The CCFC error signal you analyzed is Q phase signal which is not used for CCFC lock. How about the detuning change with I phase?

[Aritomi, Yuhang, Michael]

Today we found that there was a 250Hz noise in CCFC signal and this noise came from low UGF OPO lock due to OPO demodulation phase change. After reloading the DDS1, this problem was solved.

After that, we found that FC lock was unstable even when Z correction, pointing, AA were engaged. In the end, FC could not be locked at all. We will investigate it next week.

Today I brought the KAGRA spare viewport that was sent together with the 2 dirty ETMY viewports.

It seems to be 10 cm diameter and 1 cm thick.

I tried to install it inside the holder for the shinkosha evaluation plate (similar thickness) BUT

- only one stabilizing screw can be used making me afraid that the viewport might move during a measurement

- from the lowest point of the mirror up to ~3.5 cm will be hidden by the holder.

I guess a possible quick fix could be to buy ~6cm stabilizing screw and use the TAMA size holder or drill new holes in the SHINKOSHA holder to move the screws and metallic rode to a good position.

In elog2187, we measured FDS with CCFC loop locked. Meanwhile, there were no GR automatic alignment (AA) and input mirror feed back (InputFB). And we have seen detuning changed by about 10Hz, which should not happen since CCFC works to remove the detuning change with precision of 1Hz (could be a bit worse in elog2187). I have thought about this in more detail, and I found misalignment can impact on CCFC lock.

I have a code Aritomi-san provided about CCFC error signal calculation. By using this code, as elog2300, I added mode matching influence for this error signal and found the shape change of this signal, which was confirmed with experiment. I made this simulation again with mode matching level of 100%, 95%(our current situation), and 90%. The simulation result is shown in the attached figure (figure 1 is the zoom in of figure 2). We can see that the CCFC error signal zero crossing point changes with different mode matching level. Let's remind that when CCFC is used to lock filter cavity, we lock filter cavity to the zero point of CCFC error signal. Since CCFC zero crossing point changes with mode matching level, a misalignment for squeezing field will translate into FDS detuning change.

Attached figures show zero crossing point changes from 1.954 to 1.895, which are normalized by detuning frequency 54Hz. Therefore, this corresponds to a frequency change of 3.2Hz. Without automatic alignment, and considering the experience of our mirror suspension system change, this misalignment induces detuning change and prevents us from the goal of 1Hz detuning stabilization. Even with GR automatic alignment, the alignment is actually fluctuating and results in ~95% misalignment, which should introduce already 3.2Hz detuning change.

To achieve really detuning change less than 1Hz, IR automatic alignment will be necessary!

On Thursday morning I measured the opto-mechanical transfer function of the pointing loop with FC locked with all loops.

For BS pitch the amplitude was 9000 and BS yaw 3000 (higher would cause unlock).

Results are presented in figure 1 and 2.

Marc, Michael, Yuhang

On wednesday morning we started the AA characterization.

First, we measured the SNR of one QPD segment (here QPD1_I1) but similar results are expected for other segments.

You can see in figure 1 that we have reasonable SNR up to about 100 Hz.

Then we measured the input opto-mechanical transfer function by injecting white noise in input pitch (amplitude 9000) and yaw (amplitude 6000) and checking the signal reconstructed by the AA.

Note that to make these measurements with diagui we had to close the mirror damp loops.

The results are presented in figure 2 and 3.

Then when we tried to inject noise to end we unlocked the FC.

Actually, even if the damp loops are not used (gain to 0), there were several filters and large offsets that caused issues.

So on Thursday morning I repeated these measurements for end mirror  with identical amplitude without particular troubles.

Next step is to combine these measurements with the filter that we are using to check the bandwidth, UGF, phase margin to see if we can improve the AA filter.

[Aritomi, Yuhang, Michael]

Today we found that the SMA cable for GRMC demod was not fixed well. After we fixed it, the 100Hz bump disappeared!

The attached figure shows FDS with CCFC. CCFC gain was 2000 and CC2 mass feedback gain was 2.7. Although the 100Hz bump disappeared, there is still a small bump at 150Hz and many 50Hz harmonics. We might have another unfixed cables.

