
rate transmission. This fact drove the development of IEC 60793-1-49 for determining the effective modal
bandwidth of laser optimized 50 μm fibres. See IEC 60793-2-10:2004 or later and IEC 61280-4-1:2003 or later for
more information.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60793-1-20, Optical Fibres – Part 1-20: Measurement methods and test procedures –
Fibre geometry
IEC 60793-1-42, Optical fibres – Part 1-42: Measurement methods and test procedures –
Chromatic dispersion
IEC 60793-1-43, Optical fibres – Part 1-43: Measurement methods and test procedures –
Numerical aperture
IEC 60793-1-49:2006, Optical fibres – Part 1-49: Measurement methods and test procedures
– Differential mode delay
60793-1-41 © IEC:2010 – 7 –
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
bandwidth (–3 dB)
value numerically equal to the lowest modulation frequency at which the magnitude of the
baseband transfer function of an optical fibre decreases to a specified fraction, generally to
one half, of the zero frequency value. The bandwidth is denoted in this document as f .
3 dB
NOTE It is known that there can be various calculations, sometimes called markdowns, to avoid reporting
extremely high values associated with “plateaus”. For example the 1,5 dB frequency, multiplied by is one
treatment used in IEC 60793-1-49. If such a calculation is used it should clearly be reported.
3.2
transfer function
discrete function of complex numbers, dependent on frequency, representing the frequency-
domain response of the fibre under test
NOTE Method A determines the frequency response by processing time domain data through Fourier transforms.
Method B can only measure the transfer function if an instrument which measures phase as well as amplitude is
used. Method C is similar to Method A as it uses Fourier transforms in a similar manner. The transfer Function is
denoted in this document as H(f).
3.3
power spectrum
discrete function of real numbers, dependent on frequency, representing the amplitude of the
frequency-domain response of the fibre under test
NOTE Methods A and C determine the power spectrum from the transfer function. Method B determines the
transfer function by taking the ratio of the amplitude measured through the fibre under test and the reference. The
power spectrum is denoted in this document as |H(f)|.
3.4
impulse response
discrete function of real numbers, dependent on time, representing the time-domain response
of the fibre under test to a perfect impulse stimulus. The impulse response is derived, in all
methods, through the inverse Fourier transform of the transfer function. The impulse response
is denoted in this document as h(t).
4 Apparatus
4.1 Radiation source
4.1.1 Method A – Time domain (pulse distortion) measurement
Use a radiation source such as an injection laser diode that produces short duration, narrow
spectral width pulses for the purposes of the measurement. The pulse distortion measurement
method requires the capability to switch the energy of the light sources electrically or
optically. Some light sources shall be electrically triggered to produce a pulse; in this case a
means shall be provided to produce triggering pulses. An electrical function generator or
equivalent can be used for this purpose. Its output should be used to both induce pulsing in
the light source and to trigger the recording system. Other light sources may self-trigger; in
this case, means shall be provided to synchronize the recording system with the pulses
coming from the light source. This may be accomplished in some cases electrically; in other
cases optoelectronic means may be employed.

– 8 – 60793-1-41 © IEC:2010
4.1.2 Method B – Frequency domain measurement
Use a radiation source such as a continuous wave (CW) injection laser diode for the purposes
of the measurement. The frequency domain measurement method requires the capability to
modulate the energy of the light sources electrically or optically. Connect the modulation
output of the tracking generator or network analyzer through any required driving amplifiers to
the modulator.
4.1.3 Method C – Overfilled launch modal bandwidth calculated from differential
mode delay (OMBc)
Use a radiation source as described in IEC 60793-1-49.
4.1.4 For methods A and B
a) Use a radiation source with a centre wavelength that is known and within ± 10 nm of the
nominal specified wavelength. For injection laser diodes, laser emission coupled into the
fibre shallexceed spontaneous emission by a minimum of 15 dB (optical).
b) Use a source with sufficiently narrow linewidth to assure the measured bandwidth is at
least 90 % of the intermodal bandwidth. This is accomplished by calculating the
normalized intermodal dispersion limit, NIDL (refer to Annex A). For A4 fibre, the linewidth
of any laser diode is narrow enough to neglect its contribution to bandwidth measurement.
c) For A1 and A3 fibres, calculate the NIDL (see Annex A) for each wavelength’s
measurement from the optical source spectral width for that wavelength as follows:
IDF
NIDL = , in GHz·km
Δλ
where:
Δλ is the source Full Width Half Maximum (FWHM) spectral width in nm,
IDF is the Intramodal Dispersion Factor (GHz·km·nm) from Annex A according to the
wavelength of the source.
