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ID Date Author Type Categorydown Subject
  8597   Fri May 17 18:24:04 2013 AnnalisaUpdate40m upgradingETMY - progress

I aligned the green beam into the Faraday. I needed an HWP to have the right polarization for the light entering the Faraday itself.

I tried to dump as much beams as possible with razor dumps, but eventually I had to use some "temporary solutions" for higher beams, because I didn't find the right mounts for razor dumps.

I measured the beam waist after the Faraday with the beam scan. Analysis and MM calculation to follow.

  8622   Thu May 23 00:16:32 2013 AnnalisaUpdate40m upgradingETMY - progress

 [Annalisa, Koji] 

 GREEN

I aligned back the beam (we lost part of the alignment after we put back the box and after the posts were installed). The green beam out from the crystal is still low, but anyway I get about 1.2 mW of green out from the Faraday. 

TO DO 

Mode Matching calculation (tomorrow)

Fix the dumping situation

Replace some of the mounts with more solid ones (in the future)

TRANSMON PATH

 QPD, PD and Camera have been rotated as Rana suggested last Wednesday. A 1m focal length lens is on the main beam transmitted path (before the harmonic separator), and the beam diameter on the QPD is about 5mm. We put another lens with a shorter focal length to put the PD very close to the beam waist and in way to have a reasonable beam size on the camera. Tomorrow I will write down all the correct sizes of the beams.

OPLEV 

(for Steve) I marked a possible beam path for the Oplev (the laser is not in the right place in the picture, but I left it in the correct place on the table). I also put the QPD for the IP-ANG, so we know in which part of the table the beam can be steered.

The space in the red rectangle (right corner) has to be left empty to put a PD for the rejected beam from the green Faraday.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Attachment 1: TransMonAndOplev.jpg
TransMonAndOplev.jpg
  8637   Fri May 24 02:12:50 2013 AnnalisaUpdate40m upgradingETMY - Mode Matching for green

 

 Mode Matching calculation for green beam - Yarm

After measuring the beam radius out from the Faraday for the green, I made the calculation to match the green beam mode with the IR mode inside the arm.

The beam waist after the Faraday is elliptical, and I found the following value for the waist:

w0x = 3.55e-5 m @ z0x = -0.042 m

w0y = 2.44e-5 m @ z0y = -0.036 m

(the origin of the z axis is the output of the Faraday, so the waist is inside the Faraday itself)

I did the calculation using a la mode, using as beam waist and its position the following values:

w0 = sqrt(w0x*w0y) = 2.943e-5 m @ z0 = (z0x+z0y)/2 = -0.039 m

The results are shown in the attached plots.

                      Focal length (m)             position (m)

lens1            0.125                                0.1416

lens2            0.100                                0.5225

L                    1.000                                1.5748 (fixed lens used to focus transmitted beam)

 

As the first plot shows, the green beam size on the ETMY is about 6mm. My concern is that it could be too big.

The third plot shows the X and Y section of the beam. It is strongly elliptical, but nevertheless the coupling factor calculated with Koji's formula  gives C=0.936 for the astigmatic beam, and C=0.985 for the non astigmatic beam, so it seems to be still ok.

 

 

Attachment 1: ModeMatchingGreen.jpg
ModeMatchingGreen.jpg
Attachment 2: ModeMatchingGreenZoom.jpg
ModeMatchingGreenZoom.jpg
Attachment 3: XYpath.jpg
XYpath.jpg
  8638   Fri May 24 11:38:00 2013 KojiUpdate40m upgradingETMY - Mode Matching for green

I got confused. Is the mode calculation in the cavity correct?
Are you sure the wavelength in the code is 532nm?

The first plot says "the waist radius at ITMY is 2.15mm". This number is already very close to
the waist size of the cavity mode (2.1mm@ITM, 3.7mm@ETM), but the spot radius at ETMY is 6mm.
They are inconsistent.

 

  8639   Fri May 24 12:50:25 2013 AnnalisaUpdate40m upgradingETMY - Mode Matching for green

Quote:

I got confused. Is the mode calculation in the cavity correct?
Are you sure the wavelength in the code is 532nm?

The first plot says "the waist radius at ITMY is 2.15mm". This number is already very close to
the waist size of the cavity mode (2.1mm@ITM, 3.7mm@ETM), but the spot radius at ETMY is 6mm.
They are inconsistent.

 

 Jenne and I just realized that a la mode has 1064e-9 m as default value. I'll change it and make the calculation again.

  8645   Sat May 25 02:03:48 2013 AnnalisaUpdate40m upgradingETMY - Mode Matching for green - new calculation

 Mode matching calculation for green - Yarm

I did again the mode matching calculation. The previous one was using 1064nm as wavelength, so it was wrong.

The seed beam waist and its position are the same as in elog 8637. The new results are shown in the attached graphs.

I got the following values for focal lengths and positions of the two Mode Matching lenses:

 

                    Focal length (m)             Distance from the Faraday output (m)

lens1            0.125                                                  0.1829

lens2           -0.200                                                  0.4398

L                   1.000                                                  1.4986 (fixed)

The position of the lens L has changed because the path lengh has been slightly  reduced. 

The Coupling factor for he astigmatic beam is C = 0.959 (it is C = 0.9974 if we consider the beam as non astigmatic).

I put the lenses and aligned the beam up to the shutter, which has been moved from its initial position because the beam size on it was too large. 

TO DO

The green beam needs to be aligned and sent into the arm cavity. 

Polarization has to be checked.

Many beams still have to be dumped, both in IR and Green paths. 

 

 

 

 

Attachment 1: ModeMatchingGreenNEW.jpg
ModeMatchingGreenNEW.jpg
Attachment 2: ModeMatchingGreenZoomNEW.jpg
ModeMatchingGreenZoomNEW.jpg
Attachment 3: XYpathNEW.jpg
XYpathNEW.jpg
Attachment 4: photo1.JPG
photo1.JPG
Attachment 5: photo2.JPG
photo2.JPG
  8646   Mon May 27 21:38:53 2013 AnnalisaUpdate40m upgradingETMY - Beam Dumps

 

I put many razor dumps along the IR/green path. The rejected beam from the IR Faraday needs to be dumped (about 1.5 mW). I used all the new razor blade I had, so I need one more for that beam.

The IR reflection of the Harmonic separator right after the doubler needs to be dumped in a better way. At the moment there is a black screen, but we need something suitable to dump more than 300 mW.

