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.

We recovered the IR injection alignment. The IR injection power was 430uW. The mode matching is 94.3%. Note that the misalignment is combination of pitch and yaw (top right and bottom left sides are bright).

I brought in PCI the cleaned viewport. Aso-san helped me confirm that the cleaned surface is the one where there is no more indium at the edge.

I installed this viewport on the holder with the correct surface facing the lasers.

I realigned DC and started the measurement.

The procedure I'm planning to do is to start from 28 mW and increase step by step up to 600 mW (eg 100,200,30, etc..)

I did the first measurement with 28 mW and got the result attached in figure 1.

It seems that we don't have clear signal on most of the map. But there are few points where the absorption seems quite high.

Sadly I was not present in the PCI during this measurement so I can not confirm that there was no saturation of the lockin so these maximal values of absorption are not reliable.

A possibility for further inspection is to repeat this measurement with same power but only around the dirty spots and make sure that the lockin is not saturating.

Depending on the absorption value of these spots, include them or not in the maps with higher laser power.

I got a bit sceptical of the spare viewport measurement reported in entry 2580.

Indeed the absorption seems too uniform with really low value..

• Discussing with Matteo, a possibility could be that the spare viewport is tilted (indeed it is placed inside a tama size holder with the addition of 2 spacers to compensate the 1 cm thickness of the viewport). That would make the surface calibration not reliable.

So I reinstalled spare viewport and aligned the DC with Pin = 0.6248W.

First, I checked the tilt of the viewport by doing Z scan and checking the the Z center of the absorption (computed from the 2 positions where the phase is 0 deg) .

From these measurements, it seems that the difference of the Z center over the radius of the absorption map is ~0.35 mm. This is far smaller than the interaction length ~2 mm (see Manuel PhD).

So we can conclude that there is negligeable tilt with this holder.

• Another possibility could be that the lockin sensitivity was saturating at a low value, making a constant noise level fakes an absorption signal.

However, an easy check is to double the power and check if the AC signal is also doubled. That would confirm that the signal we are seeing is coming from absorption.

Therefore, I decided to do a new measurement with Pin = 1.2545 W (radius 30mm and step size 2 mm) which is roughly twice larger than the measurement reported in entry 2580.

The mean ac value went from 8.1 e-4 to 1.6e-3. This confirms that we are seeing absorption signal !

Finally I also took a new absorption map with this higher power as reported in figure 1.

In figure 2, you can see the same map but with the colorscale constraint between 15 and 20 ppm and we can see the (small)  non-uniformity of the viewport absorption.

I fitted the power as a function of the HWP angle.

Quite useful to know how to increase the power step by step.

[Aritomi, Yuhang, Michael, Marc]

Today the new mirror mounts (FMD MM1000S) arrived and we started the replacement work.

We replaced 1 mirror mount before AMC and 3 mirror mounts in IR injection path.

We also replaced 5 mirrors in the IR injection path with layertec mirror (106910) whose reflectivity is more than 99.9%.

While I mounted the mirror, I tightened the screw too much and broke one mirror...

After the replacement work, we completely lost the IR alignment, but we recovered it. Now the IR transmission is more than 400 with 450uW injection. The IR transmission should be more than 500 with 450uW injection, so we still need to optimize it.

From tomorrow, we will optimize the injection alignment and replace mirrors and mirror mounts in the reflection path and try to optimize the reflection mode matching.

[Aritomi, Michael]

We measured squeezing and anti-squeezing for different green power with CCFC. We changed the green power from 10mW to 30mW with 5mW step. The OPO temperature and p pol PLL frequency are summarized in elog2577.

For green power larger than 40mW, we couldn't find squeezing (~ -130dBm at minimum) and the noise level suddenly changed from -130dBm to -110dBm...

The result is shown in the attached figure. Since we couldn't inject large green power, the phase noise information cannot be extracted.

I measured the round trip losses with new beam spot which has more stable IR transmission. The round trip losses are 139 ppm.

P_res = 245; % BAB reflection power on resonance (uW)

P_in = 362;   % BAB reflection power off resonance (uW)

R_gamma = P_res/P_in;

gamma = 0.05; % uncoupled power to the cavity: 5% from mode mismatch

R = (R_gamma-gamma)/(1-gamma);

T = 0.00136; % input mirror transmissivity

L = (T/2)*(1-R)/(1+R) = 139 ppm