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RESEARCHES OF BEND INFLUENCES ON BRILLOUIN REFLECTOGRAMS OF DIFFERENT TYPES OF OPTICAL FIBERS
To ensure the operation of fiber optical communication line (FOCL) for many years it is important to monitor the optical fiber (OF) integrity in the optical cable (OC). Remote fiber test systems (RFTSs) based on optical time domain reflectometers (OTDRs) are used for this purpose. But it is also necessary to detect "problem" fiber segments: segments with unauthorized access (UA), increased mechanical strain or transformed temperature. The longitudinal mechanical strain in OFs in FOCL can lead to decrease of the OC operation. Moreover, the "trouble" segments of OCs can be originated by the temperature transformations in the OFs. The crucial task in monitoring of the FOCLs is the premature diagnostics of such segments in OFs, that helps us to solve the problem before a breakdown of OC. The monitoring systems for the OFs based on the OTDR operation are not monitor the OF segments with transformed temperature and strain. For that purpose, the Brillouin optical time-domain reflectometers (BOTDR) operating at the analysis of a Mandelstam - Brillouin backscatter spectrum (MBBS) are used. FOCLs have more tamper resistance than other communication lines. In spite of the heavy expenses and development complexity, the UA to an OF can be organized, but it requires a close contact to the OF for interception, since the protective sheaths and the construction units of an OC essentially decrease the sidelight radiation.
The segments with enhanced level of sidelight radiation are applied in many UA means. These means have a high privacy because of the small changes in features of signal propagating along the OF. The sidelight optical radiation from the lateral OF area can appear in the presence of distributed and local loads on the OF, as well as on the OF bends because of the transformation of the guided modes into the leaking modes. Long-haul seamless interception can be implemented on a gentle OF bend or on exposure to the low temperatures changing a refraction index of the glass due to the increase in the level of scattering. The change of the total internal reflection due to a longitudinal strain is possible either when OF bend or the local attack on the OF are occurred. Experimental tests with the BOTDR were organized to elucidate the bend effect of various optical fibers on BOTDR-traces. The bend is a half-loop (semicircle) of the specific diameter which was gradually decreased, and MBBS in the peak area for the tested OF was observed at a minimum frequency step. As a result, the influence of a bend diameter on the level of a backscattered signal was clearly demonstrated in the trace of the MBBS. The changes in the BOTDR-traces are observed at the bend diameter in the form of a half-loop of 20-25 mm in most tested OFs. The lowest sensitivity to such bends is found in the fibers "Panda" and "ULTRA". The highest sensitivity is observed in the dispersion-shifted fiber. The changes of the traces from a standard enable a statement about a prospective UA to the FOCL to be made. The BOTDR allows us to implement the premature diagnostics of OFs in the FOCL, diagnosticate the unfeatured means of UA to the OF connected with temperature transformations, longitudinal strain and bends of the OF. The BOTDR in the monitoring system can increase its efficiency.
Information about author:
Igor V. Bogachkov, Associate professor (docent) of "Communication means and information security" department of Omsk State Technical University (OmSTU), Senior Member IEEE, Omsk, Russia
Для цитирования:
Богачков И.В. Исследование влияния изгибов оптических волокон различных типов на бриллюэновские рефлектограммы // T-Comm: Телекоммуникации и транспорт. 2019. Том 13. №3. С. 75-79.
Bogachkov I.V. (2019). Researches of bend influences on brillouin reflectograms of different types of optical fibers. T-Comm, vol. 13, no.3,
DOI 10.24411/2072-8735-2018-10253
Igor V. Bogachkov,
Omsk State Technical University (OmSTU), Omsk, Russia, bogachkov@mail.ru
Keywords: optical fiber, early diagnostics, unauthorized access, monitoring, bend, strain, Brillouin reflectometer.
For citation:
pp. 75-79.
The important task in monitoring of fiber optical communication lines (FOCLs) is a premature diagnostics and recovery of "problem" segments in optical libers (OF) - places with changed strain and temperature, segments with bends and microbends and unauthorized acccss (UA) to the OFs 111.
FOCLs have more advantages than other communication lines as a high privacy due to a particular behavior ot'propagation of the optical signal in the OF [1 — 3].
To ensure the operation of FOCL for many years ¡1 is important to do the premature monitoring of the OF integrity in the optical cable (OC) and diagnostics the "trouble" FOCL segments having heightened mechanical strain or temperature transformations [1 -4].
