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J. Phys. D: Appl. Phys. 32 (1999) 1618–1625. Printed in the UK                                                PII: S0022-3727(99)01288-7



      Measurement of optical fibre
      parameters using an optical
      polarimeter and Stokes–Mueller
      formalism
P Olivard, P Y Gerligand, B Le Jeune, J Cariou and J Lotrian
Laboratoire de Spectrom´ trie et Optique Laser, Universit´ de Bretagne Occidentale, 6,
                       e                                 e
Avenue Le Gorgeu BP 809, 29285 Brest Cedex, France
E-mail: Pascal.Olivard@univ-brest.fr

Received 26 January 1999

Abstract. Although the method based on the Mueller matrix for the experimental
determination of optical-device polarization behaviour is a powerful tool, it has rarely been
applied to optical fibre. This paper introduces an experimental methodology for measuring
the Mueller matrices of monomode fibre under uniform strains. Using a theoretical model
derived from coupled-mode equations, we were able to first estimate the physical parameters
of the fibre, then to use them to both test the model validity and assess their influence on the
polarimetric behaviour of the fibre.




1. Introduction                                                           2. Theoretical foundations

The polarization characteristics of optical fibre are of                   In the ideal case of an isotropic monomode optical fibre two
particular interest in systems like optical sensors or                    degenerate modes can propagate. This degeneracy indicates
interferometers using fibre. Although the Mueller matrix-                  that their propagation constants are identical, and so all the
based technique has rarely been used to describe polarization             states of polarization launched at the input face of the guide
phenomena in monomode optical fibres, Mueller matrix                       will not be transformed during the propagation process. In
polarimetry is powerful for the experimental determination                the case of external perturbations or of imperfections in the
of the polarization properties of optical devices like                    fibre core the degeneracy is lifted, the value of the propagation
linear or circular birefringence, dichroism, losses, rotary               constant is modified and then the two modes are coupled. If
power or also depolarization. Thus, an experimental                       ax and ay are the amplitudes of these modes which are now
Mueller matrix enables one to predict the transformation                  z-dependent, the total electric field in the weakly guiding
of every state of polarization by propagation through                     approximation can be described by the following expression
the medium characterized by the matrix. Furthermore,                      where the time dependence exp(−jωt) is assumed and not
the coupled-mode theory well known in the Jones                           expressed:
space (amplitude terms), transformed into Stokes space
(energetic terms), has rarely been investigated experimentally                    E(x, y, z) = ax (z)Ex (x, y) + ay (z)Ey (x, y).     (1)
[1, 2].
     The present paper is organized as follows: the theoretical           The coupling process can be described [3] by a set of two
development permitting the description of polarization                    first-order differential equations on the hypothesis that the
evolution in a uniformly perturbed fibre is recalled in                    variations be small versus z, the distance travelled along the
section 2 where the theoretical Mueller matrix used is                    guide. This set is written as follows
presented. Section 3 reports on some simulation results in the
case of elliptical birefringence, whereas section 4 describes                        d     ax (z)      k        k12    ax (z)
                                                                                                  = −j 11                             (2)
the experimental method applied to measure the Mueller                               dz    ay (z)      k21      k22    ay (z)
matrix. Some experimental results and the methodology
to extract physical parameters, i.e. coupling coefficients, are            where kij are the coupling coefficients and d/dz indicates the
presented in sections 5 and 6. Conclusions are presented in               derivative with regard to z. This system can be directly solved
section 7.                                                                [4, 5], and its solutions are then expressed as a function of

0022-3727/99/141618+08$30.00       © 1999 IOP Publishing Ltd
Measurement of optical fibre parameters

both the initial conditions, i.e. the input state of polarization,
and the coupling coefficients:                                                           y         y'

                   ax
                        = f (ax (0), ay (0), z, kij ).          (3)                                                         τ x'
                   ay

Unfortunately, this formulation has some drawbacks for                                            z                         x
carrying out experimental investigations: first, the quantities
are complex and thus difficult to measure and second,
depolarization is not considered. These considerations led us         Figure 1. A representation of the laboratory (x, y) and rotary
to integrate this system in Stokes space where all quantities         (x , y ) frame.
have an energy dimension. In consequence, the four Stokes
                                                                                                                      C
parameters can be expressed as a function of amplitude terms                                                          S
                                                                                                                        D
                                                                                                                       3
by the following relations:                                                                                  P1

                                 ∗              ∗
                 S0 (z) = ax (z)ax (z) + ay (z)ay (z)                                       -45


                              ∗              ∗
              S1 (z) = ax (z)ax (z) − ay (z)ay (z)
                                                                                    H                             O
                                                                                                                                         V
                                 ∗              ∗                                   S
                                                                                                       2θ
                 S2 (z) = ax (z)ay (z) + ay (z)ax (z)                                   1


                               ∗        ∗                                                               P0
             S3 (z) = j[ax (z)ay (z) − ax (z)ay (z)].           (4)
                                                                                                                                   +45
Introducing the system (4) into (2) and assuming insignificant                                                                       S
                                                                                                                                     2

losses, which is consistent with the conservation of energy
along the fibre, i.e. dS0 /dz = 0 ⇒ S0 (z) = S0 (0) =                                                          C
                                                                                                                  G
constant, the differential system becomes
          dS1                                                       Figure 2. Evolution of the eigenpolarization in the rotary frame
            dz           0      2k2 2k1      S1 (z)                   versus the external twist rate.
          dS2  = −2k2          0           S2 (z)       (5)
            dz
            dS3        −2k1 −          0     S3 (z)                   along the fibre or at its output versus perturbation variation.
            dz
                      ∗                                               At this level of development, thanks to an experimental
where K = k12 = k21 = k1 + jk2 and = k22 − k11 in the
                                                                      measurement of the Stokes vectors, one can determine the
lossless case.
                                                                      physical parameters of the fibre, i.e. the coupling coefficients,
     Without depolarization, i.e. S0 = S1 + S2 + S3 the
                                   2      2     2     2

