When the polarization direction of incident light corresponds to either of the fast or slow axis, the polarization direction of propagating light in the fiber does not change. However, the polarization direction of incident light is tilted against both the fast and slow axis, the polarization changes continuously since the light power is periodically exchanged between X  and Y polarized modes of PMF. The phase difference between the propagating X and Y polarized light can be 2π at some distance. The distance (for the polarization to return to the original) is called as beat length, Lb, or coupling length.

Figure schematically shows the series of polarization change in PANDA fiber when a linearly polarized light is incident along the direction tilted from X (or Y) polarized light by 45 degree.



Fig. Change in polarized wave for incidence of linearly polarized light tilted by 45 degree to fast axis of PANDA fiber

The beat length represents the amount of birefringence. The relationship between beat length, Lb, and birefringence, B, is expressed as follows.