IEC 60793-1-42:2007

IEC 60793-1-42
Edition 2.0 2007-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Optical fibres –
Part 1-42: Measurement methods and test procedures – Chromatic dispersion

Fibres optiques –
Partie 1-42: Méthodes de mesure et procédures d’essai – Dispersion
chromatique
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IEC 60793-1-42
Edition 2.0 2007-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Optical fibres –
Part 1-42: Measurement methods and test procedures – Chromatic dispersion

Fibres optiques –
Partie 1-42: Méthodes de mesure et procédures d’essai – Dispersion
chromatique
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
V
CODE PRIX
ICS 33.180.10 ISBN 2-8318-9204-X
– 2 – 60793-1-42 © IEC:2007
CONTENTS
FOREWORD.4

1 Scope.6
2 Normative references .7
3 Overview of methods .7
3.1 Method A, phase shift.7
3.2 Method B, spectral group delay in the time domain.7
3.3 Method C, differential phase shift .7
3.4 Method D, interferometry.8
4 Reference test methods.8
4.1 Category A1 and category A4f, A4g and A4h multimode fibres .8
4.2 Class B single-mode fibres .8
5 Apparatus.8
5.1 Launch optics.8
5.2 High-order mode filter (single-mode) .8
5.3 Input positioning apparatus .9
5.4 Output positioning apparatus.9
5.5 Computation equipment.9
6 Sampling and specimens.9
6.1 Specimen length.9
6.2 Specimen end face.9
6.3 Reference fibre .9
7 Procedure .9
8 Calculations .10
8.1 Category A1 and A4f, A4g, A4h multimode and B1.1 and B1.3 single-mode
fibres.10
8.2 Category B1.2 single-mode fibres.10
8.3 Category B2 single-mode fibres.10
8.4 Category B4 and B5 single-mode fibres.11
9 Results .11
10 Specification information .11

Annex A (normative) Requirements specific to method A, phase-shift .12
Annex B (normative) Requirements specific to method B, spectral group delay in the
time domain .17
Annex C (normative) Requirements specific to method C, differential phase-shift .21
Annex D (normative) Requirements specific to method D, interferometry .26
Annex E (normative) Chromatic dispersion fitting.30

Figure A.1 – Chromatic dispersion measurement set, multiple laser system (typical) .13
Figure A.2 – Typical delay and dispersion curves.13
Figure A.3 – Chromatic dispersion measurement set, LED system (typical) .15
Figure B.1 – Block diagram, fibre Raman laser system .18
Figure B.2 – Block diagram, multiple laser diode system.18

60793-1-42 © IEC:2007 – 3 –
Figure C.1 – Chromatic differential phase dispersion measurement set, multiple laser
system .22
Figure C.2 – Chromatic differential phase dispersion measurement set, LED system .23
Figure C.3 – Chromatic dispersion measurement set, differential phase by dual
wavelength method.23
Figure C.4 – Chromatic dispersion measurement set, differential phase by double
demodulation .24
Figure D.1 – Fibre chromatic dispersion test set – Interferometry by fibre reference
path .28
Figure D.2 – Fibre chromatic dispersion test set – Interferometry by air reference path.28
Figure D.3 – Examples of delay data .29

Table E.1 – Definition of fit types and fit coefficients; equations for group delay and
dispersion coefficient .30
Table E.2 – Slope equations .30
Table E.3 – Zero-dispersion wavelength and slope equations .31

– 4 – 60793-1-42 © IEC:2007
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
OPTICAL FIBRES –
Part 1-42: Measurement methods and test procedures –
Chromatic dispersion
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
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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 60793-1-42 has been prepared by subcommittee 86A: Fibres and
cables, of IEC technical committee 86: Fibre optics.
This second edition cancels and replaces the first edition published in 2001. It constitutes a
technical revision. The main changes in this second edition concern the addition of a new
Annex E on chromatic dispersion fitting and the applicability to A4 fibres.
This bilingual version replaces the monolingual version (2007) and its corrigendum (2007).
This standard is to be read in conjunction with IEC 60793-1.
This bilingual version, published in 2007-04, corresponds to the English version.

