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13238   Tue Aug 22 02:19:11 2017 gautamUpdateALSALS OLTFs

Attachment #1 shows the results of my measurements tonight (SR785 data in Attachment #2). Both loops have a UGF of ~10kHz, with ~55 degrees of phase margin.

Excitation was injected via SR560 at the PDH error point, amplitude was 35mV. According to the LED indicators on these boxes, the low frequency boost stages were ON. Gain knob of the X end PDH box was at 6.5, that of the Y end PDH box was at 4.9. I need to check the schematics to interpret these numbers. GV Edit: According to this elog, these numbers mean that the overall gain of the X end PDH box is approx. 25dB, while that of the Y end PDH box is approx. 15dB. I believe the Y end Lightwave NPRO has an actuator discriminant ~5MHz/V, while the X end Innolight is more like 1MHz/V.

Not sure what to make of the X PDH loop measurement being so much noisier than the Y end, I need to think about this.

More detailed analysis to follow.

 Quote: I am now going to measure the OLTFs of both green PDH loops to check that the overall loop gain is okay, and also check the measurement against EricQ's LISO model of the (modified) AUX green PDH servos. Results to follow.

Attachment 1: ALS_OLTFs.pdf
Attachment 2: ALS_OLTF_Aug2017.zip
13244   Tue Aug 22 23:27:14 2017 ranaUpdateALSALS OLTFs

Didn't someone look at what the OLG req. should be for these servos at some point? I wonder if we can make a parallel digital path that we switch on after green lock. Then we could make this a simple 1/f box and just add in the digital path (take analog control signal into ADC, filter, and then sum into the control point further down the path to the laser) for the low frequency boost.

13246   Wed Aug 23 17:22:36 2017 gautamUpdateALSFiber ALS - reinstalled

I completed the revamp of the box, and re-installed the box on the PSL table today. I think it would be ideal to install this on one of the electronic racks, perhaps 1X2 would be best. We would have to re-route the fibers from the PSL table to 1X2, but I think they have sufficient length, and this way, the whole arrangement is much cleaner.

Did a quick check to make sure I could see beat notes for both arms. I will now attempt to measure the ALS noise with this revamped box, to see if the improved power supply and grounding arrangement, as well as fiber cleaning, has had any effect.

Photos + power budget + plan of action for using this box to characterize the green PDH locking to follow.

For quick reference: here is the AM/PM measurement done when we re-installed the repaired Innolight NPRO on the new X endtable.

13254   Fri Aug 25 15:54:14 2017 gautamUpdateALSFiber ALS noise measurement

[Kira, gautam]

Attachment #1 - Photo of the revamped beat setup. The top panel has to be installed. New features include:

• Regulated power supply via D1000217.
• Single power switch for both PDs.
• Power indicator LED.
• Chassis ground isolated from all other electronic grounds. For this purpose, I installed all the elctronics on a metal plate which is only connected to the chassis via nylon screws. The TO220 package power regulator ICs have been mounted with the TO220 mounting kits that provide a thin piece of plastic that electrically insulates its ground from the chassis ground.
• PD outputs routed through 20dB coupler on front panel for diagnostic purposes.
• Fiber routing has been cleaned up a little. I installed a winding fixture I got from Johannes, but perhaps we can install another one of these on top of the existing one to neaten up the fiber layout further.
• 90-10 light splitter (meant for diagnostic purposes) has been removed because of space constraints.

Attachment #2 - Power budget inside the box. Some of these FC/APC connectors seem to not offer good coupling between the two fibers. Specifically, the one on the front panel meant to accept the PSL light input fiber seems particularly bad. Right now, the PSL light is entering the box through one of the front panel connectors marked "PSL + X out". I've also indicated the beat amplitude measured with an RF analyzer. Need to do the math now to confirm if these match the expected amplitudes based on the power levels measured.

Attachment #3 - We repeated the measurement detailed here. The X arm (locked to IR) was used for this test. The "X" delay line electronics were connected to the X green beat PD, while the "Y" delay line electronics were connected to the X IR beat PD. I divided the phase tracker Hz calibration factor by 2 to get IR Hz for the Y arm channels. IR beat was at ~38MHz, green beat was at ~76MHz. The broadband excess noise seen in the previous test is no longer present. Indeed, below ~20Hz, the IR beat seems less noisy. So seems like the cleaning / electronics revamp did some good.

Further characterization needs to be done, but the results of this test are encouraging. If we are able to get this kind of out of loop ALS noise with the IR beat, perhaps we can avoid having to frequently fine-tune the green beat alignment on the PSL table. It would also be ideal to mount this whole 1U setup in an electronics rack instead of leaving it on the PSL table.

 Quote: Photos + power budget + plan of action for using this box to characterize the green PDH locking to follow.

GV Edit: I've added better photos to the 40m Google Photos page. I've also started a wiki page for this box / the proposed IR ALS  system. For the moment, all that is there is the datasheet to the Fiber Couplers used, I will populate this more as I further characterize the setup.

Attachment 1: IMG_7497.JPG
Attachment 2: FOL_schematic.pdf
Attachment 3: 20170825_IR_ALS.pdf
13255   Fri Aug 25 17:11:07 2017 ranaUpdateALSFiber ALS noise measurement

Is it better to mount the box in the PSL under the existing shelf, or in a nearby PSL rack?

 Quote: Further characterization needs to be done, but the results of this test are encouraging. If we are able to get this kind of out of loop ALS noise with the IR beat, perhaps we can avoid having to frequently fine-tune the green beat alignment on the PSL table. It would also be ideal to mount this whole 1U setup in an electronics rack instead of leaving it on the PSL table

13257   Sun Aug 27 11:57:31 2017 ranaUpdateALSFiber ALS noise measurement

It seems like the main contribution to the RMS comes from the high frequency bump. When using the ALS loop to lock the arm to the beat, only the stuff below ~100 Hz will matter. Interesting to see what that noise budget will show. Perhaps the discrepancy between inloop and out of loop will go down.

13266   Tue Aug 29 02:08:39 2017 gautamUpdateALSFiber ALS noise measurement

I was having a chat with EricQ about this today, just noting some points from our discussion down here so that I remember to look into this tomorrow.

• I believe that currently, the channels C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ and the Y arm analog read out the frequency of the green beat, in Hz.
• In the comparison I plotted, I WRONGLY divided the spectrum of the IR beat by 2, instead of multiplying in by 2, which is what should actually be done for an apples-to-apples comparison.
• The deeper question is, what should this channel actually readout?
• Looking at my codes from past arm scans etc, I see that I am dividing the downloaded data by 2 in order to convert the X-axis of these scans to "IR Hz". But this should really be all we care about.
• So I think I will have to re-do the cts-to-Hz calibration in the ALS models. It should be possible to do ~10FSR scans with the IR beat, and then we can use the sideband resonances (presumably the sideband frequencies are known with better precision than the arm length, and hence the FSR) to calibrate the phase tracker.
• I don't think this changes the fact that the Fiber ALS situation has been improved - but I will have to repeat the measurement to be sure. The improvement may not be as stellar as I tried to sell in my previous elog .

Other thoughts:

• Can we make use of the Jetstor raid array for some kind of consolidated 40m CDS backup system? Once we've gotten everything of interest out of it...

13288   Fri Sep 1 19:15:40 2017 gautamUpdateALSFiber ALS noise measurement

Summary:

I did some work today to see if I could use the IR beat for ALS control. Initial tests were encouraging.

I will now embark on the noise budgeting.

Details:

• For this test, I used the X arm
• I hooked up the X-arm + PSL IR beat to the X-arm DFD channel, and used the Y-arm DFD channels to simultaneously monitor the X-arm green beat.
• I then transitioned to ALS control and used POX as an out-of-loop sensor for the ALS noise.
• Attachment #1 shows a comparison of the measurements. In red is the IR beat, while the green traces are from the test EricQ and I did a couple of nights ago using the green beat.
• I also wanted to do some arm cavity scans with the arm under ALS control with the IR beat - but was unsucessful. The motivation was to fix the ALS model counts->Hz calibration factors.
• I did however manage to do a 10 FSR scan using the green beatnote - however, towards the end of this scan, the green beat frequency (read off the control room analyzer) was ~140MHz, which I believe is outside (or at least on the edge) of the bandwidth of the Green BBPDs. The fiber coupled IR beat photodiodes have a much larger (1GHz) spec'd bandwidth.

I am leaving the green beat electronics on the PSL table in the switched state for further testing...

Attachment 1: IR_ALS_noise.pdf
13325   Thu Sep 21 01:32:00 2017 gautamUpdateALSAUX X Innolight AM measurement running

[rana,gautam]

We set up a measurement of the AUX X laser AM today. Some notes:

• PDA 55 that was installed as a power monitor for the AUX X laser has been moved into the main green beam path - it is just upstream of the green shutter for this measurement.
• AUX X laser power into the doubling crystal was adjusted by rotating HWP upstream of IR Faraday (original angle was 100, now it is 120), until the DC level of the PDA 55 output was ~2.5V on a scope (high impedance).
• BNC-T was installed at the PZT input of the Innolight - one arm of the T is terminated to ground via 50 ohms. The purpose of this is to always have the output of the power splitter from the network analyzer RF source drive a 50 ohm load.
• The output of the Green PDH servo to the Innolight PZT was disconnected downstream of the summing Pomona box - it is now connected to one output of a power splitter (borrowed from SR function generator used to drive the PZT) connected to the RF source output of the AG4395.
• Other output of power splitter connected to input R of AG4395.
• PDA55 output has been disconnected from CH5 of the AA board. It is connected to input A of the AG4395 via DC block.

