Monday

• Place the bars on the in-vac tables to mark the positions of the test mass suspensions. (ITMX/ITMY/ETMX/ETMY)
• Check the table leveling again (ITMX/ITMY/ETMX/ETMY/BSC)
• Align the whole interferometer.
• Check the OPLEV spots by either QPDs or apertures
• Check the OSEM values (MC1/2/3, BS, PRM, SRM, ITMX, ITMY, ETMX, ETMY)
• Energize OMC PZTs.
  • We have removed the cards at the back of the HV driver.
  • Insert the card and check the connection.
  • Adjust the DC values at the middle of the range.
    -> They have an internal bias circuit to provide +75V at the outputs. (D060287).
    The actual voltages confirmed.
  • Adjust the physical knobs of the PZTs such that we can see the spots at the OMCR cam
• If everything is fine, attach the access connector.
• If still everythig is fine, put the BS heavy door.

Tuesday

- Do the following list for all of the testmass chambers.

• Check if the OSEMs and the OPLEV are still fine.
• Inspect the surface of the mirror with a laser pointer or a fiber coupled halogen light.
• Blow the mirrors by the ionization gun.
• Inspect the mirror surface again.
• Move the suspension tower close to the door.
• Make a single drag-wipe with iso
• Move the SOS tower at the original place.
• Check the OSEMs and the OPLEVs. Adjust the alignment.
• Put the heavy door.

- Start slow pumping

• Check EQ-stops
• clamp down counter weights
• check other components are clamped
• remove all tools
• check cabling is not is not shorting out seismic stack or blocking beam
• confirm well centered spots on mirrors

Tuesday

• If still everythig is fine, put the BS heavy door.

- Do the following list for all of the testmass chambers.

• Check if the OSEMs and the OPLEV are still fine. (ITMX and ITMY were not done on Monday, so need extra care.)
• Inspect the surface of the mirror with a laser pointer or a fiber coupled halogen light.
• Blow the mirrors by the ionization gun.
• Inspect the mirror surface again.
• Move the suspension tower close to the door.
• Make a single drag-wipe with iso
• Move the SOS tower at the original place.
• Check the OSEMs and the OPLEVs. Adjust the alignment.
• Put the heavy door.

- Start slow pumping

What was the point:

I twiddled with several different things this evening to increase the power into the Mode Cleaner.  The goal was to have enough power to be able to see the arm cavity flashes on the CCD cameras, since it's going to be a total pain to lock the IFO if we can't see what the mode structure looks like.

Summed-up list of what I did:

* Found the MC nicely aligned.  Did not ever adjust the MC suspensions.

* Optimized MC Refl DC, using the old "DMM hooked up to DC out" method.

* Removed the temporary BS1-1064-33-1025-45S that was in the MC refl path, and replaced it with the old BS1-1064-IF-2037-C-45S that used to be there.  This undoes the temporary change from elog 3878.  Note however, that Yuta's elog 3892 says that the original mirror was a 1%, not 10% as the sticker indicates. The temporary mirror was in place to get enough light to MC Refl while the laser power was low, but now we don't want to fry the PD.

* Noticed that the MCWFS path is totally wrong.  Someone (Yuta?) wanted to use the MCWFS as a reference, but the steering mirror in front of WFS1 was switched out, and now no beam goes to WFS2 (it's blocked by part of the mount of the new mirror). I have not yet fixed this, since I wasn't using the WFS tonight, and had other things to get done.  We will need to fix this.

* Realigned the MC Refl path to optimize MC Refl again, with the new mirror.

* Replaced the last steering mirror on the PSL table before the beam goes into the chamber from a BS1-1064-33-1025-45S to a Y1-45S.  I would have liked a Y1-0deg mirror, since the angle is closer to 0 than 45, but I couldn't find one.  According to Mott's elog 2392 the CVI Y1-45S is pretty much equally good all the way down to 0deg, so I went with it.  This undoes the change of keeping the laser power in the chambers to a nice safe ~50mW max while we were at atmosphere.

* Put the HWP in front of the laser back to 267deg, from its temporary place of 240deg.  The rotation was to keep the laser power down while we were at atmosphere.  I put the HWP back to the place that Kevin had determined was best in his elog 3818.

* Tried to quickly align the Xarm by touching the BS, ITMX and ETMX.  I might be seeing IR flashes (I blocked the green beam on the ETMX table so I wouldn't be confused.  I unblocked it before finishing for the night) on the CCD for the Xarm, but that might also be wishful thinking.  There's definitely something lighting up / flashing in the ~center of ETMX on the camera, but I can't decide if it's scatter off of a part of the suspension tower, or if it's really the resonance.  Note to self:  Rana reminds me that the ITM should be misaligned while using BS to get beam on ETM, and then using ETM to get beam on ITM.  Only then should I have realigned the ITM.  I had the ITM aligned (just left where it had been) the whole time, so I was making my life way harder than it should have been.  I'll work on it again more today (Tuesday). 

What happened in the end:

The MC Trans signal on the MC Lock screen went up by almost an order of magnitude (from ~3500 to ~32,000).  When the count was near ~20,000 I could barely see the spot on a card, so I'm not worried about the QPD.  I do wonder, however, if we are saturating the ADC. Suresh changed the transimpedance of the MC Trans QPD a while ago (Suresh's elog 3882), and maybe that was a bad idea? 

Xarm not yet locked. 

Can't really see flashes on the Test Mass cameras. 

- Previously MC TRANS was 9000~10000 when the alignment was good. This means that the MC TRANS PD is saturated if the full power is given.
==> Transimpedance must be changed again.

- Y1-45S has 4% of transmission. Definitively we like to use Y1-0 or anything else. There must be the replaced mirror.
I think Suresh replaced it. So he must remember wher it is.

