EN 61746:2001

EUROPEAN STANDARD EN 61746
NORME EUROPÉENNE
EUROPÄISCHE NORM November 2001
ICS 17.180.30;33.180.99
English version
Calibration of optical time-domain reflectometers (OTDRs)
(IEC 61746:2001)
Etalonnage des réflectomètres optiques Kalibrierung optischer
dans le domaine de temps (OTDR) Rückstreumessgeräte (OTDR)
(CEI 61746:2001) (IEC 61746:2001)
This European Standard was approved by CENELEC on 2001-10-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and
notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands,
Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2001 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61746:2001 E
Foreword
The text of document 86/175/FDIS, future edition 1 of IEC 61746, prepared by IEC TC 86, Fibre
optics, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61746 on 2001-10-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2002-07-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2004-10-01
Annexes designated "normative" are part of the body of the standard.
Annexes designated "informative" are given for information only.
In this standard, annexes A, B, C and ZA are normative and annex D is informative.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61746:2001 was approved by CENELEC as a European
Standard without any modification.
__________
- 3 - EN 61746:2001
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
IEC 60050-731 1991 International Electrotechnical --
Vocabulary (IEV)
Chapter 731: Optical fibre
communication
IEC 60617-10 1996 Graphical symbols for diagrams EN 60617-10 1996
Part 10: Telecommunications:
Transmission
IEC 60793-1 Series Optical fibres --
Part 1: Generic specification
IEC 60794-1 Series Optical fibre cables EN 60794-1 Series
Part 1: Generic specification
IEC 60825-1 1993 Safety of laser products EN 60825-1 1994
Part 1: Equipment classification, + corr. February 1995
requirements and user's guide + A11 1996
A1 1997 - -
A2 2001 A2 2001
IEC 61300-3-2 1999 Fibre optic interconnecting devices and EN 61300-3-2 1999
passive components - Basic tests and
measurement procedures
Part 3-2: Examinations and
measurements - Polarization
dependence of attenuation in a single-
mode fibre optic device
ISO 1993 International vocabulary of basic and--
general terms in metrology
ISO 1995 Guide to the expression of uncertainty--
in measurement
ITU-T 1997 Definition and test methods for the--
Recommendation relevant parameters of single-mode
G.650 fibres
NORME CEI
INTERNATIONALE IEC
INTERNATIONAL
Première édition
STANDARD
First edition
2001-09
Etalonnage des réflectomètres optiques
dans le domaine de temps (OTDR)
Calibration of optical time-domain
reflectometers (OTDRs)
© IEC 2001 Droits de reproduction réservés ⎯ Copyright - all rights reserved
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

61746 © IEC:2001 – 3 –
CONTENTS
FOREWORD . 9
1 General. 11
1.1 Scope . 11
1.2 Normative references. 11
2 Definitions. 13
3 Calibration test requirements. 23
3.1 Preparation . 23
3.2 Test conditions. 23
3.3 Traceability . 23
4 Distance calibration – General. 25
4.1 Location error model . 25
4.2 Using the calibration results . 29
4.3 Measuring fibre length. 29
5 Distance calibration methods. 31
5.1 External source method . 31
5.1.1 Short description and advantage. 31
5.1.2 Equipment. 31
5.1.3 Measurement procedure. 35
5.1.4 Calculations and results . 37
5.1.5 Uncertainties. 37
5.2 Concatenated fibre method . 41
5.2.1 Short description and advantages. 41
5.2.2 Equipment. 41
5.2.3 Measurement procedures . 43
5.2.4 Calculations and results . 45
5.2.5 Uncertainties. 47
5.3 Recirculating delay line method . 49
5.3.1 Short description and advantage. 49
5.3.2 Equipment. 49
5.3.3 Measurement procedures . 51
5.3.4 Calculations and results . 53
5.3.5 Uncertainties. 53
6 Loss calibration – General. 57
6.1 Determination of the displayed power level F . 57
6.2 Selection of an appropriate reference loss A . 59
ref
6.3 Development of a test plan. 59
6.4 Polarization dependence . 63
6.5 Calculation of the calibration results . 65
6.6 Using the calibration results . 67

61746 © IEC:2001 – 5 –
7 Loss calibration methods. 67
7.1 Loss calibration with fibre standard. 67
7.1.1 Short description and advantage. 67
7.1.2 Equipment. 67
