15 hours

[Rana, Jenne]

We have looked at a few things that do and don't affect the out of loop noise of the ALS X beat, and found that cavity alignment and beatnote RF frequency had the strongest effects.

Possible causes of noise:

1.  Air currents from A/C or flowbench.  No effect. 

        * When table lid is on, turning on and off the flow bench air did not qualitatively change the out of loop beatnote time series signal.

2.  Scattered light from other beams hitting green PDH PD.  No effect.

        * There are a few spots of green light that are hitting the case of the PDH photodiode, but when I put an iris in place to block those spots, there was no change in the beatnote spectra.  This makes sense to me since none of those spots were close to hitting the diode itself. 

        * Rana did notice that the beam was not well centered on the PD, so he steered the beam onto the center of the diode.  Also, the PD is now tilted a little bit so that the reflection from the diode doesn't go back into the beam path.  Neither of these things had an effect that we noticed in the beatnote noise.

3.  Oplev laser light getting to PDH PD.  Not tested.

       * We don't see any red light over by the PDH PD, so we did not try turning off the oplev's laser to see if that had an effect, but we suspect that it is not the cause of our noise.

4.  Clipping of main IR / green beam on Xend table.  Not tested.

      * We should still go have a look at this, but we no longer think that this is the main cause of the elevated noise.

5.  Scattered light all over Xend table.  Not tested.

     * We should still work on dumping extraneous beams on the table, but we do not think that this is the main cause of the elevated noise.

     * Rana took some photos so that we can see how truely bad the situation is.

6.  Amplitude modulation dip in NPRO.  Not tested.

    * It is probably still a good idea to check this, in case the dip in the amplitude modulation has changed over the year or two since it was last measured, but we also don't think that this is the main problem.

7.  Check PDH servo.  Not done.

     * I think this is still on Q's long-term todo list, but we should give the PDH servos a once-over.

8.  Arm cavity longitudinal motion.  No effect.

     * While the Xarm was locked with IR, we put a line at 1.7 Hz with 325 counts into the ITMX position.  To keep lock, the ETM had to move as well.  When we turned on this line (and increased its amplitude up to the final value of 325 cts), we did not see any qualitative change in the beatnote time series noise.

9.  Arm cavity alignment.  Significant DC effect.

    * When the alignment of one of the arm cavity mirrors is changed, the DC value of the beatnote signal changes. 

           * ITMX moved in yaw, we see a 7kHz/15urad DC shift in the BEATX_FINE_PHASE_OUT_HZ time series.

          * ETMX moved in yaw, we see an 8kHz/5.5urad DC shift in the time series.  We aren't sure why this is about a factor of 3 times larger effect (same shift for smaller misalignment) than the ITM.

    * We want to do a Yuta-style analysis to see what the angle to length coupling looks like, so that we can measure the angular motion of our cavity mirrors and put the expected noise into our ALS noise budget.  Perhaps this will help us understand the low frequency difference between our in-loop beatnote error signal and our in-loop PDH error signals (red vs. maroon on the ALS noise budget posted above Pianosa). 

    * I've asked Manasa to take some transfer functions in the morning, so that we can start to have an idea of what is going on with this.

10.  Beatnote RF frequency.  Significant broadband effect.

     * We have found that when the Xarm beatnote is at low RF frequencies, the noise is high, and when the beatnote is at high RF frequencies, the noise is low! 

     * Low RF freqs are below about 40 MHz, while high RF freqs are above about 90 MHz.  This has not been tested for the Yarm.  Also, these are for the case of "temp slider up, beatnote up".  I have not checked if the same is true for the other side of the PSL frequency, although I don't have reason to believe that it would be.

     * Maybe we are saturating some amplifiers?  We need to check this out.  One thought that Den mentioned was the harmonics, and that perhaps they are causing trouble in the electronics.

     * Den is going to think about implementing a frequency divider so that we can directly digitize the beatnote signal. 

    * Here are spectra for different cases:

        * And here is a spectrogram showing us going back and forth between the high and low noise states:

                     *  A:  First noticing that noise is good when RF frequency is high.

                     * B:  Not locked on TEM00 mode, so extra noisy.  Disregard.

                     * C:  Bad noise time.  Xbeat was 21 MHz (dark purple on DTT spectrum above), Ybeat was 118 MHz (sea green on DTT spectrum above).

                    * D:  Good noise time. Xbeat was 89 MHz (light purple on DTT plot), Ybeat was still 118MHz (turquoise on DTT plot).

                     * E:  Bad noise time.  Xbeat was 37.5 MHz, Ybeat was still 118 MHz.

                     * F:  Good noise time.  Xbeat was 113 MHz, Ybeat was still 118 MHz.

So, I really should have done this as soon as Manasa measured the arm lengths... I've updated my MIST model with the real arm lengths, but still am using assumed identical losses of 75ppm on each mirror. (I've tried measuring the arm losses for real, but got numbers in the hundreds of ppms, so I need to reexamine things...) 

Here's a simulation of the fields in a perfectly locked PRC when CARM is swept (Normalized to input power = 1). 

More importantly, here's the latest simulation of MICH vs. PRCL demodulation angle separation in the 3F signals. It seems that we may be getting burned by using REFL33 for the PRC lock. REFL165, on the other hand looks much more robust. We should try this out. 

(Some of my previous simulations incorrectly implemented MICH excitations; I only moved the ITMS, not the ETMS along with them, so some other stuff slipped in... )

 Here's the angles of MICH and PRCL from the my earlier plot by themselves; this shows that the individual demod angles in REFL165 aren't changing much either. 

Since Q has found that REFL165 will be better for holding the PRMI while we reduce the CARM offset, I had a look at locking PRMI sideband locking with both 3f PDs.

I checked the REFL165 demod phase, and changed it from -142.5 deg to -138.5 deg. to minimize the Q signal while driving PRM length.

I found that keeping the MICH and PRCL loop gains the same, and using matrix elements +0.1 for both I and Q for REFL165, rather than +1 for both I and Q for REFL 33.

MICH gain is +0.8, PRCL gain is -0.02.  FMs 4,5 on for both, FM 2 triggered for MICH, FMs 2,3,6 triggered for PRCL.

