Green beam characterization

The first plot shows the measurement of the green beam out of the SHG cavity and the fit result of it. The red spot is the position(-7.3cm) and the waist size of the green beam(26um). Origin is set at the first line of hole in front of the cavity.

In the second attachment, we put the position and the focal length of the three lenses we using now. We used the result of the first plot as initial value of Matlab program to get these result. We did some measurements of the beam size near the lenses this morning and compared them to the theoretical values, seems they fit well. Also if we considering the threshold of causing the aberration from the report, with 2cm beam waist, convex-plano lens should has a focal length large than 2m. The lenses we are using now is 100mm, 200mm, 1000mm, the beam size we have now is in the safe range.

BS local controls restored

In the past days I have restored the local control on the BS mirror (used as steering mirror after the telescope). The optical scheme (similar to that described here) is shown in picture 1. We have noticed that there are two reflected beams (from the first and the second surface) which are very close. In this configuration the two reflections cannot be separated and they arrive almost superposed on the PSD. Moreover the beam is impinging at a distance of about 4.5 cm from the mirror center. According to my computation (reported here), it should induce an error on the yaw measurament of about 3.5%.

I had some trouble in diagonalizing the driving. After many tries, I have found that the best diagonalization is achieved without using the two lower coils (see coils disposition in the last attached picture). This fact is very strange and should be better investigated. With the following driving matrix 

	YAW	PITCH
COIL 1	1	0.95
COIL 2	-1	1.1
COIL 3	0	0
COIL 4	0	0

I found the mechanical TFs shown in picture 2 and 3 (when exiciting yaw and pitch respectively). They look similar to those measured last november.

The open loop transfer functions are shown in picture 4. Due to the second narrow resoance in pitch at about 10 Hz, the UGF is crossed two times. I'm not sure about the phase of the pitch open loop TF for the second crossing point. Anyway I was able to close the two loops and they look stable. We also added offsets and we were able to observe the change in the beam position on the target at 290 (both in vertical and orizontal direction according to the loop to which we add the offset) 

The comparison between the open and closed loop spectra are shown in picture 5. Maybe the UGF can be increased a bit.

The calibration is  0.37 mrad /V. It has been computed as done here. where V_SUM of the PSD is 13.5 V and the lever arm is about 0.7 m

Some remarks

1) In order to investiate the driving issue, I have injected a line at 5 Hz with the same amplitude in each coil (one by one). The spectra in the four cases are shown in the attached pdf and seem pretty much the same.

2) I have observed that while measuring mechanical TFs, at the begining I was not able to find a good coherence at low frequency (below 1 Hz) and the resonances both in pitch and yaw where always very excited. I found out that this was due to the air flow inside the cleanbooth. I have temporarily disconected it to make the measurements. 

3) I have observed an oscillation at 50 Hz in the four signals sent to the coil driver. The amplitude is quite high (about 600 mV pp) and it is not present if i look at the signals just at the output of DAC. I'm not sure about how to get rid of this. 

4) While glueing the magnets we checked that the north-south polarization was in agreement with the convention reported in the last attached picture. Now, observing the driving matrix it seems that the sign should be inverted. I remember that this happened also for the dummy BS. 

Check the beam shape

Since last Friday we found out when we sent the beam to the end, the shape of the beam is very bad, much worse than the beam shape we saw at the beginning of last week, both the green and the infrared, the beam both are large and long in the horizontal direction. Last Friday we tried to align the beam to the center of both the suspension mirrors, but the situation did not become better, so we tried to move the 2inch mirror a little bit by the picomotor, and found out to one direction the beam gets better but not very obvious, it seems we did not have enough range of that picomotor, so we decided to move 2inch mirror by hand.

