ETMY UL epoxy soaking dish. All teflon in glass.

I'm not sure what happened to ETMY SUS, but it was in a pretty bad state.  Bad burt restore, I would guess.

Most egregiously, the inputs to all of the coil output filters were switched off.  This is a bit insidious, since these inputs being off doesn't show up on the overview screen at all.  This explains why ETMY had not been damping for the last couple of day, and why my binary whitening switching measurements were nonsense.

I also found that ETMYs damping filter was a 30 Hz high pass, instead of the 3 Hz high pass in all the other suspension controllers.  Unfortunately a messed up burt restore can't explain that.

I normalized the ETMY controller to match all of the other controllers (ie. gave it a nice new 3 Hz high pass), adjusted gains accordingly, and now ETMY is behaving nicely.

Actually, ETMY was the only good one. They should all have the 30 Hz High pass as the damping filter. I think these details are in the elog entry that we originally made while doing ETMY.

They should all also have a 3:30 in the XXSEN to compensate the whitening. The logic is supposed to be that FM1 is ON when the hardware whitening is ON. This is the opposite of the old logic and its why the damping filter has to be moved from 3 to 30 Hz.

ETMY sus damping was found to be tripped.

It was retored.

All fluorecent light were turned off. Please try to conserve some energy.

The Caltech gasoline storage tank is being upgraded.

They are jack hammering and digging with bulldozer 50 yards south of  ETMY

[Kiwamu / Steve]

We checked some electronics noise on the ETMY shadow sensor system.

Noise from the WF, AA board and ADC are below the shadow sensor spectra on ETMY.

It means something funny is going on in the upstream side (including the satelight box and shadow sensors)

OR the coil drivers side are going crazy ??

As Rana pointed out in his entry (#5025), the spectra of the shadow sensors on ETMY were quite bad below 3 Hz. The floor are higher than that of ETMX by factor of 10 or so.

To check if the noise comes from some of the electronics, we disconnected D15-sub from pd to whitening in.

The spectra with/without shadow sensors are attached below.

The curves in brown and green are the ones taken when the shadow sensors were disconnected from the WF board.

So these two curves represent the summed noise of the WF, AA and ADC.

This tells us to look toward the OSEM.

Today morning, I suspended ETMY and made the same checks dscribed below. The clamping went smoothly, 5 in. lb. of torque seems sufficient, in the limited observation time, there has been no evidence of wire sag. Today afternoon, we will go about putting the OSEM coils in, setting their equilibrium points etc. This may need to be re-done once the optic is in the chamber and the first contact has come off, but at least we can coarsely place them in the relative convenience of the cleanroom. 

GV EDIT 9.15pm 22 Aug: Eric had a look at both towers and pointed out that I had neglected to use washers on the wire stops. After consultation with Steve, I decided that it is not worth it to remove the clamp and re-suspend the optic - it is likely that the current suspension process will have caused new grooves in the suspension block, which will have to be removed, and the sanding process did not work so well last time. In any case, the net effect of this will be that the actual torque with which the clamp is tightened will be slightly different from 5 in. lb., but since there is no evidence that the clamp isn't tight enough / is too tight, I think it is okay to push ahead. 

Here is my analysis. I think there are still some problems with this suspension.

Attachment #1: Time domain plots of the ringdown. The LL coil has peak response ~half of the other face OSEMs. I checked that the signal isn't being railed, the lowest level is > 100 cts.

Attachment #2: Complex TF from UL to the other coils. While there are four peaks now, looking at the phase information, it isn't possible to clearly disentangle PIT or YAW motion - in fact, for all peaks, there are at least three face shadow sensors which report the same phase. The gains are also pretty poorly balanced - e.g. for the 0.77 Hz peak, the magnitude of UR->UL is ~0.3, while LR->UL is ~3. Is it reasonable that there is a factor of 10 imbalance?

