Over the last couple of days, I've been working towards getting the infrastructure ready to test out the scheme of sensing (and eventually, controlling) the homodyne phase using the so-called RF44 scheme. More details will be populated, just quick notes for now before I forget.
- Loose fiber coupler: Sorry about that. I could not detect something was loose there, although some of the locks were not tightened.
- S incident instead of P: Sorry about that too. I completely missed that the IMC takes S-pol.
Attachment #1 shows the optical setup currently being used to send the LO field with RF sidebands on it to the air BHD setup.
Attachment #2 shows spectra of the relative phase drift between LO and IFO output field (from the Dark Michelson).
Attachment #3 shows the signal magntiude of the signals used to make the spectra in Attachment #2, during the observation time (10 minutes) with which the spectra were computed. The dashed vertical lines denote the 1%, 50% and 99% quantiles.
Attempts to close a feeddback loop to control the homodyne phase:
As promised some time ago, I've obtained input noise spectra from the sites calibrated to physical units. They are located in a new subdirectory of the BHD repo: A+/input_noises. I've heavily annotated the notebook that generates them (input_noises.ipynb) with aLOG references, to make it transparent what filters, calibrations, etc. were applied and when the data were taken. Each noise term is stored as a separate HDF5 file, which are all tracked via git LFS.
So far there are measurements of the following sources:
These can be used, for example, to make more realistic Hang's bilinear noise modeling [ELOG 15503] and Yehonathan's Monte Carlo simulations [ELOG 15539]. Let me know if there are other specific noises of interest and I will try to acquire them. It's a bit time-consuming to search out individual channel calibrations, so I will have to add them on a case-by-case basis.
Turns out what was causing the instability in the aLIGO plots were the lock commands which I forgot to remove before running the simulation. Removing these also made the simulation much faster.
Other than that I improved other stuff in the simulations:
Still need to do:
Feel free to add to the todo list.
After more trials, I think the phase tracker part used to provide the error signal for this scheme needs some modification for this servo to work.
Attachment #1 shows a block diagram of the control scheme.
I was using the "standard" phase tracker part used in our ALS model - but unlike the ALS case, the magnitude of the RF signal is squished to (nearly) zero by the servo. But the phase tracker, which is responsible for keeping the error signal in one (demodulated) quadrature (since our servo is a SISO system) has a UGF that is dependent on the magnitude of the RF signal. So, I think what is happening here is that the "plant" we are trying to control is substantially different in the acquisition phase (where the RF signal magnitude is large) and once the lock is enabled (where the RF signal magnitude becomes comparitively tiny).
I believe this can be fixed by dynamically normalizing the gain of the digital phase tracking loop by the magnitude of the signal = sqrt(I^2 + Q^2). I have made a modified CDS block that I think will do the job but am opting against a model reboot tonight - I will try this in the daytime tomorrow.
I'm also wondering how to confirm that the loop is doing something good - any ideas for an out-of-loop monitor? I suppose I could use the DCPD - once the homodyne phase loop is successfully engaged, I should be able to drive a line in MICH and check for drift by comparing line heights in the DCPD signal and RF signal. This will requrie some modification of the wiring arrangement at 1Y2 but shouldn't be too difficult...
The HEPAs, on the PSL table and near ITMY, were dialled down / turned off respectively, at ~8pm at the start of this work. They will be returned to their previous states before I leave the lab tonight.
I don't think the proposed scheme for sensing and controlling the homodyne phase will work without some re-thinking of the scheme. I'll try and explain my thinking here and someone can correct me if I've made a fatal flaw in the reasoning somewhere.
Field spectrum cartoon:
Attachment #1 shows a cartoon of the various field components.
So is there a 90 degree relative shift between the signal quadrature in the simple Michelson vs the DRFPMI? But wait, there are more problems...
Closing a feedback loop using the 44 MHz signal:
We still need to sense the 44 MHz signal with a photodiode, acquire the signal into our CDS system, and close a feedback loop.
I don't have any bright ideas at the moment - anyone has any suggestions?🤔
I wanted to check what kind of signal the photodiode sees when only the LO field is incident on the photodiode. So with the IFO field blocked, I connected the PDA10CF to the Agilent analyzer in "Spectrum" mode, through a DC block. The result is shown in Attachment #2. To calculate the PM/AM ratio, I assumed a modulation depth of 0.2. The RIN was calculated by dividing the spectrum by the DC value of the PDA10CF output, which was ~1V DC. The frequencies are a little bit off from the true modulation frequencies because (i) I didn't sync the AG4395 to a Rb 10 MHz signal, and (ii) the span/BW ratio was set rather coarsely at 3kHz.
