Aritomi and Yuhang

Since the unity gain frequency of CC2 was only ~400Hz because of some resonance. We decide to put some more damping material. 

So we put some double layers bent rubber(as shown in the attached photo1) in the position where there may be some spring resonance(as shown in attached photos 2 and 3).

Then we measured OLTF, which is shown in the attached photo 4. And it is shown that the unity gain frequency is around 2kHz now. We also tried to increase the gain, we measured the resonance frequency when there is oscillation. As shown in the attached photo5, the oscillation is at ~23kHz. 

We also measured OMTF. By comparing the measurement we did before putting this new damping rubber, we found the peak around 1 and 2kHz disappeared(as shown in the attached figure 6).

[Aritomi, Yuhang]

First we measured squeezing when filter cavity is locked/unlocked (Pic. 1). We expected more squeezing since we improved reflectivity of dichroic mirror by 4%, but squeezing level is still around 6.1dB when filter cavity is unlocked. Squeezing level when filter cavity is locked/unlocked is similar, but phase noise seems suppressed when filter cavity is locked.

Here are some information of this measurement.

[Eleonora, Matteo]

Today we did good progress on the dithering.

We gave up on measuring the sensing matrix and we tried to close the loops by simply feeding back each demodulated signal to "its" mirror. 

This seemed to work surprisingly well. (See pic 1-2). The transmitted power increased and became quite stable as well as the lock.

I tried to move one by one the BS, the INPUT mirror and the END mirror when dithering is engaged and the loops seem able to move the cavity mirrors to recover the good alignment. (See pic 3, 4, 5)

From the last two plots we can also check the level of coupling of our error signals, and possibly improve the driving.

The main problem I see is that the error signals are not oscillating around zero but they have an offset and the dithering lines are still well visible in transmission. It shouldn't be like this but I'm not sure about the reason.

Currently the corrector filter is a simple pole at 0.001 Hz. Maybe we should move it to zero. I will also try to put a offset in the loops and see if it can improve the transmission and reduce the dithering line in transmission.

Even if the transmitted power is very stable because we were mostly affected by pitch misalignment, I also tried to close the loops in yaw. They seem not to work well. The error signal is always around zero, so it is not easy to tell the difference when I close the loops, but when tried to misalign the BS in yaw they didn't recover. I didn't spend much time to investigate the problem but I will do it soon.

Since the error signals get crazy if the cavity unlocks I decided it was safer to stop the lock for the night. The cavity kept the lock for more than 5 hours (best record ever) and the lock was stopped on purpose before I go to sleep. It will be good to implement some kind of guardian that will open the dithering loops when the cavity unlocks.

Some details:

- The gain of the input and end loops are respectively -1 and -0.5.

- I reduced the amplitude of both dithering lines from 10000 to 5000 counts.

The wire inside the AOM driver box for supplying DC voltage was disconnected.
I fixed it by soldering, then it was conducting.

Also I soldered the power supply cable with 2 capacitors.

I forgot to take pictures.
I will upload them tomorrow.

We have one flat silicon mirror which will be used for measurement.
It has a wedge, 50arcmin.

On the other hand, fused silica input and output mirrors' wedge are 30min.
We have to take it into account to make a mirror cap.

Aritomi and Yuhang

We measured the green power reflected from the quadrant(used for the AA system). The incident power is measured as 1.2mW while the reflected power is measured as 0.46mW.

Almost 40% of power is reflected from it.

I took a picture of the green beam shape from the filter cavity transmission more than one week ago (11 days ago). Shown in attached figure 1.

I took the same picture last Thursday after the flatting of green beam height. Shown in attached figure 2. 

I also took the same picture again today after correcting the beam cut issue on AOM/iris. Shown in attached figure 3.

I think it is clear that the filter cavity transmission has less astigmatism.

[Aritomi, Yuhang]

Current mode matching is as follows. Mode matching is around 90%. TEM00 is fluctuating a lot due to alignment drift of suspended mirrors and it's difficult to improve the alignment without auto alignment.

Then we checked IR reflection and it was cutted. We measured BAB reflection from filter cavity when BAB is on/off resonance.

Reflectivity when BAB is on/off resonance is 44% and 77%. "real" cavity reflectivity which is ratio of these reflectivity is 57% and this is too low compared with ~80% in paper we published last year.

[Eleonora, Matteo]

MODULATION

Excitations are sent in pitch

INPUT MIRROR: 15.5 Hz 

END MIRROR: 18.5 Hz  

Amplitude: 10000 count

Both lines are well visible in the transmitted power. (Pic1, bottom)

DEMODULATION

Before the demodulation the transmitted power is filtered with a resonant filter at the modulation frequency (see pic 2-3)

The demodulation phase is chosen by looking at the transfer function between the injected line (seen on the oplev) and the transmitted power. This phase is ~12 deg for both input and end (pic 4-5) 

The demodulated signal is filtered with a first order low pass (simple pole) at 0.3 Hz.