procedure developed by Franceschetti and Smith [6] enables            and then find whether the model used is valid or not. It should
one to solve this system and obtain the evolution of the              be underlined that this measurement will be valid only in the
Stokes parameters versus the distance z. When the coupling            case of a given incident state of polarization. For example,
coefficients are independent of z, and only in that case, the          predicting from one measurement how another polarization
solutions take the following form                                     state will behave will be quite impossible. The Mueller
                                                                      matrix has allowed us to get round this disadvantage.
S0 (z) = S0 (0) = 1
                                                                           Let us note that the output Stokes vector is linearly bound
         ˆ                  ˆ
S1 (z) = S1 N + (S1 (0) − S1 N) cos(δβz)                              to the input one, leading thus directly to the Mueller matrix
     ±(Sˆ3 S2 (0) − S2 S3 (0)) sin(δβz)
                    ˆ                                                 in the following form:
         ˆ                   ˆ                                                    
S2 (z) = S2 N + (S2 (0) − S2 N) cos(δβz)                                            1             0                        0
        ˆ1 S3 (0) − S3 S1 (0)) sin(δβz)
                    ˆ                                                             0       ˆ2        ˆ2
                                                                                          S1 + (1 − S1 )C         ˆ ˆ             ˆ
                                                                                                                 S1 S2 (1 − C) ± S3 S
     ±(S                                                              [M(z)] =          ˆ ˆ
                                                                                   0 S1 S2 (1 − C) S3 S  ˆ         ˆ         ˆ
                                                                                                                   S2 + (1 − S2 )C
                                                                                                                      2         2
            ˆ                 ˆ
S3 (z) = S3 N + (S3 (0) − S3 N) cos(δβz)                                                 ˆ ˆ              ˆ       ˆ ˆ
                                                                                    0 S1 S3 (1 − C) ± S2 S S2 S3 (1 − C) S1 S     ˆ
          ˆ             ˆ
     ±(S2 S1 (0) − S1 S2 (0)) sin(δβz)                       (6)                                                               
                                                                                                                    0
                      ˆ          ˆ           ˆ
where N = S1 S1 (0) + S2 S2 (0) + S3 S3 (0) and S =        ˆ                                             ˆ ˆ               ˆ
                                                                                                         S1 S3 (1 − C) S2 S 
    ˆ ˆ ˆ
[1, S1 , S2 , S3 ] is the Stokes vector of the eigenpolarization                                         ˆ2 S3 (1 − C) ± S1 S  (9)
                                                                                                         S   ˆ             ˆ
mode, i.e. the states of polarization that propagate without                                                ˆ2          ˆ2
                                                                                                            S3 + (1 − S3 )C
                                                  ˆ
transformation between the fibre ends. The Si components
are expressed as follows:                                             with C = cos(δβz) and S = sin(δβz).
                                                                           This matrix describes the most general case of elliptical
       ˆ                    ˆ          2k1         ˆ      2k2         birefringence without losses and depolarization. As the
       S1 = ±               S2 =                   S3 = ±       (7)
                  δβ                   δβ                 δβ          measurement of the Mueller matrix allows one to predict
                                                                      the behaviour of each incident state of polarization, the
                        δβ =       2   + 4|K|2 .                (8)   Poincar´ sphere representation can also be used to evaluate,
                                                                              e
In equation (8), δβ is the propagation constant difference            for example, the influence of various parameters such as the
of the two eigenpolarization modes. The set of equations (6)          external strain on polarization evolution.
permits us to describe the evolution of the state of polarization          Two particular cases will be discussed in the following:

                                                                                                                                             1619
P Olivard et al
                              1

                                                                                                                                m11
                           0.8

                                                                                                                                m12
                           0.6

                           0.4                                                                                                  m13


                           0.2                                                                                                  m21

                              0                                                                                                 m22

                           -0.2   0
                                                                                                                                m23

                           -0.4                                                                                                 m31

                           -0.6
                                                                                                                                m32

                           -0.8
                                                                                                                                m33

                             -1
                                         0.25       0.5            0.75         1         1.25             1.5
                                                          Twist rate (turns per meter)


                   Figure 3. Evolution of the Mueller matrix elements in the rotary frame versus the external twist rate.


2.1. Linearly-birefringent fibre                                                                                                 C
                                                                                                                                  D
                                                                                                                                S
                                                                                                                                 3
                                                                                                                       P1
Let us suppose a monomode optical fibre in which there
is a linear birefringence β expressed in radians per length                                          -45

unit, so that its fast axis makes an angle θ with respect to
the horizontal x-axis, the coupling coefficients are then [7]
                                                                                                                            O
2k11 = −2k22 = β cos 2θ and 2k12 = −2k21 = β sin 2θ .
                                                                                            H
                                                                                                                                            V
                                                                                            S                    2θ
                                                                                                 1
One can easily compute the parameters contained in the
                                                                                                                  P0
theoretical Mueller matrix defined in (9) so that
                                                                                                                                      +45
                                                                                                                                       S
                  2k1 = β sin 2θ         k2 = 0                                                                                         2




                                                                                                                        C
                   = −β cos 2θ          δβ = β.                                                                             G




The eigenpolarization Stokes vectors are then [1, cos 2θ,                     Figure 4. Evolution of the eigenpolarization in the laboratory
                                                                              frame versus the external twist rate.
  sin 2θ, 0] and the Mueller matrix takes the following well
known form with C = cos 2θ and S = sin 2θ :
                                                                              takes the following form:
           
             1          0                  0                                                                             
            0 C 2 + S 2 cos βz CS(1 − cos βz)                                                  1       0         0     0
[M(z)] =                                                                                      0 cos(gτ z) sin(gτ z) 0 
             0 CS(1 − cos βz) S 2 + C 2 cos βz                                     [M(z)] =                              .                    (11)
             0      −S sin βz          C sin βz                                                 0 − sin(gτ z) cos(gτ z) 0
                                                                                               0       0         0     1
                                           0
                                       S sin βz                              This is an ideal case. However, if the fibre exhibits a small
                                                 .     (10)
                                      −C sin βz                               intrinsic birefringence because of core imperfection, it will
                                        cos βz                                have significant effects on the polarization properties of the
                                                                              guide. Elliptical birefringence will occur which must be
This matrix corresponds to a linear birefringent network.
                                                                              carefully considered.
This form is valid only in an invariant frame and with a
uniform birefringence, i.e. with independence of z. This
matrix can be used to study the effect of bending with regard                 3. Elliptical birefringence
to the internal intrinsic birefringence of the fibre. However,
                                                                              Elliptical birefringence results from the superposition of
in the calculation of the birefringence, one should be cautious
                                                                              linear and circular birefringence. This becomes the case
of the beat length (transition at 2π ) and take into account the
                                                                              when a linearly-birefringent monomode fibre is uniformly
possible rotation of the fibre [8] local axis (see section 3).
                                                                              twisted. It then requires one to take into account the rotation
                                                                              of the fibre axes which, in the laboratory frame x, y, z, makes
2.2. Circular-birefringent fibre                                               the coupling coefficients dependent on z. As previously
                                                                              mentioned, the integration method giving the Mueller matrix
An isotropic monomode fibre under uniform twist τ (in                          is not valid in this case. To circumvent this problem, one
radians per length unit) holds a circular birefringence gτ ,                  must refer to the local frame of the fibre (x , y , z) depicted
where g is the elasto-optic coefficient of the material. In                    in figure 1. The coupling coefficients are then independent
such a case, the coupling coefficients can be expressed [9]                    of z and the Mueller matrix can be computed by the method
                                 ∗
as k11 = k22 = 0 and k12 = k21 = jgτ/2, so that k1 = 0,                       described above.
2k2 = gτ ,     = 0 and δβ = gτ . The eigenpolarization                             Let us consider a monomode fibre with a linear uniform
Stokes vectors are then [1, 0, 0, ±1] and the Mueller matrix                  retardance β expressed in radians per length unit so that its

1620
Measurement of optical fibre parameters
                               1

                                                                                                                            m11
                             0.8

                                                                                                                            m12
                             0.6

                             0.4                                                                                            m13


                             0.2                                                                                            m21

                               0                                                                                            m22

                            -0.2   0
                                                                                                                            m23

                            -0.4                                                                                            m31

                            -0.6
                                                                                                                            m32

                            -0.8
                                                                                                                            m33

                              -1

                                           1      2          3      4        5        6         7         8
                                                                 Twist rate (turns per meter)


                 Figure 5. Evolution of the Mueller matrix elements in the laboratory frame versus the external twist rate.




