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DETERMINATION OF MECHANICAL STRESSED PLACES OF OPTICAL FIBERS IN OPTICAL CABLES USING BRILLOUIN REFLECTOMETERS
DOI 10.24411/2072-8735-2018-10205
The work was performed with the financial support of the Ministry of Education and Science of the Russian Federation within the scope of the base part of a State Assignment within the sphere of scientific activity (Project No. 8.9334.2017/8.9).
Igor V. Bogachkov,
Omsk State Technical University (OmSTU), Omsk, Russia, bogachkov@mail.ru
Keywords: optical fiber, strain, optical cable, Mandelstam - Brillouin scatter, early diagnostics, Brillouin reflectometry.
An important task of an early diagnostics of optical fibers (OFs) is the timely detection and suppression of fiber segments with mechanical stress in optical cables (OC). The local inho-mogeneities of quartz (including microcracks) may occur in the processes of fiber production. They create internal stresses, so the fiber becomes vulnerable to bending or vibration. The increased OF strain in the laid OC affects durability of them, although modern modular structures of OCs have a certain stock of OC protection against the dangerous tension. To detect such "problem" segments of an OF the Brillouin reflectometry or phase shift method is used. This method underlies the operation of Brillouin optical time domain reflectometers (BOTDR), which are able to provide correct information on a distribution of a strain degree along OF length. Also, this information allows us to predict the degradation of fibers.
It is necessary at various stages of OC production to control the characteristics of an OF, including such as OF strain. Investigation results of the strain in stressed optical fiber and optical cable in various processes of production using BOTDR are described in this article. In the production process of OFs and OCs it is not difficult to get access to both ends of the OF. The phase shift method is effectively used to find the total optical fiber elongation. During the construction and operation of the OCs, the access is possible only to one OF end, therefore only the reflectometry methods (BOTDR) can be used. When laying OCs, the OF strains will be redistributed (all OF strains along the line will be averaged while meeting the requirements of cable laying), but obvious defects, such as segments with high strain are likely to continue, and therefore the OC life can be significantly reduced. It is recommended to include the BOTDR in the control systems of the OF characteristics to detect OF segments with high mechanical stress and temperature changes. This will make it possible to identify the potentially harmful segments in the OC at various stages of OC production and improve the technologies used in the production process.
Information about author:
Igor V. Bogachkov, Associate professor (docent) of "Communication means and information security" department of Omsk State Technical
University (OmSTU), Omsk, Russia
Для цитирования:
Богачков И.В. Определение механически напряжённых мест оптических волокон в оптических кабелях с помощью бриллюэновских
рефлектометров // T-Comm: Телекоммуникации и транспорт. 2018. Том 12. №12. С. 78-83.
For citation:
Bogachkov I.V. (2018). Determination of mechanical stressed places of optical fibers in optical cables using brillouin reflectometers. T-Comm,
vol. 12, no.12, рр. 78-83.
An important task of an early diagnostics of optical fibers (OFs) is the timely detection and suppression of OF segments with mechanical stress in optical cables (OC) [1-7J.
The local inhomogeneities of quartz (including microcracks) may occur in the processes of OF production. They create internal stresses, so the fiber becomes vulnerable to bending or vibration. As a result, the tensile strength of OF decreases [1-4, 7].
The increased OF strain in the OC affects durability of them. According to data of the "Fujikura" and "Coming" companies when a longitudinal strain is more than 0.3%, a liber life of 5 ... 10 years is reduced instead of the expected 25 years, and at the strain of order of \%~ less than a year 11-4].
To detect such "problem" segments of an OF the Brillouin reflectometry or phase shift method is used [1-4J. This method underlies the operation of Brillouin optical time domain refleetometer (BOTDR) and Brillouin optical time domain analyzer (BOTDA), which are able to provide correct information on a distribution of a strain degree along OF length. Also, this information allows us to predict the degradation and breakout of the OF f 1-5, 8].
The past experience made the manufacturers of OF and OC envisage measures, which minimize an effect of bending and deformation of an OC on an OF strain and increase a reliability ofthe OC as a whole.
The control transpooling ofthe fiber under load is used to detect OF defects in the process of production [3, 4, 7], which allows "brittle" segments to be found.
Note that in the production of OC the OFs should be laid in it w ith excess fiber length lo have a certain margin of OC strain. This is presented by OC manufacturers in modularized design. If these conditions are not met, the dangerous levels of a strain will be appeared.
