Birefringence assessment of single-mode optical fibers

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1 Birefringence assessment of single-mode optical fibers Fernando Treviño-Martínez Universidad Autónoma de Nuevo León, FIME, Ciudad Universitaria, San Nicolás de los Garza, N.L., México Diana Tentori, César Ayala-Díaz, Francisco Javier Mendieta-Jiménez Centro de Investigación Científica y de Educación Superior de Ensenada, Div. Física Aplicada, km 107 carretera Tijuana-Ensenada, Ensenada, B.C., México diana@cicese.mx, cayala@cicese.mx, jmendiet@cicese.mx Abstract: Using the Poincaré sphere and wavelength scanning it is possible to determine if the fiber birefringence corresponds to that of a linear, circular or elliptical retarder, as well as to obtain an approximate measurement of the polarization beatlength. This method is useful for low birefringence single-mode fibers. It is applied to erbium-doped fibers Optical Society of America OCIS codes: ( ) Fiber characterization; ( ) Fibers, erbium References and links 1. A.M. Smith, Automated birefringence measurement system J. Phys. E 12, , S. Lacroix, M. Parent, J. Bures, J. Lapierre, Mesure de la biréfringence linéaire des fibres optiques monomodes par une méthode thermique, Appl. Opt. 23, , A. J. Barlow, Optical-fiber birefringence measurement using a photo-elastic modulator, J. Lightwave Technol. LT-3, , R. Ulrich, A Simon, Polarization optics of twisted single-mode fibers, Appl. Opt., 18, , M. Monerie, P. Lamouler, Birefringence measurement in twisted single-mode fibres, Electron. Lett. 17, , H.Y. Kim, E. H. Lee, B.Y. Kim, Polarization properties of fiber lasers with twist-induced circular birefringence Appl. Opt. 36, , E.A. Kuzin, J.M. Estudillo Ayala, B. Ibarra Escamilla, J.W. Haus, Measurements of beat length in short low-birefringence fibers Opt. Lett. 26, , T. Chartier, A. Hideur, C. Özkul, F. Sanchez, G. M. Stéphan, Measurement of the elliptical birefringence of single-mode optical fibers, Appl. Opt. 40, , M. Wegmuller, M. Legré, N. Gisin, Distributed beatlength measurement in single-mode fibers with optical frequency-domain reflectometry, J. Lightwave Technol. 20, , C. Tsao, Optical Fibre Waveguide Analysis (Oxford University Press, New York, p.101, 1992). 11. K. Kikuchi, T. Okoshi, Wavelength-sweeping technique for measuring the beat length of linearly birefringent optical fibers, Opt. Lett. 8, , S.C. Rashleigh, Measurement of fiber birefringence by wavelength scanning effect of dispersion, Opt. Lett. 8, , T.I. Su, L. Wang, A cutback method for measuring low linear fibre birefringence using an electro-optic modulator, Opt. Quantum Electron. 28, , W. Eickhoff, Y. Yen, R. Ulrich, Wavelength dependence of birefringence in single-mode fiber Appl. Opt. 20, , V. Ramaswamy, R.D.Standley, D.Sze, W.G.French Polarization effects in short length, single mode fibers Bell Sys. Tech. J. Vol. 57, No.3, , D. S. Kliger, J. W. Lewis, C. E. Randall, Polarized Light in Optics and Spectroscopy (Academic Press, Inc., San Diego 1990). 17. S.C. Rashleigh, Origins and control of polarization effects in single-mode fibers, J. Lightwave Technol. LT-1, , D. Tentori, C. Ayala-Díaz, F. Treviño-Martínez, M. Farfán-Sánchez, F.J. Mendieta-Jiménez, Birefringence evaluation of erbium doped optical fibers Proc. SPIE, 5531, , B. L. Heffner Accurate, Automated Measurement of Differential Group Delay Dispersion and Principal State Variation Using Jones Matrix Eigenanalysis, IEEE Photon. Technol. Lett. 5, , (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2556

2 1. Introduction Real fibers present various internal perturbations, such as core ellipticity and internal stress that cause characteristic differences of the propagation constants of the orthogonal polarization modes. For short distances where statistical polarization mode coupling can be neglected, the polarization properties of single mode fibers are often described by its phase birefringence, characterized by the birefringence beat length L b. In addition to this parameter, to be able to describe the evolution of polarization along the single-mode fiber it is almost generally assumed that the fiber residual birefringence is linear [1-3]. Linear birefringence is considered to be so dominant that circular birefringence or fiber twist are completely neglected and, only when the fiber is twisted it is assumed that in addition to the residual linear birefringence there is a circular birefringence contribution. When the twisting rate is high it is considered that circular birefringence can become dominant [4,6]. Another approach used by