We are currently using the SUS wiring diagram found on Ben Abbott's page (link here) to determine the ADC/DAC/BO channel numbers for each individual optics inputs and outputs. Basically it involves tracking the paths back from the Pentek's, XY220, and IC110Bs to a point where we can identify it as a Coil UL or a PD whitening filter control or whatever it might be.
Once done we will have a nice wiki page describing what the final wiring is going to be, along with which ADC effectively plugs into which analog board and so forth.
The mode cleaner FF static filtering is by no means done. More work has to be done in order to succefuly implement it, by the means of fine tuning the IIR fit and finding better MISO Wiener filters.
I have begun to look at implementing FF to the YARM cavity for several reasons.
1) Even if the mode cleaner FF is set up as best as we can, there will still be seismic noise coupling into the arm cavities.
2) YARM is in the way of the beam path. When locking the IFO, one locks YARM first, then XARM. This means that it makes sense to look at YARM FF first rather than XARM.
3) XARM FF can't be done now since GUR2 is sketchy.
I'm planning on using this eLOG entry to document my Journey and Adventures (Chapter 2: YARM) to the OPTIMAL land of zero-seismic-noise (ZSN) at the 40m telescope.
donatella was one of our last workstations running ubuntu12. we installed SL7 on there today
Then there are ~20 errors related to gds-crtools that look like this:Error: Package: gds-crtools-2.18.12-1.el7.x86_64 (lscsoft-production) Requires: libMatrix.so.6.14()(64bit)
Batteries + power cables replaced, and computers back on UPS from today ~3pm.
The UPS is now incessantly beeping. I cannot handle this constant sound so I shut down all the control room workstations and moved the power strip hosting the 4 CPUs to a wall socket for tonight. Chub and I will replace the UPS batteries tomorrow.
We got a new computer from Xi computer corp. I am currently installing Ubuntu 10.04 LTS on to it to start with and then will move on to 12 if we can figure out a way to test it besides "I guess it should work?"
Rosalba has been removed and put onto the old Jamie desk. Old Jamie desk also has a Mac Mini running on there.
At the meeting tomorrow we need to decide on a new Italian baby girl name for this new machine.
I've finished setting up the fstab on Chiara and the upgrade to Ubuntu 12 seems to have gone well enough. She's fast:
but I forgot to make sure to order a dual head graphics card for it. So we'll order some dual DVI gaming card that the company recommends. Until then, its only one monitor.
Still, its ready for testing control room tools on. If everything works OK for a couple weeks, we can go to 12 on all the other ones.
1)The PDFR scripts have all been migrated into /scripts/PDFR/
2) The MEDM screen to run PDFR is /medm/MISC/PDFR.adl
3) A new button has been added on sitemap to open the above medm window.
4) All data and plots generated will sit in /scripts/PDFR/"PD Name"/
5) All features are working after the migration and absolute file paths are being used.
Work Remaining : Manual for others to make changes and keep using my system.
We dither locked the X arm and then aligned the green beam to it using the PZTs. Everything looks ready for us to do a mode scan tomorrow.
We got buildup for Red and Green, but saw no beat in the control room. Quick glance at the PSL seems OK, but needs more investigation. We did not try moving around the X-NPRO temperature.
Tomorrow: get the beat, scan the PhaseTracker, and get data using pyNDS.
Den and I spent some time with the interferometer last night with hopes of bringing in the AO path, but were stymied by the (re)occurrence of the anomalously high low frequency motion of the Xarm, as seen by fluctuations of TRX from .9 to .2 while "held" on resonance.
Jenne reported that they weren't seeing it earlier in the evening, and then it started again when I showed up. Holding the arms on IR, we could see a fair amount of excess low frequency noise in the BEATX_FINE_PHASE_OUT_HZ channel, as compared to BEATY, bringing its RMS to 5 times that of the Y arm. From the shape of the excess noise (broad slope from DC to tens of Hz), Rana suspected air currents and/or scattering effects being the culprit.
