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ID Date Author Type Category Subject
1892   Sat Sep 2 13:01:22 2017 awadeDailyProgressBEATRe: Sept 1st Calibrated Spectrum

Great. Its probably ok to use a (slightly) larger slope on the Marconi ext modulation input for now.  I guess we should have some number estimates on the noise at different settings to 'calibrate' our understanding on how much benefit there is in going to lower and lower values. A quick nasty fix would be to hook up an ethernet box to the marconi and send GPIB commands to adjust the abolute frequency of the carrier to bring the PLL actuation signal back to the middle of range; I looked around and there weren't any python modules ready made so maybe not worth the time.

The factor of two that you are seeing between the Aglient and the SR785 may have to do with their input impedance.  Aglient is 50 Ω and SR785 is 1 MΩ so the aglient will report a voltage times two from a 50 Ω output of the SR560 preamplifier.

The calibration looks fancy but maybe unnecessary for a first pass? G/(1-G) => 1 well within the bandwidth of the PLL. So if we raise the PLL bandwidth to a decent frequency our in loop actuation signal should be a faithful representation of Hz/sqrt{Hz}.  I think just multiply the Hz/Vrms on the front panel display by 1/sqrt{2} is enough for a rough calibration.

Do we have any of the band passing filters turned on on the SR560. I'm not sure I understand why OLG is converging to two there.

1949   Thu Oct 12 16:57:51 2017 Craig, awadeDailyProgressBEATTransmission Beatnote Recovered

Just a quick update.  We recovered our beatnote and (maybe?) lowered ringing in our FSS boxes with some specific gain values.

South Laser Slow Control Voltage: -6.13 V

North Laser Slow Control Voltage: 3.54 V

North Cavity Temperature Control Power Supply Voltage: 11.25 V  (This swings our beatnote by about 250 MHz/V.  Also this setup is terrible, we need to remove all clipdoodles from the main setup.)

Slow PID Loop Time Step: 0.1 seconds

South Common Gain Slider: -3.5

South Fast Gain Slider: 2.28 (This one is very sensitive... moving it slightly causes ringing)

North Common Gain Slider: -3.13

North Fast Gain Slider: -1.29

The peak is still moving since we recently changed the North Cavity Temperature by 0.75 volts to put the locked region of the two cavities within 125 MHz of each other.  The cavity has a time constant of ~1 hour... we can and should measure this sometime.

Next step is to watch the slow control voltages over a long period of time along with the vac can temperature sensors to see if vac can temperature flucutations are still affecting our lock to the PLL.

Attachment 1: October12Beatnote.jpg
1950   Mon Oct 16 12:57:43 2017 Craig, awadeDailyProgressBEATPicture of beatnote off of HP

We need to figure out GPIB for the HP8560E so we can directly get data off of there.  In the meantime here is a picture of our transmission beatnote and the messy spectrum around it.

Attachment 1: 2017Oct16_TransmissionSpectrum.jpg
1951   Mon Oct 16 20:16:15 2017 Craig, awadeDailyProgressBEATPrecav Beatnote Dirty Spectra Facts

Total power on precav PD - 643 µW

South power on precav PD - 173 µW

North Power on precav PD - 470 µW

When both PZT paths for the FSS boxes are plugged in, we see 5 MHz sidebands on our transmission beatnote.  When either path is unplugged from the laser PZT, the 5 MHz sidebands go away.  This is weird because the PZT shouldn't be that fast...

We also have sidebands around 430 kHz.  We opened the loop to the lasers and still see this modulation on DC...

Tried grounding all power supplies together to avoid ground loops.  Found out that our negative high voltage supply was shorted to ground.  Spent day making new cords.  Will test tomorrow.

1963   Mon Oct 30 16:15:09 2017 Craig, awadeDailyProgressBEATPLL locked at ~82 MHz

awade and I managed to lock both cavities today after diagnosing the acromag communications issues.  Some stats:

South Slow Voltage: -6.12 V

North Slow Voltage: 3.46 V

Vaccan Temp: 30 Celcius

Marconi FM Devn: 25 kHz

Marconi RF Lvl: +12.6 dBm

Trans Beatnote Frequency: 82.4 MHz (and drifting up at ~5 kHz/s)

Channels dumped into acromag1 at /home/controls/CTNWS/data/20171030_161251_ChannelDump.txt

awade is currently playing with the Marconi to try and get the FM Devn down to around 1 kHz so we can get a spectrum that is not limited by actuation noise.  Beatnote frequency drift is settling down significantly as I type this, so it could be possible soon.

∆f/f = ∆L/L for the Fabry Perot cavity.  f = 282 THz, L = 3.68 cm, and ∆f = ~ 5 kHz/s implies that ∆L ~ 6.5e-13 m/s for our relative cavity length changes.

From elog 1874, we have ∆T ~ ∆L / (alpha * mirror thickness), where alpha = 5.5e-7 [1/Kelvin] and mirror thickness = 6.35 mm.  We get a relative temperature drift of ∆T ~ 0.2 mK / s

We seem to have underestimated how stable our cavities must be to temperature drift.  In any case our beatnote is still drifting rapidly despite the vaccan stabilization.

1983   Wed Nov 15 14:53:06 2017 CraigDailyProgressBEATFSS Fast Gain Effect on Beatnote Spectrum

I was wondering what is causing our huge mountain at 10 kHz.  awade said it might be the PZT EOM crossover with gain peaking.  I did a quick test in which I turned down the Fast gain sliders to -2V, then incremented them up by 1V while taking spectra at every point.

We start to get diminishing returns from increasing the Fast gain at around 4V.  Before then, we can see some FSS gain peaks moving around and breathing at around 2-3 kHz.

We have a broad peak at 350 Hz.  This one is weird in that it likes to breath, or come in an out of existence, on a timescale of a few seconds.

Similar descriptions for the 640 Hz and 1040 Hz lines, but to a lesser extent.  There is some sort of creepy-crawly noise floor we're hitting between 300-2000 Hz once we're no longer FSS limited.  I blame the australian.

Also the mountain at 10 kHz is not affected by any of this.

Next step: FSS error signal spectra, FSS OLGs, and FSS calibration for both cavities.  This will help with determining our current limiting noise floor.

Some interesting points:

The North cavity has weird breathing in it's error signal on the order of a few milliseconds.

The South cavity does not like a Fast gain of 4V: the error signal tends to blow up randomly, then be fine for a minute, then do it again.  Could be the crossover between the EOM and PZT acting up.

The North cavity and South cavity have significantly different responses to extremely high Fast gain.  The South cavity, when the Fast gain is too high, causes the EOM to start fighting it, increasing the EOM response until it rails.  The North cavity has no limit to how high you can turn the Fast gain; the EOM response never increases in response to a huge PZT gain.  Cause unknown, maybe the North has good phase at the crossover while the South is bad?

