Matteo, Satoshi

We measured the transfer function of RFPD used for PDH locking.
I have thought it be a resonant RFPD, but not.
Attached picture shows the TF of the RFPD.
The gain is 33.5db at 29.1MHz which is the modulation frequency of EOM.

Here I attach the measurement of PLL phase noise. The measurement has some disconnect points although I used V/sqrt(Hz) as a unit. The result is summarized as following.

The lock of filter cavity make RMS phase noise increase by a factor of 1.5/3.5

Aritomi and Yuhang

Today, we checked together about the polarization again with PBS plate.

• The mirror was mounted as flipped(the uncoated side was facing laser). We flipped it back to the correct side.
• The angle of the PBS plate was quite far from 45 deg. We checked today this angle. When we have all the light reflected, the angle was quite far from 45 deg(at that time, around 35 deg). When we turn the mirror to the direction of 45 deg, this reflection disappeared.

So we concluded that the beam we were using is p polarization. And we also need to pay attention next time about the surface of mirror and the angle of PBS plate.

Matteo, Simon

Since the DC value seems to be depending on the longitudinal position of the mirror-substrate (Z), we repeated taking a XY-map at Z=112 with re-calibrated DC-value. It turned out that also the absorption-values increased. We now think that we would have to readjust the DC value every time the Z-stage moves, which would make it basically impossible to receive any quantitative results on XZ and YZ maps (however, qualitative results are anyway achievable).

We run another map at the center position with adjusted DC value to have the three maps (Z=46, 79, 112) all adjusted.

At the same time we started to reconfigure the absorption bench for measuring polarization maps.
Therefore, we aquired some PBS and a 2" lens (f=100) which we could put in the outgoing beam path. In a first test, we already recognized that the main part of the pump beam is p-polarized with ~10% s-polarization, although we thought it were merely s-polarized!
So, we put another PBS in the incoming beam-path to make it almost completely p-polarized and checked the effect on the photometer. When moving the mirror into the beam-path, there seems to be some changes happening but without more precise analysis with photo-diodes, we cannot be sure.
Anyway, we let it like this and will continue tomorrow when the other absorption measurement is finished.

At last, we removed also the first-contact from the ETMY substrate which we put there to clean its surface after the laser-burning accident last week. It seemed to be successful and we could not see any dirt anymore. In this state, we put the container over the substrate to protect it.

[Aritomi, Yuhang]

First we tried DC balance of homodyne with s pol, but power unbalance of BS was large for s pol. Reflection of BS is 630 uW and transmission is 600 uW for s pol while it's almost balanced for p pol. So we decided to use p pol for the moment. We made LO p pol with HWP and confirmed its polarization with cubic PBS (Newport,10BC16PC.9). However, when we put plate PBS (thorlabs, PBSW-1064) instead of cubic PBS, most of the LO was reflected by plate PBS. Actually I didn't check the direction of plate PBS and maybe it's wrong, but even in that case we cannot explain this behavior. We'll check the direction of plate PBS and check the polarization with AMC tomorrow.

Anyway, today we managed to make LO DC balanced with "p pol".

I checked the current assumption as suggested by Matteo. It is fine. The situation now is the same with entry 1426. At the same time, for -19V channel, the current assumption is 0.063A. Although we don't have reference for this negative channel, I think it is also fine.

After the optimization of locking servo parameter, we locked PLL_CC. We are interested in this PLL because this phase noise is coupled into squeezing measurement directly. However, the other PLL phase noise changes the parametric amplification and de-amplification factor. How much can they affect squeezing needs to be considered further.

Anyway, the measurement result is attached. There is two main difference:

• The high-frequency PLL noise when the filter cavity is locked is lower
• The low-frequency PLL noise when the filter cavity is locked is higher

I think this is because we are making the main laser to follow filter cavity. Filter cavity suspension makes low frequency noisier while high frequency quieter.

Yuhang and Aritomi

Last week, we found we were using FC lock mixer in an inappropriate way. Then we talked with Matteo and Matteo said that he and Marco did it on purpose.

So I checked what is the difference between amplifying LO and amplifying RF. The result is shown in the attached figure 1 and 2. It is obvious that, by amplifying RF, both signal amplitude(10 times larger) and signal to noise ratio become better.

Then I tuned FC locking servo input attenuator and gain, finally, both 7 seems a good choice(see attached figure 3 and 4). The measurement of the open-loop transfer function is shown in figure 5. Now we have a unity gain frequency of 22kHz and phase margin of 55 deg. But there is a very broad peak after unity gain frequency, maybe we should make unity gain frequency a bit lower.

Simon

Please find attached the absorption maps of GT's ETMX mirror substrate that is currently being analyzed in the absorption bench.
For now, we have three XY maps taken at different positions along the c-axis (Z = 46, 79, 112, where "79" marks the center of the mirror related to the bench-alignment; given in [mm]).

I also calculated some histograms which give statistics on the distribution of the absorption-values.

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.

Matteo, Simon

Today, we got the first results of the absorption measurements which we started Yesterday. The map showing a circular area around the center of the substrate (taken in the middle of the bulk-body) can be seen in the attachement. The mean absorption coefficient of the whole map is around 75ppm/cm.

Meanwhile, I prepared a python script than can be used as an alternative to the Matlab-code written by Manuel to calculate the absorption-map from the data-file written by the Labview program. I upload it to the dropbox-folder of the PCI PC.

