SIST EN 60793-1-41:2004
(Main)Optical fibres -- Part 1-41: Measurement methods and test procedures - Bandwidth
Optical fibres -- Part 1-41: Measurement methods and test procedures - Bandwidth
Describes two methods for determining and measuring the modal bandwidth of multi-mode optical fibres (see EN 60793-2-10, EN 60793-2-30 and EN 60793-2-40).The baseband frequency response is directly measured in the frequency domain by determining the fibre response to a sinusoidally modulated light source, it can also be measured by observing the broadening of a narrow pulse of light. Method A - Optical time domain measurement method (pulse distortion). Method 2 - Frequency domain measurement method. Each method can be performed using one of two launches: an overfilled launch (OFL) condition or a restricted mode launch (RML) condition.
Lichtwellenleiter -- Teil 1-41: Messmethoden und Prüfverfahren - Bandbreite
Fibres optiques -- Partie 1-41: Méthodes de mesure et procédures d'essai - Largeur de bande
Deux méthodes pour mesurer la largeur de bande sont présentées: la réponse impulsionnelle, et la réponse fréquentielle. Les deux méthodes s'appliquent à la mesure de la largeur de bande des fibres multimodales de catégorie A1. L'application aux autres catégories de fibres multimodales de classe A est à l'étude. Aucune des méthodes ne s'applique à la mesure de la larguer de bande sur fibres unimodales de type B.
Optična vlakna – 1-41. del: Metode merjenja in preskusni postopki Pasovna širina (IEC 60793-1-41:2003)*
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SIST EN 60793-1-41:2004SLOVENSKIseptember 2004
STANDARDOptična vlakna – 1-41. del: Metode merjenja in preskusni postopki Pasovna širina (IEC 60793-1-41:2003)*Optical fibres - Part 1-41: Measurement methods and test procedures - Bandwidth (IEC 60793-1-41:2003)©
Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljenoReferenčna številkaSIST EN 60793-1-41:2004(en)ICS33.180.10
EUROPEAN STANDARD
EN 60793-1-41 NORME EUROPÉENNE EUROPÄISCHE NORM
December 2003 CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2003 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60793-1-41:2003 E
ICS 33.180.10 Supersedes EN 60793-1-41:2002
English version
Optical fibres Part 1-41: Measurement methods and test procedures –
Bandwidth (IEC 60793-1-41:2003)
Fibres optiques Partie 1-41: Méthodes de mesure
et procédures d'essai –
Largeur de bande (CEI 60793-1-41:2003)
Lichtwellenleiter Teil 1-41: Messmethoden
und Prüfverfahren –
Bandbreite (IEC 60793-1-41:2003)
This European Standard was approved by CENELEC on 2003-11-01. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Lithuania, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United Kingdom.
EN 60793-1-41:2003 - 2 - Foreword The text of document 86A/841/FDIS, future edition 2 of IEC 60793-1-41, prepared by SC 86A, Fibres and cables, of IEC TC 86, Fibre optics, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 60793-1-41 on 2003-11-01. This European Standard supersedes EN 60793-1-41:2002. It updates and completes the 2002 edition in particular by the restricted mode launch intended for the laser launch transmission system. This standard is to be read in conjunction with EN 60793-1-1:2003. The following dates were fixed: – latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement
(dop) 2004-08-01 – latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow) 2006-11-01
Annexes designated "normative" are part of the body of the standard.
Annexes designated "informative" are given for information only.
