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!

With Manuel's help I managed to change the sensitivity initialization of the lock-in. Now it is set at 5mV/mA.
I restarted the measurement around 23:45 after measuring the stability of the pump laser: dP/P < 2.5% (see attached fig.1)

I logged in to monitor the measurement and found that the labview crashed. I tried to restart the measurement but the computer was unbearably slow so I reboted it.
The crash of the labview and subsequent restart resetted the LockIn parameters, therefore the measurement now seems unfeasible.

We will investigate tomorrow how to solve this problem.

Pierre, Aritomi and Yuhang

We found that when we just connect the source port of network analyzer to servo, the error signal presents a very very large 50Hz noise.

Today at 20:15 I checked the status of the map and found that the computer rebooted (apparently due to some windows updates even if the auto-update was switched off).
Luckily the laser was still operating at the previously set value, so I restarted the map at 20:17.

Matteo, Eleonora, Simon

With the alignment set yesterday and the day before (see log entries 1448 and 1445), we were able to start with inserting a sapphire mirror (SHINKOSHA, No. 7). We are using this mirror as a kind of a test-sample to check whether the system is giving reliable data since this sample has been measured already in the past.
 

• We removed the silica mirror from the mirror-holder (that one is now stored in the small shelf inside the clean-room - be very careful when moving around as it is easy to hit!!!)
• We took the sapphire mirror from its container and put it inside the mirror-holder (see attached pictures)
• For placing the mirror-holder inside the transition-stage, we removed the protection-shield around the detector-bench, relocated the imaging units of both red and IR (red -> 6.7mm), and moved the translation-stage to a convenient position
• Then, we placed the mirror-holder on the translation-stage
• After that, we put the protection shield

Note: In order to have enough clearance for the mirror (thickness ~15cm), we removed the IR imaging-unit later on

Once the mirror was set, we started a checkup measurement to compare our setup with measurements from the past:

• Setting the border-limits to define where the mirror starts and ends in Y and X direction
• Readjusting the imaging-unit to have a maximum in DC
• Run horizontal measurements to obtain the coordinates of input and output surface with ~1W of input power in the center
• Run horizontal measurements with 10.47 W of input power to check all initial settings (see screenshot)

With the results, we confirmed an absorption coefficient of ~130 ppm/cm which is in agreement with Manuel's measurements (~119 ppm/cm mean value over an entire map)

Then, in the afternoon, we started to take a X-Y absorption map in the center of the mirror (anticipated duration ~14h). That should be finished by tomorrow.

Pierre and Yuhang

Since Matteo designed the new base plate for the new mirror mount (RadiantDyes), I replaced the old base plate with this new one. This new one has two holes in the plate, so we expect it to have a better performance.

After the replacement, I tried to lock again with the new MZ/GRMC servo realized by Pierre. But at that moment, we didn't succeed. The reason was found out that somehow GRMC transmission was reduced. Now the highest power we can have for GRMC transmission is around 55mW.

Then we tried to find the coherent control error signal for CC1. But we failed. We will try to figure out the reason tomorrow.