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R&D (FilterCavity)
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YuhangZhao - 17:56, Thursday 22 August 2019 (1559)Get code to link to this report
OPO escape efficiency issue and HR coating of PPKTP

If we consider an error of reflectivity of HR coating of PPKTP(0.02%). We could have a very different estimation of OPO round trip loss and OPO escape efficiency.

Note that here we use transmissivity of OPO of 0.2% as a prior (as entry 1538).

Images attached to this report
1559_20190822105624_loss.png 1559_20190822105630_escape.png
R&D (FilterCavity)
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NaokiAritomi - 14:59, Thursday 22 August 2019 (1557)Get code to link to this report
Comment to OPO escape efficiency (Click here to view original report: 1538)

Escape efficiency is T/(T+L) where T is transmission of output coupler and L is intra cavity loss. So escape efficiency should be 0.08/(0.08+0.0065) = 92.5%. Calculation in Marco' s thesis seems wrong.

R&D (FilterCavity)
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YuhangZhao - 14:30, Thursday 22 August 2019 (1556)Get code to link to this report
Comment to OPO escape efficiency (Click here to view original report: 1538)

From Marco thesis, the escape efficiency is 0.92/(0.92+0.0065). It is 99.3%, it seems fine in that case.

R&D (FilterCavity)
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EleonoraPolini - 00:57, Thursday 22 August 2019 (1555)Get code to link to this report
Comment to Injection and reflection telescope for SQZ into filter cavity (Click here to view original report: 1546)

I was using a database considering all the focal lenghts commercially available, not only the ones on Thorlabs. Did you check also the robustness of the injection telescope with the two lenses that close?

The last version of injection telescope was version 3 of entry #1366.  

KAGRA MIR (Absorption)
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SimonZeidler - 18:27, Thursday 15 August 2019 (1554)Get code to link to this report
Repeating mapping from yesterday and first spectra of new calibration samples

Simon

For some reasons (probably Windows related), the absorption-bench PC had a reboot Yesterday evening (around 19:08). Therefore, the running measurement of ETMY abs. map was interrupted. Fortunately, nothing bad happened to the system itself and I could easily recover the setup and start another measurement today.
Fingers crossed...

Meanwhile, I have started to take spectra of the Sapphire samples that we will use as new calibration samples to have a measurement on their absorption index. Actually, I did such a measurement already on Tuesday and wanted to finish it today with the other samples but for some reason, the settings in the spectrometer were totally different from Tuesday and I couldn't get any stable signal from the spectrometer. I asked Mitsui-san from ATC for help but he is now in vacation...
Anyway, attached to this report is the preliminary result of Blue-Sapphire No1.
According to the measurement, we can expect an absorbance of 0.106 at 1064nm.

Images attached to this report
1554_20190815112706_bluesapphire1.png
KAGRA MIR (Absorption)
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SimonZeidler - 17:01, Wednesday 14 August 2019 (1553)Get code to link to this report
Calibration of reseted absorption bench and ETMY absorption measurements

Simon

From the measurements last week I obtained the calibration of the system:

R_surf = AC_surfref/(DC_surfref*P_in*abs_surfref) = 24.9 [1/W]
where AC_surfref = 0.75V, DC_surfref = 4.15V, P_in = 0.033W and abs_surfref = 0.22

R_bulk = AC_bulkref/(DC_bulkref*sqrt(T_bulkref)*P_in*abs_bulkref) = 0.535 [cm/W]
where AC_bulkref = 0.083V, DC_bulkref = 4.7V, T_bulkref = 0.55, P_in = 0.033W and abs_bulkref = 1.04W/cm

As can be seen, the calibration factor of the surface reference seems to be a lot higher than for the measurements done in July (-> 16). Probably, because I did not pay attention to the orientation of the surface sample cheeky
The bulk-sample, however, is not so far away from the values of past calibration measurements (-> 0.604 in July). That gives me at least some confidence about the calibration so far.

Furthermore, today I restarted the laser and put the ETMY test-mass on the translation stage again.
I started a Z-scan with ~2W input power to check the positions of the surfaces. Also from this measurements, the crossing point seems to be shifted by 2mm (center is now at 77!), which is consistent with the calibration.

