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ID Date Author Type Category Subject
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.

15211   Thu Feb 13 21:30:55 2020 shrutiUpdateALSALS OOL noise with arms locked

[Meenakshi, Gautam, Shruti]

Summary:

- 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:

users/Templates/ALS/ALS_outOfLoop_Ref.xml
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...
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.

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.

 Quote: 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.

15217   Wed Feb 19 22:20:22 2020 ranaUpdateALSALS OOL noise with arms locked

Could you please put physical units on the Y-axis and also put labels in the legend which give a physical description of what each trace is?

It would also be good to a separate plot which has the IR locking signal and the green locking signal along with this out of loop noise, all in the same units so that w can see what the ratio is.

15218   Fri Feb 21 10:59:08 2020 shrutiUpdateALSPDH error signals?
Here are a few PDH error signals measured without changing the servo gain or phase from that optimized for 231.25 kHz. This was done by keeping the X arm cavity and laser unlocked but keeping the shutter for green open; so I did not force a frequency sweep but saw the unhindered motion of cavity wrt the laser using the PDH servo error monitor channel from the box (not sure if this is the best way to do it?).

Koji mentioned that there is a low pass filter with a cutoff frequency probably lower than 700 kHz which at the moment would hinder the efficacy of the locking at higher frequencies. The transfer function on the wiki suggests the same, although we are yet to investigate the circuit.

I measured the maximum range in the linear region of the signal, and here are the results:
• Attachment 1: 231.25 kHz (current PDH sideband mod freq): 1.7 V
• Attachment 2: 225.642 kHz: 1.2 V
• Attachment 3: 100 kHz: 900 mV
• Attachment 4: 763.673 kHz: 220 mV
Right now we have only inverted the phase to try locking at different frequencies (no finer adjustments were performed so elog 15216 cannot be an accurate representation of the true performance)

Ideas from the 40m meeting for adjusting the phase:
1. Delay line for adding extra phase (would require over 40m of cable for 90 deg phase shift)
2. Using two function generators for generating the sideband, clocked to each other, so that one can be sent to the PZT and the other to the mixer for demodulation.
3. Use a different LPF (does not seem very useful for investigating multiple possible frequencies)

Once we adjust the phase we may be able to increase the servo gain for optimal locking. Unless it may be a good idea to increase the gain without optimizing the phase?

15219   Fri Feb 21 13:02:53 2020 KojiUpdateALSPDH error signals?

Check out this elog: ELOG 4354

If this summing box is still used as is, it is probably giving the demod phase adjustment.

15220   Fri Feb 21 20:44:18 2020 shrutiUpdateALSALS OOL noise and PDH

[Meenakshi, Shruti]

In order to adjust the relative phase for PDH locking, we used the Siglent SDG 1032X function generator which has two outputs whose relative phase can be adjusted.

This Siglent function generator was borrowed from Yehonathan's setup near the PSL table and can be found at the X end disconnected from our setup after our use.

Initially, we used the Siglent at 231.250 kHz and 5 Vpp from each output with zero relative phase to lock the green arm cavity. By moving the phase at intervals of 5deg and looking at the PDH error signals when the cavity was unlocked we concluded that 0deg probably looked like it had the largest linear region (~1.9 V on the yaxis. Refer elog 15218 for more information) as expected.

Then we tried the same for 225.642 kHz, 5 Vpp, and found the optimal demod phase to be -55deg, with linear region of ~3 V (Ref. Attachment 2). A 'bad' frequency 180 kHz was optimized to 10deg and linear region of ~1.5 V.

The error signals at higher frequencies appeared to be quite low (not sure why at the moment) and tuning the phase did not seem to help this much.

For the noise measurement, the IFO arms were locked to IR and green, but even after optimizing the transmission with dither, we couldn't achieve best locking (green transmission was around ~0.2). Further, the IMC went out of lock during the experiment after which Koji helped us by adjusting the gains a locking point of the PMC servo. Attachment 1 contains some noise curves for the 3 frequencies with a reference from an earlier 'good' time.

15221   Sun Feb 23 18:15:22 2020 ranaUpdateALSALS OOL noise and PDH

to make the comparisons meaningfully

one needs to correct for the feedback changes

faithfully

15233   Thu Feb 27 22:45:40 2020 gautamUpdateALSALS noise high

There was some UNELOGGED work at EX today. The DFD outputs were also hijacked for loss measurement. Unclear who the culprit was, but there is now a broad noise bump centered around ~180 Hz in the ALS X noise curve, which certainly wasn't there yesterday. Maybe let's keep the few working systems working, it is annoying to have to deal with these auxiliary issues every night. I'll push ahead with locking, hopefully the ALS noise is "good enough".

15315   Fri May 1 01:49:55 2020 gautamUpdateALSASY commissioning

Summary:

It appears that the EY green steering PZTs have somehow lost their bipolar actuation range. I will check on them the next time I go to the lab for an N2 switch.

Details:

• Yuki installed the EY green PZTs and did some initial setup of the RTCDS model.
• But we don't have a functional dither alignment servo yet, which is mildly annoying. So I thought I'll finally finish my SURF project.
• There were several problems with the signal flow, MEDM screens etc.
• I rectified these, and set up some operational scripts, burt snapshots etc in \$SCRIPTS/ASY. The c1asy and c1als models were also modified, recompiled and restarted, everything appears to have come back online smoothly.
• The LO frequencies/amplitudes, demod filter gains and demod phases were chosen to have a signal mostly in the _I quadrature of the demodulated signal when the alignment is slightly disturbed from optimal (monitored after the post-demod LPF).
• While trying to close the integrator loops, I found that I appear to only have monopolar actuation ability (positive DAC output changes the alignment, negative DAC output does nothing).

Could be that the power outage busted something in the drive electronics.

15316   Fri May 1 22:44:17 2020 gautamUpdateALSASY M2 PZT damaged

I went to EY and saw that the HV power supply was only putting out 50 V and had hit the current limit of 10 mA (nominally, it should be 100 V, drawing ~7mA). This is definitely a problem that has come up after the power shutdown event, as when I re-energized the HV power supply at EY, I had confirmed that it was putting out the nominal values (the supply was not labelled with these nominal numbers so I had to label it). Or maybe I broke it while running the dither alignment tests yesterday, even though I never drove the PZTs above 50 Hz with more than 1000cts (= 300 mV * gain 5 in the HV amplifier = 1.5 V ) amplitude.

The problem was confirmed to be with the M2 PZT (YAW channel) and not the electronics by driving the M2 PZT with the M1 channels. Separately, the M1 PZT could be driven by the M2 channels. I also measured the capacitance of the YAW channels and found it to be nearly twice (~7 uF) of the expected 3 uF - this particular PZT is different from the three others in use by the ASX and ASY system, it is an older vintage, so maybe it just failed? 😔

I don't want to leave 100 V on in this state, so the HV supply at EY was turned off. Good GTRY was recovered by manual alignment of the mirror mounts. If someone has a spare PZT, we can replace it, but for now, we just have to live with manually aligning the green beam often.