                                                  (a)                                               (b)
Figure 6. Evolution of the state of polarization for two different incident states (in the laboratory frame). (a) Horizontal incident state:
[1, 1, 0, 0]. (b) Elliptical incident state: [1, 0.578, −0.21, 0.788] (θ = −10◦ , ε = 26◦ ).

                                        vertical Quarter-wave                     Quarter-wave       Horizontal
                                       polarizer    plate                            plate           polarizer


                                                        ν                                  ν'


                             Laser                                      Sample                                          Detector

                                           P1           L1                                L2                   P2


                                                Figure 7. The schematic layout of the polarimeter.


fast axis makes an angle θ with respect to the horizontal                        A similar procedure was also followed by Sakai and Kimura
x -axis. If this fibre is submitted to an external twist τ (in                    [4] in Jones space.
radians per length unit), the coupling coefficients can be                             Let [M] be the Mueller matrix in the laboratory frame
expressed in the rotary frame by                                                 and [M ] that in the rotary frame. The following relation can
                                                                                 then be written:
                   2k11 = −2k22 = β cos 2θ
                      ∗                                                                             [M(z)] = [R(−2τ z)][M (z)]
              2k12 = k21 = β sin 2θ − jτ (2 − g)                   (12)
and the theoretical Mueller matrix takes the general form                                            [M (z)] = [R(2τ z)][M(z)].                      (13)
of equation (9). In the most general case, experimental
                                                                                 In the same way, let [S] be the Stokes vector in the laboratory
Mueller matrices are measured in the laboratory frame, so
                                                                                 frame and [S ] that in the rotary frame. One can thus write
they are not directly comparable to the theoretical model
and this does not allow an easy estimation of the physical                                                    [S(z)] = [R(−2τ z)]                   (14a)
parameters. There are two possible ways of solving this
problem: either the theoretical matrices are converted from                      and consequently
the rotary to the laboratory frame or, conversely, experimental
results are converted from the laboratory to the rotary frame.                                  [S(z)] = [R(−2τ z)][M (z)][S (0)].                  (14b)

                                                                                                                                                     1621
P Olivard et al

                            P        L   P1   L1     O1       F       O2          L2 P 2
            M3
                                                                                                      CAMERA




                                M1
            M2




                        LASER




                           P, P1, P2 : Linear polarizers                               M 1 , M 2 , M 3 : Dielectric mirrors
                            L1 , L2 : Quarter - wave plates                            O1 , O2 : Injection optics
                            L : Half - wave plate                                      F : Optical fiber

                                                    Figure 8. Experimental set-up.

                             1.20


                             0.80                                                                       m11

                                                                                                        m12

                             0.40                                                                       m13

                                                                                                        m21

                             0.00                                                                       m22

                                                                                                        m23

                             -0.40                                                                      m31

                                                                                                        m32

                             -0.80                                                                      m33




                             -1.20
                                 0.00              3.14            6.28    α z (rad)    9.42


                      Figure 9. Evolution of experimental Mueller matrice coefficients (in the laboratory frame).


When considering that the rotary frame coincides with the             monomode optical fibre of 1 m in length was considered and
laboratory frame at τ = 0 where there is no twist, the previous       the following physical parameters were used: g = 0.16,
relation becomes                                                      β = 0.52 rad m−1 , θ = 0.52 rad. The Mueller matrix
                                                                      coefficients were computed by introducing equation (12) into
              [S(z)] = [R(−2τ z)][M (z)][S(0)].            (15)
                                                                      (9) and applying relation (13). Equations (14) and (15)
Introducing equations (12) into (9) with the help of equations        were used for the Stokes vectors. The simulation results
(13)–(15) enables one to simulate the evolution of every              are presented by a couple of figures. Figures 2 and 3 were
coefficient of the Mueller matrix or of the Stokes vectors             obtained in the rotary frame and figures 4 and 5 in the
in the local or laboratory frame. These simulations can be            laboratory frame.
made versus z, the distance propagated in the guide, versus                The arrows in figures 3 and 5 indicate some characteristic
the physical parameters such as β or versus the external twist        points where the fibre can be considered as a pure polarization
strain τ . Thus, the Poincar´ sphere can be used to draw
                              e                                       rotator. One should note that all the coefficients of the last row
the trajectory of the polarization state using the normalized         and last column of the matrix (m33 being excepted) become
Stokes vector coefficients.                                            rapidly negligible, and the rotation characteristics are then
                                                                      preponderant. This fact was corroborated by the evolution
4. Simulations                                                        of the eigenstate of polarization which converged toward the
                                                                      north of the sphere. The two trajectories simulated on the
To illustrate the developments described in the previous              Poincar´ sphere (figure 6) indicate a reduction of oscillation
                                                                               e
section we will now present some simulation results. A                with twist and the rotation also appears to be preponderant.

1622
Measurement of optical fibre parameters
      500                                                                   Therefore, the 64 measured intensities were expressed by the
               (δβ)²
                                Experimental points
                                                                            following matrix
      400                       Quadratic interpolation
                                                                                                                         k = [0.15]
                                                                                     [Ik ] = [B][m1 ]           with                       (18)
                                                                                                                         l = [4i + j ]
      300
                                                                            where the coefficients of matrix B depend on the orientation
                                                                            angles of the quarter-wave plates L1 and L2 . The mij (i, j =
      200
                                                                            0.3) were represented by

