NAOJ GW Elog Logbook 3.2
Since last week, we tried to set up a working telescope to be able to lock the filter cavity with the AOM.
Our design is the following : one lens (L1) before the AOM, two lenses (L2 and L3) after the AOM and before the Faraday isolator.
With the configuration f1=100mm , f2 = 100mm and f3 = 150mm we were able to align the beam into the filter cavity but we saw Laguerre Modes.
By amplifying 5 times the Pound-Drever-Hall signal we were able to lock the cavity but this lock was really unstable.
Indeed, either the input beam in the PR chamber was at a good size but the reflected beam from the IM was too big, either the input beam was too small and the reflected beam was at the good size.
Despite many attents to slightly move the lenses, we couldn't manage to obtain a better beam size.
We also check the beam size on the bench which confirm that this telescop can't work.
We then tried to use a f2=100mm f3=175mm configuration. But this telescop couldn't work neither. We think it was because the beam wasn't collimated at all.
In order to be able to use a bigger value of f3, we decide to move the AOM before the sterring mirror on the bench which allows us to have a bigger range for our new telescop. Indeed we mesure a beam size of 1.3mm just after the beam-splitter close to L1. Because L1 shrinks the size of the beam, we can have a small enough beam size for the AOM between this beam-splitter and the sterring mirror.
Then we choose a f2=100mm and f3=250mm configuration.
With this telescop, we were able to obtain a good beam size on the 2'' mirror and send the beam trough the filter cavity.
We obtain transmission peaks around 1V (better than from the first telescop) and gaussian modes. By amplifying 5 times the Pound-Drever-Hall signal, we could obtain a quite stable lock of the filter cavity on the green with the AO
We check the power exiting the optical table and found out that the AOM first order efficiency is now 67%. We are now trying to improve this value as well as the beam shape ( a bit astimatic) which might come either from the AOM or because the beam size entering the Faraday might be a little big.
We put back a camera on the IR path on the end room optical table.
Because the green power is quite smaller than before, we are able to see the IR modes flashes.
When the filter cavity is locked on the green, it seems that the IR flashes disappear and we can see a lock on some higher modes. We need to check if it is reamaining green or really IR.
Members: Manuel, Kuroki
- Tanioka-san helped us with the crane moving one clean-booth from the first floor to downstairs.
- We washed the prefilter of the clean booth and wipe the walls of the booth inside and outside.
- We stuck the mats on the floor at the entrances of the pre-clean area and put a hanger for the clean suits
- We cut an aperture to enter in the clean booth from the pre-clean booth
- Using some anti-static sheet, we covered the space between the pre-clean booth and the clean booth
- We installed an in-line filter for the air shower, between the membrane-air-dryer and the pressure regulator.
- We moved the electronic parts of the experiment in the pre-clean area (computer, display, keyboard, mouse, and lock-in amplifier) and reconnected the cables
- We wiped inside the clean-booth using wipers and sticky rollers.
- We cover almost all the unused optical table holes with the anti-static sheet. So when we make an air shower of the experiment, the dust doesn't come from the holes.
1. Taking the PR as an example, magnets 1 and 3 for pitch, 2 and 4 for yaw, we checked this by sending noise to each magnet. Also with the same method, we made sure that the magnet we lost is the 3rd one. The original driving matrix for yaw is [0,1,0,-1], the four numbers are the voltage send to 1~4 magnets, which means when we try to move yaw, we don't send any voltage to the magnets for pitch. So even we lost the 3rd magnets, when we send noise to yaw, we should not see the coherence in pitch we can see now.
2. When I tried to change the matrix from [0,1,0,-1] to [0,1,0.1,-1], the coherence should not change since we lost the 3rd magnet, but now it changes.
This situation now make me really confused, so I am not sure how should I change the matrix, so I just did some random try. The picture shows the least coherence I can get when sending noise through yaw.
So the aperture before the FI, the orders are not seperated enough there, the aperture after the FI, we can open a bit to let the beam go through it, but then the reflection beam will be cut. So finally we used the diaphragm we put in front of the window to do the fitler job, showed in the first picture.
