IEC 60793-1-44:2023

IEC 60793-1-44 ®
Edition 3.0 2023-07
COMMENTED VERSION
INTERNATIONAL
STANDARD
colour
inside
Optical fibres –
Part 1-44: Measurement methods and test procedures – Cut-off wavelength
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IEC 60793-1-44 ®
Edition 3.0 2023-07
COMMENTED VERSION
INTERNATIONAL
STANDARD
colour
inside
Optical fibres –
Part 1-44: Measurement methods and test procedures – Cut-off wavelength
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.10 ISBN 978-2-8322-7323-4
– 2 – IEC 60793-1-44:2023 CMV © IEC 2023
CONTENTS
FOREWORD .5
1 Scope .7
2 Normative references .7
3 Terms and definitions .8
4 Background .8
5 Overview of methods .8
6 Reference test method .9
7 Apparatus .9
7.1 Light source .9
7.2 Modulation . 10
7.3 Launch optics . 10
7.4 Support and positioning apparatus . 10
7.5 Deployment mandrel . 10
7.5.1 General . 10
7.5.2 Cable cut-off wavelength deployment, method A . 10
7.5.3 Cable cut-off wavelength deployment, method B . 11
7.5.4 Fibre cut-off wavelength deployment, method C. 12
7.6 Detection optics . 13
7.7 Detector assembly and signal detection electronics . 14
7.8 Cladding mode stripper . 14
8 Sampling and specimens . 14
8.1 Specimen length . 14
8.2 Specimen end face . 14
9 Procedure . 14
9.1 Positioning of specimen in apparatus . 14
9.1.1 General requirements for all methods . 14
9.1.2 Deployment requirements for each method . 14
9.2 Measurement of output power . 14
9.2.1 Overview . 14
9.2.2 Bend-reference technique . 15
9.2.3 Multimode-reference technique . 16
10 Calculations . 16
10.1 Bend-reference technique . 16
10.2 Multimode-reference technique . 16
10.3 Curve-fitting technique for improved precision (optional) . 14
10.3.1 General .
10.3.2 Step 1, define the upper-wavelength region .
10.3.3 Step 2, characterize the attenuation curve .
10.3.4 Step 3, determine the upper wavelength of the transition region .
10.3.5 Step 4, determine the lower wavelength of the transition region .
10.3.6 Step 5, characterize the transition region with the theoretical model .
10.3.7 Step 6, compute the cut-off wavelength, λ .
c
11 Mapping functions . 21
12 Results . 21
13 Specification information . 22

Annex A (normative) Requirements specific to method A – Cable cut-off wavelength,
λ , using uncabled fibre . 23
cc
A.1 Specimen length . 23
A.2 Procedure – Position specimen on deployment mandrel . 23
Annex B (normative) Requirements specific to method B – Cable cut-off wavelength,
λ , using cabled fibre . 24
cc
B.1 Specimen length . 24
B.2 Procedure – Position specimen on deployment mandrel . 24
Annex C (normative) Requirements specific to method C – Fibre cut-off wavelength,
λ . 25
c
C.1 Specimen length . 25
C.2 Procedure – Position specimen on deployment mandrel . 25
Annex D (informative) Cut-off curve artifacts . 27
D.1 Description of curve artifacts . 27
D.2 Curve-fitting technique for artifact filtering . 27
D.2.1 Overview . 27
D.2.2 General . 28
D.2.3 Step 1: define the upper wavelength region . 29
D.2.4 Step 2: characterize the spectral transmittance . 29
D.2.5 Step 3: calculate the deviation of the spectral transmittance from the
linear fit . 29
D.2.6 Step 4: determine the end wavelength of the transition region . 30
D.2.7 Step 5: determine the start wavelength of the transition region . 30
D.2.8 Step 6: characterize the transition region with the theoretical model . 30
D.2.9 Step 7: compute the cut-off wavelength, λ . 31
c
D.3 Fibre deployment method for artifact attenuation . 32
Bibliography . 34
List of comments . 35

