PDs dark noise and laser noise

I decided to use the usual VIGO pre-amps, since the Maurizio pre is too noisy. So I connected each PD to its own VIGO pre-amp and power supply. I first noticed that the pre of PD12595 is initially unstable, with DC output rapidly fluctuating between -10 and 10 volts. After some time (10min approx) it becomes stable as the pre of PD12599.

First of all, I measured the dark noise of the two PDs, shown in the next figure.



As it can be see, PD12595 has a better noise floor, but many more peaks wrt PD12599.
Then I turned on the laser, but still no beam is going on the PDs. Nevertheless, the outputs on the oscilloscope changed a lot from being flat to being modulated at something like 67kHz. The tracks are shown in the next figure.



But even if the time series look so different, it seems that this fact does not affect the noise measurement, as shown in the next figure, where the dark noise of the PD12599 is plotted when the laser is off and on.



Finally, I shined the PDs with some laser light. It appears that PD12595 is measuring much more low frequency noise wrt PD12599, even if the DC output of the two PDs are almost the same. This must be understood better. I must say that I did not check the alignment of the PDs.