Marc, Yuhang
As for any waveplate, theta should have a 90 degrees periodicity with maybe one of the 2 orthogonal axis of better quality than the other.
We performed large rotation of the mirror while injecting s polarization and results are reported in the attached figure.
We estimated the rotation angle by measuring the arc length between each rotation. The errorbar comes from uncertainty in this estimation (+/-5 mm).
We measured the Ip and Is intensities at the center of the mirror.
We found out that we have, as expected, a 90 degrees periodicity in the minimum of Ip (or equivalently the s to p polarization losses).
Furthermore, it seems that a +90 degrees rotation (+ means clockwise rotation from the laser side), gave better results than around 0 degrees.
We started measurement at this position that are attached in the following figures.
They show the only small improvement on the losses but this might be due to the remaining theta.
The mean theta at the center position is -2.09 deg.
Marc, Michael, Yuhang
Today we finished the cleaning of #5 and the 1.5 inch sapphire.
Then, we tried to fine tune the rotation angle of the mirror.
This time, we placed a flexible ruler on the mirror and could precisely (+/-0.2 mm) estimate the arc length of the rotation.
The results are attached in figure 1.
Actually I made a mistake and rotated the HWP by +0.7 deg before this measurement so we need to add this value to the effective rotation of the mirror.
Note that for some points the mirror was a bit tilted which creates stronger coupling between s and p polarizations.
That's why we repeated measurements at some rotation angle.
We started measurement at -2.9 deg rotation with HWP = 0 deg.
In the end, before starting polarization measurement, we checked the HWP position that minimized the Ip power to be +0.8 deg.
We did measurement with input polarization angles of 0 deg, 15 deg and 30 deg.
We computed delta n and theta from them.
Especially, we found that theta = 1.6 deg in the center of the mirror.
This is exactly the value we expected from the HWP rotation angle.
We will use this rotation of the mirror for the polishing.