I grab the data we recorded yesterday from the scope and plot it in normalized units (remove noise level and divide by maximum). See attachment.

It can be seen that the measured ringdown time is ~ 17us while the shut-off time is ~12us.

I plan to model the PD+AOM as a lowpass filter with an RC time constant of 12us and undo its filtering action on the PMC trans ringdown measurement to get the actual ringdown time.

Is this acceptable?

{Yehonathan, Rana, Jon}

To check whether we laser is being shut fast enough for the ringdown measurement we put a PD55 in the path that leads to the beat note setup. The beam is picked off from the back steering mirror after AOM and before the PMC.

@Shruti the PD is now blocking the beam to your setup.

As before, we drive the AOM to deflect the beam. The deflected beam is coupled to the PMC cavity. We lock the PMC and then shut the beam by turning off the output of the function generator that provides voltage to the AOM driver.

We measure the transmitted light of the PMC together with the light that is picked off before the PMC. In Attachment 1, the purple trace is the PMC transmission, the green trace is the peaked-off beam and the yellow trace is the function generator signal.

Rana was pointing out that the PDs, the function generator and the scope were not carefully impedance matched, which could lead to erroneous measurements. He also mentioned that the backscattered beam was too bright which might indicate that the PMC is oscillating. To remedy this we lowered the gain of the PMC lock to ~8.

We repeat the measurement after setting all the components to 50ohm (attachment 2). We then realize that the BNC T junction connected on the function generator is splitting the signal between the 50ohm AOM driver and 1Mohm oscilloscope channel which causes distortions as can be seen. We remove the T junction and get a much cleaner measurement (see next elog).

It seems like either the shutting speed or the PDs are only slightly faster than the PMC. I also check the AOM driver RF output fall time doing the same kind of measurement (attachment 3).

We suspect the PDs' bandwidth is to blame (although they are quoted to have 10MHz bandwidth).

In any case, this is fast enough for the IMC and arm cavities whose lifetime should be much longer than the PMC.

I will post an elog with some numbers tomorrow.

Megatron is now running Ubuntu 18.04 LTS.

We should probably be able to load all the LSC software on there by adding the appropriate Debian repos.

I have re-enabled the cron jobs in the crontab.

The MC Autolocker and the PSL NPRO Slow/Temperature control are run using 'initctl', so I'll leave that up to Shruti to run/test.

Make sure to measure the power drop of the beam downstream of the AOM but before the PMC. Need to plot both together to make sure the chop time is much shorter than the 1/e time.

[Jon, Yehonathan]

We carried out a set of cavity ringdown measurements of the PMC. The 1/e decay time scale is found to be 35.2 +/- 2.4 (systematic) μs. The statistical error is negligible compared to the systematic error, which is taken as the maximum absolute deviation of any measurement from the average value.

To make the measurement, we injected the first order deflection beam of an 80 MHz AOM, then extinguished it quickly by cutting the voltage offset to the AOM driver provided by an RF function generator. A 100 MHz oscilloscope configured to trigger on the falling voltage offset was used to sample the cavity in transmission as sensed by a PDA55. We found the detector noise of the DC-coupled output of the 35.5 MHz REFL PD to be too high for a reflection-side measurement.

Further loss analysis is forthcoming.

{Yehonathan, Jon}

We are able to lock the PMC on the TEM00 mode of the deflected beam.

However when we turn off the driving voltage to the AOM and back on the lock is not restored. It get stuck on some higher order mode.

There are plethora of modes present when the PZT is scanned, which makes us believe the cavity is misaligned.

To lock again on the TEM00 mode again we disconnect the loop (FP Test point 1), find a TEM00 mode using the DC output adjust and close the loop again.

{Jon, Yehonathan}

We burt-restored the PSL and the PMC locked immediately.

The PMC is now locked on the AOM first order mode.

nvm the PZT can scan over many GHz.

Just realized that the diffracted beam is frequency shifted by 80MHz. It would shift the PZT position in the PMC lock acquisition, wouldn't it?