7.1.3 Measurement procedure. 71
7.1.4 Calculations and results . 71
7.1.5 Uncertainties. 73
7.2 External source method . 75
7.2.1 Short description and advantage. 75
7.2.2 Equipment. 75
7.2.3 Measurement procedure. 77
7.2.4 Calculations and results . 79
7.2.5 Uncertainties. 81
7.3 Splice simulator method . 81
7.3.1 Short description and advantage. 81
7.3.2 Equipment. 83
7.3.3 Procedure . 85
7.3.4 Calculations and results . 87
7.3.5 Uncertainties. 89
7.4 Power reduction method. 89
7.4.1 Short description and advantage. 89
7.4.2 Equipment. 91
7.4.3 Measurement procedure. 95
7.4.4 Calculations and results . 95
7.4.5 Uncertainties. 95
8 Reflectance calibration . 97
9 Documentation. 97
9.1 Measurement data and uncertainties . 97
9.2 Test conditions. 99
Annex A (normative) Recirculating delay line for distance calibration. 101
A.1 Construction . 101
A.2 Calibration . 101
A.3 Uncertainties. 105
A.4 Documentation. 107
Annex B (normative) Optical fibre standard for loss calibration . 109
B.1 Fibre requirements. 109
B.2 Suitability check of the fibre. 109
B.3 Preparation and calibration of the fibre standard. 113
B.4 Recalibration of the optical fibre standard. 115
B.5 Uncertainty of the fibre standard. 115
B.6 Documentation. 115
Annex C (normative) Standard splice simulator for loss calibration . 117
C.1 Structure. 117
C.2 Preparation of the standard splice simulator . 119
C.3 Calibration procedure. 119
C.4 Uncertainties. 121
C.5 Documentation. 123

61746 © IEC:2001 – 7 –
Annex D (informative) Mathematical basis . 125
D.1 Deviations. 125
D.2 Uncertainties type A . 125
D.3 Uncertainties type B . 127
D.4 Accumulation of uncertainties. 129
D.5 Reporting . 131
Figure 1 – Definition of attenuation dead zone. 13
Figure 2 – Representation of the location error ∆L(L) . 27
Figure 3 – Equipment for calibration of the distance scale – External source method. 31
Figure 4 – Set-up for calibrating the system insertion delay . 33
Figure 5 – Concatenated fibres used for calibration of the distance scale . 41
Figure 6 – Distance calibration with a recirculating delay line. 49
Figure 7 – OTDR trace produced by recirculating delay line. 51
Figure 8 – Determining the reference level and the displayed power level. 57
Figure 9 – Measurement of the OTDR loss samples. 59
Figure 10 – Region A, the recommended region for loss measurement samples. 61
Figure 11 – Possible placement of sample points within region A . 63
Figure 12 – External source method for testing the polarization dependence of the OTDR . 63
Figure 13 – Reflection method for testing the polarization dependence of the OTDR. 65
Figure 14 – Loss calibration with a fibre standard. 69
Figure 15 – Placing the beginning of section D outside the attenuation dead zone. 69
Figure 16 – Loss calibration with the external source method . 75
Figure 17 – Location and measurements for external source method. 79
Figure 18 – Set-up for loss calibration with splice simulator. 83
Figure 19 – OTDR display with splice simulator (the smaller circle represents the OTDR
response to the reference loss). 83
Figure 20 – Measurement of the splice loss . 85
Figure 21 – Loss calibration with "fibre-end" variant of the power reduction method. 93
Figure 22 – Loss calibration with "long-fibre" variant of the power reduction method . 93
Figure A.1 – Recirculating delay line . 101
Figure A.2 – Measurement set-up for loop transit time T . 103
b
Figure A.3 – Calibration set up for lead-in transit time T . 105
a
Figure B.1 – Determination of a highly linear power range . 111
Figure B.2 – Testing the longitudinal backscatter uniformity of the fibre standard. 113
Figure C.1 – Splice simulator and idealized OTDR signature . 117
Figure C.2 – Determination of the reference loss A . 121
ref
Figure D.1 – Deviation and uncertainty type B, and how to replace both
by an appropriately larger uncertainty. 127
Table 1 – Attenuation coefficients defining region A . 61

61746 © IEC:2001 – 9 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDRs)
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 61746 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
86/175/FDIS 86/177/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
Annexes A, B and C form an integral part of this standard.
