I simulated the signal of the absorption system, in order to be able to compare the measurement reported in this post .

The reference sample is made of silica and absorbs 116%/cm. Thickness is 3.6mm. Chopper frequency 400Hz.

I attach an animated gif to show three aspects:

1. The colored image represents the temperature distribution inside the bulk reference sample, heated by the horizontal pump beam. The shadow represents the probe beam at an angle of 7° to the pump.

Moving the sample along the z axis (along the pump beam) make the crossing point of pump and probe to scan the depth of the sample, and it is equivalent to move the probe vertically respect to the pump.

2. The interference pattern is the difference between the intensity of the probe passing through the heated sample and the intensity of the probe not passing through the sample. This is because the heating is modulated. The signal is given by the integral of the interference pattern in the area of the photodetector. The area of the photodetector is represented by the black square.

3. The plot is the calculated signal from the lock-in as a function of the sample depth. The 0 is when the pump-probe crossin point is at the surface of the sample.