Yesterday before we start to move the 2inch, we checked the beam shape at different place of the beam path with beam profiler,the beam seems circular, although at some place the beam is fluctuated very quickly, but during the fluctuation there are some moments that the beam is round. Then we tried to adjust the beam more center on the lens of green beam, and finally we had to adjust from the very beginning, we aligned all the component to let the beam go straight and checked each time we put back a component,almost circular everywhere,(except there seems have some dust on one of the component of green, so when the beam reached the window, there is one line shape shadow on the top of the beam, we tried to clean all the component with gas, but cannot remove that) after everything was done, the beam shape at the 290m target did not get any better. So then we can focus on the two vacuum chamber.

We tried to push or pull the 2 inch mirror by hand and align the green beam to see it on the target, although the beam shape is still strange but at some point the beam is much smaller than the other place, and except the main beam there is another small beam on the side of it which is quiet good shape, maybe we can find out where it comes. Then when we cut the green and tried to see the infrared, the infrared is almost what we want to see, a small beam with almost circular shape, so this means with moving the 2 inch mirror, we can get the situation better. Today we are going to find a good position of that mirror to get a better green beam.

When we tried to move the BS yaw picomotor yesterday, it does not work with the computer command again, it is better to change the picomotor before we close the chamber.

Comment to wedge measurement and magnets/standoff glueing on the input mirror (Click here to view original report: 471)

Participants: Eleonora, Marc

Few days ago we observed that on one of the stand-off we glued on the input mirror the glue seemed to have overfowded on the groove where the wire is supposed to stay (picture1). Today we made some tests with a wire of the same diameter of those used in the suspension and verified that it was the case. After trying to remove the excess of glue without good results, we decided that was safer to remove the stand-off and to glue a new one. (picture 2)

Filter cavity end mirror successfully installed

In the past days we worked in order to suspend the filter cavity end mirror in the end room vacuum chamber. 

As a preliminary activity we performed a major cleaning of the end room.

On Thursday 25th, we opened the vacuum chamber and removed the dummy mirror (an old TAMA PR mirror).

On Friday 26th, we installed the final filter cavity mirror (end #1 substrate)

The procedure followed to change the mirror is sketched in the attached pdf.

After the mirror substitution, we sent the beam from the central bulding to the end mirror and tried to move it with picomotrs in order to send the beam back. Unfortunately the dispalcement in yaw achievable with picomotors was not enough to allow it and we were forced to slightly turn the whole suspension. (The problem of the small yaw range was already observed for the PR telescope mirror. Also in this case we solved it by slighty moving the whole suspension). After this operation the reflected beam seems reasonably centered on the small gate window (that cannot be open utill the end chamber is evacuated).

We remark that the green beam is quite big, still too astigmatic and also moving a lot. The work in the next days will be devoted to improve it.

OTHER REMARKS

1) Picomotors for controlling pitch and yaw are working fine. They can be remotely controlled with labview (IP adress 133.140.121.15) and also locally using the joystick (pictures 2-3)

2) Thanks to Yoshizumi-san, we now have working wi-fi (naoj-open) and working phone (number 3472) in the end room!  (picture 4)

3) Dummy mirror (old TAMA PR) is currently stored in the filter cavity mirror case of substate #1 (picture 5)

Comment to Sesnsitivity curve with crystalline coatings and squeezing (Click here to view original report: 476)

I calculate the horizon of BBH and BNS  for the sensitivity curves:

- Amorfous coating ; no squeezing      BBH = 3.28 GPc, BNS = 360 MPc;

- Amorfous coating ; squeezing           BBH =  4.42 GPc, BNS = 509 MPc; 

- Crystalline coating ; no squeezing     BBH =  3.46 GPc, BNS =  378 MPc; 

- Crystalline coating ; squeezing          BBH = 4.90 GPc, BNS = 566 MPc;  

d_H = (G^5/6 * M^1/3 * mu^1/2) / (c^3/2 * pi^2/3 * rho) * sqrt( 5/6 * int_f1^f2 f^(-7/3) / S(f) df)

M is the sum of the 2 masses, mu is the reduced mass, rho is the SNR, f1 and f2 are the frequency range for the event signal, S(f) is the noise spectrum (square of the equivalent strain)

