EN 61280-1-4:2003

SLOVENSKI SIST EN 61280-1-4:2004

STANDARD
september 2004
Preskusni postopki komunikacijskega podsistema optičnih vlaken – 1-4. del:
Splošni komunikacijski podsistemi - zbiranje in zmanjševanje dvo-
dimenzionalnih bližnjih podatkov za laserske oddajnike za večrodnavlakna
(IEC 61280-1-4:2003)*
Fibre optic communication subsystem test procedures - Part 1-4: General
communication subsystems - Collection and reduction of two- dimensional nearfield
data for multimode fibre laser transmitters (IEC 61280-1-4:2003)
ICS 33.180.01 Referenčna številka
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

EUROPEAN STANDARD EN 61280-1-4
NORME EUROPÉENNE
EUROPÄISCHE NORM April 2003
ICS 33.180.01
English version
Fibre optic communication subsystem test procedures
Part 1-4: General communication subsystems -
Collection and reduction of two-dimensional nearfield data
for multimode fibre laser transmitters
(IEC 61280-1-4:2003)
Procédures d'essai des sous-systèmes  Prüfverfahren für Lichtwellenleiter-
de communication à fibres optiques Kommunikationsuntersysteme
Partie 1-4: Procédures d'essai des sous- Teil 1-4: Allgemeine
systèmes généraux de télécommunication - Kommunikationsuntersysteme -
Recueil et réduction de données Erfassung und Reduzierung
à deux dimensions de champs proches zweidimensionaler Mehrmodenfasern
pour les émetteurs de laser à fibres für Nahfelddaten von Lasersendern
multimodales (IEC 61280-1-4:2003)
(CEI 61280-1-4:2003)
This European Standard was approved by CENELEC on 2003-03-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.

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

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

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

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

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

© 2003 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61280-1-4:2003 E
Foreword
The text of document 86C/465/FDIS, future edition 1 of IEC 61280-1-4, prepared by SC 86C, Fibre
optic systems and active devices, of IEC TC 86, Fibre optics, was submitted to the IEC-CENELEC
parallel vote and was approved by CENELEC as EN 61280-1-4 on 2003-03-01.

The following dates were fixed:

– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2003-12-01

– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2006-03-01

Annexes designated "normative" are part of the body of the standard.
Annexes designated "informative" are given for information only.
In this standard, annex ZA is normative and annex A is informative.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61280-1-4:2003 was approved by CENELEC as a
European Standard without any modification.
__________
- 3 - EN 61280-1-4:2003
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
1) 2)
IEC 60793-1-20 - Optical fibres EN 60793-1-20 2002
Part 1-20: Measurement methods and
test procedures - Fibre geometry

1) 2)
IEC 60793-1-41 - Part 1-41: Measurement methods and EN 60793-1-41 2002
test procedures – Bandwidth
1) 2)
IEC 60793-1-43 - Part 1-43: Measurement methods and EN 60793-1-43 2002
test procedures - Numerical aperture

1) 2)
IEC 60825-2 - Safety of laser products EN 60825-2 2000
Part 2: Safety of optical fibre
communication systems
1)
Undated reference.
2)
Valid edition at date of issue.