In short:

Steps undertaken:

1. Normally I would unlock the IMC (Disabling the servo between the 'Filter' and 'Polarity' on the Mode Cleaner Servo Screen), but today I did not have to since Rana had kept it unlocked.

2. Misaligned the ITMX. This is to prevent cavity resonances from returning to the laser

3. Turned up the air on the HEPA at the PSL table to 100% during the measurement

4. Cables were connected as before (diagram shown in attachment of elog 15069)

5. The X end laser NPRO was actuated for the TF measurement using a long cable connected to TO AUX_X LASER PZT

Thoughts and observations:

- Reading out the error signal after amplification cannot distinguish between a locked loop or one out of its range. The error signal would be very small in both cases.

- Looking at the beat note on an oscilloscope, there also seemed to be an additional amplitude modulation that I had not noticed earlier. Rana suggested that it may have something to do with the pre-mode cleaner and the AOM being driven at 80 MHz

- Even though the TF was attempted, it seemed too noisy, suggesting that the PLL did not seem to work

- Rana also suggested that it may be a better idea to use the PZT of one of the lasers as the VCO for the PLL feedback instead of the Marconi.

{Yehonathan, Rana}

In order to setup a ringdown measurement with perfect extinction we need to align the first order beam from the AOM to the PMC instead of the zeroth order.

We connected a signal generator to the AOM driver and applied some offset voltage. We spot the first order mode and align it to the PMC. The achieved transmitted power is roughly as it was before this procedure.

Along the way few changes has been made in the PSL table:

1. Some dangling BNCs were removed.

2. Laser on the south east side of the PSL table was turned off.

3. DC power supplies were removed (Attachment 1 & 2). The rubber legs on the first one are sticky and leave black residue.

4. The beam block that orginally blocked the AOM high order modes was raised to block the zeroth order mode (Attachment 3).

5. The unterminated BNC T junction (Attachment 4 - before picture). from the PMC mixer to the PMC servo was removed.

However, we are currently unable to lock the PMC on high gain. When the gain is too high the PZT voltage goes straight to max and the lock is lost.

I check the seismometers in the last 14 hours (Attached). Seems like the coherenece is restored in the x direction.

I noticed recently that Megatron was running Ubuntu 12, so I've started its OS upgrade.

1. Unlocked the IMC + disabled the autolocker from the LockMC screen + closed the PSL shutter (IMC REFL shutter doesn't seem to do anythin)
2. Disabled the "FSS" slow servo on the FSS screen
3. did sudo apt-get update, sudo apt-get upgrade, and then sudo apt-get do-release-upgrade which starts the actual thing
4. According to the internet, the LTS upgrades will go in series rather than up to 18 in one shot, so its now doing 12 -> 14 (Trusty Tapir)

Megatron and IMC autolocking will be down for awhile, so we should use a different 'script' computer this week.

Mon Dec 9 14:52:58 2019

upgrade to Ubuntu 14 complete; now upgrading to 16

The IBM laptop at EX was running Ubuntu 14, so I allowed it to start upgrading itself to Ubuntu16 as it desired. After it is done, I will upgrade it to 18.04 LTS. We should have them all run LTS.

I have re-centered the EX (and EY) seismometers. They are Guralp CMG40-T, and have no special centering procedure except cycling the power a few times. I turned off the power on their interface box, then waited 10s before turning it back on.

The fist atm shows the comparison using data from 8-9 PM Saturday night:

1. there seems to be a factor of 2 calibration diff between the T240 near the BS, and the Guralp seismometers at the end. Which one is right? When was the last time they were cross calibrated?
2. The low coherence between BS_X and EX_X shows the problem. They should be very coherent (> 0.9) for 0.1-1 Hz.

Yehonathan, please center the EX seismometer.