Figure 1 – Cut-off measurement system block diagram .9
Figure 2 – Deployment configuration for cable cut-off wavelength λ , method A . 11
cc
Figure 3 – Deployment configuration for cable cut-off wavelength λ , method B . 12
cc
Figure 3 – Default configuration to measure λ
c.
Figure 4 – Deployment configurations for fibre cut-off measurement .
Figure 4 – Standard deployment for fibre cut-off wavelength measurement . 13
Figure 5 – Cut-off wavelength using the bend-reference technique . 15
Figure 6 – Cut-off wavelength using the multimode-reference technique . 15
Figure 7 – Cable cut-off vs fibre cut-off for a specific fibre (multimode reference) . 21
Figure A.1 – Alternative cable cut-off deployment . 23
Figure C.1 – Alternative fibre cut-off deployment – Sliding semi-circle. 25
Figure C.2 – Alternative fibre cut-off deployment – Multi-bend . 26
Figure C.3 – Alternative fibre cut-off deployment – Large curve . 26
Figure D.1 – Cut-off curve with linear fit error (multimode reference) . 27
Figure D.2 – Fibre cut-off curve fitting technique (multimode reference) . 28
Figure D.3 – Curve fitting regions . 28

– 4 – IEC 60793-1-44:2023 CMV © IEC 2023
Figure D.4 – Fibre cut-off curve with artifacts (multimode reference) . 32
Figure D.5 – Fibre cut-off curve with artifacts (bend reference) . 32
Figure D.6 – Fibre deployment with large diameter bends for mode filtering . 33
Figure D.7 – Fibre cut-off curve with mode attenuation (multimode reference) . 33

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-44: Measurement methods and test procedures –
Cut-off wavelength
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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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.
This commented version (CMV) of the official standard IEC 60793-1-44:2023 edition 3.0
allows the user to identify the changes made to the previous IEC 60793-1-44:2011
edition 2.0. Furthermore, comments from IEC SC 86A experts are provided to explain the
reasons of the most relevant changes, or to clarify any part of the content.
A vertical bar appears in the margin wherever a change has been made. Additions are in
green text, deletions are in strikethrough red text. Experts' comments are identified by a
blue-background number. Mouse over a number to display a pop-up note with the
comment.
This publication contains the CMV and the official standard. The full list of comments is
available at the end of the CMV.

– 6 – IEC 60793-1-44:2023 CMV © IEC 2023
IEC 60793-1-44 has been prepared by subcommittee 86A: Fibres and cables, of IEC technical
committee 86: Fibre optics. It is an International Standard.
This third edition cancels and replaces the second edition published in 2011. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) used the diameter of the fibre loops to describe deployment;
b) added Annex D related to cut-off curve artifacts;
c) reorganized information and added more figures to clarify concepts.
The text of this International Standard is based on the following documents:
Draft Report on voting
86A/2314/FDIS 86A/2327/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
This document is to be read in conjunction with IEC 60793-1-1.
A list of all parts of the IEC 60793-1 series, published under the general title Optical fibres –
Measurement methods and test procedures, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.

