TROUBLESHOOTING
Several problems which can arise with the detector modules and
procedures to check for them are outlined below.
Problems:
Damaged photodiode/Excessive offset voltage:
The photodi-
ode can be damaged by electrostatic discharge or excessive
optical power, and both causes lead to an increased dark (or offset)
voltage. A damaged photodiode can result in a degraded respon-
sivity and frequency/impulse response. See DC Offset Voltage
below.
Low bias voltage:
If the photodiode bias voltage is too low, the
frequency/impulse response of the detector will degrade, as will
the responsivity. See Checking the Battery below.
Dirty fiber-optic connector:
A dirty fiber-optic connector can
cause an apparent degradation of responsivity. See Basic Optical
Test below.
Tests Procedures and Solutions:
DC Offset Voltage
With no light on the photodetector, turn the detector on and use a
voltmeter to measure the Bias Monitor output voltage. This voltage
is the DC offset plus dark current. If the output is >10 mV, then the
detector is probably damaged and will need to be returned to
New Focus.
Checking the Battery
1. Turn on the module using the power switch.
2. Connect the “Bias Monitor” port to a voltmeter.
3. Press and hold the “Batt Chk” button and observe the bias mon-
itor output. The photodiode bias voltage is momentarily applied
to the “Bias Monitor” SMA connector. A reading of 5 V on this
connector is typical with a new battery; the battery should be
replaced when the voltage reaches 3.5 V.
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