Formatted and re-installing OS on rossa for the 3rd or 4th time this year. I suggest that whoever is installing software and adjusting video settings please stop.

If you feel you need to tinker deeply, use ottavia or zita and then be prepared to show up and fix it.

While I was moving the UPS around, the network lights went out for Rossa, so I may have damaged the network interface or cable. Debugging continues.

Summary:

I was able to lock the FPMI. The lock was quite stable. However, the fluctuations in the ASDC power suggest that it will be difficult to make a DC measurement of the contrast defect in this configuration. This problem can be circumvented in part by some electronics tuning. However, the alignment jitter couples some HOM light which is an independent effect. Can this be a good testbed for the proposed AS WFS system? 

Details:

• First, the arm cavities were locked and TRX/TRY were maximized using ASS.
• Next, AS55_Q-->MICH_A (MICH-->BS) matrix element was set to 1 in the LSC input (output) matrix. The trigger was set to always on.
• AS55 digital demod phase was -37 degrees.
• I was then able to increase the gain on the MICH servo and turn on some integrators without any problem.
• Some guesswork had to be done to get the correct sign. Final servo gain used was -0.8. 

I didn't do any serious budgeting yet - need to think about / do some modeling on how this configuration can be made useful.

Today, I found out that this type of "0x2bad" DC error is connected to the 1e+20 cts output. The solution was to bite the bullet and stop/start the c1oaf model (at the risk of crashing the vertex FEs). Today, I was lucky and the model came back online with all CDS indicators green. At which point I was able to engage length feedforward to MC2 (with some admittedly old filter). Some subtraction is happening, see Attachment #1. This was just meant to test whether the signal routing is happening - the feedforward signal goes to the "ALTPOS" input of the suspension CDS block, which AFAIK does not have a corresponding MEDM EPICS indicator. So I couldn't figure out whether the feedforward control signal was in fact making it to the suspension. On the evidence of the suppression of MCL in the 1-3 Hz band, I would conclude that it is. Useful to be able to engage these FF filters for better lockability.

wonder if its possible to do variable finesse locking

Gabriele mentioned that Virgo used arm trans PDH for this, but I guess we could possibly use POX/POY to start and bring in the PRM with 50% MICH trans

I set up the spectrum analyzer to make the WFS head RF transfer function measurement (V/W) on WFS1. I placed the Jenne laser on the AP table, along with the reference PD power supply, laptop, and laser power supply. The Agilent output AM modulates the laser; the reference PD is again NewFocus 1611, with its AC output sent to Agilent's R channel and DC output sent to an oscilloscope;

I closed the PSL shutter while checking for a location to place the breadboard, and opened it while writing this. Headed back to Cryo to pick up the large incandescent bulb we'd borrowed over the summer.

I propose the following re-organization of the PDFR measurement breadboard. We have all the parts on hand, just needs ~30mins of setup work and some characterization afterwards. The fiber beamsplitter will not be PM, but for this measurement, I don't think that matters (the patch fiber from the diode laser head isn't PM anyways). We have one spare 1 GHz BW NF1611 that is fiber coupled (used to live on the ITMY in-air table, and is (conveniently) labelled "REF DET", but I'm not sure what the function of this was). In any case, we have at least 1 free-space NF1611 photodiode available as well. I suggest confirming that the FC version works as expected by calibrating against the free space PD first.

Update 245pm: Implemented, see Attachment #2. Aaron is testing it now, and will post the characterization results.

I'm curious to see if we really need the 1611, or if we can calibrate the diode laser vs. the 1611 one time and then just use that calibration to get the absolute cal for the DUT.

I'm afraid that the RF modualtion of the laser is nonlinear and the electrical and optical resoponse is dependent on the LD pumping current and RF input power. So I feel safe if we keep the reference PD. Of course, this is my feeling and it should be quantitatively tested.

Summary:

There is an imbalance between the POX and POY detector outputs reported in the CDS system. Possibilities are (i) the POX PD has a uncoated glass window whereas POY does not or (ii) there is some problem in the elctronics.

Details:

1. Nominally, we run the POX/POY locking with +18dB whitening gain on POY and +30 dB on POX. This is a factor of 4 difference.
2. The DC levels reported in C1:LSC-POXDC_OUT and C1:LSC-POYDC_OUT differ by a factor of 10 (24 cts for POY vs 2.4 cts for POX with 0dB whitening gain). These channels come from the P2 connector on the back of the PD Interface board into the fast CDS system.
3. The levels reported by the Acromag system (which come out of the P1 connector) are 60mV for POY  vs 15 mV for POX.
4. I confirmed that this imbalance is not due to clipping on the POX photodiode - I tweaked the steering mirror and observed the plateau (I did not, however, look at the beam on the PD active area with an IR viewew which would be a more conclusive test).
5. I measured the power incident on either PD (using Ophir power meter, filter OFF). They were both ~10uW, as expected since the beam extraction for POY and POX are identical - a single HR mirror and the vacuum viewport.

Update 820pm: 

1. I checked that there is no glass window on the PD.
2. It is hard to see the beam on a viewer - but with the PRM aligned, I think I convinced myself that the beam is pretty well centered on the PD. 

So increasingly, it looks like the electronics are the source of the problem.

I measured the RF response of the fiber-coupled NewFocus 1611, calibrating out the cable delay. The laser current was set to 20.0 mA, and the RF power going into the splitter was -10 dBm. The DC voltage was 1.87 V, and Gautam and I measured the power from the fiber at 344uW.

Something still looks very wrong -- the PD is supposed to be flat out to 1GHz, and physical units pending, need food.

