40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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ID Date Author Type Category Subject
13674   Thu Mar 8 23:50:27 2018 gautamUpdateALSFirst look at new ALS electronics
• Locked single arms, dither aligned, and saved offsets to EPICS (slow) sliders in anticipation of having to reboot all vertex FEs.
• Shutdown ETMY watchdog, stopped all models on c1iscey, and shutdown that frontend.
• Walked down to Y-end, powered of c1iscey expansion chassis, and removed the ADC adaptor card.
• Stopped all models on c1lsc. Shutdown watchdog on all optics in anticipation of c1sus model failing. Shutdown the c1lsc frontend.
• Powered off the c1lsc expansion chassis. Installed the borrowed adapter card from c1iscey in c1lsc expansion chassis. Connected it to the "spare" ADC card Koji and I had installed in c1lsc expansion chassis last Wednesday.
• Connected differential output of demod board to differential input of AA chassis. Connected SCSI connector from output of AA chassis to the newly installed adapter card.
• Powered the c1lsc expansion chassis back on. Then powered c1lsc FE on.
• Walking back out to the control room, I saw that all vertex FEs had crashed. I had to go back in and hard-reboot c1sus.
• Before bringing back any models, I backed up the existing c1lsc model, and then modified c1x04 and c1lsc to use the newly acquired ALS signals for the X arm ALS signal chain.
• Restarted all vertex FE models. Everything came back up smooth. DC light is still red on c1oaf but I didn't bother trying to rectify it tonight for these tests.
• Reset appropriate LSC offsets with PSL shutter closed. Locked X arm on IR. Reset phase tracker servo gain for X arm ALS. Engaged slow temperature servo on EX laser.

Then I looked at  the spectrum, see Attachment #1. Disappointingly, it looks like the arm PDH servo is dominating the noise, and NOT unsuppressed EX laser frequency noise,. Not sure why this is so, and I'm feeling too tired to debug this tonight. But encouragingly, the performance of the new ALS signal chain looks very promising. Once I tune up the X arm loop, I'm confident that the ALS noise will be at least as good as the reference trace.

I am leaving c1iscey shutdown until this is fixed. So ETMY is not available for the moment.

Random factoid: Trying to print a DTT trace with LaTeX in the label text on pianosa causes the DTT window to completely crash - so if you dont save the .xml file, you lose your measurement.

 Quote: I made a LISO fit of the measured TF of the daughter board, so that I can digitally invert the daughter board whitening. Results attached. (Inverse) Filters have been uploaded to the ALS X Foton filter banks.

13675   Fri Mar 9 01:07:01 2018 gautamUpdateALSFirst look at new ALS electronics

[koji, gautam]

I was going to head out but then it occurred to me that I could do another simple test, which is to try and lock the X arm on ALS error signal (i.e. actuate on MC length to keep the beat between EX laser and PSL fixed, while the EX frequency is following the Xarm length). Comparing the in loop (i.e. ALS) error signal with the out-of-loop sensor (i.e. POX), it seems like POX is noisy. The curves were lined up by eye, by scaling the blue curve to match the red at the ~16Hz peaks. This supports my hypothesis in the previous elog. On the downside, could be anything. Electronics in the POX chain? The demod unit itself? Will look into it more tomorrow..

As an aside, controlling the arm with ALS error signal worked quite well, and the lock was maintained for ~1 hour.

13679   Mon Mar 12 22:08:31 2018 gautamUpdateALSNoisy POX

[kevin, gautam]

we tested my noisy POX hypothesis tonight. By locking the single arm with POX, the arm length is forced to follow PSL frequency, which is itself slaved to IMC length. From Attachment #1, there is no coherence between the arm control signal and MC_F. This suggests to me that the excess noise I am seeing in the arm control signal above 30 Hz is not originating from the PSL. It also seems unlikely that at >30Hz, anything mechanical is to blame. So I am sticking with the hypothesis that something is wonky with POX. For reference, a known "normal" arm control signal spectrum looks like the red curve in this elog.

13680   Mon Mar 12 23:57:31 2018 gautamUpdateALSNoisy POX

[kevin, gautam]

Kevin suggested I shouldn't be so lazy and test the POY spectrum as well. So we moved the timing card back to c1iscey, went through the usual dance of vertex machine reboots, and then got both single arm locks going. Attached spectrum shows that both POX and POY are noisy. I'm not sure what has changed that could cause this effect. The fact that both POX and POY appear uniformly bad, but that there is no coherence with MC_F, suggests to me that perhaps this has something to do with the work I did with Koji w.r.t. the power situation at the LSC rack. But we just checked that

1. All the demod board front panel LED indicators are green.
2. Marconi and all RF amplifier boxes are on (but we didn't actually measure any RF power levels yet tonight).
3. We checked the KEPCO power supplies in the little cabinet along the Yarm, and all of them are reporting the correct voltages/currents as per Steve's (recently updated) labels.
4. Checked the expansion chassis at the LSC rack for any red lights, there were none.

Another observation we made: note the huge bump around 70Hz in both arm control signals. We don't know what the cause of this is. But we occassionally noticed harmonics of this (i.e. 140, 210 Hz etc) appear in the control signal spectra, and they would grow with time - eventually, the X arm would lose lock (though the Y arm stayed locked).

I'm short on ideas for now so we will continue debugging tomorrow.

Unrelated to this work: Kevin reminded me that the high-pitched whine from the CRT TVs in the control room (which is apparently due to the flyback transformer) is DEAFENING. It's curious that the "chirp" to the eventual 15kHz whine is in opposite directions for the QUAD CRTs and the single display ones. Should be a Ph6 experiment maybe.

Update 2:30pm Mar 13: The furthest back I seem to be able to go in time with Frames is ~Jan 20 2018. Looking for a time when the arms were locked from back then, it seems like whatever is responsible for a noisy POX and POY was already a problem back in January. See Attachment #2. So it appears that the recent work at 1Y2 is not to blame...

13688   Mon Mar 19 15:02:29 2018 gautamUpdateALSNoisy MC sensing

The working hypothesis, since the excess noise in single arm locks is coherent between both arms, the excess sensing noise is frequency noise in the IMC locking loop (sensing because it doesn't show up in MC_F). I've started investigating the IMC sensing chain, starting with the power levels of the RF modulation source. Recall that we had changed the way the 29.5MHz signal was sent to the EOM and demod electronics in 2017. With the handheld RF power meter, I measured 13.2dBm coming out of the RF distribution box (this is routed straight from the Wenzel oscillator). This is amplified to 26dBm by an RF amplifier (ZHL-2-S) and sent to the EOM, with a coupled 16dBm part sent to a splitter that supplies the LO signal to the demod board and also the WFS boards. Lydia made a summary of expected RF power levels here, and I too seem to have labelled the "nominal" LO level to the MC_REFL demod board as +5dBm. But I measured 2.7dBm with the RF power meter. But looking closely at the schematic of the splitting circuitry, I think for a (measured) 16.7dBm input to it, we should in fact expect around 3dBm of output signal. So I don't know why I labelled the "nominal" signal level as 5dBm.

Bottom line: we are driving a level 17 mixer with more like +14dBm (a number inferred from this marked up schematic) of LO, which while isn't great, is unlikely to explain the excess noise I think (the conversion loss just degrades by ~1dB). So I will proceed to check further downstream in the signal chain.

13749   Thu Apr 12 18:12:49 2018 gautamUpdateALSNPRO channels hijacked

## Summary:

1. Today, the measured IR ALS noise for the X arm was dramatically improved. The main change was that I improved the alignment of the PSL pickoff beam into its fiber coupler.
2. The noise level was non-stationary, leading me to suspect power modulation of the RF beat amplitude.
3. I am now measuring the stability of the power in the two polarizations coming from EX table to the PSL table, using the PSL diagnostic connector channels.
4. The EX beam is S-polarized when it is coupled into the fiber. The PSL beam is P-polarized. However, it looks like I have coupled light along orthogonal axes into the fiber, such that when the EX light gets to the PSL table, most of it is in the P-polarization, as judged by my PER measurement setup (i.e. the alignment keys at the PSL table and at the EX table are orthogonal). So it still seems like there is something to be gained by trying to improve the PER a bit more.

## Details:

Today, I decided to check the power coupled into the PSL fiber for the BeatMouth. Surprisingly, it was only 200uW, while I had ~3.15mW going into it in January. Presumably some alignment drifting happened. So I re-aligned the beam into the fiber using the steering mirror immediately before the fiber coupler. I managed to get ~2.9mW in without much effort, and I figured this is sufficient for a first pass, so I didn't try too much more. I then tried making an ALS beat spectrum measurement (arm locked to IMC length using POX, green following the arm using end PDH servo). Surprisingly, the noise performannce was almost as good as the reference! See Attachment #1, in which the red curve is an IR beat (while all others are green beats). The Y arm green beat performance isn't stellar, but one problem at a time. Moreover, the kind of coherence structure between the arm error signal and the ALS beat signal that I reported here was totally absent today.

Upon further investigation, I found that the noise level was actually breathing quite significantly on timescales of minutes. While I was able to successfully keep the TEM00 mode of the PSL beam resonant inside the arm cavity by using the ALS beat frequency as an error signal and MC2 as a frequency actuator, the DC stability was very poor and TRX was wandering around by 50%. So my new hypothesis is that the excess ALS noise is because of one or more of

• Beam jitter at coupling point into fiber.
• Polarization drift of the IR beams.

While I did some work in trying to align the PSL IR pickoff into the fiber along the fast (P-pol) axis, I haven't done anything for the X end pickoff beam. So perhaps the fluctuations in the EX IR power is causing beatnote amplitude fluctuations. In the delay line + phase tracker frequency discriminator, I think RF beatnote amplitude fluctuations can couple into phase noise linearly. For such an apparently important noise source, I can't remeber ever including it in any of the ALS noise budgets.

