The measurement is done by measure in DGS with 50 Ohm inserted into the channel.

The unit of measurement is counts, so we need to do calibration. The calbration factor is 0.61V for 1000 counts (see entry #1315). So we have

Noise_ADC = ~ 3e-6  [V/sqrt(Hz)] 

or alternatively

Noise_ADC  = 20*log(3e-6) = -110 dBVrms

This is almost 40dB higher than the noise of Network analizer we always use. So if we use DGS to record data, we will use at least 100 amplification given by pre-amplifier (SR560).

[Aritomi, Eleonora, Matteo]

To reduce LO backscattering, we reduced LO power by a factor of 10. We also increased CC2 gain from 3 to 10.

Attached picture is FDS when CC2 demodulation phase is 20deg and 40deg. There is still large bump below 60Hz.

We thought the bump is changing when CC2 demodulation phase is changed, but it seems that bump itself doesn't change with CC2 demodulation phase.

[Aritomi, Eleonora, Matteo]

To reduce the effect of locking accuracy, we reduced green power from 40mW to 25mW.

OPO temperature: 7.172 kOhm, p pol PLL: 200 MHz

Then we measured FDS. CC2 demodulation phase is 20deg for anti squeezing and 130deg for squeezing.

Simon Pengbo

Today, we continue the measurement of the beam waist. First we changed the position of the first lens, putting it bewteen the FI and the PBS, and make it adjustable.

By changing the position of the first lens, we did two x axis scans. It seems that as the first lens moves away from the FI, the waist become lager and closer to laser bench direction. 

We will do more tests next week.

I re-aligned another double-pass AOM.
The total diffraction efficiency is about 70% which is reasonable value.

Then I modified the position of BS for ISS.
The next step is align the HOM's beams.

In addition, I ordered pillars for composing stage inside the cryostat chamber.
The drawing will be uploaded on wiki.

Given the number Aritomi-san used (which are maybe too optimistic about MM), if we reduce the amount of initial squeezing  from 16 dB to 6 dB. The effect of lock accuracy is much reduced and in priciple we should be able to go below shot noise.

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.