New Focus 1480, User Manual

Page: 14 / 20

14 • Troubleshooting NEW FOCUS, Inc.
2.
Using a voltmeter or oscilloscope, measure the
output voltage from the Bias Monitor on the front
panel of the bias supply.
With no light on the detector, the Bias Monitor
voltage should be <10 mV.
3.
Illuminate the photodetector.
4.
With the voltmeter or oscilloscope, you should
observe a DC output voltage.
If you know the optical power and wavelength,
you can calculate the expected output voltage
(
V
out
) using the expression:
V
out
=
P
in
•
R
•
G
, where
P
in
is the input optical power (watts),
R
is the
photodetector’s responsivity (A/W), and
G
is the
amplifier’s transimpedance gain (V/A). The gain of
the bias monitor port is 1000 V/A.
If the output voltage is low, then the fiber connector
may be dirty (see “Cleaning the Fiber Connector,”
below). If you suspect the unit is damaged, contact
New Focus to arrange for a repair (see “Customer
Service” on page 17).
Cleaning the Fiber Connector
The high-speed photodetectors have an internal fiber
connector and a fiber pigtail that brings the input light
to the detector. If the fiber connector is dirty, this can
block the input light so that light sent into the detector
will produce little or no output voltage.
Note also that if the fiber types are not compatible (e.g.
multimode to single mode), this can also be the cause
of a very low output voltage.
To clean the fiber:
1.
Turn off the module.
2.
Blow filtered air or another inert gas (such as
nitrogen) into the input fiber connector. This will
1480 FC Det revA.fm Page 14 Monday, March 13, 2000 2:55 PM