Page 32 · 6300 Series ComboSource User’s Manual
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 front 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
The ComboSource offers two current ranges: a high range, which is the
maximum rated current for the driver (e.g., 500mA on a 6305) and low range,
which is one-half the maximum rated current for the driver (e.g., 250mA on a
6305). The lower range offers improved accuracy (in Io (ACC) and Io HiBW
modes) and improved stability in all modes.
Using the Laser Limits
Both the laser current and voltage limits are implemented in hardware, providing
for fast response to changes in laser diode operation. When a voltage limit is
detected, the output is immediately shutdown. Because of the sensitivity of the
voltage limit, operating near the limit (within one to two hundred millivolts) is not
recommended. In general, you should set the voltage limit to 0.1V to 0.2V higher
than any anticipated operating point. The voltage limit is tested against the
voltage at the connector, unless in remote voltage sense mode. Any Laser
Cable R value is ignored, as Laser Cable R is a software only calculation, and
the voltage limit is implemented in hardware. See Using the Cable R Setting,
above, for more information on the Laser Cable R setting.
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