[Manasa, EricQ, Gabriele]

Today we changed the PRC length translating PR2 by 27 mm in the direction of the corner. After this movement we had to realign the PRC cavity to get the beam centered on PRM, PR2, PR3, BS (with apertures) and ITMY (with aperture). To realign we had to move a bit both PR2 and PR3. We could also see some flashes back from the ETMY . //Edit by Manasa : We could see the ETMY reflection close to the center of the ITMY but the arm is not aligned or flashing as yet//. 

After the realignment we measured again the PRC length with the same method of yesterday. We only had to change one of the length to measure, because it was no more accessible today. The new map is attached as well as the new script (the script contains also the SRC length estimation, with random numbers in it).

The new PRC length is 6753 mm, which is exactly our target!

The consistency checks are within 5 mm, which is not bad.

We also measured some distances to estimate the SRC length, but right now I'm a bit confused looking at the notes and it seems there is one missing distance (number 1 in the notes). We'll have to check it again tomorrow.

 [EricQ, Gabriele]

We could carry out the measurement of PRC length. The AS110 photodiode was plugged into REFL11. So REFL11 is giving us the AS11 signal. Here is the procedure.

1. Lock MICH.
2. Add a line in MICH (amplitude 20000 counts)
3. Tune AS11 demod phase to have the line in I.
4. Change the demod phase by steps of 1 degree around the rough optimum, taking one minute of data at each point
5. Lock PRMI on sidebands
6. Add a line in MICH (amplitude 500 counts)
7. Tune AS11 demod phase to have the line in I.
8. Change the demod phase by steps of 1 degree around the rough optimum, taking one minute of data at each point

We repeated the same measurement also using AS55, with the same procedure.

Roughly, the phase difference for AS11 was 11 degrees and for AS55 it was 23 degrees. A more detailed analysis and a calibration in terms of PRC length will follow.

 I analyzed the data we took yesterday, both using AS11 and AS55. For each value of the phase I estimated the Q/P ratio using a demodulation code. Then I used a linear regression fit to estimate the zero crossing point.

Here are the plots of the data points with the fits:

The measurements a re more noisy in the PRMI configuration, as expected since we had a lot of angular motion. Also, the AS11 data is more noisy. However, the estimated phase differences between PRMI and MICH configurations are:

• AS11 = -10.9 +- 1.0 degrees
• AS55 = -21.1 +- 0.4 degrees

The simulation already described in slogs 9539 and 9541 provides the calibration in terms of PRC length. Here are the curves

The corresponding length errors are

• AS11 = 1.44 +- 0.13 cm
• AS55 = 0.59 +- 0.01 cm

The two results are not consistent one with the other and they are both not consistent with the previous estimate of 4 cm based on the 55MHz sideband peak splitting.

I don't know the reason for this incongruence. I checked the simulation, repeating it with Optickle and I got the same results. So I'm confident that the simulation is not completely wrong.

I also tried to understand which parameters of the IFO can affect the result. The following ones have no impact

• Beam matching
• ITM curvatures
• Schnupp asymmetry
• Distance PR-BS
• ITM and PRM misalignments

The only parameters that could affect the curves are offsets in MICH and PRCL locking point. We should check if this is happening. A first quick look (with EricQ) seems to indicate that we indeed have an offset in PRCL. However, tonight the PRMI is not locking stably on the sidebands. 

If possibile, we will repeat the measurement later on tonight, checking first the PRCL offset.

 [Manasa, EricQ, Gabriele]

We managed to measure the PRC length using a procedure close to the one described in slog 9573.

We had to modify a bit the reference points, since some of them were not accessible. The distances between points into the BS chamber were measured using a ruler. The distances between points on different chambers were measured using the Leica measurement tool. In total we measured five distances, shown in green in the attached map.

We also measured three additional distances that we used to cross check the results. These are shown in the map in magenta.

The values of the optical lengths we measured are:

LX    = 6828.96 mm
LY    = 6791.74 mm
LPRC  = 6810.35 mm
LX-LY = 37.2196 mm  

The three reference distances are computed by the script and they match well the measured one, within half centimeter:

M32_MP1  = 117.929 mm   (measured = 119 mm)
MP2_MB3  = 242.221 mm   (measured = 249 mm)
M23_MX1p = 220.442 mm   (measured = 226 mm)

See the attached map to see what the names correspond to.

The nominal PRC length (the one that makes SB resonant without arms) can be computed from the IMC length and it is 6777 mm. So, the power recycling cavity is 33 mm too long w.r.t. the nominal length. This is in good agreement with the estimate we got with the SB splitting method (4cm).

According to the simulation in the wiki page the length we want to have the SB resonate when the arms are there is 6753 mm. So the cavity is 57 mm too long.

Attached the new version of the script used for the computation.

Ah, AWESOME. Indefinite PRMI lock was finally achieved.

POP setup

- Looked at the POP setup. Checked the spot on POP110 PD. Found some misalignment of the beam.
  The beam spot was aligned to the PD with PRMI locked. The value of POP110I almost doubled by the alignment
  and recovered previous value of 400. Therefore previous normalization values of MICH 0.01 / PRCL 100 were restored.

- Placed PDA36A (Si 3.6mmx3.6mm) on the POP path that Jenne prepared. The gain knob was set to 40dB.
  Since the original spot had been too small, a lens with f=50mm was inserted in order to expand the beam.
  Connected the PD output to the SMA feedthrough on the ITMX table enclosure.
  I found the BNC cable labeled "PO DC" hanging. Connected this cable to the enclosure SMA.

- Went to the LSC rack. Found the corresponding PO DC cable. Stole the POPDC channel from POP110I Bias T to this PO DC cable.

- Razor blade setup: Machined a junk Al bracket in order to fix a razor blade on it. Attached the Al bracket to a sliding stage.

Locking

- Locked the PRMI with REFL33I&AS55Q. Cut the beam into half by the razor blade.

- Made a temporary PRM_ASC_YAW filter.
  Zero: 0Hz Pole: 2kHz
  Resonant Gain 3.2Hz Q:2 Height 30dB
  Butterworth 2nd-order 60Hz

  => Expected UGF 0.1Hz&10Hz

- CDS: By the work described in this entry, the POPDC signal was connected to the "MC" bank of the LSC.
            BTW, the 11th row of the LSC output matrix is connected to the PRM_ASC_YAW.

- The "MC" servo input (i.e. the POPDC signal) was normalized by POP110I (without SQRTing).

- Engaged the PRM ASC path. Gradually increased the gain of PRM_ASC_YAW. G=+100 seemed to be the best so far.
  It was visible that the spot on the POP CCD was stablized in yaw.

- The lock lasted for ~40min. Took several measurements, alignment adjustment, etc.

- Tweaking the PRM ASC unlocked the PRMI.

- Locked again. Switched from REFL33I/AS55Q (x1/x1) combination to REFL55I/REFL55Q (x1/x0.3) combination.
  This also kept the lock more than 20min.