Some settings:

The Fast and Common gain sliders are calibrated to 32 dB/V.

The Common gain sliders were set to -3.5V.

The number of averages was 25.

The PLL was locked at FM Devn of 1 kHz/Vrms, Preamp Gain of 2000, and Carrier Freq around 109 MHz.

1984   Wed Nov 15 14:54:30 2017 CraigDailyProgressBEATFSS Fast Gain Effect on Beatnote Spectrum

I was wondering what is causing our huge mountain at 10 kHz.  awade said it might be the PZT EOM crossover with gain peaking.  I did a quick test in which I turned down the Fast gain sliders to -2V, then incremented them up by 1V while taking spectra at every point.

We start to get diminishing returns from increasing the Fast gain at around 4V.  Before then, we can see some FSS gain peaks moving around and breathing at around 2-3 kHz.

We have a broad peak at 350 Hz.  This one is weird in that it likes to breath, or come in an out of existence, on a timescale of a few seconds.

Similar descriptions for the 640 Hz and 1040 Hz lines, but to a lesser extent.  There is some sort of creepy-crawly noise floor we're hitting between 300-2000 Hz once we're no longer FSS limited.  I blame the australian.

Also the mountain at 10 kHz is not affected by any of this.

Next step: FSS error signal spectra, FSS OLGs, and FSS calibration for both cavities.  This will help with determining our current limiting noise floor.

Some interesting points:

The North cavity has weird breathing in it's error signal on the order of a few milliseconds.

The South cavity does not like a Fast gain of 4V: the error signal tends to blow up randomly, then be fine for a minute, then do it again.  Could be the crossover between the EOM and PZT acting up.

The North cavity and South cavity have significantly different responses to extremely high Fast gain.  The South cavity, when the Fast gain is too high, causes the EOM to start fighting it, increasing the EOM response until it rails.  The North cavity has no limit to how high you can turn the Fast gain; the EOM response never increases in response to a huge PZT gain.  Cause unknown, maybe the North has good phase at the crossover while the South is bad?

Some settings:

The Fast and Common gain sliders are calibrated to 32 dB/V.

The Common gain sliders were set to -3.5V.

The number of averages was 25.

The PLL was locked at FM Devn of 1 kHz/Vrms, Preamp Gain of 2000, and Carrier Freq around 109 MHz.

Edit: Remade plot with more resonable labels and ylabel.

Attachment 1: TransBeatnote_BothPathsFastGain_0V_15-11-2017_134517_Spectra.pdf
Attachment 2: TransBeatnote_BothPathsFastGain_0V_15-11-2017_134517.tgz
2012   Sat Dec 16 16:39:31 2017 awadeDailyProgressBEATTune up: Minor PLL and FSS tuning, lowering beat note amplitude

### Adjustments to the BN amplitude: fixing saturation

I just checked the BN amplitude.  Its around 10 dBm (2.06Vpp) @ 170 Mhz which is way too high. This reduced down to about 6 dBm at about 59 MHz, not sure what changed.  Could have been the ISS or me changing the alignment somewhere in the beat board. I can see harmonics at x2 main peak, we are saturating.

Optical power measured just after the combining beam splitter on the beat board was 86 uW North and 95 uW South.  After the beam splitter and a OD1.3 ND filter the total combined power was reduced to 19 µW total on the BN detector. For beat note signal of

$\small BN = 20\log_{10}[2\eta RG\sqrt{P_NP_S}] - 10log_{10}[50 \Omega]+30 dBm$

where R = 0.8 A/W, G = 40e3, PN = PS = 10e-6 W, this would suggest an overlap effiecency of eta = 0.7. Ok I guess. However, RF power is too high for NF1811 detector,

I have changed ND filter to 2.0OD before the beat detector to bring the RF beat note power down to -3 dBm.  Its a little bit on the low side but plenty to lock the PLL. I didn't have a single ND filter handy between these values that was clean enough to use.  Stacking multipe ND filters is a bad idea, given we are concerned with scatter etc.

I also checked power incident of the cavities: south = 1.39 mW and north  = 1.17 mW.

### CCD cameras on the beat board

I inserted right angle connectors on the two CCD cameras to get the BNC cable out of the way of the NF1811 area. We need to angle the detector into the board so a black glass capture beam dump can be installed.  Right now the prompt beam from the NF1811 is reflected off the beat board and is not dumped.

### Relocking PLL with -3 dBm BN, adding PID to heater shield

For now we want to optimize scatter, we can address issues of PLL noise floor later.  With the lower beat note the SR560 preamp has been turned up to gain = 100 (low noise mode) with no filters engaged.  Marconi slope was turned back up to 10kHz/V, we are nowhere near the noise floor for this actuation range in the <3 kHz band so there is no point in lowering this slope yet.  The loop is the standard 1/f OLG shape with a UGF of 30.5 kHz and phase 60 degrees.  Phase drops off pretty quickly after this point with 90 degrees at 49.8 kHz and 180 just above 100 kHz.   This will  limit turning it up further I guess, but is not a bit issue for the main band of interest.

I added some soft channels to implement a PID loop on the North cavity heater.  I have switched out the bench top power supply for a variable heater driver I built last week.  The bench top power supply was driving the beat note all over the place.  When I pointed a heat gun at the bench supply for 30 seconds, the beat note frequency responded strongly, moving by 10s of MHz. Although this is less scientific than comparing sensors from the vacuum can and room temp (we are measuring this now), it is good enough evidence for immediately ditching what should have always have been a temporary solution.

I haven't documented this new heater driver on the elog yet. I will try to get to this in the next few days.  It is essentially a Sallen-Key 2nd order LP filter set at 159 mHz corner followed by an op amp buffer wrapped around a IRF630 MOSFET.  So similar to Kevin and Kira's heater driver (PSL:1903) but with an active LP filter.  I used FET input op amps and used a 10 Ω sensing resistor (20 ppm/K), so the conversion is 0.1 A/V.  We need to heat the north cavity with about 0.8 W to get a beat note <150 MHz.  North heater resistance is 158 Ω.

Craig's Marconi netgpib re ranging script keeps the PLL locked, but periodically adjusts the carrier frequency of the Marconi. The software carrier frequency change kicks the PLL every time it reranges, so its not ideal. Knowing the VCO slope setting and and actuation voltage applied to the Marconi, provides enough information to estimate the current BN frequency.  There is already a soft channel dedicated to this (being continuously logged). The plan is to implement a PID to pull the BN to a set point.  The PLL can then be locked at all times with large deviations handled by the Marconi locker script and a PID pulling the BN to a desired value. Playing around by hand, it is possible to hold the BN to a value around 70 MHz with just the north cavity heater driver.

