[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.

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.

1. With REFL_DC coupled into the CM board through an SR560 (with an offset subtractor), we were able to transition to use it as the CARM error signal.
2. We reduced the CARM offset until the arm powers went up to ~13.
3. We had the AO path turned on and the MCL/AO crossover was ~150 Hz.
4. We saw the double cavity pole come in from HF down to ~1-2 kHz. The lock stayed stable like this.
5. We've set the IMC overall gain higher by +4dB in the mcup script. That's -4 dB from Eric's max gain earlier today.
6. We have some scripts now for this scripts/PRFPMI/ :   camr_cm_down.sh and carm_cm_up.sh
7. The sequence was ALS -> SqrtInv while digital with CARM -> MC2. Then we digital transition to REFL_DC using the CM board switch to put REFL_DC into the REFL11_I socket.
8. REFL_DC is noisy, so we upped the SR560 gain by 10 and compensated.

Also, we found the PRM OL off and turned it back on. The ETMY was swinging a lot after lock loss, so we set its SUSPOS damping gain to match the ETMX and it stopped swinging so much.

Next up: more of the same, make this sequence more stable, turn on CARM OSC and watch the LOCKI outputs while we slowly ramp between signals.

Also, what should be the sign of the CARM offset ???

 I have done a sweep of CARM, while looking at the fields inside of one arm (I've chosen the Xarm), to see where any resonances might be, that could be causing us trouble in keeping the PRMI locked as we bring the arms into resonance. 

Since Gabriele pointed out to me that we're using the 3x55MHz signal for locking, we should be most concerned about resonances of the higher orders of 55, and not of 11.  So, on this plot, I have up to the 6th order 55 MHz sidebands, which are 332 MHz.  Although the Matlab default color chart has wrapped around, it's clear that the carrier is the carrier, and the +4f2, which is the same blue, is not the giant central peak.  So, it's kind of clear which trace is which, even though the legend colors are degenerate.  Also, the main point that I want to show here is that there is nothing going on near the carrier, with any relevant amplitude.  The nearest things are the plus and minus 55 MHz sidebands themselves, and they're more than 50 nm away from the carrier. 

Recalling from elog 9122, the PRFPMI and DRFPMI linewidths are about 40pm.  50pm away from the resonant point is ~1/10 the power, and 100pm away from the resonant point is ~1/100 the power.  So, 50 nm is a looooong ways away. 

Just for kicks, here is a plot of all the resonances of the 1f and 2f modulation frequencies, up to 30*f1, which is the same 6*f2:

The resonances which are "close" to the carrier are the 9th order 11 MHz sidebands, and they're 280pm from the carrier, so twice as far as we need to be, to get our arm powers to ~1/100 of the maximum, and, they're a factor of ~1e4 smaller than the carrier.

The interferometer can nearly be locked again. I was unable to fully hand off control from ALS-->RF, I suspect I may be using the wrong sign on the AO path (or some such other sub-optimal CM board settings). I'll hook up the SR785 and take some TFs tomorrow, that should give more insight into what's what. With the arms held off resonance, the PRMI acquires lock nearly instantly (REFL165 I for PRCL, REFL165 Q for MICH), and can stay locked nearly indefinitely, which is what I need so I can get the RF lock going. However the sensing matrix (for vertex DoFs, arms held off resonance) still makes no sense to me. The MICH loop has ~50 Hz UGF and the PRCL loop ~150 Hz. I think the MICH loop shape can be optimized a little for better low frequency suppression, but this isn't the show-stopper at the moment. For record-keeping, the ALS performance was excellent and other subsystems were nominal tonight.

1. PRM satellite box indeed seems to have been the culprit - shortly after I swapped it to the SRM, its shadow sensors went dark. I leave the watchdog tripped.
2. I still was unable to realize the RF only IFO
   • Clearly my old settings don't work, so I tried to go about it systematically. First, try and transition CARM to RF, leave DARM on ALS.
   • As usual, I can realize the state were the arm powers are ~100, and the two paths are blended. 
   • But I'm not able to completely turn off the CARM_A path without blowing the lock.

Pity really, I was hoping to make it much further tonight. I think I'll have to go back to the high BW POX/POY lock, and also check out the conversion efficiency / noise of the daughter board on the REFL11 demod board. Compared to before my work on the RF source, the demod phase for the PRMI lock using REFL11 as an error signal has basically necessitated a change of the digital demod phase by 180 degrees - so I made the appropriate polarity changes in the CM_SLOW and AO paths (the assumption is that CARM in REFL11 would require the same change in digital demod phase, and I think this is a reasonable assumption - indeed, with the arm powers somewhat stable ~100, if I look at the PDH signal in REFL11 I and Q, it does seem to show up largely in the I quadrature (pre digital phase rotation). Anyway, with so many weird effects (wonky PRM suspension, strange PRMI sensing etc etc, who knows what's going on. This will take a systematic effort.

I defer the electronics characterization for the daytime (if I feel like I need it tomorrow I'll do it, else. Koji has said he can do it on Friday).

