We lock FC with green (and dithering alignment on pitch), set BAB on resonance and monitored both IR and transmitted power. See attached plot.

IR transmission starts to decrease after ~15 min.

[Aritomi, Eleonora, Yuhang]

First we measured locking accuracy as shown in FIG. 1. Locking accuracy is 8 Hz which corresponds to 8.8 pm.

Then we changed green pump power from 50mW to 40mW so that we can reproduce FDS measurement before. OPO temperature and p pol PLL is as follows. 

Then we measured FDS around 400 Hz and 75 Hz detuning (FIG. 2 and FIG. 3). The parameters are as follows. Degradation budget is shown in FIG. 4.

It seems that locking accuracy is limiting squeezing level at low frequency.

sqz_dB = 16;                    % produced SQZ

L_rt = 150e-6;                    % FC losses

L_inj = 0.20;                     % Injection losses

L_ro = 0.11;                      % Readout losses

A0 = 0.05;                         % Squeezed field/filter cavity mode mismatch losses

C0 = 0.05;                         % Squeezed field/local oscillator mode mismatch losses

ERR_L =   8.8e-12;                  % Lock accuracy [m]

ERR_csi = 80e-3;                  % Phase noise[rad]

phi_Hom = [-4/180*pi, -27/180*pi];       % Homodyne angle [rad] 

det =  [423 446];                       % detuning frequency [Hz]

sqz_dB = 16;                    % produced SQZ

L_rt = 150e-6;                    % FC losses

L_inj = 0.20;                     % Injection losses

L_ro = 0.11;                      % Readout losses

A0 = 0.05;                         % Squeezed field/filter cavity mode mismatch losses

C0 = 0.05;                         % Squeezed field/local oscillator mode mismatch losses

ERR_L =   8.8e-12;                  % Lock accuracy [m]

ERR_csi = 80e-3;                  % Phase noise[rad]

phi_Hom = 90/180*pi;       % Homodyne angle [rad] 

det =  75;                       % detuning frequency [Hz]

Pengbo, Simon

Today we were struggeling hard to align the beam in the sample-area horizontal to the optical table while crossing the probe-beam.
After some hours of work fine-tuning the lenses and steering-mirror, and constant measurements with the blade, we could achieve a good alignment in the end.

Then, we started to check the power-adjustment with the motorized HWP by turning it for 45 degrees. However, we recognized that there is no beam anymore coming out of the FI, from any of its openings.... So, something must have been wrong.
Therefore, we checked the beam-path to the FI again with much lower intensity and saw that the FI became misaligned somehow.
We think that our initial strategy by using adjustable posts may not be enough for the FI...

As a strategy, I will try to find a suitable micrometer-stage for the FI tomorrow and rebuild it.

[Yuhang, Aritomi, Eleonora]

We measured FC round trip losses with the usual lock-unlock technique. (See pic 1). We found about 150 ppm. The associated expected sqz degradation in plotted in fig 2.

Below I reported the computation. The formulas used are from this paper.

P_res = 425;

P_in = 640;

R_gamma = P_res/P_in;

gamma = 0.06; 

R = (R_gamma-gamma)/(1-gamma);

T = 0.00136;

L = (T/2)* (1-R)/(1+R) = 148 e-6

I adjusted the beam size at AOMs by changing the positions of lenses.
I aligned one of the AOMs and I could get about 70 % diffaction efficiency with double-pass.
Then I put a BS for ISS.

The next step is align another AOM and install a BS.

with help of Simon

We injected Helium gas into the cryostat chamber.
Current temperature is about 200 K.
It is rising now.

The relation between the CC2 demodulation phase and the measured squeezing phase can be found in this document, uploaded on the Virgo logbook. More details can be found in the entry #44285 of the Virgo logbook.

Simon, Pengbo

Yesterday we maesured the beam profile again. We moved the first lens for 2.5 cm, make it closer to the FI. As can be see from the attachment, the result we get is 0.042 mm, which is better than before.

As I replaced some mirror mounts yesterday, the beam alignment needs to be adjusted.
In addition, the beam profile was also changed and the beam size at AOMs are relatively larger, about 800 um.
Therefore, I tried to minimize the beam size at AOMs with changing the lenses and their positions.

I could somehow adjust the beam size about 500 um at AOMs.

The next step is adjust the alignment in order to inject beams to AOMs.
Then play with STMs to maximize the diffracted beam power.

Eleonora and Yuhang

We tried to tilt lens by more than 10 degrees to avoid back scattering (see attached figure 1). Beside, we tried to put beam dump (see attached figure 2) or iris(SM1D12 with thread and can be mounted to the lenses before homodyne) to block the scattered light.

The comparison of shot noise with homodyne open to filter cavity(but only vacuum come from FC) is shown in the attached figure 3. Although we could see there are no obvious difference between use beam dump (iris) or not, the level of back scattering around 10Hz is less than the shot_FC measurement we did one month before. This means anyway, the tilt of lens helps to reduce back scattering.

However, the coating of lens is not optimal for non-normal incidence. So we tried to measure squeezing again. And the result is attahed as figure 4. We could see we have only 5.63dB squeezing. This means we have 3% more loss caused by the tilt of lens.

Anyway, we measured also the FIS from FC, the result is attached in figure 5. Although we have only 2.17dB squeezing from FC, but the low frequency squeezing is much better now.