60793-1-42 © IEC:2007 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
86A/1136/FDIS 86A/1146/RVD
Full information on the voting for the approval of this part can be found in the report on voting
indicated in the above table.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The list of all parts of the IEC 60793 series, under the general title Optical fibres, can be
found 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.
– 6 – 60793-1-42 © IEC:2007
OPTICAL FIBRES –
Part 1-42: Measurement methods and test procedures –
Chromatic dispersion
1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the chromatic
dispersion of optical fibre, thereby assisting in the inspection of fibres and cables for
commercial purposes.
Chromatic dispersion varies with wavelength. Some methods and implementations measure
the group delay as a function of wavelength and the chromatic dispersion and dispersion
slope are deduced from the derivatives (with respect to wavelength) of this data. This
differentiation is most often done after the data are fitted to a mathematical model. Other
implementations can allow direct measurement (of the chromatic dispersion) at each of the
required wavelengths.
For some categories of fibre, the chromatic dispersion attributes are specified with the
parameters of a specific model. In these cases, the relevant recommendation or standard
defines the model appropriate for the definition of the specified parameters. For other fibre
categories, the dispersion is specified to be within a given range for one or more specified
wavelength intervals. In the latter case, either direct measurements may be made at the
wavelength extremes or some fitting model may be used to allow either group delay
measurement methods or implementations or storage of a reduced set of parameters that may
be used to calculate the interpolated dispersion for particular wavelengths which may not
have actual direct measurement values.
Annex E gives a general description of chromatic dispersion fitting and outlines a number of
fitting equations suitable for use with any of the measurement methods or fibre categories.
This standard gives four methods for measuring chromatic dispersion:
– method A: phase shift;
– method B: spectral group delay in the time domain;
– method C: differential phase shift;
– method D: interferometry.
Methods A, B, and C apply to the measurement of chromatic dispersion of the following fibres
over a specified wavelength range:
– class A1 graded-index multimode fibres;
– category A4f, A4g and A4h multimode fibres;
– class B single-mode fibres (all categories).
Method D applies to the measurement of chromatic dispersion values of single-mode fibres
categories B1, B2, B4 and B5 over the 1 000 nm to 1 700 nm wavelength range.
The methods can be applied to laboratory, factory and field measurements of chromatic
dispersion, and the wavelength range of the measurements can be tailored as required.
Measurements are made at temperature as stated in IEC 60793-1-1, Table 1 – Standard
range of atmospheric conditions (Temperature 23 °C ±5 °C).

