Today I decided to further increase the laser power. Indeed my concern was coming from the fact that we could not clearly see the effects of the viewport 2 surfaces on the ac nor the phase signal.

I choose HWP angle = 55 degrees which translate to Pin = 3.135 W.

Figure 1 shows the results of a large Z scan of the translation stage : The 2 surfaces are now visibles (spikes in the AC/DC and phase jump) !

The surface with the smaller Z is the surface we want to measure (it is close to the expected value of 41.5 +30 mm ).

I checked the tilt of the viewport and it is still around 0.3 mm over the entire map area (30 mm radius).

A result of absorption measurement is presented in figure 2.

The strange thing is that it is really coherent with previous measurements....

On this measurement, one spot was saturating the AC signal so I started a new measurement with sensitivity 1 V (max)  instead of 50 mV.

I really suspect 2 high absorption spots to be due to dust as it is quite visible by eye and seems different than the other drop like stains.

If this assumtion is correct (maybe can be checked with  another measurement after applying first contact), it means that after cleaning, most of the remaining dirty things are mainly generating absorption below 100 ppm.