NIDL is not defined for wavelengths from 1 200 nm to 1 400 nm. The source spectral
width for these wavelengths shall be less than or equal to 10 nm, FWHM.
NOTE The acceptability of a NIDL value depends upon the specific user's test requirements. For example, a
0,5 GHz·km NIDL would be satisfactory for checking that fibres had minimum bandwidths greater than some value
less than 500 MHz·km, but would not be satisfactory for checking that fibres had minimum bandwidths greater than
500 MHz·km. If the NIDL is too low, a source with smaller spectral width is required.
d) The radiation source shall be spectrally stable throughout the duration of a single pulse
and over the time during which the measurement is made.
4.2 Launch system
4.2.1 Overfilled launch (OFL)
4.2.1.1 OFL condition for A1 fibre
Use a mode scrambler between the light source and the test sample to produce a controlled
launch irrespective of the radiation properties of the light source. The output of the mode
scrambler shall be coupled to the input end of the test sample in accordance with Annex D.
The fibre position shall be stable for the complete duration of the measurement. A viewing
system may be used to aid fibre alignment where optical imaging is used.
The OFL prescription in Annex D, based on the allowed variance of light intensity on the input
of the fibre under test, can result in large (>25 %) variations in the measured results for high
bandwidth (>1 500 MHz·km) A1a fibres. Subtle differences in the launches of conforming
equipment are a cause of these differences. Method C is introduced as a means of obtaining
an improvement.
60793-1-41 © IEC:2010 – 9 –
Provide means to remove cladding light from the test sample. Often the fibre coating is
sufficient to perform this function. Otherwise, it will be necessary to use cladding mode
strippers near both ends of the test sample. The fibres may be retained on the cladding mode
strippers with small weights, but care shall be taken to avoid microbending at these sites.
NOTE Bandwidth measurements obtained by the overfilled launch (OFL) support the use of category A1
multimode fibres, especially in LED applications at 850 nm and 1 300 nm. Some laser applications may also be
supported with this launch, but could result in reduced link lengths (at 850 nm) or restrictions on the laser sources
(at 1 300 nm).
4.2.1.2 OFL condition for A3 and A4 fibres
OFL is obtained with geometrical optic launch in which the maximum theoretical numerical
aperture of the fibre is exceeded by the launching cone and in which the diameter of the
launched spot is in the order of the core diameter of the fibre. The light source shall be able to
excite both low-order and high-order modes in the fibre equally.
NOTE A mode scrambler excites more or less all modes. Mode excitation is very sensitive to the source/mode
scrambler alignment and the interaction with any intermediary optics such as connectors or optical imaging
systems. A light source with large NA and core diameter will only excite meridional modes or LP modes.

0,m
4.2.2 Restricted mode launch (RML)
4.2.2.1 RML condition for A1b fibre
The RML for bandwidth is created by filtering the overfilled launch (as defined by Annex D)
with a RML fibre. The OFL is defined by Annex D and it needs to be only large enough to
overfill the RML fibre both angularly and spatially. The RML fibre has a core diameter of
23,5 μm ± 0,1 μm, and a numerical aperture of 0,208 ± 0,01. The fibre shall have a graded-
index profile with an alpha of approximately 2 and an OFL bandwidth greater than
700 MHz·km at 850 nm and 1 300 nm. For convenience, the clad diameter should be 125 μm.
The RML fibre should be at least 1,5 m in length to eliminate leaky modes; and it should be
less than 5 m in length to avoid transient loss effects. The launch exiting the RML fibre is then
coupled into the fibre under test.
Provide means to remove cladding light from the test sample. Often the fibre coating is
sufficient to perform this function. Otherwise, it will be necessary to use cladding mode
strippers near both ends of the test sample. The fibres may be retained on the cladding mode
strippers with small weights, but care shall be taken to avoid microbending at these sites.
NOTE 1 In order to achieve the highest accuracy, tight tolerances are required on the geometry and profile of the
RML fibre. In order to achieve the highest measurement reproducibility, tight alignment tolerances are required in
the connection between the launch RML fibre and the fibre under test to ensure the RML fibre is centred to the
fibre under test.