INTERNATIONAL IEC
STANDARD 61280-1-4
First edition
2003-01
Fibre optic communication subsystem
test procedures –
Part 1-4:
General communication subsystems –
Collection and reduction of two-dimensional
nearfield data for multimode fibre laser
transmitters
Procédures d'essai des sous-systèmes
de télécommunication à fibres optiques –
Partie 1-4:
Procédures d'essai des sous-systèmes généraux
de télécommunication – Recueil et réduction de données
à deux dimensions de champs proches pour les
émetteurs de laser à fibres multimodales
© IEC 2003 ⎯ Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
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Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch  Web: www.iec.ch
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– 2 – 61280-1-4 © IEC:2003(E)
CONTENTS
FOREWORD . 3
1 General . 4
1.1 Scope and object. 4
1.2 Assumptions . 4
2 Normative references. 5
3 Apparatus . 5
3.1 Sources . 5
3.1.1 Calibration source. 5
3.1.2 Laser under test. 5
3.2 Test jumper assembly. 6
3.3 Fibre shaker . 6
3.4 Micropositioner . 6
3.5 Microscope objective . 7
3.6 Detector. 7
4 Sampling and specimens . 7
5 Procedure. 7
5.1 Overview of the measurement procedure. 7
5.2 Camera calibration . 8
5.2.1 Camera geometric calibration . 8
5.2.2 Camera optical calibration. 9
5.3 Measuring 2D nearfield flux distributions . 9
5.4 Finding the optical center of the test jumper assembly. 9
5.5 Finding the nearfield distribution of a laser under test.10
6 Calculations or interpretation of results.10
6.1 Coordinate transforms .10
6.2 Centroid computation.11
6.3 Computation of radial data functions.12
7 Documentation.14
8 Specification information.15
Annex A (informative) Camera data reduction .16
Bibliography.20

61280-1-4 © IEC:2003(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –
Part 1-4: General communication subsystems –
Collection and reduction of two-dimensional nearfield data
for multimode fibre laser transmitters
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61280-1-4 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics
The text of this standard is based on the following documents:
FDIS Report on voting
86C/465/FDIS 86C/494/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.
The committee has decided that the contents of this publication will remain unchanged until 2008.
At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
– 4 – 61280-1-4 © IEC:2003(E)
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –
Part 1-4: General communication subsystems –
Collection and reduction of two-dimensional nearfield data
for multimode fibre laser transmitters
1 General
1.1 Scope and object
This part of IEC 61280 sets forth a standard procedure for the collection of two-dimensional
fibre optic nearfield grayscale data and subsequent reduction to one-dimensional data
expressed as a set of three sampled parametric functions of radius from the fibre’s optical
center. The object of this standard is to reduce measurement errors and inter-laboratory
variation, supporting accurate mathematical prediction of minimum guaranteed link length in
gigabit and ten gigabit fibre optic data communications systems.
These radial functions are intended to characterize fibre optic laser sources for use in
mathematical models predicting the minimum guaranteed length of a communications link.
Although available as a byproduct, estimation of the nearfield diameter is not an objective.
1.2 Assumptions
The 50-micron or 62,5-micron core near-parabolic graded-index multimode fibre used as the
“test jumper assembly” is treated as if it possessed perfect circular symmetry about its optical
center, as asymmetries in the launched optical flux distributions will dominate any
lopsidedness of the test jumper assembly. It is further assumed that all cladding modes will be
stripped by passage through the specified ten meters or more of fibre. The modes of a mode
group need not carry equal flux. (In fact, with such short fibres, one thousand meters or less,
unequal distribution of flux in the modes of a group is the norm, not the exception.)
The fibre micropositioner that moves the fibre in the receiving camera's field of view, being
used to calibrate the camera for geometric distortions, is used as a reference standard. The
microscope objective, used to project the magnified nearfield onto the CCD chip, is treated as
an optically perfect thick lens.
The flux detectors are required to be both linear and memoryless; this excludes for instance
lead sulphide vidicon detectors. Detectors shall meet the detector requirements of
IEC 60793-1-43. Absolute radiometric measurement of flux (optical power flow) is not
required. A computer is required to perform the needed computations, which are too extensive
to be performed manually. Although the present measurement method assumes a CCD
camera, mechanically-scanned “slitscan” and pinhole cameras may also be used.
Safety: all procedures in which an LED or laser source is used as the optical source shall be
carried out using safety precautions in accordance with IEC 60825-2.