I've updated the PLL medm screen and added a heater driver PID as a service to acromag1.  The only issue is that the Marconi software locker is run as a bash line script, rather than a service with epics channels to configure it. The PID is blind to whether the PLL is actually locked and will rail if the Marconi locker dies.  Its something that will need babysitting in case PLL, FSS or other loops die. Also there is a sqrt(2) error in the kHz/V conversion that needs fixing.

2015   Mon Dec 18 13:49:14 2017 awadeDailyProgressBEATTune up: Minor PLL and FSS tuning, lowering beat note amplitude

I've changed the NCav heater slider to units of Watts. Voltage applied to the heater driver is computed using a soft IOC channel. This is so we have an actuator slider that is linear injected heat power rather than quadratic.

However, when I rebooted the acromag1 IOC the channel didn't come up.  Craig had a look at the db file and found that I was using a calcout record for the intermediate power to volts conversion. To the error was that the power and voltage were both being written to the acromag which and output of zero volts regardless of the slider value.  Heater was down for about 60 minutes and has taken a long time get it back to <100MHz range of the south cavity.

2017   Mon Dec 18 15:45:26 2017 ranaDailyProgressBEATTune up: Minor PLL and FSS tuning, lowering beat note amplitude

best to detect all the optical power and just have a lower transimpedance gain somehow

using any ND filters or power attenuators is against the rules of low noise measurement

2018   Mon Dec 18 19:00:47 2017 awadeDailyProgressBEATTune up: Minor PLL and FSS tuning, lowering beat note amplitude

Yeah, it would be much better to lower the detector gain. As things are I think the dark noise we measured today on the NF1811 was on the order of 200-300 nV/sqrtHz. This was measured from low passed output of the level 13 mixer used in our PLL.  We might have been a factor of 2 off there from 50 Ω into SR785.  With the factor of two this would put us at 20-30 mHz/sqrtHz in our noise budget for the current preamp-Gain = 20, Marconi=10kHz/V configuration. That will not let us clear Brownian noise of 10 mHz/sqrtHz @ 100 Hz.

It looks like these 1811 have a fixed 7kΩ all-in-one transimpedance amplifier chip strait after the ac coupling capacitor.  So no option to modify that, unless there is a pin-for-pin replacement on the market.  Do we have any non-resonant ~100 MHz photo-detectors or designs that are good? We don't need it right, scatter hunting is the task at hand, but would be good to have something lined up to switch in, if we have it.

 Quote: best to detect all the optical power and just have a lower transimpedance gain somehow using any ND filters or power attenuators is against the rules of low noise measurement

2019   Tue Dec 19 10:26:47 2017 ranaDailyProgressBEATTune up: Minor PLL and FSS tuning, lowering beat note amplitude

Brittany & Jon have some holometer 1811 notes that would be helpful. I think they lowered the transimpedance there for this reason.

2020   Tue Dec 19 11:28:47 2017 awadeDailyProgressBEATTune up: Minor PLL and FSS tuning, lowering beat note amplitude

I have Brittany's notes. It looks like their main modifications were to the biasing of the PD (I think), and adjusting the AC coupling network -- including the inductor -- before the NE5210 7kΩ all-in-one transimpedance chip.  I'll put these notes into the PSL electronics section of the ATF wiki.

It doesn't seem like we have have access to transimpedance gain as a knob to turn without compleatly removing the first stage of the AC path and making some PCB 'art'. Maybe there is an equivalent chip with some gain selector pins out there...

 Quote: Brittany & Jon have some holometer 1811 notes that would be helpful. I think they lowered the transimpedance there for this reason.

2021   Tue Dec 19 14:30:05 2017 awadeDailyProgressBEATPID control of north cavity heater, up for 24 hours

I've implemented the PID control of the north cavity heater through the heater driver box.  Its been running for almost 24 hours continuously now. It takes the computed beat note frequency from the PLL carrier and control frequency to the Marconi and feeds back to the heater power IOC channel.

Physical parameters are:

North cavity heater impedance = 158 Ω

Gain heater box = 0.1 A/V with 159 mHz LPF

Mean heater level  = 0.75 W

Thermal response = TBC with a model and a step function response.

---

I've gently tuning the PID parameters to improve settling time and reduce oscillations in the beat note frequency. It seems to have a response on the order of 4-8 minutes.  Current settings are too slow in reaching a set point from, say, a 10 MHz change; but at least it doesn't push the PLL so hard it drops. Settings are:

P = -1.0000; I  = -0.00006; D = -0.00050

Setpoint = 50 MHz; Average heating power = 0.75 W

Wander about set point is less than 10.0 kHz at the moment -- equivalent to 10kHz/(155MHz/K) = 64 µK -- but we would like much less for the final measurement of Brownian noise with small VCO slope.

There is nothing particularly systematic about the way I came to these numbers. I started with a little bit of I and then added some P once it got close to the set point.  I added some D to help damp the overshoot, but its not clear if this objectively improved things much. Everything moves on such a slow time scale that its hard to see the whole picture of the loop dynamics without a systematic approach. We want something that reaches the set point much faster and with more dampening but it will do for now.  I'm just noting the settings here so there is something to go back to.

 Quote: I've changed the NCav heater slider to units of Watts. Voltage applied to the heater driver is computed using a soft IOC channel. This is so we have an actuator slider that is linear injected heat power rather than quadratic.  However, when I rebooted the acromag1 IOC the channel didn't come up.  Craig had a look at the db file and found that I was using a calcout record for the intermediate power to volts conversion. To the error was that the power and voltage were both being written to the acromag which and output of zero volts regardless of the slider value.  Heater was down for about 60 minutes and has taken a long time get it back to <100MHz range of the south cavity.

2025   Thu Dec 21 16:59:21 2017 awadeDailyProgressBEATTune up: FSS settings checkup

The values on the FSS sliders have been tweaked and changed over the last few weeks.  This wasn't done systematically so I've done a tune up so we have a checkpoint of some good parameters with known loop properties.

## North

North FSS + PID settings
Setting Value
Common gain 2.29899 V
Fast gain 1.71654 V
Offset -0.33850 V
Slow Freq offset 3.4597 V
Slow P 0.00217
Slow I 0.00026
Slow D 0.00158

This gave a common loop UGF of 177 kHz (-145 degree phase) and a cross over frequency of about .  Hear I've used the direct Out1/Out2 method for speed and convenience.

## South

South was at Common UGF of 80 kHz and a crossover of 6.2 kHz. New values are below.

South FSS + PID settings
Setting Value
Common gain 2.94295 V
Fast gain 2.26449 V
Offset -0.2095 V
Slow Freq offset -6.1263 V
Slow P 0.00217
Slow I 0.00026
Slow D 0.00158

This gave a common loop UGF of 128 kHz (-146 degrees) and a crossover frequency of 10.5 kHz (-104 degrees). Again, I've used the direct out1/out2 method.