Remote fiber test systems (RFTS) using the optical time domain re Hectometers (OTDR) are applied to diagnosticate the troubles of OFs as well as unauthorized access to the OF in the FOCL.
The longitudinal mechanical strain in OFs in FOCL can decrease of the OC operation. Therewith, the "trouble" segments of FOCL can be originated by temperature transformations in the OFs [I -31.
Subsequently, the premature diagnostics of aforementioned segments helps us to recover the "trouble" segments before a breakdown of the OC. Therefore, to evaluate a FOCL operation we need to have the right information on strain of the OF in OC. The monitoring systems for the OFs based on the OTDR operation arc not diagnosticate the OF segmenls with transformed temperature and strain.
To diagnosticate such segments in the light-pipe the Brillouitl optical time-domain reflectometers (BOTDR) operating at the analysis of a Mandelstam - Brillouin backscatter spectrum (MBBS) are applied [4 - 8J. By considering the MBBS in OF and finding the Brillouin frequency shift (fB) localized by the MBBS peak we can get the distribution pattern of strain [6 - 8|.
In spite of the heavy expenses and development complexity, the unauthorized access to an OF can be organized, but it requires a close contact to the OF for interception. The protection from UA to the OF in connection or bifurcation of FOCL segments, etc. is performed by administrative procedures, that will not be discussed in this paper.
In the passive UA means the segments with enhanced level of sidelight radiation are used. These means have a high privacy due to the small changes in features of radiation propagating along the OF.
The sidelight optical radiation from the lateral OF area can appear in the presence of distributed and local loads on the OF and the OF bends because of the transformation of the guided modes into the leaking modes |4 - 5, 9].
By using the active means it is possible to get a radiation of higher power through the lateral OF area. Note that the changc of features of radiation propagating along the OF can be diagnosticated. These means are contained in mechanical bends of the OFs, probe embossing into the OF sheath, thermal and mechanical strain of the geometrical features of fibers [4-5,9].
The protective sheaths and Ihe design details of an optical cable (OC) essentially decrease the sidelight radiation, therefore, an interception by any listed above means can be organized only in integrity breakdown of an external protective sheath of an OC and a close contact to an OF.
Long-haul seamless interception can be implemented on a gentle OF bend or on exposure to the low temperatures changing a refraction index of the glass due to the increase in the level of scattering [4-5,9j.
Transformation of the grazing angle of the beam at the boundary of the core-cladding takes place because of the OF bend. The grazing angle is less than the limiting angle, so the some beams of the electromagnetic radiation cross the line on the OF. Thus, the possibility of unauthorized interception at the localized segment is formed. According to tests information ¡5, 9], the OF bends with diameter from 20 mm to 18 mm pave the way to the change of an attenuation index from 0.1 to 1.0 dB, respectively, The mode monitoring with unnoticeable mutilation of reflected signal coming into the OTDR is detected [5].
Note that the UA in the FOCL leads to losses. Estimating a relative wave strength, we obtain a signal power loss and evaluate a percent of the radiation, which can be removed by a violator from the FOCL [4].
The change of the traces from a standard (new "events", a level transformation of a reflected signal, etc.), enable a statement about a prospective UA to the FOCL to be made.
The change of the total internal reflection due to a mechanical attack is possible cither when OF bend or the local attack on the OF are occurred causing an unmonitored scanering (unlike bend) at the breakdown area.
Another external influence is mechanical attack without the OF shape transformation as stretching, changing the ratio of a refractive sheath index to the refractive index of the OF core.
Thus, the BOTDR in the monitoring system can diagnosticate the hardly noticeable means of an UA to the OF connected with mechanical, acoustic and thennal attacks on the OF. In the following, we describe that the BOTDR-t races are sensitive to bends and microbends of the OFs.
Experimental tests with CJSC "Moskabel-Fujikura" BOTDR "Ando AO 8603" were organized to elucidate the bend effect of different types of optical fibers on Brillouin refleetograms.
The results of tests with standard single-mode fibers (G. 652) are proposed in preceding papers [5, 10-12].
In the following, for each kind of OF under test, the bend segment was formed at a distance of at least 2 m from the OF joint. The bend is a half-loop (semicircle) of a specific diameter
(Fig. 1).
The diameter of the half-loop gradually was decreased, and MBBS in the peak area for the tested OF was observed at a minimum frequency step. As a resuli. the influence of a bend diameter (0) on the level of a backscattered signal was clearly demonstrated in the Brillouin trace of the MBBS distribution along the OF,
77
BOTDR-trace for an OF "Panda" (polarization maintaining fiber-PMF - PS887-A270318 [10]) with a decrease in diameter of the bend half-loop from 35 mm to 5 mm is shown in Ihe Fig. 7.