[Jon, Gautam, Johannes]

We did the following today:

1. Dither align arms such that ITMs were reliable arm references.
2. Configure the IFO such that ITMX single bounce was the only visible beam reaching the AS port from the symmetric side - ITMY, both ETMs, PRM and SRM were misaligned.
3. Do coarse alignment on the AS table using the usual near field / far field overlap technique, with "near" and "far" dictated by arm reach on the AS table. In this way, the ingoing AUX beam and the PSL single bounce from ITMX were collimated on the AS table.
4. Lock the AUX / PSL PLL. We expected a beatnote on AS110 at eithe (80-50)=30 MHz or (80+50)=130 MHz. 80 MHz is the AOM driver frequency, while 50 MHz is the PLL offset. (Marconi was actually set to 60 MHz, prolly Keerthana forgot to reset it after some remote experimentation).
5. Beat was found at 30 MHz. 
6. Input steering of AUX beam into the IFO was tweaked to maximize the beat. Johannes claims he saw -35 dBm on AS110 last week. But Jon reported a best effort of ~-60 dBm today. Not sure how to square that circle.
7. Once we were confident that the input of the AUX and PSL beams were well aligned, we decided to do a scan. PRC was chosen as PRMI can be locked but I don't yet know the correct settings for SRMI locking, and DRMI seemed too ambitious for daytime.
   • PRMI was locked on carrier.
   • Jon can comment more here, but the measurement with AM sidebands does not rely on any beatnote on the AS110 PD, it is just looking for coupling of the AM sideband into the IFO from the AS port at resonant frequencies of the PRC.
   • For a coarse sweep, we swept from 1-60 MHz, 801 points, and the IF bandwidth was set at 30 kHz on the AG4395.
   • Transfer function being measured was the ratio of AM signal detected at AS110 PD, to RF drive applied to the AOM driver.
   • We were expecting to see dips separated by the PRC FSR (~25 MHz, since the PRC RT length is ~12.5m), when the AM sideband becomes resonant in the PRC.
   • But we saw nothing. Need to think about if this is an SNR problem, or if we are overlooking something more fundamental in the measurement setup.

This measurement seems like a fine candidate to trial the idea of looking for the FSRs (and in general, cavity resonances) of the PRC in the phase of the measured TFs, rather than the amplitude.

The PRC FSR is, of course, very close to twice of our f1 moudlation frequency (11MHz x 2 = 22MHz) .

I still don't understand what response the measurement is looking for. I understood the idea of using the subcarrier as a stablized carrier to the PRC with a certain freq offset from the main carrier. I suppose what was swept was the AOM modulation frequency (i.e. modulation frequency of the AM applied to the subcarrier). If that is the case, the subcarrier seemed fixed at an arbitorary frequency (i.e. 50MHz) away from the carrier. If one of the AM sidebands hits the PRC resonance (i.e. 22, 44, 66MHz away from the main carrier), you still have the other sideband reflected back to the AS. Then the RF signal at the AS is still dominated by this reflected sideband. I feel that the phase modulation is rather suitable for this purpose.

If you are talking about ~MHz AM modulation by the AOM and scanning the PLL frequency from 1MHz to 60MHz, the story is different. And this should involve demodulation of the AS signal at the AM modulation frequency. But I still don't understand why we don't use phase modulation, which gives us the PDH type signal at the reflection (i.e. AS) port...

Here's a Finesse modeling of what we're expecting to observe with this test. It uses Gautam's base model of the 40m IFO with appropriate modifications for the needed configuration.

The idea is to lock the IFO in the SRMI configuration, with the phase-locked AUX beam injected from the AS port. The AUX beam is imprinted with AM sidebands and slightly misaligned relative to the SRC so as to transfer power into HOM1. The RF network analyzer provides the drive signal for the AOM, and its frequency is swept to coherently measure the transfer function [reflected AUX beam / drive]. The reflected AUX beam is sensed by the AS110 PDA10CF.

It is also possible to drive PM sidebands as Koji suggests, but the squeezer group has encouraged using AM for practical advantages. The SNR with AM is a bit higher (less power lost into harmonics at large modulation index), there is a broadband AOM already available aligned to the SQZ beam at LLO, and there is also concern that driving strong PM could interfere with the SQZ control loops.

Expected SRMI Response

Attachment #1 shows the expected response to swept-AM in SRMI. Resolving just the FSR and the first-order mode splitting is sufficient to extract the SRC Gouy phase.

Expected DRMI Response

Since the 40m has not been opearted in SRMI since ~2016 (last done by Eric Q.), Gautam believes it may take some time to relock this configuration. However, the modeling indicates that we can likely obtain sufficient sensitivity in DRMI, which would allow us to proceed faster. Attachment #2 shows the expected response to swept-AM in DRMI. The PRC leakage signal turns out to be significantly smaller than the SRC reflection (a factor of ~30 in amplitude), so that the signal still retains its characteristic shape to a very good approximation. The tradeoff is a 10x reduction in SNR due to increased PSL shot noise reaching AS110.

Based on this, we should proceed with DRMI scans instead of PRMI next week.

Just some plots. There is nothing new here except for the fact that I learned how to analyze phase maps myself and how to prepare them for Finesse. In other words, everything is ready for a Finesse simulation.

These phase maps show the raw measurement of ITMY, ITMX and PRC:

Subtracting out the tilt from all phase maps, and the curvature from the PRC (I found the fit 121m consistent with previous fits), the one obtains the following residuals that can be used in Finesse (order is again ITMY, ITMX and PRC):

[Koji, Jenne]

* Found that IPANG was no longer centered, so we used PZT2's sliders to get the spot back on the center of the QPD.  Koji points out that I should have moved the lens even farther away, to have a larger beam (many mm, not just ~1) on the QPD.

* Found that MICH alignment had drifted, so used ITMX to realign MICH.

* Aligned PRM, got REFL beam through viewport.  Just made sure reflected beam was colinear with incident beam.

* PRC flashes were visible on AS camera. 

* PRM was more precisely aligned to have good interference with ITM reflections, by looking at AS camera.

* Decided to align SRM.  Spot was ~5mm too far to the north on the SRM....so we were off from center by ~5mm.

* Moved SR2 yaw a little bit to get spot centered on SRM.

*  Couldn't align SRM within bias slider range, so moved SRM in yaw to get reflected beam colinear with incident beam.

* Centered the spot on the steering mirrors.  The 2nd steering mirror after the SRM was moved by ~1 inch.  All mirrors after that were aligned to match this new beam.

* Found spot on AS table, aligned AS table mirrors so that beam hits AS55 PD window.  Haven't actually centered beam on PD.

* Transmission of 99% reflector was too weak to use with a card to get the beam back on the AS camera, so we moved the camera over to the AS110 path.

* Precisely aligned PRM and SRM by watching AS camera.

* Both the PRC and SRC look kind of funny.  Koji agrees.  Seriously.  They're a little weird. We can't align either recycling cavity, one ITM at a time (so PRM with ITMX, PRM with ITMY, SRM with either single ITM) to get rid of all the fringes.  Something is definitely funny.  It's got to be in the recycling cavities, since the weirdness is common between both ITMs for a given recycling mirror.  We need to take Sensoray views of these tomorrow.=

* There is some clipping on the right side of the AS camera view.  We have determined that it is not clipping at the viewport exiting the vacuum, but we aren't sure where it is.  It is at least before PZT4 (the 2nd PZT in the output AS path). 

NOTE: There was a small bug in my initial calculation.  The plots and numbers have been updated with the fixed values.  The conclusion remains the same.

