What I did

As the temperature became enough low, I tried to lock the laser to the cryogenic cavity and measure the finesse.
First, I scanned the laser frequency to measure the linewidth.
In addition to the linewidth, I could see the splitting peak probably due to the difference of polarization.
Then I did ringdown measurement several times.

Next step

I will analyze the obtained data.

Notes

As there will be a power outage work on next Sunday, I turned off the refrigerator to raise the temperature.
I will cool down to 10 K next time after improving the alignment.

This issue is originated by the fact that the servo galvo and the DGS are following two different convention for the WFS segment order:

The galvo servo from initial TAMA is following the convention in pic 1 (take from this document), while the DGS is following the convention on pic 2 (KAGRA convention).

The cables from the WFS DC segments have been connected into the galvo servo followig the convention from KAGRA/DGS and this resulted in a swap between X and Y error and correction signals.

We decided to keep the current configuration to be coherent with the DGS beam position monitor. Note that for WFS1 the cable for the correction signal for X and Y were originally inverted by mistake.

[Aritomi, Yuhang, Eleonora]

First we changed the demodulation phase for WFS1 from 17.5 deg to 107.5 deg in order to have all signals in I phase.

Then we measured spectrum of WFS signals (attached picture). As you can see, 11Hz BS pitch peak appears in WFS1_I_YAW which means there is large pitch and yaw coupling in WFS1. Eleonora will make rotation matrix to rotate WFS1 I pitch and yaw.

[Aritomi, Yuhang]

We measured closed loop transfer function (G_CL) of galvo by measuring ADD_OUT/ADD_IN in galvo servo. From G_CL, open loop transfer function (G_OL) can be calculated as follows.

G_OL = -1+1/G_CL

Note that sign of formula (3) in the attached galvo document should be opposite.

We measured OLTF of QPD2 DC pitch (X) (attached figure). UGF is 800Hz and phase margin is 30 deg. Compared with design (figure 8 in the galvo document), shape is similar, but UGF is smaller (design is 2kHz).

Today I measured the finesse of the cavity at room temperature by using ringdown method.
I also measured by cavity scan.
The results will be reported tomorrow or day after tomorrow...

Then I turned on the refrigerator and started cooling down the cavity.

As they will do works which involve a power outage on next Sunday, the target temperature is 120 K at this moment.

[Aritomi, Yuhang]

First we found that optimal demodulation phase for WFS1 has changed. It was 20 deg, but today it is 17.5 deg after we optimized it.

We measured sensing matrix by injecting a line to INPUT PIT/YAW and END PIT/YAW (attached pictures). The line frequency is 12Hz and the amplitude is 2000.

In the attached pictures, the left peak is BS pitch 11Hz peak and the right peak is the 12Hz injected line.

We have to think about how to decouple pitch and yaw in driving/sensing.

When we have test timed-out in DGS, we need to restart standalone.

This is a memo for restart of standalone.

1. take snapshot

2. turn off the power button of standalone

3. turn on the power button of standalone

4. restore snapshot

What I did

Today, I attached two temperature sensors using indium foil on the breadboard and mirror holder, respectively.
Then I did alingment of input optics and confirmed that the cavity can be locked.

After that, I closed the chamber and started roughing.

Next Step

I will measure the finesse of the cavity at room temperature tomorrow.
Then I will start cooling down.

Aritomi, Yuhang

It was reported in elog2130 that there is discrepancy between measurement in the path for QPD1 and QPD2.

This is due to the implementation is not the same with design. Due to the space limitation, QPD2 is not placed in the correct position. Therefore, we should move QPD2 further three holes.

As reported in entry #2117, the large offset in WFS2_Q3 is due to a broken channel (n°14) in the Anti-Aliasing board (AA1). It can be solved by using some spare AA channels but so far we considered that we won't use Q signal for AA. So we decided to wait untill the BNCtoDsub issue (that is likely to cause this kind of problem) is solved.

[Aritomi, Matteo]

After we locked both of galvo loops, we checked WFS2 signals.

We found that WFS2_Q3 had large offset (~4000) compared with others(<10). Even when we put 50 Ohm to DGS input of WFS2_Q3, WFS2_Q3 is still very large. So this problem seems to come from DGS.

To avoid using WFS2 Q phase signals, we injected 12Hz pitch line in input mirror and tried to minimize the 12Hz peak in WFS2_Q1 (maximize WFS2_I1) by changing the LO demodulation phase from DDS. The measured result is as follows.

The original DDS demodulation phase is 90 deg and it is already good demodulation phase. So as long as we use WFS2 I phase, this will not be a problem.

What I did

I re-connected Q-mass to the flange and tightened the bolts in order to improve the vacuum level.
This improved the vacuum level as before.
Then I tried to measure the residual gas by Q-mass.
Q-mass, hoever, showed a fatal error as shown in fig. 2 --- filament 2 defect.
Actually I could not measure the residual gas as shown in fig. 1 though I could do it yesterday...
It's like "yak shaving".

