I made a script (/users/aaron/40m/GPIB/tds_dump.py) that grabs data from a Tektronix scope and packages into a pickled dict with the following structure:

1. ch1
   1. times ("ts")
   2. values ("vals")
   3. channel info ("info")
2. ch2
   1. ""
3. etc

I made a python notebook that does the following:

1. Grab the data from the pickle above
2. Fit a triangle wave to the drive signal
3. Determine the (change in Volts) / second from the triangle wave, as well as define the times of a single sweep of the PDH error signal
4. Trim the error signal data to contain the PDH signal from the carrier and two sidebands only (the original trace was for three periods).
5. Fit the functional form of the PDH signal to the trimmed error signal. 
   1. The sideband frequency is fixed at 35.5 MHz, and the scaling of Volts-to-Hz is left free, so this fit gives the calibration of IF volts to Hz.
6. Grab the spectra (already saved from the Agilent with the netgpib scripts) and apply this V-to-Hz scaling
7. Plot the spectra

The fit in step (5) is still looking quite bad, despite the fitted values being close to the expected. Since we really just want a calibrated spectrum, I'll instead fit a line to the linear portion of the PDH error signal for the carrier and both sidebands, then determine the scaling from that.

Here are the results from the fit. Data can be found on nodus in /users/aaron/40m/data/PMC/190617/. I've put a jupyter notebook with the analysis in /users/aaron/40m/analysis/PDH_calibrate.ipynb (might be some filename issues due to different directory structure on my laptop).

Here's a summary of the current measurement. I'll be referencing the diagram for the PMC servo card.

1. With the PMC servo loop open, sweep the PMC PZT by sending a triangle wave in to J5 (external drive on the servo card).
   1. I used a 100Hz drive, but should use something slower so my drive isn't filtered out by the 100Hz pole on the servo card and the 10Hz pole on the PZT.
2. Monitor the voltage at the HV drive, as well as at "mixer out" (J8 on the servo board)
   1. Note that I took this PDH error signal from FP2TEST rather than "mixer out", which means my error signal was not low pass filtered.
3. Calibrate the HV mon in units of Hz by fitting the PDH signal. The sidebands should be 35.5Mhz away from the carrier peak.
   1. This part needs to be done differently to account for thermal locking in the PMC.
4. You now have the PDH error signal as a function of PMC resonance in Hz, and can use that to calibrate the PDH error spectrum.
   1. The spectrum is taken when the PMC is locked, so the Hz/V scaling is the slope of the PDH error signal.

In the figures below, I obesrved that for fast (100Hz) drives, the PDH error signal had a pi/2 phase shift relative to the triangle wave, which means even though the resonance appears near the turnaround of the triangle, it is actually occuring near the center of the range.

There are several problems with this data:

• PMC error signal spectrum is not properly calibrated, even according to the process described above
• The drive was faster than the response of the PZT.
• I was driving with a ~1V excitation, so I've lost a factor of 10 somewhere on the way to the external drive curve. Probably just a problem with how I've read the data dump from the scope.

The IMC REFL error signal was measured to compare it with the other spectra (if we have).

The blue curve is the in-loop IMC error and the red is the dark noise. So they are not an apple-to-apple comparison. But the red noise is going to be suppressed by the loop, and still the red is below blue. This means that the blue curve is the measured noise rather than the readout noise.

We suspect that the current issue is the PC drive saturation (as usual). Does this indicate that the laser freq noise is actually increased?

----

Another suspect was that the degradation of the LO level. We used to have the issue of slowly dying ERA-5 (ERA-5SM indeed). The RF levels on the demod board were measured using an active probe.

The LO input: 0dBm, ERA-5 input: -2.7dBm and -2.1dBm for I and Q. I found that the outputs of the ERA-5SM were +10.5dBm and +10.6dBm.
This lead me to replace the chips but the situation was not changed. Then I realized that the LO levels should have been measured with the load replaced from the mixers to a 50Ohm load. Somehow these mixers lower the apparent LO levels. So I decided to say this is OK.

aI went to repeat these measurements using the mixer out channel from the servo box, and with a slower sweep for the PDH calibration.

I had trouble getting the PDH signal, here are some notes:

• I added a 50 Ohm terminator to BNC T on the mixer box. This had been terminated before I started, but I noticed no terminator today.
• Noticed some distortion of my driving triangle wave if I measured it on ch3 and 4 of the tektronix scope, not present on ch 1/2
• Initially wasn't finding a signal because I was opening the loop by turning off the Test 1 switch, but this meant the mixer mon on the servo box also did not receive the PDH signal. Instead, I cut the loop with the "BLANK" switch on the PMC screen, which instead blanks out the op amp between the mixer mon and the PMC drive conditioning (so the external drive still reaches the PZT).

attachment 1 is the configuration of the PMC screen when I was trying to get some PDH signal; I did move the DC output adjust to 0V, but found that this led to the output being railed; this makes sense, the op amp at U9 has a negative bias at GND.

Rana came by and gave me some tips.

• I'd been using the wrong servo board diagram, it should be in D1400221
• We removed the LP filter from the mixer output (before going to FP1TEST on the servo board), since the board itself already is filtering the IF.
• We might have observed the thermal locking? See for yourself, the trans and refl signals while sweeping the PZT drive at 5 Hz and 30 Hz respectively are in attachments 2 and 3.
• Rather than using an SR560, I should use an RF coupler between the mixer and FP1TEST to measure the error signal spectrum. I found a ZFDC-20-5-S+ (0.1-2000 MHz) and sent an SMA cable from the coupled port to channel R of the Agilent 4395.

We finally got the PDH signal again, and I recorded the PDH signal while driving with the following settings on the Siglent function generator.

• 1.1 Hz triangle wave, 6Vpp, -7Vdc offset, high impedance mode

I tried getting a spectrum using the coupler, the mixer mon is seeing a DC offset though and causing the PZT to rail. Will try to understand why, but in the meantime removing the coupler (still no LP filter) lets us lock the PMC again.

RXA: Kruthi thinks all of our subsequent IMC locking problems are Aaron's fault (she was quick to give him up as soon as the thumb screws were tightened...)

The latest in my fling with the PMC. Though PMC trans is back to nominal levels (~0.713 V), we'd still like to understand the PMC noise.

Last time, I took some spectra with the RF probe (Agilent 41800A). I had already measured the PDH error signal by sweeping the PZT at ~1 Hz. The notebook I used for analysis has been updated in /users/aaron/analysis/PDH_calibrate.ipynb. The analysis was the following:

• fit the PDH error signal, assuming a 35.5MHz modulation frequency. Here are the (approximate) fit parameters:
  • Mapping of PZT mon voltage to Hz: 5.92 Hz/V_{PZT_mon}
  • P_carrier*P_signal: 0.193 W^2
  • HV mon voltage on resonance: 0.910 V
  • Error signal far off resonance: 0.249 V
  • Transmission: 0.00238
    • ​yikes. The nominal transmission is T=0.003. I let this parameter be free as a check, and to avoid overconstraining the data; is this consistent with measurements of the PMC optics' transmission?
  • Length: 0.0210 m
    • This is consistent with the nominal PMC length
• Using the fit of the full PDH error signal, I am able to plot error signal vs frequency, and fit the linear portion of the carrier PDH signal. The results of this fit are:
  • -9.75e-7 V_PDH per Hz
  • 0.105 V error signal at DC
• I then divide the power spectra by the squared slope of the linear fit above (V_PDH^2/Hz^2) to get the spectra in Hz^2/rtHz
  • I've plotted both the spectrum I took directly at the mixer I using the agilent probe, as well as the spectrum taken by sending the PMC servo card's mixer mon to an SR560 (G=2) then to the spectrum analyzer

There are a few problems remaining:

• There should be a gain of 100 between the mixer I and the servo board's mixer out. It's not clear to me that this is reflected in the spectra. Moreover, the header files on the spectra I grabbed from the Agilent say that the R (mixer I) channel has 20dB of input attenuation, which is also not reflected. If I have swapped the two spectra and not accounted for either the gain of the servo card or the attenuation of the spectrum analyzer, these two gains would cancel, but I'm not confident that's what's going on.

The IMC often was making that scratchy noise when first catching lock and sometimes breaking. Thinking of the crappy crossover sit that EQ showed in his latest plots, I decided that it didn't make sense to acquire lock with an unstable PZT/EOM crossover, so I have changed mcdown to acquire with +13 dB Fast Gain and its much fast now and no longer makes that sound.