Using Nic's a la mode mode matching program, I've calculated the PRC mode and g-parameter for various PR2/3 scenarios.  I then looked at the overlap of the resultant PRC eigenmodes with the ARM eigenmode.  Here are the results:

NOTE: each optical element below (PR2, ITM, etc.) is represented by a compound M matrix.  The z axis in these plots is actually just the free space propagation between the elements, not the overall optical path length.

ARM

This is the ARM mode I used for all comparisons:

PRC, nominal design (flat PR2/3)

This is the nominal "as designed" PRC, with flat PR2/3 folding mirrors.

 ARM mode matching: 0.9998

PRC, both PR2/3 flipped

This assumes both PR2 and PR3 have a RoC of -600 when not flipped, and includes the affect of propagation through the substrates.

ARM mode matching: 0.9806

PRC, only PR2 flipped

In this case we only flip PR2 and leave PR3 with it's bad -600 RoC:

ARM mode matching: 0.9859

Discussion

I left out the current situation (PR2/3 with -600 RoC) and the case where only PR3 is flipped, since those are both unstable according to a la mode.

I guess the main take away is that we get slightly better PRC stability and mode matching to the arms by only flipping PR2.

This surprises me. I am curious to know the reason why we can't make the cavity stable by flipping the PR3 as PR3 was supposed to have more lensing effect than PR2 according to my statement.

 I would guess that either flipping PR2 or PR3 would give nearly the same effect (g = 0.9) and that flipping both makes it even more stable (smaller g). But what we really need is to see the cavity scan / HOM resonance plot to compare the cases.

The difference of 0.5% in mode-matching is not a strong motivation to make a choice, but sensitivity to accidental HOM resonance of either the carrier or f1 or f2 sidebands would be. Should also check for 2*f2 and 2*f1 resonances since our modulation depth may be as high as 0.3. Accidental 2f resonance may disturb the 3f error signals.

You would guess, and I would have guessed too, but the calculations tell the story.   Flipping both does not increase the stability.  The main issue is that flipping PR3 induces considerable astigmatism.  This is why flipping PR3 alone does not make the cavity stable.  I will do some simple calculations today that will demonstrate this effect.

But again, we should only change one thing at a time and understand that before moving on.  Given that the calculations show that flipping only PR2 should alone have a positive affect, we should do just that first, and verify that we understand what's going on, before we move on to making more changes.

I will try to make some more arbcav runs as well, for just the flipped PR2.

Yes, at 45degrees PR3 will only have a curvature of about 850m for the vertical mode of the beam, apparently not enough to stabilize the cavity.

I intended to post a long analysis of the PRC/arm mode matching for the various TT situations using Nic's a la mode mode matching program, but I seem to have encountered what I think might be a bug.  I'll talk to Nic about it first thing in the AM.  Once the issue is resolved I should be able to post the full analysis fairly quickly.  Sorry about the delay.

I have added the PRCL ASS to the main ASS screen, and created the servo and lockin screens.  The filters loaded are the same as those used for the arms (bandpasses and lowpasses for the lockins, and an integrator for the servo).

I'm going to try to lock, and get the ASS to work.

 I have added an IPC sender from the LSC model, to send POPDC to ASS.  I have copied over the structure of the arms' ASS, to do the same for PRCL.  I have set it up to dither the PRM, and feed back to the PRM.  I did not include an LSC set, since I'm assuming that we'll set the input pointing with the arms, and just want to move the PRM to maximize POPDC.

Models have been compiled, installed, and restarted, and the daqd was restarted.

Why POPDC???

 I guess I was thinking that POPDC was a proxy for any type of PRCL lock.  Even if we're sideband locked, there is still some signal in POPDC (although it is very small relative to a carrier lock - ~40cts vs. 1,000cts).  However, as soon as this question was asked of me, I realized that one of the 2f demodulated signals made more sense. 

Since I want the ability to choose between POP110 and POP22, I have put a little 1x3 input matrix before the PRCL lockins in the ASS model.  Since POPDC was already there, I included it as an option in the matrix (in case we ever want to do some PRCL ASS after we have some carrier resonating as well). 

Koji is working on PRMI locking with different photodiodes, and rather than typing different numbers into the input matrix, it is more convenient to just be able to click on/off buttons for different filter banks.  So, the CARM filter bank in the LSC model is currently being borrowed as a secondary PRCL filter bank.  I have copied all of the current PRCL filters over to the CARM filter bank. 

Just for reference, although we have not yet used CARM for CARM, the previous filters were the "default" set, +6dB, 0:1, 1:5, 1:50, 1000:10, RG3.2, RG16.5, RG24, empty, empty.  These are currently the same in the DARM and MC filter banks, so we can copy them back over in the future.

I'm sad.  And frustrated. 

The PRCL angular feed forward is not working, and without it I am having a very difficult time keeping the PRMI locked while the arms are at high power (either buzzing, or the one time I got stable high power partway through the transition).  Obviously if the PRMI unlocks once CARM and DARM are mostly relying on the REFL signals, I lose the whole IFO. 

Q and I had been noticing over the last few weeks that the angular feed forward wasn't seeming quite as awesome as it did when I first implemented it.  We speculated that this was likely because we had started DC coupling the ITM optical levers, which changes the way seismic motion is propagated to cavity axis motion (since the ITMs are reacting differently).

Anyhow, today it does not work at all.  It just pushes the PRM until the PRMI loses lock. I am worried that, even though Rana re-tuned the BS and PRM oplev servos to be very similar to how they used to be, there is enough of a difference (especially when compounded with the DC coupled ITMs) that the feed forward transfer functions just aren't valid anymore.

Since this prevents whole IFO locking, I spent some time trying to get it back under control, although it's still not working. 

I remeasured the actuator transfer function of how moving PRM affects the sideband spot at the QPD, in the PRMI-only situation.  I didn't make a comparison plot for the yaw degree of freedom, but you can see that the pitch transfer function is pretty different below ~20Hz, which is the whole region that we care about.  In the plot below, black is from January (PRMI-only, no DC-coupled ITMs) and blue is from today (PRMI-only, with DC-coupled ITMs, and somewhat different BS/PRM oplev setup):

Pitch_oldVsNew.pdf

I calculated new Wiener filters, and tried to put them in, but sometimes (and I don't understand what the pattern is yet) I get "error" in the Alternate box, rather than the zpk version of my sos filter.  It seems to go away if you use fewer and fewer poles for fitting the Wiener filters, but then the fit is so poor that you're not going to get any subtraction (according to the residual estimation plot that uses the fitted filters rather than the ideal Wiener filters). The pitch filters could only handle 6 poles, although the yaw filters were fine with 20.

The feed forward just keeps pushing the PRM away though.  I flipped the signs on the Wiener filters, I tried recalculating without the actuator pre-filtering, I don't know why it's failing.  But, I'm not able to lock the interferometer.  Which sucks, because I was hoping to finally get most of my noise coupling measurements done today.

This is a mid-evening update, so I don't forget all the stuff I've already done.

Aligned PRMI, no nice flashes on POP110.  Aligned and locked PRM-ITMY half-cavity on the carrier, and used that POP beam to center the beam on the POP110 PD.  I also turned on the new QPD and centered the beam on it.

Notes about QPD setup:  The "zero/cal" switch is OFF, so none of the small knobs on the front (basically, everything but the gain knob) should be bypassed.  The gain knob is set to position 3.  This is the highest gain that I can have without the "too much light" saturation light blinking on the front panel.  (During this time, POP110I is flashing around 200 counts).