Attachment #1 shows a preliminary scan from tonight - we looked at the region 10kHz-10MHz, with an IF bandwidth of 100Hz, 16 averages, and 801 log-spaced frequencies. The idea was to get an idea of where some promising notches in the AM lie, and do more fine-bandwidth scans around those points. Data + code used to generate this plot in Attachment #2.

Rana points out that some of the AM could also be coming from beam jitter - so to put this hypothesis to test, we will put a lens to focus the spot more tightly onto the PD, repeat the measurement, and see if we get different results.

There were a whole bunch of little illegal things Rana spotted on the EX table which he will make a separate post about.

I am running 40 more scans with the same params for some statistics - should be done by the morning.

 Quote: I borrowed the HP impedance test kit from Rich Abbott today. The purpose is to profile the impedance of the NPRO PZTs, as part of the AUX PDH servo investigations. It is presently at the X-end. I will do the test in the coming days.

Update 12:00 21 Sep: Attachment #3 shows schematically the arrangement we use for the AM measurement. A similar sketch for the proposed PM measurement strategy to follow. After lunch, Steve and I will lay out a longish BNC cable from the LSC rack to the IOO rack, from where there is already a long cable running to the X end. This is to facilitate the PM measurement.

Update 18:30 21 Sep: Attachment #4 was generated using Craig's nice plotting utility. The TF magnitude plot was converted to RIN/V by dividing by the DC voltage of the PDA 55 of ~2.3V (assumption is that there isn't significant difference between the DC gain and RF transimpedance gain of the PDA 55 in the measurement band) The right-hand columns are generated by calculating the deviation of individual measurements from the mean value. We're working on improving this utility and aesthetics - specifically use these statistics to compute coherence, this is a work in progress. Git repo details to follow.

There are only 23 measurements (I was aiming for 40) because of some network connectivity issue due to which the script stalled - this is also something to look into. But this sample already suggests that these measurement parameters give consistent results on repeated measurements above 100kHz.

TO CHECK: PDA 55 is in 0dB gain setting, at which it has a BW of 10MHz (claimed in datasheet).

Some math about relation between coherence $\gamma_{xy}(f)$ and standard deviation of transfer function measurements:

$\mathrm{SNR}(f) = \sqrt{\frac{\gamma_{xy}^{2}(f)}{1-\gamma_{xy}^{2}(f)}}$

$\sigma_{xy}^{2} = \frac{1-\gamma_{xy}^{2}(f)}{2N\gamma_{xy}^{2}(f)}|H(f)|^2$  --- relation to variance in TF magnitude. We estimate the variance using the usual variance estimator, and can then back out the coherence using this relation.

$\sigma_{\theta_{xy}} = \mathrm{tan}^{-1}\left [ \sqrt{\frac{1-\gamma_{xy}^{2}(f)}{2N\gamma_{xy}^{2}(f)}} \right ]$ --- relation to variance in TF phase. Should give a coherence profile that is consistent with that obtained using the preceeding equation.

It remains to code all of this up into Craig's plotting utility.

Attachment 1: Innolight_AM.pdf
Attachment 2: Innolight_AM.tar.gz
Attachment 3: IMG_7599.JPG
Attachment 4: 20170921_203741_TFAG4395A_21-09-2017_115547_FourSquare.pdf
13326   Thu Sep 21 01:55:16 2017 ranaUpdateALSX End table of Shame

Image #1: No - we do not use magnetic mounts for beam dumps. Use a real clamp. It has to be rigid. "its not going anywhere" is a nonsense statement; this is about vibration amplitude of nanometers.

Image #2: No - we do not use sticky tape to put black glass beam dumps in place ever, anywhere. Rigid dumps only.

Image #3: Please do not ruin our nice black glass with double sticky tape. We want to keep the surfaces clean. This one and a few of the other Mickey Mouse black glass dumps on this table were dirty with fingerprints and so very useless.

Image #4: This one was worst of all: a piece of black glass was sticky taped to the wall. Shameful.

Please do not do any work on this table without elogging. Please never again do any of these type of beam dumping - they are all illegal. Better to not dump beams than to do this kind of thing.

All dumps have to be rigidly mounted. There is no finger contacting black glass or razor dumps - if you do, you might as well throw it in the garbage.

Attachment 1: 20170921_003143.jpg
Attachment 2: 20170921_002430.jpg
Attachment 3: 20170921_002243.jpg
Attachment 4: 20170921_001906.jpg
13327   Thu Sep 21 15:23:04 2017 gautamOmnistructureALSLong cable from LSC->IOO

[steve,gautam]

We laid out a 45m long BNC cable from the LSC rack to the IOO rack via overhead cable trays. There is ~5m excess length on either side, which have been coiled up and cable-tied for now. The ends are labelled "TO LSC RACK" and "TO IOO RACK" on the appropriate ends. This is to facilitate hooking up the output of the DFD for making a PM measurement of the AUX X laser. There is already a long cable that runs from the IOO rack to the X end.

13333   Tue Sep 26 19:10:13 2017 gautamUpdateALSFiber ALS setup neatened

[steve, gautam]

The Fiber ALS box has been installed on the existing shelf on the PSL table. We had to re-arrange some existing cabling to make this possible, but the end result seems okay (to me). The box lid was also re-installed.

Some stuff that still needs to be fixed:

1. Power supply to ZHL amplifiers - it is coming from a table-top DC supply currently, we should hook these up to the Sorensens.
2. We should probably extend the corrugated fiber protection tubing for the three fibers all the way up to the shelf.

Beat spectrum post changes to follow.

Quote:

Is it better to mount the box in the PSL under the existing shelf, or in a nearby PSL rack?

 Quote: Further characterization needs to be done, but the results of this test are encouraging. If we are able to get this kind of out of loop ALS noise with the IR beat, perhaps we can avoid having to frequently fine-tune the green beat alignment on the PSL table. It would also be ideal to mount this whole 1U setup in an electronics rack instead of leaving it on the PSL table

Attachment 1: IMG_7605.JPG
13335   Wed Sep 27 00:20:19 2017 gautamUpdateALSMore AM sweeps

Attachment #1: Result of AM sweeps with EX laser crystal at nominal operating temperature ~ 31.75 C.

Attachment #2: Tarball of data for Attachment #1.

Attachment #3: Result of AM sweeps with EX laser crystal at higher operating temperature ~ 40.95 C.

Attachment #4: Tarball of data for Attachment #2.

Remarks:

• Confirmed that PDA 55 is in the "0dB" setting - the actual dial is unmarked, and has 5 states. I guessed that the left-most one is 0dB, and checked that if I twiddled the dial by one state to the right, the DC level on the scope increased by 10dB as advertized. Didn't check all the states.
• DC level is ~2.3V on a high-impedance scope. So it will be ~1.15V to a 50ohm load, which is what the DC block is. The inverse of this value is used to calibrate the vertical axis of the TF measurement to RIN/V.
• Input R (split RF source signal) attenuation: 20dB. Input A (PDA55 output) attenuation: 0dB.
• Main problem is still network hangups when trying to do many sweeps.
• Seems to persist even when I connect the GPIB box to one of the network switches - so don't think we can blame the WiFi.
• Need to explore possibility of speedup - takes >2hours to run ~50scans!

To-do:

• Overlay median and uncertainty plots for the two temp. settings. There is a visible diference in both the locations and depths/heights of various notches/peaks in the AM profile.
• Repeat test with a fast focusing lens to focus the beam more tightly on the PD active area to confirm that the measured AM is indeed due to the PZT drive and not from beam-jitter (presently, spot diameter is ~0.5x active area diameter, to eye).
• Get the PM data.
• Depending on what the PM data looks like, do a more fine-grained scan around some promising AM notches / PM peaks.
Attachment 1: TFAG4395A_26-09-2017_202344_FourSquare.pdf
Attachment 2: lowTemp.tgz
Attachment 3: TFAG4395A_26-09-2017_231630_FourSquare.pdf
Attachment 4: highTemp.tgz
13337   Wed Sep 27 23:44:45 2017 gautamUpdateALSProposed PM measurement setup

Attachment #1 is a sketch of the proposed setup to measure the PM response of the EX NPRO. Previously, this measurement was done via PLL. In this approach, we will need to calibrate the DFD output into units of phase, in order to calibrate the transfer function measurement into rad/V. The idea is to repeat the same measurement technique used for the AM - take ~50 1 average measurements with the AG4395, and look at the statistics.

Some more notes:

• Delay line box is passive, just contains a length of cable.
• IQ Demodulation is done using an aLIGO 1U chassis unit, with the actual demod board electronics being D0902745
• The RF beatnote amplitude out of the IR beat PD is ~ -8dBm.
• The ZHL-3A amplifiers have gain of 24dB, so the amplified beat should be ~16dBm
• At the LSC rack, the amplified beat is split into two - one path goes to the LO input of D0902745 (so at most 13dBm), the other goes through the delay line.
• On the demod board, the LO signal is amplified with a AP1053, rated at 10dB gain, max output of 26dBm, so the signal levels should be fine for us, even though the schematic says the nominal LO level is 10dBm - moreover, I've ignored cable losses, insertion losses etc so we should be well within spec.
• The mixer is PE4140. The datasheet quotes LO levels of 17dBm for all the "nominal" tests, we should be within a couple of dBm of this number.
• There is no maximum value specified for the RF input signal level to the mixer on the datasheet, but I expect it to be <10dBm.
• We should park the beatnote around 30MHz as this should be well within the operational ranges for the various components in the signal chain.
Attachment 1: IMG_7609.JPG
13346   Fri Sep 29 11:16:52 2017 SteveUpdateALSY End table corrected

The first Faraday isolater rejected beam path from the NPRO is fixed.