After the second steering mirror along the green beam path there is a very small transmission (about 6 uW), which is difficult to dump because there is no space enough. Can it be dumped with a black screen?

 

The Oplev has a lot of reflection hitting the central BS (The BS for the transmitted beam). It is very difficult to dump them without intercepting the main beam path. Maybe we have to slightly change the Oplev beam angle to avoid so many reflections.

 

  8652   Tue May 28 22:11:23 2013 AnnalisaUpdate40m upgradingETMY - Yarm shutter cable

 

 For some strange reason the Yarm shutter cable runs up to the POY table, where it is connected to another cable going to the rack. It has to be put off from the table, at least. It would be better to have only one cable going directly to the rack.

I roughly aligned the green into the Yarm and I've seen the green beam flashing on the PSL table, but the mode matching is not so good and I get an higher order mode, so I'm going to fix the mode matching tomorrow.

  11521   Thu Aug 20 18:08:28 2015 IgnacioFrogs40m upgradingFatality. Something broke.

So I made coffee at 1547 and was astonished to find the above. Its a sad, very sad day.

At first I thought that something (a gravity wave?) or someone, accidentally hit the thing and it fell and broke. But Koji told me that the janitor was cleaning around the thing and it did indeed fell accidentally.

  12625   Fri Nov 18 00:25:08 2016 JohannesOmnistructure40m upgradingAcromag Chassis Development

I had Rich show me his approach to a chassis for the Acromag modules. The document tree for his design can be found on the DCC. Note that he's using the high densitymodel ES series, which is available as a bare board variant with pluggable screw terminals:

He can fit up to 4 of these in a 2U chassis and has outsourced the wiring from front panel Dsubs to the board connectors to an external company. At the 40m (and in West Bridge) we currently only have the rail mounted XT series

At first glance the specs are very similar. Both A/D and D/A flavors have 16-bit precision in both cases. The high density ES series with Rich's layout can achieve 128 A/D per 2U, 64 D/A per 2U, or 384 DIO per 2U. Into a 4U chassis of the type we have currently we can fit ~32 XT modules (assuming two rows), which results in very similar numbers, except for the DAC, of which we could fit more.

XT1221-000 (8 diff. channel 16-bit ADC)                          $495.00      $61.88/ch
XT1541-000 (8 channel 16-bit DAC and 4 discrete I/O )    $525.00      $65.63/ch
XT1120-000 (16 channel DIO)                                         $320.00     $20.00/ch

ES2162-0010 (32 diff. channel 16-bit ADC)                     $2050.00    $64.06/ch
ES2172-0010 (16 channel 16-bit DAC)                           $1400.00    $87.50/ch
ES2113-0010 (96 channel DIO)                                      $1100.00    $11.46/ch

It's cheaper to stick with the current XT models, but they need the bulkier 4U chassis. The good news is that actually all these models have 16 bit precision, which wasn't clear to me before. Lydia and I will work out what connectors we want on the boxes, and how many modules/channels we need where. Rich also got me in touch with Keith Thorne, who handles the analog I/O Acromag at LLO, and I will ask him for advice. From his documents on the DCC it seems that he is using yet another series: EN. The 968EN-4008 for example is a rail-mounted ADC with pluggable connections, but looses quite clearly in price per channel.

For a generic multipurpose DAQ interface box the ES series is the best approach in my opinion, because it offers a more compact design. We could for example fit 1 ADC, 2 DAC, 1 DIO in a 2U chassis for 32/32/96 channels. The combined price tag for this scenario would be ~$6k.

 

 

  12634   Tue Nov 22 13:55:32 2016 JohannesOmnistructure40m upgradingAcromag Chassis

Current Acromag chassis status:

I found out that Acromag offers DIN rail mounting kits for the open boards, so we can actually fit both XT series and ES/EN series in the same boxes, depending on the signal needs. The primary design driver will be the ES footprint, but if we find we don't need that many channels in some of the units, it's interchangable. For the wiring to the front panel - for which we will have a standard front panel express design, but may order modified ones for the custom needs of the 40m, I will contract the same company that Rich used for the wiring in his DIO box (Panel mount connectors terminating in loose wires/pre-routed plugs for Acromag units). We will either run a single DIN rail along the length of the chassis, or have two in parallel across.

Lydia and I took close looks at the breakout arrangements on the rack sides, and determined that because of the many cross-connects between non-DAQ ports it is not possible to redo and debug this in a reasonable amount of time without essentially shutting down the interferometer. So instead, we will connect the chassis directly to the slots that were previously leading to the slow machines. They come in two different flavors: The ADC modules have 64 pins, while the DIO and DAC ones have 50. There are a couple things we can do:

  • For ADC: Put favorite 64+ pin connector on front panel. I would advocate for the 68 pin VHDIC (SCSI-5). This standard ist widely used, has a sturdy connector, and usually off-the-shelf cables have twisted pair leads.
  • For DAC+DIO: Either use favorite 50 pin connector (there are 50-pin DSUB connectors, and also 50-pin IDC connectors with backshell), or also send the signals through VHDIC connectors, tolerating a few unused lanes. I would prefer the second option, after all it all goes to some 64 pin VME-crate backplane connector in the end, so if we ever get rid of the rack-side breakouts the wiring will much more uniform.
  • For good measure, we will add a few (16 maybe) BNC connectors to the front panel.
  • A standardized front panel could have a variety of different connectors by default: DSUBs, BNCs, etc., to be used when needed with some initial default wiring.
  • Note that THEORETICALLY we could even connect all backplane EUROCARD ports to the Acromag chassis and do the cross-connect wiring entirely inside, although that would make the inside extremely messy.

Based on Rich's design I will get started on a parts list and wiring diagrams to send out to the cable company.