Den poked around a bit on the PSL table, which didn't really change much. He then went down to the X end table to inspect the table, and while he was there, I noticed the noise go down to being in line with the Yarm. I joined him at the end, and we found the beat phase noise in the frequency region of concern to be hugely sensitive to tapping on the enclosure, air current, etc. There is also a ton of green light everywhere, and multiple spots of green light around the green refl PD.
At that point however, the quiescent noise was acceptable (TRX fluctuations of <.2), so we went back to the control room to try to lock. Unfortunately, after a few attempts, the noise was back. At this point, we went home. The layout of the end table likely needs some attention to try and minimize our susceptibility to excess scatter effects.
Turn off the AC and flow bench please.
Leaving LSC mode OFF for now while CDS is still under investigation
Not really related to this work: We saw that the safe.snap file for c1oaf seems to have gotten overwritten at some point. I restored the EPICS values from a known good time, and over-wrote the safe.snap file.
My motivation tonight was to get an up-to-date spectrum of a calibrated measurement of the out-of-loop displacement of an arm locked on ALS (using the PDH signal as the out-of-loop sensor) to compare the performance of ALS control noise with the Izumi et al green locking paper.
I was able to fish out the PSD from the paper from the 40m svn, but the comparison as plotted looks kind of fishy. I don't see why the noise from 10-60Hz should be so different/worse. We updated the POX counts to meters conversion by looking at the Hz-calibrated ALSX signal and a ~800Hz line injected on ETMX.
It seems that just repeating the measurement was enough to get a good transfer function of the x arm cavity. Here's what I got.
I'm going to fit the data on matlab, but at first sight, the pole seems to be at about 1.7KHz (that is where the phase is 45deg): as expected.
Probably it was useful to realign the beam on the Transmission PD. (btw, I'm using the PDA255 that was still on the X end table since the AbsL experiemtn that measured the arm length)
I measured the transfer function between MC_TRANS and TRX and I'm attaching the result.
That looks quite strange. Something's wrong. I'll repeat it tomorrow.
for the night I'm putting everything back. I'm also reconnecting the OMC_ISS_EXC and opening again the test switch on the ISS screen.
The RFAM monitor remains disable
I would have guessed that you have to calibrate the detectors relative to each other before trying this. Its also going to be tricky if you use 2 different kinds of ADC for this (c.f. today's delay discussion in the group meeting).
I think Osamu used to look at fast transmission signals by making sure the PD at the end had a 50 Ohm output impedance and just drive the 40m long cable and terminate the receiving end with 50 Ohms. Then both PDs go into the SR785.
On Rana's suggestion I measured the trasfer function between the two photodiodes PDA255 that I'm using.
I took the one that I had on the end table and put it on the PSL table. I split the MC transmitted beam with a 50% beam splitter and sent the beams on the two diodes. (Rana's idea of picking off the beam and interposing the PDs before the ISS PDs was not doable: ISS PDs would be too close and there would be no room to install the PDA255 before them). See picture with the final setup.
The transfer function also includes the 40m long cable that I was using for the Arm Cavity measurement.
Here's what I got. It looks rather flat. Yesterday the calibration was probably not the problem in that measurement.
I'm now going to install the PD back on the end table and measure the TFs between the excitation and several points of the loop.
(Trivia. At first, the PDs were saturating so Koji attached attenuation filters on to them. Suddenly the measurement got much nicer)
Rana suspects that the lack of X beatnote is related to the PSL laser temperature change (ELOG 11294).
I used the information on the wiki and old elogs (wiki-40m, ELOG 6732), to deduce that the new end laser temperatures should be:
I went out to the X end and found the laser crystal temperature set to 40.87, which is not what the measurements I linked to suggest would be the ideal temperature for the previous NPRO laser temperature of 30.89, which would be 37.02. I could not find any elog describing the choice of this setpoint.
I've changed the X end laser crystal temperature to the value above. I've hooked up the X IR and green beatnotes to go the control room analzyer, and have been looking for the beatnote as I adjust the digital temperature offset, but haven't found it yet...