 The spectra of the demodulated signal after and before lowpass look as expected. (Pic 1 top. Blue and Red)

ERROR SIGNALS

Error signals are very noisy (see some examples in the attached pdf). To investigate their goodness we have misaligned in turn the input and end mirror of a known amount of counts and measured the change in count of the two error signals. The results are not very clear, nevertheless we have tried to compute a sensing matrix.

It seems quite unbalanced and not very reasonable. Another suspicious thing is that error signal behavior is not very reproducible.

It seems they are affected by some variables which we are not considering and controlling.

Yuhang and Aritomi

Power reference form

We set the power after AOM by changing the AOM modulation depth. The maximum RF signal we should give to AOM is 30dBm(1W). The signal is generated from a signal generator then pass through a RF amplifier(amplify 32dB). So the maximum RF signal power we should generate should be -2dBm. Then we measured the GR power after AOM while changing AOM modulation depth(see attached form).

[Aritomi, Yuhang]

We recovered IR flash and alignment. However, when IR is aligned, IR reflection seems a bit cutted. We measured BAB reflectivity from filter cavity. Filter cavity was not locked with IR. IR injection is 343uW and reflection is 286uW. Reflectivity is 83% which is lower than before.

Current mode matching is as follows. Mode matching is 1706/(1706+343) = 83%. Pump green power is 60mW and green phase is scanned.

I tried to solder the cable to another AOM driver.
But the wire inside the box was disconnected for some reasons.

Tomorrow I will repair this.

I soldered the twisted cables as power supply for AOM driver with two capacitors, 1nF and 100nF, which are implemented for high frequency noise reduction.
Then I connected it to DC power supply and added the DC voltage, 28V.
However, the AOM did not work properly at first.
This was due to the current limit of DC power supply was not enough high.
I increased the current limit, then the AOM driver worked properly.

Then I injected the laser light into the AOM and I could see the diffracted beam as shown in 2nd picture.
I have not yet tuned the shifting frequency, alingment, and so on.
Anyway, I could confirm the AOM can diffract the laser beam.

[Yuhang, Yuefan, Matteo, Eleonora]

Motivated by the transmission issue reported in entry # 1672 we have investigated the goodness of green beam alignment and matching into FC.

First, we have checked that the beam was centered on both input and end mirror. To do this we have steered the beam with the BS on each side of the mirrors in order to make it more visible since it was hitting the suspension frame. We have recorded the BS position for the beam at the two sides and choose the middle point at a reference position. We confirmed that the beam position was already good even if the precision of this measurement is limited to, let's say, ~5mm. This was done to center the yaw, but it also allowed to assess the pitch centering while the beam was on a side. We could also have done the opposite and steer the beam from top to bottom with BS using the mirror top and bottom frame as reference.

Note that for the end mirror we could reach only the right (back toward input mirror) leg of the suspension, while the beam was hitting the pipe before reaching the left leg. So we used the earthquake stoppers as side references. The camera at the end was aligned to make the beam centered in this configuration. Actually, it was already in a good position.

Then, we scanned the cavity after having aligned it at our best. We kicked the end mirror by applying a large offset to the coils and check with the length oplev the excitation of the pendulum motion.

Pic 1 shows the flashes in transmission (top) and the length op lev (bottom). As expected the height of the peaks is smaller and the density is higher when the cavity is moving faster. Note that there seems to be a delay of the oplev signal wrt the transmission one. Actually they arrive from the end room with two different fiber system (old tama fiber system for oplev and new fast optical fiber for transmission) 

We zoomed in a region when the velocity is constant so that it is easier to identify FSR. (See pic 2). It seems that HOM are reasonably small (the fact the higher peak is TEM 00 is confirmed by the flashed in the camera).

We have misaligned on purpose the pitch and observed that the HOM become higher as expected. (Pic 3-4). I think this measurement confirms that we don't have a major problem with matching and alignment that can justify a drop of the transmitted power to half of the previous power.  Anyway a more refined measurement, with the identification of HOM would be nice.

We reconstructed the history of the cavity transmission. 

At the beginning of June we locked again the FC, after more than 6 months (entry #1385), at that time the injected power was 34 mW and the PD in transmission read a bit more than 4 V (which corresponds to ~ 7000 counts). (entry #1404)

After then we decided to reduce the injected power to 12 mW and the transmitted power was reduced accordingly ( ~2300 counts). (Not reported on logbook!)

Recently Yuhang found that by reducing the gain of the locking servo the transmission could reach 5000 counts. (entry #1644) We didn't remeasure the power at that time. 

Few days ago, after some realignment work on the bench (entry #1647), we found out the transmission was back to 2200 counts. (entry #1655). In that occasion (even if it is not reported on the logbook)  the AOM was also realigned and this brought to a large increasing of the power injected into FC (30 mW). The power was reduced by tweaking the half waveplate after the main laser.

