IEC 60793-1-48:2017

IEC 60793-1-48 ®
Edition 3.0 2017-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical fibres –
Part 1-48: Measurement methods and test procedures – Polarization mode
dispersion
Fibres optiques –
Partie 1-48: Méthodes de mesure et procédures d’essai – Dispersion de mode de
polarisation
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IEC 60793-1-48 ®
Edition 3.0 2017-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical fibres –
Part 1-48: Measurement methods and test procedures – Polarization mode

dispersion
Fibres optiques –
Partie 1-48: Méthodes de mesure et procédures d’essai – Dispersion de mode

de polarisation
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.10 ISBN 978-2-8322-4686-3

– 2 – IEC 60793-1-48:2017 © IEC 2017

CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions, symbols and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Symbols and abbreviated terms . 8
4 General . 10
4.1 Methods for measuring PMD . 10
4.2 Reference test method . 13
4.3 Applicability . 13
5 Apparatus . 14
5.1 General . 14
5.2 Light source and polarizers . 14
5.3 Input optics . 14
5.4 Input positioner . 14
5.5 Cladding mode stripper . 14
5.6 High-order mode filter . 15
5.7 Output positioner . 15
5.8 Output optics . 15
5.9 Detector . 15
5.10 Computer . 15
6 Sampling and specimens . 15
6.1 General . 15
6.2 Specimen length . 16
6.3 Deployment. 16
6.3.1 General . 16
6.3.2 Uncabled fibre . 16
6.3.3 Optical fibre cable . 16
7 Procedure . 17
8 Calculation or interpretation of results . 17
9 Documentation . 17
9.1 Information required for each measurement . 17
9.2 Information to be available . 17
10 Specification information . 18
Annex A (normative) Requirements specific to method A (FA) – Fixed analyser
measurement method . 19
A.1 Apparatus . 19
A.1.1 Block diagrams . 19
A.1.2 Light source . 19
A.1.3 Analyser . 20
A.2 Procedure . 20
A.2.1 Wavelength range and increment . 20
A.2.2 Complete the scans . 20

A.3 Calculations . 23
A.3.1 Approaches of calculating PMD . 23
A.3.2 Extrema counting . 23
A.3.3 Fourier transform . 23
A.3.4 Cosine Fourier analysis . 25
Annex B (normative) Requirements specific to method B (SPE) – Stokes parameter
evaluation method . 30
B.1 Apparatus . 30
B.1.1 Block diagram . 30
B.1.2 Light source . 30
B.1.3 Polarimeter . 31
B.2 Procedure . 31
B.3 Calculations . 32
B.3.1 Principle . 32
B.3.2 Jones matrix eigenanalysis (JME) . 33
B.3.3 Poincaré sphere analysis (PSA) . 34
B.3.4 State of polarization (SOP) . 35
Annex C (normative) Requirements specific to method C (INTY) – Interferometry
method . 36
C.1 Apparatus . 36
C.1.1 Block diagram . 36
C.1.2 Light source . 39
C.1.3 Beam splitter . 39
C.1.4 Analyser . 39
C.1.5 Interferometer . 39
C.1.6 Polarization scrambler . 39
C.1.7 Polarization beam splitter . 40
C.2 Procedure . 40
C.2.1 Calibration . 40
C.2.2 Routine operation . 40
C.3 Calculations . 44
C.3.1 General . 44
C.3.2 TINTY calculations . 44
C.3.3 GINTY calculations . 45
Annex D (informative) Determination of RMS width from a fringe envelope . 47
D.1 Overview. 47
D.2 RMS calculation for TINTY . 47
D.3 RMS calculation for GINTY . 49
Bibliography . 51

Figure A.1 – Block diagrams for method A . 19
Figure A.2 – Typical results from method A . 22
Figure A.3 – PMD by Fourier analysis . 25
Figure A.4 – Cross-correlation and autocorrelation functions . 29
Figure B.1 – Block diagram for method B . 30
Figure B.2 – Typical random mode coupling results from method B. 32
Figure B.3 – Typical histogram of DGD values. . 33
Figure C.1 – Schematic diagram for method C (generic implementation) . 36

– 4 – IEC 60793-1-48:2017 © IEC 2017
Figure C.2 – Other schematic diagrams for method C . 38
Figure C.3 – Fringe envelopes for negligible and random polarization mode coupling of
TINTY-based measurement system . 41
Figure C.4 – Fringe envelopes for mixed, negligible and random polarization mode
coupling of GINTY-based measurement system . 43
Figure D.1 – Parameters for interferogram analysis . 47

Table A.1 – Cosine transform calculations . 28

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL FIBRES –
Part 1-48: Measurement methods and test procedures –
Polarization mode dispersion
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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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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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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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 60793-1-48 has been prepared by subcommittee 86A: Fibres and
cables, of IEC technical committee 86: Fibre optics.
This third edition cancels and replaces the second edition published in 2007. It constitutes a
technical revision. This edition includes the following significant technical change with respect
to the previous edition:
a) removal of the SOP approach.