- We must confirm the beam pointing on the MC mirrors with A2L.

- We must check the MCWFS path alignment and configuration.

- We should take the picture of the new PSL setup in order to update the photo on wiki.

I replaced the final steering mirror (Y1-1037-45-S) in the zig-zag path on the PSL table by a 0 deg mirror Y1-1037-0.

With a sensor card I confirmed the transmission reduced a lot after the replacement.

As we expected, the replacement of the mirror caused a mis-alignment of the incident beam axis to the MC, so I compensated it by touching the angle of the mirror a little bit.

After the alignment of the mirror, the MC is still resonating at TEM00.

We will check the spot positions more accurately by A2L technique.

 Decreased the gain of MC-Trans-Mon QPD ckt

The resistors R1, R2, R3, R4 are now 49.9 kOhm. The previous elog on this subject 3882 has the ckt details. 

I undid Yuta's temporary setup, and put beam back on both WFS.  Since Koji had just aligned the Mode Cleaner, I centered the beam on the WFS using the WFS QPD screen, while watching the WFS Head screen, to make sure that the beam was actually hitting the QPD, and not off in lala land. 

I connected PZT1 and PZT2 to a slow front end machine c1iscaux.

Now we are able to align these PZTs from the control room via epics.

   Since we removed C1ASC that was controlling the voltage applied on the PZTs, we didn't have the controls for them for a long time.

So Rana and I decided to hook them up to an existing slow front end machine temporarily.

(probably the best solution is to connect them to C1LSC, which is fast enough to dither them.)

We actually found that c1iscaux is the proper machine, because it looked like it used to control the PZTs a long long time ago.

Moreover, c1iscaux still has DAC channels named like C1:LSC-PZT1_X, and so on.

  Below shows a screen shot of the medm screen for controlling the PZTs, invoked from a button on sitemap.adl ( pointed by a black arrow in the picture below)

The current default values are all zero at the right top sliders.

I checked the spot positions on MC1 and MC3 by running Yuta's A2L script.

The amounts of the off-centering were good except for YAW of MC1.

 So we have to adjust the YAW alignment of the beam axis by steering the mirrors at the PSL table.

- - - (measured off-centering) - - - 

MC1_PIT  =  -0.711 mm

MC1_YAW =  1.62 mm

MC3_PIT  =   -0.0797 mm

MC3_YAW  =  - 0.223 mm

This is a connection diagram for the input PZTs (i.e. PZT1 and PZT2).

As drawn in the diagram, the signals don't go through the anti-imaging filter D000186 in the current configuration.

[Kiwamu, Jenne]

We have a measely 465mW going into the MC.  We lose a boatload of power on the PZT mirror that is part of the last zigzag for steering into the MC.  Just before this mirror, we measure 1.21W .  Just after this mirror, we measure ~475mW.  Then a teeny bit gets picked off for the PSL POS/ANG.  But we're losing a factor of 3 on this one mirror.  Need to fix!!!!!!!!!

 I checked the triple resonant box for the broadband EOM this afternoon, and found it was healthy.

So I installed it again together with a power combiner and succeeded in locking the MC.

Since the box has a non-50 Ohm input impedance at 29.5 MHz, so it maybe needed to adjust the phase of the LO for the demodulation of the PDH signal.

A good thing is that now we are able to impose the other sidebands (i.e. 11 MHz and 55MHz) via the power combiner.

[Larisa and Jenne]

We wanted to get rid of the awkward cart that was sitting behind the 1Y1 rack.  This cart was supplying a +5V offset to the PZT driver, so that we could use the MC length signal to feedback to lock the laser to the MC cavity.  Instead, we put the offset on the last op amp before the servo out on the Mc Servo Board.  Because we wanted +5V, but the board only had +5, +15, -15V as options, and we needed -5 to add just before the op amp (U40 in the schematic), because the op amp is using regular negative feedback, we made a little voltage divider between -15V and GND, to give ourselves -5V.  We used the back side of the voltage test points (where you can check to make sure that you're actually getting DC voltage on the board), and used a 511Ohm and 1.02kOhm resistor as a voltage divider. 

Then we put a 3.32kOhm resistor in ~"parallel" to R124, which is the usual resistor just before the negative input of the op amp.  Our -5V goes to our new resistor, and should, at the output, give us a +5V offset. 

Sadly, when we measure the actual output we get, it's only +2.3V.  Sadface.

We went ahead and plugged the servo out into the PZT driver anyway, since we had previously seen that the fluctuation when the mode cleaner is locked was much less than a volt, so we won't run into any problems with the PZT driver running into the lower limit (it only goes 0-10V).

Suresh has discovered that the op amp that we're looking at, U40 on the schematic, is an AD829, which has an input impedance of a measely 13kOhm.  So maybe the 3.32kOhm resistors that we are using (because that's what had already been there) are too large.  Perhaps tomorrow I'll switch all 3 resistors (R119, R124, and our new one) to something more like 1kOhm.  But right now, the MC is locked, and I'm super hungry, and it's time for some arm locking action.

I've attached the schematic.  The stuff that we fitzed with was all on page 8.

I can not think of any reason that the input impedance of 13kOhm between the pos/neg inputs produces such a big change at the output. In any case, the differential voltage between the pos/neg inputs produces a big output. But the output was just 2V or so. This means that the neg input was actually about zero volt. This ensures the principle of the summing amplifier of this kind.

Because the input impedance of the summing node (the additional resister you put at the negative input) is not infinity, the voltage divider is not perfect and shows 10% reduction of the voltge (i.e. the output will have +4.5V offset instead of +5V). But still it is not enough to explain such a small output like 2.3V.

What I can think of is that the earlier stages somehow have the offset for some reason. Anyway, it is difficult to guess the true reason unless all of the nodes around the last stage are checked with the multimeters.

