[Aritomi, Yuhang]

First we fixed DDS AA phase to 150 deg and optimized I/Q demodulation phase again. As long as FC alignment is good, changes of the optimal demodulation phase are within a few degrees. During this measurement, we checked that WFS1 I3/Q3 coupling is less than 3%.

Then we injected a 12Hz line to INPUT PIT and measured sensing matrix, but there is still large PY coupling. We found that 12Hz peak heights on each QPD1 segment are quite different (following table).

This gain unbalance may cause the coupling problem. So we compensated this gain unbalance in matrix in DGS (attached picture). Each number in the matrix is decided by 10/(12Hz peak height on the segment). In this case, there is no coupling in WFS1 I YAW for INPUT PIT driving, but there is still 16% coupling in WFS1 I PIT for INPUT YAW driving.

After that we aligned FC well and measured 12Hz peak height again. This time the gain unbalance is different from previous measurement.

To decide the gain unbalance precisely, we will check PDH signal on each segment and calibrate it by sending a 12 kHz line to PZT as we did for FC PDH signal. 12kHz is within DGS bandwidth and it is around UGF of FC loop. Since what only matters is ratio of gain of each segment, it is not a problem even if the injected line is suppressed by FC control loop.

There was strange coupling observed in elog2206, but I think it may due to the not proper excitation singal sent to END mirror.

Excitation: the excitation is a sine wave, with amplitude from 700 to 11000, sent to END mirror pitch. (An example shotscreen is shown in attached figure 2)

Measurement: the spectrum of end mirror optical lever pitch/yaw were observed. (An example shotscreen is shown in attached figure 3)

The response information is extracted by using the cursor at 6Hz on each spectrum, and read the value of cursor. The coupling is the ratio of yaw/pitch peaks.

The coupling was always around 5.2%, which is the minimum could be found now.

The method used to find minimum was by adjusting the coefficient for H1 and H3 coils (giving them same/slightly different values from 0.02 to 0.06). The minimum is around -0.05 for H1 and 0.05 for H3. 

What I did

I replaced the PD at transmitted port in order for precise measurement of decay time.
Current PD has the bandwidth of 150 MHz.

Then I measured the ringdown of transmitted beam around 8 K.

Results

The PD has fast response such that the measurement can be done well.

The finesse was obtained by fitting the measured data, and it was about 1.65*10^4  though 1.67*10^4 at room temperature.

Next step

As the UGF of PDH servo is about 3 kHz and cannot achieve stable lock, I have to modify the servo to set higher UGF.
In addition, I will monitor the finesse behavior for a while.

Aritomi and Yuhang

As reported in elog2173, the driving for end mirror has some coupling between pitch and yaw. To decouple them, we decide to modify the driving matrix.

However, we found out that the coupling between pitch and yaw is different for different excitation strength. The coupling situation is shown in the attached figures and sumarized in the following table.

This measurement is done after optimizing the coupling with excitation of 1000. The pitch driving matrix is as following:

I compared green, BAB, CCFC locking accuracy. 

As pointed out by Aritomi-san, the formula used to calibrate the measurement had some problem (check entry642). After correcting that, the measurement result becomes reasonable.

Today, we find that we were injecting 25kHz noise inside the PZT.

After removing the injected signals, the cavity scan was performed again. The diaggui file for cavity scan (green) is saved in Desktop/cavity as cavity_green_scan.xml.

This time, the spectrum is good.

Apart from this, FC green locking is normal again.

As pointed out in the last FC meeting, the error signal for green and infrared around 10kHz is similar. This is actually strange for me. Due to the cavity pole for infrared and green has a factor around 25 difference. Above their pole frequency, the green error signal should be around 25 times larger than infrared.

However, I checked several times this entry and compared with elog642, I couldn't find what is wrong. I will try to measure it again.

Just to add a bit more information about my understanding. If CC1 loop is locked first, the lock of filter cavity causes the light from main laser having a large phase change. Then CC1 loop needs to give a large correction to keep the same phase. In the end, it causes the saturation of CC1 correction signal.