A decrease in the level of the back reflected signal is observed only at small diameters ofbend half-loop (less than 10 mm).
fig, 7. BOTDR-trace in the place of bending in the "Panda" OF
Thus, the BOTDR detects the fiber bends and finds the "problem" place in all considered cases (Fig. ! - Fig. 7).
Major change in the MBBS pattern because of the fall of reflected signal level is presented as in lateral loads of about 0.2 N leading to the microbends [6],
Table 1 shows the features of tested OF kinds obtained in experimental tests from the BOTDR-traces: value offm is the initial level of Brillouin frequency shift (fB) at room temperature and in the absence of mechanical strains [7-12], as well as additional attenuation (A, dB) of the level ofthe back reflected signal in the bend place depending on the half-loop diameter.
Table I
The measured characteristics G.652 LUI. (G.652) ULTRA (G.652) G.652-200 дпл G.653 (DSF) G.655 (NZDSF) LEAF (G.655) G.6S7 "Panda"
fm. GHz 10.85 10.84 10.82 10.77 10.47 10.63 10.65 10.79 10.5 6
A diainelcr of some visibility of the betid 25 24 20 16 27 26 23 24 И
A diameter ofthe hend visibility 20 16 19 15 25 23 22 19 10
A, 0.5 1» 10 18 14 24 19 21 15 8
dB 1.0 16 8 17 13 20 ¡7 20 14 7
1.5 15 6 16 12 17 16 19 ! 3 6
2.0 14 5 15 It 15 ¡5 18 12 5
3.0 13 4 12 10 13 13 16 10 4
Moreover, the similar features for OFs (G.652, G.653 (Dispersion-shifted fiber - DSF), G.655, G.657 considered in earlier papers are added to table I for reference [8 - 12].
In conclusion, the Brillouin reflectometry method helps us to implement the premature diagnostics of OFs in the FOCL, allows to detect the hardly noticeable means of 1JA to the OF connected
with temperature transformations, longitudinal strain and bends ofthe OF.
The changes in the Brillouin traces are observed at the bend diameter in the form of a half-loop of 20-25 mm in most tested optical fibers.
The lowest sensitivity to such bends is found in the libers "Panda" and "ULTRA".
The highest sensitivity is observed in the DSF.
The "initial level" offm should be found for each kind of Ihe
OF.
References
1. Bogachkov I.V., Gorlov N.l. (2013). Methods and means of monitoring and early diagnostics of fiber-optical transmission lines. Omsk: Publishing house OmSTU. 192 p.
2. Agraval G. Nonlinear Fiber Optica. (201 I). SPb, Lan*. 592 p.
3. Bogachkov I.V., Trukhina A,I., Gorlov N.l. (2018), Improvement ofthe Monitoring Systems of Fiber Optical Communication Lines. Nth International Conference on Actual Problems of Electronic Instrument Engineering Proceedings. Novosibirsk. Vol. I,p. 1, pp. 192-197.
4. Lutchenko S.S., Bogachkov I.V. (2018). Influence ofthe optical fiber strain on the availability of fiber-optic communication lines. Systems of Signal Synchronization. Generating and Processing in Telecommunications (SINKHROINFO-2018). Minsk, pp. 1-4,
5. Bogachkov I.V., Gorlov N.l. (2016). Investigation of effects of longitudinal stretching of optical fibers on Brillouin hack scattering spectrum. 13th International Conference on Actual Problems of Electronic Instrument Engineering Proceedings. Novosibirsk. Vol. 1, p. 1, pp. 162-168.
6. Bogachkov I.V., Gorlov N.l. (2014). Experimental Rcscarchcs of the Transverse Pressures Influences on Optical Fibers, Brillouin Backscattering Spectrum and Strain Characteristics. 12th Internationa! Conference on Actual Problems of Electronic Instrument Engineering Proceedings. Novosibirsk. Vol. I, pp. 228-233.
7. Bogachkov I.V., Trukhina A.I, (2017). Detection of sections with slightly changed optical characteristics in fiber optical communication lines. International Siberian Conference on Control and Communications (SIBCON-2017) Proceedings. Astana, pp. 1-6.
8. Bogachkov I.V., Trukhina A.I. (2018). A Detection of Bends of the Optical Fibers by Using Hrillouin Rcflectometer. Systems of Signal Synchronization, Generating and Processing in Telecommunications (SINKHROINFO-20I8). Minsk, pp. 1-4.
