I got linear relation between these. The results and method are below.

RESULTS

METHOD

I added offset (C1:ASS-YARM_ETM_PIT_L_DEMOD_I_OFFSET and C1:ASS-YARM_ETM_YAW_L_DEMOD_I_OFFSET) to shift the beam spot on ETMY. For each data point, I measured the difference in angle of ETMY with oplev before and after adding offset. The precedure for each measurement I employed is following:

-- run dither

-- wait until error signals of dithering settle down

-- freeze dither

-- measure angle (10s avg)

-- add offset

-- wait until error signals of dithering settle down

-- freeze dither

-- measure angle (10s avg)

The reason why I measured the angle without offset for each measurement is that the angle which oplev shows changed with time (~several tens of minutes or something). 

DISCUSSION

At the maximum values of offsets, the arm transmission power started to degrade, so the range where I can do this measurement is limited by these values as for now. However, we have to shift the beam spot more in order to make loss maps of sufficiently broad area on the mirror (the beam width (w) on ETM; w ~ 5 mm). Then, we have to find out how to shift the beam spot more.  

In preparation for the armloss map I checked the calibration of the Y-Arm ITM and ETM OpLevs with the method originally described in https://nodus.ligo.caltech.edu:8081/40m/1247. I was getting a little confused about the math though, so I attached a document at the end of this post in which I work it out for myself and posteriority. Stepping through an introduced offset in the control filter for the corresponding degree of freedom, I recorded the change in transmitted power and the reading of the OpLev channel with the current calibration. One thing I noticed is that the calibration for ITM PIT is inverted with respect to the others. This can of course be compensated at any point in any readout/feedback chain, but it might be nice to establish some sort of convention where positive feedback to the mirror will increase the OpLev reading.

The calibration factors I get are within ~10% of the currently stored values. The table (still incomplete, need to relate to the current values) summarizes the results:

The individual graphs:

ETM PIT

ETM YAW

ITM PIT

ITM YAW

The math:

I poked around a bit thinking about what we need for a single AS WFS. 

New things that we would need:

• The WFS head itself: aLIGO ASC RF Photodetector (WFS) - E1400103 (Which will need some modification for 55MHz)
• Interface board for DC power and DC signals: aLIGO WFS Interface Chassis - D1101906
• Quad Demod chassis: Quad IQ Demodulator Chassis -  D0902796 [Variant B (WFS)] (Needs LO, but we have unused outputs in the LSC LO distributor)

Things we have:

• Many Eurocard-style Whitening chassis, such as we use for all of the LSC PDs. 
• Enough ADC (c1ioo has two ADCs, but doesn't even use 32 channels, so we can steal one, and throw it into c1lsc)

We'd have 12 new signals to acquire: 4 quadrants x DC, I, Q. In principle the DC part could go into a slow channel, but we have the ADC space to do it fast, and it'll be easier than mucking around with c1iscaux or whatever.

Open question: What to do about AA? A quick search didn't turn up any eurocard AA chassis like the ones we use for the LSC PDs. However, given the digital AA that happens from 64kHz->16kHz in the IOP, we've talked about disabling/bypassing the analog AA for the LSC signals. Maybe we can do the same for the QPD signals? Or, modify the post-demod audioband amplifer in the demod chassis to have some simple, not too agressive lowpass.

Motivation:

1. I want to use the QPD at POP, calibrate it into physical units, and quantify the amount of angular jitter in the PRC (which I claim is what limits DRMI stability atm).
2. I want to revive the PRC angular feedforward to try and mitigate this a bit. But is feedforward even the best approach? Can we use feedback using the POP QPD?

POP QPD checkout:

• The POP QPD sits on the ITMX optical table. 
• It is interfaced to the CDS system via an OT301 and then a Pentek whitening stage (z:p = 15:150). 
• The OT301 claims to have a switchable offset nulling capability - but despite my best efforts tonight, I couldn't use the knobs on the front to null the offset (even with the PRC locked on carrier and a strong POP beam on the QPD).
  • We don't have readbacks of the individual quadrants available.
  •  
• So I moved the QPD with the PRC locked, to center the CDS readback of the spot position at (0,0).
• Next step is to calibrate the POP QPD readback into physical units.
  • I'm thinking of using the EricG diode laser for this purpose.
  • I can calibrate counts to mm of displacement on the QPD active area.
  • After which I can use the estimated position to PR2 (from which POP is extracted) to convert this to angular motion.
• I guess I should check for coherence between the POP QPD signal and all angular sensors of PRM/BS/MC1/MC2/MC3 to try and confirm the hypothesis that the folding mirrors are dominating the angular noise of the cavity. Unfortunately we don't have readbacks of the angular positions of TT1 and TT2.
• I moved the POP camera a bit in YAW so that the POP spot is now better centered on the CCD monitor.
• I also wanted to check the centering on the other POP QPD (POP22/POP110/POPDC?) but I think the POPDC signal, used for triggering the PRCL LSC servo, is derived from that PD, so everytime I blocked it, the lock was lost. Need to think of another strategy.
• MC3 has been rather glitchy tonight.
  • So I will wait for a quieter time when I can collect some data to train the WF for angular FF.

[Yuki, Gautam]

Attachments #1 is the current setup of AUX Y Green locking and it has to be improved because:

• current efficiency of mode matching is about 50%
• current setup doesn't separate the degrees of freedom of TEM01 with PZT mirrors (the difference of gouy phase between PZT mirrors should be around 90 deg) 
• we want to remotely control PZT mirrors for alignment
  (Attachments #2 and #3)

About the above two: 

One of the example for improvement is just adding a new lens (f=10cm) soon after the doubling crystal. That will make mode matching better (100%) and also make separation better (85 deg) (Attachments #4 and #5). I'm checking whether we have the lens and there is space to set it. And I will measure current power of transmitted main laser in order to confirm the improvement of alignment.

About the last:

I am considering what component is needed. 

Reference:

• current setup of Y: elog:40m/11897

[ Yuki, Koji, Gautam ]

An alignment of AUX Y end green beam was bad. With Koji and Gautam's advice, it was recovered on Friday. The maximum value of TRY was about 0.5.

[ Yuki, Gautam ]

The setup I designed before has abrupt gouy phase shift between two steering mirrors which makes alignment much sensitive. So I designed a new one (Attached #1, #2 and #3). It improves the slope of gouy phase and the difference between steering mirrors is about 100 deg. To install this, we need new lenses: f=100mm, f=200mm, f=-250mm which have 532nm coating. If this setup is OK, I will order them.

There may be a problem: One lens should be put soon after dichroic mirror, but there is little room for fix it. (Attached #4, It will be put where the pedestal is.)  Tomorrow we will check this problem again.