Attachment #3: Nevertheless, I assumed the following mapping of the peaks (quoted f0 is from a lorentzian fit) and attempted to find the input matrix that best convers the Sensor basis into the Euler basis.

Unsurprisingly, the elements of this matrix are very different from unity (I have to fix the normalization of the rows).

Attachment #4: Pre and post diagonalization spectra. The null stream certainly looks cleaner, but then again, this is by design so I'm not sure if this matrix is useful to implement.

Next steps:

1. Repeat the actuator diagnonality test detailed here.
2. ???

In case anyone wants to repeat the analysis, the suspension was kicked at 1828 PDT today and this analysis uses 15000 seconds of data from then onwards.

​Update 18 May 3pm:  Attachment #5 better presentation of the data shown in Attachment #2, the remark about the odd phasing of the coils is more clearly seen in this zoomed in view.  Attachment #6 shows Lorentzian fits to the peaks - the Qs are comparable to that seen for the other optics, although the Q for the 0.77 Hz peak is rather low.

To investigate my mapping of the eigenfrequencies to eigenmodes, I checked the Oplev spectra for the last few hours, when the Oplev spot has been on the QPD (but the optic is undamped).

1. Based on Attachment #1, I can't figure out which peak corresponds to what motion.
   • The most prominent peak (judged by peak height) is at 0.771 Hz for both PITCH and YAW
   • Assuming the peak at 0.92 Hz is the other angular mode, the PIT/YAW decoupling is poor in both peaks, only ~factor of 2 in both cases.
2. Why are the POS and SIDE resonances sensed so asymmetrically in the PIT and YAW channels? There's a factor of 10 difference there...

So, while I conclude that my first-contact residue removal removed a constraint from the system (hence the pendulum dynamics are accurate and there are 6 eigenmodes), more thought is needed in judging what is the appropriate course of action.

I have connected ETMY sus electronics to megatron ADC/DAC.
We continue this state until 15:00 of today. (Restored 13:00)

0) Now the connection for the ETMY suspension was restored in a usual state. It damps well.

1) I thought it would be nice to have dataviewer and DTT working.
   So far, I could not figure out how to run daqd and tpman.
   - I tried to configure
    /cvs/cds/caltech/target/fb/daqdrc
    /cvs/cds/caltech/target/fb/master
    /cvs/cds/caltech/chans/daq/C1TST.ini (via daqconfig)

   - I also looked at
    /cvs/cds/caltech/targetgds/param/tpchn_C1.par
   but I don't understand how it works. The entries have dcuids of 13 and 14 although C1TST has dcuid of 10.
   The file is unmodified.

   I will try it later when I got a help of the experts.

2) Anyway, I went ahead. I tried to excite suspension by putting some offset.

It seems to have no DAC output. I checked the timing signal. It seems that looks wrong clock.

   I looked at DAC output by putting 5000,10000,15000,20000,25000cnt to UL/UR/LR/LL/SD coils.
   I could not find any voltage out of the DAC in any channels.

   Then, I checked the timing signal. This clock seems to have wrong frequency.
   What we are using now is a clock with +/-4V@4MHz. (Differential)
   Maybe 4194304Hz (=2^22Hz)?

   I went to 1Y3 and checked the timing signal for 16K. This was +/-4V@16kHz. (Diffrential)

   The possible solution would be
   - bring a function generator at the end and try to input a single end 4V clock.
   - stretch a cable from 1Y3 to 1Y9. (2pin lemo)

OK. Now, Timing/ADC/DAC are working. It's almost there.

1) As a temporaly clock, I put a function generator at the back side of the ETMY.
Set it to the rectangular +/-4V@16384Hz. Connect it to D060064 PCIX Timing Interface Board in the IO Chasis.
That is a line receiver to feed the TTL signal into ADCs/DACs.

I confirmed the actual sampling clock is supplied to the ADC/DAC boards by looking at the SMB output of the D060064.

2) Restarted the realtime code.

