We found the filter cavity green transmission DC is changed to ~2400 at maximum after last week's work. Although we tried to change servo gain and aligned filter cavity as well as possible, we couldn't bring it back to 5000.

Yuefan and Yuhang

We found the beam height of the green beam between SHG and filter cavity has very large beam height change after the work of beam center on each lens.

We measured the beam height at several points. (The beam height at Faraday isolator is a very important parameter. We may send a tilted beam if the beam height is wrong. Also, Astigmatism beam could be cut if the beam is not center in this FI.)

Note: Lens1~5 is the lens in the sequance from SHG to PR chamber. (Details are also shown in the attached figure 1)

We also checked the beam shape. Attached picture 2: beam shape after AOM. Attached picture 3: beam shape before injection to PR chamber (magnified by a 50mm lens). Attached picture 4: beam shape seen at the end camera. Attached picture 5: reflected beam shape seen before FI.

We measured the beam parameter and fit with basic Gaussian beam function before and after today's work. The comparison is attached in the last two figures.

In the past days I spent some time to modify again the damping loops.

The main reason was that the loops that I have implemented before (see entry #1641) had the first UFG at too low frequency (<100 mHz) and it took to much time to change DC offset to align the cavity. Also I'm not sure if the signal in the 10-100 mHz corresponds to a real motion of the suspensions or is oplev noise.

In the attached PDF I report the new filter performances. I tried to avoid any overshoot but I think they can be further improved.

Some remarks:

- I observed that sometimes an eccess of noise shows up in the whole BS YAW spectrum when closing the BS PITCH loop. I have slightly decreased the BS pitch gain and it has disappeared.

- The pitch peaks just above 10 Hz are quite hard to damp as they are coming from the stack resonance. (See attached a plot of the spectrum of an oplev for a mirror placed directly on the stack.)

Up to now, all the loops are closed one by one by, manually changing the gain from 0 to 1 in the MEDM interface.

Since this is inconvinient especially if we will need to close alignment loops, I prepared a python script to close (and open) the damping loop just by clicking on a button on the MEDM screen. (see attached picture) 

The scripts, which use EZCA module to change values of epics channels, are located in /home/controls/FDSscripts.

A strange issue we found (while checking with Shoda-san) is that the script doesn't work if I open MEDM interface using the desktop icon but only if I open it from terminal.

I contacted Yamamoto-san and he suggested that it can be due python environmental variables and he suggested to try with a bash script that launch a python script. I tried this solution but also in this case it only works if I open MEDM from terminal. I will investigate more.

For the moment, if we want to use the button to open and close all the damping loops in one click please open MEDM from terminal (just open a terminal and type "sitemap").

On Friday afternoon we realizied that we were not able to use diaggui software to perform for fourier analysis anymore. After launching any measurements we got the error message: " test timed out".

I contacted Yamamoto-san in KAGRA and he explained me that this problem is caused by a difference between a system time of the client workstation (desktop1) and a timing signal on the standalone. The system time of the client workstation is synchronized to a correct time via NTP (probably within 1 second accuracy). But the accuracy of a timing signal on the standalone depends on the 65kHz-clock signal from a function generator. So the time on the standalone deviate from the correct time by the long term operation because the function generator is not so accurate for the real-time system.

Yuefan and Yuhang

This work is inspired by astigmatism we had for green reflection from the filter cavity. Astigmatism can be generated by misalignment or refractive error.

If it is because of misalignment, this can be improved by doing the alignment. If it is because of refractive error, we can move a bit beam on the lens to correct the refractive error.

The filter cavity reflection always has the same direction astigmatism no matter what is the alignment condition. So we think most of astigmatism comes from the refractive error.

We always check the beam shape on the first and second iris(along the vacuum tube), but we should notice that the beam shape is difficult to tell. The reason is that we look at the iris while the beam hits on the iris with angle. Also, we checked the beam we see on the filter cavity transmission camera when we misalign the input mirror. Always this filter cavity transmission has astigmatism(like filter cavity reflection). So we think the astigmatism is mainly because of refractive error.

To solve this problem, we tried to center beam on every lens. We have five lenses between SHG and FC. However, we found most of them are not centered in the lens. But this is based on a clear phenomenon that SHG reflection was seen to have some fringes if we reflect it far and large enough to see clearly. The elongation is the diffraction fringes at one side of the SHG hole and caused by the clipping of the SHG hole side. We know that the beam direction of SHG reflection is defined by the axis of SHG crystal and the SHG in-coupling mirror. We need to adjust the crystal and in-coupling mirror to have a better axis.

Although we centered beam on each lens, the filter cavity reflection still has astigmatism. Maybe astigmatism comes from SHG, and we should assemble it again. Maybe astigmatism happens inside the vacuum chamber, and we need to consider how to improve this situation.