And another problem; one steering mirror on the corner of the box is not easy to access. (Attached #5) I have to design a new seup with considering this problem.

[ Yuki, Steve ]

With Steve's help, we checked a new lens can be set soon after dichroic mirror.

[ Yuki, Gautam ]

We want to remotely control steeing PZT mirrors so its driver is needed. We already have a PZT driver board (D980323-C) and the output voltage is expected to be verified to be in the range 0-100 V DC for input voltages in the range -10 to 10 V DC.
Then I checked to make sure ir perform as we expected. The input signal was supplied using voltage calibrator and the output was monitored using a multimeter. 
But it didn't perform well. Some tuning of voltage bias seemed to be needed. I will calculate its transfer function by simulation and check the performance again tommorow. And I found one solder was off so it needs fixing.  

Reference:
diagram --> elog 8932
 

Plan of Action:

• Check PZT driver performs as we expected
• Also check cable, high voltage, PZT mirrors, anti-imaging board
• Obtain calibration factor of PZT mirrors using QPD
• Measure some status value before changing setup (such as tranmitted power of green laser)
• Revise setup after a new lens arrives
• Align the setup and check mode-matching
• Measure status value again and confirm it improves
• (write programming code of making alignment control automatically)

[ Yuki, Gautam ]

I fixed the input terminal that had been off, and made sure PZT driver board performs as we expect. 

At first I ran a simulation of the PZT driver circuit using LTspice (Attached #1 and #2). It shows that when the bias is 30V the driver performs well only with high input volatage (bigger than 3V). Then I measured the performance as following way:

1. Applied +-15V to the board with an expansion card and 31.8V to the high voltage port which is the maximum voltage of PS280 DC power supplier C10013.
2. Terminated input and connectd input bias to GND, then set offset to -10.4V. This value is refered as elog:40m/8832.
3. Injected DC signal into input port using a function generator.
4. Measured voltage at the OUT port and MON port.

The result of this is attached #3 and #4. It is consistent with simulated one. All ports performed well.

• V(M1_PIT_OUT) = -4.86 *Vin +49.3 [V]
• V(M1_YAW_OUT) = -4.86 *Vin +49.2 [V]
• V(M2_PIT_OUT) = -4.85 *Vin +49.4 [V]
• V(M2_YAW_OUT) = -4.86 *Vin +49.1 [V]
• V(M1_PIT_MON) = -0.333 *Vin +3.40 [V]
• V(M1_YAW_MON) = -0.333 *Vin +3.40 [V]
• V(M2_PIT_MON) = -0.333 *Vin +3.40 [V]
• V(M2_YAW_MON) = -0.333 *Vin +3.40 [V]

The high voltage points (100V DC) remain to be tested.

[ Yuki, Gautam, Steve ]

Results:
I calibrated a QPD (D1600079, V1009) and made sure it performes well. The calibration constants are as follows:

X-Axis: 584 mV/mm
Y-Axis: 588 mV/mm

Details:
The calibration of QPD is needed to calibrate steeing PZT mirrors. It was measured by moving QPD on a translation stage. The QPD was connected to its amplifier (D1700110-v1) and +-18V was supplied from DC power supplier. The amplifier has three output ports; Pitch, Yaw, and Sum. I did the calibration as follows:

• Center beam spot on QPD using steering mirror, which was confirmed by monitored Pitch and Yaw signals that were around zero.  
• Kept Y-axis micrometer fixed, moved X-axis micrometer and measured the outputs. 
• Repeated the procedure for the Y-axis. 

The results are attached. The main signal was fitted with error function and I drawed a slope at zero crossing point, which is calibration factor. I determined the linear range of the QPD to be when the output was in range -50V to 50V, then corresponding displacement range is about 0.2 mm width. Using this result, the PZT mirrors will be calibrated in linear range of the QPD tomorrow. 

Comments:

• Some X-Y coupling existed. When one axis micrometer was moved, a little signal of the other direction was also generated.
• As Gautam proposed in the previous study, there is some hysteresis. That process would bring some errors to this result.
• A scale of micrometer is expressed in INCH!
• The micrometer I used was made to have 1/2 inch range, but it didn't work well and the range of X-axis was much narrower. 

Reference:
previous experiment by Gautam for X-arm: elog:40m/8873, elog:40m/8884

I assume this QPD set is a D1600079/D1600273 combo.

How much was the SUM output during the measurement? Also how much were the beam radii of this beam (from the error func fittings)?
Then the calibration [V/m] is going to be the linear/inv-linear function of the incident power and the beam radus.

You mean the linear range is +/-50mV (for a given beam), I guess.

• The SUM output was from -174 to -127 mV.
• The beam radii calculated from the error func fittings was 0.47 mm.
• Total optical path length measured by a ruler= 36 cm.
• Beam power measured at QPD was 2.96 mW. (There are some loss mechanism in the setup.)

Then the calibration factor of the QPD is

X axis: 584 * (POWER / 2.96mW) * (0.472mm /  RADIUS) [mV/mm]
Y axis: 588 * (POWER / 2.96mW) * (0.472mm /  RADIUS) [mV/mm].

We need to set up a copy of the c1asx model (which currently runs on c1iscex), to be named c1asy, on c1iscey for the green steering PZTs. The plan discussed at the meeting last Wednesday was to rename the existing model c1tst into c1asy, and recompile it with the relevant parts copied over from c1asx. However, I suspect this will create some problems related to the "dcuid" field in the CDS params block (I ran into this issue when I tried to use the dcuid for an old model which no longer exists, called c1imc, for the c1omc model).

From what I can gather, we should be able to circumvent this problem by deleting the .par file corresponding to the c1tst model living at /opt/rtcds/caltech/c1/target/gds/param/, and rename the model to c1asy, and recompile it. But I thought I should post this here checking if anyone knows of other potential conflicts that will need to be managed before I start poking around and breaking things. Alternatively, there are plenty of cores available on c1iscey, so we could just set up a fresh c1asy model...

[ Yuki, Gautam ]

I calibrated PZT mirrors. The ROUGH result was attached. (Note that some errors and trivial couplings coming from inclination of QPD were not considered here. This should be revised and posted again.) 

The PZT mirrors I calibrated were:

• A 2-inch CVI mirror (45 degree, HR and AR for 532nm)
• A 1-inch Laseroptik mirror (45 degree, HR and AR for 532nm)

I did the calibration as follows:

• +-15V was supplied to PZT driver circuit, +100V to PZT driver bias, and +-18V to QPD amplifier.
• Optical path length was set to be same as that when I calibrated QPD, which is 36cm.
• The full range of CVI mirror is 3.5mrad according to its datasheet and linear range of QPD is 0.2mm, so I set the distance between PZT mirrors and QPD to be about 6cm. (I realized it was wrong. When mirror tilts 1 deg, the angle of beam changes 2 deg. So the distance should be the half.)
• After applying 0V to PZT driver input (at that time 50V was applied to PZT mirror), then centered beam spot on QPD using steering mirror, which was confirmed by monitored Pitch and Yaw signals of QPD that were around zero.  
• In order to avoid hysteresis effect, I stared with an input signal of -10V. I then increased the input voltage in steps of 1V through the full range from -10V to +10V DC. The other input was kept 0V.
• Both the X and Y coordinates were noted in the plot in order to investigate pitch-yaw coupling.