I made a super hacky ASC screen, which is accessible from the ASC button on the sitemap.  While there is a pitch path in the model, I only put in the yaw elements (except for the QPD readouts) in the screen, since that's what I'll be using for now. 

I added filter banks to the front side of the ASC subblock in the ASS model, so that I have a place to monitor the QPD signals on the screen and with striptool. 

Using the settings that Koji recorded in elog 8521 in the "Locking with SQRT(POP110I)" section (and no ASC engaged so far), I can lock the PRMI for ~10 or 20 seconds, at 150 or 200 counts on POP110I.  So, I'm doing well so far, and next up is to copy the ASC filters Koji made in elog 8562, and try the new ASC.

With Rana's help/supervision/suggestions, I have closed the loop on the PRMI ASC servo with the new QPD.  I think I've had it locked for ~30+ minutes now.  It was locked for ~45 minutes, but then the MC momentarily lost lock.  I immediately recovered the PRMI+ASC (after small PRM yaw tweaking, since the ASC isn't triggered yet, so the MC lockloss caused a big yaw step function to go to the PRM, which displayed a bit of hysteresis.).

My biggest problem was that I didn't really understand Koji's servo filter choices, so I wasn't using the right ones / doing good things.  In particular, I need to compensate for the oplev servo filters.  The oplev servo shape is something like ^, so the 1/(1+G) shape is something like =v= (ignoring the lower horizontal lines there).  For tonight, we just turned off the PRM oplevs, but clearly this isn't a permanent solution.  (Although, after Rana went in and roughly centered the PRM oplev, we noticed that turning the oplev on and off doesn't make a huge difference for the PRM....we should investigate why not.  Also, we turned off the FM2 3.2Hz resonant gains in the PRM oplevs, since the Q of those filters is too high, much higher than our actual stacks). 

Rana and I also locked the PRM-ITMY half cavity, and used that beam to realign the beam onto the POP QPD, POP110 PD, and the camera. 

The POP QPD pitch and yaw signals with the half cavity have some noise, that looks like 60Hz crap.  Since this goes away (rather, is much less noticeable) with the regular sideband-locked PRMI, we suspect this is a problem with perhaps the normalization, with the sum very low, and having some noise on it.

Once we had our ASC filters set up (not the 10Hz boost yet though, I think), if I increased the gain from -0.02 to -0.03, we start to get some gain peaking.  With a gain of -0.04, the peak is very noticeable around 250Hz.  We aren't sure where this is coming from, since it shouldn't be coming from the ASC loop.  The UGF of that loop is much lower (I measured it, to check, and the UGF is ~5Hz). Anyhow, this is still a mystery, although the gain of -0.02 holds the cavity pretty well.

I measured the power spectra of the POP QPD pit, yaw, sum, as well as POPDC and POP110I, with the ASC loop on and off (dashed lines are with the loop on.  You can see that the yaw motion as seen on the QPD was reduced by almost 2 orders of magnitude below 1Hz.  It also looks like we can win some more by turning on the equivalent pitch ASC servo (this is also something we see when looking at the dataviewer traces).

I also tried to measure the PRMI sensing matrix, but I get some weird results, even after I double the drive actuation.  I need to be checking whether or not my drive is actually coherent with the error signals that I'm seeing, because right now I'm not sure that I believe things. I'm going to leave that on the to-do list for tomorrow night though.

Next up:

* Engage POP QPD -> pitch loop, copying yaw loop.

* enable ASC triggering

* model PRMI sensing matrix and error signals, bringing one arm into resonance

* Lock the PRMI, and bring the Xarm into IR resonance using the ALS system.

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

Here are some numbers and plots from the night:

Right now, I'm locking the LSC with:

MICH LSC with AS55Q, FMs 4 and 5 on, FM 3 is triggered, gain = -40.0, normalized by sqrt(POP110I)*0.1

PRCL LSC with REFL33I, FMs 4 and 5 on, FM 9 is triggered, gain = +2.5, normalized by sqrt(POP110I)*10

(FM3 of MICH and FM9 of PRCL are the same, just in different spots).

The ASC (only POP yaw -> PRM yaw right now) has:

FMs 1,2,5,6 on (1 = integrator [0:0.1], 2 = 3.2 res gain, 5 = [1000,1000:1 and gain of 0.01], 6 = 10Hz boost).  Gain = -0.020,  Limit=5000.

Turn off the input, turn on the output and the gain, clear the histories (to clear out the integrator in FM1), then turn on the input.

PRM oplev is OFF. (need to put in a filter to compensate for it in the ASC servo, but for tonight, we just turned it off.)

We measured the spectra of the POP QPD signals with the ASC loop on and off:

I also measured the ASC loop (with the PRM oplev still off):

(sorry about the separate plots - I can't make DTT give me more than 2 plots on a page at a time right now, so I'm giving up, and just making 3 separate pages)

Weird sensing matrix, unsure if I'm really getting good coherence:

Rana had the epiphany that I didn't have any antiwhitening for my POP QPD.  Ooops. 

We looked at the schematic for the Pentek Generic board (pdf), and saw that it has a Zero @ 15Hz, and Poles @ 150Hz and 1500Hz, times 2 stages.  We determined from the TF that I posted that probably both stages are engaged, so I made an antiwhitening filter consisting of the inverse (so, 2 poles at 15Hz, 2 zeros at 150Hz and 2 zeros at 1500Hz).  [Rana points out that for this low frequency system we may not want to include the 1500Hz compensation, since it is probably just enhancing ADC noise].  The ASC system worked really well, really easily, after that.

Another note though, the AA stage of the Pentek Generic boards have 4 poles at 800Hz, which are not compensated.

Rana also added a 60Hz comb to the filter bank with the AntiWhitening, since the QPD has an unfortunately large amount of 60Hz noise.  Also, the 60Hz lowpass in the ASC loop was engaged for both pitch and yaw.

Rana, Lisa and Manasa also found that the ASC system was *more* stable with the PRM oplev ON. 

So, the ASC locking situation is:

PRM oplev loops on.

AS-POP_QPD_[PIT/YAW] filter banks with FM1, FM6 on.

ASC-PRCL_[PIT/YAW] filter banks with FM1, FM5, FM6 and FM9 on.

ASC-PRCL_YAW_GAIN = -0.040

ASC-PRCL_PIT_GAIN = +0.030

(No triggering yet).

The ASC Up and Down scripts (which are called from the buttons on the ASC screen) have all of these gain settings, although they assume for now that all the filters are already on.

Here's a screenshot of the power spectra showing the angular motion suppression. The PDF is attached so you can zoom in and see some details.  The dashed lines are the "PRMI locked, ASC off" case, and the solid lines are the "PRMI locked, ASC on" case.  You can see that according to the QPD, we do an excellent job suppressing both the pitch and yaw motion (although better for yaw), but there isn't a huge effect on POPDC or POP110I.  While we could probably do better if we had a 2 QPD system with the QPDs at differet gouy phases, this seems to be good enough that we can keep the PRMI locked ~indefinitely. 

I would like to compile the ASC model, so that I can implement triggering.  For tonight, we did not have the ASC engaged during our PRMI+Xarm tests (see Manasa's elog), but I think it'll make things a little easier if we can get the ASC going automatically.

