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How crystal mounting was done.

We did "in-between" method (alog 88900) and inserted two pieces of 3/8IDx5/8ODx0.025" thick shim washers between the front plate and the input side plate.  shim_washers_spacers.jpg shows the washers and the front plate before the rest of the EOM structure is placed on top. Note that the front plate still has the alumina piece on top, not the RTP. Important points in this picture:

1. We're using two pieces of alumina pieces as a convenient 4mm thick shim to raise the front plate. It doesn't have to be alumina pieces, but the front plate must be raised enough so the shim washers "clear the ground".
2. Three set screws are pre-adjusted so each sticks out just shy of 3.5mm. This is the height of the crystal (4mm) minus the depth of the shallow groove (nominally 0.02" or 0.508mm). This made it somewhat easier to to make sure that there's a good contact between the crystal and the metals.

(Added later: If I were to do this again, I'll set the set screws with alumina piece in place such that all touch the board when there's a good contact between board/plate/alumina (of course no indium). Insertion of indium foil with the real RTP later will automatically ensure that the screws are just shy.)

Next we cut a 40mmx4mm piece of indium sheet with a clean pair of scissors and installed it in a groove in the front plate. I tried to set the edge of the sheet to be 7.6mm from the outside edge of the front panel (this is 7mm plus the thickness of 0.025" washer). Judging from indium.jpg, I was mildly successful, maybe it's more like 7.4mm but it's not 7mm nor 8mm.

rtp.jpg shows the side view of the crystal. Maybe it's hard to see but it's wedged, in this picture the shortest face is down, the longest side is up.

I placed the crystal on top of the indium sheet, making sure that the edge of the crystal is well aligned with the edge of the indium as good as I can. We also made sure that the shortest face mates with the front panel. You cannot see any of that in the crystal_installed.jpg but you can at least see that the crystal is there.

Then we went through the same procedure we've already done more than several times, i.e. tighten the screws on the input panel until the panel touches the washer and the washer touches the front panel, tighten the scrwes on the output panel so the screws touch that panel, then go balanced tightening, moving just a tiny amount at a time, always applying small downward pressure for the EOM side/board/bottom assy otherwise the assy will shift when the screws are tightened. When all of the screws are tightened stronger than finger tight, screws on the input side are tightened just a bit more. After this, neither Elena nor I weren't able to undo screws by finger.

Sorry no picture of the assembled unit.
 

Tuning is done, larger capacitance than obtained before with alumina, looks good. (But why do the dips have to be so narrow?)

We noticed that the frequencies were lower than what we have previously obtained with alumina, i.e. the capacitance is bigger. This is probably a good sign even though we don't know if this is due to the indium or something else. Especially, 118MHz dip was a MHz or two lower than nominal.

We were able to tune all four frequencies using trim cap, except that the trim cap for 45.5MHz hit the minimum and we could not increase the frequency any more, so we bent the winding of the coil a bit to spread loops apart. Below is the table of center frequencies (actual vs nominal). In the attached photos, cursor is placed close to the nominal frequency. They all look good in that the frequency is close enough to nominal that we're only loosing less than a dB, but the resonances are all very narrow (for my preference, anyway). The Q values are from 640 for 9MHz to 1300 for 118MHz, going higher as the frequency. A small change in frequency will result in a big degradation in the modulation depth.

(Added later: Read Valera's entry below, thanks Valera! The numbers here are not the real Q, they should be smaller. To make it more embarrassing, I somehow mixed up 3dB and 6dB. Given that all dips are smaller than -20dB, a quick thing to do is probably to define the width of the peak as full width of the -3dB points in S11, not +3dB points from the bottom but really -3dB measured from 0dB full reflection. If you do that the numbers are more like 100 instead of 1000.)

If you look at the pictures, you'll also notice that the reflection dips at the center are -23dB (9MHz), -24.5dB (24MHz), -23dB (45MHz) and is -25dB (118MHz) so a bit smaller than 10% in amplitude is coming back. It's not really matched to 50 Ohm transmission line, and that on its own is OK, but because of that, I wonder if we can add a bit of resistance to bring down Q values without any negative impact (like worse matching with increased reflections) in the future design. 

After this first round of tuning, three set screws on the front plate were all extracted, and there was no change in the tuning of 118MHz dip.