60793-1-42 © IEC:2007 – 7 –
The methods are suitable for fibre or cable lengths greater than 1 km. They may also be
applied to shorter lengths, but accuracy and repeatability may be compromised. Method D is
the preferred method for shorter piece fibres (1 m to 10 m).
Information common to all methods is contained in Clauses 1-8, and information pertaining to
each individual method appears in Annexes A, B, C, and D, respectively.
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 60793-1-1:2002, Optical fibres – Part 1-1: Measurement methods and test procedures –
General and guidance
IEC 60793-1-41, Optical Fibres – Part 1-41: Measurement methods and test procedures –
Bandwidth
3 Overview of methods
3.1 Method A, phase shift
This method describes a procedure for determining the chromatic dispersion of all categories
of type B single-mode fibres, category A1 graded-index multimode fibres and category A4f,
A4g and A4h fibres, over a specified wavelength range using the relative phase shifts among
sinusoidally modulated optical sources of different wavelengths. The sources are typically
laser diodes or filtered light emitting diodes or filtered amplified spontaneous emission (ASE)
sources. Relative phase shifts are converted to relative time delays, and the resultant spectral
group delay data are then fitted to an equation defined for each fibre type.
3.2 Method B, spectral group delay in the time domain
This method describes a procedure for determining the chromatic dispersion of all categories
of type B single-mode fibres, category A1 graded-index multimode fibres and category A4f,
A4g and A4h fibres with the use of a Nd:YAG/fibre Raman laser source or multiple laser
diodes operating at a number of wavelengths both greater than and less than the typical zero-
dispersion wavelength.
In this method, the time difference of optical pulse delay through a known length of fibre at
several wavelengths is measured. A reference set of measurements shall also be taken
through a short reference fibre and data are subtracted from data taken from the fibre under
test to obtain relative spectral group delay. The resultant spectral group delay data are then
fitted to an equation defined for each fibre type.
3.3 Method C, differential phase shift
This method describes a procedure for determining the chromatic dispersion of all categories
of type B single-mode fibres, category A1 graded-index multimode fibres and category A4f,
A4g and A4h fibres. The dispersion coefficient at a particular wavelength is determined from
the differential group delay between two closely spaced wavelengths.
In this procedure, a modulated light source is coupled into the fibre under test, and the phase
of the light exiting the fibre at a first wavelength is compared with the phase of the light
exiting at a second wavelength. Average chromatic dispersion over the interval between the
two wavelengths is determined from differential phase shift, wavelength interval and fibre
length.
– 8 – 60793-1-42 © IEC:2007
The chromatic dispersion coefficient at a wavelength medial to the two test wavelengths is
assumed to be equal to the average chromatic dispersion over the interval between the two
wavelengths. The resultant chromatic dispersion data are then fitted to an equation defined
for each fibre type.
3.4 Method D, interferometry
This method describes a procedure for determining the chromatic dispersion of single-mode
fibres categories B1, B2, B4 and B5 over the 1 000 nm to 1 700 nm wavelength range. By
using this test method, the chromatic dispersion of a short piece of fibre can be measured.
In this test method, the wavelength-dependent time delay between the test sample and the
reference path is measured by Mach-Zehnder interferometer. The reference path can be an
air path or a single-mode fibre with known spectral group delay.
It should be noted that extrapolation of the chromatic dispersion values derived from the
interferometric test in fibres of a few metres length to long fibre sections assumes longitudinal
homogeneity of the fibre. This assumption may not be applicable in every case.
4 Reference test methods
4.1 Category A1 and category A4f, A4g and A4h multimode fibres
For category A1 and category A4f, A4g and A4h multimode fibres, method B, spectral group
delay in the time domain, is the reference test method (RTM), which shall be the one used to
settle disputes.
4.2 Class B single-mode fibres
For all categories of class B single-mode fibres, method A, phase shift, is the reference test
method (RTM). Method C, differential phase shift, may also be used to resolve disputes.
5 Apparatus
The following apparatus is common to all measurement methods. Annexes A, B, C, and D
include layout drawings and other equipment requirements that individually apply for each of
the methods, A, B, C, and D, respectively.
5.1 Launch optics
The output from the signal sources shall be coupled to the fibre under test or the reference
fibre such that the physical path length for each source is held constant during the
measurement. (This requirement ensures that the relative phases of the sources do not
change due to path-length changes.) Suitable devices may include multichannel single-mode
optical switches or demountable optical connectors.
For measurement of category A1, A4f, A4g, A4h multimode fibre, launch conditions shall
comply with method A, impulse response, of IEC 60793-1-41.
5.2 High-order mode filter (single-mode)
For measurement of single-mode fibre, use a method to remove high-order propagating
modes in the wavelength range of interest. An example of such a high-order mode filter is a
single loop of radius sufficiently small to shift cut-off wavelength below the minimum
wavelength of interest.
60793-1-42 © IEC:2007 – 9 –
5.3 Input positioning apparatus
Provide means to couple the input of the specimen to the light source. Examples include the
use of x-y-z micropositioner stages, or mechanical coupling methods such as connectors,
vacuum splices, three-rod splices, etc. The position of the fibre shall remain stable over the
duration of the test.
5.4 Output positioning apparatus
Provide means of positioning the output end of the specimen such that the guided optical
power is coupled to the system detector. Such coupling may include the use of lenses, or may
be a mechanical connection to a detector pigtail.
5.5 Computation equipment
A digital computer may be used for purposes of equipment control, data acquisition, and
numerical evaluation of the data.
6 Sampling and specimens
6.1 Specimen length
Methods A, B, and C require the specimen to be a fibre or cable of known length sufficiently
long to produce adequate phase measurement accuracy. A typical minimum length is 1 km.
Because category A4f, A4g and A4h fibres have higher loss than category A1 fibres, for these
A4 fibres a minimum length of 100 m is acceptable.
NOTE Reproducibility is affected when using shorter measuring length. Longer lengths generally yield better
reproducibility.
Method D (interferometry) requires a typical specimen length in the range of 1 m to 10 m.
6.2 Specimen end face
Prepare a flat end face, orthogonal to the fibre axis, at the input and output ends of each
specimen.
6.3 Reference fibre
A single mode fibre with known dispersion characteristics shall be used to compensate for
chromatic delays in the optical sources and other equipment components. The length of this
fibre shall be less than or equal to 0,2 % of the specimen length.
In case of A4f, A4g and A4h fibres, the length of the reference fibre shall be less than or
equal to 2 m. If this length is longer than 0,2 % of the length of the specimen under test, the
chromatic dispersion of the reference fibres shall be taken into account by subtracting its
chromatic dispersion value from the results measured on the specimen length.
NOTE The temperature of the specimen should be stable during the measurement within 0,1 °C to 1 °C,
depending upon the temporal behaviour due to this change.
7 Procedure
See Annexes A, B, C and D for the procedures for methods A, B, C and D, respectively.
Reference fibre measurements are required for all methods. Reference fibre data can be
stored for use in making measurements on the specimens. The reference fibre measurement
procedure should be repeated when equipment changes on the source or receive optics or
electronics occur.
– 10 – 60793-1-42 © IEC:2007
8 Calculations
The calculation of relative delay appropriate for each method is given in Annexes A, B, C and
D, respectively.
The remainder of this clause describes the numerical fit that can be applied for all methods to
the spectral group delay data normalized by length, τ(λ), see also Annex E.
λ   is the wavelength   [nm]
τ(λ) is the normalized spectral group delay data fit   [ps/km]
D(λ) is the chromatic dispersion coefficient, with D(λ) = dτ(λ) /dλ   [ps/(nm.km)]
λ is the zero-dispersion wavelength [nm]

