EN 61746:2005

SLOVENSKI SIST EN 61746:2005
STANDARD
november 2005
Kalibriranje optične časovne domene reflektometrov (OTDR) (IEC 61746:2005)
Calibration of optical time-domain reflectometers (OTDR) (IEC 61746:2005)
ICS 17.180.30; 33.180.99 Referenčna številka
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

EUROPEAN STANDARD EN 61746
NORME EUROPÉENNE
EUROPÄISCHE NORM March 2005
ICS 33.180.01 Supersedes EN 61746:2001

English version
Calibration of optical time-domain reflectometers (OTDR)
(IEC 61746:2004)
Etalonnage des réflectomètres optiques Kalibirerung optischer
dans le domaine de temps (OTDR) Rückstreumessgeräte (OTDR)
(CEI 61746:2004) (IEC 61746:2004)

This European Standard was approved by CENELEC on 2005-02-01. 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, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, 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

© 2005 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61746:2005 E
Foreword
The text of document 86/230/FDIS, future edition 2 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 2005-02-01.
This European Standard supersedes EN 61746:2001.
Specific changes to EN 61746:2001 include the development of Clause 9, “Reflectance calibration”,
and the introduction of Annexes E, F and G.
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) 2005-11-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2008-02-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61746:2005 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60825-1 NOTE Harmonized as EN 60825-1:1994 (not modified).
IEC 60825-2 NOTE Harmonized as EN 60825-2:2004 (not modified).
IEC 61300-3-6 NOTE Harmonized as EN 61300-3-6:2003 (not modified).
__________
- 3 - EN 61746:2005
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE Where an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
IEC 60793-1 Series Optical fibres EN 60793-1 Series
Part 1: Measurement methods and test
procedures
1) 2)
IEC 60793-1-40 - Optical fibres EN 60793-1-40 2003
(mod) Part 1-40: Measurement methods and
test procedures – Attenuation
1) 2)
IEC 60794-1-2 - Optical fibre cables EN 60794-1-2 2003
Part 1-2: Generic specification - Basic
optical cable test procedures
1) 2)
IEC 61300-3-2 - 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
ITU-T 2004 Definitions and test methods for linear, - -
Recommendation deterministic attributes of single-mode
G.650.1 fibre and cable
ITU-T 2002 Definitions and test methods for statistical - -
Recommendation and non-linear attributes of single-mode
G.650.2 fibre and cable
1)
Undated reference.
2)
Valid edition at date of issue.

NORME CEI
INTERNATIONALE IEC
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2005-01
Etalonnage des réflectomètres optiques
dans le domaine de temps (OTDR)
Calibration of optical time-domain
reflectometers (OTDR)
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For price, see current catalogue

61746¤ IEC:2005 – 3 –
CONTENTS
FOREWORD . 9
1 Scope .13
2 Normative references .13
3 Terms, definitions and symbols.15
4 Calibration test requirements .27
4.1 Preparation .27
4.2 Test conditions .27
4.3 Traceability .29
5 Distance calibration – General .29
5.1 Location error model.29
5.2 Using the calibration results.33
5.3 Measuring fibre length .33
6 Distance calibration methods .35
6.1 External source method .35
6.2 Concatenated fibre method.45
6.3 Recirculating delay line method .53
7 Loss calibration – General .63
7.1 Determination of the displayed power level F .63
7.2 Selection of an appropriate reference loss A .65
ref
7.3 Development of a test plan .65
7.4 Polarization dependence .69
7.5 Calculation of the calibration results .71
7.6 Using the calibration results.73
8 Loss calibration methods .73
8.1 Loss calibration with fibre standard.73
8.2 External source method (see Figure 16).81
8.3 Splice simulator method .89
8.4 Power reduction method .97
9 Reflectance calibration .105
9.1 Reflectance measurements (see Figure 23) .105
9.2 Use of the backscatter parameter, K .107
9.3 Range of reflectance measurement.109
9.4 Development of a test plan .111
9.5 Equipment .113
9.6 Measurement procedure .115
10 Documentation .117
10.1 Measurement data and uncertainties .117
10.2 Test conditions .119
Annex A (normative) Recirculating delay line for distance calibration.121
Annex B (normative) Optical fibre standard for loss calibration .129
Annex C (normative) Standard splice simulator for loss calibration .137
Annex D (informative) Mathematical basis.147