The attached PDF shows the seismometer signals (I'm assuming that they're already calibrated into microns/s) during the lab tour for the art students on 11/1. The big spike which I've zoomed in on shows the time when we were in the control room and we all jumped up at the same time. There were approximately 15 students each with a mass of ~50-70 kg. I estimate that out landing times were all sync'd to within ~0.1 s.

[Jordan, gautam]

We did the following:

• Route the fiber from the control room to 1Y2.
• Plug fiber in to FiBox at either end, turned FiBoxes ON.
• Tested the optical connection by driving a 1Vpp 440 Hz sine wave from a function generator - Yehonathan hears it loud and clear in the control room.
• Tested that both CH1 and CH2 work - only CH1 is connected to the speakers in the control room at the moment.
• There is some cross-coupling between the channels - not sure if this is happening in the multi-mode fiber or in the electroncis, but I estimate the isolation to be >30dB.
• Connected CH8 and CH9 of DAC0 in the c1lsc expansion chassis to CH1 and CH2 respectively of the FiBox in 1Y2. 
• Restarted the c1daf model on c1lsc, came up smooth.
• Routed the POY11 error signal through the various matrices in c1daf, and we could 👂 the Y-arm cavity 🔐 😎 
• Channels are muted for now - I'll give this a whirl while doing the PRFPMI locking.

Summary:

I made it to 0 CARM offset, PRMI locked a bunch of times today. However, I could not successfully engage the AO path.

Details:

Much of the procedure is scripted, here is the rough set of steps:

• Transition control of the arms from IR signals to ALS signals.
• DC couple the ITM oplev servos
• Burt-restore the settings for PRMI locking with REFL165I-->PRCL, REFL165Q-->MICH, and then enable the MICH_B / PRCL_B locking servos.
• Add some POPDC to the PRMI triggering (nominally only POP22_I) to let these loops be locked while POP22_I fluctuates wildly when we are near the CARM=0 point.
• Zero the CARM offset.
• Adjust the CARM_A/DARM_A offsets such that CARM_B/DARM_B are fluctuating symmetrically about 0.
• CARM_B gain --> 1.0, to begin the RF blend.
• Prepare to hand the DC control authority to ALS by turning off FM1 in the CARM filter bank, and turning ON an integrator in the CARM_B filter.

As I type this out, I realized that I was incorrectly setting offsets to maximize the arm powers by adjusting CARM/DARM offsets as opposed to CARM_A / DARM_A offsets. Tried another round of locking, but this time, I can't even turn the integrator on to get the arms to click into somewhat stable powers. 

One thing I noticed is that depending on the offsets I put into the 3f locking loops, the mean value of REFL11 and AS55 when the ALS CARM/DARM offsets are zeroed changes quite significantly. What is the correct condition to set these offsets? They are different when locking the PRC without arm cavities, and also seem to change continuously with CARM offset. I am wondering if I have too much offset in one of the vertex locking loops?

In preparation for locking tonight, I re-centered the spots on the Oplev QPDs for the ITMs, BS and PRM after locking and running the dither alignment for the arms and also the PRMI carrier. In the past, DC coupling the ITM Oplevs helped the angular stability a bit, let's see if it still does.

I tried starting the c1oaf model, but got a DQ error (I want the option of running feedforward during locking even if the filters aren't particularly well tuned yet). Note that this isn't "just a warning light" - some channels are initialized to +/- 1e20, so if you try turning some filters on, you will deliver a massive kick to the optics. Restarting it crashed c1lsc (this is not unexpected behavior - the only way to clear the DQ error is to restart the model, and empirically, the success rate is ~50%). The reboot script brought everything back online smoothly, and the second, time, c1oaf started without any issues.

While looking at the CDS overview screen, I noticed that the c1scy model was reporting frequent RFM errors for the C1:SCY-RFM_ETMY_LSC channel (but none of the others). On the sender model (c1rfm), no errors were being reported. The diag reset button / mxstream restart didn't really work either. See Attachment #1. Just restarting the c1scy model didn't fix the error - I had to reboot the machine and restart the models, and now no errors are being reported.

