this morning, I added a DET36 photodiode in reflection (I used a thin beam splitter) to better measure the coupling over the full beam size.
this DET36 photodiode is sent to the "laser locking" oscilloscope with the other signals :
CH1 :transmission
CH2 : reflection
CH3 : error signal  or  FP-cavity PZT (depending on the Laselock Monitor 1 signal configuration)
CH4 : laser PZT (Laselock Monitor 2 signal)

I kept the DET10 photodiode in reflection, but slightly misaligned to get the 500MHz laser harmonic without too much amplitude variation when locked (the lock  behavior changes almost the center of the beam).
the DET10 photodiode is sent to the "RF locking" oscilloscope on CH1