Yuhang and Yuefan

While designing the telescope of the automatic alignment, we had one question is that how close we could  put the galvo from the quadrant to have decent range. To answer this question, we should at least have the idea how much the reflection beam could move.

So in order to do this, we tried to do a long term monitor of the DC signal of the quadrant without closing the galvo loop.

The problem is that the filter cavity could not stay lock during the time we record the DC signal, so we only could have data of less than one hour.

• In the first try(fig7), during around 1h of locking, the recombined DC pitch signal has a maximum movement of 0.1, first we tried if we manually moved the beam in pitch to have 0.1 movement, the galvo was totally able to bring the beam back to the center.(fig6) But then we thought maybe it is better to do a rough calibaration to have an idea about how much the beam moved on the quadrant, with the chaning we saw on the dataviewer.

1. Record the DC pitch and yaw signal with Dataviewer (fig 1), we could see that for pitch and yaw, the reading are both around 0.2.
2. For the pitch, we record the beam height at that moment by put a ruler in front of the quadrant. The position of the ruler also recorded. (fig 2)
3. Move the screw of the galvo to move the beam around 1mm in pitch. Actually the movement done by the screw has a coupling between pitch and yaw, but because we are only checking pitch this time, we didn't care about the yaw value changing.
4. Checking the difference in the signal in Dataviewer again. (fig 3) Pitch moved to 0.1
5. Starting from the position of last step, where yaw reading is -0.14, Repeat the steps above on yaw (fig 4), then it changes -0.22. (fig 5)

         So the calibration results are like below,        

• For the second(fig8) and third try(fig9), we compared the DC signals with the cavity green power tranmission. There were several short time unlock and relock, so we could see that everytime the cavity relock, it seems lock again at different position. And while the alignment of the cavity is drifting away, the tranmission power is lower and lower, and finally cavity unlock, we could see the DC signals also goes to zero slowly, from this two fact we had some guesses.

1. Since everytime we tried to overlap the input and reflection green by only checking the viewport, the beam may not always go to the same direction, which results in the reflection beam height changing we obsered yesterday and also in the past weeks. But of course closing the galvo loop could make the situation better, but the galvo mirror is quite small, if the beam pointing change is too much, the beam could totally miss the galvo. So probably we should do another long term monitor with galvo loop close.
2. What we expected the DC signal should be like when the cavity slowly misalign is that in one or both direction the signal should firstly increase and then a sudden drop to zero when the beam is totally off the quadrant. But what we saw is quite different. After discussing with Matteo and Eleonora, we found out this could  be caused by the fact we didn't normalise the pitch and yaw signal with the total power, so the slowly drop we saw is because the power reduction when the beam is moving out of the quadrant.

Simon

I finished the spectroscopic measurements of the colored Sapphire samples from 1600 to 300 nm wavelengths (respective figures are attached).

I measured the transmittance (T) and the reflectance (R) for both blue and green sapphire samples, and calculated the absorbance by 1-R-T = A.
In short, the green samples have the lowest absorbance at 1064 nm wavelength (A = 0.05), while the absorbance of the blue ones ranges from 0.143 to 0.158.

This is the scheme of the AA telescope now

According to the measurement yesterday, we made the new design for the telescope, this time seems we could use one telescope to get gouy phase around 90 degree difference in two axis of the beam.

Firstly, we calculated the reflection beam waist position according to the measuremnt. (colors refer to different curves in last entry)

The size of the waist in two direction seems much closer than all the measurement we did before.

Then because the space limitation, we fixed the two quadrant position as before, which is around 1.9m and 2.3m from the FI, and both the positions have some margin of 5cm.

The beam we used to first determine the lens is the orange one, because it has smaller divergence than the other one. So with the orange as the initial beam, we tried to put different focal length, and move the lens position to have the beam waist around 2.1m which is in the middle of the two quadrants. By checking the gouy phase difference, we could know that we need longer focal length or shorter.

The final results we got is to put a lens with focal length of 1m, and 0.82m from the FI. 

Then the quadrants information are as below

Then use the distance to calculate the gouy phase of the other beam axis (blue). By slightly adjusting the lens position and the quadrants position, we could get the gouy phase difference around 90 degree for both axises. In this case the lens position is 0.75m.

Yuefan and Yuhang

We did beam height measurement on the bench and the result is reported here. However, we could see that the beam is still not flat at the height of 76mm. So we adjusted beam height again. After the adjustment, the beam height after AOM and until the last steering mirror is maintained at the level of 76mm.

After this activity, we aligned the filter cavity again. And the reflected green beam seems to be better. We think at this point, we should turn the work to design and implement the final telescope for AA telescope. We also measured the reflected beam. The result is attached and Yuefan will use this result to design the new AA telescope.

Note that if reflectivity of HR coating of PPKTP is 99.995%, we can explain 10% loss from OPO in loss and phase noise measurement.

I discussed with Shoda-san and we looked better into the issue about python scritp launching from MEDM (see entry #1651). We couldn' t solve the problem.

So I decided that the best solution is to use only a bash script, as for the moment we simply need to change loops gains.

I prepared two bash scripts to open and close the loops repectively ( open_loops.sh, close_loops.sh), just using "caput" command. The scripts are located in /home/controls/FDSscripts.

I modified the shell command MEDM to launch them and now we can close and open the loops by pressing the button, even when we open MEDM from desktop icon.

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.