NAOJ GW Elog Logbook 3.2
Figures were missed in the entry. They are shown here.
Open-loop transfer function
On 12/21, we measured open-loop transfer function of SHG control, because a new servo was designed and installed by Matteo Leonardi. For the previous situation, please refer to an entry 585.Fig.1 shows open loop transfer function of the SHG control. We have three switches on the new servo, each of them is corresponding to a different integrator. Measurements were conducted for three combinations of these integrators. Noise level inserted were different depending on the conditions in order to keep the cavity being locked. PZT resonance point is located aroun 25 kHz. Unity gain frequency is shown on each graph by a black line (770 Hz, 1380 Hz, and 1610 Hz). You can see a high phase margin (approx. 90 degree) when the mid integrator ON.
We also measured power spectra of an error signal on the loop and DC signal from the photo detector which is monitoring transimitted light from the SHG (Fig.2). You can see a spike around 600 Hz. I am not sure what it means.
All spectra were taken by Agilent 36540A.
Output power stability
Fig.3 shows a long term (1000 second) stability of the SHG output. Stability of the SHG is much better than before. In this moment, we do not find a large fluctuation in the SHG output (532 nm) as we did so far.
Figures were missed in the entry. They are shown here.
A comment on the local controls large output: the voltage range at the output of the DAC is +/- 10 Volts. So If the correction signal is out of this range (as in the case of picture 1) the output is saturating and the control loop is likely not to be working properly (as can be seen from the large RMS in picture1).
More in general, looking at the error singnal, it seems to me that we are still affected by the "spikes issue". It could be useful to compare the open loop rms with the reference values in the absence of spikes.
We know for PDH control, the local oscillator phase and signal phase must match. But the cable length can affect this phase. So after changing cables, we need to set the phase of signal generator again.
Firstly, we need to close the loop for local control to make the light beam interferes well inside the cavity. However, I found the local control goes oscillation even with the previous value. The previous value means the value we used for locking yesterday. I should mention this abnormal case happened the day before yesterday.
After a very long time of adjusting, I found the the local can be fairly stable while the output of local is large(Fig 1). I adjusted it a little and make the output much smaller(but it is still around 40). Then I could find the beam through the end room camera.
Secondly, I took the photo of beam on the first iris(Fig 2,3) and the second iris(Fig 4).
Thirdly, I went to set the PDH phase. I set different value to see how the error signal looks like. I took three pictures(Fig 5,6,7) for this processes. We can deduce from them the worst case should between 80 and 90 degrees. So I set the phase as 175 degrees.
Finally, I found the transmitted signal decreases from 1V to 0.6V(Fig 8). And I found this is not caused by the SHG. Now the SHG locking is pretty pretty good! This is caused by the bad local control. The position of mirror drifts away fast. However, this maybe also caused by the large output of local control. Besides, I and Matteo measured the TF of BS pitch. Matteo said it is not a good control system according to this TF. So the local control definitely should be improved.
A comment on the local controls large output: the voltage range at the output of the DAC is +/- 10 Volts. So If the correction signal is out of this range (as in the case of picture 1) the output is saturating and the control loop is likely not to be working properly (as can be seen from the large RMS in picture1).
More in general, looking at the error singnal, it seems to me that we are still affected by the "spikes issue". It could be useful to compare the open loop rms with the reference values in the absence of spikes.
see the attached document
While I check the first iris, I find the pattern on it is really similar with the pattern we get from infrared. So I think the problem should come from the bench. Refer to figure 2.
But the locking is not stable enough, I will check the phase of two signals sending to EOM. Refer to figure 3.
In the entry 613, I made a miscalculation. This entry is a correct one.
I re-calculated optimal lens pairs for a telescope for the mode cleaner (green) with accurate dimensions (see here for the drawing). Figures attached show lens pairs that seem acceptable and the parameters I used.
In the case of the first figure, the mode matching factors are 99.718% and 99.601% for vertical and horizontal axes, respectively.
In the case of the second figure, the mode matching factors are 100% and 99.727% for vertical and horizontal axes, respectively.
The lens with 75.6 mm focal length that we already have is one without any surface coating thus low transmissivity. So I think the first case is preferable.
After the installation of the coverage on optical bench, we are now checking the situation.
Note that our AOM is very easily misaligned by touching the cable.
Temporarily, I put a post right next to the cable in order not to make it move a lot (see a figure attached).
In order to allow their work all the cables from the bench were disconnected and the racks were moved away.