In this standard, annexes A, B, C and ZA are normative and annexes D and E are informative. Annex ZA has been added by CENELEC. EN 60793-1-4X consists of the following parts, under the general title: Optical fibres: - Part 1-40: Measurement methods and test procedures – Attenuation - Part 1-41: Measurement methods and test procedures – Bandwidth - Part 1-42: Measurement methods and test procedures – Chromatic dispersion - Part 1-43: Measurement methods and test procedures – Numerical aperture - Part 1-44: Measurement methods and test procedures – Cut-off wavelength - Part 1-45: Measurement methods and test procedures – Mode field diameter - Part 1-46: Measurement methods and test procedures – Monitoring of changes in optical transmittance - Part 1-47: Measurement methods and test procedures – Macrobending loss - Part 1-48: Measurement methods and test procedures – Polarization mode dispersion - Part 1-49: Measurement methods and test procedures – Differential mode delay __________ Endorsement notice The text of the International Standard IEC 60793-1-41:2003 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following note has to be added for the standard indicated:
IEC 60793-1-40 NOTE Harmonized as EN 60793-1-40:2003 (modified). __________
- 3 - EN 60793-1-41:2003
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications This European Standard incorporates by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies (including amendments). NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies. Publication Year Title EN/HD Year IEC 60793-1-20 2001 Optical fibres Part 1-20: Measurement methods and test procedures - Fibre geometry
EN 60793-1-20 2002 IEC 60793-1-42 2001 Part 1-42: Measurement methods and test procedures - Chromatic dispersion
EN 60793-1-42 2002 IEC 60793-1-43 2001 Part 1-43: Measurement methods and test procedures - Numerical aperture
EN 60793-1-43 2002 IEC 60793-2-10 2002 Part 2-10: Product specifications - Sectional specification for category A1 multimode fibres
EN 60793-2-10 2002 IEC 60793-2-30 2002 Part 2-30: Product specifications - Sectional specification for category A3 multimode fibres
EN 60793-2-30 2002 IEC 60793-2-40 2002 Part 2-40: Product specifications - Sectional specification for category A4 multimode fibres
EN 60793-2-40 2002
NORMEINTERNATIONALECEIIECINTERNATIONALSTANDARD60793-1-41Deuxième éditionSecond edition2003-04Fibres optiques –Partie 1-41:Méthodes de mesure et procédures d'essai –Largeur de bandeOptical fibres –Part 1-41:Measurement methods and test procedures –BandwidthPour prix, voir catalogue en vigueurFor price, see current catalogue IEC 2003
Droits de reproduction réservés
Copyright - all rights reservedAucune partie de cette publication ne peut être reproduite niutilisée sous quelque forme que ce soit et par aucun procédé,électronique ou mécanique, y compris la photocopie et lesmicrofilms, sans l'accord écrit de l'éditeur.No part of this publication may be reproduced or utilized in anyform or by any means, electronic or mechanical, includingphotocopying and microfilm, without permission in writing fromthe publisher.International Electrotechnical Commission,
3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, SwitzerlandTelephone: +41 22 919 02 11
Telefax: +41 22 919 03 00
E-mail: inmail@iec.ch
Web: www.iec.chCODE PRIXPRICE CODEUCommission Electrotechnique InternationaleInternational Electrotechnical Commission
60793-1-41 IEC:2003– 3 –CONTENTSFOREWORD.51Scope.92Normative references.93Definition.114Apparatus.114.1Radiation source.114.2Launch system.134.3Detection system.154.4Recording system.174.5Computational equipment.174.6Overall system performance.175Sampling and specimens.175.1Test sample.175.2Reference sample.195.3End preparation.195.4Test sample packaging.195.5Test sample positioning.196Procedure.196.1Method A – Pulse distortion method (optical time domain) input pulsemeasurement.196.2Method B – Frequency domain measurement method.217Calculations or interpretation of results.237.1–3 dB frequency, f3 dB.237.2Calculations for optional reporting methods.238Length normalization.239Results.239.1Information to be provided with each measurement.239.2Informations available upon request.2510Specification information.25Annex A (normative)
