Eleonora and Yuhang

Today we tried to monitor detuning drift but filter cavity always got unlock(almost every 5 minutes). And we noticed that the length correction signal was always drifting. Considering the change of 3000counts, we can convert it to the voltage we send to laser PZT is drifting by about 3.66V(3000count/1000count*0.61V*(100/50)).  Maybe it drifts out of range and made filter cavity unlock. However, from the manual of laser, the PZT driving coefficient is 1MHz/V. And the PZT driving range is +/-65MHz. It seems they are not consistent.

Besides, the reason for this correction drift is till under investigation.

Since the parts for fixing the cavity have been delivered, I started the installation of a folded cavity.

1. Installed a stage and put a spacer on it.
2. Fix the position of the spacer.
3. Install a mirror and injected a beam.
4. Adjusted the alignment roughly.

Now the "transmitted" beam can be picked off with two steering mirrors.

Pengbo, Simon

after the successful polarization test, we started to do also the absorption tests in order to see whether we can reproduce our results from before the upgrade.

Actually, the first test was performed with the TAMA#1 sample but we repeated the test with Shinkosha#7 since we don't have any direct reference for TAMA#1.
We can say that the actual results in terms of the absorption coefficient are well reproducible (see attached figures from before [Z = 74.1] and after [Z = 80] the upgrade).
However, there is an issue with the "effectivity of absorption", meaning that we get a factor of 2-3 smaller AC-values than before. The reason is still not quite clear to us. It may be that the size of the beam-waist, which is 10% larger than before, has a bigger influence on the AC-value than anticipated. We will try to further reduce the waist and see what may happen.

At least we can exclude that the polarization of the beam has any significant influence on the absorption. We have tested this yesterday as well.

I installed 2 lenses for mode matching of TEM00 mode beam.
Their positions can be optimized by the stage.

The beam is collimated less than 100 um dia.
The requirement is about 50 um and I need optimization.

Tomorrow, the rest parts for folded cavity installation will be delivered and I will install the cavity with these parts.

This is my fitting result.

Eleonora and Yuhang

We tried to measure filter cavity FIS with scanning LO phase to get squeezing and anti-squeezing level. But since the suspended mirrors are always moving a lot, we couldn't see the expected M shape.

Thank you for the measurement.

Can you add the fitting result of homodyne angle? 

Eleonora and Yuhang

We fixed the CC2 demodulation phase and measure FDS during more than one hour.

During these measurement, filter cavity got unlock twice. Also, for the last measurement, we misaligned filter cavity yaw.

The detuning was stable within ~10min. Even without unlock, detuning changes and unlock almost doesn't change detuning if green is recovered. But detuning will change after a bit misalignment.

Aritomi, Eleonora and Yuhang

Today we monitored BAB PDH signal for more than one hour, which is taken by collecting filter cavity BAB reflection on an RF PD. During this time, filter cavity got unlock several times. 

The orange color line is normalized PDH signal.

The cyan color line is PDH signal before normalization. 

The dark blue color line is filter cavity control loop correction signal.

• We could see basically the PDH signal doesn't drift and it means the BAB is always on resonance. And there is fluctuation, by taking the pk-pk value of this thick line(let's say 8count pk-pk) and take calibration factor of 2Hz/count. We should have RMS locking accuracy of 11Hz. And this is roughly consistent with the measurement of locking accuracy spectrum which give result of 6Hz.
• We could see that in the middle, there is a time filter cavity got unlock, and normalized PDH has an offset from zero.
• There are many times filter cavity got unlock and everytime when it got lock again, the PDH went back zero again.

But we observed at least twice a strange behavior. The PDH becomes smaller and BAB transmission becomes also smaller while GR transmission is kept. After we tune AOM frequency, the BAB transmission was recovered and the detuning was changed a bit. 

Earthquake of magnitude 7 occurred at 19:44 JST in the north of Hokkaido (https://www.jma.go.jp/en/quake/20200213104426395-13193432.html). Mirrors are shaking very much and the lock is difficult to recover.

By monitoring the long term drift of the supesnsions, we observed a  ~24h cycle likey related to the temperature. It is especally visible in pitch. The "inversion points" are at about 5 am and 5 pm. Shorter period oscillations seem to appear in yaw.

Non normalized:  0.011 Hz/count

Normalized:  1.98 Hz/count

[Aritomi, Yuhang]

We locked FC with green and implemented dithering to be sure that alignment of FC is good. We locked IR off resonance (detuning is 1kHz). Unfortunately, today there was a large mode mismatch peak in AMC, which was ~50mV out of 1V. This becomes ~10% readout loss.

Measured loss is 40(1) % and phase noise is 30(5) mrad.

K1:FDS-WFS2_DC_SEG1_OUT    ->  PDH IR 

K1:FDS-WFS2_DC_SEG2_OUT    ->  IR TRA

K1:FDS-WFS2_DC_PIT_IN1         ->  normalized PDH IR  (i.e PDH IR/IR TRA)

Aritomi and Yuhang

The loss measurement doesn't include intra-OPO loss (roughly 8%) in injection.

This measurement also doesn't include homodyne loss (roughly 4%), PD quantum efficiency (roughly 1%) in readout.

total loss:

[Aritomi, Yuhang]

[Aritomi, Eleonora, Yuhang]

Recently we couldn't lock FC and we thought it is due to strong wind, but today we found that green injection power was 11.5 mW which was a bit lower than usual. When we increased FC gain, FC locked. Then we changed SHG temperature from 3.078 to 3.09 and green injection power became 15.2 mW. We tuned the FC gain so that UGF of FC loop is 20kHz.

I am trying to align the TEM00 beam to the folded cavity.
In order to do that I am modifiying the beam path which allows us to align with 2 STMs which have some amount of separation distance.

Since a mount for folded cavity has not been delivered yet, I cannot clapm the cavity which withdraws me to play with silicon mirror.
So I am using tentative mirrors.

Tomorrow I will measure the beam profile and tweak the mode matching and consider about the output layout.

This entry is a log of yesterday's work.
I installed a number of mirrors in TEM00 path to inject the beam into the folded cavity only with sample mirror inside the chamber.
Then I install 2 mirrors inside the chamber for picking off the ''transmitted'' beam.
I confirmed that the transmitted beam can come back on the optical bench.

The next step is optimize the position of the folded cavity and mode matching.