– 6 – IEC 60793-1-48:2017 © IEC 2017
The text of this standard is based on the following documents:
CDV Report on voting
86A/1678/CDV 86A/1766/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.
This International Standard is to be read in conjunction with IEC 60793-1-1:2008. 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 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.
INTRODUCTION
Polarization mode dispersion (PMD) causes an optical pulse to spread in the time domain.
This dispersion could impair the performance of a telecommunications system. The effect can
be related to differential phase and group velocities and corresponding arrival times δτ of
different polarization components of the signal. For a sufficiently narrow band source, the
effect can be related to a differential group delay (DGD), ∆τ, between pairs of orthogonally
polarized principal states of polarization (PSP) at a given wavelength. For broadband
transmission, the delays bifurcate and result in an output pulse that is spread out in the time
domain. In this case, the spreading can be related to the average of DGD values.
In long fibre spans, DGD is random in both time and wavelength since it depends on the
details of the birefringence along the entire fibre length. It is also sensitive to time-dependent
temperature and mechanical perturbations on the fibre. For this reason, a useful way to
characterize PMD in long fibres is in terms of an average DGD value over an appropriately
large optical frequency range, either RMS <∆τ>, the rms DGD over this frequency range, or
MEAN <∆τ>, the (linear) mean of the DGD over this same frequency range. In principle, the
average DGD value (RMS <∆τ> or MEAN <∆τ>) does not undergo large changes for a given
fibre from day to day or from source to source, unlike the parameters δτ or ∆τ. In addition, the
average DGD value is a useful predictor of lightwave system performance.
The term "PMD" is used both in the general sense of two polarization modes having different
group velocities, and in the specific sense of the average DGD value (RMS <∆τ> or MEAN
<∆τ>). Although the DGD ∆τ or pulse broadening ∆δ is preferably averaged over frequency,
for certain situations it may be averaged over time, or temperature.
The coupling length l is the length of fibre or cable at which appreciable coupling between
c
the two polarization states begins to occur. If the fibre length L satisfies the condition L << l ,
c
mode coupling is negligible, and <∆τ> scales with fibre length. The corresponding PMD
coefficient is
short-length PMD coefficient = <∆τ>/L.
Fibres in practical systems nearly always have fibre lengths much greater than the coupling
length and random mode coupling. When mode coupling is random, <∆τ> scales with the
square root of fibre length, and
long-length PMD coefficient = <∆τ>/ L .

– 8 – IEC 60793-1-48:2017 © IEC 2017
OPTICAL FIBRES –
Part 1-48: Measurement methods and test procedures –
Polarization mode dispersion
1 Scope
This part of IEC 60793 applies to three methods of measuring polarization mode dispersion
(PMD), which are described in Clause 4. It establishes uniform requirements for measuring
the PMD 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 60793-1-1, Optical fibres - Part 1-1: Measurement methods and test procedures - General
and guidance
IEC TR 61292-5, Optical amplifiers - Part 5: Polarization mode dispersion parameter -
General information
ITU-T Recommendation G.650.2, Definitions and test methods for statistical and non-linear
related attributes of single-mode fibre and cable
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ITU-T Recommendation
G.650.2 apply.
NOTE Further explanation of their use in this document is provided in IEC TR 61282-9.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.2 Symbols and abbreviated terms

