IEC 61746-2:2010
(Main)Calibration of optical time-domain reflectometers (OTDR) - Part 2: OTDR for multimode fibres
Calibration of optical time-domain reflectometers (OTDR) - Part 2: OTDR for multimode fibres
IEC 61746-2:2010 provides procedures for calibrating multimode optical time domain reflectometers (OTDR). It covers OTDR measurement errors and uncertainties. The test of the laser(s) source modal condition is included as an optional measurement. This standard does not cover correction of the OTDR response. Keywords: calibrating multimode optical time domain reflectometers (OTDR)
Étalonnage des réflectomètres optiques dans le domaine temporel (OTDR) - Partie 2: OTDR pour fibres multimodales
L'IEC 61746-2:2010 fournit des procédures destinées à l'étalonnage des réflectomètres optiques dans le domaine de temps pour fibres multimodales (OTDR). Elle traite des erreurs de mesure et incertitudes de l'OTDR. L'essai des conditions modales des sources laser est une mesure facultative. La présente norme ne couvre pas la correction de la réponse de l'OTDR. Mots clés: l'étalonnage des réflectomètres optiques dans le domaine de temps pour fibres multimodales (OTDR).
General Information
Standards Content (Sample)
IEC 61746-2 ®
Edition 1.0 2010-06
INTERNATIONAL
STANDARD
Calibration of optical time-domain reflectometers (OTDR) –
Part 2: OTDR for multimode fibres
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IEC 61746-2 ®
Edition 1.0 2010-06
INTERNATIONAL
STANDARD
Calibration of optical time-domain reflectometers (OTDR) –
Part 2: OTDR for multimode fibres
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
X
ICS 33.180.01 ISBN 978-2-88912-026-0
– 2 – 61746-2 © IEC:2010(E)
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references.7
3 Terms, definitions and symbols .7
4 Preparation for calibration.13
4.1 Organization .13
4.2 Traceability.13
4.3 Preparation.13
4.4 Test conditions .13
4.5 Documentation .13
5 Distance calibration – General .14
5.1 General .14
5.2 Location deviation model .14
5.3 Using the calibration results.16
5.4 Measuring fibre length .17
6 Distance calibration methods .17
6.1 General .17
6.2 External source method .17
6.2.1 Short description and advantage .17
6.2.2 Equipment .17
6.2.3 Calibration of the equipment .19
6.2.4 Measurement procedure .20
6.2.5 Calculations and results .20
6.2.6 Uncertainties .21
6.3 Concatenated fibre method (using multimode fibres) .23
6.3.1 Short description and advantages .23
6.3.2 Equipment .23
6.3.3 Measurement procedures.24
6.3.4 Calculations and results .24
6.3.5 Uncertainties .25
6.4 Recirculating delay line method.26
6.4.1 Short description and advantages .26
6.4.2 Equipment .27
6.4.3 Measurement procedure .28
6.4.4 Calculations and results .28
6.4.5 Uncertainties .29
7 Vertical scale calibration – General .30
7.1 General .30
7.2 Loss difference calibration .31
7.2.1 Determination of the displayed power level F.31
7.2.2 Development of a test plan.31
7.3 Characterization of the OTDR source near field .33
7.3.1 Objectives and references .33
7.3.2 Procedure.33
8 Loss difference calibration method.34
61746-2 © IEC:2010(E) – 3 –
8.1 General .34
8.2 Long fibre method.34
8.2.1 Short description.34
8.2.2 Equipment .34
8.2.3 Measurement procedure .36
8.2.4 Calculation and results.36
Annex A (normative) Multimode recirculating delay line for distance calibration.37
Annex B (normative) Mathematical basis .41
Bibliography .44
Figure 1 – Definition of attenuation dead zone .8
Figure 2 – Representation of the location deviation ΔL(L).15
Figure 3 – Equipment for calibration of the distance scale – External source method .18
Figure 4 – Set-up for calibrating the system insertion delay.19
Figure 5 – Concatenated fibres used for calibration of the distance scale.23
Figure 6 – Distance calibration with a recirculating delay line .27
Figure 7 – OTDR trace produced by recirculating delay line .28
Figure 8 – Determining the reference level and the displayed power level .31
Figure 9 – Region A, the recommended region for loss measurement samples .32
Figure 10 – Possible placement of sample points within region A .33
Figure 11 – Linearity measurement with a long fibre .35
Figure 12 – Placing the beginning of section D outside the attenuation dead zone .35
Figure A.1 – Recirculating delay line.37
Figure A.2 – Measurement set-up for loop transit time T .38
b
Figure A.3 – Calibration set-up for lead-in transit time T .39
a
Table 1 – Additional distance uncertainty.16
Table 2 – Attenuation coefficients defining region A.32
– 4 – 61746-2 © IEC:2010(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 2: OTDR for multimode 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.