Attachment #2 shows the current nominal CDS status - the red light on c1lsc is due to some missing c1dnn channels (I'll remove these at the next c1lsc model change because I don't want to un-necessarily reboot the vertex FEs), and the c1omc model is obsolete I guess. c1daf isn't running right now but once I get the new fiber (ordered), I'm gonna restart this model as well.

P.S. The ALS temperature sliders are not SDF-ed. So when the model was restarted, I had to change the sliders back to their old values to get the beat back in the usable range.

I symlinked the SRmeasure and AGmeasure commands to /usr/bin/ on donatella (as it is done on pianosa) so that these scripts are in $PATH and may be run without having to navigate to the labutils directory.

[Yehonathan, Gavin]

Measuring POX11_Q_MON and injecting a signal into the ITMX_UL_IN port a signal could not be seen on the function generator. After debugging the source of the issue was two fold:

• By using the quadrant drives for coils (UL, UR etc) a signal has to pass through a switch before reaching the driver. To resolve this the signal input was switched to POS_IN (driving the entire coil at once rather than in quadrants) which has no switch to bypass.
• The averaging on the Stanford SR785 was set too low. By increasing the averages from 10 to 25 the signal became more visible.

Unrelated to these issues the signal input was switched to POY11_Q_MON and ITMY_POS_IN as part of the debugging process. The function generator used was switched from the Stanford to the Siglant SDG 1032X.

An unrelated issue but note worthy was the Lenovo 40m laptop used to measure the IFO state (locked or unlocked) ran out of battery in a very short timespan.

To gauge where the resonance of the test masses are FEA model of a simple 40m test mass was computed to give an esitimate at what frequency the eigenmodes exist. For the first two modes the model gave resonances at 20.366 kHz (butterfly mode) and 28.820 kHz (drumhead mode). Then by measuring with an acquisition time of 1 s at they frequencies on the SR785 and injecting broad band white noise with a mean of 0 V and a stdev of 2 V, small peaks were seen above the noise at 20.260 kHz and 28.846 kHz. By then injecting a sine wave at those frequencies with 9 Vpp, the peak became clearly visible above the noise floor.

The last step is to measure the natural decay of these modes when the excitation is turned off. It is difficult to tell currently if these are indeed eigenmodes or just large cavity injections with an associated stabilisation time (what could appear as a ringdown decay). More investigation is required.

• The arm powers could be stabilized somewhat once the CM_SLOW path to MC2 was engaged.
• However, I was never able to get the AO path to do anything good.
• Took a bunch of CM board TFs, need to think about what I need to do differently to get this next bit to work.
• An SR785 is sitting next to the LSC rack hooked up to the CM board. I also borrowed the GPIB unit from the AG4395 to grab data from said SR785.
• One thing I noticed that the CARM_B (=CM_SLOW) and DARM_B (=AS55_Q) signals both had a DC offset, so maybe this is indicative of some DC offset in the PRMI 3f signals? Right now, I lock the PRMI without any offsets, and as I reduce the CARM offset, I can see the DC value of REFL11_I and AS55_Q changing significantly. To be investigated in tonight's locking.

Were some cables from the ALS beat setup modified? I can't see the beat on the scope, and this elog doesn't say anything about cable connection rearrangement. At ~2311, I am reverting the setup to as it should be.

Trawling through some past elogs, I saw that the ALS noise increase as a function of CARM offset reduction is not really a new thing (see e.g. this elog). In the past, when we were able to lock, when the CARM offset is reduced to zero, the arms would "buzz" through resonance. It just wasn't clear to me how much the buzzing was - in all the plots we presented, we were not looking at the fast 16k output, so it looked like the arm powers had stabilized. But today, looking at the frame data at 16k from back in 2016, it is clear to me that the arm transmission was in fact swinging all the way from 0 to some maximum. Once the IR signal (=REFL11) blending is turned on, we were able to stabilize the arm power somewhat. What this means is that we are in a comparable state as to when we were able to lock in the past (since I'm able to sit at 0 CARM offset with the PRMI locked almost indefinitely).

