IEC 60793-1-45:2024

IEC 60793-1-45 ®
Edition 3.0 2024-04
REDLINE VERSION
INTERNATIONAL
STANDARD
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Optical fibres –
Part 1-45: Measurement methods and test procedures – Mode field diameter

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IEC 60793-1-45 ®
Edition 3.0 2024-04
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Optical fibres –
Part 1-45: Measurement methods and test procedures – Mode field diameter
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.10 ISBN 978-2-8322-8812-2

– 2 – IEC 60793-1-45:2024 RLV © IEC 2024
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions and abbreviated terms . 7
3.1 Terms and definitions . 7
3.2 Abbreviated terms . 7
4 General consideration about mode field diameter . 8
5 Reference test method . 8
6 Apparatus . 8
6.1 General . 8
6.2 Light source . 9
6.3 Input optics . 9
6.4 Input positioner . 9
6.5 Cladding mode stripper . 9
6.6 High-order mode filter . 9
6.7 Output positioner . 9
6.8 Output optics . 10
6.9 Detector . 10
6.10 Computer . 10
7 Sampling and samples . 10
7.1 Sample length . 10
7.2 Sample end face . 10
8 Procedure . 10
9 Calculations . 10
9.1 Basic formulae . 10
9.2 Method A – Direct far-field scan . 10
9.3 Method B – Variable aperture in the far field . 11
9.4 Method C – Near-field scan . 12
10 Results . 13
10.1 Information available with each measurement . 13
10.2 Information available upon request . 13
11 Specification information . 13
Annex A (normative) Requirements specific to method A – Mode field diameter by
direct far-field scan . 14
A.1 Apparatus . 14
A.1.1 General . 14
A.1.2 Scanning detector assembly – Signal detection electronics . 14
A.1.3 Computer. 15
A.2 Procedure . 15
A.3 Calculations . 15
A.3.1 Determine folded power curve . 15
A.3.2 Compute the top (T) and bottom (B) integrals of Formula (1) . 15
A.3.3 Complete the calculation . 16
A.4 Sample data . 16
Annex B (normative) Requirements specific to method B – Mode field diameter by
variable aperture in the far field . 17

B.1 Apparatus . 17
B.1.1 General . 17
B.1.2 Output variable aperture assembly . 17
B.1.3 Output optics system . 18
B.1.4 Detector assembly and signal detection electronics . 18
B.2 Procedure . 18
B.3 Calculations . 18
B.3.1 Determine complementary aperture function . 18
B.3.2 Complete the integration . 19
B.3.3 Complete the calculation . 19
B.4 Sample data . 19
Annex C (normative) Requirements specific to method C – Mode field diameter by
near-field scan . 20
C.1 Apparatus . 20
C.1.1 General . 20
C.1.2 Magnifying output optics . 20
C.1.3 Scanning detector . 21
C.1.4 Detection electronics . 21
C.2 Procedure . 21
C.3 Calculations . 21
C.3.1 Calculate the centroid . 21
C.3.2 Fold the intensity profile . 22
C.3.3 Compute the integrals . 22
C.3.4 Complete the calculation . 23
C.4 Sample data . 23
Annex D (normative) Requirements specific to method D – Mode field diameter by
optical time domain reflectometer (OTDR) . 24
D.1 General . 24
D.2 Apparatus . 24
D.2.1 OTDR . 24
D.2.2 Optional auxiliary switches . 24
D.2.3 Optional computer . 25
D.2.4 Test sample . 25
D.2.5 Reference sample . 25
D.3 Procedure – Orientation and notation . 25
D.4 Calculations . 26
D.4.1 Reference fibre mode field diameter . 26
D.4.2 Computation of the sample mode field diameter. 26
D.4.3 Validation . 27
Annex E (informative) Sample data sets and calculated values . 29
E.1 General . 29
E.2 Method A – Mode field diameter by direct far-field scan . 29
E.3 Method B – Mode field diameter by variable aperture in the far field . 30
E.4 Method C – Mode field diameter by near-field scan . 30
Bibliography . 31