I also changed the caput command from 'caput -l' to 'caput -c -l' to see if the async 'wait for callback' feature will insure that the commands get sent. I witnessed the mcdown not actually writing all of its commands once or twice tonight. With the MC Boost left on its never going to lock.

mcdown has been committed to SVN. Please, if you have recently edit mcup and Autolock, commit them to the SVN or else I will delete them and do an svn up.

This is probably just a confirmation of something we discussed a couple of weeks back, but I wanted to get more familiar with using the multi-coherence (using EricQs nice function from the pynoisesub package) as an indicator of how much feedforward noise cancellation can be achieved. In particular, in light of our newly improved WFS demod/whitening boards, I wanted to see if there was anything to be gained by adding the WFS to our current MCL feedforward topology.

I used a 1 hour data segment - the channels I looked at were the vertex seismometer (X,Y,Z) and the pitch and yaw signals of the two WFS, and the coherence of the uncorrelated part of these multiple witnesses with MCL. I tried a few combinations to see what is the theoretical best achievable subtraction:

1. Vertex seismometer X and Y channels - in the plot, this is "Seis only"
2. Seis + WFS 1 P & Y
3. Seis + WFS 2 P & Y
4. Seis + WFS 1 & 2 P
5. Seis + WFS 1 & 2 Y

The attached plot suggests that there is negligible benefit from adding the WFS in any combination to the MCL feedforward, at least from the point of view of theoretical achievable subtraction. 

I also wanted to put up a plot of the current FF filter performance, for which I collected 1 hour of data tonight with the FF on. While the feedforward does improve the MCL spectrum, I expected better performance judging by previous entries in the elog, which suggest that the FIR implementation almost saturates the achievable lower bound. The performance seems to have degraded particularly around 3Hz, despite the multi-coherence being near unity at these frequencies. Perhaps it is time to retrain the Weiner filter? I will also look into installation of the accelerometers on the MC2 chamber, which we have been wanting to do for a while now...

Based on the observation of the PMC error signal, I started measuring the IMC OLTF. Immediately, it was found that the overall IMC loop gain was too low.
The UGF was ~40kHz, which was really marginal. It had been >100kHz when I have adjusted it about a year ago. (Next entry for the detail)

The first obvious thing was that the SMA cables around the IMC servo have visible degradation (Attatched photos).
I jiggled the signal cable from the demodulator Q_out to the MC servo. The openloop gain seemed fluctuating (increased) based on the cabling.
I decided to repair these cables by adding solder on the shield.

Even after the repair, the open loop TF didn't show any improvement. I checked the LO level and found that it was -16.7dBm.

I traced the problem down to the frequency generator unit (T1000461). The front panel of the unit indicates the output power for the 29.5MHz output is 13dBm,
while measurement showed it was 6~8dBm (fluctuating). The T1000461 document describes that there is only a wenzel oscillator inside. Does this mean the oscillatorwas degraded??? We need to open the box.

I was not sure what was the LO level. I naively assumed the input is 0dBm. Reducing the attenuation of the dial on the AM Stabilizer unit from 12dB attn to 0dB.
This made the LO level -3.3dBm.

Later at home, I thought this nominal LO level of 0dBm could have been wrong.

The demodulator circuit (D990511) has the amplifier ERA-5 (G=~20dB) at the input. Between the input and the ERA-5 there is a pattern for an attenuator.
Assuming we have no attenuator, the ERA-5 has to spit out 20dBm. That is too much for this chip. I need to pull out the box to see how much is the nominal LO for this box using an active probe.

This decrease/increase of the LO level affects the WFS demod too. According to D980233-B, the input stage has the comparator chip AD96687, which can handle differential voltage of 5.5V.
Therefore the effect is minimal.

On the demod board there is a 10 dB attenuator (AT1), which lowers the level to -10 dBm before the ERA-5. Then it should be 10 dBm before going to the rest of the parts. But I guess the ERA-5 chips which come later on in the circuit could be decaying like the ones in the PMC LO board.

TO DO

Done (Nov 23, 2015) - Check if the attenuator is still there in the input chain

Done (Nov 23, 2015) - Check if the actual LO levels at the 17dBm mixers are reasonable.

- Check if the actual LO levels for the LSC demods are OK too

A tmux session named "MCFreeSwingTest" will run on Rossa. This session is running script scripts/SUS/freeSwingMC.py (also attached) which will trigger at 5:00 am to impart 30000 counts kick to MC1, MC2, and MC3 after shutting PSL shutter and disabling the MC autolocker. It will let them freely swing for 1050 sec and will repeat 15 times to allow some averaging. In the end, it will undo all the changes it does and switches on autolocker on IMC. The script is set to restore any changes in case it fails at any point or a Ctrl-C is detected.

[Paco, Anchal]

# Initial State
- MC is locked. The PRM monitor shows some oscillations.
- POP monitor shows light flashing once in a while.
- AS monitor shows one beam along with some other flashing beam around it.
- PRM Watchdog is tripped and shutdown. Everything else is normal except for overload on SRM OpLevs.
- Donatella got a mouse promotion

# Reenabling PRM watchdog:
- The custom reEnablePRMWatchdog.py has been deleted.
- Tried enabling the coil outputs manually and switching watchdog to Normal.
- Again saw large fluctuations like yesterday.
- Probably still the same issue of how current calculated actuations to the coils is in range -600 to -900 and gives and impulse to the optics when suddenly turned on.
- Waiting for PRM to damp down a little.
- Today we plan to change the position bias on PRM C1:SUS-PRM_POS_OFFSET instead of changing biases in pitch and yaw.
- Changing C1:SUS-PRM_POS_OFFSET from 0 to +/- 100 without enabling the coils, it seems upper and lower coils are anticorrelated with just changing the position. So going back to changing pitch.
- Changing C1:SUS-PRM_PIT_OFFSET from 0 -> 780. Switched on watchdog to normal.
- PRM damped down. OpLev errors are also within range.
- Enabled both OpLevs.

# Try locking Y-Arm
- IFO>CONFIGURE>YARM>Restore YARM (POY) using Donatella. See a bunch of python error messages in the call complaining about unable to find some python 2 files. Closed it with Ctrl-C after a stuck state.
- Tried running it on Pianosa, the script ran without error but Y-Arm didn't lock.

# Try locking X-Arm
- IFO>CONFIGURE>XARM>Restore XARM (POX) on Donatella. Again a bunch of OSError messages. Donatella is not configured properly to run scripts.
- Tried running it on Piasnosa, the script ran without error but X-Arm didn't lock.
- This might mean that both arms are misaligned or the BS/PRM is misaligned.
- Moving around C1:SUS-PRM_PIT_OFFSET and C1:SUS-PRM_YAW_OFFSET in order to see if the transmitted light is misalgined. Both arms are set to acquire lock if possible. No luck.

# Hypothesis: The Arm cavity is not aligned within itself (ITM-ETM)
- Will try to lock X-Arm with green light while tuning the ETMX. Hopefully the BS and ITM are aligned so that once we align ETMX to get a green lock, the IR will also lock from the other side.
- Running IFO>CONFIGURE>XARM>Restore XARM (ALS) on Pianosa. No lock, moving forward with tunning ETMX pitch and yaw offsets. Nothing changed. Brought back to same values.

[Rana joined, Anchal moved to Rossa from Pianosa]

# Moving on to IMC suspensions characterization:
- Closed the PSL shutter, to our suprise, the MC was still locked. We thought this would take away any light from IMC but it doesn't. Maybe the IFO Overview needs to show the schematic in a way where this doesn't happen: "No light from any laser entering the MC but it still is locked with a resonating field inside."
- Shutting IMCR shutter (hoping that would unlock the IMC), still nothing happend.
- Tried shutting PSL shutter from Rossa, nothing happened to MC lock still.
- Closed shutter IOO>Lock MC> Close PSL and this unlocked the IMC. Found out that this shutter channel is C1:PSL-PSL_ShutterRqst while the one from the sitemap>Shutter>PSL changes C1:AUX-PSL_ShutterRqst. Some clarification on these medm screens would be nice.
- Disabled the MC autolocked from IOO>Lock MC screen (C1:IOO-MC_LOCK_ENABLE).
- Checked the scripts/SUS/freeswing.py to understand how kick is delivered and optic is left to swing freely.
- Next, we are looking at the C1SUS_MC1 screen to understand what channels to read during data acquisition.
- In sensor matrix, we see INMON for each sensor which is probably raw counts data from the OSEMs. Rana mentioned that OSEM data comes out in units of microns. These are C1:SUS-MC1_ULSEN_OUTPUT (and so on for UR, LL, LR, SD).