3) I looked at DAC output by putting 5000,10000,15000,20000,25000cnt to UL/UR/LR/LL/SD coils again.
Yes! I could see the DAC channels are putting DC voltages.

4) Then I connected DAC CH0 to ADC CH0 using SCSI breaking up boards.
Yes! I could see the coil output switching change the ADC counts!

Now, we are ready to see the suspension damped. Check it out.

Koji, Rana

The megatron DAC was temporaly connected to the suspension electronics for the DAC test. We went down to ETMY as we could not excite the mirror.

The DAC is putting correct voltages to the channels. However, the anti imaging filter test output does not show any signal.
This means something wrong is there in the DAC I/F box or the cables to the AI circuit. We will check those things tomorrow.

The ETMY was restored to the usual configuration.

It appears the front panel for the DAC board is mis-labeled.  Channels 1-8 are in fact 9-16, and 9-16 are the ones labeled 1-8.  We have put on new labels to reduce confusion in the future.

Hurraaaah!
We've got the damping of the suspension.
The Oplev loops has also worked!

The DAC channnel swapping was the last key!

DataViewer snapshot to show the damping against an artificial excitation was attached

/cvs/cds/caltech/target/fb/daqd -c daqdrc

This starts the FB.

Now the dataviewer and DTT work!

Attachment #1 shows the main result - there are 4 peaks. The frequencies are a little different from what I have on file for ETMY and the Qs are a factor of 3-4 lower (except SIDE) than what they are in vacuum, which is not unreasonable I hypothesize. The fits suggest that the peak shape isn't really Lorentzian, the true shape seems to have narrower tails than a Lorentzian, but around the actual peak, the fit is pretty good. More detailed diagnostic plots (e.g. coil-to-coil TFs) are in the compressed Attachment #2. The condition number of the matrix to diagonalize the sensing matrix (i.e. what we multiply the "naive" OSEM 2 Euler basis matrix by) is ~40, which is large, but I wouldn't read too much into it at this point.

I see no red flags here - the PIT peak is a little less prominent than the others, but looking back through the elog, this kind of variation in peak heights doesn't seem unreasonable to me. If anyone wants to look at the data, the suspension was kicked every ~1100seconds from 1288673974, 15 times.

For the input matrix diagonalization, it seemed to me that when we had a significant seismic event or a re-alignment of the optic with the bias sliders, the input matrix also changes.

Meaning that our half-light voltage may not correspond to the half point inside the LED beam, but that rather we may be putting the magnet into a partially occluding state. It would be good to check this out by moving the bias to another setting and doing the ringdown there.

The results from the ringdown are attached - in summary:

• The peak positions have shifted <50 mHz from their in-air locations, so that's good I guess
• The fitted Qs of the POS and SIDE eigenmodes are ~500, but those for PIT and YAW are only ~200
• The fitting might be sub-optimal due to spurious sideband lobes around the peaks themselves - I didn't go too deep into investigating this, especially since the damping seems to work okay for now
• There is up to a factor of 5 variation in the response at the eigenfrequencies in the various sensors - this seems rather large
• The condition number of the matrix that would diagonalize the sensing is a scarcely believable 240, but this is unsurprising given the large variation in the response in the different sensors. Unclear what the implications are - I'm not messing with the input matrix for now

I looked at the free-swinging sensor data from two nights ago, and am struggling with the interpretation. 

[Attachment #1] - Fine resolution spectral densities of the 5 shadow sensor signals (y-axis assumes 1ct ~1um). The puzzling feature is that there are only 3 resonant peaks visible around the 1 Hz region, whereas we would expect 4 (PIT, YAW, POS and SIDE). afaik, Lydia looked into the ETMY suspension diagonalization last, in 2016. Compared to her plots (which are in the Euler basis while mine are in the OSEM basis), the ~0.73 Hz peak is nowhere to be seen. I also think the frequency resolution (<1 mHz) is good enough to be able to resolve two closely spaced peaks, so it looks like due to some reason (mechanical or otherwise), there are only 3 independent modes being sensed around 1 Hz.