I then locked the PRMI on sideband with REFL 33 and then REFL 165, and measured the other one as an out of loop sensor of the motion.  I find that REFL33 and 165 are both comparable, and so we shouldn't have any trouble using REFL165 for locking.

[Rana, Jenne]

We spent some time tonight looking at locking the PRMI with REFL165 vs. REFL33, while reducing the CARM offset. 

We were not able to lock the PRMI on REFL165 I&Q at small CARM offsets.  When locking at larger CARM offsets (about 100 counts, which is about 100nm) and then re-adjusting the REFL165 demod phase as I reduced the CARM offset, I saw that I had to significantly rotate the phase.  For PRMI only (no arms), the REFL165 demod phase was -138.5 deg.  When the PRMI was locked with a -100 count CARM offset, the optimal demod phase was -123 deg.  Then at -90 counts the phase was -113 deg.  At -70 counts, the phase was -108 deg, at -50 counts it was -98 deg, and at -40 it was -93 deg.  We want to go back and look at these more carefully, and in a more continuous way, by watching the sensing matrix calibration lines.  It's unclear to me right now why we're seeing this, but it's possible that we're getting some kind of extra 55MHz resonances.

REFL DC looks like it should be good - same slope and gain as sqrtTR, extra 20 or 30 deg of phase margin, so we think that we should be able to transition over to it, and then try engaging the AO path.  Tonight we had Den's new 1kHz lowpass engaged, and with this, everything looks nice and stable.

Game plan:  Bring CARM in until transmissions are at about 10ish, then try keeping CARM on sqrtInvTrans for the DC part, and engage the AC AO part with REFL DC.  We probably just need to try this for a while more to find just the right way to turn it on.

Need to think about demod phase rotation vs CARM offset as well as extra resonances, but this may take a while, and if we can just get the AO path engaged, that would be good.

Attached is the measurement of the transfer function from ITMX oplev error in yaw to the ALSX error signal.

The arm was locked to the IR using POX and the green beat frequency (between X arm trans in green and PSL green) in this case was 27MHz.

The transfer function looks mostly flat between 1Hz - 30Hz at 700-800 Hz/urad. The DC shift that Jenne measured from the time series is ~500 Hz/urad.

So far I have not been able to measure the TF below 1Hz without the arm losing its lock. Updates will follow.

Data xml file can be found in /users/manasa/data/140521/

Below are the transfer functions measured between the angular (pit, yaw) motion of X arm mirrors and the ALSX error signal. The measurements were again made for 1Hz-30Hz.

The transfer functions are mostly flat.

ITMX P - 200-300 Hz/urad (beat freq = 45 MHz)

ITMX Y - 700-800 Hz/urad (beat freq = 27MHz)

ETMX P - 500-600 Hz/urad (beat freq = 56 MHz)

ETMX Y - 1000-2000 Hz/urad (beat freq = 62.5MHz)

Data xml files can be found in /users/manasa/data/140521/

Grr.  I am very frustrated.  After lunch I redid alignment for both X and Y green systems (Yarm both at the end and on the PSL table, Xarm just on the PSL table).  After that realignment work, I cannot find a beatnote for the Xarm!!! 

The Ybeat, after aligment, was up to -5.5 dBm when the beat was at 11 MHz. Last week it was something like -20 dBm, so alignment makes a big difference.  After doing IR alignment I had noticed that the green transmitted through the Yarm didn't look very bright on the camera, and the power was around 0.2, so I went to the Yend and gently touched the green input steering mirrors, and got the Ygreen trans PD back to more than 0.9 with the PSL green shutter closed.  Awesome.  Then I touched up the Ygreen PSL alignment, and then saw that the beatnote was nice and large.  Hooray.  I measured the out of loop noise, and it was even better than the best we saw last week:  (greenish was best last week for Yarm, teal blue is new Ygreen):

At this point, I still hadn't touched anything on the X path (except the PZT input steering mirrors, remotely from the control room).  The beatnote was about the same size as it was on Friday, around -27dBm.  I went onto the PSL table and did the same alignment procedure that I had just done for the Yarm:  Remove the green trans PD and the accompanying lens so that I get far-field spots on the wall, and then steer the PSL green and the X green spots until they are nicely overlapped at both the camera (near-field) and on the wall.  I looked at the DC output of the beat PD, and centered the beam on the diode.  I put back the thorlabs DC transmission PD and the lens, and centered the beam on that.  However, after this work, I cannot find a beatnote for the X arm!  I still see the nice big Ygreen beatnote, and I have the PSL and Xend temperatures where they usually are (  abs(FSS Slow) < 0.1, and X end Slow around 10,090. )  I scanned -10,000 counts, and +5,000 counts from there, and still don't find a beatnote!

I went back inside, and I don't see an RF signal coming into the beatbox from the Xarm.  It's not the cable's fault though, since I then hooked the RF output of the beat PD to a 'scope, and still didn't see any beatnote.  The DC path of the PD is definitely seeing things, because when I switch the 'scope over to the DC output of the Xbeat PD, and I block/unblock the beam, I see the voltage step up and down as expected. 

I have not pulled out the Xgreen broadband PD, but unless someone else has a good idea of what to check, that might be one of the next things to do. 

Ideas of things I could try:

* Put the X broadband PD on the Y beatnote path to see if I see the same Y beatnote (use the port where the Y green trans PD is, since it has the coaligned beams, and a lens).

* Open the PD and see if anything on the RF path is fried.

* Move the Y PD over to the X path, to see if it sees the beatnote.

* ????

I made my attempts trying to figure out what was wrong.

Checking if we are at the right X end laser temperature: 
I aligned the arms and found the Y beatnote.I blocked the light falling on the X beat PD so that the RF analyser was only looking at the output from the Y beat PD. AT the RF analyser, I found the strong Y-PSL beatnote, the X-Y beat note and a weak  X-PSL beatnote. This confirmed that we have the X end laser at the right temperature to be able to detect the beatnote. Unblocking the light on the X beat PD did not bring in any additional peaks. 

