After the implementation of CC1 mirror mount, we used the 1/f integrator and locked this loop(with unity gain frequency of ~2.2kHz). We measured open loop transfer function and the noise spectrum of the error signal. The result is as follows.

But the measurement of the noise spectrum when the turbopump was on/off was not so reasonable.

We decide to characterize this loop again. Including the measurement of free-running noise, noise while locking, PD dark noise and also the optomechanical transfer function.

We always need many people to check several checking points to align the filter cavity. Also, the channels for monitoring video signals from the end room were not enough(two channels were working at that time).

This means: (1) camera is not enough   (2) channels are not enough

• For the camera, we set up a new camera to take real-time video inside PR chamber. By using this camera, we could check the overlap of injection/reflection beam.
• For the not enough channels, we found two cables(actually is fiber) were broken from west-south corner to control area(we can also call it 'short part' of fiber). We replaced them(Broken fibers are 1-13 and 1-14. 1-13 was replaced with a working fiber and the label was changed as well. Now the 3-11 short part is broken). Then we found one of the channels of board1 was broken after one-day work. We changed it to a good channel later.

In the end, we have four real-time videos on the screen and an additional one (first target) which can be monitored after we switch from channel A to channel B. The organization of four videos are as follows

 The connection situation at the end room is shown in the attached figure 1 and 2.

[Eleonora, Matteo]

Up to now we have been using only 16 of the 32 channels available in the ADC of the standalone DGS recently installed in TAMA.

Today we received from Kamioka the BNC->Dsub converter that we missed in order to use them all. We have installed it and connect 4 more cables from the AA to the converter (each of them as 4 channels) (pic1).

We borrow these 4 cables from ATC after asking Akutsu-san (pic 2).  The cables are much longer than what we need (pic3) so in the future it would be good to change them with shorter ones.

I have modified the realtime model to include these additional channels and I verified that it is working fine.

In conclusion, we have now 32 ADC channels in our DGS.

According to Emil's thesis P. 42 figure 2.10 or P.48 figure 2.13 (b), 200 mrad of phase noise seems to degrade 15 dB of squeezing to almost 0 dB of squeezing. Our situation seems around 100 mrad of phase noise. Did you consider the effect of control bandwidth when you calculated rms phase noise? As you noted, it's better to measure the spectrum and integrate it within control bandwidth.   

[Aritomi, Eleonora P]

You are right. We'll scan AOM frequency in the range of 1MHz.

It is expected that you don't find a TEM00 each 0.5 MHz frequency shift. As reported in entry #661, since the AOM is put on the green path, the change in the frequency which it induces is compensated by the servo with a change on the IR which is half of the frequency change in the AOM. This means that a shift of 1 MHz in the driving frequency of the AOM corresponds to a shift of 500 kHz in the frequency of the IR light. 

I think that we can now start to tweak the lens position to improve the matching.  It would be also good to do a cavity scan as the one done in entry #776.

[Aritomi, Eleonora P]

Today we managed to align IR into filter cavity and lock both green and IR again. First we locked IR with 109.03569MHz of AOM frequency as last measurement, but we found that TEM00 is much brighter at 109.03535MHz (Pic 1).

The power of each mode is as follows. Pic 2-7 shows shape of each mode other than HG10. Since green phase lock is not stable, we scanned green phase and measured maximum and minimum of IR transmission. Green phase is scanned at 20Hz. 

Now largest mode is TEM00 and main higher order modes are IG20 (Pic 5) and HG10.

I measured END oplev signal with dataviewer. The signal somehow oscillates at around 1.5Hz...

Beam spot fluctuation seems at several Hz. I measured PR and BS closed loop spectrum. This also looks fine.

Both the open loop and close loop end mirror TF look fine to me. There is no oscillation at 0.3 Hz.  Note that striptool used EPIC channels are sampled at 64 Hz, so the oscillation could be an artifact (down-coversion of higher frequency noise?) You can double-check with dataviewer.

From the movie of the transmitted beam it is not clear the frequncy of the oscillation but it likely to be from a steering mirror (BS or PR). It would be useful to take a spectrum of their motion.

[Aritomi, Eleonora P]

We found that green beam spot at trans is fluctuating (Mov 1) and drifting. So we measured oplev signal of each mirror. From Pic 1 we can see that END mirror is fluctuating at around 0.3Hz. We opened END mirror control loop and closed it again, but it didn't change. We also measured open and closed loop END spectrum (Pic 2,3). Open loop spectrum is similar to previous result. Anyway, since beam spot fluctuation is much faster than 0.3Hz, this is not due to END mirror fluctuation.

Mov 1 green beam spot at trans

Pic 1 time series of oplev signal

Pic 2 open loop END spectrum

Pic 3 closed loop END spectrum

Then we maximized IR reflection and could recover 00 flash once, but problem is that green alignment keeps drifting during IR alignment. Beam spot of green reflection at PR viewport is going away mainly in pitch direction and centering of trans beam by BS is also drifting. We're not sure the reason.

