A novel method of polarization dispersion measurements using an interferometric loop is presented. It can be carried out using a particularly simple set-up and provides a representation of the probability distribution of the polarization dispersio

Robert, P.

Thévenaz, Luc

Infoscience - École polytechnique fédérale de Lausanne

Simple Method for Polarization Dispersion Measurements inLong Single-Mode FibresLuc Thévenaz, Philippe RobertEPFL, Swiss Federal Institute of Technology, Metrology Laboratory, 1015 Lausanne, SwitzerlandAbstract: A novel method of polarization dispersion measurements using an interferometric loop is presented.It can be carried out using a particularly simple set-up and provides a representation of the probability distributionof the polarization dispersion.IntroductionThe study of polarization dispersion in standard single-mode fibres takes on a growing importance,because the delay between polarization modes causes pulse broadening and may becomepredominant in multigigabit long-span optical links. The combined effects of weak birefringence andfluctuating mode coupling result in a stochastic behaviour of the polarization dispersion and of theoutput polarization state. The so-called principal states, a particular set of orthogonal polarizationstates, experience the greatest difference in propagation time and are therefore very convenient forthe description of polarization dispersion [1]. Recent contributions use the properties of these statesto demonstrate that the evolution of polarization is quite similar to a random walk and the probabilitydensity function of the fluctuating polarization dispersion can therefore be described by a Gaussian-like distribution [2,3].Several results have been reported so far on the measurement of polarization dispersion in longsingle-mode fibres, mainly using a technique analysing pathes over the Poincaré sphere [4,5]. Thistechnique requires a high control of the laser source wavelength and the determination of the outputpolarization state using analyzing elements. Measurements without using any polarizing element canbe performed by an interferometric technique [6], which is of little use for the study of randomcoupling, because it remains restricted to meter-length samples. In this contribution we present anovel method which allows the measurement of polarization dispersion in long single-mode fibresusing a particularly simple set-up, without polarizing elements or retardation plates. Furthermore itmakes the evaluation of the probability density function of polarization delay possible.Basic principlesA schematic diagram of the experimental set-up is shown in Fig.1. It simply consists of a choppedwhite-light source spectrally filtered by a monochromator, a wavelength-independent single-modeLampIFig. 1 Schematic diagram of the experimental set-up.ECOC ‘90 - 5810.01100Cu0.2Li4-C’,0.1 C)4-E0.01. 0.2 0.4 0,6 0.8 1.0 1.2Polarization D&ay, PS4- U.UO0.050.04. 0.030.020.01E< 0.000.0 0.2 0.4 0.6 0.8Polarization Delay, psFig.3 Fourier transform of the detected intensities shown in Fig.2, representing the distribution ofpolarization delay, and corresponding Gaussian fit with its standard deviation.As mentioned above the succession rate of interferences with increasing wavelength is directlyrelated to polarization dispersion, so that a Fourier transform performed over the detected intensitiesas a function of the optical frequency yields a representation of the probability density function of thepolarization delays. The result of such a transform is shown in Fig.3. The stochastic nature of the delayis confirmed and the numerical fitting shows that the observed distribution is compatible with aGaussian function. On the other hand a measurement on a short sample (3m), for which modecoupling is negligible, shows a regular succession of interferences and therefore the absence of astochastic behaviour, as shown in Fig.4.The minimum observable delay is limited by the spread of the investigated spectral range, becauseat least one interference succession must be measured, that is:3r> _.2jnaxAmin_C max - mmwhere min 2max are the lower and upper bounds of the investigated spectrum, respectively. On theother hand the maximum observable delay is bounded by the source spectral width, because it must1.20.80,21200 1300 1400 1500 1600 0.0 0.2 0.4 0.6 0.8Wavelength, nm Polarization Delay, PSFig,4 Detected intensity at the inter’ferometric loop output for a 3-rn sample of standard single-modefibre showing no polarization fluctuation, and corresponding Fourier transform which reduces toa single peak in this case.1.0ECOC 90 - 583remain shorter than the coherence time in order to observe interferences. HenceBwhere 4A. is the spectral width and 2. the center wavelength. When this bound is reached, theinterference contrast becomes very poor and the measurement can therefore at once be discarded.With our set-up (amin—l000nm, Amax’’17OOnm, zlA.=4nm) the absolute measurable delay isbounded this way:O.OO8ps<&r< 1.4psConclusionThis novel method of measuring polarization dispersion in long single-mode fibres differs from othertechniques by its simplicity and easy handling. Furthermore it enables a straightforward evaluation ofthe statistical properties of the polarization delay of a given fibre. This method promises to become animportant tool for the investigation of polarization properties in single-mode fibres and other guided-wave optical devices.Acknoledgements: The authors are grateful to P.A.Nicati for helpful comments and to OPG SA,Câbleries de Cossonay SA and Oerilkon-BUhrle SA (project CERS 1790) for their support.References1. C.D.Poole, R.E.Wagner, “Phenomenological approach to polarisation dispersion in long single-mode fibres”,Electron. Letters, 22 (1986), pp.1029-1030.2. C.D.Poole, “Statistical treatment of polarization dispersion in single-mode fiber”, Optics Letters, 13 (1988),pp.687-689.3. F,Curti, B.Daino, G.de Marchis, F.Matera, “Statistical treatment of the evolution of the principal states ofpolarization in single-mode fibers”, IEEE J. Lightwave Technol., LT-8 (1990), in press.4. D.Andresciani, F.Curti, F.Matera, B.Daino, “Measurement of the group delay difference between the principalstates of polarization on a low-birefringence terrestrial fiber cable”, Optics Letters, 12 (1987), pp.844-846.5. C.D.Poole, N.S.Bergano, R.E.Wagner, H.J.Shulte, “Polarization dispersion and principal states in a 147-kmundersea lightwave cable”, IEEE J. Lightwave Technol., LT-6 (1988>, pp.1 185-1190.6. L.Thévenaz, J.P.Pellaux, N.Gisin, J.P.von der Weld, “Birefringence measurement in fibers without polarizer”,IEEE J. Lightwave Technol., LT-7 (1989), pp.1207-1212.7. J.P.von der Weld, L.Thévenaz, J.P.Pellaux, “Interferometric measurements of chromatic and polarizationmode dispersion in highly birefringent single-mode fibres”, Electron. Letters, 23 (1987>, pp.1 51-152.ECOC 90 - 584

Simple method for polarization dispersion measurements in long single-mode fibres

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Simple method for polarization dispersion measurements in long single-mode fibres

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