IEC 61280-1-4:2009

IEC 61280-1-4 ®
Edition 2.0 2009-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic communication subsystem test procedures –
Part 1-4: General communication subsystems – Light source encircled flux
measurement method
Procédures d’essai des sous-systèmes de télécommunication à fibres
optiques –
Partie 1-4: Sous-systèmes généraux de télécommunication – Méthode de
mesure du flux inscrit de la source lumineuse

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IEC 61280-1-4 ®
Edition 2.0 2009-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Fibre optic communication subsystem test procedures –
Part 1-4: General communication subsystems – Light source encircled flux
measurement method
Procédures d’essai des sous-systèmes de télécommunication à fibres
optiques –
Partie 1-4: Sous-systèmes généraux de télécommunication – Méthode de
mesure du flux inscrit de la source lumineuse

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
W
CODE PRIX
ICS 33.180.01 ISBN 978-2-88910-473-4
– 2 – 61280-1-4 © IEC:2009
CONTENTS
FOREWORD.4
0 Introduction .6
0.1 General .6
0.2 Changes from previous edition .6
0.3 Assumptions applicable to the characterization of data sources .6
0.4 Assumptions applicable to the characterization of measurement sources .6
1 Scope.7
2 Normative references .7
3 Terms and definitions .7
4 Symbols .8
5 Apparatus.9
5.1 Common apparatus .9
5.1.1 General .9
5.1.2 Computer .10
5.1.3 Image digitizer.10
5.1.4 Detector .10
5.1.5 Magnifying optics.11
5.1.6 Attenuation .11
5.1.7 Micropositioner (optional) .11
5.1.8 Input port.12
5.1.9 Calibration light source.12
5.2 Transmission source apparatus .12
5.2.1 General .12
5.2.2 Test jumper assembly.13
5.2.3 Fibre shaker .13
5.3 Measurement source apparatus .14
6 Sampling and specimens.14
7 Geometric calibration .15
8 Measurement procedure.15
8.1 Safety .15
8.2 Image acquisition .15
8.2.1 Raw image acquisition.15
8.2.2 Dark image acquisition .16
8.2.3 Corrected image .16
8.3 Optical centre determination.16
8.3.1 General .16
8.3.2 Centroid image .16
8.3.3 Centroid computation .17
8.4 Test source image acquisition .17
9 Computation of encircled flux .17
9.1 Computation of radial data functions .17
9.2 Integration limit and baseline determination.19
9.2.1 Integration limit.19
9.2.2 Baseline determination .19
9.2.3 Baseline subtraction .19

61280-1-4 © IEC:2009 – 3 –
9.3 Computation of encircled flux .19
10 Results .20
10.1 Information available with each measurement .20
10.2 Information available upon request.20
11 Specification information .20
Annex A (informative) Measurement sensitivity considerations .22
Annex B (informative) Theory of geometric calibration using the micropositioner .27
Annex C (normative) Procedure for geometric calibration using the micropositioner.32
Bibliography.34

Figure 1 – Apparatus block diagram.10
Figure 2 – Typical set-up for transmission source measurement .13
Figure 3 – Fibre shaker example.14
Figure 4 – Pixel and ring illustration.18
Figure A.1 – Core images from instrument A and instrument B .22
Figure A.2 – Compressed core images from instrument A and instrument B.22
Figure A.3 – Intensity versus radius for Instruments A and B .23

– 4 – 61280-1-4 © IEC:2009
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –

Part 1-4: General communication subsystems –
Light source encircled flux measurement method

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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6) All users should ensure that they have the latest edition of this publication.
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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 61280-1-4 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics.
This second edition cancels and replaces the first edition published in 2003. This second
edition constitutes a technical revision. The significant technical changes with respect to the
previous edition are described in the introduction.
The text of this standard is based on the following documents:
FDIS Report on voting
86C/920/FDIS 86C/932/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.

61280-1-4 © IEC:2009 – 5 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 61280 series can be found, under the general title Fibre optic
communication subsystem test procedures, on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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 publication using a colour printer.