61280-1-4 © IEC:2003(E) – 5 –
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 60793-1-41: Optical fibres – Part 1-41: Measurement methods and test procedures –
Bandwidth
IEC 60793-1-43: Optical fibres – Part 1-43: Measurement methods and test procedures –
Numerical aperture
IEC 60825-2: Safety of laser products – Part 2: Safety of optical fibre communication systems
3 Apparatus
As the objective of this international standard is to optically characterize laser sources, many
different laser sources will be used, while the rest of the apparatus is held constant. The
apparatus is calibrated using a broadband incoherent calibration source (such as a light-
emitting diode (LED) or a xenon arc lamp) in place of the lasers.
3.1 Sources
There are two kinds of sources used in the present measurement method: the incoherent
broadband overfilled source used for calibration, and the various laser sources being tested,
as described in the following paragraphs.
There is always an optical connector between the source and the test jumper assembly.
3.1.1 Calibration source
The purposes of the calibration source are to find the optical center of the test jumper
assembly, and also to determine the geometric corrections needed to convert 2D nearfield
measurements taken in camera (“TV”) coordinates into the equivalent true geometric
measurements, compensating for non-square pixels, imprecisely known magnification factors,
and the like. For these purposes, an incoherent broadband source that overfills the modes of
the test jumper assembly is used in place of the laser sources under test.
Any spectrally broad non-coherent light source, such as a tungsten-halogen lamp, a xenon
arc lamp or a light-emitting diode (LED) may be used to overfill the test jumper assembly’s
fibre. The chosen calibration source shall be stable in intensity over a time period sufficient to
perform the measurements.
Optionally, an IEC 60793-1-41 mode scrambler may be used with the chosen calibration
source to ensure more uniform overfilling of the fibre.
3.1.2 Laser under test
The only requirements on the lasers under test are that they have an operating wavelength
compatible with the test jumper assembly and the detector, and have optical connectors or
splices compatible with those of the test jumper assembly. The construction details of the
laser sources are otherwise unspecified.
The laser drive current shall be sufficient to ensure that the laser always acts as a laser,
rather than an LED.
– 6 – 61280-1-4 © IEC:2003(E)
3.2 Test jumper assembly
The purpose of the test jumper assembly is to strip cladding modes, and to allow speckle to
be averaged out by mechanical flexing of a portion of the test jumper assembly.
The test jumper assembly shall be at least ten meters in length, made of germanium-doped
near-parabolic graded-index fused-silica multimode “glass” category A1 fibre with a core
diameter of either 50 µ or 62,5 µ and an overall glass diameter of 125 µs. The test jumper
assembly shall consist of a single, uncut length of fibre with connectors at each end. The test
jumper assembly connectors shall have single-mode mechanical tolerances, even though the
fibre is multimode.
3.3 Fibre shaker
The purpose of the fibre shaker is to ensure that optical speckle is averaged out, with only a
few percent of residual ripple or noise due to speckle being allowed to remain in the
measured nearfields. Manual shaking of the fibre is generally not sufficient.
Part of the test jumper assembly shall be mechanically shaken continuously in each of three
nominally orthogonal directions (using three independent shaker mechanisms) during the
measurement, making at least one hundred shake cycles in each of the three directions
during the measurement period. The shake frequencies in the three directions shall be chosen
such that the three shake cycles synchronize no more often than once every five hundred
cycles of the middle shake frequency.
A fibre shaker mechanism may be of any design as long as it induces large amplitude
movements and flexing in the optical fibre. Fibre transverse displacements of more than
25 mm are suggested. The fibre shakers shall include a fibre-holding fixture for securely
holding the fibre.
One exemplary mechanism design has three turns of fibre coiled into a 3-ply figure-eight
arrangement, with the loops each being approximately 120 mm in diameter. A motor-driven
eccentric drives a slider back and forth at about one stroke per second, alternately flattening
and stretching one loop of the figure eight with 25 mm amplitude. Three such mechanisms in
series will consume about 3 × 3 ×(2 ×π × 0,120) = 6,8 meters of the test jumper assembly’s
fibre.
The fibre ends leading into and out of the fibre shakers shall be mechanically fixed or
stabilized to prevent movement of fibres at connection points. In addition, the fibre shakers
shall be mechanically isolated from the rest of the test setup so that vibrations are not
transmitted to connection points throughout the apparatus, or to the micropositioner, camera,
or microscope objective.
NOTE 1  Vibration reduction is easier if the fibre shaker is both statically and dynamically balanced, and if all
moving components are light in weight.
NOTE 2 There is no required relation between the measurement period (containing the one hundred strokes) and
the duration of a CCD camera exposure. Typically, in each measurement period, many exposures are taken and
later summed, to avoid saturation of the CCD, and to ensure that speckle is in fact averaged out. Too short a total
exposure time will prevent the desired averaging out of speckle.
3.4 Micropositioner
The purpose of the micropositioner is to bring the projected image of the fibre face into focus
on the CCD chip within the camera, and also to support geometric calibration of the apparatus
by making calibrated moves in X and Y, these axes being perpendicular to the optic axis Z.
The X-axis and Y-axis accuracy and resolution shall be one micron or less (finer), and it shall
be possible to sweep the centroid of the calibration-source nearfield image from one edge of
the CCD chip to the other, in both X and Y directions, by adjustment of the X and Y axes
alone, with the nearfield image remaining substantially in focus on the CCD chip. The X-axis

