I found one of the integrator was not working due to the insufficient soldering.
I remedied it, then the TF of the servo seems what I intented.

However, I found that the gain was too high for stable lock...
I will modify the servo or locking scheme.

[Aritomi, Yuhang]

We replaced a flipping mirror for CCFC with flipping 50:50 BS. We originally had 36% loss and we have 50% additional loss from BS. The total loss will be 68%.

We measured FDS with CCFC lock (attached picture). We also tried to measured anti squeezing, but we cannot have anti squeezing larger than ~11dB so it is not shown in this plot. It seems the nonlinear gain is not optimized.

For HOM angle -1,-14,-28 deg, detuning is around 60Hz and more or less stable, but for HOM angle -38 deg, detuning is 44Hz. I'm sure that green alignment is fine for HOM angle -1,-14,-28 deg, but not sure for HOM angle -38 deg. 

We will measure FDS again and check green alignment at the same time.

Aritomi, Matteo and Yuhang

Matteo implemented the RF amplifiers and filters.

The comparison of each segment is attached. Two orders of magnitude amplification is observed at low frequency.

This file is saved in DGS system in Desktop/AA with name WFS_amplify.

Just a reminder, sometimes we still suffer from the unlock of CC1 loop. This is mainly due to the saturation of CC1 correction signal. We need to investigate how to improve this.

I plotted the obtained ringdown data at room temp., 120 K, and 8 K, respectively.
The results shown in this entry are preliminary though, the decay time at 8 K is less than that at room temp.

It should be noted that the measured data at 120 K was 3days after the cavity reached 120 K.

I will analyze the optical loss of the folded cavity.
In addition, I am planning to compute the dynamical response of the cavity.

[Matteo, Eleonora]

In order to transform one of the broken TAMA demodulation board into a simpler phase shifter (to be used for CCFC lock) we removed the mixer and shortcut the output of the phase shifter to RF IN. 

We measured the trasfer function between LO in and RF IN which is now an output. See attached picture. The TF is in agreement with what we expect from the phase shifter data sheet (SPH-16+). 

[Aritomi, Yuhang]

Carrier and CC AOM frequency are as follows.

We changed a cable length for CCFC LO and checked the CCFC error signal. The result is as follows (Pic.1,2,3 are 0,2,4 m cable length).

Then we added the CCFC I phase error signal to perturb of green FC servo. We used SR560 with lowpass filter 0.1 Hz and gain 200 before injecting the perturb. Then CCFC stably locked!

When SR560 gain is 1000, it oscillates with 88Hz (Pic. 4).

We measured CCFC phase noise. For CCFC calibration, we used Pic. 5. Since AOM scan speed is 1600 Hz/s, the calibration factor (Hz/V) is 1600 Hz/s*70 ms/40 mV = 2800 Hz/V. Measured CCFC phase noise is shown in Pic. 6. Note that free run CCFC error signal is out of linear range and the spectrum could be wrong. I also compared with IR locking accuracy we usually use (Pic. 7).

We measured the ratio of CCFC OLTF and green OLTF with SR560 (Pic. 8). The crossover frequency is 40 Hz.

When the CCFC demodulation phase is optimal, CCSB frequency separation can be obtained from the distance of two dips in Q phase signal since the two dips correspond to CCSB resonance (Pic. 9). From Pic. 2, time difference of the two dips is about 120 ms and therefore frequency separation of CCSB is 120 ms*1600 Hz/s = 192 Hz. This is a bit larger than optimal value 108Hz. Note that when the CCFC demodulation phase is not optimal, it will also change the distance of the two dips.

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.