Figure 1 – Transform relationships between measurement results . 8
Figure A.1 – Far-field measurement set . 14

– 4 – IEC 60793-1-45:2024 RLV © IEC 2024
Figure B.1 – Variable aperture by far-field measurement set . 17
Figure C.1 – Near-field measurement set-ups . 20
Figure D.1 – Optical switch arrangement . 25
Figure D.2 – View from reference fibre A . 26
Figure D.3 – View from reference fibre B . 26
Figure D.4 – Validation example – Comparison of methods . 27

Table 1 – Abbreviated terms . 7
Table E.1 – Sample data, method A – Mode field diameter by direct far-field scan . 29
Table E.2 – Sample data set, method B – Mode field diameter by variable aperture in
the far field . 30
Table E.3 – Sample data set, method C – Mode field diameter by near-field scan . 30

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-45: Measurement methods and test procedures –
Mode field diameter
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition IEC 60793-1-45:2017. A vertical bar appears in the margin
wherever a change has been made. Additions are in green text, deletions are in
strikethrough red text.
– 6 – IEC 60793-1-45:2024 RLV © IEC 2024
IEC 60793-1-45 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 2017. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Modification of the minimum distance between the fibre end and the detector for the direct
far field scan (Annex A).
b) Generalization of the requirement for the minimum dynamic range for all fibre types
(Annex A).
The text of this International Standard is based on the following documents:
Draft Report on voting
86A/2300/CDV 86A/2366/RVC
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.
A list of all parts in the IEC 60793 series, published under the general title Optical fibres, 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, or
• revised.
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 1-45: Measurement methods and test procedures –
Mode field diameter
1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the mode field diameter
(MFD) of single-mode optical fibre, thereby assisting in the inspection of fibres and cables for
commercial purposes.
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 60793-1-40:2001, Optical fibres – Part 1-40: Attenuation measurement methods and test
procedures – Attenuation
IEC 60793-2, Optical fibres – Part 2: Product specifications – General
3 Terms, definitions and abbreviated terms
3.1 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
3.2 Abbreviated terms
The abbreviated terms are given in Table 1.
Table 1 – Abbreviated terms
Abbreviated term Full term
CCD charge-coupled devices
FWHM full width half maximum
MFD mode field diameter
OTDR optical time domain reflectometer
RTM reference test method
– 8 – IEC 60793-1-45:2024 RLV © IEC 2024
4 General consideration about mode field diameter
The mode field diameter measurement represents a measure of the transverse extent of the
electromagnetic field intensity of the guided mode in a fibre cross section, and it is defined from
the far-field intensity distribution as a ratio of integrals known as the Petermann II definition.
See Formula (1).
The definitions of mode field diameter are strictly related to the measurement configurations.
The mathematical equivalence of these definitions results from transform relationships between
measurement results obtained by different implementations summarized in Figure 1.