Attachment 1: acroplan.pdf
acroplan.pdf
  10130   Sat Jul 5 04:18:45 2014 AndresUpdate40m Xend Table upgradeAdding Two Lenses After the Second Steering Mirror in Order Two Increase the Gouy Phase Difference Between the Sterring Mirrors

I had been working on the Xend table optical layout update. Since the two steering mirrors in the Xend green are too close to each, there is a very small Gouy Phase different between these two mirrors. It was suggested to place two lenses so that we can increase the Gouy Phase. I have been working with Nick on this problem, and we had found a solution by using a la mode. We had written an a la mode code that optimize the Gouy Phase and the Mode Matching at the same time. After trying different lenses, we found the following results: a mode matching of 0.9939 as it is show in the first attachment below, and we found a Gouy Phase different between the two mirrors of about 60 degrees. I took photos of the Xend Table. The first photo is the Xend table as we had it right now. In the second photo, I moved the 2nd lens, and I placed the two more lenses that we need it, with more or lenses the correct position where they will be placed. The three old lenses will be replaced by three lenses of different focal length as it can be seen in the first attachment below. The first lens and third lens will stay in the same position where the old first lens and old third lens are, and the second lens will be moved by about half of an inch. We might have one or two of the lenses that we need, but we will have to order the rest of the lenses that need. My plan is to verify the lenses that we already have. Then, I need to let Nick know with lenses we need to order. Hopefully, we will be able to update the table by the end of this week if everything turn out fine.

Attachment 1: OverlapAndComponentsOfTheSolution.png
OverlapAndComponentsOfTheSolution.png
Attachment 2: CloseLookToTheGouyPhaseBetweenMirr1AndMirr2.jpg
CloseLookToTheGouyPhaseBetweenMirr1AndMirr2.jpg
Attachment 3: EntireRangeOfBeamPath.jpg
EntireRangeOfBeamPath.jpg
Attachment 4: XendTableWithTwoNeedLensesAdding.JPG
XendTableWithTwoNeedLensesAdding.JPG
Attachment 5: SchematicOfSolutionForTheLensesGouyPhase.jpeg
SchematicOfSolutionForTheLensesGouyPhase.jpeg
Attachment 6: XendGreenModeMatchingAndGouyPhaseOptimization.m
clear all
% In this code we are using a la mode to optimatize the mode matching and
% to optimatize the Gouy phase between mirror 1 and mirror 2. All the units
% are in meter

w0=2.943*1e-5; % The Waist of the laser measured before the faraday
z0_laser=-0.039; % position measured where the waist is located 
lamb= 532*10^-9; % wavelength of green light in mm
lFaraday=.0638; % Length of the faraday

... 148 more lines ...
Attachment 7: BeforeIncludingLensesORMovingLenses.JPG
BeforeIncludingLensesORMovingLenses.JPG
  10191   Sun Jul 13 17:06:35 2014 AndresUpdate40m Xend Table upgradeXarm Table Upgrade Calculation and Diagrams of possible new table layout

 Current Mode Matching and Gouy Phase Between Steering Mirrors

We found in 40m elog ID 3330 ( http://nodus.ligo.caltech.edu:8080/40m/3330a documentation done by Kiwamu, where he measured the waist of the green. The waist of the green is about 35µm. Using a la mode, I was able to calculate the current mode matching, and the Gouy phase between the steering mirrors. In a la mode, I used the optical distances,which is just the distance measured times its index of refraction. I contacted someone from ThorLabs (which is the company that bought Optics For Research), and that person told that the Faraday IO-5-532-LP has a Terbium Gallium Garnet crystal of a length of 7mm and its index of refraction is 1.95. The current mode matching is 0.9343, and the current Gouy phase between steering mirrors is 0.2023 degrees. On Monday, Nick and I are planning to measure the actual mode matching. The attached below is the current X-arm optical layout. 

 

 

Calculation For the New Optical Layout

 

Since the current Gouy phase between the steering mirror is essentially zero, we need to find a way how to increase the Gouy Phase. We tried to add two more lenses after the second steering mirror, and we found that increasing the Gouy phase result in a dramatically decrease in mode matching. For instance, a Gouy phase of about 50 degrees results in a mode matching of about .2, which is awful. We removed the first lens after the faraday, and we added two more mirrors and two more lenses after the second steering mirror. I modified the photo that I took and I place where the new lenses and new mirrors should go as shown in the second pictures attached below. Using a la mode, we found the following solution:

 label                         z (m)            type                       parameters         

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

 lens 1                       0.0800          lens                      focalLength: 0.1000

 First mirror              0.1550          flat mirror            none:            

 Second mirror         0.2800          flat mirror            none:            

 lens 2                      0.4275           lens                      focalLength: Inf   

 lens 3                     0.6549            lens                      focalLength: 0.3000

lens 4                      0.8968            lens                      focalLength: -0.250

Third mirror           1.0675            flat mirror            none:            

Fourth mirror         1.4183            flat mirror            none:            

lens 5                      1.6384            lens                     focalLength: -0.100

Fifth mirror            1.7351            flat mirror           none:            

Sixth mirror           2.0859            flat mirror           none:            

lens 6                     2.1621            lens                     focalLength: 0.6000

ETM                      2.7407            lens                    focalLength: -129.7

ITM                       40.5307          flat mirror          none:             

The mode matching is 0.9786. The different Gouy phase different between Third Mirror and Fourth Mirror is 69.59 degrees, Gouy Phase between Fourth and Fifth 18.80 degrees, Gouy phase between Fifth and Sixth mirrors is 1.28 degrees, Gouy phase between Third and Fifth 88.38 degrees, and the Gouy phase between Fourth and Sixth is 20.08 degrees. Bellow attached the a la Mode code and the Plots.

 

 

Plan for this week

I don't  think we have the lenses that we need for this new setup. Mostly, we will need to order the lenses on Monday. As I mention, Nick and I are going to measure the actual mode matching on Monday. If everything look good, then we will move on and do the Upgrade.

 

Attachment 1: CurrentOpticalLayout.png
CurrentOpticalLayout.png
Attachment 2: NewSetUp.PNG
NewSetUp.PNG
Attachment 3: AlaModeSolutionplots.png
AlaModeSolutionplots.png
Attachment 4: EntireScaleRangeAlaModeSolution.png
EntireScaleRangeAlaModeSolution.png
Attachment 5: NewXarmOptimizationFromFaraday.m
close all
clear all
% In this code we are using a la mode to optimatize the mode matching and
% to optimatize the Gouy phase between mirror 1 and mirror 2. All the units
% are in meter

w0=(50*1e-6)/sqrt(2); % The Waist of the laser measured after SHG
z0_laser=-0.0083; % position measured where the waist is located 
lamb= 532*10^-9; % wavelength of green light in mm
lFaraday=.0638; % Length of the faraday
... 209 more lines ...
  10195   Mon Jul 14 16:19:41 2014 AndresUpdate40m Xend Table upgradeTook the measurement for the Mode Matching

 Nick and I measured the reflected power of the green light in locked and unlocked. I'm working on the calculation of the mode matching. Tonight, I'll be posted my calculation I'm still working on it.

