NAOJ GW Elog Logbook 3.2
I removed the blades and reinstalled the mirror on the translation stage.
I decrease the input power to about 240 uW so that the maximum voltage given by the PSD is about 0.8V (lock-in maximum being 1V).
I tuned the hwp and qwp together to get a linearized polarization.
For that, I roughly went to the s polarization input and then adjusted by hand the qwp angle and by 0.1 deg the hwp angle so to minimize the p polarization readout.
I could finally reach s polarization = 0.7808 V and p polarization = 187.5 uV that corresponds to an ellipticity of sqrt(1 - (s pol / p pol)^2) = 7N
The s polarization corresponds to hwp angle of 45 deg and p polarization to 90 deg (note that the round values come from some reset of the hwp angle readout that is solved by disconnecting/reconnecting the cable).
I started measurements in that condition.
They last about 1h30 hour with 0.5 mm step size and should be finished tomorrow.
While waiting for measurement, I started to analyze data and found out that our measurements are clipped.
Indeed I forgot to change the Y center position of the sample while, to switch from absorption to birefringence, we add 2 steering mirrors that shift quite a lot the beam.
I checked the sample surfaces by looking at the position that minized both the s and p polarization power.
I measured Y_center = 160.37 mm and restarted measurements.