I installed the another double-pass AOM — to be precise, I installed a lens, QWP, and mirror for composing double-pass AOM system.
The attached picture shows the installed optics.
The following is the procedure.

1. Adjusted the alignment to maximize the -1st order diffracted beam power.
   The diffraction efficiency was about 85%.
2. Then put a lens (f=75mm) and QWP to collimate the beam.
3. Put a mirror and adjusted the alignment to let the diffracted beam enter the AOM.
4. Adjusted the alignment with two STMs and one end mirror by monitoring double-passed beam power.
5. I got about 72% double-pass diffraction efficiency at the end.

I have not investigated the beam jitter of this system.
So I gonna measure the beam jitter with frequency scan and then install optics for ISS.

Today I restarted pumping down.
I turned on the scroll pump at 15:40 and waited until the pressure level became less than 10 Pa.
Then I turned on the turbo pump at 16:00 and the pressure is getting lower.

I will leave them running until tomorrow morning.

I measured the beam jitter induced by AOM frequency scan by using beam profiler.
I set the requirement of beam jitter as 0.05mrad/MHz .
This value corresponds to 0.1mm shift at folded cavity input mirror though there exists 1MHz frequency shift (I assumed that the distance from AOM to folded cavity is 2m).
This beam jitter is small enough from the view point of alignment during the DC locking.

The measured beam shifts at 0.5m distance from the AOM was about 0.2mm with 15MHz frequency shift.
So the beam jitter is about 30urad/MHz with current configuration.
This seems small enough for the moment.

The next step is implement another double-pass AOM and measure the beam jitter.
Then construct ISS using AOMs.

Yaochin, Yuhang

In the attached file, I calculated the loss comes from a spatial mismatch between OPO and IRMC. The calculation is revised by following Eleonora's suggestion. The result shows a loss of 0.000014% from this issue.

I tried to improve the efficiency of double-pass AOM.
Before this work, the efficiency was about 65%.

Today, I modified the position of the mirror which is used to re-inject the beam to the AOM.
Now, the mirror is located closer position to the AOM than before.
After that, the double-pass efficiency became about 72% which is satisfactory value.

So I will tackle on the beam jitter measurement.

I stopped the pumps around 16:00.
At that time, the pressure was 1.9*10^-3 Pa.

After stopping the pumps, the pressure level increased gradually.
In this time, I did not record the pressure.
I wiil do the vacuum test one more time and take the data.

[Takahashi-san, Tanioka]

Takahashi-san kindly installed the another roughing pump yesterday.
We turned it on this morning.
After several minutes, the pressure became below 10 Pa.
Then we turned on the turbo pump.
Now the two pumps are working and I plan to leave the pumps running for a while.

Aritomi, Yaochin, and Yuhang

Since we found there may be an issue of scattering from OPO(entry 1842), we did this check.

From the thesis of Emil,  they had 8% of power loss due to the higher-order modes from the damaged OPO. If we have the same situation with GEO600, we should have the problem reported in entry 1842. At a farther distance, the measurement of power will go down since the higher-order modes are much larger than TEM00. An easy check to tell if we have the same problem is to check OPO transmission by the camera.

We checked two positions, one is just after the dichroic mirror. Another is at some distance after the new Faraday isolator. They give almost the same result. However, as shown in the attached figures(the first is a picture taken by the camera by using the OD5 filter, the second is a picture taken by the camera by using the OD3 filter). OPO transmission doesn't have a special problem. (There are some points in the photo, the reason to have this is still unknown)

Simon, Pengbo

This week we finished the characterization of the KAGRA-size Shinkosha sample #7.  We did several polarization maps with different incident polarization angles.( 0 degrees represents the pure p polarization)

As can be seen, both S- and P- polarization maps show an apparent offset, which is reasonable due to the birefringence effect. And the other three mixtured maps are well-matched with the incident polarization angles.

Noted that, each time when I adjusted the polarization, the DC signal keeps a relatively stable state but the AC signal seems very unstable, either goes up or goes down for a long time. 

[Aritomi, Yaochin, Yuhang]

Yesterday we found that loss from dichroic mirror is 6.7%, so we investigated loss of dichroic mirror. We used two dichroic mirrors: one is HBSY11 from thorlabs which we are using and another one is from newport.

We measured BAB transmission of dichroic mirror and tried to minimize it. To remove green generated by BAB inside OPO, we put OPO temperature in the region with no parametric gain. This time we set 9.36kOhm of OPO temperature. We tweaked angle of dichroic mirrors a lot, but reflection power almost didn't change. By looking at transmission of dichroic mirror, we somehow found the minimum point and measured loss. We measured BAB power in two ways: one is to scan OPO and measure the BAB peak height, another one is to lock OPO by hand. For both cases, we had still ~3% loss from dichroic mirrors.

To characterize HBSY11, we checked reflectivity of HBSY11 with LO. We measured reflection from HBSY11 with power meter and PD. The result is as follows. Reflection power with power meter didn't depend on angle of HBSY11, but reflection power with PD changed from 380mV to 392mV by tweaking angle. Note that distance between HBSY11 and PD is 4cm.

For both cases, reflectivity of HBSY11 is consistent with specification which is 99.3%. From these results, we concluded that dichroic mirror (HBSY11) is fine.

Then we put HBSY11 back to after OPO and measured reflection and transmission of HBSY11 at the same time. The result is as follows and shown in Pic.1. The sum of reflection and transmission doesn't match with injection just after OPO.