NOTE 2 Bandwidth measurements obtained by a restricted mode launch (RML) are used to support 1 Gigabit
Ethernet laser launch applications. The present launch is especially proven for 850 nm sources transported over
type A1b fibres.
4.2.2.2 RML condition for A3 fibre
RML condition for A3 fibre is created with geometrical optic launch which corresponds to
launch NA = 0,3.
Spot size shall be larger or equal to the size of core.
4.2.2.3 RML condition for A4 fibre
The RML for A4 fibre shall correspond to NA = 0,3. It can be created by filtering the overfilled
launch with a mandrel wrapped mode filter, shown in Figure 1. The mode filter shall be made
with the fibre of the same category as the fibre under test. In order to avoid redundant loss,
the length of fibre should be 1 m. The diameter of the mandrel should be 20 times as large as
that of the fibre cladding and the number of coils may be 5.

– 10 – 60793-1-41 © IEC:2010
NOTE Do not apply any excessive stress in winding fibre on to the mandrel. The wound fibre may be fixed to the
mandrel with an adhesive. Unwound parts of fibre should be set straight.

OFL condition
Fibre under test
IEC  2012/10
Figure 1 – Mandrel wrapped mode filter
4.2.3 Differential mode delay (DMD) launch
The DMD launch shall comply with the launch requirements of IEC 60793-1-49.
4.3 Detection system
The output optical detection apparatus shall be capable of coupling all guided modes from the
test sample to the detector active area such that the detection sensitivity is not significantly
mode-dependent.
A device shall be available to position the specimen output end with sufficient stability and
reproducibility to meet the conditions of 4.6 below.
An optical detector shall be used that is suitable for use at the test wavelength, linear in
amplitude response, spatially uniform to within 10 %, and sufficiently large to detect all
emitted power. An optical attenuator may be used to control the optical intensity on the
detector. It shall be mode-independent as well.
The detection electronics as well as any signal preamplifier shall be linear in amplitude
response (nonlinearities less than 5 %) over the range of encountered signals.
The detection system for Method C shall comply with the requirements of IEC 60793-1-49.
4.4 Recording system
For the time domain (pulse distortion) measurement (method A), use an oscilloscope suitably
connected to a recording device, such as a digital processor, to store the received pulse
amplitude as a function of time. For temporal measurements, data taken from the oscilloscope
display shall be considered secondary to those derived from the recorded signal.
For the frequency domain measurement (method B), use a tracking generator-electrical
spectrum analyzer combination, scalar network analyzer, vector network analyzer or an
equivalent instrument to detect, display and record the amplitude of the RF modulation signal
derived from the optical detector. This shall be done in such a manner as to reduce harmonic
distortion to less than 5 %.
The recording system for Method C shall comply with the requirements of IEC 60793-1-49.

60793-1-41 © IEC:2010 – 11 –
4.5 Computational equipment
For the time domain (pulse distortion) method (method A) and overfilled launch bandwidth
calculated from differential mode delay (method C) or if impulse response is required from
method B, computational equipment capable of performing Fourier transforms on the detected
optical pulse waveforms as recorded by the waveform recording system shall be used. This
equipment may implement any of the several fast Fourier transforms or other suitable
algorithms, and is useful for other signal conditioning functions, waveform averaging and
storage as well.
4.6 Overall system performance
NOTE This subclause provides a means of verifying system stability for the duration of a measurement or the
system calibration period, depending on the method used (A, B or C, see subclauses 6.1, 6.2 and IEC 60793-1-49,
respectively).
The measurement system stability is tested by comparing system input pulse Fourier
transforms (method B) or input frequency responses (method A) over a time interval. As
shown in Annex B, a bandwidth measurement normalizes the fibre output pulse transform by
the system calibration transform. If a reference sample is substituted for the fibre sample, the
resultant response, H(f), represents a comparison of the system to itself over the time
interval. This normalized system amplitude stability is used to determine the system stability
frequency limit (SSFL).
The SSFL is the lowest frequency at which the system amplitude stability deviates from unity
by 5 %. If method A-1 or B-1 is employed, it shall be determined on the basis of one re-
measurement at a time interval similar to that used for an actual fibre measurement. If method
A-2 or B-2 is employed, it shall be determined over substantially the same time interval as
that which is used for periodic system calibration (see 6.1.2). In this latter case, the time
interval may influence the SSFL.