some authors is to treat the fiber as an elliptic retarder [7-10]; i.e. to assume that the fiber presents simultaneously linear and circular birefringence. Circular birefringence can be produced by optical activity and/or axial rotation (twist). In this work we use the Mueller matrix formalism and wavelength scanning to identify the type of retarder suitable to describe the fiber birefringence, as well as to determine the approximate value of the polarization beat length. Wavelength scanning has been previously used to evaluate the beat length of high birefringence fibers with a low birefringence dispersion [11,12]. We work with low birefringence fibers with high birefringence dispersion (erbium-doped fibers). We present experimental results of the assessment of the residual anisotropy of two commercial erbium-doped fiber samples. Fig. 1. Optical set-up used to work with the Poincaré sphere. Monochromatic signals come from a tunable diode laser. The azimuth angle of the input linear polarization is modified rotating the input linear polarizer. The Stokes vector of the output state of polarization is measured by a polarization analyzer. 2. Cutback and wavelength scanning In the experiment, the input polarization is fixed (it is usually linear). When the length of the fiber is varied (cutback technique) or the input signal wavelength is modified (wavelength scanning method) the output polarization state moves along some trajectory on the Poincaré sphere [11-13]. The phase change introduced by the birefringence is 2πs Φ= n ; (1) λ where n is the birefringence (linear, circular, or elliptical), λ is the signal wavelength and s is the fiber length. For Φ = 2π the input state of polarization is restored, and the length s = L b associated to this phase change is the polarization beat length. Equation (1) shows us that cutback and wavelength scanning methods produce similar results if the wavelength dependence of birefringence can be neglected. To satisfy this requirement in this work we use closely spaced signal wavelengths to perform the measurements [14]. In this case wavelength scanning is a better option because in practice it is easier to keep the same orientation of the fiber. In addition to it, it is a non-destructive technique. (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2557

3 3. Birefringence assessment using the Poincáre sphere Since the characterization of the ellipticity of the output radiation alone does not supply enough information on the birefringence parameters of the sample [15], numerous analytical and graphic methods have been devised, based on the evolution of polarization under different conditions. Among them, the graphic methods permit a clear insight, even in complex situations. In this section, we simulate the trajectory described on the Poincaré sphere by the output polarization state, when the signal input polarization is linear (Fig. 1). We assume that the cutback technique or the wavelength scanning method have been used to modify the phase change [Eq. (1)]. To work with the Poincaré sphere we represent the electric field using Stokes vectors and the fiber birefringence is described using Mueller matrices. A linearly polarized signal from a monochromatic light source is launched into the single-mode fiber sample. At the fiber output, the Stokes vector of the signal is S out = M S in, (2) where M is the Mueller matrix (fast axis azimuth equal to zero) that describes the fiber birefringence and the input linearly polarized signal with azimuth angle ϕ is ( 1 cos2 sin2 0) t S = ϕ ϕ ; (3) in where t indicates transpose. The explicit form of matrix M is given in Table I for each type of retarder. Substituting Eq. (3) and the proper Mueller matrix in relation (2), we find that for each type of retarder the output state of polarization is given by the relations shown in Table II. Equations (4) to (6) in Table II were used to simulate the path described on the Poincaré sphere when the fiber length or the signal wavelength are modified; i.e. when the phase retardation changes. The results we obtained are shown in Figs. 2 to 5. Table I. Mueller matrices of the retarders used to describe the birefringence of single-mode fibers Linear retarder with zero azimuth axis; total retardation γ [16] M = 0 0 cosγ sinγ 0 0 sinγ cosγ Circular retarder (right or left); total retardation θ [16] cos θ ± sin θ 0 M = 0 sin θ cos θ Elliptical retarder with zero azimuth axis, angle between retardations σ, total retardation δ [ 10] cos σsin δ cos σsin 2δ sin 2σsin δ M = 0 cos σsin 2δ cos 2δ sin σsin 2δ sin 2σsin δ sin σsin 2δ 1 2sin σsin δ (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2558