Matteo also suggested to feedback correction signal to the end mirror, which will offload the large correction sent to main laser.

We have a problem of CC1 saturation and we found that the CC1 saturation is caused by FC lock/unlock. We should lock FC first and then lock CC1.

I checked the splitting ratio of pickoff BS (BSW11) for CCFC with BAB.

This BS is roughly 55:45 and gives 45% loss for squeezing. 

Aritomi and Yuhang

Recently we found the lock of filter cavity always has problems. For example, after we try to remote lock, it takes a while to lock or it doesn't lock.

So we checked several setting for that. Firstly, the injected green power confirmed to be 24.2mW. According to elog1886, we checked the setting of remote lock.

If I understand well entry1886, I think they are fine. (But it seems the offset from DGS(unlock) should be 1.5V. Although it is different from what we measured, it should not make difference for the remote lock performance)

Then we checked the Green_tra_DC with osilloscope, the transmission peak was only around 1V. So we decide to scan FC_green by sending a ramp signal to END mirror length. The measurement is shown in the attached figure.

It is clearly shown in the figure that there are visible sidebands around Green TEM00. Surprisingly, these sidebands are about half the magnitude of TEM00. Compared with elog1674, this is so much different.

We also checked the sideband of modulation from EOM by looking at the spectrum of GRMC reflection DC channel on oscilloscope. For GRMC ref_DC, the sideband is barely visible. So it should not be the problem of EOM modulation.

I started some analysis for finesse of the folded cavity.
The finesse was obtained by fitting the transmitted signal.

The finesse at room temp. was about 1.678*1e4(+/- 11) though that at 8 K was 1.579*1e4(+/- 26).

Further analysis is ongoing...

Aritomi and Yuhang

There was END mirror pitch/yaw coupling problem found in AA sensing matrix, as reported in elog2165. Besides, it was found the END mirror driving from coil/magnets has already been not clean, as reported in elog2173.

So we decide to find a good driving matrix for END mirror. Before doing that, the check about if the END mirror PSD is sensing well the pitch/yaw was done.

The method is to excite yaw of END mirror and check the response of  signals in pitch/yaw channel relative to this excitation. If the sensing is not good, the resonant peak will go from yaw channel to pitch channel.

The test result is attached. By comparing the resonant peak at 1.58Hz, the sensing coupling was found to be around only 1%. This means that sensing coupling of oplev is not a problem.

What I did

• Ringdown and doppler measurements @room temp.
• Open loop TF measurement of PDH locking loop

Results

The obtained data are still in USB and floppy disk...
But the UGF of TF was about 3.1 kHz and phase margin was much larger than 30 deg.
Therefore, the lock seems to be stable one.

Next

• Cooling down to 10 K

I am planning to cool down the cavity to 10 K which is the designed temperature for the ET-LF.
And then monitor the finess in order to estimate the optical loss at 10 K.

[Aritomi, Yuhang]

We found that FC misalignment changes the shape of CCFC error signal and/or adds offset. This can cause the detuning fluctuation and we think that auto alignment is necessary to obtain the stable detuning fluctuation.

We measured CCFC error signal again (attached picture). CCFC error signal today is larger than last week at 100Hz-10kHz region. This may be related to the fact that we turned on the lasers today.

What I did

• Ringdown and doppler measurements
• Injected He gas to raise the temperature

Results

The attached file shows the very preliminary results of the ringdown measurements.
As shown in the figure, the finesse at 134 K, and 165 K is smaller than that of 170 K (judged by my eyeballs).
This might imply that the molecular layer i.e., amorphous ice can desorb around between 165 K and 170 K.

It should be noted that the fitting has not yet done...

Next

• TF measurement of PDH loop

Tomorrow, I will measure the TF of PDH locking loop.
Then I will re-cooling the cavity.
At this moment, the target temperature is 10 K and monitor the behavior of finesse in order to estimate the loss induced by cryogenic molecular layer.
I think this result would provide some implications to the Einstein Telescope.