PMC_cal (m/V) = (1064 nm)/2 / V_FSR
The alignment of the PMC adjusted on the PSL table: Trans 0.737->0.749
The alignment of the IMC adjusrted on the sliders: Trans 14300->15300
WFS offset has been reset by /opt/rtcds/caltech/c1/scripts/MC/WFS/WFSoffsets
For a few days, I've noticed that the PSL overview StripTool panel shows PMC transmission and FSS RMTEMP channels with variation that is too large to be believable. Looking at these signals on an oscilloscope, there was no such fuzziness in the waveform. I ruled out flaky connections, and while these are the only two channels currently being acquired by the temporary Acromag setup underneath the PSL enclosure, the Acromags themselves are not to blame, because once I connected a function generator to the Acromag instead of the PMC transmission photodiode, both channels are well behaved. So the problem seems to be with the PMC transmission photodiode, perhaps a grouding issue? Someone please fix this.
Gautam showed me how the PMCTRANSPD signal was reading zero, and he suspected it might have to do with the acromag wiring. Disconnected the acromag box underneath the PSL table and checked the ADC wiring. Side note: When benchtesting the c1psl acromag chassis there was excess noise in the AI channels, and grounding the minus pin of the ADC channel eliminates the noise.
So I grounded the (-) pins on the ADC1 (192.168.113.122), which PMCTRANSPD is connected to and that seemed to fix the problem. As of right now PMCTRANSPD is reading ~.75 V.
See attached pictures
gautam: While this fix seems to have worked, I wonder why this became necessary only in the last month. Note that the problem was a noisy readback on the PMC transmission PD, which also made the FSS_RMTEMP channel noisy, leading me to suspect some kind of ground loop issue.
Rana and I connected the PMC_trans output to the BNC connector board on the west end of the PSL table (the channel is labeled). I took a few spectra off of PMC_trans and the SR785 was connected directly to the PMC_trans output for about an hour.
Data will follow.
The attached PDF shows a possible gain / input noise config for the POP 22/110 that we would use to detect the RF power in the DRMI. Design is in the SVN.
If Kiwamu/Jenne say that this has good enough sensing noise for the lock triggering than we will build it. This is using a 2mm diode.
If we can get away with 1 mm, we might as well use a PDA10CF for now.
I was in the lab, near the south end of the ITMX oplev table, looking for something, and I bumped the POP ASC QPD's power supply. I thought that it was fine, but did not adequately check it. When EricQ asked me just now about why the PRC is so wobbly today, I checked, and the power for the QPD wasn't properly connected (it's kind of a crappy connector, that if you nudge, contacts or loses contact). Anyhow, I restored power to the QPD, and the PRC looks a little more stable now. My fault for not checking more carefully, and my apologies to Q and Gabriele for their frustrations this afternoon.
I was thinking about POP today, and wanted to know if there was something to be done to allow us to use the PRCL ASC for at least a little bit farther into arm power buildup.
Anyhow, I checked, and while PRMI is locked on sidebands (ETMs misaligned), POP DC is about 80 counts, and the power measured by the Ophir power meter is 24 microWatts.
We were on the 3rd gain setting for the QPD's power amplifier. I turned it down to the "2" option. (When at 4, the front panel light indicates saturation).
It's not clear to me what the gain settings mean exactly. I think that "1" means 4*10^3 V/A, and "6" means 4*10^6 V/A (On-Trak OT301 info site), but I don't know for sure how the gain changes for the settings 2-5. Anyhow, I have changed the digital gain for the ASC to be -0.063 from -0.023 for both pitch and yaw.
Tega and I went in to adjust the POP being in the ITMX Table. The beam entered the table high, so we adjusted the this by adding mirrors (The highlighted in Turqoise are mirrors which adjust the pitch of the beam). All the mirrors are set and we are now in the process of adjusting the PD.
Got POP beam centered on camera and nominally on the two PDs. Attachment #1 shows "carrier" camera.
I have some data for how much motion of any PRMI-relevant optic affects the beam seen by the POP QPD.
For this, I am using the QPD calibration from the micrometer (elog 8851) to get me from counts to mm of motion. Note that the pitch calibration hasn't been redone (I tried locking the PRMI this afternoon, but ITMX kept drifting away from me**, so I didn't get any more data.) The pitch calibration is obviously very rough, since I only have 2 points defining my fit line.
Anyhow, if we assume that's close enough to get us started, I now have a calibrated QPD spectrum:
As detailed in elog 8854, I took single frequency transfer functions, to determine the effect at the QPD from shaking any single PRMI optic. These transfer functions gave me a conversion factor between the optics' oplev readings (in microradians) to the counts seen at the QPD. I used this number, as well as the QPD calibration from the micrometer data, to convert each optics' oplev spectra to motion that one would expect to see at the QPD.