Intramodal dispersion factor and the normalized intermodaldispersion limit.27Annex B (normative)
Fibre transfer function, H(f).35Annex C (normative)
Calculations for other reporting methods.37Annex D (informative)
Comparison between this test method and ITU requirements.39Annex E (informative)
Mode scrambler requirements for overfilled launching conditionsto multimode fibres.41Bibliography.53
60793-1-41 IEC:2003– 5 –INTERNATIONAL ELECTROTECHNICAL COMMISSION____________OPTICAL FIBRES –Part 1-41: Measurement methods and test procedures –BandwidthFOREWORD1)The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprisingall national electrotechnical committees (IEC National Committees). The object of the IEC is to promoteinternational co-operation on all questions concerning standardization in the electrical and electronic fields. Tothis end and in addition to other activities, the IEC publishes International Standards. Their preparation isentrusted to technical committees; any IEC National Committee interested in the subject dealt with mayparticipate in this preparatory work. International, governmental and non-governmental organizations liaisingwith the IEC also participate in this preparation. The IEC collaborates closely with the International Organizationfor Standardization (ISO) in accordance with conditions determined by agreement between the twoorganizations.2)The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, aninternational consensus of opinion on the relevant subjects since each technical committee has representationfrom all interested National Committees.3)The documents produced have the form of recommendations for international use and are published in the formof standards, technical specifications, technical reports or guides and they are accepted by the NationalCommittees in that sense.4)
In order to promote international unification, IEC National Committees undertake to apply IEC InternationalStandards transparently to the maximum extent possible in their national and regional standards. Anydivergence between the IEC Standard and the corresponding national or regional standard shall be clearlyindicated in the latter.5)
The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for anyequipment declared to be in conformity with one of its standards.6)
Attention is drawn to the possibility that some of the elements of this International Standard may be the subjectof patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.International Standard IEC 60793-1-41 has been prepared by subcommittee 86A: Fibres andcables, of IEC technical committee 86: Fibre optics.This second edition cancels and replaces the first edition published in 2001. This editionconstitutes a technical revision.This new edition updates and completes the earlier edition in particular by the restricted modelaunch intended for the laser launch transmission system.The text of this standard is based on the following documents:FDISReport on voting86A/841/FDIS86A/853/RVDFull information on the voting for the approval of this standard can be found in the report onvoting indicated in the above table.This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.This standard is to be read in conjunction with IEC 60793-1-1.
60793-1-41 IEC:2003– 7 –IEC 60793-1-4X consists of the following parts, under the general title Optical fibres:Part 1-40:Measurement methods and test procedures – AttenuationPart 1-41:Measurement methods and test procedures – BandwidthPart 1-42:Measurement methods and test procedures – Chromatic dispersionPart 1-43:Measurement methods and test procedures – Numerical aperturePart 1-44:Measurement methods and test procedures – Cut-off wavelengthPart 1-45:Measurement methods and test procedures – Mode field diameterPart 1-46:Measurement methods and test procedures – Monitoring ofchanges in optical transmittancePart 1-47:Measurement methods and test procedures – Macrobending lossPart 1-48:Measurement methods and test procedures – Polarisation mode dispersion 1)Part 1-49:Measurement methods and test procedures – Differential mode delay 2)The committee has decided that the contents of this publication will remain unchanged until2007. At this date, the publication will be•reconfirmed;•withdrawn;•replaced by a revised edition, or•amended.___________1)
To be published.2)
To be published.
