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ID Date Author Type Category Subject
2428   Mon Sep 16 17:05:00 2019 anchalSummaryBEATBeatnote Frequency stabalization

Over the weekend, I ran Relay Tuning method for the PID of beatnote frequency control. After CTN:2426 this needed to be done to fix the PID constants to appropriate value. The results of the tuning were:

Critical period Tc = 45.900000000000006
Critical gain Kc = 18.382414309527164
Suggested kp, ki, kd are 3.676482861905433, 0.16019533167343933, 56.25018778715313

RXA: Wah!  precision help here

The relay amplitude was set to 0.5 W and I could see very good sustained oscillations which the code used to get above calculated values.

### Testing performance

I tested the performance of PID today. Attached is the convergence of beatnote frequency, which happened in about 20 minutes only to 40 kHz offset value. After that point, the proportional gain of the PID is so high, that the actuator response essentially copies the fluctuations in the beatnote frequency itself. So no more stabilization happens. The integral constant is very low (I think it is required for quick convergence with no overshoot), so to travel this 40 kHz distance, it will probably take hours. But that's fine with us as our photodetectors work well enough with this offset too.

If you see the second plot, the beatnote did not drift beyond +/- 2kHz for over 40 min. I want to see if tonights beatnote will get any better due to this good stabilization.

Code And Data

Attachment 1: BeatnotePIDPerformance.pdf
2429   Mon Sep 16 18:03:33 2019 anchalNotesBEATQuick note: Detector changed to Sn101
• Beatnote is stabalized to 27.3 MHz. Changing detector to SN101.
• Reduced South PMC Variable Gain to -2dB from -1dB as it was oscillating. That's probably why 16th september BN was bad.
• Changed bandwidth of Moku for frequency nosie measurement to 2.5 kHz.
2430   Fri Sep 20 18:34:19 2019 anchalNotesVacuumPossible reflected lights on upgrade

Attached is a first attempt at tracing the rays on reflection from the wedged and tilted window together with the cavity mirror.

### traceit

I used Sean Leavey's zero and created a ray tracing module for simple purposes which is fast and easy to use. Check out the examples to see the capabilities.

To use, git pull labutils to update and keep labutils /traceit in your python path.

More info about each ray can be seen by layout['R4'] kind of statements. Or just write layout.rays to see info about all rays. This includes their vector positions, the origin phenomenon, and ray etc.

I know a lot can be done to make this look better. But I'm not going to dive into developing this module right now. However, suggestions on how to make the ray trace diagram more useful are welcome so that I can make it more informational.

Seems like most of the reflections would be bunched together in two directions where we should put the beam dumps.

2431   Tue Sep 24 15:40:33 2019 anchalDailyProgressNoiseBudgetAdding specifics of discepancy 2

Today I ran the two codes with the same parameter values to check if the effective reflectivities calculated during the calculation of thermo-optic nose matches. They do match exactly actually. Attached is an over plot.

Coating effective coefficient of thermo-refractive effect comes out to be:

Old Code: 8.61(46)e-05 K**-1

New Code: (8.59+/-0.21) e-05 K**-1

So this discrepancy was not really there. I was just comparing apples with oranges earlier.

### Discrepancy #2

The effective coating CTR in the previous code was 7.9e-5 1/K and in the new code, it is 8.2e-5 1/K. Since this value is calculated after a lot of steps, it might be round off error as initial values are slightly off. I need to check this calculation as well to make sure everything is right. Problem is that it is hard to understand how it is done in the previous code as it used matrices for doing complex value calculations. In new code, I just used ucomplex class and followed the paper's calculations. I need more time to look into this too. Suggestions are welcome.

Attachment 1: ComparisonEffReflectivities.pdf
2432   Tue Sep 24 17:11:13 2019 anchalDailyProgressNoiseBudgetAdding specifics of discepancy 2 (More)

Also, I found that the effective reflectivities from the top surface of layers calculated in Evans et a. PRD 78, 102003 (2008) were different from reflectivities calculated by matrix method in Hong et al. PRD 87, 082001 (2013). It turned out that the sign before phase in Eq. B3 and B4 in Evans et a. PRD 78, 102003 (2008) were opposite to what comes from the matrix method. After discussions with Gabriele, I came to the conclusion that this sign only creates a difference of giving complex conjugates of effective reflectivity. But to have consistency, I have corrected the sign of phase in the new code. Attached is a comparison of old effective reflectivity and the new one. One can see that the imaginary values are opposite in sign but everything else matches perfectly. Also, this has no effect on the effective coating coefficient of thermo-refraction.

Attachment 1: ComparisonEffReflectivitiesSignChange.pdf
2433   Tue Sep 24 19:39:06 2019 anchalDailyProgressNoiseBudgetChecking variation in estimated Coating Brownian Noise with Loss Angles

Gabriele suggested that I plot the estimated coating Brownian noise for different values of bulk and shear loss angles and compare with the old code. The expectation is that the estimated noise should vary smoothly and should agree with old result where bulk and shear loss angles are taken the same. In the first plot, I have plotted estimate coating Brownian noise for CTN experiment over the nominal range of bulk and shear loss angles. Since this range doesn't really have an overlap with equal bulk and shear loss angle case, I made another plot in which such a square region is chosen. We can see that the estimated coating Brownian noise is slightly higher (roughly 2.5% from CTN:2390 figure 2).

Let me know if someone can think of a better viewing angle or more insights to take out from this. Code is attached.

Attachment 1: VariationOfCoatingBrownianNoiseEstimate.pdf
Attachment 2: VariationOfCoatingBrownianNoiseEstimate.zip
2434   Fri Sep 27 16:40:49 2019 anchalDailyProgressNoiseBudgetChecking variation in estimated Coating Brownian Noise with Loss Angles Better Plots

Better Plots attached.

Attachment 1: VariationOfCoatingBrownianNoiseEstimate.pdf
Attachment 2: VariationOfCoatingBrownianNoiseEstimate.zip
2435   Mon Sep 30 11:14:41 2019 anchalDailyProgressNoiseBudgetChecking variation in estimated Coating Brownian Noise with Loss Angles 2D Plots

2D Heat Maps

Edit: Added comparison plot as well.

Attachment 1: VariationOfCoatingBrownianNoiseEstimate2.pdf
Attachment 2: VariationOfCoatingBrownianNoiseEstimate2.zip
2437   Mon Sep 30 16:47:15 2019 anchalDailyProgressNoiseBudgetAdding links again; Comparison with Old Estimates on same plot

With comparison with old estimates:

CTN_Latest_BN_Spec_With_OldEst.pdf

CTN_Daily_BN_Spec_With_OldEst.pdf

CTN_Latest_BN_Spec.pdf

CTN_Daily_BN_Spec.pdf

2438   Mon Sep 30 20:08:42 2019 anchalDailyProgressNoiseBudgetChecking variation in estimated Coating Brownian Noise with Beam Radius

Variation of the estimate of coating Brownian noise as beam spot radius on the mirror changes. Plotted in displacement noise as the frequency conversion factor is dependent on cavity length and beam radius also depends on that.

