IEC 61746-1:2009

Calibration of optical time-domain reflectometers (OTDR) - Part 1: OTDR for single mode fibres

IEC 61746-1: 2009 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. This first edition of IEC 61746-1 cancels and replaces the second edition of IEC 61746, published in 2005. It constitutes a technical revision. The main technical changes are the adaptation of Clause 4, the suppression of Clause 10, the improvement and the addition of some definitions, the change of some calculations and the change of graphical symbology to IEC/TR 61930. Keywords: calibrating single-mode optical time domain reflectometers (OTDR)

Etalonnage des réflectomètres optiques dans le domaine temporel (OTDR) - Partie 1: OTDR pour fibres unimodales

L'IEC 61746-1:2009 fournit des procédures destinées à l'étalonnage des réflectomètres optiques dans le domaine de temps pour fibres unimodales (OTDR). Elle ne traite que des erreurs de mesure et incertitudes de l'OTDR. Cette norme ne couvre pas la correction de la réponse de l'OTDR. Cette première édition de la CEI 61746-1 annule et remplace la deuxième édition de la CEI 61746, publiée en 2005. Elle constitue une révision technique. Les principales modifications techniques sont l'adaptation de l'article 4, la suppression de l'article 10, l'amélioration et l'ajout de certaines définitions, le changement de certains calculs et le changement de symbologie graphique IEC / TR 61930. Mots clés : l'étalonnage des réflectomètres optiques dans le domaine de temps pour fibres unimodales (OTDR)

General Information

• Status: Published
• Publication Date: 16-Dec-2009
• ICS: 33.180.01 - Fibre optic systems in general
• Technical Committee: TC 86 - Fibre optics
• Drafting Committee: WG 4 - TC 86/WG 4
• Current Stage: PPUB - Publication issued
• Start Date: 15-Jan-2010
• Completion Date: 17-Dec-2009

Relations

• Replaces: IEC 61746:2005 - Calibration of optical time-domain reflectometers (OTDR)
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61746-1 ®
Edition 1.0 2009-12
INTERNATIONAL
STANDARD
Calibration of optical time-domain reflectometers (OTDR) –
Part 1: OTDR for single mode fibres

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IEC 61746-1 ®
Edition 1.0 2009-12
INTERNATIONAL
STANDARD
Calibration of optical time-domain reflectometers (OTDR) –
Part 1: OTDR for single mode fibres

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XC
ICS 33.180.01 ISBN 978-2-88910-553-3
– 2 – 61746-1 © IEC:2009(E)
CONTENTS
FOREWORD.6
INTRODUCTION.8
1 Scope.9
2 Normative references.9
3 Terms, definitions and symbols .9
4 Preparation for calibration.16
4.1 Organization .16
4.2 Traceability.16
4.3 Preparation.16
4.4 Test conditions .16
4.5 Documentation .16
5 Distance calibration – General .17
5.1 General .17
5.2 Location deviation model .17
5.3 Using the calibration results.19
5.4 Measuring fibre length .19
6 Distance calibration methods .20
6.1 General .20
6.2 External source method .20
6.2.1 Short description and advantage .20
6.2.2 Equipment .20
6.2.3 Calibration of the equipment .21
6.2.4 Measurement procedure .22
6.2.5 Calculations and results .23
6.2.6 Uncertainties .24
6.3 Concatenated fibre method.25
6.3.1 Short description and advantages .25
6.3.2 Equipment .25
6.3.3 Measurement procedures.27
6.3.4 Calculations and results .27
6.3.5 Uncertainties .28
6.4 Recirculating delay line method.29
6.4.1 Short description and advantage .29
6.4.2 Equipment .29
6.4.3 Measurement procedure .31
6.4.4 Calculations and results .31
6.4.5 Uncertainties .32
7 Loss calibration – General .33
7.1 General .33
7.2 Determination of the displayed power level F .33
7.3 Selection of an appropriate reference loss A .34
ref
7.4 Development of a test plan .35
7.5 Polarization dependence.37
7.6 Calculation of the calibration results.38
7.7 Using the calibration results.38
8 Loss calibration methods .38