- In prep for finishing, recovered Autolocker by first opening the PSL mechanical shutter, then re-enabling the Autolocker. The IMC lock didn't immediately recover, and we saw some fuzz on the PSL-FSS_FAST trace, so we closed the shutter again, waited a minute, then re-opened it and MC caught its lock.
 

Did you fix this issue? It is helpful to post a screenshot of the offending MEDM screen in addition to witticisms. The elog says "sitemap>Shutter>PSL" but I can't find PSL under the dropdown for shutters from Sitemap.

No we didn't fix the issue. We'll post some screenshots tomorrow. From "sitemap>Shutter>PSL" we meant in Shutter medm window, we clicked on the PSL close button. As pointed later, it switches C1:AUX-PSL_ShutterRqst while the PSL shutter switch on Lock MC medm screen switches C1:PSL-PSL_ShutterRqst. We were not sure if this was intentional, so we didn't change anything.

I lowered the (FAST) PZT gain on the IMC/FSS servo today.

I noticed that the MC locks looked unstable a lot of the day, and during lock the PCDRIVE channel is above 1 Vrms (which means the loop is oscillating, ttypically at the PZT/EOM crossover frequency).

I changed the default setting from 22 to 20 19 dB in the PSL Settings screen so the mcup script will use this for now. Feel free to revert if this turns out to be a Fluke (which you would think is a terrible name for a company, but...)

It has happened multiple times today that IMC has tripped on its own. Yehonathan and I have had to come back to manually lock IMC multiple times.

Wed Apr 26 10:24:07 2023 [EDIT]

[Paco] I aligned the MC by hand, let it run locked for 30 minutes without angular controls, and then switched on the WFS loops yesterday at ~ 6 PM. IMC has been locked ever since.

Attachment #1 - In loop error spectra, measured as Koji posted end of last week.

• Main difference is that the line noise seems much lower.
• For the "dark" measurement, I set the IN1 gain of the servo board to the value of +4 dB, which is what it is in lock.
• As Koji mentioned, this isn't an apple-to-apple comparison as the IMC loop will squish the plotted orange trace.
• Nevertheless, the fact that the blue trace is above orange everywhere gives confidence that we are in fact measuring frequency noise.
• For the higher frequency measurement, I used the AG4395 analyzer, which has 50 ohm input impedance. So to get the measurements with the SR785 to line up, I multiplied these by x2.
• For the frequency axis calibration, I used the value of 13 kHz/V for the PDH discriminant, which was what I measured it to be last year (but I didn't check again today).
• Note that the IMC locking loop OLTF has not been undone, so this isn't the actual laser frequency noise on the transmitted beam. In order to measure the latter, we'd have to use (for example) an arm cavity as an analyzer.

Attachment #2 - OLTF of the IMC loop.

• Measurement was made using the IN1/IN2 method, injection was done at the "A EXC" front panel BNC input.
• For comparison, I've overlaid a measurement from the 2017 IMC loop investigations. Doesn't seem to be significantly different.
• UGF and phase margin are in the ballpark of what they were reported to be in the past.

Attachment #3 - Photo courtesy Koji showing the bank of BNC connectors used for these measurements.

Clearly, these measurements were taken in a time when the IMC was "well behaved". How to characterize what's happening when this isn't the case?

[valera, gautam]

Last night, Valera and I looked into two aspects of the IMC:

1. How can we accurately set the offset at the error point of the PDH servo such that we lock to the true center of the resonance?
2. What's up with the large common mode offset on the WFS?

I will post a more detailed elog about last night's work, but Valera also thought it might be a good idea to try and improve the mode-matching into the IMC. I couldn't find anything on the wiki/elog about the mode matching situation on the PSL table, so I quickly went over yesterday to measure some lengths. From looking at the MCREFL DC levels when the mode cleaner is locked (~0.37V) and unlocked (~5.7V), the current mode matching efficiency seems to be about 88%, so there is definitely some headroom for improvement.

Here is my cartoon of the situation on the PSL table. All lengths are measured in mm, and I would say correct to +/- 5 mm, so there could be considerable error here...

  (L1 : f=+200mm. L2: f=-150mm. L3:  f=+400mm)

I extracted the lengths from the edge of the PSL table to IM1 and MC1 from (what I think are) the latest CAD drawings on the DCC. I then put all this into an a la mode script [Attachment #5] - I assumed a waist of 370um at the PMC output mirror, and a waist of 1.78mm at MC1. I neglected the passage through the in-vac Faraday, EOM and BS1 (on the sketch above) and the MC1 substrate. I was able to achieve a theoretical mode-matching efficiency of 1 by just moving the positions of L2 and L3. 

Given that there are probably errors of the order 0.5cm in the lengths on the PSL table, and also the in-vacuum distance to MC1, I figured it would be ideal to just move one lens and see if we can improve the efficiency. It looks like it may be more effective to move L2 than L3. The plot on the right shows that the sensitivity is approximately equal to the positioning of L2 and L3. Judging by this plot, looks like w.r.t. the coordinates in this plot, we are somewhere around (0.02,-0.02).

It looks like if we want to do this, moving L2 (f = -150mm) may be the best way to go.

[valera, gautam]

We implemented the plan outlined in the previous elog. The visibility (Pmax-Pmin)/(Pmax+Pmin) calculated with the MC REFL PD levels with the MC locked/unlocked is now ~96% (up from 88%). The MC REFL DC level in lock is now ~0.12V (compared to 0.4V). Assuming a modulation depth of 0.1 @ 29.5MHz, about 25% of this (i.e. 0.03V) is from sideband light.

The procedure followed was (see sketch in previous elog for various optic labels):

1. Move L2 back (towards PMC) by ~2cm.
2. Walk the beam using M3 and M4 to minimize MCREFL, re-lock IMC, run WFS. 
3. Move L3 back (towards PMC) by ~2cm.
4. Repeat steps 2 and 3, the latter with smaller steps, monitor MCREFL DC level.

We could probably tweak the fine positioning of L2 and L3 and improve the efficiency a little more, but the primary objective here was to see if there was any effect on the large common mode offset on the WFS demodulated "SUM" output. Unfortunately, we saw no effect.

Here are two photos of the relevant section of the PSL table before (left) and after (right) our work there:

[Paco, Yuta]

We have increased the output power from the PSL table to 951 mW (it was 96.7 mW).
IMC was recovered including WFS, and both arms are flashing nicely in IR.
We tweaked the alignment of GRX and GRY injection to align them with IR, but it was hard.
Right now IR beams are not centered on TMs. We should center them first.

What we did:
Power increase and IMC recovery
 - Replaced a beam splitter which splits the beam into IMC REFL RF PD path and WFS path from R=98% to R=10% one. Reflection goes to RF PD.
 - Put a R=98% beam splitter back into WFS path.
 - We also tried to put a window in front of IMC REFL camera to recover the arrangement in 40m wiki, but the beam reflected from the window was too weak for us to align. So, we decided not to place a window in front of the camera.
 - Attached photos are the IMC REFL path before and after the work.
 - Measured the PSL output power as Koji did in elog 40m/16672. It was measured to be 96.7+/- 0.5 mW.
 - Rotated the HWP using the Universal Motion Controller (it was not possible for us to do it from the MEDM screen). The position was changed from 73.99 deg to 36.99 deg. Output power was measured to be 951 +/- 1 mW
 - IMC locked without any other changes.
 - Changed C1:IOO-WFS_TRIGGER_THRESH_ON to 5000 (was 500). IMC WFS also worked.
 - After running MC WFS relief script, WFS DC offsets and RF offsets are adjusted following the steps in elog 40m/16835. Below are the results.