[Attachment #2] - Koji arrived and we looked at some transfer functions to see if we could make sense of all this. During this investigation, we also think that the UL coil actuator electronics chain has some problem. This test was done by driving the individual coils and looking for the 1/f^2 pendulum transfer function shape in the Oplev error signals. The ~ 4dB difference between UR/LL and LR is due to a gain imbalance in the coil output filter bank, once we have solved the other problems, we can reset the individual coil balancing using this measurement technique.

[Attachment #3] - Downsampled time-series of the data used to make Attachment #1. The ringdown looks pretty clean, I don't see any evidence of any stuck magnets looking at these signals. The X-axis is in kilo-seconds.

We found that the POS and SIDE local damping loops do not result in instability building up. So one option is to use only Oplevs for angular control, while using shadow-sensor damping for POS and SIDE.

I did some tests of the electronics chain today.

1. Drove a sine-wave using awggui to the UL-EXC channel, and monitored using an o'scope and a DB25 breakout board at J1 of the satellite box, with the flange cable disconnected - while driving 3000 cts amplitude signal, I saw a 2 Vpp signal on the scope, which is consistent with expectations.
2. Checked resistances of the pin pairs corresponding to the OSEMs at the flange end using a breakout board - all 5 pairs read out ~16-17 ohms.
3. Rana pointed out that the inductance is the unambiguous FoM here: all coils measured between 3.19 and 3.3 mH according to the LCR meter...

Hypothesising a bad connection between the sat box output J1 and the flange connection cable. Indeed, measuring the OSEM inductance from the DSUB end at the coil-driver board, the UL coil pins showed no inductance reading on the LCR meter, whereas the other 4 coils showed numbers between 3.2-3.3 mH. Suspecting the satellite box, I swapped it out for the spare (S/N 100). This seemed to do the trick, all 5 coil channels read out ~3.3 mH on the LCR meter when measured from the Coil driver board end. What's more, the damping behavior seemed more predictable - in fact, Rana found that all the loops were heavily overdamped. For our suspensions, I guess we want the damping to be critically damped - overdamping imparts excess displacement noise to the optic, while underdamping doesn't work either - in past elogs, I've seen a directive to aim for Q~5 for the pendulum resonances, so when someone does a systematic investigation of the suspensions, this will be something to look out for.. These flaky connectors are proving pretty troublesome, let's start testing out some prototype new Sat Boxes with a better connector solution - I think it's equally important to have a properly thought out monitoring connector scheme, so that we don't have to frequently plug-unplug connectors in the main electronics chain, which may lead to wear and tear.

The input and output matrices were reset to their "naive" values - unfortunately, two eigenmodes still seem to be degenerate to within 1 mHz, as can be seen from the below spectra (Attachment #1). Next step is to identify which modes these peaks actually correspond to, but if I can lock the arm cavities in a stable way and run the dither alignment, I may prioritize measurement of the loss. At least all the coils show the expected 1/f**2 response at the Oplev error point now. The coil output filter gains varied by ~ factor of 2 among the 4 coils, but after balancing the gains, they show identical responses in the Oplev - Attachment #2.

I've installed Guralp readout box back and it turned out that it does not work with voltage provided from the rack (+13.76 0 -14.94).  +/-12 voltage regulators inside the box convert it to -0.9 0 -12. I've connected the box to +/-15 DC voltage supply to measure seismic motion at the ETMY table. Readout box works fine with +/- 15.

Seismic noise on the ETMY table measured to be a few times higher then on the floor in horizontal direction in the frequency range 50 - 200 Hz. Attached are compared spectrums of X, Y and Z motions.

Accelerometers were installed on the ETMY table and nearby ground to measure amplification of the seismic noise due to the table. During this experiment ground and table motions were measured simultaneously.

