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.
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.
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.
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.
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.
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.
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:
Based on Rich's design I will get started on a parts list and wiring diagrams to send out to the cable company.
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.
Current Mode Matching and Gouy Phase Between Steering Mirrors
We found in 40m elog ID 3330 ( http://nodus.ligo.caltech.edu:8080/40m/3330) a 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.
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.
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.
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.
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.
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.
That was super fast! Great job, Andres and Nic!
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.
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.
oops, forgotten the third attachment...
here it is
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.
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'.
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.
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.
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.
The details about the modifications are contained in this dedicated wiki page (Upgrade_09 / RF System / Upgraded RF Photodiodes)
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).
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).
I have to figure out what are the sources of such noises.
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:
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:
The full noise spectrum looks like this:
A big bump is present at ~275MHz. it could important if it also shows up on the shot noise spectrum.
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.
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%.
Putting my hands ahead, I know I could have taken more measurements around the 3dB point, but the 40m needs the PDs soon.
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.
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.
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)
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.
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.
Result: the transimpedance of REFL11at 11 MHz is about 4000 Ohm.
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.
Here's the (calibrated) transimpedance of the new REFL55 PD.
T(55.3) / T_(11.06) = 93 dB
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.
How much is the width?
The transfer function phase drops by 180 degrees in about 2MHz. Is that a good way to measure the width?
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.
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.
The following is a simplified illustration of the resonant circuit:
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
I started putting together the components that are coint to go inside the frequency generation box. Here's how it looked like:
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.
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.
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.
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.
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.
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:
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:
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.
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.
This is how the RF generation box might soon look like:
A dedicated wiki page shows the state of the work:
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:
And this is how it looks like for 37.5m:
Or, god forbid, we change the modulation frequencies...
For both sidebands to be antiresonant in the arms, the first modulation frequency has to be:
f1 = (n + 1/2) c / (2*L)
where L is the arm length and c the speed of light. For L=38m, we pick to cases: n=3, then f1a = 13.806231 MHz; n=2, then f1b = 9.861594 MHz.
If we go for f1a, then the mode cleaner half length has to change to 10.857m. If we go for f1b, the MC length goes to 15.200m. A 2 meter change from the current length either way.
And the mode cleaner would only be the first of a long list of things that would have to change. Then it would be the turn of the recycling cavities.
Kind of a big deal.
[Alberto, Kiwamu, Kevin, Rana]
Today we tried to measured the beam shape after the MC MMT1 that Jenne installed on the BS table.
The beam scan showed a clipped spot. We tracked it down to the Farady and the MCT pickoff mirror.
The beam was getting clipped at the exit of the Faraday. But it was also clipping the edge of the MCT pick-off mirror. I moved the mirror.
Also the beam looked off-center on MC2.
We're coming back on Sunday to keep working on this.
Now things are bad.
I checked the effect of the arm length to the reflectance of the f2(=5*f1) sidebands.
Conclusion: If we choose L_arm = 38.4 [m], it looks sufficiently being away from the resonance
We may want to incorporate small change of the recycling cavity lengths so that we can compensate the phase deviation from -180deg.
f1 of 11.065399MHz is assumed. The carrier is assumed to be locked at the resonance.
Attachment 1: (Left) Amplitude reflectance of the arm cavity at f2 a a function of L_arm. (Right) Phase
Horizontal axis: Arm length in meter, Vertical Magnitude and Phase of the reflectance
At L=37.93 [m], f2 sidebands become resonant to the arm cavity. Otherwise, the beam will not be resonant.
Attachment 2: close-up at around 5 f1 frequency.
The phase deviation from the true anti resonance is ~0.7deg. This can be compensated by both PRC and SRC lengths.
[Jenne, Kiwamu, Rana, Eric Gustafson]
The SRM and PRM have been re-hung, and are ready for installation into the chambers. Once we put the OSEMs in, we may have to check the rotation about the Z-axis. That was not confirmed today (which we could do with the microscope on micrometer, or by checking the centering of the magnets in the OSEMs).
Also, Eric and Rana inspected the Tip Tilt magnets, and took a few that they did their best to destroy, and they weren't able to chip the magnets. There was concern that several of the magnets showed up with the coatings chipped all over the place. However, since Rana and Eric did their worst, and didn't put any new chips in, we'll just use the ones that don't have chips in them. Rana confiscated all the ones with obvious bad chips, so we'll check the strengths of the other magnets using a gaussmeter, and choose sets of 4 that are well matched.
Eric, photographer extraordinaire, will send along the pictures he took, and we'll post them to Picasa.
[Jenne, Steve, Nancy, Gopal]
We made an attempt at hanging some of the Tip Tilt eddy current dampers today.
Photo 1 shows the 2 ECDs suspended.
(1) Loosen the #4-40 screws on the side of the ECDs, so the wire can be threaded through the clamps.
(2) Place the ECDs in the locator jigs (not shown), and the locator jigs in the backplane (removed from main TT structure), all laying flat on the table.
(3) Get a length of Tungsten wire (0.007 inch OD = 180um OD), wipe it with acetone, and cut it into 4 ~8cm long segments (long enough to go from the top of the backplane to the bottom).
(4) Thread a length of wire through the clamps on the ECDs, one length going through both ECDs' clamps.
(5) One person hold the wire taught, and straight, and as horizontal as possible, the other person tightened the clamping screws on the ECDs.
(6) Again holding the wire in place, one person put the clamps onto the backplane (the horizontal 'sticks' with 3 screws in them).
(7) The end. In the future, we'll also clip off extra pieces of wire.
When we held up the backplane to check out our handy work, it was clear that the bottom ECD was a much softer pendulum than the top one, since the top one has the wire held above and below, while the bottom one only has the wire held on the top. I assume we'll trim the wire so that the upper ECD is only held on the top as well?
* This may be a 3 person job, or a 2 people who are good at multitasking job. The wire needs to be held, the ECDs need to be held in place so they don't move during the screwing/clamping process, and the screws need to be tightened.
* Make sure to actually hold the wire taught. This didn't end up happening successfully for the leftmost wire in the photo, and the wire is a bit loose between the 2 ECDs. This will need to be redone.
* We aren't sure that we have the correct screws for the clamps holding the wire to the backplane. We only have 3/16" screws, and we aren't getting very many threads into the aluminum of the backplane. Rana is ordering some 316 Stainless Steel (low magnetism) 1/4" #4-40 screws. We're going for Stainless because Brass (the screws in the photo), while they passed their RGA scan, aren't really good for the vacuum. And titanium is very expensive.
The 2nd photo is of the magnet sticking out of the optic holder. The hole that the magnet is sitting in has an aluminum piece ~2/3 of the way through. A steel disk has been placed on one side, and the magnet on the other. By doing this, we don't need to do any press-fitting (which was a concern whether or not the magnets could withstand that procedure), and we don't need to do any epoxying. We'll have to wait until the ECDs are hung, and the optic holder suspended, to see whether or not the magnet is sticking out far enough to get to the ECDs.