Got the network to work again just by unplugging the power cord and letting it sit for awhile. But corrupted OS by trying to install Nvidia drivers.

https://www.advancedclustering.com/act_kb/installing-nvidia-drivers-rhel-centos-7/

The 1GHz PD has a bit more flat response, but the laser and the driving network have more frequency dependence as you saw.

I think the metric of interest here is the consistency of the AC transimpedance of the proposed new "Reference PD" (= fiber coupled NF1611) vs the old reference (free space NF1611), since everything will be calibrated against that.

Summary:

I managed to achieve a few transitions of control of the XARM length using the ALS error signal. The lock is sort of stable, but there are frequent "glitches" in the TRX level. Needs more noise hunting, but if the YARM ALS is also "good enough", I think we'd be well placed to try PRMI/DRMI locking with the arms held off resonance (while variable finesse remains an alternative).

Details:

Attachment #1: One example of a lock stretch. 

Attachment #2: ASD of the frequency noise witnessed by POX with the arm controlled by ALS. The observed RMS of ~30pm is ~3-4 times higher than the best performance I have seen, which makes me question if the calibration is off. To be checked...

The AA filter for ASDC was fixed.

== Test Status ==

[done] Whitening gain switching test
[done] AA enable/disable switching
[0th order] LO Det Mon channel check
[none] PD I/F board check
[done] QPD I/F board check
[none] CM Board
[none] ALS I/F board

The AA filter for the 4th section of the LSC analog electronics bank (D000076) was pulled out for the test. On the workbench, questionable CH8 was checked. It tuned out that the filter amplifier module for the 8th-order elliptic filter at 7.5kHz was not properly working and exhibited unusual attenuation. This filter module (Frequency Devices Inc D68L8E-7.50kHz) was desoldered and replaced with a module from a spare board. Note that Gautam and I had tried to use this spare board instead of the current one, but it didn't give us any signal for an unknown reason. Since the desoldering required a lot of force and had a risk of damaging the PCB, a socket was made from an IC socket (see Attached 1). This change made CH8 functioning equally to the other channels do.

I took this opportunity to ckech the performance of the AA filters. For each channel, the input signal was injected from J3 using a pomona clip. The output was taken from pin 1, 5, 9, ... of J2. This is the + side of the differential output. The - side just has the equivalent performance but the signal polarity. The digital signals for the AA bypass switches were not connected. Fortunately, this was just fine as it made the anti-aliasing filters engaged.

Attachment 2 shows the transfer functions of all the channels. All the channels showed an identical response (at least visually). The transfer function for CH1 was fitted by LISO. The ZPK values are listed here:

pole 5.2860544577k 503.1473053928m
pole 5.9752193716k 1.0543411596
pole 8.9271953580k 3.5384364788
pole 8.2181747850k 3.4220607928
pole 182.1403534923k 1.1187869426 # This has almost no effect
zero 13.5305051680k 423.6130434049M
zero 15.5318357741k 747.6895990654k
zero 23.1746351749k 1.5412966100M

factor 989.1003181564m
delay 24.4846075283n

Attachment 3 shows the ASD of the output voltage noise measurement. Note the input was shorted for this measurement. The nominal output voltage was found to be 0.1 uV/rtHz and the 1/f noise corner freq was about 100Hz. Only CH3 showed a deviation from the typical values. It looks like this is neither an artifact nor transient noise. Fortunately, nothing is connected to this channel right now.

Summary:

The fiber-coupled PD seems to have a factor of ~1.5 difference in responsivity compared to the free-space PD. There are some differences in the two ways I made the measurement that I don't yet understand.

Details

I measured relative responsivities of the fiber and free coupled NewFocus 1611 PDs (scaled by the Jenne AM transfer function).

I made the measurement in two ways, see attachment three. In attachment one, I show the response for separately measuring the two PDs relative to a pickoff of the source (two-port thru calibration). In attachment two I measure the relative responses directly, without picking off a reference (three-port calibration). I scaled the transfer functions by their DC voltages; both PDs have transimpedances of 700 V/A.

However, there are some clear differences in the response (overall factor of 0.5dB offset that may be explained by a miscalibrated DC level; apparent periodicity in attachment 1) that I don't yet understand.The free path of the non-fiber PD is ~5-6 inches, which accounts for the ~45 degrees of phase advance of the fiber relative to free coupled PD signal. (12.7cm / (c / 300 MHz) * 360 degrees ~ 45 degrees)

[KA, GV]

This elog is meant to be a summary of some of the many subtleties on the CM board. The latest schematic of the version used at the 40m can be found at D1500308 .

Latch logic:

• There are several Binary Outputs and one Binary Input to the CM board.
• The outputs control ENABLE/DISABLE switches and gains of amplifier stages, while the input reports whenever the limiter has been reached.
• The variable gain feature is implemented by enabling/bypassing several cascaded fixed gain stages. So in order to change the gain of a single composite amplifier stage, multiple individual amplifier stages have to be switched.
• This is implemented by the user interacting with the hardware via a "control word", consisting of a number of bits depending on the number of cascaded stages that have to be switched. 
• This control word is sent to the device via modbus EPICS, which is an asynchronous communication protocol. Hence, it may be that the individual bits composing the control word get switched asynchronously. This would be disastrous, as there can be transient glitches in the gain of the stage being controlled. 
• To protect against such problems, there is a latch IC in the hardware between the Binary Inputs to the board (= Binary Outputs from Acromags), and the actual switches (= MAX333) that enable/bypass the cascaded gain stages. The latch IC used is a SN74ALS573. This device acts as a bus, which transmits/blocks changes for multiple bits (= our control word) from propagating, depending on the state of a single bit (= the LATCH ENABLE bit). Thus, by controlling a single bit, we can guarantee that multiple bits get switched synchronously. 
• In order to use this latch capability, we need some software logic that sets/disables the LATCH ENABLE bit. For our system, this logic is implemented in the form of a continuously running python 🐍 script, located at /cvs/cds/caltech/target/c1iscaux/latch.py. It is implemented as a systemctl service on the c1iscaux Supermicro. The logic implemented in this script is shown in Attachment #1. While the channels referred to in that attachment are for REFL1_GAIN, the same logic is implemented for REFL2_GAIN, AO_GAIN, and the SuperBoosts.
• Some FAQ:
  1. Q: Why do we need the soft channels C1:LSC-REFL1_SET_LSB and C1:LSC-REFL1_SET_MSB?
     A: These soft channels are what is physically linked to the Acromag Binary Outputs. In order for our latch logic to be effective, we need to detect when the user asks for a change, and then disable the LATCH ENABLE bit (which is on by default, see FAQ #3) before changing the physical acromag channels. The soft channels form the protective layer between the user and the hardware, allowing latch.py to function.
  2. Q: Why is there an "_MSB" and "_LSB" soft channel? 
     A: This has to do with the mbboDirect EPICS channel type, which is used to control the multiple bits in our control word using a single input (= an MEDM gain slider). The mbboDirect data-type requires the bits it controls to have consecutive hardware addresses. However, the Acromag hardware addressing scheme is not always compatible with this requirement (see pg 33 of the manual for why this is the case). Hence, we have to artifically break up the control word into two separate control words compatible with the Acromag addressing scheme. This functionality is implemented in latch.py.
  3. Q: Why is the default state of LATCH ENABLE set to ON? 
     A: This has to do with the fact that all Binary Inputs, not just the multi-bit ones, to the CM board are propagated to the control hardware via a latch IC. For the single-bit channels, there is no requirement that the switching be synchronous. Hence, rather than setting up ~10 more single-bit soft channels and detecting changes before propagating them, we decided to leave the LATCH ENABLE ON by default, and only disable it when changing the multi-bit gain channels. This is the same way the logic was implemented in the VME state code, and we think that there are no logic reasons why it would fail. But if someone comes up with something, we can change the logic.

Acromag BIO testing:

During my bench testing of the Acromag chassis, I had not yet figured out mbboDirect and the latch logic, so I did not fully verify the channel mapping (= wiring inside the Acromag box), and whether the sitching behavior was consistent with what we expect. Koji and I verified (using the LED tester breakout board) that all the channels have the expected behavior 👏. Note that this is only a certification at the front-panel DB37 connectors of the Acromag chassis  testing of the integrated electronics chain including the CM board is in progress...

[Gautam, Koji]

Input gain part of the CM servo board D1500308 was tested. A couple of problems were detected. One still remains.

== Test Status ==

[done] Whitening gain switching test
[done] AA enable/disable switching
[0th order] LO Det Mon channel check
[none] PD I/F board check
[done] QPD I/F board check
[in progress] CM Board
[none] ALS I/F board

We started to test the CM Servo board from the input stages. Initially, DC offsets were provided to IN1 and IN2 to check the gain on the oscilloscope or a StripTool plot. However, the results were confusing, AC measurements with SR785 was carried out in the end. It turned out that both IN1 and IN2 had some issues. IN1 showed an increment of the gain by 2dB every two gain steps, having suggested that the 1dB gain stage had a problem. IN2 showed sudden drop of the signal at the gain +8~+15dB and +24~+31dB, having suggested that a particular 8dB stage had a problem. The board was exposed with the extender and started tracing the signals.

CH1: The digital signal to switch the 1dB stage reached Pin 1A of the DIN96 connector. However, the latch logic (U47 74ALS573) does not spit out the corresponding level for this bit. Note that the next bit was properly working. We concluded that this 74ALS573 had failed and need to be replaced. We have no spare of this wide SOIC-20 chip, but Downs seems to have some spares (see Todd's spare parts list). We will try to get the chip on Monday.

CH2: The stage only used between +8dB and +15dB and between +24dB and +31dB is the +8dB stage (U9 and U2A). I found that the amped output signal did not reach the FET switch U2A (MAX333A). Therefore it was concluded that the opamp U9 (AD829) has an issue. In fact, the amp itself was working, but the output pin was not properly soldered to the pad.  Resoldering this chip made the issue gone. Note that this particular channel has some OP27s soldered instead of AD829. Gautam mentioned that there was some action on the board a few years back to deal with the offset issue. Next time when the board is polled out, I'll take the photos of the board.

Using SR785, the swept sine measurements between 100 and 100kHz were taken for all the gain settings for each channel. Between -31dB and -11dB, the input signal amplitude of 300mV was used. Between -10dB and +10dB, it was reduced to 100mV. For the rest, the amplitude was 10mV. Note that the data for +11dB for CH1 and +2dB for CH2 are missing presumably due to a data transfer issue.

The results are shown in Attachments 1~4.

Attachments 1 and 3 show the gain at each slider value. The measured gain was represented by the average between 1kHz and 10kHz. The missing 1dB every two slide values are seen for CH1. The phase delay at 100kHz is show in the lower plot. There is some delay and delay variation seen but it is in fact less than 1deg at 10kHz (see later) so it's effectfor CM servo (IMC AO path) is minimum. The gain for CH2 tracks the slider value nicely. The phase delay is larger than that of CH1, as expected because of OP27.

Attachments 2 and 4 show the transfer functions. The slider value was subtracted from the measured gain magnitude to indicate the deviation between them. The missing 1dB is obviously visible for CH1 in addition to the overall gain offset of ~0.2dB. CH2 also shows the gain offset of 0.1dB~0.2dB. The phase delay comes into the play around 20kHz particularly at higher gains where the UGF of the AO path is.

gautam: Here is the elog thread for IN2 opAmps going AD829-->OP27. Also, I guess Attachment #1 and #3 x-axes should be "Gain [dB]" rather than "Frequency [Hz]".

