IEC 61744:2001

INTERNATIONAL IEC
STANDARD
First edition
2001-02
Calibration of fibre optic chromatic
dispersion test sets
Etalonnage des ensembles d'essai de la dispersion
chromatique des fibres optiques

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edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the
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INTERNATIONAL IEC
STANDARD
First edition
2001-02
Calibration of fibre optic chromatic
dispersion test sets
Etalonnage des ensembles d'essai de la dispersion
chromatique des fibres optiques

 IEC 2001  Copyright - all rights reserved
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– 2 – 61744  IEC:2001(E)
CONTENTS
Page
FOREWORD . 4

INTRODUCTION .5

Clause
1 Scope . 7

2 Normative references . 7
3 Terms and definitions. 8
4 Calibration . 14
4.1 Rationale for calibration of CD test sets. 14
4.1.1 (Full) calibration . 14
4.1.2 Calibration checking . 14
4.2 Preparation for calibration . 15
4.2.1 General advice and organization. 15
4.2.2 Test environmental requirements. 16
4.2.3 Measurement equipment requirements . 16
4.2.4 Traceability . 16
4.3 Calibration procedure. 16
4.4 Calibration checking procedure . 17
5 Wavelength calibration procedure. 17
5.1 General. 17
5.2 Discrete sources . 17
5.3 Tunable sources . 18
5.3.1 Method A . 18
5.3.2 Method B . 19
5.3.3 Method C . 20
5.4 Uncertainties and reporting . 20
6 Delay [dispersion] calibration procedure . 20
6.1 General. 20
6.2 Equipment and preparation . 20
6.2.1 Pulse delay CD test sets. 21

6.2.2 Phase shift CD test sets . 22
6.2.3 Interferometric CD test sets. 22
6.2.4 Differential phase shift CD test sets. 22
6.3 Calibration procedure. 23
6.4 Uncertainties and reporting . 23
7 Calibration checking procedure . 23
7.1 General. 23
7.2 Equipment and preparation . 24
7.3 Procedure . 24
7.4 Uncertainties and reporting . 25
7.5 Generation of infant reference fibre . 25
8 Documentation. 26
8.1 Specifications, measurement data and uncertainties. 26
8.2 Traceability information . 27

61744  IEC:2000(E) – 3 –
Annex A (normative) Mathematical basis. 28

A.1 Deviations. 28

A.2 Uncertainties type A . 28

A.3 Uncertainties type B . 29

A.4 Accumulation of uncertainties. 30

A.5 Reporting . 31

Annex B (normative) Assessment of operational uncertainties. 32

B.1 Wavelength calibration uncertainties . 32

B.2 Fibre length uncertainty. 33
B.3 Optical delay variation . 33
B.4 Instrumentation uncertainties.34
B.5 Effect of dispersion modelling. 37
B.6 Fibre related uncertainties. 37
B.7 System dispersion uncertainties . 38
Annex C (informative) Chromatic dispersion. 39
C.1 Chromatic dispersion in fibres . 39
C.2 Description of chromatic dispersion test sets . 39
C.3 Measurement techniques.40
C.4 Fibre chromatic dispersion specifications . 41
Annex D (informative) CD test set calibration compensation. 42
D.1 Calibration compensation . 42
D.2 Calibrated reference fibres . 43
D.3 Calibration compensation procedure. 43
Bibliography . 45

– 4 – 61744  IEC:2001(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
CALIBRATION OF FIBRE OPTIC CHROMATIC

DISPERSION TEST SETS
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 61744 has been prepared by IEC technical committee 86: Fibre
optics.
The text of this standard is based on the following documents:
FDIS Report on voting
86/170/FDIS 86/173/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 3.
Annexes A and B form an integral part of this standard.
Annexes C and D are for information only.
The committee has decided that the contents of this publication will remain unchanged until
2002. At this date, the publication will be
reconfirmed;
withdrawn;
replaced by a revised edition, or
amended.
61744  IEC:2000(E) – 5 –
INTRODUCTION
Chromatic dispersion in optical fibres