61746¤ IEC:2005 – 5 –
Annex E (normative) Reflectance standard.155
Annex F (normative)  Simple version of reflectance standard.169
Annex G (informative)  OTDR basis: Backscatter theory – Reflectance measurements
using an OTDR – Determination of fibre backscatter parameter.177
Bibliography .189
Figure 1 – Definition of attenuation dead zone .15
Figure 2 – Representation of the location error ∆L(L) .31
Figure 3 – Equipment for calibration of the distance scale – External source method .37
Figure 4 – Set-up for calibrating the system insertion delay .39
Figure 5 – Concatenated fibres used for calibration of the distance scale .47
Figure 6 – Distance calibration with a recirculating delay line .55
Figure 7 – OTDR trace produced by recirculating delay line .57
Figure 8 – Determining the reference level and the displayed power level.63
Figure 9 – Measurement of the OTDR loss samples .65
Figure 10 – Region A, the recommended region for loss measurement samples .67
Figure 11 – Possible placement of sample points within region A .69
Figure 12 – External source method for testing the polarization dependence of the OTDR .69
Figure 13 – Reflection method for testing the polarization dependence of the OTDR.71
Figure 14 – Loss calibration with a fibre standard.75
Figure 15 – Placing the beginning of section D outside the attenuation dead zone .77
Figure 16 – Loss calibration with the external source method .83
Figure 17 – Location and measurements for external source method .87
Figure 18 – Set-up for loss calibration with splice simulator.91
Figure 19 – OTDR display with splice simulator.91
Figure 20 – Measurement of the splice loss .93
Figure 21 – Loss calibration with "fibre-end" variant of the power reduction method.101
Figure 22 – Loss calibration with "long-fibre" variant of the power reduction method .101
Figure 23 – Parameters involved in reflectance measurements.107
Figure 24 – The same reflectance at the end of three fibres with different values of
the backscatter parameter shows different pulse amplitudes .109
Figure 25 – Maximum and minimum values for the pulse amplitude, ∆F .111
Figure 26 – Range of reflectance measurement .111
Figure 27 – Determining the default displayed power level and the default location.113
Figure 28 – Set-up for reflectance calibration .115

61746¤ IEC:2005 – 7 –
Figure A.1 – Recirculating delay line .121
Figure A.2 – Measurement set-up for loop transit time T .123
b
Figure A.3 – Calibration set-up for lead-in transit time T .125
a
Figure B.1 – Determination of a highly linear power range .131
Figure B.2 – Testing the longitudinal backscatter uniformity of the fibre standard.133
Figure C.1 – Splice simulator and idealized OTDR signature .137
Figure C.2 – Determination of the reference loss A .141
ref
Figure D.1 – Deviation and uncertainty type B, and how to replace both by an
appropriately larger uncertainty.149
Figure E.1 – Reflectance standard description and trace .155
Figure E.2 – Calibration set up and reference points for calibration .163
Figure F.1 – Reflectance standard description and trace .169
Figure F.2 – Calibration set up and reference points for calibration.175
Figure G.1 – OTDR signals used for determining reflectance.181
Figure G.2 – Set-up for measurement of the backscatter coefficient .185
Table 1 – Attenuation coefficients defining region A .67

61746¤ IEC:2005 – 9 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR)
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
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). 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. 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 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 IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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 61746 has been prepared by IEC technical committee 86: Fibre
optics.
This second edition cancels and replaces the first edition published in 2001. It constitutes a
technical revision. Specific technical changes include the development of Clause 9,
“Reflectance calibration,” and the introduction of Annexes E, F and G.
The text of this standard is based on the following documents:
FDIS Report on voting
86/230/FDIS 86/232/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.

61746¤ IEC:2005 – 11 –
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.
61746¤ IEC:2005 – 13 –
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR)
1 Scope
This International Standard provides procedures for calibrating single-mode optical time
domain reflectometers (OTDR). 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
must 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).
f) the ability to evaluate the reflectance of a reflective event.
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 (all parts), Optical fibres – Part 1: Measurement methods and test procedures
IEC 60793-1-40, Optical fibres – Part 1-40: Measurement methods and test procedures –
Attenuation
IEC 60794-1-2, Optical fibre cables – Part 1-2: Generic specification – Basic optical cable
test procedures
IEC 61300-3-2, 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
ITU-T Recommendation G.650.1:2004, Definitions and test methods for linear, deterministic
attributes of single-mode fibre and cable
ITU-T Recommendation G.650.2:2002, Definitions and test methods for statistical and non-
linear attributes of single-mode fibre and cable