I have not yet completely figured out how to make an estimate of the PR folding optics' affect on the POP QPD spot position, if I know their motion. The current plan is to do as Den did in elog 8451, and infer the PR2/3 motion from the ITMX/BS motion measured by the oplevs. My plan was to take the spectra of the oplev signals while the BS/ITMX are undamped, divide by the SOS pendulum transfer functions, then multiply by the TT transfer functions (which I finally wrote down in elog 8564). I'm planning on using the undamped data, since the oplev signals are still within the linear range of the oplev QPDs, and I won't have to take the SUS damping into account. Anyhow, after I do that, I'll have an idea of how much the tip tilts are moving, but not what that does to the cavity axis.
However, after looking at the plots below, it seems like the PRM is the main culprit causing the PRC axis motion, although the BS (and to a smaller extent the ITMs) are not innocent. Since the plots get very busy very quickly, I have many plots, each plot comparing one of the above QPD spectra (either pitch or yaw) with a single optics' oplev inferred motion.
EDIT: After talking with Koji, I realize that, since the ASC was engaged during the PRM oplev spectrum measurement, I cannot yet say whether the motion is due to PRM, or if it is from PR2 or PR3, and imprinted on the PRM via the ASC servo. The lump where the PRM-caused motion is greater than the QPD spectra is entirely in the region where the ASC is active. So, the QPD motion I expect without the ASC would be something like the green trace in the PRM comparison plots. The blue trace is then the closed loop measurement. Since the ITMs and BS are below the closed loop values, they aren't the ones causing the big lump. I should retake all of these spectra at a time when the PRMI is locked, but the ASC is not engaged. I'm not sure if I'll have a chance to do that tonight or not. If I can find some GPS times when the PRMI was locked, before we had ASC, I can get the oplev data.
I think part of the reason PRM is dominating is that it's damped motion is ~10x greater than any other optics', most noticeably the BS'. I'll write a quick separate elog about this. Also, note that the ~3Hz resonant gain had been turned off in the PRM oplev loop, but not in any other loops. This is why there isn't the sharp dip in the PRM's oplev motion. Also, since the PRM ASC was engaged for this measurement, and the ASC pushes on the PRM to minimize the QPD motion, it isn't totally crazy that the PRM's motion is greater than what we actually see at the QPD, if it is compensating for the motion of other optics.
** Re: PRMI locking this afternoon, it was almost as if ITMX were bi-stable. I aligned both arms, to set the ITM positions. Then, I would lock and tweak up the michelson to get the AS port nice and dark (usually touching ITMX today, since it seemed like the drifter....ITMX at this point was usually between -7 and -15 microradians in pitch from the center of the oplev QPD). When I then brought the PRM back into alignment, ITMX was starting to drift away. As soon as I hit the LSC Enable switch, and looked back over to the OpLev screen, ITMX was misaligned, usually around -65 urad in pitch. I did this circus probably 3 or so times before giving up. Koji said that he had seen this bi-stability before, but he didn't remember what fixed it. The drifting that Koji mentioned in elog 8801 seems to have been fixed by centering all the PRMI oplevs every day, but I had already done that, and was still seeing ITMX drift.
It was not actually easy to see from the entry what signal was taken in what condition but from the shape of the spectra
I had the impression that the ASC & OPLEV signals were measured under the presence of the ASC control.
That is (moderately to say) tricky as the ASC control imprints the angular noise
from unkown mirror on the PRM, and then the oplev observes it. The original stability of the oplev is
obscured by the injection from the servo and the fair comparison of the stability is almost impossible.
So the true comparison between the ASC and oplev signals should be done without the control loop.
We can recover the free running spectrum of the ASC signals by compensating the loop transfer functions
because the ASC signals are the in-loop error signals. The oplev signals should be measured without
the ASC loop engaged.
Power not on to the POP QPD yet though. Also, still need to reconnect POPDC.
I was bad, and forgot to elog the most important part of my work yesterday - that I had rotated the POP QPD by 90 degrees, so that I could fit the micrometer onto the table. There is a sticker on the front of the QPD to indicate which direction is "X" and "Y" for the output of the readout box. Right now (and the way that I will mount the QPD to the table, after I redo the calibration today), X is PITCH, and Y is YAW. Koji and Nic swapped the cables to the ADC to make this all consistent.
Yesterday, I locked the PRM-ITMY half cavity, and tried to take calibration data. However, with no ASC servo engaged, the beam was still moving. Also, with only the half-cavity, I had very little light on the QPD, and since it has internal normalization, the outputs can get a little funny if there isn't enough light. I had checked, and even with the gain cranked up to maximum, the "light level too low" LED was illuminated. So, my calibration data from yesterday isn't really useful.