2439   Tue Oct 1 13:46:49 2019 anchalDailyProgressNoiseBudgetUsing bayesian inference to estimate loss Angles

I used the method described in Section V of https://arxiv.org/abs/1406.4467, to do a bayesian inference using mechanical ring-down measurements of coating loss angles by Steve Penn (recently reported in https://dcc.ligo.org/LIGO-G1901676) as a prior. I assumed all probability distributions to be gaussian.  We are clearly not close to measuring the Brownian noise as with present uncertainty limits reported by Penn et al, the prior distribution and the likelihood distribution do not overlap with each other at all, giving us no meaningful result. ​Note the different limits in x-axis on first and second plot.

This was a quick run of this though, just to set up the method. Ideally, I should use integrated noise in some frequency range around 300 Hz to use as the measurement. Also, Steve Penn actually did not report any uncertainty value with his measurements, so I assumed the uncertainty the same as his previous measurement. Looking forward to the more formal release of his data.

Attachment 1: BayesianAnalysis.pdf
Attachment 2: BayesianAnalysis.zip
2440   Tue Oct 1 17:28:25 2019 anchalNotesBEATQuick note: Moku connected with USB

Liquid Instrument's Application Engineer at La Jolla told me that connection with moku might be mroe stable if it is directly connected to the computer through a USB cable. It still gets identified with Name, Serial Number or IP address, just the connection is mroerobust. So today, I have connected our moku with USB. I have seen in past couple of weeks that every few days the moku data transfer gets stuck or it fails to connect through LAN. So trying this out.

2441   Wed Oct 2 12:22:44 2019 anchalDailyProgressBEATBeatnote Frequency long time series data

After the new PID parameters were tuned (CTN:2428), I waited for some time and the beatnote was stably locked to its setpoint of 27.34 MHz for over 2 weeks now. It is a good time to assess the beatnote frequency stabilization. Here I took data of 10 days and plotted it in three different timescales. The standard deviation plotter in light blue is calculated by standard deviation over 10 s of averaging of data. Green background means everything was locked at that time. Other than green would mean that either something was unlocked or there is a gap in the channel data (this case).

### Conclusions:

• Over 10 days, the beatnote hardly left +/- 2 kHz zone from the setpoint. Even with one standard deviation far away, the beatnote does note leave +/- 2.5 kHz zone. We are using Moku at 2.5 KHz bandwidth right now.
• Over a day, here it was Sep 28th (Saturday), the beatnote is within +/- 2kHz even up to one standard deviation point.
• Over 1 hour which was between 1 am to 2 am on Sep 28th, the beatnote was similarly calm.
• In the last plot, I have plotted drift over 60s in beatnote frequency which is our measurement duration. This drift doesn't even cross 1.5 kHz mark.

### But

How good is good? We were so bad, I never did this calculation. Are we hitting boundaries of how good the thermal controls can anyway do? Is the remaining noise in beatnote spectrums just scatter noise or there is still room for improvement in beatnote stabilization. Food for thought.

Code and Data

Attachment 1: BeatnoteLongTimeSeries.pdf
2442   Wed Oct 2 15:13:27 2019 anchalDailyProgressNoiseBudgetChecking variation in estimated Coating Brownian Noise with Beam Radius

I updated this calculation by adding curves for contribution through bulk loss angle and shear loss angle separately. Rana suspected that shear loss and bulk loss should behave differently with the change in beam radius on the mirrors. But apparently the Hong et al. calculations do not suggest that way. I checked this analytically too. The definitions of Eq. (96) of power spectral densities of coating thickness fluctuation of a particular layer due to Bulk or Shear loss angles have the same dependence on the effective area of the beam on the layer.

The only thing different between the final contribution from these different fluctuations is in the transfer functions mentioned in Table I from bulk and shear noise fields to layer thicknesses and surface height of coating-substrate interface. These are also plotted in a second curve to get an idea of these transfer functions.

Along with the effects of layer thickness and surface height changes to final mirror displacement (effective) via phase change of reflected light is given by parameters q^B and q^S as defined in Eq. (94). These are plotted in the third plot and show the real difference in contribution from bulk and shear. This is in stark contradiction with what Ian just told me. They believe that for a Gaussian pressure profile, the energy stored in shear strain is 3 times higher than that stored in bulk strain. For comparison with a figure of the paper (fig.7.), I plotted the square root of these transfer functions in the fourth plot. However, the paper plots these for Silica and Tantla, not AlGaAs.

My conclusion is that at least in Hong et al.'s treatment, the effect of beam area on the mirror is equal to both bulk and shear contributions (Eq. 96).

2443   Wed Oct 2 19:09:31 2019 anchalDailyProgressFSSTTFSS OLTFs; Repeated this 5 year old measurement

I repeated the measurement done by Tara and Evan to update residual NPRO noise in our noise budget.

Quote:

Acutally it does look like it's a 50 Ω loading issue. I find that when 50 Ω inline terminators are added to OUT1 and OUT2, the measured OLTF is reduced by a factor of 1.6. This explains the discrepancy between the SR785 and HP4395 measurements. I've attached the corrected OLTF plots, along with plots of a vector fit, and the expected residual frequency noise [assuming a free-running NPRO noise of 104 Hz/Hz1/2 × (1 Hz / f)].

### Conclusions:

• Clearly, the North side's boost isn't working as was reported in CTN:2384
• South side looks as healthy as North in CTN:1504
• Tomorrow I'll update the noisebudget with new residual NPRO noise, for now, the second plot gives the prediction.
• It matches very well with our observed floor of noise in 100Hz to 1 kHz region in the beatnote spectrum.

Attachment 1: FSS_OLTF.pdf
2444   Fri Oct 4 13:41:44 2019 anchalNotesFSSFSS Plant Model

I and Ian discussed what the transfer functions would look like. Then today, using some old calculations, I put up this notebook which does the calculations for us. The notebook has the calculations typed up in latex too.

This is the first attempt. We have to work on making the EOM path's transfer function closer to the expected model transfer function. And we should use a faster opamp as well, I think.

Edit Fri Oct 4 14:44:33 2019 anchal:

• Added a buffer at the input of PZT to isolate PZT capacitance from the adder circuit.
• Updated and better circuit schematic coming up by Ian. Removed this image.
Attachment 1: FSS_Plant_Zero_Sim.pdf
Attachment 2: PlantModel.pdf
Attachment 3: PlantModel.zip
2447   Tue Oct 8 10:41:43 2019 anchalNotesFSSFSS Plant Model

The nodes at input and output of buffer in the PZT path are connected together. That is wrong. Also, If possible, you should name the elements same as the zero model in the notebook. Anyways, I think we are ready to solder a circuit board.