τ(λ ) is the relative delay minimum at the zero-dispersion wavelength [ps/km]
S(λ)  is the dispersion slope, with S(λ) = d D(λ) /dλ. [ps/(nm ⋅km)]
S   is the dispersion slope at the zero-dispersion wavelength [ps/(nm ⋅km)]
NOTE τ(λ) and D(λ) may either be direct measurements or the result of fitting the direct measurements to a
specified function.
Where, for example, a data fitting function is specified, the parameters of the expression on the right side of the
equation are determined so as to minimize the sum of squared errors with regard to the direct measurements.
Once determined, this expression is used to determine the values of other various parameters.

The fit parameters are given as the variables A, B, C, D, or E, see also Annex E.
8.1 Category A1 and A4f, A4g, A4h multimode and B1.1 and B1.3 single-mode fibres
The following applies to category A1 and A4f, A4g and A4h multimode fibres, and to category
B1.1 and B1.3 single-mode fibres around 1 310 nm.
The delay or dispersion data fit shall be fitted with the 3-term Sellmeier fit type, see Annex E.
Calculations for the chromatic dispersion coefficient D(λ), the zero-dispersion wavelength λ
and the dispersion slope at the zero-dispersion wavelength S are shown in Annex E.
In the 1 550 nm region only, the chromatic dispersion can be approximated as a linear
function with wavelength (quadratic fit type to the delay data), see Annex E.
8.2 Category B1.2 single-mode fibres
The following applies to category B1.2 single-mode fibres.
Depending on accuracy requirements, for wavelength intervals of up to 35 nm, the quadratic
fit type is allowed in the 1 550 nm region. This fitted equation should not be used to predict
chromatic dispersion at wavelengths outside the range used for the fit. For longer wavelength
intervals, either the 5-term Sellmeier fit type or the 4th order polynomial fit type is
recommended. It is not meant to be used in the 1 310 nm region.
Calculations for the chromatic dispersion coefficient D(λ) and the dispersion slope S(λ) are
shown in Annex E.
8.3 Category B2 single-mode fibres
The following applies to category B2 single-mode fibres.
Depending on accuracy requirements, for wavelength intervals of up to 35 nm, the quadratic
fit type is allowed in the 1 550 nm region. The fitted equation should not be used to predict
chromatic dispersion at wavelengths outside the range used for the fit.