Today, hopefully after lunch, I will lock the PRMI with the new AC-coupled ASC servo, so that I can have the servo on, and the PRMI locked on the sideband, so that I have more light on the QPD.
After that, it seems that the final thing we need to do before we vent is hold an arm near, but off resonance, lock the PRMI, and then swing the arm in and out of resonance a bit.
Calibration data for the POP QPD has been taken, with the PRMI locked on sideband (with AS55Q and REFL33I, since it stayed locked longer with those 2). ASC was on, AC coupled.
We didn't get too far on either side of center of the QPD, since the ASC servo would go unstable, so we only explored the roughly linear region. Data / plots / analysis to follow.
These are the data, one plot for when the vertical QPD position was changed, and one for when the horizontal (yaw) QPD position was changed.
The micrometer is in inches, so 1 unit is 0.1 inches, I believe.
Clearly, I need to redo the measurement and take more data in the linear region.
I tried to retake POP QPD calibration data again today. The MC was mostly fine, but whenever the PRMI unlocked, both ITM watchdogs would trip. I'm not sure what was causing this, but the ITM alignment wasn't perfect after this kind of event, so I felt like I was continuously locking and realigning the arms to get the alignment back. Then, after turning on the ASC and tweaking up the PRM alignment for maximum POP110I signal, I had to recenter the QPD, so none of my previously taken data was useful. Frustrating. Also, I had recentered the PRMI-relevant oplevs, but I had these weird locklosses even with nicely centered oplevs.
I have given up for the daytime, and will come back to it if there's a spot in the evening when arm measurements aren't going on.
Here is the data from last week, and the data from today. The micrometer readings have been calibrated into mm, and I have fit a line to the linear-looking region. Obviously, for the Pitch calibration, I definitely need to take more data.
I took POP QPD calibration data with a new method, on Rana's suggestion. I locked the PRMI, and engaged the ASC servo, and then used awggui (x8) to put dither lines on all of the PRMI-relevant optic's ASCPIT and ASCYAW excitation points. I then took the transfer function of the suspensions' oplev signals (which are already calibrated into microradians) to the POP_QPD signals (which are in counts). This way, we know what shaking of any optic does to the axis translation as seen by the POP QPD. We can also infer (from BS or PRM motion for PR3, and ITMX motion for PR2) what the folding mirrors do to the axis translation. Note that we'll have to do a bit of matrix math to go from, say, PRM tilt effect to PR3 tilt effect on the axis motion.
The data is saved in /users/jenne/PRCL/July152013_POP_TFs.xml . There is also a .txt file with the same name, in the same folder, listing the frequencies used by the awg.
I'll analyze and meditate tomorrow, when my brain is not so sleepy.
I am prepping to do the POP QPD calibration, and so have turned off the POP QPD, and put it onto a micrometer stage. My plan is to (after fixing the ASC servo filters to make the servo AC coupled, rather than DC coupled) lock the PRM-ITMY half cavity, and use that beam to calibrate the QPD. While this isn't as great as the full PRMI, the PRMI beam moves too much to be useful, unless the ASC servo is engaged.
While on the table, I noticed 2 things:
* In order to place the micrometer, I had to temporarily move the POP55 RFPD (which has not been used in quite a long time). I think it's just that the panel-mount SMA connector isn't tight to the panel inside, but the RF out SMA cable connector is very loose. I have moved the POP55 RFPD to the very very south end of the SP table, until someone has time to have a quick look. (I don't want to get too distracted from my current mission, since we haven't put beam onto that PD for at least a year).
* The ITMX oplev beam setup isn't so great. The last steering mirror before the beam is launched into the vacuum is close to clipping (in yaw... pitch is totally fine), and the steering mirror outside of vacuum to put the beam on the QPD is totally clipping. The beam is falling off the bottom of this last steering mirror. Assuming the beam height is okay on all of the input optics and the in-vac table, we need to lower the last steering mirror before the oplev QPD. My current hypothesis is that by switching which in-vac steering mirror we are using (see Gautam's elog 8758) the new setup has the beam pointing downward a bit. If the problem is one of the in-vac mirrors, we can't do anything about it until the vent, so for now we can just lower the out of vac mirror. We should put it back to normal height and fix the oplev setup when we're at atmosphere.
After aligning the PRC, I centered the POP QPD.
I locked the PRMI, and tried to turn on the ASS, but this caused PRMI to lose lock.
Since this is similar to what happened the other night (see elog 9243, 2nd big paragraph), I looked into it a little further. I noticed that the POP QPD pitch was very close to the edge of the QPD, so I went out and (while PRMI was locked) recentered the POP QPD. After doing so, I was able to run the PRM ASS, and it worked very nicely, just as it has before. So, it looks like something drifted, such that the optimal PRM alignment caused the POP beam to not be fully on the QPD. Since the ASC loop is triggered by PRMI lock, and is constantly on, falling off the QPD causes lockloss.
