[Aritomi, Yuhang, Eleonora]

The strong earthquake of last friday night ( https://www.jma.go.jp/en/quake/20200131171237393-01020757.html) moved INPUT pitch too much and we could not recover with local controls.

So we used picomotors. Almost all the suspensions show a jump. See attached plot. Anyway the cavity has been realigned and looks more stable than usual.

What I did

• Modified the beam path of TEM00
• Measured the beam profile of TEM00 mode
• Adjusted the polarization of TEM00
• Started the installation of stage inside the chamber

Modification of optical path

As I replaced some screws before, I had to modify the beam path of TEM00 mode a little bit.
I aligned the EOM and lens after that in order to inject the beam into Faraday Isolator.

Measurement of TEM00 beam profile

After aligned the beam, I measured the beam profile of TEM00 which will be used for mode matching.
The result will be shown tomorrow.

Adjustment of polarization

Then I re-installed the Faraday isolator which is used for picking off the reflected beam for PDH locking.
So far, the polarization of TEM00 was adjusted to P-pol which is not appropriate one.
So I changed the polarization of TEM00 from P-pol to S-pol.
Now that TEM00 beam has S-pol.
HOMs' beam have P-pol.

Installation of stage

As the beam height from the table of cryostat chamber is too high (about 160 mm), I designed the stage for compensation.
Since some parts were delivered, I started the installation of this stage as shown in attached picture.

Next step

• Modify the layout where close to the chamber
• Assemble the folded cavity

Actually, I think those results are not correct.

I re-calculated the S-pol map and got a much smoother distribution with those stripes being vanished almost completely.

As for P-pol, the map-data are unfortunately not reliable as the lock-in's sensitivity wasn't set correctly resulting in saturation on many occasions.

Pengbo, Simon

I changed the HWP holder for the input-polarization control to a motorized one. Now, we have full remote control of the measurements once the system is set up.
However, I noticed that with two motorized stages, the software to control both stages sometimes seems to be a bit slow in responding or does not respond at all (maybe due to the USB-hub?). Apparently, loading the controllers one by one helps and also running a short diagnostics seems to have some effect.

Also attached is the S-pol map (now corrected) from last week Thursday.

Compared with the former results (see elog 1807), we now see a significat reduction in abundance and strength of the vertical stripes. Our initial assumption that those are a result of polarization instability seems to be correct. Althought they are not fully removed, the system with the FI produces now much more presentable results.

Currently, the P-pol measurements are done again with both HWPs motorized.

Aritomi, Eleonora, Yuhang

We tried to measure FDS with correct detuning (this time produced squeezing level is only 5 dB and we are thinking temperature may be responsible for this change). However, low frequency part is dominated by back scattering as expected.

Good thing is that the rotation part of the curve is still visible from the noise curve. We are also thinking to increase the average times to further reduce the spectrum curve thickness, so that the rotation part can be more clear. It's also better to analyze the data as soon as we take it and if it's not correct detuning, we should tune the detuning and take the data again.

Simon, Pengbo

We reconfigured the polarization system and did a test measurement on tama-size #1.It was place on the mount with the mark point to the top.

As can be seen from the result, both the maps show a smaller offset compared with the result measured before.

Also we can see some structure pattern from the bottom left corner to the top right corner.

In the last FDS measurement, average detuning is 119.6 Hz and the standard deviation is 11.6 Hz. This is kind of consistent with the measurement of locking accuracy which is 6.4 Hz.

I modified the pitch damping filter for the INPUT mirror to reduce the 9 Hz peak that was showing up in the IR lock accuracy plot.

New filter is ACdamp3 and seems to work better.

To reduce the effect of locking accuracy, I reduced the pump green power. First I reduced green power from 26.7mW to 20mW by changing MZ offset, but I couldn't lock GRMC and MZ.

Then I put OD0.2 after GRMC and green power reduced from 26.7mW to 18 mW.

OPO temperature and p pol PLL frequency are 7.173kOhm and 220MHz with 18mW green power.

[Aritomi, Yuhang, Eleonora, Matteo]

By reducing green pump power from 26.7mW to 18mW, we managed to reproduce the last Friday's FDS measurement. The detuning is around 100-130Hz. It seems that squeezing level at low frequency is still limited by locking accuracy.