To determine the SSFL, attenuate the optical signal reaching the detector by an amount equal
to or greater than the attenuation of the test sample plus 3 dB. This may require the
introduction of an attenuator into the optical path, if an attenuator, such as might be used for
signal normalization and scaling, is not already present. Also, normal deviations in the
position and amplitude of the pulse or frequency response on the display device shall be
present during the determination of the SSFL.
5 Sampling and specimens
5.1 Test sample
The test sample shall be a known length of optical fibre or optical fibre cable.
5.2 Reference sample
The reference sample shall be a short length of fibre of the same type as the test sample, or
cut from the test sample. Except A4 fibre, the reference length shall be less than 1 % of the
test sample length or less than 10 m, whichever is shorter.
For A4 fibre, the reference length shall be 1 to 2 m. In case of RML, the output of the mode
filter is the reference.
5.3 End face preparation
Prepare smooth, flat end faces, perpendicular to the fibre axis.

– 12 – 60793-1-41 © IEC:2010
5.4 Test sample packaging
For A1 fibres, the deployment (spool type, wind tension, and other winding characteristics)
can affect the results by significant values. It is normal to conduct most quality control
measurements with the fibre deployed on spools in a manner that is suitable for shipment.
The reference deployment, however, is one in which the fibre is stress-free and in which
microbending is minimized. Mapping functions can be used to report the expected value that
would be obtained from a reference deployment measurement based on measurements of the
fibre as deployed on a shipping spool. The mapping function shall be developed from
measurements of a set of fibres that have been deployed both ways and which represent the
full range of bandwidth values of interest.
For A4 fibre, test sample shall be wound into coils with diameter of at least 300 mm, free from
any stress. It shall be certain that the test sample is free from both macro- and microbending
and that the energy distribution at the output of the launching system is substantially constant.
5.5 Test sample positioning
Position the input end of the test sample such that it is aligned to the output end of the launch
system to create launching conditions in accordance with sub-clause 4.2.
Position the output end of the test sample such that it is aligned to the optical detector.
6 Procedure
6.1 Method A – Time domain (pulse distortion) measurement
6.1.1 Output pulse measurement
a) Inject power into the test fibre and adjust the optical attenuator or detection electronics, or
both, such that one entire optical pulse from the fibre is displayed on the calibrated
oscilloscope, including all leading and trailing edges having an amplitude greater than or
equal to 1 % or -20 dB of the peak amplitude.
b) Record the detected amplitude and the calibrated oscilloscope sweep rate.
c) Record the fibre output pulse and calculate the Fourier transform of this pulse, per Annex
B.
d) Determine the input pulse to the test sample by measuring the signal exiting the reference
sample. This may be accomplished by using a reference length cut from the test sample
or from a similar fibre.
6.1.2 Input pulse measurement method A-1: reference sample from test sample
a) Cut the test fibre near the input end according to 5.2. Create a new output end face, per
5.3, and align the end with respect to the optical detector as outlined in 6.1.1 a). Do not
disturb the input end.
b) Apply the cladding mode stripper, if used (see 5.2).
c) If an optical attenuator is used, read just for the same displayed pulse amplitude as
outlined in 6.1.1 a).
d) Record the system input pulse using the same oscilloscope sweep rate as for the test
sample, and calculate the input pulse Fourier transform per Annex B.
6.1.3 Input pulse measurement method A-2: periodic reference sample
a) The following system calibration procedure employing the periodic reference sample shall
be performed over substantially the same time interval as used to determine the SSFL
(see 4.6). In most cases where adequate preparation of mode scrambler, laser diode, and
alignment equipment has been made, it is acceptable to use a reference sample not taken
from the test sample.
60793-1-41 © IEC:2010 – 13 –
b) Prepare input and output ends per 5.3 on a reference sample of the same fibre class and
same nominal optical dimensions as the test sample.
c) Align the input and output ends as outlined in 5.5 and, if an optical attenuator is used,
adjust to obtain the correct displayed pulse amplitude.
d) Record the system input pulse using the same oscilloscope sweep rate as for the test
sample, and calculate the input pulse Fourier transform per Annex B.
6.2 Method B – Frequency domain measurement
6.2.1 Output frequency response
a) Sweep the modulation frequency, f, of the source from a low frequency, to provide an
adequate DC zero reference level, to high frequency in excess of the 3 dB bandwidth.
