6.1 INTRODUCTION • An ideal fiber has a perfectly circular cross section, a truly rotational symmetric index profile ( not disturbances).
• An idealfiber would support two degenerate and orthogonal polarization modes. All real single mode fibers support two nearly degenerated polarization modes and should be labeled as single mode doublepolarization fibers.
The two orthogonal degenerate modes in an axially symmetric single-mode fiber can be termed HE11x and HE11y , with their principal electric field components lying along the x and ydirections respectively. Alternatively, the two modes can be represented as clockwise and counterclockwise circularly polarized HE11 modes. A more common notation used to represent the modes—which abandonsspecific mention of the electric and magnetic fields—is the linearly polarized (LP) notation. In this notation, HE11 becomes LP01.
However…. • Imperfections are introduced in the manufacturingprocess. • Perturbation are introduced in the cabling process. • Additionally, the fiber may be pressed, bent, or twist during the installation or in the experiment. In real single mode fibers, theeffective index (nx, ny) of the two polarizations modes are slightly different. Therefore, single mode fibers are slightly birefringence.
a) Symmetrical fiber, where the index ellipsoid is a circleb) Fiber with birefringence (X,Y slow and fast axes)
Polarization characteristics of transmission
Ideal single mode fiber
Conventional single mode fiber
LB6.2 The origin of the optical fiber birefringence
Non circular cross section Internal (structural)
Bending and Twist External Externally applied stress or fields
6.2.1Polarization parameters for linear birefringent fibers
nx n y
Beat length 2 LB x y Bm Polarization Extinction ratio
PER (dB) ...