If this proves totally fruitless, we can just put the lasers back to their original temperatures, since it's unclear if it helped the PC drive noise levels.
Meh. I've searched in steps of 20 counts in C1:GCX-SLOW_SERVO2_OFFSET units (16 bit +\- 10V DAC, and 1GHz/V coeffecient for the Xgreen aux laser means this is ~0.6MHz per 20 count step). I went from -400cts to +800 cts and haven't found the beatnote yet. Meh.
Both PSL green and Xgreen beams are going to the Xgreen BBPD. Both beams are easily visible, so while I didn't actually measure the power, it should be sufficient. The arm is being re-locked in green for each step, but it's not locked in IR, but that doesn't matter for just finding the beatnote.
I've got the output of the BBPD directly connected to the 50 ohm input of the HP8591E spectrum analyzer, with the freq span from 10MHz to 120MHz. The BBPD is supposed to be good up to ~100MHz, so I should catch any beatnote that's there. I have to head out, so I guess I'll continue the search tomorrow.
One of Kiwamu's suggestions was that, since no one is using the Ygreen concurrent with my fiddling, I rotate the waveplate after the PSL doubling oven so that max power goes to the Xgreen path, thus giving myself a bigger signal. I'll try that tomorrow. Today, I didn't ever touch the waveplate.
There are no more double sided tape on this table.
Image #1: No - we do not use magnetic mounts for beam dumps. Use a real clamp. It has to be rigid. "its not going anywhere" is a nonsense statement; this is about vibration amplitude of nanometers.
Image #2: No - we do not use sticky tape to put black glass beam dumps in place ever, anywhere. Rigid dumps only.
Image #3: Please do not ruin our nice black glass with double sticky tape. We want to keep the surfaces clean. This one and a few of the other Mickey Mouse black glass dumps on this table were dirty with fingerprints and so very useless.
Image #4: This one was worst of all: a piece of black glass was sticky taped to the wall. Shameful.
Please do not do any work on this table without elogging. Please never again do any of these type of beam dumping - they are all illegal. Better to not dump beams than to do this kind of thing.
All dumps have to be rigidly mounted. There is no finger contacting black glass or razor dumps - if you do, you might as well throw it in the garbage.
I walked down to the X end and found that the entire AUX laser electronics rack isn't getting any power. There was no elog about this.
I couldn't find any free points in the power strip where I think all this stuff was plugged in so I'm going to hold off on resurrecting this until tomorrow when I'll work with Steve.
The X arm green does not stay locked to the cavity - the alignment looks fine, and the green flashes are strong, but the lock does not hold. This shouldn't be directly connected to anything we did today since the Green PDH servo is entirely analog.
I'm going to guess that this was me: I was disconnecting some octopus power strip nonsense down there (in particular illuminators and cameras), so I might have turned off the AUX rack by mistake.
On Friday, while Udit and I were doing some characterization of the EX+PSL IR beat at the LSC rack, I noticed that there were sidebands around the main beat peak at 20dBm lower level. These were offset from the main peak by ~200kHz - I didn't do a careful characterization but because of the symmetric nature of these sidebands and the fact that they appeared with the same offset from the main peak for various values of the central beat frequency, I hypothesize that these are from the modulation sidebands we use for PDH locking the EX laser to the arm cavity. So we can estimate the modulation depth from the relative powers of the main beat peak and the ~200kHz offset sidebands.
Since the IR light is used for the beat and we directly couple it to the fiber to make the beat, there is no green or IR cavity pole involved here. 20dBm in power means . And so the modulation depth, . I will do a more careful meaurement of this, but this method of measuring the modulation depth can give us a precise estimate - for what it's worth, this number is in the same ballpark as the measurement I quote in elog12105.
What is the implication of having these sidebands on our ALS noise? I need to think about this, effectively the phase noise of the SR function generators we use to do the phase modulation of the EX laser is getting imprinted on the ALS noise? Is this hurting us in any frequency range that matters?
Something funky is happening with the green light locked to the X arm. The green transmitted power is drifiting around. Maybe something weird is happening with the doubler? The digital thermal feedback loop is not on.