One problem of this configuration is that all the orders pass through all the optics after AOM, actually since all the optics are aligned with the first order, so I think for the beam deformation maybe there will be no problem, but I am not sure if this will affect the FI.
After aligning everything and having a reasonable beam, we tried to lock the cavity again. Using the Stanford to amplify the PDH signal with gain of 10, the cavity can be locked again(pic 2), but not very stable. The alignment seems cannot improve more, so I think it is still have something to do with the mode matching.
Then I tried to change the matrix. The situation is when we send noise in yaw, we have high coherence in pitch. But actually the matrix we use for this situation, the coefficient of two coils for pitch is zero. So it seems a bit strange to have this situation. Then I tried to change the coefficient for the three still-working coils little by little, the coherence of pitch got smaller, but still there.
https://www.dropbox.com/s/nerrc4i4yx3bw8v/VID_1.MOV?dl=0
Also last week we found out the lens combination did not work well because I made some mistake in the calculation, so we tried to measure the better shape first order again, and found a new solution. The second lens should be put 40.1cm from the MZ beam splitter and the third 62cm. In this new configuration, all the lenses are in front of Faraday, so we don't need to worry about the affect to the reflection beam.
The other problem we mentioned in entry 457 is that all the optics on the 2nd FI path are not along the hole, so we can not screw the rail on the bench, so we used clamps. The path finished shows in the pictures, the way of clamping the rail now does not look very fixed, we will improve it later.
Since the new beam splitter has been installed, the green power injected into the cavity now is around 8mW.
With everything settling down, we could inject the beam into the chamber and align the cavity again. But today we are not able to lock the cavity, one reason is that the input mirror local control cannot hold the mirror fixed for long time, so while we were trying to find better mode matching, we continued to lose the alignment of the input. The other side, the beam produced by the new configuration is different from before, according to the calculation, the beam size arrive at 2 inch mirror should be close to our previous one, but the beam from the bench is much smaller than before and reflection of the input is larger than before. It could be the reason why we always have some Laguerre modes. So tomorrow we can try to change the driving matrix of the input and PR mirror, get better control of them and try to see if we are able to lock or not.
https://www.dropbox.com/s/nerrc4i4yx3bw8v/VID_1.MOV?dl=0
When we tried to study the MC finesse we did a mistake.
As we can see on the picture, it seems that the two TEM00 peaks are on the same part of the ramp (meaning same sign of cavity change length). But there was an offset as the TEM00 peaks show some small peaks symetric with respect to TEM00 peak.
So we took new data (second plot).
Finally we could extract the following parameters using the following equations : T0 / ( 1 + 4r/(1-r²) * sin ( PI (x - x0)/ FSR ) ² )
T0 = 0.98
FSR = 8.2072 ms
bg = 0.0205
r = 0.9902
x0 = -1.617 ms
with a R² factor = 0.9929
We could mesure the FMWH of the peaks at 0.026 ms which give us a finesse = 315.66
With all this reference today we started to install the AOM. The first lens has 100mm focal length which put as close as the original beam splitter. Then installed a new beam splitter after this lens and align both the transmission and reflection path of it.
The AOM was put after the mirror to get a good beam size, with the steering mirror, I aligned the first order straight first and then by adjusting the stage to increase the efficiency of the first order. It took long time for us to adjust the AOM, but still cannot get a perfect round beam, then we check the beam entering the AOM, from there the beam already has some strange shape halo around it. But before the beam splitter, we did not see this strange things, I think this is what we expected to see here, since when we installed the original third lens of the telescope, we already found out the shape affected a lot by the lens position. We will try to align better.
Just for testing the new telescope works or not, we put the other two lenses at their position, but it seems this combination does not works well as we tested it. One of the reason I think it may because when we did the test, the first lens position is not as close as now to the beam splitter, but I preferred to keep the first lens position like it is now, since this one has smaller divergence. This means we need to find other lenses, so we measured the beam size of the first order again after the AOM. Now the result I got is that we need to put the other two 250mm lens, one is at 40.5cm, the other at 88cm, which is doable.
Another problem we found it is that all the optics on the second Faraday path is not along the holes. So if we put the lens on the rail, the beam will be quite off-center and the range we can move with the screw is not enough to let the beam pass through the lens center.
