This afternoon we started to work on the calibration of the BPL.
We used a substrate in fused silica which thickness is 10 mm and placed it after EIB_M1c at around the same distance than the EOM entrance (the point to be stabilized). This corresponds to about 12.5 cm from PZT2.
Then we rotated it both around the X and Y axis in order to induced a known shift towards the QPD.
The plots are attached to this entry. We will analyze it in details tomorrow.
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The calibration is done for the displacement at the stabilized point : the entrance of the EOM.
For the shift, the vertical one was quite clean and gave a calibration of -1900 (V/V)/m, which means 526 um/(V/V) (plot 1)
For the horizontal shift, it was not as clean as the vertical one (plot 2), we might need to redo it. For now we decided to use the same calibration than the vertical one since they are not supposed to be different.
Then we did the calibration of the tilt by using the PZT on EIB (plot 3 and 4)
We know that the PZT has a dynamic of 3.5 mrad for 140 V, we did a scan of 3 V and found extreme values of 0.39 V/V and -0.73 V/V on Vnorm
We found 3.5e-3/140 *3/(0.39+0.73) = 67 urad / (V/V)
Similarly we found 68 urad / (V/V) for the horizontal.
We added the calibrated channels to ACL.
The results with the loop closed is showed in plot 5 and are quite in agreement with what we were expecting : few nrad and few nm of residual noise.
We keep the loop closed to monitor is long term behavior
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