 I've added xml file with measurement settings and data to 40m svn at directory 40m_seismic/etmy.

 The ETMY optical table 4' x 2' with all optical components was placed on two carts and rolled out of the end.  The 4' x 3' x 4" TMC 78-235-02R breadboard was placed into position and marked for anchoring bolt locations.

It will be drilled, tapped, shimmed, leveled and bolted it into final position tomorrow morning.  I'm planning to bring  the acrylic enclosure to the east end tomorrow afternoon.

 The new table is tapped and leveled, but not ready for final anchoring. The existing 8 mm shims on the top of the support legs will be moved to the bottom of the legs, where more torque is available on the 1/2" bolts

The coated tin film sheets are being installed at the shop. The table and the enclosure will be ready on Friday.

Steve's suggestion for how to level the end table using "swivel leveling mounts":

 The cables are Not disconnected. I have people coming to do the lifting at 9:45am  Email is not working. This will cause a delay. I need conformation that we still moving on with the swap

Yes! We are swapping.

I'll be there very soon!

I put the ETMY trans QPD in. 

The ETMY trans beam was already going toward the TRY DC PD, and a CCD camera.  I put a beam splitter in that beam (reducing the power to TRY and the CCD by 50%), and sent my picked-off beam to the ETMY QPD.  Since there is a lens in this path to focus the beam onto TRY and the camera, I put the QPD ~the same distance from the lens as the camera.  Due to space requirements (because of all the green stuff on the table now), I had to put a Y1 turning mirror between the beam splitter and the QPD.  The beam is aligned onto the PD, although the signal isn't super strong.  When the PD is blocked, the sum is ~(-92 counts).  When the beam is on the PD, the sum is ~(-78 counts). 

It seemed that the ETMY watchdog tripped early Sunday morning.
The reason is not known. I just looked at ETMX, but it seemed fine.

I called the control room just in case someone is working on the IFO.
Also I did not see any elog entry to indicate on going work there.

So, I decided to reset the watchdog for ETMY. And it is working fine again.

I checked the state of the whitening filters for the ETMY shadow sensors.

Result : They've been OFF  (i.e. flat response).

(measurement and setup)

 I measured the transfer functions of the whitening board (D000210) by looking at the signal before and after the whitening stage.

 The whitening board handles five signals; UL, UR, LR, LL and SD, and there are five single-pin lemo outputs for each signal on the front panel.

A good thing on those lemo monitors is that their signals are monitored before the whitening stages.

Rana suggested me to use these signals for the denominator of the transfer functions and consider the sensor signals as excitation signals.

So I plugged those signals into extra ADC channels via an AA-board and measured the transfer functions.

In the measurement the coherence above 4 Hz was quite small while the suspension was freely swinging.

Therefore I had to excite the ETMY suspension by putting random noise in a frequency band from 5 Hz to 35 Hz to obtain better coherence.

(results)

 The response is flat over frequency range from ~ 0.2 Hz to ~40 Hz, see the plot below. 

According to the spectrum of each signal the measurements above 10 Hz are just disturbed by the ADC noise.

If the whitening filters are ON, a pole and zero are expected to appear at 30 Hz and 3 Hz respectively according to the schematic, but no such features.

I acquired a second full pair of Host interface board cards (one for the computer and one for the chassis) from Rolf (again, 2nd generation - the bad kind).

However, they exhibited the same symptoms as the first one that I was given. 

Rolf gave a few more suggestions on getting it to work.  Pull the power plugs.  If its got slow flashing green lights, just soft cycle, don't power cycle.  Alex suggested turning the IO chassis on before the computer.

None of it seemed to help in getting the computer talking to the IO chassis.

I finally decided to simply take the ETMX IO chassis and place it at the Y end.  So for the moment, ETMY is working, while ETMX is temporarily out of commission. 

We also made the necessary cables (2x 37 d-sub female to 40 pin female and 40 pin female to 40 pin female) .  Kiwamu also did nice work on creating a DAC adapter box, since Jay had given me a spare board, but nothing to put it in.