C1:IOO-WFS1_SEG1_DC.AOFF => -0.0008882080010759334
C1:IOO-WFS1_SEG2_DC.AOFF => -0.0006527877490346629
C1:IOO-WFS1_SEG3_DC.AOFF => -0.0005847311617496113
C1:IOO-WFS1_SEG4_DC.AOFF => -0.0010395992663688955
C1:IOO-WFS2_SEG1_DC.AOFF => -0.0025944841559976334
C1:IOO-WFS2_SEG2_DC.AOFF => -0.003191715502180159
C1:IOO-WFS2_SEG3_DC.AOFF => -0.0036688060499727726
C1:IOO-WFS2_SEG4_DC.AOFF => -0.004011172490815322

IOO-WFS1_I1         :  +1977.7 ->    +2250 (Significant change)
IOO-WFS1_I2         :  +3785.8 ->  +3973.2
IOO-WFS1_I3         :  +2014.2 ->  +2277.7 (Significant change)
IOO-WFS1_I4         :  -208.83 ->  +430.96 (Significant change)
IOO-WFS1_Q1         :  +2379.5 ->  +1517.4 (Significant change)
IOO-WFS1_Q2         :  +2260.4 ->  +2172.6
IOO-WFS1_Q3         :  +588.86 ->  +978.98 (Significant change)
IOO-WFS1_Q4         :  +1654.8 ->  +195.38 (Significant change)
IOO-WFS2_I1         :  -1619.9 ->  -534.25 (Significant change)
IOO-WFS2_I2         :  +1610.4 ->  +1619.8
IOO-WFS2_I3         :  +1919.6 ->  +2179.8 (Significant change)
IOO-WFS2_I4         :    +1557 ->  +1426.6
IOO-WFS2_Q1         :   -62.58 ->  +345.56 (Significant change)
IOO-WFS2_Q2         :  +777.01 ->  +805.41
IOO-WFS2_Q3         :  -6183.6 ->  -5365.8 (Significant change)
IOO-WFS2_Q4         :  +4457.2 ->  +4397.

IFO Alignment
 - Aligned both arms using IR. Both arm flashes at the following, which is consistent with the power increase.
 C1:SUS-ETMX_TRX_OUT_DQ ~1.1
 C1:SUS-ETMY_TRY_OUT_DQ ~0.6
 - With this, we tried to tweak GRX and GRY injection. The following is after the work. We could increase GTRX to 0.204 when the Xarm is aligned to green. This suggests that GRX injection is not aligned nicely yet. But the beams are also not centered on TMs. We should center them first.
 C1:ALS-TRX_OUT_DQ ~0.13
 C1:ALS-TRY_OUT_DQ ~0.07
 - GTRX and GTRY cameras are adjusted to have nicer images. In GRX path, the second and last lens before the PD and CCD was pulled ~ 1 cm behind its original position and both beams realigned. Then, on GRY path, the beam was re-centered on the first and only lens, the whole assembly pushed forward by ~ 2 cm and the beams re-centered.

Next:
 - Center the IR beam on TMs (first by our eyeballs; better to use A2L after arm locking is recovered and coils are balanced)
 - Tweak GRX and GRY injection (restore GRY PZTs?)
 - Install ETMXT camera (if it is easy)
 - Lock Xarm and Yarm (C1:LSC-TRX/Y_OUT needs to be fixed for triggering. Can we use other PDs for triggering?)
 - MICH locking (REFL and AS PDs might need to be re-aligned; they are not receiving much light)
 - RTS model for BHD needs to be updated

After typing up the elog, I decided to try locking the IMC again - now it locks again with the "OLD" gain settings. I tested it ~5 times, the autolocker brings the lock back and the PC drive levels are normal. IMC transmission and MC REFL DC light levels in lock are normal. The PC Drive RMS voltage is <1V. What's more, there is no longer any evidence of 60 Hz line harmonics any more in the PMC diagnostics channels. Compare attachment #1 to this elog.

WTF.

I undid the changes Koji made to the autolocker gains, and am trying the old settings again. Let' see how stable or otherwise the config is. I must've jiggled some poor cable connection back into a good spot while working on the PSL table?

Anyway, this helps Kruthi and Milind.

Note that I have removed an SR785, an oscilloscope, some SRS instruments from the PSL and PMC last night.

But they (and RF Network Analyzer) were not there when the problem started.

We should record the IMC error (at test point monitor) too? If the IMC locks on Monday too, I'll do it.

Over the last 24 hours, the IMC autolocker was able to keep the MC locked ~60% of the time. This is not particularly good, but is an improvement on ~2 weeks ago when the IMC couldn't be locked.

There are two periods, which I've indicated by vertical cursors, between which the autolocker was doing something strange - usually this kind of trend is caused by one or more of the VME crates being unresponsive and the autolocker gets stuck, but I confirmed that both c1psl and c1iool0 are telnet-able. So I conclude that the stability and reliability of the IMC loop is still not as good as it used to be.

Note also that while the PC drive RMS level mostly hovers around 1 V, there are several excursions above that level. This in itself isn't a new phenomenon. I will do some more characterization by measuring the in-loop error signal spectrum and maybe the OLTF of the IMC locking loop.

I was a little confused why the In1 Gain had to be as high as +10dB - before the changes to the RF chain, we were using +27dB, and we expect the changes made to have increased the modulation depth by a factor of ~25, so I would have expected the new In1 Gain to be more like 0dB.

While walking by the PSL table, I chanced upon the scope monitoring PMC transmission, and I noticed that the RIN was unusually high (see the scope screenshot below). We don't have the projector on the wall anymore, but it doesn't look like this has shown up in the SLOW monitor channel anyways. Disabling the MC autolocker / closing the PSL shutter had no effect. I walked over to the amplifier setup in 1X2, and noticed that the SMA cable connecting the output of the amplifier to the EOM drive was flaky. By touching the cable a little, I noticed that the trace on the scope appeared normal again. Turning off the 29.5MHz modulation source completely returned the trace to normal.

So I just made a new cable of similar length (with the double heat shrink prescription). The PMC transmission looks normal on the scope now. I also re-aligned the PMC for good measure. So presumably, we were not driving the EOM with the full +27dBm of available power. Now, the In1 Gain on the MC servo board is set to +2dB, and I changed the nominal FSS FAST gain to +18dB. The IMC OLTF now has a UGF of ~165kHz, though the phase margin is only ~27 degrees.. 

29.5 MHz RF Modulation Source

• The +13dBm from the Wenzel oscillator gets amplified to +27dBm by a ZHL-2-S. There is a 5dB attenuator on the input to the amplifier to avoid compression/saturation.
• The amplified output goes to the EOM (+26dBm measured at the rack, no measurement done at the input to the triple-resonant circuit box yet), while a 10dB coupled part goes to the RF distribution box which splits the input into 16 equal parts. The outputs were measured to spit out +5dBm.
• 2 of these go to the WFS demod boards - it was verified that this level of drive is okay for the comparator chips on the demod board.
• A third output goes to the IMC Demod board. The demod board was modified so that the nominal LO input level is now +5dBm (details below).
• The remaining outputs are all terminated with 50ohms.

IMC Demodulation Board

• The input attenuator, amplifier and power splitter were removed.
• Schematic with changes marked and power levels measured, along with a high-res photograph (taken with our fancy new Macro lens + LED light ring) has been uploaded to a page I made to track changes for this part on the DCC (linked to 40m document tree).
• After making the changes, it was verified that the power levels in the signal chain were appropriate up till the input to the ERA-5SM amplifier directly before the LO. These levels were deemed appropriate, and also scaled in a predictable manner with the input power. As Koji mentions in the previous elog, the dynamically changing input impedance of the mixer makes it difficult to measure the LO level at this point, but I am satisfied that it is within ~1dBm of the nominal +17dBm the mixer wants.
• The board was further checked for gain imbalance and orthogonality of the I and Q outputs. The graphic below show that there is negligible gain imbalance, but the relative phase between the I and Q channels is ~78 degrees (they should be 90 degrees). Of course this doesn't matter for the IMC locking as we only use the I phase signal, but presumably, we want to understand this effect and compensate for it. 
  
  
• The label on the front panel has been updated to reflect the fact that the nominal LO input is now +5dBm
• The demodulation phase had changed since the RF signal change was modified - Rana and I investigated this effect on Monday morning, and found that a new ~1.5m long cable was needed to route the signal from the RF distribution box to the LO input of the demod board, which I made. Subsequent modifications on the demod board meant that an extra ~10cm length was needed, so I just tacked on a short length of cable. All of the demodulated signal is now in the I output of the demod board (whereas we had been using the Q output).
• The graphics below confirms that claim above. Note the cool feature on the digital scopes that the display persistence can be set to "infinity"!
      

I wanted to do a quick check to see if the observed signal levels were in agreement with tests done on the workbench with the Marconi. The mixers used, JMS-1H, have an advertised conversion loss of ~7dB (may be a little higher if we are not driving the LO at +17dBm). The Lissajous ellipse above is consistent with these values. I didn't measure powers with the MC REFL PD plugged into the demod board, but the time series plot above suggest that I should have ~0dBm power in the MC REFL PD signal at 29.5MHz for the strongest flashes (~0.3Vpp IF signal for the strong flashes). 

