In order to calculate the calibration in the case of thick samples, I simulated the scan of the bulk reference silica sample, then I simulated the same thing but adding a 6cm-thick sapphire sample on the probe path.

I already calculated the probe beam size on the detector for different sample thicknesses (Elog entry 263)

In the  first plot, there is the scan of the bulk reference sample (red line), and also the same scan but with a 60mm thick sapphire after the sample (blue line). Adding 60mm of sapphire after the sample changes the optical path of the probe and makes a different signal.

The calibration value is taken at z=2mm, and there is a factor of 5 of difference between the two cases.

The second plot is a 2x2 matrix of plots and it shows the probe beam profile at the detector, when the sample is at z=2mm. First column of plots is the beam profile. Second column is the interference pattern, from which the AC signal is calculated. The first row is the case with only the bulk reference sample and the second row is the case with the  bulk reference plus the tama-sized sapphire sample after it. The white rectangle is the profile of the photodetector. The unit of axis is m, the photodetector is 1mm large.

I think it is necessary to adjust the reimaging of the detection unit. I will try to get better signal in the simulations by changing some distances among the components and also changing the focal lengths of the lens and of the sphere. 

I will also reproduce the same experimental configuration of this simulation, putting the tama-size sapphire sample after the reference sample, and making a real scan. This will also be a test to see how reliable is the simulation.