PDF of this tutorialScientists and engineers for years have expressed the fear that as optical networking systems get faster and send signals longer distances, major physics-related problems would become a limiting force.
The technology for years had been given a free ride as it grew from 90 Mpbs to 270 Mbps to 435 Mbps to 2.5 Gbps. A problem began to manifest itself in 10 Gbps systems and threatens major dislocation at 40 Gbps networking. For the first time, the fiber-optics industry was faced with a networking killer.
The problem, which itself was not even discovered until the early 1990s, is PMD. It can distort signals, render bits inaccurate, and destroy the integrity of a network.
Approximately 20 percent to 30 percent of the single-mode fiber manufactured before the mid 1990s has a property that has become more problematic as bit rates and span lengths increase. The problem is that the core of this fiber is not perfectly round.
Of course, no fiber's core is perfectly symmetrical, but this fiber is off by enough that it causes dispersion to the degree that it renders signals unreadable.
When light travels down a single mode fiber toward the receiver, it has two polarization modes that follow the path of two axes. They move toward the receiver at right angles to each other.
When the core of the fiber that bounds the light is asymmetrical, the light traveling along one polarization axis moves slower or faster than the light polarized along the other axis. This effect can spread the pulse enough to make it overlap with other pulses or change its own shape enough to make it undetectable at the receiver (see Figure 1).
Figure 1. Graphical Representation of the Effect of PMD on an Optical Pulse
In Figure 1, the optical pulse and its constituent photons travel from the source, or transmitter, at distance =0, along the single-mode optical fiber. At some distance after PMD has affected the pulse, the polarized energy is separated by some time. This time is known as differential group delay (DGD). DGD is the fundamental measure of PMD and is measured in picoseconds (10-12 seconds). If DGD is severe, the receiver at some distance L cannot accurately decode the optical pulse, and bit errors can result.
The optical eye pattern of a PMD–limited signal exhibits the effects of DGD by “closure” of the eye. The effect of the eye closure is caused by the separation of the polarized axes of photons, as the DGD becomes higher, separation becomes greater, and optical pulses start to interfere with each other, causing the eye to close (see Figure 2).
Figure 2. Sample Eye Pattern at Various Levels
