filter cavity input mirror wedge

The input mirror wedge sign was studied with an autocollimator callibrated with the reflection of the front surface (by superposing the reticle and the front face reflection).
To understand which reflection was the one from the front surface, we looked at the reflections due to a roof light. When this light hitted only the front surface, its reflection was red. When it was hitting both surfaces, we could see two shifted reflections, one green and one red. So the red reflection corresponds to the front surface reflection.

The callibration of the autocollimator means that the incident beam and the front surface reflection are on the same direction. Then, depending on the wedge sign, the back surface reflection will either be seen on the top or on the bottom of the front surface reflection.

By looking at the autocollimator, we were able to tell that the wedge sign corresponds to the second configuration on the scheme. This means that the "#4" sign on the mirror corresponds to the lower part of the wedge.

Pump beam profile

In order to place the waist of the pump beam on the right position, I characterized the pump beam using the beam profiler and moving it with the translation stage. 

Laser settings: I = 1A, P = 210mW , hwp_angle=0, not chopped.

I aligned the pump beam to be parallel to the z-axis translation stage within 0.2mm on the whole translation range 250mm (±0.8mrad).

The beam profiler position is set 0 at the position of the KAGRA-size sample first surface (see picture).

Without the focusing lens, the waist is 377um at -0.88m

Using the focusing lens f=125mm placed at -60mm, the waist becomes w0=53um  at 0.081m

According to this calculation the deepest measurable point is 0.08*1.81=0.149m for sapphire (and less for silica).  Since KAGRA-size thickness is 15cm, the cross point (which will be at the pump waist) cannot go beyond the second surface.  There is not enough space to place the lens closer to the sample, therefore I need to change the focal lens.

Making a quick simulation with JamMt, a focal lens of 150mm will move the waist to 0.111m (w0=62.8um) without sample, With a 15cm-thick sapphire substrate the waist will move to 0.178m, and with a 15cm-thick silica substrate, the waist moves to 0.16m, which is enough to measure the entire thickness of the sample.

Comment to Green telescope on bench changed and simulation of the beam (Click here to view original report: 490)

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Green telescope on bench changed and simulation of the beam

Yesterday after tried moving many times all three lenses in the bench, it seems the first 100mm lens has very large effect on the beam shape. So we decided to change this lens into a large focal length one and re-design the telescope.

This is the new telescope parameters:(origin still at the first line of hole out of the cavity)
L1 f1=200mm z1=8cm
L2 f2=200mm z2=56.5cm
L3 f3=175cm z3=99cm

With this new telescope we had a better beam at the window of PR chamber and also before the beam goes inside the arm, the previous one produced a beam a little bit longer in horizontal direction. So we decided to go on with this configuration.

Then today after we realign everything on the bench, took some measurement to check the difference between the theoretical value and practical one is inside the tolerance or not. All the points we measured are equal or smaller than the result of the calculation. We also added the two telescope mirror inside the chamber to check if the beam in the tube has the size we want or not. At the two inch mirror we had a beam radius about 1mm which is good, and the largest size inside the arm is about 1.3cm.

The other thing we did is we measured several points on the bench after the third lens. With these data, we calculated the beam waist and simulate the propagation of beam in 300mm, but without the two telescope mirror. From the picture we can see that the beam shape on the bench can not produce astigmatism at 300m.

investigation on the green beam astigmatism: effects of the windows

Today we keep investigating on the beam astigmatism. As a first thing we realign all the optics on the bench and measured the beam in different position. There results will be posted in a dedicated entry by Yuefan which has also calculated the astigamism expected at 300 m on the basis of the measuremts done on the bench. (Very small).

Then we sent the beam in the pipe and open the first windows between the BS and NM2. The change in the shape when the windows is open is shown in this video.

After that we let the beam propagate for about 30 m in the corridor and move the small telescope mirror in order to have the beam as collimated as possible. We succeded in finding a position allowing to have a beam which remains pretty small (2-3 cm) up to the end of the tunnel.