60793-1-42 © IEC:2007 – 11 –
For longer wavelength intervals, either the 5-term Sellmeier fit type or the 4th order
polynomial fit type is recommended. It is not meant to be used in the 1 310 nm region.
The corresponding chromatic dispersion coefficient D(λ), the zero-dispersion wavelength λ
and the dispersion slope at the zero-dispersion wavelength S are shown in Annex E.
8.4 Category B4 and B5 single-mode fibres
The following applies to category B4 and B5 single-mode fibres.
For normal use over longer wavelength intervals (> 35 nm), either the 5-term Sellmeier fit type
or the 4th order polynomial fit type is recommended. The fitted equation should not be used to
predict chromatic dispersion at wavelength outside the range used for the fit.
NOTE For B4 fibres only, the quadratic fit type may be used in case of a short wavelength interval (≤ 35 nm). The
fit type should not be used to predict chromatic dispersion at wavelengths outside the range used for the fit.
The corresponding chromatic dispersion coefficient D(λ) and the dispersion slope S(λ) are
shown in Annex E.
9 Results
9.1 Report the following information with each measurement:
– date and title of measurement;
– equation(s) used to calculate the results;
– identification of specimen;
– length of specimen used for length normalization;
– measurement results as required by the detail specification.
NOTE Examples of the information that the detail specification may require:
a) Dispersion coefficient values measured at certain specified wavelengths.
b) Dispersion minimum and / or maximum over a specified range of wavelengths.
c) The zero-dispersion wavelength and dispersion slope at this wavelength.
9.2 The following information shall be available upon request:
– method used: A, B, C or D;
– description of optical source(s) and measurement wavelengths used;
– modulation frequency (if applicable);
– description of signal detector, signal detection electronics, and delay device;
– description of computational techniques used;
– date of latest calibration of measurement equipment.
10 Specification information
The detail specification shall specify the following information:
− type of fibre to be measured;
− failure or acceptance criteria;
− information to be reported;
− any deviations to the procedure that apply.

– 12 – 60793-1-42 © IEC:2007
Annex A
(normative)
Requirements specific to method A, phase-shift

A.1 Apparatus
A.1.1 Light source
The light source shall be stable in position, intensity, and wavelength over a time period
sufficiently long to complete the measurement procedure. Multiple laser diodes (for an
example, see Figure A.1), wavelength-tunable laser diodes, light-emitting diodes (for
example, see Figure A.3), or broadband sources (for example, a Nd:YAG laser with a Raman
fibre or an ASE source) may be used, depending on the wavelength range of the
measurement.
The wavelength launched into the fibre under test may be selected using an optical switch, a
monochromator, dispersive devices, optical filters, optical couplers, or by tuning the laser,
depending on the type of light sources and measurement set-up. The wavelength selector
may be used either at the input or at the output of the fibre under test.
For category B1 fibres measured with a three-wavelength system in which the source
wavelengths bracket the zero-dispersion wavelength, λ (see Figure A.2), the tolerance or
instability, δλ, in center wavelength will lead to maximum errors of 3δλ in measuring λ .
Maximum errors in dispersion slope, S , are directly proportional to δλ/Δλ (where Δλ = source
wavelength spacing) and will be approximately 0,012 ps/(nm ·km) for δλ/Δλ = 1 nm/30 nm.
Errors smaller than the above maximum errors can be achieved by selecting optical sources
with an average wavelength close to the expected λ of the specimen and by using more than
three wavelengths, or both.
When laser sources are used, typically, a temperature-controlled, single longitudinal-mode
laser diode with output power stabilization (e.g., PIN feedback) is sufficient. An additional
laser may be required for the reference link for field measurement sets (see A.1.4).
A.1.2 Spectral width
The spectral width of the source, as measured in the specimen, shall be less than or equal to
10 nm at 50 % power points (FWHM).