It is necessary at various stages of OC production to control the characteristics of an OF, including such as OF strain (elongation).
The light signal under increased longitudinal strain in the OF causes the development of microcracks, which inevitably occur due to the crystallization of quartz glass during the OF production [5, 6], This eventually leads to a microcrack growth during the operation of the OC (slow microcrack growth at a longitudinal strain of about 0.2% or fast growth at a strain of more than 1 %), and as a result - lo the degradation ofthe OF and its premature destruction [2-5].
The dependence of life for single-mode OF (G. 652) on the strain according to the data from tlie "Fujikura" company allows us to highlight a safe range of strain is less than
0.2 %, an unacceptable is more than 0.6 %, an intermediate range of strain values, requiring an additional analysis | I-4J as well as is not valid for OFs laid in the OC as in this case an OC durability is reduced. [1-6]. The strain of the OF should not exceed 0.26 % to have the warranty life of 25 years. With an increase in the strain of the OF to 0.45%, a 50 percent probability ofthe OF breakage can be appeared during the OF life [3-4].
Although a number of factors (additional OF mie rod a mages or penetration of dampness lo the OF surface) were not taken into account in these estimates, it follows that even a slight increase in the longitudinal OF strain can lead to a multiple decrease in its life [2].
Since OF. materials of modules, strength elements and OC sheaths have different elongation rates and coefficients of thcr-
T-Comm Vol.12. #12-2018
mal expansion, when the temperature changes the significant stresses inside the OF may occur due to imperfect expansion of (he contacting materials [6, 8].
The control of mechanical stresses in OF with the possibility of spatial localization of "problem" OF segments is one ofthe major features of the Brillouin reflectometry method.
This feature is useful even in bench-test of OC in laboratory (production) conditions, since the elongation of an OF can significantly exceed the maximum permissible value in some segments, although fhe average value obtained using IFC 60794-1 methods, such as the attenuation increment method and the phase shift method, can remain within permissible values [2-4].
The phase shift method is based on the measurement (control) ofthe phase shift ofthe optical signal relative lo the formed reference channel, which enables us to obtain the value ofthe OF elongation by phase increment [3, 5, 7, 11 ].
Also, il is not difficult to access to both ends ofthe OF in the process of OF and OC production. Therefore, the phase shift or BOTDA method is used, BOTDR method owns the feature of one-end access, which is very important for measurements in the construction and operation ofthe OC [1-6].
The Brillouin reflectometry method is the basis of BOTDR, the principle of this method is based on the analysis ofthe Mandelslam - Brillouin backscalter spectrum (MBBS) in fhe fight-pipe that can be observed when the increased radiation is launched into the OF.
The components ofthe MBBS have the shifted frequency by a value proportional io the OF strain and its temperature. By determining ihe distribution ofthe MBBS along the light-pipe and calculating the Brillouin frequency shift (the frequency of MBBS maximum), we can get a distribution pattern of strains in the OF, find their characteristics and analyze the causes of these changes in MBBS [2-6].
Through analyzing the BOTDR reflectograms of the MBBS, the distribution pattern of the OF strain along the longitudinal coordinate can be got.
Experimental studies using BOTDR "Ando AQ 8603" and "Viavi MTS-8000" were carried out to investigate the features of Brillouin reflectograms of the OFs laid in the OCs, which are subject to additional strain because ofthe effect of different factors.
In fhe experimental studies, fhe results of which are presented below, OCs containing the single-mode optical fiber (G.652) have been investigated.
The coils ofthe OFs made of a single workpiece have nearly identical MBBS profile, Brillouin frequency shift, and therefore the characteristics of strains, but for OFs of different parties, some spread in performance is observed even from one manufacturer [3,4].
Figure 1 shows a pattern of the OF strain located on the coi! obtained by BOTDR "Ando AQ 8603".
Although the strains ofthe OF of different segments (center or edges) and layers (external or internal) are slightly different, it is completely harmless for the OF durability. The strain pattern between the markers "1" and "2" transforms almost into a straight line passing through the level of "-0.002%" when unwinding the OF into a line (or rewinding the OF into the hank), so the reflectogram of the strain of the OF located on the coi! helps lo measure the quality ofthe OF before the next stages of OC production.