While I was out there, I tweaked up the PMC pitch alignment yet again. The FSS numbers all looked reasonable, however PMC transmission was ~0.75 . I did a tiny bit of work in pitch, and now we're back to 0.83 transmission.
In addition to the simulation described in my previous elog, I simulated the signal on a quadrant photodetector demodulated at 2F. The input laser beam is modulated at 11MHz up to the fifth order. There is no additional 55 MHz modulation.
The QPD demodulated at 2F shows good signals for PRC control for all CARM offsets, as expected from the previous simulation.
This is nice. Can we test this idea with POP22 + a razor blade?
Just to take transfer functions in PRMIsb between the PRM angle to POP QPD/POP22+razor blade
as well as the noise spectrum measurement are already useful.
We want to figure out the requirement for the 2f QPD.
(Transimpedance / Noise level / Beam size / etc)
Depending on the requirement we'll see if we need demodulation or just a power detector.
The POP QPD X/Y/SUM signals, which are acquired in c1ioo, are now being broadcast over dolphin. c1ass was modified to pick them up there as well:
Here are the new IPC entries:
controls@fb ~ 0$ egrep -A5 'C1:IOO-POP' /opt/rtcds/caltech/c1/chans/ipc/C1.ipc
desc=Automatically generated by feCodeGen.pl on 2014_Apr_30_17:33:22
desc=Automatically generated by feCodeGen.pl on 2014_Apr_30_17:33:22
desc=Automatically generated by feCodeGen.pl on 2014_Apr_30_17:33:22
controls@fb ~ 0$
Both c1ioo and c1ass were rebuild/install/restarted, and everything came up fine.
The corresponding cruft was removed from c1rfm, which was also rebuild/installed/restarted.
I have added a few things to the ASS model, and the ASC sub-block, so that we can send POP QPD information down to the ETMs for CARM angular control after we've reduced the CARM offset and gotten some carrier buildup. I did not remove our ability to actuate on PRM, so that we can still play with it in PRMIsb cases.
The input matrix has been expanded so that it can send signals to new CARM_YAW and CARM_PIT filter banks. The corresponding filter banks have been created. The output matrix was also expanded to take in the 2 new servo outputs, and so it can send signals to both ETMs, pitch and yaw. I did not include any triggering logic for this new CARM situation, since I assume we'll just turn it on and off with our scripts. (We haven't really been using the triggering capability of the PRM ASC either lately, although it's all still there). I added the inputs and outputs of the CARM servos to the list of acquired channels.
The ASC sub-block:
I also modified the top level of the ASS model. This was just a simple addition of summing nodes for the ETMs, similar to what was already in place for the PRM, so that we can send both the ASS dither alignment signals and the ASC servo control signals to the optics.
The ASS top level:
I also quickly modified the ASC screen to expose all of the new options:
The ASS model was compiled, and restarted. As usual, this temporarily removes the biases on the input pointing tip tilts, but the pointing seems to have come back without any trouble.
Using the data I collected yesterday, the POP angular FF filters have been trained. The offline time-domain performance looks (unbelievably) good, online performance will be verified at the next available opportunity(see update).
The sequence of steps followed is the same as that done for the MCL FF filters. The trace that is missing from Attachment #1 is the measured online subtraction. Some rough notes:
Update Apr 5 1145pm:
This afternoon, I kept the PRM locked for ~1hour and then measured transfer functions from the PRM angular actuators to the POP QPD spot motion for pitch and yaw between ~1pm and 4pm. After this work, the PRM was misaligned again. I will now work on the feedforward filter design.
that's pretty great performance. maybe you can also upload some code so that we can do it later too - or maybe in the 40m GIT
I wonder how much noise is getting injected into PRC length at 10-100 Hz due to this. Any change the PRC ERR?
I don't have a recent measurement of the optical gain of this config so I can't undo the loop, but in-loop performance doesn't suggest any excess in the 10-100 Hz band. Interestingly, there is considerable improvement below 10 Hz. Maybe some of this is reduced A2L noise because of the better angular stability, but there is also improvement at frequencies where the FF isn't doing anything, so could be some bilinear coupling. The two datasets were collected at approximately the same time in the evening, ~5pm, but on two different days.
We first aligned the single arm cavity resonance for both arms to get maximum flashing. As we opened the chamber, I found that the POP beam was mostly hitting the POP_SM4 mirror but was clipping about 2 mm on the top edge.