The calibration factor was

CVI-pitch: 0.089 mrad/V

CVI-yaw: 0.096 mrad/V

Laseroptic-pitch: 0.062 mrad/V

Laseroptic-yaw: 0.070 mrad/V

Comments:

• I made sure that PZT mirrors move linearly in full input range (+-10V).
• PZT CH1 input: Yaw, CH2: Pitch, CH3: +100V bias
• The calibration factor of PZT mirrors [mrad/V] are not consistent with previous calibration (elog:40m/8967). I will check it again.
• I measured the beam power in order to calibrate QPD responce with a powermeter, but it didn't have high precision. So I used SUM output of QPD to the calibration.
• Full range of PZT mirrors looks 2 times smaller.

Reference:
Previous calibration of the same mirrors, elog:40/8967

Interim Procedure Report:

Purpose

The current setup of AUX Y-arm Green locking has to be improved because:

• current efficiency of mode matching is about 50%
• current setup doesn't separate the degrees of freedom of TEM01 with PZT mirrors (the difference of gouy phase between PZT mirrors should be around 90 deg) 
• we want to remotely control PZT mirrors for alignment

What to do

• Design the new setup and order optices needed (finished!)
    - As the new setup I designed, adding a new lens and slightly changing the position of optics are only needed. The new lens was arrived here.
• Check electronics (PZT, PZT driver, high voltage, cable, anti-imaging board) (finished!)
    - All electronics were made sure performing well.
    - The left thing to do is making a cable. (Today's tasks)
• Calibrate PZT mirror [mrad/V] (finished!)
    - The result was posted here --> elog:40m/14224.
• Measure the status value of the current setup (power of transmitted light ...etc) (Tomorrow, --> finished!)
• Install them in the Y-end table and align the beam (Will start from Tomorrow) (The setup has a probrem I found on 10/04)
• Measure the status value of the new setup
    - I want to finish above during my stay.
• Prepare the code of making alignment automaticaly

[ Yuki, Gautam ]

I improved Anti-Imaging board (D000186-Rev.D), which will be put between DAC port and PZT driver board.

It had notches at f = 16.6 kHz and 32.7 kHz, you can see them in the plot attached. So I replaced some resistors as follows:

• R6 and R7 replaced with 511 ohm (1206 thin film resistor)
• R8 replaced with 255 ohm (1206 thin film resistor)
• R14 and R15 replaced with 549 ohm (1206 thin film resistor)
• R16 replaced with 274 ohm (1206 thin film resistor)

Then the notch moved to 65.9 kHz (> sampling frequency of DAC = 64 kHz, good!). 
(The plot enlarged around the notch frequency and the plot of all channels will be posted later.)

All electronics and optics seem to be ready. 

Reference, elog:40m/8857
Diagram, D000186-D.pdf

[ Yuki, Gautam ]

I made a cable which connects DAC port (40 pins) and AI board (25 pins). I will check if it works.

Tomorrow I will change setup for improvement of AUX Y-end green locking. Any optics for IR will not be moved in my design, so this work doesn't affect Y-arm locking with main beam. 
While doing this work, I will do:

• check if the cable works
• make another cable which connects AI board (10 pins) and PZT driver (10 pins).
• check if eurocate in Y-rack (IY4?) applies +/-5V, +/-15V and +/-24V. It will be done using an expansion card.
• improve alignment servo for X-end.
• setup alignment servo for Y-end.
• about optical loss measurement.  

Before changing setup at Y-end table, I measured the status value of the former setup as follows. These values will be compared to those of upgraded setup.

• beam power going into doubling crystal (red12): 20.9 mW with filter, 1064nm
• beam power going out from doubling crystal (red12): 26.7 mW with filter, 532nm
• beam power going into faraday isolator (green5): 0.58 mW without filter, 532nm
• beam power going out from faraday isolator (green5): 0.54 mW without filter, 532nm
• beam power going to ETMY: 0.37 mW without filter, 532nm
• beam power of transmitted green light of Y-arm, which was measured by C1:ALS-TRY_OUT: 0.5 (see attachment #1)

(These numbers are shown in the attachment #2.)

The setup I designed is here. It can bring 100% mode-matching and good separation of degrees of TEM01, however I found a probrem. The picture of setup is attached #3. You can see the reflection angle at Y7 and Y8 is not appropriate. I will consider the schematic again.

???

The SHG crystal has the conversion efficiency of ~2%W (i.e. if you have 1W input @1064, you get 2% conversion efficiency ->20mW@532nm)

It is not possible to produce 0.58mW@532nm from 20.9mW@1064nm because this is already 2.8% efficiency.

I measured it with the wrong setting of a powermeter. The correct ones are here:

• beam power going into doubling crystal (red12): 240 mW, 1064nm
• beam power transmitted dichroic mirror (Y5): 0.70 mW, 532nm
• beam power going into faraday isolator (green5): 0.58 mW, 532nm
• beam power going out from faraday isolator (green5): 0.54 mW, 532nm
• beam power going to ETMY: 0.37 mW, 532nm
• beam power of transmitted green light of Y-arm, which was measured by C1:ALS-TRY_OUT: 0.5 (see attachment #1)

The calculated conversion efficiency of SHG crystal is 1.2%W.

What about just copying the Xend layout? I think it has good MM (per calculations), reasonable (in)sensitivity to component positions, good Gouy phase separation, and I think it is good to have the same layout at both ends. Since the green waist has the same size and location in the doubling crystal, it should be possible to adapt the X end solution to the Yend table pretty easily I think.

I designed a new layout. It has good mode-matching efficiency, reasonable sensitivity to component positions, good Gouy phase separation. I'm setting optics in the Y-end table. The layout will be optimized again after finishing (rough) installation.  (The picture will be posted later)

After installation I measured these power again.

• beam power going into doubling crystal: 241 mW, 1064nm
• beam power transmitted dichroic mirror: 0.70 mW, 532nm
• beam power going into faraday isolator: 0.56 mW, 532nm
• beam power going out from faraday isolator: 0.53 mW, 532nm
• beam power going to ETMY: 0.36 mW, 532nm

There is a little power loss. That may be because of adding one lens in the beam path. I think it is allowable margin.

