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  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]
  • ADC noise

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. 


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
Attachment 1: DFDnoise.pdf
  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.

Attachment 1: IMG_7393.JPG
Attachment 2: greenModeMatch.pdf
  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.

Attachment 1: errMonCalib.pdf
Attachment 2: errMon.png
Attachment 3: OLTF.pdf
Attachment 4: EX_frequencyNoises.pdf
  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


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.


PDH discriminant (40 uV/Hz, see this elog) 

Attachment 1: cavityFlashes.pdf
Attachment 2: modelPDH.pdf
  14528   Tue Apr 9 19:07:12 2019 gautamUpdateALSIR ALS noise budget

Updated the noise budget to include the unsuppressed frequency noise from the EX laser. It does not explain the noise between 10-100 Hz, although the 1-3 Hz noise is close.

Actually, I think the curve that should go on the budget is when the X arm length is locked to the PSL frequency, whereas this is when the X arm is just locally damped. I will update it later tonight.

Update 1010pm: I've uploaded the relevant plot as Attachment #2. Predictably, the unsuppressed frequency noise of the EX laser is now higher, because the MC length is a noisier frequency reference than the arm cavity. But still it is a factor of 10 below the measured ALS noise.


Next noises to budget:

  1. In-loop X arm length noise
  2. In-lop EX laser frequency noise
Attachment 1: ALS_noiseBudget.pdf
Attachment 2: ALS_noiseBudget.pdf
  14533   Thu Apr 11 01:10:05 2019 gautamUpdateALSLarge 2kHz peak (and harmonics) in ALS X

These weren't present last week. The peaks are present in the EX PDH error monitor signal, and so are presumably connected with the green locking system. My goal tonight was to see if the arm length control could be done using the ALS error signal as opposed to POX, but I was not successful.

Attachment 1: EX_PDH_2kNoise.pdf
  14548   Wed Apr 17 00:50:17 2019 gautamUpdateALSLarge 2kHz peak (and harmonics) in ALS X no more

I looked into this issue today. Initially, my thinking was that I'd somehow caused clipping in the beampath somewhere which was causing this 2kHz excitation. However, on looking at the spectrum of the in-loop error signal today (Attachment #1), I found no evidence of the peak anymore!

Since the vacuum system is in a non-nominal state, and also because my IR ALS beat setup has been hijacked for the MZ interferometer, I don't have an ALS spectrum, but the next step is to try single arm locking using the ALS error signal. To investigate whether the 2kHz peak is a time-dependent feature, I left the EX green locked to the arm (with the SLOW temperature offloading servo ON), hopefully it stays locked overnight...


These weren't present last week. The peaks are present in the EX PDH error monitor signal, and so are presumably connected with the green locking system. My goal tonight was to see if the arm length control could be done using the ALS error signal as opposed to POX, but I was not successful.

Attachment 1: EX_PDHnoise.pdf
  14549   Wed Apr 17 11:01:49 2019 gautamUpdateALSLarge 2kHz peak (and harmonics) in ALS X no more

EX green stayed locked to XARM length overnight without a problem. The spectrogram doesn't show any alarming time varying features around 2 kHz (or at any other frequency).

Attachment 1: EX_PDH_specGram.pdf
  14643   Wed May 29 18:13:25 2019 gautamUpdateALSFiber beam-splitters are now PM

To maintain PM fibers all the way through to the photodiode, I had ordered some PM versions of the 50/50 fiber beamsplitters from AFW technologies. They arrived some days ago, and today I installed them in the BeatMouth. Before installation, I checked that the ends of the fibers were clean with the fiber microscope. I also did a little cleanup of the NW corner of the PSL table, where the 1um MZ setup was completely disassembled. We now have 4 non-PM fiber beamsplitters which may be useful for non polarizaiton sensitive applications - they are stored in the glass-door cabinet slightly east of the IY chamber along the Y arm, together with all the other fiber-related hardware.

Anjali had changed the coupling of the beam to the slow axis for her experiment but I ordered beamsplitters which have the slow axis blocked (because that was the original config). I need to revert to this config, and then make a measurement of the ALS noise - if things look good, I'll also patch up the Y arm ALS. We made several changes to the proposed timeline for the summer but I'd like to see this ALS thing through to the end while I still have some momentum before embarking on the BHD project. More to follow later in the eve.


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.

  14645   Fri May 31 15:55:16 2019 gautamUpdateALSPSL + X beat restored

Coupling into the fast axis of the fiber:

The PM couplers I bought require that the light is coupled to the fast axis. The Thorlabs part that Andrew ordered, and which Anjali was using for the MZ experiment, was the opposite configuration, and so the input coupler K6XS mount was rotated to accommodate this polarization. The HWP was also rotated to cut the power into the fiber. I undid these changes. Mode-matching is ~65% (2.42mW/3.70mW) which isn't stellar, but good enough. The PER is ~15dB (ratio of power in fast axis to slow axis is ~40), which I verified using another collimator at the output, and a PBS + two photodiodes. Again isn't stellar but good enough.

EX laser temperature adjustment:

Rana adjusted the temperature of the main laser to 30.61 C. According to the calibration, the EX laser temperature needed to be ~32.8 C. It was ~31.2 C. I made the change by rotating the dial on the front panel of the EX laser controller. Fine adjustment was done using the temperature slider on the ALS screen. With an offset of ~+610 counts, I found a beat at ~80 MHz.

First look at PM beamsplitters:

From my initial test, the beat amplitude was stable to my moving of the fibers yes. The NF1611 DC monitor reports 2.6 V DC with only the EX light, and 3.15 V DC with only the PSL light. So I should probably cut the PSL power a little to improve the contrast. Assuming the 10 kohm DC transimpedance spec can be believed, this means the expected signal level is 4*sqrt(260uA * 315uA)*700V/A ~0.8 Vpp, and I see ~0.9 Vpp, so roughly things add up (this is actually more consistent with an RF transimpedance of 800V/A, which is maybe not unreasonable). The RF amps for routing this signal to the delay line has been borrowed for the 2um frequency noise experiemnt - I will reacquire it today and check the ALS noise performance.

So overall, I am happy with the performance of the current iteration of the BeatMouth.

  14740   Tue Jul 9 18:42:15 2019 gautamUpdateALSEX green doubling oven temperature controller power was disconnected

There was no green light even though the EX NPRO was on. I checked the doubling oven temperature controller and found that its power cable was loose on the rear. I reconnected it, and now there is green light again. 

  14800   Mon Jul 22 23:53:16 2019 gautamUpdateALSIR ALS locking attempt


My goal tonight was to lock the PSL frequency to be resonant in the XARM cavity, using the PSL+EX beat as the error signal. I was not successful - mainly, I was plagued by huge BR mode coupling in the error signal, and I could not enable the BR notch filter in the control loop without breaking the lock. Need to think about next steps.


