Jones birefringence in twisted single-mode optical fibers

Diana Tentori; A. Garcia-Weidner

doi:10.1364/OE.21.031725

1. Introduction

In a series of studies performed on twisted single-mode optical fibers we have found that the effect of torsion is far more complex than the initial proposal of twist-induced circular birefringence developed by Ulrich and Simon in 1979 [1]. In an earlier investigation, making use of classical polarization optics we demonstrated that for a very low torsion (~4 rad/m) the fiber could be represented as the concatenation of multiple segments with the same residual anisotropy (elliptical) whose birefringence axes rotate uniformly along the fiber length. Under this scope, the birefringence matrix is equal to the product of two operators, the elliptical rotator describing the residual anisotropy of the fiber, multiplied by a circular rotator whose rotation angle is equal to the twist angle [2]. Since these matrices are rotation operators describing gyration around two different axes, the fiber no longer behaves as a homogeneous retarder; i.e. it cannot become circularly birefringent.

For larger rotation angles, and working with standard telecommunication fibers, we found that the effect included not only a rotation of the birefringence axes but also a photoelastic contribution. Due to photoelastic effect, the retardation angle varied linearly with the applied torsion while fiber ellipticity remained constant [3]. In these experiments, we analyzed the evolution of the output polarization state with the applied torsion using the Stokes parameters (S₁, S₂, S₃ vs. applied twist). We obtained nearly sinusoidal curves whose frequencies and amplitudes remained fixed for positive and negative twist angles. The frequency and amplitude of parameters S₁ and S₂, had the same values; while the frequency of parameter S₃ was higher, and its oscillation amplitude lower. Due to the fact that the output polarization state was maintained on the surface of the Poincaré sphere, the effect of torsion was described in terms of unitary rotation matrices with different orientations on their gyrating axes and/or responses to torsion: 1) A rotation matrix associated with rotation of birefringence axes, which depends on the applied torsion, 2) A rotation matrix with a constant delay, and, 3) The matrix of an elliptical retarder in which delay between polarization eigenmodes depends on torsion. Such semi-empiric model predicted with high accuracy the results obtained using single-mode fibers for optical communications (Corning SMF-28 and SMF-28e) [3], even in the case of fibers having residual torsion (Nufern 1060-XP) [4]. Nevertheless, this model does not predict observed changes of the polarization state in single-mode erbium-doped fibers (EDFs).

In order to extend the application of the model offered in [3], it was translated into Jones differential matrices [5], which allowed identifying the origin of observed changes and facilitated its numerical adjustment. The present work submits an amplified form of the model, which makes use of the linear birefringence component known as Jones birefringence. We should point out that we used short fiber samples (< 2 m) whose absorption was negligible, therefore, along this work signals are considered as fully polarized.

This work also discusses the adjustment obtained within values predicted by the model and experimental data obtained using single-mode fibers doped with low and medium erbium concentrations. As a form of comparison, in this study we include some aspects of the twist-induced polarization behavior of a standard fiber (SMF-28e) used in communications.

2. Matrix differential analysis of a twisted-fiber

In 1948 Jones published an investigation in which he described the evolution of the polarization state of a plane wavefront that propagates through an anisotropic medium. The differential matrices used in his infinitesimal description were derived from the birefringence matrix of the optical medium M (expressed as a 2 × 2 Jones matrix) using the relation:

N = \frac{d M}{d z} M^{- 1};

where M⁻¹ is the inverse matrix of M. In his work, Jones demonstrated that there are eight infinitesimal 2 × 2 matrices that describe the linear optical effects of an anisotropic medium. Four of such matrices correspond to absorption media; the remaining four describe the effects observed in homogeneous retarders. The analysis of the N matrix of an anisotropic medium facilitates the identification of the basic optical effects that define its birefringence.

2.1 Jones matrix (M_T) of a twisted-fiber

It has been shown recently that the birefringence of a twisted-fiber can be described using the matrix [3]:

M_{T} (τ, δ_{τ}, σ, θ, β) = R (- θ) [R (β) R (b τ) M_{τ} (δ_{τ}, σ)] R (θ)

In Eq. (2) the fast birefringence axis of the fiber is rotated by an angle θ with respect to x axis of the laboratory reference frame, β is a fixed rotation angle between the system coordinates and the fiber birefringence axes, and bτ describes the geometric rotation of the principal axes due to torsion, b is a constant with a value close to one unit. R is used to represent a matrix of rotation that in the space of the Poincaré sphere takes place around the axis crossing the poles of the sphere (it should be noted that in this space, rotation angles are doubled and measured from S1 axis). In Eq. (2), M_τ(δ_τ,σ) is the matrix of an elliptical retarder with eigenmode ellipticity ε (here described in the space of the Poincaré sphere by the angle σ = π/2 - 2ε) and zero azimuthal angle. The retardation δ_τ induced between eigenmodes by the applied torsion is:

δ_{τ} = δ_{0} + c τ;

where

δ_{0} = \frac{2 π}{λ} Δ n z = c_{0} z

is the retardation between elliptical eigenmodes in a non-twisted fiber from 0 to z (Δn, the elliptical birefringence of the untwisted fiber is constant; i.e. c₀ is a constant); angle τ = Tz = τ(z) is the torsion angle from 0 to z, here T is the twist rate (T is constant); and the constant coefficient c is characteristic, for each type of fiber, of the influence of the torsion on the retardation. Therefore Eq. (3) has the form: δ_τ = c₀ z + c Tz.