Checking the RF cabling from the X beat PD to the beat box:
I swapped the RF cables such that the signal from the Y beat PD output was going to the X input on the beatbox. I could still see the beatnote on the RF analyser. This confirmed that there aren't any broken RF cables along the X path.

Checking X green PSL alignment:
I replaced the X beat PD with the Y beat PD to check if the alignment of X&PSL green are alright. I could find the X beat note this way without any alignment tweaking.

I suspect we probably have some RF component burnt in the X beat PD. Do we have any spares lying around? There is a Koji's box with a PD having the same serial number.

IFO status report for anyone who is looking to do some locking tonight : 
The Y beat PD is back along the Y path and I have confirmed the presence of Y-PSL beat note after replacing the PD.
The X beat PD has been removed and now rests on the electronics bench for checking. 

While aligning the arms today, I noticed that enabling LSC would cause misalignment of the ETMY suspension. I haven't tried to find out what has been causing this. Could be something similar to what was noticed with the ETMX suspension a couple of weeks ago elog9969.  

If the PD is the suspect, just pull it from the table and bring it to the PD testing setup.

The transimpedance of the PD should be ~2000 Ohm for both of the RF and DC outputs.

The test setup gives you the systematic opportunity for examination of the signal line.
Check the signal level with the active probe.

Once the broken component is found replace it. You are supposed to have the replacement
components on the blue tower.

I'll put more detail in the morning, but I was able to get the PRM/ITMY/ETMY coupled cavities locked with 32kHz bandwidth using the AO path. (However, this is a pretty low-finesse situation, since the BS is dumping so much power out of the PRC. Full buildup is only 3 or 4 times the single arm power)

Since our ALS is better than it was a month ago when I last played with this, I was able to hop straight from ALS to REFL11 I on resonance, with the PRY locked on 3f.

Here are some quick OLTF plots I took along the way.

I'm using this configuration to validate my loop modelling for the full double arm case. Right off the bat, this tells me that the "minus" polarity on the CM servo is the correct one. I didn't use REFLDC at all tonight, but I figure I can check it out by doing the transition backwards, so to speak.

Wait. It is not so clear.

Do you mean that the IFO was locked with REFL11I for the first time?

Why is it still in the "low finesse" situation? Is it because of misalignment or the non-zero CARM offset?

Sorry, the X arm is completely misaligned. This is the configuration I first tried in ELOG 9859, that is: a PRM->ITMY recycling cavity and ITMY->ETMY arm cavity. ITMX is completely misaligned, so the BS is dumping much of the recycling cavity light out, which is why I wrote "low finesse." This is the first time I've used REFL11 to control any of our cavities, though.

I noticed today, and Rana said that he saw Saturday, that the MC refl value when the MC is unlocked is unusually high.  It typically goes to about 4.5 V, but now is going up to 6.5V.  Since the PMC output is the same as usual (max seen has been about 0.82 today), something must have happened between the PMC and the IMC. 

Late last week, EricG and Nichin were looking at things on the AS table.  Was anything touched on the AS table?  Was anything touched on the PSL table?  'Fess up please, so that we can pinpoint what the change was.

Also, this afternoon, I touched up the MC alignment a bit, although it still needs work (I've asked Manasa to look at it tomorrow).  Rana centered the WFS to my MC alignment (this will need to be redone after the MC is truely aligned), and we turned the WFS on.  I also locked both arms individually, and locked MICH and PRMI sideband.  The PRMI wasn't especially stable unless I turned on the POP ASC.  I assume (hope) that this is just because I was doing it during the day, and not because there is something actually different about the PRMI since the computer meltdown.

Rana and I also took some notes on things that need to be done, starting tomorrow (the first line and the yellow line are scribbles):

 1. Recovered MC alignment and locked it manually after the ottavia cron failed to help.

2. Measured the MC spots and could not get the MC spots better looking than this.

spot positions in mm (MC1,2,3 pit MC1,2,3 yaw):
[1.6609930611758554, -1.4655939584833202, 1.3133188677545831, -1.9483764375494121, -1.6539541350121174, -0.8862744551298497]

3. Realigned the beams to the MC WFS and enabled WFS servo.

MC Trans SUM is ~17000 counts and MC REFL is ~0.5 counts.

To-do list

MC spots

MC WFS

IOO QPDS center

BBPD char

Recover REFL 33

MC REFL

MC autolocker cron

PRMI locking with REFL33 is fine.  As it was yesterday, it's a little wobbly without the ASC (just PRM oplev), but I don't know that it's any different than it used to be.  It'll hold for long periods of time, so I feel okay about it. 

When the PRMI is locked, you can push the "up" button on the ASC screen, and it'll turn down the PRM oplev gain by a large factor, and engage the ASC.  When you lose lock, press the "down" button to undo these changes.  (Probably the ifoconfig script should include the ASC down script).  These up and down scripts for the ASC are already included in the carm_up script (the ASC up), and the watch scripts which run a down script (including ASC down) for the whole IFO when ALS loses lock.  If the ASC is engaged, I get bored of watching it before the PRMi loses lock on its own, so I think it's okay.  (Let's say that means I've watched it stay locked for at least a few tens of seconds, but it looks like it always has with the ASC - like it'll stay forever).

The only thing that seems different about the PRMI is that I've increased the PRCL gain from -0.02 to -0.04.  This is a value that it was at some weeks ago, and then we turned it down for loop osc reasons, but now it doesn't want to catch lock with the lower value, and if I turn it down after it's locked, it has trouble holding on.  I have included this change into the PRMI sideband configure script.

I haven't tried anything creative like locking with REFL 165.  I also didn't lock with 11 or 55, since 33 just worked.

 Nope, we did not touch any of the PDs other than AS55. I have mentioned in  my elog:10037 what we did exactly.

We just looked at all the other PDs to check if they were being illuminated by the correctly labeled fiber. Nothing other than that.

Arms locked in comm and diff with ALS. PRMI locked with REFL33 I&Q while arms off resonance. Having trouble reducing CARM offset, even to get to arm powers of 1.

