Tell me whether it is correct or not. Otherwise I won't be able to sleep tonight.
According to these results, these would be the proposed adjustements to the cavity lengths:
dl(PRC) = -0.0266 m; dl(SRC) = 0.612 m
According to these results, these would be the proposed adjustements to the cavity lengths:
dl(PRC) = -0.0266 m; dl(SRC) = 0.612 m
Sorry. I was in a rush to go to the LIGO "all hands" meetings when I posted that elog entry, that I forgot a zero in the SRC length value. The correct values are:
dl(PRC) = -0.0266 m; dl(SRC) = 0.0612 m
The cavity absolute lengths are then:
L(PRC) = 0.5/2/f1*c - 0.0266 = 6.7466 m
L(SRC) = c/f2 + 0.0612 = 5.4798 m
where c is the speed of light; f1 = 11065399 Hz; f2 = 55326995 Hz
Today we measured the missing distance to reconstruct SRC length.
I also changed the way the mirror positions are reconstructed. In total for PRC and SRC we took 13 measurements between different points. The script runs a global fit to these distances based on eight distances and four incidence angles on PR2, PR2, SR2 and SR3. The optimal values are those that minimize the maximum error of the 13 measurements with respect to the ones reconstructed on the base of the parameters. The new script is attached (sorry, the code is not the cleanest one I ever wrote...)
The reconstructed distances are:
Reconstructed lengths [mm]:
LX = 6771
LY = 6734
LPRC = 6752
LX-LY = 37
LSX = 5493
LSY = 5456
LSRC = 5474
The angles of incidence of the beam on the mirrors are very close to those coming from the CAD drawing (within 0.15 degrees):
Reconstructed angles [deg]:
aoi PR3 = 41.11 (CAD 41)
aoi PR2 = 1.48 (CAD 1.5)
aoi SR3 = 43.90 (CAD 44)
aoi SR2 = 5.64 (CAD 5.5)
The errors in the measured distances w.r.t. the reconstructed one are all smaller than 1.5 mm. This seems a good check of the global consistency of the measurement and of the reconstruction method.
NOTES: in the reconstruction, the BS is assumed to be exactly at 45 degrees; wedges are not considered.
Last night, I collected ~30mins of data for the vertex seismometer channels and the POP QPD PIT/YAW signals with the PRMI locked on carrier (angular FF OFF). The ITM Oplev loops weren't DC coupled, as they are in the full IFO locking sequence, but I feel like the angular FF filters can be improved - there are frequent sharp dives in the AS110 signal level which are correlated with large amplitude motion of the POP spot on the control room CCD monitor.
Repeating the frequency domain multicoherence analysis using BS_X and BS_Y seismometer channels as witnesses suggest that we can win significantly (see Attachment #1).
I've never really implemented feedforward filters - I was planning on using ericq's latest entry on this subject as a guide. From what I gather, the procedure is as follows:
Some notes from Rana from some years ago: https://nodus.ligo.caltech.edu:8081/40m/11519
If anyone has pointers / other considerations I should take into account, please post here.
I'd like to revive the PRC angular feedforward system. However, it looks like the coherence between the vertex seismometer channels and the PRC angular motion witness sensor (= POP QPD) is much lower than was found in the past, and hence, the stabilization potential by implementing feedforward seems limited, especially for the Pitch DoF.
I found that the old filters don't work at all - turning on the FF just increases the angular motion, I can see both the POP and REFL spots moving around a lot more on the CRT monitors.
I first thought I'd look at the frequency-domain weiner filter subtraction to get a lower bound on how much subtraction is possible. I collected ~25 minutes of data with the PRC locked with the carrier resonant (but no arm cavities). Attachment #1 shows the result of the frequency domain subtraction (the dashed lines in the top subplot are RMS). Signal processing details:
The coherence between target signal (=POP QPD) and the witness channels (=seismometer channels) are much lower now than was found in the past. What could be going on here?