  • POX and POY locking was easily restored.
  • EX green alignment was tweaked at the end-table. A large YAW correction was required, which I opted to apply on the mechanical mirror mounts rather than the PZTs. GTRX ~0.4 was recovered.
  • The arm cavity length was first locked using POX as an error signal 
    • Then I looked at the out-of-loop ALS noise, trusting the DFD's V/Hz calibration (red-trace in Attachment #1).
    • I judged it to be close enough to the benchmark reference (green-trace in Attachment #1), and so decided that I could go ahead and try locking.
  • A modified version of the script /opt/rtcds/caltech/c1/scripts/XARM/Lock_ALS_XARM.py was used to transition control from POX to the ALS error signal
    • I found that I had to change the sign of the CARM loop gain for the servo to remain stable (in this config, CARM-->MC2 length, thereby modifying the IMC frequency to keep the PSL resonant in the XARM cavity).
    • I don't know why this sign change was required - we are still sticking to the same convention that the beat frequency increases when the temperature slider for the EX laser is incremented in counts.
    • The script failed multiple times at the BOOST/BR notch filter enabling step.
    • Doing these steps manually, I found that turning the BR notch, FM6, ON destroyed the lock immediately.
    • Motivated by this observation, I looked at the in-loop error signal spectrum, see Attachment #2. Here, the PSL frequency is servoed by the ALS error signal, but the BR notch filter isn't enabled.
    • The Bounce-mode peak is huge - where is this coming from? It is absent in the ALS spectrum when the XARM is locked with POX. So it is somehow connected with actuating on the MC2 suspension? Or is it that the FM6=BounceRoll filter of the XARM loop is squishing the noise when looking at the ALS spectrum in POX lock, i.e. Attachment #1? In which case, why can't I engage FM6 for the CARM loop???

Anyway, now that I have a workable set of settings that gets me close to the ALS lock of the XARM, I expect debugging to proceed faster.

Update 2019 July 23: I looked at the control loop shape today, see Attachment #3. I'm not sure I understand why the "BounceRoll" filter  in this filter bank looks like a resonant gain rather than a notch, as it does for the Oplev or SUSPOS loops for example - don't we want to not actuate at these frequencies because the reason the signal exists is because of the imperfect OSEM/magnet positioning? This does not explain the spectrum shown in Attachment #2 however, as that filter was disabled.

Attachment 1: ALS_X_outOfLoopnoise.pdf
Attachment 2: ALS_X_inLoopnoise.pdf
Attachment 3: CARM_loopShape.pdf
  14802   Wed Jul 24 00:22:24 2019 gautamUpdateALSPSL frequency locked to XARM length using ALS


I succeeded in locking the PSL frequency to the XARM cavity length, with 9 pm RMS (Attachment #1) motion below 1 kHz, by actuating on MC2 to change the IMC length. The locks were pretty stable (~20 minutes) - the dominant cause of lockloss was the infamous ETMX drifting problem.


  1. I did not need to do anything to fix the anomalosly high BR mode coupling I reported yesterday surprise.
    • To test where this could be coming from - I looked at the ALS spectrum again with the XARM length locked to the PSL frequency using POX.
    • Then I compared the spectra with the BR filter in the XARM servo enabled/disabled, see Attachment #2
    • There bounce/roll peak heights even with the BR filter disabled is ~x100 smaller than what I reported yesterday (it remained the case today, because without enabling the BR filter in the CARM servo bank, the TRX level was fluctuating wildly at ~16 Hz). 
  2. The CARM loop (which is what the PSL frequency was slaved to) had ~150 Hz UGF with ~40 degrees phase margin, see Attachment #3.
  3. The quoted RMS sensing noise is if we trust the old POX calibration - may be off by a factor of a few, but probably not an order of magnitude. I'll recalibrate using the free-swinging Michelson technique in the coming days.
  4. The two broad humps in Attachment #1, centered at ~180 Hz and ~300 Hz, are present in the XARM lock as well - so it is somehow imprinted on the arm cavity length. Fixing that will improve the RMS noise performance significantly.

My main motivation here is to make some measurements and investigate the SoCal idea using a toy system, i.e. a single arm cavity controlled using ALS, so that's what Craig and I will attempt next.

Attachment 1: ALS_X_noise_POX.pdf
Attachment 2: BR_comparison.pdf
Attachment 3: ALS_CARM_OLG.pdf
  14811   Thu Jul 25 12:25:56 2019 gautamUpdateALSIR ALSX noise


  1. There are some broad peaks in the ALS out-of-loop noise, centered at ~145 Hz, ~245 Hz and ~570 Hz which are absent in both the POX in-lock error signal and in the green PDH error signals (see Attachment #1). So I conclude they originate in the IR ALS beat chain somewhere. Needs more investigation, in the general quest to improve the ALS noise.
  2. This measurement also shows that the ALS noise is limited by unsuppressed EX green PDH frequency noise above ~400 Hz (100 Hz if you ignore the unexplained broad humps).

These spectra were taken with the arm cavity length locked to the PSL frequency using POX as an error signal, and the EX laser frequency locked to the XARM cavity length by the analog PDH servo at EX, so there is no feedback control with the ALS beat signal as an error signal.

Other details:

  • The transition of arm resonance control from POX to ALS error signal is more robust now - I am able to do this during daytime, and also maintain the lock for >20 minutes at a time.
  • Rana encouraged me not to spend too much time on this - so my next goal here will be to get the Y arm IR ALS working, and then we can control the two arms using ALS error signals in the CARM/DARM basis instead of the X/Y basis.
  • I still think it's worth getting the ALS good enough that the locking becomes repeatable and reliable.
    • The main task here is going to be re-doing the EY green layout to match the EX layout, get good MM into the cavity etc.
    • The IR light also has to be coupled into the fiber at EY.
Attachment 1: ALS_broadPeaks.pdf
  14823   Fri Aug 2 11:37:38 2019 gautamUpdateALSEY IR ALS Assay


I'd like to confirm that the IR ALS scheme will work for locking. The X-arm performance so far has been encouraging. I want to repeat the characterization for the Y arm. So I inspected the layout on the EY table, and made a list of characterization tasks. The current EY beam routing is difficult to work with, and it will definitely benefit from a re-do. However, I don't know how much time I want to spend re-doing it, so for a start, I will just try and couple some amount of light into a fiber and bring it to the PSL table, and see what noise performance I get.


Attachment #1: Photo of the current beam layout. The powers indicated were measured with the Ophir power meter.

  • I measure an SHG conversion efficiency of 0.87 %/W, which is considerably lower than the ~3.7%/W that is theoretically expected, and 1.5%/W that is realized at EX.
  • Of the 0.5 mW of green light that is generated, I measure ~0.375 mW at the viewport into the EY chamber. So there is ~25 % loss in the green beam path on the EY table. Seems high to me.
  • The previous solution of coupling IR light into the fiber realized at EY was to use the SHG leakage IR beam. While there isn't a measurement showing that this dirty beam is noisier than a cleaner pickoff, I'd like to adopt the solution used at EX, which is to use the leakage beam from the first steering mirror in the NPRO beam path. This will allow better mode-matching and polarization control of the beam being coupled into the fiber, which at least in principle, translates to less phase noise.
  • However - the beam layout at the EY table offers much less freedom to work with this idea than EX. A constraint is the clamp that secures the enclosure to the optical table, labelled in the photo. Further behind it, the green steering optics occupy all available space. A more comprehensive photo of the EY table can be found here.
  • Off the top of my head, I don't see any other good open spots on the EY table where we could couple IR light into the fiber. 
  • One other change I'd like to make is to replace the first steering mirror after the NPRO head, which is currently a Y1 HR mirror, with a R=99% BS. This will make it easier to control the amount of power coupled into the fiber, which is something we'd like.