JCD:  Andres forgot to mention that they closed the PSL shutter, so that they could look at the green light that is reflected off the harmonic separator toward the IR trans path.  Also, the Xarm (and the Yarm) were aligned to IR using the ASS, and then ASX was used to align the green beam to the cavity.

  10207   Tue Jul 15 22:23:51 2014 AndresUpdate40m Xend Table upgradeScan the Xarm for the mode matching

 Nick and I with the help of Jenne scan the green light when the cavity is unlocked. Nick placed a Beam dump on the IR so that we can just scan the green, but it was removed as soon as we finished with the measurement. I'm working on the calculation, and i'll be posted solution tonight.

  10226   Thu Jul 17 02:57:32 2014 AndresUpdate40m Xend Table upgradeFInish Calculation on Current X-arm mode Matching

Data and Calculation for the Xarm Current Mode Matching

Two days ago, Nick, Jenne, and I took a measurement for the Green Transmission for the X-arm. I took the data and I analyzed it. The first figure attached below is the raw data plotted. I used the function findpeaks in Matlab, and I found all the peaks. Then, by taking close look at the plot, I chose two peaks as shown in the second figure attached below. I took the ratio of the TEM00 and the High order mode, and I average them. This gave me a Mode Matching of 0.9215, which this value is pretty close to the value that I predicted by using a la Mode in http://nodus.ligo.caltech.edu:8080/40m/10191, which is 0.9343. Nick and I measured the reflected power when the cavity is unlocked and when the cavity is locked, so we measured the PreflUnLocked=52+1µW and PreflOnLocked=16+2µW and the backgroundNoise=0.761µW. Using this information we calculated  Prefl/Pin=0.297. Now, since Prefl/Pin=|Eref/Ein|2, we looked at the electric fields component by using the reflectivity of the mirror we calculated 0.67. The number doesn't agree, but this is because we didn't take into account the losses when making this calculation. I'm working in the calculation that will include the losses.

Today, Nick and I ordered the lenses and the mirrors. I'm working in putting together a representation of how much improvement the new design will give us in comparison to the current setup.

Attachment 1: RawDataForTheModeGreenScan.png
RawDataForTheModeGreenScan.png
Attachment 2: ResultForModeMatching.png
ResultForModeMatching.png
Attachment 3: DataAndCalculationOfModeMismatch.zip
  10237   Fri Jul 18 16:52:56 2014 AndresUpdate40m Xend Table upgradeFInish Calculation on Current X-arm mode Matching

Quote:

Data and Calculation for the Xarm Current Mode Matching

Two days ago, Nick, Jenne, and I took a measurement for the Green Transmission for the X-arm. I took the data and I analyzed it. The first figure attached below is the raw data plotted. I used the function findpeaks in Matlab, and I found all the peaks. Then, by taking close look at the plot, I chose two peaks as shown in the second figure attached below. I took the ratio of the TEM00 and the High order mode, and I average them. This gave me a Mode Matching of 0.9215, which this value is pretty close to the value that I predicted by using a la Mode in http://nodus.ligo.caltech.edu:8080/40m/10191, which is 0.9343. Nick and I measured the reflected power when the cavity is unlocked and when the cavity is locked, so we measured the PreflUnLocked=52+1µW and PreflOnLocked=16+2µW and the backgroundNoise=0.761µW. Using this information we calculated  Prefl/Pin=0.297. Now, since Prefl/Pin=|Eref/Ein|2, we looked at the electric fields component by using the reflectivity of the mirror we calculated 0.67. The number doesn't agree, but this is because we didn't take into account the losses when making this calculation. I'm working in the calculation that will include the losses.

Today, Nick and I ordered the lenses and the mirrors. I'm working in putting together a representation of how much improvement the new design will give us in comparison to the current setup.

We want to be able to graphically see how much better it is the new optical table setup in comparison to the current optical table setup. In other words, we want to be able to see how displacement of the beam and how much angle change can be obtained at the ETM from changing the mirrors angles independently. Depending on the spread of the mirrors' vectors we can observe whether the Gouy phase is good. In the plot below, the dotted lines correspond to the current set up, and we can see that the lines are not spread from each other, which essentially mean that changing the angles of the two mirrors just contribute to small change in angle and in the displacement of the beam at the ETM, and therefore the Gouy phase is not good. Now on the other hand. The other solid lines correspond to the new setup mirrors. We can observe that the spread of the line of mirror 1 and mirror 4 is almost 90 degrees, which just implies that there is a good Gouy phase different between these two mirrors. For the angles chosen in the plot, I looked at how much the PZT yaw the mirrors from the elog http://nodus.ligo.caltech.edu:8080/40m/8912. In this elog, they give a plot in mrad/v for the pitch and yaw, so I took the range that the PZT can yaw the mirrors, and I converted into mdegrees/v and then I plotted as shown below. I plot for the current setup and for the new setup in the same plot. The matlab code is also attached below.

Attachment 1: OldAndNewSetupPlotsOfDisplacementAndAngleAtTheETM.png
OldAndNewSetupPlotsOfDisplacementAndAngleAtTheETM.png
Attachment 2: OldSetUpDisplacementAndNewSetup.m.zip
  10290   Tue Jul 29 20:14:08 2014 AndresUpdate40m Xend Table upgradeXarm Green steering mirror upgrade

 Xarm Green Steering Mirror Upgrade

Nick and I did the upgrade for the green steering mirror today. We locked in the TEM00 mode.
We placed the shutter and everything. We move the OL, but we placed it back. Tonight, I'll be doing a more complete elog with more details.

  10291   Tue Jul 29 20:14:10 2014 KojiUpdate40m Xend Table upgradeXarm Green steering mirror upgrade

That was super fast! Great job, Andres and Nic!