61746-1 © IEC:2009(E) – 3 –
8.1 General .38
8.2 Fibre standard method.39
8.2.1 Short description and advantage .39
8.2.2 Equipment .39
8.2.3 Measurement procedure .40
8.2.4 Calculations and results .41
8.2.5 Uncertainties .41
8.3 External source method (see Figure 16).42
8.3.1 Short description and advantage .42
8.3.2 Equipment .42
8.3.3 Calibration of the reference loss.43
8.3.4 Measurement procedure .44
8.3.5 Calculations and results .45
8.3.6 Uncertainties .45
8.4 Splice simulator method.46
8.4.1 Short description and advantage .46
8.4.2 Equipment .46
8.4.3 Procedure.47
8.4.4 Calculations and results .49
8.4.5 Uncertainties .49
8.5 Power reduction method .50
8.5.1 Short description and advantage .50
8.5.2 Equipment .51
8.5.3 Measurement procedure .52
8.5.4 Calculations and results .53
8.5.5 Uncertainties .53
9 Reflectance calibration.54
9.1 Objective .54
9.2 Reflectance measurements (see Figure 23) .54
9.3 Use of the backscatter parameter, K .54
9.4 Range of reflectance measurement.55
9.5 Development of a test plan .56
9.6 Equipment .57
9.7 Measurement procedure .58
9.7.1 Preparation.58
9.7.2 Taking reflectance measurements.58
9.7.3 Calculation and results.58
9.7.4 Uncertainties .58
Annex A (normative) Recirculating delay line for distance calibration .60
Annex B (normative) Optical fibre standard for loss calibration.64
Annex C (normative) Standard splice simulator for loss calibration.68
Annex D (normative) Mathematical basis.72
Annex E (normative)  Reflectance standard .75
Annex F (normative)  Simple version of reflectance standard.81
Annex G (informative)  OTDR basis: Backscatter theory – Reflectance measurements
using an OTDR – Determination of fibre backscatter parameter .85
Bibliography .90

– 4 – 61746-1 © IEC:2009(E)
Figure 1 – Definition of attenuation dead zone .10
Figure 2 – Representation of the location deviation ΔL(L).18
Figure 3 – Equipment for calibration of the distance scale – External source method .21
Figure 4 – Set-up for calibrating the system insertion delay.22
Figure 5 – Concatenated fibres used for calibration of the distance scale.26
Figure 6 – Distance calibration with a recirculating delay line .30
Figure 7 – OTDR trace produced by recirculating delay line .30
Figure 8 – Determining the reference level and the displayed power level .34
Figure 9 – Measurement of the OTDR loss samples .35
Figure 10 – Region A, the recommended region for loss measurement samples .36
Figure 11 – Possible placement of sample points within region A .36
Figure 12 – External source method for testing the polarization dependence of the
OTDR .37
Figure 13 – Reflection method for testing the polarization dependence of the OTDR .37
Figure 14 – Loss calibration with a fibre standard .39
Figure 15 – Placing the beginning of section D outside the attenuation dead zone.40
Figure 16 – Loss calibration with the external source method.43
Figure 17 – Location and measurements for external source method .44
Figure 18 – Set-up for loss calibration with splice simulator .46
Figure 19 – OTDR display with splice simulator .47
Figure 20 – Measurement of the splice loss.48
Figure 21 – Loss calibration with "fibre-end" variant of the power reduction method .51
Figure 22 – Loss calibration with "long-fibre" variant of the power reduction method.52
Figure 23 – Parameters involved in reflectance measurements.54
Figure 24 – The same reflectance at the end of three fibres with different values of the
backscatter parameter shows different pulse amplitudes .55
Figure 25 – Maximum and minimum values for the pulse amplitude, ΔF .56
Figure 26 – Range of reflectance measurement.56
Figure 27 – Determining the default displayed power level and the default location .57
Figure 28 – Set-up for reflectance calibration.58
Figure A.1 – Recirculating delay line.60
Figure A.2 – Measurement set-up for loop transit time T .61
b
Figure A.3 – Calibration set-up for lead-in transit time T .62
a
Figure B.1 – Determination of a highly linear power range.65
Figure B.2 – Testing the longitudinal backscatter uniformity of the fibre standard .66
Figure C.1 – Splice simulator and idealized OTDR signature.68
Figure C.2 – Determination of the reference loss A .70
ref
Figure E.1 – Reflectance standard description and trace.75
Figure E.2 – Calibration set up and reference points for calibration.78
Figure F.1 – Reflectance standard description and trace.81
Figure F.2 – Calibration set up and reference points for calibration .83
Figure G.1 – OTDR signals used for determining reflectance .86
Figure G.2 – Set-up for measurement of the backscatter coefficient .88

61746-1 © IEC:2009(E) – 5 –
Table 1 – Attenuation coefficients defining region A.35

– 6 – 61746-1 © IEC:2009(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 1: OTDR for single mode fibres

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61746-1 has been prepared by IEC technical committee 86: Fibre
optics.
This first edition of IEC 61746-1 cancels and replaces the second edition of IEC 61746,
published in 2005. It constitutes a technical revision.
The main technical changes are the adaptation of Clause 4, the suppression of Clause 10, the
improvement and the addition of some definitions, the change of some calculations and the
change of graphical symbology to IEC/TR 61930.

61746-1 © IEC:2009(E) – 7 –
The text of this standard is based on the following documents:
FDIS Report on voting
86/347/FDIS 86/362/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.
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.
A bilingual version of this standard may be issued at a later date.

– 8 – 61746-1 © IEC:2009(E)
INTRODUCTION
In order for an Optical time-domain reflectometer (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.