Assuming the birefringence axes of the anisotropic medium and the laboratory system are aligned (θ = 0) and using Jones’s matrices (they are defined for a right orthogonal coordinate system to be able to follow [5]), Eq. (2) is reduced to:

\begin{array}{l} M_{T} = R (β) R (b τ) M_{τ} (δ_{τ}, ε) \\ = [\begin{matrix} \cos (β + b τ) & \sin (β + b τ) \\ - \sin (β + b τ) & \cos (β + b τ) \end{matrix}] [\begin{matrix} \cos (δ_{τ} / 2) + j \cos 2 ε \sin (δ_{τ} / 2) & sin 2 ε \sin (δ_{τ} / 2) \\ - \sin 2 ε s i n (δ_{τ} / 2) & \cos (δ_{τ} / 2) - j \cos 2 ε \sin (δ_{τ} / 2) \end{matrix}] . \end{array}

2.2 Differential matrix N_T of a twisted fiber

Considering that for any normal section of the fiber, z is its coordinate along the fiber axis, with the monochromatic wave propagating to the positive z, and that only the twist angle τ and the retardation angles δ₀ and δ_τ are functions of z, we obtain substituting (4) in (1), N_T, the differential matrix of a twisted fiber

N_{T} = A [\begin{matrix} j \frac{c}{2} \cos 2 (β + b τ) \cos 2 ε & b + \frac{c}{2} \sin 2 ε - j \frac{c}{2} \sin 2 (β + b τ) \cos 2 ε \\ - b - \frac{c}{2} \sin 2 ε - j \frac{c}{2} \sin 2 (β + b τ) \cos 2 ε & - j \frac{c}{2} \cos 2 (β + b τ) \cos 2 ε \end{matrix}],

Here

A = T \frac{d δ_{0}}{d z} = T c_{0}

, is a constant. The differential matrix N_T in Eq. (5) can be broken down into three elemental matrices:

N_{T} = A {\frac{c \cos 2 (β + b τ) \cos 2 ε}{2} [\begin{matrix} j & 0 \\ 0 & - j \end{matrix}] - \frac{c \sin 2 (β + b τ) \cos 2 ε}{2} [\begin{matrix} 0 & j \\ j & 0 \end{matrix}] - \frac{2 b + c \sin 2 ε}{2} [\begin{matrix} 0 & - 1 \\ 1 & 0 \end{matrix}]} .

This last expression shows clearly the contribution of the three types of birefringence: the first term describes a linear birefringence coefficient at zero degrees, the second term describes a linear birefringence coefficient at 45 degrees and the third term describes a circular birefringence coefficient. It should be noted that terms cos2(β + bτ) and sin2(β + bτ) in Eqs. (5) and (6) are typical of a twisted fiber [3]. Applying Jones’s notation [5] we may define,

β_{0} (c, β, b, ε, τ) = A \frac{c \cos 2 (β + b τ) \cos 2 ε}{2} = \frac{η_{y} - η_{x}}{2}, linear birefringence coefficient at 0 °

β_{45} (c, β, b, ε, τ) = - A \frac{c \sin 2 (β + b τ) \cos 2 ε}{2} = \frac{η_{- 45} - η_{45}}{2}, linear birefringence coefficient at 45 °

β_{c} (c, β, b, ε) = - A \frac{2 b + c \sin 2 ε}{2} = \frac{η_{R} - η_{L}}{2}, circular birefringence coefficient,

where

η = 2 π n / λ

is the propagation constant. Substituting relations (7)-(9) in Eq. (6), we find that matrix N_T may be written as:

N_{T} = β_{0} [\begin{matrix} j & 0 \\ 0 & - j \end{matrix}] + β_{45} [\begin{matrix} 0 & j \\ j & 0 \end{matrix}] + β_{c} [\begin{matrix} 0 & - 1 \\ 1 & 0 \end{matrix}],

where the term β_c is independent from torsion τ (A, b, c and ε are constants); whereas the linear birefringence contributions β₀ and β₄₅ have a periodic variation in relation to τ.

In order to understand the birefringence changes introduced when an elliptical retarder is twisted, the differential matrix of a fiber with elliptical birefringence is calculated below, and compared with the mathematical description of Eq. (10).