1. Integrated >1 hour at RF only control, high circulating powers tonight.
   • All of the locklosses were due to me typing a wrong number / turning on the wrong filter.
   • So the lock seems pretty stable, at least on the 20 minute timescale.
   • No idea why given the various known broken parts.
2. Did a bunch of characterization.
   • DARM OLTF - Attachment #1. The reference is when DARM is under ALS control.
   • CARM OLTF - Attachment #2. Seems okay.
   • Sensing matrix - Attachment #3. The CARM and DARM phases seem okay. Maybe the CARM phase can be tuned a bit with the delay line, but I think we are within 10 degrees.
3. TRX/TRY between 300-400, with large fluctuations mostly angular. So PRG ~17-22, to answer Koji's question in the meeting today.
   • This is similar to what I had before the vent of Sep 2020.
   • Not surprising to me, since I claim that we are in the regime where the recycling gain is limited by the flipped folding mirrors.
4. Tried to tweak the ASC (QPD only) by looking at the step responses, but I could never get the loop gains such that I could close an integrator on all the loops.

I need to think a little bit about the ASC commissioning strategy. On the positive side

1. REFL11 board seems to perform at least as well as before.
2. ALS performance made me (as Pep would say), so so happy.
3. Whole lock acquisiton sequence takes ~5mins if the PRMI catches lock quickly (5/7 times tonight).
4. Process seems repeatable.

Things to think about:

1. How to get the AS WFS in the picture?
2. What does the (still) crazy sensing matrix mean? I think it's not possible to transfer vertex control to 1f signals with this kind of sensing.
3. What does it mean that the PRM actuation seems to work, even though the coils are imabalnced by a factor of 3-5, and the coil resistances read out <2 ohms???
4. What's going on at the ALS-->CARM transition? The ALS noise is clearly low enough that I can sit inside the CARM linewidth. Yet, there seems to be some offset between what ALS thinks is the resonant point, and what the REFL11 signal thinks is the resonant point. I am kind of able to "power through" this conflict, but the IMC error point (=AO path) is not very happy during the transition. It worked 8/8 times tonight, but would be good to figure out how to make this even more robust.

Summary:

I am inclined to believe that the arm cavity losses are such that the IFO is overcoupled. Some calculations, validated with Finesse modeling also suggest that there isn't a sign change for the CARM error signal when the IFO goes from being undercoupled to overcoupled, but I may have made a mistake here?

Details:

• We’d like to gain some insight into whether the interferometer is undercoupled, critically coupled, or overcoupled. Factors that determine which of these is true include:
  • Arm cavity losses
  • Recycling cavity losses
• The proxy by which we determine the recycling gain is usually the arm cavity transmission. Assuming T_PRM = 5.637 % according to the wiki, and assuming the arm cavity transmission is normalized to 1 when locked in the POX/POY state, we can say that the PRG is given by G_PRC = TRX × T_PRM, assuming that the (i) the RF sideband fields are perfectly rejected by the arm cavities and (ii) mode-matching efficiency between the input beam and the arm mode is the same as that between the input beam and the CARM mode.
• Apart from this, the other measurement we have available to us is the buildup of the sideband fields, namely POP22 and POP110. We can compare the values in the PRMI lock vs the PRFPMI to make some inference.
• I started off with an analytic calculation of the reflectivity of the compound arm cavity mirror.
  • Attachment #1 suggests we will have an over-coupled IFO for arm cavity losses below ~200 ppm, which is a regime we are almost certainly in now.
• Then, I repeat the analysis for the coupled CARM cavity, with the end mirror as the compound arm mirror and the input mirror as the PRM.
  • I assume 2 % loss in the PRC.
  • Attachment #2 shows that while the carrier field goes through a sign change in amplitude reflectivity (as expected), the sideband fields dont.
  • Per equation 4.2 of Koji's thesis, the error signal for CARM depends on the (signed) IFO reflectivity, and the absolute value of the derivative of the arm cavity reflectivity for the carrier w.r.t. CARM phase.
  • So, we don't expect the REFL11 signal to show a sign change.
  • The situation is more complicated for PRCL in REFL11, because as explicitly evaluated in Eq 4.3 of Koji's thesis, there are two terms that contribute, and their relative magnitudes will dictate the overall sign. 
• For a Finesse validation, I use a simplified 3 mirror coupled cavity to approximate the PRFPMI. I also retained the RF sidebands for diagnostic purposes. The idea was to study these PRG proxies and what their expected behavior is.
  • Attachment #3 shows the PDH error signal in the (arbitrarily defined) REFL11 I quadrature. While the optical gain changes as a function of the arm cavity loss, the actual slope does not change sign. The fact that the zero crossing doesn't happen at exactly 0 CARM offset is because of higher order mode light at the REFL port (in my model, I tried to preserve the flipped folding mirror situation so the mode matching between the arm cavity and PRC in my model is ~96%).
  • In fact, this may explain why a CARM_B offset is required to do the ALS-->IR handoff - the ALS servo wants to keep the arm offset to zero, but at that point, the PDH error signal isn't zero, and so the two loops end up fighting each other?
  • Attachment #4 is a more detailed study of the recycling gain as a function of arm cavity loss, but now including losses in the recycling cavity.

Conclusions:

1. I think the arm cavity losses are in the 60-80 ppm round-trip region. I don't see how we can explain the arm cavity transmission of ~350 otherwise.
2. The fact that REFLDC decreases as the arm transmission increases is because the input beam is getting better matched to the CARM mode, and there is less junk carrier light. 

Thoughts from others?