I used TT2-PR3 to lower the injection beam angle and moved pairs of ITMY-ETMY, and ITMX-ETMX to recover as much flashing as I could in the both arms. Then, I moved PR2 in pitch from 49 to 71 to maximize the arm flashing again. After these steps, the POP beam was clearly within the POP_SM4 mirror but still in the upper half of the optic and there was maybe just a mm of clearance from the top edge. I decided to raise POP_SM4 mirror by 0.14" spacer. Now the beam is still in upper half of the mirror but has a good clearance from the edge.
The POP beam is coming outside in the in-air table at as a rising beam in the nominal path near the center of the window. This beam needs to be directed to the POP camera and RFPD on the far-side of the table.
Some obvious things to be fixed
- We need POP55 and POP CCD for diagnoses.
Done. The beam is also going vaguely in the direction of POP110, but I can't see the beam, so it's tricky.
Order of operations:
1. Find POP on the table, place iris so I wouldn't forget. Find beam by putting big IR card where I think beam should be, look at IR card with IR viewer.
2. Move and re-clamp 2" lens so beam is on center of lens.
3. Move and re-clamp 1st (2") mirror so that beam is on center of mirror.
4. Remove BS-33% so that all the beam goes to POP55, steer that 1st mirror so beam is on POP55's little mirror. Align little mirror so beam is centered on POP55 (as seen by looking at PD with viewer, finding "edges" of PD, going back to center).
5. Put BS-33% back in place. The reflected portion of this beam is not possible to see using card+viewer technique.
6. Remove BS-50% that reflects half of this beam to POP110. Find beam reflected from BS-33% by waving POP camera around. Steer BS-33 until beam goes back in the direction that the camera used to be mounted. Adjust camera mount and BS-33 so that beam is on camera.
7. Put BS-50% back in place. Steer it around with voltmeter on PD to see if beam ever hits PD. Unsuccessful. Give up, since we have POP55, and POP camera.
8. Make a youtube video: POP, AS, REFL, ITMXF (all on Quad3) - PRMI coarsely aligned, no IFO parts locked. MICH was locked earlier, but not during video time.
We needed to sort out the POXDC signal so we could work on X-arm alignment. Given that POXDC channel value was approx 6 compared to POYDC value of approx. 180, we decided to open the ITMX chamber to see if we could improve the situation. We worked on the alignment of POX beam but could not improve the DC level which suggests that this was already optimized for. As an aside, we also noticed some stray IR beam from the BS chamber, just above the POX beam which we cold not identify.
Next we moved on to the POP beam alignment, where we noticed that the beam level on LO1 and POP_SM4 was a bit on the high side. Basically, the beam was completely missing the 1" POP_SM4 mirror and was close to the top edge of LO1. So we changed TT2 pitch value from 0.0143 to -0.2357 in order to move the beam position on POP_SM4 mirror. This changed the input alignment, so we compensated using PR2 (0.0 -> 49.0) and PR3 (-5976.560 -> -5689.800). This did not get back the alignment as anticipated, so we moved ITMY pitch from 0.9297 to 0.9107. All of these alignment changes moved the POP beam down by approx 1/5 of an inch from outside the mirro to the edge of POP_SM4 mirror, where about half of the beam is clipped.
We need to repeat these aligment procedures with say 1.5 time the change in TT2 pitch to center the beam on POP_SM4 mirror.
It still looks like we might be seeing some clipping in the in-vac POP steering mirrors - we haven't gotten to them yet.
We fixed up, as best we can, the in-vac POP alignment. We are entirely limited in yaw by the aperture size of the 2" 45deg mirror launching the beam out of the vacuum. The main centroid of the beam is well centered, but the inflated weird part of the beam is totally clipped. There's nothing we can do about it except use a much larger mirror, install a fast lens inside the chamber, or just fix the damn PRC. I vote for the third option there.
How did we work our magic?
We put a green laser pointer where the POP DC PD was, and injected it into the vacuum, just like we normally do. However, this time, we made sure the green laser was centered on all of the out of vacuum mirrors, so that there was no real work to do once we turned off the laser pointer. We locked the cavity, and confirmed that we are well centered on all of the in and out of vacuum mirrors, and discovered our aperture problem with the last in-vac mirror.
Here is a snapshot of the POP camera:
We put a green laser pointer at ~4 inches on the POX table, and steered it using a mirror on the POX table to hit the center of the last in-vac mirror that POP sees. I then steered that mirror so we were hitting the center of the other POP in-vac steering mirror, and hitting the same spot as the main IR beam. It is easy to hold an IR card in front of PR2 and see the IR and green beams simultaneously. I aligned both of the POP in-vac steering mirrors such that the green beam is co-aligned with the IR beam at PR2, as well as as far as I could reach toward the face of PRM from the ITMX door.