61746¤ IEC:2005 – 15 –
3 Terms, definitions and symbols
For the purposes of this document, the following definitions apply.
NOTE For more precise definitions, the references to IEC 60050-731 should be consulted.
3.1
attenuation
A
loss
optical power decrease in decibels (dB)
NOTE If P (watts) is the power entering one end of a segment of fibre and P (watts) is the power leaving the
in out
other end, then the attenuation of the segment is
§ ·
P
in
¨ ¸
A = 10log dB (1)
¨ ¸
P
© out¹
[IEV 731-01-48, modified]
3.2
attenuation coefficient
α
attenuation of a fibre per unit length
[IEV 731-03-42, modified]
3.3
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  1627/04
Figure 1 – Definition of attenuation dead zone
Displayed power F  dB
61746¤ IEC:2005 – 17 –
3.4
backscatter parameter
K
at a given point along the fibre, the backscattered propagating power per unit incident energy
NOTE 1 K is given by the following formula:
ν
−1
K = Sα  s (2)
s
where
−
α  is the scattering coefficient, e.g.; in m
s
S  is the backscatter capture fraction. It depends on other standard fibre parameters such as the mode field
diameter in single mode fibre;
ν  is the group velocity, in metres per second;
= c / N where c is the speed of the light in vacuum, N the group index of the fibre.
NOTE 2 See also Annex G.
3.5
backscatter coefficient
C
for a given pulse, the ratio of backscattered power at the input side of the fibre to the pulse
input power
NOTE 1 It represents the backscatter parameter for a given pulse width. The backscatter coefficient is defined
from the backscatter parameter using the following formula:
C()∆T = K∆T (3)
where ∆T is the pulse width, e.g. in seconds.
Usually the backscatter coefficient is expressed in dB for a given pulse width, ∆T.
C ()∆T = 10log (K∆T) (4)
dB 10
NOTE 2 The pulse width, ∆T in the previous formula is used to normalise C()∆T . Usual values for ∆T are
1 ns and 1 µs. See also Annex G.
3.6
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
NOTE See ISO Guide International vocabulary of basic and general terms in metrology.
3.7
centre wavelength
λ
centre
power-weighted mean wavelength of a light source in vacuum, in nanometres (nm)
NOTE For a continuous spectrum, the centre wavelength is defined as:
λ = p(λ)λdλ (5)
centre
³
P
total
For a spectrum consisting in discrete lines, the centre wavelength is defined as:
PȜ
¦ i i
i
λ = (6)
centre
P
i
¦
i
61746¤ IEC:2005 – 19 –
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.
3.8
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.
3.9
distance
spacing (actual or simulated) between two features in a fibre, for example in metres
3.10
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.
3.11
distance scale deviation
∆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
otdr ref
distance, for example in m/m
NOTE 1 ∆S is given by the following formula:
L
< D >− D < D >
otdr ref otdr
∆S = = − 1 (7)
L
D D
ref ref
where < D > is the displayed distance between two features on a fibre (actual or simulated) averaged over at
otdr
least one sample spacing
NOTE 2 It is assumed that a relatively long distance, for example 2 000 m, is used in this formula.
3.12
distance scale factor
S
L
average displayed distance divided by the correspondent reference distance
NOTE 1 S is given by the following formula:
L
< D >
otdr
S = (8)
L
D
ref
where < D > is the displayed distance between two features on a fibre (actual or simulated) averaged over at
otdr
least one sample spacing
NOTE 2 It is assumed that relatively long distances are used in this formula.
3.13
distance scale uncertainty
σ
∆SL
uncertainty of the distance scale deviation, for example in m/m

61746¤ IEC:2005 – 21 –
NOTE 1 σ is given by the following formula:
∆SL
§ · § ·
< D > < D >
otdr otdr
¨ ¸ ¨ ¸
σ = σ −1 = σ (9)
∆SL ¨ ¸ ¨ ¸
D D
© ref ¹ © ref ¹
NOTE 2 It is assumed that the distance is relatively long, because short distances may lead to larger
uncertainties.
NOTE 3 In the above formula, σ() is understood as the standard uncertainty of ().
3.14
dynamic range (one-way)
amount of fibre attenuation that causes the backscatter signal to equal the noise level
NOTE 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)
3.15
expanded uncertainty
range of uncertainties within which the true value of the measured quantity lies, at the given
confidence level
NOTE 1 For further information, see Annex D and the ISO Guide for the expression of uncertainty in
measurement.
NOTE 2 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.
3.16
group index
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
3.17
location
L
spacing (actual or simulated) between the front panel of the OTDR and a feature in a fibre,
for example in metres
3.18
location error
∆L
displayed location of a feature L minus the reference location L , for example in metres
otdr ref
NOTE This error is a function of the location
3.19
location offset
∆L
(constant) additive term of the location error model used in this standard, for example, in
metres
NOTE 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).
3.20
location offset uncertainty
σ
∆L0
uncertainty of the location offset expressed in metres