Attachment 1: ETMYf1.jpg
13366   Fri Oct 6 17:08:09 2017 SteveUpdateALSX End table beam traps corrected

There are no more double sided tape on this table.

Attachment 1: c1.jpg
Attachment 2: c2.jpg
Attachment 3: c3.jpg
Attachment 4: c4.jpg
13502   Thu Jan 4 12:46:27 2018 gautamUpdateALSFiber ALS assay

Attachment #1 is the updated diagram of the Fiber ALS setup. I've indicated part numbers, power levels (optical and electrical). For the light power levels, numbers in green are for the AUX lasers, numbers in red are for the PSL.

I confirmed that the output of the power splitter is going to the "RF input" and the output of the delay line is going to the "LO input" of the demodulator box. Shouldn't this be the other way around? Unless the labels are misleading and the actual signal routing inside the 1U chassis is correctly done :/

• Mode-matching into the fibers is rather abysmal everywhere.
• In this diagram, only the power levels measured at the lasers and inputs of the fiber couplers are from today's measurements. I just reproduced numbers for inside the beat mouth from elog13254.
• Inside the beat mouth, the PD output actually goes through a 20dB coupler which is included in this diagram for brevity. Both the direct and coupled outputs are available at the front panel of the beat mouth. The latter is meant for diagnostic purposes. The number of -8dBm of beat @30MHz is quoted using the direct output, and not the coupled output.

Still facing some CDS troubles, will start ALS recovery once I address them.

Attachment #2 is the svg file of Attachment #1, which we can update as we improve things. I'll put it on the DCC 40m tree eventually.

Attachment 1: FiberALS.pdf
Attachment 2: FiberALS.svg.zip
13519   Tue Jan 9 21:38:00 2018 gautamUpdateALSALS recovery
• Aligned IFO to IR.
• Ran dither alignment to maximize arm transmission.
• Centered Oplev reflections onto their respective QPDs for ITMs, ETMs and BS, as DC alignment reference. Also updated all the DC alignment save/restore files with current alignment.
• Undid the first 5 bullets of elog13325. The AUX laser power monitor PD remains to be re-installed and re-integrated with the DAQ.
• I stupidly did not refer to my previous elog of the changes made to the X end table, and so spent ages trying to convince Johannes that the X end green alignment had shifted, and turned out that the green locking wasn't going because of the 50ohm terminator added to the X end NPRO PZT input. I am sorry for the hours wasted
• GTRY and GTRX at levels I am used to seeing (i.e. ~0.25 and ~0.5) now. I tweaked input pointing of green and also movable MM lenses at both ends to try and maximize this.
• Input green power into X arm after re-adjusting previously rotated HWP to ~100 degrees on the dial is ~2.2mW. Seems consistent with what I reported here.
• Adjusted both GTR cameras on the PSL table to have the spots roughly centered on the monitors.
• Will update shortly with measured OLTFs for both end PDH loops.
• X end PDH seems to have UGF ~9kHz, Y end has ~4.5kHz. Phase margin ~60 degrees in both cases. Data + plotting code attached. During the measurement, GTRY ~0.22, GTRX~0.45.

Next, I will work on commissioning the BEAT MOUTH for ALS beat generation.

Note: In the ~40mins that I've been typing out these elogs, the IR lock has been stable for both the X and Y arms. But the X green has dropped lock twice, and the Y green has been fluctuating rather more, but has mangaged to stay locked. I think the low frequency Y-arm GTRY fluctuations are correlated with the arm cavity alignment drifting around. But the frequent X arm green lock dropouts - not sure what's up with that. Need to look at IR arm control signals and ALS signals at lock drop times to see if there is some info there.

Attachment 1: GreenLockStability.png
Attachment 2: ALS_OLTFs_20180109.pdf
Attachment 3: ALS_OLTF_data_20180109.tar.bz2
13531   Thu Jan 11 14:22:40 2018 gautamUpdateALSFiber ALS assay

I did a cursory check of the ALS signal chain in preparation for commissioning the IR ALS system. The main elements of this system are shown in my diagram in the previous elog in this thread.

Questions I have:

1. Does anyone know what exactly is inside the "Delay Line" box? I can't find a diagram anywhere.
• Jessica's SURF report would suggest that there are just 2 50m cables in there.
• There are two power splitters taped to the top of this box.
• It is unclear to me if there are any active components in the box.
• It is unclear to me if there is any thermal/acoustic insulation in there.
• For completeness, I'd like to temporarily pull the box out of the LSC rack, open it up, take photos, and make a diagram unless there are any objections.
2. If you believe the front panel labeling, then currently, the "LO" input of the mixer is being driven by the part of the ALS beat signal that goes through the delay line. The direct (i.e. non delayed) output of the power splitter goes to the "RF" input of the mixer. The mixer used, according to the DCC diagram, is a PE4140. Datasheet suggests the LO power can range from -7dBm to +20dBm. For a -8dBm beat from the IR beat PDs, with +24dB gain from the ZHL3A but -3dB from the power splitter, and assuming 9dB loss in the cable (I don't know what the actual loss is, but according to a Frank Seifert elog, the optimal loss is 8.7dB and I assume our delay line is close to optimal), this means that we have ~4dBm at the "LO" input of the demod board. The schematic says the nominal level the circuit expects is 10dBm. If we use the non-delayed output of the power splitter, we would have, for a -8dBm beat, (-8+24-3)dBm ~13dBm, plus probably some cabling loss along the way which would be closer to 10dBm. So should we use the non-delayed version for the LO signal? Is there any reason why the current wiring is done in this way?

13534   Thu Jan 11 20:51:20 2018 gautamUpdateALSFiber ALS assay

After labeling cables I would disconnect, I pulled the box out of the LSC rack. Attachment #1 is a picture of the insides of the box - looks like it is indeed just two lengths of cabling. There was also some foam haphazardly stuck around inside - presumably an attempt at insulation/isolation.

Since I have the box out, I plan to measure the delay in each path, and also the signal attenuation. I'll also try and neaten the foam padding arrangement - Steve was showing me some foam we have, I'll use that. If anyone has comments on other changes that should be made / additional tests that should be done, please let me know.

20180111_2200: I'm running some TF measurements on the delay line box with the Agilent in the control room area (script running in tmux sesh on pianosa). Results will be uploaded later.

 Quote: For completeness, I'd like to temporarily pull the box out of the rack, open it up, take photos, and make a diagram unless there are any objections.

Attachment 1: IMG_5112.JPG
13552   Tue Jan 16 21:50:53 2018 gautamUpdateALSFiber ALS assay

With Johannes' help, I re-installed the box in the LSC electronics rack. In the end, I couldn't find a good solution to thermally insulate the inside of the box with foam - the 2U box is already pretty crowded with ~100m of cabling inside of it. So I just removed all the haphazardly placed foam and closed the box up for now. We can evaluate if thermal stability of the delay line is limiting us anywhere we care about and then think about what to do in this respect. This box is actually rather heavy with ~100m of cabling inside, and is right now mounted just by using the ears on the front - probably should try and implement a more robust mounting solution for the box with some rails for it to sit on.

I then restored all the cabling - but now, the delayed part of the split RF beat signal goes to the "RF in" input of the demod board, and the non-delayed part goes to the back-panel "LO" input. I also re-did the cabling at the PSL table, to connect the two ZHL3-A amplifier inputs to the IR beat PDs in the BeatMouth instead of the green BBPDs.

I didn't measure any power levels today, my plan was to try and get a quick ALS error signal spectrum - but looks like there is too much beat signal power available at the moment, the ADC inputs for both arm beat signals are overflowing often. The flat gain on the AS165 (=ALS X) and POP55 (=ALS Y) channels have been set to 0dB, but still the input signals seem way too large. The signals on the control room spectrum analyzer come from the "RF mon" ports on the demod board, and are marked as -23dBm. I looked at these peak heights with the end green beams locked to the arm cavities, as per the proposed new ALS scheme. Not sure how much cable loss we have from the LSC rack to the network analyzer, but assuming 3dB (which is the Google value for 100ft of RG58), and reading off the peak heights from the control room analyzer, I figure that we have ~0dBm of RF signal in the X arm. => I would expect ~3dBm of signal to the LO input. Both these numbers seem well within range of what the demod board is designed to handle so I'm not sure why we are saturating.

Note that the nominal (differential) I and Q demodulated outputs from the demod board come out of a backplane connector - but we seem to be using the front panel (single-ended) "MON" channels to acquire these signals. I also need to update my Fiber ALS diagram to indicate the power loss in cabling from the PSL table to the LSC electronics rack, expect it to be a couple of dB.

 Quote: After labeling cables I would disconnect, I pulled the box out of the LSC rack. Attachment #1 is a picture of the insides of the box - looks like it is indeed just two lengths of cabling. There was also some foam haphazardly stuck around inside - presumably an attempt at insulation/isolation. Since I have the box out, I plan to measure the delay in each path, and also the signal attenuation. I'll also try and neaten the foam padding arrangement - Steve was showing me some foam we have, I'll use that. If anyone has comments on other changes that should be made / additional tests that should be done, please let me know. 20180111_2200: I'm running some TF measurements on the delay line box with the Agilent in the control room area (script running in tmux sesh on pianosa). Results will be uploaded later.