2449   Wed Oct 9 16:08:39 2019 anchalDailyProgressBEATBeatnote Frequency long time series data

Whoever commented last, suggested a good idea. So I've here plotted the NPRO slow control voltage signals, converted into the inferred temperature of the cavities (see CTN:2415). I'm not so sure which CMRR the anonymous commentator is talking about. More clarification on that would be helpful.

### Conclusions:

• As the anonymous commentator said, the ideal temperature difference between the cavities would be around 20 uK, but that is clearly not the case as North Cavity varies more than 0.1 K while the South Cavity is relatively more stable.
• This could also indicate that some external low-frequency noise source drifts the North Lasers frequency by corrupting the FSS.
• The thermal dynamics of the two cavities should be similar unless same driven current causes starkly different emission of heat from the coil heaters (which are supposedly of same length).
• Some more help in understanding these results are required, or atleast a person who can bounce back ideas with me.
• So the can upgrade might be useful afterall. If anything, it will reduce the mysteries related to the cavities locked inside there for the last 5 years.

Code and Data

Attachment 1: BeatnoteLongTimeSeriesPart2.pdf
2450   Mon Oct 14 12:54:41 2019 anchalNotesFSSFSS Plant Model v2

I have updated the plant model to contain the cavity pole also. Cavity pole is a pair of positive and negative real poles, so it is hard (or maybe impossible) to imitate it exactly with an electronic circuit. Or maybe, my analysis is wrong.

Nevertheless, I have for now made this circuit which has a second-order pole, so it correctly matches the magnitude of the model transfer function up to 1 MHz for both PZT and EOM paths. Note that the elliptical filter is not included in this as we can connect the circuit to Test port 1 which injects just before the filter in LIGO-D0901894. Also, for the gains in EOM path, I had to add some factors to make it the same as the model transfer function. All components are calculated for E12 series resistors and capacitors.

Attached is a pdf of the notebook which contains all the mathematics in latex and a zip file with all files to recreate and further work on this. Ian can use these as support to learn zero further.

Attachment 1: FSS_Plant_Zero_Sim.pdf
Attachment 2: PlantModelv2.pdf
Attachment 3: PlantModelv2.zip
2451   Mon Oct 14 17:01:43 2019 anchalNotesLaserSouth Laser Switched Off!

On Oct 11th at 15:04:04, the south laser switched off on its own. I would like to know if anyone entered the lab around this time. Koji did mention that our Laser Safety sign outside was blinking, but I have no more information than that. Attached is the data of south PMC reflection DC, which is the first photodiode that measures the laser. It suddenly went to zero, indicating the laser was switched off and the locks did not drive it to this point. I'm also finding that the laser intensity is reducedasit used to saturate the South PMC reflection photodiode when unlocked but presently shows around 5V. I'm trying to put the experiment back to same parameters as before.

Code and Data

Attachment 1: SouthLaserStopped.pdf
2452   Mon Oct 14 18:30:14 2019 anchalNotesPMCSouth PMC Error Signal Oscillations

South PMC Error signal is showing huge weird capacitor charging type oscillations. Attached is an oscilloscope measurement of it. A more weird thing is that this peak is appearing at random frequencies, that is, if I take single sequence measurements of 1 ms length, I see these peaks occurring at a difference from 100 us to 500 us, randomly.

### Changes:

• I had to re-adjust the power going to the PMC to get 9.6mW power falling on the cavities. But I think the knob was more or less in same place.
• I had to restart whole lab (all channels and scripts) as I suspected some channels are not responsive.
• I can't increase the fast gain on southside beyond -7 dB (while this was 14 dB before). so this SPMC oscillations are really hindering badly.
• I also suspect that the NPRO temperature control is not as stable as before on the south side, probably because it was switched off for more than 2 days.
• I'm hoping by tomorrow, this should get better. but still need to investigate the issue on south PMC.

Data

Attachment 1: SPMCMixerOutOscillations.pdf
2453   Tue Oct 15 13:23:16 2019 anchalNotesSafetyQuick note: Shutting Down Lasers as Safety Sign is not working

Following up after CTN:2452, the laser safety sign is not working. Hence the lab has been shut down now. All lasers are switched off with key turned to off position. I'll fix the laser safettysign before turning the experiment back one. Possible reasons might be an interlock glitch or bulb fuse.

2454   Tue Oct 15 18:53:00 2019 anchalNotesSafetyQuick note: Lasers started again

I have taken the bulb from ATF lab inside sign for now. I'm ordering a new one to replace that soon.

2455   Tue Oct 15 18:54:16 2019 anchalNotesPMCSouth PMC further problems

With Laser actuation not connected, I see that the South PMC Servo board is acting up. Firstly, it is not responding to changes made on the ramp when engage is off. This shows that maybe the engage DAC channel is faulty and PMC lock is always on. Need to investigate more on this so for now I have disconnected the PMC PZT from the servo board so that nothing further happens. North side is completely happy and sound.

2456   Wed Oct 16 16:56:43 2019 anchalNotesPMCSouth PMC won't work with it's own Servo Card only

I have a wierd observation. The following two combination work:

• North PMC locked through South PMC Servo Card and then the frequency locked through North FSS Box.
• South PMC locked through North PMC Servo Card and then the frequency locked through South FSS Box.

But when I connect the South PMC to its own South PMC Servo Card, the PZT output voltage does not change with changes made to Ramp. The other side, works.

I checked the capacitance of the South PMC PZT and it came to about 395 nF which is same as specifications with the error bars. So the PZT isn't bad.

But if I disconnect the South PMC PZT from the South PMC Servo Card, the output voltage at the servo card changes as expected with ramp voltage.

This is very perplexing. I think I need second opinion here to do sanity checks otherwise I'll go mad in the basement.

Updated schematics for reference: South PMC Servo Card

2458   Wed Oct 16 18:46:58 2019 anchalNotesPMCSouth PMC started working somehow :(

Yeah, the cables are isolated and no inversion could happen.

 Quote: Are you sure that all the cables involved are isolated and there is no polarity inversion? e.g. The unfunctional combination provides HV to GND directry at the cable, for example.

Even more bizzarething is that it works now!  I'm not halucinating here but the same thing was not working before.I even have lab notes from yesterday when it wasn't working.

