Yuhang and Yuefan

While designing the telescope of the automatic alignment, we had one question is that how close we could  put the galvo from the quadrant to have decent range. To answer this question, we should at least have the idea how much the reflection beam could move.

So in order to do this, we tried to do a long term monitor of the DC signal of the quadrant without closing the galvo loop.

The problem is that the filter cavity could not stay lock during the time we record the DC signal, so we only could have data of less than one hour.

• In the first try(fig7), during around 1h of locking, the recombined DC pitch signal has a maximum movement of 0.1, first we tried if we manually moved the beam in pitch to have 0.1 movement, the galvo was totally able to bring the beam back to the center.(fig6) But then we thought maybe it is better to do a rough calibaration to have an idea about how much the beam moved on the quadrant, with the chaning we saw on the dataviewer.

1. Record the DC pitch and yaw signal with Dataviewer (fig 1), we could see that for pitch and yaw, the reading are both around 0.2.
2. For the pitch, we record the beam height at that moment by put a ruler in front of the quadrant. The position of the ruler also recorded. (fig 2)
3. Move the screw of the galvo to move the beam around 1mm in pitch. Actually the movement done by the screw has a coupling between pitch and yaw, but because we are only checking pitch this time, we didn't care about the yaw value changing.
4. Checking the difference in the signal in Dataviewer again. (fig 3) Pitch moved to 0.1
5. Starting from the position of last step, where yaw reading is -0.14, Repeat the steps above on yaw (fig 4), then it changes -0.22. (fig 5)

         So the calibration results are like below,        

• For the second(fig8) and third try(fig9), we compared the DC signals with the cavity green power tranmission. There were several short time unlock and relock, so we could see that everytime the cavity relock, it seems lock again at different position. And while the alignment of the cavity is drifting away, the tranmission power is lower and lower, and finally cavity unlock, we could see the DC signals also goes to zero slowly, from this two fact we had some guesses.

1. Since everytime we tried to overlap the input and reflection green by only checking the viewport, the beam may not always go to the same direction, which results in the reflection beam height changing we obsered yesterday and also in the past weeks. But of course closing the galvo loop could make the situation better, but the galvo mirror is quite small, if the beam pointing change is too much, the beam could totally miss the galvo. So probably we should do another long term monitor with galvo loop close.
2. What we expected the DC signal should be like when the cavity slowly misalign is that in one or both direction the signal should firstly increase and then a sudden drop to zero when the beam is totally off the quadrant. But what we saw is quite different. After discussing with Matteo and Eleonora, we found out this could  be caused by the fact we didn't normalise the pitch and yaw signal with the total power, so the slowly drop we saw is because the power reduction when the beam is moving out of the quadrant.