I used f1=10Hz; f2_BBH=73Hz; f2_BNS=1571Hz

M_BH = 30M_sun (M=60 M_sun)

M_NS = 1.4M_sun (M=2.4 M_sun)

rho = 8

Sesnsitivity curve with crystalline coatings and squeezing

I calculated how the coating brownian thermal noise will change in the case KAGRA mirrors will employ crystalline coatings. The mechanical loss I used is 4.5e-6 at cryogenic temperature (from G.Cole, et.al, "Tenfold reduction of brownian noise in high-reflectivity optical coatings", Nature photonics, 2013)

I took the LCGT design sensitivity curve contributions from KAGRA website. I replaced the brownian coating thermal noise with the one for crystalline coatings, and I replaced the quantum noise with the one calculated by Eleonora with the frequency dependent squeezing.

Telescope installed for green and infrared

Today we received the optics we need so we started to install the telescope for both infrared and green path. Since green is easier to see and two beam are more or less superposed at the 2inch mirror, so we use the green for the reference.

Then we started to installed the 150mm and 175mm lens at the calculation place with rails and moved a little bit along the rail to make the beam have a good size at the 2inch mirror.

For the green path, we tried to remount everything. But according to my calculation, the first lens should be at 7.5cm from the cavity output, but we discovered today, the limited we can do is 8cm, cannot get closer because of the screw of the dichroic mirror. So we put it at 8cm, and did the simulation again, the result shows below,

L1: z=8cm f1=100mm
L2: z=49cm f2=200mm
L3: z=117cm f3=1000mm

In the order of L1, two dichroic, Faraday Isolator, half-wave plate, L2, L3, Faraday Isolator, half-wave plate, we put all of them and align the beam again. During the installation, one problem is that the third lens is very close to one mirror, and also there is an aperture very close to this mirror, so we put another aperture firstly just after the previous one but provide enough space for the rail of the lens.

Then we tried to measure both the green and infrared beam with the beam profiler, did some adjustment to have the size we want(1mm in radius). But the infrared beam looks have some astignatism. Checked the beam at 290m target, the green is larger than the infrared, and also there are two green beams, one is larger, one is smaller but more circular(pic 1), we tried to move the picomotor of BS to make this two green beam superpose(pic 2). The infrared is much smaller(pic3), when it moves to some position of the target, it looks good.

End mirror optical lever signal affected by rotary pump and status of vacuum

Yesterday I switched on the optical lever of the end mirror. The signals in time were showing a clear excess of noise with respect to the last time I have looked at them (july 2016).

Since the end chamber was put under vacuum few weeks ago, we suspeced the pumps. We first switched of the turbo pump (after closing the valve) and wait for the frequency to go to zero without notice any remarkable improvement in the noise. After that, we switched off the rotary pump and we could observe that many lines have disapeared in the spectrum. Indeed the signal in time was much better than before.

We also monitored the change in the vacuum level after closing the valve. Its trend in the first 45 minutes is plotted in the attached figure. 

The value today (after about 18 h)  is  2.1e-4 torr.

wedge measurement and magnets/standoff glueing on the input mirror

This morning I have dismounted the gluening jigs that we used to glue magnets and standoff on the end mirror 2 days ago. Magnets and standoff seem well attached but while trying to put the mirror inside its case a magnet came off. We reglued it just after cleaning the mirror. After few hours we removed the jig and let the glue cure without support (picure1)

We then proceeded measuring the wedge (pictures 2-3) and glueing the magnets on the input mirror (support number 4), following the same procedure reported here.

Status of vacuum

On Wed May the 17th I check the status of the vacuum in the south arm and in the south end vacuum chamber.
There are a total of six pressure measurements along the south arm.

