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ID Date Author Type Categoryup Subject
  10492   Wed Sep 10 22:17:29 2014 KojiSummaryLSCX/Y green beat mode overlap measurement

[Koji Manasa]

We made quantitative inspection of the X/Y green beat setup on the PSL table.

DC output of the BBPD for each arm was measured by blockiing the beams at either or both side of the recombination BS.

The power over lap for the X arm beat note setup was 7.8% and is now 53%.
There is 3dB of headroom for the improvement of the mode overlap.

The power over lap for the Y arm beat note setup was 1.2% and is now 35%.
There is 4dB of headroom for the improvement of the mode overlap.

The RF analyzer monitor for the beat power is about 10dB lower than expected. Can we explain this only by the cable loss?
If not it there something causing the big attenuation?

             XARM   YARM
o BBPD DC output (mV)

 V_DARK:   -  3.3  + 1.9
 V_PSL:    +  4.3  +22.5
 V_ARM:    +187.0  + 8.4

o BBPD DC photocurrent (uA)

I_DC = V_DC / R_DC ... R_DC: DC transimpedance (2kOhm)

 I_PSL:       3.8   10.3
 I_ARM:      95.0    3.3

o Expected beat note amplitude
I_beat_full = I1 + I2 + 2 sqrt(e I1 I2) cos(w t) ... e: mode overwrap (in power)

I_beat_RF = 2 sqrt(e I1 I2)

V_RF = 2 R sqrt(e I1 I2) ... R: RF transimpedance (2kOhm)

P_RF = V_RF^2/2/50 [Watt]
     = 10 log10(V_RF^2/2/50*1000) [dBm]

     = 10 log10(e I1 I2) + 82.0412 [dBm]
     = 10 log10(e) +10 log10(I1 I2) + 82.0412 [dBm]

for e=1, the expected RF power at the PDs [dBm]
 P_RF:      -12.4  -22.6

o Measured beat note power (before the alignment)     
 P_RF:      -23.5  -41.7  [dBm] (38.3MHz and 34.4MHz) 
    e:        7.8    1.2  [%]                         
o Measured beat note power (after the alignment)      
 P_RF:      -15.2  -27.1  [dBm] (26.6MHz and 26.8MHz) 
    e:       53     35    [%]                         

Measured beat note power at the RF analyzer in the control room
 P_CR:      -25    -20    [dBm]
Expected    -17    - 9    [dBm]

Expected Power:
Pin + External Amp Gain (0dB for X, 20dB for Y)
    - Isolation trans (1dB)
    + GAV81 amp (10dB)
    - Coupler (10.5dB)

  10494   Thu Sep 11 02:08:32 2014 JenneUpdateLSCHigher transmission powers

No breakthroughs tonight. 

DRMI didn't want to lock with either the recipe that we used a year ago (elog 9116) or that was used in May (elog 9968).  Being lazy and sleepy, I chickened out and went back to PRFPMI locking. 

Many attempts, I'll highlight 2 here.

(1) I had done the CARM -> sqrtInvTrans transition, and reduced the CARM offset to arm powers of about 7, and lost lock.  I don't remember now if I was trying to transition DARM to AS55, or if I was just prepping (measuring error signal ratio and relative sign).


(2) I stopped the carm_cm_up script just before it wanted to do the CARM -> sqrtInvTrans transition, and stayed with CARM and DARM both on ALS.  I got to reasonably high powers, and was measuring the error signal ratios I needed for CARM -> REFL DC and DARM -> AS55.  Things were too noisy to get good coherence for the DARM coefficient, but I thought I was in good shape to transition CARM to REFL DC (which looks like REFL11I, since REFLDC goes to the CM board, and the OUT2 of that board is used to monitor the input to the board. )  Anyhow, I set the offset such that it matched my current CARM offset value, and started the transition, but lost lock about halfway through.  CARM started ringing up here, and I think that's what caused this lockloss.  Could have been the CARM peak, which I wasn't considering / remembering at the time.


Daytime activity for Thurs:  Lock DRMI, maybe first on 1f signals, but then also on 3f signals.

  10497   Fri Sep 12 00:28:04 2014 ericqUpdateLSCHoly sensitivity, Batman!

I took a quick measurement of the ALS stability, using POX and POY as out of loop sensors, using a CARM calibration line to line POX and POY up to the calibrated PHASE_OUT channels at 503Hz. 

  • X arm RMS ~1kHz
    • Could use more low frequency suppression
  • Y arm RMS ~200Hz


  10498   Fri Sep 12 00:40:23 2014 JenneUpdateLSCDRMI locking

 Tonight I worked on DRMI locking.  

I think the reason the May2014 DRMI recipe wasn't working for me is because I wasn't including the REFL11 -> SRCL element.  I had left it out because (a) I didn't think we should need it and (b) REFL11 is going through the CM board.  

Tonight, I flipped the switch on the CM screen so that OUT2 was seeing REFL11I, not REFLDC, so I had REFL11 in the usual place.  I reset the demod phase, since we had left it at zero for CM stuff. 

Setting demod phases for PRMI:

I locked PRMI on sideband, REFL 33 I&Q and drove PRM.  REFL55 was at 55deg, and I changed it to 33deg to minimize the peak in the Q-phase.  REFL11 was a 0deg, and I set it to 17deg.  I also checked the AS55 phase in the MICH-only case, and changed it from 14.75deg to 24.75 deg.

The May 2014 recipe (elog 9968) calls for adding 25 degrees to the REFL55 phase, so I put REFL55 at 58deg for DRMI locking.

After that, using the parameters in the May2014 recipe, the DRMI just locked.  Awesome!

I checked the demod phases with DRMI lock.  REFL11 stays at 17 degrees.  If I actuate the SRM, I get the largest peak in the I-phase of REFL55 with a phase of -143deg, but the acquisition is best with phase around 55deg.  [Note, as Q points out, I wonder if SRCL is mostly locked with REFL11I for some magical reason, which is why it didn't matter so much that I put a sign flip into REFL55...I wonder if fixing our macroscopic length offset in SRCL will fix this].  I also changed the REFL165 phase from -155.5deg to +145deg.

By looking at transfer functions at an excitation frequency, I expected that I should be able to hold SRCL and MICH on REFL165, with matrix elements -0.085 for REFL165I->SRCL and -0.23 for REFL165Q->MICH.  I was not able to acquire with these values, nor was I able to ramp the matrix elements while keeping lock.

So, I tried moving PRCL to REFL33I, which did work.  I used 1.245*REFL33I->PRCL, but left SRCL and MICH on REFL55 I&Q, with the REFL11I->SRCL element also there. This is where I started trying to get rid of the REFL11I element, but couldn't maintain lock most times, and could never acquire lock without it.

Next up, checking the MICH->SRCL coupling due to the output matrix.  I did as Koji did in elog 8816 , but first I copied the notches in FM10 of MICH over to PRCL and SRCL (old notch freqs were SRCL=566.1Hz, PRCL=675.1Hz, now they're all 475.1Hz).  I drove BS, and checked that the PRM element minimized the peak in REFL33I, the PRCL error signal.  I also added an SRM element to reduce the peak in REFL55I, the SRCL error signal.  I ended up with 0.5*BS, -0.284*PRM, -1.5*SRM for MICH drive, and unity in the PRM and SRM elements for PRCL and SRCL, respectively.

I measured the SRCL open loop gain, and the UGF was pretty low, so I increased the SRCL gain from 0.2 to 0.5 to make the UGF be around 70Hz.  I measured PRCL and MICH also, and they matched their references.

I worked a little bit on trying to remove REFL11 from the SRCL error signal, but didn't get anywhere.  I'm leaving the IFO to Q for the rest of the night.

To sum up, here is the set of parameters that worked for DRMI locking.  (These are saved as the template on the IFO Config screen.):


REFL11:  17 deg

REFL33: 140.5 deg (not changed tonight)

REFL55: 58 deg  (58deg for DRMI, 33deg for PRMI)

REFL165: 145 deg

AS55: 24.75 deg


MICH = 0.15 * REFL55Q

PRCL = 1.245 * REFL33I

SRCL = -0.09 * REFL11I + 1.0 * REFL55I

DOF Triggers

MICH, PRCL, SRCL all on POP22I, 50:10


MICH = 1.0

PRCL = -0.02

SRCL = 0.5

FM triggers 

MICH:  35:2, 2 sec delay, FM 2, 3, 6, 9

PRCL:  35:2, 0.5 sec delay, FM 2, 3, 6, 9

SRCL:  35:2, 5 sec delay, FM 3, 6, 9  (always lose lock trying to engage FM2).