Figure 1 – Transform relationships between measurement results
Four methods are described for measuring mode field diameter:
• method A: direct far-field scan;
• method B: variable aperture in the far field;
• method C: near-field scan;
• method D: bi-directional backscatter using an optical time domain reflectometer (OTDR).
All four methods apply to all categories of type B single-mode fibre shown in IEC 60793-2 and
operating near 1 310 nm or 1 550 nm. Method D is not recommended for the measurement of
fibres of unknown type or design.
Information common to all four methods is contained in Clause 1 to Clause 11, and information
pertaining to each individual method appears in Annex A, Annex B, Annex C, and Annex D
respectively.
5 Reference test method
Method A, direct far-field scan, is the reference test method (RTM), which shall be the one used
to settle disputes.
6 Apparatus
6.1 General
The following apparatus is common to all measurement methods. Annex A, Annex B, Annex C,
and Annex D include layout drawings and other equipment requirements for each of the four
methods, respectively.
6.2 Light source
For method A, method B and method C, use a suitable coherent or non-coherent light source,
such as a semiconductor laser or a sufficiently powerful filtered white light source. The source
shall produce sufficient radiation at the intended wavelength(s) and be stable in intensity over
a time period sufficient to perform the measurement.
A monochromator or interference filter(s) may be used, if required, for wavelength selection.
The detail specification shall specify indicate the wavelength of the source. The full width half
maximum (FWHM) spectral line width of the source shall ≤10 nm, unless otherwise specified.
The source power level shall be chosen so it is not impacting the repeatability of the mode
diameter measurement.
The source power shall be stable for the complete duration of the measurement.
See Annex D for method D.
6.3 Input optics
For method A, method B, and method C, an optical lens system or fibre pigtail may be employed
to excite the specimen sample. It is recommended that the power coupled into the specimen
sample be relatively insensitive to the position of its input end face. This can be accomplished
by using a launch beam that spatially and angularly overfills the input end face.
If using a butt splice, employ index-matching material between the fibre pigtail and the specimen
sample to avoid interference effects. The coupling shall be stable for the duration of the
measurement.
See Annex D for method D.
6.4 Input positioner
Provide means of positioning the input end of the specimen sample to the light source.
Examples include the use of x-y-z micropositioner stages, or mechanical coupling devices such
as connectors, vacuum splices,or three-rod splices. The position of the fibre shall remain stable
over the duration of the measurement.
6.5 Cladding mode stripper
Use a device that extracts cladding modes. Under some circumstances, the fibre coating will
perform this function.
6.6 High-order mode filter
Use a means to remove high-order propagating modes in the wavelength range that is greater
than or equal to the cut-off wavelength of the specimen sample. For example, a one-turn bend
with a radius of 30 mm on the fibre is generally sufficient for most B1.1 to B6 B-652, B-653, B-
654, B-655, B-656 and B-657 fibres. For some B6 B-657 fibres, smaller radius, multiple bends,
or longer specimen sample length can be applied to remove high-order propagating modes.
6.7 Output positioner
Provide a suitable means for aligning the fibre output end face to allow an accurate axial
adjustment of the output end, such that, at the measurement wavelength, the scan pattern is
suitably focused on the plane of the scanning detector. Such coupling may include the use of
lenses or may be a mechanical connector to a detector pigtail.

– 10 – IEC 60793-1-45:2024 RLV © IEC 2024
Provide means such as a side-viewing microscope or camera with a crosshair to locate the fibre
at a fixed distance from the apertures or detectors. It may can be sufficient to provide only
longitudinal adjustment if the fibre is constrained in the lateral plane by a device such as a
vacuum chuck (this depends mainly upon the size of the light detector).
6.8 Output optics
See the appropriate annex: Annex A, Annex B, Annex C or Annex D.
6.9 Detector
See the appropriate annex: Annex A, Annex B, Annex C or Annex D.
6.10 Computer
Use a computer to perform operations such as controlling the apparatus, taking intensity
measurements, and processing the data to obtain the final results. For individual details, see
the appropriate annex: Annex A, Annex B, Annex C or Annex D.
7 Sampling and specimens samples
7.1 Specimen Sample length
For method A, method B and method C, the specimen sample shall be a known length, typically
2 m ± 0,2 m for most B1.1 to B6 B-652, B-653, B-654, B-655, B-656 and B-657 fibres. For some
B6 B-657 fibres, longer specimen sample length can be used to avoid high-order propagating
modes, 22 m for example.
For method D, OTDR, the sample shall be long enough to exceed (or be positioned beyond)
the dead zone of the OTDR, with both ends accessible, as described in the backscatter test
method in IEC 60793-1-40.
7.2 Specimen Sample end face
Prepare a flat end face, orthogonal to the fibre axis, at the input and output ends of each
specimen sample.
8 Procedure
See Annex A, Annex B, Annex C and Annex D for method A, method B, method C, and
method D, respectively.
9 Calculations
9.1 Basic formulae
The basic formulae for calculating mode field diameter are Formula (1) for method A, Formula (2)
for method B and Formula (6) for method C. For additional calculations, see the appropriate
annex: Annex A, Annex B, Annex C or Annex D. Sample data sets for method A, method B and
method C are included in Annex E.
9.2 Method A – Direct far-field scan
The following formula defines the mode field diameter for method A in terms of the
electromagnetic field emitted from the end of the specimen sample.