61280-1-4 © IEC:2003(E) – 7 –
and Y-axis repeatability error shall be no larger than one third of a micron. It shall be possible
to mechanically lock both the X and Y axes, to prevent drift in the apparent location of the test
jumper assembly’s optical center as tests are performed.
The Z-axis accuracy, repeatability, and resolution are unspecified, but shall be sufficient to
bring the system into focus, and it shall be possible to mechanically lock the Z axis once
focus is achieved, to prevent drift in the system magnification as tests are performed.
3.5 Microscope objective
Suitable optics shall be provided which project the magnified image of the output end of the
test jumper assembly onto the receiving CCD chip such that the CCD can measure the entire
nearfield flux distribution. These optics shall not restrict the numerical aperture of the formed
image. (Based on IEC 60793-1-43.)
NOTE The actual magnification of the microscope objective as used in the present apparatus generally will not be
the same as the nominal magnification factor engraved into the side of the objective, because the present
apparatus differs from the standard microscope for which that nominal magnification factor was computed.
3.6 Detector
The flux detectors shall be both linear and memoryless; this excludes for instance lead
sulphide vidicon detectors. Detectors shall satisfy the detector requirements of
IEC 60793-1-43. Absolute radiometric measurement of flux (optical power flow) is not
required.
Automatic gain control (AGC), if present, shall be disabled.
In CCDs with anti-blooming provisions, “saturation” is considered to occur at the “white-clip”
level, not ultimate saturation, to preserve linearity of response.
If more than one in one thousand of the CCD’s pixels are bad, or if the camera's offsets and
pixel crosstalk are too large to allow accurate measurements, replace the camera. See 5.2.2
for details.
NOTE 1 Detector saturation may often be avoided by taking a number of very short exposures and summing them
pixel for pixel.
NOTE 2 Neutral-density (ND) filters, optionally used to prevent detector saturation, are most conveniently placed
between the microscope objective and the detector, and should be slightly tilted (by a few degrees of angle) to
prevent reflections from the filter from reaching the source.
4 Sampling and specimens
Laser sources to be tested shall be chosen and prepared as defined by the user of this
standard, who shall document the sampling and preparation procedures used, as described in
Clause 7 of this standard. See Clause 3 for technical requirements on sources.
5 Procedure
5.1 Overview of the measurement procedure
This procedure consists of the following steps:
a) calibrate the camera,
b) measure the calibration source’s 2D nearfield flux distribution,
c) measure one or more laser launch 2D nearfield flux distributions,
d) perform the calculations, and
e) report the results.
– 8 – 61280-1-4 © IEC:2003(E)
Note that calibration of the apparatus is critical to the accuracy of this measurement
procedure. (See A.5 for description of the kinds of noise and errors which calibration can
correct.) There is one calibration procedure and one nearfield measurement procedure, each
being used multiple times. The following paragraphs first describe these two basic
procedures, and then describe how these two procedures are used to implement the overall
procedure.
The receiver end of the test jumper assembly shall be firmly attached to the camera and
micropositioner assembly and left undisturbed during this entire process. All three micro-
positioner axes shall be locked once calibration is complete, so that the fibre optical center
and geometric scale factors (magnifications) found with the calibration source will continue to
apply to measurements of the laser-source nearfields, without undue drift.
Calibrate the camera setup again, after taking all the laser data, to detect any drift in the
camera or setup. Drift in geometric calibration can cause severe errors in the computed radial
data functions.
The equipment must remain stable over the course of all measurements. Unless it can be
shown not to be required, the laboratory ambient temperature shall be stable to within 2 °C,
the equipment shall be allowed to warm up for at least fifteen minutes before calibrations or
measurements are made, and any automatic gain control (AGC) features shall be disabled.
NOTE The tight temperature tolerance is required to counter the temperature sensitivity of the optical flux
detectors in the camera, particularly the dark current. See A.5 for details.
5.2 Camera calibration
Any data taken shall be conditioned before use is made of that data. Conditioning involves
pixel-by-pixel removal of offsets (due to dark current and fixed-pattern noise and the like),
normalization for differences in pixel sensitivity (responsivity), possible identification of bad
pixels and correction for the camera's geometric distortions. These issues are discussed
individually in the following paragraphs.
5.2.1 Camera geometric calibration
The purpose of geometric calibration is to obtain the measurement data needed to compute
the transform matrix. The transform matrix will be used to compensate measured 2D nearfield
data for the actual size and shapes of the pixels in the CCD camera, and to calculate the
actual magnification of the microscope objective lens as used in the present apparatus.
To calibrate cameras for these geometric effects, a fibre micropositioner, which is mechanical
and built for precision, will be used as the reference standard.
Perform the following steps.
a) Overfill the fibre with light from the calibration source.
b) Move the test jumper assembly’s receiver end to three well-separated non-collinear
positions (calibration points) in the camera’s field of view.
c) Record both the fibre position in true space (micropositioner X and Y coordinates) and the
location of the corresponding centroid of flux in TV space (camera coordinates).
d) Solve for the 3x3 transform matrix mapping from the one 2D space to the other, as
detailed in 6.1 and 6.2.
e) The “three well-separated non-collinear positions” can be in a rough equilateral or right
triangle; any reasonable triangle will work, but the closer to equilateral, the better. The
triangle should be as large as possible without having any part of the nearfield clipped off
by the encroaching edges of the TV frame. The broadband incoherent source’s intensity
should be set such that the peak intensity is at about 75 % of camera saturation.