While pointing Yuki to the c1asx servo system, I noticed that the filter file for c1asx is missing in the usual chans directory. Why? Backups for it exist in the filter_archive subdirectory. But there is no current file. Clearly this doesn't seems to affect the realtime code execution as the ASX model seems to run just fine. I copied the latest backup version from the archive area into the chans directory for now.

Setting up c1asy:

• Backed up old c1tst.mdl as c1tst_old_bak.mdl in /opt/rtcds/userapps/release/cds/c1/models
• Copied the c1tst model to /opt/rtcds/userapps/release/isc/c1/models/c1asy.mdl as this is where the c1asx.mdl file resides.
• Backed up original c1rfm.mdl as c1rfm_old.mdl in /opt/rtcds/userapps/release/cds/c1/models (since the old c1tst had an RFM block which is unnecessary).
• Deleted offending RFM block from c1rfm.mdl.
• Recompiled and re-installed c1rfm.mdl. Model has not yet been restarted, as I'd like suspension watchdogs to be shutdown, but c1susaux EPICS channels are presently not responsive.
• Removed c1tst model (C-node91) from /opt/rtcds/caltech/c1/target/gds/param/testpoints.
• Removed /opt/rtcds/caltech/c1/target/gds/param/tpchn_c1tst.par (at this point, DCUID 91 is free for use by c1asy).
• Moved c1tst line in /opt/rtcds/caltech/c1/target/daqd/master to "old model definitions models" section.
• Added /opt/rtcds/caltech/c1/target/gds/param/tpchn_c1asy.par to the master file.
• Edited/diskless/root.jessie/etc/rtsystab to allow c1asy to be run on c1iscey.
• Finally, I followed the instructions here to get the channels into frames and make all the indicators green.

Now Yuki can work on copying the simulink model (copy c1asx structure) and implementing the autoalignment servo.

Final Procedure Report for Green Locking in YARM:

Purpose

The current setup of AUX Y-arm Green locking has to be improved because:

• current efficiency of mode matching is about 50%
• current setup doesn't separate the degrees of freedom of TEM01 with PZT mirrors (the difference of gouy phase between PZT mirrors should be around 90 deg) 
• we want to remotely control PZT mirrors for alignment

What to do

• Design the new setup and order optices needed (finished!)
    - As the new setup I designed, adding a new lens and slightly changing the position of optics are only needed. The new lens was arrived here.
• Check electronics (PZT, PZT driver, high voltage, cable, anti-imaging board) (finished!)
    - All electronics were made sure performing well.
    - The left thing to do is making a cable. (Today's tasks)
• Calibrate PZT mirror [mrad/V] (finished!)
    - The result was posted here --> elog:40m/14224.
• Measure the status value of the current setup (power of transmitted light ...etc) (finished!)
• Install them in the Y-end table and align the beam (Almost finished!) (GTRY signal is 0.3 which means Mode-Matching efficiency is about 30%. It should be improved.)
• Measure the status value of the new setup (finished!)
• Prepare the code of making alignment automaticaly
  • see sitemap.adl>ASC>c1asy. I prepared medm. If you move PZT SLIDERS then you can see the green beam also moves.
  • Preparing filters is needed. You can copy them from C1ASX.
  • Note that now you cannot use C1ASX servo because filters are not applied.

To do for Green Locking in YARM:

The auto-alignment servo should be completed. This servo requires many parameters to be optimized: demodulation frequency, demodulation phase, servo gain (for each M1/2 PIT/YAW), and matrix elements which can remove PIT-YAW coupling. 

Summary:

To practise the dither alignment servo tuning, I decided to make the ASX system work again (mainly because it has fewer DoFs and so I thought it'd be easier to manage). Setup is: dither PZT mirrors on EX table-->demodulate green transmission at the dither frequencies-->Servo the error signals to 0 by an integrator.

Details:

1. Started by checking the dither lines are showing up with good SNR in GTRX. They are, see Attachment #1. The dither lines are at 18.23 Hz, 27.13 Hz, 53.49 Hz and 41.68 Hz, and all of them show up with SNR ~100.
2. Hand-aligned the beam till I got a maximum of GTRX ~ 0.35. This is lower than the usual ~0.5 I am used to - possibilities are (i) in the process of plugging in the BNC cable to the rear of the EX laser for my PLL investigations, I disturbed the alignment into the SHG crystal ever so slightly and I now have less green light going into the cavity or (ii) there is an iris on the EX table just before the green beam goes into the vacuum on which it is getting clipped. IIRC, I had centered the GTRX camera view such that the spot was well centered in the field of view, but now I see substantial mis-centering in pitch. So the cavity alignment for IR could also be sub-optimal (although I saw TRX ~1.15). Anyways, I decided to push on.
3. Introduced a deliberate offset in a given DoF, e.g. M1 PIT. Then I looked at the demodulated error signals (filtered through an RLP0.5 filter post demodulation, so the 2f component should be attenuated by 100 dB at least), and tuned the demod phase until most of the signal appeared in the I-phase, which is what is used for servoing. The Q-phase signals were ~x10 lower than their I-phase counterparts after the tuning.
4. Checked the linearity of the error signal in response to misalignment of a given DoF. I judged it to be sufficiently linear for all four DoFs about the quadratic part of the GTRX variation.
5. Tweaked the overall servo gains to have the error signals be driven to 0 in ~10 seconds.
6. There was quite significant cross-coupling between the DoFs - why should this be? I can understand the PIT->YAW coupling because of imperfect mounting of the PZT mounted mirror in a rotational sense, but I don't really understand the M1->M2 coupling.
7. Nevertheless, the servo appears to work - see Attachment #2.

The adjusted demod phases, servo gains were saved to the .snap file which gets called when we run the "DITHER ON" script. Also updated the StripTool template.

I plan to repeat similar characterization on the IR dither alignment servos. I think the tuning of the ASS settings can be done independently of figuring out the mystery of why the TRY level is so low.

I tried implementing a basic PRMI ASC using the POP QPD as a sensor. The POP22 buildup RMS is reduced by a factor of a few. This is just a first attempt, I think the loop shape can be made much better, but the stability of the lock is already pretty impressive. For some past work, see here.

I made a change to the c1ass model to normalize the PIT and YAW POP QPD outputs by the SUM channel. A saturation block is used to prevent divide-by-zero errors, I set the saturation limits to [1,1e5], since the SUM channel is being recorded as counts right now. Model change is shown in the attached screenshots. I compiled and installed the model. Ran the reboot script to reboot all the vertex FEs to avoid the issue of crashing c1lsc.

Attachment #1 - comparison of the POP QPD PIT and YAW output signal spectra with and without them being normalized by the SUM channel. I guess the shape is different between 30-100 Hz because we have subtracted out the correlated singal due to RIN?