After Manasa installed the new Xgreen PD, Koji looked at the PSL table alignment with me.  I saw only a very weak beatnote with the X BBPD, even though I could see the beatnote on the Y PD from the leakage of the X beam to the Y PD  (Yend shutter was closed, so just PSL and X greens were on the table).  I had thought that my near-field and far-field alignments were pretty good (actually, I checked them, but didn't feel that I needed to tweak them since Manasa did the alignment this afternoon).  Anyhow, it was just a matter of tweaking up the alignment a bit, and then the X beatnote got up to about -25dBm at a few tens of MHz.  I am starting to question myself if the other BBPD is broken, or if I just not well enough aligned.  Anyhow, the spare is in, we can still have a look at the previous X BBPD, but it may be okay, and it's just me embarrassing myself by not catching an alignment problem.

Anyhow, after the X beat was found, I was able to (on my first try) lock the arms using ALS comm and diff.  (I already had a nice strong Y beatnote, so that didn't need finding, other than temp adjustment of the end laser).  I ran the carm_cm_up.sh script, and it did everything nicely.  I did a quickie check of the phase tracker loop gains, but that should be redone in the morning.

PRMI was a little reluctant to lock, so I played around with the MICH and PRCL gains, but didn't really find any combination that was any better than the usual (+0.8 for MICH, -0.04 for PRCL as I had last night, although I needed to reduce the PRCL gain back to -0.02 to eliminate loop osc). 

After an arm lockloss, I relocked just the PRMI and used awggui to put a line into C1:LSC-PRM_EXC to check the RF PD phasing.  I changed REFL33 from 133.5 to 138.5, and REFL 165 from -142.5 to -152.5.  I didn't think that REFL11 needed changing, and I didn't check REFL55.  I also checked that I could lock PRMI without arms, using both REFL33 and REFL165 - they seemed about the same to me, both stable.  REFL33 has 1's in the input matrix, and I was using 0.07's for REFL165.  The demod phase adjustment didn't really improve PRMI locking while the arms were held off resonance, even if I moved the arms even farther from resonance (usually we do 3nm, I went out to 5nm to see if that helped - it didn't).  I tried REFL165 locking, but that wasn't any good either.  I tried using REFL165 with the arms held off resonance, but that didn't seem to catch at all (at least with REFL33 I was getting short lock blips). 

Anyhow, of the 3 or 4 times that I caught REFL33 PRMI lock and tried to reduce the CARM offset, only one time did I even get to arm powers of about 1 (CARM digital offset of -0.1, with CARM held on sqrt transmission signals), and then it didn't stay for more than a few tens of seconds.  The other few times, it broke lock on the way up to arm powers of 1. 

So, carm_cm_up.sh works pretty well, although perhaps the arm powers of 1 offset reduction needs to be a little slower.  PRMI doesn't catch and hold lock very easily with REFL33, and even less so with REFL165.  It may be useful to try catching lock with REFL11 or 55, and doing a transition over to 3f.  No real progress forward, but we're pretty much recovered.

Still no real luck getting the beam back aligned to the IFO.

Koji and I tried a few minutes of wiggling the input pointing tip tilts (TT1 and TT2) around, and then tried doing some thinking.

We note that the beam propagates (modulo a few pickoffs):

IMC -> Faraday -> TT1 -> MMT1 -> MMT2 -> TT2 -> PRM.

Since moving TT1 to the rails does make beam reflections in the BS chamber move (as seen by movement of the general illumination on the PRM face camera), I posit that the beam is getting through the Faraday.  It is certainly getting at least mostly through the Faraday, although since the MC locked so easily, I assume that we didn't have too much movement after the ~2pm Alaskan earthquake & aftershocks, so we're at pretty much the same alignment as usual, in terms of beam pointing coming from the IMC.

The plan is then to see the position of the beam on MMT1, and steer using TT1 to get the beam to roughly the center.  Then, see the beam propagate to MMT2 (if possible) and TT2 (if possible).    From here, we should be able to see the spot on PRM.  We should be able to use TT2 to tweak things up, and get the beam back to about the right place on POP, or REFL, or somewhere farther along.  Hopefully at this point, we'd see some flashes in the Yarm. 

Using a spare Watek camera, I was able to capture a shot of the face of MMT1.  This is when the TTs were restored to their values that were saved last Monday.  I checked, and this is also roughly the center of the actuation range of TT1, for both pitch and yaw.

I am not able to see the face of MMT2, or TT2.  If I leave TT1, and move TT2, I am not able to see any movement of any beam or reflections seen in the PRM face camera.

Koji and I are checking the MC spot positions, but it may be time to leave this for the morning crew.

EDIT:  The MC spots were actually pretty bad, and the WFS were working really hard.  Koji realigned the MC suspensions, and now the MC spots are slightly better, although quite similar, to what Manasa measured last week.  The restored TT values still don't give us any flashes in the arms.

[Jenne, EricQ, Manasa]

We are still not able to get the beam to go to the interferometer, which is super frustrating. 

We tried using cameras and viewers to look into several ports, however all we can see is the face of MMT1, which I  posted a still of last night.  I have a camera looking at the back of the PRM, in hopes that we could see the beam on PRM, but no luck.  The thought was that the lever arm between TT2 and PRM is so short that as long as TT2 is reasonably in the center of its range, it doesn't need to be precise.  However, it seems that no matter how much I move TT1, I am not able to get a nice beam spot on PRM.  Part of this is that we have to be close enough to the right alignment to hit MMT1, MMT2 and TT2. 

Anyhow, we're frustrated, and I'm not sure what our next step is.  I would very much prefer it if we didn't have to open any chambers, but I don't currently have any new ideas on how to see the spot on MMT2 or TT2.

I need to figure this out before it's worth trying any acrobatic AO path turn-on scenarios.

Also this evening, I went to the Yend and did another tweak-up of the green beam alignment. 

Never mind.  This is just the low pass filter that Den put in to try to deal with the moving cavity pole. 

Before I realized this, just in case it was a computer quirk, Koji and I rebooted the end station front end machines.  This had no effect other than to keep me searching and measuring until I figured it out.  If I turn off the low pass, the phase pops back up to very close to the reference.  The low pass currently comes on automatically as part of the carm_cm_up script.

I'm not sure why the c1cal model didn't come up the last time c1lsc was rebooted, but I did an "rtcds start c1cal" on the lsc machine, and it's up and running now.