61746-1 © IEC:2009(E) – 9 –
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 1: OTDR for single mode fibres

1 Scope
This part of IEC 61746 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.
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 60793-1-40, Optical fibres – Part 1-40: Measurement methods and test procedures –
Attenuation
IEC 60793-2-50, Optical fibres – Part 2-50: Product specifications – Sectional specification for
class B single-mode fibres
ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
ITU-T Recommendation G.650.1:2002, 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
3 Terms, definitions and symbols
For the purposes of this document, the following terms, definitions and symbols apply.
NOTE For more precise definitions, the references to IEC 60050-731 should be consulted.
3.1
attenuation
loss
A
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
out
⎝ ⎠
[IEV 731-01-48, modified]
– 10 – 61746-1 © IEC:2009(E)
3.2
attenuation coefficient
α
attenuation ( 3.1) 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 ΔF
NOTE The attenuation dead zone (see Figure 1 below) 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
ΔF
Attenuation
dead zone
Location  km
IEC  1627/04
Figure 1 – Definition of attenuation dead zone
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 m/s);
= 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.
Displayed power F  dB
61746-1 © IEC:2009(E) – 11 –
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 ( 3.4) using the following formula:
C()ΔT = KΔT (3)
where ΔT is the pulse width, e.g. in seconds.
Usually the backscatter coefficient is expressed in dB for a given pulse width, ΔT.
C ()ΔT = 10log (KΔT) (4)
dB 10
NOTE 2 The pulse width, ΔT in the previous formula is used to normalise C()ΔT . Usual values for ΔT 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/IEC Guide 99 [11] in the bibliography.
3.7
centroidal wavelength
λ
avg
power-weighted mean wavelength of a light source in vacuum
[IEC 61280-1-3, definition 2.1.4]
3.8
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.
3.9
distance
D
spacing between two features
NOTE Usually expressed in metres.
3.10
distance sampling error
ΔL
sample
maximum distance ( 3.9) error attributable to the distance between successive sample points
NOTE 1 Usually expressed in metres.
NOTE 2 The distance sampling error is repetitive in nature; therefore, one way of quantifying this error is by its
amplitude.
– 12 – 61746-1 © IEC:2009(E)
3.11
distance scale deviation
ΔS
L
difference between the average displayed distance ( 3.9) < D > and the correspondent
otdr
reference distance ( 3.27) D divided by the reference distance ( 3.27)
ref
NOTE 1 Usually expressed in m/m
NOTE 2 ΔS is given by the following formula:
L
< D >− D < D >
otdr ref otdr
ΔS = = − 1 (5)
L
D D
ref ref
where < D > is the displayed distance on a fibre averaged over at least one sample spacing.
otdr
3.12
distance scale factor
S
L
average displayed distance ( 3.9) distance divided by the correspondent reference distance
( 3.27)
NOTE S is given by the following formula:
L
< D >
otdr
S = (6)
L
D
ref
where < D > is the displayed distance between two features on a fibre averaged over at least one sample
otdr
spacing.
3.13
distance scale uncertainty
u
ΔSL
uncertainty of the distance scale deviation ( 3.11)
NOTE 1 Usually expressed in m/m.
NOTE 2 u is given by the following formula:
ΔSL
⎛ ⎞ ⎛ ⎞
< D > < D >
otdr otdr
⎜ ⎟ ⎜ ⎟
u = u − 1 = u (7)
ΔSL
⎜ ⎟ ⎜ ⎟
D D
⎝ ref ⎠ ⎝ ref ⎠
NOTE 3 In the above formula, u() is understood as the standard uncertainty of ().
3.14
dynamic range at 98 %(one-way)
amount of fibre attenuation ( 3.1) that causes the backscatter signal to equal the noise level at
98 % ( 3.24)
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 60793-2-50).
3.15
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
61746-1 © IEC:2009(E) – 13 –
3.16
location
L
spacing between the front panel of the OTDR and a feature in a fibre
NOTE Usually expressed in metres.
3.17
location deviation
ΔL
displayed location ( 3.16) of a feature L minus the reference location ( 3.28) L
otdr ref
NOTE 1 Usually expressed in metres.
NOTE 2 This deviation is a function of the location.
3.18
location offset
ΔL
constant term of the location deviation ( 3.17) m odel
NOTE 1 Usually expressed in metres.
NOTE 2 This is approximately equivalent to the location of the OTDR front panel connector on the instrument's
distance scale.
NOTE 3 For a perfect OTDR, the location offset is zero.
3.19
location offset uncertainty
u
ΔL0
uncertainty of the location offset ( 3.18)
3.20
location readout uncertainty
u
Lreadout
uncertainty of the location ( 3.16) measurement samples caused by both the distance sampling
error ( 3.10) and the uncertainty type A of the measurement samples
3.21
loss deviation
ΔA
difference between the displayed loss of a fibre component A and the reference loss ( 3.29) ,
otdr
in dB
NOTE 1 ΔA is given by the following formula:

ΔA = A − A (8)
otdr ref
NOTE 2 The loss deviation usually depends on the displayed power level, F.
3.22
loss uncertainty
u
ΔA
uncertainty of the loss deviation ( 3.21), in dB