I would expect only 44 MHz and 66 MHz peaks, from the interference between the 11 MHz and 55 MHz sideband fields, all other field products are supposed to cancel out (or are in orthogonal quadratures). This is most definitely not what I see - is this level of RIN normal and consistent with past characterization? I've got no history in this particular measurement.
I got some feedback from Koji who pointed out that the phase tracker is not required here. This situation is similar to the phase locking of two lasers together, which we frequently do, except in that case, we usually we offset the absolute frequencies of the two lasers by some RF frequency, and we demodulate the resulting RF beatnote to use as an error signal. We can usually acquire the lock by simply engaging an integrator (ignoring the fact that if we actuate on the laser PZT, which is a frequency actuator, just a proportional feedback will be sufficient because of the phase->frequency conversion), the idea being that the error signal is frequently going through a zero-crossing (around which the sinusoidal error signal is approximately linear) and we can just "catch" one of these zero crossings, provided we don't run of actuation range.
So the question here becomes, is the RF44 signal a suitable error signal such that we can close a feedback loop in a similar way? To try and get more insight, I tried to work out the situation analytically. I've attached my thinking as a PDF note. I get some pretty messy complicated expressions for the RF44 signal contributions, so it's likely I've made a mistake (though Mathematica did most of the heavy lifting), it'll benefit from a second set of eyes.
Anyways, I definitely think there is some additional complications than my simple field cartoon from the preceeding elog would imply - the relative phases of the sidebands seem to have an effect, and I still think the lack of the PRC/SRC make the situation different from what Hang/Teng et. al. outlined for the A+ homodyne phase control analysis. Before the HEPA failed, I had tried closing the feedback loop using one quadrature of the demodulated RF44 signal, but had no success with even a simple integrator as the loop (which the experience with the PLL locking says should be sufficient, and pretty easily closed once we see a sinusoidally oscillating demodulated error signal). But maybe I'm overlooking something basic conceptually?
The simple interferometer, composed of a single bounce reflection from ITMY and the LO beam deilvered via fiber to the AS table, can be locked - i.e. the phase of the LO beam can be controlled such that the DC light level on the DCPDs after the two beams are interfered can be stabilized. This test allows us to confirm that various parts of the sensing and actuation chain (e.g. PI PZT for homodyne phase control, Trek amplifier etc etc) are working.
I will post more quantitative analysis tomorrow.
Attachment #2 shows the servo topology.
Attachment #1: spectra of the phase noise between LO and IFO output fields sensed using the RF44 signal.
Closing a feedback loop:
Summary of discussion between Koji and gautam on 14 Oct:
I tried all of these last night / overnight, here are my findings.
Analog locking of the homodyne phase:
See Attachment #1.
Relative stability of two IFR2023s synchronized to the same FS725 Rb standard:
The electrical LO signal for demodulation of the 44 MHz photocurrent is provided by an IFR2023 signal generator. To maintain a fixed phase relation between this signal, and the phase modulation sidebands imprinted on the interferometer light via a separate IFO2023 signal generator, I synchronize both to the same Rb timing standard (a 10 MHz signal from the FS725 is sent to the rear panel frequency standard input on the IFR). We don't have a direct 44 MHz electrical signal available from the main IFO Marconi at the LSC rack (or anywhere else for that matter). So I decided to do this test at 55 MHz.
A look at the time domain signal:
With the Michelson locked on the dark fringe, the RF44 I and Q signals in the time domain are shown in Attachment #3 for a 1 minute stretch.
Pushed another update to MCMC simulation. This includes:
The DOFs<->RFPD associations I use are:
However, one thing that bothers me is that for some reason ~ 15 out of 160 aLigo simulations are discarded while none for A+. It can also be seen that the A+ simulations are more spread-out which might be related.
The new simulation results are attached.
I found this H1 alog entry by Izumi confirming that the calibrated channels CAL-CS_* need the same dewhitening filter.
This encouraged me to download the PRCL and MICH data and using Jon's example notebook. I incorporated these noise spectra into the MCMC simulation. The most recent results are attached.
I am still missing:
Also, now the MCMC repeats a simulation if it doesn't pass the RF PDs test so the number of valid simulations stays the same. I'm still not sure about why the A+ simulations are much more robust to these tests than aLigo simulations.