MICH = 0.5 * BS +  (-0.284)*PRM + (-1.5)*SRM



  10499   Fri Sep 12 03:49:57 2014 ericqUpdateLSCSome more PRFPMI efforts

 Since DRMI didn't get fully commissioned, I tried my hand at PRFPMI locking with the newly improved ALS performance. 

ALS seemed reliable, I think my main limiting factor was the PRMI locking. We should set up a restore script for PRFPMI that is a superset of the ALS CARM DARM, because the current restore script doesn't put all the vertex settings back, so I was trying to lock for a while without the FM boosts on PRCL and MICH, which really hurt my stability. 

Transitioning to SqrtInv works fine; a couple of times I've gotten to arm power of ~10, and have been able to sit there for a while as I set up excitation line comparisons with the CM board's REFLDC, but the PRC would always lose it before I did anything interesting. 

The PRMI locks with a reasonable MICH offset, I found that adding a offset of 20 to 40 makes the AS spot visibly dimmer, and ASDC falls to ~0.05 from .1-.2. 

I looked into adding a boost to the CARM loop after transitioning to sqrtInv, but we only have 30 degrees of margin, and the error signal is already fairly white, so there isn't much to do, really. 

The ALS locking script is sporadically hanging a fair while, as well, which is strange. Otherwise, not much to report...

  10500   Fri Sep 12 11:25:42 2014 KojiUpdateLSCDRMI locking

This is great.

And I got confused. Is REFL11 going through the CM board?
If so how the demod phase for REFL11 take an effect for the sensing?

Maybe I understood. CM SERVO SLOW has been connected to REFL11I? whitening.
Therefore using REFL11 in the CM SERVO gives us REFL11I at the usual channels.
And then how can we ensure the gain matching between I & Q?

Then is the next step 3f DRMI? How is REFL165 healthy?
I also wonder how the relative phase and modulation depths improves the sensing matrix.

  10501   Fri Sep 12 12:00:59 2014 ericqUpdateLSCDRMI locking

REFL11 I, as seen in digital land, is connected to the slow output of the CM board. I tuned the demod angle of the REFL11 demodulator board by cable length back in ELOG 9850. It would be good to check that the phase is still good. If the CM board gains are at 0dB, we should be able to used the digital angle adjustment as normal. 

  10502   Fri Sep 12 14:11:17 2014 ericqUpdateLSCDRMI locking

We need to get an interferometric estimation of the SRC length error / SRC sideband splitting, because if the 7.5cm hand-measured error is true, it looks like it might be hard to control the DRMI on 3F. 

I did some DRMI sensing simulations, to get an idea if sensing matrix elements might change as the CARM offset changes. Last night, I tried just going to zero CARM offset on ALS, and was having problems keeping the PRMI locked on REFL33, so I wanted to confirm that it should at least work in theory. 

Thus, I simulated what happens to the sensing matrix element in the vertex DoFs as the CARM offset is reduced, in both the PR and SR cases. I normalized all of the elements to PRCL at zero carm offset, to get an idea of what the good relative gains should be for MICH and SRCL. 

In the end, there don't seem to be significant DC gain changes, or demod angle fluctuations, in either the PRFPMI or DRFPMI case, as the CARM offset changes, which is good.

However, the SRC length as hand-measured, seems to mess up the MICH angle in the DRFPMI case, and really lowers the SRCL signal amplitude. 

To be fair, past efforts of simulating demodulation angles haven't always been borne out on the IFO, so we should still forge ahead experimentally until it becomes apparent that there is a real problem. 

Here are the simulations for the IFO as-is:

(A note on the plots. Though they kind of look like Bodes, they're just the sensing element represented as a complex number in the I-Q plane,I being phase=0 and Q = 90)


All three signals are along the I axis in the DRMI case, which seems like it would be tough to control, since we only have 2 3F diodes... We've been using REFL33Q when PRMIing, which is simulated at around 45 deg; it should be easy to verify this empirically. 

Here are the same plots with the SRC length corrected. Now MICH shows up mostly in the Q phase as desired in the DRMI case. SRCL in REFL165 also wins 20dB of optical gain, as well. 



To drive the point home, here's a simulated scan of AS110 and REFL55 Q to show the effect of the measured length error:



  10512   Wed Sep 17 01:40:55 2014 JenneUpdateLSCCloser to REFL DC? Maybe?

I tried a bunch of times to reduce my CARM offset so I could jump to REFLDC digitally, but I think I'm maybe being a little ambitious with the arm power I'm trying to get to.

I have modified the carm_cm_up script so that it does my new procedure.  Everything is the same through locking the PRCL and MICH on 3f.  Then it reduces the CARM offset to 1.5 nm.  This is where we *used* to transition to sqrtInvTrans.  Now I have it going a bit farther to 0.5 nm, and arm powers of about 1 before doing that transition.  Also, before it transitions it lowers the CARM gain and engages the 1kHz lowpass in FM9.  A gain of about 4 is fine to keep the gain peaking in the CARM loop to only about 10dB, and sets a UGF of 100Hz which is the peak of the phase bubble with the lowpass engaged. 

Once I got to this point (several times tonight), I turned on CARM and DARM oscillations and looked at the transfer functions between (CARM and REFLDC) and (DARM and AS55Q). I have 2 DTT templates setup for this, in /users/Templates/PRFPMI.  These templates assume that you have your new DARM signal (AS55) going to SRCL_IN1 and your new CARM signal (REFLDC, which is actually REFL11I coming through the CM board) going to MC_IN1. 

I'm not sure why I'm losing lock. I don't see anything terribly telling on the time series plots, in particular none of the loops look like they are oscillating.  Here is one of the better examples from this evening:


Other notes:

* I realigned the Xgreen on the PSL table (again) to maximize the beatnote amplitude.  Y was fine, but X was very poorly overlapped on the camera.

* I put the SR785 back by the LSC rack and plugged it into the CM board for transfer functions.  Didn't take any tonight.

* We have a small wishlist for scripting things:  (1) DRMI restore script should reset REFL11 to "normal" REFL11.  (2) CARM/DARM acquisition restore script should reset REFL11 to REFLDC.  (3) CARM/DARM acquisition restore should also set PRMI parameters (as Q noted last week).

  10514   Wed Sep 17 15:40:00 2014 ericqUpdateLSCDRMI locking

I have not had any success the past two days in getting an interferometric measurement of the SRC length. 

So, the question posed at today's meeting was: "How precisely do we need to change the SRC length to be able to lock the DRMI on 3F"

The two ways I could think to quantify this are:

  • How much MICH -> [S,P]RCL cross coupling is ok?
  • How much [S,P]RCL ->  MICH cross coupling is ok?

REFL33 should have its phase set to put PRCL along I, and REFL165 should have SRCL along I, so the simulation result that matters is the angle of MICH in these planes. The cross couplings are then given by the appropriate trigonometric projections. In the following plots, I used 10% as the acceptable cross coupling in either direction. 




  • To limit the MICH -> [S,P]RCL coupling to 10%, we must hit the ideal length within +- 1.2cm.
  • To limit the SRCL -> MICH coupling to 10%, we must hit the ideal length within +- 2mm.
  • It doesn't look like we can get the REFL33 angle totally to 90 degrees, REFL165 looks more promising.

Code (finesse + pykat + ipython notebook) and plots are attached. 

Attachment 2: drfpmiVertexSensing.zip
  10516   Thu Sep 18 02:42:28 2014 JenneUpdateLSCAO path partly engaged

Tonight was a night of trying to engage the AO path.  The idea was to sit at arm powers of a few on sqrtInvTrans for CARM and ALS for DARM, and try to increase the gain for REFLDC->AO path.

No exciting nit-picky details in locking procedure.  Mostly it was just a night of trying many times. 

The biggest thing that Q and I found tonight was that the 2-pin lemo cable connecting the CM board's SERVO OUT to the MC board's IN2 is shitty.  The symptom that led to this investigation was that I could increase the AO path gain arbitrarily, and have no change in the measured analog CM loop transfer function. We checked that the CM board servo out spit out signals that were roughly what we expected based on our ~2kHz excitation.  However, if we look at digitized signals from the MC board, the noise level was very high, with loads of 60Hz lines, and a teensy-tiny signal peak.  We put a small drive directly into the MC board and could see that, so we determined that the cable is bad.  We have unplugged the white 2-pin lemo, and ran a long BNC cable between the 2 boards.  Tomorrow we need to make a new 2-pin lemo cable so that we can have the lower noise differential drive signal.

After putting in the temporary cable, we do see an excitation sent to the CM board showing up after the MC board.  For this monitoring, the MC_L cable to the ADC has been borrowed, so instead of being the OUT1, the regular length signal, MC_L is currently the OUT2 monitor right after the board inputs. 