At the beginning of this week we proceed some extra checks of local control problems.
First of all we get some transfer functions of some mirrors. Attached to this entry are some strange TF. For example the PR TF shows that when we inject noise in yaw, there is a correlation with pitch. But this correlation doesn't seem to appear when we inject noise in pitch. Also, IM TF shows correlation between pitch and yaw wherever we send noise.
This confirms our first idea that one magnet of PR has probably fallen.
We then proceed to do some others tests.
First we used the "normal vi" and not the latest version which is not able to get a proper feedback.
Second, we check what happend when sending noise to each coil :
The coil number 3 of PR doesn't respond.
All BS coils seem ok but every coil impact both pitch and yaw (mainly yaw)
IM coils (2,4) move only yaw , coil 3 move only pitch but coil 1 doesn't respond.
EM coils seem good : coils (1,3) move only yaw, coils (2,4) move only pitch
Third we manage to align roughly the cavity again using picomotors.
Finally, after removing offset of local control, we could align the cavity using local control.
We locked the cavity but the stability doesn't seem to last for very long.
Finally, we could get all the references needed to be able to align the beam with the AOM.
Yesterday (August 7th 2017) , in order to get references, we tried to lock the cavity. However, it was impossible to control some of the mirrors due to saturation of the correction signal.
Last lock was effected the previous monday ( July 31st 2017 ).
There were some earthquake in the mean time ( August 3d, 4th 2017) which we suspect removed one magnet from the PR suspended mirror at least ( if we excite one of the coil of this mirror there is no change in error signal from local control, 3 others coil shows some changes )
We also had some trouble with the BS suspended mirror but it seems to be due to an offset (-0.2 on yaw, 4.1 on pitch). Once removed, the results were coherent.
Down are listed the results of some of the test we did to chack the saturation problem :
| open-loop (YAW;PITCH) | correction with open-loop offset value (YAW;PITCH) | max/min values for saturating the correction | |
| PR | 0.242 ; -2.6 | 0 +/- 0.1 ; 0 +/- 1.5 | 2.945/-5 ; -1.2/3.9 (high correction values before + every changes saturates (peak) ) |
| BS | -0.44 ; 0.54 | 0+/- 0.1 ; 0 +/- 0.5 | 1.72/-2.52 ; 1.065/-0.835 |
| IM | -0.356 ; 0.145 | 0 +/- 0.01 ; 0+/- 0.2 | ok/ok ; ok/-1.14 |
| EM | 0.55 ; 3.04 | 0.27 +/-0.1 ; 2.1 +/-0.1 | ok/ok ; ok/ok |
It also seems that the EM has some trouble ( pitch correction with open-loop offset is really different from 0 )
For both IM and EM there was "PSD tilt angle = -0.04 "
As usual, we tested the AOM on the transmission of the beam splitter. From our last test the AOM should be put 17.5cm from the beam splitter, but after checking, there is not enough space to put the AOM before the mirror, as close as we could to the mirror we can put the AOM 25cm from the beam splitter. And then if we put the second and third lens more or less at the same position as our last test, when the beam arrives at the position of FI, the diffraction orders are not separated enough to filter by the aperture. Then we tried to move the two lenses, situation did not get better.
So our idea is to filter the beam after the AOM but before the second lens. So we tried to do this and wanted to check the beam size after so we can find another proper combination of lens. When we measure the beam with the beam profiler, we found out the first order is very elliptical. We tried to align the AOM better, in the picture you can see there are some black lines which cut the beam into many pieces. Actually we had this kind of lines from the very beginning, but before we can get rid of them and get more or less a round first order with good alignment. This time we tired again and again, but still cannot get a good beam shape. We already sent an email to the company to ask if the black lines are only caused by bad alignment or something else. Meanwhile we checked the beam and found out the beam transmitted by the beam splitter itself has some tilt, and also the first 100mm lens could bring some astigmatism.