Chromatic dispersion is the variation with optical light wavelength of the light propagation delay

time in a length of fibre. This variation can cause bandwidth limitation in the fibre when used to

transmit communication signals. For a more detailed explanation, refer to annex C and

IEC 60793-1-1.
Chromatic dispersion (CD) test sets

CD test sets are used to measure the chromatic dispersion properties of optical fibres and
typically comprise an optical source of known wavelength(s), a fibre light input coupling and
output coupling means, optical detection means, and electronic or optical means of determining
the optical delay or dispersion at the source wavelength. There are several variants each
requiring slightly different calibration techniques. Refer to annex C for further details.
In general, all CD test sets produce an output of fibre delay or dispersion versus the light
wavelength, typically in graphical form. Thus, wavelength constitutes the ‘x-axis’ and delay or
dispersion the ‘y-axis’.
Overview of calibration procedures described in this standard
The requirement to calibrate the CD test set, traceable to known standards, is essential for
quality control in fibre optic production, fibre research and similar activities. This standard
describes the detailed procedures used to establish calibration of a CD test set.
Calibration of a CD test set is established by applying known artefacts or standards
(themselves calibrated to reference standards) to the CD test set, measuring its response and
adjusting (correcting) the CD test set to achieve results that match the standards used. In this
way the CD test set results will be brought to close agreement with other CD test sets also
calibrated in the manner described in this standard.
Primarily the artefacts or standards used are as follows:
a) wavelength artefact(s) used to calibrate the light source wavelength(s) used by the CD test
set. This is to establish the correct excitation wavelength for the system (the ‘x-axis’) in
order that the correct delay or dispersion (the ‘y-axis’) be determined subsequently;
b) delay or dispersion artefact(s) used to calibrate the delay or dispersion response of the CD
test set (the ‘y-axis’).
Calibration can only be carried out using these artefacts. After a calibration has been

completed, a calibration period is defined over which the CD test set is deemed to remain
calibrated. At the end of this period, it would be necessary to establish if the CD test set
calibration requires updating (changing); this can be performed using the artefact described
above, or by use of a known standard fibre (reference fibre) whose chromatic dispersion is
known. This is referred to as calibration checking. The fibre forms a stable source of known
dispersion and may be used as a simple dispersion artefact.
If it is found that the calibration has not changed within the required uncertainty limits, then it is
possible to simply extend the calibration period again by a defined amount.
If, however, it is found that the CD test set measurement results have changed significantly
compared to the user requirements (i.e. the test set has drifted), then calibration using the
artefacts (if not already carried out at this time) should be carried out and the calibration
renewed.
– 6 – 61744  IEC:2001(E)
The above rationale ensures that the CD test set calibration is only ever performed using known

standards (artefacts), but that if the CD test set is sufficiently stable over the calibration period

selected, then a simple check of calibration can suffice to ascertain this and to (justify) allow

the extension of the calibration period. The extension can be repeated indefinitely over many

calibration periods, provided the CD test set continues to remain within uncertainty limits over

the entire set of calibration periods.

In order to be considered calibrated and in conformance with this standard, a CD test set must

have its calibration adjusted based on comparison to artefacts for the primary parameters of
wavelength and delay [dispersion]. In all cases, this calibration of primary parameters is
necessary, but may or may not be sufficient, to ensure calibration of the CD test set to the

required uncertainty.
In addition, it may be necessary to also confirm or compensate the calibration state of a CD test
set using a calibrated reference fibre. The CD test set calibration compensation is explained
more fully in annex D. It should be noted that use of a calibrated reference fibre alone is not
sufficient to ensure calibration of a CD test set.
It should also be noted that if a calibrated CD test set undergoes calibration compensation
using a calibrated reference fibre, the scope and extent of its calibration is limited to the
conditions used at the time of calibration compensation (i.e. wavelength, fibre type, loss regime,
etc.) Care should be exercised that test sets calibrated and compensated in this manner are
used only within the appropriate limits of their calibration extent. The adjustments required to
effect compensation on one set of wavelengths for one fibre type may increase the uncertainty
of measurement of other fibre types with different minimum dispersion wavelengths.