Attachment #2: A candidate mode-matching solution, given the constraints outlined above. It isn't great, with only 85% modematching even theoretically possible. The lenses required are also pretty fast lenses. But I think it's the best possible without a complete overhaul of the EY layout. I'm still waiting for the lens kit to arrive, but as soon as they get here, I will start this work.

Characterization tasks:

  • Characterize SHG at EY [done 7/28]
  • Characterize gPDH at EY (loop TFs, improve MM, PDH discriminant, check the polarization)
  • Couple IR light into fiber with good MM at EY [done with 36% MM 8/9]
  • Clean fiber at EY, and at the PSL table [done 8/9]
  • Make the PSL + Y IR beat [done 8/9]
  • Noise budget
Attachment 1: IMG_7780.JPG
Attachment 2: Ey_MM_20190802.pdf
  14835   Tue Aug 6 23:09:20 2019 gautamUpdateALSEY table work
  • Removed power monitoring PD (It was off anyways)
  • Installed Steering mirror and collimator in K6XS mount (fast axis = p-pol to best effort by eye)
  • Installed lens mounts in approx position
  • Cleaned fiber at EY and connected to the collimator
  • Coupled EY--->PSL and spare PSL-->EY fibers together at the PSL table to facilitate coupling.
  • tbc tomorrow...

Couple IR light into fiber with good MM at EY

  14838   Fri Aug 9 16:37:39 2019 gautamUpdateALSMore EY table work


  1. 220 uW / 600 uW (~36 % mode-matching) of IR light coupled into fiber at EY.
  2. Re-connected the RF chain from the beat mouth output on the PSL table to the DFD setup at 1Y2.
  3. A beat note was found between the PSL and EY beams using the BeatMouth.


We want to know that we can lock the interferometer with the ALS beat note being generated by beating IR pickoffs (rather than the vertex green transmission). The hope is also to make the ALS system good enough that we can transition the CARM offset directly to 0 after the DRMI is locked with arms held off resonance.


Attachment #1: Shows the layout. The realized MM is ~36 %. c.f. the 85% predicted by a la mode. It is difficult to optimize much more given the tight layout, and the fact that these fast lenses require the beam to be well centered on them. They are reasonably well aligned, but I don't want to futz around with the pointing into the doubling crystal. Consequently, I don't have much control over the pointing.

Attachment #2: Shows pictures of the fiber tips at both ends before/after cleaning. The tips are now much cleaner.

The BeatMouth NF1611 DC monitor reports ~580 mV with only the EY light incident on it. This corresponds to ~60 uW of light making it to the photodiode, which is only 25% of what we send in. This is commensurate with the BS loss + mating sleeve losses.

To find the beat between PSL and EY beams, I had to change the temperature control MEDM slider for the EY laser to -8355 cts (it was 225 cts). Need to check where this lies in the mode-hop scan by actually looking at the X-tal temperature on the front panel of the EY NPRO controller - we want to be at ~39.3 C on the EY X-tal, given the PSL X-tal temp of ~30.61 C. Just checked it, front panel reports 39.2C, so I think we're good.

Next steps:

  • Fix the IMC suspension
  • Measure the ALS noise for the Y arm
  • Determine if improvements need to be made to the IR beat setup (e.g. more power? better MM? etc etc).

EY enclosure was closed up and ETMY Oplev was re-enabled after my work. Some cleanup/stray beam dumping remains to be done, I will enlist Chub's help on Monday.

Attachment 1: IMG_7791.JPG
Attachment 2: fiberCleaning.pdf
  14846   Thu Aug 15 18:54:54 2019 gautamUpdateALSALS sensing noise due to IMC


I came aross an interesting suggestion by Yutaro that KAGRA's low-frequency ALS noise could be limited by the fact that the IMC comes between the point where the frequencies of the PSL and AUX lasers are sensed (i.e. the ALS beat note), and the point where we want them to be equal (i.e. the input of the arm cavity). I wanted to see if the same effect could be at play in the 40m ALS system. A first estimate suggests to me that the numbers are definitely in the ballpark. If this is true, we may benefit from lower noise ALS by picking off the PSL beam for the ALS beat note after the IMC.


Even though the KAGRA phase lock scheme is different from the 40m scheme, the algebra holds. I needed an estimate of how much the arm cavity moves, I used data from a POX lock to estimate this. The estimate is probably not very accurate (since the arm cavity length is more stable than the IMC length, and the measured ALS noise, e.g. this elog, is actually better than what this calculation would have me believe), but should be the right order of magnitude. From this crude estimate, it does look like for f<10 Hz, this effect could be significant. I assumed an IMC pole of 3.8 kHz for this calculation.

I've indicated a "target" ALS performance where the ALS noise would be less than the CARM linewidth, which would hopefully make the locking much easier. Seems like realizing this target will be touch-and-go. But if we can implement length feedforward control for the arm cavities using seismometers, the low frequency motion of the optics should go down. It would be interesting to see if the ALS noise gets better at low frequencies with length feedforward engaged.

* Some updates were made to the plot:

  1. Took data from Kiwamu's paper for the seismic noise
  2. Overlaid measured ALS noise
Attachment 1: ALSsensingNoise.pdf
  14847   Fri Aug 16 04:24:03 2019 ranaUpdateALSALS sensing noise due to IMC

What about just use high gain feedback to MC2 below 20 Hz for the IMC lock? That would reduce the excess if this theory is correct.

  14918   Mon Sep 30 18:20:26 2019 gautamUpdateALSALS OOL noise - a first look

Attachment #1 shows a first look at the IR ALS noise after my re-coupling of the IR light into the fiber at EY. 

Measurement configuration: 

  • Each arm length was individually stabilized to the PSL frequency using POX/POY locking.
  • The respective AUX laser frequencies were locked to the arm cavity length using the AUX PDH loops.
  • GTRX ~0.3 (usually I can get ~0.5) and GTRY ~ 0.2 (the mode-matching to the arm cavities is pretty horrible as suggested by the multitude of bullseye modes seen when toggling the shutter).
  • The control signal to the AUX PZT had the DC part offloaded by the slow temperature control servos to the AUX laser crystal temperature.

CDS model changes:

  • The c1lsc model was modified to route the input signals to the Y phase tracker servo from ADC1_2 and ADC1_3 (originally, they were ADC0_20 and ADC0_21).
  • This change was necessary because the DFD output is sent differentially to the ADC1 card in the c1lsc expansion chassis (bypassing the iLIGO whitening and AA electronics, for now just going through an aLIGO AA board with no whitening available yet).
  • I chose to use the differential receiving (as opposed to using the front-panel single ended BNC connectors) as in principle, it is capable of delivering better noise performance.
  • After making the model changes, I compiled and restarted the model. Apart from the missing path issue, the compile/restart went smoothly.