Attachment #1 shows that the ITMX, ETMY and beamsplitter Oplev light levels have decayed significantly from their values when installed. In particular, the ETMY and ITMX sum channels are now only 50% of the values when a new HeNe was installed. ELOG search revealed that ITMY and ETMX HeNes were replaced with newly acquired units in March and September of last year respectively. The ITMX oplev was also replaced in March 2019, but the replacement was a unit that was being used to illuminate our tourist attraction glass fiber at EX.

We should replace these before any vent as they are a useful diagnostic for the DC alignement reference.

Adjusted focus on ETMYF camera so that the IR beam is in focus.

The camera that Yuta means in his elog from last night/this morning is the scattering camera at the Yend.  The reason (I think) that he had to do this is that Manasa and Jan took the cable for the ETMYT camera, and were using it for their scattering camera.  They mention in elog 8072 that they installed a camera, but they didn't say anything about having taken the ETMYT cable.  This is the kind of thing that is useful to elog!

Anyhow, I have removed the Watec that belongs with the scattering setup, that Yuta borrowed, and put it back on the scattering table-on-a-pedestal. I then realigned the usual ETMYT camera (that Yuta moved out of the way to install the borrowed Watec), and put the ETMYT cable back to its usual place, connected to the Sony camera's box on the floor.

tl;dr: ETMYT camera is back to original state.

EDIT later:  I put the Watec back, since it is more sensitive to IR, so now we have a Watec in the regular ETMYT place.

I've applied first contact to both the ETMs. They're now ready to be suspended. I've also cut up some lengths of the new wire and put them in the oven for a 12 hour 70C bake. 

• For both ETMs, I first applied first contact to the bulk of the HR and AR surfaces (all the way out to the edge for the HR, for the AR as large an area as possible without getting too close to the magnets). Calum recommended pouring first contact onto the horizontal optic, but since I had no practise with this method, I opted not to try it out for the first time on our ETMs
• After allowing this to dry for 24 hours, I peeled this layer off. Visual inspection suggests that the whole film came off cleanly. 
• I then applied first contact to a smaller area around the center of the optic for only the HR surface. This will only be peeled off once the suspended optic is back in the vacuum chamber. This way, we keep the HR face protected for as long as possible.
• Even though we applied F.C to both faces of the ITMs, I don't think its so important to keep a film on the AR side of the ETMs till we take it in. So I didnt re-coat the AR side with a smaller area of F.C. This way, if we want, we can do the OSEM assembly in the cleanroom without having to worry about peeling the F.C off with limited access to the rear of the optic.
• I also opted to bake some lengths of the newly arrived steel wire for suspension. Not sure how important/useful this bake will be.

Unless we want the AR surface to also have a small F.C coat until the optic is in the vacuum chamber, I think I will proceed with re-suspending the ETMs..

David and I were thinking about changing the non-polarizing beam splitter in the EUCLID setup from 50/50 to 33/66 (ref picture).  It serves as a) a pickoff to sample the input power and b) a splitter to send the returning beam to a photodetector 2 (it then hits a polarizer and half of this is lost.  By changing the reflectivity to 66% then less (1/3 instead of 1/2) of the power coming into it would be "lost" at the ref photodetector 1, and on the return trip less would be lost at the polarizer (1/6 instead of 1/4).