I have put beam dumps in front of both of the end transmission QPDs so that I could measure the dark noise.  They are still there.

I checked that the Yend QPD sliders and switches were doing things as I expected.  I couldn't do the Xend since c1auxex is still lost (elog 10165).  I'll post plots and actual information on this checkout, as well as my calculation of what this dark noise means in terms of meters for CARM when we're using 1/sqrt(trans) signals tomorrow morning. 

The other day, I took spectra of the Yend transmission QPD dark noise for several different configurations of the transimpedance gain and the whitening gains. 

I also calculated with Optickle the expected sensitivity vs. CARM offset for the sqrtInv CARM sensor. 

I put these things together to get a plot of transmission QPD noise vs. CARM offset, for a chosen set of analog settings.

Measurement of Yend transmission QPD dark noise

Since EricQ had already checked out the whitening filters (see elog 9637 and elog 9642), I didn't check on them.  I just left them (the analog whitening, and the digital antiwhitening filters) on. 

First, I checked the noise vs. transimpedance gain. There are a few too many settings to put them all on one plot, so I have them sorted by the original transimpedance:  0.5 kOhms vs 5 kOhms.  It's a little tricky to see, but all of the spectra that begin with the 5k transimpedance have a little extra noise around 10 Hz, although I don't know why.  In the legend I have made note of what the settings were.  x1 x1 is my representation of the "inactive" setting.

I then looked at the noise with different whitening gain slider settings.  All but one of the traces are the 20 kOhm setting.  

These .xml files are in /users/jenne/Arms/TransQPDnoise_July2014/

Calculation of inverse sqrt transmission sensitivity

I used Optickle to give me the power transmitted through the ETMs.  I first find the transmission in the FPMI case.  I use that to normalize the full PRFPMI transmission, so that the output units are the same as our C1:LSC-TR[x,y]_OUT units. 

I take the square root of the transmitted power (sum of transmissions from each arm) at each CARM offset point, add 1e-3 as we do in the front end model to prevent divide-by-zero problems, and then take the inverse.

I find the slope by taking the difference in power between adjacent points, divided by the CARM offset difference between those points. 

In this plot, I have taken the absolute value of the sensitivity, just for kicks.  I also display an arbitrarily scaled version of the log of the transmitted power, so that we can see that the highest sensitivity is at half the maximum power.

 Calculate the QPD dark noise in terms of meters

Finally, I put it all together, and find the dark noise of the QPD in terms of meters.  Since the spectra were measured in units where the single-arm transmission is unity, the already match the units that I used to calculate the sqrtInv sensitivity. 

I take the spectra of the QPD dark noise for the 20 kOhm case, and multiply it by the sensitivity calibration number at several different CARM offsets.  As we expect, the noise is the best at half-max transmission, where the sensitivity is maximal. 

I took some data tonight for a quick look at what combinations of DC signals might be good to use for DARM, as an alternative to ALS before we're ready for RF.

I had the arms locked with ALS, PRMI with REFL33, and tried to move the CARM offset between plus and minus 1.  The PRMI wasn't holding lock closer than about -0.3 or +0.6, so that is also a problem.  Also, I realized just now that I have left the beam dumps in front of the transmission QPDs, so I had prevented any switching of the trans PD source.  This means that all of my data for C1:LSC-TR[x,y]_OUT_DQ is taken with the Thorlabs PDs, which is fine, although they saturate around arm powers of 4 ever since my analog gain increase on the whitening board.  Anyhow, the IFO didn't hold lock for much beyond then anyway, so I didn't miss out on much.  I need to remember to remove the dumps though!!

Self:  Good stuff should be between 12:50am - 1:09am.  One set of data was ./getdata -s 1089445700 -d 30 -c C1:LSC-TRX_OUT_DQ C1:LSC-TRY_OUT_DQ C1:LSC-CARM_IN1_DQ C1:LSC-PRCL_IN1_DQ

I removed the dumps in front of the trans QPDs.  The Yend QPD needed re-normalization, so I did that.

I realized while I was looking at last night's data that I had been doing CARM sweeps, when really I wanted to be doing DARM sweeps.  I took a few sets of data of DARM sweeps while locked on ALSdiff.  However, Rana pointed out that comparing ALSdiff to TRX-TRY isn't exactly a fair comparison while I'm locked on ALSdiff, since it's an in-loop signal, so it looks artificially quiet. 

Anyhow, I may consider transitioning DARM over to AS55 temporarily so that I can look at both as out-of-loop sensors. 

Also, so that I can try locking DARM on DC transmission, I have added 2 more columns to the LSC input matrix (now we're at 32!), for TRX and TRY.  We already had sqrt inverse versions of these signals, but the plain TRX and TRY were only available as normalization signals before.  Since Koji put in the facility to sqrt or not the normalization signals, I can now try:

Option 1:  ( TRX - TRY ) / (TRX + TRY)

Option 2:  ( TRX - TRY ) / sqrt( TRX + TRY )

DARM does not yet have the facility to normalize one signal (DC transmission) and not another (ALS diff), so I may need to include that soon.  For tonight, I'm going to try just changing matrix elements with ezcastep.

Since I changed the c1lsc.mdl model, I compiled it, restarted the model, and checked the model in.  I have also added these 2 columns to the AUX_ERR sub-screen for the LSC input matrix.  I have not changed the LSC overview screen.

New script called scripts/ALS/findRedResonance.py finds the IR resonance in less than 20 seconds.

1. Do a ~2 FSR scan of the arm over 10 seconds using the Phase Tracker servo offset ramp. (dt = 10 s)
2. Load the frame data for the TR_OUT and the Phase Tracker phase. (dt = 2 s)
3. Use Python (modern) SciPy to find all peaks with moderate SNR (dt = 3 s)
4. Take the top 3 peaks (all presumably TEM00) and choose the one closest to zero and go there.

The above is the gist of what goes on, but there were several issues to overcome to get the code to run.

