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R&D (FilterCavity)
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MarcEisenmann - 19:52, Tuesday 07 August 2018 (935)Get code to link to this report
Green beam reflected by the FC

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.

Images attached to this report
935_20180807125132_reflgbeamfcwithlens.jpg
Comments related to this report
YuhangZhao - 13:31, Wednesday 08 August 2018 (937)

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.

MarcEisenmann - 22:38, Monday 27 August 2018 (958)

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.

YuhangZhao - 09:45, Wednesday 29 August 2018 (962)

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.