 Quote: Could be that the power outage busted something in the drive electronics.
15317   Sat May 2 02:35:18 2020 KojiUpdateALSASY M2 PZT damaged

Yes, we are supposed to have a few spare PI PZTs.

15319   Wed May 6 00:31:09 2020 gautamUpdateALSOptomechanics during CARM offset reduction

Summary:

The apparent increase in the ALS noise (witnessed in-loop, e.g. Attachment #2 here) during the CARM offset reduction may have an optomechanical origin.

Details:

• A simplified CARM plant was setup in Finesse - 3 mirror coupled cavity with PRM, ITM and ETM, 40m params for R/T/L used.
• For a sanity check, DC power buildup and coincident resonance of the PRC and arm cavity were checked. PRG and CARM linewidth also checks out, and scales as expected with arm losses.
• To investigate possible optomechanical issues - I cut the input power to 300 mW (I estimate 600 mW incident on the PRM), set a PRG of ~20, to mimic what we have right now.
• I drive the ITM at various CARM offsets, and measure the m/m transfer function to itself and the ETM.
• Attachement #1 shows the results.

Interpretation:

• ericq had similar plots in his thesis, but I don't think the full implications of this effect were investigated, the context there was different.
• The optomechanical resonance builds up at ~10 Hz and sweeps up to ~100 Hz as the CARM offset approaches zero, with amplification close to x100 at the resonance.
• What this means is that the arm cavity is moving by up to 100x the ambient seismically driven dispalcements.
• The EX/EY uPDH servos have considerable gain at these frequencies, and so the AUX laser frequency can keep up with this increased motion (to be quantified exactly what the increase in residual is).
• However, the ALS loop that maintains the frequency offset b/w the PSL and the AUX lasers is digitial, and only has ~20 dB gain at 30 Hz. - so the error signal for CARM control becomes noisier as we see.
• I speculate that the multiple peaky features in the in-loop error signal are a result of some dynamical effects which Finesse presumably does not simulate.
• The other puzzler is: this simulation would suggest that approaching the zero CARM offset from the other side (anti-spring) wouldn't have such instabilities building up. However, I am reasonably sure I've seen this effect approaching zero from both sides, though I haven't checked in the last month.
• Anyways, if this hypothesis is correct, we can't really take advantage of the ~8pm RMS residual noise performance of the IR ALS system sadly, because of our 250g mirrors and 800mW input power
• Possible workarounds:
• High BW ALS - this would give us more gain at ~30 Hz and this wouldn't be a problem anymore really. But in my trials, I think I found that the IN2 gain on the CM board has to be inverted for this to work (the IN1 path and the IN2 path share a common AO path polarity, and we need the two paths to have the opposite polarity).
• Cut the input power - this would reduce the optomechanical action, but presumably the vertex locking becomes noisier. In any case, this isn't really practical without some kind of motorized/remote-controlled waveplate for power adjustment.

Update 415pm 5/6: Per the discussion at the meeting, I have now uploaded as Attachment #2 the force-->displacement (i.e. m/N) transfer functions. I now think these are appropriate units. For the ALS case, we could convert the m/N to Hz/N of extra frequency noise imprinted on the AUX laser due to the increased cavity motion. Is W/N really better here, since the mechanism is extra frequency noise on a beatnote, and there isn't really a PDH or DC error signal?

15482   Wed Jul 15 17:46:05 2020 anchalSummaryALSNoise budget for ALS

I started my attempt on noise budgeting of ALS by going back to how Kiwamu did it and adding as many sources as I could find up till now. This calculation is present in ALS_Noise_Budget notebook. I intend to collect data for noise sources and all future work on ALS in the ALS repo.

The noise budget runs simulink through matlab.engine inside python and remaining calculations including the pygwinc ones are done in python. Please point out any errors that I might have done here. I still need to add noise due to DFD and the ADC after it. For the residual frequency noise of AUX laser, I have currently used an upper limit of 1kHz/rt Hz at 10 Hz free-running frequency noise of an NPRO laser.

15496   Mon Jul 20 19:21:16 2020 anchalSummaryALSFew proposals for Voyager ALS

I've added 4 proposed schemes for implementing ALS in voyager. Major thing to figure out is what AUX laser would be and how we would compare the different PSL and AUX lasers to create an error signal for ALS. Everywhere below, 2um would mean wavelengths near 2 um including the proposed 2128nm. Since it is not fixed, I'm using a categorical name. Same is the case for 1um which actually would mean half of whatever 2 um carries.

### Higher Harmonic Generation:

• We can follow the current system of ALS with using 1.5 times PSL frequency as AUX instead of second harmonic as 1 um is strongly absorbed in Si.
• To generate 1.5 times PSL frequency, three stages would be required.
• SHG: Second Harmonic Generation mode matched to convert 2um to 1um. If we are instead making 2 um from 1um to start with, this stage will not be required.
• SFG: Sum Frequency Generation mode matched to sum 2um photon and 1um photon to give 0.65 um photon.
• DPDC: Degenerate Parametric Down Conversion mode matched to convert 0.65 um to 1.3 um (which would be 1.5 times PSL frequency).
• To compare, we can either convert pick-off from PSL to AUX frequency by doing the above 3 stages (Scheme II).
• Or we can just do SHG and SFG at PSL pick-off and do another SHG at AUX end (Scheme I) to compare the AUX and PSL both converted to 0.65 um (which would be 2 times AUX and 3 times PSL frequency).
• This method would have added noise from SHG, SFG and DPDC processes along with issues to be inefficiency of conversion.

### Arbitrary AUX frequency:

• We can get away with using some standard laser near 1.5 um region directly as AUX. Most probably this would be 1550 nm.
• What's left is to devise a method of comparing 1.5 um and 2um frequencies. Following are two possible ways I could think of:

#### Using a frequency comb:

• Good stable frequency combs covering the wavelength region from 1.5 um to 2 um are available of the shelf.
• We would beat PSL and transmitted AUX separately with the frequency comb. The two beat note frequencies would be:
$\Delta_\text{PSL} = \nu_\text{PSL} - \nu_{CEO} - m_1 \nu_\text{Rep}$
$\Delta_\text{AUX} = \nu_\text{AUX} - \nu_{CEO} - m_2 \nu_\text{Rep}$
• Here, m1 and m2 represent the nearest modes (comb teeth) of frequency comb to PSL and AUX respectively.
• Carrier Envelope Offset frequency ($\nu_{CEO}$) can be easily generated by using an SHG crystal in front of the Frequency comb. This step is not really required since most of the modern frequency combs now comb with inbuilt zero $\nu_{CEO}$ stabilization.
• Mixing above beatnotes with $\nu_{CEO}$ would remove $\nu_{CEO}$ from them along with any noise associated with $\nu_{CEO}$.
• Further, a Direct Digital Synthesis IC is required to multiply the AUX side RF signal by m1/m2. This finally makes the two RF signals to be:
$\nu_{A} = \nu_\text{PSL} - m_1 \nu_{Rep}$
$\nu_{B} = \frac{m_1}{m_2}\nu_\text{AUX} - m_1 \nu_{Rep}$
• Which on mixing would give desired error signal for DFD as :
$\nu_\text{PSL} - \frac{m_1}{m_2}\nu_\text{AUX}$
• This method is described in Stenger et al. PRL. 88, 073601 and is useful in comparing two different optical frequencies with a frequency comb with effective cancellation of all noise due to the frequency comb itself. Only extra noise is from the DDS IC which is minimal.
• This method, however, might be an overkill and expensive. But in case (for whatever reason) we want to send in another AUX at another frequency down the 40m cavity, this method allows the same setup to be used for multiple AUX frequencies at once.