EDIT: These numbers are for a perfect, non-lossy arm cavity. So, a half real, half imaginary world.
Carrier uses arm cavity reflectivity for perfectly resonant case.
PRC carrier gain, flipped PR2, PR3 = 61
PRC carrier gain, regular PR2, PR3 = 68 (same value, within errors, for no folding at all).
Carrier gain loss = (68-61)/68 = 10%
SB uses arm cavity reflectivity for perfectly anti-resonant case.
PRC SB gain, flipped PR2, PR3 = 21
PRC SB gain, regular PR2, PR3 = 22 (same value, within errors, for no folding at all). <--- yes, this this "regular PR2, PR3 = 22..."
SB % gain loss = (22-21)/22 = 4.5%
I claim that we will be fine, recycling gain-wise, if we flip the folding mirrors. If we do as Yuta suggests and flip only one folding mirror, we'll fall somewhere in the middle.
We have both calculated, and agree on the numbers for, the PRC gain for carrier and sideband.
We are using the measured arm cavity (power) loss of 150ppm....see elog 5359.
We get a PRC gain for the CARRIER (non-flipped folding) of 21, and PRC gain (flipped folding) of 20. This is a 4.7% loss of carrier buildup.
We get a PRC gain for the SIDEBANDS (non-flipped folding) of 69, and PRC gain (flipped folding) of 62. This is an 8.8% loss of sideband buildup.
The only difference between the "flipped" and "non-flipped" cases are the L_PR# values - for "non-flipped", I assume no loss of PR2 or PR3, but for the "flipped" case, I assume 1500ppm, as in Rana's email. Also, all of these cases assume perfect mode matching. We should see what the effect of poor mode matching is, once Jamie finishes up his calculation.
Why, one might ask, are we getting cavity buildup of ~20, when Kiwamu always quoted ~40? Good question! The answer seems, as far as Yuta and I can tell, to be that Kiwamu was always using the reflectivity of the ITM, not the reflectivity of the arm cavity. The other alternative that makes the math work out is that he's assuming a loss of 25ppm, which we have never measured our arms to be so good.
For those interested in making sure we haven't done anything dumb:
ppm = 1e-6;
% || | | || ||
% PRM PR2 PR3 ITM ETM
T_PRM = 0.05637;
t_PRM = sqrt(T_PRM);
L_PRM = 0 *ppm;
R_PRM = 1 - T_PRM - L_PRM;
r_PRM = sqrt(R_PRM);
T_PR2 = 20 *ppm;
t_PR2 = sqrt(T_PR2);
L_PR2 = 1500 *ppm;
R_PR2 = 1 - T_PR2 - L_PR2;
r_PR2 = sqrt(R_PR2);
T_PR3 = 47 *ppm;
t_PR3 = sqrt(T_PR3);
L_PR3 = 1500 *ppm;
R_PR3 = 1 - T_PR3 - L_PR3;
r_PR3 = sqrt(R_PR3);
T_ITM = 0.01384;
t_ITM = sqrt(T_ITM);
L_ITM = 0;%100 *ppm;
R_ITM = 1 - T_ITM - L_ITM;
r_ITM = sqrt(R_ITM);
T_ETM = 15 *ppm;
t_ETM = sqrt(T_ETM);
L_ETM = 0 *ppm;
R_ETM = 1 - T_ETM - L_ETM;
r_ETM = sqrt(R_ETM);
rtl = 150*ppm; % measured POWER round trip loss of arm cavities.
rtl = rtl/2; % because we need the sqrt of the exp() for ampl loss....see Siegman pg414.
eIkx_r = exp(-1i*2*pi);
r_cav_res = -r_ITM + (t_ITM^2 * r_ETM * eIkx_r * exp(-rtl)) / (1 - r_ITM*r_ETM * eIkx_r * exp(-rtl) );
eIkx_ar = exp(-1i*pi);
r_cav_antires = -r_ITM + (t_ITM^2 * r_ETM * eIkx_ar * exp(-rtl)) / (1 - r_ITM*r_ETM * eIkx_ar * exp(-rtl) );
%% PRC buildup gain
g_antires = t_PRM*eIkx_ar / (1-r_PRM*r_PR2*r_PR3*r_cav_antires*eIkx_ar);
G_ar = g_antires^2;
G_ar = abs(G_ar) % Just to get rid of the imag part that matlab is keeping around.