Summary:

I improved the alignment of the green beam into the Y arm cavity.

• GTRY went from ~0.2 to ~0.25, see Attachment #1.
• This resulted in improvement of the Y arm ALS noise above 💯Hz by a factor of ~5, see Attachment #2.
• I tried controlling the two arm cavities in the CARM/DARM basis using ALS error signals - but didn't manage to successfully execute this transition today - this will be the commissioning goal for the upcoming week.

Details:

• I had to do the alignment by tweaking the steering mirrors at EY - the PZTs didn't give me anywhere near enough range.
• While I was at EY, I tried moving the two MM lenses mounted on translation stages to try and improve the mode-matching into the arm cavity - wasn't successful, still see a bunch of bullseye modes when I toggle the shutter.
• They EY green layout would benefit from a do-over (basically just copy the EX layout), but this isn't the priority right now, the ALS noise RMS is dominated by low frequency noise (as usual). 
• There is a ~5% leakage of the GTRX beam onto the GTRY photodiode.
• One thing to try would be to revive the MCL loop to reduce the <1 Hz laser frequency noise and see if that helps - basically testing this hypothesis.
• I had done some careful noise-budgeting of the EX green PDH system, the EY system would benefit from the same, but not critical.
• The improvement of the high-frequency noise is clear, and now we are consistent with the "known good reference" level from the time the DRFPMI locking was working back in early 2016.

Other changes made today:

1. /opt/rtcds/caltech/c1/scripts/general/videoscripts/videoswitch was modified to be python3 compatible - for some reason, there were many syntax errors being thrown (even though I was using python2.7) and I wasn't able to change the displays in the VEA using the MEDM screen, but now it works again 👍.
2. The LSC overview and several daughter MEDM screens were edited to remove references to channels that no longer exist. All screens I edited have a backup stored in the MEDM directory with today's date as a suffix.
3. Input pointing into the PMC was tweaked.
4. Noted that some pump is noisy at pumpspool - also noted that the annuli are no longer pumped. Some event seems to have triggered an interlock condition that closed off the annular volume from TP3, needs investigation...

Summary:

I managed to execute the first few transitions of locking the arm lengths to the laser frequency in the CARM/DARM basis using the IR ALS system 🎉 🎊 . The performance is not quite optimized yet, but at the very least, we are back where we were in the green days.

Details:

1. Locking laser frequency to Y arm cavity length using MC2 as a frequency actuator
   • This is the usual diagnostic done to check the single-arm ALS noise using POY as an out of loop sensor.
   • The procedure is now scripted - I had to guess the sign and optimize the gains a few times, but this works deterministically now. 
   • Script lives at /opt/rtcds/caltech/c1/scripts/YARM/Lock_ALS_YARM.py.
   • Attachment #1 shows the result. If we believe the POY sensor calibration, the RMS displacement noise is ~6 pm
2. Encouraged by the good performance of the Y arm, I decided to try the overall transition from the POX/POY basis to the CARM/DARM basis using ALS error signals.
   • The procedure starts with the arm cavities locked with POX/POY, and the respective green frequencies locked to the arm cavity length by the end PDH servos.
   • The DFD outputs serve as the ALS error signals - the PSL frequency is adjusted to the average value of DFD_X_OUT and DFD_Y_OUT.
   • I changed the LSC output matrix element for DARM-->ETMX from -1 to -5, to make it symmetric in actuation force w.r.t. ETMY (since the series resistane on ETMX is x5 that on ETMY).
   • After some guesswork, I fould the right signs for the gains. After enabling the boosts etc, I was able to keep both arms (approximately) on resonance for several minutes. See Attachment #2 for the time series of the transition process - the whole thing takes ~ 1 minute. 
   • A script to automate this procedure lives at /opt/rtcds/caltech/c1/scripts/ALS/Transition_IR_ALS.py.
   • The transition isn't entirely robust when executed by script - the main problem seems to be that in the few seconds between ramping off the IR servos and enabling the CARM/DARM integrators/boosts, the DARM error-point offset can become rather large. Consequently, when the integrator is engaged, ETMX/ETMY get a large kick that misalign the cavity substantially, degrade the green lock, and destroy the CARM lock as well. The problem doesn't seem to exist for the CARM loop. 
   • Anyways, I think this is easily fixed, just need to optimize sleep times and handoff gains etc a bit. For now, I just engage the DARM boosts by hand, putting in a DARM offset if necessary to avoid any kicking of the optic.
   • Attachment #3 shows the length noise witnessed by POX/POY when the arm cavities are under ALS control. If we believe the sensor calibration, the RMS displacement noise is ~15 (20) pm for the Y (X) arm.
   • This is rather larger than I was hoping would be the case, and the RMS is dominated by the <1 Hz "mystery noise".
   • Nevertheless, for a first pass, it's good to know that we can achieve this sort of ALS performance with the new IR ALS system.

Over the week, I'll try some noise budgeting, to improve the performance. The next step in the larger scheme of things is to see if we can lock the PRMI/DRMI with CARM detuned off resonance.

Mon Oct 7 14:51:53 2019. I closed the PSL shutter to measure the WFS head responsivity.

I made a thru calibration as in this elog, treating laser, reference PD, and WFS RF output as a three-port device. The DC current supplied to the laser is 20.0 mA in all cases. The Agilent spectrum analyzer supplies a -10 dBm excitation to Jenne laser's AM port, and A/B is measured with 20dB attenuation on each input port. Results are in /users/aaron/WFS/data/191007/. The calibration had 100 averages, all other measurements 32 averages; other parameters found in the yml file, same folder as the data.