Record the relative optical power exiting the test specimen as a function of f; denote this
power as P (f). If a network analyzer and the impulse response is desired, the high
out
frequency should exceed -15 dB point and the phase φ (f) should be recorded.
out
NOTE A function related to P (f), such as log P (f), may be recorded to finally obtain |H(f)| in 7.1.
out out
b) Determine the input modulated signal to the test sample by measuring the signal exiting
the reference length of the fibre. This may be accomplished using a reference length from
the test sample (method B-1; preferred method to be used in case of conflict in test
results) or from a similar fibre (method B-2).
6.2.2 Method B-1: Reference length from test specimen
a) Cut the test sample near the input end and prepare flat end faces (see 5.3) at this newly
created output end. Strip the cladding modes from the output end if necessary. Do not
disturb the launching conditions to this short length.
b) Sweep the modulation frequency, f, of the source from a low frequency, to provide an
adequate DC zero reference level, to a high frequency in excess of the 3 dB bandwidth.
Record the relative optical power exiting the reference length as a function of f; denote
this power as P (f).
in
6.2.3 Method B-2: Reference length from similar fibre
a) If the apparatus exists to position a fibre at the same place in the mode scrambler output
as was the input of the test sample, then another short length of fibre having the same
nominal properties of the test sample may be substituted as the reference. Use the
reference fibre to replace the test sample. Apply a cladding mode stripper, if necessary,
and align the output end in front of the detector.
b) Sweep the modulation frequency, f, of the source from a low frequency, to provide an
adequate DC zero reference level, to a high frequency in excess of the 3 dB bandwidth.
Record the relative optical power exiting the reference length as a function of f; denote
this power as P (f).
in
NOTE A function related to P (f), such as log P (f), may be recorded to finally obtain |H(f)| in 7.2.
in in
6.3 Method C – Overfilled launch modal bandwidth calculated from differential mode
delay (OMBc)
a) Measure the differential mode delay of fibre in accordance with IEC 60793-1-49.
b) Calculate the overfilled modal bandwidth according to the formulae B2 of IEC
60793-1-49:2006” using weights given here in Table 1. Linear interpolation of the weight
value shall be applied for any radial position of the actual scan that is known to lie
between the integer positions listed in Table 1.
NOTE Table 1 weightings are only applicable for A1a fibres at 850 nm.

– 14 – 60793-1-41 © IEC:2010
Table 1 – DMD weights for calculating overfilled modal bandwidth (OMBc)
from DMD data for 850 nm only
DMD weights for
r (μm)
OMBc
0 0
1 0,00073
2 0,00157
3 0,00253
4 0,00362
5 0,00487
6 0,00631
7 0,00795
8 0,00983
9 0,01198
10 0,01443
11 0,01725
12 0,02046
13 0,02414
14 0,02836
15 0,03317
16 0,03869
17 0,04500
18 0,05221
19 0,06047
20 0,06992
21 0,08073
22 0,09310
23 0,10725
24 0,12345
25 0,14197
7 Calculations or interpretation of results
7.1 -3 dB frequency, f
3 dB
Calculate the frequency response, H(f). Calculate the -3 dB fibre bandwidth, f , in accordance
3 dB
with Annex B.
If the measured -3 dB frequency exceeds the NIDL (as calculated in 4.1.4) divided by the fibre
length, L, in km, report the measured result. In this case, it is preferable to show that the
measurement result may have been limited by the equipment, as shown in Example 1.
EXAMPLE 1 A fibre 2,2 km long has a length-normalized measured -3 dB frequency of 2,2 GHz·km, but the
measurement system has a NIDL of 2 GHz·km at this wavelength. Preferably, the result is reported as
">normalized measured value" (">2,2 GHz·km", in this example). Similarly, the actual measured value is preferably
reported as "> {measured value}" (">1,0 GHz", in this example). The ">" sign shows that the measured value may
have been limited by the test set. If the measured -3 dB frequency exceeds the SSFL (as determined in 4.6), report
the result as being greater than the SSFL as shown in Example 2.

60793-1-41 © IEC:2010 – 15 –
EXAMPLE 2 A fibre 2,2 km long has a measured -3 dB frequency of 0,95 GHz (2,09 GHz·km), which is greater
than the SSFL for the test set, 0,9 GHz (1,98 GHz·km for this fibre length). Report the result as "> (SSFL)" (">
0,9 GHz", here). Report the length-normalized result as ">(SSFL times the sample length in km" ("> 1,98 GHz·km",
here). The ">" sign is required to show that the measured value is limited by the test set.
7.2 Calculations for optional reporting methods
Other reporting methods may be required by a detail specifica
...