[Eleonora P, Yuhang]

We replaced the FI's stage along the green path, after a hole has been drilled on it as explained in entry #1367. 

In fig.1 the picture of the stage on the bench.

I installed one silicon mirror, which was labeled "No.1", inside the cryostat and did rough alignment.
Then I measured beam profile for mode matching.

Today, I re-installed some optics for higher-order-modes.
I wanted to put a iris for the mark of alignment, but I could not find it...
From tomorrow, I'm going to install silicon cavity inside the cryostat.

I changed optical layout to achieve better alignment.
I will update the schematic figure of optical layout later...
Anyway, the alignment seems ok so far.
 

As shown in entry #1363, the error on the beam waist position is large ( around 8%).

I checked if the solution #3 of entry #1366 would be good also considering the border positions of beam waist in the error interval, running again the simulations (fig 1, 2, 3) and analyzing the robustness on Python.

Both focal lengths are 750 mm.

I summarize the results in the following table:

where the positions of the lenses is referred to the PBS after the OPO.

I computed on Python the position of the two lenses corresponding to the condition of minimum total mismatch.

In fig. 4, 5 and 6 are shown the robustness of the telescope with z=0.081 (min position), 0.089 (real position), 0.097 (max position), that are still pretty good (less than 10% mismatch).

[Yuhang, Chien-Ming]

Today we tried to decrease CC laser power. But when we just decreased it to half, we couldn't lock the CC1 loop. 

The cc error signals that we can use to lock and cannot be used to lock are attached in Fig1.

The RMS dark noise for CC1 and CC2 PD is measured and attached in Fig2. We can see that the CC1 PD has RMS dark noise almost 5 times higher than CC2 PD. 

We also suspected that this noisy PD for CC1 locking brings also phase noise.

I installed a FI for TEM00 PDH locking, and confirmed that one can pick off reflected beam with simple setup (QWP and mirror).
The attached picture shows the reflected beam picked up by FI.

Then, I continued the installation of optics.
However, I found that the height of FI was not good, which is not the one I mentioned but already installed one before.
So I modified the alignment.
Actually, the beam height is still higher than the target height, though I cannot adjust anymore.

So I am planning to install STMs in front of the EOM, which can adjust the beam height.

[Yuhang, Chien-Ming, EleonoraP]

After the improvement of homodyne suggested by Chien-Ming, we measured squeezing again.

This time, we could see only two peaks for homodyne shot noise limited down to 10Hz when LO incident. The 34Hz peak is related to some unknown device(maybe air-conditioner). The characterization of this 34Hz peak is done by Federico and Irene. While another peak is a 50Hz power line. (We didn't perform a low-frequency high-resolution measurement, we may see some peaks at low frequency in that case. We will do it soon.)

For SQZ, we could see the noise floor(5.5dB SQZ) down to 10Hz with several narrow peaks. But notice that now there is still 2.4mV DC signal(for SQZ path) not balanced on homodyne. We could possibly improve CMRR for SQZ path soon.

For anti-SQZ, we could see 13.25dB. And it is also very flat.

The result is shown in the attached figure. We didn't pay attention to the measurement bandwidth of anti-SQZ and LO-shot noise. But we may do it again after the refinement of BAB balance.

The local controls for the input mirror have been implemented in the new DGS.

As usual,  we amplified and filtered the PSD signals for pitch and yaw with a SR560 (2nd order lowpass, cutoff 100 Hz, gain 100)

TF for yaw and pitch are reported in pic 1 and 2. (injected noise: 5000 and 7000 counts respectively)

As for PR and BS the pitch TF and coherence is not very good.

Comparison between open and closed loop spectra are shown in pic 3 (yaw) and 4 (pitch)

I attach the filters values (model and damp for both yaw and pitch):

Fig 5  yaw model

Fig 6  yaw damp

Fig 7 pitch model

Fig 8 pitch damp

I will compute the calibration soon.

Chien-Ming, Yu-Hang, and Aritomi

Yesterday we adjusted the BS horizontal angle to make the LO beam reach the DC balance at homodyne detector. We measured the CMRR, it is better than 80 dB at 1 kHz.

However, in this case, if we let that BAB and LO have a good overlapping, we need to change the optical set up of BAB a lot.

So today we keep the BAB beam in its original condition and adjust the BS angle to make the BAB beam reach the DC balance at homodyne. Then we adjust the LO beam to achieve a good overlapping with BAB beam. In this case, when the BAB is DC balanced, the homodyne DC offset of the LO shows -10mV.

We repeatedly adjust the BS angle and the alignment of both beams a little bit, finally, we make the LO beam become DC balanced and the DC Offset of BAB is 2.4 mV.