Because of my stupid work, the optics located very upper stream part of the system were not tightly clamped.
So I decided to modify them and did re-alignment work.

It is still ongoing and the next step is the alignment of double-pass AOMs in HOMs.

I replaced 2 mirrors to BSs which are used for pick off the beam power in order to intensity stabilization.
In addition, I installed a HWP for adjust the polarization of the beam because the polarization is changed by double-pass AOM configuration.
I checked the transmitted beam power with 3.2 mW input.
The result was 0.5 mW and this value was consistent with the spec.
At that moment the angle of HWP was 300 deg.

[Shoda, Eleonora]

Yesterday we found that dataviewer and diaggui were not working properly. In particular, diaggui was showing the error: "Unable to obtain measurement data"

With the help of Shoda-san, we ssh into the standalone, check the disk status and discovered that the disk for data storage was full.

We deleted some of the old data stretch and the system started to work again.

We should delete the data stored in /frames/trend/second

Here some useful commands:

df                                                # to check the disk status

ls -l                                             # to check the data folder with information on dates when they were acquired

du -h --max-depth=1              # to check files size

rm -r (number of the folder)       # to remove the data stretch in the selected folder

Command example:

ssh standalone

cd /frames/trend/second

ls -l (or ls -la to show hidden files)

rm -r (number of the folder)

Simon, Pengbo

Last friday and today, we measured the beam profile after install the FI.

First time we didn't change the distance between the telosocpe, the result is: w = 0.0582+/- 0.0009mm, z_position = 56.54+/- 0.34 mm. ( Figure 1.)

Then we adjust the teloscope distance from 22cm to 23 cm by moving the first lens, the result is : 0.0512 +/-0.0012 mm, z_position = 30.45 +/- 0.34 mm.

the waist position is closer to the optical bench than before but the waist radius is just a little bit smaller than before.(Figure 2)

I successfully glued another fused silica mirror.
Now that I have a set of fused silica mirrors and I will start installation of folded cavity.

Yesterday I tried to glue a fused silica mirror to a mirror holder using a jig as show in attached picture.
I used DP-190 for gluing which was borrowed from Tomaru-san.

This morning I checked the situation of the mirror and it seemed glued to some extent which can be used for foled optical cavity.
The AR side was enough clean.
I will glue another mirror.

[note]
The procedures are as follows:

1. Apply First Contact on HR surface for protection.
2. Set a mirror holder and a jig.
3. Apply glue on the  mirror holder.
4. Attach the mirror to the mirror holder.

Eleonora and Yuhang

As we tested on Thursday, CVI mirror is ideal for our requirement. So we decide to use CVI BS. Although it takes us a lot of time to find a good position for it. We succed in balancing homodyne in the end and to find a good compromise between the position of a lot of optical components. The change of optical components is summarized in the last attached picture(the latest change is updated to naoj_wiki).

We verified that no backreflected beams could be found with the new BS. 

After balancing homodyne, we measurde back scattering and squeezing again. The comparison is between Friday and Thursday result (attached figure 1, 2 and 3).

Note that the back-scattering is measured by placing a sensor card on the squeezing path. Since the peaks observed at low frequency are at the bench resonant frequencies, we assume that the sensor card (which is vibrating with the bench), is reflecting back the scattered light towords homodyne. If we put a beam dump instead of the sensor card the peak at low frequency disapperad.

In attached figure 1, we can see the back scattering is even worse. Based on the fact that this back scattering is not stable, we think that the change of BS doesn't improve the situation.

In attached figure 2, we can see the squeezing level is improved from 5.90dB to 6.06dB. This means CVI BS really has less loss. From the loss and phase noise information measured in entry 1916, we can infer we reduce loss by ~1% by replacing BS.

In the attached figure 3, we can see there are some new peaks appear around 3kHz to 10kHz. These peaks was there several months ago. They were removed after the use of Faraday. We will investigate why they show up again.

Conclusion: the back scattering peaks at low frequency observed when we place a sensor card on the sqz path are still present even after replacing BS with a better one, with no detectable back reflection, and even after replacing homodyne lenses with superpolished ones. The source of back scattering was not the cube BS. So, what can it be? Homodyne?

Next steps: Put beam dumps around Homodyne. Check the situation with the filter cavity aligned.

After 2.5 days cooling, the cryostat reached the temperature ~60 K.
This value is my guess because the sensor had some problems and it showed higher temperature than actual.

Anyway, the refrigerator worked well though longer cooling time than before.
This is due to some insulators were not installed inside the chamber...
But it can be cooled down at some extent and I gonna do with this configuration.

I turned off the refrigerator around 20:20.

[Yuhang, Eleonora]

Before Christmas we decide to replace the cube BS before homodyme (where SQZ and LO are recombined) with a plate BS in order to reduce back-reflection as much as possible.

The BS ratio must be as much as possible close to 50/50 to get optimal homodyne balance. We purchased 5 plated BS and characterized them with Simon's set up (entry #1931). We use s-pol.

Here the results:

We note that the power mesured by the power meter is quite sensitive to the incidence angle. So, we adjusted power meter orientation every time to optimize it. Newport BS has the most balanced splitting ratio at 45 deg. and it is very much insensitive to the angle. Anyway CVI has no ghost beam, and the mirror quality should be better so we decided to go on with CVI BS and adjust the angle to get optimal balance.