At some point in the evening, around 1:15am, ETMX started exhibiting the annoying behavior of wandering off sometimes.  I went in and pushed on the SUS cables to the satellite box, and I think it has helped, although I still saw the drift at least once after the cable-squishing.

Other than that, it has just been many trials.

The best was one where I was holding the arm powers around 4, and got the CM board's AO gain to -8 dB and the MC board's IN2 AO gain to -4 dB. I lost lock trying to increase the CM board gain to -7 dB. 

I took several transfer functions, and used Q's nifty "SRmeasure" script to gather data, and Q made a plot to see the progress.

TF progress plots:


Time series of that lockloss:


I don't know yet if the polarity of the CM board should be plus or minus.  This series was taken with "minus".  But,  since the phase looked opposite of Q's single arm CM board checkout from several months ago, we did a few trials with the polarity switched to "plus".  I thought we weren't getting as high of AO path gains, so I switched back to "minus", but the last few trials didn't get even as far as the plus trials did.  So, I still don't know which sign we want.

  10519   Thu Sep 18 17:44:55 2014 JenneUpdateLSCOld AO cable pulled

[Q, Jenne]

We pulled the old 2-pin lemo cable after I had a look at the connectors.  When I unscrewed the connector on the MC side, one of the wires came off.  I suspect that it was still hanging on a bit, but my torquing it finally killed it. 

We pulled the cable with the idea of resoldering the connectors, but there are at least 2 places where the cable has been squished enough that the shielding or the inner wires are exposed.  These places aren't near enough the ends to just cut the cable short.

Downs doesn't have a spool of shielded twisted single-pair cable, so Todd is going to get me the part number for the cable they use, and I've asked Steve to order it tomorrow. 

For now, we will continue using the BNC cable that we installed last night - I don't think it's worth resoldering and putting in a crappy 2-pin lemo cable that we'll just throw out in a week.

  10520   Fri Sep 19 04:05:05 2014 ericqUpdateLSCAO path partly engaged

More AO efforts. No huge news. 

Came at AO from each side. For each sign, I lost lock just a few dB from the AO portion of the loop crossing unity gain. Both attempts were about arm powers of 1, which should correspond to ~300pm CARM offset, which I have simulated the crossover as possible with my current loop models (including latest MC loop). The gain steps were usually 6dB in between measurements. 

Positive polarity on CM board screen:

I made it to +5 dB of the last plot here, but the 6th broke it open. Gains on CM In2, CM AO, and MC In2 were -6, -4, -2 on that last, lock breaking, step. 


Negative polarity on CM board screen:

Lost it just 2dB above the last trace. Gains were -6, +1, -2 (So, overall 5dB higher than the other polarization)


Many things happened in between these two lock stretches, but I'm not sure what may or may not have affected things. They include:

  • Jenne mentioned PRMI being fussy earlier in the evening. I adjusted REFL33 and POP22 angles during a PRMI lock, while CARM was held away with ALS. My simulations suggest that there are small changes to the 3F sensing when the arms are totally absent, but doing it at a finite CARM offset is closer to where we want it, it seems. 
  • I tried using REFL165Q for MICH, since my simulations suggest a better MICH/PRCL angle, which would stave off cross couplings. Lined up excitations, etc., but no luck. 
  • I measured the PRMI loops
    • found PRCL to have ~200Hz UGF, 8dB gain peaking. Maybe a little high, but didn't seem terrible. 
    • MICH had UGF of around 20Hz, with the FM gain at 0.8. By the shape of the phase bubble, the loop seems designed for higher bandwidth. I raised the gain to 2.5 for a 70ishHz UGF, and called in FMs 7 and 9 for additional triggered boosts. Things seemed to stay locked pretty well. 
  • Lower excitation amplitude the second time around, measuring the AO loop. Looking at the CM output spectra, you can see the excitation wailing away; I wanted to avoid it.

The location of the CARM resonance peak lines up with my simulation, which is good, but there appears to be less phase than expected... I tried making sure that we don't have any whitening uncompensated for, but it looked ok. All my AO path loop model contains is the CM board TF (measured and fitted), the IMC seen as an actuator(measured and fitted), and the REFLDC optical TF (simulated in MIST). Maybe the DC path of whatever diode this is coming from needs to be included...

Discontinuities / glitches could be seen in the CM board fast output when MC board gains were changed, which isn't so nice. Incidentally, I notice now that each lock loss corresponded to a step of AO gain on the CM board.

  10521   Fri Sep 19 13:12:07 2014 JenneUpdateLSCAO path glitches


Discontinuities / glitches could be seen in the CM board fast output when MC board gains were changed, which isn't so nice. Incidentally, I notice now that each lock loss corresponded to a step of AO gain on the CM board.

Back in May I looked at all the glitches that happen when we change the AO gain slider on the CM board - see elog 9938.   I wonder if the MC IN2 gain slider has the same issues.  I think I'll look at this this afternoon. Maybe we can set the CM board gain someplace, and just use the MC IN2 slider (if it's not as glitchy) for the delicate part where we're just about to cross unity, and then later we can again use the CM board's AO gain.

EDIT:  Yes, the glitches on the CM board AO path are *much* bigger, and more frequent.  Interestingly, the biggest glitches were every 4 dB.  When I went from -29 to -28, again from -25 to -24, -21 to -20, etc.  I saw the largest glitches on the MC IN2 slider going -29 to -28 and -17 to -16, but if there were small glitches at other transitions, they didn't hit my trigger levels.  I think next time I try engaging the AO path I'll try to do the delicate stuff by upping the MC IN2 gain rather than the CM board AO gain.

  10527   Tue Sep 23 17:37:10 2014 ericqUpdateLSCDRMI locking

Rather than using a CAD drawing, I used Gabriele's code from ELOG 9590 to try and judge if we could shorten the SRC by the appropriate length, without clipping the SR3-SR2 beam. 

Specifically, I used these lines:

% Move SRM 7.5 towards SR2, parallel to beam


dAS = BS2-AS; Vector from SRM to SR2

dASmag = sqrt(dAS(1)^2+dAS(2)^2);

dMove = delta*dAS/dASmag;  delta times unit vector

CS = CS+dMove;

draw_sos(CS, 180/pi*angles)

to help generate this plot:



As a reminder, Gabriele's code used the following logic:

  • We know the nominal dimensions of all of the suspensions
  • We hand measured various distances between features of the suspension structures. (Corner to corner)
  • A global fit, minimizing the maximum error, reconstructed the positions of the suspensions. 
  • Beam positions assumed to be ideally aligned. 
  • Beam trajectories traced out, and optical path lengths estimated (taking into account changing indices of refraction due to flipped mirrors)

In my opinion, this is the best estimate of beam trajectory that we currently have.

Thus, from looking at the plot above, I claim we can correct the SRC length without clipping the beam by moving the SRM forward by the required 7.5cm.

Although the measured distance may be off on the order of a cm (since our PRC correction had a 0.5cm disagreement between interferometric and hand distance measurements), this will nevertheless markedly improve our 3F DRMI sensing, based on my previous ELOG. 


Hence, given our discussions last week, Jenne and I will proceed to ready the interferometer for venting in the morning, by following the vent checklist.

Our sole objective for this vent is this move of the SRM. 

Steve, please check the jam nuts, and begin the vent when you get in.  Thanks!

  10531   Wed Sep 24 11:02:38 2014 manasaUpdateLSCMoving SRM

I looked at the CAD layout and it seems like we will clearly be clipping POY if we move SRM by 7.5cm. Since POY is not visible at low power, we cannot be sure about the clipping.

We should have a plan B before we move everything. I suggest we move a combination of SRM and SR2 to get the desired SRC length.
Moving SR2 will require extra effort to walk the beam unclipped through all the 6 output steering mirrors that follow; but there will be little room for error if we use irides to propagate the beam through the first 4 mirrors that are in the BS and ITMY chamber.

  10538   Thu Sep 25 11:33:41 2014 JenneUpdateLSCPOY alignment laser


I looked at the CAD layout and it seems like we will clearly be clipping POY if we move SRM by 7.5cm. Since POY is not visible at low power, we cannot be sure about the clipping.

 I was bad and forgot to elog this yesterday (bad grad student!), but I setup a laser pointer to show us where the POY beam is. 

To do this, I removed the tiny mirror that sends the beam to the POY RF PD (so we do not have POY to lock the Yarm right now.  I think Q has successfully been using AS).  The laser pointer goes through 2 temporary steering mirrors, then passes through the place that the tiny mirror usually sits, and then travels along the POY path into the vacuum system.  The idea here is that we should be able to adjust the laser pointer and the temp steering mirrors, and not touch any of the actual POY mirrors, but still get the green beam to go all the way to ITMY.  Yesterday I confirmed that the laser pointer was hitting the in-vac POY pickoff mirror, and today Q and Manasa are doing final adjustment to get the beam all the way to the ITM. 