Note: The drawings by Koji have the POP "forward" beam (transmission through PR2 of the beam from PRM to PR2) dumped, while the POP "backward" beam (transmission through PR2 of the beam from PR3 to PR2) leaving the vacuum. I aligned the steering mirrors such that the 'forward' beam would come out, although no dump is in place to dump the other beam. I can't think of a reason why we care one way or the other, but I feel like Koji has perhaps mentioned something in the past. I need to figure this out before we put doors on.
Like yesterday with POX, we used the Watec with the aperture fully open to look at the POY pickoff, while I held the IR card in front of the mirror, to confirm that the beam was ~on the center of the optic. Then we took the lens off the camera, and made sure that the POY beam hit the CCD on the POY table.
To do list for Monday: While we are putting the heavy doors on, someone needs to wave an IR card in front of the IPANG steering mirrors in the ETMY chamber, while someone else takes a photo / still snapshot with the Watec. Also, Manasa wanted to retake in-vac photos of at least the ITMY chamber, since SR2 was moved a very slight amount. Also, also, someone tall needs to put the regular EQ stops on the PRM face (we have the old spring ones in there now).
Before pumpdown, we also need to get the IPANG beam centered on the PD. The beam is cleanly coming out of the vacuum and hitting the first out of vac steering mirror, I just haven't centered it onto the QPD.
Barring any other thoughts that people have of things that *must* be done before we pump down, I think we're ready to start putting heavy doors on the chambers on Monday.
Other thoughts, for next vent: We need to re-look at the ITMY table. POY's pickoff is just too close to the main beam. Is it possible to move the AS steering mirrors and get POY from the BS table? VENT CZAR: please put looking at this on the next vent to-do list.
What was the reasoning / resolution of the POP forward/backward beam? Are we going to have the right beam for DRMI locking?
From Koji's email to me:
"With the backward beam you can see the returning beam even when the PRM is misaligned. That's the only difference. Once the PRM is aligned both beams have the same information."
So, we should be fine.
We aligned the PRMI. We definitely can lock MICH, but we're not really sure if PRCL is really being locked or not. I don't think it is.
Anyhow, we found 2 different places on the AS camera that we can align the PRMI. One (middle, right hand side of the camera), we see the same weird fringing that we've been seeing for a week or two. The other (lower left side of the camera), we see different fringing, almost reminds me more of back in the day a few months ago when the beam looked like it was expanding on each pass. As I type, Evan is uploading the movies to youtube. I *still* don't know how to embed youtube videos on the elog!
Also, we found both forward-going and backward-going POP beams coming out onto the POX table. We placed the 2" lens in the path of the backwards beam, so that we can find it again. We can't see it on an IR card, but if we put some foil where we think the beam should be, we can use a viewer to see the spot on the foil. Poking a hole in the foil made an impromptu iris.
Lower left on camera
Middle right on camera
How can you lock the PRMI without the REFL beams? c.f. this entry by Kiwamu
Which signals are you using for the locking?
I think the first priority is to find the fringes of the arms and lock them with POX/POY.
As for the POP, make sure the beam is not clipped because the in-vac steering mirrors
have been supposed to be too narrow to accommodate these two beams.
- At the end of the session, Jenne told me that the POP PD still has a large diameter beam. (and a steering mirror with a peculiar reflection angle.)
==> THIS SHOULD BE FIXED ASAP because the normalization factor can be too much susceptible to the misalignment of the spot.
Koji set the IFO in a PRM-ITMY configuration for me, while I went to put a lens on the POP path. Before putting the lens, the maximum average output that I saw from the diode (on a 'scope) was 4.40mV. After putting in the lens and realigning the beam onto the diode, the new max DCvalue that I saw was 21.6mV. This is a factor of 4.9.
EDIT: The dark value was -3.20mV, so actually the ratio is ~3.25 .
I have not yet done anything to fix the situation of the large angle of incidence on the first out-of-vac steering mirror.
I did some re-alignment of the POP beam on the IX in air table. Here are the details:
Tangentially related to this work - I took the nuclear option and did a hard reboot of the c1susaux Acromag crate on Sunday to fix the EPICS issue - it seems to be gone for now, see Attachment #5.
We set up POP camera and POPDC PD, and centered REFL PDs.
We also tried to center AS55 PD, but AS55 seems to be broken.
What we did:
1. POP path alignment:
Shot green laser pointer from ITMX table at where POPDC PD was sitting and centered green beam at optics in the POP path. Steered POPM1/M2 mirrors in the ITMX chamber to make green laser overlap with the PRM-PR2 beam as far as I can reach from ITMX chamber. We removed some ND filters and a BS for attenuating POP beam because POP power was somehow so low. Currently, POP is pick-off of the beam which goes from PRM to PR2.