Next steps:

  • Get the easy fixes done (better GTRX, GTRY).
  • Test the noise with POX and POY as the OOL sensors, and the arms controlled using the ALS error signal - this is the relevant metric for how ALS will be used in locking.
  • Noise budget. Need to double-check the DFD output calibration into Hz.
  • For the general interferometer recovery, I think I will push ahead with trying to lock some other configurations like the PRMI (should be easy to recover), DRMI (potentially more difficult to find the right settings), and the FPMI (I'd like to use this config to get an estimate for how much contrast defect we have in the interferometer, but I think it'll be pretty challenging to lock in this configuration).
Attachment 1: ALS_OOL_20190930.pdf
  14938   Fri Oct 4 00:32:24 2019 gautamUpdateALSMore locking updates


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


Attachment #1One example of a lock stretch. 

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

Attachment 1: ALS_singleArm.png
Attachment 2: ALS_OOL_20191003.pdf
  14943   Sat Oct 5 21:26:34 2019 gautamUpdateALSY-end green alignment tweaked


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

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


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

Other changes made today:

  1. /opt/rtcds/caltech/c1/scripts/general/videoscripts/videoswitch was modified to be python3 compatible - for some reason, there were many syntax errors being thrown (even though I was using python2.7) and I wasn't able to change the displays in the VEA using the MEDM screen, but now it works again 👍.
  2. The LSC overview and several daughter MEDM screens were edited to remove references to channels that no longer exist. All screens I edited have a backup stored in the MEDM directory with today's date as a suffix.
  3. Input pointing into the PMC was tweaked.
  4. Noted that some pump is noisy at pumpspool - also noted that the annuli are no longer pumped. Some event seems to have triggered an interlock condition that closed off the annular volume from TP3, needs investigation...
Attachment 1: ALSY_alignment.png
Attachment 2: ALSY_OOL.pdf
  14944   Sun Oct 6 15:23:27 2019 gautamUpdateALSArm control using error signals achieved


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


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

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

Attachment 1: ALSY_20191006.pdf
Attachment 2: transitionIRALS.png
Attachment 3: arms_ALS.pdf
  14979   Fri Oct 18 20:21:33 2019 shrutiUpdateALSAM measurement attempt at X end

[Shruti, Rana]

- At the X end, we set up the network analyzer to begin measurement of the AM transfer function by actuation of the laser PZT.

- The lid of the PDH optics setup was removed to make some checks and then replaced.

- From the PDH servo electronics setup the 'GREEN_REFL' and 'TO AUX-X LASER PZT' cables were removed for the measurement and then re-attached after.

- The signal today was too low to make a real measurement of the AM transfer function, but the GPIB scripts and interfacing was tested. 

  14981   Mon Oct 21 12:25:46 2019 gautamUpdateALSDFD electronics checkout


There are no unexpected red-flags in the performance of the DFD electronics. The calibration factors for the digital phase tracker system are 71.291 +/- 0.024 deg/MHz for the X delay line and 70.973 +/- 0.024 deg/MHz for the Y delay line, while the noise floor for the frequency noise discrimination is ~0.5 Hz/rtHz above 1 Hz (dominated by ADC noise).


  1. Attachment #1 - This observation is what motivated my investigation.
    •  found that for certain beat frequencies between the PSL + EX lasers, the frequency noise reported by the DFD system was surprisingly low.
    • The measurement condition was: EX laser frequency locked to the arm cavity length by the uPDH servo at EX, arm cavity length locked to PSL frequency via POX locking.
  2. To investigate further, I disconnected the output of the NF1611 PDs going to the ZHL-3A amplifiers on the PSL table (after first blocking the PSL light so that the PDs aren't generating any RF output).
    • An RF function generator (IFR2023B) was used to generate an RF signal to mimic the ALS beat signal.
    • I used a power splitter to divide the signal power equally between the two DFD paths.
    • The signal level on the Marconi was set to -5 dBm, to mimic the nominal power level seen by the DFD system.
    • I then performed two tests - (i) to calibrate the Phase Tracker output to deg / MHz and (ii) to measure the frequency noise reported by the DFD system for various signal frequencies.
    • Test (i): sweep the marconi frequency between 10 MHz - 200 MHz, measure the I and Q channels for each phase tracker servo, and figure out the complex argument of the signal using the arctangent. A linear polynomial was fit to the measured datapoints to extract the desired slope.
    • Test (ii): Sample frequencies uniformly distributed between 20 MHz - 80 MHz (nominal range of ALS beat frequencies expected). Reset the phase tracker servo gain, clear the output histories, wait for any transients to die out, and then collect the phase tracker servo output for 1 minute. Compute the FFT to figure out the frequency noise.
    • Attachment #2: Shows the phase tracker calibration, i.e. the results of Test (i). I took this opportunity to update the EPICS calibration fields that convert phase tracker servo output to Hz, the correction was ~7%. These numbers are consistent with what I measured previously - but the updated values weren't registered with SDF so everytime the LSC model was restarted, it reverted to the old values.
    • Attachment #3: Shows the spectra for the various measurements from Test (ii).
    • Attachment #4: Shows the gain of the phase tracker servo as a function of the RF signal frequency. This is a proxy for the signal strength, and the observed trend suggests that the signal power seen after digitization of the demodulated delay line output goes down by ~20% at 80 MHz relative to the level at 20 MHz. Seems reasonable to me, given frequency dependent losses of the intervening electronics / cabling.

Conclusion and next steps:

I still don't know what's responsible for the anomalously low noise levels reported by the ALS-X system sometimes. Next test is to check the EX PDH system, since on the evidence of these tests, the problem seems to be imprinted on the light (though I can't imagine how the noise becomes lower?).

Attachment 1: ALSnoiseAnomaly.pdf
Attachment 2: DFDcalib.pdf
Attachment 3: spectra.pdf
Attachment 4: PTgains.pdf
  14984   Tue Oct 22 15:32:15 2019 gautamUpdateALSEX uPDH electronics checkout


The EX PDH setup had what I thought was insufficient phase and gain margins. So I lowered the gain a little - the price paid was that the suppression of laser frequency noise of the end laser was reduced. I actually think an intermediate gain setting (G=7) can give us ~35 degrees of phase margin, ~10dB gain margin, and lower residual unsuppressed AUX laser noise - to be confirmed by measurement later. See here for the last activity I did - how did the gain get increased? I can't find anything in the elog.

Attachment 1: uPDH_X_OLTFs.pdf
  14986   Wed Oct 23 10:23:26 2019 ranaUpdateALSEX uPDH electronics checkout

During our EX AM/PM setups, I don't think we bumped the PDH gain knob (and I hope that the knob was locked). Possible drift in the PZT response? Good thing Shruti is on the case.

Is there a loop model of green PDH that agrees with the measurement? I'm wondering if something can be done with a compensation network to up the bandwidth or if the phase lag is more like a non-invertible kind.

  14987   Wed Oct 23 11:11:01 2019 gautamUpdateALSEX uPDH electronics checkout

The closest thing I can think of is here.


Is there a loop model of green PDH that agrees with the measurement? I'm wondering if something can be done with a compensation network to up the bandwidth or if the phase lag is more like a non-invertible kind.