60793-1-41 IEC:2003– 9 –OPTICAL FIBRES –Part 1-41: Measurement methods and test procedures –Bandwidth1 ScopeThis part of IEC 60793 describes two methods for determining and measuring the modalbandwidth of multi-mode optical fibres (see IEC 60793-2-10, IEC 60793-2-30 andIEC 60793-2-40). The baseband frequency response is directly measured in the frequencydomain by determining the fibre response to a sinusoidally modulated light source. Thebaseband response can also be measured by observing the broadening of a narrow pulse oflight. The two methods are the following:Method A – Optical time domain measurement method (pulse distortion)Method B – Frequency domain measurement methodEach method can be performed using one of two launches: an overfilled launch (OFL) conditionor a restricted mode launch (RML) condition.NOTE
These test methods are commonly used in production and research facilities and are not easilyaccomplished in the field.2 Normative referencesThe following referenced documents are indispensable for the application of this document. Fordated references, only the edition cited applies. For undated references, the latest edition ofthe referenced document (including any amendments) applies.IEC 60793-1-20:2001, Optical Fibres – Part 1-20: Measurement methods and test procedures –Fibre geometryIEC 60793-1-42:2001, Optical fibres – Part 1-42: Measurement methods and test procedures –Chromatic dispersionIEC 60793-1-43:2001, Optical fibres – Part 1-43: Measurement methods and test procedures –Numerical apertureIEC 60793-2-10:2001, Optical fibres – Part 2-10: Product specifications – Sectionalspecification for category A1 multimode fibresIEC 60793-2–30:2002, Optical fibres – Part 2-30: Product specifications – Sectionalspecification for category A3 multimode fibresIEC 60793-2-40:2002, Optical fibres – Part 2-40: Product specifications – Sectionalspecification for category A4 multimode fibres
60793-1-41 IEC:2003– 11 –3 DefinitionsFor the purposes of this part of IEC 60793, the following definition applies.bandwidth (–3 dB)the value numerically equal to the lowest modulation frequency at which the magnitude of thebaseband transfer function of an optical fibre decreases to a specified fraction, generally to onehalf, of the zero frequency value.NOTE
Various methods of reporting the results are described in the annexes, but the results shall be expressed interms of the –3 dB (optical power) frequency unless otherwise specified by the detail specification.4 Apparatus4.1 Radiation source4.1.1 Optical time domain measurement method (pulse distortion measurement)(Method A)Use a radiation source such as an injection laser diode that produces short duration, narrowspectral width pulses for the purposes of the measurement. The pulse distortion measurementmethod requires the capability to switch the energy of the light sources electrically, optically ormechanically.4.1.2 Frequency domain measurement method (Method B)Use a radiation source such as a continuous wave (CW) injection laser diode for the purposesof the measurement. The frequency domain measurement method requires the capability tomodulate the energy of the light sources electrically, optically or mechanically.4.1.3 For both methodsa) Use a radiation source with a center wavelength that is known and within ±10 nm of thenominal specified wavelength. For injection laser diodes, laser emission coupled into thefibre must exceed spontaneous emission by a minimum of 15 dB (optical).b) Use a source with sufficiently narrow linewidth to assure the measured bandwidth is atleast 90 % of the intermodal bandwidth. This is accomplished by calculating the normalizedintermodal dispersion limit, NIDL (refer to Annex A). For A4 fibre, the linewidth of the laserdiode is narrow enough to neglect its contribution to bandwidth measurement.c) For A1 and A3 fibres, calculate the normalized intermodal dispersion limit (NIDL, seeAnnex A) for each measurement wavelength from the optical source spectral width for thatwavelength as follows: NIDL = IDF/∆λ where: NIDL is the normalized intermodal dispersion limit in GHz·km; ∆λis the source full width half maximum (FWHM) spectral width in nm; IDF is the intramodal dispersion factor (GHz·km·nm) from Annex A according to thewavelength of the source. NIDL is not defined for wavelengths from 1 200 nm to 1 400 nm. The source spectral widthfor these wavelengths shall be less than or equal to 10 nm, FWHM.