Last night before dinner, I copied over the ASC yaw servo filters to the ASC pitch filter bank.  Using ASC gain of +0.001, I was getting the ~250Hz oscillations that Rana and I had seen with yaw. 

Rana pointed out to me that my measured TF of the yaw loop doesn't look right up in the several hundred Hz region:

As you can see on the right side, which is all of the PRCL ASC yaw filter banks, multiplied by a simulated pendulum filter, the magnitude should just keep decreasing.  However, on the measured plot on the left, you can see that I have a little gain hump.  I'm not sure what this is from yet.

I didn't have any success with the ASC tonight.  I copied over the filters that Koji had used in elog 8562, and put them in the new ASC filter banks (and turned them off in the SUS-PRM_ASCYAW bank).  I also moved all the old scripts that were in .../scripts/ASC to an OLD subdirectory (the most recent edit is from 2009 sometime).  I then copied over the up and down scripts that Koji had written for his ASC test into the ..../scripts/ASC directory, and modified them to work with my new channels. 

I then tried locking, and wasn't very successful.  Actually, my best lock, ~4 minutes, including tweaking up the PRM alignment, was when the ASC path was off (even though I thought it was on).  After discovering my mistake, I tried locking for another hour or so, but haven't really gotten anywhere.  The lock stretches I'm getting are rarely long enough for me to get to the terminal and run my up script, and the maybe ~6 or 7 times I've been able to run it, I haven't converged toward finding a good gain value for the PRC yaw loop.  At some point, I redid the MICH alignment since it had drifted away a bit, but that didn't really help.

I think that one of the next things I might try is carrier-locking the PRMI, to find okay loop gain settings for the ASC path.  Since the QPD output is already normalized (I'd have to custom-make some electronics to make it non-normalized), I think the gain should be the same for both carrier and sideband lock cases.

_______________________________

Once I finally get a good, stable, PRMI sideband lock, I think I need to take the following measurements:

* CTRL and ERR spectra for MICH and PRCL

* TFs for MICH and PRCL loops

* Sensing matrix, including AS55, REFL11, REFL33, REFL55, POX and POY.

---->> Are there any others?

Attachment #1 shows the measured PRCL loop shape. The blue line is meant to be the "expected" loop shape. While the measured loop shape tracks the expectation down to ~100 Hz, I cannot explain the shape below it. I am also not sure what to make of the fact that there is high coherence down to 10 Hz fron IN2 to IN1, but no coherence between EXC/IN2. I confirmed that the low-frequency boost filters were ON during the measurement. I don't understand how a pendulum TF + the digital filters we used can account for the shape below 100Hz.

gautam 11pm: After discussing with Koji, I conclude that the low frequency loop shape is consistent with the excitation amplitude being insufficient below 100 Hz. Coherence is good between In1/In2 because they are the same signal effectively - what we need is coherence between In1 and EXC, which isn't plotted. It is still strange that Coherence between In2/EXC is ZERO....

don't use IN_1/IN_2: recall pizza meeting from a few weeks back: use IN1/EXC + Al-Gebra

Jenne, Den

We suspect PRM shows significant length to angle coupling due to large oplev beam angle in yaw.  Tonight we locked PRCL with ITMs.

We could lock PRCL on carrier to power recycling gain of 15. Lock continued for a few hours but power rin RMS was 0.15.

We triggered and normalized on POP_DC. MICH gain was -1 (filters FM3-5), PRCL gain was -8 (filters FM2,4,5,6,9).

MC_L was OFF during locking.

 The PRCL once again doesn't want to lock on sidebands for me.  I can lock on the carrier just fine (using the IFO Config settings, along with some hand-alignment of the PRM). 

However, I can't convince it to lock on sidebands.  Using the configs that I used on Dec 18th (elog 9491), I'm not getting it.  I've done the arm ASS alignment, and I've run LSCoffsets, both of which seemed to do their things appropriately. 

I'm going to attribute this today to not being in the groove yet, and I'll look at it again in the morning.

Inspired by a comment by Koji the other day, I spent some time yesterday and today working on locking a (very lossy) power recycled Y-arm. ITMX was misaligned, to save myself the headache of dealing with ITMY getting a sign flip and ITMX staying the same when the arm resonates. 

My main goal was to achieve high bandwidth control with the analog CARM servo. 

TL,DR: Transisitoned 90% to REFLDC through CM_SLOW at TRY = 2.1 twice. Couldn't make it all the way over. 

PRCL settings:

• Input: REFL165 I.
• Actuate on PRM +1
• Control: G=-.32 (~100Hz UGF); Acq on FM 4,5; Trig 1,2,3,6,9 (I modified the +10dB in FM1 to a 1kHz ELP)
• Trig: POP 110 I: 1.5 up, 0 down (max was around 4 counts, very weak PRC!)

The PRC was very stable in this configuration, which doesn't surprise me due to its simplicity. I was honestly a little surprised there was enough light to lock on 3f. REFL33 didn't work. 

My efforts to bring the Y-arm into lock were very similar to the CARM procedure we've been using recently. (Which is the motivation for this exercise)

At first I was actuating on ETMY, and got to the point where I wanted to start bringing in the CARM servo slow output, then realized that I didn't want to actuate both on the ETM and MC AO. (Maybe this would be doable, but in the end, not what I'm interested in learning about in terms of overlap with CARM locking)

From then on, I only actuated YARM on MC2. (Heads up, my lock-losses will show up in the trends of the MC2 Trans addition to the WFS.)

Transitioning the arm to SqrtInv TRY control was just as straightforward as it has been for CARM. However, engaging the LSCLock FM (FM4), would sometimes work beautifully, and sometimes kick the hell out of MC2. Keeping an eye on the error signal spectrum and UGF gave no indication which outcome would happen. Once FM4 could be engaged, the transmitted power was very stable. Without FM4, reducing the offset didn't get very far without losing lock. 

I tried a few times to bring in CM_slow (set to just IN2, i.e. offset adjusted REFLDC), at arbitrary arm powers, with little success. I didn't know how much arm power to expect at resonance, and thus didn't really know where on the line width I was.

I knew I was mostly outside of the linear regime of the PDH signals, since, even though I had good coherence between, say, REFL11 I and SqrtInvTry, with an ETMY excitation on; when I would turn TRY normalization on/off, I would see the sign of the TF change. 

I then realized that I could actively keep an eye on the trend of POY11, to see when I got to the PDH "hump", which is where REFLDC starts being usable, and SqrtInv is reaching its limit. 

This brought me to a YARM offset of .115, with a steady TRY of about 2.1. I adjusted the analog offset of the REFLDC input to the CARM board, and the digital gain of the CMSLOW input filter to get 1:1 correspondence between CMSLOW and the SQRTINVY channels. Their spectra were neigh identical, with CMSLOW having slightly more high frequency noise. 

I started stepping SQRTINV down by .1, and upping CMSLOW by .1. This shifted the offset around, so I opted for taking away gain before bringing it back, because I didn't want to get so close to resonance that SQRTINV would freak out. I got to .1*SQRTINVY + .9*CMSLOW, and lost lock. TRY was getting noisier as I made the transition. 

I'm not sure what exactly was the reason for failure. I'm going to go back over some of the data to try to get an idea.. Maybe I should've loosened up some of the gain/boosts during the transition. 