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61746-2 has been prepared by IEC technical committee 86: Fibre
optics.
The text of this standard is based on the following documents:
CDV Report on voting
86/336/CDV 86/359/RVC
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.
61746-2 © IEC:2010(E) – 5 –
A list of all parts of IEC 61746 series, under the general title Calibration of optical time-domain
reflectometers (OTDR), can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability 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 publication may be issued at a later date.
– 6 – 61746-2 © IEC:2010(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) the ability to measure type A1a or A1b IEC 60793-2-10 fibres;
b) a programmable index of refraction, or equivalent parameter;
c) the ability to present a display of a trace representation, with a logarithmic power scale and
a linear distance scale;
d) two markers/cursors, which display the loss and distance between any two points on a trace
display;
e) the ability to measure absolute distance (location) from the OTDR's zero-distance reference;
f) the ability to measure the displayed power level relative to a reference level (for example, the
clipping level).
Calibration methods described in this standard may look similar to those provided in Part 1 of
this series. However, there are differences: mix of different fibre types, use of mode conditioner
or different arrangement of the fibres. This leads to different calibration processes as well as
different uncertainties analysis.
61746-2 © IEC:2010(E) – 7 –
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 2: OTDR for multimode fibres
1 Scope
This part of IEC 61746 provides procedures for calibrating multimode optical time domain
reflectometers (OTDR). It covers OTDR measurement errors and uncertainties. The test of the
laser(s) source modal condition is included as an optional measurement.
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 60793-2-10, Optical fibres – Part 2-10: Product specifications – Sectional specification for
category A1 multimode fibres
IEC 60793-2-50, Optical fibres – Part 2-50: Product specifications – Sectional specification for
class B single-mode fibres
IEC 61280-1-4, Fibre optic communication subsystem test procedures – Part 1-4: General
communication subsystems – Light source encircled flux measurement method
IEC 61280-4-1, Fibre optic communication subsystem test procedures – Part 4-1: Installed
cable plant – Multimode attenuation measurement
IEC 61745, End-face image analysis procedure for the calibration of optical fibre geometry test
sets
ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
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
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
– 8 – 61746-2 © IEC:2010(E)
⎛ ⎞
P
in
⎜ ⎟
A = 10log dB (1)
⎜ ⎟
P
⎝ out⎠
[IEV 731-01-48, modified]
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 1424/10
Figure 1 – Definition of attenuation dead zone
3.4
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
NOTE See ISO Guide International vocabulary of basic and general terms in metrology.
3.5
centroidal wavelength
λ
avg
power-weighted mean wavelength of a light source in vacuum
[IEC 61280-1-3, definition 2.1.4]
Displayed power F (dB)
61746-2 © IEC:2010(E) – 9 –
3.6
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 power scale displays five times the logarithm of the received power, plus a constant
offset.
3.7
distance
D
spacing between two features
NOTE Usually expressed in metres.
3.8
distance sampling error
ΔL
sample
maximum distance ( 3.7) 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.
3.9
distance scale deviation
ΔS
L
difference between the average displayed distance ( 3.7) < 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 (2)
L
D D
ref ref
where < D > is the displayed distance on a fibre averaged over at least one sample spacing.
otdr
3.10
distance scale factor
S
L
average displayed distance ( 3.7) divided by the correspondent reference distance ( 3.27)
NOTE 1 S is given by the following formula
L
< D >
otdr
S = (3)
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.
3.11
distance scale uncertainty
u
ΔSL
uncertainty of the distance scale deviation ( 3.9)
NOTE 1 Usually expressed in m/m.
– 10 – 61746-2 © IEC:2010(E)
NOTE 2 u is given by the following formula
ΔSL
⎛ < D > ⎞ ⎛ < D >⎞
otdr otdr
⎜ ⎟ ⎜ ⎟
u = u −1 = u (4)
ΔSL ⎜ ⎟ ⎜ ⎟
D D
⎝ ref ⎠ ⎝ ref ⎠
NOTE 3 In the above formula, u() is understood as the standard uncertainty of ().
3.12
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 A fibre (see
IEC 60793-2-10).