60793-1-42 © IEC:2007 – 13 –
Multiple laser
diodes
Optical
Test sample or
switch
calibration fibre
Detector
Spliter
Connector Connector
Variable
Amplifier
Fibre reference link
attenuator
(non co-located equipment)
Detector
Fibre reference link
Amplifier
(non co-located equipment)
Phasemeter
Signal
generator
Signal
Electrical reference Reference
(co-locoted equipment)
Computer
IEC  516/07
Figure A.1 – Chromatic dispersion measurement set, multiple laser system (typical)

+ 6
τ (λ)
Relative
λ = 1321 m
delay, ns/km
+ 30
s = 0,0834 ps/nm × km
D (λ)
Dispersion
+ 4 ps/nm × km
λ × μm
0,8 1,0 1,2 1,4 1,6
– 30
A = – 38,360 ns/km
+ 2 2
B =  10,421 ns/km × μm
C =  31,735 ns × μm /km
– 60
– 90
– 120
– 2
IEC  517/07
Figure A.2 – Typical delay and dispersion curves

– 14 – 60793-1-42 © IEC:2007
A.1.3 Modulator
The modulator shall amplitude modulate the light sources to produce a waveform with a
single, dominant Fourier component. For example, a sinusoidal, trapezoidal or square wave
modulation shall be acceptable. The frequency stability shall be a minimum of one part in 10 .
It is essential to prevent ambiguities of 360(n) degrees, where n is an integer, in measuring
phase shift. This can be accomplished by means such as tracking 360° phase changes, or by
choosing a modulator frequency sufficiently low to limit the relative phase shifts to less than
360°. Determine the maximum frequency for a 360° shift for category B1 fibres as:

−1
⎡ ⎤
6 2 2
⎛
⎛ ⎞
8⋅10 λ ⎞ λ
0 0
⎜
⎟
⎜ ⎟
f = λ − − λ − (A.1)
⎢ ⎥
max i j
S L ⎝ λ⎠ λ
⎝ ⎠
0 i j
⎣ ⎦
where
f is the maximum frequency for a 360° shift for category B1 fibres (MHz);
max
L  is the maximum expected specimen length (km);
S is the expected typical dispersion slope at λ (ps/nm × km);
0 0
λ is the expected typical zero-dispersion wavelength (nm);
λ and λ comprise the wavelength pair, used in the measurement, that minimizes f
i j max.
The frequency of the modulator shall be sufficiently high to ensure adequate measurement
precision.
The following is an example of the dependence of precision on test system parameters: for
category B1 fibres and a three-wavelength system, in which the source wavelengths span Δλ,
maximum errors will be 0,0012 ps/nm · km for S , and 0,4 nm for λ if the minimum
0 0
modulator frequency, f (MHz), is:
min
Δφ ⋅10
f = (A.2)
min 2
L(Δλ)
where
f is the minimum modulator frequency (MHz);
min
Δφ is the overall measurement equipment phase instability (degrees);
L is the minimum expected specimen length (km);
Δλ is the average wavelength spacing between adjacent sources (nm).
Hence for Δφ = 0,1°, L = 10 km, and Δλ = 32 nm, a minimum frequency of approximately 100
MHz is required.
NOTE 1 Equation (A.2) above was developed by repeatedly solving for λ and S in the three-term Sellmeier
0 0
expression for group delay in Annex E with various values of wavelength spacing and phase instability.
NOTE 2 Errors smaller than the above maximum errors can be achieved by selecting sources having an average
wavelength close to the expected λ of the specimen, and by using more than three wavelengths, or both.
The phase modulation at each light source may be adjustable to facilitate measurement-set
calibration.
60793-1-42 © IEC:2007 – 15 –
A.1.4 Signal detector and signal detection electronics
Use an optical detector that is sensitive over the range of wavelengths to be measured in
conjunction with a phase meter. An amplifier may be used to increase the detection system
sensitivity. A typical system might include a PIN photodiode, FET amplifier, and a vector
voltmeter.
The detector-amplifier-phase meter system shall respond only to the fundamental Fourier
component of the modulating signal and shall introduce a signal phase shift that is constant
over the range of received optical powers encountered. The received power range may be
controlled by a variable optical attenuator.
A.1.5 Reference signal
Provide a reference signal with the same dominant Fourier component as the modulating
signal to the phase meter against which to measure the phases of the signal sources. The
reference signal should be phase-locked to the modulating signal and is typically derived from
the modulating signal.
Examples of reference signal configurations (see Figures A.1 and A.3 for examples a, b,
and c):
a) Where the signal sources and detector are co-located, such as in a laboratory test or
during calibration, an electrical connection can be used between the signal generator and
the reference port of the phase meter.
b) An optical splitter, inserted before the specimen, and a detector may also be used for co-
located equipment.
c) For field testing of optical cables (sources and detector not co-located), an optical link can
be used, typically comprising a modulated light source, fibre, and detector similar to those
used for the specimen.
d) A reference signal for field testing can also be transmitted on the fibre under test using
wavelength division multiplexing.

