Telescope adjusted

With the data measured at the end of the bench, we found the beam waist and position. Then we did the simulation with the 2-inch mirror, PR suspension, input and output mirror. The position and the focal length of these mirrors are as below:(The origin is set as the steering mirror which is 85cm from the end of the bench)

2-inch mirror: f=-30cm z=4.51m
PR suspension: f=3m z=7.235m
Input mirror: f=-218.35m z=12.135m
End mirror: f=-218.35m z=312.135m

With these values, the simulation result shows that the beam will keep diverging in the arm which is not what we want(pic1). So I did some calculation, we need to move the 2-inch 3.9cm backward to get the beam waist around 150m inside the arm. (pic2)

The other reason why we should move 2-inch mirror is after we installed the real input mirror, we got green reflect back from the from surface of that mirror. But it seems the reflected beam is much larger the original one.

Before moving the mirror, we set two reference point, one is the position of the beam in front of the input mirror, the other is the beam reflected back to the bench. Also after the input mirror we put two aluminum mirror to reflect the beam into the corridor to check the beam propagation. At first we only tried to move the picomotor of the 2-inch, but even we when we finished all the range, nothing changed. So we moved by hand little by little. Every time we moved, we recovered the beam with pitch and yaw of the 2-inch on two reference point and check the beam size at the entrance of the corridor and bench. After moving about 3cm, the reflected beam size seems fine, so we followed the beam inside the corridor, found it has beam waist around 150m, and the size is from 1cm to 2cm which is acceptable.

Then we took the reference of the BS chamber like what we did to the PR, with two aluminum mirror, the beam transmitted by the 2-inch and the BS suspension have been reflect out of the chamber.

Comment to HEPA filter check for replacement (Click here to view original report: 426)

The filter's fan part was fine, so it was enough to buy only the filter part. New HEPA filters were delivered to Tama last friday.  I washed the prefilters with water, I cleaned the fans, replaced the filters and placed them back to the top of the absorption bench clean booth.

Vacuum in EM2, Gate valve opening and pressure

On Monday June the 12th after lunch I started the evacuation in the EM2 chamber using the large rotative pump.
After 30'-45' the pressure was at 100 mTor so I started the turbo pump.
On Friday evening (June 17th) the pressure in the EM2 chamber reached 3.1e-7 Torr.
On the other side of the gate valve the pressure was 1.1 e-7 Torr.
So I open the large gate valve and immediately after the valve between the turbo pump and the tube
(the valve between the turbo pump and the EM2 chamber was already open).
The pressure went up a little bit in the 1e-6 Torr range and then came down again.
This morning (Monday June the 19th) the pressures are:

EM2 chamber = 1.6e-7 Torr
EM2 tube = 2.3e-7 Torr
Middle of the tube: 4.7e-8 Torr
Middle of the tube: 2.7e-8 Torr
NM2 tube = 8.6e-8 Torr (this sensor was not working but is working again now)
NM2 chamber = in air

Overall it looks reasonable to me even if there is probably some out-gassing coming
from the EM2 chamber.

VI bug solved

Setting the limits at the translation stage generated a bug in the part of the VI where it sends the positioning commands: if the position exceeds the axis range, the VI goes in loop. I fixed it making a subVI "Read limits axis.vi" that reads the limits from the translation stage controller and set the minimum and maximum positions in the property node of the position controls of the VI.

Telescope of the mode cleaner cavity

I measured the beam transmitted by the beam splitter of the MZ, did the fitting and found the beam waist position of that beam.

The origin was set at the front surface of the beam splitter. The beam waist size is w0=51.2um and waist position is z=-9.5cm.

Then I took this as the initial value to calculate the lens we are going to use for the telescope. So the condition is, the MC input mirror is 60cm away from the origin, and the beam waist should be around the position of the input mirror and has a size of 277um.

I let the program chose the lenses with focal length we have now and got this result.

L1: f1=100mm z1=5cm
L2: f2=200mm z2=37.5cm
L3: f3=200mm z3=55cm

So with this design the final beam waist is w1=279um, z1=60cm, which is the result we want.

The other thing is before we were using a 200mm lens to focalize the green beam we distracted from the Faraday to the PD. But yesterday we found out, if we want to give power supply and the signal output, we need more space for cable. But the position of the PD before was too close to the post of the steering mirror. So today I changed the lens into a 150mm one, and moved the PD 5cm forward.

HeNe probe Imaging unit assembled

I assembled the imaging unit for the HeNe probe laser. I placed it on a 50mm translation stage.