This did not have the effect I desired - I was hoping that by normalizing the signals, I wouldn't need to change the gain of the ASC servo as the buildup in the PRC changed, but I found that the settings that worked well for PRMI locked with the carrier resonant (no arm cavities, see Attachment #2, buildup RIN reduced by a factor of ~4) did not work for the PRMI locked with the sideband resonant. Moreover, Koji raised the point that there will be some point in the transition from arms off resonance to on resonance where the dominant field in the PRC will change from being the circulating PRC carrier to the leaking arm carrier. So the response of the actuator (PRM) to correct for the misalignment may change sign. 

In conclusion, we decided that the best approach to improve the angular stability of the PRC will be to revive the PRC angualr feedforward, which in turn requires the characterization and repair of the apparently faulty vertex seismometer.

Summary:

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.

Details:

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:

• Data was downloaded and downsampled to 64 Hz (from 2kHz for the POP QPD signals and from 128 Hz for the seismometer signals). The 'FIR' option of scipy decimate was used.
• FFT time used was 16 seconds for the multi-coherence calculations

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?

This afternoon, I kept the PRM locked for ~1hour and then measured transfer functions from the PRM angular actuators to the POP QPD spot motion for pitch and yaw between ~1pm and 4pm. After this work, the PRM was misaligned again. I will now work on the feedforward filter design.

Summary:
Using the data I collected yesterday, the POP angular FF filters have been trained. The offline time-domain performance looks (unbelievably) good, online performance will be verified at the next available opportunity(see update).

Details:

The sequence of steps followed is the same as that done for the MCL FF filters. The trace that is missing from Attachment #1 is the measured online subtraction. Some rough notes:

• The "target" channels for the subtraction are the POP QPD PIT/YAW signals, normalized by the QPD sum. For the time that the PRMI was locked yesterday, the QPD readouts suggested that the beam was well centered on the QPD, but the POP QPD (OT-301) doesn't give me access to individual quadrant signals so I couldn't actually verify this.
• I used 64s impulse time on the FIR filter for training. Maybe this is too long, but anyways, the calculation only takes a few seconds even with 64^2 taps.
• I found that the Levinson matrix algorithm sometimes failed for this particular dataset. I didn't bother looking too much into why this is happening, the brute force matrix inversion took ~4 times longer but still was only ~5 seconds to calculate the optimal filter for 20 mins of training data sampled at 64 Hz.
• The actuator TF was measured with >0.9 coherence between 0.3 Hz - 10 Hz and fitted, and the fit was used for subsequent analysis. Fit is shown in Attachment #2.
• FIR to IIR fitting took considerable tweaking, but I think I got good enough fits, see Attachments #3, #4. In fact, there may be some benifit to making the shape smoother outside the subtraction band but I couldn't get IIRrational to cooperate. Need to confirm that this isn't re-injecting noise.

Update Apr 5 1145pm:

• Attachment #1 has now been updated to show the online performance. The comparison between the "test" and "validation" datasets aren't really apple-to-apple because they were collected at different times, but I think there's enough evidence here to say that the feedforward is helping.
• Attachment #5 shows that the POP DC (= PRC intracavity buildup) RMS has been stabilized by more than x2. This signal wasn't part of the training process, and I guess it's good that the intracavity power is more stable with the feedforward on. Median averaging was used for the spectral densities, there were still some abrupt glitches during the time this dataset was collected.
• _{The next step is to do the PRFPMI locking with all of these recently retuned feedforward loops engaged and see if that helps things}.

that's pretty great performance. maybe you can also upload some code so that we can do it later too - or maybe in the 40m GIT

I wonder how much noise is getting injected into PRC length at 10-100 Hz due to this. Any change the PRC ERR?

I don't have a recent measurement of the optical gain of this config so I can't undo the loop, but in-loop performance doesn't suggest any excess in the 10-100 Hz band. Interestingly, there is considerable improvement below 10 Hz. Maybe some of this is reduced A2L noise because of the better angular stability, but there is also improvement at frequencies where the FF isn't doing anything, so could be some bilinear coupling. The two datasets were collected at approximately the same time in the evening, ~5pm, but on two different days.

Summary:

I've been thinking about the IMC WFS. I want to repeat the sort of analysis done at LLO where a Finesse model was built and some inferences could be made about, for example, the Gouy phase separation b/w the sensors by comparing the Finesse sensing matrix to a measured sensing matrix. Taking the currently implemented output matrix as a "measurement" (since the IMC WFS stabilize the IMC transmission), I don't get any agreement between it and my Finesse model. Could be that the model needs tweaking, but there are several known issues with the WFS themselves (e.g. imbalanced segment gains).

Building the finesse model:

• I pulled the WFS telescopes from Andres elogs/SURF report, which I think was the last time the WFS telescopes were modified.
• The in-vacuum propagation distances were estimated from CAD diagrams.
• According to my model, the Gouy phase separation between the two WFS heads is ~70 degrees, whereas Andres' a la mode simulations suggest more like 90 degrees. Presumably, some lengths/lenses are different between what I assume and what he used, but I continue the analysis anyway...
• The appropriate power attenuations were placed in each path - one thing I noticed is that the BS that splits light between WFS1 and WFS2 is a 30/70 BS and not a 50/50, I don't see any reason why this should be (presumably it was to do with component availability). see below for Rana's comments.

Simulations:

• The way the WFS servos are set up currently, the input matrix is diagonal while the output matrix encodes the sensing information.
• In finesse, I measured the input matrix (i.e. response sensed in each sensor when an optic is dithered in angle). The length is kept resonant for the carrier (but not using a locking signal), which should be valid for small angular disturbances, which is the regime in which the error signals will be linear anyways.
• Then I inverted the simulated sensing matrix so as to be able to compare with the CDS output matrix. Note that there is a relative gain scaling of 100 between the WFS paths and the MC2T QPD paths which I added to the simulation. I also normalized the columns of the matrix by the largest element in the column, in an attempt to account for the various other gains that are between the optical sensing and the digitizaiton (e.g. WFS demod boards, QPD transimpedance etc etc).
• Attachment #1 shows the comparison between simulation and measurement. The two aren't even qualitatively similar, needs more thought...

Some notes about the WFS heads:

• The transimpedance resistor is 1.5 kohms. With the gain stages, the transimpedance gain is nominally 37.5 kohms, and 3.75 kohms when the attenuation setting is engaged (as it is for 2/4 quadrants on each head).
• Assuming a modulation depth of 0.1, the Johnson noise of the transimpedance resistor dominates (with the MAX4106 current noise a close second), and these heads cannot be shot noise limited when operating at 1 W input power (though of course the situation will change if we have 25 W input).
• The heads are mounted at a ~45 deg angle, mixing PIT/YAW, but I assume we can just use the input matrix to rotate back to the natural PIT/YAW basis.