Since we decided to have gouy phase 90 degree different on two quadrants instead of exactly 0 and 90 degree, I calculated again the telescope based on the beam waist reported in last entry.

In the case of two different axis has different beam waist size, I made two different telescopes. 

The telscope was designed to have more or less same quadrant position as before, and to have two quadrants one before and one after the beam waist.

1. beam waist of 711um

500mm lens 1.5m from the Faraday Isolator including the height increasing in the periscope.

2. beam waist of 1274um

1m lens 0.975m from the Faraday Isolator including the height increasing in the periscope.

We will check the green injection path these days, if we could have a more round beam in the reflection, we can measure the beam again. 

Yuefan and Yuhang

As planned last week, today we measured the reflection beam again after the 500mm lens with cavity locked. The fitting shows in the attached figure. 

Same as last time, the position of the waist in two axis are more or less the same, but they both moved around 8cm from 59cm from the lens to 51cm to the lens this time.

The waist size is very different. In last measurement the waist was 64um and 92um, this time is 60um and 119um.  This gives the reflection beam waist size and position.

1. 711um. 0.863m before the lens, which should be between the periscope and first mirror.

2. 1274um, 5 m before the lens.

The difference in these two measurements, we guess is caused by the alignment condition. Because when we installed the mirrors and lens last week, they were well centered, but today when we aligned the cavity, the beam looks obviously lower than the center of the lens, which will induce some more astigmatism. But since the lens has a focal length of 500mm, it didn't affect so much, but in the design the second lens has a focal length of -50mm, if everytime we realign the cavity, the beam is off center on that lens, it could be a problem.

The solution we were thinking about is to put one or two iris on the reflection beam path when we centered all the optics, then they will act as reference for future alignment.

Anyway, apparently the beam has astigmatism, which may bring some problem to the automatic alignment system. So tomorrow if it is possible, we will move the last lens on the green input path because it is really bad algined now.

Yuefan and Yuhang

Recently, we always have a problem of filter cavity green transmission power drop. It dropped from 5000 to 1500 recently. We figured out this problem today although we are always checking with the same method. The reason for GRtra power drop is too high gain.

After reducing the gain of filter cavity control loop while looking at filter cavity green transmission, we found the indicator of GRtra goes to 5000.

The incident power to filter cavity is 12.5mW, setting of the filter cavity loop is attached in figure 1 and 2(attenuation:1.68, gain:2.28). The locking bandwidth is 21kHz.

Yuhang and Yuefan

Since now we have the reflected beam coming from the filter cavity, it has astigmatism of ratio almost 2 between two axes. And also, it was jittering while testing.

It is reasonable that the range is larger at yaw direction while smaller at pitch direction due to astigmatism.

Anyway, the galvo works well with some oscillation. I attach here a video. https://drive.google.com/open?id=1sxK1SRzyOSfxDVE6_rPHDkVkjddDbnPS

While working remotely on loops optimization, I noticed that the BS oplev high frequency spectrum suddenly increased. See attached picture. 

After some time (about ~20 min) it went back to the previous value. I suspect it might be due to an increase of environmental vibration maybe bought by the activation of some device.

Unfortunately I was not there to check. To be monitored.

I spent some time to optimize the new damp loops, whitout DC control, to be used when we want to engage AA or dithering control.

In the attached file the spectra with open and close loops for BS, INPUT and END mirror are reported together with the poles/zeros of the filters.

In some cases, an optimization (espacially to reduce overshoot) is still necessary.

To separate devices we put beneath the in-air bench, Matteo prepared a new shelf, we replaced it. The situation is now like the attached figure.

Yuhang and Yuefan

We tested the galvo for AA system.

We put a mirror after Faraday because there is still gate valve inside the vacuum tube. In this case(see attached figure 1), we could get a stable reflection. As mentioned by yuefan in the last entry, we had a problem of astigmatism in the beginning. We removed the second lens(-50mm lens), and then astigmatism disappeared.

Couldn't lock situation: If we found the light ERROR X or ERROR Y becomes light on(attached figure 2 and 3), it means we have a large offset in X or Y direction. So we couldn't lock loop and center beam.

Could lock situation: We found if error x is within 290mV and error y is within 730mV(attached figure 4 and 5), we could lock loop and center beam. But we found oscillation of around 30Hz or 1Hz after lock(attached figure 6 and 7).

The moment of lock is attached in figure 8.

Next step:

1. Get the real jittering beam and see if we could center beam.

2. Test galvo even with astigmatism.

Yuefan and Yuhang

Last Friday, for better understanding to beam situation, we decided to check again the simulation.

According to the beam size measured by Yuhang and Aritomi-san a while ago after the 500mm lens, we could get the original beam waist size and position of the reflection green beam. The beam waist is 2.5m before the 500mm lens, and the beam waist size is 440um which is different from what we expected (~1mm) and mearsured very long time ago by Yuhang and Marc(1008um).

