Michael and Yuhang

This Friday (20220114), we changed optical set-up from what described in elog2788. This is because we found one of the steering mirrors in front of OPO cannot effectively align laser beam inside OPO. We checked beam parameters in elog2486, which shows that the non-effective steering mirror is located just around the beam waist after 40mm lens. This explains why this mirror shows anomaly while OPO alignment procedure.

To solve this problem, we modified the optical set-up. We add one steering mirror just before OPO while leaving enough space for a BS between it and OPO (Fig.1). To confirm this set-up is capable of aligning beam and the OPO still has good internal alignment, we aligned OPO while laser beam is injected from the crystal side of OPO. We found the newly add steering mirror helps to optimize alignment.

In the end, we achieved a decent alignment by moving the position of lenses. The TEM02 mode is optimized by moving position of lenses. The position of lens are shown in Fig.2 and Fig.3.

We also characterize the mode matching level by measuring the TEM00, TEM01 and TEM02 modes. Their seperate power is shown in Fig.4, 5, and 6. This indicates a mode-mismatch of (5.6+18.8)/(5.6+18.8+282) = 8%

Using parameters of OPO, we simulate the beam parameters as Fig.7. From this simulation, we see the beam waist should be located in front of crystal when injected from the crystal side. But if we inject from in-coupling mirror side, the beam waist should be located inside OPO. Therefore, to measure optical losses and flip OPO housing, we need to flip OPO and move it OPO by 54mm closer to 75mm lens.

[Aritomi, Yuhang]

Last year, we found that the BS picomotor in yaw direction does not move as reported in elog2767.

Today, when we tried to move the BS picomotor in pitch direction with step of 50, the picomotor moved a lot and got stuck. We will open BS chamber to check the picomotor next Monday.

Michael and Yuhang

As discussed in elog2784, we need to inject laser from OPO in-coupling mirror side to distinguish different OPO internal losses values. This is the reason of work reported in this elog. To achieve the measurement, we have done the followings:

1. Remove periscope and rotation stage for OPO

2. Prepare a 50:50 BS placing in front of OPO to extract OPO reflection signal information

3. Lower the height of OPO transmission BS, PD and camera

(these works are shown in Fig.1)

4. Confirm and adjust the flatness of laser beam after removing periscope

(this work is shown in Fig.2,3,4)

5. Put OPO in place and optimize its position by hand. Try to get a reasonable OPO transmission mode. We have found a TEM01 mode now.

Next step: Find OPO transmission signal on PD. Optimize OPO injection beam alignment.

The camera used in this set-up is found to be tilted today. Fig.1 shows that this tilt is introduced through the interface between camera and post.

During the alignment, we use camera to characterize cavity HG modes shapes. When the beam and camera height is aligned, we found beam appears not in the center of screen as Fig.2. We suspect the tilt between camera and post is the cause of the cavity modes mis-center as Fig.2.

Note that Fig.2 shows the non-ideal alignment of crystal, which has a black defect.

This entry reports the measurement of polarization angle and birefringence of the shinkosha 7 sample measured in the same orientation as Manuel absorption measurement (ie arrow at the top pointing towards the imaging unit).

It is in quite good agreement with the previous measurement.

Note that the birefringence results reported here neglect the angle of incidence of the pump beam

Considering the CCFC+green OLTF shown in elog2758 and the IR locking accuracy (CCFC error signal) without CCFC, we can estimate the expected IR locking accuracy with CCFC. I compared the estimated IR locking accuracy with CCFC and the measured one as shown in the attached figure. As you can see, the two spectra agree well. The discrepancy between 10-100 Hz could be because the measured spectrum would be limited by the spectrum analyzer noise between 10-100 Hz.

Since OPO is finally closed, the next step is to characterize the intra-cavity losses. This is important for us because we are suspecting some of the optical losses are from OPO (current estimated loss budget for FDS). So this is an important step to understand the loss budget in the frequency dependent squeezing experiment.

I modified a bit the code to see the difference of measurement for different OPO intra-cavity losses.