So, I think what I'll try for the next 3 days is to get this blending going, I think I couldn't enable the CM_slow path because when I was experimenting with the high bandwidth Y arm cavity locking, I had increased the whitening gain of this channel, but REFL11 has much more optical gain (=larger signal) than POY11, and so I'll start from 0dB whitening gain and see if I can turn the magic integrator on. Long term, we should try and compensate the optomechanical plant that changes as our CARM offset gets reduced, as this would further reduce the lock acquisition time and simplify the procedure (no need to fiddle with the integrator, offsets etc). A relevant thread from the past.

It was way more annoying without a script and took longer than the 4 minutes it does now.

You can fix the requirement to enter password by changing the sshd settings on the FEs like I did for pianosa.

After running the script, you should verify that there are no red flags in the output to console. Yesterday, some of the settings the script was supposed to reset weren't correctly reset, possibly due to python/EPICS problems on donatella, and this cost me an hour of searching last night because the locking wasn't working. Anyway, best practise is to not crash the FEs.

After the CDSs crashed we run the rebootC1LSC.sh script.

The script is a bit annoying in that it requires entering the CDSs' passwords multiple times over the time it runs which is long.

The resulting CDS screen is a bit different than what was reported before (attached). Also, not all watchdogs were restored.

We restore the remaining watchdogs and do XARM locking. Everything seems to be fine.

{Yehonathan, Gavin}

Yesterday we tried to shake ITMX with a function generator in order to observe the 28.8kHz drum mode.

We laid a long BNC cable that runs from the YARM to the XARM. This cable either needs to be collected back to the BNC big plastic cable box under the IMC or be labeled so that it could be found easily in the future.

First, we tried to shake it at a lower frequency (100's of Hz) where the shaking should be easily observed in the POSX channel. We try driving the POS channel on the ITMX servo but nothing happens. Most likely it is disconnected.

While setting up for shaking the individual OSEM channels 4 CDSs crashed (c1lsc, c1ass, c1oaf, c1cal).

I worked on the setup up for the phase modulation measurement of the X end NPRO PZT. A previous similar measurement can be found here (12077). The setup was assembled based on the schematic in Attachment1.

Mixer used: Level 7, Mini circuits ZP-3+
LPF: up to 1.9MHz

Cables exiting the PSL table:
1. LO (Marconi -> Mixer)
2. RF (PSL+X beat note -> Mixer) The cable for this was taken from the Beat Mouth (otherwise connected to the oscilloscope)
3. Ext modulator (SR560 -> Marconi)

The long cable labled 'X Green Beat' was used to connect to the PZT (from the network analyzer).

Observations: The beat note kept floating between 0 and ~100 MHz

The PLL part of the circuit was tested coarsely with the spectrum analyzer function of the Agilent, where the loop was seen to stabilize when the carrier frequency of the Marconi was close to the instantaneous beat frequency.

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. 

Recently, accordian to Gautam, the NDS2 server has been dying on Megatron ~daily or weekly. The prescription is to restart the server.

1. I could find no instructions (that work) in the elog or wiki. We must remove the misleading entries from the wiki and update it with whatever works as of today.
2. There is a line (which has been commented out) in the Megatron crontab which is close to the right command, but it has the wrong path.
3. Running the command from the CRON (/home/nds2mgr/nds2-megatron/test_restart), gives several errrors.
4. when I run the init.d command which is in the script, it seems to run fine
5. the server then takes several minutes to get itself together; i.e. just because it is running doesn't mean that you can get data. I recommend waiting 5-10 minutes.

Also, megatron is running Ubuntu 12 !! Let's decide on a day to upgrade it to a Debian 18ish....word from Rolf is that Scientific Linux is fading out everywhere, so Debian is the new operating system for all conformists.

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.