The green has been locked on a TM00 mode this whole time. The step in power is me closing the PSL green shutter, but I'm not doing anything during the smooth changes in power. IR power is steady, so the alignment should be ok. I can't recover full power with the end PZT alignement either.
Have you tried a different set of laser temperatures? I don't remember the value for the Xgreen, but whatever the value that matches PSL of 0.62ish and above seems to put the Xgreen laser at a bad temperature. I think this is the mode-hopping region, and we sometimes lock to the wrong mode.
So, FSS values of above 0.5ish are good, but they should be below 0.61ish.
Have you tried a different set of laser temperatures?
Yep, that is how I got back to stable powers.
The X-end IR Trans path was cleaned up.
I have been investigating the Xarm ASS issue. The Xarm ASS sensors behaved not so straight forward.
I went to the X-end table and found some suspect of clipping and large misalignmnet in the IR trans path.
Facing with the usual chaos of the end table, I decided to clean-up the IR trans path.
The optical layout is now slightly better. But the table is, in general, still dirty with bunch of stray optics,
loose cables and fibers. We need more effort to make the table maintained in a professional manner.
- Removed unnecessary snaking optical path. Now the beam from the 1064/532 separator is divided by a 50-50 BS before the QPD without
any other steering mirrors. This means the spot size on the QPD was changed as well as the alignment. The spot on the QPD was aligned
with the arm aligned with the current (=not modified) ASS. This should be the right procedure as the spot must be centered on the end mirror
with the current ASS.
- After the 50-50 BS there is an HR steering mirror for the Thorlab PD.
- A VIS rejection filter was placed before the 50-50 BS. The reflection from the filter is blocked with a razor blade dump.
Important note to everyone including Steve:
The transmission of the VIS rejection filter at 1064nm is SUPER angular sensitive.
A slight tilt causes significant reduction of 1064nm light. Be careful.
- As we don't need double VIS filter, I removed the filter on the QPD.
- X-End QPD was inspected. There seemed large (+/-10%) gain difference between the segments.
They were corrected so that the values are matched when the beam is only on one segment.
The corrections were applied at C1:SUS-ETMX_QPDx_GAIN (x=1, 2, 3, or 4).
I decided to put "-20dB" filters on C1:SUS-ETMi_QPD_SUM and C1:SUS-ETMi_TRY (i = X or Y)
in order to make their gain to be reasonable (like 0.123 instead 0.000123 which is unreadable).
Jenne's normalization script reads relative values and the current gains instead of the absolute values.
Therefore the script is not affected.
We cleared up some optics and optomechanics at the X end table that are not being used and moved them to the SP table. [Ed by KA: They seemed to be leftover of the other projects. I blame them]
From fitting the arm cavity transfer functions I got the following values for the cavity pole frequencies.
X ARM: fp_x = (1720 +/- 70) Hz
Y ARM: fp_y = (1650 +/- 70) Hz
Attached are the plots from the fitting.
The current status of the dither alignment system:
- Both Xarm and Yarm alignment are working. The scripts are: scripts/autoDither/alignX(Y). Each script sets up the respective arm, turns on the dither lines and servos for 66 sec, offloads the control signals to TM alignment biases and PZT sliders in case of Yarm, and to TM and BS alignment biases in case of Xarm, and finally turns off and clears the servo filters and turns off the dither lines.
- Jammie witnessed the final tests of both scripts - both X and Y arm power went up from 0.6-0.7 to close to 1 and the AS beam became symmetric. Also Jammie wanted me to leave the ETMY oplev in its current non-nominal but more stable state i.e. the oplev signals go to the ADC from the D010033 card not the D020432 one. The scripts can now run from the CONFIGURE medm screen.