Update 215 pm 5/6: adding in some comments from Rana raised during the meeting:

1. The transimpedance is actually done by the RLC network (L6 and C38 for CH 3), and not 1.5 kohms. It just coincidentally happens that the reactance is ~1.5 kohms at 29.5 MHz. Note that my LTspice simulation using ideal inductors and capacitors still predicts ~4pA/rtHz noise at 29.5 MHz, so the conclusion about shot noise remains valid I think... One option is to change the attenuation in this path and send more light onto the WFS heads.
   The transimpedance gain and noise are now in Attachment #2. I just tweaked the L values to get a peak at 29.5 MHz and a notch at twice that frequency. For this I assumed a photodiode capacitance of 225pF and the shown transimpedance gain has the voltage gain of the MAX4106 stages divided out. The current noise is input referred.
2. The imbalanced power on WFS heads may have some motivation - it may be that the W/rad TF for one of the two modes we are trying to sense (beam plane tilt vs beam plane translation) is not equal, so we want more light on the head with weaker response.
3. The 45 degree mounting of the heads is actually meant to decouple PIT and YAW.

This is the doc from Keita Kawabe on why the WFS heads should be rotated.

OK so the QPD segments are in the "+" orientation when the 40m IMC WFS heads are mounted at 45 deg. I thought "+" was the natural PIT/YAW basis but I guess in the the LIGO parlance, the "X" orientation was considered more natural.

Summary:

I implemented an ASC servo for the PRC, with the POP QPD as a sensor, and the PRM as the actuator. This has improved the stability of the lock (longer locks are possible), and also reduced the RIN of the arm transmission.

Details:

Attachment #1 shows the in-loop error signal suppression, and some out-of-loop monitors (POP22 and POPDC).

• To practise and get some workable servo settings, I locked the PRMI with carrier resonant (no ETMs).
• Then, I compare the beam motion witnessed by the POP QPD with and without the feedback loop enabled.
• I also look at the spectra of the POPDC and POP22 signals, as out-of-loop proxies, to get an estimate of how much noise is being injected out of band.
• In this toy study, both the in-loop and out of loop monitors show good performance.
• However, when repeating the same diagnostics with the PRFPMI locked, I note that while the in-loop suppression looks good, POPDC and POP22 report elevated noise, relative to the PRMI carrier case.
• I don't have a comparison to the PRFPMI locked with the feedback disabled, because of stability reasons. Plus, for the PRMI, the angular feedforward loops were engaged, but for the PRFPMI traces, they were disabled.
• Nevertheless, the arm RIN goes down by ~2.5 in RMS, so this is doing something good.

Attachment #2 compares the arm transmission RIN with the PRFPMI locked, with and without PRC ASC. The 3 Hz bump is definitely squished, but I think we can do better yet. 

Attachments #3-5 are in the style of elog15361. No Oplev signals yet, I'll add them soon.

Summary:

In ELOG 15368, I had claimed that the POP QPD based feedback servo actuating on the PRM stabilized the lock. I now believe this scheme of sensing using the POP QPD and feeding back to the PRM is not a good topology for stabilizing the PRC angular motion.

Details:

• I was never able to get a measurement of the OLTF of this loop that made sense 
  • the loop was initally commissioned with the PRMI locked on the carrier, and the settings hence inferred to give a ~5 Hz UGF loop were used in the PRFPMI lock.
  • In the PRFPMI configuration, however, the loop gain seemed way too low when I measured using the usual IN1/IN2 method.
  • So it is critical for the lock stability that the angular feedforward works well, which it kind of does now (not that I have changed anything, but the glitches in the seismometer have not resurfaced recently).
  • Hopefully, this becomes less of an issue once we replace the TTs with SOS and OSEM based damping.
• To get some more insight, I did some finesse modeling
  • Attachment #1 shows the sensing response at the QPDs we have available currently (POP and TR). 
  • I included the telescopes (propagation distances, in-air lenses) to these QPDs as best as I could.
  • A simplified model (3 mirror coupled cavity) is used, so there isn't really a common/differential mode in this picture, but we still get some insight I think.
  • Specifically, once the full lock is realized, the PRC optic motion isn't sensed well with our QPDs, and so it was some fluke that turning on these PRC angular feedback loops worked. 
  • Attachment #2 shows the same info as Attachment #1, but with the pendulum transfer functions (and radiation pressure effects) included. The SOS suspensions are modelled as f0=0.7/0.8 Hz (for P/Y), Q=5, while the tip-tilts have f0~5 Hz, Q~10. The high frequency phase is 0 degrees and not 180 as expected because of the pendulum complex pole pair because of the way the quantity is computed in Finesse.
• The current scheme I use is:
  • DC couple the ITM oplevs, using their individual Oplev QPDs.
  • Use the TR QPDs, mixed to actuate on the ETMs in a common/differential way.
  • I think the system is under-determined with the sensors we currently have - we wan't to sense the 10 angular modes - PIT and YAW for the PRC, Csoft, Chard, Dsoft and Dhard (using the terminology from Kate's thesis), but we only have 6 sensors of the same field (POP, TRX and TRY QPDs, PIT and YAW from each).
  • So we need more sensors?
• One thing that can easily be improved I think is to make the ASS system work at high power. 
  • I think this should be as simple as scaling the gain for the loops to work for the high power.
  • Then we can counteract the input pointing drift at least.
  • But the ITM Oplev DC coupling would need to be turned OFF and then ON again, I'm not sure if this will introduce some transient that will destroy the lock...

I would also like to bring up the topic of implementing some WFS for the interferometer fields again, there doesn't seem to be any mention of this in the procurement/planning for the BHD. It is not obvious to me yet that we need WFS and not just DC QPDs from a noise point of view, but at least we should discuss this.

Summary:

I want to be able to run the dither alignment servo with the PRFPMI locked - I've been thinking about what the scheme should be, and I list here some questions I had while thinking about this.

