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CORRELATION OTDR FOR TRANSOCEANIC CABLES

Ruggeri Stephane , Ait Sab Omar, Mongardien Dominique

(Alcatel-Lucent Submarine Networks)

Email: Stephane.ruggeri@alcatel-lucent.com

Alcatel Lucent Submarine Networks, route de Villejust, 91620 Nozay, France.

Abstract: A C-OTDR system permits submarine operators to monitor the continuity of their repeatered links. C-OTDR based on correlation techniques is a reliable and powerful tool to analyze and locate rapidly a cable fault, such as a broken fiber, with high level of precision. This functionality has been integrated into the submarine line terminating equipment for deployment on submarine cables. With the cooperation of a cable owner, Alcatel Lucent recently installed an integrated C-OTDR solution on an in-service cable system. Tests of the solution demonstrated the C-OTDR capability with a reach of up to 12 000 km. To achieve this performance, a very low noise technology combined with a very fast digital signal processing delivers different signatures using a range of binary sequences. In addition, this paper presents the results of an experiment using both repeated and unrepeatered sections with Raman amplification.

1 INTRODUCTION

Coherent Optical Time Domain Reflectometer (Coherent OTDR) helps submarine systems operators to monitor their submerged transmission lines. COTDR is mainly used for fault detection and localization. Coherent OTDR has a good sensitivity (compared to traditional OTDR) and allows the fiber attenuation measurement even with a high level of noise. To achieve this sensitivity, Coherent OTDR uses a complex and thus expansive coherent technology which requires a very narrow light source. For a very long submarine network, the Coherent detection shows some limitations due to the loss of backscattered light coherence. To analyze transoceanic cables, we really need to monitor and localize any fiber default over up to 12000kms with the same dynamic and performance whatever the distance. This paper introduces Correlation OTDR (C-OTDR) concept. C-OTDR is based on

direct detection and uses binary sequences having appropriate and good correlation properties. Therefore C-OTDR presents a very good sensitivity and has no performance limitation versus the length of the link. In this paper we demonstrate the performance of the C-OTDR detection over an 11500kms cable recently activated. In addition we have also tested the C-OTDR over a hybrid transmission system which concatenates a Raman amplified section with several EDFA amplified ones.

2 DESCRIPTION OF THE CORRELATION OTDR

The C-OTDR transmission signal is inserted at the 5% input of the amplifier at the transmission side. The received signal is recovered from the 1% output monitoring of the amplifier at the reception side. This configuration depicted in Figure 1 has therefore no impact on an existing system. The results are available from the first acquisitions and the details can be

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read (with or without smoothing) the measurement which has no time limit.

Figure 1

3 FIELD TRIAL OVER 11500KMS TRANSOCEANIC CABLE.

With a cooperation of a cable owner, awas carried out on a submarine of 11500Km composed of 180 repeaters. The goal was to characterize the 180 spanof +D/-D fiber in minimum of time and to simulate a fiber cut by shutting down the last repeater. The cable was loaded with dummy channels and working channels stabilizing the line correctly. The OTDR signal is positioned between the working channels and the dummy channel on a 50GHz grid as shown in Figure 2. After 39 hours of analysis, the result shresponse of the cable with the same visibility of the line at the beginning than at the end, thus demonstrating the effectiveness of the correlation detection technique which appears to be independent of the distance.

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smoothing) during the measurement which has no time limit.

11500KMS TRANSOCEANIC CABLE.

cooperation of a cable owner, a test submarine line cable

500Km composed of 180 repeaters. the 180 spans

in minimum of time and to simulate a fiber cut by shutting down the

. The cable was loaded with dummy channels and working channels stabilizing the line correctly. The OTDR signal is positioned between the working channels and the dummy channel on a 50GHz grid as shown in Figure 2. After 39 hours of analysis, the result showed the response of the cable with the same visibility of the line at the beginning than at the end, thus demonstrating the effectiveness of the correlation detection technique which appears to be independent

Figure 2: spectra example transmission side

Figure 3 shows the 180 spans11500kms. Figure 4 shows a zoom on the first spans while figure 5 shows a zoom on the last spans localized at 11500kms.

Figure 3: full line

Figure 4: first spans

Figure 5: last spans

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example at the

transmission side.

180 spans over shows a zoom on the

shows a zoom on 11500kms.

Figure 3: full line

Figure 4: first spans

Figure 5: last spans

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The figure 6 presents the last spans attenuation when we had simulatecut by shutting down the last repeater. this case, as the repeater receivedtraffic, it had increased its gain which amplify the back scattering signal 12dB. This additional gain reduceconsiderably the detection time. In this example, a non reflective fiber cut is detected with a good accuracy at the end of the line in less than 10 minutes.

