What I did

Today, I tried to measure the finesse of the cavity by locking the laser.
However, I could not lock and found the servo had some problems at low-pass parts.
I tried to remedy the circuit but could not solve the problem.
So I decided to make another circuit.
Actually this work has not finished yet...

Besides it, I did some measurements without locking the laser, i.e. doppler method which depends on the dynamical response of the cavity.
Also I measured the frequency split between transmitted p- or s- polarized beam.
As shown in the attached picture, there is a frequency shift between the s- and p- polarization.
The injected beam is pure s-polarized beam, but by playing with a HWP one can inject p-polarized beam. 
This frequency spilt measurement corresponds to a cavity enhanced ellipsometry.

Results

To be honest, the analysis has not yet done...
The frequency spit was about 150 usec by 100 Hz 3 Vpp triangle wave.

Notes

The temperature of the cavity was about 120 K.

At first, the reflected (and transmitted) power was about 70 % less than I expected.
I doubted that the misalignment, but it seemed not.
Though I optimized the alignment, the reflected power did not improve.
After one or two minuites illumination, suddenly both of them recovered.
I could not figure out the reason, but the desorption of moleuclar layer is one of the possible reason.
Further investigations are needed.

[Aritomi, Yuhang]

We replaced a TAMA mixer with a new mixer (ZX05-1L-S+) and a low pass filter (SLP-1.9+) for CC1. CC1 error signal with the new mixer and TAMA mixer are shown in Pic. 1,2. SNR with the new mixer seems much better and there is no offset in the new mixer. Note that CC1 servo offset should be 5.

Then we locked CC1. CC1 gain is 2 for 25mW green. Pic. 3 shows CC1 OLTF. UGF is 3.7 kHz and phase margin is 50 deg. Pic. 4 shows CC1 phase noise. CC1 rms phase noise is 84.2 mrad and squeezing phase noise from CC1 is 84.2/2 = 42.1 mrad.

A similar test for input mirror with elog2173 was done.

Almost no coupling was found. This is consistent with the result reported in elog2172.

This entry is just a memomrandum.

Last week I made a sum-amp circuit to add the offset to PDH feedback signal.
But the output from the circuit did not contain the input signal, i.e. insensitive to the input signal and could not use though I confirmed all the conduction was fine.

Actually, there was a problem at one op-amp port labeled 5 on the board.
By changing the port, this problem was solved, but what caused this was uncertain...

One possible reason is due to the poor soldering around the op-amp but not sure.

[Aritomi, Yuhang, Eleonora]

First we amplified CCFC RF signal by 33.6dB with RF amplifier port which was used for AOM before. We put a DC block (BLK-89-S+) before the RF amplification. The result is as follows.

We demodulated it with a new mixer and measured CCFC signal when CC1 is scanned and CCSB are off resonance. The signal is 120 mVpp and no offset (Pic. 1).

Then we locked CC1 (CC1 gain: 0.5 for 56 mW green) and changed PLL setting as before. Carrier and CC AOM frequency are as follows. Frequency difference between carrier and CC is 60Hz which is good.  We scanned AOM around CC resonance. The CCFC error signal seems I phase (Pic. 2). Theoretical CCFC error signal with 56mW of green is Pic. 3.

We tried to change the CCFC demodulation phase by changing 14MHz DDS phase, but CCFC error signal didn't change. Since changing 14MHz DDS phase changes both CC1 LO and CCFC LO, this may cancel out the phase change.

So we tried to change the CCFC LO demodulation phase only by adding a 2.5 m cable to CCFC LO. Then CCFC error signal became Q phase (Pic. 4) although it is not crossing 0. Theoretically 90deg phase delay of 14MHz signal should be 3.5 m (lambda/4 = c/sqrt(2.3)/14MHz/4) where 2.3 is the relative permittivity of polyethylene. There is a bit difference between theory and experiment. We need a phase shifter for fine tuning of CCFC demodulation phase.

Anyway it seems we can use this error signal to lock FC.

Aritomi and Yuhang

As reported in entry2156, different HWP has different optical losses. Especially the dirty ones have more optical losses.

So we decide to clean these HWP and measure again the optical losses of them.

The experiment set-up is the same with entry2156, but the measurement is different this time. This time, the HWP was moved slightly by hand around center or tilt a bit to find the maximum/minimum transmission. Let's remind that HWP was placed randomly last time.

The result of optical losses max/min value is summarized as following:

One important point is that the minimum losses is the same before and after cleaning, which indicates that there is always a good point on the HWP to minimize optical losses.

But it was a bit strange that the maximum losses became larger after cleaning.

This entry is a log on the last weekend.
I injected He gas in order to raise the temperature inside the chamber.
Now the temperature is about 293 K.

Let's also check driving for INPUT to compare.

Eleonora and Yuhang

Since the problem of AA's pitch/yaw coupling is more severe when END mirror is driven, we decide to check the driving/control of END mirror.

1. The first test was to check the pitch/yaw motion peak with oplev when one set of coil/magnet is driven. The test was done for 4Hz and 10Hz. (figure 1-8)

Therefore we can derive

2. The second test was to send directly the pitch/yaw driving signal. After that, we checked the time series and also the spectrum. (figure 9-12)

[Aritomi, Yuhang]

This is work on Aug 17th.

First we maximized WFS2 I3 12Hz INPUT PIT by DDS demodulation phase. We set WFS2 segment 3 DGS phase 0. The optimal DDS demodulation phase for WFS2 I3 was 160 deg.