This is pretty bad as I don't want to be unaware of something in the lab that caused this. The only other clue here in all this is that the laser intensity changed. We control the intensity of light going into the experiment at (24,110) through the half waveplate before a PBS. Rana told me that the polarization direction of laser coming from NPRO shouldn't change, but since the incident of last Friday, I have had to change the rotation of this half waveplate in both directions to ensure same amound of light reaches the cavity as on North. since no alignment was changed at the south PMC, there is no reason for the mode matching to change drastically there or during the day. but this is the only fishy clue I've got for now.

### Presently

Both cavities are locked. with same amount of gains in the FSS and PMC loops in both paths as before. The can's temperature as reached to the setpoint and the beatnote frequency PID is working to take it to 27.34 MHz. I'll set trigger for tonight for beatnote frequency noise measurement if the frequency reaches in the range of the photodiode.We then will know what is the impact on the experiment noise.

2460   Wed Oct 16 19:11:11 2019 anchalNotesBEATQuick note: Cavities locked; Changed BN Freq Counter Poll Rate
• Changed polling rate of beatnote frequency counter to 1 Hz and the script is updating the EPICS channel at the same rate now.
• Changed BNFreqSignUpdate and BNFreqSlopeCalc scripts accordingly.
2461   Fri Oct 18 17:47:49 2019 anchalNotesBEATQuick note: BN Noise bakc to where it was.

Latest BN Spectrum: CTN_Latest_BN_Spec.pdf

Daily BN Spectrum: CTN_Daily_BN_Spec.pdf

• Changed bandwidth of Moku PLL 10 10 kHz until beatnote frequency control settles back normally.
• We are back to same position in noise but there are some extra sharp peaks which need to be looked at. But they are very small.
2463   Wed Oct 23 13:21:12 2019 anchalDailyProgressNoiseBudgetFixed all discrepancies.

I ran the code for aLIGO coating structure and tried to reproduce fig5 and fig7 of Hong et al. paper. It turned out that the derivatives of the complex reflectivity were not matching with the paper. I rewrote the code, with a fresh mind without looking at previous code and voila, after increasing computation time slightly due to more brute force calculations, I was able to reproduce the figures. These figures are attached. Since I do not have access to the data of the figures in the paper (I tried to email the authors but got no replies), I could only try to plot it on the same scale and limits and check the values by eyes. The values seem to match. So I am more confident now to declare that this code completely follows the paper's calculations.

However, this does not change the coating Brownian noise. I have updated the noise budgets at the Daily and Latest plots.

Attachment 1: aLIGOComplexReflectivityVariationFig5.pdf
Attachment 2: aLIGOBreakDownThermalNoiseFig7.pdf
Attachment 3: aLIGOCoatingBrNoiseCalculations.zip
2464   Wed Oct 23 17:39:44 2019 anchalNotesPMCSouth PMC started working somehow :(

The same thing happened again. This time, not just with the SPMC actuation voltage, but the South Laser slow voltage control was also unresponsive. However, I am not very sure about the latter. This was resolved once the restarted the whole lab. This narrows down the problem to following possibilities:

• One or more channels in an acromag card become unresponsive. It could be the 10.0.1.49 acromag used for South PMC Controls.
• Some device in TCS or QIL lab is interfering by using an IP address reserved for docker services on which the EPICS channels are hosted and python scripts are run.

These still don't explain CTN:2456. Again, since this is an irreproducible error, I will just have to wait for it to happen to gather more clues. Right now, everything is fine and beatnote is traveling towards set frequency.

2466   Fri Oct 25 16:50:12 2019 anchalNotesPMCSouth PMC problem debugging efforts

Today, this problem happened again (check history for details). I have done the following investigative steps:

• Situation: The South PMC PZT Ramp was changing by the EPICS voltage but the PZT displacement value was stuck. Also, the South Laser Slow voltage channel also goes back to some value even after changing it.
• I took out the South PMC box from the rack. During this:
• I had to disconnect the 9V supply used by acromag DACs for setting gain values and switch values.
• I also had to disconnect the ethernet cable too.
• I also disconnected RFPD input to the box, HV input, and PMC PZT.
• The first thing I noticed is that the PMC PZT not following the ramp problem is fixed.
• However, the slow output of the laser was going back to a set value (-6.1046V) without any reason.
• I attached the SPMC RFPD RF (Downconverted) signal back to FPTest1 port the PMC just got locked! This happened even though the engage button was switched off.
• And again, the PMC PZT ramp stopped causing any changes to the PZT displacement value. This is common when the PMC is locked in a normal manner.
• I detached the SPMC RFPD, the changes were happening normally. So the presence of a strong signal might be the issue.
• But, when I connected the RFPD back, the problem was not there anymore. At this point, I found the following reproducible issue:
• Engage button is off.
• PZT displacement is following the ramp normally.
• Slowly change the ramp and at the sweet spot, the PMC gets locked. Remember the engage is still off.
• Now, changing the ramp doesn't change the PZT displacement anymore.
• Switching on the engage ON doesn't change anything.
• Switching it back OFF unlocks the PMC and the PZT displacement starts responding to ramp voltage again.
• That's weird right. Since this was reproducible, I did this a few times and found that the problem doesn't necessarily happen at the locking point. It can happen anywhere. And in that case, the 5th step of switching on the engage does show a difference in locked mode. And again, switching it back OFF resolves the issue.
• At this point, I checked back the slow voltage and it had changed to -6.1086. Now apparently this was the new point it liked to stay on regardless of any changes made to the EPICS channel controlling it.
• I checked if the signal for 'PMC Engage' to the U5A AD602. This signal works as expected. The gain setting signal also works as expected. The Ramp setting signal also works as expected. So there is no evidence that the acromag card or the EPICS channel is causing this issue (at least in the PMC problem).
• It could be that the AD602 switches ON itself due to some reason and doesn't switch off until it explicitly sees a low-high change on its pin 4.
• Also, it could be that the U9 PA85 is the one causing trouble. But I doubt that as the behavior matches more with engaging the lock upstream at U5A AD602.
• I restarted the lab by restarting the channels and scripts (this is done in a preset protocol by restartAll command).
• This resolved the issue with the slow voltage of the laser. But the PMC problem persists. In fact, it is not locking now.

I'll take another inspection with a fresh mind next time. This problem needs to be resolved as we can't leave some unexplained phenomenon to keep happening in the lab.

2467   Tue Oct 29 19:10:59 2019 anchalNotesPMCSouth PMC problem debugging efforts

I reduced the power falling on the PMC to ensure the high signal level isn't causing this problem. It was not. The problem still persisted.

Then, I did this reproducible step (quoted below)again, but this time I had a small 10 mV signal from SR785 going into FP2TEST and I was taking transfer function to TP2. If the U5A AD602 is switched off by the Blanking pin, the transfer function should remain null. This gave me a way of checking if the AD602 is wrongly getting switched on on its own.