Arg Argument function
ASE Amplified spontaneous emission
BBS Broadband source
CFT Cosine Fourier transform
c/c Velocity of light in vacuum/in free space
DGD Differential group delay
DGD Maximum DGD value
max
DOP Degree of polarization
E Number of extrema in R(λ) (method A)
EC Extrema counting
f(∆τ) Maxwell probability distribution
FA Fixed analyser (method A)
FCFT Fast cosine Fourier transform
FT Fourier transform
FUT Fibre under test
GINTY General analysis for method C
INTY Interferometry method (method C)
I/O Input/output
JME Jones matrix eigenanalysis (method B)
k Mode coupling factor
l Coupling length
c
L Length of fibre/fibre cable test sample
LED Light emitting diode
N Total number of measurements/population of mode-coupled
fibres/wavelength intervals
P (λ) Optical power recorded with analyser in place (method A)
A
P Probability of exceeding DGD
F max
o
P (λ) Optical power recorded with analyser rotated 90 (method A)
B
P (λ) Optical power recorded with analyser removed (method A)
TOT
P (τ)/P (τ) Received power in the two orthogonal SOP axes corresponding to the
x x
fringes in method C
PBS Polarization beam splitter
PDL Polarization dependent loss
PDV Polarization dispersion vector
PMD Polarization mode dispersion
PMD Link design PMD value
Q
PSA Poincaré sphere analysis (method B)
PSP Principle states of polarization
R(λ) Output ratio from PMD measurement system (method A)
RBW Resolution bandwidth
RTM Reference test method
s Normalized output Stokes vectors
SOP State of polarization
SPE Stokes parameter evaluation (method B)
T Jones matrix
-1
T Inverse of the Jones matrix
Optical source coherence time (method C)
t
c
TINTY Traditional analysis for method C
α Single parameter which specifies a Maxwell distribution
Χ Chi-squared variable
– 10 – IEC 60793-1-48:2017 © IEC 2017
ˆ
ˆ
ˆ ˆ
h v
∆ /∆ /∆ q /∆ c Finite differences computed from the Stokes vectors
δλ Wavelength step size
∆λ Optical source spectral width (full width half maximum (FWHM) unless
otherwise noted)
δν Optical frequency step size
∆θ Rotation angle on Poincaré sphere
δτ Arrival time of different polarization components of a signal or pulse
broadening
∆δτ Maximum δτ value that can be measured
max
∆δτ Minimum δτ value that can be measured
min
∆τ DGD value
∆τ Maximum DGD
max
<∆τ> Average DGD over wavelength scan range or PMD value
< ∆τ > r.m.s. DGD over wavelength scan range or PMD value (method C)
<∆τ> Maximum PMD specification that each fibre shall meet in a population of
mode-coupled fibres
<∆τ> Average DGD over time
t
<∆τ> Average DGD over temperature
T
<∆τ> Average DGD over wavelength
λ
∆ω Angular frequency variation in method B
λ Test wavelength used to measure PMD
λ Central wavelength of the light source
λ /λ maximum or
1 2 First/last wavelength in set of test wavelengths (or position of first/last
minimum in R(λ) in method A)
ν Optical light frequency
–1
ρ /ρ Complex eigenvalues of T(ω + ∆ω)T (ω)
1 2
σ One-standard-deviation uncertainty
σ
r.m.s. width of the squared envelope of the autocorrelation interferogram
(method C, GINTY)
σ r.m.s. width of the autocorrelation envelope (method C)
A
σ Second moment of FT data (method A)
R
r.m.s. width of the squared envelope of the cross-correlation interferogram
σ
x
(method C, GINTY)
σ r.m.s. width of the cross-correlation envelope (method C, TINTY)
ε
ω Angular optical frequency
Ω PDV
4 General
4.1 Methods for measuring PMD
Three methods are described for measuring PMD (see Annexes A, B and C for more details).
The methods are listed below in the order of their introduction. For some methods, multiple
approaches of analysing the measured results are also provided.

a) Method A
1) Fixed analyser (FA)
2) Extrema counting (EC)
3) Fourier transform (FT)
4) Cosine Fourier transform (CFT)
b) Method B
1) Stokes parameter evaluation (SPE)
2) Jones matrix eigenanalysis (JME)
3) Poincaré sphere analysis (PSA)
c) Method C
1) Interferometry (INTY)
2) Traditional analysis (TINTY)
3) General analysis (GINTY)
The PMD value is defined in terms of the differential group delay (DGD), ∆τ, which usually
varies randomly with wavelength, and is reported as one or another statistical metric.
Equation (1) is a linear average value of the DGD values and is used for the specification of
optical fibre cable. Equation (2) is the root mean square value which is reported by some
methods. Equation (3) can be used to convert one value to the other if the DGDs are assumed
to follow a Maxwell random distribution.
PMD = ∆τ (1)
AVG
PMD = ∆τ (2)
RMS
1/ 2
 8 
∆τ = ∆τ (3)
 