2.3 Differential matrix N_Τ of a fiber with elliptical birefringence

Jones’s matrix M_e describing a fiber with elliptical retardation (using again a right coordinate system) is:

M_{e} = [\begin{matrix} \cos (δ_{0} / 2) + j \cos 2 ε \sin (δ_{0} / 2) & \sin 2 ε \sin (δ_{0} / 2) \\ - \sin 2 ε \sin (δ_{0} / 2) & \cos (δ_{0} / 2) - j \cos 2 ε \sin (δ_{0} / 2) \end{matrix}];

where δ₀ is retardation between polarization eigenmodes and ε is ellipticity of the anisotropic medium.

Applying Eq. (1), we find that the differential matrix of a fiber with elliptical birefringence is:

N_{e} = \frac{1}{2} c_{0} [\begin{matrix} j \cos 2 ε & - \sin 2 ε \\ \sin 2 ε & - j \cos 2 ε \end{matrix}] = c_{0} {\frac{\sin 2 ε}{2} [\begin{matrix} 0 & - 1 \\ 1 & 0 \end{matrix}] + \frac{\cos 2 ε}{2} [\begin{matrix} j & 0 \\ 0 & - j \end{matrix}]} .

The first term corresponds to circular birefringence and the second term to linear birefringence. The coefficients indicate the relative values of such birefringences: $η_{y} - η_{x} = c_{0} \sin 2 ε$ and $η_{R} - η_{L} = c_{0} \cos 2 ε$ . Comparing Eqs. (10) and (12) we notice that the description of linear birefringence requires two components (shown in Fig. 1) only when dealing with a twisted fiber.

Fig. 1 Geometrical relations between total birefringence coefficient Γ and the circular and linear (0 and 45°) birefringence coefficients. This representation holds in the space of the Poincaré sphere.

Download Full Size | PDF

From relations (7) and (8) we observe that for a fixed torsion, the geometric sum of the linear birefringence components generates a linear birefringence (β_l) whose fast axis in general does not match axes x, y or ± 45°. Therefore, the applied torsion produces a change of the value of the azimutal angle of the linear birefringence fast axis. The resulting linear birefringence coefficient is (Ac cos2ε)/2.

It should be pointed out that despite such simplification, since the coefficient of circular birefringence of a set value of torsion is -A(2b + sin2ε)/2, the linear and circular birefringence components of the twisted medium do not correspond to the components of a matrix N of elliptical birefringence [Eq. (12)]. Furthermore, expressing the result in terms of Jones’s matrices M, shows the product of a rotation matrix with a circular birefringence coefficient (β + bτ) (where β is a constant), multiplied by an elliptical birefringence matrix.

2.4 Mueller matrix of a twisted fiber with two linear birefringence components: a deduction from its Jones matrix

In this section we describe the result developed in the previous section, in terms of a Mueller matrix. To be able to use the Mueller matrix of an elliptical retarder developed by C. Tsao [6], we used a left coordinate system [7]. Therefore, we base our description on the following Jones’s matrix of elliptical birefringence:

M_{J e} = [\begin{matrix} \cos Γ - j \frac{β_{0}}{Γ} \sin Γ & - \frac{β_{c} - j β_{45}}{Γ} s i n Γ \\ \frac{β_{c} + j β_{45}}{Γ} \sin Γ & \cos Γ + j \frac{β_{0}}{Γ} \sin Γ \end{matrix}] = [\begin{matrix} T_{11} & T_{12} \\ T_{21} & T_{22} \end{matrix}] = T .

In this unitary matrix [Eq. (13)], Γ = δ is equal to δ_τ /2 modulo 2π, and linear birefringence has two components: one running along the horizontal axis and another one at 45 degrees (Fig. 1). The transformation rule to solve Mueller matrix (M_Me), is [6]:

M_{M e} = U (T \times T^{*}) U^{- 1};

where

U = [\begin{matrix} \begin{matrix} 1 & 0 \\ 1 & 0 \end{matrix} & \begin{matrix} 0 & 1 \\ 0 & - 1 \end{matrix} \\ \begin{matrix} 0 & 1 \\ 0 & - j \end{matrix} & \begin{matrix} 1 & 0 \\ j & 0 \end{matrix} \end{matrix}]

and

T \times T^{*} = [\begin{matrix} \begin{matrix} T_{11} T_{11}^{*} & T_{11} T_{12}^{*} \\ T_{11} T_{21}^{*} & T_{11} T_{22}^{*} \end{matrix} & \begin{matrix} T_{12} T_{11}^{*} & T_{12} T_{12}^{*} \\ T_{12} T_{21}^{*} & T_{12} T_{22}^{*} \end{matrix} \\ \begin{matrix} T_{21} T_{11}^{*} & T_{21} T_{12}^{*} \\ T_{21} T_{21}^{*} & T_{21} T_{22}^{*} \end{matrix} & \begin{matrix} T_{22} T_{11}^{*} & T_{22} T_{12}^{*} \\ T_{22} T_{21}^{*} & T_{22} T_{22}^{*} \end{matrix} \end{matrix}] .