– 6 – 61280-1-4 © IEC:2009
0 Introduction
0.1 General
This part of IEC 61280 is used to measure the encircled flux of a multimode light source.
Encircled flux is a measure, as a function of radius, of the fraction of the total power radiating
from a multimode optical fibre’s core.
The basic approach is to collect 2D nearfield data using a calibrated camera, and to
mathematically convert the 2D data into three normalized functions of radial distance from the
fibre’s optical centre. The three functions are intensity, incremental flux and encircled flux.
Intensity has dimension optical power per area; incremental flux has dimension power per
differential of radius; and encircled flux has dimension total optical power, all three being
functions of radius.
These three radial functions are intended to characterize fibre optic laser sources either for
use in mathematical models predicting the minimum guaranteed length of a communications
link, or to qualify a light source to measure insertion loss in multimode links.
0.2 Changes from previous edition
This edition of the standard differs from its predecessor in both scope and content. Many of
the content changes improve the measurement precision. Several changes have been made
to the computation procedure:
• the integration methodology of the radial functions was simple summation, and is now
specified to use trapezoidal integration or other higher-order techniques (see 9.3);
• a baseline subtraction step is specified to improve immunity to DC drifts (see 9.2.2 and
9.2.3);
• the ring width parameter is explicitly specified (see 9.2.1);
• the integration limit is specified (see 9.3).
The geometric calibration of the apparatus microscope now specifies either (depending on the
application) the methodology of IEC 61745 or the original technique using the
micropositioning stage (see Clause 7). Pixel sensitivity uniformity correction is now optional.
0.3 Assumptions applicable to the characterization of data sources
The 50-μm or 62,5-μm 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
centre, 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 metres or more of fibre. The modes of a mode
group need not carry equal flux. (In fact, with such short fibres, one thousand metres or less,
unequal distribution of flux in the modes of a group is the norm, not the exception.)
0.4 Assumptions applicable to the characterization of measurement sources
Measurement sources are assumed to be sufficiently broadband and incoherent that speckle
is not a problem, and to have a sufficiently symmetrical nearfield distribution that the
truncated centroid of that nearfield indicates the location of the optical centre of the fibre with
sufficient accuracy for the purposes of this standard.

61280-1-4 © IEC:2009 – 7 –
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –

Part 1-4: General communication subsystems –
Light source encircled flux measurement method

1 Scope
This part of IEC 61280 is intended to characterize the encircled flux of two types of light
sources: transmission light sources, which are usually coherent and substantially under-excite
the mode volume of a multimode fibre, and measurement light sources, which are incoherent
and excite most of the mode volume of a multimode fibre.
This part of IEC 61280 sets forth a standard procedure for the collection of two-dimensional
fibre optic nearfield greyscale data and subsequent reduction to one-dimensional data
expressed as a set of three sampled parametric functions of radius from the fibre’s optical
centre. This revision of IEC 61280-1-4 continues to fulfil its original purpose, characterization
of transmission light sources, which enables the accurate mathematical prediction of minimum
guaranteed link length in 1 gigabit per second or greater fibre optic data communication
systems. New to this revision is support for improved measurement precision of insertion loss
in multimode fibre optic links through the characterization of measurement light sources.
Estimation of the fibre core diameter is not an objective of this standard.
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 60793-2-10, Optical fibres – Part 2-10: Product specifications – Sectional specification for
category A1 multimode fibres
IEC 60825-1, Safety of laser products – Part 1: Equipment classification and requirements
IEC 61745:1988, End-face image analysis procedure for the calibration of optical fibre
geometry test sets
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
calibration light source
light source used to find the optical centre of a multimode fibre
3.2
centroid image
image used to determine the optical centre of the multimode fibre core

