It seems that the bulk calibration was overestimated. This is especially apparent when computing its transmission that was 45% instead of the expected 55%.

I performed again the bulk calibration and got :

AC_bulkref = 0.062;
DC_bulkref = 4.14;
P_in = 26.4e-3;
P_t = 13.1e-3;
T_bulkref = P_t/P_in
abs_bulkref = 1.04;
R_bulk = AC_bulkref/(DC_bulkref*sqrt(T_bulkref)*P_in*abs_bulkref) = 0.7743 W/cm

I used this new calibration to compute again the absorption map of the sample (see the 3 attached figures.

In the figure, the absorption is extracted from a fit using 2 normal distributions.

Here I also add the overall mean and standard deviation of each map (ie without any fittting) :

I created a page in our wikipage to share information of the old measurement of optical losses and phase noise for FIS and FDS.

https://gwpo.mtk.nao.ac.jp/wiki/FilterCavity/losses%20and%20Phase%20noise

Mode mismatch between filter cavity and LO was found to be relatively high. And this results in homodyne detection effeciency to drop by about 13.3%. Together with the bad mode matching inside filter cavity reported in elog2503, we could explain worse FDS measurement.

As shown in the attached two figures, the TEM00 peak is 1.19V while the LG01 peak is 0.104V. This corresponds to 8.0% mode mismatch. Note that this spectrum is after the optimization of alignment and filter cavity half-detuned.

We have tried to reduce this LG01 mode by moving the mode matching lenses. However, the mode matching can be barely improved.

To search for the reason of this mode mismatch, we checked the beam position on every in-air optics, we found no clipping issue.

We have also tried to measure the power loss, the total power loss from after the in-air Faraday to before homodyne is about 19%. This is in-agreement with the old measurments.

For the in-vacuum part, we could try to scan the injection steering mirror yaw or pitch slightly and see if there will be a clear power drop. We will try with this method to check if there is in-vacuum clipping.

If there is not clipping found, we need to first understand why this could happen. In principle, for our optical system, there should not be such large mode matching change. In the worst case, if we couldn't figure out what is causing this problem, we will need to measure the beam parameter again and redesign the telescope.

I measured CCFC error signal with 20% pick off. The CCFC calibration amplitude is 452mVpp. Fig 1 shows the CCFC error signal with different demodulation phases. The CCFC error signal agrees well with theory, but the CC detuning changed by 20 Hz from the previous measurement. This means that the filter cavity length changed. The CC PLL frequency can be written as follows:

CC PLL frequency = 14*FSR + CC detuning

From this formula, the CC detuning change of 20 Hz corresponds to the FSR change of 1.4 Hz and the filter cavity length change of delta L = delta FSR /FSR * L = 0.8 mm. We need to tune the CC PLL frequency.

Note that I fixed the mode mismatch between OPO/FC to 6% in the calculation.

Then I locked CCFC. The filter setting is gain of 10000 and LPF of 0.03Hz. The CCFC can lock only for a few minutes due to the CC1 saturation.

Fig 2 shows the IR locking accuracy with/without CCFC. Now the IR locking accuracy with CCFC is 1.2 Hz and the high frequency noise shape looks similar to the best locking accuracy we obtained on 20201211. I compared the IR locking accuracy on 20210517 with the one on 20210514. The difference of these is whether the laser is kept on for more than one day or not. It seems that keeping laser on makes the high frequency noise better.

I used strip tool to check how long time AA/pointing loop needs to use to go from unlocked point to locked point. This tells us rouhgly the bandwidth information of these loops.

The AA loop filter and gain are as following:

Note that these filters and gain are not optimized yet. The time to go from unlock to lock is shown in the attached figure 1. We can see it took about 5 second, which means the bandwidth is about 200mHz.

The beam pointing loop filter and gain are as following:

Note the pointing loop gain is not optimized yet. The time to go to the good point is shown in the attached figure 2. We can see it took about 2 min, which means the bandwidth is about 8mHz.

The absorption distribution is fitted with 2 normal distributions.

I thought it could be useful for the case of XZ and YZ maps (where there are measurement points outside the sample) because it allows to remove the effects of absorption outside the sample and point defects/dust on the surface.

But I agree that it might not be the most suitable distribution, especially for the shinkosha samples...

I checked the IR mode matching to filter cavity and improved it. The injected BAB was 440uW.

We must check TEM00 power and injected BAB power at the same time to confirm the mode matching.

Mode matching before improvement: 77%

Mode matching after improvement: 94%

I replaced a 50% pick off BS (BSW11) with 20% one (BSS11).

I checked the reflectivity of them. For BSW11, injection, transmission, reflection powers were 74uW, 42uW, 31uW, respectively. So the reflectivity is 42%.

For BSS11, injection, transmission, reflection powers were 375uW, 293uW, 72uW, respectively. So the reflectivity is 19%.

Both are consistent with the reflectivity for p pol in their specification.

Marc and Yuhang

We have put two lenses for OPO mode matching telescope. They are LA1422 and LA1608, exactly the lenses suggested from simulation in elog2486.

After putting lens, we checked with sensor card and found the beam waist is about 5 holes after the second lens. This agrees with simulation.

According to the calibration factors in elog1874, I plotted all our suspended mirrors oplev signal.

As shown in the attached figure, bascially all mirrors pitch or yaw have bascially the same behavior. But there is an expectation of PR yaw.