So, no great success story yet, but this configuration is a lot simpler than the full PRFPMI, and I feel that I should soon be able to get it fully controlled, and figure out a systematic way to make the digital to analog transition for this PRFP cavity, and thus have a much more informed basis for doing the same for CARM control. 

I forgot that I had already done some investigation into recovering the PRFPMI lock after my work on the RF source. I don't really have any ideas on how to explain (or more importantly, resolve) the poor seperation of MICH and PRCL sensed in our 3f (but also 1f) photodiodes, see full thread here. Anyone have any ideas? I don't think my analysis (=code) of the sensing matrix can be blamed - in DTT, just looking the spectra of the _ERR_DQ channels for the various photodiodes while a ssingle frequency line is driving the PRM/BS suspension, there is no digital demod phase that decouples the MICH/PRCL peak in any of the REFL port photodiode spectra.

I've extended my analysis to the PRFPMI case, with the current working knowledge of radii of curvature and cavity lengths. However, losses were not included.

I do not see any HOM activity within about 20nm of the carrier TM00 resonance. 

Basically, what I did was use the standard formulae for the reflection and transmission coefficients of FB cavities viewed as compound mirrors. However, I modified the normal spatial propagation terms to include the additional Guoy phase accumulated by the HOMs. I created these coefficients for each arm individually, and then used (rX + rY)/2 as a mirror in the PRC, and used that to create the transmission coefficient for the PRFPMI as a whole, as a function of frequency offset from the carrier, spatial mode order and CARM offset. As a check, this produced the correct finesse for the carrier lock to the single arm and PRFPMI. 

Here is a PRFPMI CARM FSR of all of the fields' power transmission coefficients, up to order n+m=5. 

One can observe some split peaks. There are two causes, the biggest effect is the mismatch between ETM radii of curvatures (ETMX:59.48, ETMY:60.26):, followed by asymmetric arm length(X:37.79, Y:37.81). (I judged this by the visual change of the plot when changing different factors). 

In the following plot, I broke down the peaks by mode order:

Code, plots attached!

A day late but here it is.

Eric and I turned on my SISO MCL Wiener filter elog:11535 during his PRFPMI 40min lock. We looked at the CARM_IN and CARM_OUT signals during the lock and with the MCL FF on/off. Here is the spectra:

2 weeks ago I took some data, and remembered today at the 40m meeting that I hadn't posted it.  Bad grad student.

All I'm trying to show here is that we see flashes in the arms that are larger than the ~50 units that we see saturate the Thorlabs transmission PDs. For arm power values below ~50, the QPD sum and Thorlabs PDs give approximately the same values.  So, 1 unit on the Thorlabs PDs is equivalent to 1 unit on the QPD sum, and 50 units on the Thorlabs diode is equivalent to 50 units on the QPD sum.

The situation was arms held on resonance with ALS, and the PRMI was flashing.

Arm powers of ~140 imply a power recycling gain of ~7.

PRFPMI locking has been revived.

I've had 6 5min+ locks so far; arm powers usually hit ~125 for a recycling gain of about 7; visibility is about 75%

The locking script takes a little under 4 minutes to take you from POX/POY lock to PRFPMI if you don't have to stop and adjust anything.

At Koji's suggestion, I used digital REFL11 instead of CM_SLOW, which got me to a semistable lock with some RF, at which time I could check the CM_SLOW situtation. It seemed like the whitening Binary IO switch got out of sync with the digital FM status somehow... 

I've been making the neccesary changes to the carm_cm_up script. I also added a small script which uses the magnitude of the I and Q signals to set the phase tracker gain automatically based on some algebra Koji posted in an ELOG some years ago. 

The RF transition seems much smoother now, most likely due to the improved PRC and ALS stability. In fact, it is possible to hold at arm powers of >100 solely on the digital servos; I don't think we were able to do this before until the AO had kicked in. 

Right now I'm losing lock when trying to engage the CARM super boost. I also haven't switched the PRMI over to 1F signals yet. Would be good to hook the SR785 back up for a loop TF, but I'll stop here for tonight since our SURFs are presenting bright and early tomorrow morning. 

More PRFPMI locks tonight. Right now, it's been locked for 22+ minutes, though with the PRMI still on 3F signals. I think the MC2/AO crossover needs some reshaping; there's a whole bunch of noise injected into CARM around 600 Hz, which is where the two paths differ by 180deg. (Addendum: broke lock at ~27 minutes, 4:16AM)

For most of this lock, sensing matrix excitations have been running for daytime analysis. 

The nominal IMC loop gain / EOM crossover were making the AO path very marginal. I've adjusted the nominal settings and autolocker scripts. 

There was some weird behavior of X green PDH earlier... Broadband RIN seen in ALS-TRX, coherent with the DC output of the beat PD, so really on the light. I fiddled with the end setup, and it mostly went away, though I didn't intentionally change anything. Disconcerting. 

Got to a 40 minute lock tonight. All other locks broke because of me poking something. 

I redid some sensing excitations, right after carefully measuring the CARM OLG at its excitation frequency, so I can get at the open loop PD response. 

I also used a MCL feedforward filter of Ignacio's which did not inject any observable noise into the CARM error signal during PRFPMI lock. He will make some elog about this. 

[ericq,gautam]

Given that most of the post vent recovery tasks were done, and that the ALS noise performance looked good enough to try locking, we decided to try PRFPMI locking again last night. Here are the details:

PRM alignment, PRMI locking

• We started by trying to find the REFL beam on the camera, the alignment biases for the 'correct' PRM alignment has changed after the vent
• After aligning, the Oplev was way off center so that was fixed. We also had to re-center the ITMX oplev after a few failed locking attempts
• The REFL beam was centered on all the RFPDs on the ASDC table

Post the most recent vent, where we bypass the OMC altogether, we have a lot more light now at the AS port. It has not yet been quantified how much more, but from the changes that had to be made to the loop gain for a stable loop, we estimate we have 2-3 times more power at the AS port now.

PRFPMI locking

• We spent a while unsuccessfully trying to get the PRMI locked and reduce the carm offset on ALS control to bring the arms into the 'buzzing' state - the reason was that we forgot that it was established a couple of weeks ago that REFL165 had better MICH SNR. Once this change was made, we were readily able to reduce the carm offset to 0
• Then we spent a few attempts trying to do blend in RF control - as mentioned in the above referenced elog, the point of failure always was trying to turn on the integrator in the CARM B path. We felt that the appearance of the CARM B IN1 signal on dataviewer was not what we are used to seeing but were unable to figure out why (as it turns out, we were locking CARM on POY11 and not REFL11 , more on this later)
• Eric found that switching the sign of the CARM B gain was the solution - we spent some time puzzling over why this should have changed, and hypothesized that perhaps we are now overcoupled, but it is more likely that this was because of the error signal mix up mentioned above...
• We also found the DC coupling of the ITM Oplev loops to be not so reliable - perhaps this has to do with the wonky ITMY UL OSEM, more on this later. We usually turn the DC coupling on after dither aligning the arms, and in the past, it has been helpful. But we had more success last night with the DC coupling turned off rather than on.
• Once the sign flip was figured out, we were repeatedly able to achieve locks with CARM partially on RF - we got through about 3 or 4, each was stable for just tens of seconds though. Also, we only progressed to RF on CARM on 1 attempt, the lock lasted for just a few seconds
• Unfortunately, the mode cleaner decided to act up just about after we figured all this out, and it was pushing 4am so we decided to give up for the night.
• The arm transmissions hit 300! We had run the transmission normalization scripts just before starting the lock so this number should be reliable (compare to ~130 in October last year). The corresponding PRG is about 16.2, which according to my Finesse models suggest we are still undercoupled, but are close to critical coupling (this needs a bit more investigation, supporting plots to follow). => Average arm loss is ~150ppm! So looks like we did some good with the vent, although of course an independent arm loss measurement has to be done...
• Lockloss plot for one of the locks is Attachment #1