3π
 
Equation (3) applies only when the distribution of DGDs is Maxwellian, for instance when the
fibre is randomly mode coupled. The generalized use of Equation (3) can be verified by
statistical analysis. A Maxwell distribution may not be the case if there are point sources of
elevated birefringence (relative to the rest of the fibre), such as a tight bend, or other
phenomena that reduce the mode coupling, such as a continual reduced bend radius with
fibre in tension. In these cases, the distribution of the DGDs will begin to resemble the square
root of a non-central Chi-square distribution with three degrees of freedom. For these cases,
the PMD value will generally be larger relative to the PMD that is indicated in
RMS MEAN
Equation (3). Time domain methods such as method C and method A, cosine Fourier
transform, which are based on PMD , can use Equation (3) to convert to PMD . If
RMS MEAN
mode coupling is reduced, the resulting reported PMD value from these methods can exceed
those that can be reported by the frequency domain measurements that report PMD ,
MEAN
such as method B.
The PMD coefficient is the PMD value normalized to the fibre length. For normal transmission
fibre, for which random mode coupling occurs and for which the DGDs are distributed as
Maxwell random variables, the PMD value is divided by the square root of the length, and the
PMD coefficient is reported in units of ps km . For some fibres with negligible mode
coupling, such as polarization maintaining fibre, the PMD value is divided by the length, and
the PMD coefficient is reported in units of ps/km.

– 12 – IEC 60793-1-48:2017 © IEC 2017
All methods are suitable for laboratory measurements of factory lengths of optical fibre and
optical fibre cable. For all methods, changes in the deployment of the specimen can alter the
results. For installed lengths of optical fibre cable that may be moving or vibrating, either
method C or method B is appropriate (in an implementation capable of millisecond
measurement time scales).
All methods require light sources that are controlled at one or more states of polarization
(SOPs). All methods require injecting light across a broad spectral region (i.e. 50 nm to
200 nm wide) to obtain a PMD value that is characteristic of the region (i.e. 1 300 nm or
1 550 nm). The methods differ in:
a) the wavelength characteristics of the source;
b) the physical characteristics that are actually measured;
c) the analysis methods.
Method A measures PMD by measuring a response to a change of narrowband light across a
wavelength range. At the source, the light is linearly polarized at one or more SOPs. For each
SOP, the change in output power that is filtered through a fixed polarization analyser, relative
to the power detected without the analyser, is measured as a function of wavelength. The
resulting measured function can be analysed in one of three ways:
a) by counting the number of peaks and valleys (EC) of the curve and application of a
formula that has been shown [1] to agree with the average of DGD values, when the
DGDs are distributed as Maxwellian. This analysis is considered as a frequency domain
approach;
b) by taking the FT of the measured function. This FT is equivalent to the pulse spreading
obtained by the broadband transmission of method C. Appropriate characterisation of the
width of the FT function agrees with the average of DGD values, when the DGDs are
distributed as Maxwellian;
c) by taking the CFT of the difference of the normalized spectra from two orthogonal
analyser settings and calculating the r.m.s. of the squared envelope. The PMDRMS value
is reported. This is equivalent to simulating the fringe pattern of the cross-correlation
function that would result from interferometric measurements.
Method B measures ∆τ(ω) by measuring a response to a change of narrowband light across a
wavelength range. At the source, the light is linearly polarized at one or more SOPs. The
Stokes vector of the output light is measured for each wavelength. The change of these
Stokes vectors with angular optical frequency, ω, and with the (optional) change in input SOP
yields the DGD as a function of wavelength through relationships that are based on the
following definitions:
ds(ω)
= Ω (ω)× s(ω) (4)
dω
∆τ (ω) = Ω(ω) (5)
where
s is the normalized output Stokes vector;
Ω is the polarization dispersion vector (PDV) in the direction of the PSPs;
∆τ is the DGD.
For both the JME and PSA analysis approaches, three linear SOPs at nominally 0 °, 45 °, and
90 ° (orthogonal on the Poincaré sphere) shall be launched for each wavelength.
___________
Numbers in square brackets refer to the Bibliography