MC Servo Board

• As mentioned above, we now use the I phase signal for lMC PDH locking.
• This has resulted in an overall sign change of the servo. I have updated the MEDM screen to reflect that "MINUS" is the correct polarity now..
• To set the various gains, I measured the OLTF for various configurations using the usual IN1/IN2 prescription on the MC Servo Board (using the Agilent analyzer). 
• I started at 0dBm "In1 Gain", and the nominal (old) values for "VCO gain", "FSS Common Gain" and "FSS FAST gain"  and found that though I could lock the MC, I couldn't reliably turn on the boosts.
• After some tweaking, I settled on +10dB "In1 Gain". Here, locking was much more reliable, and I was able to smoothly turn on the Super Boosts. The attached OLTF measurement suggests a UGF of ~118kHz and phase margin of a little more than 30 degrees. There is room for optimization here, since we have had UGFs closer to 200kHz in the recent past. 
  
• I didn't get around to measuring the actual PZT/EOM crossover yesterday. But I did measure the OLTF for various values of the FSS gains. At the current value of +20dBm, the PC drive signal is hovering around 1.5V. This bit of optimization needs to be done more systematically. 
• I've edited mcup and mcdown to reflect the new gains. 
  

Some general remarks

• The whole point of this exercise was to increase the modulation depth for the 29.5MHz signal. 
• By my estimate, assuming 8mrad/V modulation index for the EOM and a gain of 0.6 at 29.5 MHz in the triple resonant box, we should have 100mrad of modulation after installing the amplifier (compared to 4mrad before the change). 
• The actual RF power at 29.5 MHz at the input/output of the triple resonant box has not yet been measured. 
• The WFS input error signal levels have to be re-measured (so I've turned off the inputs to the digital WFS filters for now)

I would think that we want to fix the I/Q orthog inside the demod board by trimming the splitter. Mixing the Q phase signal to the I would otherwise allow coupling of low frequency Q phase junk from HOMs into the MC lock point.

[valera, gautam]

Motivation: see this elog

I was fiddling around for a few days trying to implement the method outlined in this paper to null this offset - I will post a separate elog about my efforts but Valera pointed out that we could try injecting an AF modulation at the IN2 input of the MC Servo Board. Last night, we hooked up an SR function generator (f = 312Hz, A = 0.01Vpp, IN2 gain = -5dB) to the unused BNC IN2 input of the MC Servo board. To avoid any additional offsets from the AO path during this measurement, I disconnected the LEMO cable (it is labelled).

We looked at the spectrum of the MC transmission around 312Hz and also 2*f = 624Hz. As a result of this modulation, we expect in the transmitted power, dP/P, a 2f term with amplitude ~(X_mod/X_0)^2 and a term at f with amplitude ~(X_offset * X_mod / X_0^2) - I may have missed out some numerical factors of order 1. So the latter should vanish if the offset at the error point is truly zero and the lock-point is the center of the resonance. Last night, we found that an offset in the range of -0.25 V to -0.19 V nulled this peak in the DTT spectrum. Today, the number was -0.05V. So the true offset seems to vary from lock to lock. Here are spectra around f=312Hz for a few different values of the offset slider (the center of the resonance seems to be -0.05V on the MEDM slider at this time).

Do these numbers make sense? Some time ago, I had pulled out the MC Servo board to find out what exactly is going on at this offset summing point. The MEDM slider goes from -10V to 10V, and by measuring the voltage at TP5 (see schematic below), I found that there is a 1/40 scaling factor between what is actually applied and the number on the MEDM slider (so for example, the numbers in the legend in the above plot have to be divided by 40). I've modified the MC Servo Board MEDM screen to reflect this. When I had pulled the board out, I noticed that in addition to the offset voltage applied via the backplane connector, there was also a potentiometer (R50 in the schematic below). I had nulled the voltage at TP5 using this potentiometer, but I guess drifts of ~5mV are possible. 

Discussion on calibration of offset slider in Hz/V:

I've yet to do a rigorous calibration of this slider into Hz, but looking at the spectrum of the transmitted intensity at 2f, we estimated the coefficient (X_mod/X_0) ~ 3e-3 for an offset of 0.2V. dP/P ~1 when the applied modulation equals the linewidth of the cavity, which is 3.6kHz. So 0.2V of offset slider corresponds to ~ 10Hz frequency offset. In other words, I estimate the slider calibration to be 50Hz/V. So with the full range of +/- 10V, we should be able to scan ~1kHz of frequency offset. What does this imply about the variation of the offset slider value that removes the peak at 1f between locks? As mentioned above, this variation is ~0.2V over a day - with the calibration mentioned above, this corresponds to a change in cavity length of ~10um, which seems reasonable to me...

So how did all of this tie in with WFS SUM offsets? We did the following:

• After nulling the length offset using the procedure detailed above, we noticed non-zero offsets on both WFS1 and WFS2 "I" SUM outputs
• So we set the dark offsets and RF offsets for the WFS, with no light incident on the WFS (PSL shutter closed). 
• Re-locking the IMC and closing the WFS loops, we noticed that WFS2 SUM offset was still hovering around 0, but WFS1 SUM offset was ~ -2000cts.
• Looking at some trends on dataviewer, this offset seems to drift around over a few days timescale by a few thousand counts - for example, the WFS1 offset today was +2000cts. Moreover, the WFS1 offset seems to drift around by ~factor of 3 times as much as WFS2 offset in the 24 hour period I looked up (plot to follow)...
• Misaligned MC2 and looked at the sum offset with just the single bounce beam off MC1 onto the WFS

I neglected to screenshot the StripTool from the times we were doing these trials but I have the times, I will pull up some dataviewer plots and upload them here tomorrow...

Now that all the front end models are running, I re-aligned the IMC, locked it manually, and then tweaked the alignment some more. The IMC transmission now is hovering around 15300 counts. I re-enabled the Autolocker and FSS Slow loops on Megatron as well.

IMC is locking now. There was nothing wrong: just a careful alignment + proper gain adj

=== Primary Alignment ===

- I used WFS error signals as the indicator of the PDH error signals. Checked C1:IOO-WFS1_(I/Q)n_ERR and ended up C1:IOO-WFS1_I4_ERR as it showed the largest PDH error PP.

- Then used MC2 and MC3 to align the IMC by maximizing the PDH error and the MC trans (C1:IOO-MC_TRANS_SUM_ERR)

=== Locking procedure ===

Note that the MC REFL path is still configured for the full power input

- (Only at the beginning) Run scripts/MC/mcdown for initialization / Run scripts/MC/MC2tickleOFF just in case

- Enable IOO-MC-SW1 (MC SERVO switch right after "IN1 Gain (dB)").
- Disable 40:4000 boost
- Increase VCO Gain from -15 to 0
- Jiggle IN1 Gain from low to +31 until the lock is achieved

- As soon as the lock is acquired, enable 40:4000
- Increase VCO Gain to +10
- Turn up "SUPER BOOST" from 0 to 3

=== Lock loss procedure ===

Note that the MC REFL path is still configured for the full power input

- Disable IOO-MC-SW1
- Disable 40:4000 boost
- Reduce VCO Gain 0
- Turn down "SUPER BOOST" to 0

- Then jiggle IN1 Gain again to lock the IMC

=== MC2 spot ===

- It was obvious that the MC2F spot was not on the center of the optic.
- I tried to move the spot on the camera as much as possible, but this did not make the trans beam to the center of the MC end QPD
- I had the impression that the trans beam started to be clipped when the beam was moved towards the end QPD,

We need to reestablish the reasonable/consistent MC2 spot on the mirror, the MC end optics, and the QPD.
We will need to use MC2 dithering and A2L coupling to determine the center of the mirror

But as long as the transmission is maximized, the transmitted beam thru MC1 and MC3 follows the input beam. So we can continue the vent work

The current maximized transmission was ~1300. MC1 refl CCD view was largely off -> The camera path was adjusted.

=== MC2 alignment note ===

During the alignment, I noticed a sudden change of the MC2 alignment. There might be some hysteresis in the MC2 suspension. If you are locking the IMC and noticed significant misalignment, the first thing to try is to touch MC2 alignment.

See trend. This is NOT symptomatic of some frozen slow machine - if I disable the WFS servo inputs, the lock holds just fine.

Turns out that the beam was almost completely missing the WFS2 QPD. WTF 😤. I re-aligned the beam using the steering mirror immediately before the WFS2 QPD, and re-set the dark offsets for good measure. Now the IMC remains stably locked. 

Please - after you work on the interferometer, return it to the state it was in. Locking is hard enough without me having to hunt down randomly misaligned/blocked beams or unplugged cables.