I normalized the result by the difference between the dark and bright DC levels of each segment.

Mon Oct 7 17:29:58 2019 opened PSL shutter.

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?

Now with the CM board tested with the signal injected, it turned out that the latch logic was flipped. As the default state locked the digital levels, the buttons other than the mbbo channels were inactive.

By giving 0 to C1:LSC-CM_LATCH_ENABLE, the modification of the digital state is enabled. And with the value of 1, the digital bits on the board is locked.

In order to reflect this, latch.py was modified and now the controls are all activated.

[Koji]

The logic chips 74ALS573 were replaced. And now the gain sliders are working properly.

== Test Status ==

[done] Whitening gain switching test
[done] AA enable/disable switching
[0th order] LO Det Mon channel check
[none] PD I/F board check
[done] QPD I/F board check
[done] CM Board
[none] ALS I/F board

Last week we found that the logic chip for the REFL1 gain switching was not transmitting the input logic. I went to Downs and obtained the chips. After some inspection some other latch chips were suspicious. Therefore U46, U47, and U48 (#1, #3, and #4 from the top) were replaced. After the replacement, the gain measurements were repeated. This time the test for the AO gain was also performed. Now all three slideres show the gain as expected except for the consistent -0.2dB deficit.

Note that the transfer functions for the REFL gains were measured with the input at IN1 or IN2 and the output at TESTA1. The TFs for the AO gain was measured with the excitation at EXC B, the input at TESTB2 and the output at the SERVO output. The gain and phase variantions for the AO gain at low frequency is the effect of AC coupling existing between the excitation and the servo output.

[Update on Oct 14, 2019]

The measured transfer functions show the phase delay determined by the opamps involved. The phase delay well below the pole frequencies can be represented well by a simple time delay (a phase delay linear to the frequency). Attachment 7 shows the time delay estimated by LISO for each gain setting of each gain stage. REFL2 has particularly large phase delay because of the use of OP27s. The delay is even larger when the gain is high presunmably because of the limited GBW.

Yesterday, Koji and I noticed (from the wall StripTool traces) that the vertex seismometer RMS between 0.1-0.3 Hz in the X-direction increased abruptly around 6pm PDT. This morning, when I came in, I noticed that the level had settled back to the normal level. Trending the BLRMS channels over the last 24 hours, I  see that the 0.3-1 Hz band in the Z direction shows some anomalous behaviour almost in the exact same time-band. Hard to believe that any physical noise was so well aligned to the seismometer axes, I'm inclined to think this is indicative of some electronics issues with the Trillium interface unit, which has been known to be flaky in the past.

Summary:

There is ~ 7% variation in the power seen by the MC2 trans QPD, depending on the WFS offsets applied to the MC2 PIT/YAW loops. Some more interpretation is required however, before attributing this to spot-position-dependent loss variation inside the IMC cavity.

Analysis:

Attachment #1: This shows a scatter plot of the MC2 transmission and IMC REFL average values after the WFS loops have converged to the set offset positions. The size of the points are proportional to the normalized variance of the quantity. The purpose of this plot is to show that there is significant variation of the transmission, much more than the variance of an individual datapoint during the course of the averaging (again, the size of the circles is only meant to be indicative, the actual variance in counts is much smaller and wouldn't be visible on this plot scale). For a critically coupled cavity, I would have expected that the TRANS/REFL to be perfectly anti-correlated, but in fact, they are, if anything, correleated. So maybe the WFS loops aren't exactly converging to optimize the inoput pointing for a given offset? 

Attachment #2: Maps of the transmission/reflection as a function of the (YAW, PIT) offset applied. The radial coordinate does not yet mean anything physical - I have to figure out the calibration from offset counts to spot position motion on the optic in mm, to get an idea for how much we scanned the surface of the optic relative to the beam size. The gray circles indicate the datapoints, while the colormaps are scipy-based interpolation. 

Attachment #3: After talking with Koji, I explicitly show the correlation structure between the IMC REFL DCMON and MC2 TRANS. The shaded ellipses indicate the 1, 2 and 3-sigma bounds for the 2D dataset going radially outwards. The correlation coefficient for this dataset is 0.46, which implies moderate positive correlation. 🤔 

Scan algorithm:

The following was implemented in a python scipt:

1. Choose 2 independent random numbers from the uniform distribution in the interval [-0.5, 0.5] (in uncalibrated counts).
2. One of these numebrs is set as the error point offset for the QPD spot-centering PITCH WFS loop, while the other is the YAW offset.
3. Wait for 600 seconds - this long wait is required because the step-response time for these loops is long. 
4. If there is an MC unlock event - wait till the MC relocks, and then another 600 seconds, to give the WFS loops sufficient time to converge.
5. Once the WFS loops have converged, average a few data channels (MC TRANS, REFL, WFS loop error points etc) for 10 seconds, and write these to a file.

I am now setting the offsets to the WFS QPD loop to the place where there was maximum transmission, to see if this is repeatable. In fact it was. Looking at the QPD segment outputs, I noticed that the MC2 transmission spot was rather off-center on the photodiode. So I went to the MC2 in-air optical table and centered the beam till the output on the 4 segments were more balanced, see Attachment #4. Then I re-set the MC2 QPD offsets and re-enabled the WFS servos. The transmission is now a little lower at ~14,500 counts (but still higher than the ~14200 counts we had before), presumably because we have more of the brightest part of the beam falling on the gap between quadrants. For a more reliable measurement, we should use a single-element photodiode for the MC2 transmission.

I simulated this circuit with zero, but haven't gotten the results to match the measurements above.

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.