61744  IEC:2000(E) – 7 –
CALIBRATION OF FIBRE OPTIC CHROMATIC

DISPERSION TEST SETS
1 Scope
This International Standard provides standard procedures for the calibration of optical fibre

chromatic dispersion (CD) test sets. It also provides procedures to perform calibration checking

on CD test sets whereby an extension to the test set calibration period may be obtained.
This standard is applicable to all types of CD test sets, with the exception that measurements
on multimode optical fibres are excluded.
The purpose of this standard is to define a standard procedure for calibrating optical fibre
chromatic dispersion (CD) test sets. The detailed calibration steps used vary according to the
measurement technique used in the CD test set.
Whilst it is acknowledged that chromatic dispersion also occurs in multimode fibre and this fibre
may be measured on many CD test sets, this standard will restrict discussion to single mode
fibre measurements only.
The purpose of the procedures outlined in this standard is to focus manufacturers and users of
CD test sets toward the reduction of measurement uncertainty in chromatic dispersion
determination in optical fibres under all applicable conditions. The procedures apply to
calibration laboratories and to the manufacturers or users of CD test sets for the purpose of
a) calibrating CD test sets,
b) setting specifications of CD test sets,
c) extending the calibration period of an already calibrated CD test set.
Use of the procedures also allows correct evaluation of CD test set uncertainty, relative and
traceable to appropriate (for example, National) standards.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this International Standard. For dated references, subsequent
amendments to, or revisions of, any of these publications do not apply. However, parties to

agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. For undated
references, the latest edition of the normative document referred to applies. Members of IEC
and ISO maintain registers of currently valid International Standards.
IEC 60050(731):1991, International Electrotechnical Vocabulary (IEV) – Chapter 731: Optical
fibre communication
1)
IEC 60793-1-1:1995, Optical fibres – Part 1: Generic specification – Section 1: General
IEC 60825-1:1993, Safety of laser products – Part 1: Equipment classification, requirements
2)
and user’s guide
___________
1)
A consolidated edition 1.1 exists (1999) that includes IEC 60793-1-1 (1995) and its amendment 1 (1998).

– 8 – 61744  IEC:2001(E)
3)
IEC 62129, Calibration of optical spectrum analyzers

ISO 9000 (all parts), Quality management and quality assurance standards

ISO 10012-1:1992, Quality assurance requirements for measuring equipment – Part 1:

Metrological confirmation system for measuring equipment

ISO 10012-2:1997, Quality assurance for measuring equipment – Part 2: Guidelines for control

of measurement processes
Guide to the Expression of Uncertainty in Measurement, 1993, ISO, ISBN 02-67-10188-9

EN 45001:1989, General criteria for the operation of testing laboratories
3 Terms and definitions
For the purpose of this International Standard, IEC 60050(731) and the following definitions
apply.
3.1
accredited calibration laboratory
calibration laboratory authorized by the appropriate national standards laboratory to issue
calibration certificates with a minimum specified uncertainty, which demonstrate traceability to
national standards
3.2
adjustment
modifying the hardware or firmware of a CD test set with the intention of making the
measurement result of the CD test set equal to that of a national standard or a similar
calibrated CD test set. This has the effect of correcting all subsequent measurements on that
CD test set
3.3
artefact
device, instrument or equipment used in the process of calibrating a CD test set, for both
wavelength and delay [dispersion]. The artefact is a means of transferring calibration of these
parameters to a CD test set
3.4
calibration
process by which the relationship between the values indicated by the infant CD test set and
known values of the calibration standard is established under specified conditions. The
intention of calibration is to bring all CD test sets into substantial agreement with a suitable
national standards laboratory. This may be performed by first comparing the relevant parameter
of a measurement artefact with that produced by the CD test set, followed by transfer of that
result, either by adjustment of the CD test set or by documentation of a calibration factor(s) in a
calibration certificate. The pertaining environmental conditions and instrument state are usually
recorded. Calibration includes estimation of all uncertainties. The use of reference fibres is for
calibration checking only
2)
A consolidated edition 1.1 exists (1998) that includes IEC 60825-1 (1993) and its amendment 1 (1997).
3)
To be published.
61744  IEC:2000(E) – 9 –
3.5
calibration chain
unbroken chain of transfers from a primary standard to the CD test set via reference standards,

intermediate and/or working standards (see figure 1)

