1、CCITT RECMN*L.L4 92 m 48b259L 0573959 458 m CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE CONSTRUCTION, INSTALLATION AND PROTECTION OF CABLE AND OTHER ELEMENTS OF OUTSIDE PLANT L.14 MEASUREMENT METHOD TO DETERMINE THE TENSILE PERFORMANCE OF OPTICAL FIBRE CABLES UNDER LOAD Re
2、commendation L.14 Geneva, 1992 CCITT RECMN*L.L4 92 = 4862591 0573960 L7T INTERNATIONAL TELECOMMUNICATION UNION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE CONSTRUCTION, INSTALLATION AND PROTECTION OF CABLE AND OTHER ELEMENTS OF OUTSIDE PLANT L.14 MEASUREMENT METHOD TO DETE
3、RMINE THE TENSILE PERFORMANCE OF OPTICAL FIBRE CABLES UNDER LOAD Recommendation L.14 Geneva, 1992 FOREWORD The CCIIT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCm is responsible for studying technical,
4、operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approva
5、l of Recommendations by the members of CCIT between Plenary Assemblies is covered by the procedure laid down in CCIIT Resolution No. 2 (Melbourne, 1988). Recommendation L.14 was prepared by Study Group VI and was approved under the Resolution No. 2 procedure on the 31st of July 1992. CCIIT NOTE In t
6、his Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication Administration and a recognized private operating agency. O ITU 1992 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic
7、 or mechanical, including photocopying and microfilm, without permission in writing from the RU. CCITT RECMN*L.34 92 W 4862593 0573b2 T42 Recommendation L.14 MEASUREMENT METHOD TO DETERMINE THE TENSILE PERFORMANCE OF OPTICAL FIBRE CABLES UNDERLOAD The CC, considering (a) that optical fibres are susc
8、eptible to str important herein is the strain of the fibre after installation of the cable; (c) that in cables with loosely packaged constructions, an elongation of the fibre, invoked by an elongation of the cable, may lead to an increase in attenuation as soon as the fibre touches the inner wall of
9、 the secondary protection; there is, however, no simple relation between the applied elongation and the increase in attenuation, the latter also being dependent on the material characteristics of both primary and secondary coating; (d) that in cables with a central tube construction (as well with a
10、single fibre as with a fibre bundle or ribbon package), a considerable elongation of the cable and the fibres can be reached without any increase in attenuation; (e) that in cables with tightly assembled constructions, the elongation of the fibres is directly foliowing the elongation of the cables w
11、ithout any increase in attenuation below the specified allowable elongation which can be easily detected; (0 that it is necessary to evaluate the tensile performance of optical fibre cables; (g) that guidelines for testing need to be formulated, recommends (1) that in order to gain insight in the me
12、chanical behaviour of an optical fibre cable under tensile load, fibre elongation andor optical attenuation shall be measured as a function of cable elongation; (2) that for measurement of the behaviour of the attenuation of an optical fibre as a function of the tensile load on a cable, IEC 794-1 me
13、thod El “Tensile performance” shall be followed; (3) that for measurement of the fibre elongation as a function of cable tensile load, the following measuring method shall be applied. 1 Introduction Various cable constructions are based on a concept in which the cable wil have a certain strain margi
14、n, The cable structure shail be such as to protect the optical fibre against the strain induced during the installation and the operational cable lifetime. It shall ensure that during pulling operations under restricted pulling forces, a minimum of elongation is applied on the fibres to prevent sign
15、ificant fibre crack growth and attenuation increase. Recommendation L.14 1 A method is given for the direct measurement of the tensile performance of optical fibres. This method can provide information on both the maximum allowable pulling force for field installation, as weil as information about t
16、he strain margin of the cable. The method is based on the phase shift of a modulated signal launched into the fibre. 2 Measuring principle For evaluating the fibre elongation strain in the range of loads specified for the optical fibre cable under test, a phase shift method is used. The elongation o
17、f optical fibres in a cable that is loaded with a tensile force, is measured by use of a modulating light source. In the frequency domain the change in phase of the modulating signal is a function of the change in length of the fibre. Depending on modulation frequency, the fibre elongation can be me
18、asured with high accuracy, even with short cable test lengths. Depending on the cable structure and the desired information (maximum pulling force andlor cable strain margin), the way of clamping the cable may require a different setup. An optical fibre cable of a suitable length is mechanically loa
19、ded with a tensile force. The puiling force on the cable is measured with a loadcell. The cable elongation caused by this force is measured. The elongation of the fibres is measured by using a light source of suitable wavelength, modulated with a stable frequency; the measurement accuracy depends on