9. Bogachkov I.V., Trukhina А.1., Gorlov N.l. (2018). Study of Bend Influences of Optical Fibers on Brillouin Rcflectograms. 14th Internationa! Conference on Actual Problems of Electronic Instrument Engineering Proceedings. Novosibirsk. Vol. I, p. 2, pp. 140-144.
10. Trukhina A.!., Bogachkov I.V., Gorlov N.l. (2019). Rcscarchcs ihe influence of bends in optical fibers of various kinds on the Brillouin traces. Systems of signals generating and processing in the field of onboard communications - Proceedings. Moscow, pp. 1 -5,
11. Bogachkov I.V., lnivatov D.P.. Kireev A.P., Gorlov N.l. (2018). A Determination of Optical Fibers Types on the Spectrum Profile ofthe Mandelstam - Brillouin Scatter. 14th International Conference on Actual Problems of Electronic Instrument Engineering Proceedings. Novosibirsk. Vol. I, p.. 1, pp. 317-321.
12. Bogachkov I.V., Trukhina A.I., lnivatov D.P., Kireev A.P., Gorlov N.l. (2018). A classification of optical fibers types on the spectrum profile ofthe Mandelstam - Brillouin backscattering. Dynamics of Systems. Mechanisms and Machines (Dynamics—2018} Proceedings. Omsk, pp. 1-6.
ИССЛЕДОВАНИЕ ВЛИЯНИЯ ИЗГИБОВ ОПТИЧЕСКИХ ВОЛОКОН РАЗЛИЧНЫХ ТИПОВ НА БРИЛЛЮЭНОВСКИЕ РЕФЛЕКТОГРАММЫ
Богачков Игорь Викторович, Омский государственный технический университет (ОмГТУ), Омск, Россия,
bogachkov@mail.ru
Аннотация
Для обеспечения долголетней надежной работы волоконно-оптических линий связи (ВОЛС) необходимо не только осуществлять своевременный контроль за целостностью оптических волокон (ОВ) ВОЛС (для этого используются системы удаленного контроля ОВ на основе оптических импульсных рефлектометров - OTDR), но и обнаруживать "проблемные" участки ВОЛС: участки с несанкционированным доступом (НСД), участки с повышенным механическим натяжением, участки с изменённой температурой. Наличие продольных механических натяжений в ОВ ВОЛС может привести к существенному сокращению срока службы оптического кабеля (ОК). Температурные изменения в ОВ также могут сигнализировать о проблемах на трассе прокладки ОК. Важной задачей систем мониторинга ВОЛС является своевременное обнаружение таких участков в ОВ, что позволяет принять необходимые меры по устранению проблемы до начала разрушения ОК. Системы мониторинга ОВ, построенные с использованием обычных OTDR, не способны обнаруживать участки ОВ с изменёнными температурой или натяжением. Для этого используются брюллиэновские рефлектометры (BOTDR), принцип работы которых основан на анализе спектра рассеяния Мандельштама - Бриллюэна (СРМБ). ВОЛС имеют высокую степень защищенности информации от НСД по сравнению с другими линиями связи. Несмотря на большие затраты и сложность, НСД к ОВ возможен, хотя при этом требуется непосредственный физический контакт с ОВ, так как защитные оболочки и элементы конструкции ОК существенно ослабляют боковое излучение. Во многих способах НСД используют участки ОВ с повышенным уровнем бокового излучения. Такие способы обладают высокой скрытностью, так как лишь незначительно изменяют параметры распространяющегося по ОВ сигнала. Появление побочных оптических излучений с боковой поверхности ОВ возможно при возникновении распределенных и локальных давлений на ОВ, а также на изгибах ОВ из-за преобразования направляемых мод в вытекающие. Протяженный безразрывный съем информации можно осуществить на пологом изгибе ОВ или при воздействии на ОВ низких температур, при которых происходит изменение коэффициента преломления стекла, в результате чего может повыситься уровень рассеяния. Нарушение полного внутреннего отражения при механическом воздействии (продольном растяжении) возможно не только при изгибе ОВ, но и при локальном давлении на ОВ.