After the QPD fix, both arms report consistent buildup - see Attachment #1. The peak values touch ~250, corresponding to a PRG of ~13. The IFO becomes critically coupled at PRG=15. I am finding that the 3f signal offsets are changing as a function of the CARM offset, and this could be responsible for the lock breaking as I approach 0 CARM offset. I found that I could maintain a more stable and deterministic transition to zero CARM offset by dynamically adjusting the 3f PRCL error signal offset to keep the REFL11 signal approximately at 0. Some shaking seems to have commenced so I am breaking for now.

Note that I find scattered throughout the elog references to a similar problem of the PRMI losing lock as the CARM offset is reduced, e.g. here. But haven't stumbled across what the resolution was, the PRFPMI could be locked pretty easily in 2015 I remember.

Today Angelina and I looked at the PRM OL with an eye towards installing a 2nd QPD. We want to try out using 2 QPDs for a single optic to see if theres a way to make a linear combination of them to reduce the sensitivity to jitter of the HeNe laser or acoustic noise on the table.

The power supply for the HeNe was gone, so I took one from the SP table.

There are WAY too many optics in use to get the beam from the HeNe into the vacuum and then back out. What we want is 1 steering mirror after the laser and then 1 steering mirror before the QPD. Even though there are rumors that this is impossible, I checked today and in fact it is very, very possible.

More optics = more noise = bad.

 Atm 1,  PRM oplev inward path with 2 lens solution: 14 cm gap between F 1145 and F 1545 mm lenses. 

Atm 2,   The PRM beam size 3 mm and the beam  quality is still bad. The BS path only needed alignment.

 The PRM sus gains checked OK

All other suspension oplev gains setting were checked out OK

[Jenne, Suresh]

We've aligned the guiderod and wire standoff to the PRM, each partly.  They have both been aligned to the correct distance above the scribe lines, but they have not yet been centered forward/backward along the thickness of the optic.  So, we're working on it...

 2" G&H mirror is installed on a DLC mount just in front of the BS.  I had to remove one of the 4 BS dog clamps, so we must put it back when we are finished with this test.

I aligned the G&H mirror such that the reflected beam is overlapped with the incident beam, and I aligned the PRM such that the regular REFL beam is retro-reflected.  This is the same as getting the beam bouncing off the PRM back to the G&H to be overlapped.

I then saw flashes of the cavity, when I held a card with a hole in the cavity, so the beam was going through a small aperture in the card, but I still saw flashes.  I was not able to see flashes on the IR card transmitted through the G&H mirror.

I also cannot see any flashes or scattered light on the face of PR2 camera.

I do, however, see flashes on the face of the PRM.  Movie saved, will post soonly.

Light is coming out of REFL on the AS table, but it's clipped somewhere....needs investigation/work before we can lock.

I also didn't see anything at the POP port with a card, but I'm hopeful that perhaps with a camera I'll see something.

 Dang it.  I didn't confirm that the movie was good, just that it was there.  It's corrupted or something, and won't play.  I'll just have to make a new movie today after I realign the cavity.

Game plan:

* Put 2" G&H mirror into BS chamber, in front of BS.

* Align it, lock cavity using an existing REFL PD.

* Align POP setup so I can use POP camera to take image of transmitted cavity mode, and actually take that image.

* Take image of face of PR2.

* Measure finesse of cavity using POP, or a Thorlabs PD at POP (looking at transmission through PR2) by scanning PRM, and infer cavity gain....compare with values in elog 7905.

* If time / inclination allow, take beam scan measurements of the REFL port.

I will not be able to do as was done in elog 6421 to look at the beam size at POP for non-resonating beams.  I expect ~0.1uW of light at POP in the non-resonant case:  100mW * 5.5% * 20ppm = 0.11microwatts.

Jenne and I noticed high pitch sound from our acoustic interferometer noise diagnostic system.
The frequency of this narrow band noise was 1256Hz, which is enough close to twice of the PRM violin mode freq.
After putting notch filter at 1256+/-25Hz at the violin filters, the noise is gone. Just in case I copied the same filters to all of the test masses.

Later, I found that the 4th violin modes are excited. Additional notch filters were added to "vio3" filter bank to mitigate the oscillation.

I have turned off the 3.2Hz res gains in the PRC ASC loops, since those seem to make the loops unstable. 

Right now the pitch gain is -0.001, with FM1,3,9 on.  Yaw gain is -0.004, with FM1,3,9 on. 

Pitch gain can't increase by factor of 2 without oscillating. 

I tried to take transfer functions, but I think the ASC situation is really confusing, since I have OSEM damping, oplev damping, and this POP QPD damping on the PRM.  It's hard to get coherence without knocking the PRC out of lock, and it keeps looking like my gain is 0dB, with a phase of 0 degrees, from ~1 Hz to ~10 Hz.  Outside that range I haven't gotten any coherence.  Moral of the story is, I'm kind of puzzled. 

Anyhow, as it is right now, the ASC helps a bit, but not a whole lot.  I increased the trigger ON value, so that it shouldn't kick the PRM so much.  I wish that I had implemented a delay in the trigger, but I'm not in the mood to mess with the simulink diagrams right now.

Ignoring the OSEM damping loops, the oplev servo loops make it so that the POP ASC loops do not see a simple pendulum plant, but instead see the closed loop response. Since the filter in the OL bank is proportional to f, this means that the open loop gain (OLG):

Which means that the CLG that the ASC sees is going to dip below unity in the band where the OL is on. For example, if the OL loop has a UGF of 5 Hz, it also has a lower UGF of ~0.15 Hz, which means that the ASC needs to know about this modified plant in this band.

