Kubo-san, Yoshiyuki, Simon

I inspected today the big furnace at the ATC (located besides the exhaust hood) to see how we can use it and whether it is available or not. Please see also the pictures attached.

The model is: ISUZU DSTR-314K
max. temp.: 1220 degrees Celsius

The inner area seems not very clean. So, in order to use it for KAGRA substrates, we need to create some kind of enclosure to save the substrates from pollutions (especially penetrating atoms/ions).

I will try to find a manual but it seems that the actual usage is relatively simple. Also, since almost nobody uses it now, we can take our time.
Before using it, however, we have to write an Email to Kubo-san and Yoshiyuki.

Here I attach the auto-alignment telescope design circulated by Yuefan. Just copy Yuefan design. 

The injection telescope for squeezing injection is changed(entry 1546) because of the existing CVI lens we have is limited. It is good to know the robust of this new telescope. Thanks to the code EleonoraPolini sent me, I easily performed this test.

Since only the injection telescope is changed, I only did the test for this injection telescope. Compared with the previous design(entry 1366), the robust is similar. So I think we can be happy.

Aim: We made some measurements with the quadrants to check if both RF and DC are working fine

For testing the quadrant, we took a beam from the main laser with a beam splitter (BS1). Scheme in figure 4.

At the first tried, we just put another beam splitter (BS2) after the first one, then EOM in one of the path with a modulation frequency of 23MHz. One lens was put between BS1 and BS2 in order to have the beam waist inside the EOM. After combining two pathes with BS3, we put a filter with OD=1 to avoid injecting too much power on the quadrant. Then we place the quadrant with the beam more or less centered.

But we could not see any beats from the RF output of the quadrant. We thought one possible reason could be the phase modulator makes 2 sidebands (one above and one below the light frequency), both of them interfere with the non-modulated beam. Probably two beat signals cancel each other almost perfectly because the beam path is the same (there is no such thing as a cavity that can treat the two sidebands differently).

So we decided to add the AOM on the non-modulated path with modulation frequency of 80MHz. With two beams of similar size and well aligned on the quadrant, we could see the 80MHz beat in the spectrum. 

Then we demodulated the RF signal with the box. By checking the in-phase(I) and quadrature(Q) of one of quadrant output , we could see that not only the power between I and Q is changing, but also the total power (both signals are moving up and down together).

Since our oscilloscope was not able to calculate the total power of two signals, we decided to put the second quadrant at the other output of BS3 and accquired one group of data while we knocked on the bench. The raw data we got show in figure 1. From this figure, we confirmed the quadrants are working fine. The I and Q are perfectly out of phase for two quadrants. The fact that the I and Q signals of the same QPD do not have the same peak-to-peak amplitude could be explained by a path length change of less than one wavelength.

Then Martin did some analysis of the data.  In figure 2, he calculated the I^2+Q^2 for both quadrants, and also the sum of them. The amplitude of two quadrants have more or less each others inverted signal. The fluctuation in the sum is smaller than that of the individual amplitudes. Hence the total power is almost conserved. The remaining fluctuation in the sum could be due to unequal beat signal amplitude (for instance due to a difference in ND filter, or a difference in overlap of the beams on the different QPDs).

In figure 3, the phase of both quadrants are moving in the same way, it could be caused by the path length difference before BS3, so it has same effect for both the quadrants. But there is this factor of 2 difference in the phase changing, which we didn't really understand. Part of it could be caused by the small DC offset of the raw signal.

After testing the RF signal of the quadrants, we also tested the DC with the galvo. We simply put the galvo in front of one of the quadrant (see the bench picture in figure 5), and did some rough alignment to make sure that the beam is inside the diode. Then by sending four quadrant DC outputs to the galvo controlling board, two outputs of this board will be used to control the x and y direction of the galvo. Then we checked the x and y error signal through the monitor port of this board. As long as the galvo loop is closed, the galvo is able to bring the error signal back to 0, which means the beam is centered on the quadrant.

Conclusion: With two quadrants, we got figure 1 that confirm both of them are working as we expected. For the DC, the galvo is able to center the beam on the quadrant. Now everything has been removed from the bench and packed. We will send them to NAOJ today or latest next Monday.

I checked the CVI lens we should buy if we want to replace the thorlabs lens now we are using between OPO and homodyne.

The result is attached in the figure. Since we could find a lens with the focal length similar to thorlabs focal length, the simulation result is similar to the previous simulation. In this case, the lens we should buy from CVI is PLCX-25.4-46.4-UV and PLCX-25.4-70-UV.

Also if we have again almost 100% match from OPO to AMC, the telescope design for filter cavity injection should be not be influenced.

If we just consider this is a factor of 11 between green and IR filter cavity locking accuracy as pointed in entry 760 figure 6, we could estimate the IR locking accuracy of the filter cavity. (in green locking case)

As reported in entry 1486 and entry 1513, we have green filter cavity locking accuracy of ~4Hz. This is corresponding to 4e-12m.

Then by considering the factor 11(because of cavity pole of green and IR is different), the locking accuracy of IR should be 0.36e-12m. In this case, it meets the requirement of filter cavity length fluctuation.

But we still need to confirm the measurement of entry 1486and 1513.

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).

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+0.00425) = 95%. Calculation in Marco' s thesis seems wrong.

From Marco thesis, the escape efficiency is 0.92/(0.92+0.00425). It is 99.5%, it seems fine in that case.

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.  

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.

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 = 1.04/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
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.

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.

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.

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.

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.

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.

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. 

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.

[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 blue curve is seen by PZT corr, the red one by END length OPLEV.

Driving matrix for length:

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.