1. None of our workstations have a modern enough Python distro to run either the ipython notebooks or the new SciPy with wavelets, so I installed Anaconda Python in the controls home directory.
2. In order to get the peak finder function to work well I gave it a range of peak widths to look for. If we change the speed of the ALS scan by more than 3x, we probably have to change the width array in the function.
3. I've used the find_peaks_cwt function in SciPy. This uses a Ricker ('Mexican Hat') wavelet to find peaks by doing multi-res transforms and looking for ridges in the wavelet space (I think)
4. To make the code run fast, I downsample from 16 kHz to 1 kHz right at the top. Would be better if we had downsampling in v2 of the getData function.

From the last plot:

- Subtracting the offset of 0.0095, the modulation depth were estimated to be 0.20 for 11MHz, 0.25 for 55MHz

- Carrier TEM00 1.0, 1st order 0.01, 2nd order 0.05, 3rd order 0.002, 4th order 0.004

==> mode matching ~93%, dominat higher order is the 2nd order (5%).

Eric: now we have the number for the mode matching. How much did the cavity round-trip loss be using this number?

[Rana, Jenne]

We had a look at the RIN of the transmission signals TRX and TRY, when the arms were individually locked on IR.  If the intensity noise is very bad, then the transmission signals aren't really a good option to use for locking.  So, if the RIN is bad, we need to work on our intensity stabilization. 

We need to understand what the situation is with the AOM, and why it isn't working as expected, so that we can reinstall it.  We also need to decide if we're going to use the SR560 setup, or if the Chas ISS is sufficiently characterized for us to use. 

The RIN is certainly bad.  Also, I don't know why the Xarm's RIN is worse between 10 Hz and a few hundred Hz than the Yarm.

I looked at what the RIN contribution of the sqrtInv sensor is by locking the arms individually on IR using POX and POY.  I then took spectra of the sqrtInv channels.  For the Xarm, I had forced the triggering so that the QPD was being used as the transmission PD, while the Yarm was using the regular Thorlabs PD.  I also had the green lasers locked to the arms, and took beatnote spectra to see what the sensing noise of the beatnotes is, all at the same time.

For the sqrtInv channels, I used the Optickle calibration from elog 10187. For today's plot, I am using the calibration at about 1nm, since that is about where we are when we transition to the sqrtInv Thorlabs signal usually.

For the ALS channel, I was using the _FINE_PHASE_OUT signal, which is in units of degrees of phase for a single green wavelength.  So, since k * x = phi, I want the phase data to be converted to radians (2*pi/360), and use k = 2*pi / lambda_green.  So, doing some algebra, this gives me x = phi_degrees * lambda / 360 for my calibration. 

What I see in the plot is that the ALS sensing noise is pretty bad compared to the sqrtInv channels, so maybe we don't have to work so hard on the ISS this next week.  Also, the Thorlabs PD is much better than the QPDs, which maybe isn't so surprising since we have them set so that they have good SNR at higher power.

Anyhow, here's the plot:

Also, here is the Thorlabs PD only, with single arm locked on RF, with the noise calibrated to different CARM offsets:

Your calibration of the ALS signal should be revised.

The phase for the ALS is not an optical phase of the green but the phase of the phase tracker servo output.

The calibration of the phase tracker depends on the cable length of the delay line in the beat box.
It seems that we are based on this calibration. Which gives up ~19kHz/deg.

Or, equivalently, use C1:.....PHASE_OUT_HZ instead.

Using these numbers for both arms (Modulation takes away .2*.25 = 5% power, mode matching takes away 7% after that), I get the following from my data from March:

Xarm loss is 561.19 +/- 14.57 ppm

Yarm loss is 130.67 +/- 18.97 ppm

Obviously, the Xarm number looks very fishy, but its behavior was qualitatively very different when I took the data. ASDC would change from ~0.298 to ~0.306 when the Yarm was locked vs. misaligned, whereas the xarm numbers were .240 to .275. 

In any case, I'll do the measurement again tomorrow, being careful with offsets and alignment; it won't take too long. 

As Koji pointed out, I messed up the calibration.  However, fixing it doesn't change things that much.

From this calibration by Yuta, the Xarm ALS calibration is 54 deg / MHz, or 19.17 kHz / deg.  So, I multiply my data which is in these degree units by 19.17e3 to get Hz.  Then I use delta_f / f = delta_L / L to convert to meters.  f = c / lambda_green, and L = 37.5 meters. 

This only changes the calibration by about 10-15%.  It still looks like the ALS noise is well above the RIN level of the sqrtInv signal.

Alone with the IFO. Started from some conversation with it.

Some ALS trials: Found the Y-end green alignment was terrible. In fact the end green set up is terrible.
Unfixed optics, clipping/fringing in the faraday, unstable suprema mounts which is unnecessarily big.

Eventualy I stopped touching the end alignment. Run ALS to see the stability of the things.
This is a performance confirmation after some touching of the ALS electronics by Manasa/SURFs

The sensing noise levels of the ALSs looks the same as before.

The intensity noise of the transmission was also checked. They are not RIN but very close to RIN
as the DC was the unity for both arms.

The X arm has worse ALS noise level and RIN.
Although I forgot to turn off the HEPA flow at the south bench during the measurement. Gurrr.

 [Koji, ericq]

We were working on getting back into the locking groove tonight.

The POP2F and REFL3F demod angles needed some tuning to lock the PRC reliably. The green alignments were mostly fine, the X end PZT ASS works reasonably well. Suspensions, especially the ITMs, seemed to be drifting a fair deal; today was fairly hot out, I guess.

We only got to the point of attempting the SqrtInv handoff once (which failed because I forgot to check the filter bank offsets). This was because the Mode Cleaner refused to stay locked longer than ~5-10 minutes at a time. We adjusted the MC and FSS servo offsets by the usual means, but this didn't make a difference.

We discussed and decided that the time is right to roll up our sleeves and dig into the MC loop, and try to figure out why these intermittent times of unreliability keep cropping up. We will check out the servo board, and see if we can find the missing phase than Evan observed, as well as characterize the FSS/PZT crossover, and investigate what kind of conditions we may create that cause the PC to saturate.

The MC openloop gains were measured with several conditions
- MC fast/PC crossover was measured to be ~30kHz.
- No feature found in the fast path above 10kHz.