  10554   Tue Sep 30 17:26:18 2014 ericqUpdateLSCNew AO cable in place

I've installed a new 2pin lemo cable going from the CM servo out to in2 of the MC servo board, and removed the temporary BNC. I used some electrical tape to give the cable some thickness where the lemo head screws on to try to strain relieve the solder joints; hopefully this cable is more robust than the last. 

I put an excitation into the CM board, and saw it come out of MC_F, so I think we're set. 

  10558   Wed Oct 1 19:40:46 2014 ericqUpdateLSCArms IR aligned
  • Beamsplitter was put into MC refl path.
  • HWP was rotated to maximize power into PMC. 
  • MC autolocker locked, small alignment tweak led to WFS taking over
  • Light present on REFL, AS and POP!
  • After small adjustments to TTs and ETMY, locked Yarm with AS55, ran ASS. 
  • Adjusted AS camera and RFPD alignment for ASS'd AS beam. 
  • Left arm locked on AS55, aligned new POY beam onto POY11. Centered ITMY oplev while I was there. 
  • After adjusting digital POY11 demod angle with an excitation into ETMY, arms were POX/POY locked and ASS'ed.
  • PRM and SRM eyeball aligned

The IFO is ready for 3F DRMI comissioning 

  10562   Fri Oct 3 03:02:17 2014 ericqUpdateLSCNo luck locking DRMI

I haven't been able to lock the DRMI tonight, neither with 1F and no arms nor 3F and arms held off with ALS... I tried previous recipes, and new combinations informed by simulations I've run, to no avail. 

I touched the alignment of the green beat PD on the PSL table, since the X beatnote was rather low, but wasn't able to improve it by much. I never took a spectrum, since it wasn't my main focus tonight, but the low frequency motion of both arms on ALS, as observed by RIN, was good as I've ever seen it. 

In our WFS work earlier today, Koji and I reset the WFS offsets, and it actually seems to have helped a good deal, in terms of the "fuzz" of MC REFL on the wall striptool. I had previously presumed this to be due to excess angular motion, but perhaps it is more accurately described as an alignment offset that let the nominal angular motion couple into the RIN more. 

  10574   Tue Oct 7 00:18:12 2014 JenneUpdateLSCYgreen PSL alignment, ETMX strain relief

No exciting progress today.  I did PSL green alignment for the Yarm, although I now think that the Xarm green needs realigning too.

Also, I was foiled for a while by ETMX jumping around.  I think it's because the adapter board on the Xend rack didn't have any strain relief.  So, I zip tied the heavy cable in a few places so that it's no longer pulling on the connector.  Hopefully we won't see ETMX misbehaving as often now, so we won't have to go squish cables as often.

  10580   Tue Oct 7 19:40:58 2014 ericqUpdateLSCCM, REFL11 Wiring

I've changed the LSC rack wiring a little bit, to give us some flexibility when it comes to REFL11. 

Previous, the REFL11 demod I output was fed straight to the CM servo board, and the slow CM board output was hooked up to the REFL11I ADC channel. Thus, it wasn't really practical to ever even look at sensing angles in REFL11, since the I and Q inputs were subject to different signal paths/gains. (Also, doing LSC offsets would do wonky things to refl11 depending on the state of the switches on the CM board screen.)

Thus, I've hooked up the CM board slow output into the the previously existing, aptly named, CM_SLOW channel. The REFL11 demod board I output is split to IN1 of the CM board, and the REFL11 I ADC channel. 

So, there is no longer hidden behavior in behind the REFL11 input filters, channels are what they claim to be, and the CM board output is just as easily accessible to the LSC filters as before. 

  10581   Wed Oct 8 03:20:46 2014 JenneUpdateLSCDo we need AO for acquisition?

As part of trying to determine whether we require the AO path for lock acquisition, or if we can survive on just digital loops, I looked at the noise suppression that we can get with a digital loop.

I took a spectrum of POX, and calibrated it using a line driving ETMX to match the ALSX_FINE_PHASE_OUT_HZ channel, and then I converted green Hz to meters. 

I then undid the LSC loop that was engaged at the time (XARM FMs 1,2,3,4,5,8 and the pendulum plant), to infer the free running arm motion. 

I also applied the ALS filters (CARM FMs 1,2,3,5,6) and the pendulum plant to the free running noise to infer what we expect we could do with the current digital CARM filters assuming we were not sensor noise limited.

In the figure, we see that the free running arm displacement is inferred to be about 0.4 micrometers RMS.  The in-loop POX signal is 0.4 picometers RMS, which (although it's in-loop, so we're not really that quiet) is already better than 1/10th the coupled cavity linewidth.  Also, the CARM filters that we use for the ALS lock, and also the sqrtInvTrans lock are able to get us down to about 1 pm RMS, although that is not including sensor noise issues. 


For reference, here are the open loop gains for the LSC filters+pendulum and ALS filters+pendulum that we're currently using.  The overall gain of these loops have been set so the UGF is 150Hz.


It seems to me that as long as our sensors are good enough, we should be able to keep the arm motion down to less than 1/10th or 1/20th the coupled cavity linewidth with only the digital system.  So, we should think about working on that rather than focusing on engaging the AO path for a while.

Attachment 3: CARMnoise_7Oct2014.zip
  10582   Wed Oct 8 03:37:44 2014 ericqUpdateLSCPRFPMI, other sign of CARM offset

 [ericq, Jenne]

We attempted some of the same old CARM offset reduction tonight, but from the other direction. (We have no direct knowledge of which is the spring and which is the anti-spring side)

We we able to get to, and sit at, arm powers on the order of 5. Really, we kind of wanted just to push things to try and inform our current ideas of what our limiting factor is, so as to appropriately expend our efforts. 

Candidates include:

  • ALS noise causing excess DARM motion
    • Means we need to DRMI to widen DARM linewidth, avoid sign flip in AS55, IR lock DARM sooner
  • Intolerable sensor noise makes CARM wander too much, changing our plant more than our loops can handle
    • We should work on having live calibrated CARM spectra during lock attempts, to compare with Jenne's noise estimates, and see where/how/why we exceed it. 
  • detuned CARM pole causes loop instability
    • Maybe some sort of notching can get us by
    • AO path could extend bandwidth, getting the pole into the control band 
  • SqrtInv signals losing low frequency sensitivity due to radiation pressure, or DC sensitivity due to transmission curve flattening out
    • Bring in AO path for supplementary bandwidth, which lets us turn up loop gain / engage big boosts
    • Or, switch to REFLDC in digital land, which is nontrivial, due to different optical plant shapes.

We took many digital CARM OLTFs at different offsets; it never really looked like a burgeoning pole was about to make things unstable. The low frequency OLTF data had bad SNR, so it wasn't clear if we were losing gain there. We weren't at arm powers where we would expect the DC transmission curve to flatten out yet, from simulations (which is above a few tens).

My impression from at least our last lock loss was a DARM excursion. However, using the DRMI won't get rid of the second two points.


  10583   Wed Oct 8 03:49:42 2014 JenneUpdateLSCPRFPMI, other sign of CARM offset

Other thoughts from talking with Rana earlier:

  • Is it possible to suppress CARM motion enough that we can use just a digital loop?  Can we do without the AO path?  What would said digital loop have to look like?
  • Q points out that there is a zero in the relative transfer function between CARM to transmission, and CARM to REFLDC.  Is that zero invertible?
  • We should look at some limits, like saturation limits.  How much will we need to actuate?
  • Rana is looking at making a more detailed CARM loop model in simulink to see if we can stay stable throughout our CARM offset reduction journey.

Also, Q and I squished on the suspension connectors earlier tonight.  MC2 was going wonky, which we feared might be because we were in that area working on Chiara earlier.  Then, after squishing the MC connectors, the PRM started misbehaving, so we went and gave all the corner suspension connectors another squish.  No suspension glitching problems since then.

  10589   Thu Oct 9 16:31:53 2014 ericqUpdateLSCCARM W/N TFs

In my previous simulation results, I've always plotted W/m, which isn't exactly straightforward. We often think about the displacement that a given mirror actuator output will induce, but when we're locking the full IFO, radiation pressure effects modify the mechanical response depending on the current detuning, making the meaning of W/m transfer functions a little fuzzy.

So, I've redone my MIST simulations to report Watts of signal response due to actual actuator newtons, which is what we actually control with the digital system. Note, however, that these Watts are those that would be sensed by a detector directly at the given port, and doesn't take into account the power reduction from in-air beamsplitters, etc.