I increased the laser power to ~10W and ran a rough map to check that the test-mass can hold that power in the whole map area. Fortunately, there seems to be no problem so far. Apparently, we were successful in removing all the soot and FC remains.

I started the mapping at the test-mass' center.

KAGRA MIR (Absorption)
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SimonZeidler - 20:27, Friday 09 August 2019 (1552)Get code to link to this report
Resetting the absorption bench

Simon

I have successfully reset the absorption bench by removing all the additional optics which we put for the polarization measurements.
After that I adjusted the optical-unit (putting it back to calibration position @70mm) and exchanged the ETMY test-mass with the calibration-sample holder. I used partly the empty space on the optical-unit table to lift the heavy test-mass.

During calibration, I recognized that the bulk-sample is still inside the holder. However, by that time, I already finished to take some calibration measurements. So, I changed it with the surface-sample and took the respective measurement. Anyway, in both cases the calibration looks very good.
The crosspoint, however, seems to be shifted by 2mm compared to our calibration for ETMX, but is in very good agreement to measurements taken in February this year. I did not do a further analysis of the calibration measurements yet (maybe I do this during weekend, let's see). But just by the eye, the AC/DC values look good.

During the weekend, I shut-down the laser-power as there is nothing to do. For the actual test-mass, I need to be there for carefully increasing the laser-power, which will take some time, I guess.

KAGRA MIR (Absorption)
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SimonZeidler - 10:40, Thursday 08 August 2019 (1551)Get code to link to this report
Polarization map of ETMY with circular polarized light

Simon

Yesterday, I slightly changed the setup and put the QWP after the HWP so to be able to make the input beam circular polarized. The results can be seen in the pictures attached.

Obviously, the map is much more homogeneous than with linear polarized light, although there is a shift towards being more elliptical as the mean angle is ~52 degrees and not 45 degrees as expected with pure circular polarized light.
Interestingly, the prominent region in the upper left part visible in the other maps, vanished completely.

Images attached to this report
1551_20190808034037_mapcircpolinput.png 1551_20190808034042_distcircpolinput.png
KAGRA MIR (Absorption)
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SimonZeidler - 10:41, Wednesday 07 August 2019 (1550)Get code to link to this report
Polarization measurements on ETMY with 30 and 60 degrees input polarization

Simon

The last measurements on the polarization maps for ETMY test-mass have been finished (with linear polarized input beam).
The results are attached as figures below, showing the maps with each 30 degrees and 60 degrees input polarization angle (0 degree is pure P-polarization -> horizontal with respect to optical table).

While in average there is a 5~6 degree offset for the 30 degree case, we have almost no offset in the 60 deg case. The reason is yet unknown. However, the already discussed prominent area in the upper-left region still shows ca. 10 deg offset with respect to the average, which is in agreement with all the other measurements.

In total, the polarization angle distributes in a range of ~20 deg over the entire map whereas the most important and serious changes happen within 10 deg.

Images attached to this report
1550_20190807034106_mapsp30polinput.png 1550_20190807034110_mapsp60polinput.png 1550_20190807034114_distsp30polinput.png 1550_20190807034120_distsp60polinput.png
KAGRA MIR (Absorption)
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SimonZeidler - 13:12, Monday 05 August 2019 (1549)Get code to link to this report
Denomination Mistake for input S and P polarization

Simon

I just realized that it is nonsense to relate the input-beam S-polarization to 0 degrees while on the out-going beam it refers to P polarization in our scheme. Therefore, I changed the denomination in the pictures so it is more clear what is happening.

Images attached to this report
1549_20190805061157_distppolinput.png 1549_20190805061203_distspolinput.png 1549_20190805061209_distsppolinput.png 1549_20190805061215_mapppolinput.png 1549_20190805061220_mapspolinput.png 1549_20190805061225_mapsppolinput.png
KAGRA MIR (Absorption)
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SimonZeidler - 11:59, Monday 05 August 2019 (1548)Get code to link to this report
Mixed S and P polarization and pure P polarization input beam on ETMY

Simon

On Friday and Saturday, measurements with equally mixed S and P polarization in the input beam and pure P-polarization have been finished. The resulting maps and the statistical analysis can be found in the attached figures (45 deg refers to mixed S and P, and 90 deg to pure P-polarization).

