Marc and Yuhang

Recently, we found a new spot of filter cavity elog2573, which makes the IR locking accuracy much better (the spectrum below~3Hz reduce by up to a factor of 10). At the beginning, we thought we found a more stable optical axis. However, we did a test of GR length correction signal when using old and new spot, which shows pretty similar spectrum at frequency region below ~3Hz (attached figure 1). Since the GR length correction signal below ~3Hz tells us mirror motion information, this means the mirror motion is similar for the old and new spot.

Meanwhile, the correction measured in this time is different from elog2312. Especially, it seems more high frequency signal is sent to end mirror.

When we leave z-correction loop open, if we send 1000 excitation at 0.1Hz to channel "END-len-ex2", we get 32.4 counts from PZT mon(1620counts from figure REF4, which is amplified by 50), which means 3240 counts are sent to main laser (PZT mon is 100 times smaller than the signal sent to main laser PZT). 3240/1000*0.31=1V. It corresponds to 2MHz of main laser frequency change. It corresponds to 2um change in cavity length.

When we leave z-correction loop closed, if we send 1000 excitation at 0.1Hz to channel "END-len-ex2", we get 1934 counts from z-correction loop correction signal. (there are still 10 counts sent to PZT, but since it is so small, we neglect it). Therefore, 1934 counts corresponds to 2 um length correction.

For correction signal sent to end mirror, the calibration factor is thus 1.034 um/kcount.

(the data of this calibration is saved in standalone desktop/detuning/20210628)

Because Pin ~3W seems quite convenient to measure viewport absorption (as it makes the 2 surfaces visible on the AC signal), we performed again the absorption measurement of the spare viewport.

Figure 1 shows a long Z scan of the translation stage with the 2 surfaces visibles and at the expected position.

Figure 2 and 3 show the absorption of respectively the spare and cleaned viewport.

The spare viewport shows some dusty spots (some visible by eye+ strong green light).

The cleaned viewport shows more dirty spots  (also some visible by eye+ strong green light).

However, it seems that there is no more large stain pattern visible.

In the last week, resonant condition between GR and IR changed by around 20Hz.

To check if there is any correlation with suspended mirrors, I checked the oplev signal of all mirrors. Basically all mirrors are staying in the same orientiation, but end mirrors have quite obvious drift during the last week. In fact, this drift seems to be not really because we are not really correcting it by the coils. So we need to investigate why end mirror oplev is behaving like this.

The high frequency noise is same for old and new beam spots, but is increased for 50Hz detuning compared with the one on resonance. This noise difference could be explained by the cavity pole effect. The cavity pole effect for 50Hz detuning (half detune) is smaller than the one on resonance by a factor of ~sqrt(2). Please check P.50 of LIGO-T1800447 for the cavity pole effect of detuned cavity.

I measured FDS with CCFC with old beam spot. However, the result is similar to the one with new beam spot...

So the bump around 50Hz and larger detuning fluctuation will not be related to the beam spot position.

We had better FDS spectra before with old beam spot. I don't know why it is worse now...

I was using AOM scanning speed as 4000Hz/1.7s in the calibration. However, since the scanning speed for IR is 1/2 of the value for GR, the figure in the old elog was wrong.

Calibration for the measured spectrum should be: calibration = 2000/1.66666*11.5/11.2 #Hz/V (PDH: 11.2mV/11.5ms) (AOM: 4000Hz/1.66666s)

There was also problem for the calibration for off-center on-resonance, I modified the plot by using a more reasonable calibration. It comes from the center on-resonance. The new plot is shown in the attached figure.

We can see the new stable optical axis makes especially the low frequency length noise reduced. However, the high frequency noise is increased a bit.

Michael and Yuhang

To check the stability of detuning with stable cavity axis, GR AA and GR pointing, we decide to perform FDS measurement. To better resolve the whole rotation part of squeezing, we detuned filter cavity by 200Hz to avoid back scattered noise contamination.

The measurement result is in the attached figure. We can see the detuning fluctuation is about 10Hz from this measurement.

We still miss the homodyne angle around 90deg, we should take it soon.