Edit awade Fri Dec 22 16:11:49 2017: I've revised North common gain down to 1.93899 V as there appears to be some gain peaking around the higher PZT resonances (~200 kHz and 400 kHz), this gives a UGF of 162 kHz (phase -145 degrees). Offset adjusted to -0.31630 V.

Edit awade Fri Dec 22 16:20:14 2017: Had to back off the south common gain as well for the same reason.  Now set to 2.38295V, UGF of 115 kHz (-145 degrees).  Offset adjusted to -0.2250 V

2077   Wed Feb 7 17:37:10 2018 CraigDailyProgressBEATBeatnote Required

Some quick settings:

North visibility: 39%
Power on RFPD in trans: 200 µW (TOO MUCH)
Power on RFPD in trans with ND filter: 20 µW
Beatnote Freq: 17.945 MHz (Slewing down at 1 kHz/min)
Beatnote Strength: -1 dBm with ND filter on
South Slow Volts: -6.0375 V
North Slow Volts: 3.5596 V

Next step: Beatnote spectra, then vent and dump scatterers.

2085   Fri Feb 9 18:50:06 2018 awade, CraigDailyProgressBEATBN spectrum Feb 09, 2018

These measurments followed some dumping work around the refcav relfection photodiodes. See, PSL:2084.

## PLL

OLG was measured using out1/out2 method.  The G/(1+G) invertion to OLG was done directly on the SR785. Plotted in attachement 1.

## BN spectrum

Current beat note is at 17 MHz.  North cavity heater is set to 0.77320 W.

PLL settings are:
Marconi FM Devn: 10 kHz
SR560 Gain: 50
Beat note strength: -3.3 dBm

Calibrated BN spectrum is attached below, attachment 2

Data in third attachement and commited to the ctn_labdata git.

Attachment 1: PLLOLTF_09-02-2018_182645_TF.pdf
Attachment 2: 20180209_190340_CalibratedSpectrum_Spectrum.pdf
Attachment 3: 20180209_dailyBNSpectrum.tar.gz
2090   Wed Feb 14 02:21:55 2018 CraigDailyProgressBEATws3 is making beatnote ASDs every five minutes

Made crontab on ws3 make calibrated beatnote spectra every 5 minutes.  One example is below.

Plot is made in ~/Git/cit_ctnlab/ctn_noisebudget/dailyNoisebudgets/dailyNoisebudgetPlotter.py. Calibration is just the Marconi FM Deviation multiplied by the ASD.  Full calibration requires the whole PLL open loop gain.

Made a bash script nbPlotterRunner.sh which runs dailyNoisebudgetPlotter.py.  It runs the plotter, and can be called by crontab.

Overall Hz/rtHz levels seem okay.

Attachment 1: Beatnote_ASD_20180214_022002_ASD.pdf
2095   Sat Feb 17 00:23:38 2018 Craig, awadeDailyProgressBEATBeatnote Stabilization PID results from today

Top plot is smoking gun that awade's new PID beatnote controller works on a timescale of half a day.

I've been trying to relock the North path tonight, so I looked at the North slow volts to get an idea of where the resonance frequency should be.  I also plotted the beatnote for good measure, to see if there was any slewing going on.

Turns out, the nice damped oscillation in the beatnote frequency you see is due to awade's PID loop controlling the North cavity shield temperature this morning.  Pretty cool.

This is incredibly useful, because it means that when we get a beatnote again, once it settles down, we'll always be able to take low-range, low noise PLL measurements because the Ncav shield temp is being actuated to keep the beatnote from moving around.

It seems like awade can probably do even better, what with all the overshooting.  Get down from under-coupled to critically coupled PID control.

EDIT:  The NCAV Shield Heater PID loop settings were
P = 0.02500
I  = 0.00001
D = 0.00000

Attachment 1: Screenshot_from_2018-02-17_00-23-28.png
2118   Mon Mar 5 22:55:37 2018 Craig, awadeDailyProgressBEATNorth and South Cavities Relocked

While Craig was messing around on computers all day, awade got to work on the optics table aligning the North path.  He managed to lock the North at 60% visibility without even touching our new mode matching lens positions.  We think we can do better in the near future.

But while we had two TEM00 modes, we decided to get a betnote measurement.
Beatnote strength: +1 dBm (This is including an ND filter on our optics)
Beatnote frequency: 103.3 MHz

Also, apparently our new vaccan temp control PID script had a sign flip in it, so we've been heating our can maximally for a while today, up to 40 degrees C.  We fixed this, which will cause our can to go to 30 C and the beatnote will slew violently.  This made getting a beatnote ASD difficult.

Attachment 1: NorthCavityAndSouthCavityTogetherInPerfectHarmony.jpg
Attachment 2: SlowVoltsAndPIDSettings.jpg
Attachment 3: StitchedSpectrum_TransBeatnote_FMDevn_10kHz_SR560Gain_20_Avgs_20_Span_102p4kHz_05-03-2018_231449_Spectrum.pdf
Attachment 4: StitchedSpectrum_TransBeatnote_FMDevn_10kHz_SR560Gain_20_Avgs_20_Span_102p4kHz_05-03-2018_231449.tgz
2137   Thu Mar 15 15:58:02 2018 CraigDailyProgressBEATCavity Power fluctuations vs Temp Fluctuations

Btw when I propped up the vaccan using shims in preparation for venting the air springs, our old friend in the beatnote ASD reappeared: the broad hump from 100-10000 Hz.
This was a problem for us in Dec-Jan, but it went away and we really didn't understand why at the time.  Turns out it's probably upconversion of seismic activity coupling into our cavities.

 Quote: Our initial impression of cavity power fluctuations was that temperature fluctuations in the EAOMs were the cause.  To check this, I made some REFL DC monitors yesterday. Plotted is one hour of data from today.  Some notes: 1) TRANS DC and REFL DC for both cavites are breathing together every fifteen minutes, and are anticorrelated (one goes up, the other goes down).  2) The temperature monitors are not fluctuating with the same regularity as the power monitors. 3) The REFL DC for the north PMC is fluctuating with temperature This leads me to believe our power fluctuations are caused by changes of alignment into the cavities from the air springs holding up our vacuum can.  Right now, the air springs are hooked up to the wall.  There is probably some pressure regulator which switches on and off every fifteen minutes.  To fix this, I'm going to switch the vacuum air springs over to our nitrogen cylinder we have in the lab and see if the fluctuations go away.