– 14 – 61746-1 © IEC:2009(E)
3.23
loss scale deviation
ΔS
A
difference between the displayed loss of a fibre component A and the reference loss ( 3.29)
otdr
A , divided by the reference loss ( 3.29), in dB/dB
ref
NOTE 1 ΔS is given by the following formula:
A
A − A
otdr ref
ΔS = (9)
A
A
ref
NOTE 2 Refer to 7.1 for more details.
3.24
noise level at 98 %
upper limit of a range which contains at least 98 % of all noise data points
3.25
non-linearity
NL
loss
difference between the maximum and minimum values of the loss deviation ( 3.21) ΔA for a
given range of power levels, in dB
NOTE 1 This is the non linearity of a logarithmic power scale.
NOTE 2 Non-linearity is one contribution to loss deviation; it usually depends on the displayed power level and the
location.
3.26
received power level
P
power received by the OTDR's optical port
3.27
reference distance
D
ref
distance ( 3.9) precisely determined by measuring equipment with calibration traceable to
international or national standards
NOTE Usually expressed in metres.
3.28
reference location
L
ref
location ( 3.16) precisely determined by measuring equipment with calibration traceable to
international or national standards
NOTE Usually expressed in metres.
3.29
reference loss
A
ref
loss of a fibre optic component precisely determined by measuring equipment with calibration
traceable to international or national standards
3.30
reflectance
R
the ratio, expressed in dB, of the reflected power (in watts), to the incident power (in watts), at
a discrete location in a fibre optic component

61746-1 © IEC:2009(E) – 15 –
NOTE 1 R is given by the following formula:
⎛ P ⎞
refl
⎜ ⎟
R=10log (10)
⎜ ⎟
P
⎝ inc⎠
where P is the reflected power, e.g. in watts;
refl
P is the incident power, e.g. in watts.
inc
NOTE 2 In this standard, reflectance is expressed in decibels.
NOTE 3 Reflectance values are negative.
NOTE 4 For the purpose of this standard, the reflectivity, ρ, is defined as the linear value of the reflectance:
P
refl
ρ= (11)
P
inc
3.31
reflectance deviation
ΔR
difference between the reflectance ( 3.30) of a fibre component R and the reflectance R ,
otdr ref
in dB
ΔR=R - R  dB (12)
otdr ref
3.32
rms dynamic range (one-way)
amount of fibre attenuation ( 3.1) that causes the backscatter signal to equal the rms noise
level ( 3.33)
NOTE Assuming a Gaussian distribution of noise, the rms dynamic range can be calculated adding 1,56 dB to the
one way dynamic range.
3.33
rms noise level
quadratic mean of the noise
NOTE 1 On a general basis, the rms noise level can not be read or extracted from the logarithm data of the
OTDR. This is because of the linear to logarithm conversion used to display the power level on a dB scale removes
the negative part of the noise.
NOTE 2 Assuming a Gaussian distribution of noise, a relation between the noise level and the rms noise level can
be found using the following formula:
noise − noise =5 ×log()2,05375 = 1,56 dB (13)
98 rms 10
where noise  is the noise level at 98 %, e.g. in dB;
noise  is the rms noise level, e.g. in dB;
rms
2,053 75 is the value of the reverse standard normal distribution for 98 %.
3.34
sample spacing
distance of consecutive data points digitized by the OTDR
NOTE 1 Usually expressed in metres.
NOTE 2 Sample spacing may be obtainable from instrument set-up information. Sample spacing may depend on
the measurement span and other OTDR instrument settings.

– 16 – 61746-1 © IEC:2009(E)
3.35
spectral width
Δλ
FWHM
full-width half-maximum (FWHM) spectral width of the source
[IEC 61280-1-3, modified]
4 Preparation for calibration
4.1 Organization
The calibration laboratory should satisfy requirements of ISO/IEC 17025.
There should be a documented measurement procedure for each type of calibration performed,
giving step-by-step operating instructions and equipment to be used.
4.2 Traceability
The requirements of ISO/IEC 17025 should be met.
All standards used in the calibration process shall be calibrated according to a documented
program with traceability to national standards laboratories or to accredited calibration
laboratories. It is advisable to maintain more than one standard on each hierarchical level, so
that the performance of the standard can be verified by comparisons on the same level. Make
sure that any other test equipment which has a significant influence on the calibration results is
calibrated. Upon request, specify this test equipment and its traceability chain(s). The re-
calibration period(s) shall be defined and documented.
4.3 Preparation
Perform all tests at an ambient room temperature of 23 °C ± 3 °C, 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 instructions.
4.4 Test conditions
The test conditions usually include the following OTDR external conditions: date, temperature,
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
width, sample spacing, centroidal wavelength. Unless otherwise specified, set the OTDR group
index to exactly 1,46.
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.
4.5 Documentation
Calibration certificates shall include the following data and their uncertainties:

