I aligned again the pump and the probe to maximize the AC signal. For several positions of the surface reference sample.

z AC DC

33.9    0.393   3.94

34.4    0.405   3.95

34.9 0.416 3.96

35.4    0.418   3.96

35.9    0.421   3.95

36.4    0.426   3.94

36.9    0.424   3.93

37.4    0.421   3.93

37.9    0.410   3.89

The maximum is at 36.4mm which means it is the closest position to the pump waist.

I adjusted the position of the imaging unit to maximize the signal.

Then I repeated the calibration, but there is not a lot of difference from the last time.

I repeated the sapphire measurement and compared with other measurements. Nothing has changed. Very strange.

Today I'll try changing the angle and size of the probe. It's the last try, then I'll write to Alexandrovski.