[Aritomi, Eleonora P]

Today we tried to improve the alignment of IR to filter cavity with last two steering mirrors on the bench, but unfortunately we lost TEM00 flash. We recovered the reference on PR chamber, but we couldn't find 00 flash (we didn't maximize IR reflection and check second target). Tomorrow we'll maximize IR reflection and try to recover 00 flash and improve the alignment.

[Aritomi, Eleonora P]

When we changed gain of filter cavity lock, glitch appeared in GRMC transmission and variable gain out of MZ like an attached picture and GRMC and MZ lost lock.

You could use some remote "smart" PDUs as done in virgo.

See as example

https://tds.virgo-gw.eu/?content=3&r=15139

(the name of the PDU is ENERGENIE EG-PMS2-LAN, see attached PDF file)

Based on the measurement we did before, we have dark noise of CC PD. We use this PD to lock the green pump phase, at the same time, we have bandwidth lower than 80Hz. So we could use this dark noise to evaluate the phase noise we have for coherent control loop 1. The calibration method is to use pk-pk value when we scan the green phase, which corresponds to radians of pi/2. In the case of that entry, 25mV corresponds to pi/2. The estimated phase noise(RMS) is attached as follows.

While it will be better to measure the spectrum and do RMS integration. And according to Emil thesis, this 200mrad of phase will degrade ~15dB of squeezing to ~5dB. And it seems to be close to the squeezing situation we saw in the past few days.

[Aritomi, Eleonora P, Yuhang]

Today we replaced a green phase shifter with a new thick phase shifter (picture 1). After re-alignment of GRMC, we got good mode matching (picture 2) and succeeded in locking of GRMC with lower gain. Green power is 146mW before GRMC and 90mW after GRMC when MZ is maximized. So transmission of GRMC is 62% which is good.

Lock of green phase with BAB transmission seems stable with this new phase shifter.

We'll measure transfer function of green phase lock loop with new green phase shifter tomorrow.

Eleonora P, Yuhang, and Aritomi

By keeping green always locked, we measured the power of all the higher order modes we could see from camera. The reason to keep green locked is that we have 'two different FSR' for IR. We found a lot of higher order modes. But the main higher order mode is HG10. This means our alignment needs to be improved for yaw direction. The power for each mode is reported as follows.(picture attached insequence without HG10 and TEM00)

Some modes I didn't write name seem so-called ince-gaussian modes. Maybe it comes from the combination of yaw misalignment and mode-mismatch.

We found the second HG10 at 109.4313MHz, which means FSR is 0.499MHz which is in agreement with our expectation.

We will do alignment of yaw for the next step.

Akutsu-san, Tanioka-san, Simon

It has been a long time since I wrote something in this logbook!

Anyway, we (that means AOS) are now relocating the scatterometer, which is still in one of JASMINE's laboratories at the ATC, to our lab on the first floor of ATC.
The reason is mainly that the JASMINE group needs the space in their lab and removed some desks which we used for the scatterometer's PC.

On the other hand, all of our optics-related stuff is more or less already in that lab where we are going to put the scatterometer in (including the back-scatterometer). So, it seems logical to relocate it there.

The first step in this week was therefore to find a suitable place in the lab, and we decided to use Torii-san's former space for that.
So, we cleaned it up and moved a theoretically usable clean-booth (still without walls) into the free space and a black-painted optical table underneath that booth (see pictures).

By doing all the cleaning-up thing, we discarded a lot of old carton-boxes and plastic garbage.
Now, it looks much more usable.

Next step will be to move the actual scatterometer.

The day before yesterday, we found a water leakage point in the middle station of TAMA arm (north, the arm we are using). See entry #1403. Yesterday I found it was filled fully by the leaked water.

Eleonora P, Yuhang, Matteo, and Aritomi

Several months ago, we have already found the channel for AOM amplification was broken. And the work we did yesterday was without AOM working. Fortunately, Eleonora C remembers where is the old RF amplifier. And today we first implemented the old RF amplifier(ZHL-2). The amplification factor of it is 16dBm. The optimal RF signal for AOM should be 27dBm(while it converts too much to the first order). so usually, we use a lower value. So I give 7dBm from the signal generator and amplify it by 16dBm to have totally 23dBm of driving signal to AOM. The conversion efficiency is about 80% now.

Then, for sure, we need to do the alignment for green again. And then also for IR.

Another very important thing is the amplification of BAB(after OPO transmission). In the beginning, we were thinking to lock it with the same method for CC locking. But we observed noise level brought by the beat between BAB and CC. And this noise is almost 200 times larger than the coherent control error signal we have. So it is not possible to lock with CC beam. By following Matteo's suggestion, we used the leakage power from OPO s-pol transmission through PBS and to p-pol locking PD. Then we feedback this signal after giving an offset. And then we can basically lock the green phase and have a more stable IR beam going inside the filter cavity.

After all the work above, we could lock both green and IR again by changing the AOM driving frequency. We found make higher order modes and they are listed as follows.

The task of tomorrow will be to maximize the TEM00 by moving the last two steering mirrors on the bench for IR. And measure the height of each higher order modes. Also, maximize mode matching. We will follow the entry.