At least, we can remove the voltage divider and instead put a 10k between -15V and the neg input in order to impose +5V offset at the output. This costs 1.5mA instead of 10mA.

[Koji Suresh Kiwamu]

Suresh modified the MC board to have +5V offset at the output. (To be reported in the separated elog)

The MC lock has not been obtained at this point. An investigation revealed that there was very small (~5mVpp) PDH signal.

Kiwamu removed his triple resonant adapter and put the 50Ohm termination insted.
This restored the signal level normal although this changed the demodulation phase about 20deg.
We left the demodulation phase as it is because this is a temporary setup and the loss of the signal is not significant.

Now the MC is steadily locked with the single super boost.

[Koji, Suresh]

    We looked at the board and found that the resistor R119 (the feed back) is 1.65k instead of the 3.32k that was needed for unity gain.  The gain has been intentionally reduced to 0.5 so that output range would be close to the 0-10V that is required at the input range of the PZT driver which follows.   A note to this effect is already present in the D040180-B, page 8.

    The voltage divider with 1k and 0.5k provides 4.5V (ref Koji's note above) this provides 2.25V at the output due to the gain of 0.5.  To get to the original goal of introducing a 5V offset on the output, we introduced the modification shown in the  'D040180-B with 5V offset.pdf' uploaded below.  Please check page 8, the changes are marked in red.  We checked to make sure that the output is 5V when the input is disconnected. 

The PCB pics at the end are also attached.  The 4.99k resistor is glued onto the PCB with epoxy and placed as close to the opamp possible.

My goal this afternoon was to measure the quantum efficiency of the MC WFS.  In the process of doing this, I discovered that when I reverted a change in the MCWFS path (see elog 4107 re: this change), I had not checked the max power going to the WFS when the MC unlocks.

Current status:

MC locks (is locked now).  No light going to WFS at all (to prevent MC WFS french-fry action).  Quantum Efficiency measured.

The Full Story:

Power to WFS:

Rana asked me to check out the quantum efficiency of the WFS, so that we can consider using them for aLIGO.  This involves measuring the power incident on the PDs, and while doing so, I noticed that WFS1 had ~160mW incident and WFS2 had ~240mW incident while the mode cleaner was unlocked.  This is bad, since they should have a max of ~10mW ever.  Not that 200mW is going to destroy the PD immediately, but rather the current out, with the 100V bias that the WFS have, is a truckload of power, and the WFS were in fact getting pretty warm to the touch.  Not so good, if things start melting / failing due to extended exposure to too much heat.

The reason so much power was going to the WFS is that it looks like Yuta/Koji et. al., when trying to use the WFS as a MC1 oplev, changed out 2 of the beam splitters in the MC WFS / MC Refl path, not just one.  Or, we've just been crispy-frying our WFS for a long time.  Who knows?  If it is option A, then it wasn't elogged.  The elog 3878 re: BS changeout only mentions the change of one BS.

Since the MC Refl path has a little more than ~1W of power when the MC is unlocked, and the first BS (which was reverted in elog 4107) is a 10% reflector, so ~100mW goes to the MC Refl PD, and ~900mW goes to the MC WFS path.  In front of a Black Hole beam dump was sitting a BS1-33, so we were getting ~300mW reflected to be split between the 2 WFS, and ~600mW dumped.  The new plan is to put a W2 window in place of this BS1-33, so that we get hopefully something like 0.1% reflected toward the WFS, and everything else will be dumped.  I could not find a W2-45S (everything else is S, so this needs to be S as well).  I found a bunch of W2-0deg, and a few W2-45P.  Does anyone have a secret stash of W2-45S's???  To avoid any more excessive heat just in case, for tonight, I have just left out this mirror entirely, so the whole MC WFS beam is dumped in the Black Hole.  The WFS also have aluminum beam dumps in front of them to prevent light going in.  None of this affects the MC Refl path, so the MC can still lock nice and happily.

Quantum Efficiency Measurement:

I refer to Jamie's LHO elog for the equation governing quantum efficiency of photodiodes: LHO 2 Sept 2009

The information I gathered for each quadrant of each WFS was: [1] Power of light incident on PD (measured with the Ophir power meter), [2] Power of light reflected off the PD (since this light doesn't get absorbed, it's not part of the QE), and [3] the photo current output by the PD (To get this, I measured the voltage out of the DC path that is meant to go to EPICS, and backed out what the current is, based on the schematic, attached). 

I found a nifty 25 pin Dsub breakout board, that you can put in like a cable extension, and you can use clip doodles to look at any of the pins on the cable.  Since this was a PD activity, and I didn't want to die from the 100V bias, I covered all of the pins I wasn't going to use with electrical tape.  After turning down the 100V Kepco that supplies the WFS bias, I stuck the breakout board in the WFS.  Since I was able to measure the voltage at the output of the DC path, if you look at the schematic, I needed to divide this by 2 (to undo the 2nd op amp's gain of 2), and then convert to current using the 499 Ohm resistor, R66 in the 1st DC path.  

I did all 4 quadrants of WFS1 using a 532nm laser pointer, just to make sure that I had my measurement procedure under control, since silicon PDs are nice and sensitive to green.  I got an average QE of ~65% for green, which is not too far off the spec of 70% that Suresh found.

I then did all 8 WFS quadrants using the 1064nm CrystaLaser #2, and got an average QE of ~62% for 1064 (58% if I exclude 2 of the quadrants....see below).  Statistics, and whatever else is needed can wait for tomorrow.

Problem with 2 quadrants of WFS2?