For i/eLIGO, we dealt with this in this way: anti-OL in iLIGO

A series of measurements / calculations for the PRM ASC characterization and servo design

1) Actuator characterization

The actuator responses of the PRM in pitch and yaw were measured (attachment figure 1). I believed the calibration of the oplev QPD to be
1 count/urad. The oplev servo loops were turned off at the FM inputs, and the filter banks were turned off so that the response has the open
loop transfer function except for the servo filter.

The measured transfer functions were fitted with LISO. The LISO results (c.f. the source codes) were shown in the figure. The responses also
include the 60Hz comb filter present in the input filters. The responses are well approximated by the single pendulum with f0 of 0.6-0.8 and q of 3.5 and 6.3.

From this measurement, the actuator responses of the PRM at DC are estimated to be 2.2 urad/cnt and 1.8 urad/cnt in pitch and yaw, respectively.

2) Sensor response of the POP QPD

As we already know how the actuators respond, the QPD optical gain can be characterized by measuring the actuator response of the QPD
(attachment figure 2). The QPD signals are such noisy that the response above 1Hz can't be measured with sufficient coherence. Below 1Hz,
the response is well represented by the actuator response measured with the oplev. From this measurement, the optical gains of the QPD
with respect to the PRM motion are 650 cnt/urad and 350 cnt/urad.

3) Open loop transfer function of the current ASC servo

By combining the above information with the servo setting of the servo filters, the open loop transfer functions of the PRM QPD ASC loops
were estimated (attachment figure 3). Actually the expected suppression of the fluctuation is poor. The yaw loop seems to have
too low gain, but in fact increasing gain is not so beneficial as there is no reasonable phase margin at higher frequency.

With the estimated openloop transfer functions and the measured free-running angular fluctuation, the suppressed angular spectra can be
estimated (attachment figure 4). This tells us that the suppression of the angular noise at around 3Hz is not sufficient in both pitch and yaw.
As there is no mechanical resonance in the actuator response at the frequency, intentional placement of poles and zeros in the servo filter is necessary.

4) Newly designed ASC filter

Here is the new design of the QPD ASC servo (attachment figure 5). The target upper UGF is 10Hz with the phase margin of 50 to 60deg.
The servo is AC coupled so that we still can tweak the alignment of the mirror.

As this servo is conditionally stable, at first we should close the loops with stable filter and then some boosts should be turned on.
Estimated suppressed fluctuation is shown in the attachment figure 6. We can see that the fluctuation was made well white between 0.5Hz to 10Hz.

The filter design is shown as follows:

Pitch
FM1: zero at 0Hz, pole at 2000Hz, gain at 2000Hz = 2000

FM3: (boost)
zero: f: 0.5Hz q: 1  /  4.5Hz, q: 1 / f: 1Hz, q: 3
pole: f: 2Hz q: 3  / f: 2.7Hz, q: 2  / f: 1Hz, q: 15

FM9: (HF Roll-off)
pole: f: 40Hz q: 1.7
 
Servo gain: -0.028

Yaw
FM1: zero at 0Hz, pole at 2000Hz, gain at 2000Hz = 2000

FM3: (boost)
zero: f: 0.7Hz q: 2  /  3Hz, q: 7 / f: 2Hz, q: 6
pole: f: 1.02Hz q: 10  / f: 4.5Hz, q: 0.8  / f: 1.5Hz, q: 10

FM9: (HF Roll-off)
pole: f: 40Hz q: 1.7
 
Servo gain: -0.0132

[Paco, Hang]

Yesterday afternoon Paco and I measured the PRM L2P transfer function. We drove C1:SUS-PRM_LSC_EXC with a white noise in the 0-10 Hz band (effectively a white, longitudinal force applied to the suspension) and read out the pitch response in C1:SUS-PRM_OL_PIT_OUT. The local damping was left on during the measurement. Each FFT segment in our measurement is 32 sec and we used 8 non-overlapping segments for each measurement. The empirically determined results are also compared with the Fisher matrix estimation (similar to elog:16373).

Results:

Fig. 1 shows one example of the measured L2P transfer function. The gray traces are measurement data and shaded region the corresponding uncertainty. The olive trace is the best fit model. 

Note that for a single-stage suspension, the ideal L2P TF should have two zeros at DC and two pairs of complex poles for the length and pitch resonances, respectively. We found the two resonances at around 1 Hz from the fitting as expected. However, the zeros were not at DC as the ideal, theoretical model suggested. Instead, we found a pair of right-half plane zeros in order to explain the measurement results. If we cast such a pair of right-half plane zeros into (f, Q) pair, it means a negative value of Q. This means the system does not have the minimum phase delay and suggests some dirty cross-coupling exists, which might not be surprising. 

Fig. 2 compares the distribution of the fitting results for 4 different measurements (4 red crosses) and the analytical error estimation obtained using the Fisher matrix (the gray contours; the inner one is the 1-sigma region and the outer one the 3-sigma region). The Fisher matrix appears to underestimate the scattering from this experiment, yet it does capture the correlation between different parameters (the frequencies and quality factors of the two resonances).