As an example, here are the SqrtInv and REFLDC CARM TFs for the anti-spring case:



The units of the SqrtInv plot are maybe a little weird, these TFs are the exact shape of the TRX W/N TFs with the DC value adjusted by the ratio of the DC sweep derivatives of TRX and SqrtInv. 

All of the results live in /svn/trunk/modeling/PRFPMI_radpressure/


  10591   Thu Oct 9 18:30:59 2014 JenneUpdateLSCCARM W/N TFs

Okay, here (finally) is the optickle version.

I have the antispring case, starting at 501pm and going roughly every 10pm down to 1pm.  I also have the spring case, starting at -501pm and going down every 10pm to roughly -113pm.  Rossa crashed partway through the calculation, which is why it's not all the way.

In the .zip is a .mat file called PDs_vs_CARMoffset_WattsPerNewton.mat, which has (a) a list of the 50 CARM offsets, (b) a frequency vector, and (c) several transfer function arrays.  The transfer function arrays are supposed to be intuitively named, eg. REFLDC_antispring. 

In the .zip file are also the original .mat files that are a result of the tickle calculations, as well as a .m file for loading them and making the plots, etc.  For anyone who is trying to re-create the transfer function variables, I by-hand saved the variable called PD_WperN to the names like REFLDC_antispring.  Just kidding.  Those original mat files are over 100Mb each, and that's just crazy.  Anyhow, I think the .zip has everything needed to use the data from these plots.

Anyhow.  Here are plots of what are in the various transfer function arrays:




Attachment 6: ForElog.zip
  10593   Fri Oct 10 00:20:37 2014 ranaUpdateLSCCARM W/N TFs


 Assuming that these Watts/Newtons TFs are correct, I've modeled the resulting open loop gain for CARM. The goal is to design a loop that is stable under a wide range of offsets and also has enough low frequency gain.

The attached PDF shows this. I used a CARM OLG Simulink model:


I've replaced the 'armTF' block with a digital gain of zero. After measuring the open loop gain of all but this piece, I multiply that 'OLG' with the W/N that Jenne extracted from Optickle for CARM->TR (not sqrtInv)

I plot the resulting estimate of the actual OLG in the following plot. Since the CARM-RSE peak is moving down, we use the LP filter that Den installed for us several months ago. To account for the radiation pressure spring, we use some low frequency boosts but not the crazy FM4 filter.

As you can see, the loop is stable from 500 to 200 pm, but then goes unstable around 110 pm. I expect that we will want to do some fancy shaping there or switch from TRX+TRY into something else.

This assumes we have filters 0, 1, 3, 5, and 7 on in the CARM filter bank - still need to add the digital AA/AI to make the loop phase lag a little more accruate, but I think this is looking promising.


Attachment 2: carm.pdf
  10594   Fri Oct 10 03:05:09 2014 ericqUpdateLSCWhich side of optical spring are we on?

 I made some measurements to try and see if any difference could be seen with different CARM offset signs. 

Specifically, at various offsets, I used a spare DAC channel to drive IN1 of the CM board, as an "AO Exciter." I used CM_SLOW to monitor the signal that was actually on the board. I used the CARM_IN1 error signal to see how the optical plant responded to the AO excitation. Rather than a swept sine, I used a noise injection kind of TF measurement. 

Here are plots of CARM_IN1 / CM_SLOW at different CARM FM offsets; I chose to plot this in an attempt to divide out some of the common things like AA and delays and make the detuned CARM pole more evident). The offsets chosen correspond roughly to powers of 2, 2.5, and 3. I tried to go higher than that, but didn't remain locked for long enough to measure the TF. 


By eye, I don't see much of a difference. We can zpk fit the data, and see what happens. 


  10595   Fri Oct 10 03:25:11 2014 JenneUpdateLSCWhich side of optical spring are we on (simulation)

I have a simulated version of the differences that we expect to see between the 2 different sides of the CARM resonance.  The point is that we can try to compare these results with Q's measured results (elog 10594) to see if we know if we are on the spring or antispring side.

I calculated the same transfer functions vs CARM offset again, although tonight I do it in steps of 20pm because I was getting bored of waiting forever.  Anyhow, this is important because my previous post (elog 10591) didn't have spring side calculations all the way down to 1pm.

This is similarly true for that elog 10591, but here are some notes on how I am currently getting the W/N units out of Optickle.  First of all, I am still using old Optickle1.  I don't know if there are significant units ramifications for that, but just in case I'll write it down.  Nic tells me that to get [W/N] out of Optickle1, I need to multiply sigAC (units of [W/m]) by my simple pendulum (units of [m/N]).  Both of these "meters" in the last sentence are "mevans meters", which are the meters you would get per actuation if radiation pressure didn't exist.  So, I guess they're supposed to cancel out?  I need to camp out in Nic's office until I figure this out and get it untangled in my head.

Plots of transfer functions for both sides of CARM resonance (same as prev. elog), as well as the ratio between the spring and antispring transfer functions at each CARM offset:





The take-away message from the 3rd column is that other than a sign flip, we don't expect to see very much difference between the 2 sides of the CARM resonance, particularly above a few hundred Hz.  (Note that we do not see the sign flip in Q's measurements because he is looking at CARM_IN1, which is after the input matrix, and the input matrix elements have opposite signs between the signs of the CARM offsets.  So, the sign flip between spring and antispring around the UGF is implied in the measurements, just not explicit).

Also, something that Rana pointed out to me, and I still don't know why it's true:  The antispring transfer functions (at least for the transmission) don't have all the phase features that we expect to see based on their magnitudes.  If you look at the TRX antispring plot, blue trace (which is about 500pm from resonance), you'll see that the magnitude starts flat at DC, has some slope in an intermediate region, and then at high frequencies has 1/f^2.  However, the phase seems to not know about this intermediate region, and magically waits until the 1kHz resonance to flip the full 180 degrees. 

Attachment 10: ForElog_9Oct2014.zip
  10598   Mon Oct 13 12:01:28 2014 ericqUpdateLSCWhich side of optical spring are we on?

 I went back into the DQ channels to look at the TF from AO injection to REFLDC (which is easy to do with this kind of noise injection TF).  


I fear that REFL does not seem to have as much phase under the resonance as we have modeled, lacking about 10-20 degrees. This could result from the zero in the REFL DC response that we've modeled at ~200ish Hz is actually higher. I'll look into what affects the frequency of that feature. 

It is, of course, possible, that this measurement doesn't properly cancel out the various digital effects, but the REFLDC phase curves do seem to settle to (+/-) 90 after the pole as expected. 

DTT XML file is attached. 

Attachment 2: AOinjection_SqrtInv_REFLDC.xml.zip
  10603   Mon Oct 13 21:20:56 2014 JenneUpdateLSCWhich side of optical spring are we on? (No progress)

[Jenne, Diego]

In order to distinguish between the spring and antispring sides of the CARM resonance, we need to have transfer function measurements down to at least 100 Hz (although lower is better). 

We tried to get some transfer functions the same way Q did, but noticed that (a) we couldn't get any low frequency coherence, and (b) that when we increased the amplitude of the white (well, lowpass at 5kHz) noise, the coherence between the AO injection and REFL DC went down.  Not clear why this is.

Anyhow, we tried taking good ol' fashioned swept sine transfer functions, although eventually the lightbulb came on that the AO path has a highpass in it.  Duh, Jenne.  So, we started trying to actuate on MC2 position rather than the AO path laser frequency.  We didn't get too far though before El Salvadore decided to have a few 7.4 earthquakes.  We're bored of aftershocks knocking us out of lock, so we're going to come back to this tomorrow.


  10607   Wed Oct 15 02:58:03 2014 JenneUpdateLSCWhich side of optical spring are we on?

Some measurements.  Unclear meaning.  

We tried both positive and negative numbers in the CARM offset, and then looked at transfer functions at various arm powers. The hope is to be able to compare these with some simulation to figure out which side of the CARM resonance we are on.

The biggest empirical take-away is that we repeatedly (3 times in a row) lost lock when holding at arm powers of about 5 with negative CARM offsets.  However, we were repeatedly (2+ times tonight) able to sit and hold at arm powers of 10+ with positive CARM offsets.




I am not sure that we get enough information out of these plots to tell us which side of the CARM resonance we are really on.  Q is working on taking some open loop CARM measurements (actuating and measuring at SUS-MC2_LSC) to see if we can compare those more directly to Rana's plots.

Positive number in the digital CARM offset:



Negative numbers in digital CARM offset:



  10608   Wed Oct 15 02:59:04 2014 ranaUpdateLSCCARM W/N TFs

 In my previous elog in this thread, I showed the CARM OLG given some new digital filters and the varying CARM plant (spring side, not anti-spring). Jenne has subsequently produced the TFs for all of the rest of the CARM offsets.