Details:

1. ITM Oplev DC coupling
   • In the current scheme, I DC couple the ITM Oplev servos after the arms have been aligned to maximize POX/POY transmission.
   • However, looking back at data from when the CARM offset is reduced (e.g. Attachment #1), it looks like the ITMs are being torqued quite a bit during this process (ITMX PIT changes by ~20urad, ITMY YAW by ~10urad in this particular lock attempt). 
   • So the spots are not actually being centered on the test-masses? I guess the spot position on ITMX isn't actually controlled because we have only one actuator (BS) for the XARM beam axis. Is it unexpected that ITMY gets torqued so much? 
   • It is unclear what would happen if the ITM Oplev servos are not DC coupled. I wonder if I'd still be able to reach the high circulating powers and then rely solely on the TR QPDs for the arm cavity angular control.
   • Another possibility is to offload the DC part of the control signal to the optic's slow bias voltage slider, and then turn off the DC coupling. After that, the dither alignment can optimize the cavity alignment.
2. Dither alignment at high circulating power
   • I think that the system should work with the same settings as for the POX/POY locking, with the following changes:
     • Scale the overall loop gain by the arm transmission.
     • Change LSC2ASS matrix element from XARM/YARM ---> DARM.
       Does this sound right?
   • In light of the above, I was thinking that we should introduce a gain scaling field in the c1ass RTCDS model (like we have for the LSC power normalization matrix). Is it worth to go through the somewhat invasive process of model recompilation etc?
   • With the PRFPMI locked, I am wondering if I can simultaneously run the dither alignment loops for all the DoFs. Probably not, especially for MICH, since the actuator is the BS, which is also the actuator for the XARM loop?

Where do we want to install the interface and readout electronics for the AS port WFS? Options are:

• 1Y1 / 1Y3  (i.e. adjacent to the LSC rack) - advantage is that 55 MHz RF signal is readily available for demodulation. But space is limited (1Y2, where the RF signal is, is too full so at the very least, we'd have to run a short cable to an adjacent rack), and we'd have a whole bunch of IPC channels between c1lsc and c1ioo models.
• 1X1/1X2. There's much more space and we can directly digitize into the c1ioo model, but we'd have to route the 55 MHz signal back to this rack (kind of lame since the signal generation is happening here). I'm leaning towards this option though - thinking we can just open up the freq generation box and take a pickoff of the 55 MHz signal...

There isn't much difference in terms of cable length that will be required - I believe the AS WFS is going to go on the AP table even in the new optical layout and not on the ITMY in-air oplev table? 

The project requires a large number of new electronics modules. Here is a short update and some questions I had:

1. WFS head and housing. Need to finalize the RF transimpedance gain (i.e. the LC resonant part), and also decide which notches we want to stuff. Rich's advise was to not stuff any more than is absolutely necessary, so perhaps we can have at first just the 2f notch and add others as we deem necessary once we look at the spectrum with the interferometer locked. Need to also figure out a neat connector solution to get the signals from the SMP connectors on the circuit board to the housing - I'm thinking of using Front-Panel-Express to design a little patch board that we can use for this purpose, I'll post a more detailed note about the design once I have it.
2. WFS interface board + soft-start board (the latter provides a smooth ramp up of the PD bias voltage). These go in a chassis, the assembly is almost complete, just waiting on the soft-start board from JLCPCB. One question is how to power this board - Sorensens or linear? If we choose to install in 1X1/1X2, I guess Sorensen is the only option, unless we have a couple of linear power supplies lying around spare.
3. Demod board (quad chassis). Assembly is almost complete, need to install the 4 way RF splitter, some insulating shoulder washers. (to ensure the RF ground is isolated from the chassis), and better nuts for the D-sub connectors. A related question is how we want to supply the electrical LO signal for demodulation. The "nominal" level each demod board wants is 10 dBm. This signal will be sourced inside the chassis from a 4-way RF splitter (~7 dB insertion loss). So we'd need 17dBm going into the splitter. This is a little too high for a compact amplifier like the ZHL-500-HLN to drive (1dB compression point is 16 dBm), and the signal level available at the LSC rack is only ~2 dBm. So do we want a beefy amplifier outside the chassis amplifying the signal to this level? Or do we want to use the ZHL-500-HLN, and amplify the signal to, say 13 dBm, and drive each board with ~6 dBm LO? The Peregrine mixer on these boards (PE4140) are supposed to be pretty forgiving in terms of the LO level they want... In either case, I think we should avoid having an amplifier also inside the chassis, it is rather full in there with 4 demod boards, regulator board, all the cabling, and an RF splitter. It may be that heat dissipation becomes an issue if we stick an RF amplifier in there too...
4. Whitening chassis. Waiting for front panels to arrive, PCBs and interface board are in hand, stuffed and ready to go. A question here is how we want to control the whitening - it's going to be rather difficult to have fast switchable whitening. I think we can just fix the whitening state. Another option would be to control the whitening using Acromag BIO channels.
5. AI chassis - will go between whitening and ADC.
6. Large number of cables to interconnect all the above pieces. I've asked Chub to order the usual "Deluxe" shielded Dsub cables, and we will get some long SMA-SMA cables to transmit the RF signals from head to demod board from Pasternack (or similar), do we need to use Heliax or the Times Microwave alternative for this purpose? What about the LO signal? Do we want to use any special cable to route it from the LSC rack to the IOO rack, if we end up going that way? 

Approximately half of the assembly of the various electronics is now complete. The basic electrical testing of the interface chassis and demod chassis are also done (i.e. they get power, the LEDs light up, and are stable for a few minutes). Detailed noise and TF characterization will have to be done.

Attachment #1 - Proposed mods for 40m RF freqs. 

• I followed Rich's suggestion of choosing an inductor that has Z~100 ohms at the frequency of interest.
• The capacitor is then chosen to have the correct resonant frequency.
• Voltronix trim caps are used for fine tuning the resonances. 2 variants are used, one with a range of 4-20 pF, and a Q of 500 per spec, while the other has range of 8-40 pF, and a Q of 200 per spec.
• In the table, the first capacitance is the fixed one, and the second is the variable one. We're not close to the rail for the variable caps.
• For the first trials, I think we can try by not populating all of the notches - just the 2f notch. We can then add notches if deemed necessary. Probably these notches are more important for a REFL/POP port WFS.
• One thing I noticed is that the aLIGO WFS use ceramic capacitors for the LC reactances. i haven't checked if there is any penalty we are paying in terms of Q of the capacitor. anyways, i'm not going to redesign the PCB and maybe ceramic is the only option in the 0805 package size?

Attachment #2 - Modelled TFs for the case where all the notches are stuffed, and where only the 2f notch is stuffed.

• The model uses realistic composite models for the inductors from coilcraft, but the capacitors are idealized parts.
• I also found the library part for LMH6624, so this should be a bit closer to the actual circuit than Rich's models which subbed in the MAX4107 in place.
• The dashed vertical lines indicate some frequencies of interest.
• Approx 1 kohm transimpedance is realized at 55 MHz. I don't have the W/rad number for the sensitivity at the AS port, but my guess is this will be just fine.
• If the 44 MHz and 66 MHz notches are stuffed, then there is some interaction with the 55 MHz notch, which lowers the transimpedance gain somewhat. So if we decide to stuff those notches, we should do a mroe careful investigation into whether this is problematic.

Attachment #3 - Modelled TFs for the case where all the notches are stuffed, and where only the 2f notch is stuffed.