Attachment 1: Beatnote_ASD_gpstime_1205189250.pdf
2151   Thu Mar 22 17:41:24 2018 CraigDailyProgressBEATBeatnote Spectrogram jupyter notebook

I've created a spectrogram (Fig 1) ipython notebook in ~/Git/cit_ctnlab/ctn_labdata/scripts/SeismicAndScatteringStudy.ipynb. I also attached a median beatnote ASD (Fig 2) for reference.
This is the beginnings of a study to look for coherence between our accelerometers and beatnote to figure out the velocity of our scatterers.

It takes a little while for nds2 to acquire the data from Gabriele's cymac3, because the data is sampled at 65536 Hz.  For 300 seconds of data it takes the script 172 seconds to retrieve the data.

I'm currently thinking about ways to make this spectrogram plot on the order of days as opposed to minutes so we can get a long-term idea about our beatnote.  Even looking at this, it seems like the scattering shelf does oscillate a bit at around 10 Hz.  Our 500 Hz hump seems pretty constant on this time scale as well.

Attachment 1: CTNLabBeatnoteSpectrogram_TimeLength_300s_gpsStart_1205785966.pdf
Attachment 2: CTNLabBeatnoteASD_gpsStart_1205785966.pdf
2153   Fri Mar 23 18:22:53 2018 CraigDailyProgressBEATFloated vs Landed Table Spectrum

I landed the table today to diagnose our TRANS and REFL power fluctuations and enable easier scattering searches.  (It seems that our TRANS and REFL power fluctuations have ceased, need more time to be sure, but this points to cavity misalignments from slight table motion causing the power fluctuations)

I noticed right away that the spectrum changed, in particular, the scattering shelf extended to even higher frequency.  I made a plot comparing the beatnote from just before landing the table and just after using ~/Git/cit_ctnlab/ctn_labdata/scripts/SeismicAndScatteringStudy.ipynb
Data GPSTimes for Floated Table = 1205884032 to 1205884332
Data GPSTimes for Landed Table = 1205888438 to 1205888738

Attachment 1: CTNLabBeatnoteASD_gpsStart_1205884032.pdf
2157   Thu Mar 29 02:51:35 2018 KojiSummaryBEATThe design for a new beat PD

DC photocurrent:
a few mA => A transimpedance of 1k Ohm will realize the output of ~a few V.
A few mA of DC current produces the shot noise photocurrent of 20~30pA/rtHz

RF photocurrent:
Resonant circuit resistance @25MHz: 100~300 Ohm.
Reduced to 100 by the shunt resistance of 150 Ohm
MAX4107 required minimum gain of 10 for the stability
-> Total Transimp. R=1K
-> Andrew said R=1K gives 13dBm output (1.4Vpk @25MHz)
-> This corresponds to 200 V/us of slewrate. This is ~40% of the full scale. (Too big)

input referred current noise ~ 12pA/rtHz ==> Shot noise intercept current ~0.44mA

If we reduce the shunt resistance to get the total transimpedance of 500Ohm
=> Beat output: 7dBm (0.7Vpk@25MHz, ~20% of the full slewrate)

input referred current noise ~24pA/rtHz ==> Shot noise intervept current ~ 1.8mA

Questions:

- Can we torelate this noise level (24pA/rtHz)?

- Or do we have MAX4106 (min gain 5), or something else for a replacement?

Attachment 1: PD_circuit.pdf
Attachment 2: PDmodel_CTN_25MHz_opamps_run5.pdf
Attachment 3: PDmodel_CTN_25MHz_opamps_run6.pdf
Attachment 4: PDmodel_CTN_25MHz_opamps_run7.pdf
Attachment 5: PDmodel_CTN_25MHz_opamps_run8.pdf
Attachment 6: IMG_3608.JPG
2161   Mon Apr 2 00:15:34 2018 ranaSummaryBEATThe design for a new beat PD

MAX4106 slew rate is 275 V/us, so almost half of 4107. LMH6611 is 460 V/us.

What about THS4271-EP ? (1000 V/us, can be used with a gain = 2, Vn = 3 nV, i_n = 3 pA)

 Quote: - Or do we have MAX4106 (min gain 5), or something else for a replacement?

Update: The "EP" model doesn't come in SOIC-8, so I just ordered the regular THS4271. It wants a 'PowerPad' heat sink, but we can try it without and see what happens as a test.

2162   Wed Apr 4 02:06:45 2018 KojiSummaryBEATFirst article of the CTN beat PD

Made the first article of the 25MHz resonant PD for CTN experiment.

Performance:

Resonant frequency f_res = 26.0MHz
Transimpedance R_res = 2.3kOhm
Input referred curent noise = 10pA/rtHz
Shot noise intercept current = 0.29mA

Remarks:

MAX4107 at G=10 gives a gain peaking at 260MHz. It is not tamed as the dark spectrum shows the peak does not have significant RMS.
But this might cause a trouble later. We can consider to replace the opamp with ones suggested by Rana. Rana bought "THS4271" to give it a try.

Attachment 1: PD_circuit.pdf
Attachment 2: CTN25_schematic_180403_KA.pdf
Attachment 3: CTN25_PHOTO.JPG
Attachment 4: liso_model.zip
Attachment 5: CTN25_transimpedance.pdf
Attachment 6: current_noise_CTN25.pdf
Attachment 7: idet_CTN25.pdf
2164   Thu Apr 5 02:04:49 2018 KojiSummaryBEATCompensation for MAX4107 at G=4.5

MAX4107 is stable only when G>+10. Because of this fact, the 25MHz PD has a too-high transimpedance for our purpose. To lower the gain without losing the stability of the amp, I have implemented a compensation network.

Attachment 1 shows the compensation network. This 10Ohm+100pF pair effectively makes the high freq noise gain higher (~10) while the low freq gain is G=4.5.

The actual implementation is found in Attachments 2 and 3. The overall schematic can be found in Attachment 5.
I also chose the resistor values so that their noise contributions are reduced.

The resulting output spectrum is compared with the one before the modification (Attachment 4).
Even with the lower gain, the gain peaking of the amplifier output is reduced.

LISO model (not shown here) indicates the input referred noise is ~1nV/rtHz. Considering the voltage division by the 50Ohm termination, the output voltage is as low as 2.25nV/rtHz. This is not a high number. Thus, we practically need a mid power / low noise amplifier attached to the output of the unit.

The total performance of the PD unit will be tested again shortly.

Attachment 1: max_circuit.pdf
Attachment 2: IMG_3667.JPG
Attachment 3: IMG_3668.JPG
Attachment 4: IMG_3665.JPG
Attachment 5: CTN25_schematic_180404_KA.pdf
2165   Fri Apr 6 04:48:02 2018 KojiSummaryBEATCompensation for MAX4107 at G=4.5

Performance of the modified photodetector unit:

Performance:

- Resonant frequency f_res = 26.0MHz
- Transimpedance gain at 26MHz is 1.27kOhm (560Ohm resistance of the resonant circuit x G=4.5 x 1/2 by 50Ohm termination)
- Input referred curent noise = 9pA/rtHz
- Shotnoise intercept current is 0.24mA

Remarks:

- There is a gain peaking at 280MHz as explained in the prev elog. If one does not like this, remove 700Ohm resistor from the max4107 stage. It will increase the amplifier gain from 4.5 to 5, and the gain peaking is reduced.