Before Ph237 today, I decided to use my polarization monitoring setup and check the "RIN" of power in the two polarizations coming out of the fiber on the PSL table. For this purpose, I decided to hijack the Acromag channels used for the PSL diagnostics connector Attachment #2 shows that there is fluctuations at the level of ~10% in the p-polarization. This is the "desired" polarization in that I aligned the PSL beam into the fiber to maximize the power in this polarization. So assuming the power fluctuations in the PSL beam are negligible, this translates to sqrt(10) ~3% fluctuation in the RF beat amplitude. This is at best a conservative estimate, as in reality, there is probably more AM because of the non PM fibers inside the beatmouth.

All of this still doesn't explain the coherence between the measured ALS noise and the arm error signal at 100s of Hz (which presumably can only happen via frequency noise in the PSL).

Another "mystery" - yesterday, while I was working on recovering the Y arm green beat signal on the PSL table, I eventually got a beat signal that was ~20mVpp into 50ohms, which is approximately the same as what I measured when the Y arm ALS performance was "nominal", more than a year ago. But while viewing the Y arm beats (green and IR) simultaneously on an o'scope, I wasn't able to keep both signals synchronised while triggering on one (even though the IR beat frequency was half the green beat frequency). This means there is a huge amount of relative phase noise between the green and IR beats. What (if anything) does this mean? The differential noise between these two beats should be (i) phase noise at the fiber coupler/ inside the fiber itself, and (ii) scatter noise in the green light transmitted through the cavity. Is it "expected" that the relative phase noise between these two signals is so large that we can't view both of them on a common trigger signal on an o'scope? Also - the green mode-matching into the Y arm is abysmal.

Anyways - I'm going to try and tweak the PER and mode-matching into the X end fiber a little and monitor the polarization stability (nothing too invasive for now, eventually, I want to install the new fiber couplers I acquired but for now I'll only change alignment into and rotation of the fiber coupler on the EX table). It would also be interesting to compare my "optimized" PSL drift to the unoptimized EX power drift. So the PSL diagnostic channels will not show any actual PSL diagnostic information until I plug it back in. But I suspect that the EPICS record names and physical channel wiring are wrong anyways - I hooked up my two photodiode signals into what I would believe is the "Diode 1 Power" and "Laser crystal temperature" monitors (as per the schematic), but the signals actually show up for me in "Diode 2 Power" (p-pol) and "Didoe 1 Temperature" (s-pol).

Annoyingly, there is no wiring diagram - on my todo list i guess...

@Steve - could you please take a photo of the EX table and update the wiki? I think the photo we have is a bit dated, the fiber coupler and transmon PDs aren't in it...

13751   Fri Apr 13 11:02:41 2018 gautamUpdateALSEX fiber polarization drift

Attachment #1 shows the drift of the polarization content of the light from EX entering the BeatMouth. Seems rather large (~10%). I'm going to tweak the X end fiber coupling setup a bit to see if this can be improved. This performance is also a good benchmark to compare the PSL IR light polarization drift. I am going to ask Steve to order Thorlabs K6XS, which has a locking screw for the rotational DoF. With this feature, and by installing some HWPs at the input coupling point, we can ensure that we are coupling light into one of the special axes in a much more deterministic way.

13752   Fri Apr 13 16:59:12 2018 gautamUpdateALSEX green mode-matching

THIS CALCULATION IS WRONG FOR THE OVERCOUPLED CAV.

Summary:

Mode-matching efficiency of EX green light into the arm cavity is ~70*%, as measured using the visibility.

Details:

I wanted to get an estimate for the mode-matching of the EX green beam into the arm cavity. I did the following:

1. Locked arm cavities to IR. Ran dither alignment servos to maximize the transmission of IR on both arms. The X arm dither alignment servo needs some touching up, I can achieve higher TRX by hand than by running the dither.
2. Aligned green PZT mirrors so as to maximize GTRX. Achieved level as 0.47.
3. Went to EX table and tweaked the two available mode-matching lens positions on their translational stages. Saw a quadratic maximum of GTRX about some equilibrium position (where the lenses are now).
4. Measured average value of the green PDH reflection DC level whiel green TEM00 mode was locked. $P_{\mathrm{locked}} = 716 \mathrm{cts}$.
5. Misaligned ITMX macroscopically. Measured the average value of the green PDH reflection DC level again. P_{\mathrm{misaligned}} = 3800 \mathrm{cts}.
6. Closed EX Green shutter. Measured the average value of the green PDH reflection DC level. $P_{\mathrm{dark}} = 30 \mathrm{cts}$.
7. Modulation depth of the EX PDH was determined to be 90mrad. Based on this, power in sideband is negligible compared to power in the carrier, so I didn't bother correcting for sideband power in reflection.
8. Mode-matching efficiency calculated as \frac{P_{\mathrm{misaligned}} - P_{\mathrm{locked}}}{P_{\mathrm{misaligned}} + P_{\mathrm{locked}} - 2P_{\mathrm{dark}} }.

This amount of mode-matching is rather disappointing - using a la mode, the calculated mode-matching efficiency is nearly 100%, but 70% is a far cry from this. The fact that I can't improve this number by either tweaking the steering or by moving the MM lenses around suggests that the estimate of the target arm mode is probably incorrect (the non-gaussianity of the input beam itself is not quantified yet, but I don't believe this input beam can account for 30% mismatch). For the Y-arm, the green REFL DC level is actually higher when locked than when ITMY is misaligned. WTF?? Only explanation I can think of is that the PD is saturated when green is unlocked - but why does the ADC saturate at ~3000cts and not 32000?

This data is almost certainly bogus as the AA box at 1X9 is powered by +/-5VDC and not +/-15VDC. I didn't check but I believe the situation is the same at the Y-end.

3000 cts is ~1V into the ADC. I am going to change the supply voltage to this box (which also reads in ETMX OSEMS) to +/-15V so that we can use the full range of the ADC.

gautam Apr 26 630pm: I re-did the measurement by directly monitoring the REFLDC on a scope, and the situation is not much better. I calculate a MM of 70% into the arm. This is sensitive to the lens positions - while I was working on the EX fiber coupling, I had bumped the lens mounted on a translational stage on the EX table lightly, and I had to move that lens around today in order to recover the GTRX level of 0.5 that I am used to seeing (with arm aligned to maximize IR transmission). So there is definitely room for optimization here.

13753   Fri Apr 13 17:56:26 2018 gautamUpdateALSFibers switched out

I swapped the EX fiber for the PSL fiber in the polarization monitoring setup. There is a lot more power in this fiber, and one of the PDs was saturated. I should really have put a PBS to cut the power, but I opted for putting an absorptive ND1.0 filter on the PD instead for this test. I want to monitor the stability in this beam and compare it to the EX beam's polarization wandering.

13754   Sat Apr 14 14:42:09 2018 gautamUpdateALSFibers switched out

It looks like the drift in polarization content in the PSL pickoff is actually much higher than that in the EX pickoff. Note that to prevent the P-pol diode from saturating, I put an ND filter in front of the PD, so the Y axis actually has to be multiplied by 10 to compare power in S and P polarizations. If this drift is because of the input (linear) polarization being poorly matched to one of the fiber's special axes, then we can improve the situation relatively easily. But if the polarization drift is happening as a result of time-varying stress (due to temp. fluctuations, acoustics etc) on the (PM) fiber from the PSL fiber coupler to the BeatMouth, then I think this is a much more challenging problem to solve.

I'll attempt to quantify the contribution (in Hz/rtHz) of beat amplitude RIN to phase tracker output noise, which will tell us how much of a problem this really is and in which frequency bands. In particular, I'm interested in seeing if the excess noise around 100 Hz is because of beat amplitude fluctuations. But on the evidence thus far, I've seen the beat amplitude drift by ~15 dB (over long timescales) on the control room network analyzer, and this drift seems to be dominated by PSL light amplitude fluctuations.

13757   Tue Apr 17 14:08:29 2018 gautamUpdateALSFibers switched out

A follow-up on the discussion from today's lunch meeting - Rana pointed out that rotation of the fiber in the mount by ~5degrees cannot account for such large power fluctuations. Here is a 3 day trend from my polarization monitoring setup. Assuming the output fiber coupler rotates in its mount by 5 degrees, and assuming the input light is aligned to one of the fiber's special axes, then we expect <1% fluctuation in the power. But the attached trend shows much more drastic variations, more like 25% in the p-polarization (which is what I assume we use for the beat, since the majority of light is in this polarization, both for PSL and EX). I want to say that the periodicity in the power fluctuations is ~12hours, and so this fluctuation is somehow being modulated by the lab temperature, but unfortunately, we don't have the PSL enclosure temperature logged in order to compare coherence.

Steve: your  plots look like temperature driven

The "beat length" of the fiber is quoted as <=2.7mm. This means that a linearly polarized beam that is not oriented along one of the special axes of the fiber will be rotated through 180 degrees over 2.7mm of propagation through the fiber. I can't find a number for the coefficient of thermal expansion of the fiber, but if temperature driven fluctuations are changing the length of the fiber by 300um, it would account for ~12% power fluctuation between the two polarizations in the monitoring setup, which is in the ballpark we are seeing...

13773   Fri Apr 20 00:26:34 2018 gautamUpdateALSFibers switched out

Summary:

I think the dominant cause for the fact that we were seeing huge swing in the power coupled into the fiber was that the beam being sent in was in fact not linearly polarized, but elliptically polarized. I've rectified this with the help of a PBS. Fiber has been plugged into my polarization monitoring setup. Let's monitor for some long stretch and see if the situation has improved.