== Test Status ==

[done] Whitening gain switching test
[done] AA enable/disable switching
[0th order] LO Det Mon channel check
[none] PD I/F board check
[done] QPD I/F board check
[done] CM Board
[none] ALS I/F board

The photos of the latest board can be found as Attachments 3/4

With some input signals, the functionarities of the CM servo switches were tested.

• Latch logic works. But latch alive signal is missing.
• IN1 enable/disable, IN2 enable/disable are properly working
• OUT2 toggle switch for REFL1/REFL2 mon is wokring
• Boost / Super Boosts are working
• EXC A enable/disable, EXC B enable/disable switches are working
• Option 1 and Option 2 now isolate the input when either is enabled (as there is no option board)
• 79Hz-1.6kHz pole zero pair works fine
• OUT1 works fine
• Disable/Enable switch for the fast path works
• Polarity switch works
• AO Gain property changes the gain
• Limitter switch works (Attachments 4/5). The limitter clipps the output at 4~4.5V. The Limitter indicator also works.

After the tests the LSC cables were reconnected (Attachment 6)

In preparation for some locking work tonight, I did the following at the POP in air table with the PRMI locked on carrier:

1. Raised the POP camera by ~5mm. The POP spot is now well centered on the CCD view.
2. Tweaked alignment onto the PDA10CF photodiode that serves as (i) POP22, (ii) POP110, and (iii) POP DC. In lock the POPDC level went from ~800 cts to ~1200 cts.
3. Moved the QPD that witnesses part of the POP beam such that the spot was centered on the photodiode. This may be useful for collecting some FF data or if we want to try feedback to stabilize the PRMI.

TBC...

The boost filters of the CM servo board were tested. Their ZPK models were made.

The transfer functions of the boost filters were measured with the SG output of a SR785 connected to IN1. The IN1 gain was set to be 0dB. The transfer function was taken between the IN1 input and the TEST1A output.
With no boost and normal boost, the input signal amplitude was fixed to 20mVpk. For the other boosts, however, I could expect large gain variation through a single sweep. Therefore automatic SG amplitude tracking was used. The target was to have the output to be 1V with maximum amplitude of 100mV.

Attachment 1 shows the measured transfer functions.

The pole and zero frequencies of the boosts were estimated using LISO. Here the TFs were normalized by the TF of 'no boost' to cancel the delay of the other stages including that of the monitor channel.

ZPK model of Normal Boost:

pole 44.0597566447
zero 4.3927650910k
factor 98.8275377818

ZPK model of Super Boost (State1):

pole 878.5368382789
zero 17.5107366335k
factor 20.0840668188
 

ZPK model of Super Boost (State2):

pole 714.8112014271
pole 1.0147609373k
zero 13.2470941080k
zero 22.2259701828k
factor 404.5411036031
 

ZPK model of Super Boost (State3):

pole 886.3650348470
pole 420.4089305781
pole 887.8490768202
zero 8.3635166134k
zero 15.7953592754k
zero 20.5144907279k
factor 8.2051379423k

Looking at the old latch.st code, looks like this is just a heartbeat signal to indicate the code is alive. I'll implement this. Aesthetically, it'd be also nice to have the hex representation of the "*_SET" channels visible on the MEDM screen.

I installed nds2 on donatello with yum, but still can't import nds2.

I installed nds2 again, this time successfully with

It would be good if you and Shruti can look at how to change the parameters in Zero so as to do a fit to the measured data. Usually, in scipy.optimize we give it a function with some changeable params, so maybe there's a way to pass params to a zero object in that way. I think Ian and Anchal are doing something similar to their FSS Pockel's cell simulator.

After making sure the beams were hitting the 3f photodiodes on the "AP" table, I was able to lock the PRMI with the sidebands resonant inside the RC using 3f error signals. This would be the config we run in when trying to lock some more complicated configuration, such as the PRFPMI (i.e. start with the arms controlled by ALS, held off resonance). Tonight, I will try this (even though obviously I am not ready for the CARM transition step). The 3f lock is pretty robust, I was able to stay locked for minutes at a time and re-acquisition was also pretty quick. See Attachment #1. Not sure how significant it is, but I set the offsets to the 3f paths by averaging the REFL33_I and REFL33_Q signals when the PRMI was locked with the 1f error signals.

As usual, there's a lot of angular motion of the POP spot on the CCD monitor, but the lock seems to be able to ride it out.

Lock-settings (I modified the .snap file accordingly):

REFL33_I --> PRCL, loop gain = -0.019, Trigger on POP22, ON @ 20cts, OFF@0.5cts.

REFL33_Q --> MICH, loop gain = +1.4, Trigger on POP22, ON @ 20cts, OFF@0.5 cts.

This problem has re-surfaced. Is this indicative of some problem with the on-board VGA? Even with 0dB of whitening gain, I see PDH horns that are 10,000 ADC counts in amplitude, whereas the nominal whitening gain for this channel is +18dB. I'll look at it in the daytime, not planning to use REFL55 for any locking tonight.

Summary:

1. ALS control of arms in the CARM/DARM basis seems pretty robust - I was able to hold lock for >40mins tonight. The scripted transition from POX/POY control to ALS control is pretty deterministic now.
2. The PRMI could be locked with the arms detuned from resonance by applying an offset to the CARM loop error point.
3. Much daytime work remains to be done before attempting any sort of reliable locking.

