Simulating the absorption of the surface reference, I optimized the Imagin Unit distances to have the maximum signal in the two cases, pump waist 40um and pump waist 80um. See the first plot, it shows the signal as a function of the distance d2 from the lens and the small sphere.

Using the optimum value of d2 in the two cases, I repeated the simulation of elog entry 905.

In the case of pump size 40micron, the absorption is 14.7ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 4.09
In the case of pump size 80micron, the absorption is 19.7ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 3.03

the probe size is still 180um in both cases, next step is to reduce it as well to be 3 times larger than the pump