Polarization Mode Dispersion or PMD is the broadening of pulse due to the difference in velocities of two orthogonally polarized optical signals. Difference in speed to the orthogonally polarized optical signals happen depending on the obstacles and the characteristics of the route they travel.
The characteristics of the route, i.e., the fiber geometrical parameters such as a Core concentricity error is controlled by fiber manufacturer. Other external factors that affect the speed of light is birefringence. Birefringence is a state where the medium exhibits slightly different refractive indices that causes the polarized lights to travel at different velocities ultimately leading to dispersion. External factors causing birefringence include bending, stress, strain, twists on the fiber.
Optical fiber is wound on a reel with sufficient tension to avoid loose winding. During winding, fiber undergoes bending, twists and strain. Winding and overlapping creates stress. All these factors contribute negatively to the PMD when measured at the reeled state.
In loose tube manufacturing process, reeled fibers are unwound and are placed loosely inside the loose tube. Fiber gets relaxed inside the loose tube. Therefore PMD comes down or reduces in the loose tube stage. Hence measurement of reeled fiber PMD becomes meaningless to specify as it may give unrealistic values. Measuring PMD of a tension free coil is almost equal to the state of fiber in a loose tube after cabling. In a stable process and good cable design, variation of PMD would be limited and well within limits, but if the fibers undergo stress, twists, strain and or excessive bends, PMD may change.