While doing all of this, I noticed that quadrants 3 and 4 of WFS2 seem to be different than all the rest.  You can see this on the MEDM screens in that all 6 other quadrants, when there is no light, read about -0.2, whereas the 2 funny quadrants read positive values.  This might be okay, because they both respond to light, in some kind of proportion to the amount of light on them.  I ended up getting QE of ~72% for both of these quadrants, which doesn't make a whole lot of sense since the spec for green is 70%, and silicon is supposed to be less good for infrared than green.  Anyhow, we'll have to meditate on this.  We should also see if we have a trend, to check how long they have been funny.

Actually, I just found out that there are different flavors of 'YAG-444'.

There's a YAG-444AH and also a YAG-444-4AH. I'm not sure which one we have or even which is better. The diode's internal resistance is not listed.

They also say explicitly that he 'YAG Enhancement' is just using thicker Silicon. Since the absorption of 1064 nm light in Silicon is very small, most of the light just goes in and then comes back out without depositing much of the power.

The MC WFS have had beam dumps in front of them for the past ~2 weeks, until I could find the appropriate optic to put in the WFS path, to avoid melting the WFS' electronics. 

Koji noted that Steve had a W2-45S in a secret stash near his desk (which Steve later had put into the regular optics storage shelves down the Yarm), so I used that in front of the black hole beam dump on the AS table.  Now the beam is ~1W reflected from the unlocked mode cleaner, and ~100mW goes to the MC REFL PD.  The other 900mW now goes to this W2, and only ~5mW is reflected toward the MC WFS.  Most of the 900mW is transmitted through the window and dumped in the black hole.  There is a ghost beam which is reflected off the back surface of the wedged window, and I have blocked this beam using a black anodized aluminum dump.  I will likely change this to a razor dump if space on the table allows.  I have aligned the beam onto WFS1 and WFS2, although I did not re-align the mode cleaner first, so this alignment of the WFS will likely need to be redone. 

WFS1 has about 2mW incident, and WFS2 has about 3mW incident, when the mode cleaner is unlocked.  I have not yet measured the power incident when the MC is locked, although obviously it will be much smaller.

Except that I might temporarily remove one of the WFS for more quantum efficiency measurements later today, the WFS should be ready to turn back on for alignment stabilization of the mode cleaner. 

Mystery solved!

I removed WFS2 from the AP table (after placing markers so I can put it back in ~the same place) so that I could take some reflectivity as a function of angle measurements for aLIGO WFS design stuff.

I was dismayed to discover, upon glancing at the diode itself, that half of the diode is covered with some kind of oil!!!.  The oil is mostly confined to quadrants 3 and 4, which explains the confusion with their quantum efficiency measurements, as well as why the readback values on the MEDM WFS Head screen for WFS2 don't really make sense. 

The WFS QPD has a piece of glass protecting the diode itself, and the oil seems to be on top of the glass, so I'm going to use some lens tissue and clean it off.

Pre-cleaning photos are on Picasa.

Update:  I tried scrubbing the glass with a Q-tip soaked with Iso, and then one soaked in methanol.  Both of these failed to make any improvement.  I am suspicious that perhaps whatever it is, is underneath the glass, but I don't know.  Rana suggested replacing the diode, if we have spares / when we order some spares.

[Larisa and Jenne]

A few weeks ago (on the 28th of January) I had tried to measure the quantum efficiency of one quadrant of the WFS as a function of angle.  However, Rana pointed out that I was a spaz, and had forgotten to put a lens in front of the laser.  Why I forgot when doing the measurement as a function of angle, but I had remembered while doing it at normal incidence for all of the quadrants, who knows?

Anyhow, Larisa measured the quantum efficiency today.  She used WFS2, quadrant 1 (totally oil-free), since that was easier than WFS1.  She also used the Jenne Laser (with a lens), since it's more stable and less crappy than the CrystaLasers.  We put a 50 Ohm terminator on the RF input of the Jenne Laser, since we weren't doing a swept sine measurement.  Again, the Ophir power meter was used to measure the power incident on the diode, and the reflected power, and the difference between them was used as the power absorbed by the diode for the quantum efficiency measurement.  A voltmeter was used to measure the output of the diode, and then converted to current as in the quote below. 

Still on the to-do list:  Replace the WFS2 diode.  See if we have one around, otherwise order one.  Align beams onto WFS so we can turn on the servo.

QE = (h*c)/(lambda*e) * (I/P)

Where I = (Volts from Pin1 to GND)/2 /500ohms
P = Power from laser - power reflected from diode.
h, c, e are the natural constants, and lambda is 1064nm.

Also, I/P = Responsivity

Larissa is going to put her data and plots into the elog shortly....

[Valera, Jenne]

After Steve and Valera switched out the PMC, the Mode Cleaner resonance needed to be brought back.  We spent some time playing with the 2 steering mirrors directly after the PMC, to get the beam through the EOM, and to achieve flashing in the MC.  Valera then adjusted those 2 steering mirrors to minimize MC_REFL_DC.  I did a little bit more, and it's kind of close now, but we're only at ~half normal transmitted power.  Since the 2 steering mirrors after the PMC are so close together, the beam alignment is pretty sensitive to even small touches.  So it's probably time to move on to using the last zigzag steering mirrors on the PSL table, since they're farther apart. 

I have to head out for a little while, but I'll be back in a few hours. Kiwamu said he might continue the alignment into the MC, if he needs the IFO.  Also, we should measure the power before and after the EOM, just to confirm that we're getting through it optimally.  The beam looks good after the EOM, and the MC is resonating, so it should be fine, but it can't hurt to check.

I forgot to elog that last night I touched up the MC2_TRANS QPD setup. I was perplexed by it always going out of alignment so I investigated.

I found that the fork clamp for the steering mirror for the QPD was not tightened. Shame. The beam diameter was equal to the aperture of the QPD and was clipping. Double shame.