#### Using a Transfer Cavity:

• We can make another more easily controlled and higher finesse cavity with a PZT actuator on one of the mirrors.
• In the schematic, I have imagined it has a triangular cavity with a back end mirror driven by PZT.
• Shining PSL from one side of the transfer cavity and employing the usual PDH, we can lock the cavity to PSL.
• This lock would require to be strong and wide bandwidth. If PZT can't provide enough bandwidth, we can also put an EOM inside the cavity! (See this poster from Simon group at UChicago)
• Another laser at AUX frequency, called AUX2 would be sent from the other side of the cavity and usual PDH is employed to lock AUX2 to the transfer cavity.
• So clearly, this cavity also requires coatings and coarse length such that it is resonant with both PSL and AUX frequencies simultaneously.
• And, the FSS for AUX2 needs to be good and high bandwidth as well.
• The transmitted AUX2 from the transfer cavity now would carry stability of PSL at the frequency of AUX and can be directly beaten with transmitted AUX from the 40m cavity to generate an error signal for DFD.
• I believe this is a more doable and cheaper option. Even if we want to do a frequency comb scheme, this could be a precursor to it.

#### Using Mode Cleaner cavity as Transfer Cavity:

• If we coat the mode cleaner cavity mirrors appropriately, we can use it to lock AUX2 laser (mentioned above).
• This will get rid of all extra optics. The only requirement is for FSS to be good on AUX2 to transfer PSL (MC) stability to AUX frequency.
• I've added suggested schematic for this scheme at the bottom.

15531   Mon Aug 17 23:36:10 2020 gautamUpdateALSWhitening and ALS noise

finally managed to install a differential-receiving whitening board in 1Y2 - 4 channels are available at the moment. As I claimed, one stage of 15:150 Hz z:p whitening does improve the ALS noise a little, see Attachment #1. While the RMS (from 1kHz-0.5 Hz) does go down by ~10 Hz, this isn't really going to make any dramatic improvement to the 40m lock acquisiton. Now we're really sitting on the unsuppressed EX laser noise above ~30 Hz. This measurement was taken with the arm cavities locked with POX/POY, and end lasers locked to the arm cavities with uPDH boxes as usual. This was just a test to confirm my suspicion, the whitening board is to be used for the air BHD channels, but when we get a few more stuffed, we can install it for the ALS channels too.

15533   Tue Aug 18 13:55:23 2020 ranaUpdateALSWhitening and ALS noise

No, there should be no unscheduled visits from any inspector, marshal, tech, or vendor. They all have to be escorted or they don't get in. If they have a problem with that, please give them my cell #.

For the ALS, in addition to the beat note spectrum, I think we need to know the loop gain use to feedback to the ETM to determine the true cavity length fluctuation. w/o ALS, the noise would be only due to the seismic noise, OSEM damping noise, and the IR-PDH residual. Those are all suppressed by the ALS loop, but then the ALS loop puts its sensing noise onto the cavity. So, if I'm thinking about this right, the ALS beat noise > 200 Hz doesn't matter so much to the CARM RMS. So the whitening seems to be doing good in the right spot, but we would like to have another boost in the green PDH to up the gain below ~300 Hz?

15587   Sat Sep 19 23:59:22 2020 anchalSummaryALSALS noise budget update

### Setting the record straight

I found out an error I did in copying some control model values from Kiwamu's matlab code. On fixing those, we get a considerably reduced amount of total noise. However, there was still an unstable region around the unity gain frequency because of a very small phase margin. Attachment 3 shows the noise budget, ALS open-loop transfer function, and AUX PDH open-loop transfer function with ALS disengaged. Attachment 4 is the yaml file containing all required zpk values for the control model used. Note that the noise budget shows out-of-loop residual arm length fluctuations with respect to PSL frequency. The RMS curve on this plot is integrated for the shown frequency region.

### Trying to fix the unstable region

Adding two more poles at 100 Hz in the ALS digital filter seems to work in making the ALS loop stable everywhere and additionally provides a steeper roll-off after 100 Hz. Attachment 1 shows the noise budget, ALS open-loop transfer function, and AUX PDH open-loop transfer function with ALS disengaged. Attachment 2 is the yaml file containing all required zpk values for the control model used. Note that the noise budget shows out-of-loop residual arm length fluctuations with respect to PSL frequency. The RMS curve on this plot is integrated for the shown frequency region.

But is it really more stable?

• I tried to think about it from different aspects. One thing is sure that  $1+G_{OL}$ remains greater than 1 in all of the frequency region plotted for. This is also evident in the common-mode to residual noise transfer function which shows no oscillation peaks and is a clean mirror image of the open-loop transfer function (See Attachment 1, page 2).
• Another way is to look for the phase margin. This is a little controversial way of checking stability. For clarity, the open-loop transfer function I'm plotting does not contain the '-1' feedback in it. So the bad phase value at unity gain frequency is -180 degrees (or 180 degrees) for us. I've taken the difference from the closest side and got 76.2 degrees of phase margin.
• Another way I checked was by plotting a Nyquist plot for the open-loop transfer function. It is said that if the contour does not encircle the point '-1' in the real axis, then the loop would be stable even if the $f_{180} < f_{UGF}$ where $f_{180}$ is the frequency where phase lag becomes -180 degrees at the lowest frequency. For us, $f_{180}$ is at 1 Hz because of the test mass actuator pole. But I have verified that the Nyquist contour of the open-loop transfer function does not encircle '-1' point. I have not uploaded the Nyquist plot as it is not straight forward to plot. Because of large dc gain, it covers a large region and one needs to zoom in and out to properly follow what the contour is really doing. I didn't get time to make insets for it.

### Is this close to reality?

For that, we'll have to take present noise source estimates but Gautum vaguely confirmed that this looked more realistic now 'shape-wise'. If I remember correctly, he mentioned that we currently can achieve 8 pm of residual rms motion in the arm cavity with respect to the PSL frequency. So we might be overestimating our loop's capability or underestimating some noise source. More feedback on this welcome and required.