I took this opportunity to do some WFS offset updates.

• First I let the WFS servo settle to some operating point, and then offloaded the DC offsets to the IMC suspensions.
• Then I disabled the WFS servo.
• I hand-tweaked MC1 and MC3 PIT/YAW (while leaving MC2 untouched) to minimize IMC REFL (a more sensitive indicator of the optimal cavity alignment than the transmission).
• Once I felt the IMC REFL was minimized (~1-2% improvement), I set the RF offsets for the WFS while the IMC remained locked. I chose this way of setting the RF offsets as opposed to unlocking the cavity and having the high-power TEM00 mode incident on the WFS QPDs.
• Overnight, I'm going to run the MC2 spot position scanning code (in a tmux session on pianosa, started ~945pm) to see if we can find a place where the transmission is higher, looking at Kruthi's code now to see it makes sense...
• The convergence time of the MC2 spot position loop is pretty slow, so the scan is expected to take a while... Should be done by tomorrow morning though, and I expect no work with the IFO tonight.
• Does this loop have to be so slow? Why can't the gain be higher?

I think this offset setting thing is not so good. People do this every few years, but putting offsets in servos means that you cannot maintain a stable alignment when there are changes in the laser power, PMC trans, etc. The better thing is to do the centering of the WFS spots with the unlcoked beam after the control offsets have been offloaded to the suspensions.

Motivation:

To test the hypothesis that the IMC lock duty cycle is affected by the PRM alignment. Rana pointed out today that the input faraday has not been tuned to maximize the output->input isolation in a while, so the idea is that perhaps when the PRM is aligned, some of the reflected light comes back towards the PSL through the Faraday and hence, messes with the IMC lock.

A script to test this hypothesis is running over the weekend (in case anyone was thinking of doing anything with the IFO over the weekend).

Methodology:

I've made a simple script - the pseudocode is the following:

• Align PRM
• For the next half hour, look for downward transitions in the EPICS record for MC TRANS > 5000 cts - this is a proxy for an MC lockloss
• At the end of 30 minutes, record number of locklosses in the last 30 minutes
• Misalign PRM, repeat the above 3 bullets

The idea is to keep looping the above over the weekend, so we can expect ~100 datapoints, 50 each for PRM misaligned/aligned. The times at which PRM was aligned/misaligned is also being logged, so we can make some spectrograms of PC drive RMS (for example) with PRM aligned/misaligned. The script lives at /opt/rtcds/caltech/c1/scripts/SUS/FaradayIsolationTest/FaradayIsolCheck.py. Script is being run inside a tmux session on pianosa, hopefully the machine doesn't crash over the weekend and MC1/CDS stays happy.

A more direct measurement of the input Faraday isolation can be made by putting a photodiode in place of the beam dump shown in Attachment #1 (borrowed from this elog). I measured ~100uW of power leaking through this mirror with the PRM misaligned (but IMC locked). I'm not sure what kind of SNR we can expect for a DC measurement, but if we have a chopper handy, we could put a chopper (in the leaked beam just before the PD so as to allow the IMC to be locked) and demodulate at that frequency for a cleaner measurement? This way, we could also measure the contribution from prompt reflections (up to the input side of the Faraday) by simply blocking the beam going into the vacuum. The window itself is wedged so that shouldn't be a big contributor.

I stopped the test earlier today morning around 11:30am. The log file is located at /opt/rtcds/caltech/c1/scripts/SUS/FaradayIsolationTest/PRM_stepping.txt. It contains the times at which the PRM was aligned/misaligned for lookback, and also the number of MC unlocks during every 30 minute period that the PRM alignment was toggled. This was computed by:

• continuously reading the current value of the EPICS record for MC Trans.
• comparing its current value to its values 3 seconds ago.
• If there is a downward step in this comparison greater than 5000 counts, increment a counter variable by 1.
• Reset counter at the end of 30 minute period.

I think this method is a pretty reliable proxy, because the MC autolocker certainly takes >3 seconds to re-acquire the lock (it has to run mcdown, wait for the next cavity flash, and run mcup in the meantime).

Preliminary analysis suggests no obvious correlation between MC lock duty cycle and PRM alignment.

I leave further analysis to those who are well versed in the science/art of PRM/IMC statistical correlations.

Summary:

There seems to be significant phase loss in the TTFSS path, which is limiting the IMC OLTF to <100 kHz. 

Details:

See Attachment #1 and #2. The former shows the phase loss, while the latter is just to confirm that the optical gain of the error point is roughly the same, since I noticed this after working on and replacing the RF frequency distribution unit. Unfortunately there have been many other changes also (e.g. the work that Rana and Koji did at the IMC rack, swapping of backplane controls etc etc - maybe they have an OLTF measurement from the time they were working?) so I don't know which is to blame. Off the top of my head, I don't see how the RF source can change the phase lag of the IMC servo at 100 kHz. The only part of the IMC RF chain that I touched was the short cable inside the unit that routes the output of the Wenzel source to the front panel SMA feedthrough. I confirmed with a power meter that the power level of the 29.5 MHz signal at that point is the same before and after my work.

The time domain demod monitor point signals appear somewhat noisier in todays measurement compared to some old data I had from 2018, but I think this isn't significant. Once the SR785 becomes available, I will measure the error point spectrum as well to confirm. One thing I noticed was that like many of our 1U/2U chassis units, the feedthrough returns are shorted to the chassis on the RF source box (and hence presumably also to the rack). The design doc for this box makes many statements about the precautions taken to avoid this, but stops short of saying if the desired behavior was realized, and I can't find anything about it in the elog. Can someone confirm that the shields of all the connectors on the box were ever properly isolated? My suspicion is that the shorting is happening where the all-metal N-feedthroughs touch the drilled surfaces on the front panel - while the front and back surfaces of the panel are insulating, the machined surfaces are not.

This is an unacceptable state but no clear ideas of how to troubleshoot quickly (without going piece by piece into the IMC servo chain) occur to me. I still don't understand how the freq source work could have resulted in this problem but I'm probably overlooking something basic. I'm also wondering why the differential receiving at the TTFSS error point did not require a gain adjustment of the IMC servo? Shouldn't the differential-receiving-single-ended-sending have resulted in an overall x0.5 gain?

Update 8 Dec 1200: To test the hypothesis, I bypassed the SR560 based differential receiving and restored the original config. I am then able to run with the original gain settings, and you see in Attachment #4 that the IMC OLTF UGF is back above 100 kHz. It is still a little lower than it was in June 2019, not sure why. There must be some saturation issues somewhere in the signal chain because I cannot preserve the differential receiving and retain 100 kHz UGF, either by raising the "VCO gain" on the MC servo board, setting the SR560 to G=2, or raising the "Common Gain Adjust" on the FSS box by 6 dB. I don't have a good explanation for why this worked for some weeks and failed now - maybe some issue with the SR560? We don't have many working units so I didn't try switching it.

So either there is a whole mess of lines or the frequency noise suppression is limited. Sigh.

[koji, gautam]

1. I began my investigations by measuring the voltage noise of the demod board outputs with an SR560 (G=100) and SR785 in the audio band.
   • Measurement made with PSL shutter closed, LO input of demod board driven with the nominal level of ~2.5dBm, RF input terminated.
   • Motivation was to look for any noise features.
   • Expected noise level is ~2nV/rtHz (Johnson noise of 50ohm) since there are no preamp electronics post SCLF-5 LP filter on this board.
   • Attachment #1 shows the results of the measurement for a few scenarios. Spectra only shown for the I channel but the Q channel was similar. The LO=+5dBm curve corresponds to driving the input at 5dBm with a marconi, to see if the label of the nominal level being +5dBm had anything to it.
   • The arches above 1kHz seemed suspicious to me, so I decided to investigate further.
2. Looking at the IMC Demod board schematic, I I saw that there were 2 ERA-5SMs in there which are responsible for amplifying the 29.5MHz signal which serves as the LO to the oscillators.
3. I pulled the demod board out and tested it on the electronics workbench. Koji and I couldn't make sense of the numbers we were seeing (all measurements made with Agilent analyzer and active FET probe with 100:1 attentuator).
4. We eventually concluded that the ERA-5SMs were not exhibiting the expected gain of ~20dB. So we decided to swap these out.
5. This sort of measurement is not ironclad as the output of the ERA-5SM goes to the mixer whose input impedance is dynamically varying as the diodes are switching. So even after replacing the suspect amplifiers with new ones, we couldn't make the numbers jive.
6. We suspected that the new amplifiers were getting saturated. The 3dB saturation point for the ERA-5SM is spec'ed as ~19dBm.
   • We "measured" this by varying the input signal level and looking for deviation from linearity.
   • We saw that there was ~1dB compression for ~13dBm output from the ERA-5SM (after correcting for all attenuators etc). But this number may not be accurate in the absolute sense because of the unknown input impedance of the mixer.
   • Moreover, looking at the spec sheet for the mixer, JMS-1H, we found that while it wasn't ideal to operate the mixer with the LO level a few dBm below the expected +17dBm, it probably wasn't a show stopper.
7. So we figured that we need 10dB of attenuation between the "nominal" LO input level of 2.7dBm and the input of the demod board in order to keep the ERA-5SM in the linear range. This has now been implemented in the form of an SMA attenuator.
8. IMC locked straight away. But I noticed that PC drive RMS level was unusually large.
9. I found that by increasing the "IN1" gain of the CM board to 12dB (from 2dB) and the "VCO gain" to 10dB (from 7dB), I could recover a transfer function with UGF ~140kHz and PM ~30degrees (need more systematic and wider span measurements of this, and also probably need to optimize the crossover gains). See Attachment #2 for my quick measurement tonight.
10. Updated mcup to reflect these new gains. Tested autolocker a few times, seems to work okay.
11. While it presumably was a good thing to replace the faulty amplifiers and prevent them from saturating, this work has not solved the primary problem of excess frequency noise on the PSL.