These attached plots for several CARM offsets show that the anti-spring side is much more stable than the spring side and so we should use that. Annecadotedally, we think that positive CARM offsets are more stable when going to arm powers of > 10, so perhaps this means that +CARM = -SPRING.

The first PDF shows the spring OLGs and the 2nd one shows the antispring OLGs. I have put in some gain changes to keep the UGF approximately the same as the offset is changed.

The PDF thumbnails will become visible once Q and Diego install the new nodus.

 UPDATE OCt 16: this is all wrong! bad conversion of filters within the invbilinear.m function.

Attachment 1: spring.pdf
Attachment 2: antispring.pdf
  10609   Wed Oct 15 13:38:33 2014 JenneUpdateLSCCARM W/N TFs

Here are the same plots, but the legend also includes the arm power that we expect at that CARM offset.  

Here is what the arm powers look like as a function of CARM offset according to Optickle.  Note that the cyan trace's maximum matches what Q has simulated in Mist with the same high losses.  For illustration I've plotted the single arm power, so that you can see it's normalized to 1.  Then, the other traces are the full PRFPMI buildup, with various amounts of arm loss.  The "no loss" case is with 0ppm loss per ETM.  The "150 ppm loss" case is with 150 ppm of loss per ETM.  The "high loss" case is representative of what Q has measured, so I have put 500 ppm loss for ETMX and 150 ppm loss for ETMY.


And, the transfer functions (all these, as with all TFs in the last week, use the "high loss" situation with 500ppm for ETMX and 150ppm for ETMY).




  10612   Wed Oct 15 19:56:38 2014 JenneUpdateLSCWhich side of optical spring are we on? Meas vs Model

 I have plotted measured data from last night (elog 10607) with a version of the result from Rana's simulink CARM loop model (elog 10593).

The measured data that was taken last night (open circles in plots) is with an injection into MC2 position, and I'm reading out TRX.  This is for the negative side of the digital CARM offset, which is the one that we can only get to arm powers of 5ish.

The modeled data (solid lines in plots) is derived from what Rana has been plotting the last few days, but it's not quite identical.  I added another excitation point to the simulink model at the same place as the "CARM OUT" measurement point.  This is to match the fact that the measured transfer functions were taken by driving MC2.  I then asked matlab to give me the transfer function between this new excitation point (CARM CTRL point) and the IN1 point of the loop, which should be equivalent to our TRX_OUT.  So, I believe that what I'm plotting is equivalent to TRX/MC2.  The difference between the 2 plots is just that one uses the modeled spring-side optical response, and the other uses the modeled antispring-side response.



I have zoomed the X-axis of these plots to be between 30 Hz - 3 kHz, which is the range that we had coherence of better than 0.8ish last night in the measurements.  The modeled data is all given the same scale factor (even between plots), and is set so that the lowest arm power traces (pink) line up around 150 Hz. 

I conclude from these plots that we still don't know what side of the CARM resonance we are on. 

 I have not plotted the measurements from the positive side of the digital CARM offset, because those transfer functions were to sqrtInvTRX, not plain TRX, whereas the model only is for plain TRX. There should only be an overall gain difference between them though, no phase difference.  If you look at last night's data, you'll see that the positive side of the CARM offset measured phase has similar characteristics to the negative offset, i.e. the phase is not flat, but it is roughly flat in both modeled cases, so even with that data, I still say that we don't know what side of the CARM resonance we are on.



  10613   Wed Oct 15 20:10:29 2014 ericqUpdateLSCInterim DARM Signal

I've done some preliminary modeling to see if there is a good candidate for an IR DARM control signal that is available before the AS55 sign flip. From a DC sweep point of view, ASDC/(TRX+TRY) may be a candidate for further exploration. 

As a reminder, both Finesse and MIST predict a sign flip in the AS55 Q control signal for DARM in the PRFPMI configuration, at a CARM offset of around 118pm.


The CARM offset where this sign flip occurs isn't too far off of where we're currently losing lock, so we have not had the opportunity to switch DARM control off of ALS and over to the quieter IR RF signal of AS55. 

Here are simulated DC DARM sweep plots of our current PRFPMI configuration, with a whole bunch of potential signals that struck me. 

Although the units of most traces are arbitrary in each plot (to fit on the same scale), each plot uses the same arbitrary units (if that makes any sense) so slopes and ratios of values can be read off. 


In the 300 and 120pm plot, you can see that the zero crossing of AS55 is at some considerable DARM offset, and normalizing by TRX doesn't change much about that. "Hold on a second," I hear you say. "Your first plots said that the sign flip happens at around 120pm, so why does the AS55 profile still look bad at 50pm?!" My guess is that, probably due to a combination of Schnupp and arm length asymmetry, CARM offsets move where the peak power is in the DARM coordinate. This picture makes what I mean more clear, perhaps:


Thus, once we're on the other side of the sign flip, I'm confident that we can use AS55 Q without much problem. 

Now, back to thoughts about an interim signal:

ASDC by itself does not really have the kind of behavior we want; but the power out of AS as a fraction of the ARM power (i.e. ASDC/TRX in the plot) seems to have a rational shape, that is not too unlike what the REFLDC CARM profile looks like.

Why not use POPDC or REFLDC? Well, at the CARM offsets we're currently at, POPDC is dominated by the PRC resonating sidebands, and REFLDC has barely begun to decline, and at lower CARM offsets, they each flatten out before the peak of the little ASDC hill, and so don't do much to improve the shape. Meanwhile, ASDC/TRX has a smooth response to points within some fraction of the DARM line width in all of the plots. 

Thus, as was discussed at today's meeting, I feel it may be possible to lock DARM on ASDC/(TRX+TRY) with some offset, until AS55 becomes feasible.

(In practice, I figure we would divide by the sum of the powers, to reduce the influence of the DARM component of just TRX; we don't want to have DARM/DARM in the error signal for DARM)

Two caveats are:

  • The slope of this signal actually looks more quadratic than linear. Is this ok/manageable?
  • I have not yet made any investigation into the frequency dependent behavior of this thing. Transmission in the denominator will have the CARM pole in it, might get complicated. 

[Code and plots live in /svn/trunk/modeling/PRFPMI_radpressure]


  10614   Wed Oct 15 22:39:17 2014 JenneUpdateLSCThe Plan

 [Rana, Jenne]

We're summarizing the discussions of the last few days as to the game plan for locking.  

  1. PRMI on REFL165.  The factor of 5 in frequency will give us more MICH signal.    We want this.
    1. Drive CARM, measure coupling to PRCL, MICH while locked on REFL33.
    2. Switch to REFL165, re-measure CARM coupling.
    3. Hopefully this will reduce the AS port fluctuations, and reduce the POP22 power decrease as CARM offset decreases. 
  2. DARM transition from ALSdiff to an intermediate signal.  Simulate, and try empirically.
    1. Maybe try ASDC normalized by sum of transmissions?
    2. Maybe try difference of transmissions divided by sum of transmissions?  
  3. Look at data on disk.
    1. Do we have anything specific causing our locklosses (lately there haven't been obvious loop instabilities causing the locklosses)?
    2. How much do we think our lengths are actually changing right now (particularly DARM on ALSdiff)?
    3. Are there better ways of combining error signals that could be useful?
    4. Do we need to work on angular loops?
      1. Oplevs
      2. POP ASC for sidebands
      3. POP QPD or Trans QPDs for arms
  4.  Think about what could be causing ETMX to be annoying.  The connection that is most suspect has been ziptied, but we're still seeing ETMX move either at locklosses or sometimes just spontaneously.
  5.  RAM.  What kind of RAM levels do we have right now, and how do they affect our locking offsets?  Do we have big offsets, or negligible offsets?
  10615   Thu Oct 16 03:13:23 2014 JenneUpdateLSCPRMI on REFL165, and more

 The first thing I looked at tonight was locking the PRMI on REFL 165.

I locked the PRMI (no arms), and checked the REFL 165 demod phase. I also found the input matrix configuration that allowed me to acquire PRMI lock directly on REFL165.  After locking the arms on ALS, I tried to lock the PRMI with REFL 165 and failed.  So, I rechecked the demod phase and the relative transfer functions between REFL 165 and REFL 33.  The end of the story is that, even with the re-tuned demod phase for CARM offset of a few nanometers, I cannot acquire PRMI lock on REFL 165, nor can I transition from REFL 33 to REFL 165.  We need to revisit this tomorrow.