Then we optimized other WFS2 segments by DGS demodulation phase. We found that optimal demodulation phases for other WFS2 segments were around 40deg. This is quite different from WFS2 segment 3 demodulation phase which is 0 deg. Maybe this is related to broken WFS2 Q3 channel and it will be solved by fixing the channel.

We also optimized WFS1 segments by DGS phase. The optimal DGS demodulation phases for WFS1 were around 0 deg. Here is a summary of optimal DGS demodulation phases with 160deg of DDS demodulation phase.

We measured sensing matrix. We still have pitch and yaw coupling...

(Pic. 1-4: INPUT PIT, INPUT YAW, END PIT, END YAW)

After that we found that WFS1 pitch and yaw coupling for INPUT PIT changed in several minutes (Pic. 5)...

We also injected a 8Hz line to INPUT PIT, but WFS1 pitch and yaw coupling is similar with 12Hz line (Pic. 6).

Yuhang, Eleonora

In order to acquire the WFS2_Q3 channel priviously connected to a broken AA channel (see entry #2117) we moved the cable from AA 13-16 to AA 29-32 which was unused. See Pic1. We modified real time model accordingly. See pic2.

I turned on the refrigerator on Friday to see the temperature the cavity can reach.
Actually, the mirror temperature was below 10 K.
Therefore, we can measure the mirror properties from room temp. to around 10 K where the ET's test mass target temperature.
Fig. 1 shows the measured temperature, A represents the temperature of the mirror and B represents that of the table.

In addition, I could lock the laser to the cavity.
The red line in Fig. 2 shows the transmitted beam power though this picture is taken when the cavity was not locked.

I will make the servo to add the offset to feedback signal.
After that I will implement it to stabilize the lock.

Aritomi, Yuhang

We are having issue of offset and bad SNR.

We checked the 14MHz peak from TAMA PD (as shown in figure 1 and 2). Usually it is amplified to -42dBm after an amplification of 21dB from -63dBm. The SNR before amplification is about 14dB, which is increased to about 20dB after the amplification.

The local oscillator used for demodulation is -7dBm, as shown in figure 3 .

The demodulated signal before and after amplification are shown in figure 4 and 5. Compared with the linewidth shown in figure 6, the SNR for demodulated signal is only about 1.5.

We could see that there is a almost constant offset around 8mV. Therefore, a larger LO may solve the offset problem.

(TAMA1991 demodulator was used and shown in this entry. We also tried other two demodulators, which give much larger offset)

[Aritomi, Yuhang]

Yesterday we optimized QPD1,2 demodulation phase and WFS1 rotation phase. Today we checked them if they change from yesterday or not. WFS1,2 signals for INPUT PIT measured today is shown in the attached picture. For WFS1, I and Q decoupling is fine, but it is not good for WFS2. Also there are pitch and yaw coupling for both WFS1 and WFS2. We need also WFS2 pitch and yaw rotation.

We optimized WFS2 demodulation phase by DGS. WFS2 DGS demodulation seems working. Optimal WFS2 demodulation phase is 10 deg while it was 0 deg yesterday.

We optimized pitch and yaw WFS1 rotation. WFS1 pit and yaw rotation phase is now 20 deg while it was 10 deg yesterday.

Aritomi, Yuhang

We had issue of pitch and yaw sensing coupling, as reported in elog2141, especially from end mirror motion. We see from that entry that the driving of end mirror pitch/yaw goes into yaw/pitch sensing mainly in WFS1.

Therefore we tried a new design of AA telescope as shown in attached figures. The first figure is contain the phase information. The second figure shows how components are organized on top of bench.

[Aritomi, Yuhang]

[Aritomi, Yuhang]

Today we found CC1 error signal is only 35.2 mVpp with old mixer while TAMA mixer gives similar value (~150mVpp) as yesterday. We need good mixers for CC1 and CCFC.

We swapped LO and RF of CC1 with TAMA mixer, but there is still offset and the CC1 error signal got smaller (first attached picture: before swap, second attached picture: after swap).

We checked RF power in LO and RF for CC1.

LO: -7.3 dBm

RF: -42 dBm

Note that this 14 MHz CC1 LO should be divided for CCFC LO.

This entry is a log on Aug. 11.

I opened the chamber in order to improve the alignment of the cavity.
The transmitted power was improved by a factor of 2.

Then I closed the chamber and started evacuating.

[note]

One temperature sensor shows fluctuation.
It may come from the connection of the cable inside the chamber.
I think it does not have serious impact on the experimet because the fluctuation is within 0.2 K though our requirement is about 1 K.

[Aritomi, Yuhang]

Today we found WFS1 optimal demodulation phase changed again. The optimal demodulation phase for WFS1 is changed from 107.5deg to 127.5deg.

We tried to decouple pitch and yaw in WFS1 I by changing the rotation matrix of pitch and yaw in WFS1 (elog2149). When the rotation angle is 20deg, 11Hz BS pitch peak in WFS1_I_YAW minimized.

Then we injected a 12Hz line to INPUT/END PIT/YAW and measured sensing matrix again (previous measurement). Although 11Hz BS pitch peak and 12Hz INPUT PIT peak are decoupled in WFS1, coupling in WFS1 for INPUT YAW and END PIT got worse and coupling in WFS1 for END YAW is still bad.