Details:

• The new fiber mount I ordered, K6XS, arrived today. I like it - it has little keys with which all DoFs can be locked. Moreover, it is compatible with the fixed collimators which IMO is the easiest way to achieve good mode-matching into the fiber. It is basically a plug-and-play replacement for the mounts we were using. Anyways, we can evaluate the performance over the coming days.
• I installed it on the PSL table (started work ~10pm, HEPA turned up to maximum, PSL shutter closed).
• But even with the new rotational DoF locking feature, I saw that slight disturbances in the fiber caused wild fluctuations in my polarization monitoring setup PD outputs. This was a useful tool through the night of checking the polarization content in the two special axes - Aidan had suggested using a heat gun but shaking the fiber a bit works well too I think.
• The PM980 fiber has an alignment key that is aligned with the slow axis of the fiber - so it is a useful alignment reference. But even by perturbing the roational alignment about the vertical by +/-15 degrees, I saw no improvement in this behavior. So I began to question my assumption that the input beam itself had clean polarization content.
• Since my pickoff beam has gone through a QWP and two PBSs, I had assumed that the beam was linearly polarized.
• But by putting a PBS just upstream of the input fiber coupler, I could see a beam at the S-port with an IR card (while I expected the beam to be P-polarized).
• OK - so I decided to clean up the input polarization by leaving this PBS installed. With this modification to the setup, I found that me shaking the fiber around on the PSL table didn't affect the output polarization content nearly as dramatically as before!!
• The state I am leaving it in tonight is such that there is ~100x the power in the P-polarization output monitor as the S-polarization (PER ~ 20dB). I didn't try and optimize this too much more for now, I want to observe some long term trend to see if the wild power fluctuations have been mitigated.
• The output coupler is mounted on the inferior K6X mount, and so there is the possibility that some drift will be attributable to rotation of the output coupler in it's mount. Thermally driven length changes / time varying stresses in the fiber may also lead to some residual power fluctuations. But I don't expect this to be anywhere near the ~25% I reported in the previous elog.
• The rejected beam from the PBS was measured to be ~300 uW. I haven't dumped this properly, to be done tomorrow.
• HEPA turned back down to 30%, PSL enclosure closed up, PSL shutter re-opened ~0030am.
• Note that the EX and EY fiber coupled beams are also likely subject to the same problem. We have to double check. I think it's better to have a PBS in front of the input fiber coupler as this also gives us control over the amount of light coupled into the fiber.

Power budget:

Power in Measured power (Ophir, filter OFF)
@Input coupler, before PBS 4.4 mW
P-pol content @ input coupler 4.06 mW
S-pol (rejected) from PBS 275 uW
@Output coupler 2.6 mW (MM ~65%)

13779   Sat Apr 21 20:25:12 2018 gautamUpdateALSPSL fiber pickoff status

Seems like there is still a bit of variation in the power in the two polarizations, though it is much smaller now, at the ~5% level (see Attachment #1). Since the pattern is repeating itself over the day timescale, I think this effect is not because of rotation of the output coupler in the mount, but is in fact a temperature driven waveplate effect because of imperfect alignment at the input coupler (which itself is locked down). I'm going to rotate the input coupler by 5 degrees (old = 110 degrees, new=115degrees) to see if the situation improves...

gautam Apr 24 2pm: Steve suggested confirming the correlation by hooking up the PSL table temperature sensor. This used to be logged but since the c1psl ADC card failure, has not been recorded. Assuming the sensor and preamp still work fine, we can use the PSL diagnostic Acromag (whose channels I have hijacked to monitor polarization stability already) to at least temporarily monitor the temperature inside the PSL enclosure. I am in need of a DB25 breakout board for this purpose which I am missing right now, as soon as I obtain one, I'll hook this up...

13784   Tue Apr 24 11:31:59 2018 gautamConfigurationALSProposed changes to EX fiber coupling

Motivation: I want to make another measurement of the out-of-loop ALS beat noise, with improved MM into both the PSL and EX fibers and also better polarization control. For this, I want to make a few changes at the EX table.

1. Replace existing fiber collimator with one of the recently acquired F220-APC-1064 collimators.
• This gives an output mode of diameter 2.4mm with a beam divergence angle of 0.032 degrees (all numbers theoretical - I will measure these eventually but we need a beam path of ~5m length in order to get a good measurement of this collimated beam).
• I believe it will be easier to achieve good mode matching into this mode rather than with the existing collimator.
• Unfortunately, the mount is still going to be K6X and not K6XS.
2. Improve mode-matching into fiber.
• I used my measurement of the Innolight NPRO mode from 2016, a list of available lenses, and some measured distances to calculate a solution that gives theoretical 100% overlap with the collimator mode, that has beam diameter 2.4mm, located 80cm from the NPRO shutter head location (see Attachment #1).
• The required movement of components is schematically illustrated in Attachment #2.
• One of the required lens positions is close to the bracket holding the enclosure to the table, but I think the solution is still workable (the table is pretty crowded so I didn't bother too much with trying to find alternative solutions as all of them are likely to require optics placed close to existing ones and I'd like to avoid messing with the main green beam paths.
• I will attempt to implement this and see how much mode matching we actually end up getting.
3. Install a PBS + HWP combo in the EX fiber coupling path.
• This is for better polarization control.
• Also gives us more control over how much light is coupled into the fiber in a better way than with the ND filters in current path.
4. Clean EX fiber tip.
5. Dump a leakage IR beam from the harmonic separator post doubling oven, which is currently just hitting the enclosure. It looks pretty low power but I didn't measure it.
6. Re-install EX power monitoring PD.

Barring objections, I will start working on these changes later today.

13786   Tue Apr 24 18:54:15 2018 gautamConfigurationALSProposed changes to EX fiber coupling

I started working on the EX table. Work is ongoing so I will finish this up later in the evening, but in case anyone is wondering why there is no green light...

1. EX laser shutter was closed.
2. Disconnected EX input to the beat mouth at the PSL table in order to avoid accidentally frying the PDs.
3. Prepared new optomechanics hardware
• To my surprise, I found a bubble-wrapped K6XS mount (the one with locking screws for all DoFs) on the SP table. No idea where this came from or who brought it here, or how long it has been here, but I decided to use it nevertheless.
• Prepared f = 200mm and f = -200mm lenses on traveling mounts (Thorlabs DT12, lenses are also Thorlabs, AR1064).
• Made a slight translation of the beam path towards the north to facilitate going through the center of the mounted lenses.
• Temporarily removed a beam dump from next to the final steering mirror before the Green REFL PD, and also removed one of the brackets between the enclosure and the table for ease of laying out components. These will be replaced later.
4. Installed this hardware on the PSL table, roughly aligned beam path.
• Beam now goes through the center of all lenses and is hitting the collimator roughly in the center.

To do in the eve:

1. Clean fiber and connect it to the collimator.
2. Optimize mode-matching as best as possible.
3. Attenuate power using PBS and HWP so as to not damage the BeatMouth PD (Pthresh = 2mW). These are also required to make the polarizations of the EX coupled light (S-pol) and PSL (P-pol) go along the same axis of the PM fiber.
4. Re-install temporarily removed beam dump and bracket on EX table.
5. Re-install EX power monitoring PD.
6. Measure beat frequency spectrum.
 Quote: Motivation: I want to make another measurement of the out-of-loop ALS beat noise, with improved MM into both the PSL and EX fibers and also better polarization control. For this, I want to make a few changes at the EX table.  Barring objections, I will start working on these changes later today.

gautam 1245am: Fiber cleaning was done - I'll upload pics tomorrow, but it seems like the fiber was in need of a good cleaning. I did some initial mode-matching attempts, but peaked at 10% MM. Koji suggested not going for the final precisely tunable lens mounting solution while trying to perfect the MM. So I'll use easier to move mounts for the initial tuning and then swap out the DT12s once I have achieved good MM. Note that without any attenuation optics in place, 24.81mW of power is incident on the collimator. In order to facilitate easy debugging, I have connected the spare fiber from PSL to EX at the PSL table to the main EX fiber - this allows me to continuously monitor the power coupled into the fiber at the EX table while I tweak lens positions and alignment. After a bit of struggle, I noticed I had neglected a f=150mm lens in my earlier calculation - I've now included it again, and happily, there seems to be a solution which yields the theoretical 100% MM efficiency. I'll work on implementing this tomorrow..

13789   Wed Apr 25 19:09:37 2018 gautamConfigurationALSNew look EX Fiber coupling

Summary:

I implemented most of the things outlined in my previous elog. Implementing the a la mode solution after including all lenses, I managed to achieve >90% mode-matching into the fiber. Power monitor PD has not been re-installed yet, neither has the bracket I removed. The polarization monitoring setup on the PSL table has now been hooked up to the EX fiber, let's see how it does overnight. All quoted power measurements in this elog were made with the Ophir power meter (filter off).

Details:

Attachment #1 shows the implemented MM solution. I did not include the PBS substrate in the calculation, maybe that will help a little.

Attachment #2 shows the new layout. The beam is a little low on the PBS and HWP - I will swap these out to mounts with slightly lower height, that should improve the situation a little. There is no evidence of clipping, and the beam clears all edges by at least 3 beam diameters.

Attachments #3 and #4 show the EX fiber before and after cleaning respectively. Seems like the cleaning was successful.

Attachment #5 shows the beam incident on the coupler with on an IR card. This beam only goes through a QWP, lens, BS and 45 degree steering mirror, so I'm not sure what's responsible for the large halo around the main beam. There is significant power in the halo too - I measured 25mW right before the coupler, but if I use an iris to try and cut off the halo, the power is measured to be ~19mW.

