Characterization of an axisymmetric birefringent optical fiber

Anisotropy in the dielectric constant of a medium having a periodic structure whose period is less than the wavelength of light is known as "form birefringence". Axisymmetric periodic media such as annular layers are expected to have axisymmetric birefringence and mode coupling phenomena in such a waveguide is analyzed theoretically. This paper reports the experimental characterization of the axisymmetric birefringence in an axisymmetric optical fiber. An axisymmetric optical fiber is drawn from an annular layered structure preform rod, whose core diameter and the relative refractive index difference between the core and the cladding are 4 ?m and 1.14 %, respectively. There can be 6 modes in total at a wavelength of 1.319 ?m and some of these modes couple with each other during propagation along the test fiber. The polarization behavior through the mode coupling is investigated theoretically and the axisymmetric birefringence of the test fiber is evaluated experimentally to be 4.1 × 10-5 from the fiber length dependence of the output light polarization state. This value is in good agreement with the theoretical value 4.7 × 10-5 estimated from the structural parameters of the preform rod from which the test fiber is drawn. The stress induced birefringence is also evaluated to be 5.9 × 10-8, which coincides with those of the standard single mode fibers and much less than the axisymmetric birefringence by three orders of magnitude.