1. Lock IMC
2. Lock one of the arms (only) using the IR PDH signal feeding back to an ETM.
3. Excite the ITM using the SR785 near 28.8 kHz
4. Look for the resulting peak using the SR785 spectra of the POX or POY error signal from the demod board
5. Based on the calibrated noise level of the POX/POY, estimate what the SNR will be of the internal mode peak.

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? 

Summary:

1. While noisier, I was able to control the arm lengths to ~30pm RMS(!) using the green ALS beats as error signals (cf. ~10 pm RMS with the IR ALS system).
2. The PRMI could be locked with a CARM offset applied.
3. When lowering the CARM offset, I saw an increase in the in-loop ALS error signal, just as I had with the IR beat.
4. IR TRX / TRY unsurprisingly did not stabilize in any meaningful way.
5. The noise increase seems to have some periodicity along the frequency axis - need to think about what this means.
6. Since there is no apparent benefit to using the green ALS beats, I restored the IR system. The green PDs should still retain somewhat good alignment if one wishes to do a comparison measurement.
7. While the shadow sensors of the ITMs report elevated noise, it is unlikely to be responsible for the cavity moving by the amount suggested by the elevated ALS error signals because of the digital low-pass filtering and 1/f^2 of the pendulum.
8. I confirmed that the ITM shadow sensors do not report elevated noise when the PRMI is locked such that the carrier is resonant. In this config, there is comparable circulating power in the PRC as to when the CARM offset is reduced to ~0.
9. The fact that the IR and green beats both show similar increase in noise suggestes that the cavity length / laser frequency is in fact being modulated, but I still don't know what the exact mechanism is.

was worth a shot i guess.

Trawling through some elogs, I see that this kind of feature showing up in the ALS CARM is not a new problem, see for example here. But I can't find out what the resolution was.

I have been experiencing frequent crashes of DTT on pianosa in the past few weeks. This is pretty annoying to deal with when trying to characterize the interferometer loops. I attach the error log dumped to console. The error has to do with some kind of memory corruption. Recall that we aren't using a GDS version that is packaged with the SL7 lscsoft packages, we are using a pretty ancient (2.15) version that is built from source. I have been unable to build a newer version from source (though I didn't spend much time trying). pianosa is the only usable workstation at the moment, but perhaps someone can make this work on donatella / rossa for general improvement in quality of life.

                   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

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 - 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.

yes, reasonable

Summary:

I tried the following minor changes to the locking procedure to see if there were any differences in the ALS noise performance:

1. Actuate DARM only on one ETM (tried both ETMX and ETMY)
2. Enable MCL and PRC seismic feedforward
3. DC couple the ITM Oplevs for better angular stability during the lock acquisition

None of these changes had any effect - the ALS noise still goes up with arm buildup. 

I think a good way to determine if the problem is to do with the IR part of the new ALS system is to resurrect the green beat setup - I expect this to be less invasive than installing attenuators/beam dumps in front of the fiber couplers at the ends. We should at the very least recover the old ALS noise levels and we were able to lock the PRFPMI with that config. If the excess noise persists, we can rule out the problem being IR scatter into the beat-mouth fibers. Does this sound like a reasonable plan?

[Yehonathan, Gavin]

As the resonant modes of the 40m TMs are at high frequencies (starting at 28.8 kHz) we started background checks to understand if we would be able to see resonant frequency excitations in the DCPD output. We used the SR785 in the Q_OUT_DEMODULATOR port of the INPUT_MODE_CLEANER to measure around this frequency. Currently we could not see any natural excitation about the noise floor indicating it may not be possible to see such a small excitation. In any case we are conducting additional measurements in the I_MON port of 1Y2_POY11 to understand if this is a certainty.

The old MCL filters are not completely useless - I find a factor of ~2 reduction in the MCL RMS when I turn the FF on. It'd be interesting to see how effective the FF is during the periods of enhanced seismic activity we see. I also wonder if this means the old PRC angular FF filters are also working, it'd help locking, tbc with PRMI carrrier...