Alignment Procedure:

• Connect spare fiber such that I can monitor coupled power (minus fiber losses and joint loss) at EX table.
• Use Fluke fault analyzer to align input and collimator modes coarsely.
• Monitored coupled power continuously using Fiber Power Meter (although MM calculations were made with Ophir, this was more convenient for "Live" viewing).
• Tweaked one available steering mirror and K6XS axes to maximize coupled power.
• Tweaked lens positions slightly to see if significant improvement could be made.
• After optimizing, I measured 17.1mW coming out of the EX fiber at the PSL table. As mentioned earlier, the input power is tricky to measure given the large amount of junk light around the main mode. But I measured 18.6 mW after the iris. So this is ~95%. In any case, safe to say that we are waaaay better than the previous situation of 380uW out of 1.9mW.
• Added PBS and HWP to cut the incident power to 1.6mW. I measured 1.2mW on the PSL table. Probably adding the PBS screwed up the MM a bit, to be tweaked tomorrow.
• I had moved the Green shutter a bit during this work - as a result, the Green REFL was not making it back to the REFL PD. I remedied this, and EX Green TEM00 mode was locked to the arm. GTRX of ~0.4 was recovered, which is around the number I'm used to seeing.
13791   Thu Apr 26 11:24:50 2018 gautamConfigurationALSNew look EX Fiber coupling - pol stability

Here is a first look at the overnight stability. For the temperature, I used the calibration I found in the old psl database file, seems to give sensible results. It's only 15 hours of data plotted, so we don't see the full 24 hour temperature swing, but I think it is safe to say that for the EX fiber, the dominant cause of the "waveplate effect" is not in fact temperature drift. The polarization extinction is still better than 10dB in the entire period of observation though... I'm going to push ahead with a beat spectrum measurement, though there is room for improvement in the input coupling alignment to fiber special axes.

The apparent increase in these plots towards the end of the 15 hour period is because the lights on the PSL table were switched on.

Annoyingly, it seems like the PSL NPRO channels (which I have hijacked to do this test) do not have minute trend data directly accessible from NDS2. Not sure whether this is an NDS2 problem, or something missing in the way the channels are setup with Acromag. Probably the former, as I am able to generate minute trend plots with dataviewer. I forget whether this is the same as the infamous minute trend problem. Second trend data is available though, and is what I used to make these plots...

13792   Thu Apr 26 18:58:21 2018 BruceConfigurationALSNew look EX Fiber coupling - pol stability

13796   Fri Apr 27 01:36:02 2018 gautamConfigurationALSIR ALS noise performance

Summary:

My goal was to do some further characterization of the IR ALS system tonight. With POX as an OOL sensor, I measured an RMS displacement noise of  8 pm with the arm under ALS control. I calculated the CARM linewidth to be 77 Hz (=10.3 pm) for the 40m parameters, assuming 30ppm arm loss. Fuurthermore, this number is 3x better than the 24 pm RMS quoted in the Izumi et. al. paper. Of course I am quoting the best results from my efforts tonight. Conclusions:

1. [Attachment #1] --- With XARM locked using POX, the ALS beat noise (i.e. Phase Tracker output noise) lines up well with the reference we have been using for some time now (and indeed, is better in some places).
2. [Attachment #2] --- With the arm locked on ALS and POX as an OOL sensor, I measured performance comparable to this measurement we did sometime last year. Anomalies in this measurement and the one above were what precipitated the IMC noise investigation.
3. [Attachment #3] --- The above two attachments are not the whole story. During the day, I get significantly worse performance (so much so that I couldn't even do the handoff to ALS control). But in 5 minutes of measurement, the ALS noise seems quite stationary.
4. [Attachment #4] --- This is really the same as Attachment #2, but I wanted to overlay some vlines. Maybe this is a clue to some 60 Hz / ground loop issues, but the RMS has significant contribution from these harmonics. Tmrw, I will add the old measurement overlaid to this plot (and for what its worth, the Izumi et. al. spectrum as well).
5. [Attachment #5] --- With the arm under ALS control, I was able to maintain the lock for a solid hour (and more as I write up this elog). Somehow inkscape screwed up the fonts, but main point here is that TRX is stable to within 10% throughout the observation time.

Since the stability and noise seemed quite good, I decided to collect some arm scan data to give to our modeSpec SURFs to practice fitting (which is the short dip in TRX in Attachment #4). Although after the discussion with Rana today, I think it may be that we want to do this measurement in reflection and not transmission, and look for a zero crossing in the PDH signal. In any case, I was able to scan 7 FSRs without any issues. I will upload the data to some git repo. GPS start time is 1208850775, sweep was 3mins long.

I think the next step here is to noise-budget this curve. At least the DFD noises

13807   Wed May 2 21:39:33 2018 gautamConfigurationALSIR ALS for EY

The new K6XS mounts I ordered have arrived. I want to install one of them at the Y-end. I can't find a picture of the current layout but it exists as there is a hardcopy affixed to the ETMY chamber door, Steve, can we dig this up and put it in the wiki? In any case, the current beam going into the fiber is the pickoff from the post-SHG harmonic separator. I'd like to change the layout a bit, and use a pickoff before the doubling oven, but looking at the optical table, this seems like a pretty involved task and would probably require large scale optical hardware rearrangement. In any case, the MM of the green beam into the Y-arm is <50%, so I would like to redo that as well. Does anyone know of a measurement of the mode from the Lightwave NPRO that is installed at EY? I think Annalisa is the one who installed this stuff, but I can't find an actual NPRO mode measurement in her elog thread.

Found it: elog4874, elog8436. I updated the laser inventory page to link the lasers in use to the most recent mode measurements I could find on the elog. I guess ideally we should also link the AM/PM response measurements.

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

as of 3-31-2016

The oplev path was optimized with AR coated lenses and new He/Ne laser Jan 24, 2017

13817   Fri May 4 21:17:57 2018 gautamConfigurationALSBeathMouth pulled out of PSL table

I have been puzzled about the beat note level we get out of the BeatMouth for some time.

• The beat PD used is the Menlo FPD310.
• But the version we have is an obsolete version of the product, for which a manual is hard to find.
• Hence, I don't know the transimpedance/electrical characteristics of this PD.
• The optical damage threshold of the PD is quoted as 2mW, which is a number I have been careful not to exceed.
• But I've felt that the beat amplitude level we get out of this PD is far too low considering the amount of DC optical power (as measured with a fiber power meter) incident on the PD.
• After some emailing and online hunting, I've gathered some numbers for the PD which are now on the wiki.
• The fiber beam splitters we use inside the BeatMouth don't have PM fibers. There are 3 such splitters inside the BeatMouth. So the overlap efficiency on the PD is unknown.
• But even so, the beat levels I was seeing were too low.

I have pulled the box out in order to re-characterize the DC power levels incident on the PD, and also to change the gain setting on the PD to the lower gain which is more compatible with the level of optical power we have going into the BeatMouth. The modern catalog for the FPD310 (see wiki) suggests that the maximum output voltage swing of the PD is 1Vpp driving a 50ohm load. With 50% overlapping efficiency between the PSL and AUX beams, and 400 uW of optical power from each beam, I expect an output of 0.5Vpp. Even with perfect overlap, I expect 0.8Vpp. So these numbers seem reasonable.

I also plan to check the scaling of electrical beat amplitude to optical power for a couple of levels to see that these scale as expected...

13824   Tue May 8 00:40:51 2018 gautamConfigurationALSBeathMouth pulled out of PSL table

Summary:

I did some more BeatMouth characterization. My primary objective was to do a power budget. Specifically, to measure the insertion loss of the mating sleeves and of the fiber beam splitters. All power numbers quoted in this elog are measured with the fiber power meter. Main takeaways:

• Measured insertion loss of all mating sleeves, which are ADAFCPMB2, are in agreement with the < 1dB spec. 1 dB in power is ~20%.
• But there is large variance in the above number, between different supposedly identical connectors.
• Measured insertion loss from input port to coupled ports of the fiber beamsplitters are slightly larger than spec (~3.5dB), when measured after mating the fiber beamsplitter (which has Hi1060 flex fiber) and PM980 fiber (which is what brings light to the BeatMouth).
• But measured insertion loss when mating is between Hi1060 flex fiber ends is more in line with the expected value of ~3.5dB.
• Isolation of fiber beam splitters seems to match the spec of >55dB.

Results:

• I used the fiber bringing 416uW of IR light from EY for this test.
• Insertion loss was measured by injecting the fiber light at one port and measuring the transmitted power at various other ports.
• In order to couple the fiber power meter across a single mating sleeve, I used a short (~1m) patch cable from newport (F-SY-C-1FCA). Technically, this is single mode fiber with the correct type of connector, FC/APC, but is not PM.
• See Attachment #2 for the labeling of the connectors which is how I refer to them in the table below.
• Lest there be confusion, I use the definition of insertion loss  $\mathrm{Insertion ~loss [dB] }=10\mathrm{log_{10}}(\frac{P_{in}}{P_{out}})$.
Mating Sleeve # Insertion loss [dB]
1 0.38
2 0.65
3 0.71
4 0.43
5 0.95
6 0.79
7 0.5

Remarks / Questions:

1. Is there any way to systematically reduce the insertion loss? Like getting a better mating part?
2. Question for the fiber experts: How do we deal with the unused port of the beam-splitters? Right now, they are just capped. But as you can see in Attachment #1, the caps certainly don't block all the light. What are the implications of back-scattered light into the fiber on the ALS noise? I guess the beamsplitter itself has the spec'd 55dB directivity, so do we not care about this?
13886   Thu May 24 13:06:17 2018 gautamConfigurationALSDFD noises

Summary:

1. The DFD noise floor is ~0.5Hz/rtHz at 100Hz (see Attachment #2).
2. I cannot account for the measured noise floor of the DFD system. The Marconi noise and the AA noise contributions should be neglibible at 100Hz.
3. This SURF report would lead me to believe that the delay line cable length is 50m. But my calibration suggests it is shorter, more like 40m (see Attachment #1). I had made some TF measurements of the delay sometime ago, need to dig up the data and see what number that measurement yields.

