Managing Polarization Mode Dispersion

Michel W. Chbat

The increasing demand for bandwidth is driving most telecommunications operators toward the deployment of large-capacity transmission systems. Systems based on 10-Gb/s channel rates are being deployed, and suppliers have announced plans for channel rates as high as 40 Gb/s. Polarization mode dispersion represents a major impairment for high- bit-rate systems, producing pulse broadening and distortion, thus leading to performance degradation.

Polarization mode dispersion in optical fiber stems from the breakup of circular symmetry in the core and cladding. Ideally, this symmetry renders the fiber perfectly isotropic; i.e., nonbirefringent. The fiber's index of refraction is independent of the orientation of the electric field or, equivalently, the polarization of the light. Light propagation in single-mode fibers is governed by two orthogonally polarized fundamental modes, which, in the case of ideal fibers, are degenerate (indistinguishable).

The degeneracy can be lifted if, through a loss of the circular symmetry, any amount of anisotropy is introduced, leading to some birefringence. This is the general case of real fibers, where the loss of symmetry originates in the fiber manufacturing process from noncircular waveguide geometry (geometrical birefringence -- static) or in deployed fiber from nonsymmetrically distributed mechanical stress (stress birefringence -- time varying). . . .