Update: The PRC angular FF loops also do some good it seems - though the PIT loop probably needs some retuning.

i reconnected the AOM driver to the AOM in the main beam path (it was hijacked for the AOM in the AUX laser path for Anjali's MZ experiment). I also temporarily hooked up the AOM to a CDS channel to facilitate some swept-sine measurements. This was later disconnected. The swept sine will need some hardware to convert the bipolar drive signal from the CDS system to the unipolar input that the AOM driver wants (DTT swept sine wont let me set an offset for the excitation, although awggui can do this).

if the RP don't fit

u must acquit

sweep the laser amplitude

to divine the couplin w certitude

Summary:

The ITMX OSEMs report elevated noise in the 10-100 Hz band when we have high circulating power in the arm cavities, see Attachment #1. Since there is no LSC actuation on the ITMs in this state, this could be a radiation presssure effect, or could be scattered 1064nm light entering the OSEMs. The Oplevs don't report any elevated noise however. ITMY has the OSEM whitening broken for two channels, but the other two channels don't report as significant an increase as ITMX, see Attachment #2. I can't find the status of which OSEMs have the 1064nm blocking filters installed. The local damping loops are rolled off by ~100dB at 30 Hz, so the sensing noise re-injection should be attenuated by this factor, so maybe the OSEM sensor noise isn't the likely culprit. But radiation pressure didn't worsen the length noise in the past, even after our mirror cleaning and the increased PRG.

...maybe the opto-mechanical CARM plant is changing as a function of the CARM offset...

Summary:

Since we are using the POX and POY photodiodes as out-of-loop sensors for measuring the ALS noise, I decided to double-check their calibrations. I determined the following numbers (for the single arm lock):

POX_I [with 30dB whitening gain]: (8 +/- 1)e-13 m/ct

POY_I [with 18dB whitening gain]: (0.9 +/- 0.1)e-13 m/ct

With this calibration, I measured the in-loop spectra of the XARM and YARM error-points when they are locked - they line up well, see Attachment #1. Note that these numbers are close to what we determined some time ago using the same method (I drove the ITMs then, but yesterday I drove the ETMs, so maybe the more accurate measure of uncertainty is the difference between the two measurements).

Attachment #2 shows the out-of-loop spectra sensed by these photodiodes with this calibration applied, when the arms are under control using ALS beat frequencies as the error signals, and controlled in the CARM/DARM basis. Need to think about why there is such a difference between the two signals.

Methodology:

The procedure used was the same as that outlined here.

• I started by calibrating the AS55_Q output with the free-swinging Michelson.
• Next, I lock the Michelson and calibrate the BS and ITM actuators using the newly calibrated AS55_Q.
• Next, I calibrate the ETM actuator gains by measuring the ratio of response in POX/POY of driving the (unknown) ETMs and the (known) ITMs.
• Finally, I calibrate the POX/POY photodiodes by driving the ETMs by a known amount of meters (at ~310 Hz where the loop gain is negligible because of the sensing matrix measurement notches).

Summary of DC actuator gains:

The quoted values of the DC gain are for counts seen at the output of the LSC filter bank. I've attempted to show that once we account for the different series resistance and some extra gains between the output of the LSC filter bank and the actual coil, things are fairly consistent.

Some remarks:

• I do not understand why we need an extra 12dB of whitening gain on the POX channel to get similar PDH fringe height as the POY channel. The light level on these photodiodes is the same, and the RF transimpedances at 11 MHz are also close according to the wiki (3kohm for POX, 2kohm for POY).
• At night-time, the ALS noise did indeed get reduced compared to what I measured earlier in the evening.
• Even assuming 50% error in the calibration factors, it's hard to explain the swing of TRX/TRY when the CARM offset is brought to zero.
• The increase in (admittedly in-loop) CARM noise as the offset is reduced still seems to me to be correlated with the buildup of IR power in the arm cavities.

MC2 analog camera was rotated by 90 degrees. Orientation correctness was verified by exciting the MC2 Yaw degree of freedom.

Attached before and after photos of the camera setup.