The detuning is a bit high in this measurement so we will measure FDS with correct detuning again.

sqz_dB = 10.4;                    % produced SQZ (dB)

L_rt = 120e-6;                    % FC losses

L_inj = 0.31;                     % Injection losses  

L_ro = 0.24;                      % Readout losses

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

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

ERR_csi = 30e-3;                  % Phase noise (rad)

ERR_L =   1e-12;                  % Lock accuracy (m)

I have installed 2 beam dumps on spurious green beams.

Figure 1 is spurious beam due to a galvo

Figure 2 is spurious beam due to the PR window.

While I don't think it will improve performances, it will make the work more confortable as these beams were quite powerful.

If my understanding is correct, this measurement was made with the FC half detuned. So it means that the mode-mismatch computed here mixes the mode-mismatch between BAB/FC together with BAB/LO, which means that we should not use this value in FDS degradation budget.

Also note that the measurement was performed over 50s and not 200s as this 50s corresponds roughly to the FDS measurement duration.

We forgot to tune the OPO temperature during this measurement that could explain the difficulty to fit..

Marc, Michael, Yuhang

Yesterday morning we started the measurement of opto-mechanical transfer functions to check the AA filter design (results will follow in another entry)

During this activity we injected noise in input pitch and yaw using diagui.

In order to send these noises from diagui we had to close the damp loop of input.

When we tried to do the same for end mirror we first could not see any noise so we remove many filters (especially 50 and 100 Hz notches) from this loop and then had a FC unlock due to an extremely large offset that we forgot to remove.

I think it is the only modification between your last 2 measurements, but as this loop had a gain of 0, it might not affect the apparition of this peak...

I measured FDS with CCFC around optimal detuning (attached figure).

CCFC filter gain is 2000 with 30Hz LPF. CC2 mass feedback gain is 2.7.

The spectrum for squeezing quadrature (red curve) seems distorted by backscattering noise and the fitting result is not reasonable.

Now the problem is a bump around 100Hz. I tried the different LPF cut off frequency and gain of CCFC filter, but the 100Hz bump cannot be removed...

Following the cleaning of the SHINKOSHA evaluation plate with first contact, I performed absorption measurements in XY,YZ and XZ planes at the same location with the previous measurements.

The results are presented in the first 3 figures.

Using the calibration computed just before the measurement and reported in entry 2510 together with the power measurements : Pt = 2.764 W and Pin = 3.193 W.

Without any fit we got :

I think that a more precise estimate of the XZ and YZ planes measurements could be done by only considering the data inside the sample.

For instance, using the equation 3.19 of Manuel's PhD where the effective thickness of the sample is computed.

Note that I checked that the lockin was not saturing before starting the XY map. However, after the last measurement in the YZ plane I found out that the lockin was saturated...

I'm wondering if it arised because of point defect/ new dust... Anyway I started a new XY plane measurement after changing the lockin gain. Sadly, the PCI computer got a windows update and stopped this measurement...

Marc, Yuhang, Michael

We measured the OPO nonlinear gain vs input green power.

The measurement was performed by modulating green phase at about 1 Hz and looking at the BAB transmission from the OPO. The signal was triggered to maintain its position on the oscilloscope window and the "persist" function was used to keep the resonance peak on the screen. The peak oscillates up and down as a consequence of modulating the green phase. The maximum value of the peak represents the amplification and the mimnum value represents the deamplification. By plotting max and min gain vs green power we can find the OPO threshold power shown in figure 1.

The fit looks a bit odd, and is quite imprecise on the deamplification fit. I found it especially quite difficult to discern what the minimum deamplified power was on the oscilloscope - the persist function of the oscilloscope was used to keep the mimnimum and maximum values of the resonant peak visible, but it also oscilalted slightly on the frequency axis, which obscured the minimum values visible.

At face value, the nonlinear gain seems slightly reduced from the previous value of 80.56 +/- 0.14 mW (Yuhang/Aritomi thesis)

Marc, Michael, and Yuhang

When we want to lock filter cavity today, we found it was harder to lock even with filter cavity z correction loop on.

To understand why this happens, we took measurement of oplev signal and checked weather information.

1. Oplev signal. In the attached figure one, four suspended mirrors oplev spectrum is shown. We can see the mirco-seismic noise between 0.2 and 0.8Hz is increased by a factor of 5 for BS/Input/End.