Details and discussion: (diagrams to follow)

• Delay line linearity was checked by driving the input with Marconi, waiting for any transient to die down, and averaging the I and Q inputs to the phase tracker servo (after correcting for the daughter board TF) for 10 seconds. The deg/MHz response was then calculated by trigonometry. Not sure what to make of the structure in the residuals, need to think about it.
• DFD noise was checked by driving the DFD input with a Marconi at 50MHz, RF level = 8dBm, which are expected parameters for nominal ALS operation. In this configuration, I measured the spectrum of the phase tracker output. I then used the calibration factor from the above bullet to convert to Hz/rtHz.
• The electronics noise contribution of the daughter board was calibrated to Hz/rtHz by using the Marconi to drive the DFD input with a known FM signal (mod depth ~0.05), and using the SR785 to measure the power of the FM peak. This allows one to back out the V/Hz calibration constant of the delay line. I tweaked the carrier frequency until the ratio of power in I channel to Q channel (or the other way around) was >20dB before making this measurement.
• I have no proof - but I suspect that the whole host of harmonics in the noise spectrum is because the 1U AA chassis sits directly on top of some Sorensen power supplies. These Sorensens power the frequency distribution box in the LSC rack, so the simplest test to confirm would be to turn off the RF chain, and then Sorensens, and see if the peaky features persist.
13889   Thu May 24 19:41:28 2018 gautamConfigurationALSBeathMouth reinstalled on PSL table

Summary:

• DC light power incident on beat PD is ~400uW from the PSL and ~300uW from EX.
• These numbers are consistent with measured mating sleeve and fiber coupler losses.
• However, I measure an RF beatnote of 80mVpp (= -18dBm). This corresponds to a mode matching efficiency of ~15%, assuming InGaAs efficiency of 0.65A/W.

I find this hard to believe.

Details:

• I took this opportunity to clean the fiber tips on the PSL table going into the BeatMouth.
• PSL light power going into the BeatMouth is 2.6mW. Of which ~400uW reaches the Beat PD (measured using my new front panel monitor port).
• Similarly, 1mW of EX light reaches the PSL table, of which ~300uW reaches the Beat PD.
• The RF amplifier gain is 20dB, and RF transimpedance is 50 ohms.
• Using the (electrical) 20dB coupled port on the front panel, I measured a beat signal with 8mVpp. So the actual beat note signal is 80mVpp.

Discussion:

As I see it, the possibilities are:

1. My measurement technique/calculation is wrong.
2. The beat PD is broken has optoelectronic different that is significantly different from specifications.
3. The non-PM fiber lengths inside the beat box result in ~15% overlap between the PSL and EX beams. Morever, there is insignificant variation in the electrical beat amplitude as monitored on the control room analyzer. So there is negligible change in the polarization state inside the BeatMouth.

I guess #3 can be tested by varying the polarization content of one of the input beams through 90 degrees.

13890   Thu May 24 20:31:03 2018 gautamConfigurationALSDFD noises

A couple of months ago, I took 21 measurements of the delay line transfer function. As shown in Attachment #2, the unwrapped phase is more consistent with a cable length closer to 45m rather than 50m (assuming speed of light is 0.75c in the cable, as the datasheet says it is).

Attachment #1 shows the TF magnitude for the same measurements. There are some ripples consistent with reflections, so something in this system is not impedance matched. I believe I used the same power splitter to split the RF source between delayed and undelayed paths to make these TFs as is used in the current DFD setup to split the RF beatnote.

 Quote: I had made some TF measurements of the delay sometime ago, need to dig up the data and see what number that measurement yields.
13955   Wed Jun 13 12:21:09 2018 gautamUpdateALSPDFR laser checkout

I want to use the Fiber Coupled laser from the PDFR system to characterize the response of the fiber coupled PDs we use in the BeatMouth. The documentation is pretty good: for a first test, I did the following in this order:

• Removed the input fiber to the 1x16 splitter located in the rack near the OMC chamber.
• Connected aforementioned fiber to a collimator.
• Aligned the output of the collimator onto a razor beam dump.
• Turned on the laser controller - it came on with a TEC temperature of 22.5 C and I_diode 0 mA, and the "output shorted" LED was ON (red).
• Turned up the diode current to 80 mA, since the "threshold current" is stated as 75 mA in the manual. In fact, I could see a beam using an IR card at 30 mA already.
• At 80mA, I measured 3.5 mW of output power using the Ophir.

Seems like stuff is working as expected. I don't know what the correct setpoint for the TEC is, but once that is figured out, the 1x16 splitter should give me 250 uW from each output for 4mW input. This is well below any damage threshold of the Menlo PDs. Then the plan is to modulate the intensity of the diode laser using the Agilent, and measure the optoelectronic response of the PD in the usual way. I don't know if we have a Fiber coupled Reference Photodiode we can use in the way we use the NF1611 in the Jenne laser setup. If not, the main systematic measurement error will come from the power measurement using a Fiber Power Meter.

13957   Wed Jun 13 22:07:31 2018 gautamUpdateALSBeatMouth PDFR measurement

Summary:

Neither of the Menlo FPD310 fiber coupled PDs in the beat mouth have an optoelectronic response (V/W) as advertised. This possibly indicates a damaged RF amplification stage inside the PD.

Motivation:

I have never been able to make the numbers work out for the amount of DC light I put on these PDs, and how much RF beat power I get out. Today, I decided to measure the PD response directly.

Details:

In the end, I decided that slightly modifying the Jenner laser setup was the way to go, instead of futzing around with the PDFR laser. These PDs have a switchable gain setting - for this measurement, both were set to the lower gain such that the expected optoelectronic response is 409 V/W.

[Attachment #1] - Sketch of the experimental setup.

[Attachment #2] - Measured TF responses, the RF modulation was -20dBm for all curves. I varied the diode laser DC current a little to ensure I recovered identical transfer functions. Assumptions used in making these plots:

1. NF1611 and FPD310 have equal amounts of power incident on them.
2. The NF1611 transimpedance is 700V/A.

[Attachment #3] - Tarball of data + script used to make Attachment #2.

Conclusions:

• The FPD310 does not have a DC monitor port.
• So the dominant uncertainty in these plots is that I don't know how much power was incident on the PD under test.
• The NF1611 DC power level could be measured though, and seemed to scale with DC pump current linearly (I had only 3 datapoints though so this doesn't mean much).
• Neither PD has transimpedance gain as per the specs.
• The X PD shows levels ~x10 lower than expected.
• The Y PD shows levels ~x3 lower than expected.
• I will repeat the measurement tomorrow by eliminating some un-necessary patch fiber cables, and also calibrating out the cable delays.
• The setup shown in Attachment #1 was used because I didn't want to open up the BeatMouth.
• But I can pipe the port of the BS not going to the FPD310 directly to the collimator, and that should reduce the systematic uncertainty w.r.t. power distribution between FPD310 and NF1611.
13973   Fri Jun 15 14:22:05 2018 gautamUpdateALSBeatMouth PDFR measurement

I did the measurement with the BeatMouth open today. Main changes:

• Directly pipe the RF output of the Menlo PDs to the Agilent, bypassing the 20dB coupler inside the BeatMouth.
• Directly pipe the unused port of the Fiber Beamsplitter used to send light to the Menlo PD to an in-air collimator, which then sends the beam to the NF1611 reference detector.

So neglecting asymmetry in the branching ratio of the fiber beamsplitter, the asymmetry between the test PD optical path and the reference PD optical path is a single fiber mating sleeve in the former vs a collimator in the latter. In order to recover the expected number of 409 V/W for the Menlo PDs, we have to argue that the optical loss in the test PD path (fiber mating sleeve) are ~3x higher than in the NF1611 path (free space coupler). But at least the X and Y PDs show identical responses now. The error I made in the previously attached plot was that I was using the 20dB coupled output for the X PD measurement .

Revised conclusion: The measured optoelectronic response of the Menlo PDs at 10s of MHz, of ~130 V/W, is completely consistent with the numbers I reported in this elog. So rogue polarization is no longer the culprit for the discrepancy between expected and measured RF beatnote power, it was just that the expectation, based on Menlo PD specs, were not accurate.#2 of the linked elog seems to be the most likely, although "broken" should actually be "not matching spec".

While killing time b/w measurements, I looked on the ITMY optical table and found that the NF1611 I mentioned in this elog still exists. It is fiber coupled. Could be a better substitute as a Reference PD for this particular measurement.

 Quote: I will repeat the measurement tomorrow by eliminating some un-necessary patch fiber cables, and also calibrating out the cable delays. The setup shown in Attachment #1 was used because I didn't want to open up the BeatMouth. But I can pipe the port of the BS not going to the FPD310 directly to the collimator, and that should reduce the systematic uncertainty w.r.t. power distribution between FPD310 and NF1611.
14468   Wed Feb 20 23:55:51 2019 gautamUpdateALSALS delay line electronics

Summary:

Last year, I worked on the ALS delay line electronics, thinking that we were in danger of saturation. The analysis was incorrect. I find that for RF signal levels between -10 dBm and +15 dBm, assuming 3dB insertion loss due to components and 5 dB conversion loss in the mixer, there is no danger of saturation in the I/F part of the circuit.

Details:

The key is that the MOSFET mixer used in the demodulation circuit drives an I/F current and not voltage. The I-to-V conversion is done by a transimpedance amplifier and not a voltage amplifier. The confusion arose from interpreting the gain of the first stage of the I/F amplifier as 1 kohm/10 ohm = 100. The real figures of merit we have to look at are the current through, and voltage across, the transimpedance resistor.  So I think we should revert to the old setup. This analysis is consistent with an actual test I did on the board, details of which may be found here.