Last Thursday (26/09) we spent half a day to carefully check and asses the alignment level (which is reported in entry #1674). The conclusion of that work is that the cavity seems well aligned. 

It seems to me that this mysterious change in the cavity transmission is connected to the change in the injected power, which is largely affected by AOM alignment condition. Also, the only straightforward reason why a reduction in the locking servo gain should bring an increasing of the transmitted power is that the loop was oscillating, and this is likely to be due to an increased power.

Anyway, I think that from now on we should better monitor (and report on the logbook) the level of power injected into FC and the cavity transmission.

Yuefan, Eleonora and Yuhang

For DC normalization, we found out why we couldn't connect. The reason is there is a range of output for photodiode DC sum, it was not able to fit in this range. But now it is fine after some adjustment done by Eleonora(mainly just adding a -1 factor to the DC sum).

For RF signal, in the beginning, we just checked the PDH signal from each quarter of quadrant. But we couldn't find any signal. Then we did some further check, including:

1. quadrant power supply(power and bias were switched on)

2. quadrant DC signal(to make sure the beam is hitting on PD, we checked this signal and it was fine)

3. LO sent to demodulator(it was fine)

4. RF signal from quadrant(before demodulation, checked directly from spectrum analyzer) we found this RF signal was only around -40dBm

5. We amplified this RF signal by 18dBm then we could see a small signal. 

6. Check demodulator for AA system. we took the RF signal from Qubig PD(used to lock filter cavity). The green power on Qubig PD is around 200uW for now. Then we just replace of demodulator for this signal. And then compare the demodulated signal(PDH signal). We got PDH signal pk-pk as 50mV. While the signal we got from another demodulator is ~500mV. So it seems this demodulator is not as good as what we were using.

7.  By using the same demodulator, we took RF signal from Qubig PD and quadrant and got PDH signal(see attached figure 1, blue curve is signal from Qubig, yellow curve is signal from quadrant). Note that green power on quadrant is ~1mW while green power on Qubig PD is ~200uW. So is seems quadrant is not so sensitive to green light or it doesn't have enough bandwidth. 

8. We also tried to move the beam totally on one quarter of quadrant. See attached figure 2. Roughly, the PDH signal was increased by a factor of 4.

9. We also tried to check the other quadrant and the other demodulation board. I shows similar result. (Attached figure 3 shows PDH signal when beam is centered on quadrant, attached figure 4 shows PDH signal when the beam is mainly located in the first quarter of quadrant)

Above is the check we did on last Friday. Since quadrant worked well in NIKHEF, we will check again these days.

Yuhang and Yuefan

As we discussed in the last filter cavity meeting, we may have a beam cut issue in AOM/EOM/iris. We checked again these optics by putting a steering mirror/lens after it and look at beam shape far.

For the cut issue of the iris, we found a better point where the higher-order diffraction beams are more separated. Attached pictures 1 and 2 show the position where we were putting the iris and we are putting it now. This more separated point is located just after the first lens after AOM. The new set-up is shown in the attached picture 3.

We also checked the beam cut issue of Faraday isolator and AOM. The checking point is just before the injection to PR chamber. We found out the main cut comes from AOM. We could see from the attached picture 4 that the most powerful center part could go through AOM/FI but not centered. After twinking a bit the position of AOM, we made the beam go through AOM from the center part(as shown in the attached picture 5). 

So although the aperature of AOM and FI are not ideal, we confirmed that the most powerful center part is not cut.

Matteo, Simon

attached are the first results of our experimental try to map also the polarization homogeneity of fused-silica substrates.

In total, I think that we can say that the homogeneity is almost perfect. The fluctuations in the map are probably due to fluctuations risen from the input polarization which cannot be filtered out. That would explain at least the stripe-like pattern in the map along the way of the measurement.

Anyway, the next steps will be to convert the bench back to a PCI for taking an absorption map of the AQ2 sample.

Matteo, Simon

attached, please find the maps and the distribution analysis of the polarization measurements on the Tama-sized Sapphire samples from Shinkosha.
The maps are taken with different polaization angles indicated in each figure, with 0 degrees being pure P-polarization.

Obviously, the birefringence effect is relatively homogeneous and divided into three parts with different offsets. The offsets itself may have their origin basically in the non-zero incident angle of the pump-beam.

with help of Tomaru-san, Namai-san, Ueda-san from KEK, and Sato-san

We did renovation work of cryostat.
First, we detached the optical breadboard from cold head.
Then we moved cryostat chamber about 80cm to extract cold head, and now that we can implement viewport windows.

Next step is to implement viewports to 80K shield, and install modified 4K shield to the chamber.

[note]
The cryostat is located different position temporally.
Cold head was attached to optical table using apiezon grease and indium.