3.13
encircled flux
EF
fraction of cumulative near field power to total output power as a function of radial distance
from the centre of the core
3.14
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
3.15
location
L
spacing between the front panel of the OTDR and a feature in a fibre
NOTE Usually expressed in metres
3.16
location deviation
ΔL
displayed location ( 3.15) 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.
3.17
location offset
ΔL
constant term of the location deviation ( 3.16) 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.
3.18
location offset uncertainty
u
ΔL0
uncertainty of the location offset ( 3.17)
61746-2 © IEC:2010(E) – 11 –
3.19
location readout uncertainty
u
Lreadout
uncertainty of the location ( 3.15) measurement samples caused by both the distance sampling
error ( 3.8) and the uncertainty type A of the measurement samples
3.20
loss deviation
ΔA
difference between the displayed loss of a fibre component A and the reference loss ( 3.29)
otdr
A , in dB
ref
NOTE 1 ΔA is given by the following formula
ΔA = A − A (5)
otdr ref
NOTE 2 The loss deviation usually depends on the displayed power level, F.
3.21
loss uncertainty
u
ΔA
uncertainty of the loss deviation ( 3.20), in dB
3.22
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 = (6)
A
A
ref
NOTE 2 Refer to 7.1 for more details.
3.23
mode conditioner
a fibre set that converts any power distribution submitted at its input to an output power
distribution that fully comply with encircled flux limits
NOTE For the purposes of this standard, the encircled flux limits are defined by the IEC 61280-4-1.
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 ( 3.20) 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.
– 12 – 61746-2 © IEC:2010(E)
3.26
received power level
P
power received by the OTDR's optical port
3.27
reference distance
D
ref
distance ( 3.7) 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 ( 3.15) 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
rms dynamic range (one-way)
amount of fibre attenuation ( 3.1) that causes the backscatter signal to equal the rms noise
level ( 3.31)
NOTE Assuming a Gaussian distribution of noise, the rms dynamic range can be calculated adding 1,56 dB to the
one way dynamic range. See 3.31.
3.31
rms noise level
the quadratic mean of the noise
NOTE 1 On a general basis, the rms noise level cannot be read or extracted from the logarithm data of the OTDR.
This is because 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 (7)
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
2,05375 is the value of the reverse standard normal distribution for 98 %.
3.32
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.
61746-2 © IEC:2010(E) – 13 –
3.33
spectral width
Δλ
FWHM
full-width half-maximum (FWHM) spectral width of the source
[IEC 61280-1-3, definition 3.2.3 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 instruction.
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, centre 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:
– 14 – 61746-2 © IEC:2010(E)
a) the location offset ΔL and its uncertainty ± 2 u as well as the distance scale
0 ΔL0
deviation ΔS and its uncertainty ± 2 u , or the location deviations ΔL and their
ΔSL
L i
uncertainties ± 2 u
ΔLi;
b) the non-linearity NL
loss ;
c) the instrument configuration (pulse with, measurement span, wavelength, averaging
time, etc.) used during calibration;
d) other appropriate calibration data and 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 = (8)
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 multimode
fibre's group index N and taking into account the differential mode delay. The determination of
N and the analysis of the consequences of the differential mode delay are beyond the scope of
this standard. Consequently, the calibration procedures below only discuss the OTDR's ability
to measure T correctly. For the purposes of this standard, a default value N = 1,46 is used and
the uncertainty of N is considered to be 0. Also the calibration methods are built to limit
uncertainties due to the differential mode delay.
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 (9)
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
61746-2 © IEC:2010(E) – 15 –
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 (9) 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 (10)
otdr ref L ref 0 ref
where
ΔS is the slope, and
L
ΔL is still the intercept, as illustrated in Figure 2.
) respectively its half-
After finding the linear approximation, the distance sampling error f(L
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
Location L
ref
IEC 1425/10
Figure 2 – Representation of the location deviation ΔL(L)
Therefore, the result of the distance calibration shall be stated by the following parameters:
ΔS , u is the distance scale deviation and its uncertainty;
ΔSL
L
Δ L , u is the location offset and its uncertainty;
ΔL0
ΔL(L) = L – L (m)
otdr ref
– 16 – 61746-2 © IEC:2010(E)
u is the location readout uncertainty, that is the combined uncertainty due to the
L
readout
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 accessible for the user as
sample
u that includes distance calculation and displaying errors.