  10296   Wed Jul 30 10:16:54 2014 AndresUpdate40m Xend Table upgradeGreen Steering Mirror Upgrade completed

Green Steering Mirror Update

Yesterday, Nick and I completed the green steering mirrors upgrade. I attached the file that contained the procedure that we plan before we did the upgrade. We placed an iris at the input of the OL and we place another iris before the harmonic separator. We did not use the beam scanner because someone was using it, so what we did was to assume that the cavity is well align and place the iris so that we can recover the alignment. We used the measuring tape to approximate as close as we could the position where the lenses were supposed to go. I did a measurement of the derivative of the waist size in terms of the position of the lens and the derivative of the waist Position in terms of the lenses position at the optimum solution that a la mode give us. Because of this plot, we decide to mount lens 3 and lens 5 into translational stages. After mounting each lenses and mirrors we worked on the alignment of the beam into the cavity. We were able to align the green into the cavity and we were able to locked the cavity to the TEM00 mode. We started to work on the optimization of the mode matching. However, the maximum mode matching that we got was around 0.6, which we need to work a little bit more on the tuning of the mode matching. We leave the iris mounted on the table. I took a picture of the table, and I attached below. For the OL, we just make sure that the output where somehow hitting the QPD, but we didn't really I aligned it. We need to work a little bit more on the alignment of the OL and the tuning of the mirror to maximize the green mode matching.

Attachment 1: XarmUpgrade.pdf
XarmUpgrade.pdf XarmUpgrade.pdf XarmUpgrade.pdf XarmUpgrade.pdf XarmUpgrade.pdf XarmUpgrade.pdf
Attachment 2: dWaistSize_dlensVsdWaistPosition_dlens.png
dWaistSize_dlensVsdWaistPosition_dlens.png
Attachment 3: XarmNewOpticalSetup.PNG
XarmNewOpticalSetup.PNG
  2653   Wed Mar 3 18:32:25 2010 AlbertoUpdate40m Upgrading11 MHz RFPD elctronics
** Please add LISO file w/ component values.
 
I designed the circuit for one of the 11 MHz photodiodes that we're going to install in the 40m Upgrade.

This is a simple representation of the schematic:

          gnd
#          |
#          Cw2
#          |
#          n23
#          |
#          Lw2
#          |
#           n22
#          |
#          Rw2                
#                 |                   |\            
#           n2- - - C2 - n3 -  - -  - |  \          
#            |    |      |   |        |4106>-- n5 - Rs -- no
# iinput    Rd   L1     L2 R24    n6- |  /     |           |
#      nin - |    |      |   |    |   |/       |         Rload    
#           Cd   n7     R22 gnd   |            |           |          
#            |    |      |        | - - - R8 - -          gnd              
#           gnd  R1     gnd      R7 
#                 |               |
#         gnd               gnd
#                 
#
#

I chose the values of the components in a realistic way, that is using part available from Coilcraft or Digikey.

Using LISO I simulated the Tranfer Function and the noise of the circuit.

I'm attaching the results.

I'll post the 55MHz rfpd later.

Attachment 1: rfpd11_v2_TF.pdf
rfpd11_v2_TF.pdf
Attachment 2: rfpd11_v2_Noise.pdf
rfpd11_v2_Noise.pdf
  2655   Thu Mar 4 08:43:35 2010 AlbertoUpdate40m Upgrading11 MHz RFPD elctronics

Quote:
** Please add LISO file w/ component values.

oops, forgotten the third attachment...

here it is

Attachment 1: rfpd11_v2.fil
# Resonant RF diode front end
#
#		  gnd
#		  |
#		  Cw2
#		  |
#		  n23
#		  |
#		  Lw2
#		  |
... 60 more lines ...
  2656   Thu Mar 4 19:53:56 2010 AlbertoUpdate40m Upgrading11MHz PD designed adjusted for diode's resistance; 55 MHz RFPD designed
After reading this study done at LIGO MIT in 1998 I understood why it is difficult to define an effective impedance for a photodiode.

I read a few datasheets of the C30642GH photodiode that we're going to use for the 11 and 55 MHz. Considering the  values listed for the resistance and the capacitance in what they define "typical conditions" (that is, specific values of bias voltage and DC photocurrent) I fixed Rd=25Ohms and Cd=175pF.

Then I picked the tunable components in the circuit so that we could adjust for the variability of those parameters.

Finally with LISO I simulated transfer functions and noise curves for both the 11 and the 55MHz photodiodes.

I'm attaching the results and the LISO source files.

 

Attachment 1: rfpd55_Noise.pdf
rfpd55_Noise.pdf
Attachment 2: rfpd55_TF.pdf
rfpd55_TF.pdf
Attachment 3: rfpd11_v2_TF.pdf
rfpd11_v2_TF.pdf
Attachment 4: rfpd11_v2_Noise.pdf
rfpd11_v2_Noise.pdf
Attachment 5: rfpd11_v2.fil
Attachment 6: rfpd55.fil
  2657   Thu Mar 4 22:07:21 2010 ranaUpdate40m Upgrading11MHz PD not yet designed

Use 10 Ohms for the resistance - I have never seen a diode with 25 Ohms.

p.s. PDFs can be joined together using the joinPDF command or a few command line options of 'gs'.

  2704   Tue Mar 23 22:46:43 2010 AlbertoUpdate40m UpgradingREFL11 upgraded
I modified REFL11 according to the changes lsited in this schematic (see wiki  / Upgrade 09 / RF System / Upgraded RF Photodiodes ).
I tuned it to be resonant at 11.06MHz and to have a notch at 22.12MHz.
These are the transfer functions that I measured compared with what I expected from the LISO model.

2010-03-23_REFL11_model_to_meas_comparison.png

The electronics transfer function is measured directily between the "Test Input" and the "RF Out" connector of the box. the optical transfer function is measured by means of a AM laser (the "Jenne laser") modulated by the network analyzer.
The AM laser's current was set at 20.0mA and the DC output of the photodiode box read about 40mV.
The LISO model has a different overall gain compared to the measured one, probably because it does not include the rest of the parts of the circuit other than the RF out path.

I spent some time trying to understand how touching the metal cage inside or bending the PCB board affected the photodiode response. It turned out that there was some weak soldering of one of the inductors.

  2711   Wed Mar 24 14:57:21 2010 AlbertoUpdate40m UpgradingREFL11 upgraded

 

 Hartmut suggested a possible explanation for the way the electronics transfer function starts picking up at ~50MHz. He said that the 10KOhm resistance in series with the Test Input connector of the box might have some parasitic capacitance that at high frequency lowers the input impedance.

Although Hartmut also admitted that considering the high frequency at which the effect is observed, anything can be happening with the electronics inside of the box.

  2715   Thu Mar 25 17:32:42 2010 AlbertoUpdate40m UpgradingREFL55 Upgraded

I upgraded the old REFL199 to the new REFL55.

