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PMD stands for Polarization Mode Dispersion. Here, Dispersion means different traveling speeds, which also means arrival at different times. So PMD means the different traveling speeds of two different polarization modes of a light pulse. You can also understand PMD as the arrival timing difference between these two polarization modes.
So then what is polarization mode? Let's take a look at the picture at left.
Figure 3 shows a standard light pulse. It seems like a single pulse, but as shown in figure 4, this pulse is actually composed of two polarization modes which are 90 degree, perpendicular to each other, we call them as Orthogonal.
The right side picture shows the cross-section view of an optical fiber. The electrical field vector is the polarization direction of the light pulse, but it can be decomposed into two perpendicular polarization modes, one at 0 degree, and one at 90 degree. The light pulse can have a polarization at any angle, but it can always be decomposed into two orthogonal polarization modes.
In a ideal perfect optical fiber, these two orthogonal modes travel at the same speed, so there is no PMD. But in the real world, there are many imperfect things in the fiber that causes these two modes to travel at different speed, thus, the PMD.
This pictures shows how actual PMD in a fiber looks like.
A light pulse is generated and coupled into the fiber. Imperfections in the fiber causes PMD, so at the end of the fiber, these two orthogonal polarization modes arrive at different times. The timing difference is called Differential Group Delay. This phenomenon is called PMD.
If the fiber causes these two polarization modes to travel at different speeds, we say that this fiber has bi-refringence.
Birefringence is a default of material such as an optical fiber, where the effective index of refraction changes with the polarization state of the input light.
There are many things in the fiber that causes bi-refringence, such as fiber core stress, cladding eccentricity, fiber twist, fiber stress, fiber bend, etc. So in the real world, there are always bi-refringence existing in the fiber, that causes PMD.
So what kind of effect does PMD have on a high speed fiber optic communication system?
The main effect PMD causes is called pulse broadening.
This sentence clearly explains what is pulse broadening.
If there is no PMD in the fiber, these two orthogonal polarization modes arrive at the same time, so when these two modes are added together, you still get the same pulse as the original input.
But if there IS PMD, these two modes will arrive at different times, when they are added together, you get a pulse that is broadened in time, but with a lower peak power, as shown here.
Let's check out two high speed fiber optic systems: one has a lower speed at 2.5 Gbps, the other has a much higher speed at 10 Gbps.
They both have the same digital pulses input, shown as the top part, 1101001. In the 2.5 Gbps system, these pulses are well separated in time, since it has a lower speed. But in the 10 Gbps system, these pulses are much more closer to each other in time.
So after the pulses travelled a certain length of fiber, which has PMD, let's compare the outputs of these two systems.
In the 2.5 Gbps system, the pulses are broadened in time, and these pulses have lower peak power, but they can still be distinguished as the original pulses, 1101001.
However in the 10 Gbps system, since these pulses are much closer in time, the pulses are broadened and interfere to each other so badly, that they can no longer be recognized as the original digits. They are called Bit Errors. So PMD can cause so many bit errors that the system's efficiency is severely affected.
So there you have it. You can find many more fiber optic tutorials on www.fo4sale.com. Visit us today!
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