The JME approach is completed by transforming the output Stokes vectors to Jones
matrices [2], appropriately combining the matrices at adjacent wavelengths and by a
calculation using the eigenvalues of the result to obtain the DGD, by application of an
argument formula, at the base frequency.
The PSA approach is completed by doing matrix algebra on the normalized output Stokes
vectors to infer the rotation of the output Stokes vector on the Poincaré sphere at two
adjacent wavelengths, using the application of an arcsine formula to obtain the DGD. The
JME and PSA approaches are mathematically equivalent for common assumptions (see
IEC TR 61282-9).
Method C is based on a broadband light source that is linearly polarized. The cross-
correlation of the emerging electromagnetic field is determined by the interference pattern of
output light, i.e. the interferogram. The determination of the PMD delay for the wavelength
range associated with the source spectrum is based on the envelope of the fringe pattern of
the interferogram. Two analyses are available to obtain the PMD delay (see IEC TR 61282-9),
both of which measure the PMD value:
RMS
a) TINTY uses a set of specific operating conditions for its successful applications and a
basic setup;
b) GINTY uses no limiting operating conditions but, in addition to the same basic set-up as
TINTY, it uses a modified setup.
The analysis approaches represent an evolution of the understanding of PMD. The GINTY is,
for example, more complete than TINTY. The reproducibility of PMD depends on the PMD
level and the wavelength range of the measurement [3]. Better relative reproducibility is
achieved for broader wavelength ranges and higher PMD values for a given range. For
measurements of higher PMD values, for example 0,5 ps, the differences in the analysis
methods are less important than for the measurements of low PMD values.
Information common to all three methods is contained in Clauses 4 to 10, and requirements
pertaining to each individual method appear in Annexes A, B, and C, respectively.
IEC TR 61282-9 provides the mathematical formulations for all methods.
4.2 Reference test method
Method B is the reference test method (RTM), which shall be the one used to settle disputes.
4.3 Applicability
PMD in fibre is a statistical parameter. IEC 60794-3 includes a statistical requirement on
PMD, called PMD or link design value that is based on sampled measurements of optical
Q
fibre cable and calculations for concatenated links. The PMD of a cabled fibre can vary from
the PMD of the uncabled fibre due to effects of cable construction and processing. A limit on
the PMD of the uncabled fibre is, however, required to limit the PMD on cabled fibre.
Q Q
Uncabled fibre PMD less than half the cabled fibre PMD limit is generally considered as a
Q Q
conservative rule. Alternative limits may be determined for particular constructions and stable
cable processes.
The fibre or cable deployment should be selected so that externally induced mode coupling is
minimized. Sources of such external mode coupling can be:
a) excessive tension;
b) excessive bending induced from
1) fibre cross-overs on a shipping reel;
2) crimping of fibre within a cable on a spool that is too small;
3) too small a bend radius.
c) excessive twist.
– 14 – IEC 60793-1-48:2017 © IEC 2017
Reproducibility of individual measurements should be evaluated after perturbing the fibre to
allow sampling the full range of mode coupling combinations. This can be done by, for
example, changing the temperature slightly or making small adjustments in the deployment.
Gisin [3] reported a fundamental relative reproducibility limit for measurements and showed
that the relative reproducibility improves as the PMD increases and as the spectral width of
the source increases. When PMD measurements are combined to evaluate the statistical
specification of optical fibre cable (see IEC 60794-3), this variability leads to a possible
overstatement of the link design value.
Guidelines for the calculation of PMD for systems that include other components such as
dispersion compensators or optical amplifiers are given in IEC TR 61282-3. Test methods for
optical amplifiers are given in IEC 61290-11-1 and IEC 61290-11-2, and other design guides
in IEC TR 61292-5. Test methods for testing links, including amplified ones, are given in
IEC 61280-4-4. Test methods for optical components are given in IEC 61300-3-32. General
information about PMD, the mathematical formulation related to the application of the present
methods, and some considerations related to the sampling theory related to the use of
different light sources and detection systems are given in IEC TR 61282-9.
5 Apparatus
5.1 General
The following apparatus is common to all three measurement methods. Annexes A, B, and C
include layout drawings and other equipment requirements for each of the three methods,
respectively.
5.2 Light source and polarizers
See Annexes A, B, and C for detailed options of the spectral characteristics of the 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. IEC TR 61282-9
provides additional guides concerning the source input SOP, degree of polarization (DOP),
use of polarizers and polarization controllers.
5.3 Input optics
An optical lens system or fibre pigtail may be employed to excite the specimen. It is
recommended that the power coupled into the specimen 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 to avoid interference effects. The coupling shall be stable for the duration of the
measurement.
5.4 Input positioner
Provide means of positioning the input end of the specimen to the light source. Examples
include the use of x-y-z micropositioner stages, or mechanical coupling devices such as
connectors, vacuum splices, three-rod splices, etc. The position of the fibre shall remain
stable over the duration of the measurement.
5.5 Cladding mode stripper
Use a device that extracts cladding modes. Under some circumstances the fibre coating will
perform this function.
5.6 High-order mode filter
Use a means to remove high-order propagating modes in the desired wavelength range that is
greater than or equal to the cut-off wavelength (see IEC
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