Calculate the mode field diameter by scanning the far-field data and evaluating the Petermann
II integral, which is defined from the far-field intensity distribution:
1/2
π/2

P (θ )sin(θ )cos(θ )dθ
F
λ 2 ∫

(1)
2W =

π/2
π 3
P (θ )sin (θ )cos(θ )dθ

F
∫
0
where
2W is the mode field diameter in µm;
P (θ) is the far-field intensity distribution;
F
λ is the wavelength of measurement in µm;
θ is the angle in the far-field measurement from the axis of the fibre.
NOTE 1 The integration limits are shown to be from zero to π/2, but it is understood that the integrands approach
zero with increasing argument so that, in practice, the integrals can be truncated.
NOTE 2 P is F (θ) in ITU-T documents.
F
The far-field method for obtaining the mode field diameter of a single-mode fibre is a two-step
procedure. First, measure the far-field radiation pattern of the fibre. Second, use a mathematical
procedure based on the Petermann II far-field definition to calculate the mode field from far-
field data, as described in Formula (1).
Annex E provides sample data and calculated 2W values for verifying the numerical evaluation
of the Petermann II Integral. The sample data are in the form of the folded power, P (θ), as a
F
function of the angle, θ.
9.3 Method B – Variable aperture in the far field
Formula (2) defines the mode field diameter for method B in terms of the electromagnetic field

emitted from the end of the specimen sample.
Calculate the mode field diameter, 2W , as follows:
−1/2

∞
λx
 
2aW = x dx (2)
( )

0  
∫
0 2
πD
 

xD+
( )

where
2W is the mode field diameter, in µm;
λ is the wavelength of measurement, in µm;
D is the distance between the aperture and the fibre, in mm;
a(x) is the complementary aperture transmission function, calculated as
Px( )
ax 1−
( ) (3)
P max
( )
=
– 12 – IEC 60793-1-45:2024 RLV © IEC 2024

where
P is the power measured through an aperture of radius, x, or half angle, θ;
(x)
P is the maximum power, assuming an infinite aperture;
(max)
x is the aperture radius, calculated as
x =D tanθ
( ) (4)
Another equivalent expression of Formula (2) is
−1/2
∞
λ 2

2W = a(θ)sin2θθd (5)
∫

π

The variable aperture far-field method for obtaining the mode field diameter of a single-mode
fibre is a two-step procedure. First, measure the two-dimensional far-field pattern as the power
passing through a series of transmitting apertures of various size. Second, use a mathematical
procedure to calculate the mode field diameter from the far-field data.
The mathematical basis for the calculation of mode field diameter is based on the Petermann II
far-field definition from Formula (1). Formula (2) and Formula (5) can be derived from
Formula (1) by integration.
9.4 Method C – Near-field scan
The following formula defines the mode field diameter for method C in terms of the
electromagnetic field emitted from the end of the specimen sample.
Calculate the mode field diameter from the measured near-field intensity distribution, using the
following integral:
1/2
 
 
∞
r f (r)dr
 
∫
(6)
2W = 22 
 
 df r 
∞ ( )
 r dr 
 
∫
   
dr
 
 
where
2W is the mode field diameter, in µm;
r is the radial coordinate, in µm;
f (r) is the near-field intensity distribution.
NOTE The upper integration limits are shown to infinity, but it is understood that since the integrands approach
zero with increasing argument, in practice the integrals can be truncated. A smoothing algorithm can be used for the
calculation of the derivative.
The near-field scan method for obtaining the mode field diameter of a single-mode fibre is a
two-step procedure. First, measure the radial near-field pattern. Second, use a mathematical
procedure to calculate the mode field diameter from the near-field data.