Uncertainty
National wavelength
and delay standard
Uncertainty of
reference
standard
Uncertainty of
Reference wavelength
Transfer
transfer
and delay standard
uncertainty
standard
Transfer
uncertainty
Uncertainty of
working
standard
Transfer wavelength
and delay standard
Transfer Uncertainty of
uncertainty CD test set at
reference
condition
Working wavelength
and delay standard
Transfer
Total
uncertainty
uncertainty of
CD test set
CD test set at
reference conditions
Operational
uncertainty
CD test set at
normal operating
conditions
IEC  2767/2000
Figure 1 – Typical calibration chain for CD test sets
3.6
calibration checking
process of establishing that a CD test set which has been previously calibrated, but is nearing
the end of its calibration period, remains within specified uncertainty limits. If the CD test set
has drifted outside these limits, then calibration is required. Otherwise, the calibration period

can be extended for a stated period, and calibration checking may be repeated indefinitely if the
CD test set remains stable over successive calibration periods. Calibration checking is
performed using a reference fibre or working standard. Essentially calibration checking is the
first part of the process of calibration, but without the additional process of transfer or
adjustment
3.7
calibration period
interval of confirmation
time period over which a calibration performed in accordance with the procedures in this
standard is deemed to remain within the uncertainty limits set. (i.e. remain valid). The time
allotted will be governed by individual user requirements, CD test set characteristics, past
experience, environmental conditions, etc. and by monitored CD test set measurement result
experience in normal use (see also ISO 10012-1 and ISO 10012-2)

– 10 – 61744  IEC:2001(E)
3.8
calibration standard
artefact that is calibrated against a reference standard and is used to calibrate CD test sets.

The artefact may be a delay [dispersion] or a wavelength standard artefact. Proper use of the

calibration standard ensures traceability. The term includes the national standard, reference

standard, the transfer standard and the working standard in descending order of metrological

uncertainty
3.9
central wavelength
power-weighted mean wavelength of a light source in air, in units of nanometers (nm)

For a continuous source spectrum, the central wavelength λ in air is defined by the following
c
integral, where the integration limits enclose the entire spectrum of the source:
λ = (1/ P ) × [ p(λ) × λ dλ](1)
c total ∫
where
P = p(λ) dλ is the total optical source power.
total ∫
For a spectrum consisting of i discrete lines, the centre wavelength in air λ is defined as:
c
 
λ = (1/ P ) × p λ (2)
∑
c total  
i i
 i 
where
p(λ) is the spectral power density of the source in W/nm;
λ is the central wavelength in air in nanometers;
c
th
λ is the i discrete line in nm;
i
p is the power levels at λ in W;
i i
=
P p is the total power in W.
∑
total
i
i
3.10
chromatic dispersion (CD) test sets
instrument capable of measuring the chromatic dispersion of a single mode fibre at various
wavelengths in the transmission windows of interest, typically the 1 310 nm and/or 1 550 nm
wavebands
3.11
combined standard uncertainty
combination of a number of individual standard uncertainties

NOTE The term “accuracy” should be avoided in this context.
All calibration reports and technical data sheets should report the combined standard
uncertainty of the CD test set as an overall expanded uncertainty, U, with the applicable
confidence level, for example 95,5 % or 99,7 %.
3.12
confidence level
estimation of the probability that the true value of a measured parameter lies in the given range
(the expanded uncertainty)
3.13
correction offset, CO
number that is added to or subtracted from the measurement result of a CD test set to correct
for a known physical effect or systematic uncertainty

61744  IEC:2000(E) – 11 –
3.14
coverage factor, k
used to calculate the expanded uncertainty, U, from the standard uncertainty σ (see 3.15)