Now, the laser is injected from the crystal side of OPO. I did a simulation of this case as Fig. 1. In this case, we will miss the information of OPO reflection. The blue and orange curves overlap for reflection.

If laser is injected from in-coupling mirror, as shown in Fig.2, we find that although decay time is not enough to indicate optical losses. We can extract losses from reflection signal.

So we will rotate OPO next week and inject laser from the in-coupling mirror side.

Michael and Yuhang

After the in-coupling mirror cleaning as reported in elog2774 and elog2776, we tried to close OPO again. Apart from mirror cleaning, this time, we tried to align OPO better than elog2769. To make sure a good crystal alignment, we paid attention to:

1. Making sure the flatness of laser beam in pitch/yaw direction using alignment tools such as rulers. Use power of few milliwat now.

2. Change to less than microwatt. Making sure camera is at good height and angle. Mark the beam position on camera by drawing a circle for the laser beam shown on TV screen. 

3. Change back to few milliwatt. Putting OPO crystal part on rotation stage with HR side facing laser. Checking injection/reflection overlapping and transmission reaching the circle on TV screen. If not, adjusting by hand the position of OPO crystal part. 

4. Adjusting camera lens to zoom in and inspect the image of crystal transmission. This image should be a circular spot without any clipping. If not, consider to adjust the position of OPO crystal part in transversal, longitudinal and tilt DOF.

5. Fixing OPO crystal part on rotation stage gently. In my case, the OPO didn't move after this. If it is moved a bit, bring it back by adjusting rotation stage. Be careful that the longer side of rotation stage should be aligned with laser. 

6. Putting in-coupling mirror part of OPO and half-fixing it. Half-fixing means that the screws are ~20 degrees before the feeling of tight screws. This is to make sure that we can adjust in-coupling mirror while keeping it not going to far after each adjustment.

7. Zooming out camera to find roughly the flash of OPO cavity. When zooming out, we can see the inner cavity reflection more clearly.

8. Moving incoupling mirror by hand in horizontal direction to make inner cavity reflection good for horizontal. Moving vertical adjusting screws to find flash.

9. Zooming in camera to see the Hermite-Gaussian mode of OPO cavity. The scan speed was set to be 3Hz.

10. Adjuting finely the incoupling mirror position until the mode shape looks to be good enough. In the end, maybe there is still some residual modes. But it will be fine.

11. Change power to few hundred of milliwatt. Putting a BS before camera and direct the light to a PD to see cavity scan spectrum on an oscilloscope.

12. Fixing in-coupling mirror by the sequence of diagonal so that the alignment can be less affected.

13. Adjusting the injection laser to finalize the alignment.

After doing this procedure, we got OPO cavity scan spectrum as Fig.1. The zoom-in of TEM00 is Fig.2. We haven't characterized how much power is on higher order modes. But it looks decent for us.

Yuhang and Michael

We checked the incoupling mirror again after Yuhang applied and peeled First Contact on each day of the holidays. It seems to be much cleaner now. Perhaps the light spots that can still be seen are internal scattering or something like that.

Here is the result of the vertical and horizontal angles of incidence that will be used during the measurement :

vertical ; 4.75 mrad or 0.272 deg

horizontal ; 41.94 mrad or 2.403 deg

Here is the result of spare ETMY bulk absorption with the 2 ears flats.

The measured value agrees with the one measured at Caltech and reported in entry 1583

Abe, Marc and help from Yuhang to move safely the mirror

We removed the spare ETMY but did not placed it inside its box as we might put first contact before the shipping back to ICRR.

We checked the surface and bulk calibration without tuning the pump nor the probe beams alignment and got R_surface = 17.87 /W and R_bulk = 0.6503 cm/W

We measured the pump beam incidence angle and installed the SHINKOSHA 7 on the translation stage (X_center = 398 mm, Y_center = 122 mm and Z_center = 71 mm).

After the usual calibration we started the measurement with s polarization at the input.

Yuhang and Michael

We removed First Contact from the mirror and inspected the cleaning. It seems like some residue of the rubber stain remains and there is a lot of scattered light (fig 1 and 2).