61280-1-4 © IEC:2003(E) – 9 –
NOTE Beware of mechanical backlash in the micropositioner. Always approach a new position from the same
direction, overshooting and coming back, if necessary, and moving between the three calibration points always in
the same direction and order. Also beware of mechanical drift, which occurs despite locking of the micropositioner
axes. Drift will limit how many lasers can be tested before the calibration source must be used to again find the
optical center of the test jumper assembly’s fibre as seen by the CCD camera. A reasonable rate would be five or
ten laser nearfield tests per center finding, but this will depend on the actual drift rate of the apparatus.
5.2.2 Camera optical calibration
The purpose of optical calibration is to obtain the measurement data needed to compensate
measured 2D nearfield data for the actual sensitivity and offset of the individual pixels making
up the data.
Perform the following procedure to remove offsets. First record the camera output in total
darkness, then again with the nearfield to be measured illuminating the camera, and finally
subtract the darkness picture from the illuminated picture, pixel for pixel. The two pictures
should be taken at exactly the same camera temperature and exposure duration, to get
adequate cancellation of offsets. If camera gain changes, say to compensate for a brighter or
dimmer source, optical calibration shall be repeated.
Perform the following procedure to remove pixel sensitivity variation: Record the camera
output while the camera is viewing a uniformly (to within 1 %) illuminated white area bright
enough to almost saturate the camera, about 75 % of saturation, and then subtract the
darkness picture, as described above. The inside of a small integrating sphere works well as
a uniformly illuminated area. Compute the average pixel value by adding up the offset-
compensated values of all pixels and dividing the sum by the number of pixels summed.
Compute each element in the normalization matrix, which has one element per pixel, by
dividing the average pixel value by the value for the pixel corresponding to that element. The
resulting element values will typically range from 0,90 to 1,10. They are to be multiplied by
their corresponding pixels, to normalize those pixels to the average sensitivity of all pixels, for
every measurement that is made.
5.3 Measuring 2D nearfield flux distributions
The step-by-step procedure to measure 2D nearfield flux distributions is as follows.
a) Power all equipment up and allow it to warm up for at least fifteen minutes. Turn any
automatic gain control (AGC) features off.
b) Calibrate the camera, as needed, both optically and geometrically, as described in 5.2.
This process yields two 2D optical matrices, the pixel offsets and the pixel normalization
factors respectively, plus one 3x3 transform matrix.
Steps a) and b) may be done once and the results used for a number of measurements made
at the same time.
c) Without disturbing the receiver assembly or camera, take a measurement. The effective
source intensity and/or camera sensitivity shall have been adjusted so that no pixels are
allowed to saturate or bloom. The middle of the fibre shall be shaken continuously during
the measurement, making at least one hundred shake cycles during the measurement
period, to ensure that speckle is averaged out. Measured data shall be corrected for offset
and sensitivity, yielding “conditioned data”.
Report the conditioned data and the transform matrix.
5.4 Finding the optical center of the test jumper assembly
This is the step-by-step procedure to find the optical center of a graded-index multimode fibre,
specifically the test jumper assembly, from measurements on the 2D nearfield resulting from
an overfilled launch from the calibration source.

