I found that the DP (DSP250) back of the TMP was failed with "ALM06" in the south end. Though I tried to restart it some times, it was not rcovered.

I replaced the DP to the other DP (DSP500) from arm front where the TMP is not working now.

[Yuhang, Eleonora]

Filter cavity lock was recovered easily.

Then we checked green alignment into OPO: we sent BAB and maximized its alignement into OPO by moving the lens in pic 1. We scanned OPO and measure the BAB transmitted TEM00 with and without green pump. The green phase was modulated at 10 Hz. We optimized the alignment of the green beam into OPO by maximizing the BAB transmission. The ratio between BAB transmission with green (pic2) and without green (pic 3) gives the parametric gain which is ~ 25.4 for 50mW of pump. It seems fine. Note that recently we observed a reduced parametric gain but this measurement is in agreemen with the early value (see entry #1131).

We changed OPO temperature from 7.185 to 7.215 kOhm to maximize the non-linear gain, (with the PLL offset without green set to 180 MHz.)

We check the p-pol PLL offset when green is injected into OPO and optimized it by maximizing CC1 error signal. Now the good values for PLL offset are:

We found that p-pol was not well aligned into OPO, and the alignment was very unsatable and difficult to optimize. Eventually we discovered the p-pol laser temperature was 2 degree away from the optimal temperature. So the issue was likely due to mode-hop. We put it back at the nominal value and we confirmed that all the lasers are at the optimal temperature, namely:

Then we aligned p-pol and CC into OPO. Spectrum is shown in Pic 4 (blue is p-pol, yellow is CC).

Finally, we measured again squeezing with 50mW green pump. The measured squeezing level is 5.9dB which is consistent with the measurement done before. Besides, the spectrum is as flat as before.

I installed the inner shields, forgot to take pictures though...
When the vacuum level gets enough low, I gonna turn on the cryostat.

Today, I did vacuum test with the repaired scroll pump and the turbo pump.
Since the chamber had been opend for a long time, the vacuum level was not good though there were no huge leaks.
In addition, one of the vacuum gauge did not work.

I will do another vacuum test, and then do cryogenic test.

Pengbo, Simon

Yesterday, we have finally set the FI in place and tried to get the laser beam through it.
We recognized that a proper adjustment stage would be desirable as it decreases the work. However, since we couldn't find a suitable one (or one which isn't used rigth now), we installed the FI-cage on top of two scalable posts and tried or luck with them. In the end, I think we could reach a good position and ca. half of the ligth passes toward the sample chamber (which is as expected since we still don't have the second HWP in front of the second PBS).
Also, in order to have a free space for the ejecting non-used beams, we removed one of the 4 bars of the cage-system for the FI. We think that this is not harming the overall stability of the system since it makes a very sophistictated impression anyway.

Judging from the laser card, the beam shape seems to be fine but we need to investigate it more deeper with the beam-profiler.

After setting the FI, we placed some beam-dumps in those positions were a non-used beam is ejected sideways from the FI. Unfortunately, these beams are under a non-rectangular angle and thus a damping is a bit tricky. We, however, used an elongated post to fix a holder where the angular position can be freely adjusted (see also attached pictures) for the beam that goes also upwards. For the other, we are utilizing a new beam-dump with a large opening so that also the transmitted light from the coated 45 deg mirror can be covered.

Pengbo, Simon

Today, the upgrading process was continued by setting everything except the FI in its determined position, according to the draft which is attached to this report.
As for a matter of understanding, this draft is a supposed to be a first trial of the basic working principle which we are aiming for.

By re-activating the laser, I recognized that if the first PBS is set plane in its position as indicated in the draft, only a few percent of power would be P-polarized. Therefore, I rotated the PBS so that the non-used beam would be weakest. With this, I planted a beam-dump on top of the PBS cube (thanks to Thorlabs cage system) to catch that beam. In my opinion, even with this a little bit strange setup, security is maintained. In a next step, the motorized HWP was tested with the laser and confirmed to be working fine.