2. POP camera and PD:
We first used camera to find the beam at where POPDC PD was sitting because it is much easier to find focused beam. Put an iris in front of the camera, and put POP DC behind it. Steered a mirror in front of PD to maximize DC output.
3. REFL PDs:
Steered mirrors in the REFL path to center the beam and maximized DC outputs, as usual.
AS55 was not responding very much to the flashlight nor AS beam. C1:LSC-ASDC_OUT looked funny. By swapping the ribbon cables of AS55, REFL55, and REFL165, I confirmed that AS55 PD itself is broken. Not the ribbon cable nor PD circuit at LSC rack. I don't know what happened. AS55 was working on Feb 8 (elog #8030).
We aligned PRMI coarsely. POP(right above) looks much better than before. REFL (left below) still looks elliptic, but ellipticity differs with the position on the camera. Some astigmatism is happening somewhere. AS (right below) looks pretty nice with MI aligned.
1. Fix AS55? Or replace it with POP55 PD, which is currently unused.
2. Confirm we are getting the right error signals or not, and lock PRMI.
I undertook the investigation of the AS55 PD. I found the PD is not broken.
I tested the PD on the PD test bench and it works just fine.
I attatched the characterization result as there has been no detailed investigation of this PD as far as I remember.
The transimpedance gain at 55MHz is 420Ohm, and the shotnoise intercept current is 4.3mA.
I thought AS55 is broken because it was not responding to the AS beam nor flashlight in DC. What's the DC gain difference between AS55 and POP55 (or REFL55)?
10010 Ohm for POP55 vs 50 Ohm for AS55 (cf. http://nodus.ligo.caltech.edu:8080/40m/4763)
I wonder if you used an LED flash light, which emits no IR.
I didn't use LED flash light. We learned from the past (elog #7355). I checked that POP55 and REFL55/165/33/11 are clearly responding to flash flight, but I didn't expect that much difference in DC gain.
I wonder why we could align AS beam to AS55 in Feb 8 (elog #8030), but not in Feb 15 (elog #8091). I will check during the pump down.
I wonder if you used an LED flash light, which emits no IR.
There are many versions of the POP22 signal path I found on the elog, e.g. this thread. But what I saw at the LSC rack was not quite in agreement with any of those. So here is the latest greatest version.
Since the 2f signals are mainly indicators of power buildups and are used for triggering various PDH loops, I don't know how critical some of these things are, but here are some remarks:
POP QPD checkout:
We would like the option of feeding back the POP beam position fluctuations to the PRM to help stabilize the PRC since we don't have oplevs for PR2 and PR3. However, we cannot just use the DC QPD because that beam spot will be dominated by carrier light as we start to get power recycling.
The solution that we are trying as of today is to look at yaw information of just the RF sidebands. (Yaw is worse than pitch, although it would be nice to also control pitch). I have placed a razor blade occluding about half of the POP beam in front of the POP PD (which serves POPDC, POP22 and POP110). I also changed the ASS model so that I could use this signal to feed back to the PRM. Loop has been measured, and in-loop spectra shows some improvement versus uncontrolled.
Optical table work:
The POP beam comes out of the vacuum system and is steered around a little bit, then about 50% goes to the DC QPD. Of the remaining, some goes to the Thorlabs PD (10CF I think) and the rest goes to the POP camera. For the bit that goes to the Thorlabs PD, there is a lens to get the beam to fit on the tiny diode.
There was very little space between the steering mirror that picks off the light for this PD, and the lens - not enough to put the razor blade in. The beam after the lens is so small that it's much easier to occlude only half of the beam in the area before the lens. (Since we don't know what gouy phase we're at, so we don't know where the ideal spot for the razor is, I claim that this is a reasonable place to start.)
I swapped out the old 50mm lens and put in a 35mm lens a little closer to the PD, which gave me just enough room to squeeze in the razor blade. This change meant that I had to realign the beam onto the PD, and also that the demod phase angles for POP22 and POP110 needed to be checked. To align the beam, before placing the razor blade, I got the beam close enough that I was seeing flashes in POPDC large enough to use for a PRMI carrier trigger. The PRMI carrier was a little annoying to lock. After some effort, I could only get it to hold for several seconds at a time. Rather than going down a deep hole, I just used that to roughly set the POP22 demod phase (I -phase maximally negative when locked on carrier, Q-phase close to zero). Then I was able to lock the PRMI sideband by drastically reducing the trigger threshold levels. With the nice stable sideband-locked PRMI I was able to center the beam on the PD.
After that, I introduced the razor blade until both POPDC and POP22 power levels decreased by about half.
Now, the POP22 threshold levels are set to up=10, down=1 for both MICH and PRCL, DoF triggers and FM triggers.
