We change the setup a bit to have the same polarization on the photodiode and the PDH (in reflexion of the cavity). CF: schematics (futur).

Thanks to that, the symetry between reflexion and transmission is better now (as you can see on the plot).

We also put more power on PHD to have a better SNR by changing splitters. (We had 100 µW and now we have more than 1 mW, at the expense of the power on the camera).

Thanks to that we obtained a good locking, The coupling is quite good (roughtly 50%, on the green line)

In the following plot, the green line is the reflexion and the yellow lin is the transmission.