61746-1 © IEC:2009(E) – 17 –
a) the location offset ΔL and its uncertainty ± 2 u as well as the distance scale deviation
ΔL0
ΔS and its uncertainty ± 2 u , or the location deviations ΔL and their uncertainties ± 2
ΔSL i
L
u
ΔLi;
b) the loss deviations ΔA and their uncertainties ± 2 u or the loss scale deviation ΔS and
A A
Δ
their uncertainties ± 2 u
;
ΔSA
c) the instrument configuration (pulse width, measurement span, wavelength, averaging
time,…) used during calibration;
d) other calibration certificate requirement as per ISO/IEC 17025.
5 Distance calibration – General
5.1 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 = (14)
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.
5.2 Location deviation model
In order to characterize location deviations, 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 + ΔL + f()L (15)
otdr L ref 0 ref
where
S is the scale factor, which ideally should be 1;
L
ΔL 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
– 18 – 61746-1 © IEC:2009(E)
Equation (15) 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 ΔL 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 ΔL are the slope and intercept,
L 0
respectively.
Equivalently, a line may be fitted to the location deviation function, that is the difference
between L and L :
otdr ref
ΔL = L - L = ΔS × L + ΔL + f()L (16)
otdr ref L ref 0 ref
where
ΔS is the slope; and
L
ΔL 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 ΔL may be determined by measuring departures from the line for different
readout
values of L . The distance sampling error amplitude ΔL is taken as half the amplitude
ref sample
of f(L ).
ref
In this standard, the distance sampling error amplitude ΔL 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
ΔL
sample
approximation
(Slope = ΔS )
L
ΔL
0 0
Location L
ref
IEC  1628/04
Figure 2 – Representation of the location deviation ΔL(L)

ΔL(L) = L – L  m
otdr ref
61746-1 © IEC:2009(E) – 19 –
Therefore, the result of the distance calibration shall be stated by the following parameters:
ΔS , u the distance scale deviation and its uncertainty;
L ΔSL
Δ L , u the location offset and its uncertainty;
0 ΔL0
u 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 u . Note that the uncertainty
Lreadout
will depend on the distance, the displayed power level and the instrument settings.
NOTE ΔL represents the physical sampling error of the instrument. This error is acces
...

IEC 61746-1 ®
Edition 1.0 2009-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Calibration of optical time-domain reflectometers (OTDR) –
Part 1: OTDR for single mode fibres

Étalonnage des réflectomètres optiques dans le domaine temporel (OTDR) –
Partie 1 : OTDR pour fibres unimodales

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IEC 61746-1 ®
Edition 1.0 2009-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Calibration of optical time-domain reflectometers (OTDR) –

Part 1: OTDR for single mode fibres

Étalonnage des réflectomètres optiques dans le domaine temporel (OTDR) –

Partie 1 : OTDR pour fibres unimodales

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XC
ICS 33.180.01 ISBN 978-2-8322-1682-8

– 2 – IEC 61746-1:2009 © IEC 2009
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references. 9
3 Terms, definitions and symbols . 9
4 Preparation for calibration . 16
4.1 Organization . 16
4.2 Traceability . 16
4.3 Preparation . 16
4.4 Test conditions . 16
4.5 Documentation . 16
5 Distance calibration – General . 17
5.1 General . 17
5.2 Location deviation model . 17
5.3 Using the calibration results . 19
5.4 Measuring fibre length . 19
6 Distance calibration methods . 20
6.1 General . 20
6.2 External source method . 20
6.2.1 Short description and advantage . 20
6.2.2 Equipment . 20
6.2.3 Calibration of the equipment . 21
6.2.4 Measurement procedure . 22
6.2.5 Calculations and results . 23
6.2.6 Uncertainties . 24
6.3 Concatenated fibre method . 25
6.3.1 Short description and advantages . 25
6.3.2 Equipment . 25
6.3.3 Measurement procedures. 27
6.3.4 Calculations and results . 27
6.3.5 Uncertainties . 28
6.4 Recirculating delay line method . 29
6.4.1 Short description and advantage . 29
6.4.2 Equipment . 29
6.4.3 Measurement procedure . 31
6.4.4 Calculations and results . 31
6.4.5 Uncertainties . 32
7 Loss calibration – General . 33
7.1 General . 33
7.2 Determination of the displayed power level F . 33
7.3 Selection of an appropriate reference loss A . 34
ref
7.4 Development of a test plan . 35
7.5 Polarization dependence . 37
7.6 Calculation of the calibration results . 38
7.7 Using the calibration results . 38