7TT
iiromj
Stuin: 0,0012 % 1,
Markor 1-2 (hax) 0.0062 % (hih> -0<0052 % ]
2.6b09d km || 1 0.05109 Job/ f 3.16132 km
Fig. 1. Pattern of the OF strain located oil ilie coil
Manufacturers are interested in the maximum possible speed of OC production. The high speeds of OF movement are necessary to accelerate llie technological processes (when applying a protective OF sheath (painting) - about 17 m/s, when forming an optical module - about 6 m/s, etc.), which require additional force when rewinding. During rewinding the OF strain increases, and after ilia! the residual strain may be survived due to the "fixation" of some OF segments for various reasons |3,4].
For instance, after curing (drying) of the protective OF sheath the shape of the OF is "fixed" which means that the increased elongation is survived, which appeared in the process of rewinding. In addition, the curing of the paint occurs irregularly (the excentricity of sheath, the irregularity of the rate of painting and curing), which can enhance this effect and even increase the signal attenuation in the OF [2J.
As a result, the strain of the painted OF located on the coil increases by an average of 0.002 ... 0,005 %, but local segments with high strain may appear.
In the process of OF integration into an optical module the fiber kinking is possible, which also at high speeds of the cable pulling and cooling of the module sheath can lead to local "pinch" of the OFs. It can be maintained in the finished OC.
The monitoring of the strain using BOTDR or phase meter allows us to detect timely such problems and to adjust the technological process.
Manufacturers lay the excess fiber length to the tensile stresses to improve the durability of OF inside the module. In case when the optical module is stretched the OF straightens out, which leads to the fact that the elongation of the module to 0.1% practically does not affect the OF strain. The load acts on the OF only at high elongations, and then the strain of the OF begins to increase linearly with the module extension.
Threading of optical modules is carried out at the next stage of OC production. This is considered to be the most dangerous process because it can lead to the OF strain in the module and even the increase in attenuation [2].
Figure 2 shows a strain pattern of one of the OF in OC in the procedural violation of threading of optical modules, obtained by BOTDR "Ando AQ 8603".
A similar pattern is observed with an optical module under a high strain during the threading process (for example, at too high speed of the cable pulling). Bursts in the BOTDR reflectogram indicate the OF segments with increased local elongation. Such OC cannot be used, since in real conditions the fast OF degradation will occur.
2.145j5 kn
| 0-510P9 W |
7.25360 k*
Fig. 2. Pattern of an OF strain in the procedural violation of module threading
To suppress these problems, it is also necessary to lay the excess fiber length in modules (about 3...5 in/km) regularly for all OFs (deviation of not more than I m/km) [ 1,2J.
Fig. 3. Pattern of an OF strain (BOTDR "Viavi MTS-80Q0") in the area of two distributed sensors with different central load-bearing elements
Experience has shown that the use of a steel central load-bearing element (steel cable in a polyethylene sheath) can also lead to local strain after threading of modules with the OFs, The irregularities of the steel cable, the longitudinal and transverse loads on it can give it a spiral shape, which as a result will lead to additional "pinch" and elongation of the OFs in the OC remaining in the finished item. The fiber-glass central load-bearing element has much less problems with "pinch" in OC [1, 2].
Tests carried out with the OCs applied as the strain sensors using the BOTDR "Viavi MTS-8000" also are shown that the OF in OC with a fiber-glass centra! element elongates very smoothly {sensor "1" in Fig. 3), compared with the OF in OC with a steel ccntral element (sensor "2" in Fig. 3), since OF slides well on the fiber-glass surface. Although the OF strain with a gradual increase in the tensile load rises more slowly in OC with a steel centra! load-bearing element, in the sensor a strong strain difference of adjacent segments along the OF is appeared.
The operations of coating the cable armor and the aramid yam in a normal technological process (the location of wires or yam and their strain should be regular, w ithout stress of the OC workpieee) do not lead to an increase in OF strain [ 11.
The stage of coaling the protective OC sheath can also cause secondary OF strain (about 0.04...0.06%) due to the post-extrusion shrinkage of polythene, since the shrinkage conditions
T-Comm Tom 12. #12-2018
(and thus the sheath length in the segments) during the spooling differ at various levels and parts of the drum.
Figure 4 demonstrates a strain pattern of one of the OF in OC coiled on the drum after shrinkage of the polythene protective sheath.
> i I.H|
Hirkei- 1 2
(raw) o.ofifli % (Hill) o.owfc %
km I I
Fig. 4. Strain pattern of an OF in OC coiled on the drum
"Half-waves" of the OC layout (more stressed in the center and less stressed along the edges), as well as a difference in the turn lengths of the internal (marker "2") and external {marker "1") layers are clearly visible in the BOTDR retlectogram.