We may still benefit from some whitening of the signal before digitization between 10-100 Hz, need to check what is an appropriate place in the signal chain to put in some whitening, there are some constraints to the circuit topology because of the MOSFET mixer.

One part of the circuit topology I'm still confused by is the choice of impedance-matching transformer at the RF-input of this demod board - why is a 75 ohm part used instead of a 50 ohm part? Isn't this going to actually result in an impedance mismatch given our RG405 cabling?

Update: Having pulled out the board, it looks like the input transformer is an ADT-1-1, and NOT an ADT1-1WT as labelled on the schematic. The former is indeed a 50ohm part. So it makes sense to me now.

Since we have the NF1611 fiber coupled PDs, I'm going to try reviving the X arm ALS to check out what the noise is after bypassing the suspect Menlo PDs we were using thus far. My re-analysis can be found in the attached zip of my ipynb (in PDF form).

14475   Thu Mar 7 01:06:38 2019 gautamUpdateALSALS delay line electronics

Summary:

The restoration of the delay-line electronics is complete. The chassis has not been re-installed yet, I will put it back in tomorrow. I think the calculations and measurements are in good agreement.

Details:

Apart from restoring the transimpedance of the I/F amplifier, I also had to replace the two differential-sending AD8672s in the RF Log detector circuit for both LO and RF paths in the ALS-X board. I performed the same tests as I did the last time on the electronics bench, results will be uploaded to the DCC page for the 40m version of the board. I think the board is performing as advertised, although there is some variation in the noise of the two pairs of I/Q readouts. Sticking with the notation of the HP Application Note for delay line frequency discriminators, here are some numebrs for our delay line system:

• $K_{\phi} = 3.7 \ \mathrm{V/rad}$  - measured by driving the LO/RF inputs with Fluke/Marconi at 7dBm/0dBm (which are the expected signal levels accounting for losses between the BeatMouth and the demodulator) and looking at the Vpp of the resulting I/F beat signal on a scope. This is assuming we use the differential output of the demodulator (divide by 2 if we use the single-ended output instead).
• $\tau_d = \frac{45 \ \mathrm{m}}{0.75c} \approx 0.2 \mu s$ [see measurement]
• $K_{d} = K_{\phi}2 \pi \tau_{d} \approx 4 \mu \mathrm{V/Hz}$ (to be confirmed by measurement by driving a known FM signal with the Marconi)
• Assuming 1mW of light on our beat PDs and perfect contrast, the phase noise due to shot noise is $\pi \sqrt{2\bar{P}\frac{hc}{\lambda}} / 1 \ \mathrm{mW} \approx 60 \ \mathrm{nrad /}\sqrt{\mathrm{Hz}}$which is ~ 5 orders of magnitude lower than the electronics noise in equivalent frequency noise at 100 Hz.
• The noise due to the FET mixer seems quite complicated to calculate - but as a lower bound, the Johnson current noise due to the 182 ohms at each RF input is ~ 10 pA/rtHz. With a transimpedance gain of 1 kohm, this corresponds to ~10 nV/rtHz.

In conclusion: the ALS noise is very likely limited by ADC noise (~1 Hz/rtHz frequency noise for 5uV/rtHz ADC noise). We need some whiteningWhy whiten the demodulated signal instead of directly incorporating the whitening into the I/F amplifier input stage? Because I couldn't find a design that satisfies all the following criteria (this was why my previous design was flawed):

1. The commutating part of the FET mixer must be close to ground potential always.
3. The DC gain of the I/F amplifier is low, with 20-30dB gain at 100 Hz, and then rolled off again at high frequencies for stability and sum-frequency rejection. In fact, it's not even obvious to me that we want a low DC gain - the quantity $K_{\phi}$ is directly proportional to the DC transimpedance gain, and we want that to be large for more sensitive frequency discriminating.

So Rich suggested separating the transimpedance and whitening operations. The output noise of the differential outputs of the demodulator unit is <100 nV/rtHz at 100 Hz, so we should be able to saturate that noise level with a whitening unit whose input referred noise level is < 100 nV/rtHz. I'm going to see if there are any aLIGO whitening board spares - the existing whitening boards are not a good candidate I think because of the large DC signal level.

14477   Tue Mar 12 22:51:25 2019 gautamUpdateALSALS delay line electronics

This Hanford alog may be of relevance as we are using the aLIGO AA chassis for the IR ALS channels. We aren't expecting any large amplitude high frequency signals for this application, but putting this here in case it's useful someday.

14478   Wed Mar 13 01:27:30 2019 gautamUpdateALSALS delay line electronics

This test was done, and I determine the frequency discriminant to be $\approx 5 \mu \mathrm{V}/\mathrm{Hz}$ (for an RF signal level of ~2 dBm).

Attachment #1: Measured and predicted value of the DFD discriminant for a few RF signal levels.

• Methodology was to drive an FM (deviation = 25 Hz, fMod = 221 Hz, fCarrier ~ 40 MHz) with the Marconi, and look at the IF spectrum peak height on a SR785
• The "Design" curve is calculated using the circuit parameters, assuming 4dB conversion loss in the mixer itself, and 3dB insertion loss due to various impedance matching transformers and couplers in the RF signal chain. I fudged the insertion/convertion loss numbers to get this curve to line up with the measurements (by eye).
• For the measurement, I assume the value for FM deviation displayed on the Marconi is an RMS value (this is the best I can gather from the manual). I'll double checking by looking at the RFmon spectrum directly on the Agilent NA.
• X axis calibrated by reading off from the RF power monitor using a DMM and using the calibration data from the bench.
• I could never get the ratio of peak heights in Ichan/Qchan (or the other way around) to better than ~ 1/8 (by moving the carrier frequency around). Not sure I can explain that - small non-orthogonality between I and Q channels cannot explain this level of leakage.

Attachment #2: Measured noise spectrum in the 1Y2 (LSC) electronics rack, calibrated to Hz/rtHz using the discriminant from Attachment #1.

• Something funky with the I channel for X, I'll re-take that spectrum.

I'm still waiting on some parts for the new BeatMouth before giving the whole system a whirl. In the meantime, I'll work on the EX and EY green setups, to try and improve the mode-matching and better characterize the expected suppressed frequency noise of the end NPROs - the goal here is to rule out the excess low-frequency noise that was seen in the ALS signals coming from unsuppressed frequency noise.

Bottom lines:

1. The DFD noise is at the level of ~ 10mHz/rtHz above 10 Hz. This justifies the need for whitening before ADC-ing.
2. The measured signal/noise levels in the DFD chain are in good agreement with the "expected" levels from circuit component values and typical insertion/conversion loss values.
3. Why are there so many 60 Hz harmonics???
14479   Thu Mar 14 23:26:47 2019 AnjaliUpdateALSALS delay line electronics

Attachment #1 shows the schematic of the test setup. Signal generator (Marconi) was used to supply the RF input. We observed the IF output in the following three test conditions.

1. Observed the spectrum with FM modulation (fcarrier of 40 MHz and fmod of 221 Hz )- a peak at 221 Hz was observed.
2. Observed the noise spectrum without FM modulation.
3. Observed the noise spectrum after disconnecting the delayed output of the delay line.
• It is observed that the broad band noise level is higher without FM modulation (2) compared to that we observed after disconnecting the delayed output of the delay line (3).
• It is also observed that the noise level is increasing with increase in RF input power.
• We need to find the reason for increase in broad band noise .
14480   Sun Mar 17 00:42:20 2019 gautamUpdateALSNF1611 cannot be shot-noise limited?

Summary:

Per the manual (pg12) of the NF 1611 photodiode, the "Input Noise Current" is 16 pA/rtHz. It also specifies that for "Linear Operation", the max input power is 1 mW, which at 1um corresponds to a current shot noise of ~14 pA/rtHz. Therefore,

1. This photodiode cannot be shot-noise limited if we also want to stay in the spec-ed linear regime.
2. We don't need to worry so much about the noise figure of the RF amplifier that follows the photodiode. In fact, I think we can use a higher gain RF amplifier with a slightly worse noise figure (e.g. ZHL-3A) as we will benefit from having a larger frequency discriminant with more RF power reaching the delay line.

Details:

Attachment #1: Here, I plot the expected voltage noise due to shot noise of the incident light, assuming 0.75 A/W for InGaAs and 700V/A transimpedance gain.

• For convenience, I've calibrated on the twin axes the current shot noise (X) and equivalent amplifier noise figure at a given voltage noise, assuming a 50 ohm system (Y).
• The 16 pA/rtHz input current noise exceeds the shot noise contribution for powers as high as 1 mW.
• Even at 0.5 mW power on the PD, we can use the ZHL-3A rather than the Teledyne:
• This calculation was motivated by some suspicious features in the Teledyne amplifier gain, I will write a separate elog about that.
• For the light levels we have, I expect ~3dBm RF signal from the photodiode. With the 24dB of gain from the ZHL-3A, the signal becomes 27dBm, which is smaller (but close to) the spec-ed max output of the ZHL-3A, which is 29.5 dBm. Is this too close to the edge?
• I will measure the gain/noise of the ZHL-3A to get a better answer to these questions.
• If in the future we get a better photodiode setup that reaches sub-1nV/rtHz (dark/electronics) voltage noise, we may have to re-evaluate what is an appropriate RF amplifier.
14481   Sun Mar 17 13:35:39 2019 AnjaliUpdateALSPower splitter characterization

We characterized the power splitter ( Minicircuit- ZAPD-2-252-S+). The schematic of the measurement setup is shown in attachment #1. The network/spectrum/impedance analyzer (Agilent 4395A) was used in the network analyzer mode for the characterisation. The RF output is enabled in the network analyser mode. We used an other spliiter (Power splitter #1) to splitt the RF power such that one part goes to the network analzer and the other part goes to the power spliiter (Power splitter #2) . We are characterising power splitter #2 in this test. The characterisation results and comparison with the data sheet values are shown in Attachment # 2-4.