– 10 – 61280-1-4 © IEC:2003(E)
a) Using the calibration source and the test jumper assembly, measure the nearfield 2D flux
distribution, as described in 5.3, yielding conditioned data and the transform matrix.
b) Compute the centroid of flux, as described in 6.2.
c) The above step 2 yields the centroid location in TV coordinates. Using the transform
matrix, also compute the centroid location in true coordinates, as detailed in 6.1.
d) Report the centroid, in both true and TV coordinates, as the location of the fibre's optical
center.
5.5 Finding the nearfield distribution of a laser under test
The test jumper assembly’s fibre is treated as if it were perfectly circular, because practical
fibre is axisymmetric to within a few percent, which is negligible in this application. Although
the nearfield from a laser launch is not assumed to be circular or even symmetric (even after
passage through ten or more meters of the graded-index multimode fibre of the test jumper
assembly), the following process will collapse all such distributions by circular summation
around the fibre optical center.
a) Using the calibration source under test and the test jumper assembly, calibrate the
camera and find the optical center of the test jumper assembly. This step shall be
performed whenever the receiver end of the apparatus is changed or disturbed, as well as
periodically (to detect mechanical drift in scale factors or fibre center position).
b) Using the laser source under test and the test jumper assembly, without disturbing the
receiver-end setup, measure the nearfield 2D flux distribution, as described in 5.3,
yielding conditioned data. Repeat this step for each laser source to be tested.
c) Compute the radial data functions as described in Clause 6, and report the results as
described in Clause 7.
NOTE Do not use the centroid of the laser nearfield distribution as the summation center. The laser centroid is
not an accurate indicator of the test jumper assembly’s optical center if the nearfield distribution is not symmetric,
which is quite often the case in short fibres, those not exceeding one thousand meters in length. The optical center
of the fibre is required (rather than the physical center of the fibre or the laser flux centroid) because it is the
optical properties of the fibre that determine that fibre's bandwidth, and thus communications performance.
6 Calculations or interpretation of results
This clause specifies the exact procedure to be used to convert measured 2D nearfield data
into the radial data functions. There are many mathematically similar or even almost
equivalent approaches to reduction of such datasets. However, we have chosen and
documented this specific procedure to ensure that all laboratories obtain the same radial data
functions given the same 2D-nearfield dataset despite the effects of finite pixel and
summation ring sizes.
6.1 Coordinate transforms
The following procedure calculates the transformation matrix required to transform from TV
(CCD image) coordinates to true (micropositioner) coordinates. This transform will also undo
the reversed and upside-down effects of the objective lens on the nearfield image.
For simplicity, “homogeneous coordinates” are used, which means that 2D points are
represented as 3-element vectors (having only two varying elements), and transforms are
represented as 3x3 matrices (having only six varying elements). The non-varying elements
are either zero or one in value (See Foley [1] ).
___________
Numbers in brackets refer to the bibliography.