The parameters are as follows.

sqz_dB = 7;                    % 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 =   13.3e-12;                  % Lock accuracy [m]

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

Strong 50 Hz showed up again in the end oplev signals.  We went to check and tweeked a bit the cables but in all the other configurations that we tried it was worse.

I put a digital notch at 50 Hz in the optical lever signals. 

We found the rapeauto was set to 1/f instead of 1/f^4, we put it back to 1/f^4 and adjust the gain: 

INPUT ATTENUATON : 1.9

PIEZO GAIN: 4 

We measured open loop TF: UFG = 20 kHz, phase margin: 55deg.

It didn't improve the stability problem we have on cavity transmission.

When IR alignment is good, IR trans is around 290 count for 220 uW of IR injection.

When we integrate IR error signal down to 1Hz, locking accuracy is 2.3 Hz.

When we integrate IR error signal down to 0.1Hz, locking accuracy becomes 3.2 Hz which corresponds to 3.4 pm.

[Aritomi, Yuhang, Raffaele, Matteo, Eleonora]

I did a quick fitting of the FDS measuremnt we did last friday (24/01).

We have about 1 dB of FDS sqz at low frequency and 3.5 dB at high frequency.

We see that at some point during the measurement the detuning changed from ~100 Hz to ~70 Hz.

Sqz degradation paramenters used for the fit:

Attached figure is FDS curve with correct detuning (54Hz). With following parameters, frequency at which squeezing crosses shot noise is around 120Hz and frequency at which anti squeezing crosses shot noise is around 45Hz.

sqz_dB = 8;                    % produced SQZ

L_rt = 150e-6;                    % FC losses

L_inj = 0.20;                     % Injection losses

L_ro = 0.10;                      % Readout losses

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

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

ERR_L =   8.5e-12;                  % Lock accuracy [m]

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

phi_Hom = [0/180*pi, 10/180*pi, 20/180*pi, 30/180*pi, 40/180*pi, 50/180*pi, 70/180*pi ,90/180*pi];  % Homodyne angle [deg]

det = -54; % detuning frequency [Hz]

Simon, Pengbo

We remove the first PBS of the FI because it will block the reflective beam, so we put another PBS behind the first HWP, then remeasure the beam waist.  

The result shows the waist is about 0.03529 mm at Z_position = 59.96 mm, which is good, our next step will be to recover the birefringence measurement system and test the performance of the new system.

Eleonora, Matteo and Yuhang

When we measure squeezing, we need to lock phase of LO with squeezing. Since the phase of squeezing is influenced a lot by suspended mirrors, the correction signal we send to IR phase shifter is quite large. We want to know how it will influence the measurement of squeezing.

On one hand, the power of IRMC transmission will be modulated. As we reported in entry 1883, we had 10% of IRMC transmission reduction when we drive IR phase shfiter with whole range. We did calculation, if we have 10% power reduction, we will have shot noise reduction of 10log10(0.9), which is 0.46 dB of reduction. This is the maximum reduction while the reduction in the real case should be half of 10log10(0.97),  which corresponds to 0.066dB reduction of shot noise.

On the other hand, there are frequency components of noise sent to IR phase shifter(attached figure 1). We measured the spectrum of homodyne when only lock CC2. (OPO is locked with p-pol, no pump sent to OPO, CC is on resonance inside OPO by tuning PLLp-pol, then let field goes to FC and reflects to homodyne, the 7MHz on homodyne is used to lock CC2) The result is shown in the attached figure 2. We also tried to send sine wave to IR phase shifter, and measured homodyne spectrum. This result is shown in the attached figure 3. These two measurement proves that the low frequency bump has contribution from IR phase shifter noise.

Please check the attached videos.

1st video shows the old broken cable.

2nd video shows the new good cable.

Simon, Pengbo

We move the first lens back to the FI direction with 1 cm, then move the second lens by 0.5 cm to the direction of the input beam.

As can be see from the image, the waist z_position change from 80mm to 70mm, and the radius is smaller than before.

we think by moving the second lens for another 0.5 cm, the waist should exist at the z_position around 60 mm.

Also the waist radius might reduce a little bit.