What makes PM fibers maintain the polarization?
In polarization-maintaining single-mode fibers (PM fibers), the fiber symmetry is broken by integrating stress elements in the fiber cladding. The light is then guided in two perpendicular principle states of polarization with different propagation constants – the fast and the slow axis. The linear polarization of light coupled into one of these axes is maintained.
If light is guided partly in the other axis then the resulting polarization is elliptical (if the coherence length of the source is larger than the phase difference). Strain and temperature variations, however, change this arbitrary elliptical state. Thus it is important to exactly align the polarization axis of the laser source with the polarization axis of the fiber e.g. using the Polarization Analyzer SK010PA.
The linearly polarized laser radiation is conventionally coupled into the slow axis because of its lower sensitivity to fiber bending.
Different types of polarization-maintaning fibers are designed depending on the geometry of the stress elements: “PANDA“ fibers, “Bow-Tie“ fibers or “Oval-Inner Clad“ fibers.
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 (except for fibers end caps).
The fiber cables with end caps made by Schäfter+Kirchhoff typically have a polarization extinction ≥ 200:1 (21 dB) or ≥ 400:1 (23 dB) for λ ≥ 780 nm.
Polarization-maintaining single-mode fibers guide coupled radiation in two perpendicular principle states, the fiber polarization axes (also called the slow and fast axis, see figure on the left).
- Good alignment: If the connector index key is in axis with the slow axis of the fiber and the laser polarization axis the alignment is good
- Bad alignment: If the connector index key is in axis with the slow axis of the fiber but the laser polarization axis is rotated the alignment is bad and the resulting output beam is arbitrarily elliptically polarized
Characterizing Polarization-maintaining Fibers (PM Fibers)
More information on how Schäfter+Kirchhoff characterizes PM fibers can be found here.