4 Table II. Stokes vectors at the output of the fiber sample Linear retarder 1 cos 2ϕ S out = (4) cos γsin 2ϕ sin γsin 2ϕ Circular retarder (left or right) 1 cos( 2ϕ θ) Sout = (5) sin ( 2ϕ θ) 0 Elliptical retarder cos 2ϕ( 1 2cos σsin δ) sin 2ϕcosσsin 2δ S out = (6) cos 2ϕcos σsin 2δ+ sin 2ϕcos2δ 2 cos 2ϕsin 2σsin δ sin 2ϕsin σsin 2δ Linear retarder. As the phase shift γ between the two polarization modes of a fiber sample with linear birefringence is increased, only components S 2 and S 3 in Eq. (4) are modified. They describe a circle of radius sin 2ϕ, centered on (S 1,S 2,S 3 ) = (cos2ϕ,0,0); i.e. it lies on a plane perpendicular to the axis labeled as S 1. When ϕ = ±45 the radius takes its maximum value; while for ϕ = 0 or ±90, the radius of the circle is equal to zero for any value of the retardation angle γ (i.e. for any fiber length or for any signal wavelength). Fig. 2 presents the results obtained for different orientations of the input polarization state. We used continuous lines for ϕ = 5, 10, 15, 20, 25, 30 and 45, and dotted lines for ϕ = 65, 70, 75, 80 and 85. In particular, for ϕ = 0, S out = (1,1,0,0) t and, when ϕ = ±90, S out = (1,-1,0,0) t. These two opposite points on the Poincaré sphere are the intersections of the symmetry axis of any trajectory with the Poincaré sphere and correspond to the principal polarization states for a linear retarder with azimuth angle equal to zero [17]. Fig. 2. These circular paths describe the evolution of the polarization state for a linear retarder. Each circle corresponds to a different azimuth angle of the input linear polarization. The symmetry axis lies on the equator plane. The fiber sample is aligned with the laboratory system. Fig. 3. This major circle describes the evolution of the polarization state for a circular retarder. This trajectory does not depend on the azimuth angle of the input linear polarization and/or the fiber sample. The symmetry axis crosses the Poincaré sphere through the north and south poles. Circular retarder. For a circular retarder, as the retardation δ is increased, the output Stokes vector describes a major circle that matches the equator of the Poincaré sphere (Fig. 3). We can notice from Eq. (4) that S 3 = 0 and S S 2 2 = 1 for any value of ϕ. Hence, this path (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2559

5 does not depend on the orientation of the linear input polarization state of the signal with respect to the sample. The symmetry axis of this trajectory crosses through the north and south poles of the sphere (principal polarization states for a circular retarder) [17]. Elliptical retarder. For an elliptical retarder the path described on the Poincaré sphere by the polarization state of the signal as it propagates along the fiber, is also a circle (fig. 4). These circular paths share a common axis of symmetry. To analyze these trajectories [Eq. (5)] it is convenient to use the rotated axes (S 1, S 2, S 3 ), where the rotation angle with respect to the S 2 axis is (π/2 + σ). In terms of the rotated coordinates, the position on the Poincaré sphere of the output state of polarization is cos 2ϕsin σ S ' = cos 2ϕcos σsin 2δ+ sin 2ϕcos 2δ. (7) sin 2ϕsin 2δ cos2ϕcosσcos 2δ As the signal propagates along the fiber, the change in the retardation angle δ modifies components S 2 and S 3 in Eq. (7). Since S S 3 2 is constant, the trajectory is a circle. The radius of the circular path is r = cos 2ϕ cos σ+ sin 2ϕ. (8) The inclination of the axis of symmetry of these circular paths depends only on the value of σ (tanσ is the linear to circular retardation ratio [10].) Since for a given fiber sample the value of σ remains constant, we can see from Eq. (8) that for a specific fiber, the radius of each circular trajectory will be determined by the azimuth angle of the input linear polarization. This result is illustrated in Fig. 4. In this figure we can notice that when the input signal is linearly polarized, the minimum value for these radii is obtained for ϕ = 0 and using Eq. (8) we get that r min = cos σ. For the simulation in fig. 4, σ = 10 and, we used continuous lines for radii ranging from 0 to 40 (10 steps) and dotted lines when ϕ varied from 70 to 85 (5 steps). Figure 5 illustrates the change in the value of the minimum radius (ϕ = 0 ) when the elevation angle ζ, between the symmetry axis and the equator plane, varies from to 5 to 30. We can also notice from Eq. (8) that for ϕ = 45 the trajectory would be a major circle. Fig. 4. These circular trajectories describe the evolution of the polarization state for an elliptical retarder. The inclination angle between the symmetry axis and the equator plane depends on the circular to linear birefringence ratio of the fiber sample. The fiber sample is aligned with the laboratory system. Fig. 5. Fiber sample with elliptical retardation. Each circle corresponds to a different circular to linear birefringence ratio tanσ [(π/2 - σ) = ζ = 5 to 30 ]. For the trajectories shown in this figure, the azimuth angle of the input linear polarization state is ϕ = 0. (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2560

