THORLABS ITC502 Operation Manual page 45

The button PD POL repeatedly toggles between four different settings:
CG
CG + BIAS
AG
AG + BIAS
CG
. . .
Toggling values follows always this order. If the "wake up" default is AG, the setting
after pressing PD POL once will be AG + BIAS. If the "wake up" default is AG +
BIAS, the setting after pressing PD POL once will be CG, etc...
Before switching on the bias voltage make sure that the monitor
If the monitor diode is forward biased with a voltage the resulting
current can damage or even destroy the monitor diode (= the laser
(monitor diode cathode grounded)
(monitor diode cathode grounded with bias voltage)
(monitor diode anode grounded)
(monitor diode anode grounded with bias voltage)
Attention
diode polarity is set correctly.
module).
2.9 Bias voltage of the monitor diode (BIAS)
ITC500 / page 37