One caveat though is that the fitting routine is not especially robust. We used the vectfit routine w/ human intervening to get some initial guesses of the model. We then used a standard scipy least-sq routine to find the maximal likelihood estimator of the restricted model (with fixed number of zeros and poles; here 2 complex zeros and 4 complex poles). The initial guess for the scipy routine was obtained from the vectfit model.  

Fig. 3 shows how we may shape our excitation PSD to maximize the Fisher information while keeping the RMS force applied to the PRM suspension fixed. In this case the result is very intuitive. We simply concentrate our drive around the resonance at ~ 1 Hz, focusing on locations where we initially have good SNR. So at least code is not suggesting something crazy... 

Fig. 4 then shows how the new uncertainty (3-sigma contours) should change as we optimize our excitation. Basically one iteration (from gray to olive) is sufficient here. 

We will find a time very recently to repeat the measurement with the optimized injection spectrum.

[johannes, gautam]

• We took the heavy door off in the morning with Steve's help
• The problem was quickly identified as the Al foil on the back of the PRM OSEMs (placed to mitigate scattered light making it into the OSEM that was making locking difficult) shorting out the pins on the rear of the OSEM
• We decided against using a black glass beam stop behind the PRM - rather, we decided to go for Al foil hats that were
  1. More "domed" - so the back plane of the OSEM isn't in direct contact with the Al foil, though the hats themselves are secure and shouldn't simply fly off during pump down etc
  2. Have a piece of kapton (courtesy Koji from the OMC lab) in the dome so that even if the foil hats move around slightly, there should be no danger of accidentally shorting out any pins
• Without removing the PRM OSEMs, we were only able to image UR and UL unambiguously showing that they have no filters. Not a single of the 5 spare 'short' OSEMs have filters. We have to open the ITMY chamber to reposition the OSEMs in the near future, which is when we will inspect SRM for filters.
• Attachment #1 shows a picture of these foil hats - the ones actually put on are shaped slightly differently, but the idea is the same
• Attachment #2 shows the PRM with its new OSEM hats (we also used a piece of clean copper wire to tie the OSEM cables to the tower on the bottom left of the cage as viewed from the BS-PRM chamber door)
• After closing up the BS-PRM chamber, I locked the IMC to see if the input pointing had gone way off because of our work on the table and the reputation of the tip-tilts hysteresis - I can see weak flashes in the Y arm but not enough to lock - so I will tweak the alignment a little
• Once I can recover the Y arm alignment, we can move on to peeling first contact and putting the X arm optics in.

Edit 7.30pm: I have managed to recover Y-arm in air locking, and the transmission is up at ~0.6 again which is what we were seeing prior to touching anything on the BS-PRM table, so it looks like the tip-tilt has not gone badly astray... I have also restored the Satellite boxes so that both PRM and SRM have their designated boxes

Tonight, and during last week's locking, we noticed something intermittently kicking the PRM. I've determined that PRM's LR OSEM is problematic again. The signal is coming in and out, which kicks the OSEM damping loops. I've had the watchdog tripped for a little bit, and here's the last ten minutes of the free swinging OSEM signal:

Here's the hour trend of the PRM OSEMS over the last 7 days a plot of just LR since the fix on the 9th of September.

It looks like it started misbehaving again on the evening of the 5th, which was right when we were trying to lock... Did we somehow jostle the suspension hard enough to knock the foil cap back into a bad spot?

It started here

Perhaps the problem is electrical? The attached plot shows a downward trend for the LR sensor output over the past 20 days that is not visible in any of the other 4 sensor signals. The Al foil was shorting the electrical contacts for nearly 2 months, so perhaps some part of the driver circuit needs to be replaced? If so a Satellite Box swap should tell us more, I will switch the PRM and SRM satellite boxes. It could also be a dying LED on the OSEM itself I suppose. If we are accessing the chamber, we should come up with a more robust insulating cap solution for the OSEMs rather than this hacky Al foil + kapton arrangement.

The PRM and SRM Satellite boxes have been switched for the time being. I had to adjust some of the damping loop gains for both PRM and SRM and also the PRM input matrix to achieve stable damping as the PRM Satellite box has a Side sensor which reads out 0-10V as opposed to the 0-2V that is usually the case. Furthermore, the output of the LR sensor going into the input matrix has been turned off.

Looks like what were PRM problems are now seen in the SRM channels, while PRM itself seems well behaved. This supports the hypothesis that the satellite box is problematic, rather than any in-vacuum shenanigans.

Eric noted in this elog that when this problem was first noticed, switching Satellite boxes didn't seem to fix the problem. I think that the original problem was that the Al foil shorted the contacts on the back of the OSEM. Presumably, running the current driver with (close to) 0 load over 2 months damaged that part of the Satellite box circuitry, which lead to the subsequent observations of glitchy behaviour after the pumpdown. Which begs the question - what is the quick fix? Do we try swapping out the LM6321 in the LR LED current driver stage? 

GV Edit Nov 2 2016: According to Rana, the load of the high speed current buffer LM6321 is 20 ohms (13 from the coil, and 7 from the wires between the Sat. Box and the coil). So, while the Al foil was shorting the coil, the buffer would still have seen at least 7 ohms of load resistance, not quite a short circuit. Moreover, the schematic suggests that that the kind of overvoltage protection scheme suggested in page 6 on the LM6321 datasheet has been employed. So it is becoming harder to believe that the problem lies with the output buffer. In any case, we have procured 20 of these discontinued ICs for debugging should we need them, and Steve is looking to buy some more. Ben Abbot will come by later in the afternoon to try and help us debug.