Comment to HeNe probe alignment, profile characterization and waist position (Click here to view original report: 495)

I had to adjust a bit the position of some parts of  the translation stage, sothe new pin-hole position at the cross point is 

@01-axis: 2646049 2646049

@02-axis: 279284 279284

End room optics

Participants : Eleonora, Marc

Yesterday, we finished the installation of the optical components to extract the signal from the end mirror on a photodiode and a CCD camera.
The optical bench is composed of one lens (focal length = 1m) inside the end chamber, one periscope, another lens (focal length = 0,150 m), one dichroic mirror, one beam splitter, one CCD camera and a photodiode.

Instead of using a periscope fixed on an elevated optical frame, we built a higher periscope.
On this periscope, 2 Aluminium mirrors (TFA-50-C08-10) were mounted. We also checked that they were reflective for infrared : 92,08% at 45°
The 2 lenses are used to get a smaller beam size onto the photodiode and the CCD camera. Currently, the beam hitting these two components is 6,67 times smaller that the beam hitting the end mirror. We might have to change the second lens as we expect a drastic change in the beam shape when we will remove the tube windows.
The beamsplitter will also be changed as it is supposed to be working for green (instead of infrared for the moment). It is still able to split the beam in two, so we were able to detect the beam in the photodiode and send it back to the central room.

Vertical jitter of the beam at 300 m

Today I tried to measure the spectrum of the beam motion on the optical end bench, after the end mirror. The main problem is that the beam shape is pretty irregular and very elongated in the orizontal direction (almost comparable with the sensor size). In the attached plot is shown the spectrum I found for the vertical motion (both with open and closed PR and BS loops). It is just a very qualitative measurements and I'm not sure about how much it makes sense.

Some remarks

1)  I used a calibration measured here for a PSD of the same kind, with red light. I'm not sure if and how it scales with the wavelenght (to be measured..)

2) The total rms (using this calibration) seems underestimating the real motion of the beam which by eye should be of the order of few millimiters.

3) The peaks are clearly coming from the PR and BS resonances. In particular the one at 10 Hz desappears when the BS pitch loop is closed as observed on BS spectrum

4) The PSD was not covered and the measurement seems affected by acustic noise.

I'll try do more quantitative measurement (better calibration, smaller beam, box on PSD) as soon as possible.

Remote control of the 290m target

Today I finished the setting for remote controlling the 2nd target.

First thing is to turn the target into the remote mode, since on the label there are only two modes(remote and local), but actually when you try to move the handle, there are three levels.Remote is the one on the left.

I connected the power control box of the target (1st picture) to the '133.40.121.***'port, so when you want to control the target, you need connect to the laptop to the same IP address.

The first thing is to set the IP address and the netmask of your own laptop to 192.168.10.2 and 255.255.255.0.

Then go the address 192.168.10.1, you will see the webpage in pic 2. Enter your username(admin) and the password(magic). The next page (pic 3)shows the condition of every outlet now.

Then press the button on the left side with the red rectangle out of it, you will enter the page where you can control the power in and out.(pic 4)

Every column in the chart means:

1st: The number of the port which written on the top of the control box, from right to left is from 1 to 4.
2nd: The name of the outlet, this depend on the order you plug in everything, for now we did like what shows in the picture.
3rd: This column is used to control the power, the three buttons in each row are "power on","power off" and "reboot".
4th: The condition of the power supply now.

EOM, FI installation and the beam shape

We finished the EOM installation and finally compensated the effect of it with moving the third lens. We measured some data after the third lens, did the fitting and compared it to the result we got without EOM. So from the picture you can see, we even had a smaller beam waist than before. Also we sent the beam coming out from the BS chamber into the 300m corridor, check the beam from time to time, it seems the size of the beam did not change too much during the propagation.The second picture shows the comparison of size between the green and infrared before we well superpose them.

Since we are going to use the green beam reflect back from the end mirror to lock the cavity. We need to send the reflected beam into a PD, so we need to extract that beam out from the FI. But when we tried to do it, we found out the beam we need is reflected to the table direction, which is very hard to extract. Also the FI we are using, only the output polarizer can be turned to tune the frequency. Finally we found a way to do it, the final setup shows in the third and fourth pictures.

Today I tried to send the beam transmitted from the BS mirror out of the chamber. At first I used a one inch mirror, but the beam is almost the same size as the mirror, so it has been a little bit cut on the edge. So then I changed it into a two inch aluminium mirror. The beam is coming out from the spare window beside the tube which connected the PR chamber and the BS chamber.Then I put a aperture out side the window, center the beam on it and another beam damper after it.