Other remarks:

• Attachment #2 shows that the ITMY UL coil is glitchy (while the others are not). At some point last night, we turned off this sensor input to the damping servos, but for the actual locks, we turned it back on. I will do a Satellite box swap to see if this is a Sat. Box problem (which I suspect it is, the bad Sat. Boxes are piling up...)
• Just now, eric was showing me the CM board setup in the LSC rack, because for the next lock attempts, we want to measure the CARM loop - but we found that the input to the CM board was POY and not REFL! This probably explains the sign flip mentioned above. The mix-up has been rectified
• The MICH dither align doesn't seem to be working too well - possibly due to the fact that we have a lot more ASDC light now, this has to be investigated. But last night, we manually tweaked the BS alignment to make the dark port dark, and it seemed to work okay, although each time we aligned the PRMI on carrier, then went back to put the arms on ALS, and came back to PRMI, we would see some yaw misalignment in the AS beam...
• I believe the SRM sat. box is still being looked at by Ben so it has not been reinstalled...
• Eric has put together a configure script for the PRFPMI configuration which I have added to the IFO configure MEDM screen for convenience
• For some reason, the appropriate whitening gain for POX11 and the XARM loop gain to get the XARM to lock has changed - the appropriate settings now are +30dB and 0.03 respectively. These have not been updated in some scripts, so for example, when the watch script resets the IFO configuration, it doesn't revert to these values. Just something to keep in mind for now...

Great to hear that we have the PRG of ~16 now!

Is this 150ppm an avg loss per mirror, or per arm?

I realized that I did not have a Finesse model to reflect the current situation of flipped folding mirrors (I've been looking at 'ideal' RC cavity lengths with folding mirrors oriented with HR side inside the cavity so we didn't have to worry about the substrate/AR surface losses), and it took me a while to put together a model for the current configuration. Of course this calculation does not need a Finesse model but I thought it would be useful nevertheless. 

In summary - the model with which the attached plot was generated assumes the following:

• Arm lengths of 37.79m, given our recent modification of the Y arm length
• RC lengths are all taken from here, I have modelled the RC folding mirrors as flipped with the substrate and AR surface losses taken from the spec sheet
• The X axis is the average arm loss - i.e. (L_ITMX+L_ITMY+L_ETMX+L_ETMY)/2. In the model, I have distributed the loss equally between the ITMs and ETMs.

This calculation agrees well with the analytic results Yutaro computed here - the slight difference is possibly due to assuming different losses in the RC folding mirrors. 

The conclusion from this study seems to be that the arm loss is now in the 100-150ppm range (so each mirror has 50-75ppm loss). But these numbers are only so reliable, we need an independent loss measurement to verify. In fact, during last night's locking efforts, the arm transmission sometimes touched 400 (=> PRG ~22), which according to these plots suggest total arm losses of ~50ppm, which would mean each mirror has only 25ppm loss, which seems a bit hard to believe.

It is also difficult to have a high arm transmission without having high PRG.

What about to plot the arm trans and the REFL DC power in a timeseries?
Or even in a correlation plot (X: Arm Trans or PRG vs Y: REFL Reflectivity)

This tells you an approximate location of the critical coupling, and allows you to calibrate the PRG, hopefully.

As Gautam mentioned, we had some success locking the PRFPMI last night. (SRM satellite box is still in surgery...)

Unsurprisingly, changing the loss/PRG/CARM finesse means we had to fiddle with the common mode servo parameters a little bit to get things to work. However, before too long, we achieved a first lock on the order of a few minutes. Not long afterwards, we had a nice half hour lock stretch where we could tune up the AO crossover and loop UGFs. The working locking script was committed to SVN. Really, no fundamentally new tactics were used, which is encouraging. (One thing I wondered about was whether a narrower CARM linewidth would still let our direct ALS->REFL11 handoff with no offset reduction work. Turns out it does)

However, the step where we increase the analog CARM gain isn't as bulletproof as it once had been. The light levels "sputter" in and out sometimes if the gain increases are too agressive, and can cause a lockloss. Maybe this is an effect of the narrower linewidth and injecting more ALS noise at high frequencies with the higher CARM bandwidth.

The spatial profiles of the light on the cameras is totally bananas. Here's AS and REFL.

As Koji suggested, here is a 2D histogram of TRY vs REFLDC. It appears that the visibility would max out at 75% or so at arm powers around 400. Indeed, we briefly saw powers that high, but as can be seen on the plot, we were usually a little under 300. Exploring the transmon QPD offset space didn't seem to have much effect here.

One thing that I hadn't looked at in previous locks is coherence with our ground seismometers. It would be cool to have more seismic feedforward, and looking at the frequency domain multiple coherence, it looks like we can win a lot between 1 and 20 Hz. I expected more of a win at 1Hz, though.

[gautam, paco]

Gautam managed to lock PRFPMI a little before ~ 22:00 local time. The ALS to RF handoff logic was found to be repeatable, which enabled us to lock a total of 4 times this evening. Under this nominal state, we can work on PRFPMI to narrow down less known issues and carry out systematic optimization. The second time we achieved lock, we ran sensing lines before entering the ASC stage (which we knew would destroy the lock), and offline analysis of the sensing matrix is pending (gpstime = 1310792709 + 5 min).

Things to note:

(a) there is an unexpected offset suggesting that the ALS and RF disagreed on what the lock setpoint should be, and it is still unclear where the offset is coming from.

(b) the first time the lock was reached, the ASC up stage destroyed it, suggesting these loops need some care (we were able to engage the ASC loops at low gains (0.2 instead of 1) but as soon as we enabled some integrators this consistently destroyed the lock

(c) gautam had (burt) restored to the settings from back in March when the PRFPMI was last locked, suggesting there was a small but somehow significant difference in the IFO that helped today relative to last week

Take home message--> The mere fact that we were able to lock PRFPMI rules out the considerably more serious problems with the signal chain electronics or processing. This should also be a good starting point for further debugging and optimization.

gautam: the circulating power, when the ASC was tweaked, hit 400 (normalized to single arm locked with a misaligned PRM) suggesting a recycling gain of 22.5, and an average arm loss of ~30ppm round trip (assuming 2% loss in the PRC). 

Last night Gautam walked us through the algorithm used to lock PRFPMI. We tried it several times with the PSL HEPA filter off between 10:00 pm July 7th to 1:00 am July 8th. None of our attempts were successful. In between, we tried to do the locking with old IMC settings as well, but it did not change the result for us. In most attempts, the arms would start to resonate with PRMI with about 200 times the power than without power recycling while the arms are still controlled by ALS beatnote. The handover of lock controls "CARM+DARM locked to ALS beatnote" to "Main laser + IMC locked to the CARM+DARM" would always fail. More specifically, we were seeing that as soon as we hand over the DC control of CARM from ALS beatnote to IR by feeding back to MC2, the lock would inevitably fail before the rest of the high-frequency control can be transferred over.

