Keithley 7090 Instruction Manual page 42

4-6 Service Information
Insertion loss procedure
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Insertion loss notes
The second channel of the Dual Photo Detector is used to measure back facet photodiode
current for the laser diode module. This measurement is one way to verify that the laser is
operational, but it is not required to compute insertion loss and can be useful when debug-
ging a troublesome system. The back detector is intended to be used as feedback for appli-
cations that this example may not require.
The current measurements from the Dual Photo Detector used in the insertion loss calcula-
tion differ from the optical power by a factor that should remain constant and is cancelled
out in the system.
Some important points to note while running this test system:
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Allow the connected system to run and warm up for 20 minutes with the laser
power on.
Turn off the laser power.
With the laser power off, connect the laser output directly to the detector input (PC
to PC) connection. The channel 1 reading of the Dual Photo Detector is the noise
and dark current. Press the Photo Detector's REL button to zero out this level.
With the laser power off, reconnect Patch Cord 1 to laser output and Patch Cord 2
to Detector input. With no laser output, connect the APC output of Patch Cord 1 to
the APC input of Patch Cord 2.
Turn on the laser power, and record the channel 1 reading of the Dual Photo
Detector as P . This value may be an average of readings, but be sure the reading
IN
is stable.
Turn off the laser power.
With the laser power off, reconnect the Model 7090 common channel to the APC
output of Patch Cord 1.
Reconnect Model 7090 channel N to the APC input of Patch Cord 2.
Using the Model 7001 front panel keys, close Model 7090 channel N.
Turn on the laser, and record the channel 1 reading of the Photo Detector as P
This value may be an average of readings, but make sure the reading is stable.
Open Model 7090 channel N.
Turn off laser power, and compute insertion loss for this channel as outlined in Cal-
culating insertion loss above.
Repeat the procedure for the remaining Model 7090 channels.
Repeatability and uncertainty of measurements can be affected by connectors and
connection points.
For specification verification, the insertion loss calculation should be done at sev-
eral wavelengths, if possible.
Fixed positions of fiber and connectors is preferable.
Model 7090 Instruction Manual
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