Hardware issues that need addressing:

1. Both EX and EY Trans QPDs need a look. I believe the one at EY is simply blocked (on account of the mode spectroscopy project), while the one at EX shows a weird discontinuity between the Thorlabs PD and the QPD. Could be just a gain/normalization issue I guess. See Attachment #1.
2. While the PRMI stayed locked, I don't think I was using anywhere close to optimal settings. Need to run some sensing lines, measure transfer functions etc, to make the PRMI + arms lock more robust. The PRMI always lost lock when I brought the CARM offset to 0. Could also benefit from some finesse modeling I guess. I could not get a reliable estimate of what the PRG is tonight, because the PRMI didn't stay locked as I approached 0 CARM offset.
3. REFL 55 whitening board needs a checkup.

1. I removed the flip-mount that was installed on the EY in-air table for the mode-spectroscopy project (see Attachment #1). The Transmon QPD at EY sees IR light again.
2. Dark noise checkout - see Attachment #2.
3. Light-level expectations:
   • For the current config, let's say 0.8 W reaches the PRM, and we will have a PRG of 50. 
   • This implies ~5.5 kW circulating power in the arms.
   • This implies ~70mW will get transmitted through the ETM, of which at most half makes it to the QPD. 
   • In the nominal operating condition, we expect more like 6 W circulating in the arm cavity. So something like 30uW is expected to make it out onto the Trans QPDs.
   • But in this condition, we expect to run with the high-gain Thorlabs PD.
   • In reality the number is likely to be somewhat smaller. But we should set the transimpedance gain of this photodiode accordingly. Currently, there are a bunch of ND filters installed on this photodiode, which probably should be removed.
4. Angular control
   • The other purpose these QPDs are expected to serve is to stabilize the angular motion of the cavities when locked with high circulating power.
   • Need to calculate what the sensing noise requirement is.

[Jon, Yehonathan, Gautam, Aaron, Shruti, Koji]

We get together on Wednesday afternoon for cleaning the lab. Particularly, we collected e-wastes: VME crates, VME modules, old slow control cables, and other old/broken electronics. They are piled up in the office area and the cage outside rioght now (Attachments 1/2). We asked Liz to come to pick them up (under the coordination with either Gautam or Koji). Eventually this will free up two office desks.

Also, we made the acromag components organized in plastic boxes. (Attachment 3)

CM Board Slow out (digital length control) path transfer function / pole-zero filter pair (79Hz/1.6kHz) transfer function

The excitation was given from EXC A. The denominator was TESTA2, and the numerator was OUT1.

Attachment 1 shows the measured transfer function with and without PZ filter off and on. The PZ filter provides ~26dB attenuation at  high frequency. The output stage has a single order 100kHz LPF and it is visible in the transfer function.

The transfer function without the PZ filter was modelled by LISO as the following PZK representation. There looked a small step in the TF which caused the additional PZ pair (66~67Hz) but has very minor effect in the mag and phase.

pole 66.2720207366
zero 67.2660731875
pole 93.3044858160k
factor -995.5583556921m

The transfer function of the PZ filter was separately analyzed. The TF with the switch ON was normalized by the one with the switch OFF. Thus it revealed the pure effect of the switch. The PZK model of the stage was estimated to be

pole 79.7312926438
zero 1.6395485993k
factor 996.2196584165m

For the CM board modeling purpose, the transfer function from TESTA2 to TESTB2 was needed. (Attachment 1)

The ZPK model of this part is

pole 76.2369881805
zero 77.4655685092
pole 7.0761486105M
factor -993.0593433578m

The output stage (and AO GAIN stage) of the MC board was modelled. The transfer function was measured with the injection from EXC B. The denominator was TESTB2, and the numerator was SERVO OUT.

This stage is AC coupled by 2x 1st order HPFs. Firstly, this transfer function was measured with AO GAIN set to be 0dB. (Attachment 1)
This TF was used to characterize the cutoffs of the HPF stages, represented as the following ZPK:

zero 1m
zero 1m
pole 6.0502599855
pole 6.0624642854
factor -26.2725046079n

Then the AO GAIN was already measured as seen in [ELOG 14948]. The AO gain TF was then modeled by LISO with the above HPF as the preset. This allows us to characterize the time delay of the AO GAIN part.

Input referred offsets on the IN1/IN2 were tested with different gain settings. The two inputs were plugged by the 50 ohm terminators. The output was monitored at OUT1 (SLOW Length Output). The fast path is AC coupled and has no sensitivity to the offset.

There is the EPICS monitor point for OUT1. With the multimeter it was confirmed that the EPICS monitor (C1:LSC-CM_REFL1_GAIN) has the right value except for the opposite sign because the output stage of OUT1 is inverting. The previous stages have no sign inversion. Therefore, the numbers below does not compensate the sign inversion.

Attachment 1 shows the output offset observed at C1:LSC-CM_REFL1_GAIN. There is some gain variation, but it is around the constant offset of ~26mV. This suggested that the most of the offset is not from the gain stages but from the later stages (like the boost stages). Note that the boost stages were turned off during the measurements.

Attachment 2 shows the input refered offset naively calculated from the above output offset. In dependent from which path was used, the offset with low gain was hugely enhanced.

Since the input referred offset without subtracting the static offset seemed useless, a constant offset of -26mV was subtracted from the calculation (Attachment 2). This shows that the input refered offset can go up to ~+/-20mV when the gain is up to -16dB. Above that, the offset is mV level.

I don't think this level of offset by whichever OP27 or AD829 becomes an issue when the input error signal is the order of a volt.
This suggests that it is more important to properly set the internal offset cancellation as well as to keep the gain setting to be high.
 

The UPS is now incessantly beeping. I cannot handle this constant sound so I shut down all the control room workstations and moved the power strip hosting the 4 CPUs to a wall socket for tonight. Chub and I will replace the UPS batteries tomorrow.