OPTICAL FIBRES –
Part 1-44: Measurement methods and test procedures –
Cut-off wavelength
1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the cut-off wavelength
of single-mode optical fibre, thereby assisting in the inspection of fibres and cables for
commercial purposes.
This document gives methods for measuring the cut-off wavelength of fibre and cable for
uncabled or cabled single mode telecom fibre. These procedures apply to all category B and C
fibre types.
There are two methods for measuring cable cut-off wavelength, λ :
cc
• Method A: using uncabled fibre;
• Method B: using cabled fibre.
There is only one method (Method C) for measuring fibre cut-off wavelength, λ .
c
The test method in this standard describes procedures for determining the cut-off wavelength
of a sample fibre in either an uncabled condition (λ ) or in a cable (λ ). Three default
c cc
configurations are given here: any different configuration will be given in a detail specification.
These procedures apply to all category B and C fibre types (see Normative references).
There are three methods of deployment for measuring the cut-off wavelength:
• method A: cable cut-off using uncabled fibre 22 m long sample, λ ;
cc
• method B: cable cut-off using cabled fibre 22 m long sample, λ ;
cc
• method C: fibre cut-off using uncabled fibre 2 m long sample, λ .
c
All methods require a reference measurement. There are two reference-scan techniques, either
or both of which may can be used with all methods:
• bend-reference technique;
• multimode-reference technique using category A1(OM1-OM5) multimode fibre.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 60793-1-1, Optical fibres – Part 1-1: Measurement methods and test procedures – General
and guidance
IEC 60793-1-40, Optical fibres – Part 1-40: Measurement methods and test procedures –
Attenuation
– 8 – IEC 60793-1-44:2023 CMV © IEC 2023
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
4 Background
Theoretical cut-off wavelength is the shortest wavelength at which only the fundamental mode
can propagate in a single-mode fibre, as computed from the refractive index profile of the fibre.
In optical fibres, the change from multimode to single mode behaviour does not occur at an
isolated wavelength, but rather smoothly over a range of wavelengths. For purposes of
determining fibre performance in a telecommunications network, theoretical cut-off wavelength
is less useful than the lower value actually measured when the fibre is deployed.
Measured cut-off wavelength is defined as the wavelength greater than which the ratio between
the total power, including launched higher-order modes, and the fundamental mode power has
decreased to less than 0,1 dB. According to this definition, the second-order (LP ) mode
undergoes 19,3 dB more attenuation than the fundamental (LP ) mode at the cut-off
wavelength. 1
Because measured cut-off wavelength depends on the length and bends of the fibre, the
resulting value of cut-off wavelength depends on whether the measured fibre is configured in a
deployed, cabled condition or if it is short and uncabled. Consequently, there are two overall
types of cut-off wavelength:
• cable cut-off wavelength (λ ) measured in an uncabled fibre deployment condition
cc
(method A), or in a cabled condition (method B);
• fibre cut-off wavelength (λ ) measured on a short length of uncabled, primary-coated fibre
c
(method C).
Cable cut-off wavelength is the preferred attribute to be specified and measured.
5 Overview of methods
All of the methods shall use the transmitted-power technique, which measures the variation
with wavelength of the transmitted power of a fibre under test compared to a reference
transmitted-power wavelength scan. The reference scan normalizes wavelength-dependent
fluctuations in the measurement equipment so that the attenuation of the LP mode in the
specimen can be properly characterized and the cut-off wavelength precisely determined.
All of the methods shall use the transmitted-power technique. A general system block diagram
is depicted in Figure 1. A fibre specimen is scanned by a wavelength spectrum. The output
optical power is measured and stored. This stored data is then analysed against a reference
power spectrum. The reference scan normalizes any wavelength-dependent fluctuations in the
measurement equipment that is not associated with the loss of the LP mode. The resulting
attenuation will thus properly characterize the cut-off wavelength.