20、 the frequency of the modulating signal. At the receiving end of the fibre the signal passes a detector with sufficient high frequency response. The phase of the electrical signal from the detector is compared with the phase of the signal from the frequency generator. The change in the phase differe
21、nce is proportional to the elongation of the optical fibre. This phasenength dependency has to be settled once for every fibre type in a calibration measurement setup. When an optical signal is sinusoidally modulated, the phase of the modulation signal propagates with the group velocity along the fi
22、bre, so that after a length Lof the fibre the phase will be: $ = (2flc) NL where: f is the modulation frequency; c N is the free space velocity of light; is the effective group index of the fibre including dispersion effects. However, a phase change is not only induced by a fibre length change, but
23、also by a change in the group index N, due to the elasto-optic effect. A calibration measurement for every fibre type is necessary to give the precise relation between the phase change and fibre elongation. It is important that no differential movement is allowed between the cable elements at each e
24、nd of the length of cable under test. 3 Procedure 3.1 Calibration fibre A reference fibre of known length and of the same type as the test fibre, shall be used to determine the relation between the change in phase shift and the fibre elongation. 3.2 Bench calibration Install the reference fibre on t
25、he elongation bench and connect it to the optical measurement apparatus. increase progressively the fibre elongation strain within the range of elongations which are expected to occur during the tensile test of the cable. Measure and record, preferably continuously, the change in phase shift as a fu
26、nction of the reference fibre elongation. 2 Recommendation L.14 CCITT RECMN*L.14 92 4862591 0573964 15 The relation thus determined takes into account the strain induced changes of the group index. Nute 1 - It is recommended to carry out the calibration with several reference fibre samples of the sa
27、me type, preferably taken from different fibre drums, in order to avoid the effects of local irregularities in the fibre. Nute 2 - It is not necessary to repeat this calibration before each cable tensile test as long as the same type of fibre is used in the cables to be tested. 3.3 Test sample measu
28、rement Put the ends of the cable test length in the clamping devices. Connect the test fibre of the cable under tensile test to the measurement apparatus. Carry out the cable tensile test as described in the IEC 794-1-El. For most cable constructions (e.g. stranded type cables), clamping on cable el
29、ements except the fibres, is practical and sufficient to get both the maximum ailowable pulling load and the sttain margin of the cable. However, for certain cable constructions (e.g. single loosetube), it may be necessary to prevent the fibres from slipping, in order to obtain the correct excess le
30、ngth. Both the force applied on the cable and the fibre elongation have to be recorded as a function of the elongation. Care shall be taken that during the pulling of the sample the reference length do not change. During this test, measure and record, preferably continuously, the changes in phase sh
31、ift as a function of cable load and/or elongation. Calculate the corresponding fibre elongation strain, using the factor deducted from the calibration described in 0 3.2. If required, the load value where the beginning of fibre strain occurs is defined, on the plot of fibre strain versus load, as th
32、e intersection of the linear portion of the curve with the load axis. Note - As a first approximation, the length of fibre under elongation strain is taken equal to the length of cable under tensile load, except the cables with tightly assembled constructions. It shall be noted however, that the cal
33、culated value of fibre elongation strain is affected by the accuracy on the value of this cable length, and also by the excess length of fibre in the cable which depends on the cable design (loose structures). Recommendation L.14 3 - - _ - - _ - CCITT RECMN*L.L4 92 W 4Bb259L 0573965 751 W APPENDIX I
34、 (to Recommendation L.14) Presentation of the results An example of a presentation of the cable and fibre elongation is given in the case of loosely packaged construction by Figure I-lL.14. Tensile performance 0.45 h 8 .-E E? Y C O O 0.35 i 0.1 5 . 0.1 o. 0.05. 0- elongation Fibre elongation 0.25 *
35、0.20. DaN 42.0 84.0 126.0 168.0 210.0 252.0 294.0 336.0 378.0 420.0 Load TO60191 0-91 O Cable elongation O Fibre elongation Note - To corresponds to the load at which the fibre becomes under strain. T, corresponds to the maximum specified pulling load. FIGURE 1-1 / L.14 Fibre and cable elongation as
36、 a function of load 4 Recommendation L.14 CCITT RECMN*L.L4 92 486259L 0573966 698 = An example of one measurement setup is shown on Figure I-2L.14. ,+2+, generator voltmeter X - Y 8 8 X-Y Recorder /Cable / T0601920-91 FIGURE 1-2 / L.14 Set up for optical fibre strain measurements Recommendation L.14 5