The code used to calculate the transfer functions and plot them is in the repo 40m/ALS/noiseBudget

Attachment 5 here shows a block diagram for the control loop model used. Output port 'Res_Disp' is used for referring all the noise sources at the residual arm length fluctuation in the noise budget. The open-loop transfer function for ALS is calculated by -(ALS_DAC->ALS_Out1 / ALS_DAC->ALS_Out2) (removing the -1 negative feedback by putting in the negative sign.) While the AUX PDH open-loop transfer function is calculated by python controls package with simple series cascading of all the loop elements.

15589   Sun Sep 20 23:12:13 2020 ranaSummaryALSALS noise budget update

I think the digital loop in the ALS budget is too optimistic. You have to include all the digital delays and anti-aliasing filters to get the real response.

aslo, I recommend grabbing some of the actual spectra from the in-lock times with nds and using the calibrated spectra as inputs to this mode. Although we don't have good models of the stack, you can sort of infer it by using the calibrated seismometer data and the calibrated MC_F or MC_L channels (for IMC) or XARM/YARM signals for those.

15593   Tue Sep 22 00:14:43 2020 anchalSummaryALSALS noise budget update

This is not a reply to comments given to the last post; Still working on incorporating those suggestions.

### Trying out a better filter from scratch

Rana suggested looking first at what needs to be suppressed and then create a filter suited for the noise from scratch. So I discarded all earlier poles and zeros and just kept the resonant gains in the digital filter. With that, I found that all we need is three poles at 1 Hz and a gain of 8.1e5 gives the lowest RMS noise value I could get.

Now there can be some practical reasons unknown to me because of which this filter is not possible, but I just wanted to put it here as I'll add the actual noise spectra into this model now.

### Few questions:

• What anti-aliasing filters are used in ALS?
• Is the digital delay fixed to a constant upper limit or is it left to change as per filters? I have already used a 470 us delay (modeled with Pade 4th order approximation).
• I could not find a good place where channel names are listed with corresponding meaning. Where can I find them?
• Is there a channel which keeps a record of lock status? In short, how do I find the in-lock times
15594   Tue Sep 22 12:14:42 2020 ranaSummaryALSALS noise budget update

This ALS loop is not stable. Its one of those traps that comes from using only the Bode plot to estimate the loop stability. You have to also look at the time domain response - you can look at my feedback lecture for the SURF students for some functions.

15601   Wed Sep 23 11:13:49 2020 anchalSummaryALSALS noise budget update

Yes, that loop was unstable. I started using the time domain response to check for the stability of loops now. I have been able to improve the filter slightly with more suppression below 20 Hz but still poor phase margin as before. This removes the lower frequency region bump due to seismic noise. The RMS noise improved only slightly with the bump near UGF still the main contributor to the noise.

For inclusion of real spectra, time delays and the anti-aliasing filters, I still need some more information.

### Few questions:

• What anti-aliasing filters are used in ALS?
• Is the digital delay fixed to a constant upper limit or is it left to change as per filters? I have already used a 470 us delay (modeled with Pade 4th order approximation).
• I could not find a good place where channel names are listed with corresponding meaning. Where can I find them?
• Is there a channel which keeps record of lock status? In short, how do I find the in-lock times

The code used to calculate the transfer functions and plot them is in the repo 40m/ALS/noiseBudget

Related Elog post with more details: 40m/15587

15617   Wed Oct 7 16:56:23 2020 anchalSummaryALSALS noise budget update - Updated AUX PDH Loop values

### AUX PDH Loop update

I used D1400293 to get the latest logged details about the universal PDH box used to lock the green laser at X end. The uPDH_X_boost.fil file present there was used to obtain the control model for this box. See attachment one for the code used. Since there is a variable gain stage in the box, I tuned the gain of the filter model F_AUX in ALS_controls.yml to get the maximum phase margin in the PDH lock of the green laser. Unity gain frequency of 8.3 kHz can be achieved in this loop and as Gautam pointed out earlier, it can't be increased much further without changes in the box.

### ALS Noise Budget update

The ALS control model remains stable with a reduction in total estimate noise because of the above update. There are few things to change though:

• This model is for a single arm locking where the beatnote signal between green laser and frequency doubled main laser is fed back to ETM at X end. Currently, Gautam is using a different scheme to lock where the feedback is sent to PSL-MC loop and the beat is taken between IR signals.
• In the LSC controls, I couldn't find a place where the digital ALS filter I have been optimizing and Kiwamu used, was placed. From what I gathered, after demodulation of beat note signal, a digital PLL is employed and the error signal is few to the Servo Filters directly. I might be missing some script which specifically switches on a particular set of filter modules in the XARM/YARM path when arms are locked through ALS.
• Another straight forward job for me is to verify the PSL-MC loop parameters with he TTFSS used. I'll do this next.
15619   Thu Oct 8 11:59:52 2020 ranaSummaryALSALS noise budget update - Updated AUX PDH Loop values

For all the loops where we drive the NPRO PZT, there is some notch/resonance feature due to the PZT mechanical resonance. In the IMC loop this limits the PZT/EOM crossove to be less than 25 kHz. I don't have a model for this, btu it should be included.

If you hunt through the elogs, people have measured the TF of ALS NPRO PZT to phase/frequency. Probably there's also a measured ALS PDH loop somewhere that you could use to verify your model.

15622   Fri Oct 9 18:32:14 2020 anchalSummaryALSALS noise budget update - Updated AUX PDH Loop values

The only two PZT Phase modulation transfer function measurements I could find are 40m/15206 and 40m/12077. Both these measurements were made to find a good modulation frequency and do not go below 50 kHz. So I don't think these will help us. We'll have to do a frequency transfer function measurement at lower frequencies.
I'm still looking for ALS PDH loop measurements to verify the model. I found this 40m/15059 but it is only near the UGF. The UGF measured here though looks very similar to the model prediction. A bit older measurement in 2017 was this 40m/13238 where I assume by ALS OLTF gautum meant the green laser PDH OLTF. It had similar UGF but the model I have has more phase lag, probably because of a 31.5 kHz pole which comes at U7 through the input low pass coupling through R28, C20 and R29 (See D1400293)

If the green laser is not being used, can I go and take some of these measurements myself?

15626   Wed Oct 14 17:03:55 2020 anchalSummaryALSALS noise budget update - Added whitening filter for ADC

Koji recommended that I can add whitening filters to suppress ADC noise easily. I added a filter before ADC in ALS loop with 4 zeros at 1.5 Hz and 4 poles at 100 Hz and added a reversed filter in the digital filter of ALS. This did not change the performance of the loop but significantly reduced the contribution of ADC noise above 1 Hz. One can see ALS_controls.yaml for the filter description. Please let me know if this does not make sense or there is something that I have overlooked.

Now, the dominant noise source is DFD noise below 100 Hz and green laser frequency noise above that. For DFD noise, I used data dating back to Kiwamu's paper. The noise contribution from DFD in the model is lower than the latest measured ALS noise budget post on elog. I'll look further into design details and noise of DFD.