– 8 – 61280-1-4 © IEC:2009
3.3
corrected image
image which has had a dark image subtracted from it and whose elements have had
uniformity correction applied
3.4
dark image
image taken with the measured light source either turned off or not installed in the input port.
Stray light and electrical signals of the detection system will remain in the dark image
3.5
image
two-dimensional rectangular array of numbers whose elements are pixels and whose pixel
values linearly correspond to the optical power falling on the pixels
3.6
light source
something that emits light that is coupled into a fibre, the output of which can be measured
(can be a calibration light source, a transmission light source or a light source used for
attenuation measurements)
3.7
measurement light source
light source intended to be used in the measurement of attenuation
3.8
nominal core radius
half the nominal core diameter of the multimode fibre to be measured
3.9
ring smoothing
technique to reduce the two dimensional near field image into a 1-D near field intensity profile
while cancelling the effects of the periodic spacing of imager pixels of finite area
3.10
transmission light source
light source used to transmit digital data over multimode fibre optic links
3.11
uniformity correction
process to correct the sensitivity of a pixel so that it performs substantially like an average
pixel
3.12
valid pixel
optical detection element in the detector matrix whose sensitivity, when corrected, is within
5 % of the mean sensitivity of the average conversion efficiency of the detector
4 Symbols
B  the baseline intensity. This value is determined from a region of the computed near
field just outside the core boundary.
D  the distance from the centre of the centroid image to the nearest boundary of the
image.
D , D , D , D the set of distances from the centre of the centroid image to, respectively, the
L R T B
left, right, top and bottom boundaries of the image. The minimum of this set is
used to compute D.
61280-1-4 © IEC:2009 – 9 –
EF(i) the encircled flux vector.
i  the index parameter used in the parametric result vectors R(i), I(i) and EF(i) .
I the matrix of pixel intensities of a dark image as measured by the detector and
dark
digitizer.
I the matrix of pixel intensities of the light source, before correction, as measured by the
raw
detector and image digitizer.
I near-field intensity matrix. This is a matrix of pixel intensities, based on I , as
r,c raw
measured by the detector and corrected using U and I
dark.
I(i) the ring-smoothed intensity vector, each element being the arithmetic average of the
set of radial coordinates of all the pixels in a given ring.
N the number of rings used to compute the 1-D near field.
R
N the number of rows in an image. All columns in an image have the same number of
r
rows.
N the number of columns in an image. All rows in an image have the same number of
c
columns.
P the most intense valid pixel in the centroid image.
Max
P the least intense valid pixel in the centroid image.
Min
R the radial coordinate, in μm, of the centre of any pixel, referenced to the optical centre
X ,Y .
0 0
R(i) the ring-smoothed radial vector, each element being the arithmetic average of the radii
th
of all the pixels in the i ring.
S the column-weighted summation of all pixel intensities greater than T in the centroid
c
image.
S (i) the intensity summation vector used in ring smoothing.
I
S the summation of all pixel intensities greater than T in the centroid image.
P
S (i) the pixel counting vector used in ring smoothing.
N
S (i) the radius summation vector used in ring smoothing.
R
S the row-weighted summation of all pixel intensities greater than T in the centroid
r
image.
T the threshold used to determine which pixels in the centroid image will be used to
determine the optical centre. All pixels greater than or equal to T are used to compute
the centroid.
U the sensitivity correction matrix, applied to a dark-subtracted image to reduce non-
r,c
uniformity of the detector’s pixel-to-pixel conversion efficiency.
W the half-width, in μm, of the rings used to compute the 1-D near field.
X the x-axis (column) location of the centre of the centroid image.
Y the y-axis (row) location of the centre of the centroid image.
5 Apparatus
5.1 Common apparatus
5.1.1 General
The Figure 1 below shows an apparatus block diagram.

