Polarization-maintaining singlemode fibers guide coupled radiation in two perpendicular principle states, the fiber polarization axes. For the two polarization axes the speeds of propagation is different, denoted the fast and slow fiber axes.
Linearly polarized radiation not coupled exactly into one of these axes is transformed into an elliptical state of polarization because of these different speeds of propagation.
The linearly polarized laser radiation is conventionally coupled into the slow axis, because of its lower sensitivity to fiber bending.
In polarization-maintaining fibers the rotational symmetry is made void by birefringence. The birefringence is achieved by stress-inducing components in the fiber cladding, such as in “PANDA“ fibers and “Bow-Tie“ fibers or by an oval clad such as in “Oval-Inner Clad“ fibers. The slow axis is orientated in parallel with the stress-inducing elements (“PANDA“ and “Bow-Tie“ fiber) or in parallel with the larger diameter of the inner cladding (“Oval-Inner Clad“ fiber).
The polarization-maintaining fiber cables made by Schäfter+Kirchhoff typically use fibers of type PANDA. The slow axis is aligned with the index key of the FC type fiber connector with high precision (< 1.5°).
The fiber cables made by Schäfter+Kirchhoff typically have a polarization extinction > 200:1 (23 dB) or > 400:1 (26 dB) for λ > 780 nm.