I changed the orientation of the EOM and the frequency jumped a bit

Up to this point, tuning was done with the front plate facing down and put on the ceramic insulator placed on the EOM base (because it's convenient to access trim caps that way).

When I changed the orientation of the EOM so the front plate becomes upfront (i.e. like intended), the frequency of 118MHz dip shifted a bit, from 118.3055MHz to 118.31745MHz, it's just 12kHz shift so not the end of the world but it's still meaningful. Maybe it's the interaction of the magnetic field from the coil and the metals nearby?

Then I tapped the front plate and side plates and it shifted again, fortunately by smaller amount (from 118.31745 to 118.3113MHz, a negative 6kHz jump).

What we'll do tomorrow is to fully assemble the unit, tune it again as good as we can, then tap and retune if necessary. Hopefully, tapping enough and things will settle to the bottom of the potential.

Once the EOM goes into chamber we'll measure the resonances again, and we might have to retune in chamber.

I was also wondering why the S11 (return loss) is narrower then the transmission curve back when we did the prototype testing at LLO. So I did the math myself - the attached plot shows the calculated curves for voltage across the crystal for 1 W incident RF power and S11 for 9.1 MHz (similar for other f's). Initially I also made the same mistake estimating the Q - the Q is actually about 100 not ~1000 as one can see from the transmission curve (voltage on the crystal).   

This afternoon Sheila and I went into the chamber and nudged some of the cables away from the OPO - Jeff will re-check the TF measurements to look for any improvements. 

There is a cable connector sitting on the OPO (which is a new addition to the OPO). I lifted that cable connector and then Sheila looked for alignment shifts - which she confirmed. For now we left the cable connector at the same spot on the OPO. We will think of moving it away after Jeff's results.

Sheila also found some changes in the beam alignment since last week - not sure if this is due to our work (AOSEM re-centered LHO alog 88910) from yesterday - although centering aosem-flag should not change the mechanical alignment. 

EPO tagged

EOM crystal mounting practice part 2 (with a remote help from Michael)

Summary:

1. "Laxen method" (no gap between the front plate and the input side, big gap for the output side) was repeated again with an extra caution (see below), no visible gap between the alumina piece and the electrode (with a help of flashlight), alumina doesn't slip out.
2.  "Appert method" (even gaps for the input and the output) was tried twice, no visible gap between the alumina piece and the electrode, alumina doesn't slip out. 
   1. During Appert method tests we have found that the output side plate might be slightly warped.
3. RF return dips were measured for all three of the above (one Laxen and two Appert mounting method), they were consistent regardless of the mounting method and were consistently higher than nominal with the alumina. (No RTP yet). It goes progressively worse as the frequency goes up. Michael told "The resonant dip needs to sweep across the desired frequency."
   1. Lower two frequencies (9.1MHz and 24.08MHz) look reasonable in that the nominal frequency is inside the observed dip.
   2. 45.5MHz dip is just barely on the shallow part of the dip curve.
   3. 118MHz is the worst, the nominal frequency is totally out of the dip (1MHz off).
4. Michael suggested that the capacitance for the elctrorode-crystal is too small but we (Matt and I) couldn't see any light coming through the supposed gap.
5. Michael also suggested that the windings of the inductor coils might have been bent (got looser) during shipment.
   1. I tested bending the coil windings for 118Mhz inductor so they come closer together and it had a huge impact. I was able to easily come close to a reasonable frequency, but it's no good unless we know that the capacitance for the crystal-electrode is OK.  I haven't changed trim cap nor touched any other frequencies.
6. I'll measure the peaks without the alumina on Monday. Will discuss further with Michael, Stephen and Gabriele (while Masayuki is traveling to here).

Laxen method test.

In alog 88862 we left the EOM module with the alumina piece mounted using Laxen method (no gap between the input side plate and the front plate, a big gap for the output side).

Shining flashlight into the iput or output aperture in the side plates is useful to see the gap between the electrode plate and the alumina piece, and we found that there was indeed a small gap only on one side (i.e. the "crystal" was pinched at the edge).

I loosened the screws for the face plate and repeated the mounting procedure, but this time being extra careful to tighten the screws by tinier amount (than my previous attempts) at a time while applying a gentle pressure from the top. As soon as I got much tighter than finger-tight, I stopped. This resulted in what was seemingly a good contact between the alumina and the electrode, no light visible between them.