The symbol * indicates complex conjugate. Using Eqs. (15) and (16) in Eq. (14) we obtained:

M_{M e} = [\begin{matrix} \begin{matrix} 1 & 0 \\ 0 & \cos^{2} Γ + \frac{β_{0}^{2} - β_{c}^{2} - β_{45}^{2}}{Γ^{2}} \sin^{2} Γ \end{matrix} & \begin{matrix} 0 & 0 \\ - \frac{β_{c}}{Γ} \sin 2 Γ - 2 \frac{β_{0} β_{45}}{Γ^{2}} \sin^{2} Γ & \frac{β_{45}}{Γ} \sin 2 Γ - 2 \frac{β_{0} β_{c}}{Γ^{2}} \sin^{2} Γ \end{matrix} \\ \begin{matrix} 0 & \frac{β_{c}}{Γ} \sin 2 Γ - 2 \frac{β_{0} β_{45}}{Γ^{2}} \sin^{2} Γ \\ 0 & - \frac{β_{45}}{Γ} \sin 2 Γ - 2 \frac{β_{0} β_{c}}{Γ^{2}} \sin^{2} Γ \end{matrix} & \begin{matrix} \cos 2 Γ + 2 \frac{β_{45}^{2}}{Γ^{2}} \sin^{2} Γ & \frac{β_{0}}{Γ} \sin 2 Γ + 2 \frac{β_{c} β_{45}}{Γ^{2}} \sin^{2} Γ \\ - \frac{β_{0}}{Γ} \sin 2 Γ + 2 \frac{β_{c} β_{45}}{Γ^{2}} \sin^{2} Γ & \cos 2 Γ + 2 \frac{β_{c}^{2}}{Γ^{2}} \sin^{2} Γ \end{matrix} \end{matrix}] .

For simplicity, we work with a (3 × 3) version of the matrix, and based on Fig. 1, we rewrite Eq. (17) substituting the inverse ratio of total retardation coefficient Γ to one of its components (along the vertical axis or on the horizontal plane) in terms of angle σ. We also relate the components of linear birefringence through angle σ_l:

\cos σ = β_{c} / Γ; \sin σ = β_{l} / Γ = {(β_{0}^{2} + β_{45}^{2})}^{1 / 2} / Γ

β_{0} = Γ s i n σ \cos σ_{l}

β_{45} = Γ \sin σ s i n σ_{l} .

Substituting Eqs. (18)–(20) in matrix of relation (17), we obtain:

M_{R} = [\begin{matrix} \cos 2 δ + 2 \sin^{2} σ \cos^{2} σ_{l} \sin^{2} δ & - \cos σ \sin 2 δ - \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ & \sin σ \sin σ_{l} \sin 2 δ - \sin 2 σ \cos σ_{l} \sin^{2} δ \\ \cos σ \sin 2 δ - \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ & \cos 2 δ + 2 \sin^{2} σ_{l} \sin^{2} σ \sin^{2} δ & \sin σ \cos σ_{l} \sin 2 δ + \sin 2 σ \sin σ_{l} \sin^{2} δ \\ - \sin σ \sin σ_{l} \sin 2 δ - \sin 2 σ \cos σ_{l} \sin^{2} δ & - \sin σ \cos σ_{l} \sin 2 δ + \sin 2 σ \sin σ_{l} \sin^{2} δ & \cos 2 δ + 2 \cos^{2} σ \sin^{2} δ \end{matrix}]

Therefore, for a linearly-polarized entry signal with azimuth angle φ in the physical space, Stokes vector S_e-out at the elliptical retarder exit-point is:

\begin{array}{l} S_{e - o u t} = M_{R} [\begin{matrix} \cos 2 φ \\ \sin 2 φ \\ 0 \end{matrix}] \\ = [\begin{matrix} \cos 2 φ (\cos 2 δ + 2 \sin^{2} σ \cos^{2} σ_{l} \sin^{2} δ) + \sin 2 φ (- \cos σ \sin 2 δ - \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ) \\ \cos 2 φ (\cos σ \sin 2 δ - \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ) + \sin 2 φ (\cos 2 δ + 2 \sin^{2} σ_{l} \sin^{2} σ \sin^{2} δ) \\ \cos 2 φ (- \sin σ \sin σ_{l} \sin 2 δ - \sin 2 σ \cos σ_{l} \sin^{2} δ) + \sin 2 φ (- \sin σ \cos σ_{l} \sin 2 δ + \sin 2 σ \sin σ_{l} \sin^{2} δ) \end{matrix}] \end{array}

The use of the additional birefringence associated with the rotation matrix R(β + bτ) that multiplies matrix M_R in Eq. (4) (as mentioned in the last paragraph in section II.3), reveals that Stokes vector S_out at the exit-point of the twisted fiber is:

S_{o u t} = R (β) R (b τ) S_{e - o u t}

and the parameters of Stokes exit-vector are:

\begin{matrix} S 1_{o u t} = \cos 2 (β + b τ) [\cos 2 φ (\cos 2 δ + 2 \sin^{2} σ \cos^{2} σ_{l} \sin^{2} δ) - \sin 2 φ (\cos σ \sin 2 δ + \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ)] \\ + \sin 2 (β + b τ) [\cos 2 φ (\cos σ \sin 2 δ - \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ) \\ + \sin 2 φ (\cos 2 δ + 2 \sin^{2} σ_{l} \sin^{2} σ \sin^{2} δ)] \\ S 2_{o u t} = - \sin 2 (β + b τ) [\cos 2 φ (\cos 2 δ + 2 \sin^{2} σ \cos^{2} σ_{l} \sin^{2} δ) - \sin 2 φ (\cos σ \sin 2 δ + \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ)] \\ + \cos 2 (β + b τ) [\cos 2 φ (\cos σ \sin 2 δ - \sin 2 σ_{l} \sin^{2} σ \sin^{2} δ) \\ + \sin 2 φ (\cos 2 δ + 2 \sin^{2} σ_{l} \sin^{2} σ \sin^{2} δ)] \\ S 3_{o u t} = - \cos 2 φ (\sin σ \sin σ_{l} \sin 2 δ + \sin 2 σ \cos σ_{l} \sin^{2} δ) + \sin 2 φ (- \sin σ \cos σ_{l} \sin 2 δ + \sin 2 σ \sin σ_{l} \sin^{2} δ) . \end{matrix}

3. Experimental results

In this work, following the experimental procedure reported in [3], we study the effect of torsion on the birefringence of three commercial erbium-doped fibers which are listed in Table 1: Sample F1 = Photonetics EDOS 230, sample F2 = Fibercore 1500E and sample F3 = INO 402 K5. We selected these fibers because their higher rigidity introduces stronger birefringence changes when twisted.

Table 1. Fiber’s parameters.

View Table | View all tables in this article

We found that an important difference between the polarization performance of doped and undoped twisted fibers is their dependence on the magnitude of the applied twist. For undoped fibers the polarization evolution described a curve that oscillated around a fixed parallel of the Poincaré sphere, keeping almost the same amplitude and frequency [3]. This polarization evolution curve corresponds to a medium with a fixed ellipticity and fixed azimuth angle. By contrast, in Fig. 2 we present the curves depicted by the output polarization state for two twisted erbium-doped fibers (F1 and F3).

Fig. 2 Evolution of the output state of polarization for a 1540 nm linearly polarized input signal when the absolute value of the twist applied to the EDF varied from 0 to 1440°. In this figure red is used for those sections of the curve located at front. (a) F1, φ = 30°, right twist; (b) F3, φ = 60°, left twist.

Download Full Size | PDF

For these fibers the oscillation does not take place around the same parallel and the oscillation frequency and amplitude depend on the applied twist. To describe this behavior it is necessary that the linear and circular components of the elliptical birefringence vary in an independent manner. This condition is satisfied in our model with the introduction of Jones birefringence.

The model we present in this work has been used to predict the evolution of the output polarization state for the three commercial EDFs we work with. Two examples of the numerical fitting results obtained for fiber F1 using Eq. (24) are shown in Fig. 3 for two signal wavelengths (1530 and 1560 nm). Figures 3(a) and 3(c) show the numerical results obtained for Stokes parameters due to the applied twist scanning (τ_a = 0 to 1440°). Figures 3(b) and 3(d) present the output SOP evolution on the Poincaré sphere for a 600° to 1440° scanning of the applied twist τ_a (τ_a = τ(L), where L is the fiber length). The azimuth angles of the input linear polarizations are φ = 60° for 1530 nm and φ = 0° for 1560 nm. Stokes parameters labeled “out” have been obtained using the theoretical model and those labeled “n” are experimental values.

Fig. 3 Numerical fitting of Stokes parameters (a) and (c), and SOP evolution depicted on the Poincaré sphere (b) and (d) obtained for fiber F1 for two different signal wavelengths (1530 and 1560 nm). The azimuth angle of the input linear polarization φ is shown for each case. Purple line on the Poincaré sphere corresponds to theoretical prediction and the experimental data are presented with a blue line (the absolute value of the twist applied to the EDF varied from 600 to 1440°). The variation of Stokes parameters is shown for applied torsions τ_a between 0 and 1440°.

Download Full Size | PDF

For fibers F2 and F3 the experimental results show a very irregular variation for low torsion values (τ_a < 500°) as we can observe in Figs. 4(a), 4(b), 4(d), 4(e), 5(a), 5(b), 5(d), and 5(e). This behavior has been previously reported for a standard single mode fiber with residual torsion [4]. Since the EDFs we study in this work also present a different response to right and left torsions, we know these fibers have a residual torsion. Here, we must mention that the results we discuss correspond to twists with the same handedness of the residual torsion.