I added a lens and tightened the mounts and centered the beam at ~9 PM yesterday. You can see in the attached trend that the measured power went up by ~10%.

Later, there's a big gap where Valera and Steve change out the PMC. You can see that the MC REFL voltage goes from 4.5 V to 5 V (10% increase in the power delivered to the MC).

There's essentially no change in the total transmission - this indicates that although the PMC transmission is now higher by ~10%, the matching to the IMC has been degraded by an equivalent fraction.

Needs some mode matching work.

So.... Kiwamu and I were concerned (still a little concerned) that ETMY is not damping as nicely as it should be.  (It's fine, but the UL rms is ~5, rather than ~1 or less. BURT restores by Kiwamu didn't change anything.) Anyhow, I was heading out to push the annoying ribbon cables more firmly into the satellite adapter board things that are tied to the racks in various places (The back of 1X5 for the corner optics and the end station racks for the ETMs).  The point was to push in the ETMY one, but while I was out in the lab and thinking about it, I also gave all of the corner connectors (MC1, MC2, MC3, ITMx, ITMY, BS, PRM, SRM) a firm push. 

Kiwamu noticed that when I did this, the Mode Cleaner alignment got a little bit worse, as if the connection to the satellite adapter boards hadn't been great, I pushed the connectors in and the connection got better, but we also got a bit of a DC offset in the MC alignment.  Anyhow, the MC_TRANS power went down by ~2, to about the place it had been before Kiwamu adjusted the position of the lens in between the zigzag mirrors.  (I don't know if Kiwamu elogged it earlier, but he scooted the lens a teensy bit closer in the optical path to the Mode Cleaner). 

To counteract this loss in MC transmitted power as a result of my connector actions, I went back to the PSL table and fiddled with the zigzag steering mirrors that steer the beam from the PSL table over to the mode cleaner.  I got it a little better, but it's still not perfect.

Kiwamu has noted that to improve the mode matching into the Mode Cleaner with the new PMC in place, we might have to move the lens which is currently between the zigzag steering mirrors, and put it after the second mirror (so in between the last steering mirror and the pickoff window that sends a piece of the beam over to PSL_POS and PSL_ANG).  This will make the waist between MC1 and MC3 tighter. 

Moral of the story:  To improve IMC mode matching we need to move the last lens closer in the optical path to the mode cleaner waist. Twiddle with zigzag steering mirrors to optimize.

 Here is the followup on Jenne's February 14th, 2011 update on the quantum efficiency measurements of WFS2.

http://nodus.ligo.caltech.edu:8080/40m/4289

Attached is a PDF of my calculations, based on measurements ranging between 0-25 degrees in 5 degree increments.

The graph at the bottom plots these angles versus the calculated quantum efficiency at each point and the responsivity. Since quantum efficiency and responsivity only differ by a factor of some natural constants (lamda, e, h, c), I used a graph with two vertical axes, because the points would be plotted at essentially the same location if quantum efficiency (%)  and responsivity (Amps/Watts) were graphed on two separate plots.

The calculated values for quantum efficiency based on my measurements (labelled "ExpAverage") were pretty close to what Jenne had calculated in earlier attempts, which was around 60%. Just to test, I compared my quantum efficiency result against the calculation of quantum efficiency using the responsivity value for silicon, 0.5 Amps/Watt, which is labelled as "Spec". Comparison of "ExpAverage" and "Spec" shows that they differ by only about 2%, so I conclude that the theoretical quantum efficiency calculated using a given responsivity agrees with my measurement-based experimental result.

I worked a little bit more on optimizing the mode matching to the MC, but it's still not great.  I've only gotten a visibility of ~45%, but Koji said that it used to be ~87%.  So there is a long way to go.  Kiwamu said he can work with the lower-power configuration for a few days, and so my next step will be to measure the beam profile (stick a window in the path, and look at the refl from the window....that way we don't get thermal lensing from transmission through an optic), and redo the mode matching calculation, to figure out where the last lens should actually sit.

For some reason, Kiwamu forced us to change the MC servo electronics today. We are now combining it with the FSS box.

The MC Servo by itself was locking by just driving the NPRO PZT. Becuase of the ~30 kHz mechanical resonances of that system, our badnwidth is limited. To get higher bandwidth, we can either use a wideband frequency shifter like the AOM or just use the ole FSS combo of PZT/EOM. The old MC servo was able to get 100 kHz because it used the AOM.

So we decided to try going through the FSS box. The MC servo board's FAST output now goes into the IN1 port (500 Ohm input impedance) of the TTFSS box. This allows us to use the FSS as a kind of crossover network driving the PZT/EOM combo.

At first it didn't work because of the 5V offset that Jenne, Larisa, Koji, and Suresh put into there, so I cut the wire on the board that connected the power to the summing resistor and re-installed the MC Servo board.

We also removed the old Jenne-SURF 3.7 MHz LP between the MC mixer and servo. Also removed the Kevin-box (1.6:40) stuck onto the NPRO PZT.

We have yet to measure the UGF, but it seems OK. The PCDRIVE is too high (~5-6V) so there is still some high frequency oscillation. Needs some investigation.

* To get the FSS SLOW servo to work (change NPRO temperature to minimize PZT drive onto NPRO) I set the setpoint to 5V in the script so that we operate the FSS box output at 5V mean. I set the threshold channel to point to MC_TRANS_SUM instead of RC_TRANSPD. I also had to fix the crontab on op340m so that it would point to the right scripto_cron script which runs the FSSSlowServo, RCThermalPID.pl, etc. I also had to fix scripto_cron itself since it had the old path definitions and was not loading up the EpicsTools.pm library.

** Also, I was flabbergasted by the dog clamping on the last turning mirror into the MC. Barely touching the mount changes the alignment.