 Here's a fun fact: since the great computer failure of June2014, the PRM Oplev gains have been ZERO.

arrrrggggh

I've restored the gains to their old values, and measured the loop TFs.

[Suresh / Kiwamu]

 We tried adjusting the OSEMs on PRM, but we didn't complete it due to a malfunction on the coils.

The UL and LL coils are not working correctly, the forces are weak.

Tomorrow we will look into the satellite box, which is one of the suspects.

 During the adjustment we found that the POS excitation force was unequal in each sensor.

At the beginning we thought it's because of the difference of the sensitivity in each OSEM due to the bad OSEM orientations.

However it turned out that it comes from the actual force imbalance on each coil.

We checked the force of each coil by putting an offset (-2000 cnts) in each output digital filter and looked at the OSEM signals in time series.

The UL and LL coils are too weak and the responses are almost buried in the noise of the OSEMs in time series.

We briefly checked some analog electronics and found the DAC, AI board and deWhitening board were healthy.

We were able to see the right amount of voltage from the monitor pin on the front panel of the coil driver.

So something downstream are suspicious, including the satellite box, feedthrough and coils.

- - -

Although the coil issue, it could be worth trying to check the input matrix.

Adjustment of the PRM OSEMs are done. The coils turned out to be healthy.

The malfunction was fixed. It was because the UL OSEM was too deeply inserted and barely touching the AR surface of the mirror.

(OSEM adjustment)

 + Excited POS at 6.5 Hz with an amplitude of 3000 cnts by the LOCKIN oscillator.

 + Looked at the signal of each sensor in frequency domain.

 + Maximized the excitation peak for each of the four face OSEMs by rotating them.

 + Minimized the excitation peak in the SIDE signal by rotating it.

 + Adjusted the OSEM translational position so that they are in the midpoint of the OSEM range.

(POS sensitivity check)

From the view point of the matrix inversion, one thing we want to have is the equally sensitive face sensors and insensitive SIDE OSEM to the POS motion.

To check the success level of today's PRM adjusment, I ran swept sine measurements to take the transfer function from POS to each sensor.

The plots below are the results.  The first figure is the one measured before the adjustment and the second plot is the one after the adjustment.

As shown in the plot, before the adjustment the sensitivity of OSEMs were very different and the SIDE OSEM is quite sensitive to the POS motion.

So PRM used be in an extremely bad situation.

After the adjustment, the plot became much better.

The four face sensors have almost the same sensitivity (within factor of 3) and the SIDE is quite insensitive to the POS motion.

 [ericq]

Steve fixed the PRM oplev pointing. I turned on the loops and measured the OLG, then set the pitch and yaw gains such that the upper UGF was ~8Hz (motivated by Jenne's loop design in ELOG 9401)

• Pitch gain: +7
• Yaw Gain: -5

I then measured the oplev spectra of the optics as they were aligned for PRMI. (OSEMs on, oplevs on, LSC off, and ASC off)

Next, Jenne and I need to fix the ASC loop such that it properly accounts for the oplev loop. 

In an effort to stop the PRC from wiggling around so much, I recentered the POP QPD after maximizing the POPDC power when locked on carrier.  The beam was basically off the QPD in yaw, and at half-range in pitch.

For the PRM, I aligned it until the arm flashes were maximized and the REFL camera showed a centered spot with dips happening during the arm pops. AS port was more messy since the Michelson alignment wasn't perfect, but the spots were both near the center of the cam and the SRM alignment maximized the wangy fringiness of the image as well as the angry cat meow sounds that the full IFO makes as heard through the DAFI (listening to POX).

On Monday, Osamu should be back and can help with doors and then alignment recovery and locking.

[Gautam, Steve, Johannes]

We put on the remaining heavy doors on the chambers (ITMY, ITMX,ETMX, in this order) this morning. On the ITMY and ETMX tables we placed old OpLev steering mirrors that are clean and baked as witness plates such that may one day provide some insight into dust accumulation on optics.

With the heavy doors on we confirmed that we were still able to lock both IFO arms and used the dither scripts to optimize the alignment. Following that we centered all OpLevs and aligned the X and Y green beams.

Radhika reported that the PRM UL OSEM PD is not responding. This PD has been identified to have a shorting problem, but the short existed only at the bias pin of the PD. We disconnected the bias voltages and the PD was working with no (=0V) bias.

It seems that it lost the signal about 8 days ago and the signal intermittently appeared and disappeared.

I suggested to Radhika to remove the Al foil suspecting the other pin of the PD is not shorting.

While looking over Koji's shoulder earlier, I noticed the big peak in the PRM yaw spectrum (and I was starting to get annoyed by the hum....the fibox is so useful in motivating tasks that otherwise get looked over!) 

I used DTT's peak find feature (cursor tab, enable both cursors, select Peak X/Y as your 'statistic', set the 2 cursors to be on either side of the desired peak) to find the frequency of the PRM's violin mode.  It is 627.75 Hz. I adjusted FM5 of the C1:SUS-PRM_LSC filter bank (the "violin" filter) to be centered around this frequency, with the start and stop freqs +\- 4Hz.  I plotted the filter linearly in frequency to ensure that my target freq was not too close to either side of the bandstop.  After loading and engaging the new filter, the hum slowly started to go away. 