60793-1-41 IEC:2003– 13 –NOTE
The acceptability of an NIDL value depends upon the specific user's test requirements. For example, a0,5 GHz·km NIDL would be satisfactory for checking that fibres had minimum bandwidths greater than some valueless than 500 MHz·km, but would not be satisfactory for checking that fibres had minimum bandwidths greater than500 MHz·km. If the NIDL is too low, a source with smaller spectral width is required.d) The radiation source shall be stable throughout the duration of a single pulse and over thetime during which the measurement is made.4.2 Launch system4.2.1 Overfilled launch (OFL)4.2.1.1OFL condition for A1 fibreUse a mode scrambler between the light source and the test sample to produce a controlledlaunch irrespective of the radiation properties of the light source. The output of the modescrambler shall be coupled to the input end of the test sample in accordance with Annex E. Thefibre position shall be stable long enough to perform the measurement. A viewing system maybe used to aid fibre alignment where optical imaging is used.Provide means to remove cladding light from the test sample. Often the fibre coating issufficient to perform this function. Otherwise, it will be necessary to use cladding modestrippers near both ends of the test sample. The fibres may be retained on the cladding modestrippers with small weights, but care must be taken to avoid microbending at these sites.NOTE
Bandwidth measurements obtained by the overfilled launch (OFL) support the use of category A1multimode fibres, especially in LED applications at 850 nm and 1 300 nm. Some laser applications may also besupported with this launch, but could result in reduced link lengths (at 850 nm) or restrictions on the laser sources(at 1 300 nm).4.2.1.2OFL condition for A3 and A4 fibresOFL is obtained with a geometrical optic launch in which the maximum theoretical numericalaperture of the fibre is exceeded by the launching cone and in which the diameter of thelaunched spot is in the order of the core diameter of the fibre.4.2.2 Restricted mode launch (RML)4.2.2.1RML condition for A1 fibreThe RML for bandwidth is created by filtering the overfilled launch (as defined by Annex E) witha RML fibre. The OFL is defined by Annex E and it needs to be only large enough to overfill theRML fibre both angularly and spatially. The RML fibre has a core diameter of 23,5 µm ± 0,1 µm,and a numerical aperture of 0,208 ± 0,01. The fibre must have a graded-index profile with analpha of approximately 2 and an OFL bandwidth greater than 700 MHz·km at 850 and1 300 nm. For convenience, the clad diameter should be 125 µm. The RML fibre should be atleast 1,5 m in length to eliminate leaky modes; and it should be less than 5 m in length to avoidtransient loss effects. The launch exiting the RML fibre is then coupled into the fibre under test.Provide means to remove cladding light from the test sample. Often the fibre coating issufficient to perform this function. Otherwise, it will be necessary to use cladding modestrippers near both ends of the test sample. The fibres may be retained on the cladding modestrippers with small weights, but care shall be taken to avoid microbending at these sites.NOTE 1
In order to achieve the highest accuracy, tight tolerances are required on the geometry and profile of theRML fibre. In order to achieve the highest measurement reproducibility, tight alignment tolerances are required inthe connection between the launch RML fibre and the fibre under test to ensure the RML fibre is centered to thefibre under test.
60793-1-41 IEC:2003– 15 –NOTE 2 - Modal bandwidth measurements on type A1b fibres obtained by this restricted mode launch at 850 nmhave been shown to correlate with the effective modal bandwidth produced by 850-nm laser transmitters only whensuch transmitters meet certain launch conditions.
