Still looking for the sapphire calibration factor of ~3
I simulated the absorption signal for bulk reference and sapphire sample in two different conditions:
- pump size (waist radius) 40micron
- pump size (waist radius) 80micron
The absorption parameter used to simulate the sapphire absorption signal is 60ppm/cm.
To calculate the sapphire absorption from the simulated signal I applied the formula Abs = AC/DC / P / R
where the calibration factor is R = (AC/DC)_ref / P_ref / Abs_ref
with P = 10W and P_ref = 30mW.
I didn't apply the material correction because that's the final estimation of this simulation.
In the case of pump size 40micron, the absorption is 19.5ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 3.08
In the case of pump size 80micron, the absorption is 23.8ppm/cm, which, compared with the 60ppm/cm gives a material correction factor of 2.52
Conclusion:
decreasing the pump size results in a better estimation of the material correction (comparing it with the value of 3.34 given by the SPTS company),
But it is still far from the factor of ~3 discrepancy of my measurements.
If the discrepancy was all due to the pump size and the simulation were exact, the material correction in the case of 80micron pump size should have been about 1, instead of 2.52.
Comments:
- There is a strong approximation on the bulk reference material, which is schottglass#21 but in the simulation is silica (because I couldn't find the thermal properties of schottglass).
- The phase for the same material shouldn't change with the pump size. But the simulation gives different values of the phase. This may be due to a different optimal position of the Imaging Unit for different pump sizes, and I didn't optimize it for the new 40micron pump size.
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
I reduced the probe size as well, from 180um to 120um, to be 3 times larger than the pump (which is 40um), but the signal doesn't change much.