After the small telescope tuning, the beam (both with closed and open window) looks different on the 290 target. Here there is another viedo who recorded the beam during the closure of the BS-NM2 windows. The beam with open widows looks still very astigmatic while with the close window is seems aftected by diffused light and very difficult to judge.

[I have the impression than after opening the windows for the first time it becames somehow dirtier and once we close it again, it affects the beam more than it was doing before.]

Some preliminary conclusions:

1) Tuning the 2"  telescope mirror position have a great impact on the beam shape at 290 m.

2) The windows seem to have a great impact on the beam shape at 290 m.

3) The beam seems well collimated and not significantly astigmatic when it propagates in the corridor.

Picomotor remote control interface

In order to speed up the remote control of picomotors that are largely used for the beam aligment, I wrote a custum vi which avoid to inserting each time command lines to select drivers, motors, velocity and steps (pic 1).  The vi (picocontrol.vi) is in a dedicated labview project (C:Digital/picomotor_control.lvproj)

We remark that BS yaw picomotor still doesn't work.

The final configuration for the picomotors control is 

CONTROLLER 133.40.121.13

A1

1. PITCH BS
2. YAW BS
3. empty

A2

1. PITCH INPUT
2. YAW INPUT
3. empty

A3

1. PITCH 2" TELESCOPE
2. YAW    2" TELESCOPE
3. LENGTH 2" TELESCOPE

CONTROLLER 133.40.121.14

A1

1. PITCH PR
2. YAW PR
3. PITCH SM1

A2

1. PITCH SM2
2. YAW SM2
3. YAW SM1

CONTROLLER 133.40.121.15  

A1

1. PITCH END
2. YAW END
3. empty

 

Where 1, 2, 3 correspondes to motor 0, motor 1 and motor 2

Green beam astigmatism investigation

In the past days we have been working in order to improve the green beam shape at the end of the arm. 

1) We have realigned the optics on the bench many times in order to be well centered on the optics. After adding each optics we measured the beam and propagate it far (about 5-6 m) to check if we could notice any astigmatism.

2) We have tried to be as centered as possibile in the sferical mirror of the telescope and on the two gates between BS-NM2 and NM2-PIPE

3) We have change the telescope configuration for the grean beem in order to have longer focal length whit respect to the dimension of the beam on the lenses and avoid sferical aberations as much as possible.

In this condition we are able to have a beam which looks failry circular at the input of the pipe and also on the first target but which becomes very bad when observed on the  290 target. (The main effect is elongation in the orizontal direction). This video shows what we see on the far target while scanning the orizontal direction with the BS local controls.  While moving from one side to the other, at first we can observe some scattered light, than we don't see anything for a while and finally we see the beam on the target. Continuing the scan, the beam disapears and after another moment of darkness we can see again the scatterd light. From this I would say that what we observe is the direct beam and not its reflection on the pipe wall.

In order to better understand the evolution of the propagating beam we have put two steering mirrors at the level of the input mirror in order to send the beam in the corridor. (As steering mirrors we used to 5" mirrors used as PR ans END dummy mirrors, which should be both former TAMA PR mirrors with long RoC )

The beam at 300 m is shown in picure. It is moving a lot because of the air but it doesn't look elongated in the orizontal direction. There are no evident reasons why the beams propagating in the corridor and in the vacuum look so different. Here some hypothesis:

1) The air is affecting the propagation, masking the astigmatism observed in vacuum.

2) The steering mirrors are not flat enough and they modifiy the beam masking the astigmatism.

3) The astigmatism is introduced by the window at the input of the pipe and becomes evident only after a long propagation (on the target at 10 m the beam stil looks good) 

4) Effect due to possible multiple reflections from the target and the input window (it could be ruled out by looking at the beam reaching the end chamber) 

We remark that the beam on the target observed by the camera looks better than in reality. The same is true for the pictures taken to the green beam where small deviation from the circular shape are difficult to be apprieciated.

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.

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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
L2；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.