5.6 Advantages and applications of the polarization optimized PMD source
The polarization optimized PMD source, constructed with digital ternary polarization
rotators, offers the following attractive features for PMD related testing:
1)
High precision, high repeatability PMD generation, resulting from the highly
repeatable rotation angles of each ternary rotator. In its discrete digital control
mode, the PMD source can generate a total of 6561 different PMD states, of
which 256 are DGD-only, 1024 have DGD and wavelength independent SOPMD,
and the rest are wavelength dependent PMD. In its quasi-continuous operation
mode, it can generate states with independently controllable DGD and wavelength
independent SOPMD values. Users can select individual PMD states or scan
sequences of PMD states at user defined time intervals.
2)
High PMD generation speed (minimum time interval around 1 ms), resulting from
the high speed switching of the ternary polarization rotators. This high speed
operation can speed up PMD tolerance tests and can be used to test the response
time of a PMD compensator against sudden PMD changes.
3)
Automatic optimization of input polarization for worst-case 1
st
order and 2
nd
order
PMD tolerance tests, regardless of rapid polarization changes before the input to
the PMD source. The polarization optimization eliminates test uncertainties and
significantly reduces the time required to complete tests. It is, therefore, ideal for
PMD tolerance tests for transceiver production lines, as shown in Figure 31.
4)
PMD compensation using either an automatically optimized PMD value or a user
selected PMD value. The PMD compensation is accomplished by maximizing the
DOP detected by the polarimeter at the output port. Both PMD and DOP values
will be shown on the front panel LCD display. By stepping the PMD values up
and down and looking at the maximized DOP values, the user can directly see
how the PMD value chosen affects PMD compensation. When optimized PMD
mode is selected, the instrument will go through all PMD states and search for the
maximum DOP. The PMD state with the maximum DOP is selected as the
optimized PMD for PMD compensation.
5)
PMD value determination of an in-service fiber link. The optimized PMD
compensation mode can be used to determine the PMD in the link, as shown in
Figure 36, because the optimized PMD value for compensation should be close to
the PMD value of the fiber link. Therefore, the PMD value of a fiber link can be
determined by simply inserting a polarization optimized PMD source in the light
path and enabling the PMD compensation function. The optimized PMD value
identified by the instrument is then the PMD value of the link. It may also be
necessary to characterize the PMD of a particular channel route in an in-service
ROADM network to determine its feasibility for 40G operation before installing
40G transmitters and receivers. In such a situation, an ASE source can be placed
at the transmitter end and a polarization optimized PMD source at the receiver
end to perform PMD compensation. The optimized PMD value identified by the
PMD source is then the PMD value of the fiber route. With this information, it is
possible to decide whether the route is suitable for 40G transmission and whether
Document #: GP-UM-PMD-1000-21
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