Today I decided to further increase the laser power. Indeed my concern was coming from the fact that we could not clearly see the effects of the viewport 2 surfaces on the ac nor the phase signal.

I choose HWP angle = 55 degrees which translate to Pin = 3.135 W.

Figure 1 shows the results of a large Z scan of the translation stage : The 2 surfaces are now visibles (spikes in the AC/DC and phase jump) !

The surface with the smaller Z is the surface we want to measure (it is close to the expected value of 41.5 +30 mm ).

I checked the tilt of the viewport and it is still around 0.3 mm over the entire map area (30 mm radius).

A result of absorption measurement is presented in figure 2.

The strange thing is that it is really coherent with previous measurements....

On this measurement, one spot was saturating the AC signal so I started a new measurement with sensitivity 1 V (max)  instead of 50 mV.

I really suspect 2 high absorption spots to be due to dust as it is quite visible by eye and seems different than the other drop like stains.

If this assumtion is correct (maybe can be checked with  another measurement after applying first contact), it means that after cleaning, most of the remaining dirty things are mainly generating absorption below 100 ppm.

[Aritomi, Michael, Yuhang]

Today CC2 mass feedback was very unstable with gain of 2.7. We found that coil output to input mass was too large, so Yuhang offloaded the input mass with picomotor. After that, CC2 mass feedback becomes stable with gain of 2.7.

We tried to figure out the cause of 100Hz bump and found that the glitch appeared in CCFC error signal when we touched the SMA cable for CC1 mixer. We tightened it.

Then we measured FDS with CCFC (Fig 1). The degradation parameters are same as elog2597. The 100Hz bump maybe a bit better, but still present... The detuning fluctuation is ~20Hz. I noticed that the detuning seems anti-correlated to the homodyne angle.

To determine the squeezing level and generated squeezing precisely, I measured the shot noise and nonlinear gain just after the FDS measurement. The p pol PLL frequency for 20mW was 185MHz and BAB maximum was 282mV with 16mW power meter range. The BAB maximum without green is 56.8mV with 240MHz of p pol PLL frequency. This means the nonlinear gain is 5, which corresponds to the generated squeezing of 10.8dB.

For precise degradation budget, it is very important to measure the shot noise and nonlinear gain just after (before) the FDS measurement.

This measurement was done on 20210622. 

I measured the CCFC locking accuracy with the new beam spot (Fig 1). CCFC calibration amplitude is 166mVpp and CCFC filter is gain of 1000 and LPF of 30Hz. CC2 mass feedback gain is 2.7.

Compared with old beam spot, the locking accuracy without CCFC is smaller. The locking accuracy with CCFC is similar to before because it is limited by high frequency noise.

Then I measured FDS with CCFC (Fig 2). The squeezing degradation parameters are as follows. Here are some points of this result.

• Because of improved reflection mode matching, the propagation loss is lower than before and squeezing level is 2.4dB. The propagation loss is 49%, which is consistent with 36% of propagation loss in PRL paper plus 20% pick off (0.64*0.8=0.51).
• In addition to the 100Hz bump, there is a large bump around 50Hz. This bump should be related to the new beam spot because this bump was not present in the old beam spot.
• The detuning fluctuation is ~20Hz with new beam spot. I think it is better to use the old beam spot for CCFC FDS measurement due to the bump around 50Hz and larger detuning fluctuation.

sqz_dB = 10.8;                    % generated squeezing

L_rt = 120e-6;                    % FC losses

L_inj = 0.32;                     % Injection losses  

L_ro = 0.25;                      % Readout losses (propagation loss is 49%) 

A0 = 0.06;                        % Squeezer/filter cavity mode mismatch

C0 = 0.02;                        % Squeezer/local oscillator mode mismatch

ERR_L =   1e-12;                  % Lock accuracy (m)

ERR_csi = 30e-3;                  % Phase noise (rad)

Abe, Marc

Following the Z scan of the cleaned viewport (see entry 2595) we decided to cross-check the relative alignment of the pump and probe laser using the surface reference sample.

We installed it and found out that it was quite misaligned (see figure 1)....