3.15
expanded uncertainty, U
(confidence interval)
range of values within which the measurement parameter, at the stated confidence level, can
be expected to lie. It is equal to the coverage factor k times the standard uncertainty σ :

U = k × σ (3)
NOTE When the distribution of uncertainties is assumed to be normal and a large number of measurements are
made, then confidence levels of 68,3 %, 95,5 % and 99,7 % correspond to values of k of 1, 2 and 3, respectively.
The measurement uncertainty of a CD test set should be specified in the form of expanded
uncertainty U.
3.16
infant reference fibre
fibre whose dispersion is measured against a parent reference fibre. The infant reference fibre
would then be intended for calibration checking of a CD test set
3.17
instrument state
complete description of the measurement conditions and state of the CD test set during the
calibration process
NOTE Typical parameters of the instrument state are the wavelength range in use, the data fit model (as
applicable), warm-up time and other instrument settings.
3.18
measurement result
displayed or electrical output of any CD test set, in dispersion D in units of
–1 –1
• × λ
ps nm × km , lambda zero in units of nm, or zero dispersion slope S in units of
0 0
–2 –1
• ps × nm × km , after completing all actions suggested by the operating instructions, for
example warm-up.
3.19
national standard
standard whose measurement is traceable to fundamental properties, such as the speed of
light, which is recognized by an official national decision and used as the basis for fixing the
value, in a country, of all other standards of the quantity concerned

3.20
national standards laboratory
body or laboratory that maintains and operates the national standard
3.21
operating range
all conditions of, for example the dispersion, temperature and other influencing quantities, over
which the CD test set is designed to perform within the stated expanded uncertainty
3.22
parent reference fibre
reference fibre which is used as the reference for generating an infant reference fibre. The
parent reference fibre may be used for calibration checking of a CD test set

– 12 – 61744  IEC:2001(E)
3.23
reference standard
artefact calibrated against a national standard and used to calibrate CD test sets. The artefact

may be a delay [dispersion] or wavelength standard artefact. Proper use of the calibration

standard ensures traceability. The term includes the national standard, reference standard, the

transfer standard and the working standard in descending order of metrological uncertainty

NOTE In this standard, reference standard can also be taken to mean the fibre (infant or parent) which is used as

the reference for calibration checking of a CD test set.

3.24
scaling factor, SF
ratio of known standard values for a standard artefact to the values indicated by the CD test set
when no correction offsets are applied The factors can apply to wavelength, delay [dispersion]
calibration, as well as to recorded zero dispersion wavelength, slope and actual dispersion data
values when using a calibrated reference fibre (see annex D)
3.25
spectral bandwidth
full-width half-maximum (FWHM) spectral width of the source
If the source exhibits a continuous spectrum, then the spectral bandwidth, B, shall be the full-
width-half-maximum (FWHM) of the spectrum.
If the source exhibits a spectrum consisting of i discrete line (for example, a laser diode with a
multiple-longitudinal mode spectrum), then the FWHM spectral bandwidth B shall be the r.m.s.
spectral bandwidth, multiplied by 2,35 (assuming the source has a Gaussian envelope):
2 2 ½
B = 2,35 × [{ ( 1/ P ) × ( p λ )} – λ ] (4)
∑
total i i c
i
where
λ is the central wavelength (see 3.9) of the laser diode, in nm;
c
P = p is the total power, in W;
∑
total i
i
th
p is the power of i longitudinal mode, in W;
i
th
λ is the wavelength of i longitudinal mode, in nm.
i
3.26
standard uncertainty
standard deviation
uncertainty of a measurement result expressed as a standard deviation σ
For further information, refer to annex A, and the Guide to the Expression of Uncertainty in
Measurement.
NOTE In order to combine standard uncertainties from different sources (see annex A) it is important that they are
all stated at the same confidence level, i.e. for normally distributed data, at a confidence level of 68,3 %. This may
be achieved by the use of each respective coverage factor k which is determined with reference to student’s
t-distribution for each individual uncertainty component.
3.27
traceability
ability to demonstrate, for a measurement result or a CD test set, an unbroken calibration chain
originating from a national standard
CD test sets calibrated by the procedures of this standard are traceable. In the sense of this
standard, direct traceability of the measurement result to either a national standards laboratory
or to an accredited calibration laboratory is demonstrated. Such traceability includes the
calibration schedules of all artefacts in the calibration chain and detailed calculations of all
(cumulative) transfer uncertainties in the calibration chain. The use of a reference fibre or
working standard alone to compare/monitor CD test set calibration will not establish or re-
establish traceability, but only extend the duration of the traceability certification (calibration
period) if no change is found.

