OPERATION
LDC-3900 Front Panel
10_15 LDC-3900 Series 25
CHAPTER 2

Laser Mode Section
The (Laser Mode) Select switch determines which Laser mode is used to control
the output of a Laser current source module. One of the following may be selected
at one time: constant current (I), constant optical (light) power (P), or constant
current with a high bandwidth output (I
HBW
) mode.
The (Adjust) LAS indicator must be orange before changing the values of the
Laser Mode functions for that channel. Refer to Figure 2.11 on page 15 for the
discussion of the Laser Mode section features.
Constant I mode is the normal mode for driving lasers. This mode uses a low-pass
filter on the laser drive current output to significantly reduce noise. In this mode a
modulation input of up to 10 kHz may be used, via the front panel MOD External
connector.
Constant P mode is also a low bandwidth mode. It is used when it is desired to
control the optical power of the laser via a monitor photodiode feedback
arrangement.
When P mode is selected, the unit will be in either I
PD
or P
PD
modes, depending
on the setting of the CAL PD (monitor photodiode responsivity) parameter. If the
CAL PD value is non-zero, the unit will operate in constant P
PD
, monitor
photodiode power (light power) mode. (For more information on the CAL PD
parameter, see CAL PD on page 29).
The LDC-3900 Modular Laser Diode Controllers have a special mode for
operating at a constant monitor photodiode current when the CAL PD parameter
value is set to zero. This constant I
PD
mode is useful for driving a laser when the
monitor photodiode's responsivity is not known, but a desired constant light output
can be determined to produce a constant monitor photodiode current. The
constant I
PD
mode is also used for monitor feedback calibration.
Constant I
HBW
is a high bandwidth constant current mode. This mode allows the
laser drive current output to be modulated up to 500 kHz via the front and/or back
panel MODULATION connectors.
Figure 2.20 LDC-3900 Laser Mode Section
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