IFO configuration CARM offset [cts] REFL 165 demod phase [deg]
Found as-is N/A +145
PRMI, no arms N/A -135
PRFPMI +3 +110
PRFPMI +2 +110
PRFPMI +1 +110
PRFPMI +0.5 +120


IFO configuration REFL 33 I / REFL 165 I (PRCL) REFL 33 Q / REFL 165 Q (MICH)
PRMI, no arms +0.1 +0.22, although easier to acquire lock with +0.1
PRFPMI, CARM offset = +3 -0.09  (TF measured, no lock) +0.033  (TF measured, no lock)

For the PRMI-only case, I ended up using 0.1's in the input matrix, and I added an FM 1 to the MICH filter bank that is a flat gain of 2.2, and then I had it trigger along with FM2.

I turned this FM1 off (and no triggering) while trying to transition from REFL33 to REFL165 in the PRFPMI case, but that didn't help.  I think that maybe I need to remeasure my transfer functions or something, because I could put values into the REFL165 columns of the input matrix while REFL33 was still 1's, but I couldn't remove (even if done slowly) the REFL33 matrix elements without losing lock of the PRMI.  So, we need to get the input matrix elements correct.

I also recorded some time series for a quick RAM investigation that I will work on tomorrow.  

I left the PRM aligned, but significantly misaligned both ITMs to get data at the REFL port of the RAM that we see.  I also aligned the PRMI (no arms) and let it flash so that I can see the pk-pk size of our PDH signals.  I need to remember to calibrate these from counts to meters.  

Raw data is in /users/jenne/RAM/ .

I have not tried any new DARM signals, since PRMI wasn't working with 3f2.  

We should get to that as soon as we fix the PRMI-3f2 situation.

  10618   Thu Oct 16 16:21:42 2014 ericqUpdateLSCInterim DARM Signal

I've added (TRX-TRY)/(TRX+TRY) to the DC DARM sweep plots, and it looks like an even better candidate. The slope is closer to linear, and it has a zero crossing within ~10pm of the true DARM zero across the different CARM offsets, so we might not even need to use an intentional DARM offset. 



  10619   Thu Oct 16 21:20:59 2014 ranaUpdateLSCmisleading modelling

 I think these are all very helpful and interesting plots. We should see some better performance using either of the DC DARM signals.

BUT, what we really need (instead of just the DC sweeps) is the DC sweep with the uncertainty/noise displayed as a shaded area on the plot, as Nic did for us in the pre-CESAR modelling.

Otherwise, the DC sweeps mistakenly indicate that many channels are good, whereas they really have an RMS noise larger than 100 pm due to low power levels or normalization by a noisy signal.

  10620   Thu Oct 16 22:35:05 2014 ranaUpdateLSCCARM W/N TFs

In my last CARM loop modelling, all of the plots are phony, so don't trust them. The invbilinear function inside of StefanB's onlinefilter.m was making bogus s-domain representations of the digital filter coefficients.

So now I've just plotted the frequency response directly from the z-domain SOS coeffs using MattE's readFilterFile.m and FotonFilter.m.

Conclusions are less rosy. The anti-spring side is still easier to compensate than the spring side, but it starts to get hopeless below ~130 pm of offset, so there we really need to try to get to REFL_11/(TRX+TRY), pending some noise analysis.

** In order to get the 80 and 40 pm loops to be more stable I've put in a tweak filter called Boost2 (FM8). As you can see, it kind of helps for 80 pm, but its pretty hopeless after that.

Attachment 1: carm_spring.pdf
Attachment 2: carm_antispring.pdf
  10621   Fri Oct 17 03:05:00 2014 ericqUpdateLSCDARM locked on DC Transmission difference

 I've been able to repeatedly get off of ALS and onto (TRY-TRX)/(TRY+TRX). Nevertheless, lock is lost between arm powers of 10 and 20. 

I do the transition at the same place as the CARM->SqrtInv transition, i.e. arm powers about 1.0 Jenne started a script for the transition, and I've modified it with settings that I found to work, and integrated it into the carm_cm_up script. I've also modified carm_cm_down to zero the DARM normalization elements. 

I was thwarted repeatedly by the frequent crashing of daqd, so I was not able to take OLTFs of CARM or DARM, which would've been nice. As it was, I tuned the DARM gain by looking for gain peaking in the error signal spectrum. I also couldn't really get a good look at the lock loss events. Once the FB is behaving properly, we can learn more. 

Turning over to difference in transmission as an error signal naturally squashes the difference in arm transmissions:


I was able to grab spectra of the error and control signals, though I did not take the time to calibrate them... We can see the high frequency sensing noise for the transmission derived signals fall as the arm power increases. The low frequency mirror motion stays about the same. 


So, it seems that DARM was not the main culprit in breaking lock, but it is still gratifying to get off of ALS completely, given its out-of-loop-noise's strong dependence on PSL-alignment. 

  10622   Fri Oct 17 13:19:48 2014 JenneUpdateLSCPOP22 ?!?!

We've seen this before, but we need to figure out why POP22 decreases with decreased CARM offset.  If it's just a demod phase issue, we can perhaps track this by changing the demod phase as we go, but if we are actually losing control of the PRMI, that is something that we need to look into.

In other news, nice work Q!




  10625   Fri Oct 17 17:52:55 2014 JenneUpdateLSCRAM offsets

Last night I measured our RAM offsets and looked at how those affect the PRMI situation.  It seems like the RAM is not creating significant offsets that we need to worry about.

Words here about data gathering, calibration and calculations.

Step 1:  Lock PRMI on sideband, drive PRM at 675.13Hz with 100 counts (675Hz notches on in both MICH and PRCL).  Find peak heights for I-phases in DTT to get calibration number.

Step 2:  Same lock, drive ITMs differentially at 675.13Hz with 2,000 counts.  find peak heights for Q-phases in DTT to get calibration number.

Step 3:  Look up actuator calibrations.  PRM = 19.6e-9/f^2 meters/count and ITMs = 4.68e-9/f^2 meters/count.  So, I was driving PRM about 4pm, and the ITMs about 20pm.

Step 4:  Unlock PRMI, allow flashes, collect time series data of REFL RF siganls.

Step 5: Significantly misalign ITMs, collect RAM offset time series data.

Step 6: Close PSL shutter, collect dark offset time series data.

Step 7: Apply calibration to each PD time series.  For each I-phase of PDs, calibration is (PRM actuator / peak height from step 1).  For each Q-phase of PDs, calibration is (ITM actuator / peak height from step 2).

Step 8:  Look at DC difference between RAM offset and dark offset of each PD.  This is the first 4 rows of data in the summary table below.

Step 9:  Look at what peak-to-peak values of signals mean.  For PRCL, I used the largest pk-pk values in the plots below.  For MICH I used a calculation of what a half of a fringe is - bright to dark.  (Whole fringe distance) = (lambda/2), so I estimate that a half fringe is (lambda/4), which is 266nm for IR.  This is the next 4 rows of data in the table.

Step 10: Divide.  This ratio (RAM offset / pk-pk value) is my estimate of how important the RAM offset is to each length degree of freedom. 

Summary table:

  PRCL (I-phase) MICH (Q-phase)
RAM offsets    
11 4e-11 m 2.1e-9 m
33 1.5e-11 m ~2e-9 m
55 2.2e-11 m ~1e-9 m
165 ~1e-11 m ~1e-9 m
Pk-pk (PDH or fringes) PDH pk-pk from flashes MICH fringes from calculation
11 5.5e-9 m 266e-9 m
33 6.9e-9 m 266e-9 m
55 2.5e-9 m 266e-9 m
165 5.8e-9 m 266e-9 m
Ratio: (RAM offset) / (pk-pk)    
11 7e-3 8e-4
33 2e-3 7e-3
55 9e-3 4e-3
165 2e-3 4e-3


Plots (Left side is several PRMI flashes, right side is a zoom to see the RAM offset more clearly):









  10626   Mon Oct 20 17:50:30 2014 JenneUpdateLSCCARM W/N TFs (Others were all wrong!)

I realized today that I had been plotting the wrong thing for all of my transfer functions for the last few weeks! 

The "CARM offsets" were correct, in that I was moving both ETMs, so all of the calculations were correct (which is good, since those took forever). But, in the plots I was just plotting the transfer function between driving ETMX and the given photodiode.  But, since just driving a single ETM is an admixture of CARM and DARM, the plots don't make any sense.  Ooops.

In these revised plots (and the .mat file attached to this elog), for each PD I extract from sigAC the transfer function between driving ETMX and the photodiode.  I also extract the TF between driving ETMY and the PD.  I then  sum those two transfer functions and divide by 2.  I multiply by the simple pendulum, which is my actuator transfer function to get to W/N, and plot.

