5.4 Desirable features for a PMD source
High repeatability and rapid generation of 1
st
and higher order PMD values are desirable
in a PMD source because they enable fast, repeatable PMD tolerance tests and PMD
compensator evaluation.
Automatic optimization of input SOP is also highly advantageous. As described
previously, PMD tolerance tests can be greatly simplified if the input SOP to the PMD
elements inside a PMD source can be controlled and optimized such that each PMD value
has the most severe possible impact on the quality of the optical signal, regardless of
polarization changes caused by disturbances to the fiber link. Specifically, the input SOP
can be automatically maintained at an alignment of 45 degrees from the PSP of the PMD
source, or the output DOP can be automatically maintained at a minimum value by
actively controlling the input SOP.
PMD compensation is also an extremely attractive function. First, system vendors and
network operators need to know how much improvement can be made in system
performance by adding PMD compensation in order to decide whether to deploy PMD
compensators on a link with performance issues. Second, if PMD compensation
significantly improves the system performance, it can be determined that PMD is the
major cause of performance degradation; otherwise, other impairments should be
considered. Therefore, a PMD source with PMD compensation capability can be used for
the diagnosis of system problems. Finally, optimized PMD compensation can help to
determine the PMD values of the fiber link, because the PMD value used for optimal
PMD compensation is close to the real PMD value of the fiber link. Such a feature is
attractive for frequent PMD monitoring of an in-service fiber link, a task that cannot be
accomplished with PMD analyzers currently on the market.
5.5 The polarization optimized PMD source
The PMDPro meets all of the requirements described above. It is constructed with a
polarization controller (PC), a polarimeter at the input end, a PMD generator, a second
polarimeter at the output end, and a digital signal processor (DSP) based electronic
circuit, as shown in Figure 33. The two polarimeters are used to provide feedback signals
to the DSP circuit for full polarization optimization. The circuit receives the polarization
measurements from the two polarimeters, processes them, then sends commands to
control the PMD generator and the polarization controller. The circuit can use the SOP
information from the input polarimeter as the feedback to control the PC and generate
any SOP the user prefers. In particular, it can automatically align the SOP to 45 degrees
from the PSP of the PMD generator. This polarization state causes the PMD generator to
cause the worst-case 1
st
order PMD (DGD) effect on the optical signal at any DGD
setting, and is therefore the preferred SOP for DGD tolerance tests for transceivers and
fiber systems. Alternatively, the circuit can use the DOP information from the output
polarimeter as the feedback to control the PC. The input SOP that minimizes the output
DOP causes the PMD generator to have the worst-case total PMD effect on the optical
signal, and is therefore the optimal polarization state for total PMD tolerance tests of
Document #: GP-UM-PMD-1000-21
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