Code, data, and schematics

15751   Wed Jan 6 22:47:41 2021 gautamUpdateALSNoisy ALS

Summary:

I want to get back to locking the interferometer so I can test out the newly installed AS WFS. However, the ALS noise is far too high, at least the transition of arm length control from IR to ALS fails reliably with the same settings that worked so reliably previously. I worked on investigating it a bit today.

Timeline

In the latter half of last year, I was focused on the air-BHD setup, so I wasn't checking in on the ALS noise as regularly.

1. On Aug 17, the noise was fine.
2. But on Oct 29, the noise is bad (and it continues to remain so, to the point where I cannot lock the interferometer).
3. Koji and Anchal confirmed nothing was touched while they were investigating the ALS system, also on Oct 29. The spectra attached in #15650 don't make any sense to me, the noise at 100 Hz cannot be <100mHz/rtHz. So, inconclusive.

Excess noise:

All tests are done with the arm cavity length locked to the PSL frequency using POX. Then, the EX laser is locked to the arm cavity length using the AUX PDH servo. The fluctuations in the beatnote between the two lasers is what is monitored as a diagnostic. See Attachment #1. The reference traces in the top pane are from a known good time. The large excess noise between ~80-200 Hz is what I'm concerned about.

A separate issue that can improve the noise is to track down the noise in the 20-80 Hz band - probably some IMC frequency noise issue.

Noise budget:

See Attachment #2

• I am pretty confident the electronics after the beat mouth are not to blame - I injected a 50 MHz signal from a Marconi and adjusted the signal amplitude to mimic what we get from the beat mouth. The trace labelled "DFD electronics noise" is the noise in this config.
• The unsuppressed AUX frequency noise was measured with an SR785 (converted to freqnecy noise units knowing the PDH horn-to-horn voltage and the cavity linewidth). I didn't confirm the sensing noise level (dark noise of the AUX PDH loop), but I figure that at 100 Hz (voltage noise of ~100 uV/rtHz on the SR785), we are above the sensing noise level, and so are truly measuring the in-loop frequency noise of the stabilized AUX laser. I also confirmed that the loop UGF was ~10 kHz and phase margin was ~60 degrees, which are nominal numbers.
• The fact that the excess noise is only in the X arm channel means the PSL frequency is not to blame.

So what could it be? The only things I can think of are (i) the beat mouth photodiode (NF1611) or (ii) excess noise in the fiber carrying the light from EX to the PSL table (but only on this fiber, and not on the EY fiber). Both seem remote to me - I'll test the former by switching the EX and EY fiber inputs to the beat mouth, but apart from this, I'm out of ideas...

To avoid this kind of issue, we should really have scripted locks of all the basic IFO configs and record the data to summary pages or something - maybe something to do once Guardian is installed, it'd be pretty hacky to do cleanly with shell scripts.

15752   Thu Jan 7 19:16:11 2021 gautamUpdateALSNoisy ALS

I'm also wondering why the error monitors for the X and Y loops report such wildly different spectra for the suppressed frequency noise of the AUX laser relative to the cavity length, see Attachment #1. The y-axis should be approximately Hz/rtHz. In both cases, the servo's error point monitor is connected to the DAQ system via a G=10 SR560. With the SR785, I measure for EX a nice bucket-shaped spectrum, bottoming out at ~10 uV/rtHz around 40 Hz, see Attachment #2. The SR560 should have an input-referred noise much less than this at the G=10 setting. The ADC noise level is only ~5 uV/rtHz, and indeed, the EY spectrum shows the expected shape. So what's up with the EX error mon? Tried swapping out the SR560 for a different unit, no change. And both the SR560 noise, and the ADC noise, check out when everything is checked individually. So some kind of interaction once everything is connected together, but it's only present at EX...

Today, I tried switching the EX and EY fibers going into the beat mouth, but I preserved the channel mapping after the beat mouth by switching the electrical outputs as well (the goal was to make sure that the beat photodiodes weren't the issue here, I think the electronics are already exonerated since driving the channel with a Marconi doesn't produce these noisy features). The EX spectrum remains noisy. I've switched everything back to the nominal configuration now to avoid further confusion. So it would appeat that this is real frequency noise that gets added in the EX fiber somehow. What can I do to fix this? The source of coupling isn't at the PSL table, else the EY channel would also show similar features. Visually, nothing seems wrong to me at EX either. So the problem is somehow in the cable tray along which the 40m of fiber is routed? This is already inside some nice foam/tubing setup, what can be done to improve it? Still doesn't explain why it suddenly became noisy...

15753   Thu Jan 7 20:07:27 2021 KojiUpdateALSNoisy ALS

How about resurrecting the PSL table green beat for the X arm to see if the non-fiber setup shows the same level of the freq noise (e.g. the PDH locking became super noisy due to misalignment etc).

15754   Thu Jan 7 21:16:22 2021 gautamUpdateALSNoisy ALS

I thought about it, but wouldn't that show up at the AUX PDH error point? Or because the loop gain is so high there we wouldn't see a small excess? I suppose there could be some clipping on the Faraday or something like that. But the GTRX level and the green REFL DC level when locked are nominal.

 Quote: How about resurrecting the PSL table green beat for the X arm to see if the non-fiber setup shows the same level of the freq noise (e.g. the PDH locking became super noisy due to misalignment etc).
15755   Thu Jan 7 23:25:19 2021 KojiUpdateALSNoisy ALS

If the sensing noise level of the end PDH degraded for some reason, it'd make the out of loop stability worse without making the end pdh error level degraded.
It's just speculation.

15756   Fri Jan 8 20:01:11 2021 gautamUpdateALSNoisy ALS

I did this test today. The excess noise around 100 Hz doesn't show up in the green beat.

See Attachment #1. The setup was as usual:

• X-Arm cavity length stabilized to PSL frequency using the POX locking loop.
• EX laser frequency locked to the X-Arm cavity length using the AUX PDH loop.
• The "BEATX" channel records frequency fluctuations in the beat sensed on the IR beat photodiode, while the "BEATY" channel records frequency fluctuations in the beat sensed on the Green beat photodiode.
• Since the green beat frequency fluctuations are twice that of the IR beat frequency fluctuations, I scaled the former ASD by a factor of 0.5 so as to compare apples to apples.
• At low frequencies, the green beat is noisier, but that channel doesn't show the excess noise at mid frequencies you see in the IR beat. So the AUX PDH sensing noise is not to blame I think.

So, this excess appears to truly be excess phase noise on the fiber (though I have no idea what the actual mechanicsm could be or what changed between Aug and Oct 2020 that could explain it. Maybe the HEPA?

For this work, I had to spend some time aligning the two green beams onto the beat photodiode. During this time, I shuttered the PSL, disabled feedback via the FSS servo, turned the HEPA high, and kept the EX green locked to the arm so as to have a somewhat stable beat signal I could maximize. Everything has been returned to nominal settings now (obviously, since I locked the arms to get the data).