Looks like there was a mysterious loss of data overnight; since there's nothing in the elog I assume that its some kind of terrorism. I'm going to call Rolf to see if he can come in and work all night to help diagnose the issue.

As per Kiwamu's request I made a light touch to the input steering and the mode matching lens.

Here V_ref and V_trans are C1:IOO-MC_RFPD_DCMON and C1:IOO-MC_TRANS_SUM, respectively.

Result: Visibility = 1 - V_ref(resonant) / V_ref(anti_reso) = 1 - 0.74 / 5.05 = 85%

What has been done:

• Alignment of the steering mirrors before and after the last mode matching lens
     V_ref: 2.7 ==> 2.2, V_trans: 34000 ==> 39000
• Moving of the last mode matching lens away from the MC (+ alignment of the steering mirrors)
     V_ref: 2.2 ==> 0.74, V_trans: 39000 ==> 55000

Friday: 

In addition to the other fixes, Alex rebuilt the daqd process. I failed to elog this. When he rebuilt it, he needed change the symmerticom gps offset in the daqdrc file (located in /opt/rtcds/caltech/c1/target/fb). 

On Friday night, Kiwamu contacted me and let me know the frame builder had core dumped after a seg fault.  I had him temporarily disable the c1ass process (the only thing we changed that day), and then replaced Alex's rebuilt daqd code with the original daqd code and restarted it.  However, I did not change the symmetricom offset at this point.  Finally, I restarted the NDS process.  At that point testpoints and  trends seemed to be working.

Sunday:

The daqd process was restarted sometime on Sunday night (by Valera i believe).  Apparently this restart finally had the symmetricom gps offset kick in (perhaps because it was the first restart after the NDS was restarted?).  So data was being written to a future gps time.

Monday:

Kiwamu had problems with testpoints and trends and contacted me.  I tracked down the gps offset and fixed it, but the original daqd process only started once successfully, after that is was segfault, core dump non-stop. I tried Alex's rebuilt daqd (along with putting the gps offset to the correct value for it), and it worked.  Test points, trends, excitations were checked at the point and found working.

I still do not understand the underlying causes of all these segmentation faults with both the old and new daqd codes.  Alex has suggested some new open mx drivers be installed today.

[Rana / Koji]

The MC servo loop has been investigated as the MC servo was not an ideal state.

With the improved situation by us, the attached setting is used for the MC and the FSS.
The current UGF is 24kHz with phase margin is ~15deg, which is unbearably small.
We need to change the phase compensation in the FSS box some time in the next week.

- We found the PD has plenty of 29.5MHz signal in in-lock state. This was fixed by reducing the LO power and the modulation depth.

- The LO power for the MC demodulator was ~6dBm. As this was too high for the demodulator, we have reduced it down to 2dBm
by changing attenuator to 12dB (at 6 oclock of the dial) on the AM stabilization box.

- The RF power on the MC PD was still too high. The PD mush have been saturated. So the modulation slider for 29.5MHz was moved
from 0.0 to 5.0. This reduced the 29.5MHz component. (But eventually Koji restored the modulation depth after the servo shape has been modified.)

- The openloop gain of the loop has been measured using EXC A/TEST1/TEST2. The UGF was ~5kHz with the phase mergin of ~10deg. 

- This quite low phase margin is caused by the fact that the loop has f^-2 shape at around 4k-100kHz. The reference cavity has
the cavity pole of 40kHz or so while the IMC has the pole of ~4kHz. Basically we need phase lead at  around 10-100kHz.

- We decided to turn off (disable) 40:4000 boost of the MC servo to earn some phase. Then MC did not lock. This is because the LF gain was not enough.
So put Kevin's pomona box in the FAST PZT path (1.6:40). By this operation we obtain ~75deg (max) at 560Hz, ~35deg at 5kHz, ~20deg at 10kHz.

- In this setup the UGF is 24kHz. Still the phase margin is ~15kHz. This phase lag might be cause by 1)  the MC servo circut 2) PMC cavity pole

NEXT STEP

- Put/modify phase lead in the FSS box.
- Measure the PMC cavity pole
- Measure and put notch in the PZT path
- Increase the UGF / measure the openloop TF

[Koji / Rana]

- Since the MC servo had UGF up to ~20kHz and huge servo bump at 50kHz, we needed more phase between 20kHz to 100kHz.

- Today a phase compensation filter in a Pomona box has been inserted between the MC servo box and the FSS box.
  This is a passive filter with zero@14kHz and pole@140kHz. We obtain ~60deg at around 50kHz.

- After the insertion, the lock of the MC was achieved immediately. The overall gain as well as the PZT fast gain was tweaked
  such that the PC feedback is reduced down to 1~2.

- The OLTF has been measured.
  The insertion of the filter change increased the UGF to 130kHz even with "40:4kHz" and double super boost turned on.
  The phase margin is 54deg. Quite healthy.

- Rana modified the existed Auto Locker script.
  It is now continuously running on op340m!
  We made a couple of testsif it correctly relock the MC and it did. VERY COOL.

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

NEXT STEPS
- Measure the PMC cavity pole
- Measure the circuit TF and try to shave off the phase lag.
- Measure the PZT resonance of the NPRO and put notch in the PZT path
- Increase the UGF / measure the openloop TF

This is the log of the work on Wednesday 23rd.

1. Power Supply of the freq divider box

Kiwamu claimed that the comparator output of the freq div box only had small output like ~100mV.
The box worked on the electronics bench, we track down the power supply and found the fuse of the +15V line
brew out. It took sometime to notice this fact as the brown-out-LED of the fuse was not on and the power
supply terminal had +15V without the load. But this was because of the facts 1) the fuse is for 24V, and 2)
the large resistor is on the fuse for lighting the LED when the fuse is brown out.