Lreadout
5.3 Using the calibration results
The error in the location of a feature ΔL = L – L can be calculated from the calibration
otdr ref
results:
ΔL = ΔL + L ΔS (11)
0 ref L
with the uncertainty in ΔL given by the following formula, in which the recommended confidence
level of 95 % is used:
2 2 2 2
± 2u = ±2(u + L u + u ) (11a)
ΔL ΔL0 ref ΔSL 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 ΔD and its uncertainty can be
calculated from the following formula:
ΔD = D ΔS (12)
ref L
with uncertainty in ΔD given by the following formula:
2 2 2
± 2u = ±2(D u + 2u ) (12a)
ΔD ref ΔSL Lreadout
where the displayed distance D can be used instead of the reference distance D .
otdr ref
NOTE The 2 in front of u is due to combining two uncorrelated uncertainties.
Lreadout
Differential mode delay may create additional uncertainties on long fibres measurement. Such
uncertainties should be negligible for distance given in Table 1.
Table 1 – Additional distance uncertainty
Length of fibre causing additional distance uncertainty
Wavelength
IEC 60793-2- IEC 60793- IEC 60793- IEC 60793-
nm
10 2-10 2-10 2-10
A1a.1 A1a.2 A1b A1d
850 1 000 m 7 500 m 500 m 50 m
1 300 1 000 m 2 500 m 1 000 m 500 m
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.
61746-2 © IEC:2010(E) – 17 –
5.4 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 contrast to a mechanical length
measurement. This method is directly compatible with the measurement principle of the OTDR
itself. In addition, the transit time can usually be measured with better accuracy than its
mechanical length, particularly when the fibre is long. Therefore, in this standard, it is
suggested that fibre transit time instead of fibre length be used whenever accuracy is
important.
Measure the transit time of the fibre T with the help, for example, of a pulse generator, a
transit
triggerable laser source, an optical-to-electrical converter (O/E converter) and a time interval
counter. It is important that the laser source has approximately the same centre wavelength
λ as the test OTDR, because a difference in wavelength may result in a difference of
centre
transit time due to the chromatic dispersion of the fibre. An alternative to the laser source is
using the OTDR itself to produce optical pulses; in this case, the centre wavelengths
automatically coincide. Record the transit time as the difference between the arrival times with
and without the fibre inserted between the laser source and the O/E converter.
When this fibre is used for OTDR distance calibrations, then the reference distance D can be
ref
calculated by
cT
transit
D = (13)
ref m
N
In this equation, use a group index N which is identical with the OTDR's group index setting.
The time measurement principle makes it possible to use D as the reference distance.
ref
6 Distance calibration methods
6.1 General
Each of the calibration methods described below is capable of determining all of the necessary
calibration results: location offset, distance scale deviation, and their uncertainties.
6.2 External source method
6.2.1 Short description and advantage
The external source method uses a calibrated time-delay generator to simulate the time delay
in a fibre and an optical source to simulate the reflected or scattered signal from a fibre.
Each time it is possible (e.g. when operation at 1 300 nm), IEC 60793-2-50 single mode fibres
are used instead of multimode fibres for the interconnections, in order to reduce uncertainties
caused by differential mode delay.
The method is well suited to automated laboratory testing under computer control. For
simplicity, only reflective features are discussed in this standard. To calibrate the OTDR
for features other than reflection, the pulsed E/O converter described below should be replaced
by an optical source that simulates the appropriate feature.
6.2.2 Equipment
In addition to the OTDR, the measurement equipment includes, as shown in Figure 3:
a) a mode conditioner;
b) an single mode optical coupler;
– 18 – 61746-2 © IEC:2010(E)
c) an optical-to-electrical converter;
d) a digital delay generator with pulse capability;
e) an electrical to optical converter;
f) a variable optical attenuator, for reduction of the pulse amplitude to just below the clipping
level.
A1 G1
C2
E2
F4 F3
Digital delay
dB
E/O
generator
F5
DUT
MC
C1
E1
F2
F1
F0
O/E
OTDR
Out
In
IEC 1426/10
Key
F0 multimode fibre
F1, F2, F3, F4 and F5 single mode fibres
MC mode conditioner
E1 and E2 electric cables
E/O electrical-to-optical converter
O/E optical-to-electrical converter
A1 variable attenuator
Figure 3 – Equipment for calibration of the distance scale –
External source method
The OTDR is connected to the coupler through the mode conditioning multimode to single
mode adapter. The coupler routes the OTDR signal to the O/E converter (detector). The
detector triggers the delay generator, which, after a known time delay, causes an optical pulse
to be generated. This pulse is then coupled back to the OTDR.
