Polarization mode dispersion analyses with polarization OTDRs

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Reference

Polarization mode dispersion analyses with polarization OTDRs

CHAUSSE, Eric, et al.

CHAUSSE, Eric, et al . Polarization mode dispersion analyses with polarization OTDRs. In:

13th Annual Conference on European Fibre Optic Communications and Networks . 1995.

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1 / 1

Polarization Mode Dispersion Analyses With Polarization OTDRs

£ Chousse. . Gisin, University of Geneva !Geneva, CH), Ch. Zimmer,

B.

Perny, Swiss

Telecom

PTT !Bern, CH)

Summary

Theoretical and experimental results on the potential advantages and limitations of POTDR's (Polarization OTDR's) are presented and discussed. Elliptic birefringence is considered. We show that although this technique is not suited for Polarization Mode Dispersion (PMD) measurements, it can become a valuable tool for the identification of fibre sections with specially high PMD within an optical link.

!/Introduction

Optical Time Domain Reflectometry (OTDR) is a powerful and well established tool for fibre characterization. However, it does not provide information about the polarization properties of the optical link. With the insertion of a polarizer between the OTDR and the fibre under test, local birefringence of the test fibre produces an oscillating signal. The interest comes from PMD, which is nowadays recognized as a major limiting factor for advanced optical communications. In this contribution, we consider such a set-up, called Polariwion OTDR (POTDR)[J ,2,3,4]. In the next section we give a theoretical description of the response of POTDR. In section 3, measurement on installed cables are presented and discussed.

2fi'heoretical model

OTDR' s allow the monitoring of the Rayleigh backscanering. The insertion of a polarizer in the set- up, see figure 1, between the OTDR and the fibre under test pem1its both to launch fully linearly polarize-d light and to analyze the backscattered light [5]. The model presented here considers the fibre as a concatenation of homogeneous trunks ¹ each one

1 We de tine trunk as virtual sections of fibre supporting the conceptualisation.

having a constant (possibly elliptic) birefringence.

Thus, the effect on polarized light is represented on the Poincare sphere by rotations around the birefringence axis

b ;

the periods of these rotations, called the beat lengths Lb, are fixed by the birefringence of the trunks. We assume that the local birefringence is wavelength independent, hence the local modal birefringence and the beat length are related as follows[6):

(J)

Following van Deventer [7], we assume that Rayleigh backscattering is equivalent to a reflection (like a mirror), represented by a symmetry with respect to the equator plane on the Poincare sphere.

The influence of depolarization are neglected in this simple model (i.e. we assume the coherence of the laser pulse higher than the PMD of the fibre). After backscattering, the backward path is again a rotation along the same axes, but in the opposite direction.

A composition of these three basic operations (forward propagation, reflection and backward propagation) leads to a resulting polarization state which is a function of the backscattering location.

This polarization state is then analyzed by the polarizer, producing an oscillating detected intensity, as illustrated in figure 1. For a single trunk, it reads:

where z is the fibre leng_~b_ traveled before backscattering,

p

the polarizer axis (represented on the Poincare sphere on the equator), its norm P

Paper presenied ollhe Thirleenlh Annual Conference on European Fibre Opl•c Commun•colions and Nelworks. Brighlon, Englond, 1995

186

represents the degree of polarization of light,

e,

^the

vertical direction in the Poincare sphere (i.e. circular polarized light) and

b

represents the direction of the birefringence axis, its norm B represents the delay per unit length in pslkrn (i.e. B is the modal birefringence). Figure 2 shows the evolution of the polarization state on the Poincare sphere in the simplest case of a single trunk. Note that even for such a case, the curve is not a simple circle.

Formula (2) shows that both the beat length and its half are involved in the oscillations (due to the square on the right hand side). The apparition of a double frequency could simplify the identification of beat lengths. However, with different orientations of the polarizer, it is possible to obtain either one, or the other, or both frequencies. The above formula is valid for a single trunk. For realistic cases of several trunks, the formula generalizes, as illustrated below.

A simulated POTDR trace of an optical link divided into 6 trunks of homogeneous fibres is presented in figure 3. Variations of the mean backscattered intensity between different trunks of fibre can clearly be seen. Figure 4 shows that the beat lengths of the 6 trunks can be recovered from the simulated POTDR trace of figure 3. This was achieved by simple local Fourier transform analyses.

The encouraging result shown in figure 4 must, however, be balanced against the resolution problem of POTDR, which is the major limiting factor. Let us first analyze the problem qualitatively. On the one hand side, it is clear that fibres with very low PMD have smooth POTDR traces, without oscillations. On the other side, ftbres with high PMD have short beat lengths Lb, hence the oscillations are rapid and insufficient resolution will fade out the oscillations.

Hence in both cases of low and hiah PMD the POTDR traces are smooth. This

an~lysis ca~

^be

made more quantitative. Let's remember that PMD and beat lengths are related as follows (8] (assuming that the local birefringence is essentially wavelength independent, a pretty good approximation for real fibres):

( )

2

2 I t..h 21 _!!_

PMD =- -₂ (--l+e

h)

cL₀ h ⁽³⁾

where I is the fibre length and h is the mean polarization mode coupling length (more precisely, h is half the correlation length of the flucruating local birefringence). The relation between PMD and Lb depends thus on the usually unknown length h. The value of h ranges from tens of meters for fibers on small spools, up to kilometres in some cables. The table below relates Lb to PMD and the mean coupling length h for a 10 km long fibre.

Table I illustrates that even for a 5 ns pulse and h= l km, only PMD smaller than l ps for a 10 km fibre can be identified with the POTDR technique.

