50 Chapter 4 Laser Diode Driver Module Operation
In the
Io
mode, the active set point is the actual drive current. A set point of 1000
mA will cause the module to drive 1000 mA through the laser diode, assuming the
Io
Lim
is set at or above 1000 mA.
In
Im
mode, the set point is the desired amount of photodiode current, in
µ
A. Unlike
the
Io
mode,
Im
mode will drive whatever current is necessary though the laser
diode, up to the limit, to achieve the set point photodiode current.
Po
mode is simply an extension on the
Im
mode, allowing the user to operate the
system in milliwatts of power. The photodiode power set point, in mW, is converted
to photodiode current using the
PD Resp
value from the setup screen.
4.5.3.2 Bandwidth
This setting is used to control noise and laser current modulation rates. Allowable
settings are
Low
,
Low CW
, and
High
.
Low CW
allows a maximum modulation rate
of 30 Hz, and operates with the least noise.
Low
allows up to 10 kHz modulation,
while
High
allows full bandwidth modulation. Modulation is disabled in
Im
and
Po
modes. The MOPA module does not have the bandwidth setting.
4.5.3.3 Modulate (MOPA Modules Only)
This setting is used to select which laser driver is modulated by the external
modulation input. The choices are
Osc
for the oscillator driver, or
Amp
for the
amplifier driver.
4.5.3.4 Io Lim
As one of the safety features of the Laser modules, the
Io Lim
sets a maximum
allowable current drive for the laser diode. The system will also limit current set
points to this value when operating in the
Io
mode. Two conditions can be generated
when the driver reaches this limit. The lesser of the two is the soft current limit. The
soft limit, indicated by
I
on the status line of the Laser Main Screen, indicates that the
laser module is limiting the current drive to the laser diode, but otherwise operating
as normal. The second condition is a hard limit, which indicates that the current drive
attempted to exceed the current limit faster than the circuitry could limit it. This
condition causes the laser module’s output to be shutdown. Both of these conditions
are monitored in circuitry on the module itself, and in the case of the hard limit,
shutdown is within microseconds of the condition being detected. See section 4.1.3
for additional information on laser diode protection.
4.5.3.5 Vcomp
The voltage compliance setting controls the shutdown of the laser module output
when the forward voltage of the laser exceeds the compliance setting. Like the
current limit described above, the voltage compliance is monitored in circuitry on the
module itself, allowing for shutdown within microseconds of the condition. See
section 4.1.3 for additional information on laser diode protection.
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