                                                                                            [m1 ] = ([B]T [B])−1 [B]T [Ik ]                (19)
      100

                                                                            where []T indicates the transposed matrix.
       0
                                                                                 This equation can then be computed for each
            0.00              3.14          6.28   αz (rad)   9.42          measurement, and the expression ([B]T [B])−1 [B]T remains
                                                                            valid from one experience to each other.
                       Figure 10. Evolution of (δβ)2 .                           Figure 8 displays the experimental set-up. The light
                                                                            source is an ionized argon laser emitting at 514 nm. The
     These simulation results have shown that the intrinsic                 detection is made by a charged coupled device (CCD) camera.
linear birefringence of the fibre can exert a strong influence                The polarimeter was calibrated without the fibre and the
on the guided state of polarization even when this fibre is                  injection devices (O1 , O2 ). The axes of the various optical
submitted to a small twist strain.                                          elements were aligned with an accuracy of 0.01◦ using a
                                                                            dichotomous method at null intensity. A ‘χ 2 test’ [10] was
                                                                            carried out to quantify the influence of noise on the Mueller
5. Experimental methodology and measurement                                 matrix coefficients, which was thus minimized with respect
set-up                                                                      to these coefficients. The estimated standard deviation on
                                                                            each mij was then less than 0.5%.
The Mueller matrix was measured with an optical polarimeter
as schematized in figure 7. The input polarization encoding
                                                                            6. Results and analysis
system is composed of a vertical (y-axis) linear polarizer fol-
lowed by a quarter-wave plate whose fast axis makes an angle                The experimental results presented were obtained with a
ν with the y-axis. The output polarization decoding system                  0.865 m long monomode optical fibre submitted to a uniform
is composed of a quarter-wave plate whose fast axis makes                   small twist, i.e. less than 1.5 turn. Figure 9 illustrates the
an angle ν with respect to the reference y-axis, followed by                evolution of the experimental Mueller matrix coefficients
a linear horizontal polarizer. The two polarizers P1 and P2                 (birefringent elements only). Some characteristic points like
were initially crossed in order to obtain a null intensity.                 those defined in section 3 are viewed, so the fibre is then
     The Stokes vector S emerging from the last polarizer                   a pure rotator of polarization. The depolarization index of
can be expressed by the following matrix product                            each matrix was computed. As the depolarization index was
                                                                            always greater than 0.95, the depolarization was considered
                   S = [P2 ][L2 ][M][L1 ][P1 ]S(0)                   (16)
                                                                            as negligible. Moreover, it indicated that the theoretical
                                                                            model defined above could be used both to describe evolution
where [P2 ], [L2 ], [L1 ] and [P1 ] are the Mueller matrices of
                                                                            and to estimate the physical parameters.
each of the devices constituting the polarimeter, [M] is the
                                                                                 The experimental Mueller matrices represented by
unknown Mueller matrix and S(0) is the Stokes vector of the
                                                                            equation (9) were transferred from the laboratory frame (see
incident light. The measurable intensity is enclosed in the
                                                                            figure 9) to the rotary frame using equation (13). The physical
first term of the output Stokes vector and, in the case of perfect
                                                                            parameters g and β are enclosed in δβ, which was easily
optical devices, i.e. quarter-wave plate linear retardance equal
                                                                            computed to give the following expression:
to 90◦ , can be expressed as a function of the orientations of
                                                                                               3                            3
both the two quarter-wave plates (ν, ν ) and the 16 unknown
                                                                            δβ = tan−1               (mij − mj i )1/2            mii
Mueller matrix coefficients:
                                                                                            i=j =1                         i=1
S0 (ν, ν ) = I (ν, ν ) = m00 + m01 C 2 + m02 CS + m03 S                      = (β 2 + τ 2 (2 − g)2 )1/2 .                                  (20)
                          2                            2
    +(m10 + m11 C + m12 CS + m13 S)(−C )                                    These two parameters, g and β, were estimated with a
    +(m20 + m21 C 2 + m22 CS + m23 S)(−C S )                                quadratic method from the curve presented in figure 10. In
    +(m30 + m31 C 2 + m32 CS + m33 S)(S )                            (17)   this particular case we obtained g = 0.147 and β = 0.46 rad.
                                                                            The estimated elasto-optic coefficient was in good agreement
with C = cos 2ν, S = sin 2ν, C = cos 2ν and S = sin 2ν .                    with published results [5, 7, 9].
     One needs only 16 equations to obtain the Mueller matrix                    The direction of the linear birefringent axis, θ , can be
coefficients. To minimize errors we used an overdetermined                   computed in the rotary frame and takes the following form:
system of 64 equations corresponding to 64 combinations of
the angles (ν, ν ). These angles were multiples of 22.5◦ so                                  m31 − m13                    m23 + m32
                                                                              2θ = tan−1                       = tan−1                   . (21)
that they were capable of describing specific test positions.                                 m23 − m32                    m13 + m31

                                                                                                                                           1623
P Olivard et al

             1.00                                           1.00                                            1.00

             0.80                                           0.80                                            0.80

             0.60                                           0.60                                            0.60

             0.40                                           0.40                                            0.40

             0.20                                           0.20                                            0.20
       m11




                                                      m12




                                                                                                      m13
             0.00                                           0.00                                            0.00

             -0 .2 00.00   3.14   6.28         9.42         -0 .2 00.00   3.14    6.28     9.42             -0 .2 00.00         3.14   6.28   9.42

             -0 .4 0                                        -0 .4 0                                         -0 .4 0

             -0 .6 0                                        -0 .6 0                                         -0 .6 0

             -0 .8 0                                        -0 .8 0                                         -0 .8 0

             -1 .0 0                                        -1 .0 0                                         -1 .0 0




             1.00                                           1.00                                            1.00

             0.80                                           0.80                                            0.80

             0.60                                           0.60                                            0.60

             0.40                                           0.40                                            0.40

             0.20                                           0.20                                            0.20
       m21




                                                      m22




                                                                                                      m23
             0.00                                           0.00                                            0.00

             -0 .2 00.00   3.14   6.28         9.42         -0 .2 00.00   3.14    6.28     9.42             -0 .2 00.00         3.14   6.28   9.42

             -0 .4 0                                        -0 .4 0                                         -0 .4 0

             -0 .6 0                                        -0 .6 0                                         -0 .6 0

             -0 .8 0                                        -0 .8 0                                         -0 .8 0

             -1 .0 0                                        -1 .0 0                                         -1 .0 0




             1.00                                           1.00                                            1.00

             0.80                                           0.80                                            0.80

             0.60                                           0.60                                            0.60

             0.40                                           0.40                                            0.40

             0.20                                           0.20                                            0.20
       m31




                                                      m32




                                                                                                      m33
             0.00                                           0.00                                            0.00

             -0 .2 00.00   3.14   6.28         9.42         -0 .2 00.00   3.14    6.28     9.42             -0 .2 00.00         3.14   6.28   9.42

             -0 .4 0                                        -0 .4 0                                         -0 .4 0

             -0 .6 0                                        -0 .6 0                                         -0 .6 0

             -0 .8 0                                        -0 .8 0                                         -0 .8 0

             -1 .0 0                                        -1 .0 0                                         -1 .0 0




Figure 11. Measured Mueller matrices versus the external twist αz (in rad). The curves represent the theoretical constructions obtained
with equation (13).


                                                                          Experimental




                                                                          theoretical

                                         (a)                                                                              (b)
Figure 12. Evolution of the state of polarization for two different incident states. Theoretical and experimental trajectories. (a) Horizontal
incident state: [1, 1, 0, 0]. (b) Elliptical incident state: [1, 0.578, −0.21, 0.788] (θ = −10◦ , ε = 26◦ ).

However, the value obtained from this expression is not                            7. Conclusion
constant, and this implies that the assumption of uniform
linear birefringence is not valid. Consequently, to compare                        An experimental method to determine the physical
our measurements with the theoretical model, we computed                           parameters of a monomode optical fibre has been presented.
the mean value of θ and obtained θ = −0.52 rad.                                    The theoretical model derived from the coupled-mode
     With the estimated physical parameters we adjusted                            equations applied to the Stokes–Mueller formalism gave
the theoretical model and compared it to the experimental                          good results. It showed that the non-uniformity of the linear
results (figure 11). The agreement between theory and                               birefringence could influence polarization behaviour.
measurement was quite good. Nevertheless, there were                                    The Stokes–Mueller formalism is a powerful tool for
discrepancies in the coefficients m13 , m23 , m31 and m32 ; they                    carrying out experimental studies of polarization phenomena
were likely to be due to the non-uniformity of the intrinsic                       in optical devices. The Poincar´ sphere, directly connected
                                                                                                                    e
linear birefringence of the fibre. The comparison between                           to the Stokes–Mueller formalism, also constitutes a useful
theoretical and measured trajectories on the Poincar´ sphere
                                                        e                          representation to describe the evolution of the polarization
(figure 12) showed identical rotary behaviour and a decrease                        state. The results reported here have shown that these
of the ellipticity with twist. This result corroborates the non-                   formalisms are very relevant in the characterization of optical
uniformity of the linear intrinsic birefringence.                                  fibres.