You may ask, why do we care. In terms of RMS frequency noise, it would appear that this excess shouldn't matter. But in all my trials so far, I've been unable to transition control of the arm cavity lengths from POX/POY to ALS. I suppose we could try using the green beat, but that has excess low frequency noise (which was the whole point of the fiber coupled setup).

 Quote: How about resurrecting the PSL table green beat for the X arm to see if the non-fiber setup shows the same level of the freq noise (e.g. the PDH locking became super noisy due to misalignment etc).
15860   Wed Mar 3 23:23:58 2021 gautamUpdateALSArm cavity scan

I see no evidence of anything radically different from my PSL table optical characterization in the IMC transmitted beam, see Attachment #1. The lines are just a quick indicator of what's what and no sophisticated peak fitting has been done yet (so the apparent offset between the transmission peaks and some of the vertical lines are just artefacts of my rough calibration I believe). The modulation depths recovered from this scan are in good agreement with what I report in the linked elog, ~0.19 for f1 and ~0.24 for f2. On the bright side, the ALS just worked and didn't require any electronics fudgery from me. So the mystery continues.

16161   Tue May 25 17:42:11 2021 Anchal, PacoSummaryALSALS Single Arm Noise Budget

Here is our first attempt at a single-arm noise budget for ALS.

Attachment 1 shows the loop diagram we used to calculate the contribution of different noises.

Attachment 2 shows the measured noise at C1:ALS-BEATX_PHASE_FINE_OUT_HZ when XARM was locked to the main laser and Xend Green laser was locked to XARM.

• The brown curve shows the measured noise.
• The black curve shows total estimated noise from various noise sources (some of these sources have not been plotted as their contribution falls off the plotting y-lims.)
• The residual frequency noise of Xend green laser (AUX) is measured by measuring the PDH error monitor spectrum from C1:ALS-X_ERR_MON_OUT_DQ. This measurement was converted into units of V by multiplying it by 6.285e-4 V/cts. This factor was measured by sending a 43 Hz 100 mV sine wave at the readout point and measuring the output in the channel.
• This error signal is referred to AUX_Freq input in the loop diagram (see attachment 1) and then injected from there.
• All measurements were taken to Res_Disp port in the 'Out-of-Loop Beat Note' block (see attachment 1).
• In this measurement, we did not DAC noise that gets added when ALS loop is closed.
• We added ADC noise from Kiwamu's ALS paper after referring it to DFD input. DFD noise is also taken from Kiwamu's ALS paper data.

### Inference:

• Something is wrong above 200 Hz for the inclusion of AUX residual displacement noise. It is coming off as higher than the direct measured residual noise, so something is wrong with our loop diagram. But I'm not sure what.
• There is a lot of unaccounted noise everywhere from 1 Hz to 200 Hz.
• Rana said noise budget below 1 Hz is level 9 stuff while we are at level 2, so I'll just assume the excess noise below 1 Hz is level 9 stuff.
• We did include seismic noise taken from 40m noise budget in 40m/pygwinc. But it seems to affect below the plotted ylims. I'm not sure if that is correct either.

### Unrelated questions:

• There is a slow servo feeding back to Green Laser's crystal temperature by integrating PZT out signal. This is OFF right now. Should we keep it on?
• The green laser lock is very unreliable and it unlocks soon after any signal is being fed back to the ETMX position.
• This means, keeping both IR and green light locked in XARM is hard and simultaneous oscillation does not last longer than 10s of seconds. Why is it like this?
• We notice that multiple higher-order modes from the green laser reach the arm cavity. The HOMs are powerful enough that PDH locks to them as well and we toggle the shutter to come to TEM00 mode. These HOMs must be degrading the PDH error signal. Should we consider installing PMCs at end tables too?
16164   Thu May 27 11:03:15 2021 Anchal, PacoSummaryALSALS Single Arm Noise Budget

Here's an updated X ARM ALS noise budget.

## Things to remember:

• Major mistake we were making earlier was that we were missing the step of clicking  'Set Phase UGF' before taking the measurement.
• Click the clear phase history just before taking measure.
• Make sure the IR beatnotes are less than 50 MHz (or the left half of HP8591E on water crate). The DFD is designed for this much beatnote frequency (from Gautum).
• We took this measurement with old IMC settings.
• We have saved a template file in users/Templates/ALS/ALS_outOfLoop_Ref_DQ.xml . This si same as ALS_outOfLoop_Ref.xml except we changed all channels to _DQ.

## Conclusions:

• Attachment 1 shows the updated noisebudget. The estimated and measured RMS noise are very close to eachother.
• However, there is significant excess noise between 4 Hz and 200 Hz. We're still thinking on what could be the source of these.
• From 200 Hz to about 3 kHz, the beatnote noise is dominated by AUX residual frequency noise. This can be verified with page 2 of Attachment 2 where coherence between AUX PDH Error signal and BEATX signal is high.
• One mystery is how the measured beatnote noise is below the residual green laser noise above 3 kHz. Could this be just because the phase tracker can't measure noise above 3kHz?
• We have used estimated open loop transfer function for AUX from poles/zeros for uPDH box used (this was done months ago by me when I was working on ALS noise budget from home). We should verify it with a fresh OLTF measurement of AUX PDH loop. That's next on our list.
16168   Fri May 28 17:32:48 2021 AnchalSummaryALSSingle Arm Actuation Calibration with IR ALS Beat

I attempted a single arm actuation calibration using IR beatnote (in the directions of soCal idea for DARM calibration)

## Measurement and Inferences:

• I sent 4 excitation signals at C1:SUS-ITM_LSC_EXC wit 30cts at 31Hz, 200cts at 197Hz, 600cts at 619Hz and 1000cts at 1069 Hz.
• These were sent simultaneously using compose function in python awg.
• The XARM was locked to mai laser and alignment was optimized with ASS.
• The Xend Green laser was locked to XARM and alignment was optimized.
• Sidenote: GTRX is now normalized to give 1 at near maximum power.
• Green lasers can be locked with script instead of toggling.
• Script can be called from sitemap->ALS->! Toggle Shutters->Lock X Green
• Script is present at scripts/ALS/lockGreen.py.
• C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ was measured for 60s.
• Also, measured C1:LSC-XARM_OUT_DQ and C1:SUS-ITMX_LSC_OUT_DQ.
• Attachment 1 shows the measured beatnote spectrum with excitations on in units of m/rtHz.
• It also shows resdiual displacement contribution PSD of (output referred) XARM_OUT and ITMX_LSC_OUT to the same point in the state space model.
• Note: that XARM_OUT and ITMX_LSC_OUT (excitation signal) get coherently added in reality and hence the beatnote spectrum at each excitation frequency is lower than both of them.
• The remaining task is to figure out how to calculate the calibration constant for ITMX actuation from this information.
• I need more time to understand the mixture of XARM_OUT and ITMX_LSC_OUT in the XARM length node in control loop.
• Beatnote signal tells us the actual motion of the arm length, not how much ITMX would have actuated if the arm was not locked.
• Attachment 2 has the A,B,C,D matrices for the full state space model used. These were fed to python controls package to get transfer functions from one point to another in this MIMO.
• Note, that here I used the calibration of XARM_OUT we measured earlies in 16127.
• On second thought, maybe I should first send excitation in ETMX_LSC_EXC. Then, I can just measure ETMX_LSC_OUT which includes XARM_OUT due to the lock and use that to get calibration of ETMX actuation directly.