Figure 1 – Cut-off measurement system block diagram 2
The reference scan uses one of the following two techniques:
• the specimen with an additional, smaller-radius fibre bend;
• a (separate) category A1 multimode fibre.
• bend reference where a small diameter bend is added to the fibre specimen;
• multimode reference where the optical power through an A1(OM1-OM5) fibre is measured.
This procedure Either reference technique can determine the cut-off wavelength of a fibre
specimen in either a cabled or uncabled condition. Each method has its own default
configurations; the detail specification will give any different configuration required.
The fibre cut-off wavelength,λ , measured under the standard length and bend conditions
c
described in this document, will generally exhibit a value larger than the cable cut-off
wavelength, λ . For normal installed cable spans, it is common for the measured λ value to
cc c
exceed the system long fibre’s transmission wavelength. Thus Cable cut-off wavelength is the
more useful description of system performance and capability.
For short cables, e.g. a pigtail with a length shorter (and possibly a bending radius larger) than
described in this method, the cable may introduce modal noise near the cut-off wavelength
when lossy splices are present (>0,5 dB).
Cable cut-off wavelength is more useful in describing an installed network system performance
and capability, while fibre cut-off would apply to short cables or pigtails. The two cut-off
wavelengths can be mapped to each other for a specific fibre type and cut-off measurement
method. The customer and the supplier shall agree to the confidence level of each mapping
function established (see Clause 11 for details).
6 Reference test method
Method A, cable cut-off wavelength using uncabled fibre, is the reference test method (RTM).
This method shall be used to settle any disputes.
The apparatus for each method is described in Clause 7.
7 Apparatus
7.1 Light source
Provide a filtered white light source, with line width not greater than 10 nm, stable in position
and intensity. The light source should be capable of operating over the wavelength range
1 000 nm to 1 600 nm for most category B fibres. An operating range of 800 nm to 1 700 nm

– 10 – IEC 60793-1-44:2023 CMV © IEC 2023
may be necessary for some B4 B-655 fibres, B5 B-656 fibres or some category C fibres. A
scanning monochromator with a halogen bulb is one example of this kind of source.
7.2 Modulation
Modulate the light source to prevent ambient light affecting the results, and to aid signal
recovery. A mechanical chopper with a reference output is a suitable arrangement.
7.3 Launch optics
Provide launch optics, such as a lens system or a multimode fibre, to overfill the test fibre over
the full range of measurement wavelengths. This launch is relatively insensitive to the input end
face position of the single-mode fibre and is sufficient able to excite the fundamental and any
higher-order modes in the specimen. If using a butt splice, provide means of avoiding
interference effects.
When using a multimode fibre, overfilling the reference fibre can produce an undesired ripple
effect in the power-transmission spectrum. Restrict the launch sufficiently to eliminate the ripple
effect. One example of restricted launch is in method A, attenuation by cut-back, of
IEC 60793-1-40. Another example of restricted launch is a mandrel-wrap mode filter with
sufficient (approximately 4 dB) insertion loss.
7.4 Support and positioning apparatus
Provide a means to stably support the input and output ends of the specimen for the duration
of the test; vacuum chucks, magnetic chucks, or connectors may be used for this purpose.
Support the fibre ends such that they can be repeatedly positioned in the launch and detection
optics. When measuring λ in method B, provide a means to suitably support the cable ends.
cc
The mechanism used to hold the fibre ends allows for fibre positioning with respect to the launch
and detection optics. Holding and moving of the fibre should not cause micro-bends that affect
the measurement accuracy. 3
7.5 Deployment mandrel
7.5.1 General
Use a means to stably support the input and output ends of the specimen for the duration of the
measurement. Support the fibre ends so that they can be repeatedly and stably positioned with
respect to the launch and detection optics without introducing microbends into the specimen.
The fibre specimen’s two ends, input and output, are mechanically held in place during the
measurement. The deployment and length of the specimen, together with the support
apparatus, are key elements of the measurement method, and they distinguish the types of
cut-off wavelength.
Additional, alternative deployments may be used if the results obtained have been
demonstrated to be empirically equivalent to the results obtained using the standard
deployment, to within 10 nm, or they are greater than those achieved with the standard
configurations.
7.5.2 Cable cut-off wavelength deployment, method A
Provide a means to make an 80 mm diameter loop at each end of the specimen and a loop of
diameter ≥ 280 mm in the central portion. See Figure 2.
NOTE Two loops at one end can be substituted for one loop at each end.

∅ ≥ 280 mm
∅ = 80 mm ∅ = 80 mm
22 m of fibre
IEC  701/11
Dimensions in millimetres
Figure 2 – Deployment configuration for cable cut-off wavelength λ , method A
cc
7.5.3 Cable cut-off wavelength deployment, method B
Provide a means to make an 80 mm diameter loop at each end of the specimen. See Figure 3.
The cabled fibre between the two 80 mm loops has a bending diameter greater than 280 mm.
NOTE Two loops at one end can be substituted for one loop at each end.