13557   Thu Jan 18 00:35:00 2018 gautamUpdateALSFiber ALS assay

## Summary:

I am facing two problems:

1. The X arm beat seems to be broadband noisier than the Y arm beat - see Attachment #1. The Y-axis calibration is uncertain, but at least the Y beat has the same profile as the reference traces, which are for the green beat from a time when we had ALS running. There is also a rather huge ~5kHz peak, which I confirmed isn't present in the PDH error/control signal spectra (with SR785).
2. The Y-arm beat amplitude, at times, "breathes" in amplitude (as judged by control room analyzer). Attachment #2 is a time-lapse of this behaviour (left beat is X arm beat, right peak is the Y arm peak) - I caught only part of it, the the beat note basically vanishes into the control room noise floor and then comes back up to almost the same level as the X beat. The scale is 10dB/div. During this time, the green (and IR for that matter) stay stably locked to the arm - you'll have to take my word for it as I have no way to sync my video with StripTool Traces, but I was watching the DC transmission levels the whole time. The whole process happens over a few (1< $\tau$ <5) minutes - I didn't time it exactly. I can't really say this behaviour is periodic either - after the level comes back up, it sometimes stays at a given level almost indefinitely.

## More details:

• Spent some time today trying to figure out losses in various parts of the signal chain, to make sure I wasn't in danger of saturating RF amplifiers. Cabling from PSL table -> LSC rack results in ~2dB loss.
• I will upload the updated schematic of the Beat-Mouth based ALS - I didn't get a chance to re-measure the optical powers into the Beat Mouth, as someone had left the Fiber Power Meter unplugged, and it had lost all of its charge .
• The Demod boards have a nice "RF/LO power monitor" available at the backplane of the chassis - we should hook these channels up to the DAQ for long term monitoring.
• The schematic claims "120mV/dBm" into 50ohms at these monitoring pins.
• I measured the signal levels with a DMM (Teed with 50ohm), but couldn't really make the numbers jive - converting the measured backplane voltage into dBm of input power gives me an inferred power level that is ~5dBm higher than the actual measured power levels (measured with Agilent analyzer in Spectrum Analyzer mode).
• Looking at the time series of the ALS I and Q inputs, the signals are large, but we are well clear of saturating our 16-bit ADCs.
• In the brief periods when both beats were stable in amplitude (as judged by control room analyzer), the output of the Q quadrature of the phase tracker servo was ~12,000 cts - the number I am familiar with for the green days is ~2000cts - so naively, I would say we have ~6x the RF beat power from the Beat Mouth compared to green ALS.
• I didn't characterize the conversion efficiency of the demod boards so I don't have a V (IF)/V (RF) number at the moment.
• I confirmed that the various peaks seen in the X arm beat spectrum aren't seen in the control signal of the EX Green PDH, by looking at the spectrum on an SR785 (it is also supposedly recorded in the DAQ system, but I can't find the channel and the cable is labelled "GCX-PZT_OUT", which doesn't match any of our current channels).
Note to self from the future: the relevant channels are: C1:ALS-X_ERR_MON_IN1 (green PDH error signal with x10 gain from an SR560) and C1:ALS-X_SLOW_SERVO_IN1 (green PDH control signal from monitor point - I believe this is DC coupled as this is the error signal to the slow EX laser PZT temp control). I've changed the cable labels at the X end to reflect this reality. At some point I will calibrate these to Hz.
• The control room analyzer signals come from the "RF mon" outputs on the demod board, which supposedly couple the RF input with gain of -23dBm. These are then routed reverse through a power splitter to combine the X and Y signals, which is then plugged into the HP analyzer. The problem is not local to this path, as during the "breathing" of the Y beat RF amplitude, I can see the Q output of the phase tracker also breathing.

## Next steps (that I can think of, ideas welcome!):

1. For Problem #1 - usual debugging tactic of switching X and Y electronics paths to see if the problem lies in the light or in the electronics. If it is in the electronics, we can swap around at various points in the signal chain to try and isolate the problematic component.
2. For Problem #2 - hook up the backplane monitor channels to monitor RF amplitudes over time and see if the drifts are correlated with other channels.
3. There is evidence of some acoustic peaks, which are possibly originating from the fibers - need to track these down, but I think for a first pass to try and get the red ALS going, we shouldn't be bothered by these.

Attachment 1: IR_ALS_20180118.pdf
13559   Fri Jan 19 11:34:21 2018 gautamUpdateALSFiber ALS assay

I swapped the inputs to the ZHL-3A at the PSL table - so now the X beat RF signals from the beat mouth are going through what was previously the Y arm ALS electronics. From Attachment #1, you can see that the Y arm beat is now noisier than the X. The ~5kHz peak has also vanished.

So I will pursue this strategy of switching to try and isolate where the problem lies...

Somebody had forgotten to turn the HEPA variac on the PSL table down. It was set at 70. I set it at 20, and there is already a huge difference in the ALS spectra

 Quote: For Problem #1 - usual debugging tactic of switching X and Y electronics paths to see if the problem lies in the light or in the electronics. If it is in the electronics, we can swap around at various points in the signal chain to try and isolate the problematic component.
Attachment 1: IR_ALS_20180119.pdf
13562   Fri Jan 19 23:04:11 2018 gautamUpdateALSFiber ALS assay

[rana, kevin, udit, gautam]

quick notes of some discussions we had today:

1. Earlier in the day, Udit and I measured (with a 20dB coupler and AG4395) ~20dBm of RF beat power at input to power splitter (just before delay line box) at the LSC rack. This means that we have ~17dBm going into the LO input of the demod board. The AP1053 can only really handle a max of 16dBm at the input. After discussion with Rana, I put a 3dB attenuator at the input to the power splitter so as to preserve the LO/RF ratio in the demod circuit.
2. Need to make a detailed optical and RF power budget for both arms.
3. The demod circuit board is configured to have gain of x100 post demod (conversion loss of the mixer is ~-8dB). This works well for the PDH cavity locking type of demod scheme, where the loop squishes the error signal in lock, so most of the time, the RF signal is tiny, and so a gain of x100 is good. For ALS, the application needs are rather different. So we lowered the gain of the "Audio IF amplifier" stage of the circuit from x100 to x10, by effecting the resistor swaps 10ohms->50ohms, 1kohm->500ohms (more details about this later).
4. There is some subtlety regarding the usage of the whitening interface boards - I need to look at the circuit again and understand this better, but Rana advised against running with the whitening gain at low values. Point #3 above should have helped with this regard.
5. I wanted to test the new signal chain (with 3dB attenuation and modified IF gain) but ETMX is not happy now, and is making it impossible to keep the X arm locked. Will try again tomorrow.
6. Eventually: need to measure the mode of the fiber, and up the MM efficiency to at least 80%, which should be doable without using any fancy lenses/collimators.
7. Udit and I felt that the back panel RF power monitor wasn't working as expected - I will re-investigate this when I have the board out again to make the IF gain change permanent with the right footprint SMD resistors.

RXA: 0805 size SMD thin film resistors have been ordered from Mouser, to be shipped on Monday. **note that these thin film resistors are black; i.e. it is NOT true that all black SMD resistors are thick film**

13571   Wed Jan 24 00:33:31 2018 gautamUpdateALSFiber ALS assay

I did some work on the PSL table today. Main motivations were to get a pickoff for the BeatMouth PSL beam before any RF modulations are imposed on it, and to improve the mode-matching into the fiber. Currently, we use the IR light reflected by the post doubling oven harmonic separator. This has the PMC modulation sideband on it, and also some green leakage.

So I picked off ~8.5mW of PSL light from the first PBS (pre Faraday rotator), out of the ~40 mW available here, using a BS-80-1064-S. I dumped the 80% reflected light into the large beam dump that was previously being used to dump this PBS reflection. Initially, I used a R=10% BS for S-pol that I found on the SP table, but Koji tipped me off on the fact that these produce multiple reflected beams, so I changed strategy to use the R=80% BS instead.

The transmitted 20% is routed to the West edge of the PSL table via 2 1" Y1-1037-45S optics, towards the rough vicinity of the fiber coupler. For now it is just dumped, tomorrow I will work on the mode matching. We may want to cut the power further - ideally, we want ~2.5mW of power in the fiber - this is then divided by 4 inside the beat mouth before reaching the beat PD, and with other losses, I expect ~500mW of PSL power and comparable AUX light, we will have a strong >0dBm beat.

Attachment #1 is a picture of my modifications. For this work, I

• Closed PSL shutter, turned HEPA up
• Moved HP GHz spec analyzer to the side for ease of access to the table.
• Moved several optics that look to me as to have once been part of the RefCav setup - I don't think this would have been a useful alignment reference in any case as we moved the RefCav in a non-deterministic way for the PSL secondary shelf install.
• Used one 1" 45 deg S-pol optic from the optics cabinet - remaining optics were scavenged from PSL table and SP table.
• Removed an SMA cable connected to an EOM, whose other end wasn't connected to anything.
• Turned HEPA back down, IMC locks fine now.