Fig. 4 Numerical fitting and experimental data obtained for fiber F2. These graphs show the evolution of the Stokes parameters of the output state of polarization of a 1530 and a 1550 nm linearly polarized input signal (φ = 30 and 120°, respectively), when the value of the twist applied to the EDF varied from 0 to −1440°. Figures 4(a), 4(b), 4(d), and 4(e) show the Stokes parameters evolution and Figs. 4(c) and 4(f), the output SOP evolution on the Poincaré sphere for applied torsions τ_a between 700 and 1440° (purple line = theory, blue line = experimental data).

Download Full Size | PDF

Fig. 5 Numerical fitting and experimental data obtained for fiber F3. These graphs show the evolution of the Stokes parameters of the output state of polarization of a 1530 and a 1550 nm linearly polarized input signal (φ = 30 and 120°, respectively), when the value of the twist applied to the EDF varied from 0 to −1440°. Figures 5(a), 5(b), 5(d), and 5(e) show the Stokes parameters evolution and Figs. 5(c) and 5(f) the output SOP evolution on the Poincaré sphere for applied torsions τ_a between 700 and 1440° (purple line = theory, blue line = experimental data).

Download Full Size | PDF

In this work we model the evolution of the output polarization state [Eq. (24)] considering that twist-induced birefringence is the dominant effect (the parameters we used produce a good fitting for the higher values of torsion). Therefore, the initial irregular variations of Stokes parameters associated with uneven modifications of the waveguide are not predicted by our model. The parameters used to fit the results shown in Figs. 3–5 are shown in Table 2.

Table 2. Fitting parameters.

View Table | View all tables in this article

4. Discussion

4.1 Numerical fitting

The numerical fitting procedure requires the determination of parameters: c, b, β and also, initial values (when there is no torsion applied to the fiber) for: the retardation between polarization eigenmodes (δ₀), the ellipticity angle of the circular birefringence component (σ₀) and the azimuth angle of the linear birefringence component (σ_l₀).

Since the evolution of angles σ_c and σ_l must be independent, to model them we defined coefficients cc and cl such that

σ = σ_{0} + c c \cdot τ_{a}

and

σ_{l} = σ_{l 0} + c l \cdot τ_{a} .

The values determined for c and b for EDFs studied in this work and standard fibers reported in [3] and [4] are presented in Table 3. We can see that, as expected, geometrical parameter b shows values very close to unity for doped and undoped fibers. We should remember that product bτ describes the rotation of the fast birefringence axis produced by the applied torsion [2].

Table 3. Geometrical parameters

View Table | View all tables in this article

Parameter c remains close to 0.9 for standard fibers and assumes different and lower values for erbium-doped fibers. We hypothesize it must be connected with the specific waveguide structure, although further information is required to relate c to the waveguide profile and glass composition.

In Table 4 we present the values determined for wavelength dependent parameters (θ, and β) and its range of variation (Δθ and Δβ) for the three erbium-doped fibers studied in this work.

Table 4. Erbium-doped fibers wavelength dependent parameters.

View Table | View all tables in this article

Angle θ is the fiber’s fast axis azimuth angle. Its spectral variation has no parallel for bulk anisotropic media, but since we modeled the evolution of the output polarization state considering that twist-induced birefringence is the dominant effect, we can propose that torsion is the origin of the spectral dependence of angle θ. Nevertheless, we should mention that the spectral variation of the azimuth angle of the fast birefringence axis (θ) has also been reported in [8] for fibers F1 and F3 when they were kept straight. To understand the spectral behavior of θ, we must take into account the previously mentioned unequal changes observed for low twists (τ_a < 500°), and the different response observed for the right and left torsions measured in this work. Under this scope, the spectral dependence of angle θ obtained for non-twisted erbium-doped fibers can be explained in terms of the presence of a residual torsion (introduced during the manufacturing process). In the presence of a residual torsion a cold twist introduces a change in the index of refraction of the glass host that gives rise to a spectral dependence of some parameters such as θ. Furthermore, for standard optical fibers parameter θ kept constant for all the input azimuth angles used for each signal wavelength (φ = 0-150 °, with a 30 ° step) [3,4]; while for erbium-doped fibers it was φ-dependent. When the azimuth angle of the input linear polarization varied, we obtained a fixed value for θ only for some wavelengths: for fiber F1 from 1540 to 1560 nm; for Fiber F2, for 1540, 1550 and 1560 nm and for fiber F3 from 1525 to 1565 nm.

In addition, parameter β showed no wavelength or geometric dependence in standard optical fibers. Its value (180°) was constant for Corning SMF-28e [3] and nufern 1060XP [4]. In the present study β is not only wavelength dependent but also varies with the orientation of the azimuth angle of the input linear polarization (φ). The variation obtained for β follows that of angle θ (orientation of the fast birefringence axis). This higher sensitivity to optical alignment can be produced by fabrication tolerances of the waveguide structure, although further information is required to confirm this hypothesis.