As can be seen together with the results of pure S-polarization, the out-going beam is never reaching the input beam's polarization. We are off by at least 11 degrees (15 deg in average) polarization angle, which is a strong hint that the out-going beam has become elliptical. The average for the linear incoming and out-going beam is apparently shifted by 2-3 degrees. This is obvious for the mixed S and P polarization as the out-going beam clearly has its average at 47~48 deg, while the incoming beam has 45 degrees.
I can confirm, after doing some tests by rotating the incoming beam, that the maximum S and P polarization are each shifted by ~6 degrees compared to the incoming beam when the test-mass is in place.

Due to the elliptical out-going beam, the angle-distributions at both pure S and P polarization have half the standard deviation as the mixed polarization due to the fact that we see the change in the polarization angle to both sides in the mixed input-beam case, while for the pure cases this view is limited by the trigonometric functions which we are using here for the analysis.

Apart from the average, which shows the behavior as discussed above, there is a prominent region that is obviously different (top-left). Here the input beam experiences probably a rotation by ~16 degrees in addition to become elliptical. That would explain why the plarization angle changes only little from S to mixed S and P polarization.

Images attached to this report
1548_20190805044927_mapsppolinput.png 1548_20190805044934_mapppolinput.png 1548_20190805044941_distsppolinput.png 1548_20190805044950_distppolinput.png
R&D (FilterCavity)
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YuhangZhao - 23:05, Friday 02 August 2019 (1547)Get code to link to this report
Infrared reflection power fluctuation issue

Aritomi and Yuhang

As Aritomi-san said in entry 1542, we have power fluctuation issue. We are interested in the amplitude and frequency of these fluctuations. The detail investigation about this will be done later. The spectrum of this reflected signal is attached in the figure.

We could see the noise goes up from 50 Hz to 10Hz. This can be an issue for further squeezing degradation.

We are thinking to check also this signal together with filter cavity IR transmission. And GR transmission and reflection. Besides, we didn't use filter cavity length control at that moment. So we will also compare this spectrum when there will be length control. 

Images attached to this report
1547_20190802160519_fcref.png
R&D (FilterCavity)
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YuhangZhao - 22:28, Friday 02 August 2019 (1546)Get code to link to this report
Injection and reflection telescope for SQZ into filter cavity

Recently we have some worry about squeezing measurement about losses(entry 1532). Although we have the main worry about OPO intra-cavity loss, we also want to replace the normal lens we are using with the super-polished lens from CVI company.

I checked the simulation of Eleonora Polini(entry 1311), there are some discrepancies.

  • the database Eleonora was using is out of date because she is using thorlabs database while we are buying from CVI.
  • the distance from PBS to waist is not consistent with my calculation. I also asked Aritomi-san to check this distance. He has the same result as me.

According to these difference, I did the calculation again. The distance(from bench to waist close to 2-inch mirror) is estimated as attached in Figure 1. This is only an approximation. But since it is a very collimated beam, I think it is fine. In this approximation, the black number is provided by Yuefan and Eleonora (entry 1133) while the purple number is calculated according to these number. The distance is estimated by the holes on the bench (2.5cm between two holes). The distance on the bench is summarized as follows:

  • Injection: count from PBS(0cm), the first steering mirror(17.5cm), Faraday isolator(32.5cm), the second steering mirror(47.5cm), the third steering mirror(57.5cm), the fourth steering mirror(77.5cm) then goes to the edge of the bench. We need to pay attention to that squeezing reflection will be between 67.5cm to 72.5cm.
  • Reflection: first we consider the distance from the waist around the 2-inch mirror to bench edge, it is 427.3cm. count from bench edge(427.3+0cm), the first steering mirror(427.3+5cm), the second steering mirror(427.3+22.5cm), the third steering mirror(427.3+35cm), then goes into homodyne and finally into alignment mode cleaner(427.3+81.25cm).