A strain pattern "straightens" after unwinding the OC from the drum.
In order to have a value of OF strain in OC in permissible limits, it is necessary to limit speed of OC pulling and cooling.
Figure 5-7 show the strain patterns of one OF from the finished OC coiled on the drum at different temperatures (a durability test of the OC to temperature changes from -40°C to +60°C in the climatic chamber), which are obtained using BOTDR "Ando AQ 8603".
Figure 5 displays a strain pattern of the OF in OC at room temperature. The minimum and maximum strain value in the segment pointed with markers "1"—"2" are specified.
|HTGH|
©
Strain Markrr 1-? (MAX* 0.0955 % (H1H) 0,0050 %
0.00000 km
2.0I350 W
[ 20.<J<T1 km
Fig. 5. Pattern of an OF strain in OC at room temperature
Figure 6 shows a pattern of an OF strain at temperature of about +60°C. The strain of the OF increased by an average of 0.05% as shown in Fig. 6. Although it is known that the dependence of the OF strain on the temperature is linear [9-13), but there are many segments where the OF elongation due to the thermal OC expansion is limited by "fixing" factors for an OC on the drum.
J u i
Strain Marker 1- 2 (MAX) 0.1443 % (MIM) 0.0387 %
0.00000 km
| 3.0«35a kW 1
J 10.13111 km
Fig. 6. Pattern of an OF strain in OC at high temperature
The graphs of the OF strain at temperature of -40aC is presented in Fig, 7,
4.1034 4 km
0.05
Str.iin Kukm 1-2 (max] Q-011Q % (HIM) -0. 10(i3 V,
VvWxi
c
. ' ; !;. ' km
Fig, 7, Pattern of an OF strain in OC at low temperature
As shown in Fig. 7, the OF compressed by an average of 0,07%, therefore the "fixing" factors have been redistributed, Bui the significant stresses of the OF presented in all three charts at distances of 6.9 km (significant) and 13.5 km (insignificant), which indicate the problem segments of OC on the drum are noticeable.
The increase in attenuation of OF is not observed at all considered temperature tests [ i I-13J.
Thus, a BOTDR allows local OF strains to be detected and errors in the processes of OC production to be identified.
The study of MBBS of the OFs located in the finished OC's subjected to significant stress loads in laboratory tests, allowed us to draw the following conclusions.
Due to the cable central load-bearing elements protecting it and all OFs inside its modules against mechanical stresses, as well as because of the excess OF length in the optical modules, the significant changes in the MBBS under tensile loads of a certain segment investigated OC cannot be observed (if there are no obvious defects and "pinch") up to critical load levels, which lead to the destruction of protective OC elements at the area of influence, and then to the rapid breakage of the OF in OC in general. (This is observed when the longitudinal tensile load effecting on the test OC, which exceeds the maximum permissible load for the OC) [11].
The analysis of the experimental data obtained by phase methods [3, 5, 11] showed that these methods allow appearance of the OF elongation to be detected (due to the longitudinal OF strain) at a tensile load not exceeding maximum permissible limit
for the OC under test. Since destruction of the OC elements are not occurred, the OC will return to its normal state after removal of load. However, such effects lead to the redistribution of OF strain, so the control using BOTDR is also necessary to detect segments with local strain.
Figure S shows the measurement results of the OF elongation using the phase shift method.
Fig. 8. Results of the fiber elongations using the phase shift method
As shown in charts, changes in the OF elongation in OC relative to the reference channel are observed at a tensile load acting on the OC more than 11 kN. Taking into consideration the fact that the area of influence on the OC in this case is known in advance, the phase method is more preferable.
When laying OCs, the OF strains will be redistributed (all OF strains along the line will be averaged to the level of 0% while meeting the requirements of cable laying), but obvious defects, such as segments with high strain are likely to continue, and therefore the OC life can be significantly reduced.
For early diagnostics of the OFs (timely detection of segments with mechanical stress, as well as segments with changed temperature) in OC laid on the cable routing operated in real conditions, the measurements using BOTDR are the most effective, since it is enough to have one-end access in contrast with BOTDA [5,6, 11-15].
Modern modular structures of OCs have a certain slock of OC protection against the high strain.
Brillouin reflectomelry method enables early diagnostics of the OFs to be performed and local "problem" segments of the OF to be removed at an early stage.
it is recommended to include the BOTDR in the monitoring systems of the OF characteristics to detect OF segments with high mechanical stress and temperature changes. This will make it possible to identify the potentially harmful segments in the OC at various stages of OC production and improve the technologies used in the production process.