• Initially, I found the (modelled) noise level to be rather higher than expected. It persisted despite making the resistors in the model noiseless. Turns out there is some leakage from the "Test Input" path. Some documents in the DCC suggest that there should be an "RF Relay" that allows one to isolate this path, but afaik, the aLIGO WFS does not have this feature. So maybe what we should do is to remove C9 once we're done tuning the resonances. Better yet, just tune the resonance with the Jenne laser and not this current-injection path.
• Horizontal dashed lines indicate shot noise for the indicated DC photocurrent levels. It is unlikely we will have even 1 mW of light on a single quadrant at the AS port, so the AS port WFS will not be shot noise limited. But I think that's okay for initial trials.
• The noise level of ~20 pA/rtHz input referred is in agreement what I would expect using Eq 3 of the LMH6624 datasheet. The preamp has a gain of 10, so the source impedance seen by it is ~100 ohms (since the overall gain is 1kohm). The corresponding noise level per Eq 3 is ~2 nV/rtHz, or 20 pA/rtHz current noise referred to the photocurrent 👍 . 
• The LMH6624 datasheet claims that the OpAmp is stable for CLG >= 10. For reasons that aren't obvious to me, Koji states here that the CLG needs to be even higher, 15-20 for stability. Do the aLIGO WFS see some instability? Should I raise R14 to 900 ohms?

Any other red flags anyone sees before I finish stuffing the board?

Last week and this week I've been working on the characterization of the Q3000 QPDs. The QPDs were named 81, 82, 83, and 94.

• Dark current [OMC LAB ELOG 402]: All the segments looked similar and acceptable except for the seg1 of #82. It has a smaller reverse breakdown voltage (~6V) but even this is an acceptable level.
• Impedance [OMC LAB ELOG 403]: All the segments showed a ~300pF junction capacitance with no reverse bias. This looks quite normal.
• Dark noise [OMC LAB ELOG 404]: All the segments showed ~5pA/rtHz dark noise above 1Hz.

My recommendation is to use #81 and #84 as they have similar dark current characteristics between the segments. But basically, all the QPDs look fine.

The actual junction capacitance and the RF dark noise should be characterized by the actual WFS head circuit.

The QPD packages were labeled and returned to Gautam to be implemented in the WFS heads.

gautam: S/N #84 was installed as the AS WFS QPD. The remaining 3 are stored in the clean cabinet at EX (where the rest of the RF photodiodes are).

I am confused by the discussion during the call today. I revisited Hartmut's paper - the circuit in Fig 6 is essentially what I am calling "only 2f_2 notch stuffed" in my previous elog. Qualitatively, the plot I presented in Attachment #2 of the preceeding elog in this thread shows the expected behavior as in Fig 8 of the paper - the impedance seen by the photodiode is indeed lower. In Attachment #1, I show the comparison - the "V(anode)/I(I1)" curve is analogous to the "PD anode" curve in Hartmut's paper, and the "V(vout)/I(I1)" curve is analogous to the "1f-out" curve. I also plot the sensitivity analysis (Attachment #2), by varying the photodiode junction capacitance between 100pF and 200 pF (both values inclusive) in 20 pF steps. There is some variation at 55 MHz, but it is unlikely that the capacitance will change so much during normal operation? 

I understand the motivation behind stuffing the other notches, to reduce intermodulation effects. But the impression I got from the call was that somehow, the model I presented was wrong. Can someone help me identify the mistake?

I didn't bother to export the LTspice data and make a matplotlib plot for this quick analysis, so pardon the poor presentation. The colors run from green=100pF to grey=200pF.

An 8 channel whitening chassis was prepared and tested. I measured:

1. TF from input to output - there are 7 switchable stages (3 dB, 6 dB, 12 dB and 24 dB flat whitening gain, and 3 stages of 15:150 Hz z:p whitening). I enabled one at a time and measured the TF. 
2. Noise with input terminated.

In summary,

1. All the TFs look good (I will post the plots later), except that the 3rd stage of whitening on both boards don't show the expected transfer function. The fact that it's there on both boards makes me suspect that the switching isn't happening correctly (I'm using a little breakout board). I'm inclined to not debug this because it's unlikely we will ever use 3 stages of 15:150 whitening for the AS WFS. 
2. The noise measurement displayed huge (x1000 above the surrounding broadband noise floor) 60 Hz harmonics out to several kHz. My hypothesis is that this has to do with some bad grounding. I found that the circuit ground is shorted to the chassis via the shell of the 9pin and 15pin Dsub connectors (but the two D37 connector shields are isolated). This seems very wierd, idk what to make of this. Is this expected? Looking at the schematic, it would appear that the shields of the connectors are shorted to ground which seems like a bad idea. afaik, we are using the same connectors as on the chassis at the sites - is this a problem there too? Any thoughts? 

I don't think your simulation looked inaccurate (at least not to me). In my opinion, we just want to minimize any excess noise from intermodulation. Of course, its possible that stuffing too many notches will make it difficult to have the same low noise as a simple circuit, so that's worth considering.

Also, the intermodulation is mainly a problem when the other peaks are not suppressed by some feedback: e.g. POP55_I can have excess noise if POP55_Q or POP11_I are not controlled by some MICH/PRCL/SRCL loops.

For the WFS, perhaps this is not a significant issue, but I'm not sure. My suggestion is to stuff 11 & 55 for sure, and then the others depending on the amplitude of the peaks and the consequent intermodulation. IF it works with all stuffed, that seems good. If its tricky to get it to work with all stuffed, I'd back off on a couple of them...but it probably takes more careful thought to figure out which ones are least important.

I'm thinking of making some modifications to the RF distribution box in 1X2, so as to have an extra 55 MHz pickoff. Koji already proposed some improvements to the layout in 2015. I've marked up his "Possible Improvement" page of the document in Attachment #1, with my proposed modifications. I believe it will be possible to get 15-16 dBm of signal into a 4 way RF splitter in the quad demod chassis. With the insertion loss of the splitter, we can have 9-10 dBm of LO reaching each demod board, which will then be boosted to +20 dBm by the Teledyne on board. The PE4140 mixer claims to require only -7 dBm of LO signal. So we have quite a bit of headroom here - as long as we limit the RF signal to 0dBm (=0.5 Vpp from the LMH6431 opamp at 55 MHz, we shouldn't be having a much larger signal anyways), we should be just fine with 15 dBm of LO power (which is what we will have after the division into the I and Q paths, and nominal insertion losses in the transmission path). These numbers may be slight overestimates given the possible degradation of the RF amps over the last 10 years, but shouldn't be a show-stopper.

Do the RF electronics experts agree with my assessment? If so, I will start working on these mods tomorrow. Technically, the splitter can be added outside the box, but it may be neater if we package it inside the box.