I found another 24V fuse and put it there. Kiwamu is working on getting the correct fuses.

2. MC locking problem

After the hustle of the freq divider, the MC didn't lock. I tracked down the problem on the rack and found
there was no LO for the MC. This was fixed by pushing the power line cable of the AM Stabilizer for the MC LO, which was a bit loose.

An RF combiner should be included in the triple resonant box because it eases impedance mismatching and hence lowers undesired RF reflections.

Therefore we should use three cables to send the RF signals to the box and then combine them in the box.

(RF combiner)

 With proper terminations an RF combiner shows 50 Ohm input impedance.

But it still shows nearly 50 Ohm input impedance even if the source port is not properly terminated (i.e. non 50 Ohm termination).

This means any bad impedance mismatching on the source port can be somewhat brought close to 50 Ohm by a combiner.

  The amount of deviation from 50 Ohm in the input impedance depends on the circuit configuration of  the combiner as well as the termination impedance.

For example a resistive 3-way splitter shows 40 Ohm when the source port is shorten and the other ports are terminated with 50 Ohm.

Also it shows 62.5 Ohm when the source port is open and the other ports are terminated with 50 Ohm.

In this way an RF combiner eases  impedance mismatching on the source port.

(RF signal transfer at the 40m)

 According to the prototype test of the resonant box it will most likely have a non-50 Ohm input impedance at each modulation freqeucy.

If we install the resonant box apart from the combiner it will create RF reflections due to the mismatch (Case 1 in the diagram below)

The reflection creates standing waves which may excite higher harmonics and in the worst case it damages the RF sources.

 To reduce such a reflection one thing we can do is to install the combiner as a part of the resonant box (Case 2).

It will reduce the amount of the mismatching in the input impedance of the resonant circuit and results less reflections.

A rule we should remember is that a cable always needs to be impedance matched.

 The input impedance of the resonant box was measured when an RF combiner was attached to the box.

Indeed the combiner makes the impedance more 50 Ohm and reduces the reflection.

**** measurement conditions ****

* The output of box, where the EOM will be connected,  was open so that the box tries resonating with a parasitic capacitor instead of the real EOM.

* ZFSC-3-13, a 3-way combiner from mini circuit, was used.

* The S-port of the combiner was directly attached to the box with a short connector (~ 30 mm).

* Port 1 and 2 are terminated by 50 Ohm.

* The input impedance was measured on port 3 with AG4395A net work analyzer.

* Reflection coefficient 'Gamma' were calculated from the measured impedance 'Z' by using an equation Gamma = (50-Z)/(50+Z).

The resonances are found at 11, 29 and 73 MHz (55 MHz resonance was shifted to 73 MHz because of no EOM).

Note that the resonances are at frequencies where the notches appear in the reflection coefficient plot.

Don't be confused by a peak at 70 MHz in the impedance. This is an extra resonance due to a leakage inductance from the transformer in the circuit.

I realized that my impedance matching theory on an RF combiner was wrong !

In fact an RF combiner behaves more like an attenuator according to a reflection measurement that I did today.

A 3-way combiner reduces power of an input signal by a factor of 4.8 dB because it can be also considered as a 3-way splitter.

So it is just a lossy component or in other words it is just an attenuator.

(reflection measurement)

To check my speculation that I posted on #4504 I measured reflection coefficients for both cases.

In the measurement I used a heliax cable, which goes from 1X2 rack to the PSL table with a length of about 10 m. Note that this is the cable that had been used as '33 MHz EOM'.

At the input of the heliax cable it was connected to a direction coupler to pick off reflections and the reflected signal was sampled in AG4395A.

The other end of the cable (output side of the cable) was basically connected to the resonant box.

Then I did a reflection measurement for both cases as drawn in this entry (see #4504).

  - case 1 -  the combiner was inserted at the input side of the heliax cable.

  - case 2 - the combiner was directly attached to the resonant box

On the combiner, ZFSC-3-13, the port 1 and 2 were terminated with 50 Ohm, therefore the port 3 was used as an input and the source port is the output.

Here is a resultant plot of the reflection measurements.

Note that whole data are calibrated so that it gives 0 dB when the output side of the heliax is open.

There are two things we can notice from this plot:

 (1) The reflection coefficient at the resonant frequencies (where notches appear) are the same for both cases.

 (2) Over the measured frequency range the reflections were attenuated by a factor of about 9.6 dB , which is twice as large as the insertion loss of the combiner.

These facts basically indicates that  the RF combiner behaves as a 4.8 dB attenuator.

Hence the location of the combiner doesn't change the situation in terms of RF reflections.

To design a new resonant EOM box I started reviewing the prototype that I've built.

As a part of reviewing I checked an important thing that I haven't carefully done so far :

I compared the measured input impedance with that of predicted from a circuit model. I found that they show a good agreement.

So I am now confident that we can predict / design a new circuit performance.

* * * (input impedance) * * *

 Performance of a resonant circuit is close related to its input impedance and hence, in other words, determined by the input impedance.

Therefore an investigation of input impedance is a way to check the performance of a circuit. That's why I always use impedance for checking the circuit.

The plot below is a comparison of input impedance for the measured one and one predicted from a model. They show a good agreement.

(Note that the input impedance is supposed to have 50 Ohm peaks at 11, 29.5 and 55 MHz.)

 
* * * (circuit model) * * *

To make the things simpler I assume the following three conditions in my model:

 1. inductor's loss is dominated by its DC resistance (DCR)

 2. capacitor's loss is characterized only by Q-value

 3. Transformer's loss is dominated by DCR and its leakage inductance

All the parameters are quoted from either datasheet or my measurement. The model I am using is depicted in the schematic below.