Next Step

I will open the chamber and install two temperature sensors with indium sheet.
In addition, I will adjust the optics and then close the chamber and do some measurements.

What I did

Today I checked the operation of Q-mass which was installed on the cryostat.
It seems that Q-mass can measure some data as attached.
However, the pressure was not good enough to  measure reasonable data by Q-mass.
I tried to improve the vacuum level,  but could not reach below 0.3 Pa with Q-mass.

Notes

At first, there was a leakage around the tube between the cryostat and Q-mass which prohibited vacuum evacuation, though Q-mass operation requires less than 10^-2 Pa.
Actually, it could not reach below 20 Pa.
This was due to the poor workability around Q-mass.
The bolts were not tightened well.
I somehow tightened them and the vacuum level was improved but not enough.

Next Step

I will try one more time to measure the residual gass by Q-mass.
After that, since indium sheet arrived, I will install temperature sensors and adjust the input optics and then start cooling down the cavity.

[Aritomi, Matteo]

Recently we had a problem that QPD2 DC centering loop cannot be locked.

Today we found that when we move the green beam horizontally, QPD2 DC signal somehow moves vertically and vice versa. This means horizontal and vertical correction signals for QPD2 galvo should be swapped. After we swapped the horizontal and vertical correction signals, QPD2 DC centering loop locked stably even when the green is reflected from filter cavity. Maybe the cabling of QPD2 is wrong.

[Aritomi, Yuhang]

We measured homodyne AR reflection and it is around 4 uW while LO power is 840 uW. It means we have 0.5% loss for each homodyne PD. In total, we have 1% additional loss from homodyne AR reflection.

I re-installed Q-mass in order for checking of operation.
Tomorrow, I will evacuate the chamber and confirm the operation of Q-mass and vacuum gauge.

Aritomi and Yuhang

According to elog 1659, the simulation result of beam size and gouy phase for AA system is shown in the attached figure 1. We could see that there is only 1000mm lens and QPD1 is located close to beam waist while QPD2 is located far from beam waist.

The measurement was done recently to check the real beam size. The measured points and beam diameter is shown in the attached figure 2. The measurement done for QPD1 (9 points) is indicated by 'cross' wihle the measurement done for QPD2 (3 points) is indicated by 'point' 21,22,23.

Since the beam size should be symmetric for two path after BS, the 'cross' and 'point' can be imagined to be located in a single beam. These measured ponits are fit with gaussian beam as shown in figure 3. The measurement done for QPD1 and 2 seems to have a wrong distance, we will check that.

[Aritomi, Yuhang]

Yesterday we tried many galvo servos for QPD2 DC centering, but no galvo servo can lock QPD2 DC centering loop. WFS2 DC signal either oscillates or goes away when we tries to lock.

We found that QPD2 DC signal moves a lot compared with QPD1 (attached movie). This may be due to larger beam size at QPD2 (farther from waist position). We measured spectrum of QPD DC signals without DC centering loop (attached screenshot). QPD2 DC signals are larger than QPD1 by a factor of ~2.

We also characterized beam size of green around QPD1 and QPD2 (Yuhang will report).

Today, I soldered another temperature sensor.
Then I tried to install the Q-mass, but it was not successful due to the weight of Q-mass...

Anyway, I will order indium sheet or Apiezon N Grease in order for thermal contact.

[Aritomi, Yuhang]

We measured sensing matrix of WFS1. We injected a line to pitch and yaw of input mirror. The frequency of the injected line was 12 Hz and the amplitude was 2000. Then we measured WFS1_PIT and WFS1_YAW signal.

First we injected a line to pitch of input mirror and measured 12 Hz peak of WFS1_I_PIT and WFS1_I_YAW with demodulation phase R = 90 deg. With R = 90 deg, WFS1_I and WFS1_Q signals were almost the same. The measured 12 Hz peak is as follows.

This means we have about 10% coupling from pitch to yaw of input mirror. This may be due to coupling of coil-magnet actuator driving.

Then we optimized the demodulation phase R in order to maximize WFS1_Q_PIT signal. The optimal demodulation phase for WFS1_Q_PIT was R = 20 deg. After that we injected the same line to yaw of input mirror and measured WFS1_I_YAW and WFS1_Q_YAW. Measured sensing matrix of WFS1 for pitch and yaw with demodulation phase R = 20 deg is as follows. Screenshots of the 12 Hz peak for PIT and YAW are also attached.

The optimal demodulation phase R = 20 deg for pitch was not exactly optimal for yaw, but it seems we can use WFS1 Q phase signal for both pitch and yaw.

We'll replace galvo servo for QPD2 and measure sensing matrix of WFS2.