After this we applied alcohol solution from a bottle in the ATC cleanroom (isopropanol based, I think it was called "Clarity" or something like that) in an attempt to remove water soluble stains (fig 3). Then after removing we reapplied First Contact.

The BS issue has been checked again today by switching off the oplev laser since I was suspecting that BS PSD maybe broken. Indeed, after switching off BS oplev laser, the BS oplev signal is as Fig.1 which is the same as what is taken last Thursday. So the strange signal we are seeing is from the broken BS PSD.

To confirm the health of BS suspension, we need to take an another PSD and use it to characterize BS suspension.

Today, we have some progress about guardian deployment in TAMA. Thanks to the help from Yamat. The detailed information can be found in the #filter_cavity channel of gokagra in Slack. Here, I put a summary of my activity today.

1. Yamat helped us check simulink file and found no problem.

2. Yamat suggested to run caget K1:FDS-FC_GR_TRA to check potential issue on client workstation of desktop1 and k1grd0. Running it on desktop1, I got -27.7212 for K1:FDS-FC_GR_TRA. But running it on k1grd0, I got channel connect timed out for K1:FDS-FC_GR_TRA.

3. Yamat suggested to check environment values in guardian computer. To do that, I used 'env > tama_filter_cavity_k1grd0_env_out.txt' to save the environment variable of k1grd0 workstation. Then I used 'scp tama_filter_cavity_k1grd0_env_out.txt controls@192.168.11.110:/home/controls/Desktop' to copy environment variables from k1grd0 workstation to desktop1 workstation. To share with Yamat on Slack, I firstly uploaded the txt file to dropbox and then share link with him. (Note that I didn't just take screenshot because the environment variable information is large.)

4. After Yamat checking env output, he found that a variable called 'EPICS_CA_ADDR_LIST' seems to have a wrong IP address. He suggested to change this from '/home/controls/.bashrc or /kagra/apps/etc/client-user-env.sh.' and reboot k1grd0.

5. I found the channel 'EPICS_CA_ADDR_LIST' is in /kagra/apps/etc/epics-user-env.sh. So I did the modification and reboot k1grd0. But the problem is not solved.

6. I found after modifying from /kagra/apps/etc/epics-user-env.sh, the env output in k1grd0 is still as before and didn't change. Maybe this is why the problem is not solved. Now I am waiting for the answer from Yamat.

Yuhang and Michael

This is a followup to the issues discussed in 2769. 

The OPO cavity was aligned and we attempted to optimise the mode structure of the transmission spectrum. However, there were two second order HOMs we could not get rid of by altering the input steering mirrors, and inspection of the transmission camera showed a lot of scattered light around the main beam.

Today, when we looked at the setup, we saw a lot of dirt on the incoupling mirror, which was visible when shining a phone flashlight below it. So we took it out to apply First Contact on both sides. At this point, we also saw a mark from the rubber o-ring that was mounted on the mirror. There may be some inspection needed for the OPO as well. However, I do not personally recall seeing the scattered light problem when the OPO was being aligned without the incoupling mirror, the most I remember seeing was something like a diffraction pattern (attached figure).

We were also in doubt of the position of the OPO inside the holder apparatus. There is some freedom in the beam propagation direction with respect to the OPO placement inside the holder, and there was no specific instruction given in the assembly directions. Given that the beam size required is extremely small , it seems like there would be strict mode matching required here, and yet we also did not see any LG modes when scanning the cavity.

For now, we applied clear First Contact to both sides of the incoupling mirror. We also want to redo the OPO alignment. Most of previous works have ensured the beam propagation axis is very closely aligned to the OPO axis. But, the incoupling mirror position was not that finely adjusted last time.For example, if the beam is hitting the incoupling mirror off-center while not passing through the center of curvature, the wavefronts will not be perpendicular to the curved surface of the mirror, so there will be mode mismatch introduced. The micro adjusters for the incoupling mirror only adjust transverse x and y position, not tilt.  So we should optimise the beam position on the incoupling mirror, and then for locking make fine adjustments to the input beam steering mirrors.  