• To start, I kept Engage OFF. This gave a voltage of 4.41 V at pin 4 of U5A AD602. So it should be shut off.
• The PZT voltage was about 97 V at this point.
• The transfer function was flat to about -80 dB from 10Hz to 100Hz.
• Then I started scanning the RAMP voltage of PZT. As the PZT voltage reached near 70V value, the PMC got locked on its own.
• The transfer function value jumped suddenly to between -20 dB to 0 dB (an increase by about 70 dB).
• The gain on AD602 was set to 0dB. So if it got on its own, the transfer function was expected to be 0dB.
• The gate voltage at pin 4 of U5A AD602 was still 4.41 V. So ideally, it should still be off.
• I have attached the data I captured. During swept sine, the ramp was being increased and we see a jump clearly when the PMC got locked on its own.

This is good evidence in my opinion that the AD602 at U5A is faulty. I need comments on this conclusion. If I don't hear otherwise by tomorrow noon, I'll start working on replacing it.

Data

Edit Thu Oct 31 10:26:50 2019

Issue fixed. See CTN:2469.

 Quote: But, when I connected the RFPD back, the problem was not there anymore. At this point, I found the following reproducible issue: ​Engage button is off. PZT displacement is following the ramp normally. Slowly change the ramp and at the sweet spot, the PMC gets locked. Remember the engage is still off. Now, changing the ramp doesn't change the PZT displacement anymore. Switching on the engage ON doesn't change anything. Switching it back OFF unlocks the PMC and the PZT displacement starts responding to ramp voltage again. That's weird right. Since this was reproducible, I did this a few times and found that the problem doesn't necessarily happen at the locking point. It can happen anywhere. And in that case, the 5th step of switching on the engage does show a difference in locked mode. And again, switching it back OFF resolves the issue.
Attachment 1: SPMC_Diagnostics.pdf
2468   Wed Oct 30 15:15:28 2019 anchalNotesPMCReplacing U5A AD602 didn't solve the issue.

I brought a new AD602AR from 40m and replaced the U5A AD602 which from the previous post seemed like the culprit, but it wasn't.

I'll think of some new way of figuring out the point of the problem. It would be nice if someone can help me with this. All the history of the issue is on this thread starting at CTN:2451.

Edit Thu Oct 31 10:26:50 2019

Issue fixed. See CTN:2469.

2469   Wed Oct 30 18:35:31 2019 anchalNotesPMCIssue closed

It turns out I did not have a full understanding of the problem and it was not really a problem. The blanking (pin 4) on U5A AD602 doesn't shut down the channel, it just reduced the gain by 100. So if the gain in previous stage is large enough, the lock can still be acquired. And that's what was happening.

Ideally, we need to keep the AD602 on all the time and lock by scanning the offset with low gain. The loop will catch the lock (the exact same thing I thought was a problem) and once that has happened, we can just increase the loop gain the set value.

Presently, the gain behind the U5A AD602 is 101, which is kind of high. I just need to check if the above-mentioned locking method would work robustly without wrongly getting locked to any higher-order modes with the gain slider set to some threshold value within -10 dB to 30 dB. If that can't be done, I might have to reduce the gain in the first stage. For now, the cavities are locked and beatnote is traveling towards set point.

2470   Fri Nov 1 17:54:47 2019 anchalDailyProgressFSSFSS Diagnostics - RFPD RF Ouput under inspection

First step in FSS Diagnostics was to see RF output from the RFPDs in FSS when they are locked. I ran some extensive measurements to cover all the information about this signal. The RF out is sent to the FSS box through ZFDC-20-5-S+ 19.5 dB directional coupler. The coupled output's spectrum is measured at different frequency ranges using both AG4395A and SR785. The measurement configuration files are included with data for metadata of the measurement. The signal is also analyzed in time series with measurement upto 1 GSa/s with TDS 3034C and one measurement at 5 GSa/s with TDS 3052B. Both measurements were done manually setting minimum possible voltage resolution and using DC coupling with 50 Ohm impedance. All data is attached raw here for now. More interpretation and analysis to come soon.

Data

Attachment 1: NFSS_RFPD_Output_Oscilloscope.pdf
Attachment 2: SFSS_RFPD_Output_Oscilloscope.pdf
Attachment 3: NFSS_RFPD_RF_OUT_COUPLED.pdf
Attachment 4: SFSS_RFPD_RF_OUT_COUPLED.pdf
2471   Mon Nov 4 12:15:01 2019 anchalNotesPMCChanging the autolocking method

### Trying gain sliding like 40m

Rana told me that in 40m, the PMCs are autolocked by reducing the gain of the loop and changing the ramp until the lock is acquired. Then the gain is increased back to operation point. I tried this method with our South PMC as the usual method being used of 'changing Blanking state' wasn't working anymore. However, even with the gain set all the way to -10 dB, the loop was not locking exactly at the center of the TEM00 mode. And was unable to skip higher-order modes. There is a header H1 which changes the input stage gain. Removing this header pin, reducing the input stage gain by a factor of 100. Even after doing this, I was unable to robustly acquire the lock by this method. Besides, this reduced gain was the case earlier (CTN/2427) and it was too low as the VCA U5A AD602 had to be kept at maximum 30dB gain. So I did not want to reduce this first stage gain.

### New method: Switch off input while changing Ramp

Somewhat similar to our FSS loops, I find it much cleaner to just not close the loop until we have reached near the lock point. This could be done fairly easily with the existing code. I just had to change the loopStateEnable variable from Engage (which changes the Blanking pin on U5A AD602) to input switch (FP1TEST for South and FP2TEST for North). So now, when finding a lock point, the input is changed to terminated inputs and the loop is closed when lock point is found. This works very nicely, just like the FSS autolocks.

This has finally fixed any problems with PMC autolocks.$\LARGE {\color{Green} \checkmark}$

2472   Mon Nov 4 15:03:26 2019 anchalDailyProgressFSSRaised North TTFSS; Fixed the boxes.

I raised the North TTFSS box by 6 inches to make way for working on South box and to reduce the congestion of connectors in front of the two boxes. I have also clamped the boxes to a fixed position now, so they can't move. This would ensure the cables are not hitting the end of the platform and face any severe strain.

The next step towards improving lab cable hygiene and layout is to replace all RF cables with RG-405 Belden-N 1671J cables. However, the effects of this change would be less significant then fixing the sick FSS. So I'll first focus on that.

