today, we observed on the photodiode used in reflexion of the cavity, that its voltage level stops increasing after 50% of power for the laser amplifier when the cavity is not locked (the FP-cavity is just a reflective mirror).
the photodiode itself is not saturated (low power sent after a wedge and an absorbing filter) and the reading is done on 50 ohms.
so we are investigating the reason of this "saturation".

1) we did a beam power measurement just after the 2 mirrors, right after the amplifier, with the "big" powermeter:

power ratio        Power (W)
10%        =>  0.9
20%        =>  7.7
30%        =>  15.5
40%        =>  24.2
50%        =>  33
60%        =>  41
70%        =>  48
80%        =>  55.5
90%        =>  62.2
100%      =>  68

plot in attached file

2) we did the same measurement after the periscope:

power ratio        Power (W)
10%        =>  0.82
20%        =>  7.7
30%        =>  15.5
40%        =>  24.2
50%        =>  32.6
60%        =>  39
70%        =>  44
80%        =>  47.4

we clearly see a power reduction from 50% and beyond.
with the viewer we saw 2 effects : a slight clipping in the telescope and some speckles on the periscope mirrors mainly.
the laser is also slightly shifted on the 2nd mirror after the amp.
it is possible the laser beam moved due to thermal effect => we will need to realign properly the whole injection line and be aware about mirrors and lens cleaning !

3) we did some measurements using 2 wedges with the "small' powermeter in reflection of the cavity.
we remove the small aperture half-waveplate to see the effect and we used a lens to focalize the beam:
the half-waveplate has an effect on the reflectivity of the wedges, this is the reason of the difference in the measurement.

                                                               Power (µW)
power ratio        with lambda/2           w/o lambda/2             w/o lambda/2+lens
10%        =>           12.4                           4.8                               5.1
20%        =>            132                            48                                51
30%        =>            262                            96                               101  
40%        =>            370                            137                             147   
50%        =>            420                            158                             175 
60%        =>            430                            165                             181

we observe a clear "saturation" after 50%-60% which is very similar to what we observed with the photodiode in reflection when the cavity is not locked.
the small iris used in front of the injection window of the cavity is a good "candidate" for clipping the beam at high power (we observed it was still quite hot after stopping the beam).
=> we have to redo the measurements after removing it.