Optical axis (z)

A fabrication process for Fiber Bragg Gratings (FBGs) for the reduction of polarization dependent properties is proposed in this paper. The fiber is twisted, illuminated, and finally relaxed. The maximum of the transversal refractive index profile describes a spiral along the optical axis. This produces a change of the polarization axes orientation along the grating length. This change causes Polarization Mode Coupling (PMC) that leads to the reduction of the Differential Group Delay (DGD) and the Polarization Dependent Loss (PDL). We theoretically demonstrate that the higher the number of twist, the greater the reduction of both DGD and PDL. The electrical field in a single mode optical fiber is characterized by two degenerated modes 11 xHE and 11 yHE. If the optical fiber is not ideal (due to geometrical or material variations), the degeneration is broken. Consequently, the field is composed by two orthogonal modes that propagate with different velocity as they experience different refractive index. The refractive index difference of the modes is called birefringence. The birefringence value and the polarization axes orientation can vary along the optical axis of the fiber. In this case, the optical fiber can be modeled as a concatenation of dielectric layers, with each layer having its own polarization axes and birefringence values. We define the discretization length Ld as the length in which the former two characteristics can be considered as constant, Consequently, a fiber with length L can be modeled as a concatenation of L/Ld segments (see Fig.1). When the light passes through one layer to another the field is projected on the new polarization axes, this phenomenon is known as Polarization Mode Coupling (PMC)