1624
Measurement of optical fibre parameters

References                                                         [6] Franceschetti G and Smith C P 1981 Representation of the
                                                                         polarization of single-mode fibers using Stokes
[1] Eftimov T A and Bock W J 1992 Experimental investigation             parameters J. Opt. Soc. Am. 71 1487–91
       of single-mode single-polarization optical fiber J. Opt.     [7] Rashleigh S C 1983 Origins and control of polarization
       Soc. Am. 17 1061–3                                                effects in single-mode fibres IEEE J. Light. Technol. 1
[2] Brown C S, Shute M W, Williams D D and Muhammed F                    312–31
       1994 The development and calibration of an optical fibre     [8] Olivard P, Cariou J, Le Jeune B and Lotrian J 1995
       polarimeter Proc. SPIE 2265 62–9                                     e             `
                                                                         D´ termination, a partir de matrices de Mueller
[3] Yariv A 1973 Coupled-mode theory for guided-wave optics               exp´ rimentales, de la birefringence induite par courbure
                                                                              e
       IEEE J. Quantum Electron. 9 919–33                                                                        `
                                                                          dans une fibre optique monomode 15 eme Journ´ es. Nat.
                                                                                                                            e
[4] Sakai J and Kimura T 1981 Birefringence and                           Opt. Guid´ e (Palaiseau, France, November 1995) p 27
                                                                                    e
       polarization characteristics of single-mode optical fibre    [9] Ulrich R and Simon A 1979 Polarization optics of twisted
       under elastic deformations IEEE J. Quantum Electron. 17            single-mode fibres Appl. Opt. 18 2241–51
       1041–51                                                    [10] Le Jeune B, Marie J P, Gerligand P Y, Cariou J and Lotrian J
[5] Monerie M and Jeunhomme L 1980 Polarization mode                      1994 Mueller matrix formalism in imagery. An
       coupling in long single mode fibres Opt. Quantum                    experimental arrangement for noise reduction Proc SPIE
       Electron. 12 449–61                                                2265 443–51