After the company work the cables are being reconnected (ongoing).
1. For leaving space for the shield case, we moved two irises close to PR. Due to one of them is used to fix the lens rail, so we need to adjust the lens again to get good mode matching. Adjusting that lens costed a period of time, and then we can lock the cavity as it before.
2. Then we improvised a tripod to hold the camera, which is used for the near iris in the vacuum tube. Then we adjusted its position. Note that the camera we used here if found on the optical bench in the central room.
3. While we can see the EM transmitted signal, we rise up both of two irises one by one. And we cannot see a clear infrared on the iris. At the same time, the transmitted signal was blocked partly.
By the way, I found the transmitted signal of EM is on the edge of EM chamber view port.
(You can see the infrared checking video by clicking the following link.)
https://drive.google.com/open?id=1CqVLnQrDSaECMgJzHP2Fivabp3ip_E8T
https://drive.google.com/open?id=18AbcXlJ9LMBlW7SOZ7-MdkWTKyISZRya
Firstly, I checked the camera for the second iris. I brought two cameras and one cable around the second iris back. We have a PR's camera, so I used it to check. The method is to replace one thing at a time. In this way, we can diagnose the replaced component. I found the cable is fine but two cameras are broken.
Broken camera 1: SONY NO.92980
Broken camera 2:SONY NO.161274
Secondly, I found the drifting of SHG is really serious.(with the driven voltage set around 50V). Within only 30 min or less, the power decreased from 50mW to 30mW and final 10mW. The measurement point is ahead of filter cavity's EOM.
Finally, I took a picture of the beam which is sent to our filter cavity. You can see from the attached photo. The beam is a combination of a large bright beam point and a small weak point. I cannot make it a rough circle by just adjusting the turing mirror or local control offset.
Firstly, about the losing of the camera of the second target(or irises or aperture). I checked the link and cable. I also changed the driver for this camera. All of them didn't bring back our camera. But I found there was a gap on the case of camera.(As you can see in the figure one.) Maybe this is one reason, so I will check its capability further more.(Since we can see the scattered light or halo, I think there is no need to use the second irises. But brining it back will be a good thing for the future.)
Secondly, about the cable in the end room. I almost stumble because of the cable around the end room's bench. The reason is one of the cables is really short, which makes it suspended away the ground. And this happened on Thursday. On Friday, I cannot find the transmitted signal from the end mirror. I checked many things and finally realized where the problem would be. If you read previous part carefully, you can see the problem comes from the short cable. It causes the misalignment of the QPD. So I think we should do something to avoid this happens again.
Thirdly, I can find something really similar with yuefan's previous infrared beam gazing. You can find the video I took from the link in the bracket. But I tried to change the local control offset for each mirror, finding it's really different from the behavior of green beam. For example, the changing of this possible infrared beam does't follow the changing of the local control offset very well. Sometimes it looks really like the halo after changing the local control offset. I need to confirm this strictly, but the signal here is really weak! I feel the confirmation is really difficult.
(https://drive.google.com/file/d/12OmtNB93hNnNigcgu_0SNxJNy5pxSIPq/view?usp=sharing)
Fourthly, Tomura-san found we can use the sensor card to sense the infrared beam in the end room. I used it to check the light from EM chamber output port to the camera. I found almost one third of the beam is lost because the alignment on the end room bench. So I changed the alignment a little bit. The change includes one lens, one 45 degree mirror(Actually I don't know what this mirror for) and the camera. Now we can see the whole two half of halos by changing the angle and height of camera. You can see the video by clicking the attached link as following.
(https://drive.google.com/file/d/1MsWtaURQJLrXH1PSE4-g_qudrmI73w9c/view?usp=sharing)
Fifthly, Tomura-san suggested there is possibility that something is blocking on the infrared. Because you can see a black line in the spot of reference infrared beam. But I check the layout of our experiment, it's not the beam sending to cavity. However, the reference which is sending to cavity has also something like defect. The defect is the combination of a small spot and a larger spot. It's really like the scenario we can see from the infrared transmitted signal of EM, where there is a bright halo and a darker halo.(You can see this combination from the second video)
Sixthly, I checked the bright halo by changing the local control offset. As you can see from the attached second and third photo, there is angle changing of halo. And the halo is flashing. I suspect this halo can be the what we want to find. I will try to figure it out soon.
