EXFO FTB-5600 Distributed PMD Analyzer - Manual (Page 120)

Distributed Cumulative PMD Measurement Theory
114 FTB-5600
The typical OTDR pulse length used for the RS-POTDR is of 100 ns or 50 ns.
A low-PMD lead fiber (LF) is used for probing coherence noise arising from
the moderately high coherence of the laser source, allowing it to be largely
eliminated in the subsequent data processing.
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 traces measured from two photodetectors, P
xj
(z
n
) and
P
yj
(z
n
), point by point as,
In the FTB-5600, P-OTDR traces are acquired in pairs for two
closely-spaced optical frequencies, ν
k
±¾δν, with the same I/O-SOP. The
central frequency ν
k
of each pair is generally different from that of any
other pair. Although the I/O-SOP corresponding to traces within each pair
must be the same, they are generally different from one pair to another.
For each pair of P-OTDR traces, a corresponding local transmission
difference, ΔT
k
(z
n
) , can be calculated at each distance point z
n
. Typically,
the data is acquired from a large number K (≥ 100) of pairs leading to K
transmission differences for a random set of independent K combinations
of central frequency and I/O-SOP, that is, ν
k
, I/O-SOP
k
, where k = 1, 2 … K.
Closely-spaced means PMD
m
· δν < 0.1 to 0.15, where PMD
m
is the largest
value to be measured and δν is an optical-frequency difference for all pairs.
The use of an optimized maximum optical-frequency difference leads to
the best SNR; for example, to measure a maximum PMD value of 10 ps, an
optimized δν is ~12.5 GHz. It should be noted that the PMD measured with
this instrument is the rms average of the Differential Group Delay (DGD)
over a wide wavelength range.
An OTDR-based measurement is necessarily a “round-trip” measurement
from any given point z
n
along the fiber. Of course, what is of interest to
most users is the cumulative PMD in the forward direction only. The
T
j
z
n
( )
P
xj
z
n
( )
P
xj
z
n
( ) P
yj
z
n
( ) +
------------------------------------------ =