  14993   Fri Oct 25 01:04:49 2019 gautamUpdateALSALS electronics chain was saturating

[Koji, gautam]


We think we got to the bottom of this issue today. The RF signal level going into the demod board is too high. This electronics chain needs some careful gain reallocation.


I was demonstrating to Koji a strange feature I had noticed in the ALS control, whereby when applying a CARM offset to detune the arms, the two arms seemed to respond differently (based on the transmission levels). This kind of CARM-->DARM coupling seemed strange to me. Anyway, I also noticed that the EPICS indicators on the ALS MEDM screen suggested ADC saturations were going on. I had never really looked at the fast time series of the inputs to the phase tracker servos, but these showed saturating behavior on ndscope traces. I went to the LSC rack and measured these on a scope, indeed, they were ~20V pp.

The output of the BeatMouth PDs are going to a ZHL-3A amplifier - we should consider replacing these with lower gain amplifiers, e.g. the Teledyne AP1053. This is relegated to a daytime task.

Other findings tonight:

While working on the PSL table, I somehow put the IMC FSS into a bad state, reminiscent of this behavior. Seems like this is linked to some flaky connection on the PSL table. One candidate is the unstable attachment of the Pomona box between the NPRO PZT and the FSS output - we should install a short BNC cable between these to avoid the lever arm situation we have right now.

  14995   Mon Oct 28 23:20:11 2019 gautamUpdateALSALS power budget


IR ALS power budget
Photodiode PSL VDC [V] PSL IDC [uA] AUX VDC [V] AUX IDC [uA] IRF [mA pk] PRF [dBm]
PSL+EX 3 300 2.5 250 ~600  ~3
PSL+EY 3 300 0.6 60 ~270 ~ -3

In calculating the above numbers, I assumed a DC transimpedance of 10 khhms and an RF Transimpedance of ~800 V/A.

[Elog14480]: per these calculations, with the NewFocus 1611 PDs, we cannot achieve shot noise limited sensing for any power below the rated maximum for linear operation (i.e. 1mW). Moreover, the noise figure of the RF amplifier we use to amplify the sensed beat note before driving the delay-line frequency discriminator is unlikely to be the limiting noise source in the current configuration. Rana suggested that we get two Gain Blocks. These can handle input powers up to ~10dBm while still giving us plenty of power to drive the delay line. This way, we can (i) not compromise on the sacred optical gain, (ii) be well below the 1dB compression point (i.e. avoid nonlinear noise effects) and (iii) achieve a better frequency discriminant

Temporary fix: While the gain blocks arrive, I inserted a 10dB (3dB) attenuator between the PSL+EX (PSL+EY) photodiode RF output and the ZHL-3A amplifiers. This way, we are well below the 1dB compression point of said RF amplifiers, and also below the 1dB compression point of the on-board Teledyne AP1053 amplifiers on the demodulator boards we use.

Nest steps: Rana is getting in touch with Rich Abbott to find out if there is any data available on the noise performance of the post-mixer IF amplifier stage in the 0.1 -30 Hz range, where the voltage and current noise of the AD829 OpAmps could be limiting the DFD performance. But in the meantime, the ALS noise seems good again, and there is no evidence of the sort of CARM/DARM coupling that motivated this investigation in the first place. Managed to execute several IR-->ALS transitions tonight in the PRFPMI locking efforts (next elog).

No new Teledyne AP1053s were harmed in this process - I'll send the 5 units back to Rich tomorrow.

  15020   Thu Nov 7 17:46:10 2019 shrutiUpdateALSAM measurement at X end

Some details:

- There was a SR560+SR785 (not connected for measurement) placed near the X end which I moved; it is now behind the electronics rack by the X arm beam tube (~15m away).

- Also, for the AM measurement I moved the AG5395A from behind the PSL setup to the X end, where it now is.

- By toggling the XGREEN shutter, I noticed that the cavity was not resonant before I disconnected anything from the setup since the spot shape kept changing, but I proceeded anyway. 

- Because Rana said that it was important for me to mention: the ~5 USD blue-yellow crocs (that I now use) work fine for me.

The AM Measurement:

1. The cables were calibrated with the DC block in the A port (for a A/R measurement)

2. The cable to the PZT was disconnected from the pomona box and connected to the RF out of the NA, the PD output labelled 'GREEN_REFL' was also disconnected and connected to the B port via a DC block. 

3. The ITMX was 'misaligned'. (This allowed the reflected green PD output as seen on the oscilloscope to stabilize.)

4. The PZT is modulated in frequency and the residual amplitude modulation (as observed in the measured reflected green light) is plotted, ref. Attachment 1. The parameters for the plotted data in the attachment were:

# AG4395A Measurement - Timestamp: Nov 07 2019 - 17:04:07
#---------- Measurement Parameters ------------
# Start Frequency (Hz): 10000.0, 10000.0
# Stop Frequency (Hz): 10000000.0, 10000000.0
# Frequency Points: 801, 801
# Measurement Format: LOGM, PHAS
# Measuremed Input: AR, AR
#---------- Analyzer Settings ----------
# Number of Averages: 8
# Auto Bandwidth: On, On
# IF Bandwidth: 300.0, 300.0
# Input Attenuators (R,A,B): 0dB 10dB 20dB 
# Excitation amplitude = -10.0dBm




Update (19:13 7thNov19):  When the ITMX was intentionally misaligned, Rana and I checked to see if the Oplevs were turned off and they were. But while I was casually checking the Oplevs again, they were on! 

Not sure what to do about this or what caused it. 


[Shruti, Rana]

- At the X end, we set up the network analyzer to begin measurement of the AM transfer function by actuation of the laser PZT.

- The lid of the PDH optics setup was removed to make some checks and then replaced.

- From the PDH servo electronics setup the 'GREEN_REFL' and 'TO AUX-X LASER PZT' cables were removed for the measurement and then re-attached after.

- The signal today was too low to make a real measurement of the AM transfer function, but the GPIB scripts and interfacing was tested. 


Attachment 1: AMTF20191107.png
  15058   Mon Dec 2 00:27:20 2019 gautamUpdateALSGreen ALS resurrection

Attachment #1 - comparison of phase tracker servo angle fluctuations for the green beat vs IR beat.

  • To convert to Hz, I used the PT servo calibration detailed here.
  • This is only a function of the delay line length and not the signal strength, so shouldn't be affected by the difference in signal strength between the IR and green beats.
  • For the green beat - I divided the measured spectra by 2 to convert the green beat frequency fluctuations into equivalent IR frequency fluctuations.
  • There is no whitening before digitization. I believe the measured spectra are dominated by ADC noise above ~50 Hz. See this elog for the frequency discriminant as a funtion of signal strength, so 5uV/rtHz ADC noise would be ~2 Hz/rtHz for a -5dBm signal, which is what I expect for the Y beat, and ~0.5 Hz/rtHz for a +5dBm signal, which is what I expect for the X beat. Hence the brown (Green beat, XARM) being lower than the green trace (IR beat, XARM) isn't real, it is just because of my division of 2. So I guess that calibration factor I applied is misleading.
  • I did not yet check the noise in the other configuration - arm lengths controlled using ALS, and POX/POY as the OOL sensors. To be tried tonight.