ASS model work:
POP22 I and POP110 I were already going to the ASS model (where ASC lives) for the PRCL ASS dither readbacks. So, I just had to include them in the ASC block, and increased the size of the ASC input matrix. Now you can select either POP QPD pit, POP QPD yaw, POP221 or POP110I to go to either PRCL yaw, PRCL pit, CARM yaw or CARM pit.
Compiled, installed and restarted the ASS model.
Engaging the servo:
I took reference spectra of POP QPD yaw and POP 22, before any control was applied. The shapes looked quite similar, but the overall level of POP22 was smaller by a factor of ~200. I also took a reference spectra of the POP QPD in-loop signal using the old ASC loop situation.
Q looked at Foton for me, and said that with the boost on, the UGF needed to be around 9 or 10 Hz, which ended up meaning a servo gain of +2.5 (the old POP QPD yaw gain was -0.063). We determined that we didn't know why there was a high-Q 50Hz notch in the servo, and why there is not a high frequency rolloff, so right now the servo only uses FM1 (0:2000), FM6 (boost at 1Hz and 3Hz) and FM7 (BLP40).
The in-loop residual isn't quite as good with POP22 as for the QPD, but it's not bad.
Here's the loop:
And here's the error spectra. Pink solid and light blue solid are the reference traces without control. Pink dashed is the QPD in-loop. Red and blue solid are the QPD and POP22 when POP22 is used as the error signal. You can definitely see that the boosts in FM6 have a region of low gain around 1.5Hz. I'm not so sure why that wasn't a problem with the QPD, but we should consider making it a total 1-3Hz bandpass rather than a series of low-Q bumps. Also, even though the POP22 UGF was set to 9 Hz, we're not seeing any suppression above about 4Hz, and in fact we're injecting a bit of noise between 4-20Hz, which needs to be fixed still.
With the re-do of the IFO alignment last week, I think that the beam was no longer about halfway on the POP22 razor blade. To fix this, I locked the PRMI on sideband, removed the razor blade, and then put it back in such that it occluded about half of the light.
I'm not entirely sure why, but when I put the razor in, POP22 went from 104(ish) to 45(ish) but POPDC went from 5200(ish) to 1600(ish). [The 'ish'es are because the PRC wasn't angularly stabilized, so there was some motion changing the power levels that leaked out to the POP port]. The ETMs were misaligned, so this should not be a carrier vs. sideband effect, since they'll both share the cavity axis defined by the ITMs and the PRM. It is possible, although I didn't check, that there is some oplev light scattered into the POP photodiode that is now blocked by the razor blade. This light would only be at DC and not the 2f frequencies. Since the signal levels for POP22 vs. POPDC didn't change with and without the table top on (and with and without room lights on), I don't think that it is an effect of ambient light getting into the diode. To check if it is oplev light I should (a) just look, and (b) try to lock the PRMI without the ITMX oplev laser being on to see if there is a difference in the POPDC signal.
Anyhow, under the assumption that the POP22 signal level is correct, I tuned up the PRCL ASC a little bit. These changes are now in the carm_cm_up script, and the carm_cm_down script resets things. Before the PRC is locked, I have FM1 and FM7 (the basic servo shape and a 40Hz lowpass) on, the gain set to zero, and the input off. After lock is acquired, the input is turned on, and the gain ramps from 0 -> 10 in 3 seconds. Then FM2 and FM6 (boosts at 1 and 3Hz) are engaged.
In the plot below, the dark blue and red curves were taken when there was no angular control on the PRC. Pink was taken last week with the old QPD yaw ASC on. Light blue is today's version of the in-loop performance of the POP22 yaw ASC loop. I didn't save the trace unfortunately, but the DC QPD saw out-of-loop improvement between about 0.8Hz - 4 Hz.
Also, has anything happened with the LSC rack in the last few weeks that might be causing lots of 60Hz noise? I saw these large lines last week, but I don't think I remember them from the past.
After I got the PRCL ASC working, I tried several iterations of locking. ETMX is still being annoying, although the last hour or so have been okay. CARM keeps getting rung up right around the transition to the sqrtInv error signal. Since CARM and DARM are kind of entangled, it took me a few iterations to figure out that it was CARM that is ringing up, and not DARM. I'm a little worried about the phase loss from the 1kHz lowpass that we turn on just before the transition to sqrtInv. I want to keep the lowpass off until after we have transitioned DARM also over to DC transmission. I tried once, but I lost lock before starting the CARM transition. Anyhow, the ETM alignment issue is annoying.
Also, Jamie, Q, Diego and I were discussing last Friday, but none of us elogged, that we think there might be something wrong with one of the Martian network switches. I'll start a separate thread about that right now, but it slows things down when you can't trust EPICS channels to be current, and I (without evidence) am a little worried that this might also affect the fast signals.