See  nogap.jpg, this is a representative picture of GOOD contact (even though I cannot prove that the contact is really plane-to-plane not just plane-to-one edge of the crystal).

Another picture gap.jpg is an example of BAD contact. It's hard to see but there's no gap at either edges closer to input/output faces, the gap is only in the middle. I don't have a good explanation for this.

Appert method tests.

We also tested Stephen's suggestion to make a gap on both sides of the front plate. This was trickier but doable by using two Allen keys. The third attachment (EOMassembly.jpg) shows the EOM placed on top of the EOM mount parts just for picture AFTER the alumina was mounted. During the mounting process, the face plate is facing down, and two allen keys will tighten two screws with green (or red) arrows in the picture with tiniest rotation at a time. Green, red, green, red, repeat it until it feels reasonably tight but much, much looser than you'll usually do for tight mechanical connection. After this was done, neither Matt nor I were able to undo the screws by finger.

We did this twice, both times no gap between the alumina and the board, and alumina didn't slip out.

Output side plate might be warped?

In the assembly picture, can you see that the gap between the face plate and the output side panel (right on the picture) is uneven, but the gap for the input (left on the picture) is fairly even? I don't think this is an optical illusion. This might be related to the reason why the crystal ALWAYS slips out when the face plate is tightened down to the output side plate, see my alog (88862). Quoting myself, "no matter what we did, the alumina piece (i.e. fake RTP for excercize) slid out of the assembly but only after tightening the screws". Maybe it's the output side plate.

Reflection measurement.

For each of the above three practices (one with Laxen method, two with Appert method), S11 coefficient was measured for all four ports.

What we found was that all four reflection dips were higher than they are supposed to be. According to Michael, alumina should give us similar results to RTP. I don't list results for all three sets (3x4=12 numbers) because numbers were pretty consistent across the sets, maybe give or take 10kHz or so. 

In the attached pictures, green line is roughly where the center should be. 9.1 and 24.08 look reasonable to me. Not sure about 45.5MHz, it's 450kHz off. 118.3MHz is totally, totally off.

As I wrote in the summary, I tried bending the coil windings for the 118MHz (bendandsqueeze.jpg) because it was the worst but also because it was the one with the loosest of all four coils (118MHzWinding.jpg), and it had a huge effect. With just a few rounds of bending/squeezing I was able to go down to 118.53MHz (afterbending_118MHz.jpg). I could have passed 118.3 and gone to the other side easily but I stopped there.

Just in case somebody else must do this, here's what I did to measure S11 (reflection coefficient). 

If you go to the optics lab, everything is already set up like in the attached cartoon except that the dirty cable is removed from the coupler and placed on top of the optics table. You might still do the calibration again (because we turned off the analyzer at the end of the day and I cannot remember if the calibration results are kept in the analyzer). Remember that EOM is class A but your cables are dirty (even though we wiped the connectors of the dirty cable using q-tips and IPA). We're using one sacrificial SMA elbow that used to be class A to connect your dirty cable to the EOM.

Anyway, calibration. Set the frequency range to whatever you want but make sure that it covers the frequency range of main interest, like at least 9MHz to 125MHz or so while performing S11 calibration.

Connect the BNC of the dirty cable to the INPUT connector of the directional coupler, like in the attached cartoon.

Press "cal" button and select S11 calibration. Don't connect anything to the SMA of the dirty cable and press "Open" button. Next attach a hand-made short circuit plug to the dirty cable  via BNC male to SMA female connector. Press "Short". Then connect a 50Ohm SMA terminator to the dirty cable via SMA barrel. Press "Load". Then press "Done".

Now you're done with calibration. Press "Measure" and make sure that you're measuring S11.

Clean the SMA with IPA and q-tip again. Connect the dirty cable to the elbow, and the elbow to the EOM. Set the frequency range to whatever you want. That's it.

Quick Monday update.

I  measured S11 coeff without the crystal/alumina but with the front panel.

Reflection dips with/without alumina are:

So the frequencies are consistently higher without the crystal/alumina.

EPO tagged

EPO Tagged.

EPO tagged.

EPO-tagged.

Masayuki and I fully tightened the JAC body mode dampers yesterday.  We were only able to get the screws ~1/2 turn past finger-tight before we could no longer tighten them.  No changes in JAC alignment were observed while we were doing this.

EPO tagging