61280-1-4 © IEC:2003(E) – 11 –
In the following, x and y are in true coordinates, while x' and y' are in TV coordinates. The
corresponding vectors are {x,y,1} and {x',y',1}, representing the same point as seen in true
and TV coordinates respectively, both expressed in homogeneous form. To convert a
measurement location in TV coordinates to the equivalent in true coordinates, the matrix
{x',y',1} is matrix multiplied by the transform matrix M as shown:
t t
{x,y,1} = {x',y',1} •M
For this to work, the value of the transform matrix M is first determined, given the three non-
collinear calibration points, which will be numbered 1, 2, and 3. By definition,
M = {{a,b,c},{d,e,f},{0,0,1}}.
There are six unknowns, and six data pairs (in three 2D points), so M can be found by solving
the following system of equations by standard algebraic methods:
t t
{x1,y1,1} = {x1',y1',1} •M
t t
{x2,y2,1} = {x2',y2',1} •M
t t
{x3,y3,1} = {x3',y3',1} •M
t
In the above, the dot (“•”) means matrix multiplication, and the superscript “t” in {.} means
transpose. The physical interpretations of the elements of M are discussed in A.6.
If M is the identity matrix, {{1,0,0},{0,1,0},{0,0,1}}, the transform has no effect, making {x,y,1}
and {x',y',1} identical. In this case, true coordinates and TV coordinates are identical.
6.2 Centroid computation
Because computing the centroid is a linear operation, it suffices to collapse the 2D nearfield
onto the one-dimensional X and Y axes, and compute the centroid of each of the two
projections, rather than computing the centroid directly on the 2D distribution. Because the
centroid is invariant under affine transforms (see Clause A.5), the centroid can be computed
in TV coordinates, and then this centroid can be transformed into true coordinates; or,
equivalently, it can be computed in true and then transformed to TV coordinates.
To eliminate position biases due to camera noise, especially pixel crosstalk noise, centroids
shall be computed using only those pixels that exceed 10 % of the largest (peak) valid pixel
value. Only centroid computations use this threshold.
NOTE The 10 % threshold is patterned after IEC 60793-1-20.
To find the X-coordinate and Y-coordinate of the flux centroid, perform the following steps in
order. The Y-coordinate step or location is given in parentheses, for example, “X-coordinate
(Y-coordinate).”
1) For each column (row), compute the sum of all intensity values in this column (row),
yielding a 1D array of sums. This is called “collapsing” the 2D data onto the X axis
(Y axis).
2) Compute the sum of the elements of the array of sums, yielding a single scalar number,
the “sum of the sums”.
3) Compute the product of each element of the array of sums with its TV-coordinate location.
Sum these products to yield a single scalar number, the “sum of the products”.
4) The X-coordinate (Y-coordinate) of the centroid is the “sum of the products” divided by the
“sum of the sums”.
– 12 – 61280-1-4 © IEC:2003(E)
6.3 Computation of radial data functions
1) Obtain the conditioned data and transform matrix by measurements of the 2D nearfields of
the calibration source and the lasers under test.
2) Compute the raw, noisy r*I(r) function by circular summation of the conditioned data
around the optical center of the fibre. This computation is done parametrically, with the
radial index “i” being the parameter. Although tenth-micron steps are used here, the actual
requirement is that the steps be half-micron or finer. This summation is performed by
executing the following steps in the order given:
a) Zero the s(i), rs(i), and c(i) arrays, which will carry the pixel intensity and radius