g_res = t_PRM*eIkx_r / (1-r_PRM*r_PR2*r_PR3*r_cav_res*eIkx_r);
G_r = g_res^2;
G_r = abs(G_r)
Getting closer, but need to use the real measured AR reflectivity values, not the 1500 ppm guess. These should be measured at the correct angles and pol, using an NPRO.
I'm still on that!
With 1500ppm loss on both PR2 and PR3, 150ppm arm cavity loss:
With a PR2 loss of 896ppm (from the plot on the wiki), but no loss from PR3 because we didn't flip it, and the same 150ppm round trip arm cavity loss, I get:
Carrier gain = 21.0
Sideband gain = 66.7
(No loss case, with an extra sig-fig, so you can see that the numbers are different: Carrier = 21.4, Sideband = 68.8 .)
So, this is 1.6% loss of carrier buildup and 3.1% loss of sideband buildup. Moral of the story - G&H's measured AR reflectivity is less than Rana's guess, and we didn't flip PR3, so we should have even less of a power recycling gain effect than previously calculated.
Issues in PRC:
1. Power recycling gain is too low (~ 15 instead of 40, according to Kiwamu).
2. Mode matching to both arms are ~90%(see #6859), but PRC has terrible mode.
Clipping/flipping in PRC?
3. From cameras, beam spot moves so much when PRMI is locked.
Alignment? Mirrors(especially PR2/3) moves too much?
4. Error signals are glitchy when PRMI is locked.
Servo design? Mirrors moves too much?
What we have learned from the vent:
1. PRM, PR2, PR3 was not flipped.
2. Their suspensions looked OK, too.
3. We noticed clipping at BS and Faraday. They must be avoided when tip-tilts are installed on next vent.
4. Took useful photos for next vent. Positions of green optics on optical layout CAD must be updated.
5. It is not so difficult to recover the IFO state after cycling the vacuum if we use attenuator setup using PBS (see elog #6892). But, of course, we need plans before cycling.
- measure PRMI power recycling gain from POP
- FPMI using ALS
- measure PRFPMI power recycling gain from TRY/X
- correlation between beam spot motion at POP camera and glitch
- correlation between PR2/PR3 motion and glitch (how can we measure PR2/3 motion? set up oplevs?)
- mode scan for PRC, using AS AUX laser
- beam profile measurement at REFL,POP
- refine servo design of MICH and PRCL
Since we're having trouble keeping the PRC locked as we reduce the CARM offset, and we saw that the POP22 power is significantly lower in the 25% MICH offset case than without a MICH offset, Rana suggested having a look at the RF spectra of the REFL33 photodiode, to see what's going on.
The Agilent is hooked up to the RF monitor on the REFL33 demod board. The REFL33 PD has a notch at 11MHz and another at 55MHz, and a peak at 33MHz.
We took a set of spectra with MICH at 25% offset, and another set with MICH at 15% offset. Each of these sets has 4 traces, each at a different CARM offset. Out at high CARM offset, the arm power vs. CARM offset is pretty much independent of MICH offset, so the CARM offsets are roughly the same between the 2 MICH offset plots.
What we see is that for MICH offset of 25%, the REFL33 signal is getting smaller with smaller CARM offset!! This means, as Rana mentioned earlier this evening, that there's no way we can hold the PRC locked if we reduce the CARM offset any more.
However, for the MICH offset 15% case, the REFL 33 signal is getting bigger, which indicates that we should be able to hold the PRC. We are still losing PRC lock, but perhaps it's back to mundane things like actuator saturation, etc.