The E/O converter can be a simple pulsed laser that simulates a reflection. Constant pulse
amplitude and pulse width are considered adequate to calibrate the distance scale for reflective
features. However, the attenuator makes it possible to adjust the pulse amplitude based on the
distance of the reflection from the front panel of the OTDR, in order to simulate the change of
reflection amplitude caused by the attenuation of the fibre.
To allow accurate calibration of the set-up, fibres F1 and F5 should have the same length (see
below). Fibre F5 is terminated to absorb reflections.
NOTE 1 The mode conditioner is needed to make sure the OTDR receives proper launch conditions from the
electrical to optical converter. Therefore fibre F0 should be connected to the output of the mode conditioner while
fibre F1 should be connected to the input.
NOTE 2 The attenuation of the optical path between the connector of the OTDR and the optical to electrical
converter may be high. This is acceptable as the output power of the OTDR is generally sufficient.
61746-2 © IEC:2010(E) – 19 –
6.2.3 Calibration of the equipment
Before using the "external source" equipment, it shall be properly calibrated. It is assumed that
the digital delay generator is regularly calibrated. For computing the location offset ΔL from
the measured data, it is also necessary to determine the insertion delay T of the apparatus.
delay
This can be accomplished by adding a pulse generator and a calibrated time interval counter to
the equipment, as shown in Figure 4.
MC
G1
A1
C2
F0
F4 E2
F3 Digital delay
dB
E/O
Out generator
In
F5
MC
E4E4
C1
Stop Start
F2 E1
F0
F1 G2
O/E
E3
Out
In
Pulse
T--interval
generator
CT1 ccountounterer
IEC 1427/10
Key
F0 multimode fibre (two during calibration)
MC mode conditioner (two during calibration)
F1, F2, F3, F4 and F5 fibres
E1, E2, E3 and E4 electric cables
C2 electrical-to-optical converter
C1 optical-to-electrical converter
A1 variable attenuator
Figure 4 – Set-up for calibrating the system insertion delay
To properly measure the propagation delay of the mode conditioner it is recommended to
include within the optical path, a second identical mode conditioner.
To calibrate the insertion delay T , proceed as follows.
delay
Set the pulse generator to square wave, with a repetition period more than twice as long as the
delay time to be measured. Use the output pulse of the pulse generator as the start pulse on
the time interval counter, and to externally trigger the delay generator. Set the digital delay
generator for external triggering and zero delay for the leading edge of the pulse generator
signal. Set the tri
...
IEC 61746-2 ®
Edition 1.0 2010-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Calibration of optical time-domain reflectometers (OTDR) –
Part 2: OTDR for multimode fibres
Étalonnage des réflectomètres optiques dans le domaine temporel (OTDR) –
Partie 2: OTDR pour fibres multimodales
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IEC 61746-2 ®
Edition 1.0 2010-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Calibration of optical time-domain reflectometers (OTDR) –
Part 2: OTDR for multimode fibres
Étalonnage des réflectomètres optiques dans le domaine temporel (OTDR) –
Partie 2: OTDR pour fibres multimodales
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.01 ISBN 978-2-8322-3067-1
– 2 – IEC 61746-2:2010 IEC 2010
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references. 7
3 Terms, definitions and symbols . 7
4 Preparation for calibration . 13
4.1 Organization . 13
4.2 Traceability . 13
4.3 Preparation . 13
4.4 Test conditions . 13
4.5 Documentation . 13
5 Distance calibration – General . 14
5.1 General . 14
5.2 Location deviation model . 14
5.3 Using the calibration results . 16
5.4 Measuring fibre length . 17
6 Distance calibration methods . 17
6.1 General . 17
6.2 External source method . 17
6.2.1 Short description and advantage . 17
6.2.2 Equipment . 18
6.2.3 Calibration of the equipment . 19
6.2.4 Measurement procedure . 20
6.2.5 Calculations and results . 21
6.2.6 Uncertainties . 21
6.3 Concatenated fibre method (using multimode fibres) . 23
6.3.1 Short description and advantages . 23
6.3.2 Equipment . 23
6.3.3 Measurement procedures. 24
6.3.4 Calculations and results . 25
6.3.5 Uncertainties . 25
6.4 Recirculating delay line method . 27
6.4.1 Short description and advantages . 27
6.4.2 Equipment . 27
6.4.3 Measurement procedure . 28
6.4.4 Calculations and results . 28
6.4.5 Uncertainties . 29
7 Vertical scale calibration – General . 30
7.1 General . 30
7.2 Loss difference calibration . 31
7.2.1 Determination of the displayed power level F . 31
7.2.2 Development of a test plan . 31
7.3 Characterization of the OTDR source near field . 33
7.3.1 Objectives and references . 33
7.3.2 Procedure . 33
8 Loss difference calibration method . 34
8.1 General . 34
8.2 Long fibre method . 34
8.2.1 Short description . 34
8.2.2 Equipment . 34