                                                                                                                              1625

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10.1.1.115.1297

  • 1. J. Phys. D: Appl. Phys. 32 (1999) 1618–1625. Printed in the UK PII: S0022-3727(99)01288-7 Measurement of optical fibre parameters using an optical polarimeter and Stokes–Mueller formalism P Olivard, P Y Gerligand, B Le Jeune, J Cariou and J Lotrian Laboratoire de Spectrom´ trie et Optique Laser, Universit´ de Bretagne Occidentale, 6, e e Avenue Le Gorgeu BP 809, 29285 Brest Cedex, France E-mail: Pascal.Olivard@univ-brest.fr Received 26 January 1999 Abstract. Although the method based on the Mueller matrix for the experimental determination of optical-device polarization behaviour is a powerful tool, it has rarely been applied to optical fibre. This paper introduces an experimental methodology for measuring the Mueller matrices of monomode fibre under uniform strains. Using a theoretical model derived from coupled-mode equations, we were able to first estimate the physical parameters of the fibre, then to use them to both test the model validity and assess their influence on the polarimetric behaviour of the fibre. 1. Introduction 2. Theoretical foundations The polarization characteristics of optical fibre are of In the ideal case of an isotropic monomode optical fibre two particular interest in systems like optical sensors or degenerate modes can propagate. This degeneracy indicates interferometers using fibre. Although the Mueller matrix- that their propagation constants are identical, and so all the based technique has rarely been used to describe polarization states of polarization launched at the input face of the guide phenomena in monomode optical fibres, Mueller matrix will not be transformed during the propagation process. In polarimetry is powerful for the experimental determination the case of external perturbations or of imperfections in the of the polarization properties of optical devices like fibre core the degeneracy is lifted, the value of the propagation linear or circular birefringence, dichroism, losses, rotary constant is modified and then the two modes are coupled. If power or also depolarization. Thus, an experimental ax and ay are the amplitudes of these modes which are now Mueller matrix enables one to predict the transformation z-dependent, the total electric field in the weakly guiding of every state of polarization by propagation through approximation can be described by the following expression the medium characterized by the matrix. Furthermore, where the time dependence exp(−jωt) is assumed and not the coupled-mode theory well known in the Jones expressed: space (amplitude terms), transformed into Stokes space (energetic terms), has rarely been investigated experimentally E(x, y, z) = ax (z)Ex (x, y) + ay (z)Ey (x, y). (1) [1, 2]. The present paper is organized as follows: the theoretical The coupling process can be described [3] by a set of two development permitting the description of polarization first-order differential equations on the hypothesis that the evolution in a uniformly perturbed fibre is recalled in variations be small versus z, the distance travelled along the section 2 where the theoretical Mueller matrix used is guide. This set is written as follows presented. Section 3 reports on some simulation results in the case of elliptical birefringence, whereas section 4 describes d ax (z) k k12 ax (z) = −j 11 (2) the experimental method applied to measure the Mueller dz ay (z) k21 k22 ay (z) matrix. Some experimental results and the methodology to extract physical parameters, i.e. coupling coefficients, are where kij are the coupling coefficients and d/dz indicates the presented in sections 5 and 6. Conclusions are presented in derivative with regard to z. This system can be directly solved section 7. [4, 5], and its solutions are then expressed as a function of 0022-3727/99/141618+08$30.00 © 1999 IOP Publishing Ltd
  • 2. Measurement of optical fibre parameters both the initial conditions, i.e. the input state of polarization, and the coupling coefficients: y y' ax = f (ax (0), ay (0), z, kij ). (3) τ x' ay Unfortunately, this formulation has some drawbacks for z x carrying out experimental investigations: first, the quantities are complex and thus difficult to measure and second, depolarization is not considered. These considerations led us Figure 1. A representation of the laboratory (x, y) and rotary to integrate this system in Stokes space where all quantities (x , y ) frame. have an energy dimension. In consequence, the four Stokes C parameters can be expressed as a function of amplitude terms S D 3 by the following relations: P1 ∗ ∗ S0 (z) = ax (z)ax (z) + ay (z)ay (z) -45 ∗ ∗ S1 (z) = ax (z)ax (z) − ay (z)ay (z) H O V ∗ ∗ S 2θ S2 (z) = ax (z)ay (z) + ay (z)ax (z) 1 ∗ ∗ P0 S3 (z) = j[ax (z)ay (z) − ax (z)ay (z)]. (4) +45 Introducing the system (4) into (2) and assuming insignificant S 2 losses, which is consistent with the conservation of energy along the fibre, i.e. dS0 /dz = 0 ⇒ S0 (z) = S0 (0) = C G constant, the differential system becomes  dS1  Figure 2. Evolution of the eigenpolarization in the rotary frame dz 0 2k2 2k1 S1 (z) versus the external twist rate.  dS2  = −2k2 0 S2 (z) (5) dz dS3 −2k1 − 0 S3 (z) along the fibre or at its output versus perturbation variation. dz ∗ At this level of development, thanks to an experimental where K = k12 = k21 = k1 + jk2 and = k22 − k11 in the measurement of the Stokes vectors, one can determine the lossless case. physical parameters of the fibre, i.e. the coupling coefficients, Without depolarization, i.e. S0 = S1 + S2 + S3 the 2 2 2 2 procedure developed by Franceschetti and Smith [6] enables and then find whether the model used is valid or not. It should one to solve this system and obtain the evolution of the be underlined that this measurement will be valid only in the Stokes parameters versus the distance z. When the coupling case of a given incident state of polarization. For example, coefficients are independent of z, and only in that case, the predicting from one measurement how another polarization solutions take the following form state will behave will be quite impossible. The Mueller matrix has allowed us to get round this disadvantage. S0 (z) = S0 (0) = 1 Let us note that the output Stokes vector is linearly bound ˆ ˆ S1 (z) = S1 N + (S1 (0) − S1 N) cos(δβz) to the input one, leading thus directly to the Mueller matrix ±(Sˆ3 S2 (0) − S2 S3 (0)) sin(δβz) ˆ in the following form: ˆ ˆ  S2 (z) = S2 N + (S2 (0) − S2 N) cos(δβz) 1 0 0 ˆ1 S3 (0) − S3 S1 (0)) sin(δβz) ˆ 0 ˆ2 ˆ2 S1 + (1 − S1 )C ˆ ˆ ˆ S1 S2 (1 − C) ± S3 S ±(S [M(z)] =  ˆ ˆ  0 S1 S2 (1 − C) S3 S ˆ ˆ ˆ S2 + (1 − S2 )C 2 2 ˆ ˆ S3 (z) = S3 N + (S3 (0) − S3 N) cos(δβz) ˆ ˆ ˆ ˆ ˆ 0 S1 S3 (1 − C) ± S2 S S2 S3 (1 − C) S1 S ˆ ˆ ˆ ±(S2 S1 (0) − S1 S2 (0)) sin(δβz) (6)  0 ˆ ˆ ˆ where N = S1 S1 (0) + S2 S2 (0) + S3 S3 (0) and S = ˆ ˆ ˆ ˆ S1 S3 (1 − C) S2 S  ˆ ˆ ˆ [1, S1 , S2 , S3 ] is the Stokes vector of the eigenpolarization ˆ2 S3 (1 − C) ± S1 S  (9) S ˆ ˆ mode, i.e. the states of polarization that propagate without ˆ2 ˆ2 S3 + (1 − S3 )C ˆ transformation between the fibre ends. The Si components are expressed as follows: with C = cos(δβz) and S = sin(δβz). This matrix describes the most general case of elliptical ˆ ˆ 2k1 ˆ 2k2 birefringence without losses and depolarization. As the S1 = ± S2 = S3 = ± (7) δβ δβ δβ measurement of the Mueller matrix allows one to predict the behaviour of each incident state of polarization, the δβ = 2 + 4|K|2 . (8) Poincar´ sphere representation can also be used to evaluate, e In equation (8), δβ is the propagation constant difference for example, the influence of various parameters such as the of the two eigenpolarization modes. The set of equations (6) external strain on polarization evolution. permits us to describe the evolution of the state of polarization Two particular cases will be discussed in the following: 1619
