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. 

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.

[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.

[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).

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

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.

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 HR coating of PPKTP is T2 = 0.025 % 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.

If L = 0.425 % like Marco's thesis P.87 (transmissivity of HR coating of PPKTP is 0.025%, not 0.25%), escape efficiency is 95 %.

[Eleonora, Matteo, Miyakawa (remotely)]

We have finally solved the problem with DGS.

As suspecteted by Miyakawa-san, it was due to a broken ADC timing adaptor. The main reason why we took so long to realize it is that in order to simplfy the configuration I was connecting the timing box (the one that was carefully tested) to only one of the two ADC cards istalled in the PC.  Miyakawa-san pointed out that all the ADC/DAC PCie installed in the standalone have to receive a correct timing signal otherwise NONE OF THEM will work. On the other and it doesn't matter if they are not connect to AA/AI.

So I test also the second ADC timing adapter and found out that it was broken. The pin of the SMB connector on the board (pic 1) was broken and got stuck in the SMB2BNC adapter (pic2). According to Miyakawa-san this is a frequent issue for these components.

Since we don't have any spare ADC timing adapter, I removed the second ADC card (which currenty is not used) from the standalone and connected the DAC timing adapter to the DAC PCie. In this configuration the system could work again.

Miyakawa-san will send to Mitaka two more ADC timing adapter from KAGRA.

Other usefull information:

1) The correct way to connect the DAC timing box to DAC PCie and AI is shown in pic 3 (note that the connectors for the two ports are the same). The first pdf attached shows the pin assignment of the DAC timing box which is usefull to test the connections and detect a possible break of the SMB. 

2) The pin assignement for ADC timing box is show in the second pdf attached. Since in this case the connector from ADC and to AA are different it is easy to identify the correct connection configuation.

3) The correct set up for the timing signal generator is: square wave at 65536 Hz, level 2.5, offset 1.25. It provide a squared wave of 0-5 Volt.

[Yuhang, Aritomi]

We changed plate BS for homodyne to cubic BS and aligned homodyne with s pol. Visibility is 0.988 which means loss from visibility is 2.4%. However, squeeze level is same. It seems that squeeze level is limited by unknown loss. Now we suspect that loss of OPO (escape efficiency) is large.

Simon

As there are some doubts about the linearity of the input-polarization, I checked the HWP and can confirm that it is not an accidentally taken QWP (I changed the angle to confirm its periodicity of 45deg).

In order to further increase the linearity, I have put a QWP in front of the HWP and looked for the S-pol minimum (while maximizing P-pol) on the sensors when the mass was out of the beam-path. I could reach a minimum of ~1.2 mV with having ~384 mV on P-pol-sensor. Turning the HWP by 45 degrees, I reached a S-pol maximum of ~312 mV and a P-pol minimum of ~0.9 mV.
Those are much better numbers than we had before so that we can say to have a good linearization now!

With those changes on the setup and setting the initial polarization to S, I started mapping ETMY.

Matteo, Eleonora, Simon

(Report from July 30th 2019)

As we finished absorption and polarization characterizations of the ETMX spare mass, we exchanched it with the ETMY spare mass (see pictures below).
We tried a kind of new technique for making the exchanging procedure a bit more easier and safer. Therefore, we made use of the holding-structure of the container where the mirror-substrates are transported. We found that we can use those structures to flip the substrate in a horizontal position (which is required for the measurements) and to put it directly on the sample-holder which coincidentally fits very well to these holding-structures.
That way works also vice-versa.

In addition, we recognized that both substrates indeed have a mark (probably) pointing to the thicker side of their wedges.
However, we found that this mark is on different sides for ETMY and ETMX (see last pictures).

Yuhang and Aritomi

Yesterday, we tried to put ND filter so that we could characterize the squeezing level.

We put ND-0.3, which is corresponding to 0.5 loss.

When we were measuring, we were sending 40mW of green. From the measurement of yesterday, we could know the squeezing level we are sending is 13dB. By using this squeezing level, I made the plot of squeezing level change curve. The x-axis variables in the plots is losses. And there are three curves for different phase noise level.

The measurement result of squeezing corresponding to different losses is

From this plot, it seems elog 1523 result is reasonable.

[Aritomi, Yuhang]

We measured loss of each optics between OPO and homodyne BS. We put BAB on resonance of OPO by hand and measured power. The result is as follows.

Loss between OPO and homodyne is 7%. 4% is from dichroic mirror and 3% is from two lenses. According to spec of dichroic mirror (HBSY11), reflectivity for s pol should be 99.3%. We can try to optimize the angle or try another HBSY11. Anyway we need low loss dichroic mirror and superpolished lenses.

I measured the error signal of PDH lock as shown in attached pictures (green line).
Frequency scan was done with 20 Hz triangular wave (3Vpp).
Some details will be reported.