=====

I have been making a circuit to test the crossover between the PZT and PC paths.
This was supposed to allow us to inject a test signal as well as the 5V necessary to offset the voltage for the HV amp.
So far this attempt was not successful although the circuit TF looked just fine. I was wondering what was wrong.

I now suspect that the noise of the circuit was too big. It has ~65nV/rtHz noise level. This corresponds to the external
disturbance of 1~2Hz/rtHz. This is ~10 times larger noise level than the freerun frequency noise.

In the control band the circuit noise is suppressed (cancelled) by the feedback loop.
This is OK when the loop is dominated by the PZT loop. However, if the loop is dominated by the PC path,
the PC path has to work for this compensation.

So what I should do is to remove the low pass filter in the FSS and move it to the downstream of the HV amp.
This way we may be able to reduce the PC path actuation as the noise of the HV amp is also reduced by the LPF.

=====

For the meantime, I used another approach to characterize the MC crossover. I could manage to lock the MC without the PC path.
The openloop was measured with and without the PC path in this low gain setup. In fact the loop was oscillating at 6kHz
due to the low phase margin. Nevertherless, this comparison can let us find where the crossover. The loop gain was also
measured with the nominal condition.

<<Measuerement condition>>

No PC
MC IN1 Gain: +19dB
VCO Gain: +3dB
Boosts: No boost / No super boost
FSS Common Gain: +13dB
Fast Path Gain: +21.5dB
The PC path disconnected.
(Note that the loop was almost oscillating and the apparent gain may look lower than it should have been)

WIth PC
MC IN1 Gain: +19dB
VCO Gain: +3dB
Boosts: No boost / No super boost
FSS Common Gain: +13dB
Fast Path Gain: +21.5dB
The PC path connected.

Nominal
MC IN1 Gain: +19dB
VCO Gain: +15dB
Boosts: Boost On / Super boost 2
FSS Common Gain: +13dB
Fast Path Gain: +21.5dB
The PC path connected.

The ultimate goal of characterizing the temperature actuator turned to be fruitful in obtaining the calibration values for ETMX and ETMY (Calibration of ITMs were done previously  here but not for ETM). In this process, I measured the PZT response by  displacing one of the test masses in the frequency range of 20 Hz and 900 Hz  and measured the transfer functions in counts/counts. 

ETMX = [12.27  x 10 ^-9/ f² ] m/count

ETMY = [14.17  x 10 ^-9/ f²] m/count

I calculated these calibration values from the measurements that we have taken( in detail : elog)  and did the following calculations: 

The measurements I made were :PZT count/ Actuator Count separately for all the test masses.

PZT count/ Actuator count = [PZT count/ arm cavity displacement(m) ]*[ displacement of a test mass(m) / Actuator Count]

For a same laser and assuming flat response of the PZT, the term [PZT count/ arm cavity displacement(m) ] remains for all the test masses.

The fitting was done on the gain plots of the PZT Response vs Test mass displacement and a function G * f ^-2 was fitted. The resulting G values were:

ETMX: 8.007* f ^-2 

ITMX: 3.067* f ^-2

ETMY :11.389* f ^-2

ITMY : 3.745* f ^-2

To calculate the calibration of ETMX:

 [PZT count/ Actuator count : ETMX ] / [ displacement of a test mass(m) / Actuator Count :ETMX] =  [PZT count/ Actuator count : ITMX ] / [ displacement of a test mass(m) / Actuator Count :ITMX]

putting the values from the above fitting and Kiwamu's elog,

the calibrated value was calculated to be [12.27 * 10^-9 /f^-2 ]m/count.

A similar calculation was done for ETMY.

The attached are the fitting plots for the measurements taken.

 Now using these and the previously measured calibrations, I will get the complete calibrated TF of the thermal actuator.

I used an oscillator and an oscilloscope to measure the open loop transfer function at higher frequency than 100kHz.
(I remember that I tried to use Agilent 4375A for this and failed before ... due to low input impedance???)\

Here is the update. It seems that the gain margin is not so large. We should apply low pass to prevent too large servo bump.

In fact there is a pomona box between the HV amp and the laser.
It is expected that the combination of the box and the laser PZT (2.36nF by Elog #3640) provides poles at 2.9Hz and 148kHz and a zero at 32Hz.
Basically, the gain of this stage is 0.1 at 10kHz. So the injected noise is reduced by factor of 10. It is just barely OK.
I need a bit more careful design of the output stage for the MC servo.

- ALS X/Y arm stability was checked by IR locked arms.

- Basically the stability looks same as before.

Q sez: here are some ALS ASDs (in Hz/rtHz). 

The reference plots are with the arms locked on CARM/DARM with ALS. The main traces are with the arms locked on POX/POY. Alignment affects these traces a fair amount.

The X arm ALS seems no worse for the upgrade, and the PZT actuators do look pretty orthogonal when we play around with the alignment. 

After aligning the arms to IR, I aligned the Y green beam to the arm.  Also, the X green beatnote was very small, so I aligned the PSL green for X.

The ALS system is iffy tonight.

After putting the cable back to the RF spectrum analyzer (it had been taken to test the frequency counter setup, and not put back), I had a good Yarm beatnote, but again this evening the Xarm beatnote is small.  I touched up the PSL table alignment (very, very little needed, but it did double my peak height).  I *think* that this is happening because we haven't settled into a good IFO alignment place, so the arm pointing keeps changing very slightly, which means that the PSL ALS alignment needs touching.  Anyhow, even after alignment the Xarm beatnote is only -36 dBm at 81 MHz.  It should be at least -25 dBm or so, although I haven't seen it any larger than about -35 dBm since the IFO beam was lost last Friday.

I am not able to hold ALS lock long enough to scan the arms and find the IR resonances.  The only optics that I am actuating on this evening are the 2 ETMs.  When I lose lock and look at the watchdogs, the ETMs are the only optics that have largeish numbers, which comes from the ALS lockloss.  So, I don't think I am suffering from the ITM suspension kicks tonight.  Rather, I think that it's that the ALS system isn't tuned up nicely.