Attachment 1: IMG_6866.JPG
13573   Wed Jan 24 00:58:59 2018 gautamUpdateALSX Green PDH modulation depth

On Friday, while Udit and I were doing some characterization of the EX+PSL IR beat at the LSC rack, I noticed that there were sidebands around the main beat peak at 20dBm lower level. These were offset from the main peak by ~200kHz - I didn't do a careful characterization but because of the symmetric nature of these sidebands and the fact that they appeared with the same offset from the main peak for various values of the central beat frequency, I hypothesize that these are from the modulation sidebands we use for PDH locking the EX laser to the arm cavity. So we can estimate the modulation depth from the relative powers of the main beat peak and the ~200kHz offset sidebands.

Since the IR light is used for the beat and we directly couple it to the fiber to make the beat, there is no green or IR cavity pole involved here. 20dBm in power means $\frac{\beta^2}{4} \approx 10^{\frac{-20}{10}} \approx 0.01$. And so the modulation depth, $\beta \approx 0.2 \mathrm{rad}$. I will do a more careful meaurement of this, but this method of measuring the modulation depth can give us a precise estimate - for what it's worth, this number is in the same ballpark as the measurement I quote in elog12105.

What is the implication of having these sidebands on our ALS noise? I need to think about this, effectively the phase noise of the SR function generators we use to do the phase modulation of the EX laser is getting imprinted on the ALS noise? Is this hurting us in any frequency range that matters?

13574   Wed Jan 24 10:45:14 2018 gautamUpdateALSFiber ALS assay

I was looking into the physics of polarization maintaining fibers, and then I was trying to remember whether the fibers we use are actually polarization maintaining. Looking up the photos I put in the elog of the fibers when I cleaned them some months ago, at least the short length of fiber attached to the PD doesn't show any stress elements that I did see in the Thorlabs fibers. I'm pretty sure the fiber beam splitters also don't have any stress elements (see Attached photo). So at least ~1m of fiber length before the PD sensing element is probably not PM - just something to keep in mind when thinking about mode overlap and how much beat we actually get.

13583   Thu Jan 25 13:18:41 2018 gautamUpdateALSFiber ALS assay

I was looking at this a little more closely. As I understand it, the purpose of the audio differential IF amplifier is:

1. To provide desired amplification at DC-audio frequencies
2. To low pass the 2f component of the mixer output

Attachment #1 shows, the changes to the TF of this stage as a result of changing R19->50ohm, R17->500ohm. For the ALS application, we expect the beat signal to be in the range 20-100MHz, so the 2f frequency component of the mixer output will be between 40-200MHz, where the proposed change preserves >50dB attenuation. The Q of the ~500kHz resonance because of the series LCR at the input is increased as a result of reducing R17, so we have slightly more gain there.

At the meeting yesterday, Koji suggested incorporating some whitening in the preamp itself, but I don't see a non-hacky way to use the existing PCB footprint and just replace components to get whitening at audio frequencies. I'm going to try and measure the spectrum of the I and Q demodulated outputs with the actual beat signal to see if the lack of whitening is going to limit the ALS noise in some frequency band of interest.

Does this look okay?

 Quote: The demod circuit board is configured to have gain of x100 post demod (conversion loss of the mixer is ~-8dB). This works well for the PDH cavity locking type of demod scheme, where the loop squishes the error signal in lock, so most of the time, the RF signal is tiny, and so a gain of x100 is good. For ALS, the application needs are rather different. So we lowered the gain of the "Audio IF amplifier" stage of the circuit from x100 to x10, by effecting the resistor swaps 10ohms->50ohms, 1kohm->500ohms (more details about this later).
Attachment 1: preampProposed.pdf
13586   Thu Jan 25 23:59:14 2018 gautamUpdateALSFiber ALS assay

I tried to couple the PSL pickoff into the fiber today for several hours, but got nowhere really, achieved a maximum coupling efficiency of ~10%. TBC tomorrow... Work done yesterday and today:

• I changed the collimator from the fixed focal length but adjustable lens position CFC-2X-C to the truly fixed F220-APC-1064 recommended by johannes.
• Used a pair of irises to level the beam out at 4" with two steering mirrors.
• Used a connector on the PSL table to couple the EX laser light to the PSL fiber - then measured the mode using the beam-scanner (beam is ~300uW)
• Measured the mode of the PSL pickoff beam, also using the beam scanner.
• Per specs on the Thorlabs website, the F220-APC-1064 has a divergence angle of 0.032 degrees. So expected waist is ~1200um, and the Rayleigh range is ~4.3m, so this is not a very easy beam to measure and fit. I may be thinking about this wrong?
• Measured beam 1/e^2 dia over ~0.65m, and found it to be fairly constant around 1800um (so waist of 900um) - beam is also pretty symmetric in x and y directions, but I didn't attempt an M^2 measurement.
• The pickoff from the PSL also did not yield a very clean beam profile measurement, even though I measured over ~1m z-propagation distance. Nevertheless, this looked more like a Gaussian beam, and I confirmed the fitted waist size/location approximately by placing the beam profiler at the predicted waist location and checking the spot size.
• Used jammt to calculate a candidate mode-matching solution - the best option seemed to be to use a combination of a f=150mm and f=-75mm lens in front of the collimator.
• Despite my best efforts, I couldn't get more than ~500uW of light coupled into the fiber - out of the 8mW available, this is a paltry 12.5%
• Because the mode coming out of the fiber is relatively large, and because I have tons of space available on the PSL table, this shouldn't be a hard mode-matching problem, should be doable without any fast lenses - perhaps I'm doing something stupid and not realizing it. I'm giving up for tonight and will try a fresh assault tomorrow.
13587   Fri Jan 26 20:03:09 2018 gautamUpdateALSFiber ALS assay

I think part of the problem was that the rejected beam from the PBS was not really very Gaussian - looking at the spot on the beam profiler, I saw at least 3 local maxima in the intensity profile. So I'm now switching strategies to use a leakage beam from one of the PMC input steering optics- this isn't ideal as it already has the PMC modulation sideband on it, and this field won't be attenuated by the PMC transmission - but at least we can use a pre-doubler pickoff. This beam looks beautifully Gaussian with the beam profiler. Pics to follow shortly...

 Quote: I tried to couple the PSL pickoff into the fiber today for several hours, but got nowhere really, achieved a maximum coupling efficiency of ~10%. TBC tomorrow... Work done yesterday and today

13591   Wed Jan 31 15:45:22 2018 gautamUpdateALSFiber ALS assay

Attachment #1 shows the current situation of the PSL table IR pickoff. It isn't the greatest photo but it's hard to get a good one of this setup. Now there is no need to open the Green PSL shutter for there to be an IR beat note.

• The key to improving the mode-matching was to abandon my "measurements" of the input mode and the mode from the collimator.
• The best I could do with these measurements was ~25% coupling, whereas now I have ~78% (all powers measured with Ophir power meter).
• Focusing was done using two f=300mm lenses (see attachment).
• By moving the second (closer to collimator) lens through ~1inch of its current position, I was able to see a clear maximum of the coupled power.
• By moving the second lens by ~5mm, and touching up the alignment, I couldn't see any improvement.

All this lead me to conclude that I have reached at least some sort of local maximum. The AR coating of the lens has ~0.5% reflection at 8 degrees AOI according to spec, and EricG mentioned today that the fiber itself probably has ~4% reflection at the interface due to there not being any special AR coating. There is also the fact that the mode of the collimator isn't exactly Gaussian. Anyways I think this is a big improvement from what was the situation before, and I am moving on to debugging the ALS electronics.

There is 3.65mW of power coupled into the fiber - our fiber coupled PDs have a damage threshold of 2mW, and this 3.65mW does get split by 4 before reaching the PDs, but good to keep this number in mind. For a quick measurement of the PMC and X end PDH modulation depth measurements, I used an ND=0.5 filter in the beam path.

Attachment 1: IMG_6875.JPG
13593   Wed Jan 31 16:29:42 2018 gautamUpdateALSModulation depths

I used the Beat Mouth to make a quick measurement of the PMC and EX modulation depths. They are, respectively, 60mrad and 90mrad. See Attachments #1 and #2 for spectra from the beat photodiode outputs, monitored using the Agilent analyzer, 16 averages, IF bandwidth set to resolve peaks offset from the main beat frequency peak by 33.5MHz for the PMC and by ~230kHz for the EX green PDH.

For this work, I had to re-align the IFO so as to lock the arms to IR. c1susaux was unresponsive and had to be power-cycled. As mentioned in the earlier elog, to avoid saturating the Fiber Coupled beat PDs, I placed a ND=0.5 filter in the fiber collimator path, such that the coupled power was ~1mW, which is well inside the safe regime.

For the EX modulation depth, I could have gotten multiple estimates of the modulation depth using the higher order products that are visible in the spectrum, but I didn't.

Attachment 1: PMCmodDepth.pdf
Attachment 2: XPDH.pdf
13594   Wed Jan 31 16:33:53 2018 gautamUpdateALSALS electronics at LSC rack

[steve, gautam]

We installed some rails to mount the 2U chassis containing ~100m of delay line cabling, and the 1U chassis containing the FET demodulators for the ALS signals in the LSC rack. This has made it MUCH easier for a single person to work there and remove/reinstall these chassis. The delay line box has 100m of cable inside it, and so was rather heavy (~8kg) - previously, it was being supported only by a pair of brackets on the front, so the new arrangement is much more robust. Steve is looking into acquiring plastic spacers of the appropriate width, so that we can secure the units to the rack using usual rack mount screws (but the material of the newly installed rails and the screw heads holding them in place necessitate this plastic spacer).

Delay line box has been re-installed, demodulator chassis has been removed by me for characterization. Steve will put up photos once the units are re-installed.