- Both arms use signals derived from modulating ITM and ETM in pitch and yaw dofs and demodulating the arm power (TRX or TRY) and the cavity length signal (AS55I). The Yarm actuation has 8 dofs - pitch and yaw of the ITM, ETM, and two input beam PZTs so all the sensed dofs are controlled. The Xarm actuation has only 6 dofs - pitch and yaw of the ITM, ETM, and BS. The Xarm servo is set up to servo the beam position on the ETMX and the relative alignment of the cavity and the input beam. The ITMX spot position is unconstrained and provides the null test. The residual displacement on the ITMX is 0.2-0.3 mm in yaw and 0.9-1.0 mm in pitch. The I phases of the beam centering lockins, which are also the error points of corresponding DOF filters, are calibrated in mm by unbalancing the TM coils by known amount. The attached snap shot of the medm screen now has both X and Y arm calibrated beam spot positions and uncalibrated input beam indicators. The input beam angle and position signals can/should be calibrated by tapping the signals digitally and applying the proper matrix transformation - this will require the model change.
- Currently there is no lock loss catching in the model. We should add a trigger on arm power (or an equivalent mechanism) to turn off the inputs to prevent the spurious inputs.
Y arm green: Nothing much was disturbed. I touched the steering mirrors and brought GTRY from 0.2 to 0.9.
X arm green: The PDH lock was not very stable mostly because of the low power in green. I changed the oven temperature for the doubler to 36.4 corresponding to maximum green power. GTRX increased from 0.1 to 0.9
Both the X and Y arm green alignment were tuned on the PSL table to their respective beat PDs.
The PSL green shutter was not responding to the medm buttons. I found the PSL green shutter set to 'local' and 'N.O' (these are switches in the shutter controller). I do not see any elog and not sure as to why the controller was even touched in the first place. I set the shutter controls to 'remote' and 'N.C'.
ETMX green power at shutter 3.6 mW at 36.35 C doubler crystal temp. [ Innolight IR settings 2.0 A, 40.83 C, 500 mW before Faraday 1/2 plate ]
ETMY green power at shutter 0.75 mW at 35.8 C doubler crytal temp. [ NPRO IR settings 1.82A, 231 mW_ display, DT 21 C, DTEC +1V, LT 40 C, LTEC 0.1V, T +41.041 ]
We wanted to lock both the arms using ALS and get IR to resonate while arms are held using ALS. The X arm was locked using ALS and offsetter2 was used to scan the arm and find IR resonance. The Y arm was locked using ALS. But as the Y arm was brought closer to IR resonance, the X arm ALS loses lock. (attachment 1)
We believe that this comes from the X and Y transmission not being well separated at the PSL table. The PBS is not sufficient to decouple them (A strong beatnote ~35dB between the X and the Y arm green lasers can be seen on the spectrum analyzer).
Decouple the X and Y arm transmitted beams at the PSL table. I am trying to find a wedged mirror/window that can separate the 2 beams at the PSL table before the beat PD (sadly the laseroptik HR532nm optics have no wedge)
The X arm air conditioner was not regulating properly. The arm temp was warmer than usual. I requested thermistor calibration.
The mechanic reset the thermostate to 68F last week. It was 70-71F before.
The ETMX oplev laser now running 4 C lower at 30 C inside the enclousure.
The ETMX optical table top is 5 C cooler at 21 C
The ETMX concrete wall temp 20 C at 9am with flow bench on.
ETMY conrete wall temp 23 C at 9am
Stabilized ALS and beat frequency sweep realized.
1. Enable appropriate filter modules and set appropriate servo gains.
2. Clear history of C1:ALS-BEATX_FINE_PHASE
3. Enable the servo loop. I had set limits on the servo loop and ramp time for gain switching so that I don't kick the ETMY hard.
Gains were decided such that the error signal C1:ALS-BEATX_FINE_PHASE_OUT was minimized.
4. Beat frequency sweep is realized by stepping up on C1:ALS-BEATX_FINE_OFFSET_OFFSET (from 0 to 2100 in this case).
Video1 shows the difference that can be seen at the RF spectrum analyzer when ALS is enabled.
Video2 shows the beat frequency sweep as seen on the spectrum analyzer.
I could not get 'getdata' to work as I wanted. So I have attached the error signal trend before and after the ALS servo loop is enabled.