Updated Circuit Diagram and photos: https://dcc.ligo.org/D1500308-v2

- (1) and (6) of the diagram: TFs with various gain slider values for REFL1/REFL2/AO GAIN [ELOG 14948] (gain values and time delay modeling)
- Switching checks, latest photo of the board, Limiter check  [ELOG 14953]
- (2): Boost transfer functions [ELOG 14955]
- (3): Slow (aka Length) CM output path [ELOG 14965]
- (4): Pole-Zero filter TF [ELOG 14965]
- (5): TF from TESTA2 to TESTB2 [ELOG 14966]
- (6): AC coupling TF of the AO GAIN stage [ELOG 14967]
- (7): AC coupling TF of the IN2 stage on IMC servo board [ELOG 15044]

Slow path = (1)*(2 if necessary)*(3)*(4 if necessary)

Fast path = (1)*(2 if necessary)*(4 if necessary)*(5)*(6)

gautam 20191122: Adding the measured AC coupling of the IN2 input of the IMC servo board for completeness.

[Liz, Gautam, Chub, Jordan, Koji]

We removed a significant amount of e-waste from the lab. The garbage was moved to the e-waste station in WB SB and are waiting for disposal.

Batteries + power cables replaced, and computers back on UPS from today ~3pm.

Summary:

There is poor separation of the PRCL and MICH length error signals as sensed in the 3f photodiodes. I don't know why this is so - one possibility is that the MICH-->PRM matrix element in the LSC output matrix needs to be tuned to minimize the MICH -->PRCL coupling.

Details:

Over the last few days, I've been trying to make the 3f locking of the PRMI more reliable. Turns out that while I was able to lock the PRMI on 3f error signals, it was just a fluke. So I set about trying to be more systematic. Here are the steps I followed:

1. Lock the PRMI (i.e. ETMs misaligned) using REFL11 for PRCL, AS55 for MICH.
   • This is the so-called 1f scheme.
   • The servo signs are chosen such that the carrier field is resonant in the PRC.
   • Run the dither alignment to maximize POPDC, minimize ASDC. This is the main purpose of locking in this config.
   • Measure some loop TFs, make sure the servo gains are giving us ~100 Hz UGF on these loops.
2. Change the sign of the servo loops to make the sidebands resonant in the PRC.
   • The error signals are still sourced from the 1f photodiodes.
   • Measure loop TFs, and also the TF between the 1f and 3f error signals. 
   • This allowed me to determine how the servo gains (and signs) that would be appropriate when using the 3f signals in place of the 1f.
   • Determine the offsets in the 3f error signals when the PRMI is locked on 1f error signals. This allows me to set the error point offsets for the PRCL_B and MICH_B paths, which are what is used for the 3f locking.
3. Change the error signals from 1f to 3f. 
   • After much trial and error, I finally managed to get a stable (>10 mins) lock going.
   • Measured some loop TFs.
   • Turned on the notch filters in the control filter bank at the sensMat oscillator frequencies, and ran some sensing lines.

Attachment #1 is the result. I don't know what is the reason for such poor separation of the MICH and PRCL error signals in REFL165. The situation seems very different from when I had the DRMI locked in Nov last year.

After this exercise, I tried for some hours to get the 3f PRMI locking going with the arm cavities held off resonance under ALS control, but had no success. The angular motion of the PRC isn't helping, but I feel this shouldn't be a show stopper.

1. Tuning the MICH-->PRM output matrix element
   • Locked the PRMI with the carrier field resonant in the PRC.
   • REFL11 used to control PRCL, AS55 for MICH.
   • Turned on the sensing notches in the control filter bank. Drove a line in MICH at 311.10 Hz.
   • Tweaked the MICH-->PRM matrix element to minimize the coupling witnessed.
   • As shown in Attachment #1, the minimum coupling was found to be at the value -0.34 (the old value was -0.2655).
   • The minimum was very sharp. A 1% change from the optimum value increased the peak height by > x2. Is this reasonable?
2. Some sensing matrix measurements: After tuning the output matrix element, I locked the PRMI (ETMs misaligned) in four configurations:
   • PRMI locked with carrier resonant. REFL11_I used for PRCL control, AS55_Q used for MICH control.
   • PRMI locked with sidebands resonant. REFL11_I used for PRCL control, AS55_Q used for MICH control.
   • PRMI locked with sidebands resonant. REFL11_I used for PRCL control, REFL165_I used for MICH control (based on sensing matrix measurement and offsets from previous config).
   • PRMI locked with sidebands resonant. REFL33_I used for PRCL control, AS55_Q used for MICH control.
   • The attached GIF shows the evolution of the demodulated sensing lines as we move through configurations.
      
   • The actual PDFs are attached as a zip, Attachment #2.
3. PRMI locking with arms under ALS control
   • The arm cavity lengths were controlled as usual with ALS. This system needs some noise budgeting.
   • I set the CARM offset to -8 (arbitrarily chosen, approximately equal to 20nm, but anyways well above the cavity linewidth).
   • Then I re-aligned the PRM, and attemped to lock the PRMI using the 3f settings determined with no arm cavities --> no success.
   • Tried locking using the 1f error signals instead - in this config, the lock could be established.
   • However, I saw that there was significant light on the AS camera, and I had to put in an offset into the MICH loop to make ASDC go as low as possible.
   • I guess it is possible that the ALS control wasn't precise enough and the leaked light to the dark port was because of differential reflectivity of the arm cavities?
   • Anyways, I ran a sensing matrix measurement with the interferometer in this configuration, and I found that the MICH signal in REFL165 had rotated significantly.
   • I also found that the 3f DC offsets in this configuration were ~5x greater than what was the case for the lock with no arm cavities.

This is as far as I got last night. The first step is to see how reliable the settings determined last night are, today. I don't understand how changing the output matrix element can have brought about such a significant change in the MICH/PRCL separation in all the RF photodiodes.