16171   Tue Jun 1 16:55:32 2021 Anchal, PacoSummaryALSSingle Arm Actuation Calibration with IR ALS Beat

Rana suggested in today's meeting to put in a notch filter in the XARM IR PDH loop to avoid suppressing the excitation line. We tried this today first with just one notch at 1069 Hz and then with an additional notch at 619 Hz and sent two simultaneous excitations.

## Measurement and Analysis:

• We added notch filters with Q=10, depth=50dB, freq=619 Hz and 1069 Hz using foton in SUS-ETMX_LSC filter bank at FM10.
• We sent excitation signals with amplitudes 600cts and 1000 cts for 619 Hz and 1069 Hz signals respectively.
• We measured time series data of C1:SUS-ITMX_LSC_OUT_DQ and C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ for 60s.
• Then, spectrum of both signals is measured with Hanning window using scipy.welch function with scaling set to  'spectrum', binwidth=1Hz.
• The beatnote signal was converted into length units by multiplying it by 1064nm * 37.79m / c.
• The ratio of the two spectrums at teh excitation frequency multiplies by excitation frequency squared gives us teh calibration constant in units of nm Hz^2/cts.
• At 619 Hz, we got $\frac{5.01}{f^2}$nm/cts
• At 1069 Hz, we got $\frac{5.64}{f^2}$nm/cts.
• The calibration factor in use is from $\frac{7.32}{f^2}$ nm/cts from 13984.
• So, the calibration factor from this methos is about 23% smaller than measured using freeswinging MICH in 13984.
• One possiblity is that our notch filter is not as effective in avoiding suppresion of excitation.
• We tried increasing the notch filter depths to 100 dB but got the same result within 2%.
• We tried changing the position of notch filters. We put them in POX filter banks. Again the result did not change more than 2%.
• The open loop gain of green PDH at 619 Hz and 1069 Hz must be large enough for our assumption of green laser perfectly following length motion to be true. The UGF of green laser is near 11 kHz.
• The discrepancy could be due to outdated freeswinging MICH measurement that was done 3 years ago. Maybe we should learn how to do the ITMX calibration using this method and compare our own two measurements.
16192   Tue Jun 8 11:40:53 2021 Anchal, PacoSummaryALSSingle Arm Actuation Calibration with IR ALS Beat

We attempted to simulate "oscillator based realtime calibration noise monitoring" in offline analysis with python. This helped us in finding about a factor of sqrt(2) that we were missing earlier in 16171. we measured C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ when X-ARM was locked to main laser and Xend green laser was locked to XARM. An excitation signal of amplitude 600 was setn at 619 hz at C1:ITMX_LSC_EXC.

## Signal analysis flow:

• The C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ is calibrated to give value of beatntoe frequency in Hz. But we are interested in the fluctuations of this value at the excitation frequency. So the beatnote signal is first high passed with 50 hz cut-off. This value can be reduced a lot more in realtime system. We only took 60s of data and had to remove first 2 seconds for removing transients so we didn't reduce this cut-off further.
• The I and Q demodulated beatntoe signal is combined to get a complex beatnote signal amplitude at excitation frequency.
• This signal is divided by cts amplitude of excitation and multiplied by square of excitation frequency to get calibration factor for ITMX in units of nm/cts/Hz^2.
• The noise spectrum of absolute value of  the calibration factor is plotted in attachment 1, along with its RMS. The calibration factor was detrended linearly so the the DC value was removed before taking the spectrum.
• So Attachment 1 is the spectrum of noise in calibration factor when measured with this method. The shaded region is 15.865% - 84.135% percentile region around the solid median curves.

We got a value of $\frac{7.3 \pm 3.9}{f^2}\, \frac{nm}{cts}$.  The calibration factor in use is from $\frac{7.32}{f^2}$ nm/cts from 13984.

Next steps could be to budget this noise while we setup some way of having this calibration factor generated in realitime using oscillators on a FE model. Calibrating actuation of a single optic in a single arm is easy, so this is a good test setup for getting a noise budget of this calibration method.

16196   Wed Jun 9 18:29:13 2021 Anchal, PacoSummaryALSCheck for saturation in ITMX SOS Driver

We did a quick check to make sure there is no saturation in the C1:SUS-ITMX_LSC_EXC analog path. For this, we looked at the SOS driver output monitors from the 1X4 chassis near MC2 on a scope. We found that even with 600 x 10 = 6000 counts of our 619 Hz excitation these outputs in particular are not saturating (highest mon signal was UL coil with 5.2 Vpp). In comparison, the calibration trials we have done before had 600 counts of amplitude, so we can safely increase our oscillator strength by that much

Things that remain to be investigated -->

• What is the actual saturation level?
• Two-tone intermodulation?
16242   Fri Jul 9 15:39:08 2021 AnchalSummaryALSSingle Arm Actuation Calibration with IR ALS Beat [Correction]

I did this analysis again by just doing demodulation go 5s time segments of the 60s excitation signal. The major difference is that I was not summing up the sine-cosine multiplied signals, so the error associated was a lot more. If I simply multpy the whole beatnote signal with digital LO created at excitation frequency, divide it up in 12 segments of 5 s each, sum them up individually, then take the mean and standard deviation, I get the answer as:
$\frac{6.88 \pm 0.05}{f^2} nm/cts$as opposed to $\frac{7.32 \pm 0.03}{f^2} nm/cts$that was calculated using MICH signal earlier by gautum in 13984.

Attachment 1 shows the scatter plot for the complex calibration factors found for the 12 segments.

My aim in the previous post was however to get a time series of the complex calibration factor from which I can take a noise spectral density measurement of the calibration. I'll still look into how I can do that. I'll have to add a low pass filter to integrate the signal. Then the noise spectrum up to the low pass pole frequency would be available. But what would this noise spectrum really mean? I still have to think a bit about it. I'll put another post soon.

Quote:

We attempted to simulate "oscillator based realtime calibration noise monitoring" in offline analysis with python. This helped us in finding about a factor of sqrt(2) that we were missing earlier in 16171. we measured C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ when X-ARM was locked to main laser and Xend green laser was locked to XARM. An excitation signal of amplitude 600 was setn at 619 hz at C1:ITMX_LSC_EXC.