- Near NM2
P1 = None (NM2 is not evacuated)
P2 = 2.5e-8 Torr

- Mid-arm
P1 = 2.0e-8 Torr
P2 = 2.5e-8 Torr

- Near EM2
P1 = 1.2e-7 Torr
P2 = 1.0e-7 Torr

Wedge measurement and magnets/standoff glueing on the end mirror

Participants: Eleonora, Marc, Raffaele

Filter cavity mirrors are supposed to have a wedge of 400 urad. (See picture 1)

Since the wedge is not marked on the edge of the mirrors (as it happens usually), we had to measured it using an autocollimator. This tool works by projecting an image (a cross) onto the mirror and measuring the deflection of the returned image against a screen with a grid. If the mirror has a wedge, the reflection of the first and second surface are not superposed, resulting in two crosses on the screen. The line joining the crosses' center indicates the wedge direction (i.e the diameter with the maximum slope). It is not easy to precisely project the wedge line seen on the screen, on the mirror surface in order to glue the stand offs in the proper position (we want the wedge to be horizontal). In order to do this we used the following procedure:

1) We used as a reference a 4" spare mirror (used so far for gluing test) which as a marked wedge  (figure 2)

2) We put it on the glueing support and aligned the wedge with the stand off jig.

3) We moved the autocollimator in order to have the two crosses well aligned on an axis of the screen grid.

4) We replace the dummy mirror with the filter cavity mirror and rotate it in order to have the crossed aligned on the same axis as before. In this way the wedge should be aligned with the stand off glueing jig.

Since we can not distiguish the first and the second reflection of the mirror we don't know if the wedge is such that an impinging beam is reflected on the right or on the left.

We proceded glueing the magnets and stand-off. We glued the end mirror number 1 that, according to my simulation (see figure 4), is the best one to be used together with input mirror number 4.

We used a brand-new set of masterbond glue and we did all the work in the ATC clean room.

-------------------------------------------------------------------------------------------------------------------------------------------

We have measured a distance between the crossed of almost 2 division of the screen grid. We are not still sure 100% about how this data should be convert into a wedge value. Marc will report on it as soon as we find it out.

Telescope design of green path

We moved the fork for the dichroic just after the cavity, which reflect infrared in and transmit the green out, to make more space, then we align the cavity again. Now we have the output green power of 45mW. After that we measured the green beam size again, did the calculation, the beam waist and position is more or less the same with the result we had before.

Based on this result and the aperture of the optics, we design the telescope:

Origin set at the first line of holes in front of the cavity.

L1: z=7.5cm f1=100mm
L2ï¼›z=45cm f2=125mm
L3: z=132.5cm f3=1000mm

According to the optical scheme, we estimated the position of the optics and their apertures:

FI: 22.5cm 5mm
EOM:57.5cm 3mm
AOM:90cm 1.5mm
FI: 123.5 5mm

In the simulation, after we put the three lenses for the telescope, we only need to put another two tune lenses for the AOM, two 100mm, one at 82.5cm, the other at 102.5cm. Then everything will be fine.

Another thing we discovered today, it's that the telescope for infrared has been finished for very long time, but with a lens having 350mm focal length which is not available from any company, so we did the design again:

L1: z=48.5cm f1=500mm(Alreday installed)
L2: z=122.5cm f2=150mm
L3: z=154.5cm f3=175cm

We will try to look into the bench if there is enough space for the lens and order them.

Also yesterday we put everything back one by one and send the beam far away to check where exactly the astigmatism come from. Everything was find until we put the Faraday, then after the mirror we put the second lens, when we moved this lens or turned it, the beam can change from elongated in vertical to round then from round to elongated in horizontal. So finally we can be sure that the astigmatism is from this lens. Then we put it in a good position and tried to send it to the end. On the screen, we can see it on the 290m target, although it is large, but it more circular than before, we will check the size of it after we finish the telescope,if it is still large, we can move the 2inch mirror.

Recheck green beam

We checked again the green beam waist size and position, because it was a little hard to believe in the result we got last week.