In Table 5 we compare the values we determined for parameters related with the anisotropy of the fibers (cc, σ₀, cl, σ_l₀ and δ₀) and its range of variation (Δcl, Δσ_l₀ and Δδ₀) for the three EDFs we study in this work and the standard fibers investigated in [3] and [4]. The elliptical birefringence of standard optical fibers was described using angle σ, related with the total elliptical birefringence. The value of angle σ was wavelength dependent.

Table 5. Parameters related with fiber’s anisotropy.

View Table | View all tables in this article

For erbium-doped fibers we describe its elliptical birefringence in terms of the circular and linear birefringence components using σ = σ₀ + cc⋅τ_a and σ_l = σ_l₀ + cl⋅τ_a, respectively. σ₀ and cc keep constant for each fiber and the wavelength dependency is observed just for σ_l₀. cl exhibits spectral variation only for fiber F2. Here, it is interesting to remark that our value of the Jones birefringence component is much higher than the values mentioned by other authors [9–12].

The initial retardation angle between polarization eigenmodes δ₀ used for numerical fitting showed (in [3] and in this work) spectral dependence for all the fibers with the exception of nufern 1060XP [4].

4.2 Birefringence changes induced by torsion

The evolution of the azimuth angle of the output state of polarization is faster for twisted erbium-doped fibers than for undoped fibers. This result is in agreement with the notion that EDFs birefringence is higher than SMF birefringence. An example of this behavior is shown in Fig. 6, where the evolution obtained for a 1530 nm signal using an undoped fiber (SMF28e, 1.75 m) is compared with that measured for erbium-doped fiber F2 (Fibercore 1500E, 1.51 m).

Fig. 6 Evolution of the output state of polarization of a 1530 nm linearly polarized signal when the twist applied to the fiber varies from: (a) 600 to 1400° for erbium-doped fiber F2 (Fibercore 1500E, 1.51 m), (b) 0 to 1440° for a standard fiber (SMF-28e, 1.75 m).

Download Full Size | PDF

From the measurements analysis we conclude that the evolution of the output polarization state due to the applied torsion is different for optical fibers doped with similar erbium concentrations (F1 and F3, Figs. 3 and 5). This result reveals there are other contributions, such as waveguide structure and residual torsion that dictate the birefringence response of erbium doped fibers.

5. Conclusions

Torsion of fibers introduces a stronger modification of polarization properties in erbium-doped fibers than in undoped single-mode fibers. The description of the polarization performance of EDFs requires a model that includes a linear birefringence component at 45° (Jones birefringence) in addition to a residual elliptical birefringence with one linear component (at 0°) and one circular component. All these birefringence components are twist-dependent.

We have found that Jones birefringence induced by twisting the birefringent media studied in this work (optical fibers) is several orders of magnitude greater than previously reported values of Jones birefringence induced by electric and/or magnetic fields in bulk birefringent media.

As a final remark, we should mention that in erbium-doped optical fibers birefringence properties are mainly linked to the glass host composition and the fiber geometrical structure than to the erbium concentration.

Acknowledgments

We would like to thank Miguel Farfán Sánchez for his help with data collection and to Conacyt for the economical support. This work was sponsored by project SEP-CONACYT, CB-2010-01-155121.

References and links

1. R. Ulrich and A. Simon, “Polarization optics of twisted single-mode fibers,” Appl. Opt. 18(13), 2241–2251 (1979). [CrossRef] [PubMed]

2. D. Tentori, C. Ayala-Díaz, E. Ledezma-Sillas, F. Treviño-Martínez, and A. García-Weidner, “Birefringence matrix for a twisted single-mode fiber: Geometrical contribution,” Opt. Commun. 282(5), 830–834 (2009). [CrossRef]

3. D. Tentori, A. García-Weidner, and C. Ayala-Díaz, “Birefringence matrix for a twisted single-mode fiber: Photoelastic and geometrical contributions,” Opt. Fiber Technol. 18(1), 14–20 (2012). [CrossRef]

4. D. Tentori and A. Garcia-Weidner, “Right- and left-handed twist in optical fibers,” submitted for publication.

5. R. C. Jones, “A new calculus for the treatment of optical systems. VII properties of the N-matrices,” J. Opt. Soc. Am. 38(8), 671–685 (1948). [CrossRef]

6. C. Tsao, Optical Fibre Waveguide Analysis (Oxford University, 1992).

7. P. Hauge, R. Muller, and C. Smith, “Conventions and formulas for using the Mueller-Stokes calculus in ellipsometry,” Surf. Sci. 96(1-3), 81–107 (1980). [CrossRef]

8. D. Tentori, C. Ayala-Díaz, F. Treviño-Martínez, and F. J. Mendieta-Jiménez, “Evaluation of the residual birefringence of single-mode erbium-doped silica fibers,” Opt. Commun. 271(1), 73–80 (2007). [CrossRef]