61744  IEC:2000(E) – 13 –
3.28
transfer
part of the calibration process where, following comparison of the relevant parameter of a

calibrated artefact to that of a CD test set, the artefact result is applied to the CD test set

Transfer may be performed either by adjustment of the CD test set, or by documentation of a

calibration factor in a calibration certificate.

3.29
transfer standard
intermediary artefact, for example a delay [dispersion] or wavelength artefact used to calibrate

new working standards of a corresponding type

3.30
transfer uncertainty
estimate, characterizing the additional uncertainty of a CD test set caused by uncertainties in
the calibration process, at the given confidence level
These uncertainties may arise from the calibration standards or artefacts as well as from the
CD test set.
3.31
uncertainty type A
type A uncertainty is obtained by statistical analysis of a series of observations, such as when
evaluating certain random effects of measurement (see Guide to the Expression of Uncertainty
in Measurement)
3.32
uncertainty type B
type B uncertainty is obtained by means other than a statistical analysis of a series of
observations, for example an estimation of probable sources of uncertainty, such as when
evaluating systematic effects of measurement (see Guide to the Expression of Uncertainty in
Measurement)
NOTE Other means may include previous measurement data, experience with or general knowledge of the
behavior and properties of relevant materials, artefacts and instruments, manufacturer’s specifications, data
provided in calibration and other certificates, and uncertainties assigned to reference data taken from handbooks.
3.33
uncertainty limits
limits of permissible error (of a measuring instrument)
bounds or extreme values of expanded uncertainty permitted by user requirements, manu-
facturer’s specification, regulatory documentation, etc. (see ISO 10012-1)

3.34
working standard
standard which, usually calibrated against a reference standard or transfer standard, is used on
a routine basis to check CD test sets

– 14 – 61744  IEC:2001(E)
4 Calibration
This clause summarizes the action of calibrating a chromatic dispersion (CD) test set and

details the recommendations for the environmental requirements of the calibration facility.

4.1 Rationale for calibration of CD test sets

4.1.1 (Full) calibration
There are two fundamental and common aspects of the various chromatic dispersion measure-

ment techniques (refer to annex C):

a) the use of a series of known (i.e. fixed) or programmable (i.e. variable) source test
wavelengths, injected into the test fibre(s);
b) the electronic or optical measurement of the pulse delay, phase shift, differential phase shift
or interference fringe peak position (according to CD test set type) produced by the test
fibre(s). Fibre dispersion is obtained by appropriate calculations on the measured data.
In essence, all CD test sets operate with wavelength as a programmed (independent) variable,
usually the ordinate (x-axis) and dispersion or time delay as the abscissa (y-axis) as a mea-
sured (dependent) variable. By their nature, fibre chromatic dispersion measurements require
multiple wavelengths to be programmed. Even in the case of a single dispersion point obtained
using the differential phase shift method, two separate wavelength values are used. It is also
typical to expect a wide range of dispersion values over a range of wavelengths to be
measured. This makes it impossible merely to transfer calibration from one CD test set to
another by exposing them to a single appropriate dispersion source, unless the use of the CD
test set is to be restricted in the range of fibre dispersion values measured (see introduction).
Rather, it is necessary to independently calibrate wavelength and delay [dispersion] response to
establish the minimum possible uncertainty in each.
The process of CD test set calibration shall therefore be broken down into two parts:
a) ensuring that the programmed wavelengths are calibrated;
b) exposing the CD test set, to known delays [dispersions] in order to calibrate the delay
[dispersion] response.
These two separate calibration stages are generally independent but ideally should always be
carried out as unified sequential operations. The detailed procedure is given in 4.3.
In each case, calibration is achieved by exposing the test set to independent transfer standards
or artefacts of wavelength and delay [dispersion]. These standards form the calibration chain
(figure 1).
4.1.2 Calibration checking
The rationale of 4.1.1 describes (full) calibration. However, typical routine operational
calibration verification (such as may frequently be carried out on CD test sets in use) may be
sufficient to perform calibration checking of CD test sets using a reference fibre as a working
standard.
The distinction between calibration checking and calibration (i.e. adjustment of correction
offsets, etc.) shall be clearly made. While it is sufficient to establish stability of the CD test set
using the reference fibre, this is not a substitute for actual calibration (see introduction). The
use of this fibre is described in clause 7.
NOTE It is envisaged that (full) calibration would be essential in the manufacture of CD test sets, while the
reference fibre would be used mainly for calibration checking.