Note, for posterity:  The bandstop used to be centered around ~645 Hz or so.  I assume this is a copy-and-paste situation, where we hadn't gone through to check the exact frequency for each optic.

[Jenne Koji]

It seems that the PRM violin mode freqs shifted from 625-ish to 640Hz.
The peaks rang up because of the servo.

Once the notch freq was shifted to 640Hz, the violin mode started to decay.

ellip("BandStop",4,1,90,636,644) gain(1.12202)

Short summary of my Sat. Box. debugging activities over the last few days. Recall that the SRM Sat. Box has been plugged into the PRM suspension for a while now, while the SRM has just been hanging out with no electrical connections to its OSEMs.

As Steve mentioned, I had plugged in Ben's extremely useful tester box (I have added these to the 40m Electronics document sub-tree on the DCC) into the PRM Sat. Box and connected it to the CDS system over the weekend for observation. The problematic channel is LR.  Judging by Steve's 2 day summary plots, LR looks fine. There is some unexplained behavior in the UR channel - but this is different from the glitchy behaviour we have seen in the LR channel in the past. Moreover, subsequent debugging activities did not suggest anything obviously wrong with this channel. So no changes were made to UR. I then pulled out the PRM sat.box for further diagnostics, and also, for comparison, the SRM sat. box which has been hooked up to the PRM suspension as we know this has been working without any issues. 

Tracing out the voltages through the LED current driver circuit for the individual channels, and comparing the performance between PRM and SRM sat. boxes, I narrowed the problem down to a fault in either the LT1125CSW Quad Op-Amp IC or the LM6321M current driver IC in the LR channel. Specifically, I suspected the output of U3A (see Attachment #1) to be saturated, while all the other channels were fine. Looking at the spectrum at various points in the circuit with an SR785, I could not find significant difference between channels, or indeed, between the PRM/SRM boxes (up to 100kHz). So I decided to swap out both these ICs. Just replacing the OpAmp IC did not have any effect on the performance. But after swapping out the current buffer as well, the outputs of U3A and U11 matched those of the other channels. It is not clear to me what the mode of failure was, or if the problem is really fixed. I also checked to make sure that it was indeed the ICs that had failed, and not the various resistors/capacitors in the signal path. I have plugged in the PRM sat. box + tester box setup back into our CDS data acquisition for observation over a couple of days, but hopefully this does the job... I will update further details over the coming days.

I have restored control to PRM suspensions via the working SRM sat. box. The PRM Sat. Box and tester box are sitting near the BS/PRM chamber in the same configuration as Steve posted in his earlier elog for further diagnostics...

GV Edit 2230 hrs 7Nov2016: The signs from the last 6 hours has been good - see the attached minute trend plot. Usually, the glitches tend to show up in this sort of time frame. I am not quite ready to call the problem solved just yet, but I have restored the connections to the SRM suspension (the PRM and SRM Sat. Boxes are still switched). I've also briefly checked the SRM alignment, and am able to lock the DRMI, but the lock doesn't hold for more than a few seconds. I am leaving further investigations for tomorrow, let's see how the Sat. Box does overnight.

Looks like the PRM Sat. Box is now okay, no evidence of the kind of glitchy behaviour we are used to seeing in any of the 5 channels.

Now that we have increased the range of the AA to +/- 10 V I have increased the PRM side OSEM transimpedance from 29 kV/A to 161 kV/A by changing the R64 in the satellite box. The first attached plot shows the ADC input spectrum before and after the change with analog whitening turned off. The PD voltage readback went up from 0.75 to 4.2 V. The second attached plot shows the sensor, ADC, and projected shot noise with analog whitening turned on and compensated digitally. The ADC calibration is 20 V/ 32768 cts. The PRM damping loops are currently disabled.

I checked for oscillation by looking at the monitor point at the whitening board. There was no obvious oscillation on a scope - the signal was 20 mV p-p on 1 us scale which was very similar to the LL channel.

[ericq]

Today, I kicked the PRM to see the sideband splitting in POP110. 

First, we can qualitatively see we moved in the right direction! (See ELOG 9490)

I fit the middle three peaks to a sum of two Lorentzian profiles ( I couldn't get Airy peaks to work... but maybe this is ok since I'm just going to use the location parameter?), and looked at the sideband splitting as a fraction of the FSR, in the same way as in Gabriele's ELOG linked above.

This gave: c / (4 * f55) * (dPhi / FSR) = 0.014 +- .001 

Since the PRC length with simultaneous resonance (to 1mm) is given by c / (4 * f11) = 6.773, this means our length is either 6.759m or 6.787m (+- .001). Given the measurement in ELOG 9588, I assume that we are on the short side of the simultaneous resonance. Thus

The sideband splitting observed from this kick indicates a PRC length of 6.759m +- 1mm

[Murtaza, Radhika]

PRM UL counts normal

We removed the extra DB25 cable (with cut wire isolating pin 5 [Attachment 1]), restoring the original PRM UL/UR/LL connections ---> UL counts ~650

Recap:

It may have been that the foil caps were causing pins 1 and 5 to short in the first place, so removing the caps and restoring the original cable fixed the issue. Final state in Attachment 2.

I found PRM watchdog tripped.  It's all better now.