At the beginning, we found out that the beam shape in the beam profiler moved a lot, it was a ellipse with a very large e. But if we sent the beam far away from the bench, it was round,(pic 1), but only with some fringe on one side, it seems from the dichroic mirror.

Then we tried to turn the dichroic mirror a little to see if the beam change. Since the dichroic is also working for sending the infrared beam into the cavity, we moved it, so we need to align the infrared beam to get the green back. Then we also find out the green power is low, so we adjusted the thermal control to find a good temperature, finally we got 50mW over 240mW of infrared, efficiency around 20%.

When everything was ready, we found out the beam size is much smaller compared to the data we got last week, so maybe the temperature also effect the beam size of the green, we took several points, did the calculation, this time the beam waist has more or less the same position of the infrared(-7.3cm),but half of the size(26 micrometer). In picture two it is the points we got in two axis of the beam profiler, and picture three it is the data points and the fitting result in one of the axis.

Also we check again the polarization just after the cavity, we got 49mW reflected by the PBS and 1mW transmitted, so we can say that the green beam come out from the cavity is in S polarization which we want it be.

We did these measurement with the air conditioner off, since the wind from all the air conditioner meet at the bench, produced a lot of fluctuation in the green beam. Then when we left, we turned the air conditioner on,which means we need to adjust the thermal control again.

The other thing is after we realigned the cavity, we found every time we locked the cavity, there is the high frequency oscillation, so we reduced the gain of the Stanford from 500 to 50, then we are fine now.

Clean up and move the translation stage

Members: Manuel, Yuefan, Marc

- We made order in Tama central room, sorted many things from the messy plastic boxes and arranged them on the new shelf.

- We clean up some dust from the cleanbooth of the absorption bench and move the translation stage on the definitive optical table, see picture.

Measure the green beam

According to Matteo's suggestion, the beam waist and size of the green and infrared may have some different, so it is better to measure the green beam come out from the cavity.

So today we remove the two dichroic, the lens and Faraday, finally we had enough space to put the beam profiler and do some measurement, but even like this, we only can took three points,after the third point, we still had space but the beam size is too large to measure. The beam profiler has two axis,v and w, the first time we took three data points, two of them is fine, the beam size in two axis is similar, but one point had a large difference. So after the calculation, the beam waist and position in these two axis is very difference(one has half size of the other). Then we did the second measurement, this time the result is good, and we got the beam waist size was 36 micrometer instead of 52 micrometer(the beam waist size of the infrared), and the beam position is 15.5cm away from the input port of cavity instead of 7.5cm(infrared).

Although we got this result, we still have the suspect that maybe the astigmatism is from the cavity, since the beam size is too large, the measurement may have some error.

Then because we already remove the dichroic, we tried to turn it to make the two beams from two surface of the mirror can be separated. But we turned the mirror little by little almost 360 degree, the two beam still become into one. So we thought maybe the wedge is too small, even it separate the two beam, but the beam size increase too quickly, then the main beam will cover the other.

Before the holiday, we found out the picomotor which controls the yaw of BS mirror cannot be controlled by the computer. And we checked the cable, from the driver to the connected part of the picomotor and the cable, it works well. Also we exchanged the port between pitch and yaw, after pitch works well, so the problem is also not from the driver. So Marc checked again the picomotor, he thought the problem is inside the picomotor,and we planned to change the it after we finished the green path and need to align the beam.

More attempt to astigmatism

Before the holiday we tried to move the lens to improve the beam shape, but still have some limitation, so we thought it maybe because the beam size is too large at the input port of the Faraday Isolator. So we tried to move the Faraday further from the lens, to let the beam converge more, but even we move the furthest we can, the beam is still large compared to the Faraday. Then we thought maybe we can change the lens into another one with smaller focal length, now we are using 100mm one.