Lb (m) Mean Coupling Length, h (km)

PMD (ps) 0.1 0.5 l 2

0.1 51 113 158 218

0.5 10 23 31 44

I 5 II 16 21

5 1 2 3 4

Table I: The Beat Length expressed 111 metres for l)'plcal values of PMD and Mean Coupling Lengths for a fibre length of 10 km at 1..=1,5 11m.

3/Measurement results

Fields experiments have been done on a 23 km long installed cable with a relatively high PMD value, around 23 ps. This cable consists in 3 sections connected together. Figure 5 compares an OTDR trace with and without inserting a polarizer in the set- up. The effect of the polarizer can clearly be seen.

Especially, the intensity can become higher at a farther distances, interrupting thus the decreasing profile of classic OTDR traces. This is a signature that polarization effects really occur in this cable.

Two curves for different polarizer orientations are displayed in figure 5. The aim was to maximize the differences of oscillations between the two curves in order to illustrate the polarization effect.

However, the oscillations seen on the curves are not due to the birefringence effect like those presented in the simulation. Indeed, the poor resolution of POTDR coming from a large pulse (I 00 ns) smoothes the rapid variations. Consequently, the oscillations seen in section 3 are the envelope of the rapid oscillations illusrratcd by a simulation like in figure 3. They are thus linked to the mean coupling length h. This could be confirmed by measuring the PMD of each section: the second one has the lowest PMD, and the third one, the highest. By switching to another fibre in section 3, the problem can be solved in a pragmatic way.

4/Conclusjon

Presently, POTDR does not allow the measurement of PMD. First, because the resolution limits the shortest measurable beat length, hence the highest measurable local modal birefringence. Next, because PMD depends also on the mean coupling length h, which is difficult (though in principle not impossible) to measure with this technique.

However, POTDR appears as a promising tool to find out specially bad sections in a cable. This is quite interesting, since it happens that high PMD values are due to only a fe.w bad sections of a cable, or even to a single bad section.

51 References

[l] A.J. Rogers, "Polarization Optical Time Domain Reflectometry: a technique for· the measurement of fteld distributions", Appl. OPT.,-vol. 20, no 6. pp.

1060-1074, 1981

Paper p1c5ented at/he Th~rleenlh Annual Conference on European f1bre OptiC Commumcolions ond Ne/'l.tvorks, Brighton, f11gland, /995

187

(2) T. Kuwub~ra, H. Koga, Y.Kato, Y. Mistunaga.

"Optical Fiber Identification Using Rayleigh

Back.sc~ cred Light", Phownics Techno!. Len., vol.

4,n• 12.pp. 750-753,1992

(3] A. G:!.harossa, G. Gianello, C.G. Someda and M.Schiano. ·'Stress Investigation in Optical Fiber RJbbon Cable by Means of Polarization Sensitive Techniques", Photonics Techno!. Len., vol.. 6, n• I 0, pp. 12;2-1234, 1994

[4) A. Tardy, M. Jurczyn, F. Bruyere, M. Hertz, J.L.

Lang, "Fiber PMD analysis for optical-tiber cable using polarisation OTDR ', in OFC'95 Technical Digest, ThD2, pp. 236-239

[5) M. ~akazawa "Theory of Backward Rayleigh Scanerim! in Polarization-Maintaining Single-Mode Fibers a~d its Application to Polarization Optical Time Domain Reflectrometry", J. Quantum Electron.

vol. 19, nG 5, pp. 854-86 I, 1983

[6] R. P2ssy, N. Gisin J.P. Pellaux and P. Stamp, ''Simul12neous Measurements of Beat Length and Polarization Mode Dispersion with the interferometric Technique", in Conference Digest of OFMC 91, York, UK, pp. 85-88

[7) M.O. van Deventer, "Polarization Propenies of Rayleigh Backscanering in Single-Mode Fibers'.

J. Lightwave Techno!., vol. .II, n° 12, pp. 1895- 1899, 1993

(8) N. Gisin, R. Passy and J.P. von der Weid,

"Definitions and Measuremems of Polarization Mode Dispersion: Interferometric versus Fixed Analyzer Methods", Photonics Techno!. Len., vol. 6, n° 6, pp. 730-732, 1994

Polarizer

Figure I: Experimental set-up

Figure ^2: Trace of the backscanered polarization state vector before the analyzer. The vector of birefringence.

6

is (J,O,J) and the vector of the polarizer, pis [1.1.0) and the curve obtained on the sphere is not a simple circle

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Figure 3: Simulation of an optical linK composed of6 different high birefringence fibres (both orientatiorHlnd charact~ristic>J

Losses are not taken into account.

Paper presenled ollhe Thirteenlh Annual Conference on European Fibre Oplrc Commumco11ons ond Networks. Brighton, fnglond, /995

188

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Figure 4: Anempt to identify the beat length of the optical link simulation presented in figure 3, which are from left to right 50, 20, 40, 25, 40 and 20 metres. Good agreement is obtained but for the last fibre, only half of the beat length is measured.

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Paper presented ollhe Thirleenlh Annual Conference on European Fibre Optic Communicolions and N!tworks, Brighton, England, 1995

189

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Thirteenth Annual Conference on

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ACKNOWLEDGEMENTS

The European lns~tule for Communications and Networks !EICN) would like to express its appreciation for the invaluable participation and contribu~ons of the T echnicol Programme Committee and Chairpersons, the Technical Advisory Committee, Speakers and Authors.

THE TECHNICAL PROGRAMME COMMITTEE/CHAIRPERSONS:

is responsible for maintaining a high quality technical conference and promoting topical sessions within this conference.

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ore valued for their support and advice

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