Optics --> Cabinet at south end (Attachment #1)
Scanned datasheets--> wiki. It would be good if someone can check the specs against what was ordered.
Basically, they repeated our specs and showed the coating performances for HR/AR for 10deg P and PR/AR for 45deg P. There is no RoC measurement by the vendor.
Nevertheless, their RoC (paper) specs should be compared with our request.
I have rebuilt the MCMC simulation in an OOP fashion and incorporated Lance/Pytickle functionality into it. The usage of the MCMC now is much less messy, hopefully.
I made an example that calculates the closed-loop noise-coupling from SRCL sensing and displacement to DARM in A+. I used the control filters that Kevin defined in his controls example.
The resulting noise budget is in attachment 1. The code is in the 40m/bhd git.
I also investigated why aLIGO simulations behave so different than the A+ simulation (See few previous elogs in this thread). That is why aLIGO results are much less variable, and the simulations in aLIGO barely pass the validity checks, while A+ simulations almost always pass.
The way I check for the validity of a kat model is by scanning all the DOFs and checking that the corresponding sensing RFPDs demodulated signals cross zero. Attachment 2 shows these scanning for 3 such RFPDS for 3 cases:
A+ model with maxtem = 2
ALigo model with maxtem = 2
ALigo model with maxtem = 'off'
It seems like the scanning curves for A+ and ALigo with no HOMs are well behaved and look like normal PDH signals, while the ALigo with maxtem = 2 curves look funky. I believe that the aLIGO+HOMS curves indicate that the IFO is not really in a good locking point. All the IFO lockings were done by using the locking methods straight out of the PyKat package.
Cool. Can you give us Bode plots of the open loop gain for each of the 5 length control loops?
I spent a few hours monkeying around with the control filters. They are totally made up and also it's my first time trying to design control filters.
The OLTFs of the different length controls are shown in attachment 1.
The open-loop couplings of the DOFS to DARM are shown in attachment 2.
Note that in Lance/Pytickle the convention is that CLTFs are 1/(1 - G). Where G is the OLTF.
I dived into the alog to make the OLTFs in the MC_controls example more realistic. I was mainly inspired by these entries:
and Evan's and Dennis's Theses.
Attachment 1 shows the new OLTFs. I tried to make them go like 1/f around the UGF and fall as quickly as possible at higher frequencies. I didn't do more advanced stability checks.
I also noticed that imbalances and detunings in the MC simulation can change the plants significantly. Especially DARM, CARM, and sometimes PRCL. I added the option to fix some OLTFs throughout the simulation. At every iteration, the simulation computes the required control filter to fix the selected OLTFs such that it will match the OLTFs in the undetuned and balanced IFO.
I have taken transfer functions and noise measurements of the two HAM-A coil driver boxes D1100687 #S2100027 and #S2100028. All transfer functions look as expected. I'm not sure about the noise measurements. If anyone sees flaw in my measurement method, please let me know. I'm not sure why in some channels I got 10Hz harmoni peaks in the noise. That was very strange. Also let me know if my current noise estimate is wrong.
I took transfer function and noise measurement of satellite amplifier box's photodiode transimpedance circuit. For the measurement, I created a makeshift connector to convert backside DB25 into DB9 with the 4 channels for PDA input. The output was taken in differential form at the front PD Output port. To feed current to the circuit, I put in 12 kOhm resistors in series at the inputs, so the V/V transfer function measured was multiplied by 12 kOhm to get the transimpedance of the circuit.
Edit Wed Feb 10 15:14:13 2021 :
THE NOISE MEASUREMENT WAS WRONG HERE. SEE 40m/15799.
I took some steps to reduce the coupling of 60 Hz harmonics in noise measurement. The box was transferred to the floor instead of on top of another instrument. Measurement was immediately converted into single-ended using SR560 in battery mode with a gain of 10. All of the setups was covered in aluminum foil to increase isolation.
I did the recommended modifications on of the boards with serial number S2100028. These included:
I took transfer function measurements with same method as in 40m/15774 and I'm presenting it here to ensure the modifications are correct and if I should proceed to the next board as well. I didn't have the data used to make plots in here but I think the poles and zeros have landed in the right spot. I'll wait for comments until tomorrow to proceed with changes in the other board as well. I'll do noise measurements tomorrow.