  • 3. P Olivard et al 1 m11 0.8 m12 0.6 0.4 m13 0.2 m21 0 m22 -0.2 0 m23 -0.4 m31 -0.6 m32 -0.8 m33 -1 0.25 0.5 0.75 1 1.25 1.5 Twist rate (turns per meter) Figure 3. Evolution of the Mueller matrix elements in the rotary frame versus the external twist rate. 2.1. Linearly-birefringent fibre C D S 3 P1 Let us suppose a monomode optical fibre in which there is a linear birefringence β expressed in radians per length -45 unit, so that its fast axis makes an angle θ with respect to the horizontal x-axis, the coupling coefficients are then [7] O 2k11 = −2k22 = β cos 2θ and 2k12 = −2k21 = β sin 2θ . H V S 2θ 1 One can easily compute the parameters contained in the P0 theoretical Mueller matrix defined in (9) so that +45 S 2k1 = β sin 2θ k2 = 0 2 C = −β cos 2θ δβ = β. G The eigenpolarization Stokes vectors are then [1, cos 2θ, Figure 4. Evolution of the eigenpolarization in the laboratory frame versus the external twist rate. sin 2θ, 0] and the Mueller matrix takes the following well known form with C = cos 2θ and S = sin 2θ : takes the following form:  1 0 0    0 C 2 + S 2 cos βz CS(1 − cos βz) 1 0 0 0 [M(z)] =   0 cos(gτ z) sin(gτ z) 0  0 CS(1 − cos βz) S 2 + C 2 cos βz [M(z)] =  . (11) 0 −S sin βz C sin βz 0 − sin(gτ z) cos(gτ z) 0  0 0 0 1 0 S sin βz  This is an ideal case. However, if the fibre exhibits a small . (10) −C sin βz intrinsic birefringence because of core imperfection, it will cos βz have significant effects on the polarization properties of the guide. Elliptical birefringence will occur which must be This matrix corresponds to a linear birefringent network. carefully considered. This form is valid only in an invariant frame and with a uniform birefringence, i.e. with independence of z. This matrix can be used to study the effect of bending with regard 3. Elliptical birefringence to the internal intrinsic birefringence of the fibre. However, Elliptical birefringence results from the superposition of in the calculation of the birefringence, one should be cautious linear and circular birefringence. This becomes the case of the beat length (transition at 2π ) and take into account the when a linearly-birefringent monomode fibre is uniformly possible rotation of the fibre [8] local axis (see section 3). twisted. It then requires one to take into account the rotation of the fibre axes which, in the laboratory frame x, y, z, makes 2.2. Circular-birefringent fibre the coupling coefficients dependent on z. As previously mentioned, the integration method giving the Mueller matrix An isotropic monomode fibre under uniform twist τ (in is not valid in this case. To circumvent this problem, one radians per length unit) holds a circular birefringence gτ , must refer to the local frame of the fibre (x , y , z) depicted where g is the elasto-optic coefficient of the material. In in figure 1. The coupling coefficients are then independent such a case, the coupling coefficients can be expressed [9] of z and the Mueller matrix can be computed by the method ∗ as k11 = k22 = 0 and k12 = k21 = jgτ/2, so that k1 = 0, described above. 2k2 = gτ , = 0 and δβ = gτ . The eigenpolarization Let us consider a monomode fibre with a linear uniform Stokes vectors are then [1, 0, 0, ±1] and the Mueller matrix retardance β expressed in radians per length unit so that its 1620
  • 4. Measurement of optical fibre parameters 1 m11 0.8 m12 0.6 0.4 m13 0.2 m21 0 m22 -0.2 0 m23 -0.4 m31 -0.6 m32 -0.8 m33 -1 1 2 3 4 5 6 7 8 Twist rate (turns per meter) Figure 5. Evolution of the Mueller matrix elements in the laboratory frame versus the external twist rate. (a) (b) Figure 6. Evolution of the state of polarization for two different incident states (in the laboratory frame). (a) Horizontal incident state: [1, 1, 0, 0]. (b) Elliptical incident state: [1, 0.578, −0.21, 0.788] (θ = −10◦ , ε = 26◦ ). vertical Quarter-wave Quarter-wave Horizontal polarizer plate plate polarizer ν ν' Laser Sample Detector P1 L1 L2 P2 Figure 7. The schematic layout of the polarimeter. fast axis makes an angle θ with respect to the horizontal A similar procedure was also followed by Sakai and Kimura x -axis. If this fibre is submitted to an external twist τ (in [4] in Jones space. radians per length unit), the coupling coefficients can be Let [M] be the Mueller matrix in the laboratory frame expressed in the rotary frame by and [M ] that in the rotary frame. The following relation can then be written: 2k11 = −2k22 = β cos 2θ ∗ [M(z)] = [R(−2τ z)][M (z)] 2k12 = k21 = β sin 2θ − jτ (2 − g) (12) and the theoretical Mueller matrix takes the general form [M (z)] = [R(2τ z)][M(z)]. (13) of equation (9). In the most general case, experimental In the same way, let [S] be the Stokes vector in the laboratory Mueller matrices are measured in the laboratory frame, so frame and [S ] that in the rotary frame. One can thus write they are not directly comparable to the theoretical model and this does not allow an easy estimation of the physical [S(z)] = [R(−2τ z)] (14a) parameters. There are two possible ways of solving this problem: either the theoretical matrices are converted from and consequently the rotary to the laboratory frame or, conversely, experimental results are converted from the laboratory to the rotary frame. [S(z)] = [R(−2τ z)][M (z)][S (0)]. (14b) 1621
  • 5. P Olivard et al P L P1 L1 O1 F O2 L2 P 2 M3 CAMERA M1 M2 LASER P, P1, P2 : Linear polarizers M 1 , M 2 , M 3 : Dielectric mirrors L1 , L2 : Quarter - wave plates O1 , O2 : Injection optics L : Half - wave plate F : Optical fiber Figure 8. Experimental set-up. 1.20 0.80 m11 m12 0.40 m13 m21 0.00 m22 m23 -0.40 m31 m32 -0.80 m33 -1.20 0.00 3.14 6.28 α z (rad) 9.42 Figure 9. Evolution of experimental Mueller matrice coefficients (in the laboratory frame). When considering that the rotary frame coincides with the monomode optical fibre of 1 m in length was considered and laboratory frame at τ = 0 where there is no twist, the previous the following physical parameters were used: g = 0.16, relation becomes β = 0.52 rad m−1 , θ = 0.52 rad. The Mueller matrix coefficients were computed by introducing equation (12) into [S(z)] = [R(−2τ z)][M (z)][S(0)]. (15) (9) and applying relation (13). Equations (14) and (15) Introducing equations (12) into (9) with the help of equations were used for the Stokes vectors. The simulation results (13)–(15) enables one to simulate the evolution of every are presented by a couple of figures. Figures 2 and 3 were coefficient of the Mueller matrix or of the Stokes vectors obtained in the rotary frame and figures 4 and 5 in the in the local or laboratory frame. These simulations can be laboratory frame. made versus z, the distance propagated in the guide, versus The arrows in figures 3 and 5 indicate some characteristic the physical parameters such as β or versus the external twist points where the fibre can be considered as a pure polarization strain τ . Thus, the Poincar´ sphere can be used to draw e rotator. One should note that all the coefficients of the last row the trajectory of the polarization state using the normalized and last column of the matrix (m33 being excepted) become Stokes vector coefficients. rapidly negligible, and the rotation characteristics are then preponderant. This fact was corroborated by the evolution 4. Simulations of the eigenstate of polarization which converged toward the north of the sphere. The two trajectories simulated on the To illustrate the developments described in the previous Poincar´ sphere (figure 6) indicate a reduction of oscillation e section we will now present some simulation results. A with twist and the rotation also appears to be preponderant. 1622