summations, and the pixel counts, respectively. The integer index i ranges from zero
to i which is typically chosen to be ten times the maximum integration radius of the
max
test fibre, nominally 1,15 times the core-cladding boundary radius, with any factor in
the range from 1,10 to 1,20 being allowed. In the case of 62,5-micron fibre, use i =
max
(10)(62,5÷2)(1,15) = (10)(35,94) = 359,4 § 360. For 50-micron core diameter fibre,
instead use 290. This value “i” represents tenth-micron steps in distance from the fibre
optical center, with i = 0 being all radii less than 0,1 µ, i = 1 being all radii from 0,1 µ
to just under 0,2 µ, and so on.
Data from beyond i cannot possibly come from the fibre core, as essentially all
max
optical power will flow within the fibre core (and perhaps the immediately adjacent
cladding), and thus apparent flux from beyond i shall be ignored. A confirmation
max
should be made that there are no significant (non-noise) values beyond this point,
which might indicate an error in the measurement.
NOTE The purpose of the cutoff at 1,15 times the core radius is to prevent pixel crosstalk noise
(see A.5) from unduly affecting the radial data functions, especially the encircled flux.
b) For each and every pixel, compute the distance from that pixel to the optical center,
using true coordinates (as detailed in 6.1), and compute the corresponding index i
from the radius by taking the integer part of ten times the radius r. If, due to roundoff
errors, i is negative, set it to zero. If i exceeds i (for example, 360 or 290
max
respectively for 62,5- or 50-micron fibre), skip this pixel. Otherwise, increment c(i) by
one, add the conditioned pixel value to s(i), add r (the actual distance from optical
center to the current pixel) to rs(i), and continue to the next pixel.
c) When all pixels have been processed, the array s(i) contains a noisy approximation to
the function “r*I(r)”, the array rs(i) contains the sum of the distances from optical
center to the pixels, and the array c(i) contains the number of pixels that contributed to
the corresponding elements of s(i) and rs(i). The noise, here called “pixel granularity
noise”, is due to beats (Moiré patterns) between the radial sampling period (one-tenth
of a micron here) and the pixel-grid X and Y periods, and is multiplicative, not additive,
affecting s(i), rs(i), and c(i) in equal proportion. Simply put, at some values of i, more
pixels will be seen than expected from 2 π r, while at other values of i, fewer pixels
than expected will be seen. The presence of this noise requires r*I(r) to be computed
indirectly, rather than directly from s(i) alone, allowing the granularity noise to be
canceled.
3) Smoothing of pixel partition noise: (not to be confused with pixel granularity noise.) It is a
property of the present measurement procedure that a pixel can belong to at most one
summation ring. This causes severe pixel partition noise if there are too few pixels to
adequately fill all rings, because some rings will be sparse and ragged and thus will be too
sensitive to natural variations in the distribution of flux. Pixel partition noise can be largely
suppressed by allowing each sample to take contributions from multiple rings. In practice,
because of the fixed-sum nature of pixel partition noise, it suffices to pool adjacent pairs
of rings. The following procedure accomplishes this by pooling the pixel data from
adjacent overlapping rings to compute each smoothed sample, where each summation
ring (and thus pixel) contributes to two adja
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