In the end, I can ride the bicycle very well now! I need to go to end room and go back many times to find the signal, which drives me to ride bicycle many times. This is really an useful skill and a good way to calm down or feel balance. &( ^___^ )&
I screw 4 short pedestals below the M-UMR12.80 linear stage for the HeNe imaging unit, I clamped it with 4 forks on the breadboard to be parallel to the beam.
Since there was not enough space for the forks, I had to move the hole breadboard by a few cm.
I did the same with the other linear stage UMR12.40 for the 1310nm probe. Since it is imperial, I had to look for a set of 4 imperial screws in ATC.
All the parts I used, I washed them in the ultrasonic bath in ATC. Except for the stages themselves because the bearings have grease, so better not to wash them.
after moving alle the optics of the IPC I had to realign the pump laser beam on the pinhole (at the reference position of the cross point).
First I adjusted the height of the beam on the optical table from the IPC to the periscope (128mm)
Then I removed the last focusing lens on the optical breadboard (after the periscope).
I used two positions of the pinhole along z (max and min position limits), and maximized the power transmitted by the pinhole. I moved back and forth the translation stage on those positions. On the closest position I adjusted the second stearing mirror, on the further position I adjusted the periscope mirror. For the first rough alignment I also used an optical cross.
Now the beam is parallel to the z axis of the translation stage. Then i put the last focusing lens back on its mount and I adjusted the lens position to have the maximum power transmitted by the pinhole.
Since I had optimized the IPC, now, in order to have 30mW on the imaging unit table with 0.8A of laser current, the half wave plate have to have an angle of about 22deg (before optimization it was 0deg)
Then I made a scan of the reference sample with the HeNe Imaging unit, and when I went to optimize the maximum of the AC scan (adjusting the pump focusing lens position) I found that it was already at the maximum.
In the video, we can see the whole halo. But if you check something in the halo, you will find there is only something like the fiber.
I attach the video as following.
https://drive.google.com/open?id=18PApeToBRHC-tDisKg--beNVbhRKi0-E
Firstly, we try to lock the cavity with green beam. The height peak is 1V. After locking the transmitted signal is also 1V. We didn't change anything else but the EOM for filter cavity.
(Because we need to put a case on the top of our bench. The base plate of EOM for filter cavity excesses the boundary. So we change the base plate angle and align it agin.)
Secondly, we follow the procedure of yuefan. We made sure the laser hit on the right position of the membrane of PR chamber. And we also change the height and angle of camera. Then we can see the signal as attached. We supposed that it's the halo as yuefan mention in the procedure. We tried to moved the camera around to find the laser point and we failed. Note here, we can see only a part of the halo. That means we can see only the shape as a sector. I think the reason maybe the incident angle. And I tried to change the incident angle, but find nothing more. I will try to figure out how to find the beam tomorrow.
Keeping the high-power laser current at 0.8A we have 420mW right after the fiber output.
Today the best I could get from the IPC was a power maximum and minimum (as a function of the rotation angle of the halfwaveplate) of:
Right after the IPC
407mW (max)
15mW (min)
On the imaging unit breadboard (after all the optics on the path, including the chopper)
190mW max
3mW min (comparable with the power-meter sensitivity)
In order to get 30mW (the power we usually use for calibration), I increased the laser current up to 1.3A
In order to notice a waist difference (in the absorption signal) between low and high current, we have to be able to have 30mW with high current. I think we have to get the IPC better optimized, or, if not possible, we need to add another IPC
With Matteo, we worked on the red probe last week
before replacing the linear stage with the longer one, we checked what's the maximum signal we could get in this condition
we moved the linear stage to find the max of the AC/DC signal. As usual, everytime we moved the linear stage, we maximize the DC
- Pump laser current= 0.78, power= 30mW
- Reference sample surface
- DC max=3.15V
- AC/DC max =0.061
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Pump laser polarization
With Kuroki, we put a cube PBS right after the output fiber and measured the p and s polarization powers with the high-power power-meter
laser current 0.8A
420mW total power
325mW s-polarization
6-10mW p-polarization (power meter not very sensitive at this low power)
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Input Power Control
we put the half wave plate in front of the output fiber to rotate the polarization into p. Then we adjusted the angle of the first plate PBS to minimize the reflected beam.
Result: minimum of reflected (p-polarized) = 19mW (transmitted400mW)
The strange thing is that the angle is about 64deg, much larger than 56deg
Vertical: w0 = 2.67e-5 m, z0 = 4.73e-2 m
Horizontal: w0 = 2.77e-5 m, z0 = 4.62e-2 m
where w0 is the spot size at a beam waist and z0 = a position of the beam waist.