Addendum: I set also a beam-dump on the second PBS to cover the non-used S-polarization.

Simon, Pengbo

Today we reset the translation stage make sure it can work properly and then start the upgrade of the laser control part.

I meaured the laser intensity noise with free-running and estimated the requirement for ISS.
The RIN with free-run laser was the order of 10^-6.

Assuming DC lock of HOMs and common mode rejection ratio as 1/10, the requirement for RIN is estimated as 3*10^-8.

Therefore, about 100-times stabilization is necessary in order to achieve the requirement.
I will design the intensity stabilization servo which satisfies this requirement.

First I optimized p pol PLL frequency when green power is 50mW and optimal frequency is 150MHz.

Today when filter cavity is locked, a strong 70kHz noise appears in CC1 error signal. This makes it difficult to get clean CCFC error signal.

Anyway I measured CCFC error signal again when CC is locked on resonance (Pic. 1).

I found that when demodulation phase of CCFC changed, CCFC error signal also changed from Pic. 2 to Pic. 3.

113 Hz is the frequency of the line sent to the rampeauto perturb channel to test LO alignemnt with the technique proposed by Raffaele. I guess we forgot to switch it off last friday. 

I connected remotely and switched it off now. Sorry for the inconvenience!

First I increased green power from 40mW to 50mW. Carrier and CC resonance frequency is as follows. CC PLL frequency is same as last week.

CCFC error signal when AOM is scanned around CC resonance is shown in Pic. 1 channel 2. It is different from the error signal last week and seems very noisy. I also measured spectrum of CCFC error signal when CC is locked on resonance (Pic. 2). There is a very strong 113Hz peak in CCFC error signal.

[Aritomi, Yuhang, Eleonora]

To make one of CCSB on resonance inside filter cavity, we set CC PLL frequency 6.997043MHz following previous measurement. We also changed 14MHz and 21MHz accordingly in DDS board.

Binary number is as follows.

Then we split CC1 LO (14MHz) into 50:50 with Z99SC-62-S+ to use it for demodulation of CCFC, but CC1 was unstable due to low SNR. Therefore we amplified CC1 LO by 33dB and attenuated by 20dB before splitting. Since output of DDS board is -6.5dBm, CC1 and CCFC LO is -6.5+33-20-3 = 3.5dBm. Then CC1 locked stably.

We set CC PLL 7MHz as usual and scanned AOM around one of CC resonance while other CC sideband was off resonance. CCFC error signal was shown in Pic. 1 (Channel 1: IR transmission, Channel 2: CCFC error signal). It seems like usual PDH signal. Then we set CC PLL 6.99704303MHz and found CC resonance at 109.03598MHz while carrier is 109.03584MHz which means carrier is detuned by 109.03598-109.03584 = 140Hz. CCFC error signal was shown in Pic. 2. The error signal was just a dip and didn't change by demodulation phase.

On Monday, I found that the cavity remote locking was not working properly. Matteo told me that somehow the treshold for lock acquisition was changed last week.

I put it back to the original value of -0.5 V and now it seems to work quite good.

I also found the the transmission was not stable despite the dithering was engaged. The situation improveed after I increased the gain. (Pic 1) 

Current values:

INPUT ATTENUATION: 2.9

GAIN: 5.7

[Aritomi, Yuhang, Eleonora]

First we measured shot noise spectrum with sensor card in squeezing path to see LO back scattering. Then we reduced LO power from 1.86mW to 0.158mW by putting ND filter after IRMC. Pic. 1 shows shot noise with different LO power. Shot noise reduced by 10.5dB and 13Hz peak from LO back scattering reduced by 21dB as expected.

Then we measured shot noise with filter cavity. At the beginning we saw low frequency bump in shot noise spectrum, but after some alignment of homodyne, the low frequency bump somehow reduced a lot. Pic. 2 shows shot noise with filter cavity.