Aritomi, Yuhang

Today when I wanted to lock PLL of p-pol, I found p-pol and master laser beat note frequency is close to zero. Then I move this beat note frequency to 150MHz by changing p-pol laser temperature as usual. But I found that I cannot lock PLL by using the current set-up. I didn't realize at that time, actually, I have already put p-pol locking frequency to another side of 0MHz. Then I changed the PLL locking sign and in the end, although everything else was fine, I couldn't find the coherent control error signal.

So please notice next time that if you couldn't lock p-pol PLL by using the current set up, please change p-pol beat note to another side of 0MHz. (different side refers to p-pol laser frequency is higher or lower than master laser frequency)

Aritomi and Yuhang

I found beam was cut obviously at the output port of MZ. The cut position is shown in the attached figure 1.

There is also a very good point to check if the beam is cut or not. In the attached figure 2, the shown place is a round shape. But when the beam was cut, I saw clearly there is an unfilled corner.(I am sorry I didn't realize to take a picture at that time)

In the attached figure 3(cut) and 4(no cut), you can see the difference before and after solving the problem of cutting. The higher order mode becomes a lit bit lower. After that we tried to change the offset of MZ servo, then we found we could reach GRMC transmitted power of 70mW. This meets the requirement of high BAB amplification for filter cavity alignment.

After I did this replacement, I found there is a new peak appeared. Compared with the alignment Chien-ming did for GRMC, this additional peak has a much higher height.

Today I use a camera to check this peak and it shows shape like the attached figures.

But I think this is not a big issue since we just need to lock on TEM00. But we will lose some power.

In the past days Pierre and Yuhang noticed that the level of the LO to the fiter cavity mixer was not the one required by the mixer, so they amplified it. After this change we were not able to lock the cavity on the TEM00 peak anymore. We went back to the previous configuration but still we could not lock properly. It seemed that we were locking on a sidebands and, strangly enough, we could lock on the top of TEM00 only by switching off the rampeauto power supply. But of course in this configuration the gain was much lower and the lock was not stable.

We found that the problem was that the sign of the lock had changed (even after going back to the original configuration). 

Note that the sign inverter on the rampeauto doesn't change the error signal sign, the only way to change the sign is by shifting the demodulation signal sent to the mixer of 180 deg. We did it and the cavity could lock again.  

We also learnt that swiching off the reampeauto power supply somehow changes the sign of the correction that it provides.

Matteo, Simon

After the little desaster today with the ETMY substrate, we confirmed that the absorption bench was not damaged and continued to do the absorption measurements with the ETMX substrate from GT. We confirmed that this sample has approximately the same thickness as the ETMY one and set it into the sample-holder in the same way.

We learned our lessons today and did the alignment settings of the substrate without the IR-laser, just with the probe-laser.

The absorption-map is now being taken and the measurement should be finished by tomorrow (fingers crossed!).

Matteo, Simon

In order to measure absorption maps of the Sappire substrate from the "GT" company, we started to take the ETMY substrate. For this purpose, we carefully opened the container of the mirror and took-out the sample This was actually a tricky part as the sample is quite heavy and not only there were some placeholder-columns in the container which we could not remove and made it difficult to pick-up the sample, but also we realized that the sample already has the "ears" attached.

Somehow we succeeded to put the sample inside the mirror-holder and confirmed that it has a reproducable position in it by using a horizontal alignment of the ears. We can furthermore report that close to the edge, we recognized several defects which we, however, do not consider as being a problem.
Also, due to a repolishing measure which has been done after bKAGRA-phase1 (the substrates were already been used as KAGRA mirrors before), the thickness is reduced compared to the Shinkosha#7 sample (150 mm → 143 mm)

Once we set the substrate together with its holder inside the transition stage, we adjusted a little bit the position of the imaging-unit (6.7 mm → 9.7 mm) due to the reduced thickness. After that, the DC-signal was adjusted and the pump-laser power maximized.

Main Issue:

At that point we could start setting the basic limits for the transition stage with the respective LabView program.
However, the initial (default) values for the stage-position put the holder into the beam-path of the pump-laser while at maximum power!!!
→ That burnt a hole into the mirror-holder
→ Parts of the burnt hard-plastic could be found also on the sample surface

Of course, we immediately blocked the laser-power and removed the sample and its holder for inspection:

• It seems that the sample itsel is not damaged but dirty on one surface now
• With an air-blower, we removed the largest parts of the dirt), while the sample-holder has a hole on the outer side. No further damage could be reported (the sample holder is still usable).
• We put the sample back into the container-frame and put first-contact on it

Let's hope this will clean the surface enough for continuing the measurements (we will see next week...).

Pierre, Aritomi, Eleonora, and Yuhang

After we used the new base plate for mirror mount, we measured the optomechanical transfer function yesterday. (Later we realize that we lock with a bandwidth of only around 100Hz. We saw ~1kHz resonance oscillation when we tried to increase gain)

Today, Pierre designed the new CC1 locking servo. While we have the same problem of ~1kHz oscillation. We could only lock with a bandwidth of 200Hz which is reasonable. I realize only now that we should have this 1kHz oscillation problem. With the locking bandwidth of 200Hz, we could measure the open loop transfer function. The measurement result is shown as attached figures.

There is no particular explanation of why we have this new 1kHz peak. And this peak height is around 20dB. This height of peak makes the system very unstable.

Notice: we could measure open loop transfer function with servo and network analyzer again. The reason is we connect network analyzer power with ground floated.

When I connect it with the same power with rack, the 50Hz noise becomes even higher!

Another thing to be noticed is that when I use SR560 as servo, and inject noise from network analyzer to SR560, there is no 50Hz noise!