The APC loop requires an error feedback to drive the average optical output power to a
desired level. This error feedback is derived from the difference of VPhoto and an
internal reference voltage APCVRef. The APC loop adjusts the LD bias current to
minimize this error. As such, the voltage APCVRef defines the “Set Point” of the loop and
reflects the desired average optical output power from the TOSA. This set point control
is called the “APC Reference Threshold” and can be changed through the register
APCRefDACCtrl with 8 bits of resolution.