Annex D is 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.
61746 © IEC:2001 – 11 –
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDRs)
1 General
1.1 Scope
This International Standard provides procedures for calibrating single-mode optical time
domain reflectometers (OTDRs). It only covers OTDR measurement errors and uncertainties.
This standard does not cover correction of the OTDR response.
In order for an OTDR to qualify as a candidate for complete calibration using this standard, it is
to be equipped with the following minimum feature set:
a) a programmable index of refraction, or equivalent parameter;
b) the ability to present a display of a trace representation, with a logarithmic power scale and
a linear distance scale;
c) two markers/cursors, which display the loss and distance between any two points on a trace
display;
d) the ability to measure absolute distance (location) from the OTDR's zero-distance reference;
e) the ability to measure the displayed power level relative to a reference level (for example,
the clipping level).
1.2 Normative references
Les documents de référence suivants sont indispensables pour l'application du présent
document. Pour les références datées, seule l'édition citée s'applique. Pour les références non
datées, la dernière édition du document de référence s'applique (y compris les éventuels
amendements).
IEC 60050-731:1991, International Electrotechnical Vocabulary (IEV) – Chapter 731: Optical
fibre communication
IEC 60617-10:1996, Graphical symbols for diagrams – Part 10: Telecommunications –
Transmission
IEC 60793-1 (all parts), Optical fibres – Part 1: Generic specification
IEC 60794-1 (all parts), Optical fibre cables – Part 1: Generic specification
IEC 60825-1:1993, Safety of laser products – Part 1: Equipment classification, requirements
and user's guide
1)
Amendment 1 (1997)
Amendment 2 (2001)
IEC 61300-3-2:1999, Fibre optic interconnecting devices and passive components – Basic test
and measurement procedures – Part 3-2: Examinations and measurements – Polarization
dependence of attenuation in a single-mode fibre optic device
ISO:1993, International vocabulary of basic and general terms in metrology
ISO:1995, Guide to the expression of uncertainty in measurement
ITU-T Recommendation G.650:1997, Definition and test methods for the relevant parameters of
single-mode fibres
________
1)
There is a consolidated edition 1.1 (1998) that includes IEC 60825-1 (1993) and its amendment 1 (1997).

61746 © IEC:2001 – 13 –
2 Definitions
For the purpose of this International Standard the definitions given below apply. For more
precise definitions, the references of IEC 60050-731 should be referred to.
2.1
attenuation, symbol A
optical power decrease in decibels (dB). If P (watts) is the power entering one end of a
in
segment of fibre and P (watts) is the power leaving the other end, then the attenuation of
out
the segment is
⎛ ⎞
P
in
A = 10 log ⎜ ⎟ dB (1)
⎜ ⎟
P
⎝ out ⎠
An alternative for "attenuation" is "loss"
[IEV 731-01-48, modified]
2.2
attenuation coefficient, symbol α
attenuation of a fibre per unit length
[IEV 731-03-42, modified]
2.3
attenuation dead zone
for a reflective or attenuating event, the region after the event where the displayed trace
deviates from the undisturbed backscatter trace by more than a given vertical distance ∆F
NOTE  The attenuation dead zone will depend on the following event parameters: reflectance, loss, displayed
power level and location. It may also depend on any fibre optic component in front of the event.