  • 6. Measurement of optical fibre parameters 500 Therefore, the 64 measured intensities were expressed by the (δβ)² Experimental points following matrix 400 Quadratic interpolation k = [0.15] [Ik ] = [B][m1 ] with (18) l = [4i + j ] 300 where the coefficients of matrix B depend on the orientation angles of the quarter-wave plates L1 and L2 . The mij (i, j = 200 0.3) were represented by [m1 ] = ([B]T [B])−1 [B]T [Ik ] (19) 100 where []T indicates the transposed matrix. 0 This equation can then be computed for each 0.00 3.14 6.28 αz (rad) 9.42 measurement, and the expression ([B]T [B])−1 [B]T remains valid from one experience to each other. Figure 10. Evolution of (δβ)2 . Figure 8 displays the experimental set-up. The light source is an ionized argon laser emitting at 514 nm. The These simulation results have shown that the intrinsic detection is made by a charged coupled device (CCD) camera. linear birefringence of the fibre can exert a strong influence The polarimeter was calibrated without the fibre and the on the guided state of polarization even when this fibre is injection devices (O1 , O2 ). The axes of the various optical submitted to a small twist strain. elements were aligned with an accuracy of 0.01◦ using a dichotomous method at null intensity. A ‘χ 2 test’ [10] was carried out to quantify the influence of noise on the Mueller 5. Experimental methodology and measurement matrix coefficients, which was thus minimized with respect set-up to these coefficients. The estimated standard deviation on each mij was then less than 0.5%. The Mueller matrix was measured with an optical polarimeter as schematized in figure 7. The input polarization encoding 6. Results and analysis system is composed of a vertical (y-axis) linear polarizer fol- lowed by a quarter-wave plate whose fast axis makes an angle The experimental results presented were obtained with a ν with the y-axis. The output polarization decoding system 0.865 m long monomode optical fibre submitted to a uniform is composed of a quarter-wave plate whose fast axis makes small twist, i.e. less than 1.5 turn. Figure 9 illustrates the an angle ν with respect to the reference y-axis, followed by evolution of the experimental Mueller matrix coefficients a linear horizontal polarizer. The two polarizers P1 and P2 (birefringent elements only). Some characteristic points like were initially crossed in order to obtain a null intensity. those defined in section 3 are viewed, so the fibre is then The Stokes vector S emerging from the last polarizer a pure rotator of polarization. The depolarization index of can be expressed by the following matrix product each matrix was computed. As the depolarization index was always greater than 0.95, the depolarization was considered S = [P2 ][L2 ][M][L1 ][P1 ]S(0) (16) as negligible. Moreover, it indicated that the theoretical model defined above could be used both to describe evolution where [P2 ], [L2 ], [L1 ] and [P1 ] are the Mueller matrices of and to estimate the physical parameters. each of the devices constituting the polarimeter, [M] is the The experimental Mueller matrices represented by unknown Mueller matrix and S(0) is the Stokes vector of the equation (9) were transferred from the laboratory frame (see incident light. The measurable intensity is enclosed in the figure 9) to the rotary frame using equation (13). The physical first term of the output Stokes vector and, in the case of perfect parameters g and β are enclosed in δβ, which was easily optical devices, i.e. quarter-wave plate linear retardance equal computed to give the following expression: to 90◦ , can be expressed as a function of the orientations of 3 3 both the two quarter-wave plates (ν, ν ) and the 16 unknown δβ = tan−1 (mij − mj i )1/2 mii Mueller matrix coefficients: i=j =1 i=1 S0 (ν, ν ) = I (ν, ν ) = m00 + m01 C 2 + m02 CS + m03 S = (β 2 + τ 2 (2 − g)2 )1/2 . (20) 2 2 +(m10 + m11 C + m12 CS + m13 S)(−C ) These two parameters, g and β, were estimated with a +(m20 + m21 C 2 + m22 CS + m23 S)(−C S ) quadratic method from the curve presented in figure 10. In +(m30 + m31 C 2 + m32 CS + m33 S)(S ) (17) this particular case we obtained g = 0.147 and β = 0.46 rad. The estimated elasto-optic coefficient was in good agreement with C = cos 2ν, S = sin 2ν, C = cos 2ν and S = sin 2ν . with published results [5, 7, 9]. One needs only 16 equations to obtain the Mueller matrix The direction of the linear birefringent axis, θ , can be coefficients. To minimize errors we used an overdetermined computed in the rotary frame and takes the following form: system of 64 equations corresponding to 64 combinations of the angles (ν, ν ). These angles were multiples of 22.5◦ so m31 − m13 m23 + m32 2θ = tan−1 = tan−1 . (21) that they were capable of describing specific test positions. m23 − m32 m13 + m31 1623
  • 7. P Olivard et al 1.00 1.00 1.00 0.80 0.80 0.80 0.60 0.60 0.60 0.40 0.40 0.40 0.20 0.20 0.20 m11 m12 m13 0.00 0.00 0.00 -0 .2 00.00 3.14 6.28 9.42 -0 .2 00.00 3.14 6.28 9.42 -0 .2 00.00 3.14 6.28 9.42 -0 .4 0 -0 .4 0 -0 .4 0 -0 .6 0 -0 .6 0 -0 .6 0 -0 .8 0 -0 .8 0 -0 .8 0 -1 .0 0 -1 .0 0 -1 .0 0 1.00 1.00 1.00 0.80 0.80 0.80 0.60 0.60 0.60 0.40 0.40 0.40 0.20 0.20 0.20 m21 m22 m23 0.00 0.00 0.00 -0 .2 00.00 3.14 6.28 9.42 -0 .2 00.00 3.14 6.28 9.42 -0 .2 00.00 3.14 6.28 9.42 -0 .4 0 -0 .4 0 -0 .4 0 -0 .6 0 -0 .6 0 -0 .6 0 -0 .8 0 -0 .8 0 -0 .8 0 -1 .0 0 -1 .0 0 -1 .0 0 1.00 1.00 1.00 0.80 0.80 0.80 0.60 0.60 0.60 0.40 0.40 0.40 0.20 0.20 0.20 m31 m32 m33 0.00 0.00 0.00 -0 .2 00.00 3.14 6.28 9.42 -0 .2 00.00 3.14 6.28 9.42 -0 .2 00.00 3.14 6.28 9.42 -0 .4 0 -0 .4 0 -0 .4 0 -0 .6 0 -0 .6 0 -0 .6 0 -0 .8 0 -0 .8 0 -0 .8 0 -1 .0 0 -1 .0 0 -1 .0 0 Figure 11. Measured Mueller matrices versus the external twist αz (in rad). The curves represent the theoretical constructions obtained with equation (13). Experimental theoretical (a) (b) Figure 12. Evolution of the state of polarization for two different incident states. Theoretical and experimental trajectories. (a) Horizontal incident state: [1, 1, 0, 0]. (b) Elliptical incident state: [1, 0.578, −0.21, 0.788] (θ = −10◦ , ε = 26◦ ). However, the value obtained from this expression is not 7. Conclusion constant, and this implies that the assumption of uniform linear birefringence is not valid. Consequently, to compare An experimental method to determine the physical our measurements with the theoretical model, we computed parameters of a monomode optical fibre has been presented. the mean value of θ and obtained θ = −0.52 rad. The theoretical model derived from the coupled-mode With the estimated physical parameters we adjusted equations applied to the Stokes–Mueller formalism gave the theoretical model and compared it to the experimental good results. It showed that the non-uniformity of the linear results (figure 11). The agreement between theory and birefringence could influence polarization behaviour. measurement was quite good. Nevertheless, there were The Stokes–Mueller formalism is a powerful tool for discrepancies in the coefficients m13 , m23 , m31 and m32 ; they carrying out experimental studies of polarization phenomena were likely to be due to the non-uniformity of the intrinsic in optical devices. The Poincar´ sphere, directly connected e linear birefringence of the fibre. The comparison between to the Stokes–Mueller formalism, also constitutes a useful theoretical and measured trajectories on the Poincar´ sphere e representation to describe the evolution of the polarization (figure 12) showed identical rotary behaviour and a decrease state. The results reported here have shown that these of the ellipticity with twist. This result corroborates the non- formalisms are very relevant in the characterization of optical uniformity of the linear intrinsic birefringence. fibres. 1624
  • 8. Measurement of optical fibre parameters References [6] Franceschetti G and Smith C P 1981 Representation of the polarization of single-mode fibers using Stokes [1] Eftimov T A and Bock W J 1992 Experimental investigation parameters J. Opt. Soc. Am. 71 1487–91 of single-mode single-polarization optical fiber J. Opt. [7] Rashleigh S C 1983 Origins and control of polarization Soc. Am. 17 1061–3 effects in single-mode fibres IEEE J. Light. Technol. 1 [2] Brown C S, Shute M W, Williams D D and Muhammed F 312–31 1994 The development and calibration of an optical fibre [8] Olivard P, Cariou J, Le Jeune B and Lotrian J 1995 polarimeter Proc. SPIE 2265 62–9 e ` D´ termination, a partir de matrices de Mueller [3] Yariv A 1973 Coupled-mode theory for guided-wave optics exp´ rimentales, de la birefringence induite par courbure e IEEE J. Quantum Electron. 9 919–33 ` dans une fibre optique monomode 15 eme Journ´ es. Nat. e [4] Sakai J and Kimura T 1981 Birefringence and Opt. Guid´ e (Palaiseau, France, November 1995) p 27 e polarization characteristics of single-mode optical fibre [9] Ulrich R and Simon A 1979 Polarization optics of twisted under elastic deformations IEEE J. Quantum Electron. 17 single-mode fibres Appl. Opt. 18 2241–51 1041–51 [10] Le Jeune B, Marie J P, Gerligand P Y, Cariou J and Lotrian J [5] Monerie M and Jeunhomme L 1980 Polarization mode 1994 Mueller matrix formalism in imagery. An coupling in long single mode fibres Opt. Quantum experimental arrangement for noise reduction Proc SPIE Electron. 12 449–61 2265 443–51 1625