[steve, gautam]

I took this opportunity of EX downtime to change the supply voltage for the AA unit (4-pin LEMO front panel) in 1X9 from +/-5V to +/-15V. Inside the AA board are INA134 and DRV135 ICs, which are rated to work at +/-18V. In the previous state, the inputs would saturate if driven with a 2.5Vpp sine wave from a DS345 func. gen. After the change, I was able to drive the full range of the DS345 (10Vpp), and there was no saturation seen. This AA chassis is only used for the OSEM signals and also some ALS signals. Shadow sensor levels and spectra are consistent before and after the change. The main motivation was to not saturate the Green PDH Reflection signal in the digital readout. The steps we took were:

1. Confirm (by disconnecting the power cable at the back of the AA box) that the power supplied was indeed +/- 5 V.
2. Remove DIN fuse blocks from DIN rail for the relevant blocks.
3. Identify a +15 V, -15 V and GND spot to plug the wires in. 
4. Effect the swap.
5. Re-insert fuses, checked supply voltage at connector end of the cable was now +/- 15 V as expected.
6. Re-connect power cable to AA box.

Steve and I restored the power to the EX AUX electronics rack. The power strip on the lowest shelf of the AUX rack now goes to another power strip laid out vertically along the NW corner of 1X9. The EX green locks to the arm just fine now.

I worked on characterizing the green PDH setup at EX, as part of the ALS noise budgeting process. Summary of my findings:

1. Green doubling efficiency is ~ 1.5 %/W (3mW of green for 450mW of IR). This is ~half of what was measured on the PSL table. There are probably large errors associated with power measurement with the Ophir power meter, but still, seems like a big mismatch.
2. The green REFL photodiode is a Thorlabs PDA36A. 
   • It is being run on 30 dB gain setting, corresponding to a transimpedance of 47.5 kohm into high impedance loads. However, the PD bandwidth for this gain setting is 260 kHz according to the manual, whereas the PDH modulation sidebands on the green light are at twice the modulation frequency, i.e. ~560 kHz, so this is not ideal.
   • There was ~250 uW of green light incident on this photodiode, as measured with the Ophir power meter.
   • The DC voltage level was measured to be ~2.7 V on a scope (High-Z), which works out to ~280 uW of power, so the measurements are consistent.
   • When the cavity is locked, there is about 25% of this light incident on the PD, giving a shot noise level of ~25 nV/rtHz. The dark noise level is a little higher, at 40nV/rtHz.
   • Beam centering on the PD looked pretty good to the eye (it is a large-ish active area, ~3mmx3mm).
   • The beam does not look Gaussian at all - there are some kind of fringes visible in the vertical direction in a kind of halo around the main cavity reflection. Not sure what the noise implications of this are. I tried to capture this in a photo, see Attachment #1. Should an Iris/aperture be used to cut out some of this junk light before the reflection photodiode?
3. The in-going beam was getting clipped on the Faraday Isolator aperture (it was low in pitch).
   • I fixed this by adjusting the upstream steering, and then moving the two PZT mounted green steering mirrors to recover good alignment to the X arm cavity.
   • GTRX level of ~0.5 was recovered.
4. To estimate the mode-matching of the input beam to the cavity axis, I looked at the reflected light with the cavity locked, and with just the prompt reflection from the ETM:
   • DC light level on the reflection photodiode was monitored using the High-Z input o'scope.
   • Measured numbers are Plocked ~ 660 mV, Pmisaligned ~ 2.6V, giving a ratio of 0.253.
   • While locked, there was a ~ 10 Hz periodic variation in the DC light level on the green REFL photodiode - not sure what was causing this modulation.
   • However, this is inconsistent with a calculation, see Attachment #2. I assumed modulation depth of 90 mrad, round-trip loss of 100 ppm, and Titm = 1.094%, Tetm = 4.579%, numbers I pulled from the core-optics wiki page.
   • Not sure what effect I've missed out on here - to get the model to match the measurement, I have to either assume a higher cavity finesse, or a much higher round-trip loss (5000ppm), both of which seem implausible.

The main motivation was to get the residual frequency noise of the EX laser when locked to the X arm cavity - but I'll need the V/Hz PDH discriminant to convert the in-loop error signal to frequency units. The idea was to look at the PDH error signal on a scope and match up the horn-to-horn voltage with a model to back out said discriminant, but I'll have to double check my model for errors now given the large mismatch I observe in reflected power.