8.2.3 Measurement procedure . 36
8.2.4 Calculation and results . 36
Annex A (normative) Multimode recirculating delay line for distance calibration . 37
Annex B (normative) Mathematical basis . 41
Bibliography . 44
Figure 1 – Definition of attenuation dead zone . 8
Figure 2 – Representation of the location deviation DL(L). 15
Figure 3 – Equipment for calibration of the distance scale – External source method . 18
Figure 4 – Set-up for calibrating the system insertion delay . 19
Figure 5 – Concatenated fibres used for calibration of the distance scale . 23
Figure 6 – Distance calibration with a recirculating delay line . 27
Figure 7 – OTDR trace produced by recirculating delay line . 28
Figure 8 – Determining the reference level and the displayed power level . 31
Figure 9 – Region A, the recommended region for loss measurement samples . 32
Figure 10 – Possible placement of sample points within region A . 33
Figure 11 – Linearity measurement with a long fibre . 35
Figure 12 – Placing the beginning of section D outside the attenuation dead zone . 35
Figure A.1 – Recirculating delay line . 37
Figure A.2 – Measurement set-up for loop transit time T . 38
b
Figure A.3 – Calibration set-up for lead-in transit time T . 39
a
Table 1 – Additional distance uncertainty . 16
Table 2 – Attenuation coefficients defining region A . 32
– 4 – IEC 61746-2:2010 IEC 2010
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 2: OTDR for multimode fibres
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61746-2 has been prepared by IEC technical committee 86: Fibre
optics.
This bilingual version (2015-12) corresponds to the monolingual English version, published in
2010-06.
The text of this standard is based on the following documents:
CDV Report on voting
86/336/CDV 86/359/RVC
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.
A list of all parts of IEC 61746 series, under the general title Calibration of optical time-domain
reflectometers (OTDR), can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 61746-2:2010 IEC 2010
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) the ability to measure type A1a or A1b IEC 60793-2-10 fibres;
b) a programmable index of refraction, or equivalent parameter;
c) the ability to present a display of a trace representation, with a logarithmic power scale and
a linear distance scale;
d) two markers/cursors, which display the loss and distance between any two points on a trace
display;
e) the ability to measure absolute distance (location) from the OTDR's zero-distance
reference;
f) the ability to measure the displayed power level relative to a reference level (for example,
the clipping level).
Calibration methods described in this standard may look similar to those provided in Part 1 of
this series. However, there are differences: mix of different fibre types, use of mode conditioner
or different arrangement of the fibres. This leads to different calibration processes as well as
different uncertainties analysis.
CALIBRATION OF OPTICAL TIME-DOMAIN
REFLECTOMETERS (OTDR) –
Part 2: OTDR for multimode fibres
1 Scope
This part of IEC 61746 provides procedures for calibrating multimode optical time domain
reflectometers (OTDR). It covers OTDR measurement errors and uncertainties. The test of the
laser(s) source modal condition is included as an optional measurement.
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 60793-2-10, Optical fibres – Part 2-10: Product specifications – Sectional specification for
category A1 multimode fibres
IEC 60793-2-50, Optical fibres – Part 2-50: Product specifications – Sectional specification for
class B single-mode fibres
IEC 61280-1-4, Fibre optic communication subsystem test procedures – Part 1-4: General
communication subsystems – Light source encircled flux measurement method
IEC 61280-4-1, Fibre optic communication subsystem test procedures – Part 4-1: Installed
cable plant – Multimode attenuation measurement
IEC 61745, End-face image analysis procedure for the calibration of optical fibre geometry test
sets
ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
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
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
– 8 – IEC 61746-2:2010 IEC 2010
P
in
dB (1)
A = 10log
P
out
[IEV 731-01-48, modified]
3.2
attenuation coefficient
a
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 DF
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
DF
Attenuation
dead zone
Location (km)
IEC 1424/10
Figure 1 – Definition of attenuation dead zone
3.4
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
NOTE See ISO Guide International vocabulary of basic and general terms in metrology.
3.5
centroidal wavelength
λ
avg
power-weighted mean wavelength of a light source in vacuum
[IEC 61280-1-3, definition 2.1.4]
Displayed power F (dB)
3.6
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 power scale displays five times the logarithm of the received power, plus a constant
offset.