61746¤ IEC:2005 – 23 –
3.21
location readout uncertainty
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
3.22
loss deviation
∆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
NOTE 1 ∆S is given by the following formula:
A
A − A
otdr ref
∆S = (10)
A
A
ref
NOTE 2 The loss deviation usually depends on the displayed power level, F.
3.23
loss scale factor
S
A
(power level dependent) ratio of the displayed loss and the reference loss, in dB/dB
NOTE S is given by the following formula:
A
A
otdr
S = (11)
A
A
ref
3.24
loss uncertainty
σ
∆SA
uncertainty of the loss deviation, in dB/dB
3.25
noise level
upper limit of a range which contains at least 98 % of all noise data points
3.26
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.
3.27
power level
a) received power level, P
power received by the OTDR's optical port
b) displayed power level, F
level displayed on the OTDR's power scale
NOTE 1 Unless otherwise specified, F is defined in relation to the clipping level (see Figure 8).
NOTE 2 Usually, the OTDR scale displays five times the logarithm of the received power, plus a constant offset.

61746¤ IEC:2005 – 25 –
3.28
reference distance
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
3.29
reference location
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
3.30
reference loss
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
3.31
reflectance
R
the ratio of the reflected power (in watts), to the incident power (in watts), at a discrete
location in a fibre optic component
NOTE 1 R is given by the following formula:
§ P ·
refl
¨ ¸
R=10log (12)
¨ ¸
P
© inc¹
where P = the reflected power, e.g. in watts;
refl
P = the incident power, e.g. in watts;
inc
NOTE 2 In this document, reflectance is expressed in decibels.
NOTE 3 Reflectance values are negative.
NOTE 4 For the purpose of this document, the reflectivity, ρ, is defined as the linear value of the reflectance:
P
refl
ρ= (13)
P
inc
3.32
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.
3.33
spectral width
∆λ
FWHM
full-width half-maximum (FWHM) spectral width of the source
NOTE 1 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 (14)
RMS
centre
¨ ¸
P
total
© ¹
61746¤ IEC:2005 – 27 –
leading to
∆λ = M∆λ (15)
FWHM RMS
where
λ is the centre wavelength of the laser diode in vacuum;
centre
P = Σ P is the total power, in watts;
total i
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 2 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.
3.34
standard uncertainty
uncertainty of a measurement result expressed as a standard deviation
NOTE For further information, see Annex D and the ISO Guide to the expression of uncertainty in measurement.
3.35
uncertainty type A
uncertainty obtained by the statistical analysis of a series of observations
NOTE For further information, see Annex D and the ISO Guide to the expression of uncertainty in measurement.
3.36
uncertainty type B
uncertainty obtained by means other than the statistical analysis of a series of observations
NOTE 1 For further information, see Annex D and the ISO Guide to the expression of uncertainty in
measurement.
NOTE 2 "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.
4 Calibration test requirements
4.1 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.
4.2 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.

61746¤ IEC:2005 – 29 –
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 10.
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 and IEC 60825-2.
4.3 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.
5 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 = (16)
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.
5.1 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
otdr
displayed location L , using OTDR averaging to eliminate noise, depends functionally on
otdr
the reference location L in the following way:
ref
L = S . L + ∆L + f(L) (17)
otdr L ref 0 ref
61746¤ IEC:2005 – 31 –
where
S is the scale factor, which ideally should be 1;
L
∆L is the location offset, which ideally should be 0;
f(L ) represents the distance sampling error, which is also ideally 0. The distance sampling
ref
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 (17) 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.
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) (18)
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  1628/04
Figure 2 – Representation of the location error ∆L(L)
∆L(L) = L – L m
otdr ref
61746¤ IEC:2005 – 33 –
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.
5.2 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 (19)
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 ) (19a)
∆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 (20)
ref L
with uncertainty in ∆D given by the following formula:
2 2 2 ½
±2 σ = ±2 (D σ + 2σ ) (20a)
∆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 part of the calibration
result.
5.3 Measuring fibre length
As indicated above, one of the methods of OTDR distance calibration is to measure fibres of
known length with the OTDR. In several instances in this standard, it is required that fibre
length be determined using the fibre's transit time, in contra
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