Figure 6: last span fiber cut: <10 minutes measurement

The next figure shows a zoom on the last span where we can measure precisely the distance by using cursor positions. information used for the measurement given on the specific viewer on which we can access simultaneously to raw and the smooth trace.

Figure 7: measurement on last span using Alcatel-lucent Viewer.

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the last spans simulated a fiber

cut by shutting down the last repeater. In d no

its gain which amplify the back scattering signal by about

gain reduced me. In this

fiber cut is detected with a good accuracy at the end of

10 minutes

zoom on the last precisely the

distance by using cursor positions. All used for the measurement is

given on the specific viewer on which we access simultaneously to raw and the

re 7: measurement on last span using

4 C-OTDR USED ON UNREPEATERED SPAN CONCATENATED WITH REPEATERED SPANS

A test was carried out over a line of 13spans, concatenated with an unrepeatRaman amplified span. This configuration is generally used at the beginning or at the end of the line to optimize the design of concatenated terrestrial section with submarine cable. The link was made byLS fiber sections, one RS with effective area fiber section and four fiber sections. This test introduced the same constraints in terms of OSNR and perfectly simulates the characteristicslong cable. But the purpose of this experiment is to show that a Raman amplified section with the fiber integrity monitoring signal has no impact on OTDR performances and onanalysis results. The figurepresents the 13 spans and the Raman amplified section. The results show that the C-OTDR can be used for this type of architecture. The trace of the last 90kms is given by the 3rd order Raman amplification.

Figure 7: 13 repeatered spansbackward Raman amplification

The figure 8 presents the C_OTDR traces showing the 13 spans. The last is the Raman amplified section.is representing the expected responsethe total link of 1100kms and ttrace is identical to the one without Raman amplified segment

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UNREPEATERED SPAN WITH

SPANS.

as carried out over a line of 13 an unrepeatered

span. This configuration is generally used at the beginning or at the

line to optimize the design of concatenated terrestrial section with

The link was made by 8 one RS with Large

and four +D/-D introduced the

same constraints in terms of OSNR and characteristics of a

le. But the purpose of this is to show that a Raman

fiber integrity has no impact on our C-

on the cable The figure 7 below spans and the Raman The results show that the

OTDR can be used for this type of the last 90kms is order Raman

spans + 90kms

aman amplification span.

presents the C_OTDR traces . The last 90kms span

section. The trace the expected response over

the total link of 1100kms and the 13 spans is identical to the one obtained

gment.

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Figure 8: 14 spans OTDR trace using an unrepeated Raman amplified section.

The C-OTDR requires the setting of working wavelengths, the desired resolution, the cable length and the output power of the signal. The wavelengths are full C + band tunable with 50GHz grid.There is no time limit for the analysis and results can be accessed remotely at any time without interrupting the measurement. The visualization tool allowssimultaneous display of two traces on which the distances and attenuations aremeasured with a great accuracy;feature is very useful when the operator needs to compare a new characterisation of the line with a saved baseline.

5 CONCLUSION

The C-OTDR has been deeply assessed and with the cooperation of a cable owner, a link of 11500kms was successfully characterized. In our example, we demonstrate the effectiveness of this technology to properly analyze such a cable and also to detect after 11500kms a fiber break with high accuracy in a very short time. In a second phase, we also demonstrate that the 3rd order Raman amplified section does not interferthe analysis of the C-OTDR.

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OTDR trace using an section.

setting of , the desired

resolution, the cable length and the output The wavelengths are

full C + band tunable with 50GHz grid. There is no time limit for the analysis and

remotely at any time without interrupting the measurement. The visualization tool allows the simultaneous display of two traces on which the distances and attenuations are measured with a great accuracy; this

very useful when the operator a new characterisation of

OTDR has been deeply assessed and with the cooperation of a cable owner, a link of 11500kms was successfully characterized. In our example, we

ness of this technology to properly analyze such a cable and also to detect after 11500kms a fiber break with high accuracy in a very short time. In a second phase, we also demonstrate that the 3rd order Raman

section does not interfere with

6 REFERENCES

[1] M. Nazarathy, S. A. Newton, “RealTime Long Range Complementary Correlation Optical Time Domain Reflectometer”. Journal of lightwave technology, Vol 7 N°1, January 1989.

[2] O. Gautheron, J. B. Leroy, and P. Marmier, “COTDR Performance Optimization for Amplified Transmission Systems” IEEE Photonics Technology Letters, vol.9, N°7, July 1997.

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M. Nazarathy, S. A. Newton, RealTime Long Range Complementary

Correlation Optical Time Domain Journal of lightwave

Vol 7 N°1, January 1989.

O. Gautheron, J. B. Leroy, and P. COTDR Performance

for Amplified Transmission IEEE Photonics Technology

Letters, vol.9, N°7, July 1997.