We thought some scattering light comes from OPO and the scattering light is also measured just after OPO. So we removed dichroic mirror and measured OPO transmission at further point to remove the scattering as shown in Pic.2. BAB transmission after removing dichroic mirror is 399uW. BAB transmission is lost by 7% between two points. The loss seems scattering from OPO, but we are not sure the reason. Yuhang will check if the aperture of power meter is large enough.

Conclusion: Dichroic mirror is fine. Loss is coming from OPO transmission.

[Takahashi-san, Tanioka]

We did vacuum test of cryostat at ATC using scroll pump and turbo pump.
The procedure was as follows:

1. Close the chamber (without 4K shield temporary).
2. Confirmed all the valve were closed and turned on the scroll pump.
3. Opened the valve.
4. Then the pressure level was down to 3.0*10 Pa.
   However, it increased and reached about 5.5*10 Pa.
   This was due to the residual water molecules inside the chamber and it was solved by opening gas ballast of scroll pump.
5. After some iterations, the pressure level reached about 10 Pa.
   We turned on the turbo pump and the pressure level decreased less than 10^-1 Pa.

So it seems that there is no huge leakage, which was good thing.
However, after a few minutes turbo pump run, the scroll pump was turned off for some reasons.
Unfortunately I could not see what happened since I went to bathroom.
We tried to recover, but the scroll pump did not work even the power indicator did not iluminate...
One possibility is that fuse blown.
We will investigate.

[Aritomi, Yaochin]

First we checked visibility. Visibility is 98.2% and loss from visibility is 3.5% as follows. This is a bit worse than before and we should improve mode matching of BAB.

Then we found that loss from dichroic mirror is 6.7% and loss from PBS is 2.1% as follows. It's strange that loss from dichroic mirror is so large.

Tomorrow we'll align dichroic mirror and replace PBS with HR mirror. The method to align dichroic mirror is to send only BAB to scanned OPO and put HR mirror just after dichroic mirror and maximize the reflection.

I installed the another AOM as attached picture as following procedure.

1. Put a PBS and adjusted its angle.
2. Put an AOM and connected to a driver.
3. Drove the AOM and played with STMs to increase the diffracted beam power.

Current diffraction efficiency is about 80% and still there is a room for improvement.

In addition, I worked on the double-pass AOM alignment previously installed.

1. Put a lens (f=75mm) after the AOM to collimate the beam.
2. Put a mirror and PBS to pick up the double-passed beam.
3. Adjusted the alignment and lens' position.

The power of double-passed beam is 5.76mW in contrast with 8.86mW input to AOM.
Hence the double-pass diffraction efficiency is about 65% which is reasonable value.

The next step is (quantitatively) measure the beam jitter with scanning frequency and minimize it.

I ordered a folded cavity spacer and related parts from NAKAO SEIKI.
They will be made of invar (IC-DX) which can be used under cryogenic temperature.
The drawings are uploaded on Wiki page (explanation are written in Japanese though) (https://gwpo.nao.ac.jp/wiki/CryogenicThermal/ActivityNAOJ/Spacer).

[Aritomi, Yaochin]

We measured squeezing and anti squeezing with new faraday to estimate loss and phase noise.

Attached picture shows the result. Loss is 25.4% and phase noise is 21.4 mrad. Compared with previous measurement, we have 4.5% more loss. We should have 3% more loss from faraday and HWP, but we should have ~3% less loss from dichroic mirror (entry 1613). So this 25.4% loss is higher than we expected. Maybe we have worse visibility after installation of faraday. We'll check visibility tomorrow.

If I remember correctly, after improving reflectivity of dichroic mirror, squeezing level didn't change. It is also better to check the reflectivity of dichroic mirror again.

Good news is that even though we turned on lasers today, we have only 21.4 mrad of phase noise and squeezing spectrum is very flat with new faraday.

I tuned the lens position and alignment, then the diffracted beam power became 5.0mW which seemed to be enough (input power was 5.7mW).
Then I installed a PBS in front of the AOM, and adjusted the alignment.
At this moment, still the diffracted beam has 5.0mW.

Tomorrow, I gonna install convex lens and re-align the double-pass AOM.

Why the correction saturates when it becomes negative and not when it becomes positive? 

I changed the lenses' position to modify the beam profile for HOMs, especially to make the beam width larger.
The target beam width was 500um though the initial beam width was ~200um.
To ahieve this beam size, I changed the lenses and their position as attached by monitoring beam profile.
I put f=-75mm and f=200mm lenses instead of f=-50 and f=150mm ones.
Then the beam width around AOMs became about 500um and it was beam waist.

After that, I played with STMs and maximize the diffraction efficiency (but not adjusted the alignment of AOM itself).
Eventually, the input power to AOM was 5.7mW and the 1st order diffraction power was 4.5mW which corresponded to ~80% diffraction efficiency.
Actually, the lens position was slightly disturbed when I clamped them.
So I will tune their position and adjust the alignment to maximize diffracted beam power as a next step.
Then I will put a PBS, QWP, and mirror to compose double-pass AOM configuration.

I installed the another AOM (S/N:149256) as attached to check its diffraction efficiency with larger beam size compared to previously installed one.
The input beam power was 5.5mW and diffracted beam power was 3.5mW though the RF level was not tuned.
So the diffraction efficiency can be enhanced by larger beam size.

Then I put a concave lens in front of the AOM previously installed whose efficiency was about 50%.
Actually the efficiency was increased to 60%.

I gonna increase the beam size at AOMs to modify the position of lenses to enhance the diffraction efficiency (according to spec sheet, about 85% can be achieved).