We checked the cables that connect the quadrant and board, if they are long enough for us to put the board above and below the bench. We tried to put the quadrant at the edge of the breadboard which is also the furthest position from the edge of the bench. In the attached pictures, we could see it is possible to do in both way. Anyway, I also contacted Matteo, he will go to ask about the cables next week when he will be in Nikhef.

Now the Faraday is located 10~14 holes after PBS, according to the measurement of beam parameter we could predict the beam size at both ends of this Faraday isolator. The result is shown in the attached figure 1. I also checked the aperture of FI(IO-3-1064-VHP), which is 2.7mm in diameter.

In this case, the FI aperture and beam size ratio is minimum as 2.7/0.945/2 = 1.4286. We could calculate the Gaussian beam power through an aperture is P/P0 = 1-e^(-2*(1.4286)^2) = 0.98. So in our current case, even the best-aligned beam will loss 2% of the incident power. Also, I remember that we achieved almost 97% of power transmission of FI even in the case of this beam clipping issue.

Usually, we make the beam five times smaller than the aperture. But it is really difficult to have space and meet this usual requirement. However, if we move this beam one and a half hole backward, the power cut by aperture will be 0.5%. This can be realized by moving the fork of green injection mirror (the last green mirror before going inside the chamber). Because this is blocking the way of moving backward.

The reflectivity of PBS is optimized with the same method of the optimization of the dichroic mirror(HBSY11).

However, there is a strange thing, which is the reflected field is even stronger than the incident field. See attached figure 1 and 2 (incidence and reflection). So the reflection is 109/106 = 102.83%. But anyway, the reflection is maximized.

[Aritomi, Yuhang]

We measured free running and closed loop CC2 phase noise from filter cavity (attached picture). CC2 error signal is 356 mVpp. Phase noise from filter cavity is large at low frequency. Although phase noise at low frequency is suppressed by CC2, bump at low frequency in squeezing spectrum might be phase noise of CC2. Odd number harmonics of 50Hz peaks appear when CC2 is closed. These peaks are related to electronics of CC2.

[Aritomi, Yuhang]

From previous measurement, dichroic mirror we are using (HBSY11) had only 96% reflectivity while the spec reflectivity is 99.3%. We tweaked angle of the dichroic mirror and measured BAB peak height when OPO is scanned. We improved the reflection by 4% from 95.2mV to 99.2mV. However, the optimal angle is not 45deg and we lose 18% of BAB at PBS after OPO with this angle of dichroic mirror. We tried to change the angle of the PBS by hand, but it didn't improve. We need to align the PBS.

[Aritomi, Yuhang]

We measured loss from PBS after OPO to filter cavity reflection just after PR chamber. We haven't measured loss from filter cavity reflection to homodyne yet.

From this measurement, loss from PBS after OPO to filter cavity reflection is 22.5%. Since we know that loss when squeezed light is directly injected to homodyne is 21%, total loss should be at least 40%.

We found that HWP we were using before had 5% loss and seems dirty. This explains 4% additional loss when I did additional loss measurement using this HWP. After replacing with new one, HWP loss becomes 2.4%.

Faraday on the bench has 7% loss. Alignment of the faraday should be optimized.

Current squeezing level is 2.4dB. Phase noise from laser around 10kHz becomes smaller. Spectrum at low frequency is bad due to CC2 lock loss during the measurement.

[Aritomi, Yuhang]

First we maximized IR reflection from filter cavity while we couldn't find IR resonance at that point. The reflection was 84% of injection.

Then we aligned LO and BAB reflected by unlocked filter cavity into AMC and measured visibility.

Calculated visibility is somehow more than 100% but it should be good anyway. Good news is that visibility is stable although we have jittering.

Finally we measured squeezing spectrum when squeezed light is reflected by unlocked filter cavity (attached picture). Injected green is 40mW and demodulation phase for squeezing is 190deg. We have 1.8dB squeezing down to 60Hz. Since it seems we have much larger phase noise from filter cavity (It will be reported later), we may have more squeezing when pump green is lower. Large bump below 60Hz is unknown. Noise around 10kHz should be phase noise of laser since we turned it on today. 

Current problems:

1. Large bump below 60Hz

2. Large phase noise from filter cavity and CC2 control is not so stable

I aligned reflection from filter cavity to AMC to see jittering effect while IR injection is still not aligned. Attached picture shows BAB mode matching in AMC when alignment is good. Largest peak is TEM00 and small peak next to TEM00 is pitch misalignment. We can see pitch jittering effect from attached movie.

[Aritomi, Yuhang, Yuefan (remotely)]

Yesterday we found IR reflection from filter cavity was only 33% of injection and IR on PR reference was 15% of injection. The reason was that polarization after faraday on the bench we recently installed was not s-polarization due to faraday rotator and some of BAB were lost in PBS before and after faraday in PR chamber. So we put HWP just after faraday on the bench and optimized the polarization. As a result, reflection from filter cavity becomes 85%.