Attachment 1: signal-attachment-2019-11-04-161746.jpeg
2473   Mon Nov 4 20:13:44 2019 anchalNotesBEATQuick note: FSS Loop Gain Changes

South Common Gain: 24 dB! , Fast Gain: 14 dB

North Common Gain: 10 dB, Fast Gain: 10 dB

2474   Tue Nov 5 18:37:59 2019 anchalDailyProgressFSSFSS Diagnostics - TTFSS Testpoint Spectrum Data

### Measurement method

• All measurements were taken withAG4395A for high frequency and SR785 for low frequency and the corresponding measurement parameter files are attached.
• For SR785, the SRmeasureWideSP.py script was used.
• HO 41800A Active Probes were used with AG4395A.
• For SR785, I simply used clips as it already has 1 MOhm input impedance. However, I have noticed oscillations in the spectrum.
• Note, South FSS box had higher gains, Common Gain: 24 dB, Fast Gain: 14 dB, while the Northside had Common Gain: 10 dB and Fast Gain 10 dB.
• TP1,5 and 4 are on common path on D040105 (North, South).
• TP17,14,15,16,18 and 19 are on PZT Path on D040105 (North, South).
• TP 11, 12 and 13 are on EOM Path on D040105 (North, South).

Data

Attachment 1: FSS_TP_Spectrum.pdf
2476   Sat Nov 9 19:28:36 2019 anchalDailyProgressFSSFSS Diagnostics - Loop nonlinearity Test

### Method

I've wrote this script, nonlinTF.py which controls a Marconi 2023A and SR785 together. Marconi is used to providing a carrier frequency which is mixed with the Source Out signal from SR785 before feeding into the TEST2 input port on D040105 of TTFSS boxes. Then OUT1 port on D040105 of TTFSS box is used to read back at channel two of SR785 (channel one being fed with a copy of the Source Out signal). So SR785 is effectively measuring any downconversion in the loop (due to some nonlinearity) from micing of CF-IF, CF and CF+IF probe signals injected into the loop. The effectively closed-loop transfer function between TEST2 and OUT1 should be G/(1+G), so this injected signal should not suffer any suppression, nor should it affect the locks. The locks were maintained without any problems during the whole measurement. The CF frequency was stepped by 100 kHz from 100kHz to 10 MHz and then by 1 Mhz upto 100 MHz.

Mixer ZX05-1LHW (level +13 dBm) was used for the mixing and IF peak voltage was set to 30 mV. The configuration of the measurement for the transfer function is present in the configuration file in the folder.

### Results?

• I had to upload plots in png as they were aggregated result of a lot of data, pdf would have been very heavy.
• North Side looks good with only minor bumps in 60 Hz harmonics. I'm not even sure if this just came because of SR785.
• The South side however looks very dramatic. A lot of stuff happening when carrier frequency was near 50 MHz.
• But I'm not sure if this measurement even makes any sense. South side is better in performance then North side, this wasn't expected.
• Also, the measurements are long-time measurements, so a lot of things could have changed between North and South (they were taken on different days too).
• I'll take more verifying measurements near the suspected frequencies tomorrow.
• I'm also thinking of taking spectrums instead of swept sine transfer functions next time.

Data

Attachment 1: NFSS_NonLinearity_Test.png
Attachment 2: SFSS_NonLinearity_Test.png
Attachment 3: signal-attachment-2019-11-09-195635.jpeg
2477   Mon Nov 11 20:42:49 2019 anchalDailyProgressFSSFSS Diagnostics - Two Tone Test

Method

• RFPD of the FSS loops were tested through there Test input ports (Updated schematics: South SN010, North SN009)
• I first took transfer function from the Test input port to the RF out port. Ideally, the transfer function of RFPD should be 100kOhm times this transfer function.
• Moku is used to generate two RF signals with a different frequency equivalent to RFPD's resonant frequency.
• These two outputs are combined with ZFSC-2-1W-S+ two-way splitter/combiner.
• I switched around the voltage amplitude of these frequencies and took a spectrum from AG4395A near difference frequency.
• The configuration file for spectrum measurement is present with data.
• This measurement was made through twoToneTest script present in the data folder.

Inferences:

• In first plot of each measurement set, I simply plotted the measured spectrum at the different signal voltage levels.
• In second plots, I have done some back calculations, but I'm not sure if this is a good of estimating non-linearity.
• I have used following formula to calculate ratio of total power in the difference frequency to the GM of powers of probe signals.
$\LARGE Ratio = \frac{ ASD_{@f2-f2} \sqrt{IFBW} } { \sqrt{ V_1\ TF_{@f1} V_2\ TF_{@f2}}}$
• This ratio I have plot in dB in the second plot.
• It seems that the spectrum level generated is not dependent on how strong the probe signals are.
• This can be seen as along the diagonal, instead of remaining the same ratio, the ratio decreases by 20 dB in each step which is an increase in the denominator above.
• Lastly, I just connected the two ends of cable between which DUT goes and repeated the measurement.
• The non-linearity of the measurement setup itself is safely 50 dB below the measured values.

But what now?

• I need more time to make sense of these measurements. So that will come tomorrow.

Data

Attachment 1: NFSS_RFPD_SN009_TF_11-11-2019_183131.pdf
Attachment 2: SFSS_RFPD_SN010_TF_11-11-2019_203310.pdf
Attachment 3: FSStwoToneTest.pdf
2478   Tue Nov 12 11:39:48 2019 anchalDailyProgressFSSFSS Diagnostics - RFPD RF Ouput under inspection

I calculated these values by integrating in the 8 MHz neighborhood around the marked harmonic peak, the power spectral density using the frequency at the point as the lower edge of the bin. Slew rate is calculated by multiplying the rms voltage level with the frequency and the fraction is calculated against the datasheet value for Max 4107.

NFSS RFPD Output Slew Rate Usage (MAX 4107, SR: 500 V/us)
Freq (MHz) Vrms (mV) Required Slew Rate (V/us) Fraction of Slew Rate Used (%)

36

59.221
2.1319
0.4264
72
9.248
0.6659
0.1332
108
1.135
0.1226
0.0245
144
0.39
0.0562
0.0112
180
0.621
0.1117
0.0223

SFSS RFPD Output Slew Rate Usage (MAX 4107, SR: 500 V/us)

Freq (MHz) Vrms (mV) Required Slew Rate (V/us) Fraction of Slew Rate Used (%)
37
23.205
0.8586
0.1717
74
3.248
0.2403
0.0481
111
0.495
0.055
0.011
148
0.301
0.0445
0.0089
185
0.512
0.0947
0.0189

These calculations at least show that MAX 4107 should be much far away from reaching its slew rate limit in both RFPDs.

2479   Tue Nov 12 18:36:59 2019 anchalDailyProgressFSSFSS Diagnostics - Two Tone Third Order Intermodulation Test

In line with industrial practices, I did two tone third order intermodulation test today on the FSS RFPDs. This test was inspired by procedure described in this technical note by MiniCircuits and this paper at IEEE.