With this distance information and new database, I simulated again the telescope. I tried to choose the result based on the already bought lens from CVI.

The result is attached in Figure 2 and 3. In this case, we are using PLCX-25.4-149.9-UV and PLCC-25.4-515.1-UV for injection. And we are using PLCX-25.4-124.9-UV and PLCC-25.4-51.5-UV for reflection. The lens in the red color we have in the lab.

Images attached to this report
1546_20190802150304_113320181203113944chamber.png 1546_20190802152839_10.png 1546_20190802152848_10.png
Comments related to this report
EleonoraPolini - 00:57, Thursday 22 August 2019 (1555)

I was using a database considering all the focal lenghts commercially available, not only the ones on Thorlabs. Did you check also the robustness of the injection telescope with the two lenses that close?

The last version of injection telescope was version 3 of entry #1366.  

R&D (FilterCavity)
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EleonoraCapocasa - 19:42, Friday 02 August 2019 (1545)Get code to link to this report
Optimization of mirror feedback loop

[Yuhang, Aritomi, Eleonora, some remote suggestions from Matteo B.]

We succeded in feeding back a part of the filter cavity PZT correction to the end mirror, so that we could reduce the PLL phase noise.

What we have done after the preliminary results (entry #1506)

1) We amplified PZTmon with a standford. Gain 50. (so it is half of the real PZT correction). This also removed the 50 Hz oscillation we saw when the signal was sent directly to DGS.

2) We measured again the TF between Length excitation and PZT mon (pIc1, top right). The ble curve is seen by PZT corr, the red one by END length OPLEV.

Driving matrix for length:

coil 1  -1
coil 2  1.2
coil 3 -1
coil 4 -1

3) We measured the PZT correction when the cavity is locked witouth any feedback on the test mass (pic 2). The spectrum is calibrated taking into account the ADC gain (6e-4 V/cout), the piezo gain (2e6 V/Hz), the gain of stanford (50) and attenuation of pzt mon (1/100). Maybe there is a factor 2 missing from SHG. Anyway, it shows a good agreement with the expected free running laser noise (1e4/f Hz/sqrt(Hz)). Except for the region from 0.1 Hz to 5 Hz where the cavity seems to move more than the laser.

4) We tested two different filters for the test mass feedback. One with a derivator witch only damps the length resonance in the ~1Hz region and one with also a low frequency pole (0.01Hz) .The performances of the different filters are shown in pic3 (red line: no feedback, bue line: damp, green: damp+DC, purple: damp+DC (with double gain)). The time signal in the first three cases is shown in the following video: https://drive.google.com/file/d/1yqvl5w8y_eeEE88MZ77aKqHNJG7Un1Ap/view?usp=sharing

5) Yuhang measured the PLL CC2 phase noise in these configurations. The results are shown in pic 4. I'm not sure about the RMS but it seems that, as expected, the noise is lower when we engaged the feedback on the mirrors. The loop can be optimized to damp also the peak at about 3.5 Hz.

6) We also checked the low frequency phase noise of PLL CC2 in the 'damp' and the 'DCdamp' cases (pic4). We wanted to see weather the main laser noise is increased in the 'DCdamp' case, due to the fact that at low frequency the laser is maybe less stable than the cavity. However it seems that the noise is the same in the two cases. Probabily the correction to the laser from the rampeauto is anyway much stronger.

We will check in the future if one of the two configurations is better in terms of squeezing performances. 

Images attached to this report
1545_20190802174308_zcorrtf10819.png 1545_20190802174344_pztcorrspe020819.png 1545_20190802174353_zcoorcfr.png 1545_20190802174404_comparison.png 1545_20190802174417_figure1.png
R&D (FilterCavity)
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YuhangZhao - 22:12, Thursday 01 August 2019 (1544)Get code to link to this report
The comparison of new and old high voltage driver

Around two or three months ago, we found one of the high voltage drivers was broken. Also, we found that the control of phase shifter always saturates. So we decided to buy a new high voltage driver with a larger dynamic range. However, we just realized yesterday that this large dynamic range high voltage driver has a worse phase behavior.