Here is the measuremrnt of green reflection jittering at 1pm, 6pm and 9pm.

It seems that they are similar.

Aritomi and Yuhang

Today we realigned again the GR.

GR on the PR chamber target was fine. (See attached figure 1)

GR on the BS target was moved in the pitch direction. If I remember well, Eleonora intentionally removes the DC offset of PR local control. And after about two weeks, the pitch was misaligned. Maybe this means PR mirror has pitch drift without local control. (See attached figure 2)

Then we moved picomotor of PR to recover GR on the BS target.

We moved picomotor of BS to make beam go through the first iris. We also moved picomotor of BS again and made beam go through the second iris. After that, we found the transmission was not hitting the center of the camera. Since we think two irises along the filter cavity should be the best reference, we decided to move the camera to center beam on the camera. But we didn't move the fork of camera, so if this should not be done we can also recover easily. The situation before we moved camera was attached figure 3.

Aritomi and Yuhang

We found that the incident beam into the filter cavity actually it is not jittering.

But we confirm that ITM and ETM are jitter. For ITM, it is obvious that reflection is jittering.  For ETM, we found ETM reflection and it was moving.

Please check the attached video. This is the green incident beam on the second target. https://drive.google.com/open?id=1O6HZabKGtyzbuyYSPC_QGHkkhuE3W1YI

Simon, Matteo

We decided to focus on some more preciser maps on spare ETMY, especially in YZ, since in the full map XZ we could see some layer-like stuctures but not in the full YZ map, which should be the case. This is probably due to a lower contrast as in YZ we have a much larger range in the absorption coefficent.
The results of the measurements can be shown in the attached figures. We can indeed see now those layer-structures in both XZ and YZ planes, as we expected.
About the reason of these layers, we can only speculate but it seems that they have their origin in some systematic oscillations of impurity concentrations during the crystallization process.

In addition to that, I finished writing the 3D representation part of the absorption maps in Python by using Myavi. I attached also a picture of the results from the last full-map measurements.

Aritomi and Yuhang

After the simulation, we implemented the filter cavity reflection telescope. It was quite strange that I moved a lot to match this beam into AMC. Maybe this is because of the injection is not well mode matched, so the reflection is also not in good shape. But anyway, we put this telescope and tried a lot to improve the matching.

The preliminary result is as the attached figures.

In the first attached figure, we could see there is a mode mismatch peak with a height of 540mV.

In the second attached figure, we could see there is a TEM00 with a height of 6.6V.

In the third and fourth figure, you can find a peak just beside the mode-mismatch peak. It changes height because of the beam jittering in pitch direction. And it can have height up to 860mV.

In the fifth and sixth figure, you can see the TEM00 peak can also change from 5.5V to 7.5V.

Conclusion:

1. We have roughly mode mistach now for filter cavity into AMC as ~7.5%.

2. Beam jittering brings misalignment of up to ~12%.

3. The estimation is not precise because the total reflected IR power also fluctuates. From the experience of green reflection, we see much more stable green reflection after using smaller focal length lens. Maybe we are having this beam clipping issue also for this measurement in IR reflection. We should check this tomorrow.

Found on 5th September, the beam was clipped!

As we talked in the meeting, I did the measurement of beam reflection jitter by using QPD.

I did measurement first time just after the meeting, I measured the GR reflection beam jitter by QPD at yaw and pitch direction. I tried my best to center this beam and made the measurement. I also measured the beam jitter difference when the filter cavity is locked and unlocked. The result is attached in figure 1.

I did measurement again before leaving. This time I just measured when the filter cavity is unlocked. The comparison is in attached figure 2.

conclusion:

1. The lock of the filter cavity can reduce this beam jettering above ~10Hz. We know the correction signal we are sending to Main laser PZT is limited by laser noise above 10Hz. So I guess the lock of the filter cavity reduces the power coupling from higher-order modes to TEM00. And at low frequency, this is limited by the suspension. So if we engage the filter cavity length control, we may solve a bit this beam jittering problem. We should try to see this effect.

2. From the first plot, It seems pitch and yaw have different peaks. Yaw is worse at a higher frequency while the pitch is worse at a lower frequency.

3. From the second plot, it seems there is no obvious difference between 6 pm and 9.5 pm of beam jittering. I will check the situation in the morning and just after lunch.

Aritomi and Yuhang

To make sure everything is going well, we decided to perform this measurement. From the simulation, beam waist should be 53.5cm after the lens and has size 146um.

From the measurement, the beam waist is 59cm after the lens and has a size around 80um. This is out of our exception.

Shall we adjust the telescope according to this measurement?

This is the light around the wall.

Here is difference when fluorescent light in TAMA is ON and OFF. Please turn it off when you want to measure small signal.

[Aritomi, Yuhang]

We could see BAB transmission (injection is 150uW) by CCD camera without amplification when IR is aligned and AOM frequency is optimized (Pic.1). We could also see BAB transmission with 11.5uW injection (Pic.2), so we can see CC transmission with current CCD camera.