Method

• Similar to previous measurement, two RF tones are generated using Moku.
• However, now, they are separated by an audio frequency, 10 kHz in our case. This is marked as DF in the plots.
• These tones are centered around the resonant frequency of the RFPD marked as CF in the plots.
• The two signals are combined together through ZFSC-2-1W+.pdf, but this time all ports of the combiner have an attenuator on it.
• 3 dB attenuators at the input port and 6 dB at the output port. This was mentione din MiniCircuits as a step to improve impedance matching near the combiner.
• Ideally, two low pass filters also should be put at the input port, but I couldn't find any with high enough cut-off frequency.
• The combined source signal was getting attenuated by 8.75 dB. So I just drove the input signals higher by this amount.
• The Source signal is fed to the Test IN of the RFPD and RF out is read through AG4395A.
• The power in each harmonic is calculated from the spectrum and plotted together against source power.
• Since all RF signals are the near-resonant peak of the photodiode, there would be little to no difference in TF seen by them.

Inference:

• I think because of the 100 kOhm resistor at the Test In port of RFPDs makes the source signal too feeble.
• I'm just measuring instrument noise upto source power of -10 dB. I can't go above 0 dB in current setup atleast.
• At 0 dB source power, which is equivalent to 3 uApk photocurrent from the photodiode, the third-order harmonics are 60 dB lower.
• One way of reporting this is, Spurious Free Dynamic Range (SFDR) at 36(37) MHz is 60 dBc at 0 dBm source power of SN009 (SN010).
• Another way this figure is reported in industry is by extrapolating at which point the third-order harmonic power becomes the same as fundamental.
• But I do not have enough distortion showing data points to extrapolate to calculate the intercept.
• Last plot is when I connected the cables together which go to DUT. We see SFDR of 60 dBc here itself.
• The last two data points are when input range of AG4395A are very high and hence has increased noise floor.
• So probably in my measurements also, I'm just seeing distortion due to measurement apparatus itself.
• But this at least sets an upper bound of distortion to SFDR of 60 dBc at the resonant frequencies (within the errors that I, a graduate student, can make).

Datasheet for MAX4107

• The datasheeet says that between 30-40 MHz, the SFDR is approximately 50 dBc (this is actually lower than what we measured).
• Also, the third-order intercept is mentioned at 15 dBm source power.

Data

Attachment 1: FSStwoToneThirdOrderIM.pdf
2481   Tue Nov 19 18:51:02 2019 anchalDailyProgressFSSFSS Diagnostics - Second-order Distortion Calculation

I did some theoretical calculations using the datasheet value of second harmonic SFDR from MAX4107 and the transfer function I measured from Test IN ports of RFPDs (using 100 kOhm series resistance).

Calculation:

• Attached notebook has all the calculations. In all places, "Omega" refers to the modulation frequency.
• I first calculated power at 0, 1-Omega and 2-Omega from the resonance frequency in the light incident on the cavity using Bessel functions.
• I used reflection function from the cavity (using transmittance of 5 ppm, cavity length of 1.45" and assuming lossless cavity):
$\LARGE R = \frac{- r(1 - e^{\iota \omega/\nu})}{1 - r^2 e^{\iota \omega /\nu}}$
• Using this, I calculated reflected power at  0, 1-Omega and 2-Omega from the resonance frequency.
• Then I used -40 dBc SFDR for second harmonic generation mentioned in MAX4107 datasheet when used with a gain of 10. I used following formula to calculate this:
$\LARGE P_{1\Omega Dist} = \frac{(V_{1\Omega}V_{2\Omega})}{100}10^{-\frac{SFDR}{10}}$
• Finally, I also calculated ratio of 1-Omega signal (actual beat between Sideband and Carrier) and the 1-Omega Distortion signal (generated) due to the non-linearity of MAX4107.

Inferences:

• These are purely theoretical calculations but I can put in the second harmonic SFDR by measurement tomorrow.
• The suppression of 2nd harmonics by the notch in the RFPD circuits is very bad. It only suppresses the second harmonic by 25-30 dB.
• This is not enough as this 2-Omega signal would mix a lot with 1-Omega and would also go forward into the TTFSS box after downconversion.
• The ratio of Signal at 1-Omega to the distortion at 1-Omega is about just 8 at 100 Hz. This doesn't reach 100 even till 1 kHz.
• And below 10 Hz, the distortion dominates the 1-Omega signal from RF out.
• This looks like a big limitation to our FSS loop. The notch filters in the RFPDs need to be doing much better job then they are doing.

Edited Wed Nov 20 14:43:07 2019: Corrected an error in code.

Attachment 1: FSSDistortionCalc.pdf
Attachment 2: FSSDistortion.zip
2482   Wed Nov 27 13:59:24 2019 anchalDailyProgressFSSFSS Diagnostics - TTFSS TP1 TimeSeries

I took time-series data at TP1 on NFSS. This is just after the elliptical filter which is after the demodulation on board.

As also seen in the spectrum measurement at this testpoint in CTN:2474, there is a lot of power at around 435 kHz.  But this is not noise!

As seen on the oscilloscope, this is a near-perfect sinusoid. So this must be either of the following:

• A mixed down frequency due to non-linearity of MAX4107 or the Mixer JMS1-H .
• This could be an oscillation in the closed-loop of FSS at the unity gain frequency. But last I checked (CTN:2443), the UGF for the same COM Gain (10dB) and FAST Gain (10 dB) was 369 kHz.

This measurement was taken with a 500 MHz 10x Probe with a 300MHz TDS 3034C oscilloscope at 0.5 GSa/s sampling rate.

Interestingly, there is no such oscillation or peak on the South side. However, the south sides COM Gain is 24 dB and Fast Gain is 14 dB. So it could be because it is just suppressing this non-linear effect properly or just has a very high UGF.

Data

Attachment 1: NFSS_TP1_Oscilloscope.pdf
2483   Wed Nov 27 15:17:51 2019 anchalDailyProgressFSSFSS Diagnostics - Quick HF OLTF of NFSS

I quickly took a high-frequency Open Loop Gain measurement of NFSS loop at 10 dB COM Gain and 10 dB FAST gain, using the same measurement method as in CTN:2443. The UGF has not changed much but there is a dip at 435 kHz. This was there before too, I was just not paying enough attention to this part of OLTF before. So, we can say with some confidence that the 435 kHz signal seen in the oscilloscope in CTN:2482 at TP1 is actually due to some non-linear effect most probably and does not get suppressed at all. The phase margin near UGF looks about 135 degrees so there is no solid reason to believe this could be due to loop oscillation.

So I got to think of what combination of RF frequencies might be mixing down to create this oscillation and where. This oscillation is also visible in Plot 6 and 7 of NFSS_RFPD_Output_Oscilloscope.pdf of the measurements done in CTN:2470.