Takahashi-san, Sato-san, Tanioka

We removed the 4K shield of cryostat in ATC in order to ship it to KEK.
Some insulator are detached from 4K shield to remove screws.

[Yuhang and EleonoraP]

First we aligned the green beam before PR chamber.

Then we changed the stage of Faraday Isolator with a smaller one, in order to make space for the wind shields. We put some schotch to cover the borders of the FI, to avoid damages (fig 1).

We found a problem: not enough space for the mirror under the FI (fig 2). The black plastic edge of the mirror mount is 3.4mm while the beam reflected by the FI crystal is 5 mm far from the stage edge (fig 3).

Possible solutions:

1. make a hole in the mount in order to make space for the mirror. Size of the hole: 30 mm (diameter) and 12 mm (depth) in fig 4.

2. build a different stage with other components, in fig 5.

I found two good options using the lenses we already have in the lab.

Version 2:

1000 mm @ z = 0.517 m (distance from PBS)

750 mm @ z = 0.618 m

Version 3:

750 mm @ z = 0.42 m

750 mm @ z = 0.673 m

As we can see in fig 2 and fig 4, the robustness is not as good as the one of entry #1365 (fig 5), but we are still under 10% of mismatch.

In fig 6 there is the scheme on the bench of version 2 (PURPLE) and version 3 (BLUE).

After the improvement of homodyne matching entry #1354, the parameters of the beam entering the injecion telescope have changed (results of measurement in entry #1363).

Parameters changes: 

w0

126um --> 94um

z0 (with respect to PBS)

15 cm --> 8.9 cm

Focal lenghts needed for the new injection telescope are:

600 mm (instead of -1000mm)

750 mm (instead of 500mm)

Robustness is still really good (fig 2).

As I mentioned in the entry, I moved the first lens after OPO. So the beam parameter going to filter cavity is changed. To have a better design for the telescope matching OPO transmission to filter cavity. I made the beam measurement again.

The way that I did measurement is illustrated in the attached figure 1. 

By measuring seven points along a long distance, I fit the beam. The result is shown in the attached figure 2.

The beam waist is even smaller than the last measurement.

I also matched BAB into OPO. The spectrum is shown in the attached figure.

The matching of main laser optical fiber is also changed after the cleaning of some mirrors.

The good thing is we see the main laser is very stable after the installation of this second FI. The situation is definitely much better than the situation before.

Chien-Ming, Yu-Hang

We installed the new Faraday Isolator (FI) on the main laser beam. The optical intensity transmission efficiency we measured was >98% although the polarizer at the entrance of this FI was accidentally scratched yesterday.

The mode matching statuses before and after inserting this new FI are shown in the attached figure. It got a little worse after inserting FI.

Now the SHG can provide 218mW when input 580mW IR. We cleaned some optical component, then we can get more IR power (up to 880 mW) to the SHG if needed.

Other mode matching status are also shown.

Since I have been changing name of the channels and modified the RTmodel quit a lot I put here the screenshots of the filter/model current configuration of photon for BS and PR, to recover it in case it is lost.

For each d.o.f of the two suspensions there is the model and the contoller (called damp).  The d.o.f and suspension is indentifed by the "module"  field and the ZPK configuration is relative the filter selected in the "section" field.

Fig1: BS PITCH  DAMP

Fig2: BS PITCH  MODEL

Fig3: BS YAW  DAMP

Fig4: BS YAW  MODEL

Fig5: PR PITCH  DAMP

Fig6: PR PITCH  MODEL

Fig7: PR YAW  DAMP

Fig8: PR YAW  MODEL

There are two minor issues I couldn't solve so far:

1) Despite many suggestion from Oshino-san, I could not make work the medm screen botton which saves and restores the EPICS channels values (so called screenshot). It seems there is no error in the coding but it doesn't work. So I have done two simple scripts (located in /home/controls) to save and restore the snapshot from terminal. The commands are respectively

./takesnap.sh  and ./restoresnap.sh

2) I also tried to implement the "wave rotator" function to rotate the oplev signal in order to compensate a possible inclination of the PSD but I couldn't find the name of the "angle channel" to be written in the medm. I asked the help of Shoda-san and we tried to implement the function on a test model in the ATC DGS but we failed as well. We could find some EPICS channels created into the fuction on the dataviewer but when we tried to read or write them on the terminal (commands caget, caput) it said they do not exist. They cannot be shown in a "text monitor object" either. The workaround for the moment is to use a matrix and put in the already computed cos/sin values corresponding to the desired rotation.