In the production process of OFs and OCs it is not difficult to get access to both ends of the OF, which allows phase methods or BOTDA to be applied. During the construction and operation of the OCs, the access is possible only to one OF end, therefore only the rcllectometry method (BOTDR) can be used.
The work wis done with the financial assistance of the Ministry of Education and Science of the Russian Federation within the scope of the base part of a State Assignment within the sphere of scientific work (Project No. 8.9334.2017/8.9).
The author would like to thank CJSC «Moskabel-Fujikura» (Moscow) for the help in performing these experiments.
References
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ОПРЕДЕЛЕНИЕ МЕХАНИЧЕСКИ НАПРЯЖёННЫХ МЕСТ ОПТИЧЕСКИХ ВОЛОКОН В ОПТИЧЕСКИХ КАБЕЛЯХ С ПОМОЩЬЮ БРИЛЛЮЭНОВСКИХ РЕФЛЕКТОМЕТРОВ
Богачков Игорь Викторович, Омский государственный технический университет, Омск, Россия, bogachkov@mail.ru
Работа выполнена при финансовой поддержке Министерства образования и науки Российской Федерации в рамках базовой части государственного задания в сфере научной деятельности (проект № 8.9334.201718.9)
Аннотация
Важной задачей ранней диагностики оптических волокон (ОВ) является своевременное обнаружение и устранение механически напряжённых участков в ОВ, находящихся в оптических кабелях (ОК). В процессе изготовления ОВ могут возникать локальные неоднородности кварца (в том числе микротрещины), создающие внутренние напряжения, которые делают ОВ уязвимым к изгибам или вибрациям. Повышенное натяжение ОВ в проложенных ОК влияет на долговечность ОК, хотя современные модульные конструкции ОК имеют определённый запас защиты ОВ от опасного натяжения. Для обнаружения подобных "проблемных" участков ОВ применяется метод бриллюэновской рефлектометрии или метод фазового сдвига. Метод бриллюэновской рефлектоме-трии положен в основу работы бриллюэновских рефлектометров (BOTDR), которые способны предоставить точную информацию о распределении степени натяжения ОВ вдоль его длины, а на основе этой информации позволяют прогнозировать деградацию ОВ. В процессе производства ОК необходимо на разных этапах контролировать характеристики ОВ в нем, в том числе такие, как натяжение (удлинение) ОВ. В статье приведены результаты исследований натяжений оптических волокон и оптических кабелей, находящихся под действием механических нагрузок при различных производственных процессах, с помощью BOTDR. В процессе производства ОВ и ОК не сложно получить доступ к обоим концам ОВ, поэтому для определения общего удлинения ОВ эффективно использовать метод измерения фазового сдвига. При строительстве и при эксплуатации ОК доступ возможен лишь к одному концу ОВ, и это допускает применение только рефлектометрических методов (BOTDR). При прокладке ОК натяжения ОВ будут перераспределяться (при соблюдении правил прокладки все натяжения ОВ вдоль линии будут усредняться), но явные дефекты, такие как места с повышенным натяжением скорей всего сохранятся, а значит, срок эксплуатации такого ОК может существенно сократиться. Для обнаружения мест ОВ с повышенным механическим напряжением и с измененной температурой рекомендуется включать BOTDR в состав системы контроля характеристик ОВ. Это позволит выявлять потенциально опасные участки в ОК на разных стадиях изготовления ОК и совершенствовать технологии, применяемые в производственном процессе.
Ключевые слова: оптическое волокно, натяжение, оптический кабель, рассеяние Мандельштама — Бриллюэна, ранняя диагностика, бриллюэновская рефлектометрия.
Литература
1. Богачков И.В., Горлов Н.И. Экспериментальные исследования влияния продольных растягивающих нагрузок на спектр бриллюэновского рассеяния в оптических волокнах // Вестник СибГУТИ, 2015. Вып. 3 (31). С. 81-88.
2. Акопов С.Г., Васильев Н.А., Поляков М.А. Использование брилюэновского рефлектометра при испытаниях оптического кабеля на растяжение // Lightwave. 2006. №1. C. 23-25.
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Информация об авторе
Богачков Игорь Викторович, к.т.н., доцент, доцент кафедры "Средства связи и информационная безопасность", Омский государственный технический университет, Омск, Россия
7ТТ