8 Loss calibration methods . 38
8.1 General . 38
8.2 Fibre standard method . 39
8.2.1 Short description and advantage . 39
8.2.2 Equipment . 39
8.2.3 Measurement procedure . 40
8.2.4 Calculations and results . 41
8.2.5 Uncertainties . 41
8.3 External source method (see Figure 16) . 42
8.3.1 Short description and advantage . 42
8.3.2 Equipment . 42
8.3.3 Calibration of the reference loss . 43
8.3.4 Measurement procedure . 44
8.3.5 Calculations and results . 45
8.3.6 Uncertainties . 45
8.4 Splice simulator method . 46
8.4.1 Short description and advantage . 46
8.4.2 Equipment . 46
8.4.3 Procedure . 47
8.4.4 Calculations and results . 49
8.4.5 Uncertainties . 49
8.5 Power reduction method . 50
8.5.1 Short description and advantage . 50
8.5.2 Equipment . 51
8.5.3 Measurement procedure . 52
8.5.4 Calculations and results . 53
8.5.5 Uncertainties . 53
9 Reflectance calibration. 54
9.1 Objective . 54
9.2 Reflectance measurements (see Figure 23) . 54
9.3 Use of the backscatter parameter, K . 54
9.4 Range of reflectance measurement . 55
9.5 Development of a test plan . 56
9.6 Equipment . 57
9.7 Measurement procedure . 58
9.7.1 Preparation . 58
9.7.2 Taking reflectance measurements . 58
9.7.3 Calculation and results . 58
9.7.4 Uncertainties . 58

Annex A (normative) Recirculating delay line for distance calibration . 60
Annex B (normative) Optical fibre standard for loss calibration . 64
Annex C (normative) Standard splice simulator for loss calibration. 68
Annex D (normative) Mathematical basis . 72
Annex E (normative)  Reflectance standard . 75
Annex F (normative)  Simple version of reflectance standard . 81
Annex G (informative)  OTDR basis: Backscatter theory – Reflectance measurements
using an OTDR – Determination of fibre backscatter parameter . 85

– 4 – IEC 61746-1:2009 © IEC 2009
Bibliography . 90

Figure 1 – Definition of attenuation dead zone . 10
Figure 2 – Representation of the location deviation ∆L(L). 18
Figure 3 – Equipment for calibration of the distance scale – External source method . 21
Figure 4 – Set-up for calibrating the system insertion delay . 22
Figure 5 – Concatenated fibres used for calibration of the distance scale . 26
Figure 6 – Distance calibration with a recirculating delay line . 30
Figure 7 – OTDR trace produced by recirculating delay line . 30
Figure 8 – Determining the reference level and the displayed power level . 34
Figure 9 – Measurement of the OTDR loss samples . 35
Figure 10 – Region A, the recommended region for loss measurement samples . 36
Figure 11 – Possible placement of sample points within region A . 36
Figure 12 – External source method for testing the polarization dependence of the
OTDR . 37
Figure 13 – Reflection method for testing the polarization dependence of the OTDR . 37
Figure 14 – Loss calibration with a fibre standard . 39
Figure 15 – Placing the beginning of section D outside the attenuation dead zone . 40
Figure 16 – Loss calibration with the external source method . 43
Figure 17 – Location and measurements for external source method . 44
Figure 18 – Set-up for loss calibration with splice simulator . 46
Figure 19 – OTDR display with splice simulator . 47
Figure 20 – Measurement of the splice loss . 48
Figure 21 – Loss calibration with "fibre-end" variant of the power reduction method . 51
Figure 22 – Loss calibration with "long-fibre" variant of the power reduction method . 52
Figure 23 – Parameters involved in reflectance measurements . 54
Figure 24 – The same reflectance at the end of three fibres with different values of the
backscatter parameter shows different pulse amplitudes . 55
Figure 25 – Maximum and minimum values for the pulse amplitude, ∆F . 56
Figure 26 – Range of reflectance measurement . 56
Figure 27 – Determining the default displayed power level and the default location . 57
Figure 28 – Set-up for reflectance calibration. 58
Figure A.1 – Recirculating delay line . 60
Figure A.2 – Measurement set-up for loop transit time T . 61
b
Figure A.3 – Calibration set-up for lead-in transit time T . 62
a
Figure B.1 – Determination of a highly linear power range . 65
Figure B.2 – Testing the longitudinal backscatter uniformity of the fibre standard . 66
Figure C.1 – Splice simulator and idealized OTDR signature . 68
Figure C.2 – Determination of the reference loss A . 70
ref
Figure E.1 – Reflectance standard description and trace. 75
Figure E.2 – Calibration set up and reference points for calibration . 78
Figure F.1 – Reflectance standard description and trace . 81
Figure F.2 – Calibration set up and reference points for calibration . 83

Figure G.1 – OTDR signals used for determining reflectance . 86
Figure G.2 – Set-up for measurement of the backscatter coefficient . 88

Table 1 – Attenuation coefficients defining region A . 35

– 6 – IEC 61746-1:2009 © IEC 2009
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 1: OTDR for single mode fibres

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61746-1 has been prepared by IEC technical committee 86: Fibre
optics.
This bilingual version (2014-06) correponds to the English version, published in 2009-12.
This first edition of IEC 61746-1 cancels and replaces the second edition of IEC 61746,
published in 2005. It constitutes a technical revision.
The main technical changes are the adaptation of Clause 4, the suppression of Clause 10, the
improvement and the addition of some definitions, the change of some calculations and the
change of graphical symbology to IEC/TR 61930.