Marc, Michael, and Yuhang

Yesterday, we made some measurement of FDS with WFS based AA.

The measurement is flat until almost 30Hz. We have also seen more than 1dB squeezing from 30 to 60Hz. However, the high frequency squeezing level was only 2dB (can not be higher by changing LO phase). This is very different from what we understood.

Anyway, I tried to use the old code to fit FDS. The fit agrees well with some measurements, but not for all. As shown in the attached figure, especially the measurement which should have squeezing at high frequency couldn't be fit by the code.

We need to investigate more about this result.

Marc, Michael and Yuhang

We tried to tilt the lenses of homodyne to reduce back scattered noise at low frequency. However, it seems it didn't work.

After the improvement of IRPS, we confirmed that there is not amplitude noise or beam jittering noise from the LO. So the input mirror feedback loop really reduced the back scattered noise.

Note: to close input mirror feedback loop, we had to improve the gain of the old loop.

We made a few more comparison yesterday. The noise and reduction is shown in the attached figures. From this measurement, the reduction of back scattered noise is not the same at all frequencies.

Marc, Yuhang

We checked the BAB transmission and locking accuracy when AA loop is closed or open.

In the case of BAB transmission, AA makes the signal worse below 100Hz.

But, in the case of BAB locking accuracy, AA seems to make the DC value better. After integrating at all frequency, AA actually makes locking accuracy better.

Today we received two viewports of EY TM OpLev.

I placed them inside ATC cleanroom as in figure 1 with :

left : EY TM OpLev viewport, Inner shield, -X +Y side, dirty surface up

right : EY TM OpLev viewport, Inner shield, +X +Y side, dirty surface up

I left them under their protective cover as in last figure.

I have some questions.

What is the meaning of the fit in the histograms? Do you have a reason to fit with specific distributions/densities or do you want to find a systematic pattern?

Marc and Yuhang

We have also checked one week data (from 2021/04/25/3pm to 2021/05/01/3pm)

We made two plots, the first plot includes all the data in this week. But the second plot excluded the last part of the data. This is because an earthquake might happen around the end of the one week data. This makes we see a peak in the first plot. 

To compare different mirrors angular drift, we calculated RMS value of data and summarized them as the attached table. [unit: urad]

From this table, we see PR pitch moved the most. But INPUT pitch also moved a lot. However, END pitch didn't move as large as PR and INPUT pitch. Since PR, INPUT and END have the same configuration, their difference in pitch drift indicates that this is not a design problem.

We know that temperature change influences mirrors drift. But PR, BS, and INPUT are all in the TAMA central room, they should have the same temperature change. But they didn't show the same drift. Form the airconditioner location, PR chamber is almost facing an air conditioner, which may cause temperature related problem. To confirm this is not the fault of air conditioner, we plan to switch off it for one or two days during weekends to check.

According to the OPLEV signal calibration value in elog1874, I plotted the four suspended mirrors drift during a time scale of one day(from 2021/05/12 1pm to 2021/05/13 1pm JST).

In figure 1, all four suspended mirrors are compared. It is very clear that PR pitch has a far larger angular drift compared with other mirrors.

The PR mirror changed by 100urad pk-pk during one day. Considering the distance from PR to END mirrors, the 1e-4*300 = 3e-2 m = 3cm.

Other mirrors other DOF changed by 10urad pk-pk during one day, which is ten times smaller than PR pitch.

Considering this measurement result, I think we should make beam pointing loop act on PR mirror.

Abe, Marc

Following the 3 maps measurements we performed again the bulk calibration where this time we moved the imagining unit by 0.32 mm in order to compensate the thickness difference between surface and bulk reference samples. We got the following result (also see last figure of this entry) :

AC_bulkref = 0.0731;
DC_bulkref = 4.164;
ACDC = 0.01755;
P_in = 29.5e-3;
P_t = 13.3e-3;
T_bulkref = P_t/P_in;
abs_bulkref = 1.04;
R_bulk = AC_bulkref/(DC_bulkref*sqrt(T_bulkref)*P_in*abs_bulkref) = 0.852 cm/W

Using this new calibration factor and using :

P_t = 6.25 W;
P_in = 7.322 W;

the absorption of this sample seems to be around 60 ppm/ cm for all 3 maps (see attached figures)

Today we will double check the bulk calibration as the change was quite larger than expected.

Abe, Marc

We modified the analysis to better estimate the mean and standard-deviation of absorption measurements.

The corrected results are attached to this entry.

Today I will remove the first contact that we applied on this sample and cross-checked if it affected the absorption measurement.

[Aritomi, Yuhang]

First we measured nonlinear gain again. We measured BAB transmission from OPO. When we used 40mW green, we decreased CC1 gain from 2 to 1. 

The measured nonlinear gain is 18.5 with 40.9 mW green while the theoretical value should be g = 1/(1-sqrt(40.9/80))^2 = 12.3. We don't know why they are different.

We found an oscillation in FC lock with green injection of 27mW and FC gain of 1.3. The green injection power was larger so we decreased FC gain from 1.3 to 1.

We measured CCFC with 40.9 mW pump green and 50% pick off. Fig 1 shows the CCFC error signal with some demodulation phase and Fig 2 shows CCFC calibration signal when CCSB are off resonance and CC1 is scanned. The amplitude of CCFC calibration signal is 524mVpp which is 10 times larger than before.

We will try with 20% pick off and thorlabs BSS11 seems good for BS (20% reflection for p pol).