- The transimpedance gain might be still too high. Then, a shunt resistor at the location of R24 can be added to the resonant circuit. This way, the shunt resistance is seen only from the RF path. Of course, lowering the signal level has to be paid by the increase of the input referred current noise level.

Attachment 1: CTN25_transimpedance.pdf
Attachment 2: current_noise_CTN25.pdf
Attachment 3: idet_CTN25.pdf
2173   Tue Apr 17 22:11:34 2018 awadeDailyProgressBEATSome final measurments before switching out NF1811

Note: I tried to attach the plot as a pdf but there is something wrong with backend processing this particular pdf and I'm getting a 502 error.  Have attached as a zip and a png

I thought it would be a good idea to document the current state of the BN spectrum before switching out the NF1811 post cavity BN detector for Koji's newly modified resonant detector.  The new detector has an area of 2 mm x 2 mm, much bigger than the 0.3 mm x 0.3 mm of the current configuration.  The larger PD means we can increase the beam size and hopefully reduce the scatter in the vicinity of the beat setup.

To compare apples with apples, I refloated the table. Also, I turned off the hepa fans around the  lab.   The current BN is 26 MHz and PID locked using the pre-cavity BN detector (Kp = -0.005, Ki = -0.000001).  It still hadn't settled from some disturbances earlier in the day, so was oscillating around ±200 kHz range about the set point. I didn't have time to wait, so I took the measurement with Marconi slope set to 10 kHz.  PLL bandwidth was ~16 kHz with SR560 gain of 100 and Marconi on slope 10 kHz/V.  In this range previous benchmarks of Marconi noise (see PSL:1588) would suggest that the PLL noise limit is 30 mHz/rtHz.  Therefore, PLL shouldn't be the limiting noise at the moment, notwithstanding PD dark noise (which isn't really quantified in the noise budget).

When I took the first measurement I found that the some of the scatter features had been improved since the last measurement (see attached figure razor blade in trace).  The noise floor seemed to be around 0.2 Hz/rtHz with a roll up at lower frequencies.  I realized, however, that I had left the razor blade profiler from the previous day's measurement (PSL:2172) in place, just above the beam.  I retook the measurement (see figure razor blade out trace).  There were a number of peaks that popped up broadly around 70 Hz, 180 Hz and 400 Hz.  Its likely that the razor blade was deflecting and dumping some of the light that has been causing these bumps in our spectrum.

I was a little suspicious of how flat the spectrum was above the characteristic scatter hump (up to 200 Hz).  In three separate measurements I blocked the beam from the BN detector, 50 Ω terminated the PLL at the BN input and then 50 Ω terminated at the ADC input .  These traces are also included in the figure below.  It shows that the apparent limit to sensitivity is the dark noise of the NF1811 photodetector.

It should be noted that the power has been attenuated onto the NF1811 from ~ 1 mW by 3.0 OD neutral density filter down to 1 µW.  Any power much above this generates harmonics, that we are trying to avoid. So it seems that with improved dumping around the NF1811 detector we can reduce a number of mechanical resonances scatter peaks but that there is a limit to the amount of signal to noise because we are likely to saturate the RF AC path stages inside the NF1811.  Im not sure if this is the right way to calculated but dark noise (NEP) is 2.5 pW/rtHz with 0.75 A/W respositivity and 40kΩ gain PD gain the output noise of the detector is 75 nV/rtHz, For a 16 kHz loop bandwidth (preamp gain 100, VCO slope 10 kHz) the total gain, not including the mixer, is 1e6 which would place the noise floor at about 0.1 Hz/rtHz.  I need to work through exactly the right way to calculate this that treats the noise in a physically correct way at the mixer.

So it seems that this measurement is getting closer to our previous best measurements. Data and plotting is attached in a zip below as well as being committed to the git.ligo.org ctn_labdata repository.

Attachment 1: plot20180417_ComparisonBNToNoiseFloors.pdf.zip
Attachment 2: 20180417_CompairNF1811ToNoiseFloor.zip
Attachment 3: plot20180417_ComparisonBNToNoiseFloors.png
2175   Sun Apr 22 17:46:55 2018 awadeSummaryBEATCompensation for MAX4107 at G=4.5

For reference I have attached original schematic to this post.  Koji sent me the link to the original document that can be found here: https://labcit.ligo.caltech.edu/~rana/dale/Length_Sensing_and_Control/LSC_Photodiode/Version_B/D980454-01.pdf

This version of the document includes page 1 with the pinouts for the power connector.

 Quote: Performance of the modified photodetector unit: Performance: - Resonant frequency f_res = 26.0MHz - Transimpedance gain at 26MHz is 1.27kOhm (560Ohm resistance of the resonant circuit x G=4.5 x 1/2 by 50Ohm termination) - Input referred curent noise = 9pA/rtHz - Shotnoise intercept current is 0.24mA Remarks: - There is a gain peaking at 280MHz as explained in the prev elog. If one does not like this, remove 700Ohm resistor from the max4107 stage. It will increase the amplifier gain from 4.5 to 5, and the gain peaking is reduced. - The transimpedance gain might be still too high. Then, a shunt resistor at the location of R24 can be added to the resonant circuit. This way, the shunt resistance is seen only from the RF path. Of course, lowering the signal level has to be paid by the increase of the input referred current noise level.

Attachment 1: D980454-01.pdf
2177   Tue Apr 24 14:37:39 2018 Anchal and AndrewDailyProgressBEATBeam Profiling Beatnote detector

Today, we did the beam profiling for the beatnote detector just before the photodiode. I have attached the data taken. The z values mentioned are from a point which is 2.1 inch away from a marked line on the stage.

However, the analysis concludes that either the beam radius changes too slowly to be profiled properly with given method of measurement or something else is wrong. Attaching the the z vs w(z) plot from this data and few fit plots.

Attachment 2: Beam_Profile_Beatnote_-_Sheet1.csv
Ref Z: 2.1 inch,,,
Z-position (mils),Edge X Value (mils),Reading (mV),
-500,175,11,11
-500,180,11,11
-500,185,12,12
-500,190,15,16
-500,195,22,25
-500,200,39,42
-500,205,67,70
-500,210,99,105

... 175 more lines ...
Attachment 3: fitatz500.png
2178   Wed Apr 25 12:25:09 2018 awadeDailyProgressBEATBeam Profiling Beatnote detector

Please don't post plots in png, vector graphics only, preferably pdf with the correct transparency in the background. Here a note on plotting that summarizes some common sins: ATF:2137

Also SI units only.  Sometimes technical drawings and other commercial technical documents and quotes are in limey units but we don't use them in the lab.