Attachment #2 : Comparison of total loss in port 1 and 2

Attachment #3 : Comparison of amplitude unbalance

Attachment #4 : Comparison of phase unbalance

• From the data sheet: the splitter is wideband, 5 to 2500 MHz, useable from 0.5 to 3000 MHz. We performd the measurement from 1 MHz to 500 MHz (limited by the band width of the network analyzer).
• It can be seen from attachment #2 and #4 that there is a sudden increase below ~11 MHz. The reason for this is not clear to me
• The mesured total loss value for port 1 and port 2 are slightly higher than that specified in the data sheet.From the data sheet, the maximum loss in port 1 and port 2 in the range at 450 MHz are 3.51 dB and 3.49 dB respectively. The measured values are 3.61 dB and 3.59 dB respectively for port 1 and port 2, which is higher than the values mentioed in the data sheet. It can also be seen from attachment #1 (b) that the expected trend in total loss with frequency is that the loss is decreasing with increase in frequency and we are observing the opposite trend in the frequency range 11-500 MHz.
• From the data sheet, the maximum amplitude balance in the 5 MHz-500 MHz range is 0.02 dB and the measured maximum value is 0.03 dB
• Similary for the phase unbalance, the maximum value specified by the data sheet in the 5 MHz- 500 MHz range is 0.12 degree and the measurement shows a phase unbalance upto 0.7 degree in this frequency range
• So the observations shows that the measured values are slighty higher than that specified in the data sheet values.
14482   Sun Mar 17 21:06:17 2019 AnjaliUpdateALSAmplifier characterisation

The goal was to characterise the new amplifier (AP1053). For a practice, I did the characterisation of the old amplifier.This test is similar to that reported in Elog ID 13602.

• Attachment #1 shows the schematic of the setup for gain characterisation and Attachment #2 shows the results of gain characterisation.
• The gain measurement is comparable with the previous results. From the data sheet, 10 dB gain is guaranteed in the frequency range 10-450 MHz. From our observation, the gain is not flat pver this region. We have measured a maximum gain of 10.7 dB at 6 MHz and it has then decreased upto 8.5 dB at 500 MHz
• Attachement #3 shows the schematic of the setup for the noise characterisation and Attachment # 4 shows the results of noise measurment.
• The noise measurement doesn't look fine. We probably have to repeat this measurement.
14486   Mon Mar 18 20:22:28 2019 gautamUpdateALSALS stability test

I'm running a test to see how stable the EX green lock is. For this purpose, I've left the slow temperature tuning servo on (there is a 100 count limiter enabled, so nothing crazy should happen).

14498   Thu Mar 28 19:40:02 2019 gautamUpdateALSBeatMouth with NF1611s assembled

Summary:

The parts I was waiting for arrived. I finished the beat mouth assembly, and did some characterization. Everything looks to be working as expected.

Details:

Attachment #1: Photo of the front panel. I am short of two fiber mating sleeves that are compatible with PM fibers, but those are just for monitoring, so not critical to the assembly at this stage. I'll ask Chub to procure these.

Attachment #2: Photo of the inside of the BeatMouth. I opted to use the flexible RG-316 cables for all the RF interconnects. Rana said these aren't the best option, remains to be seen if interference between cables is an issue. If so, we can replace them with RG-58. I took the opportunity to give each fiber beam splitter its own spool, and cleaned all the fiber tips.

Attachment #3: Transfer function measurement. The PDFR setup behind 1X5/1X6 was used. I set the DC current to the laser to 30.0 mA (as read off the display of the current source), which produced ~400uW of light at the fiber coupled output of the diode laser. This was injected into the "PSL" input coupler of the BeatMouth, and so gets divided down to ~100 uW by the time it reaches the PDs. From the DC monitor values (~430mV), the light hitting the PDs is actually more consistent with 60uW, which is in agreement with the insertion loss of the fiber beamsplitters, and the mating sleeves.

The two responses seem reasonably well balanced (to within 20% - do we expect this to be better?). Even though judging by the DC monitor, there was more light incident on the Y PD than on the X PD, the X response was actually stronger than the Y.

I also took the chance to do some other tests:

• Inject light into the "X(Y)-ARM" input coupler of the Beat Mouth - confirmed that only the X(Y) NF1611's DC monitor output showed any change. The DC light level was ~1V in this condition, which again is consistent with expected insertion losses as compared to the "PSL" input case, there is 1 less fiber beamsplitter and mating sleeve.
• Injected light into each of the input couplers, looked at the interior of the BeatMouth with an IR viewer for evidence of fiber damage, and saw none. Note that we are not doing anything special to dump the light at the unused leg of the fiber beamsplitter (which will eventually be a monitor port). Perhaps, nominally, this port should be dumped in some appropriate way.

Attachment #4: Dark Noise analysis. I used a ZHL-500-HLN+ to boost the PD's dark noise above the AG4395's measurement noise floor. The measured noise level seems to suggest either (i) the input-referred current noise of the PD circuitry is a little lower than the spec of 16 pA/rtHz (more like 13 pA/rtHz) or (ii) the transimpedance is lower than the spec of 700 V/A (more like 600 V/A). Probably some combination of the two. Seems reasonable to me.

Next steps:

The optical part of the ALS detection setup is now complete. The next step is to measure the ALS noise with this sysytem. I will use the X arm for this purpose (I'd like to make the minor change of switching the existing resistive power splitter at the delay line to the newly acquired splitters which have 3dB lower insertion loss).

14502   Fri Mar 29 21:00:06 2019 gautamUpdateALSBeatMouth with NF1611s installed
• Newfocus 15V current limited supply was taken from bottom NE corner of the ITMY Oplev table to power the BeatMouth on the PSL table
• BeatMouth was installed on top shelf on PSL table [Attachment #1].
• Light levels in fibers were checked:
• PSL: initially, only ~200uW / 4mW was coupled in. This was improved to 2.6mW/4mW (~65% MM) which was deemed sufficient for a first test), by tweaking the alignment of, and into the collimator.
• EX: ~900uW measured at the PSL table. I remember the incident power being ~1mW. So this is pretty good.
• Fibers hooked up to BeatMouth:
• EX light only, DC mon of X PD reads -2.1V.
• With PSL light, I get -4.6 V.
• For these numbers, with the DC transimpedance of 10kohm and the RF transimpedance of 700 ohm, I expect a beat of ~0dBm
• DC light level stability is being monitored by a temporarily hijacked PSL NPRO diagnostic Acromag channel. Main motivation is to confirm that the alignment to the special axes of the PM fibers is still good and we aren't seeing large tempreature-driven waveplate effects.
• RF part of the circuit is terminated into 50ohms for now -
• there is still a quesiton as to what is the correct RF amplifier to use in sending the signal to the 1Y3 rack.
• An initial RF beat power level measurement yielded -5dBm, which is inconsistent with the DC monitor voltages, but I'm not sure what frequency the beat was at, will make a more careful measurement with a scope or the network analyzer.
• We want the RF pre-amp to be:
• Low noise, keeping this in mind
• High enough gain to boost the V/Hz discriminant of the electronic delay line
• Not too high gain that we run into compression / saturate some of the delay line electronics - specifically, the LO input of the LSC demod board has a Teledyne amp in the signal chain, and so we need to ensure the signal level there is <16dBm (nominal level is 10dBm).
• I'm evaluating options...
• At 1Y3:
• I pulled out the delay-line enclosure, and removed the (superglued) resistive power splitters with the help of some acetone
• The newly acquired power splitters (ZAPD-2-252-S+) were affixed to the front panel, in which I made some mounting holes.
• The new look setup, re-installed at 1Y3, is shown in Attachment #2.
14503   Sun Mar 31 15:05:53 2019 gautamUpdateALSFiber beam-splitters not PM

I looked into this a little more today.

1. Looking at the beat signal between the PSL and EX beams from the NF1611 on a scope (50-ohm input), the signal Vpp was ~200 mV.
2. In the time that I was poking about, the level dropped to ~150mVpp. seemed suspicious.
3. Thinking that this has to be related to the polarization mismatch between the interfering beams, I moved the input fibers (blue in Attachment #1) around, and saw the signal amplitude went up to 300mVpp, supporting my initial hypothesis.
4. The question remains as to where the bulk of the polarization drift is happening. I had spent some effort making sure the input coupled beam to the fiber was well-aligned to one of the special axes of the fiber, and I don't think this will have changed since (i.e. the rotational orientation of the fiber axes relative to the input beam was fixed, since we are using the K6XS mounts with a locking screw for the input couplers). So I flexed the patch cables of the fiber beam splitters inside the BeatMouth, and saw the signal go as high as 700mVpp (the expected level given the values reported by the DC monitor).