– 10 – 61280-1-4 © IEC:2009
Collimating
region
Computer
Attenuation
(optional)
Detector Image
digitizer
electronics
Input port
Detector
Magnifying
optics
The image digitizer may be either part of a camera or a computer add-in board.
The detector electronics are usually integral to the camera and digitizer.
Attenuation is best placed in the collimating region of the optical path, but not all optical designs
will have an accessible collimating region. When this is not possible, the attenuation should
be placed on the detector side of the optics.
When a micropositioner (not shown) is employed, the input port will be physically attached to it.
IEC  2207/09
Figure 1 – Apparatus block diagram
5.1.2 Computer
Since the acquired image contains many thousands of pixels, and the reduction of the image
to encircled flux requires substantial computation, a computer is required. The computer will
usually be connected to the image digitizer to control the acquisition of an image through
software, and may also control the micropositioner (and the source, if correlated double
sampling is implemented).
5.1.3 Image digitizer
The nearfield of the fibre core is imaged onto the detector and then digitized by the image
digitizer. The image digitizer may be an integral part of a camera which also contains the
detector, or may be an add-in frame-grabber board in the computer.
Automatic circuitry in the digitizer, for example AGC or automatic gain control often found in
video cameras, shall be disabled.
5.1.4 Detector
The detector is typically a CCD or CMOS camera. Other types of array cameras may be
considered. In any case, detectors shall be both nominally linear and memoryless; this
excludes for instance lead sulphide vidicon detectors. Absolute radiometric measurement of
flux (optical power flow) is not required.
Automatic circuitry in the detector, for example AGC or automatic gain control often found in
video cameras, shall be disabled.
The difference in conversion sensitivity from pixel to pixel in the detector will affect the
accuracy of measurement. The corrected conversion efficiency non-uniformity of the detector
shall not exceed ± 5 %. It is possible to calibrate and correct a detector whose uncorrected
uniformity is worse than 5 % by applying a pixel-by-pixel sensitivity correction matrix, U, to the
raw image. Often, this correction is part of the camera function (and so each element of U
may be taken as unity). Sometimes, the correction matrix may be supplied by the detector
supplier. In other cases, the correction matrix shall be determined by the procedure outlined
in A.2.
61280-1-4 © IEC:2009 – 11 –
Detectors can have invalid pixels, these are pixels whose corrected conversion efficiency
exceeds ± 5 % of the average conversion efficiency of the detector; invalid pixels will often
produce no signal or a completely saturated signal, or be stuck at some intermediate value.
Detectors whose invalid pixel count exceeds 0,1 % of the total number of pixels shall be
rejected.
In most cameras and image digitizers, the setting of the ‘black level’ is user adjustable. Since
the detector will be slightly noisy, it is important that the detector and digitizer do not clip
random black signals at zero (in common systems, random noise in a detector will have a
standard deviation less than 0,5 % of the saturation level). To ensure no clipping of the noise,
when settable, set the black level to produce a small positive signal (typically at least five
times the standard deviation of the noise) when no light is impinging on the detector.
5.1.5 Magnifying optics
Suitable optics shall be provided which projects the magnified image of the input port onto the
detector such that the detector can measure the entire nearfield flux distribution. The
numerical aperture of the magnifying optics shall exceed the nominal numerical aperture of
the fibres (as specified in the fibre’s family specification) used in calibration or measurement.
Microscope objectives are often appropriate for this purpose.
NOTE 1 When a microscope objective is used, its actual magnification 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.
The geometric calibration procedures outlined in Clause 7 of this standard determine the actual magnification.
NOTE 2 When characterizing measurement light sources, measurement precision is important, so optical
distortion is kept to a minimum. Care in selection and application of the lenses and other optical components
should be considered. Plan-type microscope objectives are an example of suitable optics. The procedures found in
IEC 61745 can be used to assess the optical integrity of the apparatus.
NOTE 3 Reflections from optical surfaces may seriously degrade the measurement of encircled flux. Anti-
reflection coating at the wavelength of measurement or other forms of reflection control may be considered to
reduce reflections.
It is important that the distance between the detector and all elements of the magnifying
optics be held fixed once calibration is performed. When the relationship between these
elements changes, the magnification is expected to change enough that recalibration will be
required. Focusing shall be accomplished by changing only the distance between input port
and the magnifying optics.
5.1.6 Attenuation
Often, the optical flux of the source will saturate the detector and the only effective solution is
to employ optical attenuation. Any attenuation element shall not reduce the numerical
aperture of the optical system, and shall not be the source of significant reflections or optical
distortions which will bias the resulting encircled flux.
NOTE 1 When neutral density filters are used in the optical system, geometric distortions may be introduced.
NOTE 2 Changing the attenuation between the optical centre image and the image of the measured source may
cause the location of the optical centre of the measurement source to move away from that determined using the
optical centre image, causing errors in the resulting radial data functions.
5.1.7 Micropositioner (optional)
The micropositioner is an optional part of the apparatus. Depending on the apparatus design,
it is possible to rely on connector ferrule geometry to place the image completely onto the
detector without a micropositioner. In many implementations, only a focus adjustment (Z axis)
is necessary, and in some cases, all three axes may only require alignment during
construction or maintenance of the apparatus. Using the ferrule to place the fibre core image
onto the detector does not relieve the requirement of finding the optical centre as required by
8.3.
– 12 – 61280-1-4 © IEC:2009
When used, the purpose of the micropositioner is to bring the projected image of the fibre
face into focus on the detector and to determine the magnification of the apparatus (see
Clause 7). Mechanical locking mechanisms or their equivalents are required for all three axes
to prevent mechanical drift during measurement. The micropositioner can optionally be driven
by motors, and can optionally employ feedback mechanisms to control the actual position of
the stage (and thus the fibre face). When geometric calibration is done using the
micropositioner (see Clause 7 and Annex C), the performance requirements are specified in
Annex B; otherwise, the only performance requirement is in the focal axis, which shall have
high enough resolution to bring the fibre end into sufficient focus to achieve the required
measurement precision.
5.1.8 Input port
The input port is where the calibration artefacts and measurement samples are connected to
the apparatus. The input port characteristics depend on which type of source is to be
characterized.
When characterizing transmission light sources, the input port is the distal end of the test
jumper assembly. The proximal end of the test jumper assembly will be imaged onto the
detector; when a micropositioner is used, the proximal end will be attached to the
micropositioner.
When characterizing measurement light sources, the input port is commonly a connector
bulkhead or its equivalent. When a micropositioner is employed, the bulkhead will be attached
to the micropositioner.
See 5.2 and 5.3 for particular requirements.
5.1.9 Calibration light source
The calibration light source is used when calibrating the apparatus (see Clause 7). When this
source is used to illuminate the test jumper assembly, the calibration source shall overfill the
modes of the jumper. Optionally, a mode scrambler may be used with the chosen calibration
source to ensure more uniform overfilling of the fibre. See IEC 60793-1-41, for information on
mode scramblers.
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. When calibrating the apparatus for the characterization of measurement light sources,
the centre wavelength of the calibration source shall be within 30 nm of the nominal
wavelength of the light sources to be qualified, and its spectral width (full width, half
maximum) shall be no more than 100 nm. When calibrating the apparatus for the
characterization of transmission light sources, the spectral characteristics of the calibration
source are not specified, but it is recommended that its spectrum be similar to the sources to
be characterized. The chosen calibration source shall be stable in intensity over a time period
sufficient to perform the measurements.
5.2 Transmission source apparatus
5.2.1 General
When characterizing transmission light sources, the input port of the apparatus consists of
two elements, the test jumper assembly and the fibre shaker (see Figure 2 below).