Attachment #2 - RIN of the DCPDs.

  • I noticed that over 10s of seconds, the GTRY level was fluctuating by ~5%. 
  • This was much more than any drift seen in the GTRX level.
  • Measuring the RIN on the DCPDs (Thorlabs PDA36A) supports this observation (spectra were divided by DC value to convert into RIN units).
  • There is ~120uW (1.6 VDC, compatible with 30dB gain setting) incident on the GTRX PD, and ~6uW (170 mVDC, compatible with 40dB gain setting) incident on the GTRY PD.
  • Not sure what is driving this drift - I don't see any coherence with the IR TRY signal, so doesn't seem like it's the cavity.

Characterization of the green beat setup [past numbers]:

  • With some patient alignment effort (usual near-field/far-field matching), I was able to recover the green beat signals.
  • Overall, the numbers I measured today are consistent with what was seen in the past when we had the ability to lock using green ALS.
  • The mode-matching between the PSL and AUX green beams are still pretty abysmal, ~40-50%. The mode shapes are clearly different, but for now, I don't worry about this.
  • I saw some strong AM of the beat signal (for both EX and EY beats) while I was looking at it on a scope, see Attachment #3. This AM is not visible in the IR beat, not sure what to make of it. The frequency of the AM is ~1 MHz, but it's hard to nail this down because the scope doesn't have a very long buffer, and I didn't look at the frequency content on the Agilent (yet).

o BBPD DC output (mV), all measured with Fluke DMM

             XARM   YARM 
 V_DARK:     +1.0    +2.0
 V_PSL:      +8.0    +13.0
 V_ARM:      +157.0  +8.0

o BBPD DC photocurrent (uA)
I_DC = V_DC / R_DC ... R_DC: DC transimpedance (2kOhm)
 I_PSL:       3.5    5.5
 I_ARM:      78.0    3.0

o Expected beat note amplitude
I_beat_full = I1 + I2 + 2 sqrt(e I1 I2) cos(w t) ... e: mode overlap (in power)
I_beat_RF = 2 sqrt(e I1 I2)

V_RF = 2 R sqrt(e I1 I2) ... R: RF transimpedance (2kOhm)

P_RF = V_RF^2/2/50 [Watt]
     = 10 log10(V_RF^2/2/50*1000) [dBm]

     = 10 log10(e I1 I2) + 82.0412 [dBm]
     = 10 log10(e) +10 log10(I1 I2) + 82.0412 [dBm]

for e=1, the expected RF power at the PDs [dBm]
 P_RF:      -13.6  -25.8

o Measured beat note power (measured with oscilloscope, 50 ohm input impedance)      
 P_RF:      -17.95dBm (80 mVpp)  -28.4dBm (24mVpp)   (40MHz and 42MHz)  
    e:        37%                    55  [%]                                             

I also measured the various green powers with the Ophir power meter (filter off): 

o Green light power (uW) [measured just before PD, does not consider reflection off the PD]
 P_PSL:       18    24
 P_ARM:       400     13

The IR beat is not being made at the moment because I blocked the PSL beam entering the fiber.

Attachment 1: ALSnoiseComparison.pdf
Attachment 2: ALS_TR_RIN.pdf
Attachment 3: GreemAM.pdf
  15059   Mon Dec 2 18:20:29 2019 gautamUpdateALSEY uPDH post mixer LPF

As part of characterization, I wanted to calibrate the EY uPDH error point monitor into units of Hz. So I thought I'd measure the PDH horn-to-horn voltage with the cable to the laser PZT disconnected. However, I saw no clean PDH fringe while monitoring the signal after the LPF that is immediately downstream of the mixer IF output. I then decided to measure the low pass filter OLTF, and found that it seems to have some complex poles (f0~57kHz, Q~5), that amplify the signal by ~x6 relative to the DC level before beginning to roll-off (see Attachment #1). Is this the desired filter shape? Can't find anything in the elog/wiki about such a filter shape being implemented...

The actual OLTF looks alright to me though, see Attachment #2.

Attachment 1: EY_uPDH_LPF.pdf
Attachment 2: EY_uPDH_OLTF.pdf
  15060   Mon Dec 2 20:18:18 2019 ranaUpdateALSEY uPDH post mixer LPF

                   filter Q seems too high,

but what precisely is the proper way to design the IF filter?

   seems like we should be able to do it using math instead of feelins

                              Izumi made this one so maybe he has an algorythym

  15063   Tue Dec 3 00:10:15 2019 KojiUpdateALSEY uPDH post mixer LPF

I got confused. Why don't we see that too-high-Q pole in the OLTF? 

  15064   Tue Dec 3 00:51:25 2019 gautamUpdateALSEY uPDH post mixer LPF

I'm not sure - maybe it was measurement error on my part, I will double check. Moreover, the EX and EY boxes don't seem to use identical designs, if one believes the schematics drawn on the Pomona boxes. The EY design has a 50ohm input impedance in the stopband, whereas the EX doesn't. Maybe the latter needs a Tee + 50ohm terminator at the input?

Judging by the schematics, the servo inputs to both boxes are driving the non-inverting input of an opamp, so they see high-Z.


I got confused. Why don't we see that too-high-Q pole in the OLTF?

  15066   Tue Dec 3 18:15:42 2019 gautamUpdateALSEY uPDH post mixer LPF

Rana and I discussed this alogrythym a bit today - here are some bullet points, I'll work on preparing a notebook. We are still talking about a post-mixer low pass filter.

  • We want to filter out the 2f component - attenuation relative to the 1f content and be well below the slew-rate of the first post-mixer opamp (OP27).
  • We don't want to lose much phase due to the corner of the LPF, so that we can have a somewhat high UGF - let's shoot for 30kHz.
  • What should the order of the filter be such that we achieve these goals?
  • We will use a numerical optimization routine, that makes a filter that has
    • yy dB attenuation at high frequencies
    • sufficient stability margin
    • sufficiently small phase lag at 30 kHz so that we can realize ~30kHz UGF with the existing servo electronics.

                   filter Q seems too high,

but what precisely is the proper way to design the IF filter?

   seems like we should be able to do it using math instead of feelins

                              Izumi made this one so maybe he has an algorythym

  15068   Tue Dec 3 21:28:24 2019 gautamUpdateALSEY uPDH post mixer LPF

Here are some loop transfer functions. I basically followed the decomposition of the end PDH loop as was done in the multi-color metrology paper. There is no post-mixer low pass filter at the moment (in my model), but already you can see that the top of the phase bubble is at ~10 kHz. Probably there is still sufficient phase available at 30 kHz, even after we add an LPF. In any case, I'll use this model and set up a cost function minimization problem and see what comes out of it. For the PZT discriminant, I used 5 MHz/V, and for the PDH discriminant, I used 40 uV/Hz, which are numbers that should be close to what's the reality at EY.

(i) Note that there could be some uncertainty in the overall gain (VGA stage in the servo).