The mathematical basis for the calculation of the mode field diameter is based on the Petermann
II definition from Formula (1). The near field, f(r), and the far field, F(θ), form a Hankel transform
pair. By Hankel transforming and using P = F (θ), it is possible to derive Formula (6) from
F
Formula (1), and vice versa.
10 Results
10.1 Information available with each measurement
Report the following information with each measurement:
• date and title of measurement;
• identification of specimen sample;
• optical source wavelength;
• mode field diameter(s), in micrometres.
10.2 Information available upon request
The following information shall be available upon request:
• measurement method used: method A, method B, method C or method D;
• type of optical source used and its spectral width (FWHM);
• description of equipment;
• description of high-order modes filter;
• details of computation technique;
• date of latest calibration of measurement equipment.
11 Specification information
The detail specification shall specify the following information:
• type of fibre to be measured;
• failure or acceptance criteria;
• information to be reported;
• any deviations to the procedure that apply.

– 14 – IEC 60793-1-45:2024 RLV © IEC 2024
Annex A
(normative)
Requirements specific to method A –
Mode field diameter by direct far-field scan
A.1 Apparatus
A.1.1 General
Annex A describes apparatus in addition to the requirements set down in Clause 6.
Figure A.1 illustrates a typical set-up for measurement by direct far-field scan.

Figure A.1 – Far-field measurement set
A.1.2 Scanning detector assembly – Signal detection electronics
Use a mechanism to scan the far-field intensity distribution. Use a scanning device capable of
0,5° steps or finer to scan the detector. Use a means of aligning the fibre axis with respect to
the rotation plane of the detector, and of aligning the fibre end-face with the centre of rotation
of the scan. A typical system might include a PIN photodiode, operating in a photovoltaic mode,
amplified by a current-input preamplifier, with synchronous detection by a lock-in amplifier. The
detector should be at least 10 mm from the fibre end (to ensure the detector to scan the far
field), and the detector's active area should not subtend an angle too large in the far field. To
ensure this, place the detector at a distance from the fibre end greater than 2wb/λ, where 2w is
the expected mode field diameter of the specimen and b is the diameter of the active area of
the detector.
For very accurate measurements, the minimum dynamic range of the measurement should be
50 dB. This corresponds to a maximum scan half-angle of 20° and 25°, or greater, for category
B1 and B2 fibres, respectively. Reducing the dynamic range (or maximum scan half-angle)
requirements may introduce errors. For example, restricting those values to 30 dB and 12,5°
for category B1 fibres, and to 40 dB and 20° for category B2 fibres, may result in a relative error,
in the determination of the mode field diameter, that is greater than 1 %.
2.w b
To ensure this, place the detector at a distance d from the fibre end with dK×
λ
where
2w is the expected mode field diameter of the sample,
=
b is the diameter of the active area of the detector,
λ is the wavelength,
K is the resolution factor which value is big enough to prevent the degradation of the far field
scan and its impact on the calculation of the mode field diameter.
A value of K, greater than 20, is suitable for most fibre types and guarantees less than 0,1 %
of error in the mode field diameter calculation.
For accurate measurements, the dynamic range of the measurement should be greater than
50 dB. The maximum scan half-angle depends on the fibre type and should be chosen so that
the far field scan is characterized down to 50 dB of the maximum signal.
Reducing the dynamic range (or maximum scan half-angle) requirements can introduce errors.
A.1.3 Computer
A typical system should also include a computer to process the far-field data.
A.2 Procedure
Align the fibre in the system, prepared as described in Clause 6, with its output end aligned on
the detector assembly for maximum power.
Scan the detector in 0,5° steps, equally spaced, and record the detector power.
Calculate a value of the Petermann II integral from the recorded data and use it to compute the
fibre mode field diameter as described in Formula (1), and in Clause A.3.
A.3 Calculations
A.3.1 Determine folded power curve
is
The folded power curve for 0 ≤ θ = θ
max
i
P θ +Pθ
( ) ( )
ii−
(A.1)
P (θ ) =
F i
where
P (θ ) is the folded power curve;
F i
P(θ ) is the measured power as a function of the angular position, θ (radians), indexed by i.
-i i
A.3.2 Compute the top (T) and bottom (B) integrals of Formula (1)
Use an appropriate numerical integration technique to compute the integrals of Formula (1).
The following is an example using the rectangular method. Any other integration method shall
be at least as accurate as this one.
n
TP= θ sinθ cosθ dθ
( ) ( ) ( ) (A.2)
∑
F ii i
– 16 – IEC 60793-1-45:2024 RLV © IEC 2024
n
BP= θ sin θ cosθ dθ
( ) ( ) ( ) (A.3)
∑
F i i i
where
P is the folded power curve;
F
θ is the angular position, indexed by i (radians);
i
dθ = θ – θ .
1 0
A.3.3 Complete the calculation
 