61280-1-4 © IEC:2009 – 13 –
Source control (optional)
to computer
Distal end Proximal end
Calibration
source
Fibre
Transmission
Optical connector ends
source no. 1
Shaker
Transmission
Test jumper assembly
source no. 2
Transmission
source no. N
IEC  2208/09
Figure 2 – Typical set-up for transmission source measurement
5.2.2 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 is only used when qualifying light sources for multimode transmission.
The test jumper assembly shall be at least 10 m in length, made of germanium-doped near-
parabolic graded-index fused-silica multimode “glass,” an IEC 60793-2-10 class A1 fibre with
a core diameter of either 50 μm or 62,5 μm. 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.
5.2.3 Fibre shaker
The purpose of the fibre shaker is to change the differential path length of the various modes
in the test jumper, ensuring that as the image is averaged, speckle in the averaged imaged
will be reduced. Speckle reduction can be accomplished in a variety of ways, and shall be
good enough to ensure sufficient repeatability in the measurement of encircled flux. Shaking
of the test jumper assembly with a mechanical device is required to reduce speckle.
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, shown in Figure 3, 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 m of the test jumper
assembly’s fibre.
– 14 – 61280-1-4 © IEC:2009
Fibre Fibre
in out
Elastic Elastic
120MM
±120 mm
fibre DIAMETER fibre
diameter
CIRCLE
clamp clamp
circle
±120 mm
120MM
diameter
DIAMETER
circle
CIRCLE
±25mm peak
displacement
IEC  2209/09
NOTE 1 Only one figure-eight loop of the three is shown here, for visual clarity. Fibre clips are used to keep fibre
in place, in addition to elastic fibre clamps that prevent transmission of fibre motion. Loose fibre clips not shown.
NOTE 2 Fibre is moved back and forth as shown, with a peak-to-peak amplitude of about 25 mm, distorting one
fibre loop.
Figure 3 – Fibre shaker example
Another exemplary approach is to hang large loose loops of fibre in front of a large fan which
blows these loops about, the turbulence in the stream of the fan randomizing the motion.
NOTE 1 The fibre ends leading into and out of the fibre shakers are mechanically fixed or stabilized to prevent
movement of fibres at connection points. In addition, the fibre shakers are 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, detector, or magnifying optics. 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 acquisition time of an image. Typically, in each measurement period, many individual images are taken and
later summed or averaged by the computer; this technique may help to avoid detector saturation. The detector and
digitizer may be able to perform an equivalent function independently. The image will be relatively speckle free
when one hundred shake cycles are averaged in this way.
5.3 Measurement source apparatus
The apparatus as described in 5.1 is sufficient to characterize measurement light sources.
Fibre shaking or other speckle reduction techniques shall not be employed.
NOTE This standard does not address the characterization OTDR transmitters, which will display significant
speckle. As of the drafting of this standard, the characterization of encircled flux for OTDRs remained a subject of
study.
6 Sampling and specimens
Light 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. The only
requirements on the light sources under test are that they have an operating wavelength

61280-1-4 © IEC:2009 – 15 –
compatible with the detector, and have optical connectors or splices compatible with the input
port of the apparatus. The construction details of the light sources are otherwise unspecified.
When qualifying lasers, the laser drive current shall be sufficient to ensure that the laser
always acts as a laser, rather than as an LED.
7 Geometric calibration
Calibration of the apparatus is critical to the accuracy of this measurement procedure. (See
A.4 for description of the kinds of noise and errors which calibration can correct.) Calibration
shall be performed periodically, and should be performed at least monthly. If the calibration is
known to drift significantly during a measurement interval, the source(s) of the drift
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