Initial dead zone
∆F
Attenuation
dead zone
Location  km
IEC  1421/01
Figure 1 – Definition of attenuation dead zone
2.4
calibration
set of operations which establish, under specified conditions, the relationship between the
values indicated by the measuring instrument and the corresponding known values of that
quantity (see ISO )
International vocabulary of basic and general terms in metrology
Displayed power F  dB
61746 © IEC:2001 – 15 –
2.5
centre wavelength, symbol λ
centre
power-weighted mean wavelength of a light source in vacuum, in nanometres (nm)
For a continuous spectrum, the centre wavelength is defined as:
λ = p(λ)λdλ (2)
centre
∫
P
total
For a spectrum consisting of discrete lines, the centre wavelength is defined as:
Pi i
∑
i
λ = (3)
centre
P
i
∑
i
where
p(λ) is the spectral power density of the source, for example in W/nm;
th
λ is the i discrete wavelength;
i
P is the power at λ , for example in watts;
i i
P = Σ P is the total power, for example in watts.
total i
The above integrals and summations extend over the entire spectrum of the light source.
2.6
confidence level
estimated probability that the true value of a measured quantity lies within a given expanded
uncertainty
NOTE  In this standard, the confidence level is standardized to 95 %. See "expanded uncertainty" for further
clarification.
2.7
distance
spacing (actual or simulated) between two features in a fibre, for example in metres
2.8
distance sampling error
maximum distance error attributable to the distance between successive sample points,
specified in metres
NOTE  The distance sampling error is repetitive in nature; therefore, one way of quantifying this error is by its
amplitude.
2.9
distance scale deviation, symbol ∆S
L
average error of the distance scale, that is difference between the average displayed distance
< D > and the correspondent reference distance D divided by the reference distance, for
otdr ref
example in m/m:
< D >− D < D >
otdr ref otdr
∆S = = − 1 (4)
L
D D
ref ref
where < D > is the displayed distance between two features on a fibre (actual or simulated)
otdr
averaged over at least one sample spacing
NOTE  It is assumed that a relatively long distance, for example 2 000 m, is used in this formula.

61746 © IEC:2001 – 17 –
2.10
distance scale factor, symbol S
L
average displayed distance divided by the correspondent reference distance:
< D >
otdr
S = (5)
L
D
ref
where < D > is the displayed distance between two features on a fibre (actual or simulated)
otdr
averaged over at least one sample spacing
NOTE  It is assumed that relatively long distances are used in this formula.
2.11
distance scale uncertainty, symbol σ
∆SL
uncertainty of the distance scale deviation, for example in m/m
⎛ ⎞ ⎛ ⎞
< D > < D >
otdr otdr
σ = σ ⎜ −1⎟ = σ ⎜ ⎟ (6)
∆SL ⎜ ⎟ ⎜ ⎟
D D
⎝ ref ⎠ ⎝ ref ⎠
NOTE 1 It is assumed that the distance is relatively long, because short distances may lead to larger
uncertainties.
NOTE 2  In the above formula, σ() is understood as the standard uncertainty of ().
2.12
dynamic range (one-way)
amount of fibre attenuation that causes the backscatter signal to equal the noise level. It can
be represented by the difference between the extrapolated point of the backscattered trace
(taken at the intercept with the power axis) and the noise level expressed in decibels, using a
standard category B fibre (see IEC 60793-1)
2.13
expanded uncertainty
range of uncertainties within which the true value of the measured quantity lies, at the given
confidence level. For further information, see annex D and the ISO Guide for the expression of
uncertainty in measurement
NOTE  When the distribution of uncertainties is assumed to be gaussian, and the (estimated) confidence level
is 95 %, then for a large number of measurements, the standard uncertainty is defined by ±2 times the standard
deviation.