More specifically, for type A1b fibres, 850-nm restricted modelaunch (RML) bandwidth >= 385MHz> ·km provides a minimum of 385 MHz·km effective modal bandwidth forsources meeting the following three launch conditions: Nominal operating wavelength = 850 nm; encircled flux <=25% at 4.5-µm radius; encircled flux >= 75% at 15-µm radius. Encircled flux measured per IEC 61280-1-4.4.2.2.2RML condition for A3 and A4 fibres4.2.2.2.1RML condition for A3 fibreRML condition for A3 fibre is created with geometrical optic launch which corresponds to anumerical aperture launch of 0,3.Spot size shall be larger or equal to the size of core.4.2.2.2.2RML condition for A4 fibreThe restricted mode launch (RML) for A4 fibre is created by filtering the overfilled launch with amandrel wrapped mode filter, shown in Figure 1. The mode filter shall be made with the fibre ofthe same category as the fibre under test. In order to avoid redundant loss, the length of fibreshould be 1 m. The diameter of the mandrel shall be 20 times as large as that of the fibrecladding and the number of coils shall be 5.Fibre under testOFL conditionIEC
863/03Figure 1 – A mandrel wrapped mode filterNOTE
Do not apply any excessive stress in winding fibre onto the mandrel. The wound fibre may be fixed to themandrel with an adhesive. Unwound parts of fibre should be set straight.4.3 Detection systemThe output optical detection apparatus shall be capable of coupling all guided modes from thetest sample to the detector active area such that the detection sensitivity is not significantlymode-dependent. The detector shall respond linearly over the range of power detected. Anoptical attenuator may be used to control the optical intensity on the detector.A device shall be available to position the specimen output end with sufficient stability andreproducibility to meet the conditions of 4.6.An optical detector shall be used that is suitable for use at the test wavelength, linear inamplitude response, spatially uniform to within 10 %, and sufficiently large to detect all emittedpower.The detected optical signal shall be displayed on a suitable instrument, such as a high speed,sampling oscilloscope with calibrated sweep rate for the time domain method and an electricalspectrum analyzer for the frequency domain measurement. The detection electronics as wellas any signal preamplifier shall be linear in amplitude response (non-linearities less than 5 %)over the range of encountered signals.
60793-1-41 IEC:2003– 17 –4.4 Recording systemFor the optical time domain method (Method A), use an oscilloscope suitably connected toa recording device, such as a digital processor, to store the received pulse amplitude as afunction of time. For temporal measurements, data taken from the oscilloscope display shall beconsidered secondary to those derived from the recorded signal.For the frequency domain method (Method B), use a tracking generator-electrical spectrumanalyzer combination or the equivalent to detect, display and record the amplitude of the RFmodulation signal derived from the optical detector. This shall be done in such a manner as toreduce harmonic distortion to less than 5 %.4.5 Computational equipmentFor the optical time domain method (Method A), computational equipment capable ofperforming Fourier transforms on the detected optical pulse waveforms as recorded by thewaveform recording system shall be used. This equipment may implement any of the severalfast Fourier transforms or other suitable algorithms, and is useful for other signal conditioningfunctions, waveform averaging and storage as well.4.6 Overall system performanceNOTE
This subclause provides a means of verifying system stability for the duration of a measurement or thesystem calibration period, depending on the method used (A or B, see 6.1 and 6.2, respectively).The measurement system stability is tested by comparing system input pulse Fouriertransforms (Method B) or input frequency responses (Method A) over a time interval. As shownin Annex B, a bandwidth measurement normalizes the fibre output pulse transform by thesystem calibration transform. If a reference sample is substituted for the fibre sample, theresultant response, H(f), represents a comparison of the system to itself over the time interval.This normalized system amplitude stability is used to determine the system stability frequencylimit (SSFL).The system stability frequency limit (SSFL) is the lowest frequency at which the systemamplitude stability deviates from unity by 5 %. If Method A-1 or B-1 is employed, it shall bedetermined on the basis of one re-measurement at a time interval similar to that used foran actual fibre measurement. If Method A-2 or B-2 is employed, it shall be determinedover substantially the same time interval as that which is used for periodic system calibration(see 6.1.1). In this latter case, the time interval may influence the SSFL.To determine the SSFL, attenuate the optical signal reaching the detector by an amount equalto or greater than the attenuation of the test sample plus 3 dB. This may require theintroduction of an attenuator into the optical path, if an attenuator, such as might be used forsignal normalization and scaling, is not already present. Also, normal deviations in the positionand amplitude of the pulse or frequency response on the display device shall be present duringthe determination of the SSFL.5 Sampling and specimens5.1 Test sampleThe test sample shall be a known length of optical fibre or optical fibre cable.
60793-1-41 IEC:2003– 19 –5.2 Reference sampleThe reference sample shall be a short length of fibre of the same type as the test sample, orcut from the test sample. Except A4 fibre, the reference length shall be less than 1 % of thetest sample length or less than 10 m, whichever is shorter.For A4 fibre, the reference length sha
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