Looks fine to me visually but the verdict can only be made once the z:p locations are quantitatively confirmed, and the noise tests pass. It would be interesting to see what kind of time-domain transient (in N of force) switching on the de-whitening introduces, i guess best done interferometrically.
I'll wait for comments until tomorrow to proceed with changes in the other board as well. I'll do noise measurements tomorrow.
I fitted zeros and poles in the measured transfer function of D1100687 S2100027 and got zeros at 130 Hz and 234 Hz and poles at 10Hz and 2845 Hz. These values are different from the aimed values in this doc, particularly the 234Hz zero which was aimed at 530 Hz in the doc.
I also took the noise measurement using the same method as described in 40m/15780. The noise in Acquisition mode seems to have gone up in 10 Hz - 500 Hz region compared to the measurement in 40m/15780 before the modifications.
All channels are consistent with each other.
Edit Mon Feb 1 12:24:14 2021:
Added zero model prediction after the changes. The measurements match with the predictions.
Edit Wed Feb 3 16:46:59 2021:
Added zero modeled noise in the noise spectrum curves. The acquisition mode curves are in agreement with the model. The noise in Run mode is weirdly lower than predicted by zero.
I have made the modifications on the other board D1100687 S2100028 as well. The measurements were taken as mentioned in 40m/15784. All conclusions remain the same as 40m/15784. The attached zip file contains all measurement data, before and after the modifications.
Edit Wed Feb 3 16:44:51 2021 :
I set up a working area on the table next to the south flow bench (see attachment). I also brought in a rolling table for some extra space.
I covered all the working surfaces with a foil from the big roll between 1x3 and 1x4.
I took the SOSs, SOS parts and the OSEMS from the MC2 table to the working area.
I cleaned some LN Allen keys with isopropanol and put them on the working table, please don't take them.
You can remove the components of the optical table enclosure (black ones) and use the optical table as your working area too.
I have made modifications recommended in this doc. The changes made are:
I took transfer function measurements, fitted them with zeros and poles and plotted it against the zero model of the circuit. The zeros and poles we intended to shift are matching well with 3Hz zero and 30 Hz pole. The later pole at 1500 Hz is at a higher value from what is predicted by zero.
I also took noise measurements and they are in good agreement with the noise predicted by zero.
I gathered all the components I could find from the SOS towers and the cleanroom and put it all on the table next to the flow bench (See attachment).
I combed through the cleanroom cabinet for SOS parts but didn't find all the parts listed in the procurement spreadsheet. I did find some extra items that were not listed.
This table compares the quantities in the spreadsheet to the quantities collected on the table. Green rows are items I found more than in the procurement spreedsheet while red rows are items I found less.
As suggested, I wrapped the satellite amplifier box D10028128 S2100029 in blanket and foam and took very low frequency spectrum starting from 32 mHz to 3 Hz. The results are attached along with stiched high frequency measurements from 40m/15793.
THIS MEASUREMENT WAS WRONG. SEE 40m/15799.
I measured the output DC voltage of the satellite amplifier box at PDMon port when the PDA input was shorted and got following offsets:
However, I think I'm making a mistake while measuring this offset as well as all the noise measurements of this satellite amplifier box so far. Since it is a current input, transimpedance circuit, the noise of the circuit should be measured with open input, not closed. Infact, by shorting the PDA input, I'm giving DC path to input bias current of AD833 transimpedance amplifier to create this huge DC offset. This won't be the case when a photodiode is connected at the input which is a capacitor and hence no DC path is allowed. So my issue of offset was bogus and past two noise measurements in 40m/15797 and 40m/15793 are wrong.
Why not just do this test with the dummy suspension box and CDS system? I think Rich's claim was that the intrinsic LED RIN was dominant over any drive current noise but we can at least measure the quadrature sum of the two (which is after all the relevant quantity in terms of what performance we can realize) and compare to a model.
Testing the satellite amp i.e. PD driver
- To test the noise of the PD transimpedance amps: Leave the PD input open (do not short the terminal goes to the PD)
- To test the current noise of the LED drivers: Short the output with an appropriate Rs to have the nominal current.
- To test the overall noise level together with the LED/PD pair: Connect the dummy OSEM module.
Testing the coil drivers
- Short the output with an appropriate Rs.
