APPLICATION NOTE #1
FRESNEL INSERTION LOSS
MEASUREMENT
The OFM20 and OFM130 Model OTDRs are
designed to measure, with Zero Deadzone, the
Insertion Loss (IL) of Connectors, Splices,
Switches, Isolators, Attenuators or other Devices,
using the Fresnel method, at wavelengths from 650
nm to 1550 nm, to approximately 0.5 dB with up to
0.2 dB accuracy. The one-way loss budget is
wavelength dependent with a range of 30 to 40 dB.
The method is used mostly for multi-mode
components, links and circuits, where the Return
Loss (RL) of a Device Under Test (DUT) is low.
Examples include non - PC, gap type, or expanded
beam connectors, low RL switches and couplers,
etc. If the RL of components in the circuit is higher
than approximately 40 dB the IL is measured using
the Rayleigh method. See the application note:
‘RAYLEIGH INSERTION LOSS MEASUREMENTS’
The Fresnel IL method requires two
measurements, Reference (R) and Sample (X or Y).
For both measurements the Window Size should be
adjusted to 1m or less, to have the pulse to be
measured appear on the screen alone. The pulse
height should be adjusted to 1 - 2 divisions by the
Sensitivity control.
If all DUTs in the circuit have equal RL, such as
gap type connectors with a constant RL of 11 dB,
then the reflected pulses from these DUTs may be
used for both R and X. Measuring R at the DUT,
Measuring X at the next connector downstream
from the DUT and by Calculating X - R and
pressing IL, determines the IL of a DUT. The IL
value of the DUT appears on the screen. An
example is shown in Fig 1.
If the RL of the DUT is not the same as the RL
of the next component downstream from the DUT
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then two features with identical RL have to be
created, one at the DUT and the other downstream
from it. These two features may be open flat
polished connectors or freshly cleaved fiber ends.
One example is the measurement of the IL of
non - PC (or low RL PC) connectors at the two ends
of a jumper as shown in Fig 2. Here the open launch
connector is used to Measure R and the far-end
open connector of the jumper is used to Measure X.
The calculated X - R IL value is the IL of the nearend mated connector pair of the jumper.
Another example is the measurement of the IL
of a fusion splice as shown in Fig 3. The first fiber
is connected to the OFM; its far-end is cleaved and
is Measured as R. The fusion splice is made (The
fiber upstream from the fusion splice has to be
pinched when the splice is made, to prevent
damaging the OFM detector). The far-end of the
second fiber is cleaved and is Measured as X. The
calculated X - R IL value is the IL of the fusion
splice plus the IL of the second fiber, if any.
The formula used by the OFM to calculate the
IL is IL=10Log(A(R)/A(X))/2, where A is the
measured area under the curve of the R and X pulse
on the screen.
The accuracy of an IL measurement depends on
the usual parameters of the ratio metric method, e.g.
the Signal to Noise Ratio (SNR) of R and X, etc., as
well as the possible difference between the RL
values of the DUT and the next component at which
X was measured. If the SNR is too low, increase the
Measurement Averages from the default 32, to 128
or higher. Typical measurement times are 10 - 30
seconds.
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