3.7
distance
D
spacing between two features
NOTE Usually expressed in metres.
3.8
distance sampling error
DL
sample
maximum distance (3.7) 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.
3.9
distance scale deviation
DS
L
difference between the average displayed distance (3.7) < 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 DS is given by the following formula
L
< D >− D < D >
otdr ref otdr
DS = = − 1 (2)
L
D D
ref ref
where < D > is the displayed distance on a fibre averaged over at least one sample spacing.
otdr
3.10
distance scale factor
S
L
average displayed distance (3.7) divided by the correspondent reference distance (3.27)
NOTE 1 S is given by the following formula
L
< D >
otdr
S = (3)
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.
3.11
distance scale uncertainty
u
DSL
uncertainty of the distance scale deviation (3.9)
NOTE 1 Usually expressed in m/m.
– 10 – IEC 61746-2:2010 IEC 2010
NOTE 2 u is given by the following formula
DSL
< D > < D >
otdr otdr
u = u −1 = u (4)
ΔSL
D D
ref ref
NOTE 3 In the above formula, u() is understood as the standard uncertainty of ().
3.12
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 A fibre (see
IEC 60793-2-10).
3.13
encircled flux
EF
fraction of cumulative near field power to total output power as a function of radial distance
from the centre of the core
3.14
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
3.15
location
L
spacing between the front panel of the OTDR and a feature in a fibre
NOTE Usually expressed in metres
3.16
location deviation
DL
displayed location (3.15) 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.
3.17
location offset
DL
constant term of the location deviation (3.16) 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.
3.18
location offset uncertainty
u
DL0
uncertainty of the location offset (3.17)
3.19
location readout uncertainty
u
Lreadout
uncertainty of the location (3.15) measurement samples caused by both the distance sampling
error (3.8) and the uncertainty type A of the measurement samples
3.20
loss deviation
DA
difference between the displayed loss of a fibre component A and the reference loss (3.29)
otdr
A , in dB
ref
NOTE 1 DA is given by the following formula
DA = A − A (5)
otdr ref
NOTE 2 The loss deviation usually depends on the displayed power level, F.
3.21
loss uncertainty
u
DA
uncertainty of the loss deviation (3.20), in dB
3.22
loss scale deviation
DS
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 DS is given by the following formula
A
A − A
otdr ref
DS = (6)
A
A
ref
NOTE 2 Refer to 7.1 for more details.
3.23
mode conditioner
a fibre set that converts any power distribution submitted at its input to an output power
distribution that fully comply with encircled flux limits
NOTE For the purposes of this standard, the encircled flux limits are defined by the IEC 61280-4-1.
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 (3.20) DA 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.
– 12 – IEC 61746-2:2010 IEC 2010
3.26
received power level
P
power received by the OTDR's optical port
3.27
reference distance
D
ref
distance (3.7) 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 (3.15) 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
rms dynamic range (one-way)
amount of fibre attenuation (3.1) that causes the backscatter signal to equal the rms noise level
(3.31)
NOTE Assuming a Gaussian distribution of noise, the rms dynamic range can be calculated adding 1,56 dB to the
one way dynamic range. See 3.31.
3.31
rms noise level
the quadratic mean of the noise
NOTE 1 On a general basis, the rms noise level cannot be read or extracted from the logarithm data of the OTDR.
This is because 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 (7)
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
2,05375 is the value of the reverse standard normal distribution for 98 %.
3.32
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.
3.33
spectral width
Dλ
FWHM
full-width half-maximum (FWHM) spectral width of the source
[IEC 61280-1-3, definition 3.2.3 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 instruction.
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, centre 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:
– 14 – IEC 61746-2:2010 IEC 2010
a) the location offset DL and its uncertainty ± 2 u as well as the distance scale
0 DL0
deviation DS and its uncertainty ± 2 u , or the location deviations DL and their
DSL
L i
uncertainties ± 2 u
DLi;
b) the non-linearity NL
loss ;
c) the instrument configuration (pulse with, measurement span, wavelength, averaging
time, etc.) used during calibration;
d) other appropriate calibration data and 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 = (8)
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 multimode
fibre's group index N and taking into account the differential mode delay. The determination of
N and the analysis of the consequences of the differential mode delay are beyond the scope of
this standard. Consequently, the calibration procedures below only discuss the OTDR's ability
to measure T correctly. For the purposes of this standard, a default value N = 1,46 is used and
the uncertainty of N is considered to be 0. Also the calibration methods are built to limit
uncertainties due to the differential mode delay.