∅ = 80 mm ∅ = 80 mm
1 m 20 m 1 m
IEC  702/11
– 12 – IEC 60793-1-44:2023 CMV © IEC 2023
Dimensions in millimetres
Figure 3 – Deployment configuration for cable cut-off wavelength λ , method B
cc
7.5.4 Fibre cut-off wavelength deployment, method C 4
Provide a circular mandrel as the initial fibre cut-off wavelength deployment. (See Figure 4a).
A split, semicircular mandrel with a radius of 140 mm that is capable of sliding, hence able to
take up slack fibre, is an alternative deployment . (See Figures 3 and 4b).
NOTE The introduction of a minimum bend of the cable sufficient to permit connection of the two ends of the whole
specimen to the measurement setup is allowed.

Launch Receive
r
rr
Lower semicircular mandrel able
to slide to take up slack fibre
IEC  703/11
Figure 3 – Default configuration to measure λ
c
L
r
IEC  704/11
Key
r = 140 mm
L = 2 m (entire fibre length)
Figure 4a) – Initial deployment configuration for fibre cut-off wavelength measurement – circular mandrel

L L
r r r
r r
IEC  705/11
Key
r = 140 mm
L = 2 m (entire fibre length)
Figure 4b) – Alternative deployment configuration for fibre cut-off wavelength measurement – split mandrel
Figure 4 – Deployment configurations for fibre cut-off measurement
Provide means to route a fibre specimen through one complete circular loop having a diameter
equal to 280 mm, see Figure 4.
Dimensions in millimetres
Figure 4 – Standard deployment for fibre cut-off wavelength measurement
7.6 Detection optics
Couple all power emitted from the specimen onto the active region of the detector. As examples,
an optical lens system, a butt splice with a multimode fibre pigtailed to a detector, or direct
coupling may be used.
– 14 – IEC 60793-1-44:2023 CMV © IEC 2023
7.7 Detector assembly and signal detection electronics
Use a detector that is sensitive to the output radiation over the range of wavelengths to be
measured and that is linear over the range of intensities encountered. A typical system might
include a germanium or InGaAs photodiode, operating in the photo-voltaic mode, and a
current-sensitive preamplifier, with synchronous detection by a lock-in amplifier. Generally, a
computer is required to analyse the data.
7.8 Cladding mode stripper
Provide a means to remove cladding-mode power from the specimen. Under some
circumstances, the fibre coating will perform this function; otherwise, provide methods or
devices that extract cladding-mode power at the input and output ends of the specimen.
8 Sampling and specimens
8.1 Specimen length
Choose the specimen length according to which parameter is being measured and, if the
parameter is cable cut-off wavelength, the method to be used. See the appropriate annex:
Annex A or Annex B for the cable cut-off wavelength measurement or Annex C for fibre cut-off
wavelength.
8.2 Specimen end face
Prepare a flat end face, orthogonal to the fibre axis, at the input and output ends of each
specimen. An optical fibre cleaver is often used to achieve very flat and clean end faces.
9 Procedure
9.1 Positioning of specimen in apparatus
9.1.1 General requirements for all methods
Align the input and output ends of the specimen to the launch and detection optics. Do not
change the launch and detection conditions during the course of the measurement.
Unless otherwise specified, when installing the specimen in the apparatus, and when using a
cladding-mode stripper, take care to avoid imposing any additional fibre bends smaller
than those specified in the configuration for the particular measurement being made.
9.1.2 Deployment requirements for each method
Deploy the specimen using the information in Clause 7 and the following annexes:
• cable cut-off wavelength, method A (see Annex A);
• cable cut-off wavelength, method B (see Annex B);
• fibre cut-off wavelength, method C (see Annex C).
9.2 Measurement of output power
9.2.1 Overview
Record the output power, P (λ), along the wavelength range, in increments of 10 nm or less.
s
The range shall be broad enough to encompass the expected cut-off wavelength and, as
outlined below, ultimately result in a curve similar to that in Figure 5 (using the bend-reference
technique) or Figure 6 (using the multimode-reference technique). The measured power is then

normalized by the reference power producing curves similar to Figure 5, bend reference and
Figure 6 multimode reference.
Figure 5 – Cut-off wavelength using the bend-reference technique 5