For this work, I had to disconnect a bunch of cabling, but only those connected to ALS. All cables were labelled, and I will re-connect them once I am done with the demod chassis.

 Quote: Anyways I think this is a big improvement from what was the situation before, and I am moving on to debugging the ALS electronics.

13595   Wed Jan 31 22:32:11 2018 gautamUpdateALSALS signal chain + power budget

Summary:

## I do not have an answer to the question "What is an appropriate gain for the IF amplifier stage in the D0902745 FET demod boards?", because of the following problems.

Deatils:

The plan is to lower the gain of the IF amplifier stage on the FET demodulator board from 100 to 10. As per Attachment #1, this will make the overall gain from RF beatnote from the Beat Mouth to the signal input to the D990694 whitening board +19dB, assuming "typical" values for the conversion loss of the mixer, and the various other passive components on the FET demod board. I've used numbers I measured a couple of weeks ago for the delay line loss and the cabling loss from the PSL table to the LSC rack. This in turn will set a limit on how much RF beat power we can handle, from the Beat Mouth. According to this power budget, if we have -5dBm of beat, we will have an input to the whitening board of ~6Vpp, which is about half its full range. The trouble is, I don't know what the transimpedance gain of the Fiber Beat PDs are. The datasheet suggests a "maximum gain" of 5e4 V/W, which presumably takes into account the InGaAs responsivity and the actual transimpedance gain. However, according to the last power budget I did inside the Beat Mouth, I had -8dBm of beat for a combined 400uW of PSL+EX light, which definitely does not add up. I've emailed the company to ask about the spec, haven't gotten anything useful yet...

The problem is further complicated by the fact that the fiber inside the Beat Mouth is NOT polarization maintaining, and so the actual relative polarizations of the arm IR light and the PSL IR light is unpredictable, and also uncontrolled. I suppose we could simply place a HWP before the fiber collimator at either end, and rotate the polarization until we get a desired amount of beat, but this still does not solve the problem of the polarization being uncontrolled.

I am going to characterize the demod board using E1100114. I am unsure as to the conversion loss of the mixer - the datasheet suggested a number of 8dB, but T1000044 suggests that the conversion loss is actually only 4dB. I figure it's best to just measure it. Would also be good to verify that the overall transfer function and noise of the IF amplifier stage match my expectation from the LISO model.

Option #1: Rana ordered 50ohm and 500ohm SMD resistors of the 0805 package size, I asked Steve to get a few more values just in case we want to twiddle with the gain of this stage further (specifically, I asked for values such that we can set it to x5, x3 and x1). But changing the feedback resistors modifies the overall TF shape - see e.g. Attachment #2. Need to also look at how the noise performance varies.

Another possibility is to turn down the gain of the IF amplifier stage to x10, retire the ZHL-3A, and use a lower gain amplifier in its place. We do have the recently acquired Teledyne amplifiers, but we would have to package it in such a way that it can be integrated into the existing Fiber ALS signal chain. This would allow us to handle significantly larger RF beatnote powers, which I expect we will have if we improve the mode matching into the fibers (provided the aforementioned polarization drift possibility doesn't hurt us too much).

A third possibility is to attenuate the power coupled into the fibers to lower the RF beatnote amplitude. I don't like this option so much because placing an ND filter or a PBS+HWP combo in the beam path is likely to screw up the mode-matching into the fiber collimator, which I have already spent so many hours trying to improve, but if it must be done, it must be done.

The correct option is of course the one that gives us the lowest ALS noise. It is not clear to me which one that is at this point.

Attachment 1: FiberALS_PowerBudget.pdf
Attachment 2: preampProposed.pdf
13596   Thu Feb 1 01:24:56 2018 gautamUpdateALSD0902745 revamp underway

I effected the change to the Audio IF preamp stage on channels 3 and 4 (Xarm and Yarm respectively) using the resistors Steve ordered (the ones Rana ordered don't have any labeling on them, and I couldn't tell the 50ohm and 500ohm ones apart except by looking at the label on the ziplock bag they came in, so I decided against using them). I've started a DCC page to collect photos, characterization data, and marked up schematic etc for this part. Characterization is ongoing, more to follow soon. Note that for the photo-taking, I disconnected all the on-board SMA connectors so that the cabling wouldn't block components. I have since restored them for testing purposes, and was careful to use the torque-limited SMA tightening tool when restoring the connections.

In order to test various things like conversion loss etc, I figured it would be useful to have two RF signal sources, so I scavenged the Fluke RF generator that Johannes was using from under the PSL table. In the process, I accidentally bumped the PSL interlock on the southeast corner of the PSL table. I immediately turned the NPRO back on, and relocked PMC/IMC. Everything looks normal now. Acromag may even have caught my transgression.

 Quote: I am going to characterize the demod board using E1100114. I am unsure as to the conversion loss of the mixer - the datasheet suggested a number of 8dB, but T1000044 suggests that the conversion loss is actually only 4dB. I figure it's best to just measure it. Would also be good to verify that the overall transfer function and noise of the IF amplifier stage match my expectation from the LISO model.

Attachment 1: PSLinterlock.png
13597   Thu Feb 1 15:31:12 2018 gautamUpdateALSALS signal chain + power budget

Summary:

### Details:

Stuff is beginning to look clearer now that I've done some initial characterization of the demod boards. I will upload a more detailed report of the characterization on the DCC page, but important findings are:

1. The overall conversion factor from RF to IF is ~2.3V IF per volt of RF.
• 50ohm source connected to RF input of demod board, level = 10dBm on Marconi screen, consistent with inferred value from RF mon output.
• LO driven at 14dBm by Fluke function generator.
• The ratio was calculated for IF voltage input into a High-Z load.
• So let's say we want to run at half the ADC full range of 10Vpp into the whitening board - this means we need to keep the RF input to <=11dBm.
2. The Teledyne amplifier has a rated maximum input voltage of 17dBm. If we want to stay 3dB below this, we can send in 14dBm into the LO input of the demod board, which is what my characterizations were done with.

The delay line has a loss of ~3dB. The power splitter has a loss of 3dB. So putting everything together, 17dBm at the input of the power splitter gives us just the right amount of RF power to have the LO input driven at 14dBm, and the IF output be ~5Vpp into a High-Z load, which is about half the ADC full range.

13599   Fri Feb 2 00:26:34 2018 gautamUpdateALSD0902745 revamp underway

I saw some interesting behaviour of the Audio IF amplifier stage on the demod board today, by accident. I was testing the board for I/Q orthogonality and gain balance, when I noticed a large gain imbalance between the I and Q channels for both Board #3 and #4, which are the ones we use for the IR ALS demodulation. This puzzled me for some time, but then I realized that I had only reduced the gain of this stage from x100 to x10 for the I channel, and not for the Q channel! The surprising thing though was that the output waveform still looked like a clean sinusoid on the o'scope, and there was no evidence of the voltage clipping that is characteristic of an op-amp being driven beyond its voltage rails. The conversion factor with a preamp gain on x10 was measured today to be 2V IF / 1V RF. But this means that for a preamp stage gain of x100, we expect 20V IF / 1V RF, which is well in the saturation regime of the AD829, since the Vcc is only +/-15V. I'm guessing the diodes D2 and D3 are for overvoltage protection, but given that the pre-amp gain is x100, the input signal at the inverting input of the AD829 is only 0.2V at DC, which isn't above the forward bias voltage for the switching diode BAV99. Perhaps there is some interaction between the pre-amp and the FET demodulator that I dont understand, or I am missing something about the differential to single-ended topology that would explain this behaviour.

I found it puzzling why the large preamp stage gain didn't hurt us with the green beat - even though the green optical beat signal was smaller than the current IR beat, a back-of-the-envelope calculation suggested that it would still have saturated the ADC with a x100 gain on the preamp. Perhaps this observation is part of the story, and there is also the unpredictable behaviour of the D990694 board for an input signal with large DC levels...

I did the following tests on this board today:

• Check +/-15V supplies, power reg board.
• Check DC offset on I and Q front panel output with LO driven at +10dBm, RF input terminated. Found it to be 0.
• Checked calibration of back-panel DSUB connector monitors for LO and RF powers. Data to be uploaded, looked quite linear.
• Checked conversion gain from RF input to IF output for two sets of LO/RF powers.
• Measured conversion gain as a function of the IF frequency (i.e. frequency offset between LO and RF inputs, out to ~700kHz, 8 datapoints)
• Checked orthogonality and gain balance of the I and Q outputs.
• Measured the noise of the I and Q outputs in the audio frequency range using the SR785.

I didn't really measure the transfer function of the preamp stage after the modification because there wasn't a convenient test point and I couldn't find the high impedance FET probe for the Agilent - I wonder if somebody in WB has it? Anyways, all the tests suggested the board is operating as expected, and I now have calibrations for the back panel DSUB for LO/RF power levels, and also the conversion gain from RF to IF. I will put together a python notebook with all my measurements and upload it to the DCC page for this part. I need to double check expected noise levels from LISO to match up to the measurement.

I will now proceed to the next piece (#3?) of this puzzle, which is to understand how the D990694 which receives the signals from this unit reacts to the expected DC voltage level of ~4Vpp.