I think that it is past time we tuned up and checked out the ALS PDH setup.  Q:  Can you please measure the loop TFs for both of the ALS PDH boxes tomorrow?  At the very least we want to know what we're working with. 

Evan:  What is the status with the ISS? 

I am going to try tomorrow to look at the suspensions, and see if I can track anything down.  I feel like I see the kicks more often when the arms are locked, i.e. we are sending an LSC signal to them.  The LSC POS signal is a factor of a few hundred larger than the damping SUSPOS signal is.  Are we saturating something somewhere?  Why is this a new thing?  We certainly do see kicks when the LSC is not engaged, so this may not be the right path, but it is something concrete to look at.

Yesterday, Q helped me look at the DACs for some of the suspensions, since Gabriele pointed out that the DACs may have trouble with zero crossings.  

First, I looked at the oplevs of all the test masses with the oplev servos off, as well as the coil drive outputs from the suspension screen which should go straight out to the DACs.  I put some biases on the suspensions in either pitch or yaw so that one or two of the coil outputs was crossing zero regularly.  I didn't see any kicks. 

Next, we turned off the inputs of the coil driver filter banks, unplugged the cable from the coil driver board to the satellite box, and put in sinusoidal excitations to each of the coils using awggui.  We then looked with a 'scope at the monitor point of the coil driver boards, but didn't see any glitches or abnormalities.  (We then put everything back to normal)

Finally, I locked and aligned the 2 arms, and just left them sitting.  The oplev servos were engaged, but I didn't ever see any big kicks. 

I am suspicious that there was something funny going on with the computers and RFM over the weekend, when we were not getting RFM connections between the vertex and the end stations, and that somehow weird signals were also getting sent to some of the optics.  Q's nuclear reboot (all the front ends simultaneously) fixed the RFM situation, and I don't know that I've seen any kicks since then, although Eric thinks that he has, at least once.  Anyhow, I think they might be gone for now.

This afternoon Q helped me put in some temporary PDs for checking for any mode hopping behavior in our 3 main lasers. 

Q helped me install PDA55s on each of the lasers (I did the ends, he did the PSL) so that we could do the mode hop temperature check.  For the Yend, I took the leakage transmission through the first Y1 steering mirror after the laser. This beam was dumped, so I replaced the dump with a PDA55. For the Xend, the equivalent mirrors are too close to the edge of the table, so I put in a spare Y1, and reflect most of the light to a beam dump.  The leakage transmission then goes to a PDA55.  Note that for both of these cases, no alignment of main laser path mirrors was touched, so we should just be able to remove them when we're through.  For the PSL, I believe that Q took the rejected light from one of the PBSes before the PMC. 

The end temporary PDs are using the TRX / TRY cables, so we will be looking at the C1:LSC-TR[x,y] channels for the power of the end lasers.  The PSL's temporary PD is connected to the PMC REFL cable.  For the end PDs, since I had filter banks available, I shuttered the end lasers and removed the dark offset.  I then changed the gains to 1, so the values are in raw counts.  The usual transmission normalization gains are noted in one of the control room notebooks.

I did a slow ezcastep and ramped the temperature of all 3 lasers over about an hour.  Since we usually use the PSL around FSS slow slider value of zero, I swept that from -10 to +10.  Since we usually use the Xend laser at around 10,000 counts, I swept that from 0 to 20,000.  For the Yend laser, it is usually around -10,000 counts, so I swept it from -20,000 to 0.  ezcastep -s 0.2 C1:ALS-X_SLOW_SERVO2_OFFSET +1,20000 C1:ALS-Y_SLOW_SERVO2_OFFSET +1,20000 C1:PSL-FSS_SLOWDC +0.001,20000


I was looking for something kind of similar to what Koji saw when he did this kind of sweep for the old MOPA (elog #2008), but didn't see any power jumps that looked suspicious.

Here is the PSL:

The Xend:

And the Yend:

 Based on the game plan, I have created a slew of updated pretty plots about our signals and loops. 

First: With measured arm losses, when do we start to see REFL DC dip? At what arm buildup powers? 

I updated my MIST model with the arm losses I've measured (Y:130ppm, X:530ppm), and some measured transmissions from the wiki, vs. the design parameters, as I used to have. Here is the DC sweep plot which is now hanging up in the control room. 

In this plot, I also calculated what MIST thinks the full arm power buildup will be as compared to our single arm locking, and I get something of order 200, rather than the 600 we've tossed around in discussions. Nothing else is very different in this plot from the old version; though the REFLDC dip is a little bit wider. 

Now, here are some radiation-pressure inclusive sensing transfer functions, for the anti-spring case (which in Rob's day was easier to lock for unknown reasons):

Next: Include new AO path TFs into CM model Look at possibilities for engaging AO path 

With these TFs, and the recently measured+fit new AO TF, here are the open loop gains of the slow, digital, SqrtInv-sensed MCL CARM and fast, analog, REFLDC-sensed AO CARM loops for the region of offsets we've achieved and a little lower. The slow digital loop includes the 1k LP that we have used in the past, in addition to the normal CARM filters. I still need to figure out the right sequence of ( offset reduction / crossover frequency motion / overall gain adjustment ) that gets the coupled cavity resonance solidly within the loop bandwidth. 

 

I don't know why, but TRY has somehow gotten a 0.3 count offset in the last hour. 

Rana and I are witnesses for each other that neither of us has gone into the IFO room in the last several hours (and we're the only ones here).  For some reason though, the TRY PD now has a 0.3 count offset.  We have been doing some ALS locks, but we have not run the offset script in the last several hours.  Closing the green shutter doesn't change things, and we still see the offset when the MC loses lock, so it's not to do with the end or the PSL laser.  We haven't been in there, so there hasn't been a change in the room lights. 

 We used to do violin mode and test mass body mode notches in the SUS-LSC filter modules. Now we want them balanced in the LSC and triggered by the LSC, so they're in the filter modules which go from the the LSC output matrix to the SUS.

Today, we were getting ETM violin mode ringups while doing ALS hunt and so we moved the bandstops into the LSC. I also changed the bandstop from a wide one which missed the ETMX mode to a double bandstop which gets both the ETMX and the ETMY mode. See attached image of the Bode mag.