2.14
group index, symbol N
factor by which the speed of light in vacuum has to be divided to yield the propagation velocity
of light pulses in the fibre
2.15
location, symbol L
spacing (actual or simulated) between the front panel of the OTDR and a feature in a fibre, for
example in metres
2.16
location error, symbol ∆L
displayed location of a feature L minus the reference location L , for example in metres. It
otdr ref
is a function of the location
2.17
location offset, symbol ∆L
(constant) additive term of the location error model used in this standard, for example, in
metres. This is approximately equivalent to the location of the OTDR front panel connector on
the instrument's distance scale (for a perfect OTDR, the location offset is zero)

61746 © IEC:2001 – 19 –
2.18
location offset uncertainty, symbol σ
∆L0
uncertainty of the location offset expressed in metres
2.19
location readout uncertainty, symbol σ
Lreadout
uncertainty of the location measurement samples caused by both the distance sampling error
and the uncertainty type A of the measurement samples, in the form of the half-width of a
confidence interval, in metres
2.20
loss
synonym for attenuation
2.21
loss deviation, symbol ∆S
A
difference between the displayed loss of a fibre component A and the reference loss A ,
otdr ref
divided by the reference loss, in dB/dB
A − A
otdr ref
∆S = (7)
A
A
ref
NOTE  The loss deviation usually depends on the displayed power level, F.
2.22
loss scale factor, symbol S
A
(power level dependent) ratio of the displayed loss and the reference loss, in dB/dB:
A
otdr
S = (8)
A
A
ref
2.23
loss uncertainty, symbol σ
∆SA
uncertainty of the loss deviation, in dB/dB
2.24
noise level
upper limit of a range which contains at least 98 % of all noise data points
2.25
non-linearity (of logarithmic power scale)
difference between the maximum and minimum values of the loss scale factor S for a given
A
range of power levels, in dB/dB.
NOTE 1  Changes of the displayed power level can be produced by changing the incident power to the component.
NOTE 2  Non-linearity is one contribution to loss deviation; it usually depends on the displayed power level and the
location.
2.26
power level
Two types of power levels are distinguished:
a) received power level, symbol P
power received by the OTDR's optical port
b) displayed power level, symbol F
level displayed on the OTDR's power scale. Unless otherwise specified, F is defined in
relation to the clipping level (see figure 8)
NOTE  Usually, the OTDR scale displays five times the logarithm of the received power, plus a constant offset.

61746 © IEC:2001 – 21 –
2.27
reference distance, symbol D
ref
distance between features in a fibre or the length of a fibre, actual or simulated, precisely
determined with the help of measurement equipment other than the OTDR, usually expressed
in metres
2.28
reference location, symbol L
ref
spacing between the OTDR's front panel and a feature on the fibre, actual or simulated,
precisely determined with the help of measurement equipment other than the OTDR, usually
expressed in metres
2.29
reference loss, symbol A
ref
loss of a fibre optic component, actual or simulated, precisely determined by means other than
direct usage of the OTDR's power scale, in decibels
2.30
reflectance
ratio of the reflected power (in watts) to the incident power (in watts), at a discrete location in a
fibre optic component. In this standard, reflectance is expressed in decibels
2.31
sample spacing
distance between two consecutive data points digitized by the OTDR, for example, in metres
NOTE  Sample spacing may be obtainable from instrument set-up information. Sample spacing may depend on the
measurement span and other OTDR instrument settings.
2.32
spectral width, symbol ∆λ
FWHM
full-width half-maximum (FWHM) spectral width of the source. For a non-continuous spectrum,
for example the spectrum of a Fabry-Perot type laser diode, the spectral width is defined as the
FWHM of the spectral envelope, to be calculated from the RMS spectral width, ∆λ :
RMS
1/2
⎛ ⎞
Σ P λ
i i 2
⎜ ⎟
∆λ = − λ (9)
RMS
centre
⎜ ⎟
P
total
⎝ ⎠
leading to ∆λ = M∆λ (10)
FWHM RMS
where
λ is the centre wavelength of the laser diode in vacuum;
centre
P = Σ P is the total power, in watts;
i
total
th
P is the power of the i longitudinal mode;
i
th
λ is the wavelength of the i longitudinal mode in vacuum;
i
M is the multiplication factor; for a source with a gaussian envelope, M = 2,35; for other
types of spectra, use M = 2,35 as well.
