I set the loop to control the intensity of the 1310nm laser.
first I measured the actuator transfer function with a random noise from the spectrum analizer of amplitude 200mV

Then I set a low pass filter at 100Hz, a DC offset from the waveform generator to keep the correction signal around 0, and a gain of 200.
I closed the loop and measured the open loop transfer function.

I measured the noise in the photodiode with the loop closed and without loop.
then I divided the noise without loop by (1 + the open loop transfer function) and compared with the closed loop pd noise.

If I increase the gain above 1000 the laser stops for exceeding the current limit.