Nonetheless, Paco and I got a good demo of how to do PRFPMI locking if the need appears. With more practice and attempts, we should be able to achieve the lock at some point in the future. The issues in handover could be due to any of the following:

• Although it seems like ALS beatnote fed control of arms keep them within the CARM IR linewidth as we see the IR resonating, there still could be some excess noise that needs to be dealt with.
• Gautam conjectures, that the presence of high power in the arms connects the ITMs and the ETMs with an optical spring changing the transfer function of the pendula. This in turn changes the phase margin and possibly makes the CARM loop in IR PRFPMI unstable.
• We should also investigate the loop transfer functions near the handover point for the ALS beatnote loop and the IR CARM loop and calculate the crossover frequency and gain/phase margins there.

More insights or suggestions are welcome.

Note; An earthquake came around lunch time and tripped all watchdogs. Most suspensions were recovered without issues, but ITMX appeared to be stuck. We tried the shaking procedure, but after this we couldn't restore the XARM lock. From alignment, we tried optimizing the TRX but we only got up to ~0.5 and ASS wouldn't work as usual. In the end the issue was that we had forgotten to enable the LL coil output so after we did this, we managed to recover the XARM.

Summary:

I wanted to put my optomechanical instability hypothesis to the test. So I decided to cut the input power to the IMC by ~half and try locking the PRFPMI. However, this did not improve the stability of the buildup in the arm cavities, while the control was solely on the ALS error signal. 

Details:

1. The waveplate I installed for this purpose was rotated until the MC RFPD DCMON channel reported ~half it's nominal value.
2. I adjusted the IMC servo gains appropriately to compensate. IMC lock was readily realized.
3. I increased the whitening gains on the POX, POY and REFL165 photodiodes by 6dB, to compensate for the reduced light levels.
   • One day soon, we will have remote power control, and it'd be nice to have this process be automated.
   • Really, we should have de-whitening filters that undo these flat gains in addition to undoing the frequency dependent whitening.
   • I'm not sure the quality of the electronics is good enough though, for the changing electronics offsets to not be a problem.
   • One possibility is that we can normalize some signals by the DC light level at that port, but I still think compensating the changing optical gain as far upstream as possible is best, and the whitening gain is the convenient stage to do this.
4. Recovered single arm POX/POY locking. 
5. Then I decided to try and lock the PRFPMI with the reduced input power.

Basically, with some tweaks to loop gains, it worked, see Attachment #1. Note that the lower right axis shows the IMC transmission and is ~7500 cts, vs the nominal ~15,000 cts.

Discussion:

Cutting the input power did not have the effect I hoped it would. Basically, I was hoping to zero the optical CARM offset while the IFO was entirely under ALS control, and have the arm transmission be stable (or at least, stay in the linear regime of REFL11). However, the observation was that the IFO did the usual "buzzing" in and out of the linear regime. Right now, this is not at all a problem - once the IR error signal is blended in, and DC control authority is transferred to that signal, the lock acquisition can proceed just fine. And I guess it is cool that we can lock the IFO at ~half the input power, something to keep in mind when we have the remote controlled waveplate, maybe we always want to lock at the lowest power possible such that optomechanical transients are not a problem. 

I also don't think this test directly disputes my claim that the residual CARM noise when the arm cavities are under purely ALS control is smaller than the CARM linewidth.

What does this mean for my hypothesis? I still think it is valid, maybe the power has to be cut even further for the optomechanics to not be a problem. In Finesse (see Attachment #2), with 0.3 W input power to the back of the PRM, and with best guesses for the 40m optical losses in the PRC and arms, I still see that considerable phase can be eaten up due to the optomechanical resonance around ~100 Hz, which is where the digital CARM loop UGF is. So I guess it isn't entirely unreasonable that the instability didn't go away?

After this work, I undid all the changes I made for the low power lock test. I confirmed that IMC locking, POX/POY locking, and the dither alignment systems all function as expected after I reverted the system.

Following Koji's suggestion, I decided to investigate the relation between my Finesse model and the measured data.

For easy reference, here is the loss plot again:

Sticking with the model, I used the freedom Finesse offers me to stick in photodiodes wherever I desire, to monitor the circulating power in the PRC directly, and also REFLDC. Note that REFLDC goes to 0 because I am using Finesse's amplitude detector at the carrier frequency for the 00 mode only. 

Both the above plots essentially show the same information, except the X axis is different. So my model tells me that I should expect the point of critical coupling to be when the average arm loss is ~100ppm, corresponding to a PRG of ~17 as suggested by my model.

Eric has already put up a scatter plot, but I reproduce another from a fresh lock tonight. The data shown here corresponds to the IFO initially being in the 'buzzing' state where the arms are still under ALS control and we are turning up the REFL gain - then engaging the QPD ASC really takes us to high powers. The three regimes are visible in the data. I show here data sampled at 16 Hz, but the qualitative shape of the scatter does not change even with the full data. As an aside, today I saw the transmission hit ~425!

I have plotted the scatter between TRX and REFL DC, but if I were to plot the scatter between POP DC and REFL DC, the shape looks similar - specifically, there is an 'upturn' in the REFL DC values in an area similar to that seen in the above scatter plot. POP DC is a proxy for the PRG, and I confirmed that for the above dataset, there is a monotonic, linear relationship between TRX and POPDC, so I think it is legitimate to compare the plot on the RHS in the row directly above, to the plot from the Finesse model one row further up. In the data, REFL DC seems to hit a minimum around TRX=320. Assuming a PRM transmission of 5.5%, TRX of 320 corresponds to a PRG of 17.5, which is in the ballpark of the region the model tells us to expect it to be. Based on this, I conclude the following:

• It seems like the Finesse model I have is quite close to the current state of the IFO 
• Given that we can trust the model, the PRC is now OVERCOUPLED - the scatter plot of data supports this hypothesis
• Given that in today's lock, I saw arm transmission go up to ~425, this suggests that at optimal alignment, PRG can reach 23. Then, Attachment #1 suggests the average arm loss is <50ppm, which means the average loss per optic is <25ppm. I am not sure how physical this is, given that I remember seeing the specs for the ITMs and ETMs being for scatter less than 40 25ppm, perhaps the optic exceeded the specs, or I remember the wrong numbers, or the model is wrong

In other news, I wanted to try and do the sensing matrix measurements which we neglected to do yesterday. I turned on the notches in CARM, DARM, PRCL and MICH, and then tuned the LO amplitudes until I saw a peak in the error signal for that particular DOF with peak height a factor of >10 above the noise floor. The LO amplitudes I used are 

MICH: 40

PRCL: 0.7

CARM: 0.08

DARM: 0.08

There should be about 15 minutes of good data. More impressively, the lock tonight lasted 1 hour (see Attachment #6, unfortunately FB crashed in between). Last night we lost lock while trying to transition control to 1f signals and tonight, I believe a P.C. drive excursion of the kind we are used to seeing was responsible for the lockloss, so the PRFPMI seems pretty stable.

With regards to the step in the lock acquisition sequence where the REFL gain is turned up, I found in my (4) attempts tonight that I had most success when I adjusted the CARM A slider while turning up the REFL gain to offload the load on the CARM B servo. Of course, this may mean nothing...