In previous entry ( #934 ) we reported that the BS pitch was saturating causing a lot of residual motion of the beam.
Yesterday, we found out that the BS pitch correction was not saturating anymore (~ -6V) and the motion was at the order of 10 urad both for pitch and yaw.
This is still yet ~5 times higher than other mirror motions at the exception of EM pitch which has similar motion.
To do the characterization we installed a f = 100 mm lens
In order to be sure that we had a good beam shape we installed a CCD after the periscope.
When the cavity is locked, the beam is shaking a lot preventing to take much more points.
Good news is that the beam doesn't seems astigmatic.
I used ABCD matrix to calculate the beam parameter before the lens(the lens used to perform better beam measurement). The result is as following:
beam waist position: -3.2m (relative to Faraday Isolator)
beam waist size: 877.20um
I also attached the python code, if you are interested, please have a look.
There were few mistakes made on this entry corrected in this one.
Attached to this entry is the proper fit of beam after the lens (previously a wrong wavelength was used for the plot).
The mean profile was used (w0 = 113.37 um 0.6981 m after the f = 100mm lens used for the characterization [lens is 40 cm after the Faraday Isolator])
The beam parameter is the following : w0 = 18.921 um @ 0.1168 m before the lens ie roughly 0.28320m after the faraday isolator.
Since my result is different from Marc's result, I did calculation again. I found a mistake in my calculation of ABCD matrixs.
From the calculation point of view, Marc's result is correct.
I will check in actual case to see if the calculation aggres with calculation or not. As I have already mentioned in the meeting, we can see the reflected beam is shaking while the filter cavity is locking. So if they don't agree with the actual case, I think the discrepancy comes from the beam shaking.