NOTE  If the laser emits at one wavelength only (single-line spectrum), it may be sufficient to specify an upper
limit, for example spectral width <1 nm.

61746 © IEC:2001 – 23 –
2.33
standard uncertainty
uncertainty of a measurement result expressed as a standard deviation. For further information,
see annex D and the ISO Guide to the expression of uncertainty in measurement
2.34
uncertainty type A
uncertainty obtained by the statistical analysis of a series of observations. For further
information, see annex D and the ISO Guide to the expression of uncertainty in measurement
2.35
uncertainty type B
uncertainty obtained by means other than the statistical analysis of a series of observations.
For further information, see annex D and the ISO Guide to the expression of uncertainty in
measurement
NOTE  "Other means" may include previous measurement data, experience with, or general knowledge of, the
behaviour and properties of relevant materials and instruments, manufacturers' specifications, data provided in
calibration and other certificates, and uncertainties assigned to reference data taken from handbooks.
3 Calibration test requirements
3.1 Preparation
Perform all tests at an ambient room temperature of 23 °C ± 3 °C, with a relative humidity of
(50 ± 20) %, unless otherwise specified. Give the test equipment a minimum of 2 h prior to
testing to reach equilibrium with its environment. Allow the OTDR a warm-up period according
to the manufacturer's instruction.
3.2 Test conditions
The test conditions usually include the following OTDR external conditions: date, temperature,
humidity (non-condensing), fibre type, connector-adapter combination and use of a lead-in
fibre.
Perform the calibration in accordance with the manufacturer's specifications and operating
procedures. Where practical, select a range of test conditions and parameters so as to emulate
the actual field operating conditions of the OTDR under test. Choose these parameters so as to
optimize the OTDR's accuracy and resolution capabilities (for example, view windows, zoom
features, etc.), as specified by the manufacturer's operating procedures.
The test conditions usually include the following OTDR parameters: averaging time, pulse
power, pulse shape, pulse width, pulse repetition rate, sample spacing, centre wavelength,
spectral width, use of optical and electronic masking. Unless otherwise specified, set the
OTDR group index to exactly 1,46.
Record the conditions as specified in clause 9.
NOTE 1  The calibration results only apply to the set of test conditions used in the calibration process.
NOTE 2  Because of the potential for hazardous radiation, be sure to establish and maintain conditions of laser
safety. Refer to IEC 60825-1.
3.3 Traceability
Make sure that all test equipment which has a significant influence on the calibration results is
calibrated in an unbroken chain to the appropriate national standard or natural physical constant.
Upon request, specify this test equipment and its calibration chain(s). The recalibration
period(s) shall be defined and documented.

61746 © IEC:2001 – 25 –
4 Distance calibration – General
The objective of distance calibration is to determine deviations (errors) between the measured
and actual distances between points on a fibre, and to characterize the uncertainties of these
deviations.
An OTDR measures the location L of a feature from the point where a fibre is connected to the
instrument, by measuring the round-trip transit time T for a light pulse to reach the feature and
return. L is calculated from T using the speed of light in vacuum c(2,997 924 58 × 10 m/s) and
the group index N of the fibre:
cT
L = (11)
2N
Errors in measuring L will result from scale errors, from offsets in the timebase of the OTDR
and from errors in locating a feature relative to the timebase. Placing a marker in order to
measure the location may be done manually or automatically by the instrument. The error will,
generally, depend on both the marker placement method and the type of feature (for example,
a point loss, a large reflection that saturates the receiver or a small reflection that does not).
Even larger errors in measuring L may result from the uncertainty in determining the fibre's
group index N. The determination of N is beyond the scope of this standard. Consequently, the
calibration procedures below only discuss the OTDR's ability to measure T correctly. For the
purpose of this standard, a default value N = 1,46 is used and the uncertainty of N is
considered to be 0.