С целью изучения влияния изгибов различных разновидностей одномодовых ОВ на BOTDR-рефлектограммы были проведены экспериментальные исследования с BOTDR. Изгиб представлял собой полупетлю (полуокружность) определенного диаметра, который постепенно уменьшался, при этом наблюдался СРМБ в области максимума для исследуемой разновидности ОВ при минимальном шаге по частоте. В результате на рефлектограмме СРМБ наглядно наблюдалось влияние диаметра изгиба на уровень обратно отражённого сигнала. У большинства ОВ изменения на BOTDR-рефлек-тограммах начинают наблюдаться при диаметре изгиба в форме полупетли 20 - 25 мм. Наименьшая чувствительность к подобным изгибам обнаружилась в ОВ "Panda" и разновидностях ОВ "Ultra". Наибольшая - в ОВ со смещённой дисперсией. Изменение рефлектограмм относительно эталонных позволяет сделать предположение о возможном НСД к ВОЛС. BOTDR позволяет осуществлять раннюю диагностику состояния ОВ ВОЛС, обнаруживать малозаметные способы НСД к ОВ, связанные с изменением температуры, продольным растяжением и изгибами ОВ. Включение BOTDR в систему мониторинга повышает её эффективность.
Ключевые слова: оптическое волокно, ранняя диагностика, несанкционированный доступ, мониторинг, изгиб, натяжение, бриллюэновский рефлектометр.
Литература
1. Богачков И.В., Горлов Н.И. Методы и средства мониторинга и ранней диагностики волоконно-оптических линий передачи. Омск: Изд-во ОмГТУ, 2013. 192 с.
2. Агравал Г. Применение нелинейной оптики. СПб: Лань, 2011. 592 с.
3. Богачков И.В., Трухина А.И., Горлов Н.И. Совершенствование систем мониторинга волоконно-оптических линий связи / Тр. 14-ой Междунар. конф. АПЭП. Новосибирск, 2018. Т. 3. С. 69-74.
4. Лутченко С.С., Богачков И.В. Влияние натяжения оптического волокна на коэффициент готовности ВОЛС // Системы синхронизации, формирования и обработки сигналов. М.: Медиа Паблишер, 2018. Том 9. №2. С. 125-130.
5. Богачков И.В., Горлов Н.И. Исследования влияния продольных растяжений оптических волокон на спектр бриллюэновского рассеяния // 13-ой Междунар. конф. АПЭП. Новосибирск, 2016. Т. 3, ч. 1. С. 111-117.
6. Богачков И.В., Горлов Н.И. Экспериментальные исследования влияний поперечных воздействий на оптические волокна на спектр бриллюэновского рассеяния и характеристики натяжения / Тр. 12-ой Междунар. конф. АПЭП. - Новосибирск, 2014. Т. 3. С. 124-130.
7. Богачков И.В., Трухина А.И. Выявление участков оптических волокон с измененными характеристиками // Современные проблемы телекоммуникаций: Мат. Рос. науч.-техн. конф. Новосибирск: Изд-во СибГУТИ, 2017. С. 166-172.
8. Трухина А.И., Богачков И.В. Обнаружение изгибов оптических волокон с помощью бриллюэновского рефлектометра // Системы синхронизации, формирования и обработки сигналов. М.: Медиа Паблишер, 2018. Т. 9. №2. С. 31-35.
9. Богачков И.В., Трухина А.И., Горлов Н.И. Влияние изгибов оптических волокон на бриллюэновские рефлектограммы / Тр. 14-ой Междунар. конф. АПЭП. Новосибирск, 2018. Т. 1, ч. 2. С. 140-144.
10. Богачков И.В., Трухина А.И. Изучение влияния макроизгибов оптических волокон на бриллюэновские рефлектограммы // Современные проблемы телекоммуникаций: Мат. Рос. науч.-техн. конф. - Новосибирск: Изд-во СибГУТИ, 2019. С. 177-184.
11. Богачков И.В., Иниватов Д.П., Киреев А.П., Горлов Н.И. Определение разновидностей оптических волокон по характеристикам спектра рассеяния Мандельштама - Бриллюэна// Тр. XIV-ой междунар. науч.-техн. конф. IEEE АПЭП. Новосибирск, 2018. Т. 3. С. 75-79.
12. Богачков И.В., Трухина А.И., Иниватов Д.П., Киреев А.П., Горлов Н.И. Классификация оптических волокон по профилю спектра рассеяния Мандельштама - Бриллюэна // Динамика систем, механизмов и машин. Омск: Изд-во ОмГТУ, 2018. Т. 6. № 4. С. 96-100.
Информация об авторе:
Богачков Игорь Викторович, к.т.н., доцент; доцент кафедры "Средства связи и информационная безопасность" Омского государственного технического университета (ОмГТУ), Senior Member IEEE, Омск, Россия