The text of this standard is based on the following documents:
FDIS Report on voting
86/347/FDIS 86/362/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.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The 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.
– 8 – IEC 61746-1:2009 © IEC 2009
INTRODUCTION
In order for an Optical time-domain reflectometer (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.

CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 1: OTDR for single mode fibres

1 Scope
This part of IEC 61746 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.
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 60793-1-40, Optical fibres – Part 1-40: Measurement methods and test procedures –
Attenuation
IEC 60793-2-50, Optical fibres – Part 2-50: Product specifications – Sectional specification for
class B single-mode fibres
ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
ITU-T Recommendation G.650.1:2002, 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
3 Terms, definitions and symbols
For the purposes of this document, the following terms, definitions and symbols apply.
NOTE For more precise definitions, the references to IEC 60050-731 should be consulted.
3.1
attenuation
loss
A
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
out
 
[IEV 731-01-48, modified]
– 10 – IEC 61746-1:2009 © IEC 2009
3.2
attenuation coefficient
α
attenuation ( 3.1) 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 (see Figure 1 below) 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
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 m/s);
= 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.
Displayed power F  dB
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 ( 3.4) using the following formula:
C (∆T ) = K∆T (3)
where ∆T is the pulse width, e.g. in seconds.
∆T.
Usually the backscatter coefficient is expressed in dB for a given pulse width,
( ) ( ) (4)
C ∆T = 10log K∆T
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/IEC Guide 99 [11] in the bibliography.
3.7
centroidal wavelength
λ
avg
power-weighted mean wavelength of a light source in vacuum
[IEC 61280-1-3, definition 2.1.4]
3.8
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.
3.9
distance
D
spacing between two features
NOTE Usually expressed in metres.
3.10
distance sampling error
∆L
sample
maximum distance ( 3.9) error attributable to the distance between successive sample points
NOTE 1 Usually expressed in metres.
NOTE 2 The distance sampling error is repetitive in nature; therefore, one way of quantifying this error is by its
amplitude.
– 12 – IEC 61746-1:2009 © IEC 2009
3.11
distance scale deviation
∆S
L
difference between the average displayed distance ( 3.9) < D > and the correspondent
otdr
reference distance ( 3.27) D divided by the reference distance ( 3.27)
ref
NOTE 1 Usually expressed in m/m
NOTE 2 ∆S is given by the following formula:
L
< D >− D < D >
otdr ref otdr
∆S = = − 1 (5)
L
D D
ref ref
where < D > is the displayed distance on a fibre averaged over at least one sample spacing.
otdr
3.12
distance scale factor
S
L
average displayed distance ( 3.9) distance divided by the correspondent reference distance
( 3.27)
NOTE S is given by the following formula:
L
< D >
otdr
S = (6)
L
D
ref
where < D > is the displayed distance between two features on a fibre averaged over at least one sample
otdr
spacing.
3.13
distance scale uncertainty
u
∆SL
uncertainty of the distance scale deviation ( 3.11)
NOTE 1 Usually expressed in m/m.
NOTE 2 u is given by the following formula:
∆SL
   
< D > < D >
otdr otdr
   
u = u − 1 = u (7)
∆SL
   
D D
ref   ref 

NOTE 3 In the above formula, u() is understood as the standard uncertainty of ().
3.14
dynamic range at 98 %(one-way)
amount of fibre attenuation ( 3.1) that causes the backscatter signal to equal the noise level at
98 % ( 3.24)
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 60793-2-50).
3.15
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
3.16
location
L
spacing between the front panel of the OTDR and a feature in a fibre
NOTE Usually expressed in metres.
3.17
location deviation
∆L
displayed location ( 3.16) of a feature L minus the reference location ( 3.28) L
otdr ref
NOTE 1 Usually expressed in metres.
NOTE 2 This deviation is a function of the location.
3.18
location offset
∆L
constant term of the location deviation ( 3.17) model
NOTE 1 Usually expressed in metres.
NOTE 2 This is approximately equivalent to the location of the OTDR front panel connector on the instrument's
distance scale.
NOTE 3 For a perfect OTDR, the location offset is zero.
3.19
location offset uncertainty
u
∆L0
uncertainty of the location offset ( 3.18)
3.20
location readout uncertainty
u
Lreadout
uncertainty of the location ( 3.16) measurement samples caused by both the distance sampling
error ( 3.10) and the uncertainty type A of the measurement samples
3.21
loss deviation
∆A
difference between the displayed loss of a fibre component A and the reference loss ( 3.29),
otdr
in dB
NOTE 1 ∆A is given by the following formula:

∆A = A − A (8)
otdr ref
NOTE 2 The loss deviation usually depends on the displayed power level, F.
3.22
loss uncertainty
u
∆A
uncertainty of the loss deviation ( 3.21), in dB

