LASER diode driver circuit
The pot labeled Vdc and the photodiode both supply variable amounts of current to the negative node of the op amp.  As such, the output must respond by going lower in order to compensate.  As long as the op amp is within its operating range, the output will do what is needed to keep the negative input at the same voltage as the positive input:  -5.2V.
When the op amp output goes lower, the LED current is decreased due to the reduced voltage drop across Rb.