Basically the Q-vaules for the capacitors that I used are quite low. I think higher Q capacitors will improve the performance and bring them to more 50 Ohm.

Since Suresh has installed the RF source box and changed the cable configuration somewhat,

the demodulation phase for the MC locking became off by about 10 degree.

I changed the length of some cables and obtained a good demodulation phase by the same technique as Suresh and Koji did before (see here for detail).

I maximized the Q signal. The lock of the MC looks healthy.

I found that the MC autolocker was OFF. Kiwamu says he turned it off because its slow. Suresh says that he has some feelings that maybe something is wrong. I'll let them describe what they know about the MC in an elog.

I checked the trend of the MC and PMC transmissions for the past 30 days:

Looks like the alignment has been drifitng. PMC was corrected recently by Koji, but the alignment of the input beam to the MC or the MC itself has to be fixed. Has someone been twiddling the MC SUS alignment biases??

[Valera, Suresh]

The first time I noticed that the MC was not locking was after I had finished switching the RF source installation.  Before this change the RF modulation frequency (for MC) was 29.485 MHz as read from the Marconi RF Source.  We replaced this with a Wenzel crystal source at 29.491 MHz.  This may have changed the loop gain. 

Today, I changed the MC alignment to optimise the MC lock.  Valera pointed out that this is not a desirable solution since it would shift the beam pointing for all components downstream.  However, since we are not sure what was the last stable configuration, we decided to stay with the current settings for now and see the trends of several parameters which would tell us if something is drifting and causing the autolocker to fail.

The MC Auto locker is now working okay.  However to obtain lock initially we reduced the loop gain by decreasing the VCO gain.  We then increased the gain after the autolocker had locked the MC.

 After Kiwamu set the REFL11 phases in the PRMI configuration (maximized PRM->REFL11I reesponse) I tried to measure the MC length and the 11 MHz frequency missmatch by modulating the 11 MHz frequency and measuring the PM to AM conversion after the MC using the REFL11Q signal. The modulation appears in the REFL11Q with a good snr but the amplitude does not seem to go through a clear minimum as the 11 MHz goes through the MC resonance.

We could not relock the PRMI during the day so I resorted to a weaker method - measuring the amplitude of the 11 MHz sideband in the MC reflection (RF PD mon output on the demod board) with a RF spectrum analyzer. The minimum frequency on the IFR is 11.065650 MHz while the nominal setting was 11.065000 MHz. The sensitivity of this method is about 50 Hz.

I tried to run the scripts/senseMCdecentering to check the centering of the MC beam spots on the mirrors. The script (csh) produces a lot of error messages on the control room machines. They are machine dependent combination of "epicsThreadOnce0sd epicsMutexLock failed", "Segmentation fault", "FATAL: exception not rethrown". Most of ezcawrite commands fail but not all(?). After running the mcassUp script couple of times all the dither lines came on. The MCL responses to dither lines look qualitatively similar to what it was in February (plot attached). The overall MCL spectrum looks ~100 times lower, presumably due to the analog gain reallocation.

Before that I realigned the beam into the PMC, recentered the PSL QPDs, and the beam into the MC to bring the MC RFPD_DC from ~3 to ~1.5 VDC then tweaked MC2 to bring the MC RFPD_DC from ~1.5 to ~1 VDC.

The mcass dither lines are off now and the loops are disabled.

 Kiwamu told me that the CDS matrix notation has changed and the 40m front end code has changed since February. I changed the senseMCdecentering script to reflect that. The other problems were: the "-" sign in ezcastep on ubuntu is not recognized - I used the known workaround of using "+-" instead; the echo command in csh script on ubuntu does not make a new line - but the echo " " does. The script ran on ubuntu with one error message "FATAL: exception not rethrown" but it finished nevertheless. The data appeared ok.  On centos machine the script produced "Segmentation fault'. The matlab script sensemcass.m now calculates the position on the MC mirrors in mm. The attached table shows the MC spot positions in mm:

I had to rephase the lockin digital phases by tens of degrees. I don't know why this should happen at ~10 Hz.

 

It seems like the best option would be to make the MCASS just adjust the SUS biases and center the beams on the suspended optics. Is this not possible somehow?

Kiwamu, Koji, Valera

We centered the beam on MC2 in pitch by moving the MC1,2,3 in the following combination [-9,+3,-7]. This actuation vector mostly moves the spot on MC2 vertically. The attached plot shows the dither before and after the centering. We monitored the demodulated signals and saw the reduction of the MC2 pit response from -1.0 to -0.22 which corresponds to the beam spot position change from 9 to 2 mm. Thus all the spots on MC mirrors are within 2 mm of the center. We estimate based on the distance between the MC1-MC3 of 20 cm, the distance from the center between MC1 and MC3 to the end of the Faraday isolator of 80 cm, and the aperture of the FI of 12 mm, the maximum angle out of MC of 3/200 rad. Which implies the maximum differential spot motion of 3 mm not to be limited by the FI aperture.

[Valera / Kiwamu]

The pointing of the incident beam to the interferometer has been jumping frequently.

Due to this jump the lock of the Y arm didn't stay for more than 2 min.

We turned off the strain gauge loop of PZT2-YAW and PZT2-PITCH, then the spot motion became solid and the Y arm locking became much more robust.

A cable for IP_POS has been disconnected at the LSC rack, 1Y2. Due to it currently no IP_POS signal shows up on the digital side.

It looks like we disconnected the cable together with some unused cables when we were cleaning up the wiring of the LSC rack.

The cable, a shielded flat-cable, is supposed to send DC power to the QPD and send the signals from the QPD to an interface board on the LSC rack.

I will check how it used to be and reconnect it.

 I found the disconnected cable, but I do not see the interface board at the LSC rack