Test sample or
calibration fibre
Light emitting
Monochromator
diode Detector
Connector Connector
Connector
Variable
Amplifier
attenuator
Detector
Fibre reference link
Amplifier
(non co-located equipment)
Phasemeter
Signal
generator
Signal
Electrical reference Reference
(co-locoted equipment)
Computer
IEC  518/07
Figure A.3 – Chromatic dispersion measurement set, LED system (typical)

– 16 – 60793-1-42 © IEC:2007
A.2 Procedure
A.2.1 Calibration
Insert the reference fibre (6.3) into the measurement apparatus, and establish a reference
signal (A.1.5). Measure and record the phase, φ (λ ), for each signal source.
in i
Alternatively, if the signal sources are phase adjustable, then with the reference fibre in place,
the phases of all signal sources shall be equalized. Then perform specimen measurements as
described in A.2.2. In this case φ (λ ) = 0 for the calculations of A.3.1.
in i
A.2.2 Specimen measurements
Insert the specimen into the measurement apparatus, and establish a reference signal (see
A.1.5). Measure and record the phase, φ (λ ), of each signal source.
out i
NOTE Perform all specimen and calibration or equalization measurements with the input optical power level at the
detector adjusted to a range that minimizes level-dependent phase shifts in the detector and detector electronics.
A.3 Calculations
A.3.1 Subtract the measured input phase at each wavelength from the output phase at that
wavelength. The relative group delay is:
τ(λ ) = []φ ()λ − φ ()λ for all λ
i out i in i i (A.3)
360 ⋅ f⋅ L
where
τ(λ )  is the relative group delay (ps/km);
i
φ (λ ) were measured in A.2.2 (degrees);
out i
φ λ ) were measured in A.2.1 (degrees);
in ( i
f (MHz) is the frequency of the modulation waveform;
L (km) is the specimen length minus the calibration specimen length.
A.3.2 Using the delay data of A.3.1, calculate the best fit to one of the delay equations in
Annex E.
A.3.3 Using these best-fit values of the appropriate coefficients from Clause 8, calculate the
dispersion, D(λ), or other parameters as required by the detail specification. Refer to
Figure A.2 as an example of the delay data, τ(λ), and the calculated dispersion, D(λ).
A.3.4 Dispersion can be specified through the zero-dispersion wavelength, λ , and slope, S ,
0 0
or by specifying the chromatic dispersion coefficient at one or more wavelengths, or both. In
some cases, the zero-dispersion wavelength and slope parameters are only used to calculate
the dispersion coefficient at wavelengths well away from the zero-dispersion wavelength.
When the zero-dispersion wavelength is specified, the measurement wavelengths should
bracket the zero-dispersion wavelength or include data at a wavelength within 100 nm. When
the zero-dispersion wavelength and slope are used only for the calculation of dispersion
coefficients at wavelengths far away from the zero-dispersion wavelength, the measurements
shall span the wavelengths with which the calculation is used. When the dispersion coefficient
is specified, the measurements shall span the wavelengths at which the dispersion coefficient
is specified. See Annex E for information on the wavele
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