2. Tide/seawave/wind information: We checked Yahoo tenki, as shown in the attached figure 2,3,4,5, the tide is small tide, the sea wave height is around 3m, the sea wind is around 12.5m/s, the ground wind is around 5m/s.

Attached figure is theoretical CCFC FDS curve with optimal detuning (54Hz). With the following parameters, frequency at which the anti squeezing crosses shot noise is 44Hz.

sqz_dB = 10.5;                    % produced SQZ (dB)

L_rt = 120e-6;                    % FC losses

L_inj = 0.35;                     % Injection losses

L_ro = 0.24;                      % Readout losses

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

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

ERR_L =   1e-12;                  % Lock accuracy (m)

ERR_csi = 30e-3;                  % Phase noise (rad)

phi_Hom = [0/180*pi, 30/180*pi, 60/180*pi ,90/180*pi]; % Homodyne angle (rad)

det = -54; % detuning [Hz]

According to elog2514, the current CC detuning should be ~72Hz and we have to change it by 18Hz to have the optimal detuning. Using the formula in elog1727, the CC PLL frequency should be changed by 2*18/1.91 = 18.85Hz. Since the current CC PLL frequency is 6.99701252 MHz, the optimal CC PLL frequency should be either 6.99703137 MHz or 6.99698367 MHz. By checking the CCFC error signal, I confirmed that 6.99703137 MHz is the correct one (In DDS, 6.99703139 MHz was set).

Here is the new CC PLL setting. I saved the DDS setting as "20210520_dds3_CCFC_check" for characterization of CCFC error signal and "20210520_dds3_CCFC_FDS" for CCFC FDS measurement.

Fig 1,2 show the measured CCFC error signal and locking accuracy, respectively. The CCFC calibration amplitude is 182mVpp. Now the CC detuning is 60Hz.

[Aritomi, Yuhang]

We changed the filter setting for CCFC to remove the peak at 170Hz. The new filter setting is 30Hz LPF and gain of 1000.

Then we measured FDS with CCFC (attached figure). The peak at 170Hz disappeared with the new setting.

Unfortunately, I couldn't find the anti squeezing quadrature. We will measure it again.

Degradation parameters:

sqz_dB = 10.5;                    % produced SQZ (dB)

L_rt = 120e-6;                    % FC losses

L_inj = 0.35;                     % Injection losses

L_ro = 0.24;                      % Readout losses

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

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

ERR_L =   1e-12;                  % Locking accuracy (m)

ERR_csi = 30e-3;                  % Phase noise (rad)

Marc, Michael, Yuhang

In the past, we usually check the mode matching between filter cavity (FC) reflection and homodyne LO without considering the beam jittering. However, FC reflected beam jittering is an issue which degrades homodyne detection efficiency.

To check this issue, we first lock filter cavity with green with AA/pointing/length control loops on. Then we half-detune BAB and check its spectrum on oscilloscope when it arrives AMC. Due to jittering, there are peaks going up and down in the AMC spectrum. We used oscilloscope persisit function to record the spectrum for about 200 seconds (as attached figure). In this situation, we measure the highest value of these peaks.

The peaks height are

All the peaks in the above table are taken in the same manner. But we firstly took TEM00, then we zoomed in and checked higher order modes. Since they are taken in the same manner, we do division as (all HOMs)/(all HOMs+TEM00) = 10.57%.

If we use this value, the mode mismatch in homodyne detection will be 0.8943*0.8943 = 80%. Considering the optical loss in elog2511, the total optical losses will be 1-0.8*0.807*0.904*0.99 = 42.2%. This value is larger than the optical losses we used in PRL paper, but closer to the derived optical losses from SQZ/ASQZ measurement in this link. However, it is noted that the evaluation of mode mismatch in this entry should be a pessimistic one. Because we take the highest HOMs, which only tells us the worst mode matching in the 200s measurement. We also conceived to take many instant AMC spectrum of FC reflected BAB, which should give a more reasonable evaluation.

In addition, we can also use visibility measurement to double check the mode mismatch induced homodyne in-efficiency.

The mode-mismatch was slightly over estimated as I divided by tem00 power and not total one...

The corrected values are : misalignment = 3.9%, mode-mismatch = 1.2% and total 5.3%.