But if we change to one with 50mm focal length,there is a possibility that the before the beam goes out from the Faraday it will expand. So I did the simulation today. I used the initial beam waist is 52.7 micrometer and 82.2cm away from the output of EOM, this is the value we had from the infrared beam, I am not sure if the green beam has the same size and position as the infrared one, but if I did the simulation with 100mm lens in its position now, the simulation result is close to the real beam size on the bench, so I did the after simulation with this starting point. Then I tried to change the lens to 50mm one in the simulation, and got the result in the first picture. The length of the Faraday is about 5cm, so if I put the beam waist at the center of the Faraday, the beam size at the input and output port of the it will be 762 micrometer in diameter which I think it is small enough.

But if we changed like this, there may be another problem, the aberration on the first lens will be much serious, since the beam size on the first lens does not change, but the focal length reduce.

We got the PBS for green today, so we can check the polarization of the green beam. If I remember right, we got pure P polarization went into the cavity, so the green should be S polarization, so we adjust the waveplate to maximum the S polarization.

BS suspension mirror has been changed

From yesterday afternoon we tried to remove the dummy mass on the BS suspension and change the real mirror.

Yesterday afternoon we tried to lower the inter-medium mass first, but we round out one of the top four screw is very hard to loose(pic 1), so we tried to lower the mass with one of the picomotor. But only with the picomotor the wire was not loose enough to remove the mirror, so finally we managed to turn that screw, and today when we need to fasten it, we changed to another screw.

So today we tried to put the real mirror on the wire, the stand-off and the magnet are all stick well, but since the real mirror is a little bit smaller, the white stopper on the up side of the mirror can not help a lot. Also when we try to put some intense on the wire to lift if from the stoppers, the mirror tilt a lot, so we turn the pitch picomotor to make it more or less vertical.

Then we tried with picomotor to align the beam again, while we were trying we found out the yaw picomotor of BS suspension does not work, we checked a bit, for now we can only say it is not the problem of the driver.We will try to fix this problem tomorrow morning. Then we tried to align with move the picomotor by hand, so we did align the beam, but maybe because the quality of mirror becomes higher, there are too many reflection everywhere, so the beam on the 290m target looks much worse than before.(pic 2)

The other thing is that we checked the picomotor driver for the end room yesterday and also checked again today, it works well both the top layer and the port layer. Now we have enough drivers.

Displacement of the NM2 optical bench

Participants: Takahashi, Flaminio, Guo, Eisenmann
Yesterday we move the optical bench in the NM2 chamber in order to have the beam
approximately passing on the beach axis. To do so the beam was previously positioned so to
pass approximately at the center of the two windows on the gate valves in the center area
(between the BS chamber and the NM2 chamber and between the NM2 chamber and the tube)
and reaching the target at 290 m.

Found the reason of astigmatism

Today the beam seems pretty low on the PR mirror, so we moved the pitch of the 2inch mirror first and aligned the beam everywhere, then we moved the input mirror bench about 2cm right to put the mirror in the right position.

With putting two aperture on the bench (pic 1), we set the position of the beam, and started to move the lens, at first when we just turn the screw in horizontal direction to center the beam on the lens, the beam is still elongated. Then we tried to turn the lens a little bit,then we found out the elongated changed to the vertical direction. If look at picture 2, it is not very obvious, but when we tried to send the beam far away, the beam is longer in the vertical direction a lot. We thought the reason of the beam shape is because of the large beam and short focal length(150mm), since the spherical aberration, when the beam pass the spherical surface, the light away from the axis does not focus on the same point as the light along the axis, so the more we off the center of the lens, the beam elongated more. So then we will try to center the beam better on the lens.

But the other strange thing is that the beam at 290m target is still elongated on horizontal direction(pic 3), this situation is the same when we tried to use the aperture to force the beam to beam round, then at the first target it looks good, but then on the second target it is pretty long in horizontal. We need to have a better beam and then try to find the possible reason of this.

The other things we discovered on Friday is that why when we center on all the mirror and also second target, we were off center of the first target. Then first target has a axial precession like the Earth's axis, so if we turn it to the other face it will be well centered also.