61744  IEC:2000(E) – 15 –
It is not possible to use the reference fibre for full calibration for the following reasons:

a) To correctly evaluate the full wavelength range and fibre length range of the CD test set,

several reference fibres with differing dispersions would be needed. This is expensive,

complex and will introduce a multiplicity of transfer and calibration uncertainty values.

b) The effect of the data fit and wavelength range used on the reference fibre zero dispersion

wavelength, λ , slope, S , and dispersion values means that comparison of systems shall
0 0
be made only within the wavelength range used.

c) The "λ -slope" representation, while perfectly adequate for a fibre, cannot cover the
possibility that a given CD test set exhibits non-linear delay [dispersion] or wavelength

behaviour – a more complete test of the delay [dispersion] and wavelength responses of the

CD set is required.
d) Comparison of CD test sets using a reference fibre requires a standard length value for that
fibre to be used in the measurements. Any physical reduction in the length of the fibre (for
example, by re-cleaving) shall be kept track of and accounted for and represents a source
of potential uncertainty.
Use of a reference fibre allows calibration traceability to national standards to be extended, if it
can be satisfactorily established that the instrument and existing correction offsets and scaling
factors, etc. are sufficient to provide dispersion results within the applicable uncertainty limits
without alteration. This simply means that the CD test set has remained stable since the last
actual calibration. Indeed, it is permissible to extend the calibration period indefinitely until such
time that the calibration checking indicates that the CD test set has drifted outside these
uncertainty limits. At this point, full re-calibration would be required.
It is anticipated that the reference fibre be used also for comparison, as distinct from
calibration, between CD test sets under identical (for example, controlled or reference)
conditions, test wavelengths, fibre type, fibre length ranges, etc. (see introduction).
4.2 Preparation for calibration
4.2.1 General advice and organization
The following recommendations apply:
Calibrations should be carried out where possible with facilities (refer to ISO 10012-1,
ISO 10012-2 and EN 45001) that are independent of the other functions of the laboratory/
organization. This independence should include measurement equipment.
The environmental conditions shall be commensurate with the degree of uncertainty that is
required for calibration:
a) the environment shall be clean;
b) temperature monitoring and control is required;
c) humidity monitoring and control is required;
d) all laser sources shall be safely operated (see IEC 60825-1).
All standards used in the calibration process shall be calibrated according to a documented
programme with traceability to national standards laboratories or to accredited standards (see
figure 1). It is advisable to maintain more than one standard on each hierarchical level of the
calibration chain so that the performance of standards can be verified by comparisons on the
same level.
There shall be a documented measurement procedure for each type of calibration performed,
giving step-by-step operating instructions and equipment to be used. There should be pro-
forma result sheets, uncertainty budgets and calibration certificates.

– 16 – 61744  IEC:2001(E)
The calibration laboratory should operate a quality system appropriate to the range of

measurements it performs (for example, ISO 9000). There should be an independent scrutiny

of measurement results, intermediary calculations and preparation of calibration certificates.

4.2.2 Test environmental requirements

The following requirements shall be observed:

a) all tests shall be performed at an ambient temperature of 23 °C ± 3 °C with a relative

humidity
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