Optical Backscatter Reflectometer 4600 175
User Guide
Figure 8-2. OBR optical network.
The basic idea from the previous section can be applied to device characterization
by including a device under test (DUT) in one arm of a Mach-Zender interferometer,
as shown in Figure 8-2. We can describe the DUT by its frequency domain linear
transfer function,
H( ω ) ; the linear transfer function contains information about the
amplitude,
ρ( ω ) , and phase, φ( ω ) , response of the DUT and is given by
H ( ω ) = ρ( ω )
e
i
φ
(
ω
)
. Given the amplitude and phase response, the detected power
is proportional to the intensity, I, given by
I =
E
0
(
t
)
2
+
E
0
( t – τ )
2
ρ( ω )
2
+ 2ρ( ω )
E
0
(
t
)
E
0
( t – τ ) cos [ ω (
t
)τ – φ( ω ) ]
.
Next a Fourier transform is performed allowing the three terms in the above
expression to be separated spectrally. Note that the first two terms will have only
low frequency characteristics, and thus will appear at or near
t = 0 in the time-
domain. The device information resides in the third term of this expression, called
the
interference
term. The
interference
term
oscillates
at the frequency ω(
t
)τ , while
the other terms do not oscillate. Its location in the time-domain will be determined
by the delay difference
τ between the two arms of the interferometer. Provided
that
τ is large enough, the device
response
will be
separable
from the low frequency
terms, and thus measurable. The Luna OBR is designed such that the internal delay
path through the device under test is long enough such that the low frequency terms
will not influence the measurement, even for very short devices. Thus the OBR
operates with no dead zone.
The Fourier transform of reflected optical intensity (I) for a short length of single-
mode fiber as the DUT is plotted in Figure 8-3 (a). Figure 8-3 (b) shows the selected
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