I decided to use the more direct method, of disconnecting feedback to the EX laser PZT, and then looking at the cavity flashes. 

Attachment #1 shows the cavity swinging through two resonances (data collected via oscilloscope). Traces are for the demodulated PDH error signal (top) and the direct photodiode signal (bottom). The traces don't look very clean - I wonder if some saturation / slew rate effects are at play, because we are operating the PD in the 30 dB setting, where the bandwidth of the PD is spec-ed as 260 kHz, whereas the dominant frequency component of the light on the PD is 430 kHz.

The asymmetric horns corresponding to the sideband resonances were also puzzling. Doing the modeling, Attachment #2, I think this is due to the fact that the demodulation phase is poorly set. The PDH modulation frequency is only ~5x the cavity linewidth, so both the real and imaginary parts of the cavity reflectivity contribute to the error signal. If this calculation is correct, we can benefit (i.e. get a larger PDH discriminant) by changing the demod phase by 60 degrees. However, for 230 kHz, it is impractical to do this by just increasing cable length between the function generator and mixer.

Anyway, assuming that we are at the phi=30 degree situation (since the measurement shows all 3 horns going through roughly the same voltage swing), the PDH discriminant is ~40 uV/Hz. In lock, I estimate that there is ~60 uW of light incident on the PDH reflection photodiode. Using the PD response of 0.2 A/W, transimpedance of 47.5 kohm, and mixer conversion loss of 6dB, the shot-noise limited sensitivity is 0.5 mHz/rtHz. The photodiode dark noise contribution is a little lower - estimated to be 0.2 mHz/rtHz. The loop does not have enough gain to reach these levels.

Some time ago, I had done an actuator calibration of ITMX. This suspension hasn't been victim to the recent spate of suspension problems, so I can believe that the results of those measurement are still valid. So I decided to calibrate the in-loop error signal of the EX green PDH loop, which is recorded via an SR560, G=10, by driving a line in ITMY position (thereby modulating the X arm cavity length) while the EX green frequency was locked to the arm cavity length. Knowing the amount I'm modulating the cavity length by (500 cts amplitude sine wave at 33.14159 Hz using awggui, translating to ~17.2 kHz amplitude in green frequency), I demodulated the response in C1:ALS-X_ERR_MON_OUT_DQ channel. At this frequency of ~33 Hz, the servo gain should be large, and so the green laser frequency should track the cavity length nearly perfectly (with transfer function 1/(1+L), where L is the OLG).

The response had amplitude 5.68 +/- 0.10 cts, see Attachment #1. There was a sneaky gain of 0.86 in the filter module, which I saw no reason to keep at this strange value, and so updated to 1, correcting the demodulated response to 6.6 cts. After accounting for this adjustment, the x10 gain of the SR560, and the loop suppression, I put a "cts2Hz" filter in (Attachment #2). I had to guess a value for the OLG at 33 Hz in order to account for the in-loop suppression. So I measured the OLTF using the usual IN1/IN2 method (Attachment #3), and then used a LISO model of the electronics, along with guesses of the cavity pole (18.5 kHz), low-pass filter poles (4x real poles at 70 kHz), PZT actuator gain (1.7 MHz/V) and PDH discriminant (40 uV/Hz, see this elog) to construct a model OLTF. Then I fudged the overall gain to get the model to line up with the measurement between 1-10kHz. Per this model, I should have ~75dB of gain at ~33Hz, so the tracking error to my cavity length modulation should be ~3.05 Hz. Lines up pretty well with the measured value of 4.7 Hz considering the number of guessed parameters. The measured OLG tapers off towards low frequency probably because the increased loop suppression drives one of the measured inputs on the SR785 into the instrument noise floor.

The final calibration number is 7.1 Hz/ct, though the error on this number is large ~30%. Note that these "Hertz" are green frequency changes - so the change to the IR frequency will be half.

Attachment #4 shows the error signal in various conditions, labelled in the legend. Interpretations to follow.