R&D (FilterCavity)
YuhangZhao - 00:39, Wednesday 11 November 2020 (2271)
Optimization of AA (check balance, demodulation phase, rotation angle and diagonalization matrix again)

At the beginning of AA implementation, we checked balance (between each segment of QPD), demodulation phase (this is equavilent demodulation phase set in DGS, which can rotate the input I/Q signals by a matrix. In our case, we rotate signals to I phase.) and rotation angle (this is the quavilent rotation of QPD). All of these were done without the lock of AA loop. And we found some change from day by day.

Since now we have AA locked. I checked them again and optimized accordingly. The old work gave already quite good gain/phase/angle values. For example, in the first two attached figures, we can see the balance situation of QPD1 and 2. The balance is already quite good. Since we could get exact values from figrue 1 and 2. I calculated the required gain to balance them. After applying these gain, the balance situation is shown in the attached figure 3 and 4. We can see that the balance becomes much better. But if this will change again from day by day should be examined later.

(Old gain is 1 for each segments.) New gain is shown in the following table

 

I1

Q1

I2

Q2

I3

Q3

I4

Q4

QPD1

1.000

1.000

0.802

1.661

0.673

1.059

1.204

1.557

QPD2

1.000

1.094

0.850

1.040

1.055

0.900

0.789

1.130

After that, I also optimized the demodulation phase. The result is shown in the attached figure 5 and 6.We could see that only negligible signals go to Q phase. This should also be checked again later.

New phase is shown in the following table

 

Segment1

Segment2

Segment3

Segment4

QPD1

97

100

97

99

QPD2

135

125

138

128

Then, the rotation angle was also optimized. The optimization result is shown in the attached figure 7 and 8. However, we could see that pitch to yaw coupling has become already around 15% in WFS1/2. This should also be checked later if they will change or not.

Old rotation angles are all zero. New angles are shown in the following table

QPD1

QPD2

13

5

In the end, a new matrix was developed to close AA loop. The loop filters/gains are the same with the old case. A comparison of diagonalized signals are shown in the attached figure 9. It can be seen that they don't have large difference.

pitch

WFS1

WFS2

Input

1

-0.5

End

1

2

yaw

WFS1

WFS2

Input

-1

0.8

End

-1

-3

Images attached to this report
2271_20201110164025_qpd1balance.png 2271_20201110164030_qpd2balance.png 2271_20201110164036_qpd1afterbalance.png 2271_20201110164044_qpd2afterbalance.png 2271_20201110164053_qpd1afterphasechange.png 2271_20201110164101_qpd2afterphasechange.png 2271_20201110164116_wfs1rotation.png 2271_20201110164124_wfs2rotation.png 2271_20201110164143_20201110.png
Comments related to this report
YuhangZhao - 19:22, Wednesday 11 November 2020 (2273)

Today, I checked again the balance, demodulation phase, and rotation angle. The results are shown in the attached figure.

The balance between each channel is not optimized now. The demodulation phase is also not optimized. The pitch/yaw coupling becomes a bit better.

This result means the balance problem cannot be solved by a stable alignment.