The other thing is that with the AOM position we decided today, we have only about 30cm to put lenses and recover the beam, although without the beam size measurement we cannot say if this space is enough or not, but if it is not, the other solution we considered is that after the FI and the waveplate, we have about 7.5cm space where we can put the lens. If we have to do this, one problem is that we can only take the reference outside the PR chamber, the other problem is that we are not sure if this will effect the beam reflect back to the PD which is used to lock the filter cavity.
After installed this mirror, the infrared became even weaker than before, I took a picture (pic 1)but it maybe too dim to see it. So when the cavity is not locked, we still can see the two infrared beam is flashing.( We are not sure why we can see two infrared beam very close to each other on the screen, maybe because some reflection between the two mirrors. ) When the cavity is locked the two beam both got stable. We tried to cut the infrared on the bench and we are sure what we saw is the infrared.
Then we tried to take a video of the infrared when the cavity is locked. This is the video: https://www.dropbox.com/s/gvsj0lsoei1pmqa/VID_1.MOV?dl=0
So this is the video I took when I asked Marc to jump in the central room, so you can see there are two beams moved to each other in the video.
To access to the MC finesse, we tried to use the following fit : T0/ ( 1 + 4R / (1-R)² * sin² ( pi (x-x0)/FSR ) ) + bg
Attached to this report are two results we could get : one without normalization, one with normalization.
By using Finesse = 4R / (1-R)², we could get the following results :
First case : 209
Second case (with normalization) : 360
By checking the ratio FSR over peak width : Finesse = 150.
We will try to undestand better how to fit this function
Today, it was not possible to lock the cavity because of a drift of the end mirror.
When we went to the end room to reset the local control, the temperature and humidity of the end room was quite high.
It appears that a air ventilation system was off. As soon as we turned it on, it started to feel more confortable.
After resetting the end mirror local control, it was possible to lock the cavity again.
I installed the 1310nm laser and the relative Imaging Unit
items:
- Laser controller
- fiber output
- golden half-inch mirror
- golden small prism mirror (before the cross point)
On the Imaging unit translation stage:
- golden large prism mirror (after the cross point)
- XY lens mount
- coated half ball
- Photo Detector
The Spectrum Analyzer provided data in ".DAT" file.
By using a program provided by Tatsumi-san, we were able to convert these data in ".DOT" file and then use Matlab to plot them.
This ".DOT" file is divided in 3 columns : frequency, magnitude and phase of the transfer function.
Attached to this entry are the open-loop, the electronic-loop and the optical loop.
They seem to be coherent with what the spectrum analyzer displayed during the measurement.
Before install everything, we checked the beam size after the transmission of the beam splitter, which is the same at the reflection. Take the origin at the front surface of the beam splitter, the beam waist is at -9.72cm, size is 53.36um. Then the first lens we use is 100mm, 5cm from the origin, but at the focal plane the beam diameter is larger than 1.3mm, which is the maximum the AOM can work according to the data sheet. So the AOM was put around 11cm, connected with the power, the AOM shows very clear diffraction orders.
Then another 100mm lens was put at 25cm. After putting this lens, we did some measurement of the beam size, and found the position to put the third lens in order to have a good size at the 2inch mirror of the telescope(less than 1.3mm). The third lens is 175mm, was put at 48.5cm. Then we checked the beam shape far, there is no obvious astigmatism as far as we can see. Measuring the beam again and we got the result that the beam at 2 inch mirror should be less than 1.2mm.
But then we found out if we want to change the MZ design and put another beam splitter after the one we have now, the first 100mm lens should be put further to give enough space to the BS. With Jammt, I did the simulation and found out if we move everything together 2.5cm further from the position mentioned before, the 175mm lens was not capable to focus the beam enough on the 2inch mirror. I tried other focal length, the 200mm should work.
The other problem is after moving(pic 1), the beam after the first lens will diverge more compared to the previous design(pic 2),sSince the two green dash lines in two pictures have the same distance. Divergence of the beam is one of the question we concern most, so I checked on the bench, after changing the AOM still works well.
It seems this new configuration is acceptable, we are going to install another beam splitter first and start to change the lenses.
https://www.dropbox.com/s/6pim9uxovu1oium/bb43a17cec87c2681906293ca3bf7889.mp4?dl=0