(ii) For the cavity pole, I assumed the single pole response, which Rana points out isn't really valid at ~1 MHz, which is close to the next FSR

(ii) The PZT response is approximated as a simple LPF whereas there are likely to be several sharp features which may add/eat phase. 


 I'll work on preparing a notebook.

Attachment 1: uPDH.pdf
  15130   Fri Jan 17 18:02:21 2020 gautamUpdateALSGain blocks packaged and characterized


  1. The ZHL-1010+ gain blocks acquired from MiniCircuits arrived sometime ago.
  2. I packaged them in a box prepared  (Attachment #1).
  3. Their performance was characterized by me (Attachment #2 and #3).

The measurements are consistent with the specifications, and there is no evidence of compression at any of the power levels we expect to supply to this box (<0dBm).


These "gain blocks" were acquired for the purpose of amplifying the IR ALS beat signals before transmission to the LSC rack for demodulation. The existing ZHL-3A amplifiers have a little too much gain, since our revamp to use IR light to generate the ALS beat.

Attachment #4: Setups used to measure transfer functions and noise.

For the transfer function measurement, I chose to send the output of the amplifier to a coupler, and measured the coupled port (output port of the coupler was terminated with 50 ohms). This was to avoid saturating the input of the AG4395. The "THRU" calibration feature of the AG4395 was used to remove the effect of cabling, coupler etc, so that the measurement is a true reflection of the transfer function of OUT/IN of this box. Yet, there are some periodic ripples present in the measured gain, though the size of these ripples is smaller than the spec-ed gain flatness of <0.6dB.

For the noise measurement, the plots I've presented in Attachment #3 are scaled by a factor of sqrt(2) since the noise of the ZFL-500-HLN+ and the ZHL-1010+ are nearly identical according to the specification. Note that the output noise measured was divided by the (measured) gain of the ZFL-500-HLN+ and the ZFL-1010+ to get the input referred noise. The trace labelled "Measurement noise floor" was measured with the input to the ZFL-500-HLN+ terminated with 50ohms, while for the other two traces, the inputs of the ZHL-1010+ were terminated with 50ohms.

Raw data in Attachment #5.

I will install these at the next opportunity, so that we can get rid of the many attenuators in this path (the main difficulty will be sourcing the required +12V DC for operation, we only have +15V available near the PSL table).

Attachment 1: photos.pdf
Attachment 2: gain.pdf
Attachment 3: noise.pdf
Attachment 4: measSchem.pdf
Attachment 5: zhl1010Data.zip
  15153   Fri Jan 24 17:14:01 2020 gautamUpdateALSGain blocks installed

Jordan will write up the detailed elog but in summary,

  1. Former +24V Sorensen in the AUX OMC power rack (south of 1X2) has been reconfigured to +12V DC.
  2. The voltage was routed to a bank of fusable terminal blocks on the NW corner of 1X1.
  3. An unused cable running to the PSL table was hijacked for this purpose.
  4. The ZHL-1010+ were installed on the upper shelf of the PSL table, the two gain blocks draw a total of ~600mA of current when powered.

I will install these at the next opportunity, so that we can get rid of the many attenuators in this path (the main difficulty will be sourcing the required +12V DC for operation, we only have +15V available near the PSL table).

  15157   Sun Jan 26 14:40:55 2020 gautamUpdateALSALS OOL noise

In preparation for resuming IFO locking activities, I measured the ALS noise with the arm lengths locked to IR, AUX laser frequencies locked to the arm lengths. Looks promising (y-axis units are Hz/rtHz).

Attachment 1: ALSnoise_20200126.pdf
  15206   Tue Feb 11 16:39:00 2020 shrutiUpdateALSAM/PM

The results of the AM/PM measurements:

  • Attachment 1: Traces of 9 AM TFs overlaid on top of each other, calibrated by measuring the voltage at the ‘GREEN_REFL’ output where the TF was measured (described in elog 40m:15197). This was almost exactly 2 V.
  • Attachment 2: Traces of 9 PM TFs also overlaid measured using DLFD (as described in elog 40m:15180). Calibrated using the measured ~600 mV pk-pk voltage. The phase plots were unwrapped (shifted by 180 deg if needed) so that each started from roughly 0 deg.

Both the AM and PM TFs were scaled to make them have the same average value. Manually adjusting the delay line offset for each measurement using the oscilloscope was probably not accurate enough and therefore resulted in different scaling which this should somewhat compensate.

Attachment 3:

  • The orange and green lines are the averages of the PM and AM values of Attachments 1 and 2 respectively.
  • The solid red line is at 230 kHz, which was the previously chosen value for PDH locking. The peak seems to have shifted to the left from previous measurements (elog 40m:12077).
  • A horizontal black dashed line is drawn to show where the ratio is 10^5.
  • The red regions correspond to frequencies where PM/AM > 10^5 [only shown for frequencies greater than 200kHz], these are roughly (in kHz):
    • 211.4-213.9
    • 221.4-230.7 (peak at 225.642)
    • 240.8-257.9
    • ~748.3
    • 753.3-799.8, two largest peaks at 763.673 and 770.237
    • 809.6-829.3, peak at 819.472
    • 839.2-842.4
    • 881.8-891.7

Updated Calibration

Attachment 2 and 3 were miscalibrated due to an error in my understanding of the delay line, but the net result of the change in factors is qualitatively almost the same and the position of the major peaks remain predominantly unchanged.

The new plot is in Attachment 5.

The new calibration factor used: 5 MHz/V at the output of the mixer to obtain the frequency modulation and then division by the mod. freq. to obtain PM.

5 MHz/V because changing the PZT voltage by 0.01 V=> change in beat frequency by 0.1 MHz, which was seen as a 20 mV change in the delay line mixer output.

Again, the calibration is not very precise and I will probably repeat this experiment at some point more precisely.

Attachment 1: AM.pdf
Attachment 2: PM.pdf
Attachment 3: Ratio_all.pdf
Attachment 4: Ratios_FM_PM.pdf
Attachment 5: Ratio_all_new.pdf
  15209   Thu Feb 13 01:47:39 2020 gautamUpdateALSFast ALS - delay line prep

A few years ago, Koji and I setup a delay line phase shifter, which can be used to impart a (switchable) delay to a signal path. Since we talked about reviving the fast (= high bandwidth) ALS control scheme at the meeting, I reminded myself of the infrastructure available.

  • Schematic
  • Comprehensive note on theory of operation / performance.
  • Past elog threads - #11603 and #11604.
  • Attachment #1 - my modification to the ALS screen to add a slider that controls the channel C1:LSC-BO_1_0_SET. The label is a bit misleading for now - elog11604 tells you the conversion between this slider value and the actual delay in nanoseconds, but I couldn't get a soft channel set up that correctly FLNKed to this record. In the process of trying to do so, I edited the C1_ISC-AUX_ALS.db file, and also restarted the modbus and latch processes on c1iscaux a few times.
  • Attachment #2 - frequency dependent loss for some representative delays. At ~200 MHz, I find the measured loss to be > 8dB, which is ~2dB more than what the D. Sigg note tells me to expect. This is rather a lot of loss, but I guess it's okay. Measurement cable loss was calibrated out with the AG4395A.
  • Attachment #3 - confirmation of constantness of delay as a function of frequency, for some representative delays. The "undelayed" setting corresponds to a fixed delay of ~4 nsec, which is consistent with what the D. Sigg note tells me to expect. Once again, I calibrated out the delay of the measurement setup using the AG4395A.

