Pengbo, Simon

We have started to analyze the coated OSTM from Shinkosha regading the absorption of the HR side.

First, we took out the OSTM and inspected the mirror visually. We found pencil marks on the barrel and among them an arrow that indicates the thicker side of the wedged substrate and shows toward the HR side of the mirror (see attached photos).
The actual orientation of the sample inside the sample holder was a little bit tricky mainly because of the wedge and the size of the sample-holder which is basically too large for such a mirror.

• At first, we tried to put the thicker side upside to have a somewhat parallel orientation between sample-holder and mirror-plane (as can be seen from the pictures, we are using a - with optical tissue - covered ruler as a spacer). However, we recognized that this will distract the pump-beam so that it cannot be measured anymore regarding its power.
• Therefore, secondly, we rotated the mirror by 90 degrees so that the distraction of both pump and probe beam is only parallel to the optical table which can be counter measured by a respective relocalisation of the IU and the photometer. However, with this position, the HR coating is facing the pump but also the spacer, which we originally wanted to avoid
• Especially regarding the probe, we had to change the IU position by ~3mm less than it would have been the case for a wedge-free substrate

After the alignment of the sample, we looked for the exact position of the HR-coating by applying Z-scans. We carefully increased the laser power to take care that the coating is not damaged (initially, we did this on the outer edges of the mirror of course).

Then, we ran a map-scan in the center with 15mm radius in the position were we identified the coating (Z = 48.8). The results of that scan can be seen also in the pictures attached. Our main result is a quite homogeneous mean absorption of 16 ppm (+/- 3ppm) with some point-like excesses indicating the positions of either dust or defects within the coating, most likely.