Aritomi and Yuhang

We found we couldn't lock CC1 yesterday, and we pointed out the 600Hz and 1kHz noise. Also, we complained about the 1kHz oscillation. However, we just realized that this oscillation was not 1kHz but 600Hz. Have a look of attached figure 1. We thought it was a bit more than 1kHz because the period of oscillation was more than one block of the oscilloscope time axis. While this is a totally stupid mistake, the period of oscillation larger than a block of 1ms should be frequency smaller than 1kHz.

But anyway, we tried to find out what is the difference between we could lock before and we cannot lock now. There are two different points:

• we changed the base plate of mirror mount.
• The PZT&mirror holder was accidentally touching to fix part of mirror mount. (touching problem was not realized at that time) This way of using mirror mount make it function as only a mirror holder, which means we cannot steer it. Please refer to the attached figure 2 to see how it is touching.

First, we tried to touch again. However, after this, we still could not lock CC1. But we couldn't find ~1kHz peak which appeared yesterday. So we think maybe the appearance of that 1kHz peak on yesterday is somehow wrong. I put one of the measurements of OLTF here as attached in figure 3.

Then we tried to follow the suggestion of Matteo. Put a piece of thing on the top of the mirror. See attached figure 4 for how we put this piece of thing. After that we still have oscillation, but it is damped by this additional piece of thing. See attached figure 5.

Then we replaced back the base plate to the original one. We tried to increase the gain, at that moment, we found the oscillation appears as ~500Hz. This is in agreement with the measurement before. Then we lock it first with a bandwidth of ~200Hz. The measurement of OLTF is attached in figure 6.

Since we know that we could use this configuration to lock, so we just tried to increase the gain. When we use a gain of 5.5, we could lock our servo! Then we measured the OLTF but we found a very strange peak at 3.7kHz. This is a very large peak, in principle, this peak will make our system oscillate. However, we could lock.

After that, I also want to try to make PZT&mirror holder not touch mirror mount. But this makes the beam tilt. So, in the end, I didn't succeed. But maybe we could shift this holder a bit ahead since anyway we have a gap between the holder and mirror mount(see the attached last figure). Actually, this gap maybe is because of we are touching the mount and could not put inside anymore. Maybe we could just shift.