EXFO FTB-5600 Distributed PMD Analyzer - Manual (Page 122)

Distributed Cumulative PMD Measurement Theory
116 FTB-5600
For very small values of PMD·δν, the ms-difference ΔT
ms
(z
n
) is proportional
to (PMD·δν)
2
. Hence, there the measured transmission differences may be
very small, leading to a very poor dynamic range, since the differences
may be of the same magnitude as the coherence noise. In order to use the
largest possible optical-frequency difference δν, the large-step formula
below is used to compute the cumulative PMD:
where ΔT
o
2
= 8/45 is a “saturated” value of the ms-difference when
PMD·δν → ∞ and α
dT
= 15 is a proportional constant when δν → 0.
When the PMD is obtained from a single measurement at a given time, the
cumulative PMD curve, PMD(z
n
) , will exhibit fluctuations as a function of
distance z, even if the cumulative PMD is constant. Therefore, it can be
advantageous to average over some length Δz, for example 0.5 km to 4 km,
to obtain a clear picture of the cumulative PMD curve. Moreover, the use of
a larger number of random and independent combinations of frequency
and I/O-SOP, for example K = 200 to 500, permits a corresponding
reduction in the measurement uncertainty.
ΔT
ms
z
n
( ) ΔT
o
2 1
2 πPMD z
n
( )δν
α
rt
α
dT
ΔT
o
-------------------------------------
⎝ ⎠
⎛ ⎞
–
2
⎝ ⎠
⎛ ⎞
exp – =