Anyway then we checked the telescope I already designed with this beam, the result didn't seem good. The quadrant position for gouy phase 0 and 90 degree didn't change a lot, but the size became 100um on NF quadrant and 2.5mm on FF, both are bad for our requirement.

To increase the beam size on the NF quadrant, we could change the 500mm to longer one, but then this will limited mainly by the space we have. We decided to check again the beam profile next week when we get the cavity back to decide if we need to change the design or not.

Another thing we have to check is the astigmatism of the beam, because during the installation we found that the second lens which has a focal length of 50mm could induce a lot of astigmatism on the FF quadrant even we could not see by eyes the beam is off center. And if the beam already has a very strong astigmatism as measured in entry 1598, the final beam on the FF quadrant will have a really ugly shape. So after we check the beam again next, if it is still the same, we would like to check the injection path to see where the astigmatism come from.

[Aritomi, Yuhang, Yuefan, Eleonora, Matteo, Takahashi]

We opened PR chamber and measured BAB power in PR chamber. BAB power was fluctuating a bit and BAB power after dichoic mirror seems not reasonable, but anyway loss of dichroic mirror should be small. Loss of faraday in PR chamber is around 5% and other loss is less than 1%.

Then we put mirror after dichroic mirror and reflected BAB into faraday and maximized the reflection. BAB injection inside PR chamber is 131uW and BAB reflection is 116uW. The round trip loss is 11.5%. 

Matteo, Simon

Yesterday, we finished to restart the translation stage after the power outage the day before.

After making sure that everything is working properly, we started to reconfigure the PCI bench for the polarization measurements:

• placing ND-filters in the beam-path before the shutter (-> order ~ 2)
• placing a Vantablack beam-dump at the same area to cover the ND-filter reflection
• putting a PBS after the first lens of the telescope
• putting a QWP  and a HWP after the PBS
• placing a razor beam-dump before the second lens of the telescope to cover a ghost-beam coming from the ND-filters
• putting a 1" lens (f=150mm) together with a beam-tube on the detector stage
• putting a second PBS so that the beam-focus is ~ 2 cm behind the PBS
• placing a beam-dumb behind the blind-side of the PBS to cover back-scattering
• putting each a PSD (position sensitive detector) 2 cm behind the S-pol and P-pol side of the PBS
• Connecting the PSDs to the Lock-Ins and fine-tune the position of the PSDs with an oscilloscope (use the sum-output for the Lock-Ins and the X/Y output for the fine-tuning; beam should be centered)

After the setup is done, we calibrated the QWP and HWP in an iterative manner to produce a maximum in P-pol.
The data on the calibration are as follows:
S-pol PSD maximum voltage: 285 mV
S-pol PSD minimum voltage: 0.77 mV

P-pol PSD maximum voltage: 349 mV
P-pol PSD minimum voltage: 0.2 mV

That is comparable to what we got before.

(Note, that these number might be imrovable as we didn't spent so much time in adjusting the PBS position.)

We started a map of the Shinkosha TAMA#2 sample during the night. The results can be seen from the attached figures.
The polarization is relatively homogeneous. However, there are two main areas with different polarization-angles on two sides which leaves the center of the sample with the largest gradient in the polarization angle.
It is noteworthy that the distribution of polarization angles remembers a little bit on the 3-fold splitting visible in the absorption maps (see the elog entry 1633).

[Takahashi, Yuefan, Yuhang, Eleonora]

This morning with the help of Takahashi-san we closed PR chamber and restarted the vacuum. We will wait the vacuum to reach 10-6 torr (at BS vacuum monitor) to open the main gatevalve between BS and INPUT.

I upload some pictures taken to PR chamber. 

Matteo, Simon

Attached to this report are the maps and distribution histograms of the absorption coefficient from the Shinkosha TAMA-sized TM #2.
As can be seen, we have now 7 different XY maps throughout the entire bulk. The YZ-map has a larger expansion in Y.

Although inhomogeneously distributed, the absorption is relatively low (~ 30 ppm/cm in the mean with some peaks exceeding 50 ppm/cm).
Please note that the colorscale is the same for all maps!

Yuefan and Yuhang

This morning we installed everything for the AA telescope before opening the chamber.

The final beam height is 225mm from the bench. All the optics position are only followed the design, we didn't have time to connect all the cables to check any signal. The second lens is installed on the rail, galvo and quadrants are all installed on the height changable posts.

The FF path is well aligned, the beam is hitting in the center of the quadrant. But the NF path, we still need to adjust a bit the mirrors of the galvo, to make the output beam on the same height and without any angle. It was not very easy to do because the position of the galvo is almost in the center of the bench.

The detail pictures are in the attachment. I found it was difficult to take a picture that shows all the components....I should also have taken some pictures from the other side of the bench, I will put them later...