Newport 500B Series, User Manual, Page: 29 / 64

28 System Operation
gain ranges noted below the BNC correspond to the low or high RANGE. A
bandwidth selection switch on the back panel introduces a dampening filter in
the output control loop to reduce the output noise when external modulation
is not required. If the output drive current does not respond to a modulation
frequency above 10kHz, check the switch to make sure it is not in the LOW
bandwidth position.
4.1.3 Range Switch
Each Model 500B series unit has two output current ranges. Before turning
the output on, the RANGE must be selected.
4.1.4 Mode Switch
The Model 500B Series Laser Diode Drivers have two modes of operation;
constant current (I) and constant optical power (P). Mode selection can only
be made when the OUTPUT is OFF.
In the “I” mode the output drive current to the laser diode is held at a constant
level. The output of the laser diode can be modulated in this mode by
connecting an external signal source to the MOD input on the front panel.
The Model 500B Series can be used with a monitor photodiode and can be
operated in a constant optical power mode. Reverse voltage biasing of the
photodiode is a standard feature that improves the dynamic range and
frequency response of a laser diode. In the “P” mode the output current to
the laser diode is controlled in response to the monitor photodiode current to
maintain a constant output optical power. As a laser diode ages or the
temperature of the laser diode increases more output drive current is required
to maintain a constant optical power level. The output current LIMIT
prevents the laser diode from exceeding its maximum current rating. The
laser diode cannot be modulated in the POWER mode.
4.1.5 OUTPUT ON Switch and Indicator
The switch will activate the ON LED and allow current flow to the laser
diode after an approximately 4 second delay. Current will not flow unless the
laser diode is correctly connected, the “LASER ENABLE” key switch is
activated, and pin 1 and 2 of the back panel interlock feature are shorted.
ERROR Indicator LED
The following conditions will cause the red ERROR LED and its protection
circuitry to activate, automatically shutting off the output:
a. An open circuit in the laser diode package or cabling.
b. If the forward voltage drop of a laser diode (or series arrangements of
multiple diodes) exceeds the compliance voltage specified in Table 1.
c. Over-modulating the external voltage input to cause the laser output
current to exceed the previously set limit value.
d. Some external transient events that could damage the laser.