## Signal analysis flow:

• The C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ is calibrated to give value of beatntoe frequency in Hz. But we are interested in the fluctuations of this value at the excitation frequency. So the beatnote signal is first high passed with 50 hz cut-off. This value can be reduced a lot more in realtime system. We only took 60s of data and had to remove first 2 seconds for removing transients so we didn't reduce this cut-off further.
• The I and Q demodulated beatntoe signal is combined to get a complex beatnote signal amplitude at excitation frequency.
• This signal is divided by cts amplitude of excitation and multiplied by square of excitation frequency to get calibration factor for ITMX in units of nm/cts/Hz^2.
• The noise spectrum of absolute value of  the calibration factor is plotted in attachment 1, along with its RMS. The calibration factor was detrended linearly so the the DC value was removed before taking the spectrum.
• So Attachment 1 is the spectrum of noise in calibration factor when measured with this method. The shaded region is 15.865% - 84.135% percentile region around the solid median curves.

We got a value of $\frac{7.3 \pm 3.9}{f^2}\, \frac{nm}{cts}$.  The calibration factor in use is from $\frac{7.32}{f^2}$ nm/cts from 13984.

Next steps could be to budget this noise while we setup some way of having this calibration factor generated in realitime using oscillators on a FE model. Calibrating actuation of a single optic in a single arm is easy, so this is a good test setup for getting a noise budget of this calibration method.

16333   Wed Sep 15 23:38:32 2021 KojiUpdateALSALS ASX PZT HV was off -> restored

It was known that the Y end ALS PZTs are not working. But Anchal reported in the meeting that the X end PZTs are not working too.

We went down to the X arm in the afternoon and checked the status. The HV (KEPCO) was off from the mechanical switch. I don't know this KEPCO has the function to shutdown the switch at the power glitch or not.
But anyway the power switch was engaged. We also saw a large amount of misalignment of the X end green. The alignment was manually adjusted. Anchal was able to reach ~0.4 Green TRX, but no more. He claimed that it was ~0.8.

We tried to tweak the SHG temp from 36.4. We found that the TRX had the (local) maximum of ~0.48 at 37.1 degC. This is the new setpoint right now.

16884   Wed Jun 1 11:56:28 2022 yutaUpdateALSShutter driver for GRY replaced

[JC, Yuta]

We replaced a shutter driver for GRY since it stopped working this morning.
We replaced it with a free driver which was sitting on the ITMY table.
The reverse polarity issue of C1:AUX-GREEN_Y_Shutter was fixed by switching one of the switches of the driver from N.O. to N.C.

Also, "Toggle" button was added to IFO_ALIGN.adl so that we can toggle shutters easily to find TEM00. It runs /home/controls/Git/40m/scripts/ALS/ShutterToggler.py.

 Quote: The green Y shutter now works but in reverese, meaning that sending 1 to C1:AUX-GREEN_Y_Shutter closes the shutter and vice versa. This needs to be fixed.

16945   Fri Jun 24 17:16:59 2022 PacoUpdateALSXAUX cable in control room

[JC, Paco]

We took the long BNC cable that ran from ETMX to ETMY and ran it from ETMX into the control room instead. This way Cici and Deeksha can send small voltage signals to the AUX PZT and read back using the beatnote pickoff that is usually connected to the spectrum analyzer.

16956   Tue Jun 28 16:59:35 2022 PacoSummaryALSALS beat allan deviation (XARM)

[Paco]

I took ~ 7 minutes of XALS beatnote data with the XAUX laser locked to the XARM cavity, and the XARM locked to PSL to develop an allan deviation estimator. The resulting timeseries for the channel C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ (decimated timeseries in Attachment #1) was turned into an allan variance using the "overlapped variable tau estimator":

$\sigma_y^2(n\tau_0, N) = \frac{1}{2n^2\tau_0^2(N - 2n)} \sum_{i=0}^{N-2n-1} (x_{i+2n} - 2x_{i+n} + x_i)^2$

Where $x_k$ represents the k-th data point in the raw timeseries, and $n\tau_0$ are the variable integration intervals under which two point variances are computed (the allan variance is a special case of M-point variance, where M=2). Then, the allan deviation is just the square root of that. Attachment #2 shows the fractional deviation (normalized by the mean beat frequency ~ 3 MHz for this measurement) for 100 integration times spanning the full duration (~ 7 min = 420 s).

The code used for this lives in Git/40m/labutils/measuremens/ALS/

If this estimate is any good, wherever the fractional beatnote deviation reaches a minimum value can be used as a proxy for the longest averaging time that give a statistical increase in SNR. After this timescale, the frequency comparison is usually taken over by "environmental instabilities" which I don't think I can comment further on. In our particular estimate, the 100 second integration gives a fractional deviation of ~ 0.44 %, or absolute deviation of 12.925 kHz.

16959   Tue Jun 28 18:53:16 2022 ranaSummaryALSALS beat allan deviation (XARM)

what's the reasoning behind using df/f_beat instead of df/f_laser ?

 Quote: [Paco] I took ~ 7 minutes of XALS beatnote data with the XAUX laser locked to the XARM cavity, and the XARM locked to PSL to develop an allan deviation estimator.

16962   Wed Jun 29 14:28:06 2022 PacoSummaryALSALS beat allan deviation (XARM)

I guess it didn't make sense since f_beat can be arbitrarily moved, but the beat is taken around the PSL freq ~ 281.73 THz. Attachment #1 shows the overlapping tau allan deviation for the exact same dataset but using the python package allantools, where this time I used the PSL freq as the base frequency. This time, I can see the minimum fractional deviation of 1.33e-13 happening at ~ 20 seconds.

 Quote: what's the reasoning behind using df/f_beat instead of df/f_laser ?

## Another, more familiar interpretation

The allan variance is related to the beatnote spectral density as a mean-square integral (the deviation is then like the rms) with a sinc window.

$\sigma^2_\nu = 2 \int_0^{\infty} S_\nu(f) \lvert \frac{\sin({\pi f \tau})}{\pi f \tau} \lvert ^2 df$

16965   Thu Jun 30 18:06:22 2022 PacoUpdateALSOptimum ALS recovery - part I

[Paco]

In the morning I took some time to align the AUX beams in the XEND table. Later in the afternoon, I did the same on the YEND table. I then locked the AUX beams to the arm cavities while they were stabilized using POX/POY and turned off the PSL hepa off temporarily (this should be turned on after today's work).

After checking the the temperature slider sign on the spectrum analyzer of the control room I took some out-of-loop measurements of both ALS beatnotes (Attachment #1) by running diaggui /users/Templates/ALS/ALS_outOfLoop_Ref_DQ.xml and by comparing them against their old references (red vs magenta and blue vs cyan); it seems that YAUX is not doing too bad, but XAUX has increased residual noise around and above 100 Hz; perhaps as a result of the ongoing ALS SURF loop investigations? It does look like the OLTF UGF has dropped by half from ~ 11 kHz to ~ 5.5 kHz.

Anyways let this be a reference measurement for current locking tasks, as well as for ongoing SURF projects.

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