λT2
MFD 2W
  (A.4)
 
πB
 
where
2W is the mode field diameter, in µm;
T is from Formula (A.2);
B is from Formula (A.3).
A.4 Sample data
See Table E.1 for a sample data set as calculated in Clause A.3.

==
Annex B
(normative)
Requirements specific to method B –
Mode field diameter by variable aperture in the far field
B.1 Apparatus
B.1.1 General
Annex B describes apparatus in addition to the requirements in Clause 6.
Figure B.1 illustrates a typical set-up for the measurement by variable aperture in the far field.

Figure B.1 – Variable aperture by far-field measurement set
B.1.2 Output variable aperture assembly
B.1.2.1 Principle
Place a device consisting of round, transmitting apertures of various sizes (such as an aperture
wheel) at a distance of at least 100 W /λ from the specimen sample, and use it to vary the
power detached from the fibre output far field pattern. Typically, the apertures are located
20 mm to 50 mm away from the fibre end.
Use a means of centring the apertures with respect to the pattern to decrease the sensitivity to
fibre end angle. Use a sufficient number and size of apertures such that the measurement
results are not unduly affected by the inclusion of any additional aperture. In addition, take care
to ensure that the largest apertures are of sufficient size to avoid truncation of the collected
pattern.
NOTE 1 Optical alignment is critical.

– 18 – IEC 60793-1-45:2024 RLV © IEC 2024
NOTE 2 The number and size of the apertures are critical to the accuracy of this method. The optimum can vary
depending on the design of the fibres being tested. Verification of a particular selection can be completed by
comparison with method A, direct far-field.
B.1.2.2 Equipment requirements for category B1 and B6 B-652, B-654 and B-657
fibre
The accuracy of the mode field diameter measurement given by this procedure depends on the
maximum numerical aperture of the measurement set. For category B1 and B6 B-652, B-654,
and B-657 fibre, the error is typically 1 % or less for a measurement set with a maximum
numerical aperture of 0,25. If less error is desired, or if the specimen sample has a mode field
diameter less than 8,2 µm, use either of two approaches:
a) use a measurement system with a maximum numerical aperture of 0,35 or greater; or
b) determine a mapping function that relates the measurement of category B1 and B6 B-652,
B-654, and B-657 fibre on limited aperture measurement set to that of a set with 0,35 or
greater numerical aperture.
B.1.2.3 Equipment requirements for category B2, B4, and B5 B-653, B-655, and B-
656 fibres
The maximum numerical aperture of the measurement set shall be ≥0,40 for fibres with mode
field diameters ≥6 µm.
B.1.3 Output optics system
Use an optical system, such as a pair of lenses, mirrors, or other suitable arrangement, to
collect all the light transmitted through the aperture, and to couple it to the detector.
B.1.4 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 can
include a germanium or InGaAs photodiode, operating in the photovoltaic mode, and a current-
sensitive preamplifier, with synchronous detection by a lock-in amplifier. Generally, a computer
is required to analyze the data.
B.2 Procedure
a) Place the specimen sample, prepared as described in Clause 6, in the input and output
alignment devices, and adjust it for the correct distance to the aperture assembly.
a) Set the aperture assembly to a small aperture and adjust the far field to an aperture lateral
alignment for maximum detected power.
b) Measure the detected power for each of the apertures.
c) Calculate the mode field diameter per Formula (2) and Clause B.3.
d) Repeat steps b), c) and d) for each specified measurement wavelength.
B.3 Calculations
B.3.1 Determine complementary aperture function
Determine the complementary aperture function for each aperture, from 1 to n:
P θ
( )
i
a (θ ) 1−
(B.1)
i
P θ
( )
n
where
a(θ ) is the complementary function for ea
...