Data

Attachment 1: NFSS_OLTF_HF.pdf
2486   Wed Dec 4 17:09:44 2019 anchalDailyProgressTempCtrlIncreased range of out-of-loop temperature sensor

This has happened few times now that acromag channel for the can heater driver stopped updating according to the PID script and the can gets heated to a very high temperature. This pushes the temperature out of the ranges of the current AD590 temperature sensor board. I have changed the range of channel 2 (this was being used for out-of-loop) to ensure we can still see some meaningful temperature value when such incidents happen. I have replaced R18 from 100k to 27k. The updated table is:

CH No on Board EPICS Channel Name Temperature Conversion Function (ºC) Range (ºC)
1 C3:PSL-TEMP_TABLE = V/0.810875 + 27.30248869 13.860-40.745
2 C3:PSL-TEMP_VACCAN_OOL = V/0.43875 + 33.3115385 8.450-58.155
3 C3:PSL-TEMP_VACCAN_INLOOP = V/1.625 + 33.3115385 26.604-40.020

### Weird phenomenon?

• I'm not sure this problem occurs though. Right now the out-of-loop temperature sensor shows that the temperature of the can is 55 Degree Celsius.
• This is also not cooling down fast. Last time it took days for it to cool down to set value.
• The alignment of the cavities change vertically when the can temperature is significantly different from the set value of 34.38 Degree Celcius.
• This hinders them from locking properly unless I tune the alignment back. But when the can will cool down finally, they will be misaligned again.
• Also, I just refuse to believe it is actually that hot, but I check the voltage after the transimpedance amplifier in temperature sensor boards and two independent AD590s are reporting this.
• And anyways, why is the acromag output channel getting frozen anyways.
• That being said, this is an irreproducible but non-harmful problem yet, so lower priority than the FSS saga going on.
2487   Wed Dec 4 18:11:09 2019 anchalDailyProgressFSSFree running laser frequency noise spectrum

I took a spectrum of PMC error signal when the FSS loop is not closed. This should provide a rough estimate of the free running laser noise. We had earlier seen a peak at 435 kHz in the Northside, hence I wanted to take this data with some references. First of all, this peak is very similar in the description of relaxation-oscillation peaks of these NPRO lasers mentioned on page 52 of this manual. The "Noise Eater (NE)" is supposed to suppress this peak significantly. However, in the spectrum of the PMC error signal, there is no difference when noise eater was ON or OFF.

I took a spectrum of Southside as well, just to see if I could see action of Noise eater there. For south laser, the noise eater suppressed noise only till 100 kHz or so and probably this side also has a similar relaxation-oscillation peak problem but is shadowed by a large feature at 30 kHz. Not, the absolute value of the spectrum between North and south are vastly different due to different amount of light, different transimpedances od the PDs and different gain values in the feedback circuit.

However, the noise eater is supposed to reduce relative intensity noise only. And the error signals of PMC should really be telling us noise in the frequency of the laser. So maybe I'm connecting two dots in different Hilbert spaces. But Rana suggested that a busted Noise Eater could be the reason for the 435 kHz peak, I just do not understand how RIN would cause frequency noise so badly. I thought photothermal transfer functions from RIN to frequency noise were very small.

Attachment 1: PMCErrorSignals.pdf
2490   Mon Dec 9 19:54:44 2019 anchalDailyProgressFSSFSS Diagnostics - How much distortion affects the functioning of PDH?

I'm trying to think hard with my small brain how the distortion would affect the PDH functioning and inject noise in the frequency of the laser. I have a line of reasoning which starts with a question.:

• The two RF sidebands of laser that fall on the cavity, upon reflection, do they destructively interfere? Because the calculation I did in CTN:2481 suggests that the power at second harmonic (2-Omega frequency) is so high because of the sidebands beating with each other.
• In case they do not destructively interfere, the generated second-order harmonic (2-Omega) will have laser amplitude noise on it. This when it mixes with the (1-Omega) signal which actually carries the cavity length noise at the inputs of MAX4107, the distorted 1-Omega created would have laser amplitude noise on it.
• This might be the reason why FSS loops get overwhelmed with seeing a lot of laser amplitude noise which ideally it is not supposed to see. And it tries to correct this noise in phase quadrature making the situation worse.
• Since Andrew left, I increased the laser power reaching the cavities to go above the shot-noise limit of the photodiodes. Maybe, this increased light level increased this effect to a point where we are witnessing the problem.

Of course, all this depends on the RF sidebands interfering constructively upon reflection. I remember (I don't know from where) that it is the opposite. Either there is a fault in my calculations or this is indeed what is happening. I need to understand this properly to go further. Need help.

2492   Tue Dec 10 17:03:41 2019 anchalDailyProgressLaserLaser Settings back to defaults

I put laser settings on both North and South Cavities back to default. From this point onwards, all settings about the lasers would be known and kept track of. The red values are the settings that were changed.

### NPRO Laser Settings

Property Display Symbol North South Units Notes
Laser Model - M126N-1064-700, SN 5519, Dec 2006 126N-1064-500, SN 280, Nov 1997 -
Diode Temperature DT 22.3 28.7 $^\circ C$ Informational only.
Diode TEC Voltage DTEC 0.8 0.7 V Informational only. +ve -> cooling, -ve -> heating.
Measured Laser Crystal Temperature LT 40.8 55.2 $^\circ C$ Informational only. Calibration dependent.
Laser TEC Voltage LTEC 0.0 -0.5 V Informational only. +ve -> cooling, -ve -> heating. Manual says typically should be 0.0V.
Target Laser Crystal Temperature T 40.087 -> 40.0010 48.0010 $^\circ C$ Changed back to factory set value on North Side.
Laser Head Power Level PWR 66  ->  624 92  ->  101 mW Calibration dependent. CHanged the calibration to meet the power meter but even then, power meter says a maximum 500 mW, so North Side is not entirely correct. On the south side, it was difficult to mount power meter perpendicular to the beam, so there might be some clipping loss in calibration.
Power Adjustment ADJ 0 -2  ->  0 - From -50 (off) to +10. Changed the diode current around set value.
Diode Current DC 2.06 2.04 A It can be changed to change power level. Reflects measured value.
Diode Power Monitor DPM 0.00 0.00 V

Calibration Dependent.

Noise Easter NE ON ON - -
Laser Diode Status LD ON ON -
Nominal Diode Current - All the way clockwise All the way clockwise - It can be changed by turning the left potentiometer from the back of the laser head. Factory default is all the way clockwise. I have set both North and South Lasers to this point.

Note:

While turning the nominal diode current of south laser all the way clockwise, I found that the laser power peaks before the maximum diode current is reached. This diode current is about 1.9 A. This is unexpected. Any explanations on this would be helpful.

ELOG V3.1.3-