PDF of this tutorialThe major cause of PMD is the asymmetry of the fiber-optic strand. Asymmetry is simply the fact that the fiber core is slightly out-of-round, or oval (see Figure 3). Fiber asymmetry may be inherent in the fiber from the manufacturing process, or it may be a result of mechanical stress on the deployed fiber. The inherent asymmetries of the fiber are fairly constant over time, while the mechanical stress due to movement of the fiber can vary, resulting in a dynamic aspect to PMD.
Figure 3. Cross-Sections of Optical Fibers
The mechanical stress on the optical fiber can originate from a variety of sources. One source that is very difficult to control is the diurnal (day/night) and seasonal heating and cooling of the optical fiber. Although much fiber is deployed in the ground and often within conduits, it is still subject to temperature variations and corresponding mechanical stress.
Another source of mechanical stress can originate from nearby sources of vibration. For example, much fiber is deployed alongside railroad tracks because of the ease of right-of-way and construction. However, vibration from passing trains can contribute to stress on the optical fiber. Fiber that is not buried next to railways and highways may be deployed aerially; in this scenario, wind can cause swaying of the fiber cable and can contribute to PMD.
Because of the combination of these effects, and the random way these effects add up over a section of fiber, PMD does not have a single value for a given section of fiber. Rather, it is described in terms of average DGD, and a fiber has a distribution of DGD values over time. The probability of the DGD of a fiber section being a certain value at any particular time follows a Maxwellian distribution (see Figure 4) As an approximation, the maximum instantaneous DGD is about 3.2 times the average DGD of a fiber.
Figure 4. Probability Distortion of DGD Levels in a Typical Fiber
