User`s manual

4200 Series LaserSource User’s Manual · Page 15
Control Modes
The LaserSource offers five laser control modes: Io (ACC), Io HiBW (ACC), Im
(AMC), Po (APC), and Vf (AVC).
Changing the control mode is done through the menu by changing the Mode
parameter in the menu to one of these values.
Io and Io HiBW modes (referred to collectively as ACC, or automatic current
control, modes) are used to drive a specific current through the laser diode.
When in this mode, the set point will be in milliamps, and the LaserSource will
drive the desired current through the laser diode as long as the voltage at the
chosen set point does not exceed the voltage limit. In Io mode, you will be
limited to approximately 10Hz modulation. To modulate above that rate, use the
Io HiBW, which is a high bandwidth current mode supporting modulation up to
250kHz (depending on model).
Im mode (also referred to as AMC, or automatic monitor photodiode control,
mode) is used to control the laser diode using the monitor diode feedback. You
select the target monitor diode current, and the LaserSource will drive exactly
enough forward current through the laser diode to generate the selected monitor
diode current. Only low frequency modulation (10Hz or less) is possible in Im
mode due to the feedback latencies of the photodiode itself.
Po mode (also referred to as APC, or automatic power control, mode) is simply
Im mode with a mathematical constant applied to the set point, providing a
convenient way of operating in milliwatts. Using the PD Response factor (in
μA/mW), a Po set point is internally converted to an equivalent Im set point by
the driver, which is then used to control the photodiode feedback. For example,
if the PD Response factor was 10, then a set point of 1mW would be the same
as a set point of 10μA.
Vf mode (also referred to as AVC, or automatic voltage control, mode) is used to
control the voltage driven through the device. Unlike ACC mode, AVC mode
allows the current to drive to whatever level is necessary to achieve the voltage
set point, so long as it does not exceed the current limit. As with Im mode, only
low frequency modulation (10Hz or less) is possible in Vf mode.
Modulation
The instrument supports external analog modulation using the modulation BNC
on the back panel of the instrument. Modulation rates vary by model, so see
your model’s specification for the maximum modulation rates. Only Io HiBW
mode supports high speed modulation. All other modes of operation have a
maximum modulation rate of approximately 10Hz.