[Aritomi, Yuhang, Eleonora]

sqz_dB = 11.5;                    % produced SQZ

L_rt = 100e-6;                    % FC losses

L_inj = 0.20;                     % Injection losses

L_ro = 0.11;                      % Readout losses

A0 = 0.1;                         % Squeezed field/filter cavity mode mismatch losses

C0 = 0.1;                         % Squeezed field/local oscillator mode mismatch losses

ERR_L =   5e-12;                  % Lock accuracy [m]

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

phi_Hom = [0/180*pi, pi/2*90/90];      % Homodyne angle [rad] (you can input a vector of values)

det =  [100 60];                       % detuning frequency [Hz]

[Sato-san, Tanioka]

We installed a 4K shield which was modified by Namai-san and Ueda-san.
The procedures are as following.

1. Attached the cables on the shield as heat anchors
   At this moment, we found that the cable which was used for thermometer was disconnected. This was solved by bypassing the cable. But the displayed temperature seemed not correct. Calibration is needed.
2. Installed insulator on the shield.
3. Made holes on the insulators in order to pass through the beam.
4. Put the shield on the optical breadboard.
5. Check the cabling and the status of insulators.
6. Then clamped to the table.

[note]

• We found that we forgot to install the insulator under the optical table...
  I hope that the impact will be small enough.
• I forgot to prepare appropriate screws and washers.
  The ones installed temporary should be replaced.

[next step]

• Replace screws.
• Calibrate thermometer.
• Vacuum test
• Cooling test

Aritomi, Eleonora, and Yuhang

Last Friday, we had an issue with homodyne alignment. We found the frequency of peaks in shot noise is exactly the frequency of bench EW direction motion. At some point, we were thinking this is because of a bad way of aligning homodyne, but this seems not to be the case in the end. We figured out it should come from leaked LO backscattering.

The leaked LO mainly comes from the AR coating of cubic BS, then it goes to a sensor card in the squeezing path and the sensor card causes backscattering. About the leaked LO from homodyne, we measured power as following

And the sensor card is shown in the attached figure 1, which has a plastic shining surface. It will reflect a little bit scatter light which contaminates the shot noise spectrum. Besides, this sensor card is put in the position which makes it easy to sense the motion of bench in EW direction. So we got the spectrum as in the attached figure2. (In this case, we are using our homodyne as a very sensitive sensor of reflective surface motion)

We tried to characterize the effect by putting it farther and farther, but the contamination doesn't change. Until we put it after a Faraday isolator, which means we have an FI to isolate this backscattered light. We got a reduction of peaks by 15dB. See attached figure 3. But the backscattering noise didn't disappear.

But we need to notice that if the reflected surface doesn't move, we will not have this backscattering noise. For example, when we don't put anything in-between OPO and homodyne, although there should be light back-reflected from OPO to homodyne, we don't have backscattering noise.

Then we tried to send this leaked LO to filter cavity, we got the homodyne shot noise spectrum as shown in the attached figure 4. This measurement suggests that our low-frequency bump in FDS is from this backscattering noise. Also, the phase noise of squeezing will not introduce the increase of floor by almost 30dB because the anti-squeezing is only ~15dB.

Additional: we measured the backscattering noise from the sensor card when turbopump is on and off. We could see the difference from attached figure 5, which means it really introduces more bench motion. 

The scripts to open and close dithering loop has been changed from bash to python. This allowed to change the state of the loop input swiitch easily. This should make the loop engagment smoother in the case of integrators ( as we have for dithering) 

According to the investigations done by Shoda-san and Yuhang last week, it seems that the excess of noise appearing sometimes in the BS oplev is due to the PSD itself.

Today I have replaced it with a simiar one, with the same gain. The spectra of pitch and yaw are exactly the same of the old one (when no noise was present) and for the moment no excess of noise has been observed with the new PSD. I could close the damping loop without change any gain.  I will keep monitoring the spectra in the next days.

According to elog1868, there is a peak of bench appears at 14.2Hz when we excite EW direction.