9. A. Rizzo and S. Coriani, “Birefringences: A challenge for both theory and experiment,” Adv. Quantum Chem. 50, 143–184 (2005). [CrossRef]

10. V. A. De Lorenci and G. P. Goulart, “Magnetoelectric birefringence revisited,” Phys. Rev. D Part. Fields Gravit. Cosmol. 78(4), 045015 (2008). [CrossRef]

11. D. Shcherbin, A. J. Thorvaldsen, D. Jonsson, and K. Ruud, “Gauge-origin independent calculations of Jones birefringence,” J. Chem. Phys. 135(13), 134114 (2011). [CrossRef] [PubMed]

12. M. Izdebski, W. Kucharczyk, and R. E. Raab, “Effect of beam divergence from the optic axis in an electro-optic experiment to measure an induced Jones birefringence,” J. Opt. Soc. Am. A 18(6), 1393–1398 (2001). [CrossRef] [PubMed]

Jones birefringence in twisted single-mode optical fibers

Abstract

1. Introduction

2. Matrix differential analysis of a twisted-fiber

2.1 Jones matrix (M_T) of a twisted-fiber

2.2 Differential matrix N_T of a twisted fiber

2.3 Differential matrix N_Τ of a fiber with elliptical birefringence

2.4 Mueller matrix of a twisted fiber with two linear birefringence components: a deduction from its Jones matrix

3. Experimental results

4. Discussion

4.1 Numerical fitting

4.2 Birefringence changes induced by torsion

5. Conclusions

Acknowledgments

References and links

Cited By

Figures (6)

Tables (5)

Equations (26)

Optics Express

Sample	Fiber	Long	absorption
F1	Photonetics EDOS 230	1.75 m	@1532 nm = 4 ± 1 dB/m
F2	Fibercore 1500E	1.51 m	@1535 nm = 0.76 dB/m
F3	INO 402 K5	1.51 m	@1535 nm = 3.9 ± 0.8 dB/m

fiber	λ(nm)	φ(°)	θ(°)	cc	σ₀	cl	σ_l0	δ₀	c	b	β(°)
F1	1530	60	123	1 × 10⁻⁴	18°	1 × 10⁻⁴	−0.5	−15°	−0.885	0.985	76
F1	1560	0	115	“	“	“	−0.05	−20°	“	“	90
F2	1530	30	55	−1 × 10⁻³	27°	5 × 10⁻⁴	0.6	45°	−0.83	1	15
F2	1545	120	85	“	“	2 × 10⁻⁴	−0.2	35°	“	“	20
F3	1530	30	40	−8 × 10⁻³	34°	−5 × 10⁻⁵	−0.3	35°	−0.79	−1	80
F3	1550	120	“	“	“	“	−0.2	−10°	“	“	45

Fiber		$\| c \|$	$\| b \|$
standard	Corning SMF-28e	0.92	0.99
standard	nufern 1060XP	0.9	0.97
erbium-doped	F1	0.885	0.98
	F2	0.83	1
	F3	0.79	1

Fiber	θ	Δθ	β	Δβ
F1	90 - 140	50	35 - 93	58
F2	50 - 90	40	153-210	57
F3	40 - 41	1	−150 to −90	50

Fiber		σ	σ₀	cc	σ_l₀	Δσ_l₀	cl	Δcl	δ₀	Δδ₀
standard	SMF-28e	16-23	-	-	-	-	-	-	−17 to −95	78
standard	1060XP	4-7	-	-	-	-	-	-	52	0
erbium-doped	F1	-	18	1 × 10⁻⁴	−0.05 to −0.9	0.85	1 × 10⁻⁴	0	−25 to 40	65
	F2	-	27	−1 × 10⁻³	−0.6 to 0.6	1.2	1 - 5 × 10⁻⁴	4 × 10⁻⁴	25 to 80	55
	F3	-	34	−8 × 10⁻³	−0.5 to 0.4	0.9	−0.5 × 10⁻⁴	0	−25 to 35	50

Abstract

1. Introduction

2. Matrix differential analysis of a twisted-fiber

2.1 Jones matrix (MT) of a twisted-fiber

2.2 Differential matrix NT of a twisted fiber

2.3 Differential matrix NΤ of a fiber with elliptical birefringence

2.4 Mueller matrix of a twisted fiber with two linear birefringence components: a deduction from its Jones matrix

3. Experimental results

4. Discussion

4.1 Numerical fitting

4.2 Birefringence changes induced by torsion

5. Conclusions

Acknowledgments

References and links

Cited By

Figures (6)

Tables (5)

Equations (26)

Optics Express

2.1 Jones matrix (M_T) of a twisted-fiber

2.2 Differential matrix N_T of a twisted fiber

2.3 Differential matrix N_Τ of a fiber with elliptical birefringence