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 + DL + f (L ) (9)
otdr L ref 0 ref
where
S is the scale factor, which ideally should be 1;
L
DL 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 (9) 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 DL 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 DL 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
DL = L - L = DS ⋅ L + DL + f (L ) (10)
otdr ref L ref 0 ref
where
DS is the slope, and
L
DL 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 DL may be determined by measuring departures from the line for different
readout
values of L . The distance sampling error amplitude DL is taken as half the amplitude
ref
sample
of f(L ).
ref
In this standard, the distance sampling error amplitude DL 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
DL
sample
approximation
(Slope = DS )
L
DL
Location L
ref
IEC 1425/10
Figure 2 – Representation of the location deviation DL(L)
Therefore, the result of the distance calibration shall be stated by the following parameters:
DS , u is the distance scale deviation and its uncertainty;
DSL
L
DL(L) = L – L (m)
otdr ref
– 16 – IEC 61746-2:2010 IEC 2010
D L , u is the location offset and its uncertainty;
DL0
u is the location readout uncertainty, that is the combined uncertainty due to the
L
readout
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 DL represents the physical sampling error of the instrument. This error is accessible for the user as
sample
u that includes distance calculation and displaying errors.
Lreadout
5.3 Using the calibration results
The error in the location of a feature DL = L – L can be calculated from the calibration
otdr ref
results:
DL = DL + L DS (11)
0 ref L
with the uncertainty in DL given by the following formula, in which the recommended confidence
level of 95 % is used:
2 2 2 2
± 2u = ±2(u + L u + u ) (11a)
ΔL ΔL0 ref ΔSL 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 DD and its uncertainty can be
calculated from the following formula:
DD = D DS (12)
ref L
with uncertainty in DD given by the following formula:
2 2 2
± 2u = ±2(D u + 2u ) (12a)
ΔD ref ΔSL Lreadout
where the displayed distance D can be used instead of the reference distance D .
otdr ref
NOTE The 2 in front of u is due to combining two uncorrelated uncertainties.
Lreadout
Differential mode delay may create additional uncertainties on long fibres measurement. Such
uncertainties should be negligible for distance given in Table 1.
Table 1 – Additional distance uncertainty
Length of fibre causing additional distance uncertainty
Wavelength
IEC 60793-2- IEC 60793- IEC 60793- IEC 60793-
nm
10 2-10 2-10 2-10
A1a.1 A1a.2 A1b A1d
850 1 000 m 7 500 m 500 m 50 m
1 300 1 000 m 2 500 m 1 000 m 500 m
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.
5.4 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 contrast to a mechanical length
measurement. This method is directly compatible with the measurement principle of the OTDR
itself. In addition, the transit time can usually be measured with better accuracy than its
mechanical length, particularly when the fibre is long. Therefore, in this standard, it is
suggested that fibre transit time instead of fibre length be used whenever accuracy is
important.
Measure the transit time of the fibre T with the help, for example, of a pulse generator, a
transit
triggerable laser source, an optical-to-electrical converter (O/E converter) and a time interval
counter. It is important that the laser source has approximately the same centre wavelength
λ as the test OTDR, because a difference in wavelength may result in a difference of
centre
transit time due to the chromatic dispersion of the fibre. An alternative to the laser source is
using the OTDR itself to produce optical pulses; in this case, the centre wavelengths
automatically coincide. Record the transit time as the difference between the arrival times with
and without the fibre inserted between the laser source and the O/E converter.
When this fibre is used for OTDR distance calibrations, then the reference distance D can be
ref
calculated by
cT
transit
D = (13)
ref
m
N
In this equation, use a group index N which is identical with the OTDR's group index setting.
The time measurement principle makes it possible to use D as the reference distance.
ref
6 Distance calibration methods
6.1 General
Each of the calibration methods described below is capable of determining all of the necessary
calibration results: location offset, distance scale deviation, and their uncertainties.
6.2 External source method
6.2.1 Short description and advantage
The external source method uses a calibrated time-delay generator to simulate the time delay
in a fibre and an optical source to simulate the reflected or scattered signal from a fibre.
Each time it is possible (e.g. when operation at 1 300 nm), IEC 60793-2-50 single mode fibres
are used instead of multimode fibres for the interconnections, in order to reduce uncertainties
caused by differential mode delay.
The method is well suited to automated laboratory testing under computer control. For
simplicity, only reflective features are discussed in this standard. To calibrate the OTDR
for fe
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