There was another problem we found today. The beam looks good everywhere until it enter the tube, then when we check at the first target, it was cut.There is a bright spot in the middle and a large ring outside, the cut part is the outside part. When we tried to move the telescope mirror, the beam on the first target moved in a very strange pattern which we still cannot understand. Finally we decided to send the beam out after the BS mirror, sent into the corridor, then we checked it further but did not see either the outside part or the cut. Now we suspected that this large cut beam may come from the reflection between the target and the window. We will try to move each telescope mirror again tomorrow for better understanding the situation.

Imaging unit optical board positioning

I moved the large sample to a z position such that the first surface is about 1 cm beyond the pump-probe cross point.

Using the Zaber Console, I read the position with the command "/02 0 tools storepos 1 current". Position is 1039045 1039045. Then set this position as the max translation limit for each axis with the commands "/02 1 set limit.away.pos 1039045" and "/02 2 set limit.away.pos 1039045".

I also noticed that there is a screw of the mount that risks to hit the HeNe probe mirror mount, so I also set the min limit on axis z to 50000 steps (~6.2mm) with the commands "/02 1 set limit.home.pos 50000" and "/02 2 set limit.home.pos 50000" see picture.

End mirror local controls

In the past days I have been working in order to restore end mirror local controls. I had a hard time finding a satisfying diagonalization both for the sensing and the driving. The current mechanical TFs, openloop TF and the comparison between open loop and closed loop spectra are shown in the attached pdf. See for comparison what I found for the dummy mirror last july.  

Comment to HeNe probe alignment, profile characterization and waist position (Click here to view original report: 495)

I calculated with Jammt the best position and focal length in order to have a probe waist of 180um at the cross point. So I replaced the lens with  a f=300mm lens and measured again the profile.  Then I plot the pump and probe profiles together. The waists positions are aligned well, within 1cm to the crosspoint. Next step is to align the imaging unit and maximize the absorption signal of a reference sample adjusting the pump position.

HeNe probe alignment, profile characterization and waist position

I used a 200um-diam pinhole to set the position of the crosspoint between the pump and the HeNe probe ().

First I checked the position of the pump waist, by looking for the position of the maximum transmitted power through the pinhole. I found 185mW/206mW  at 11.9cm +/- 0.5cm (14cm from the board) which is consistent with the previous measurement with the beam profiler and fit.

Then, using a crosshair, with the help of Matteo, I aligned the HeNe laser to be at 0.1rad  (5.7deg) with the pump beam and passing through the crosspoint at 14cm from the board.

I mounted the pinhole to the 2inches mount using the 1inch-to-2inch adaptor. I centered  the pinhole at the crosspoint and checked the absolute position in microsteps (1mm = 8063steps): 

@1-axis 2641973

@2-axis 279284

@3-axis 981325

Then I made a fine adjustment of the probe position to maximize the transmission through the pinhole.

I measured the beam profile of the HeNe laser without focusing lenses and with a f=150mm lens. See plot. The waist of the probe should be at the cross point (14cm from the board). There are two ways to move the probe waist: or move the lens, or change the lens focal length.  

EOM installed

Today when we tried to install the EOM, we found out since the telescope changed, the beam size at EOM input is a little bit large. The EOM should be installed between the second lens and the MZ, and we still need some space for another mirror and beam splitter of MZ. So we checked the optical scheme and found out there is enough space to push the MZ further from the lens. We pushed the MZ three holes back and installed the EOM.

Then the beam size changed a lot at the PR window and also it seems diverge very quickly after the BS suspension. We checked the manual of the EOM, did not find anything about this situation. For better understanding what happened to the beam after the EOM, we removed the third lens, take a few measurement before the EOM and after the EOM. Then did the fitting to check what changes. The origin set as the line of hole near the second lens. From the figure it seems only the beam waist shift from 27.2 to 20.2, the beam waist size changed a bit, but we should consider that the space is limited, so we need to take several points of measurement which is quiet close to each other. Also the z value is measured with meters which is not very precise. It seems this problem can be solved only with moving the third lens.

Comment to Pump beam profile (Click here to view original report: 484)

I replaced the 125mm lens with a 150mm lens. So the waist moved as expected to 0.11m. See the plot

Beam motion at 290 m possibly affected by PR resonance

This video shows the movement of the green beam on the target at 290 m when the local control loops of the suspended injection mirrors (PR and BS) are closed. It seems to me that it is moving too much in the vertical direction. 

In the attached pdf there are the spectra of the PR and BS mirrors.

When the loops are closed, there is still a narrow peak at about 7.5 Hz in the pitch of the PR, that is cleary visible also looking at the loop error signal in time domain. According to what observed here, I would say that it is due to the intermediate mass touching the magnet holder plate. In the past we observed that it happened often after moving a lot the yaw picomotor. 

We should now try to understand to what extent the beam motion is affected by this and possibly solve the problem.

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.