To do that I had to replace the old photodiode inside, switching to a 2mm one.

Electronics and optical transfer functions, non normalized are shown in the attached plot.

2010-03-25_REFL55_model_to_meas_comparison.png

The details about the modifications are contained in this dedicated wiki page (Upgrade_09 / RF System / Upgraded RF Photodiodes)

Attachment 1: 2010-03-25_REFL55_model_to_meas_comparison.png
2010-03-25_REFL55_model_to_meas_comparison.png
  2761   Sat Apr 3 19:54:19 2010 AlbertoUpdate40m UpgradingREFL11 and REFL55 PDs Noise Spectrum

These are the dark noise spectrum that I measured on the 11MHz and 55MHz PD prototypes I modified.

The plots take into account the 50Ohm input impedance of the spectrum analyzer (that is, the nosie is divided by 2).

2010-04-03_REFL11_darknoise.png 2010-04-03_REFL55_darknoise.png

With an estimated transimpedance of about 300Ohm, I would expect to have 2-3nV/rtHz at all frequencies except for the resonant frequencies of each PD. At those resonances I would expect to have ~15nV/rtHz (cfr elog entry 2760).

Problems:

  1. For the 55MHz PD the resonance peak is too small
  2. In the 55 MHz: noise is present at about 7MHz
  3. In the 11MHz PD there's a lot of noise below 10 MHz.

I have to figure out what are the sources of such noises.

Suggestions?

  2767   Mon Apr 5 10:23:40 2010 AlbertoUpdate40m UpgradingREFL11 Low Frequency Oscilaltion Reduced

After adding an inductor L=100uH and a resistor R=10Ohm in parallel after the OP547A opamp that provide the bias for the photodiode of REFL11, the noise at low frequency that I had observed, was significantly reduced.

See this plot:

 2010_04_05_REFL11_darknoise_with_100uH_coil_10ohm_res.png

A closer inspection of the should at 11MHz in the noise spectrum, showed some harmonics on it, spaced with about 200KHz. Closing the RF cage and the box lid made them disappear. See next plot:

 2010_04_05_REFL11_darknoise_wide_freq_window_lid_open-closed.png

The full noise spectrum looks like this:

2010_04_05_REFL11_darknoise_wide_freq_window_lid_open-closed.png

A big bump is present at ~275MHz. it could important if it also shows up on the shot noise spectrum.

  2782   Thu Apr 8 10:17:52 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)

The fit shown in the plot is: Vn = Vdn + sqrt(2*e*Idc) ; Vn=noise; Vdn=darknoise; e=electron charge; Idc=dc photocurrent

The estimate from the fit is 3-4 times off from my analsys of the circuit and from any LISO simulation. Likely at RF the contributions of the parassitic components of each element make a big difference. I'm going to improve the LISO model to account for that.

2010_04_05_REFL11_ShotnoiseVsPhotocurrent.png

The problem of the factor of 2 in the data turned out to be not a real one. Assuming that the dark noise at resonance is just Johnson's noise from the resonant circuit transimpedance underestimates the dark noise by 100%.

  2783   Thu Apr 8 10:24:33 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Quote:

From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)

Putting my hands ahead, I know I could have taken more measurements around the 3dB point, but the 40m needs the PDs soon.

  2784   Thu Apr 8 20:53:13 2010 KojiUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Something must be wrong. 

1. Physical Unit is wrong for the second term of "Vn = Vdn + Sqrt(2 e Idc)"

2. Why does the fit go below the dark noise?

3. "Dark noise 4 +/- NaN nV/rtHz"   I can not accept this fitting.

Also apparently the data points are not enough.

Quote:

From the measurements of the 11 MHz RFPD at 11Mhz I estimated a transimpedance of about 750 Ohms. (See attached plot.)

The fit shown in the plot is: Vn = Vdn + sqrt(2*e*Idc) ; Vn=noise; Vdn=darknoise; e=electron charge; Idc=dc photocurrent

The estimate from the fit is 3-4 times off from my analsys of the circuit and from any LISO simulation. Likely at RF the contributions of the parassitic components of each element make a big difference. I'm going to improve the LISO model to account for that.

2010_04_05_REFL11_ShotnoiseVsPhotocurrent.png

The problem of the factor of 2 in the data turned out to be not a real one. Assuming that the dark noise at resonance is just Johnson's noise from the resonant circuit transimpedance underestimates the dark noise by 100%.

 

  2785   Fri Apr 9 06:45:28 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Quote:

Something must be wrong. 

1. Physical Unit is wrong for the second term of "Vn = Vdn + Sqrt(2 e Idc)"

2. Why does the fit go below the dark noise?

3. "Dark noise 4 +/- NaN nV/rtHz"   I can not accept this fitting.

Also apparently the data points are not enough.

 1) True. My bad. In my elog entry (but not in my fit code) I forgot the impedance Z= 750Ohm (as in the fit) of the resonant circuit in front of the square root: Vn = Vdn + Z * sqrt( 2 e Idc )

2) That is exactly the point I was raising! The measured dark noise at resonance is 2x what I expect.

3) I don't have uncertainties for the fit offset (that is, for the Dark Noise). The quick fit that I used (Matlab's Non Linear Least Squares method) doesn't provide 95% confidence bounds when I constrain the offset parameter the way I did (I forced it to be strictly positive).
Sure. It's not a very good fit. I just wanted to see how the data was going.

I also admitted that the data points were few, especially around the 3dB point.

Today I'm going to repeat the measurement with a new setup that lets me tune the light intensity more finely.

  2789   Mon Apr 12 16:20:05 2010 AlbertoConfiguration40m UpgradingREFL55 improved
During the commissioning of the AS55 PD, I learned how to get a much better rejection of the 11MHz modulation.
So I went back to REFL55 and I modified it using the same strategy. (Basically I added another notch to the circuit).
After a few days of continuous back and forth between modeling, measuring, soldering, tuning I got a much better transfer function.

All the details and data will be included in the wiki page (and so also the results for AS55). Here I just show the comparison of the transfer functions that I measured and that I modeled.

I applied an approximate calibration to the data so that all the measurements would refer to the transfer function of Vout / PD Photocurrent. Here's how they look like. (also the calibration will be explained in the wiki)

2010-04-12_REFL55_TF_model_to_meas_comparison.png.