It means that over few days the alignment deteriorated quite a lot. We would really investigate how to get more stable alignment (maybe using similar mount provider as for the FC critical optics reported in entries 2583 2593)

We then spent most of the days trying to recover the proper alignment condition and finally reached the condition in figure 2.

To go from figure 1  to figure 2 we :

- checked the IU position (still 68 mm)

- checked the translation stage z position. By trying to have same amplitude of the lateral peaks in the AC signal we got z = 41.5 mm.

This gives us a new calibration factor :

ac = 0.4515 V
dc = 4.178 V
acdc = 0.108
p = 29.8 mW
R = 16.47 /W

Finally we reinstalled the cleaned viewport on the translation stage.

Abe, Marc

On Monday we decided to further increase the laser power by tuning the HWP.

In order to avoid burning dust and/or stains, we decided to do smaller map (radius = 9 mm) centered at X = 324 mm and Y = 134 mm.

Indeed, in this area no spikes were present in the previous measurement (see entry 2589).

The resulting map with Pin = 0.6236 W is presented in figure 1. In this measurement the lockin amplifier sensitivity was set to 500 uV.

There was some spikes saturating but we thought it could be good enough to estimate the viewport surface background absorption.

The resulting absorption is presented in figure 2. Again, the background level seems lower than the spare viewport one (see entry 2585). Another difference is the presence of many spikes (~2 times larger absorption than overall area).

This lower absorption could be explained by several possibilities (too low power to distinguish absorption, misalignment of probe and/or pump lasers, viewport tilted, properties of spare and cleaned viewport are differents, cleaning damaged the surface, ....)

To eliminate possibilities, we increased the power to Pin = 1.2503 W (HWP angle = 55 deg). and got the results reported in figure 3. The background absorption level stayed the same, meaning that we are indeed sensing absorption.

(note that the spike absorption level is not meaningful because the locking amplifier was saturating on purpose).

To eliminate the possibility of a viewport tilt, we then started to do Z scan to check the surface position across the viewport surface.

We changed back the laser power to Pin ~ 0.6W and did a Z scan at the top position of entry 2585. Result is presented in figure 4.

While the phase exhibits the expected behavior, I'm a bit more surprised by the AC and DC shapes...

I measured the visibility between LO and BAB with improved reflection mode matching and new beam spot. The measurement method is same as the previous measurement.

To know the BAB power during the measurement, BAB pick off power was also measured in CCFC port. Fig 1 shows the BAB power before AMC and the BAB pick off power. From this measurement, we can know the calibration factor from the BAB pick off power to the BAB power before AMC (bottom plot in Fig 1). The calibration factor is 0.25. The LO power was 1.21V. The offset of visibility was 8.5mV and the offset of pick off was 3.8mV. 

The measured visibility and BAB pick off are shown in Fig 2. The 10Hz modulation was applied to IR phase shifter. To calculate the visibility, I divided this data into 500 segments with 0.02s step (Fig 3) and calculated the visibility in each segment (Fig 4). From the histogram, the visibility is 0.98(1) which corresponds to optical loss of 4(2)%. We could recover the good reflection alignment!

[Aritomi, Yuhang, Michael]

First we replaced mirrors and mirror mounts in the IR reflection and LO path.

We replaced 1 mirror mount (FMD MM1000S) and 3 mirrors (layertec) in the reflection path. We also replaced 2 mirror mounts (FMD MM1000S) in the LO path.

Then we moved a lens in the reflection path to improve the reflection mode matching. We moved a lens which is closer to homodyne in the reflection path.

Before moving the lens, we checked reflection alignment with AMC. TEM00 was 872mV and mode mismatch was 54.4mW, which means mode matching without misalignment is 94%. This value is consistent with the previous visibility measurement.

We moved the lens and reduced the mode mismatch from 54.4mV to 4mV as follows. 

Now the effect of mode mismatch is only 0.5% and the misalignment is larger than the mode mismatch.

I checked the IR injection alignment. At the begining of today, the mode matching was 92.1% as follows. The IR injection was 455uW.

Note that the misalignment 1 means top left and bottom right are bright and the misalignment 2 means top right and bottom left are bright.

Then I aligned the IR injection. The mode matching became 94.4% as follows. The IR injection was 442uW.