I have done a quicky offline Wiener filter to check how much PRM yaw motion we can subtract using a seismometer in the corner station.  This work may be redundant since Koji got the POP beam shadow sensor feedback loop working on Friday night.

Anyhow, for now, I used the GUR2 channels, since GUR2 was underneath the ITMX chamber (at the north edge of the POX table).  Note that Zach is currently borrowing this seismometer for the week.

I used GUR2_X, GUR2_Y and GUR2_Z to subtract from the PRM_SUSYAW_IN1 channel (the filename of the figure says "GUR1", but that's not true - GUR1 is at the Yend).  All 4 of these channels had been saved at 2kHz, but I downsampled to 256 (I probably should downsample to something lower, like 64, but haven't yet).  There is no pre-filtering or pre-weighting of the data, and no lowpass filters applied at the end, so I haven't done anything to remove the injected noise at higher freqs, which we obviously need to do if we are going to implement this online.

If I compare this to Koji's work (elog 8562), at 3.2Hz, he gets a reduction of 2.5x, while this gets 10x.  At all other frequencies, Koji's work beats this, and Koji's method gets reduction from ~0.03Hz - 10Hz, while this is only getting reduction between 0.4Hz and 5Hz.  Also, this does not include actuator noise, so the actual online subtraction may not be quite as perfect as this figure.

[yehonathan, paco, anchal]

We attempted to find any symptoms for actuation problems in the PRMI configuration when actuated through BS and PRM.

Our logic was to check angular (PIT and YAW) actuation transfer function in the 30 to 200 Hz range by injecting appropriately (f^2) enveloped excitations in the SUS-ASC EXC points and reading back using the SUS_OL (oplev) channels.

From the controls, we first restored the PRMI Carrier to bring the PRM and BS to their nominal alignment, then disabled the LSC output (we don't need PRMI to be locked), and then turned off the damping from the oplev control loops to avoid supressing the excitations.

We used diaggui to measure the 4 transfer functions magnitudes PRM_PIT, PRM_YAW, BS_PIT, BS_YAW, as shown below in Attachments #1 through #4. We used the Oplev calibrations to plot the magnitude of the TFs in units of urad / counts, and verified the nominal 1/f^2 scaling for all of them. The coherence was made as close to 1 as possible by adjusting the amplitude to 1000 counts, and is also shown below. A dip at 120 Hz is probably due to line noise. We are also assuming that the oplev QPDs have a relatively flat response over the frequency range below.

I suggest plotting all the traces in the plot so we can see their differences. Also remove the 1/f^2 slope so that we can see small differences. Since the optlev servos all have low pass filters around 15-20 Hz, its not necessary to turn off the optlev servos for this measurement.

I think that based on the coherence and the number of averages, you should also be able to use Bendat and Piersol so estimate the uncertainy as a function of frequency. And we want to see the comparison coil-by-coil, not in the DoF basis.

4 sweeps for BS and 4 sweeps for PRM.

[Anchal, Paco]

We had a second go at this with an increased number of averages (from 10 to 100) and higher excitation amplitudes (from 1000 to 10000). We did this to try to reduce the relative uncertainty a-la-Bendat-and-Pearsol

where are the coherence and number of averages respectively. Before, this estimate had given us a ~30% relative uncertainty and now it has been improved to ~ 10%. The re-measured TFs are in Attachment #1. We did 4 sweeps for each optic (BS, PRM) and removed the 1/f^2 slope for clarity. We note a factor of ~ 4 difference in the magnitude of the coil to angle TFs from BS to PRM (the actuation strength in BS is smaller).

For future reference:

With complex G, we get complex error in G using the formula above. To get uncertainity in magnitude and phase from real-imaginary uncertainties, we do following (assuming the noise in real and imaginary parts of the measured transfer function are incoherent with each other):

{Paco, Yehonathan, Hang}

We measured the sensing PRMI sensing matrix. Attachment 1 shows the results, the magnitude of the response is not calibrated. The orthogonality between PRCL and MICH is still bad (see previous measurement for reference).

Hang suggested that since MICH actuation with BS and PRM is not trivial (0.5*BS - 0.34*PRM) and since PRCL is so sensitive to PRM movement there might be a leakage to PRCL when we are actuating on MICH. So there may be a room to tune the PRM coefficient in the MICH output matrix.

Attachment 2 shows the sensing matrix after we changed the MICH->PRM coefficient in the OSC output matrix to -0.1.

It seems like it made things a little bit better but not much and also there is a huge uncertainty in the MICH sensing.

{Yehonathan, Anchel}

In an attempt to fix the actuation of the PRMI DOFs we set to modify the output matrix of the BS and PRM such that the response of the coils will be similar to each other as much as possible.

To do so, we used the responses at a single frequency from the previous measurement to infer the output matrix coefficients that will equilize the OpLev responses (arbitrarily making the LL coil as a reference). This corrected the imbalance in BS almost completely while it didn't really work for PRM (see attachment 1).

The new output matrices are shown in attachment 2-3.

not sure that this is necessary. If you look at teh previous entries Gautam made on this topic, it is clear that the BS/PRM PRMI matrix is snafu, whereas the ITM PRMI matrix is not.

Is it possible that the ~5% coil imbalance of the BS/PRM can explain the observed sensing matrix? If not, then there is no need to balance these coils.