It is not clear to me why installing an attenuator to prevent amplifier saturation has necessitated a 10dB increase in the IN1 gain and 3dB increase in the VCO gain. Initially, I was trying to compensate for the gain by increasing the FSS "Common Gain" but in that setting, I found an OLTF measurement impossible. The moment I enabled the excitation input to the CM board, the lock was blown, even with excitation amplitudes as small as -60dBm (from the Agilent network analyzer).

This may also be a good opportunity to test out one of the aLIGO style FET mixer demod boards (recall we have 2 spare from the 4 that were inside the ALS demod box). I'm going to ask Steve to package these into a 1U chassis so that I can try that setup out sometime. From a noise point of view, the aLIGO boards have the advantage of having a x100 preamp stage straight after the mixer+LPF. We may need to replace the lowpass filter though, I'm not sure if the one installed is 1.9MHz or 5MHz.

I've left an SR785 and AG4395 near 1X2 in anticipation of continuing this work tomorrow.

Unrelated to this work - seems like the WFS DC and RF offsets had not been set in a while so I reset these yesterday. The frequent model restarts in recent times may mean that we have to reset these to avoid using dated offset values.

Re-measured the demod board noises after replacing the suspect ERA-5SMs, with LO driven by a marconi at the "nominal" level of 2.5dBm, and RF input terminated. Attachment #1 is the input referred voltage noise spectra. I used the FET low noise pre-amp box for this purpose. I cannot explain the shape of the spectra above 1kHz. I tried doing the measurement on a minicircuits mixer (non-surface mount) and found the shape to be flat throughout the SR785 span. Unclear what else could be going on in the demod board though, all the other components on it are passive (except the ERA-5SMs which were replaced). I considered adopting a PMC style demod setup where we do the demod using some separate Minicircuits Mixer+LowPass filter combo. But the RF flashes for the IMC monitored at the RFmon port are ~0.2Vpp, and so the RF input to the mixer is expected to be ~2Vpp. The minicircuits mixer selection guide recommends choosing a diode mixer with LO level at least 10dBm above the expected RF input signal level, and we don't have any standalone mixers that are >Level 7. I've asked Steve to package the aLIGO demod board in the meantime, but even that might not be a plug and play replacement as the IF preamp stage has ~120degrees phase lag at 1MHz, which is significantly higher than the existing board which just has a SCLF5 low pass filter after the mixer and hence has <45degrees phase lag at 1MHz.

This elog by koji inspired me to consider power supply as a possible issue.

The demod board receives +/-24V DC (which is regulated down to +/-15V DC by 7815/7915), and also +15V DC via the backplane. The ERA-5SM receives DC power from the latter (unregulated) +15V DC. I can't think of why this is the case except perhaps the regulators can't source the current the amp wants? In any case, it doesn't look feasible to change this by cutting any traces on the PCB to me. While I had the board out, I decided to replace the JMS-1H mixers in a last ditch effort to improve the demod board noise. Unfortunately I'm having trouble de-soldering these MCL components from the board. So for now, I'm leaving the demod board out, IMC unlocked. Work will continue tomorrow. 

After some persistence, I managed to get the mixers off.

• Having gotten the mxiers off, I decided to temporarily solder on 50ohms between the LO pin pad and ground on the demod board and measure the RF signal levels in the LO chain with the active probe again.
• Today, with this change, I confirmed that the ERA-5SM begins to saturate closer to the +19dBm advertised on its datasheet. So we need only 2dB of attenuation at the input to have 17dBm at the LO pin of the mixer, assuming 50ohm input impedance.
• But this begs the question - what does minicircuits mean by a Level-YY mixer? Do they expect YY dBm delivered to a 50ohm load? Or do we need to supply YY dBm accounting for the dynamically changing input impedance of the mixer, as monitored by a high impedance probe?
• I soldered on some new mixers (JMS-1H) I procured from Downs earlier today.
• Re-installed the demod board in the eurocrate.

Unfortunately, the coherent noise between the arms persists so the sensing noise injection must be happening elsewhere. IMC seems to lock fine though so I'm leving the autolocker on

I took some transfer functions of the IMC loop and crossover, being careful that the PC drive never exceeding 1V during the measurements. 

I then did some algebra to try and back out the individual loop paths, without having to make assumptions/approximations about the loop gain being high enough. This only really works in the region where both the open loop and crossover measurements have coherence. 

It seems to me that the PZT path has pretty low phase margin on its own, but maybe this is ok, since its never really meant to run solo. The EOM path shape is harder to understand.

The data I took, and code that made the above plot is attached. This afternoon, I'll post an update comparing the measured OLG and crossover to earlier measurements. 

The promised historical comparisons follow. The crossover looks mostly the same as before. There is a new feature in the OLG at 50-60kHz; what could've changed about the EOM path in that time?

We want to measure the IMC round-trip loss using the Isogai et. al. ringdown technique. I spent some time looking at the various bits and pieces needed to make this measurement today, this elog is meant to be a summary of my thoughts.

1. Inventory
   • AOM (in its new mount to have the right polarization) has been installed upstream of the PMC by Johannes. He did a brief check to see that the beam is indeed diffracted, but a more thorough evaluation has to be done. There is currently no input to the AOM, the function generator on the PSL table is OFF.
   • The Isogai paper recommends 3 high BW PDs for the ringdown measurement. Souring through some old elogs, I gather that the QPDs aren't good for this kind of measurement, but the PDA255 (50MHz BW) is a suitable candidate. I found two in the lab today - one I used to diagnose the EX laser intensity noise and so I know it works, need to check the other one. We also have a working PDA10CF detector (150 MHz BW). In principle, we could get away with just two, as the ringdown in reflection and transmission do not have to be measured simultaneously, but it would be nice to have 3
   • DAQ - I think the way to go is to use a fast scope triggered on the signal sent to the AOM to cut the light to the IMC, need to figure out how to script this though judging by some 2007 elogs by rana, this shouldn't be too hard...
2. Layout plans
   • Where to put the various PDs? Keeping with the terminology of the Isogai paper, the "Trans diode" can go on the MC2 table - from past measurements, there is already a pickoff from the beam going to the MC TRANS QPD which is currently being dumped, so this should be straightforward...
     
   • For the "Incident Diode", we can use the beam that was used for the 3f cancellation trials - I checked that the beam still runs along the edge of the PSL table, we can put a fast PD in there...
   • For the "REFL diode" - I guess the MC REFL PD is high BW enough, but perhaps it is better to stick another PD in on the AS table, we can use one of the existing WFS paths? That way we avoid the complicated transfer function of the IMC REFL PD which is tuned to have a resonance at 29.4MHz, and keeps interfacing with the DAQ also easy, we can just use BNC cables...
   • We should be able to measure and calibrate the powers incident on these PDs relatively easily.
      