After discussion with Koji, I have also decided to look into putting together a daughter board for an alternative Audio IF preamp stage. The motivation is that for the ALS application, we expect a high DC signal level all the time (because the loop does not suppress the beat note amplitude). So we would like for the preamp stage to have the usual shape of some zero around 4Hz, a pole around 40Hz, and then the LowPass profile of the existing preamp stage (to cut out the 2f frequency product, but also to minimize the possibility of the fast AD829 going into some unpredictable regime where it oscillates). So, the desired features are:

• Whitening (z,p) at (4Hz,40Hz) or (15Hz,150Hz) so that we have frequency dependent gain that can handle the large DC signal level expected. Need to measure noise of the actual IR beat signal to determine what the appropriate whitening shape is.
• Low-pass above a few 100kHz to cut out 2f modulation product
• Low-passing at input of AD829 (or just use OP27?)
13600   Fri Feb 2 13:16:55 2018 gautamUpdateALSALS signals whitening switching

While setting up for this measurement, I noticed something odd with the whitening switching for the ALS channels. For the usual LSC channels, the whitening is set up such that switching FM1 on the MEDM screen changes a BIO bit which then enables/disables the analog whitening stage. But this feature doesn't seem to be working for the ALS channels - I terminated all 4 channels at the LSC rack, and measured the spectrum of the IN1 signals with DTT in the two settings, such that I expect to see a difference in the spectra if the whitening is enabled or disabled - FM1 enabled (expected analog whitening to be engaged) and FM1 disabled (expected analog whitening to be bypassed). But I see no difference in the spectra. I confirmed that the BIO bit switching is happening at least on the software level (i.e. the bit indicator MEDM screens indicate state toggling when FM1 is ON/OFF). But I don't know if something is amiss in the signal chain, especially since we are using Hardware channels that were previously used for AS_165 and POP_55 signals.

Is the whitening shape such that we expect the terminated noise level to be below ADC Noise even when the whitening is engaged? I just checked the shape of the de-whitening filter, and it has -40dB gain above 150Hz, so the inverse shape should have +40dB gain.

 Quote: I will now proceed to the next piece (#3?) of this puzzle, which is to understand how the D990694 which receives the signals from this unit reacts to the expected DC voltage level of ~4Vpp

gautam 2.15pm: This was a FALSE ALARM, with the inputs terminated, the electronics noise really is that low such that it is buried under ADC noise even with +40dB gain. I cranked up the flat whitening gain from 0dB to 45dB for the X channels (but left the Y channels at 0dB). Attachment #2 is the comparison. Looks like the switching works just fine.

Attachment 1: ALS_whitening_switching.pdf
Attachment 2: ALS_whitening_switching_works.pdf
13606   Mon Feb 5 14:11:01 2018 gautamUpdateALSHuge harmonics in ALS channels

I've been trying to setup for the THD measuremetn at the LSC rack for a couple of days now, but am plagued by a problem summarized in Attachment #1: there are huge harmonics present in the channel when I hook up the input to the whitening board D990694 to the output of a spare DAC channel at the LSC rack. Attachment #2 summarizes my setup. I've done the following checks in trying to debug this problem, but am no closer to solving it:

• The problem is reminiscent of the one I experienced with the SR785 not too long ago - there the culprit was a switching power supply used for the Prologix GPIB-ethernet box.
• Then I remembered sometime ago rana and i had identified the power supply for the Fibox at the LSC rack as a potential pollutant. But today, I confirmed that this power supply is not to blame, as I unplugged it from its powerstrip and the spectrum didn't change.
• There are a couple of Sorensens in this LSC rack, from what it looks like, they supply power to a BIO interface box in the LSC rack. I thought we would want to keep this rack free of switching power supplies? Wasn't that the motivation behind keeping the (linear) power supplies for all the LSC rack electronics in a little separate rack along the east arm?
• I confirmed that when the D990694 input is terminated, these harmonics are no longer present.
• I plugged the output of the SOS dewhite board to an oscilloscope - there is a ~20mVpp signal there even when the DAC output is set to 0, but this level seems too small to explain the ~1000 ct-pp signal that I was seeing. The whitening gains for these channels are set to 0dB.
• I also looked at the signal in the time domain using DTT - indeed the peak-to-peak signal is a few thousand counts.
• This isn't a problem with the particular input channel either - the behaviour can be reproduced with any of the 4 ALS input channels.

Am I missing something obvious here? I think it is impossible to do a THD measurement with the spectrum in this condition...

Attachment 1: ALSpowerlineHarmonics.pdf
13608   Mon Feb 5 22:57:28 2018 gautamUpdateALSHuge harmonics in ALS channels

Did some quick additional checks to figure out what's going on here.

1. The SOS/dewhite board for which I didn't have a DCC number is D000316. It has a single ended output.
2. I confirmed that the origin of this noise has to do with the ground of the aforementioned D000316 - as mentioned in my previous elog, having one end of a BNC cable plugged into the whitening board D990694 and the other end terminated in 50ohms yields a clean spectrum. But making the ground of this terminator touch the ground of the SMA connector on the D000316 makes the harmonics show up.
3. Confirmed that the problem exists when using either the SMA or the LEMO monitor output of these D000316 boards.

So either something is busted on this board (power regulating capacitor perhaps?), or we have some kind of ground loop between electronics in the same chassis (despite the D990694 being differential input receiving). Seems like further investigation is needed. Note that the D000316 just two boards over in the same Eurocrate chassis is responsible for driving our input steering mirror Tip-Tilt suspensions. I wonder if that board too is suffering from a similarly noisy ground?

 Quote: Am I missing something obvious here? I think it is impossible to do a THD measurement with the spectrum in this condition...

13609   Tue Feb 6 11:13:26 2018 gautamUpdateALSPossible source of ground loop identified

I think I've narrowed down the source of this ground loop. It originates from the fact that the DAC from which the signals for this board are derived sits in an expansion chassis in 1Y3, whereas the LSC electronics are all in 1Y2.

• I pulled the board out and looked at the output of Ch8 on the oscilloscope with the board powered by a bench power supply - signal looked clean, no evidence of the noisy ~20mVpp signal I mentioned in my previous elog.
• Put the board back into a different slot in the eurocrate chassis and looked at the signal from Ch8  - looked clean, so the ground of the eurocrate box itself isn't to blame.
• Put the board back in its original slot and looked at the signal from Ch8 - the same noisy signal of ~20mVpp I saw yesterday was evident again .
• Disconnected the backplane connector which routes signals from the DAC adaptor box to D000316 board - noisy signal vanished .

Looking at Jamie's old elog from the time when this infrastructure was installed, there is a remark that the signal didn't look too noisy - so either this is a new problem, or the characterization back then wasn't done in detail. The main reason why I think this is non-ideal is because the tip-tilt steering mirrors sending the beam into the IFO is controlled by analogous infrastructure - I confirmed using the LEMO monitor points on the D000316 that routes signals to TT1 and TT2 that they look similarly noisy (see e.g. Attachment #1). So we are injecting some amount (about 10% of the DC level) of beam jitter into the IFO because of this noisy signal - seems non-ideal. If I understand correctly, there is no damping loops on these suspensions which would suppress this injection.

How should we go about eliminating this ground loop?

 Quote: So either something is busted on this board (power regulating capacitor perhaps?), or we have some kind of ground loop between electronics in the same chassis (despite the D990694 being differential input receiving). Seems like further investigation is needed. Note that the D000316 just two boards over in the same Eurocrate chassis is responsible for driving our input steering mirror Tip-Tilt suspensions. I wonder if that board too is suffering from a similarly noisy ground?

13612   Tue Feb 6 22:55:51 2018 gautamUpdateALSPossible source of ground loop identified

[koji, gautam]

We discussed possible solutions to this ground loop problem. Here's what we came up with:

1. Option #1 - Configure the DAC card to receive a ground voltage reference from the same source as that which defines the LSC rack ground.
2. Option #2 - construct an adapter that is differential-to-single ended receiving converter, which we can then tack on to these boards.
3. Option #3 - use the D000186-revD board as the receiver for the DAC signals - this looks to have differential receiving of the DAC signals (see secret schematic).  We might want to modify the notches on these given the change in digital clock frequency

Why do we care about this so much anyways? Koji pointed out that the tip tilt suspensions do have passive eddy current damping, but that presumably isn't very effective at frequencies in the 10Hz-1kHz range, which is where I observed the noise injection.

Note that all our SOS suspensions are also possibly being plagued by this problem - the AI board that receives signals is D000186, but not revision D I think. But perhaps for the SOS optics this isn't really a problem, as the expansion chassis and the coil driver electronics may share a common power source?

gautam 1530 7 Feb: Judging by the footprint of the front panel connectors, I would say that the AI boards that receive signals from the DACs for our SOS suspended optics are of the Rev B variety, and so receive the DAC voltages single ended. Of course, the real test would be to look inside these boards. But they certainly look distinct from the black front panelled RevD variant linked above, which has differential inputs. Rev D uses OP27s, although rana mentioned that the LT1125 isn't the right choice and from what I remember, LT1125 is just Quad OP27...

13613   Wed Feb 7 10:16:26 2018 gautamUpdateALSALS signal chain + power budget

After emailing the technical team at Menlo, I have uploaded the more detailed information they have given me on our wiki.

 Quote: The trouble is, I don't know what the transimpedance gain of the Fiber Beat PDs are. The datasheet suggests a "maximum gain" of 5e4 V/W, which presumably takes into account the InGaAs responsivity and the actual transimpedance gain.

13616   Wed Feb 7 15:51:15 2018 gautamUpdateALSD0902745 revamp complete

Summary of my tests of the demod boards, post gain modification:

• DC tests (supply voltage, DC offsets at I and Q outputs, power LEDs etc)
• RF tests
• Back panel RF and LO power level monitor calibration
• Coupling factor from RFinput to RFmon channel
• Conversion loss as a function of demodulated beat frequency
• Orthogonality and gain balance test
• Linearity of unit as a function of RF input level
• Electronics oise in the 1-10kHz band at the IF outputs.