4.1 Location error model
In order to characterize location errors, a specific model will be assumed that describes the
behaviour of most OTDRs. Let L be the reference location of a feature from the front panel
ref
connector of the OTDR and let L be the displayed location. It is assumed that the displayed
otdr
location L , using OTDR averaging to eliminate noise, depends functionally on the reference
otdr
location L in the following way:
ref
L = S L + ∆L + f(L ) (12)
otdr L ref 0 ref
where
S is the scale factor, which ideally should be 1;
L
∆L is the location offset, which ideally should be zero;
f(L ) represents the distance sampling error, which is also ideally zero. The distance
ref
sampling error is a periodic function with a mean of zero and a period equal to the
distance interval between sampled points on the OTDR. As an example, if the location
of a large reflection is measured by placing a marker on the first digitized point that
shows an increase in signal and the position of the reflection is incremented in fine
steps, then f(L ) may be shaped like a periodic ramp waveform.
ref
Equation (12) is meant to characterize known errors in location measurements, but there may
still be an additive uncertainty type A. This will affect both the distance measurements and the
accuracy with which parameters describing the errors can be determined by the procedures
below.
61746 © IEC:2001 – 27 –
S and ∆L may be determined by measuring L for different values of L , then fitting a
L 0 otdr ref
straight line to the data by the least squares method. S and ∆L are the slope and intercept,
L 0
respectively.
Equivalently, a line may be fitted to the location error function, that is the difference between
L and L :
otdr ref
∆L = L – L = ∆S L + ∆L + f(L ) (13)
otdr ref L ref 0 ref
where
∆S is the slope, and
L
∆L is still the intercept, as illustrated in figure 2.
After finding the linear approximation, the distance sampling error f(L ) respectively its half-
ref
amplitude ∆L may be determined by measuring departures from the line for different
readout
values of L . The distance sampling error amplitude ∆L is taken as half the amplitude
ref sample
of f(L ).
ref
In this standard, the distance sampling error amplitude ∆L is treated as part of the
sample
location readout uncertainty type A. The stated uncertainty result thus ignores the repetitive
nature of the sampling error, that is it does not distinguish between the relative contributions of
the sampling error and the uncertainty type A.
Linear
∆L
sample
approximation
(Slope = ∆S )
L
∆L
Location L
ref
IEC  1422/01
Figure 2 – Representation of the location error ∆L(L)
Therefore, the result of the distance calibration shall be stated by the following parameters:
∆S ,σ the distance scale deviation and its uncertainty;
L ∆SL
∆ L ,σ the location offset and its uncertainty;
0 ∆L0
σ
the location readout uncertainty, that is the combined uncertainty due to the
Lreadout
distance sampling error and the uncertainty type A of the measurement samples, in
the form of a standard deviation.
In compliance with the "mathematical basis", divide the largest excursions from the least-
squares approximation by the square root of 3 for stating σ . Note that the uncertainty
Lreadout
will depend on the distance, the displayed power level and the instrument settings.
∆L(L) = L – L m
otdr ref
61746 © IEC:2001 – 29 –
4.2 Using the calibration results
The error in the location of a feature ∆L = L – L can be calculated from the calibration
otdr ref
results:
∆L = ∆ L + L ∆S (14)
0 ref L
with the uncertainty in ∆L given by the following formula, in which the recommended confidence
level of 95 % is used:
2 2 2 2 ½
±2 σ = ±2 (σ + L σ + σ ) (14a)
∆L ∆L0 ref ∆SL Lreadout
where the displayed location L can be used instead of the reference location L without
otdr ref
serious consequences.
Similarly, the error in the distance between two features ∆D and its uncertainty can be
calculated from the following formula:
∆D = D ∆S (15)
ref L
with uncertainty in ∆D given by the following formula:
2 2 2 ½
±2 σ = ±2 (D σ + 2σ ) (15a)
∆D ref ∆SL Lreadout
where the displayed distance D can be used instead of the reference distance D .
otdr ref
NOTE  The 2 in front of σ is due to combining two uncorrelated uncertainties.
Lreadout
Additional uncertainties may have to be taken into account if the type of feature is different
from the feature used in the calibration. Specify the type of feature as par
...