EXFO FTB-5600 Distributed PMD Analyzer - Manual (Page 123)

Distributed PMD Analyzer 117
C Acquisition Data
The purpose of the acquisition data is to build a PMD cumulative curve.
This curve is build from OTDR traces taken at multiple frequencies or
wavelengths, each being launched with a particular polarisation state.
The cumulative PMD from 0 to z
n
is deduced from the mean-square (ms)
value of the K random transmission differences divided by a relative
variance of the traces. The mean-square (ms) value is computed
according to the following equation:
More precisely, in practice each of the two traces in each pair is acquired
twice consecutively in time, thus producing a repeated pair having a
difference ΔT'
k
(z
n
) which may differ somewhat from ΔT
k
(z
n
).
The P-OTDR traces P
j
(z
n
) as a function of distance z are acquired for many
different random and independent combinations of optical central
frequency ν
k
and I/O-SOP. The transmission T
j
(z
n
) of the analyzed light is
computed from the OTDR traces measured from two photodetectors,
P
xj
(z
n
) and P
yj
(z
n
), point by point as,
For the sake of simplicity, the (z
n
) notation of distance has been dropped.
The P
x
traces comes from the first branch of the PBS and the P
y
traces,
comes from the other branch of the PBS.
ΔT
ms
z
n
( )
1
σ
r
2 z
n
( )
---------------- ΔT
k
z
n
( )ΔT′
k
z
n
( ) 〈 〉
k
=
T
j
P
xj
P
xj
P
yj
+
--------------------- =
T
j
′
P
xj
′
P
xj
′
P
yj
′
+
------------------------ =