Eric, Annalisa, Gautam, Kiwamu, Tomo for TeamWFS  Report made by Annalisa and Eric, posted by Tomo

Today, we went to the IMC_REFL table to characterize the beam on the quadrant photodiodes path. We measured the beam with a wincam beam profiler. We started to measure the beam right after the beam splitter. The results are shown on figure 1 and 2. We found:

• Fitting the data taken up to QPD1 quadrant
  • Wx=197 um @2.17m after the lens.
  • Wy=312 um @2.16m after the lens.

• Fitting the data taken up to QPD2 quadrant
  • Wx=199 um @2.19m after the lens.
  • Wy=314 um @2.18m after the lens.

These 2 results are matching quite well.

We have noticed also that the beam becomes dirty after the second HR steering mirror on the quadrant beam common path (see 3rd picture). It could be due to some local defect on the optic. We tried to clean it with a dry optical tissue but it didn’t solve the issue.

Then, using the results of those measurements we have computed the Gouy phase difference between the 2 quadrants. We found 23 degrees for the horizontal direction (see 4th and 5th plots) and 24 degrees for the vertical direction (see 6th and 7th plots).
The Gouy phase separation is not optimal and will have to be reviewed once the PMC will be installed and the matching will be optimized.

With the current beam size one thing which could be done to increase the Gouy phase difference is to move QD1 closer to the waist. As you can see on the 4th plot, moving QD1 to the waist we could increase by almost 50 degrees the Gouy phase difference thus having more than 70 degrees. Note that the size of the beam on QD2 is 600 um on x and 470um on y.