Everything looks within the typical performance specs outlined in E1100114, except that the measured noise levels don't quite line up with the LISO model predictions. The measurement was made with the scheme shown in Attachment #1. I didn't do a point-by-point debugging of this on the board. I have uploaded the data + notebook summarizing my characterization to the DCC page for this part. I recommend looking at the HTML version for the plots.

*I'd put up the wrong attachment, corrected it now...

 Quote: I will put together a python notebook with all my measurements and upload it to the DCC page for this part. I need to double check expected noise levels from LISO to match up to the measurement.

gautam 9 Feb 2018 9pm: Adding a useful quote here from the LISO manual (pg28). I think if I add the Johnson noise from the output impedance of the mixer (assumed as 50ohms, I get better agreement between the measured and observed noises (although the variance between the 4 channels is still puzzling). The other possible explanation is small variations in the voltage noise at the various mixer output ports. Could we also be seeing the cyclostationary shot noise difference between the I and Q channels?

For the computation of noise, the distinction between uinput and iinput is ignored, since no input signal is assumed. The source-impedance given in the uinput or iinput instruction is assumed to be connected from the input node to ground. It will affect the gain of noise contributions from their source to the output. The impedance itself is considerednoise-free, i.e. no Johnson noise is computedfor it. If you want to compute the source impedance’s Johnson noise, you must explicitly enter it as a resistor.

In any case, I am happy with this level of agreement, so I am going to stick this 1U chassis back in its rack with the primary aim of measuring a spectrum of the beatnote, so that I have some idea of what kind of whitening filter shape is useful for the ALS signals. May need to pull it out again for actually implementing the daughter board idea though... I have updated DCC page with LISO source, and also the updated python notebooks.

Attachment 1: 6613CD37-5014-44AE-B1FE-6F6A8137DF62.jpeg
13621   Thu Feb 8 00:33:20 2018 gautamUpdateALSD990694 characterization / THD measurement plan

I decided to try doing the THD measurement with a function generator. Did some quick trials tonight to verify that the measurement plan works. Note that for the test, I turned off the z=15,p=150 whitening filter - I'm driving a signal at ~100Hz and should have plenty of oomph to be seen above ADC noise.

1. Checked for ground loops - seem to be fine, see black trace on Attachment #1 which was taken with the FnGen hooked up to the input, but not putting out any signal
2. Spectrum with 1Vpp sine wave @ ~103Hz. The various harmonic peaks are visible, and though I've not paid attention to bin width etc, the largest harmonics are ~1000x smaller than the main peak, and so the THD is ~1ppm, which is in the ballpark of what the datasheet tells us to expect around 100Hz for a gain of ~10 (=20dB). The actual gain was set at 0dB (so all opAmps in the quad bypassed)

I'm going to work on putting together some code that gives me a quick readback on the measured THD, and then do the test for real with different amplitude input signal and whitening gain settings.

**Matlab has a thd function, but to the best of my googling, can't find a scipy.signal analog.

To remind myself of the problem, summarize some of the discussion Koji and I had on the actual problem via email, and in case I've totally misunderstood the problem:

1. The "Variable Gain" feature on the D990694 boards is achieved by 4 single gain stages cascaded together in series, with the ability to engage/bypass each stage individually.
2. The 4 gain stages are constructed using the 4 OpAmps in a quad LT1125 IC, each in standard non-inverting configuration.
3. The switches unfortunately are on the output side of each op-amp. This means that even if a stage is bypassed, the signal reaches the input pin of the OpAmp.
4. For proper operation, in closed-loop, the differential voltage between the input pins of the OpAmp are 0.
5. But this may require the OpAmp to source more current than it can (just using Ohms law and the values of the resistors in the feedback path).
6. As a result, a large differential voltage develops between the input pins of the OpAmp.
7. The LT1125 is not rated to operate in such conditions (this is what Hartmut was saying in the ilog linked earlier in this thread).
8. Part of the internal protection mechanism to prevent damage to the IC in such operating conditions is a pair of diodes between the input pins of the OpAmp.
9. When a large differential voltage develops between the input pins of the OpAmp, the diodes act to short the two to bring them to the same potential (minus whatever small drop there is across the diodes). Actually, according to the datasheet, when the differential voltage between the input pins exceeds 1.4V, the input current must be limited to 25mA, to avoid damaging the protection diodes? If so, we may already have damaged a bunch of these amplifiers.
10. While the LT1125 IC is protected in this condition, the infinite input impedance of the OpAmp is reduced to the resistance between the inverting input and ground. The output voltage may still be saturated, but the output current draw is within what the IC can supply.
11. As a result, Ohms law means that the preceeding stage is overdrawn for current. This is clearly not ideal.
12. Another possible problem is that there is some sort of interaction between the 4 opamps in the quad IC, which means that even if one stage is overdrawn for current, all of them may be affected.
13. The Advanced LIGO version of this board addresses #11 and #12 by (i) placing a series resistor between the input signal and the non-inverting input of the opamp, and (ii) using single opamp ICs instead of a quad, respectively.

So my question is - should we just cut the PCB trace and add this series resistance for the 4 ALS signal channels, and THEN measure the THD? Since the DC voltage level of the ALS signal is expected to be of the order of a few volts, we know we are going to be in the problematic regime where #11 and #12 become issues.

Attachment 1: D990694THD_trial.pdf
13622   Thu Feb 8 01:27:16 2018 KojiUpdateALSD990694 characterization / THD measurement plan

> So my question is - should we just cut the PCB trace and add this series resistance for the 4 ALS signal channels, and THEN measure the THD?

13623   Thu Feb 8 12:00:09 2018 gautamUpdateALSD990694 is NOT differential receiving

Correcting a mistake in my earlier elog: the D990694 is NOT differential receiving, it is single ended receiving via the front panel SMA connectors. The aLIGO version of the whitening board, D1001530 has an additional differential-to-single-ended input stage, though it uses the LT1125 to implement this stage. So the possibility of ground loops on all channels using this board will exist even after the planned change to install series resistance to avoid current overloading the preceeding stage.

 Quote: So either something is busted on this board (power regulating capacitor perhaps?), or we have some kind of ground loop between electronics in the same chassis (despite the D990694 being differential input receiving). Seems like further investigation is needed. Note that the D000316 just two boards over in the same Eurocrate chassis is responsible for driving our input steering mirror Tip-Tilt suspensions. I wonder if that board too is suffering from a similarly noisy ground?

13625   Thu Feb 8 13:13:14 2018 gautamUpdateALSD990694 pulled out

After labeling all cables, I pulled out one of the D990694s in the LSC rack (the one used for the ALS X signals, it is Rev-B1, S/N 118 according to the sticker on it).

Took some photos before cutting anything. Attachments #1-3 are my cutting plans (shown for 1 channel, plan is to do it for both ALS channels coming into this board). #1 & #2 are meant to show the physical locations of the cuts, and #3 is the corresponding location on the schematic. These are the most convenient locations I could identify on the board for this operation.

I don't know what the purpose of resistors R196, R197, R198 are. I'm assuming it has something to do with the way the ADG333ABR switches. The aLIGO board uses a different switch (MAX4659EUA+), and doesn't have an analogous resistor (though from what I can tell, it too is a CMOS SPDT switch just like the ADG333ABR, just has a lower ON resistance of 25ohm vs 45ohm for the ADG333ABR).

As for the actual resistance to be used: Let's say we don't have signals > 5V coming into this board. Then using 301ohms (as in the aLIGO boards) in series means the peak current draw will be <20mA, which sounds like a reasonable number to me. Larger series resistance is better, but I guess then the contribution of the current noise of the OpAmp keeps increasing.

Attachment 1: IMG_5131.JPG
Attachment 2: IMG_5133.JPG
Attachment 3: D990694-B_cuttingPlan.pdf
13627   Thu Feb 8 18:10:36 2018 gautamUpdateALSD990694 pulled out

This is proving much more challenging than I thought - while Cut #1 was easy to identify and execute, my initial plan for Cut #2 seems to not have isolated the input of the second opamp (as judged by DMM continuity). Koji pointed out that this is actually not a robust test, as the switches are in an undefined state while I am doing these tests with the board unpowered. It seems rather complicated to do a test with the board powered out here in the office area though - and I'd rather not desolder the 16 and 20 pin ICs to get a better look at the tracks. This PCB seems to be multilayered, and I don't have a good idea for what the hidden tracks may be. Does anyone know of a secret place where there is a schematic for the PCB layout of this board? The DCC page only has the electrical schematic drawings, and I can't find anything useful on the elog/wiki/old ilog on a keyword search for this DCC document number. The track layout also is not identical for all channels. So I'm holding off on exploratory cuts.

*I've asked Ben Abbott/Mike Pedraza about this and they are having a look in Dale Ouimette's old drives to see if they can dig up the Altium/Protel files.

13628   Fri Feb 9 13:37:44 2018 gautamUpdateALSTHD measurement trial

I quickly put together some code that calculates the THD from CDS data and generates a plot (see e.g. Attachment #1).

Algorithm is:

4. Compute THD as $\mathrm{THD} = \frac{\sqrt{\sum_{i=2}^{N}\mathrm{V}_i^2}}{V_1}$where V_i denotes an rms voltage, or in the case of a power spectrum, just the y-axis value.