Amorphous solid materials like glass or plastics generally are isotropic materials.
Thus the common optical fiber only has one refractive index n0. To encode the plane of polarization with the help of FBGs the gratings have to be birefringent. This means, that the fiber must have two refractive indices, due to what two reflection peaks occur.
There are two possibilities to create two refractive indices. One method is to put the FBG under lateral stress.
When two different transversal strains ?x and ?y, directed along the x and y axes are applied to a FBG written in a SM fiber, the refractive index changes and splits into two values. This appearance of birefringence leads to the separation or the single reflected Bragg peak into two distinct ones, like illustrated below
The peak wavelength shift is given by
where ?x, ?y and ?zare the principal strain in the core of the SM fiber directed along the three coordinate axes. It is interesting to note that the peak separation in this case is proportional to the difference between the transversal strains ?x and ?y only.
Another possibility is to inscribe the grating in a high birefringent fiber, a so called PM fiber.
In this type of fiber two refractive indices occur due to its manufacturing process. Experiments show that putting the FBGs under lateral stress isn’t a trivial thing. To reach a adequate peak separation, forces near the fracture strength of the material have to be applied, what frequently ended in a destruction ofthe fiber.
A much more solid method is to use FBGs written in PM fiber. Here the peak separation and the symmetry of both regions of different refractive indices is much better, as well as the mechanical stability.
Here the peak separation is