6 4. Experiment Using the set-up shown in Fig. 1, carefully aligned following the procedure reported in reference 18, we scanned the sampling wavelength from λ = 1511nm to 1571nm using 6nm steps. The erbium-doped fiber sample was kept straight without introducing any torsion. The elongation was controlled using a ~2Nt weight [18]. We used two commercial single-mode erbium-doped fibers: Photonetics EDOS 103 (length 1.63 m) and INO-NOI 402K5 (length 1.61 m). The path described by the output Stokes vectors obtained for each wavelength scanning experiment is shown as a curve (continuous or dotted) in Figs. 6 to 9. In these figures we used the same color for equivalent orientations of the input linear polarization azimuth angle (such as 0, 180, 360 ). In Fig. 6 the results for ϕ = 10 to 40 (10 steps) are represented by continuous lines and we used dotted lines for input azimuth angles between 190 and 220 (10 steps). When the experiment was finished the sample was removed from the optical set up and allowed to hang free, holding it from one connector. Every time the same fiber end was placed at the input position. The lightwave polarization analyzer requires an operation wavelength to define the reference frame (laboratory system used to define the azimuth angle of the polarization state); since we are working with several optical wavelengths within a wide spectral band, to minimize systematic errors we defined the reference frame using a wavelength in the center of our working range (λ = 1541nm). The degree of polarization of the output signals varied between 97% and 100%, hence the depolarizing effect of absorption through the subsequent fluorescent emission (typical of erbium-doped fibers) is negligible. Despite the fact that the wavelength scanning results shown in Figs. 6 and 8 for some azimuth angles of the input linear polarization show clearly that both erbium-doped fibers behave as elliptical retarders, these results do not match the theory. A closer analysis of the trajectories followed by the output state of polarization shows that the paths described on the Poincaré sphere are not plane curves, as can be seen in Figs. 7 and 9, comparing segments with the same color. The simulation results shown in section 3 indicate that for linear retardation the elevation angle is ζ = 0 and the minimum value of the radius is zero (for ϕ = 0 ). For circular retardation, ζ = 90 and for any value of the azimuth angle of the input linear polarization ϕ, the trajectory is a major circle. For elliptical retardation 0 ζ 90 and the minimum value of the circular path, given by Eq. (8) is different from zero. In this work, to determine the elevation angle ζ we take into account that the circular path described on the Poincaré sphere is normal to a line (axis of symmetry) that forms an angle with the plane of the equator equal to ζ; where tan(ζ/2) is the ellipticity [15]. The cross product of the Stokes vectors of two consecutive sampling wavelengths, obtained using the same input linear polarization, L 1,2 (ϕ) = S(λ 1,ϕ) S(λ 2,ϕ), is normal to the symmetry axis of the circular path. Hence, the angle between L 1,2 (ϕ) and S 3 axis is equal to the elevation angle ζ, between the symmetry axis and the plane of the equator (Fig. 5). For the INO NOI sample ζ = -3.1 ± 2.4, and for the Photonetics sample ζ = 86 ± 1.1. We can notice in Figs. 6 to 9 that the apparent inclination of the axis of symmetry in each one of these figures is different to these values. This effect is produced by birefringence dispersion. When a wide spectral range is used, the contribution of the wavelength dependence of birefringence is no longer negligible. The results obtained for the average value of the elevation angle, calculated using consecutive values of the sampling wavelength are shown in Figs. 10 and 11 for INO NOI and Photonetics samples, respectively. The quasiperiodic behavior of the elevation angle is similar to the result reported by Heffner [19] using a different measuring system, a different type of fiber (Corning PRSM designed for use at 1300 nm, length mm) a different wavelength scanning band ( nm), and different data processing. This behavior is probably consequence of birefringence dispersion, although further research is needed to verify this (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2561