The moral of the story is that the 3f locking seems to not be as good with large MICH offsets. We need a quick Mist simulation to reproduce the plots below, to make sure this all jives with what we expect from simulation.
For the plots, the blue trace has the true frequency, and each successive trace is offset in frequency by a factor of 1MHz from the last, just so that it's easier to see the individual peak heights.
Here is the plot with MICH at 25% offset:
And here is the plot with MICH at 15% offset:
Note that the analyzer was in "spectrum" mode, so the peak heights are the true rms values. These spectra are from the monitor point, which is 1/10th the value that is actually used. So, these peak heights (mVrms level) times 10 is what we're sending into the mixer. These are pretty reasonable levels, and it's likely that we aren't saturating things in the PD head with these levels.
The peaks at 100MHz, 130MHz and 170MHz that do not change height with CARM offset or MICH offset, we assume are some electronics noise, and not a true optical signal.
Also, a note to Q, the new netgpib scripts didn't write data in a format that was back-compatible with the old netgpib stuff, so Rana reverted a bunch of things in that directory back to the most recent version that was working with his plotting scripts. sorry.
As the measurements have been done under feedback control, the lower RF peak height does not necessary mean
the lower optical gain although it may be the case this time.
These non-33MHz signals are embarassingly high!
We also need to check how these non-primary RF signals may cause spourious contributions in the error signals,
including the other PDs.
A question was raised as to how much passive filtering we benefit from if we pick off the local oscillator beam for BHD from the PRC. I did some simplified modeling of this. For the expected range of arm cavity round trip losses (20-50 ppm), I think that the 40m CARM pole will be between 75-85 Hz. The corresponding recycling gain will be 40-50, with the current PRM. I assumed 1000 ppm loss inside the PRC. The net result is that, assuming the single pole coupled cavity response, we will get ~8-9 dB of filtering at ~200 Hz of the intensity noise of the input laser field to the interferometer if we pick the LO beam off from the PRC (e.g. PR2 transmission), instead of picking it off before.
The next questions are: (i) can we do a sufficiently good job of achieving the required RIN stability on the LO field for BHD without relying on the passive filtering action of the PRC? and (ii) is the benefit of the PRC filtering ruined in the process of routing the LO field from wherever the pickoff happens to the BHD setup?
After in-vac alignment work last night, PRC is flashing brighter than PRMI alignment last week.
My hypothesis is that "we aligned PRM to junk MI fringe last week". Possibly, we used MI fringe caused by AR reflection of ITMs, or MI fringe reflected from SRM.
PRC flashing last week (youtube, elog #8085, elog #8091)
PRC flashing this time (Lens in-front of AS camera was taken out)
My hypothesis can explain:
- why we had dimmer POP last week (flash in half-PRC was way brighter even when we had more attenuators (youtube))
- why I thought AS55 is broken (AS was too dim)
Be careful of junk beams.
[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.
global sos_lx sos_ly sos_cx sos_cy tt_lx ...
tt_ly tt_cx tt_cy sos_sx sos_sy sos_dy
%% Survey of the PRC+SRC lengths %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% measured distances
d_MB2_MY = 2114 + 27 + 9;
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.
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:
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
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
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.
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.
- 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.
- 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:
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.
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.
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):
* 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.
This is the ARM mode I used for all comparisons:
This is the nominal "as designed" PRC, with flat PR2/3 folding mirrors.
This assumes both PR2 and PR3 have a RoC of -600 when not flipped, and includes the affect of propagation through the substrates.
In this case we only flip PR2 and leave PR3 with it's bad -600 RoC:
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.
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.
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.
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.
The main issue is that flipping PR3 induces considerable astigmatism.
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.
- We are ready to implement ASS for PRM
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.
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):
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.
* 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....
Measured loop TFs - PRCL is a big mystery. Used these to finalize loop gains.
don't use IN_1/IN_2: recall pizza meeting from a few weeks back: use IN1/EXC + Al-Gebra
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.
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!