Abe, Marc

On Friday we inspected the spare viewport with a green light.

By eye it is really hard to see remaining stains but we could see few dust particles. We tried to remove them with an air duster but at least one remained quite close to the viewport center.

Then we started again absorption measurement (radius 30mm and step size 2 mm) looking carefully at the lockin saturation.

We used Pin = 0.0308 W (ie HWP = 39 degrees) and started with lockin sensitivity of 100 uV.

There was 2 positions with saturation :  (X=349.8mm,Y = 125mm) and (X = 315mm,Y = 126.6 mm).

We increased the lockin sensitivity to 500 uV (still saturating) and finally 2mV where no more saturation was visible.

The absorption map with this last setting is reported in figure 1. We can see that the maximal AC value of these two spots is ~ 700 uV which corresponds to absorption of ~ 300 ppm.

With this low power, it is still not possible to see the absorption (except at the 2 dirty spots) so we increased the laser power to Pin = 90.6 mW (HWP = 41 degrees) and sensitivity to 5 mV.

Still no absorption visible.

We are not sure how to estimate the 'damage threshold' of this dust so we decided to take absorption measurements slightly shifted (X_center = 360.5 mm) with a smaller radius (21 mm) in order to avoid these dirty spots. We increased the laser power to Pin = 0.2392 W (HWP = 44 degrees) and sensitivity = 20 mV.

This absorption map is reported in figure 2.

We can see higher level of absorption at the edge of the map which corresponds actually roughly to 1 cm of the edge of the viewport.

This is reasonable if the cleaning was not performed at the edge of the viewport.

For further measurement, we would need to reduce further more the map radius to be sure to avoid burning the stains.

However, as seen in figure 3 (same measurement as in figure 2 but with constrained colorscale limits), there  seems to be absorption spots with absorption > 100 ppm appearing in this area..

To understand how stable the GR lock can provide for the detuning, I would propose the record AOM frequency few times one day. And up to few days or weeks at least.

Note: we should try to not change alignment condition and use the same pointing.

1. 2021/06/20 12:10 109.538080MHz

2. 2021/06/21 09:20 109.538070MHz

3. 2021/06/21 14:21 109.538070MHz

4. 2021/06/21 21:37 109.538060MHz

5. 2021/06/22 09:27 109.538070MHz

6. 2021/06/22 13:29 109.538075MHz

7. 2021/06/22 15:53 109.538065MHz

8. 2021/06/23 09:07 109.538055MHz

9. 2021/06/23 13:27 109.538055MHz

10. 2021/06/23 21:40 109.538064MHz

11. 2021/06/24 09:53 109.538066MHz

12. 2021/06/24 13:53 109.538070MHz

13. 2021/06/24 20:02 109.538060MHz

14. 2021/06/25 16:06 109.538060MHz

15. 2021/06/25 19:02 109.538050MHz

16. 2021/06/26 10:37 109.538047MHz

17. 2021/06/26 20:06 109.538040MHz

18. 2021/06/27 11:04 109.538045MHz

19. 2021/06/27 19:21 109.538030MHz

20. 2021/06/28 10:01 109.538050MHz

21. 2021/06/28 15:00 109.538020MHz

22. 2021/06/28 22:00 109.537910MHz

23. 2021/06/29 15:59 109.538030MHz

24. 2021/06/30 09:56 109.538070MHz

25. 2021/06/30 20:19 109.538060MHz

26. 2021/07/1 11:01 109.538065MHz

27. 2021/07/1 20:12 109.538065MHz

28. 2021/07/2 08:54 109.538060MHz

29. 2021/07/2 20:06 109.538030MHz

30. 2021/07/3 07:06 109.538024MHz

31. 2021/07/4 09:29 109.538060MHz

32. 2021/07/5 16:37 109.538050MHz

33. 2021/07/6 10:23 109.538030MHz

Yesterday, the IR transmission was more than 400, but it was only ~200 today. Although we could recover the IR transmission more than 400 easily, the stability of injection alignment might not be good even with new mirror mounts. According to Yuhang, the alignment can be changed just after the new mirror mounts are placed. Let's see the long term stability.