7 assumption. To verify the validity of results obtained with the method here proposed we used Eq. (8) and the average value determined for the inclination angle of the INO NOI sample to predict the radii of the circular paths for some of the angles used in this work. The results shown in Fig. 12 indicate that in practice it is valid to assume that birefringence dispersion is negligible when the evaluation is performed using closely spaced light wavelengths [17]. Fig. 6. Wavelength scanning results obtained for the INO NOI 402K5 single-mode erbium-doped fiber. The sampling signal was scanned from 1511nm to 1571nm using 6nm steps. Each curve corresponds to a different azimuth angle of the input linear polarization state. Fig. 7. Lateral view of the curves shown in Fig. 6. In addition to the results in fig. 6, this orientation of the sphere allows us to include some other trajectories produced using different azimuth angles of the input polarizer. It is evident that each path is not a plane curve. Fig. 8. Wavelength scanning results obtained for the Photonetics EDOS-103 single-mode erbium-doped fiber. The sampling signal was scanned from 1511nm to 1571nm using 6nm steps. Each curve corresponds to a different azimuth angle of the input linear polarization state. Fig. 9. Lateral view of the curves shown in Fig. 8. In both figures, the same color was used for equivalent orientations of the input linear polarizer. It can be observed that when the signal wavelength is scanned, the trajectory described by the output polarization state is not a plane curve. In regard with beatlength evaluation, a 2π circular angle produces a full circular path on the Poincaré sphere. From Fig. 6 we can see that scanning the sampling signal from 1511 nm to 1571 nm produces a circular angle smaller than π for a 1.61 m fiber length. Since the Poincaré sphere is a 2-sphere, the phase change is smaller than π/2; i.e. L > 4s( λ λ ) 24cm for the INO NOI sample. For the same wavelength interval the b Photonetics sample covers a circular angle smaller than 40, hence L b > 1.1m. The average value of the elevation angle was also used to determine the ratio of the linear to the circular birefringence components, tan σ [10]. The precision of this evaluation is poor. For ζ = π/2 - σ = 3.1 ± 2.4 (INO NOI, sample) it varies from -82 to -10. These values indicate us that the linear retardation is at least 10 times higher than the circular retardation, and their signs are opposite. For the Photonetics sample circular birefringence is dominant. The circular to linear birefringence ratio varies from 11 to 19. These values indicate us that the circular retardation is at least 11 times larger than the linear retardation, and they have the same sign. (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2562

8 Fig. 10. Results obtained for the average value of the elevation angle ζ, for the INO NOI sample. These values were calculated using consecutive values of the sampling wavelength ( ζ=-3.1 ± 2.4 ). Fig. 11. Results obtained for the average value of the elevation angle ζ, for the Photonetics sample. These values were calculated using consecutive values of the sampling wavelength ( ζ=86 ± 1.1 ). Fig. 12. The radii predicted by the value of the inclination angle determined from wavelength scanning data are compared with the real trajectories shown in fig. 8. The continuous line is the average value of the radius. The range of values produced by the standard deviation lies between the neighboring dotted lines. 5. Conclusion Previous to the birefringence characterization of single-mode erbium-doped fibers, it is necessary to identify the type of retardation that better describes the polarization optics of a fiber sample and to determine an approximate value for the birefringence beat-length. In this work we present the theoretical basis used to perform this assessment. In particular we propose a methodology based on the Poincaré sphere, and we apply it to two single-mode commercial erbium-doped fiber samples. Acknowledgments This work was supported by Conacyt-DAIC, project G37000-E and by the scholarship granted to Fernando Treviño Martínez by PROMEP-UANL-256. (C) 2005 OSA 4 April 2005 / Vol. 13, No. 7 / OPTICS EXPRESS 2563