This work is done with IRMC locking, I just exchanged the high voltage driver. We could see from the attached figure. The old high voltage has a better phase margin.

So we are sacrificing phase margin to have a larger dynamic range. But we have a more stable phase behavior, so we should use back the old style high voltage driver to have a better locking performance.

Images attached to this report
1544_20190801151255_comparisonofhighvoltagedriver.png
R&D (FilterCavity)
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YuhangZhao - 22:04, Thursday 01 August 2019 (1543)Get code to link to this report
An abnormal behavior of CC2 control loop

[Yuhang and Aritomi]

We tried to use different control bandwidth for CC2 locking. But the measurement result shows not a very reasonable result.

Usually, the higher the bandwidth the better control. However, we are not having this situation now. But it can also be related to the not well-designed control loop.

Images attached to this report
1543_20190801150347_ccphase.png 1543_20190801150359_sqz.png
R&D (FilterCavity)
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NaokiAritomi - 20:24, Thursday 01 August 2019 (1542)Get code to link to this report
IR back reflection problem is solved

[Aritomi, Yuhang, Eleonora]

Recently we had a problem about IR back reflection from filter cavity. After installation of faraday, green phase error signal with p pol transmission is very stable (Pic 1) and we could lock green phase with p pol transmission stably. After locking of green phase, IR transmission of filter cavity is stable (Pic 2). One problem is that IR FC reflection is fluctuating even when green phase is locked and it seems to come from motion of suspended mirrors (entry 1547).

Images attached to this report
1542_20190801132436_img7722.jpg 1542_20190801132442_img7723.jpg
R&D (FilterCavity)
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YuhangZhao - 19:35, Thursday 01 August 2019 (1541)Get code to link to this report
Comment to OPO escape efficiency (Click here to view original report: 1538)

I checked the website of KTP company(RAICOL crystal), the substrate absorption loss should be 100ppm in our case.

KAGRA MIR (Absorption)
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SimonZeidler - 17:24, Thursday 01 August 2019 (1540)Get code to link to this report
ETMY map with S-polarized input-beam

Simon

Below you can find the first results of Yesterday's measurement with an S-polarized input beam. The structure of the map is quite similar to the wavefront-error measurements done by Hirose-san last year(?).
The homogeneity in terms of the polarization angle is ~2 degrees which is almost twice as much as for ETMX. However, also this is apparently consistent with Hirose-san's measurements.

The view onto the map is as the incoming beam would "see" the substrate.

Images attached to this report
1540_20190801102451_mapspolinput.png 1540_20190801102454_distspolinput.png
R&D (FilterCavity)
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NaokiAritomi - 13:43, Thursday 01 August 2019 (1538)Get code to link to this report
OPO escape efficiency

BAB before OPO is 114.5 mW and BAB after OPO is 0.231 mW, so current transmissivity of OPO is 0.2 %. When transmissivity of incoupling mirror is T1 = 8 % and transmissivity of PPKTP is T2 = 0.25 % and round trip loss inside OPO is L, we can get L = 12 % by solving following equation.

T1*T2/(1-sqrt(1-T1)*sqrt(1-L))^2 = 0.002

Escape efficiency = T1/(T1+L) = 40 % which is very low.

Even if L = 0.65 % like Marco's thesis P.87, escape efficiency is only 92.5 %.

Comments related to this report
YuhangZhao - 19:35, Thursday 01 August 2019 (1541)

I checked the website of KTP company(RAICOL crystal), the substrate absorption loss should be 100ppm in our case.

YuhangZhao - 14:30, Thursday 22 August 2019 (1556)

From Marco thesis, the escape efficiency is 0.92/(0.92+0.0065). It is 99.3%, it seems fine in that case.

NaokiAritomi - 14:59, Thursday 22 August 2019 (1557)

Escape efficiency is T/(T+L) where T is transmission of output coupler and L is intra cavity loss. So escape efficiency should be 0.08/(0.08+0.0065) = 92.5%. Calculation in Marco' s thesis seems wrong.