I can't really tell what is going on because of the weird units, but it looks like there isn't enough propagation distance for any meaningful change in the beam size.

You can make a prediction of the expected beam waist size from the cavity waist (~180 µm) and by measuring the beam propagation path and taking into account the lens at output of the vacuum can. By propagating the Gaussian beam through the lens and along the beam path of the beat setup on the output you can make some predicted beam radius to compare to your measurements (in SI units, of course).

 Quote: Today, we did the beam profiling for the beatnote detector just before the photodiode. I have attached the data taken. The z values mentioned are from a point which is 2.1 inch away from a marked line on the stage. However, the analysis concludes that either the beam radius changes too slowly to be profiled properly with given method of measurement or something else is wrong. Attaching the the z vs w(z) plot from this data and few fit plots.

2179   Thu May 3 22:05:23 2018 awade and anchalDailyProgressBEATBeam Profiling Beatnote detector

Today we made a new mount to be able to profile the laser beam for longer distances on the table.

Attachment 1: 20180503_193329.jpg
2180   Wed May 9 00:51:45 2018 anchalDailyProgressBEATBeam Profiling Beatnote detector

PFA the results of beam profile analysis of transmitted laser from south cavity.

Description:

We are profiling the transmitted laser beam from the south cavity. All measurements of z-direction are from a reference line marked on the table. A razor blade mounted with a micrometer stand is used to profile the beam. The razor moves in the vertical direction and the whole mount is fixed using holes on the optical table so it moves in steps of 25.4 mm.

The beam is first split using a beam splitter and the other port is used as witness detector. The mean value of voltage from the photodetector over 4s time is normalized by the witness detector mean value to cancel out effects of laser intensity fluctuations. The peak to peak voltage from PD over 4 s is used to estimate the standard deviation of the signal. I assumed the error to be sinusoidal and estimated standard deviation as 1/sqrt(8) times the peak to peak voltage.

The profiles at each z point is then fitted with A*(0.5 - erf(norm_x)) + C where norm_x = (x - mu)*np.sqrt(2)/w . This gives estimates of beam radius w at each z position. This data is then fitted to w0*np.sqrt(1 + ((z-zc)*1064e-6/(np.pi*w0**2))**2) to estimate the beam wasit position and wasit size. I have also added the reduced chi-square values of the fits but I'm not sure how much it matters that our standard deviation is calculated in the manner as explained above.

Attachment 1: FitGraphs.pdf
2181   Wed May 9 08:56:54 2018 NeoDailyProgressBEATBeam Profiling Beatnote detector

2182   Thu May 10 18:18:06 2018 AnchalDailyProgressBEATBeam Profiling Beatnote detector

Today I took more measurements after reflecting off the beam by 90 degrees to another direction and using the Beam Profiler Dataray Beamr2-DD. I used the InGaAs detector with motor spee dof 11 rps and averaging over 100 values.

Following is the fit with and without the new data taken. Data1 in the graph is the earlier data taken using razor blade and Data2 is the data taken today using beam profiler.

The two fits estimate same waist positions and waist sizes within error bars of each other. However, the reduced chi-square is still pretty high.

I've also added the data file and code in the zip.

Attachment 1: ModifiedFitWithNewData.pdf
Attachment 2: DataAndCode.zip
2183   Sun May 13 21:18:55 2018 awadeDailyProgressBEATReinstalling beam splitter and beat detector

I reinstalled the beam splitter, steering mirror and PD in the transmission path.  We have the beam profiling measurement and can reposition the detector more optimally in the near future.  But for now we need to get back to the scatter hunt: specifically the 500 Hz feature that haunts the area directly around the north path pre-mode cleaner (PMC).

Some of the beam dumps were removed as they weren't bolted down or were in the way of the beam profiling (sorry Craig).

A few things to note:

• I have now permanently removed the black cube steering mirror mount from the beat setup.  There are visable damage marks on the mirror that make it a dubious choice.  Also the mount just has too much close proximity surfaces on the reflection side that can be potential scatter points.
• I replaced the south path steering mirror (previously the black cube beam mount) with a Newport 10Q20HE.1 mirror.  The surface specs are ok 10-5 scratch dig and better than lambda/10.  Its optimized to be over 99% on both polarizations but not technically ultra good HR. We're out of the really good HR reflectors so I might be time to buy some more
• After a bit of checking around I've concluded that the first beam splitters directly after the cavity are actually 50:50 splitters optimized for 45 deg on s-pol. The reflection is used to generate the beat and transmission of these splitters is used for ISS trans PDs and CCD cameras.  There is a 99% 45 s-pol splitter after the 50:50 splitters that picks most of the light off for the ISS PDs.  Its seems like we are wasting a lot of light going to the ISS here and should actually be using p-pol to get critical coupling into the InGaAs diodes.
• We're still using two separate raised breadboards here in this optical setup.  I couldn't find any larger 3' by 1' boards in the Bridge West labs, might need to make a purchase to get this together all on one board.
• I looks like John Martin will be using the ISS from the ATF lab in his SURF project.  I need to get onto getting more stuff fabricated so we can build the ISS better. Maybe a good task for Anchal would be to make a SolidWorks assembly of the new board to see how it will fit together.
• There are still a few 1/4" adjustble hight mounts in the ISS part of the board.  These should be replaced with 1" mounts as we now have them in stock.
Attachment 1: IMG_3368_DamageOnSouthBeatSteeringMirror.JPG
Attachment 2: IMG_CurrentBeatBoardLayout.JPG
2184   Mon May 14 09:25:44 2018 CraigDailyProgressBEATReinstalling beam splitter and beat detector

If I recall correctly, I placed those beam dumps everywhere because the transmission table ghost beams had gotten out of control.  Particularly bad was out of the cavity, it seems you have left those beam dumps there but I would double check their status.  There was also reflection from the first lenses back into the cavity that I tried to have the post-cav beam dumps block, unclear how successful I was.

Also bad was reflection from the ND filter we had on our pre-RFPD lens, it seems like you have removed this lens which is great.  Also, there was some backscatter from the third camera I had installed at the other end of the recombination BS that unites the North and South path light, seems like that is gone as well, you might dump that light if you aren't already.

Finally, the scatter from the IR cameras was like a diffraction grating pattern that went everywhere, it might be prudent to place those black glass beam dumps with the holes in front of them.

