User`s manual

4200-DR Series LaserSource User’s Manual · Page 17
select the target monitor diode current, and the 4200-DR 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 Resp 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
Resp 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 current 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 rear 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
modulation bandwidth of 10Hz or less.
Current Range
When operating in Io or Io HiBW modes, the 4200-DR offers two current
ranges: a high range, which is the maximum rated current for the driver (e.g.,
500mA on a 4205-DR) and low range, which is one-half the maximum rated
current for the driver (e.g., 250mA on a 4205-DR). When the set point is below
the low range limit, the driver will operate with lower noise and greater accuracy.