3. Other concerns
   • I have yet to do a thorough characterization of the AOM performance, there have been a number of elogs noting possible problems with the setup. For one, the RF driver datasheet recommends 28V supply voltage but we are currently giving it 24V. In the (not too distant) past, the AOM has been seen to not be very efficient at cutting the power, the datasheet suggests we should be able to diffract away 80% of the central beam but only 10-15% was realized, though this may have been due to sub-optimal alignment or that the AOM was receiving the wrong polarization...
4. Plan of action
   • Check RF driver, AOM performance, I have in mind following the methodology detailed here
   • Measure PMC ringdown - this elog says we want it to be faster than 1us
   • Put in the three high BW PDs required for the IMC ringdown, check that these PDs are working
   • Do the IMC ringdown

Does this sound like a sensible plan? Or do I need to do any further checks?

We expect that the MC sus are susceptible to the temperature change and the alignment drifts away with time.

Here is the proper alignment procedure.

0) Assume there is no TEM00 flash or locking, but the IMC is still flashing with higher-order modes.

1) Use the CCD camera and WFS DC spots to bring the beam to the nominal position.

2) Use only MC2 and MC3 to align the cavity to have low-order modes (TEM00,01,02 etc)

3) You should be able to lock the cavity on one of these modes. Minimize the reflection (maximize the transmission) for that mode.

4) This should allow you to jump to a better lower-order mode. Continue alignment optimization only with MC2/3 until you get TEM00.

5) Optimize the TEM00 alignment only with MC2/3

6) Look at the MC end QPD. use one of the scripts in scripts/MC/moveMC2 . Note that the spot moves opposite to the name of the scripts. i.e. MC2_spot_down moves the spot up, MC2_spot_right moved the spot left, etc...
These scripts move MC1/2/3 and try to keep the good MC transmission.

7) moveMC2 scripts are not perfect. As you use them, it makes the MC alignment gradually degraded. Use MC2 and MC3 to recover good transmission.

8) If MC2 spot is satisfactory, you are done.

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

Step 6-8 can be done with the WFS on. This way, you can skip step 7 as the WFS servo takes care of it. But if the spot move is too fast, the servo can't keep up with the change. If so, you have to wait for the settling of the servo. Once the spot position is satisfactory, MC servo relief should be run so that the servo offset (in actuation) can be offloaded to the bias slider.

I've added two curves to the NB. Both are measured (with FET preamp) at the output of the demod board, with the LO driven at the nominal level by the Wenzel RF source pickoff (as it would be when the IMC is locked) and the RF input connected to the IMC REFL PD. For one curve, I simply closed the PSL shutter, while for the other, I left the PSL shutter open, but macroscopically misaligned MC2 so that there was no IMC cavity. So barring RFAM, there should be no PDH signal on the REFL PD, but I wanted to have light on there. I'm not sure if I understand the difference between these two curves though, need to think on it. Perhaps the IMC REFL PD's optical/electrical response needs to be characterized?

Since ~ 2 hours ago, the IMC autolocker has not been able to keep the IMC locked. I don't see any obvious trends in the wall StripTool that may point to what's going on. For the brief periods in which a TEM00 mode is locked, the PC Drive RMS level is ~5x what the nominal level is, and while the autolocker is trying to lock the IMC, the PC drive RMS level is hovering around 4V DC, which is high. The PMC Error and Control signal spectra show huge 60 Hz (and harmonics) peaks, and indeed this is visible in the time domain signals as well (on ndscope or on the oscilloscope on the PSL table), but this is not a new feature in the last two hours. Usually, this kind of problem signals that either/both the c1psl or c1iool0 slow machines need to be power-cycled, but I confirmed that both machines are online and telnet-able. Possibilities: (i) some card in the c1psl / c1ioo crates have failed or (ii) something in the MC/FSS electronics chain has failed or (iii) there is a huge amount of excess high-frequency noise from the NPRO.

I am leaving the PSL shutter closed.

WFS loops were running for past 2 hours when I made the overall gain slider zero at:

PDT: 2022-10-18 20:42:53.505256 PDT
UTC: 2022-10-19 03:42:53.505256 UTC
GPS: 1350186191.505256

The output values are fixed to a good alignment. IMC transmission is about 14100 counts right now. I'll turn on the loop tomorrow morning. Data from tonight can be used for monitoing open loop noise.

Turning WFS loops back on at:

PDT: 2022-10-19 09:48:16.956979 PDT
UTC: 2022-10-19 16:48:16.956979 UTC
GPS: 1350233314.956979

Overtime the coil output filters on IMC optics have drifted into a bad configuration. Today at the meeting, Rana told us the correct configuration for these filters. I'll summarize this here and we have changes the filters on all IMC optics, MC1, MC2, and MC3 to match this configurations:

MC1 and MC3

Analog side:

Both MC1 and MC3 have a 28 Hz 5th order elliptical low pass filter as teh dewhitening filter in LIGO-D000316

Digital side:

At the coil output filters named as C1:SUS-MC1_ULCOIL, the filter module FM9 is connectd in RTCDS to the analog dewhitening filter such that only one of the two can remain ON. So For MC1 and MC3, we put a ellip("LowPass", 5, 1, 50, 28) filter on FM9 for all 5 coil output filters.

Note: We do not add a inverse dewhitening filter at FM10 like most other optics as inverting this filter will create resonant peaks at the dips of the elliptical filter which we want to avoid and we anyways do not use MC1 and MC3 optics for any kind of actuation above 20 Hz.

MC2

Analog side:

For MC2, the dewhitening filter is a 10 Hz pole, 30Hz zero like most other suspended optic.

Digital side:

At the coil output filters named as C1:SUS-MC2_ULCOIL, the filter module FM9 is connectd in RTCDS to the analog dewhitening filter such that only one of the two can remain ON. So For MC2 we put a SimDW filter which is matched to the anlog filter. We also put a InvDW filter on FM10, which is the analytical inverse of the SimDW filter. This filter does the anti-dewhitening required on the digital side and should be always ON.

To MC2 equivalent to other IMC optics in terms of overall transfer function for the local damping loops and ASC loops, we need additional 28 Hz elliptical low pass filter in these loops. But such a filter should not be in the path of LSC feedback when MC2 is used for locking CARM with a bandwidth of ~100 Hz. Thus, we put a ellip("LowPass", 5, 1, 50, 28) filter on FM6 of the following filters, which should be always ON as well:

• C1:SUS-MC2_ASCPIT
• C1:SUS-MC2_ASCYAW
• C1:SUS-MC2_SUSPOS
• C1:SUS-MC2_SUSPIT
• C1:SUS-MC2_SUSYAW
• C1:SUS-MC2_SUSSIDE

Effect on 60 Hz Noise

With the above changes, we see that the 60 Hz noise is same as the previous levels when we use the analog dewhitening filter (28 Hz elliptical filter) for MC1. We can move forward with our science experiments with that configuration but there is still something fishy about MC1 in comparison to MC3 which does not have this behavior. So this still needs to be looked at in future.

Wiki page for filter details and configurations

Information of this kind should be stored in a wiki page in my opinion. We should have a page where we list all common filter configurations for our suspensions and other loops, that can be generally classified and is useful for understanding legacy configuration for future folks who work here. I'm starting such a wiki page here, where I'll dump more information as I collect it and get time. Everyone is encouraged to update this in there free/procrastination times.

Also reply to: 40m/16125

I migrated the code used in 40m/16125 to our scripts git repo and used it to apply offsets to IMC optics and noticing the parabolic change in the transmission values. Fitting the data with parabola and using the calculations mentioned in the previous post, we get following angular actuation calibration at DC from the PIT/YAW alignment output channels (cts) to actual motion in (urad):

*Note that in the previous post, the radius of curvature of MC2 used was wrong and has been corrected in this calculation to 17.87 m taken from Gautam's thesis Table A.1

Due to lack of time, we ran test faster on MC2, hence more uncertainty in it's results. Also, during MC1 YAW test, lock for breifly lost which required me to manually throw away some data points, but it did not affect the quality of fit much. Please see attached the data plots and fit.

For calibration at AC, another test needs to be performed which I did not do right now. 40m/16125 also describes how to do that, so someone can repeat that in future.

[Paco, JC]

We came in this morning and noted the IMC was grossly misaligned, with MC3 still damped but with >= 100 rms motion in all coil monitors  (a lot but not enough to trip the WD)... Turning off the WFS didn't do much so it was obviously an issue with the recent f2A output filters, so we turned all off (though only MC3 had this excess motion). After this we aligned IMC, engaged the lock and turned WFS back on.

There was no elog about f2A beyond this test scheduled to run Friday, I guess the filters were meant to stay on long term?

Two days ago I opened the PSL shutter by switching the switch on the shutter driver. That caused the shutter's switch on the medm screen to work in reversed mode: open meant closed and closed meant open.

I fixed that. Now the medm screen switch state is correct.