– 14 – IEC 61746-1:2009 © IEC 2009
3.23
loss scale deviation
∆S
A
difference between the displayed loss of a fibre component A and the reference loss ( 3.29)
otdr
A , divided by the reference loss ( 3.29), in dB/dB
ref
NOTE 1 ∆S is given by the following formula:
A
A − A
otdr ref
∆S = (9)
A
A
ref
NOTE 2 Refer to 7.1 for more details.
3.24
noise level at 98 %
upper limit of a range which contains at least 98 % of all noise data points
3.25
non-linearity
NL
loss
difference between the maximum and minimum values of the loss deviation ( 3.21) ∆A for a
given range of power levels, in dB
NOTE 1 This is the non linearity of a logarithmic power scale.
NOTE 2 Non-linearity is one contribution to loss deviation; it usually depends on the displayed power level and the
location.
3.26
received power level
P
power received by the OTDR's optical port
3.27
reference distance
D
ref
distance ( 3.9) precisely determined by measuring equipment with calibration traceable to
international or national standards
NOTE Usually expressed in metres.
3.28
reference location
L
ref
location ( 3.16) precisely determined by measuring equipment with calibration traceable to
international or national standards
NOTE Usually expressed in metres.
3.29
reference loss
A
ref
loss of a fibre optic component precisely determined by measuring equipment with calibration
traceable to international or national standards
3.30
reflectance
R
the ratio, expressed in dB, 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 (10)
 
P
 inc 
where P is the reflected power, e.g. in watts;
refl
P is the incident power, e.g. in watts.
inc
NOTE 2 In this standard, reflectance is expressed in decibels.
NOTE 3 Reflectance values are negative.
NOTE 4 For the purpose of this standard, the reflectivity, ρ, is defined as the linear value of the reflectance:
P
refl
ρ= (11)
P
inc
3.31
reflectance deviation
∆R
and the reflectance R ,
difference between the reflectance ( 3.30) of a fibre component R
otdr ref
in dB
∆R=R - R  dB (12)
otdr ref
3.32
rms dynamic range (one-way)
amount of fibre attenuation ( 3.1) that causes the backscatter signal to equal the rms noise
level ( 3.33)
NOTE Assuming a Gaussian distribution of noise, the rms dynamic range can be calculated adding 1,56 dB to the
one way dynamic range.
3.33
rms noise level
quadratic mean of the noise
NOTE 1 On a general basis, the rms noise level can not be read or extracted from the logarithm data of the
OTDR. This is because of the linear to logarithm conversion used to display the power level on a dB scale removes
the negative part of the noise.
NOTE 2 Assuming a Gaussian distribution of noise, a relation between the noise level and the rms noise level can
be found using the following formula:
noise − noise =5 ×log (2,05375) = 1,56 dB (13)
98 rms 10
where noise  is the noise level at 98 %, e.g. in dB;
noise  is the rms noise level, e.g. in dB;
rms
2,053 75 is the value of the reverse standard normal distribution for 98 %.
3.34
sample spacing
distance of consecutive data points digitized by the OTDR
NOTE 1 Usually expressed in metres.
NOTE 2 Sample spacing may be obtainable from instrument set-up information. Sample spacing may depend on
the measurement span and other OTDR instrument settings.

– 16 – IEC 61746-1:2009 © IEC 2009
3.35
spectral width
∆λ
FWHM
full-width half-maximum (FWHM) spectral width of the source
[IEC 61280-1-3, modified]
4 Preparation for calibration
4.1 Organization
The calibration laboratory should satisfy requirements of ISO/IEC 17025.
There should be a documented measurement procedure for each type of calibration performed,
giving step-by-step operating instructions and equipment to be used.
4.2 Traceability
The requirements of ISO/IEC 17025 should be met.
All standards used in the calibration process shall be calibrated according to a documented
program with traceability to national standards laboratories or to accredited calibration
laboratories. It is advisable to maintain more than one standard on each hierarchical level, so
that the performance of the standard can be verified by comparisons on the same level. Make
sure that any other test equipment which has a significant influence on the calibration results is
calibrated. Upon request, specify this test equipment and its traceability chain(s). The re-
calibration period(s) shall be defined and documented.
4.3 Preparation
Perform all tests at an ambient room temperature of 23 °C ± 3 °C, 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 instructions.
4.4 Test conditions
The test conditions usually include the following OTDR external conditions: date, temperature,
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
width, sample spacing, centroidal wavelength. Unless otherwise specified, set the OTDR group
index to exactly 1,46.
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.
4.5 Documentation
Calibration certificates shall include the following data and their uncertainties:

a) the location offset ∆L and its uncertainty ± 2 u as well as the distance scale deviation
∆L0
∆S and its uncertainty ± 2 u , or the location deviations ∆L and their uncertainties ± 2
∆SL i
L
u
∆Li;
b) the loss deviations ∆A and their uncertainties ± 2 u or the loss scale deviation ∆S and
A
A
∆
their uncertainties ± 2 u
;
∆SA
c) the instrument configuration (pulse width, measurement span, wavelength, averaging
time,…) used during calibration;
d) other calibration certificate requirement as per ISO/IEC 17025.
5 Distance calibration – General
5.1 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 = (14)
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,
gene
...