This is a problem - such large shifts in the signal level means we have to leave sufficient headroom in the choice of RF amplifier gain to prevent saturation, whereas we want to boost the signal as much as possible. Moreover, this kind of operation of tweaking the fiber seating to increase the RF signal level is not repeatable/reliable. Options as I see it:

1. Get a fiber BS that is capable of maintaining the beam polarization all the way through to the beat photodiode. I've asked AFW technologies (the company that made our existing fiber BS parts) if they supply such a device, and Andrew is looking into a similar component from Thorlabs.
• These parts could be costly.
2. Mix the beams in free space. We have the beam coming from EX to the PSL table, so once we mix the two beams, we can use either a fiber or free-space PD to read out the beatnote.
• This approach means we lose some of the advantages of the fiber based setup (e.g. frequent alignment of the free-space MM of the two interfering beams may be required).
• Potentially increases sensitivity to jitter noise at the free-space/fiber coupling points
 Quote: An initial RF beat power level measurement yielded -5dBm, which is inconsistent with the DC monitor voltages, but I'm not sure what frequency the beat was at, will make a more careful measurement with a scope or the network analyzer.
14510   Wed Apr 3 09:04:01 2019 gautamUpdateALSNote about new fiber couplers

The new fiber beam splitters we are ordering, PFC-64-2-50-L-P-7-2-FB-0.3W, have the slow axis working and fast axis blocked. The way the light is coupled into the fibers right now is done to maximize the amount of light into the fast axis. So we will have to do a 90deg rotation if we use that part. Probably the easiest thing to do is to put a HWP immediately before the free-space-to-fiber collimator.

Update 6pm: They have an "SB" version of the part with the slow axis blocked and fast axis enabled, same price, so I'll ask Chub to get it.

14513   Wed Apr 3 12:32:33 2019 KojiUpdateALSNote about new fiber couplers

Andrew seems to have an integrated solution of PBS+HWP in a singe mount. Or, I wonder if we should use HWP/QWP before the coupler. I am interested in a general solution for this problem in my OMC setup too.

14516   Fri Apr 5 00:33:58 2019 gautamUpdateALSPromising IR ALS noise

Summary:

I set up a free-space beat on theNW side of the PSL table between the IR beam from the PSL and from EX, the latter brought to the PSL table via ~40m fiber. Initial measurements suggest very good performance, although further tests are required to be sure. Specifically, the noise below 10 Hz seems much improved.

Details:

Attachment #1 shows the optical setup.

• I used two identical Thorlabs F220APC collimators to couple the light back into free space, reasoning that the mode-matching would be easiest this way.
• Only 1 spare K6Xs collimator mount was available (this has the locking nut on the rotational DoF), so I used a K6X for the other mount. The fast axis of the Panda fibers were aligned as best as possible to p-polarization on the table by using the fact that the connector key is aligned to the slow axis.
• I cut the power coupled into the PSL fiber from ~2.6mW to ~880uW (using a HWP + PBS combo before the input coupling to the fiber) to match the power from EX.
• The expected signal level from these powers and the NF1611 transimpedance of 700 V/A is ~320 mVpp. After alignment tweaking, I measured ~310mVpp (~ -5dBm) into a 50 ohm input on a scope, so the mode-matching which means the polarization matching and mode overlap between the PSL and EX beams are nearly optimal.
• To pipe the signal to the delay line electronics, I decided to use the ZHL-3A (G=27dB, 1dB compression at 29.5dBm per spec), so the signal level at the DFD rack was expected (and confirmed via 50 ohm input on o'scope) to be ~19dBm.
• This is a lot of signal - after the insertion loss of the power splitter, there would still be ~15dBm of signal going to the (nominally 10dBm) LO input of the demod board. This path has a Teledyne AP1053 at the input, which has 10dB gain and 1dBm compression at 26dBm per spec. To give a bit of headroom, I opted on the hacky solution of inserting an attenuator (5dB) in this path - a better solution needs to be implemented.
• The differential outputs of the demod board go to the CDS system via an AA board (there is no analog whitening).

Yehonathan came by today so I had to re-align the arms and recover POX/POY locking. This alllowed me to lock the X arm length to the PSL frequency, and lock the EX green laser to the X arm length. GTRX was ~0.36, whereas I know it can be as high as 0.5, so there is definitely room to improve the EX frequency noise suppression.

Attachment #2 shows the ALS out-of-loop noise for the PSL+X combo. The main improvements compared to this time last year are electronic.

• The failed experiment of making custom I/F amplifier was abandoned and Rich Abbott's original design was reverted to.
• New power splitter was installed with 3dB less insertion loss.
• According to the RF path level monitor, the signal level at the RF input to the demod board is ~10dBm. Per my earlier characterization, this will give us the pretty beefy frequency discriminant of ~15uV/Hz.
• I estimate the frequency noise of the detection electronics + ADC noise now translate to 1/3 the frequency noise compared to the old system. With some analog whitening, this can be made even better, the electronics noise of the DFD electronics (~50nV/rtHz) is estimated to be <10mHz/rtHz equivalent frequency noise.
• Note that the calibration from phase-tracker-servo to units of Hz (~14 kHz / degree) was not changed in the digital system - this should only be a property of the delay line length, and hence, should not have changed as a result of the various electronics changes to the demod board and other electronics.

Next steps:

• Improve pointing of green beam into X arm cavity.
• I plan to recover the green beat note as well and digitize it using the second available DFD channel (eventually for the Y arm) - then we can simultaneously compare the the green and IR performance (though they will have different noise floors as there is less green light on the green beat PDs, and I think lower transimpedance too).
 Quote: Mix the beams in free space. We have the beam coming from EX to the PSL table, so once we mix the two beams, we can use either a fiber or free-space PD to read out the beatnote.  This approach means we lose some of the advantages of the fiber based setup (e.g. frequent alignment of the free-space MM of the two interfering beams may be required). Potentially increases sensitivity to jitter noise at the free-space/fiber coupling points
14519   Fri Apr 5 11:49:30 2019 gautamUpdateALSPSL + X green beat recovery

Since we haven't been using it, the PID control was not enabled on the doubling oven on the PSL table (it is disabled after every power outage event in the lab). I re-enabled it just now. The setpoint according to the label on the TC200 controller is 36.9 C. The PID paramaters were P=250, I=200, D=40. These are not very good as the overshoot when I turned the control on was 44 C, seems too large. The settling time is also too long, after 10 minutes, the crystal temperature as reported by the TC200 front panel is still oscillating. I can't find anything in the elog about what the nominal PID parameter values were. The X end PID seems much better behaved so I decided to try the same PID gains as is implemented there, P=250, I=60, D=25.

With the Ophir power meter, I measured 60mW of IR light going into the doubling oven, 110uW green light coming out, for a conversion efficiency of 2.7%/W, seems pretty great.

Next, I went to EX and tweaked the steering mirror alignment - I wasn't able to improve the transmission significantly using the PZT sliders on the EPICS screen, and the dither alignment servo isn't working. It required quite a substantial common mode yaw shift of the PZT mirrors to make GTRX ~ 0.5.

 Quote: I plan to recover the green beat note as well and digitize it using the second available DFD channel (eventually for the Y arm) - then we can simultaneously compare the the green and IR performance (though they will have different noise floors as there is less green light on the green beat PDs, and I think lower transimpedance too).
14521   Mon Apr 8 00:04:08 2019 gautamUpdateALSIR ALS noise budget

To start the noise budgeting, I decided to measure the "DFD noise", which is really the quadrature sum of the following terms:

• ZHL-3A (RF amplifier) noise, NF ~ 6dB per spec (~ 1nV/rtHz)
• Delay line demod board noise, ~30nV/rtHz [measurement]
• AA board noise [measurement]

According to past characterizations of these noises, the ADC noise level, which is expected to be at the level of a few uV/rtHz, is expected to be the dominant noise source.

The measurement was made by disconnecting the NF 1611 free space photodiode from the input to the RF amplifier on the PSL table, and connecting a Marconi (f_carrier = 40 MHz, signal level=-5dBm) instead. The phase tracked output was then monitored, and the resulting digital spectrum is the red curve in Attachment #1. The blue curve is the ASD of fluctuations of the beatnote between the PSL and EX IR beams, as monitored by the DFD system, with the X arm cavity length locked to the PSL frequency via the LSC servo, and the EX green frequency locked to the X arm cavity length by the analog PDH servo.

Conclusions:

Assuming the Marconi frquency noise is lower than the ones being budgeted:

1. the measured frequency noise is above the DFD noise - this needs to be budgeted.
2. The DFD noise level is consistent with a frequency discriminant of 15 uV/rtHz and an ADC noise level of 3 uV/rtHz at high frequencies.

Next noises to budget:

1. In-loop X arm length noise
2. In-lop EX laser frequency noise
14523   Mon Apr 8 18:28:25 2019 gautamUpdateALSEX Green PDH checkout

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

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

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

14524   Mon Apr 8 23:52:09 2019 gautamUpdateALSEX Green PDH error monitor calibrated

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

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

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

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

14525   Tue Apr 9 00:16:22 2019 ranaUpdateALSEX Green PDH error monitor calibrated

G=10 or G=100?

14526   Tue Apr 9 00:18:19 2019 gautamUpdateALSEX Green PDH error monitor calibrated

wrong assumption - i checked the gain just now, it is G=10, and is running in the "low-noise" mode, so can only drive 4V. fixed elog, filter.

Note: While working at EX, I saw frequent saturations (red led blinking) on the SR560. Looking a the error mon signal on a scope, it had a pk-to-pk amplitude of ~200mV going into the SR560. Assuming the free-swinging cavity length changes by ~1 um at 1 Hz, the green frequency changes by ~15 MHz, which according to the PDH discriminant calibration of 40 uV/Hz should only make a 60 mV pk to pk signal. So perhaps the cavity length is changing by 4x as much, plausible during daytime with me stomping around the chamber I guess.. My point is that if the SR560 get's saturated (i.e. input > 13000 cts), the DQ-ed spectrum isn't trustworthy anymore. Should hook this up to some proper whitening electronics

 Quote: G=10 or G=100?
14527   Tue Apr 9 18:44:00 2019 gautamUpdateALSEX Green PDH discriminant measurement

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

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

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

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

 Quote: PDH discriminant (40 uV/Hz, see this elog)
ELOG V3.1.3-