The mode-mismatch was slightly over estimated as I divided by tem00 power and not total one...

The corrected values are : misalignment = 3.9%, mode-mismatch = 1.2% and total 5.3%.

I measured FDS with CCFC (attached figure). The DDS setting for FDS with CCFC is saved as 20210519_dds3_CCFC. During this measurement, CC2 input test mass feedback was engaged with gain of 3.

There is a large peak around 170 Hz introduced by CCFC. I will try to change the filter for CCFC if we can remove the peak.

Unfortunately, the homodyne angles in this measurement are in the wrong side. So I will measure again with correct homodyne angles.

Marc and Michael

We measured the beam profile after the mode matching telescope simulated in 2486 and placed as shown in 2501. The measurements are plotted in figures 1 and 2. Figure 2 includes an outlier, perhaps the distance was recorded incorrectly? Figure 1 shows the result without the outlier. Either way, it seems the beam size and position is close enough to the prediction that fine tuning can be done using the lens rails of the mode matching telescope. Note that in the OPO replacement measurement, the beam will be shifted by a periscope after the f=75mm and then the OPO will be mounted on a rotation stage.

Before this measurement, we checked the alignment of the beam path. It is well centered on the steering mirror before the f=75mm. Without the lens, the beam goes along the screw holes on the table. However, when the lens is placed, the beam diverts about 1cm towards the edge of the table over a distance of about 25-30 holes. It diverts in the same direction when the lens is flipped.

Also I made a slight bump on the EOM rotation stage adjustment knob when handling the beam profiler cable, but it doesn't seem to have affected the beam path.

[Aritomi, Michael]

This work is done on 20210518.

We decreased the pump green power from 40mW to 20mW for CCFC. First, we checked the nonlinear gain with 20mW green.

The measured nonlinear gain is 5 with 20mW green. When we assume the OPO threshold is 80.6mW, the theoretical nonlinear gain is 4. It seems that the OPO threshold is lower than 80.6mW now.

Then we measured CCFC error signal and locking accuracy (Fig 1,2). The CCFC calibration amplitude is 182mVpp.

I found that a huge offset is sometimes injected in Z correction and the Z correction unlocks due to it. I also found that the huge offset appears when I touch the LEMO cable between rampeauto and SR560 in Z correction loop. I replaced the LEMO cable and the problem was solved.

Aritomi, Michael, Yuhang

We tried to optimize filter cavity reflection mode matching to homodyne LO. Now it is about 5%, which is still larger than last year.

After that, we tried to measure FDS. Considering a reasonable degradation budget, we did a fit for all the measurements.

We found ~3.3dB squeezing above rotation frequency, but almost no squeezing was observed below rotation frequency.

The degradation parameters:

squeezing level: 11.2dB

round trip loss: 120ppm

total optical loss: 38%

Matching to filter cavity: 5%

Matching to local oscillator: 5%

Locking accuracy: 5e-12m

phase noise: 30mrad

Marc, Michael, Yuhang

Yesterday we started characterizaton of the fds degradation budget,

PROPAGATION LOSSES :

First we checked the propagation losses by measuring the BAB power at various places on the bench :

after the waveplate just after the OPO : 462 uW

before homodyne : 369 uW

The ratio of these powers gives an overall propagation loss of 19.3%. This value is compatible with previous measurements.

We also compared at the edge of the bench before injection ( P=437 uW ) and just after reflection (P=374 uW ). This ratio gives the in vacuum propagation loss as 14.4%. Note that the BAB was not resonating inside the FC for this measurement.

We checked at the edge of the bench in reflection ( P = 368 uW) and just before homodyne ( P = 363 uW ). This gives losses on this part of 1.4%. There is only 5 optical components there and better quality ones are already bought.

We also checked the power after the waveplate after the OPO ( P = 462 uW ) and just before injection at the edge of the bench ( P = 446uW). This gives 3.5% of losses.

BAB/FC MODE-MATCHING :

We locked the FC with green and tuned the AOM frequency around the various resonances of BAB.

We placed a photodiode just before the one used for CCFC in order to get a larger gain.

TEM00 was scan with AOM speed 60mHz and deviation 6kHz while TEM01/10 and LG01 were scanned with same speed and deviation 600Hz. Taking into account these factors we can calibrate these signals with calibration = 2*deviation / (1 / (2/speed)) = 2 * 2 *speed  * deviation where the first '2' comes from green to IR conversion and second one to get the half-period of AOM scan.

In figure 1 you can see these 3 scans.

The mode-matching was estimated by computing the area under each curve after removing the offset (90 counts).

It gives 4.1% misalignment and 1.3% mode-mismatch and overall value of 5.4%

ROUND-TRIP LOSSES :

We did as reported in the RTL estimation paper (namely switch on/off resonance of BAB).

We got the results attached in figure 2.

In addition to the mode-matching, we also assumed 8% of RF sidebands power and 1% lost due to laser fluctuations.

It gives round-trip losse of 116 ppm in good agreement with previous estimation.

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.