Thank you Jenne for helping retrieve more sensible data!
The beat note is very strong and we can clearly see its harmonics as well. Attached is the picture showing the several harmonics.
Peak frequency(MHz) Power(dBm)
1. Obtain IR resonance.
2. Check the digital anti-whitening filter after the beatbox.
3. The effect of the harmonics should be figured out.
4. Write scripts to enable ALS and findIRresonance.
It's nice that we are now able to scan the cavity again. We got close to PRMI+one arm one step further.
The calibration of the scan frequency and the evaluation of the in-loop/out-of-loop error signal in terms of (Hz/rtHz) would be necessary.
The beat amplitude looks actually huge aIthough I don't know where you are monitoring.
Talk to Jamie to figure out how much the signal should be at the monitoring point.
If it is more than we are supposed to have, put an attenuator somewhere.
I ran a series of diagnostics on the X arm ALS to look at how the beatbox behaves after the makeover.
Diagnostic tests run:
1. X arm ALS in-loop spectrum
2. X arm ALS out-of loop spectrum
3. X ALS scan of the X arm cavity
The noise suppression looks better after the makeover at the lower frequencies. To suppress the noise at high frequencies, we would have to add more whitening filters.
After I effected the series resistance change for ETMX, the X arm ASS didn't work (i.e. IR transmission would degrade if the servo was run). Today, we succeeded in recovering a functional ASS servo .
So both arms have working dither alignment servos now. But remember that the Y arm ASS gains have been set for locking the Y arm with MC2 as the actuator, not ETMY.
We then tried to maximize GTRX using the PZT mirrors, but were only successful in reaching a maximum of 0.41. The value I remember from before the vent was 0.5, and indeed, with the IR alignment not quite optimized before we began this work, I saw GTRX of 0.48. But the IR dither servo signals indicate that the cavity axis may have shifted (spot position on the ITM, which is uncontrolled, seems to have drifred significantly, the Pitch signal doesn't stay on the StripTool scale anymore). So we may have to double check that the transmitted beam isn't falling off the GTRX DC PD.
While working on the single arm alignment, I noticed that today, i was able to get the X arm transmission back to ~1.22, and the GTRX to 0.52. These are closer to the values I remember from prior to the vent. Running the dither alignment promptly degrades both the green and IR transmissions. Since the pianosa SL7 upgrade, I can't use the sensoray to capture images, but to me, the spot looks a little off-center in Yaw on ETMX in this configuration, I've tried to show this in the phone grab (Atm #2). Maybe indicative of clipping somewhere upstream of ITMX?
Anyways, I'm pushing onwards for now, something to check out in the daytime.
C1:SUS-ETMX_QPD is removed and internal SM1 thread adapter epoxied into position as it is at the Y end
This adapter will take FL1064-10 line filter holder
Line filter is attached and qpd needs alignment.
The X arm was also aligned for the IR by hand and ASS. Also the X end green PZT was aligned to make the TEM00 mode reasonably locked.
What I did:
- Looked at the ITMXF camera. It seemed that the green beam was hitting the mirror.
- Went to the end. Looked at the X end green REFL. Tuned coarse alignment of the ETMX so that the beam was (retro-)reflected to the Faraday and the REFL PD.
- Looked at the ETMX face from the view port. Tried to locate the spot from the ITMX by shaking the ITMX alignment with 0.1 and then 0.01 increments.
- After some struggle with the ETMX and ITMX alignment, resonant fringes were found on the ETMY face while I still looked at the ETMX.
- Once the ITMX/ETMX were aligned, the BS needed to be aligned. But of course there was no IR fringe.
- Returned to the original alignment of the ITMX to find the ITMX spot on the AS camera.
Then gradually moved the ITMX to the aligned value for the green while tracking the michelson alignment with the BS.
This made the AS spots at the upper left edge of the AS video image.
- This was enough to find the IR spikes at TRX. Then the ETMX was touched to maximize the transmission.
- Lock the cavity. Use the ASS to optimize the alignement.
- Once the arm mirrors were aligned, the Xend PZT was also adjusted to have TEM00 for the green beam.