The antispring plots don't change in shape, but the spring side plots do.  I think that this means that Rana's plots from last week are still true, so we can use the antispring side of TRX to get down to about 100 pm.

Here are the revised plots:




Attachment 1: PDs_vsCARMoffset_20Oct2014.mat.zip
  10627   Tue Oct 21 00:38:40 2014 ericqUpdateLSCsweep + RMS as uncertainty


BUT, what we really need (instead of just the DC sweeps) is the DC sweep with the uncertainty/noise displayed as a shaded area on the plot, as Nic did for us in the pre-CESAR modelling.

I've taken a first stab at this. Through various means, I've made an estimation of the total noise RMS of each error signal, and plotted a shaded region that shows the range of values the error signal is likely to take, when the IFO is statically sitting at one CARM offset. 

I have not included any effects that would change the RMS of these signals in a CARM-offset dependent way. Since this is just a rough first pass, I didn't want to get carried away just yet. 

For the transmission PDs, I measured the RMS on single arm lock. I also measured the incident power on the QPDs and thorlabs PDs for an estimate of shot noise, but this was ridiculously smaller than the in-loop RIN. I had originally though of just plotting sensing noise for the traces (i.e. dark+shot), because the amount of seismic and frequency noise in the in-loop signal obviously depends on the loop, but this gives a very misleading, tiny value. In reality we have RIN from the PRC due to seismic noise, angular motion of the optics, etc., which I have not quantified at this time. 

So: for this first, rough, pass, I am simply multiplying the single transmission noise RMSs by a factor of 10 for the coupled RMS. If nothing else, this makes the SqrtInv signal look plausible when we actually practically find it to be plausible. 

For the REFL PDs, I misaligned the ITMs for a prompt PRM reflection for a worst-case shot noise situation, and took the RMS of the spectra. (Also wrote down the dark RMSs, which are about a factor of 2 lower). I then also multiplied these by ten, to be consistent with the transmission PDs. In reality, the shot noise component will go down as we approach zero CARM offset, but if other effects dominate, that won't matter. 

Enough blathering, here's the plot:


Now, in addition to the region of linearity/validity of the different signals, we can hopefully see the amount of error relative to the desired CARM offset. (Or, at least, how that error qualitatively changes over the range of offsets)

This suggests that we MAY be able to hop over to a normalized RF signal; but this is a pretty big maybe. This signal has the response of the quotient of two nontrivial optical plants, which I have not yet given much thought to; it is probably the right time to do so...

  10630   Wed Oct 22 02:35:45 2014 JenneUpdateLSCEfforts at hopping PRMI to REFL165

[EricQ, Jenne]

The first half of our evening was spent working on CARM and DARM in PRFPMI, and then we moved on to the PRMI part.

I moved the DARM ALSdiff -> TransDiff transition to be after the CARM ALScomm -> SqrtInvTrans transition in the carm_cm_up script.  After I did that, I succeeded every time (at least  10?  We did it many times) to get both CARM and DARM off of the ALS signals. 

We tried for a little while looking at transitioning to REFL11 normalized by the sum of the transmissions, but we kept losing lock.  We also several times lost lock at arm powers of a few, when we thought we weren't touching the IFO for any transitions.  Looking at the lockloss time series did not show any obvious oscillations in any of the _IN1 or _OUT channels for the length degrees of freedom, so we don't know why we lost lock, but it doesn't seem to be loop oscillations caused by changing optical gain.  Also, one time, I tried engaging Rana's "Lead 350" filter in FM7 of the CARM filter bank when we were on sqrtInvTrans for CARM, and the arm powers were around a few, but that caused the transmission signals to start to oscillate, and after one or two seconds we lost lock.  We haven't tried the phase lead filter again, nor have we tried the Boost2 that is in FM8. 

We increased the REFL11 analog gain from 0dB to 12dB, and then reset the dark offsets, but still weren't able to move CARM to normalized REFL11. Also, I changed the POP22 demod phase from 159 degrees to 139 degrees. This seems to be where the signal is maximized in the I-phase, while the arms are held off resonance, and also partway up the resonance peak. 

We then decided that we should go back to the PRMI situation before trying to reduce the CARM offset further.  We can robustly and quickly lock the PRMI on REFL33 while the arms are held off resonance with ALS.  So, we have been trying to acquire on  REFL33 I&Q, and then look at switching to REFL 165 I&Q.  It seems pretty easy to get PRCL over to REFL165 I (while leaving MICH on REFL33 I).  For REFL33, both matrix elements are +1.  For PRCL on REFL165, the matrix element is -0.08.  We have not successfully gotten MICH over to REFL 165 ever this evening. 

We went back and set the REFL165 I&Q offsets so that the outputs after the demod phase were both fluctuating around 0.  I don't know if they were around +/-100 because our dark offsets were bad or what, but we thought this would help.  We were still able to get PRCL transitioned no problem, but even after remeasuring the MICH REFL33 vs. REFL165 relative gains, we still can't transition MICH.  It seems like it's failing when the REFL33Q matrix element finally gets zeroed out, so we're not really getting enough signal in REFL165Q, or something like that, and throughout the rest of the transition we were depending entirely on REFL33Q. 

So. Plan:

  • Get PRMI on REFL165 while arms are held off resonance. 
    • May require PRCL-MICH FF decoupling, by combining error signals?
    • May require looking back at simulations to see what we expect the relative gains and signs to be.
  • Look at CARM loop stability in simulation for REFLDC, REFL11, and normalized REFL11.  Is there a stable loop path from about 100pm down to 0pm on normalized REFL11?
  10631   Wed Oct 22 06:32:29 2014 ranaUpdateLSCsweep + RMS as uncertainty


 This is looking very useful. It will be useful if you can upload some python code somewhere so that I can muck with it.

I would guess that the right way to determine the trans RMS is just to use the single arm lock RIN and then apply that as RIN (not pure TR RMS) to the TR signals before doing the sqrt operation.

  10634   Thu Oct 23 02:08:40 2014 JenneUpdateLSCIncreased DARM gain

I changed the carm_cm_up.sh script so that it requires fewer human interventions.  Rather than stopping and asking for things like "Press enter to confirm PRMI is locked", it checks for itself.  The sequence that we have in the up script works very reliably, so we don't need to babysit the first several steps anymore. 

Another innovation tonight that Q helped put in was servoing the CARM offset to get a certain arm power.  A failing of the script had been that depending on what the arm power was during transition over to sqrtInvTrans, the arm power was always different even if the digital offset value was the same.  So, now the script will servo (slowly!!) the offset such that the arm power goes to a preset value.

The biggest real IFO progress tonight was that I was able to actually measure the CARM and DARM loops (thanks ChrisW!), and so I discovered that even though we are using (TRX-TRY)/(TRX+TRY) for our IR DARM error signal, we needed to increase the digital gain for DARM as the CARM offset was reduced.  For ALS lock and DC trans diff up to arm powers of 3, we use the same ol' gain of 6.  However, between 3 - 6, we need a gain of 7.  Then, when we go to arm powers above 6 we need a gain of 7.5.  I was also measuring the CARM loop at each of these arm powers (4, 6, 7, 8, 9), but the gain of 4 that we use for sqrtInvTrans was still fine. 

So, the carm_cm_up script will do everything that it used to without any help (unless it fails to find IR resonance for ALS, or can't lock the PRMI, in which case it will ask for help), and then once it gets to these servo lines to slowly increase the arm power and increase the DARM gain, it will ask you to confirm before each step is taken.  The script should get you all the way to arm powers of 9, which is pretty much exactly 100pm according to Q's Mist plot that is posted.

The CARM and DARM loops (around the UGFs) don't seem to be appreciably changing shape as I increase the arm powers up to 9 (as long as I increase the DARM loop gain appropriately).  So, we may be able to go a little bit farther, but since we're at about 100pm, it might be time to look at whether we think REFL11 or REFLDC is going to be more promising in terms of loop stability for the rest of the way to resonance.

Here are some plots from this evening. 

First, the time I was able to get to and hold at arm powers of 9.  I have a striptool to show the long time trends, and then zooms of the lockloss.  I do not see any particular oscillations or anything that strikes me as the cause for the lockloss.  If anyone sees something, that would be helpful.




This next lockloss was interesting because the DARM started oscillating as soon as the normalization matrix elements were turned on for DARM on DC transmissions.  The script should be measuring values and putting in matrix elements that don't change the gain when they are turned on, but perhaps something didn't work as expected.  Anyhow, the arm powers were only 1ish at the time of lockloss.  There was some kind of glitch in the DARM_OUT (see 2nd plot below, and zoom in 3rd plot), but it doesn't seem to have caused the lockloss.




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