Optical Backscatter Reflectometer 4600 179
User Guide
domain axis is given by t ∈ [ 0, T ] and the data is highlighted at a cursor such that
t ∈ [ t
1
,
t
2
] , then the average return loss at that cursor is given by
RetLoss
=
t
2
∑
h
˜
j
10
log
--
t
--
1
---------
,
∑
h
˜
j
0
where h
˜
j
is the impulse response, further defined below under “Time Domain
Amplitude and Amplitude (dB)” on page 180.
The differential loss (Diff Loss (dB)) displayed in the graph area is simply the
difference between the return losses at each cursor, divided by two.
The differential loss is calculated by summing all of the power, p, within the
highlighted segment around the first cursor, and dividing this sum by the sum of
the power within the highlighted segment around the second cursor. The loss is
calculated as five times the log of this ratio (shown below) because the loss is
accumulated twice: once for the light to get to the particular segment, and once for
it to return from the segment.
N
∑
p
k
Differential Loss = 5 log
k
=C
1
∑
p
k
k
=
C
2
N is the number of points specified in the Integration Width field (or Sensing
Range field, in Sensing Mode). C
1
and C
2
are the start positions for the two
segments integrated.
Standard Parameter Calculations
As mentioned above, the OBR uses coherent interferometry to measure the linear
transfer function,
H( ω )
= ρ( ω )
e
i
φ ( ω)
, of the DUT. (See “Optical Network for
OBR” on page 174.) From this the loss, dispersion and time domain parameters are
extracted.
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