Document #: GP-UM-PMD-1000-21
Page 108 of 122
5.3 Instruments required for PMD related tests
PMD source for PMD tolerance test
No matter which approach is taken, it is desirable to have an instrument capable of testing
the performance of different methods in mitigating PMD effects. This test is called a
PMD tolerance test. The general setup is shown in Figure 31A. The key instrument in this
setup is the PMD source, which can generate precise 1
st
and higher order PMD values.
The bit-error rate (BER) of the system, or another such performance indicator, is
measured as the 1
st
order PMD (DGD) values generated by the PMD source are gradually
increased until the BER reaches the limit set for the system, as shown in Figure 31B. The
corresponding DGD is the 1
st
order PMD tolerance of the system. Both the 1
st
and 2
nd
order PMD (SOPMD) values can also be increased as the BER of the system is measured
and plotted, as shown in Figure 31C. The system outage probability can be calculated
from the data obtained.
Figure 31 A) PMD tolerance test setup using a PMD source. The computer selects PMD settings and
collects BER readings. B) BER vs. DGD curve. The DGD tolerance of the system for a given BER
threshold can be deduced using the curve. C) Map of BER vs. DGD and SOPMD. Each contour represents
a BER value. The PMD tolerance can be obtained from the contour plot.
PMD tolerance test results can be used by network operators to compare systems made
by different vendors and to verify the PMD related specifications promised by the
vendors. They can also be used by system vendors to 1) determine which PMD
mitigation approach is most suitable to adopt, 2) to fine tune the PMD mitigation related
parameters in the transceivers during system development, and 3) to perform output
quality control of the final system, including transceivers, for the PMD related
specifications.
DGD
BER
BER tolerance
DGD tolerance
SOPMD
B)
1
0
C)
10
-12
10
-1
10
-1
10
-9
10
-8
DGD
RX
TX
Computer
BER tester
PMD source
A)