Here is a proper measurement for PD transimpedance amplifier circuit in the Satellite amplifier box D1002818 S2100029. The input from rear DB25 connector was left open and measurement was taken with AC coupling with correction by the AC coupling transfer function (Zero at 0, pole at 160 mHz). I have calculated the input referred displacement noise by calculating the conversion factor of OSEM in A/m. From 40m/12470, old conversion factor of OSEM to output of sat amplifier was 1.6 V/mm. then, the transimpedance was 39.2 kOhm, so that must mean a conversion factor of 1.6e3/39.2 A/m. This I scaled with increased drive current by factor of 35/25 as mentioned in this document. The final conversion factor turned out to be around 57 mA / m. If someone finds error in this, please let me know.
There is excess noise in the low-frequency region below 5-6 Hz. If people think I should make a measurement of amplified noise to go further away from the instrument noise floor, let me know.
I expect that a single OSEM channel can't be better than 1e-10 m/rHz above 5 Hz, so probably something wrong in the calibration. 1.6 V/mm seems right to me, so could be some place else.
Gautam pointed out that there are extra Sm-Co magnets stored in the clean optics cabinet.
I took the magnet box out and put it on the rolling table next to south flow bench. The box contains 3 envelopes with magnets.
They are labelled as following:
FLUX 94 - 50 parts
FLUX 93 - 10 parts
FLUX 95 - 40 parts
(What is FLUX??)
The box also contains some procurement documents.
The clean and bake dcc says :
1. Ultrasonic clean in methanol for 10 minutes
2. Bake in vacuum at 177 C° for 96 hours
Should we go ahead with the C&B?
The curie temp of SmCo seems about x2 (in K) of the one for NdFeB. i.e. 600K vs 1000K. So I believe 177degC = 450K is not an issue. Just make sure the curie temp, referring the specific property for the magnets from this company. (You already know the company from the procurement doc). It'd be great if you upload the doc on the 40m wiki.
Also, the magnets are nickel-plated. I guess that doesn't matter for the baking (Curie temp of 355 °C)?
A summary of things that need to be fabricated/purchased/done:
Stephen and I discussed the nominal heights of the BHD platform components.
I've brought 4 DO-32L-PE cards from WB for BHD upgrade for Jon.
In the cleanroom, I opened the nickel-plated SmCo magnet box to take a closer look. I handled the magnets with tweezers. I wrapped the tips of the tweezers with some Kapton tape to prevent scratching and magnetization.
I put some magnets on a razor blade and took some close-up pictures of the face of the magnets on both sides. Most of them look like attachment 1.
Some have worn off plating on the edges. The most serious case is shown in attachment 2. Maybe it doesn't matter if we are going to sand them?
I measure the magnetic flux of the magnets by just attaching the gaussmeter flat head to the face of the magnet and move it around until the maximum value is reached.
For envelope #1 out of 3 the values are: (The magnet ordering is in attachment 3):
Going to continue tomorrow with the rest of the magnets. I left the magnet box and the gaussmeter under the flow bench in the cleanroom.
Continuing with envelope number 2
I think I have to redo envelope 1 tomorrow.
Redoing magnet measurement of envelope 1:
Moving on to inspect and measure envelope 3 (the last one):
I measure some of the dowel pins we got from Mcmaster with a caliper.
One small pin is 0.093" in diameter and 0.376" in length. The other sampled small pin has the same dimensions.
One big pin is 0.187" in diameter and 0.505" in length. The other is 0.187" in diameter and 0.506" in length.
The dowels meet our requirements.
We got some dumbells from Re-Source Manufacturing (see attached). I picked 3 in random and measured their dimensions:
1. 0.0760" in diameter, 0.0860" in length
2. 0.0760" in diameter, 0.0860" in length
3. 0.0760" in diameter, 0.0865" in length
In accordance with the Schematics.
Today I assembled the skeleton of 6 towers, without clamps and sensor assembly (attachment 1).
Some of the side plates have this weird hole that doesn't fit any of the suspension blocks (attachment 2). I didn't notice when I counted the parts and now there are exactly enough side plates to assemble 7 towers.
Also found that one of the stiffener plates has a broken threading.
We will need more parts to go beyond the necessary 7 SOSs. I will do the recounting later.
Things to do next:
1. Find the capped spring plungers and send them to C&B.
2. Assemble the clamps onto the suspension blocks.
3. Push some Viton tips into the vented screws we got to make safety stops.
4. more C&B: Magnets, dumbells, dowel pins, OSEMs.
5. Push clean dowel pins into the last suspension block.
6. Assemble 7th Tower.
7. Assemble safety stops and clamps.
8. Glue magnets to dumbells.