Photodiode Feedback Connections; Grounding Considerations - ILX Lightwave LDX-3500 Series User Manual

I N S T A L L A T I O N A N D O P E R A T I O N
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C H A P T E R
Introduction to the LDX-3500 Series Front Panel
through an earth-ground path. If the output circuit is earth-grounded at some point
(such as through the laser package and mount), the user must be careful to avoid
multiple earth grounds in the circuit. Multiple earth grounds may provide circuit
paths that induce spurious currents in the photodiode feedback circuit and output
leads.

Photodiode Feedback Connections

The 9-pin connector on the back panel contains the current supply output. The
photodiode signal is input at the connector at pins 6 and 7 (see Figure 2.3). The
LDX-3500 Series Precision Current Source provides an adjustable reverse bias of
0 - 5 V for the photodiode. To set the photodiode bias to 5 volts reverse bias, turn
the back panel PHOTODIODE BIAS ADJUST fully clockwise. To set the
photodiode bias to 0 volts reverse bias, turn the back panel PHOTODIODE BIAS
ADJUST fully counter-clockwise.
The photodiode feedback may also be connected via the PHOTODIODE (BNC)
connector, located above the main 9-pin D-sub connector.
Many laser diode modules contain an internal photodiode that monitors the back-
facet emission of the laser. Usually, this photodiode is internally connected to
either the laser anode or cathode. Figures 2.4A - 2.4D show the recommended
connections and shielding for the various configurations of laser diode modules
and photodiode feedback schemes.
The photodiode and laser inputs of the LDX-3500 Series Precision Current
Source are electrically isolated from ground and each other. So, if a 4-pin
connection is made (no common connections) no additional jumpers are required.
Figures 2.4A - 2.4D show the recommended connections and shielding for 3-pin
lasers (where the common connection is internal to the device). A 4-pin laser
should be connected with the same shielding as shown in Figure 2.4, but the
common connection (between the photodiode and the laser) is optional.

Grounding Considerations

The LASER outputs of the LDX-3500 Series Precision Current Source are
isolated from chassis ground allowing either output terminal to be grounded at the
user's option. Figure 2.4 shows the proper earth-ground shielding for laser
diode/photodiode connections.
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LDX-3500 Series
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