The ratio between the amplitude of the 55Mhz modulation over the 11MHz is ~ 90dB

The electronics TF doesn't provide a faithful reproduction of the optical response.

  2790   Mon Apr 12 17:09:30 2010 AlbertoUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Quote:

Quote:
 

 1) True. My bad. In my elog entry (but not in my fit code) I forgot the impedance Z= 750Ohm (as in the fit) of the resonant circuit in front of the square root: Vn = Vdn + Z * sqrt( 2 e Idc )

2) That is exactly the point I was raising! The measured dark noise at resonance is 2x what I expect.

3) I don't have uncertainties for the fit offset (that is, for the Dark Noise). The quick fit that I used (Matlab's Non Linear Least Squares method) doesn't provide 95% confidence bounds when I constrain the offset parameter the way I did (I forced it to be strictly positive).
Sure. It's not a very good fit. I just wanted to see how the data was going.

I also admitted that the data points were few, especially around the 3dB point.

Today I'm going to repeat the measurement with a new setup that lets me tune the light intensity more finely.

 Here's another measurement of the noise of the REFL11 PD.

This time I made the fit constraining the Dark Noise. I realized that it didn't make much sense leaving it as a free coefficient: the dark noise is what it is.

2010-04-09_REFL11NoiseMeasurements.png

Result: the transimpedance of REFL11at 11 MHz is about 4000 Ohm.

Note:
This time, more properly, I refer to the transimpedance as the ratio between Vout @11Mhz / Photocurrent. In past entries I improperly called transimpedance the impedance of the circuit which resonates with the photodiode.
  2795   Mon Apr 12 22:44:30 2010 KojiUpdate40m UpgradingREFL11 Noise Vs Photocurrent

Data looks perfect ... but the fitting was wrong.

Vn = Vdn + Z * sqrt( 2 e Idc ) ==> WRONG!!!

Dark noise and shot noise are not correlated. You need to take a quadratic sum!!!

Vn^2 = Vdn^2 + Z^2 *(2 e Idc)

And I was confused whether you need 2 in the sqrt, or not. Can you explain it?
Note that you are looking at the raw RF output of the PD and not using the demodulated output... 

Also you should be able to fit Vdn. You should put your dark noise measurement at 10nA or 100nA and then make the fitting.

Quote:

 Here's another measurement of the noise of the REFL11 PD.

This time I made the fit constraining the Dark Noise. I realized that it didn't make much sense leaving it as a free coefficient: the dark noise is what it is.

2010-04-09_REFL11NoiseMeasurements.png

Result: the transimpedance of REFL11at 11 MHz is about 4000 Ohm.

Note:
This time, more properly, I refer to the transimpedance as the ratio between Vout @11Mhz / Photocurrent. In past entries I improperly called transimpedance the impedance of the circuit which resonates with the photodiode.

 

  2882   Wed May 5 16:32:39 2010 AlbertoUpdate40m UpgradingNew REFL55 PD, 11MHz rejection

Here's the (calibrated) transimpedance of the new REFL55 PD.

T(55.3) / T_(11.06) = 93 dB

2010-05-05_REFL55_CalibratedOpticalResponse0-60MHz.png

  2886   Thu May 6 16:18:37 2010 AlbertoUpdate40m UpgradingNew improved design for the 11MHz photodiode

After munching analytical models, simulations, measurements of photodiodes I think I got a better grasp of what we want from them, and how to get it. For instance I now know that we need a transimpedance of about 5000 V/A if we want them to be shot noise limited for ~mW of light power.

Adding 2-omega and f1/f2 notch filters complicates the issue, forcing to make trade-offs in the choice of the components (i.e., the Q of the notches)

Here's a better improved design of the 11Mhz PD.

Attachment 1: pox11.pdf
pox11.pdf
  2893   Thu May 6 19:57:26 2010 AlbertoUpdate40m UpgradingNew improved design for the 11MHz photodiode

Quote:

After munching analytical models, simulations, measurements of photodiodes I think I got a better grasp of what we want from them, and how to get it. For instance I now know that we need a transimpedance of about 5000 V/A if we want them to be shot noise limited for ~mW of light power.

Adding 2-omega and f1/f2 notch filters complicates the issue, forcing to make trade-offs in the choice of the components (i.e., the Q of the notches)

Here's a better improved design of the 11Mhz PD.

 This should be better. It should also have larger resonance width.

Attachment 1: pox11.pdf
pox11.pdf
  2894   Fri May 7 11:21:49 2010 kojiUpdate40m UpgradingNew improved design for the 11MHz photodiode

How much is the width?

Quote:

 This should be better. It should also have larger resonance width.

 

  2896   Fri May 7 18:18:02 2010 AlbertoUpdate40m UpgradingNew improved design for the 11MHz photodiode

Quote:

How much is the width?

Quote:

 This should be better. It should also have larger resonance width.

 

 The transfer function phase drops by 180 degrees in about 2MHz. Is that a good way to measure the width?

  2897   Fri May 7 19:02:27 2010 ranaUpdate40m UpgradingNew improved design for the 11MHz photodiode

To measure the width of a resonance, the standard method is to state the center frequency and the Q. Use the definition of Q from the Wikipedia.

As far as how much phase is OK, you should use the method that we discussed - think about the full closed loop system and try to write down how many things are effected by there being a phase slope around the modulation frequency. You should be able to calculate how this effects the error signal, noise, the loop shape, etc. Then consider what this RFPD will be used for and come up with some requirements.

  2902   Mon May 10 16:59:35 2010 AlbertoUpdate40m UpgradingUnexpected oscilaltionin the POY11 PD

The measured transimpedance of the latest POY11 PD matches my model very well up to 100 MHz. But at about ~216MHz I have a resonance that I can't really explain.

2010-05-10_POY11_CalibratedOpticalResponse0-500MHz.png

 

 The following is a simplified illustration of the resonant circuit:

POX11.png

 

Perhaps my model misses that resonance because it doesn't include stray capacitances.

While I was tinkering with it, i noticed a couple of things:

- the frequency of that  oscillation changes by grasping with finger the last inductor of the circuit (the 55n above); that is adding inductance

- the RF probe of the scope clearly shows me the oscillation only after the 0.1u series capacitor

- adding a small capacitor in parallel to the feedback resistor of the output amplifier increases the frequency of the oscilaltion

  2924   Wed May 12 17:10:16 2010 AlbertoUpdate40m UpgradingRF frequency generation box - step 0

I started putting together the components that are coint to go inside the frequency generation box. Here's how it looked like:

DSC_1499_small.JPG

The single component are going to be mounted on a board that is going to sit on the bottom of the box.