Key
A (λ) = The spectral transmittance referenced to the multimode fibre (dB)
m
Figure 6 – Cut-off wavelength using the multimode-reference technique
9.2.2 Bend-reference technique
With input and output conditions unchanged, introduce a smaller-diameter bend between input
and the output. Record the transmitted spectral power, P (λ), over the same wavelength range
b
and with the same spectral increments as in making the original measurement on the specimen.
The exact value of the smaller diameter may be determined prior to measurement; it should be
small enough to attenuate the second-order mode, but not too small in order to avoid
macrobending effects at higher wavelengths. A radius between 10 and 30 mm is typical for most
B1.1 to B5 fibres. For some B6 fibres, the radius shall be much smaller, and this measurement
technique may not be adequate for these fibres. See Note to 10.1. A diameter between 20 mm
and 60 mm is typical for most B652 to B656 fibres.

– 16 – IEC 60793-1-44:2023 CMV © IEC 2023
For some bend insensitive fibres (category B-657), the diameter may need to be smaller, and
this measurement technique may not be adequate for these fibres. Curve artifacts can persist
and require the use of both techniques described in Annex D. 6
9.2.3 Multimode-reference technique
Replace the specimen with a short (< 10 m) length of category A1(OM1-OM5) multimode fibre
as a reference. Record the transmitted signal power, P (λ), over the same wavelength range
m
and with the same spectral increments as in making the original measurement on the specimen.
NOTE The power using the multimode-reference technique, P (λ), may be stored in a computer for use in repetitive
m
measurements on different specimens.
Ripples can occur in the reference spectrum P (λ). Provide a means to add mode mixing which can reduce this
m
effect; this can be done by wrapping a multimode fibre at least 5 times around a mandrel 10 mm to 20 mm in diameter.
A1(OM2-OM5) type multimode fibres are preferred for this technique.
10 Calculations
10.1 Bend-reference technique
Calculate the spectral transmittance of the specimen without the smaller-radius small diameter
bend, referenced to the condition where the smaller-radius bend is introduced:
P λ
( )
s
A λ = 10log (1)
( ) in dB
b 10
P λ
( )
b
where
A (λ) is the spectral transmittance referenced to the smaller-radius small diameter bend (dB);
b
P (λ) is the output power spectral optical power through the fibre specimen;
s
P (λ) is the transmitted spectral optical power through the sample with the smaller-radius
b
small diameter bend introduced.
Figure 5 shows a schematic result. The short and long wavelength edges are determined by
the specimen deployed with and without the smaller-radius bend, respectively. Determine the
longest wavelength at which A (λ) = 0,1 dB from Figure 5. This is the cut-off wavelength,
b
provided that ∆A is equal to or greater than 2 dB.
b
If ∆A < 2 dB, or if it is unobservable, broaden the wavelength scan and enlarge the single-
b
mode launch conditions, or decrease the smaller-bend radius. Repeat these adjustments and
the measurement procedure until ∆A > 2 dB.
b
NOTE For certain implementations of bend-insensitive fibres (category B6 B-657), ∆A will not
b
reach 2 dB loss, because of the very nature of these fibres. It is recommended to use the
multimode-reference technique as reference scan for these fibres. See Annex D for dealing with

curve artifacts that can affect the results.
10.2 Multimode-reference technique
Calculate the spectral transmittance of the specimen, referenced to that of the multimode fibre:

P λ
( )
s
A (λ) = 10log (2)
m 10
P λ
( )
m
where
A (λ) is the spectral transmittance referenced to the multimode fibre (dB);
m
P (λ) is the output spectral optical power through the specimen;
s
P (λ) is the transmitted signal spectral optical power through the multimode reference fibre.
m
Figure 6 shows a schematic result.
Fit a straight line to the long-wavelength portion of A (λ), displacing it upward by 0,1 dB, as
m
shown by the dashed line in Figure 6. Determine the longest wavelength at which this displaced
line intersects with A (λ). This is the cut-off wavelength, provided that ∆A is equal to or greater
m m
than 2 dB. Between measured data points, A (λ) is defined by linear interpolation.
m
If ∆A < 2 dB, or if it is unobservable, broaden the wavelength scan and enlarge the single-mode
m
launch conditions. Repeat these adjustments and the measurement procedure until ∆A > 2 dB,
m
and until the long-wavelength tail is of adequate length to be fitted by a straight line.
NOTE 1 When using the multimode-reference technique, fibres with high cut-off wavelengths, when combined with
reference fibres with high water peaks, can have erroneous values reported as cut-off wavelength.
NOTE 2 For certain implementations of bend-insensitive fibres (category B6 B-657), the bend-
reference technique is not the optimal technique as reference scan. For these fibres, the
multimode-reference technique is recommended. See Annex D for dealing with curve artifacts
that can affect the results.
10.3 Curve-fitting technique for improved precision (optional)
10.3.1 General
In the absence of spurious humps or excessive noise in the upper-wavelength region, accurate
values for cut-off wavelength can be determined without curve fitting.
If curve fitting is considered necessary for improving precision, there are six steps. The first two
steps define the LP region, or upper-wavelength region. The next two steps define the
transition region, where LP attenuation begins to increase. The fifth step characterizes this
region according to a theoretical model. The last step computes the cut-off wavelength from the
characterization parameters.
This analysis is applicable for λ and λ measured by all methods, using either the bend-
c cc
reference technique or the multimode-reference technique.
The term α(λ) represents either A (λ) or A (λ).
b m
10.3.2 Step 1, define the upper-wavelength region
10.3.2.1 Using the bend-reference technique
One method to identify the lower wavelength of the upper wavelength region is to find the
maximum attenuation wavelength. For wavelengths greater than the maximum attenuation

– 18 – IEC 60793-1-44:2023 CMV © IEC 2023
wavelength, the lower wavelength of the region is the wavelength at which the following function
is a minimum: α(λ) – 8 + 8λ, with λ in μm.
The upper wavelength of the upper wavelength region is the lowest wavelength value of the
upper wavelength region plus 150 nm.
10.3.2.2 Using the multimode-reference technique
One method to identify the lower wavelength of the upper wavelength region is to find the
maximum slope wavelength, the wavelength at which the first difference, α(λ) - α (λ + 10 nm),
is largest. For wavelengths greater than the maximum slope wavelength, the lower wavelength
of the region is the wavelength at which the attenuation is a minimum.
10.3.3 Step 2, characterize the attenuation curve
Characterize the attenuation curve, α(λ), of the upper wavelength region as a linear equation
in wavelength, λ :
α(λ) = A + B (λ in µm) (3)
u u
where
α(λ) is the attenuation curve;
A and B are median attenuation values (dB).
u u
a) Using the bend-reference technique
Set B to 0 and A to the median attenuation values in the upper wavelength region. Then define
u u
a function, a(λ), to represent the difference between the attenuation and the line fit to the upper
wavelength region:
a(λ) = α(λ) – A – B λ   (λ in µm) (4)
u u
where
a(λ) is the function representing the difference between attenuation and line fit (dB);
A and B are as defined for Equation (3).
u u
b) Using the multimode-reference technique
Fit the attenuation values using a special technique to avoid the effects of positive humps:
a) Find A and B by simplex regression so that the sum of the absolute values of error is
u u
minimum, and such that all errors are non-negative.
b) Determine
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