IEC 61280-1-4:2003

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

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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
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For price, see current catalogue

– 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
...