I'm thinking whether to mount the components on an isolating board (like they did in GEO), or on an aluminum board.

I emailed Hartmut to know more details about his motivations on making that choice.

  2925   Wed May 12 23:31:17 2010 AlbertoUpdate40m Upgrading216 MHz resonance in the POY11 PD killed
It turned out that the resonance at 216 MHz in the 11MHz PD that I showed in the elog entry 2902 was casued by an instability of the of the MAX4107 opamap' feedback loop.
As the datasheet of the opamp shows, the close-loop gain has a peak at about 200-250MHz, in presence of even small capacitive loads.
In my case, perhaps either the capacitance of the BNC cables plugged to the RF output of the PD box, or the shunt capacitance of the circuit parts after the opamap (traces and resistors) might have introduced capacitance at the output of the amplifier.
 
LISO had failed in predicting the resonance because it has only ideal transfer functions of the opamps. In particular the open-loop gain of the opamaps in the library is just a function with a simple pole.
 
At RF frequencies the output impedances of the opamp starts having a non-negligible inductance that interacts with the load capacitance, generating a typical LC-circuit resonance.
 
In cases like this, such effect can be mitigated by introducing an "isolating" resistor at the output of the opamp.
 
So I did that and modified the circuit as in this simplified schematic here:
 
POX11.png
 

The choice of 100 Ohm for the isolating resistor was mainly empirical. I started with 10, then 20 and 50 until I got a sufficient suppression of the resonance. Even just 10Ohm suppressed the resonance by several tens of dB.

2010-05-12_POY11_CalibratedOpticalResponse0-500MHz.png

 

In that way the gain of the loop didn't change. Before that, I was also able to kill the resonance by just increasing the loop gain from 10 to 17.  But, I didn't want to increase the closed-loop gain.

One thing that I tried, on Koji's suggestion, was to try to connect the RF output of the PD box to an RF amplifier to see whether shielding the output from the cable capacitance would make the resonance disappear: It did not work.

  2926   Thu May 13 05:06:43 2010 ranaUpdate40m Upgrading216 MHz resonance in the POY11 PD killed

 

 This idea was tried before by Dale in the ~1998 generation of PDs. Its OK for damping a resonance, but it has the unfortunate consequence of hurting the dynamic range of the opamp. The 100 Ohm resistor reduces the signal that can be put out to the output without saturating the 4107.

I still recommend that you move the notch away from the input of the 4107. Look at how the double notch solution has been implemented in the WFS heads.

  2957   Thu May 20 12:34:46 2010 kiwamuConfiguration40m Upgradingoptical breadboards with legs

Yesterday Steve and I revived two legs to mount some optical breadboards outside of the end table.

These legs had been used as oplev's mounts many years ago, but now they are served for 40m upgrading. These are really nice.

By putting them on the side of the end table, a mirror mounted on the top of the leg can reflect the beam outside of the end table.

Once we pick off the green beam from the end table to its outside, the green beam can propagate through the 40m walkway along the Y-arm.

So that we can measure the beam profile as it propagates.

These legs are also going to be used during mode matching of the vacuum optics.

Attachment 1: leg1_small.png
leg1_small.png
Attachment 2: leg2_small.png
leg2_small.png
  2966   Fri May 21 11:56:34 2010 AlbertoUpdate40m Upgrading40mUpgrade Field Power and RF Power Spectrum at the ports. 38m/38.55m arm length issue.

I update my old 40mUpgrade Optickle model, by adding the latest updates in the optical layout (mirror distances, main optics transmissivities, folding mirror transmissivities, etc). I also cleaned it from a lot of useless, Advanced LIGO features.

I calculated the expected power in the fields present at the main ports of the interferometer.

I repeated the calculations for both the arms-locked/arms-unlocked configurations. I used a new set of functions that I wrote which let me evaluate the field power and RF power anywhere in the IFO. (all in my SVN directory)

As in Koji's optical layout, I set the arm length to 38m and I found that at the SP port there was much more power that I woud expect at 44Mhz and 110 MHz.

It's not straightforward to identify unequivocally what is causing it (I have about 100 frequencies going around in the IFO), but presumably the measured power at 44MHz was from the beat between f1 an f2 (55-11=44MHz), and that at 110MHz was from the f2 first sidebands.

Here's what i found:

RFPower_locked_38m.png

RFPower_unlocked_38m.png

FieldPower_locked_38m.png

FieldPower_unlocked_38m.png

 

I found that When I set the arm length to 38.55m (the old 40m average arm length), the power at 44 and 110 MHz went significantly down. See here:

RFPower_3855m.png

 FieldPower_3855m.png

I checked the distances between all the frequencies circulating in the IFO from the closest arm resonance to them.

I found that the f2 and 2*f2 are two of the closest frequencies to the arm resonance (~80KHz). With a arm cavity finesse of 450, that shouldn't be a problem, though.

40mUpgrade_distanceFromResonance_38m.png

 I'll keep using the numbers I got to nail down the culprit.

Anyways, now the question is: what is the design length of the arms? Because if it is really 38m rather than 38.55m, then maybe we should change it back to the old values.

  2994   Wed May 26 17:10:09 2010 AlbertoUpdate40m UpgradingRF Generation box

This is how the RF generation box might soon look like:

Visio-frequencyGenerationBox_wiringSchematic.png

A dedicated wiki page shows the state of the work:

http://lhocds.ligo-wa.caltech.edu:8000/40m/Upgrade_09/RF_System/frequency_generation_box#preview

  2996   Wed May 26 22:22:03 2010 AlbertoConfiguration40m UpgradingArm cavity length

The second sideband is resonant in the arms for a cavity length of 37.9299m.

The nearest antiresonant arm lengths for f2 (55MHz) are 36.5753m and 39.2845m.

If we don't touch the ITMs, and we use the room we still have now on the end tables, we can get to 37.5m.

This is how the power spectrum at REFL would look like for perfect antiresonance:

reflRFpowerVsArmLength_3658m.png

And this is how it looks like for 37.5m:

reflRFpowerVsArmLength_3750m.png

Or, god forbid, we change the modulation frequencies...

ELOG V3.1.3-