Now I leave the IFO with ITMX/Y, ETMX/Y and BS aligned. As I wrote above, the AS spot is very high at the AS camera.
We need to revisit the AS steering (SR TTs?) to ensure the AS beam unclipped.
I aligned X arm so that the beam spot comes roughly on the center.
1. Use ITMX and ETMX (mainly ITMX) to make beam spot come on center of the optic using eyeball.
2. Use ETMX and BS to maximize TRX power (reached ~ 0.85)
3. Aligned green optics on X end. Transmission of X green measured at PSL table is now 255 uW and TEM00 has the most power.
It was not easy to increase X green transmission more because beam spot on green transmission PD is wiggly when X end table is opened. When closed, wiggliness is about the same for Y green and X green.
Turning off HEPA on the X end didin't helped, but there must be something bad in the X end table. If we couldn't figure out why, let's wait for PZTs to come for end tables.
Considering the laser power is different(X end 1 W, Y end 700 mW), X green transmission should reach ~400 uW. But I think we should go on to X beat search.
I placed green shutter for X end back for convenience. I put some spacers to adjust its height and avoid beam clipping.
What causing wiggly X green transmission was the air flow from the air conditioner. When we turned it off, beam spot motion became quiet. Air flow from HEPA was not effecting much.
The amounts of the X arm's beam off-centering have been measured by the A2L technique.
So now we are able to start aligning the IR beam axis in a quantitative way.
Since we saw big residual motions at 1 Hz, 16 Hz on both the green beat note signal and the IR PDH signal (see #4268 and #4211),
we are suspecting that these noise come from an angle to length coupling.
In order to minimize the angle to length coupling, one thing we can do is to bring the beam spots to the center of ITMX and ETMX more precisely.
To do it, we have to quantitatively know how well the beam spots are on the center of the optics. Therefore I started measuring the amount of the beam off-centering.
The A2L technique was used to measure the off-centering with the real-time lockin system, which has been recently embedded in the real-time code by Joe (see #4265).
The idea is the same as Yuta did before (see #3863).
But this time the excitation signal from the real-time oscillator was injected directly to the coil matrix on either ITMX or ETMX, at 18.13 Hz with the amplitude of about 400 cnt.
When the IR laser stays locked to the X arm, the LSC feedback signal is demodulated with the oscillator signal.
This demodulated signal gives the amount of the off-centering.
For this purpose I modified Yuta's A2L script such that we can use it also for the X arm.
I obtained the following values:
PIT = -1.61 mm
YAW = -0.918 mm
PIT = -3.76 mm
YAW = -2.24 mm
I used the same calibration factor as that of Koji calculated (see #3020) for MC, in order to convert the results from the coil gain to the off-centering.
These values are consistent with the spots appearing on the CCD monitors.
I found the beat note for X arm. I did not change anything this morning (to the best of my knowledge). Hooking up the spectrum analyzer, I could find the beatnote signal at the PD RF output, after the amplifier and also at the MON port of the beatbox. I still don't know what changed from the last night set of trials
Manasa has done some work to get the Xgreen aligned, so I'll switch to trying to find that beatnote for now.
Aligned X-arm green in TEM00.
It was difficult to get the X-arm to lock in TEM00 earlier. Even when it locked, it was just a TEM00 flash. The green was mainly bad in pitch.
I started aligning with the arms flashing in IR and it was still not possible to lock. The second trial was with the arms locked in IR; I lost the green lock when the arms were aligned for IR. I aligned by overlapping the ingoing green with the reflected green visible on the steering mirror. This got the green to lock in TEM00; but still it would only stay that way for 30 sec. Jenne pointed me to Yuta's suggestion of increasing the green refl PD gain. Once I increased the PD gain, the mode stabilized.
I went ahead and centered ALS TRX on the PD and GTRX camera (on the PSL table). ALS_TRX reads 2000+ counts.
EDIT by Jenne: This corresponds to a power of 550uW on the PSL table, measured before the first out-of-vac steering mirror.