Today I did 2 long Z scans (between 20 and 100 mm then 100 mm to 110 mm) with 0.05 mm step size and median/average filters order 10 and could see the phase transition at Z = 37.6 mm and Z = 104.95 mm.

It means Z_center = 71.275mm.

The XY map started with 70 mm radius, median/average filters order 10 for a ~14h long measurement Pt = 6.541 W and Pin = 7.582 W.

It means that laser will be on up to December 30th for the 3D measurements.

Abe, Marc

Here are the vertical and horizontal angle of incidences used during the briefringence measurement of the spare ETMY.

vertical : 4.78 10^-3 rad or 0.27 deg

horizontal : 4.33 . 10^-2 rad or 2.48 deg

To compare the IR locking accuracy (CCFC error signal) for different FC green gain, I compared the 1/f^4 and 1/f filters of FC green lock. The 1/f^4 filter has much larger gain than the 1/f filter at low frequency. The nominal filter is 1/f^4 and the nominal gain is 1.5. For the 1/f filter, I used the FC gain of 5 to have the UGF of 14kHz. Fig 1 shows FC OLTF for 1/f^4 and 1/f filters. Both filters have UGF of 14kHz.

Fig 2 shows the FC green error signal (EPS1, 1Vpp fixed range) with 1/f^4 and 1/f filters. The FC green error signal with 1/f^4 filter at low frequency is below the dark noise and limited by the spectrum analyzer noise. This means that increasing the green gain just reinjects the dark noise.

Note that BS pointing and AA were engaged during the measurement, but Z correction was not engaged due to the problem. I found that 1/f filter with gain 5 is stable even without Z correction, but 1/f^4 filter is unstable without Z correction.

Then I aligned BAB to FC. The optimal p pol PLL frequency without green was 275MHz and BAB power before FC was 466uW. The maximum IR transmission of FC was 490.

Then I checked the CCFC error signal. I optimized the p pol PLL frequency to maximize the CCFC error signal. The optimal p pol PLL frequency with 20mW green was 230MHz and the CCFC error signal was 122mVpp.

Fig 3 shows the CCFC error signal with 1/f^4 and 1/f filters. We can see that increasing the green gain improves the IR locking accuracy above 10Hz, but not below 10Hz. The IR locking accuracy is dominated by the FC length noise below 10Hz and the laser noise above 10 Hz. By increasing the green gain, the laser noise can be reduced because the laser noise is common for both IR and green, but the FC length noise cannot be reduced because the FC length noise is different for IR and green.  

Michael and Yuhang

Today, we have successfully closed OPO cavity. The OPO no.28 component issue is solved by replacing it with a longer screw.

After cavity closed, we optimized OPO cavity alignment relative to its incident beam. However, we found the scan spectrum is as Fig.1.

1. We can see that there are apparently two higher order modes in the scan spectrum. By scanning OPO cavity with 10mHz ramp and looking at a designated camera , we found that the higher order modes are second-order Hermit Gaussian modes. (The cavity scan video is here https://drive.google.com/file/d/1Od2jI40D_3oZuZksuI474tunwtt7znIs/view?usp=sharing , note that it seems cavity mode is not centered in a circle. But this circle is just a mirror. The cavity mode looks to be well centered when mirror is removed.)

2. We can also see that there are modes overlapping with TEM00 mode. It indicates that we are having a cavity length which offers a non-ideal Gouy phase.

3. When we misalign pitch or yaw of the incident beam, we found the first order mode appears but the existing second order mode decreases. This is wired for me.

4. We have identified a potential beam clipping in the optical path before OPO. After fixing the clipping issue, we didn't get better OPO scanning spectrum.

5. We suspect that we may fix either crystal or in-coupling mirror too tightly, so that mirror has distortion and introduces higher order modes. After loosing a bit the screws, we didn't see improvement.

6. We suspect that there maybe some contamination. Especially, the crystal was in air for several months. But we are not sure if contamination is really true because the cleanroom in ATC is designed to be a class 10 cleanroom.

Personally, I think we need to open OPO again and adjust maybe crystal position.