For a beat note in the regime 10-100 MHz, we should have plenty of range in this module to add a delay such that we zero one quadrature of the ALS DFD output (for a linear error signal). 

I then proceeded to connect the single-ended front panel BNC corresponding to the ALS_X_I DFD channel to the IN2 input of the CM board (this would be what we use for high bandwidth ALS feedback). The conventional ALS system uses the differential output from a rear-panel D-sub, so in principle, both systems could run in parallel. I confirmed that I could see a signal when the IN2 path on the CM board was engaged (monitored using ndscope at the CM_Slow output), and that this signal stabilized when the green laser was locked to the X-arm length, which itself was slaved to the PSL frequency using the usual POX locking scheme. I have not yet routed the LO leg of the ALS_X beat through the delay line phase shifter - see next elog for details.

Update about the ALS MEDM screen slider: the trick was to change the OMSL field of the C1:LSC-BO_1_0 channel to "closed_loop" instead of "supervisory". Once this is done, the DOL value of the same channel can be set to the soft channel C1:ALS-DelayCalc, which sets the 16 bit binary string that controls the delay. Because arbitrary delays are not possible, I think it's more natural for the user to interact with this 16-bit binary string rather than the actual delay itself. So the MEDM screen has been slightly modified from what is shown in Attachment #1.

Attachment 1: delaySlider.png
Attachment 2: delayLineLosses.pdf
Attachment 3: delayLineCal.pdf
  15211   Thu Feb 13 21:30:55 2020 shrutiUpdateALSALS OOL noise with arms locked

[Meenakshi, Gautam, Shruti]


- We initially aligned the arm cavities to get the green lasers locked to them. For the X arm cavity, we tweaked the ITMX and ETMX pitch and yaw and toggled the X green shutter until we saw something like a TEM00 mode on the monitor and a reasonable transmitted power.

- With the LSC servo enabled, the IR light also became resonant with the cavities.

- Then we measured the noise in different configurations. Attachment 1 shows the the ALS OOL (in the IR beat signal) noise with the arms locked inidividually via PDH.

The script for plotting the ALS beat frequency noise is:

Attachment 1: 20200213_ALS.pdf
  15212   Fri Feb 14 00:53:50 2020 gautamUpdateALSFast ALS - more setup

In the process of setting up some cabling at 1Y2, I must've bumped a cable to the c1lsc expansion chassis. Anyways, the c1lsc models crashed. I ran the reboot script around 530pm PDT. Usual locking behavior was recovered after this. The work at 1Y2 was:

  • Ran a cable from X Beat power splitter ("LO" leg of the analog delay line) to variable delay line. 
  • Ran cable from delay line to demodulator's LO input.
  • Set up the SR785 for some CM board TF measurements.

The IN2 to CM board was already connected to I single ended output of the ALS X demodulator. The ~100 Hz UGF digital locking using the CM_SLOW path is straightforward but I didn't have any success with the AO path tonight. I wonder how high BW this lock can be made without injecting a ton of noise into the IMC loop, given that the EX uPDH only has ~ 10 kHz UGF.

Attachment #1 shows the spectra of the ALS signal 

  • The two "CM Slow" traces are the digitized "SLOW" output of the common mode board, whose IN2 is connected to the demodulated I output of the analog delay line.
  • The delay in the LO line of the analog delay line is adjusted to zero the DC value of this signal to best effort.
  • These spectra are measured with the arm cavities POX/POY locked, and the EX laser locked to the arm cavity using the end PDH box.
  • I simultaneously monitor the output of the digital phase tracker servo, and scale the CM Slow signal such that the spectra line up. The scaling factor required was to multiply the CM_SLOW signal x10 (CM board IN2 gain was set to +6dB, to account for the x2 gain in going from single ended to differential inside the ALS demodulator box).
  • One puzzline feature is why switching on the ADC whitening makes the ALS spectrum noisier (even though it clearly changes the digitization noise floor). There is a peak that appears at ~ 8 kHz with the whitening on, and it may also be downconverted noise from some peak at higher frequencies I guess (if the AA isn't sharp enough). 

Attachment #2 is an OLTF measurement.

  • In the blue trace, the arm length is controlled by using the CM Slow signal as an error signal, applying feedback to IMC length via MC2.
  • In the red trace, I turned the digital MC2 violin notches off, and added upped the IMC IN2 gain to -12 dB (AO gain slider = 0dB).
  • This was as high as I could go before the PC drive RMS began to go crazy.
  • But still, there isn't any significant phase advance.
  • It is possible I need to tack on a low-pass filter to prevent noise injection at higher frequencies...
Attachment 1: CMSlow_ALSnoise.pdf
Attachment 2: OLTFmeas.pdf
  15213   Fri Feb 14 14:02:13 2020 shrutiUpdateALSALS OOL noise with arms locked

[Meenakshi, Shruti]

Even though we were not able to lock the the IR beat (by enabling LSC) during the day possibly because of increased seismic activity, we tried to the measure the ALS beat frequency noise by changing the PDH side-band frequency to different values.

I tried choosing values that corresponded to the peaks in the PM/AM as found in elog:15206 but for some reason unknown to us the cavity did not lock between 700-800 kHz.

The three attachments have data for different sideband frequencies:

Attachment 1: 819.472 kHz (peak in PM/AM, measured around noon)

Attachment 2: 225.642 kHz (peak in PM/AM, measured earlier in the morning)

Attachment 3: 693.500 kHz (not a peak in PM/AM)

We don't think these plots mean much and will do the measurement at some quieter time more systematically.


While doing the experiment, the ITMY pitch actuation was changed from -2.302 to -2.3172V because it locked better.

The ITMX, ETMX alignment was also tweaked to try to lock with different sideband frequencies and then restored to the values that were found earlier in the morning.

Attachment 1: 819472_10.pdf
Attachment 2: 225642_10.pdf
Attachment 3: 693500_10.pdf
  15214   Fri Feb 14 14:52:41 2020 gautamUpdateALSALS OOL noise with arms locked

Unlikely, the alignment was probably just not good. I restored the alignment and now the arms can be locked to IR frequency.


Even though we were not able to lock the the IR beat (by enabling LSC) during the day possibly because of increased seismic activity

  15216   Tue Feb 18 18:14:59 2020 shrutiUpdateALSALS OOL noise with arms locked

We proceeded with the trying to measure the ALS out-of-loop noise of the X arm when the X arm cavity is green-locked using different PDH sideband frequencies.

Before doing the experiment, Koji helped us with getting the arm cavities locked in IR using LSC (length) and ASC (angular).

With the arms locked in IR and green, we repeated the same measurements as before at different sideband frequencies (Refer Attachment 1 - label in Hz). We did not optimize the phase nor did we look at the PDH error signal today which is possibvly why we did not see an improvement in the noise. We will look into this possibly tomorrow.

Attachment 1: ALSNoise.pdf
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