In general, we use PM fiber to preserve plane polarized light when launched on one axis of that fiber. Interferometers can therefore be made that are free from signal fading as the recombination of signals is always in the same plane polarized state, such that interference is complete. Any rotation of plane polarization or change to elliptical polarization causes signal fading as interference occurs only between parallel components. In non PM fiber there is rotation and ellipticity due to various mechanisms that must be compensated with polarization controllers.
Having established the advantages of PM fiber there are problems associated with PM components that exhibit cross coupling, including connectors and splices. There is spilling of some of the light on the slow axis into the fast axis (or vice versa). Strings, or components in series, that have cross coupling, give rise to an accumulation of that cross coupling lowering the overall ER (extinction ratio) of the string. If the source has a long coherence length then the cross coupled signals interfere, being summed with respect to amplitude and phase, causing intensity modulation. on both axes that is temperature dependent.
If the fiber paths are kept to the shortest possible and the components to the lowest extinction ratio (lowest cross coupling) then the problem is minimized and usually an acceptable performance is achieved.
If the assembly of such strings is maintained at a constant temperature the problem is eliminated altogether, but the output may not have the optimum extinction ratio.
If individual path lengths of fiber linking PM components are thermally controlled than there are conditions where optimization can be made such that outputs are better than the original component specifications.
PM components in free space do not suffer from the birefringence effects above, but neither do they have the advantage of a polarization preserving fiber optic transmission.
Connectors and splices can be eliminated from arrays and trees between components using side polished techniques which allow the manufacturing of PM components on a continuous length of fiber. The cross coupling count and cost are reduced and the overall ER improved.