 Quote: I reinstalled the beam splitter, steering mirror and PD in the transmission path.  We have the beam profiling measurement and can reposition the detector more optimally in the near future.  But for now we need to get back to the scatter hunt: specifically the 500 Hz feature that haunts the area directly around the north path pre-mode cleaner (PMC).   Some of the beam dumps were removed as they weren't bolted down or were in the way of the beam profiling (sorry Craig). A few things to note: I have now permanently removed the black cube steering mirror mount from the beat setup.  There are visable damage marks on the mirror that make it a dubious choice.  Also the mount just has too much close proximity surfaces on the reflection side that can be potential scatter points.   I replaced the south path steering mirror (previously the black cube beam mount) with a Newport 10Q20HE.1 mirror.  The surface specs are ok 10-5 scratch dig and better than lambda/10.  Its optimized to be over 99% on both polarizations but not technically ultra good HR. We're out of the really good HR reflectors so I might be time to buy some more After a bit of checking around I've concluded that the first beam splitters directly after the cavity are actually 50:50 splitters optimized for 45 deg on s-pol. The reflection is used to generate the beat and transmission of these splitters is used for ISS trans PDs and CCD cameras.  There is a 99% 45 s-pol splitter after the 50:50 splitters that picks most of the light off for the ISS PDs.  Its seems like we are wasting a lot of light going to the ISS here and should actually be using p-pol to get critical coupling into the InGaAs diodes. We're still using two separate raised breadboards here in this optical setup.  I couldn't find any larger 3' by 1' boards in the Bridge West labs, might need to make a purchase to get this together all on one board. I looks like John Martin will be using the ISS from the ATF lab in his SURF project.  I need to get onto getting more stuff fabricated so we can build the ISS better. Maybe a good task for Anchal would be to make a SolidWorks assembly of the new board to see how it will fit together. There are still a few 1/4" adjustble hight mounts in the ISS part of the board.  These should be replaced with 1" mounts as we now have them in stock.

2185   Tue May 15 14:24:25 2018 awadeDailyProgressBEATReinstalling beam splitter and beat detector

When I orient the beam into p-pol I noticed that the beam splitters give a lot of secondary reflections.  All these mirrors need to be switched out for p-pol optimized splitters and steering mirrors.

We'll try to get all these beam dumps back in and bolted down soon.

 Quote: If I recall correctly, I placed those beam dumps everywhere because the transmission table ghost beams had gotten out of control.  Particularly bad was out of the cavity, it seems you have left those beam dumps there but I would double check their status.  There was also reflection from the first lenses back into the cavity that I tried to have the post-cav beam dumps block, unclear how successful I was.  Also bad was reflection from the ND filter we had on our pre-RFPD lens, it seems like you have removed this lens which is great.  Also, there was some backscatter from the third camera I had installed at the other end of the recombination BS that unites the North and South path light, seems like that is gone as well, you might dump that light if you aren't already.  Finally, the scatter from the IR cameras was like a diffraction grating pattern that went everywhere, it might be prudent to place those black glass beam dumps with the holes in front of them.
2187   Tue May 15 16:53:36 2018 anchalNotesBEATAnalysis for idea of sending beam at Brewster angle to Photo diode

Recently we were given the idea of sending the beam to the photodiode at Brewster angle. If we do so, ideally one particular polarization (parallel to the plane of incidence) will not reflect back. So if we send the beam polarized in this direction (or set the incidence plane such that these conditions are matched), we can minimize the reflection from PD significantly.
Sounded like a good idea, so I started reading about the InGaAs detector we have. Unfortunately, the datasheet of the C30642 detector we use does not mention either the fraction of In in InGaAs or the refractive index of it. So I went into the literature found these two papers:
Kim et al. Applied Physics Let Vol 81, 13 23 (2002) DOI: 10.1063/1.1509093
Adachi et al. Journal of Applied Physics 53, 5863 (1982); doi: 10.1063/1.331425
Using the empirical coefficients and functions from these paper, I calculated the refractive index for InGaAs for various fractions of x and the corresponding Brewster angles (Find Attached).
However, just after doing the analysis, we realized that doing this is not really possible. The Brewster angle is arctan(n2/n1) where n2 is the medium light is going into. This implies the Brewster angle would always be greater than 45 degrees and detector won't really absorb much light at this angle. So currently the conclusion is that this idea won't work.
However, there might be some error in our assumption of InGaAs as a transparent medium as the calculations do not take into account absorption of the photon at all. Attaching the python notebook too in case someone figures this out.

Attachment 1: InGaAsIncidenceAngleAnalysis.pdf
Attachment 2: InGaAsIncidenceAngleAnalysis.zip
2188   Tue May 15 18:19:05 2018 KojiNotesBEATAnalysis for idea of sending beam at Brewster angle to Photo diode
2189   Wed May 16 10:40:06 2018 anchalDailyProgressBEATInstalled Beatnote detector with known beam width at the point

Using the result of the beam profile analysis, I calculated the required position and lens to put in to decrease beam width to about 300 um at the RFPD (1/3 of detector size 2mmx2mm). Attached matlab code for the same. I used a 687.5 mm Fused Silica lens for this purpose. Finally, I installed the beatnote detector after taking beam profiles with the beam profiler and finding the right position. Attached is the screenshot of the beam profile at this position shows that beam has a width of ~300 um. I have positioned the RFPD such that beam is incident at 30 degrees from normal on it.

Attachment 1: Images.pdf
Attachment 2: RFPDpositioning.zip
2230   Fri Sep 7 23:23:00 2018 awade, anchalDailyProgressBEATBack to beat

Today we go the beat to converge on the 26 MHz resonance of the resonant transmission beat detector.  Anchal and I took a TF of the PLL and found the UGF to be 40 kHz, sufficient for a BN spectrum.  Beat not power was about -1 dBm.

Sorry no beat spectrum, we lost lock with the south path autolocker turned off and the cavity heat PID didn't realize and knocked the BN way off.  Going to take another 12 hours for it to settle down again.

It looks like the spectrum was about 0.1 Hz at its lowest point (>3 kHz).  We saw that there was a very prominent narrow peak at about 2.8 kHz.  We had a look at the mixer monitor signals and saw that the narrow peak was coming from the south path FSS.  After doing a cross spectrum between the beat and the north and south FSS it became apparent that a significant portion of our noise is coming from the south FSS somehow.  We need to track down exactly what would be coupling in a 2.8 kHz narrow peak and also the broader noise of the south FSS.

I suspect​ it might be a photodetector​ issue.  We had excess noise is one path relative to the other before and swapped detectors and field boxes.  We never quite diagnosed it but it could well be that the 14.75 MHz tuned detector that is now in the south path is responsible.  The first thing to check is the TF by injecting into the test port.  Also the dark noise should be checked around 14.75 MHz.  The optical TF can also be taken with the Jenny rig at the 40m.  Now might be a good time to switch modulation frequencies to 36 MHz and 37 MHz.  These are fine for HOM spacing and we have the Wenzel crystals ready to go.  We also have two detectors tuned to 35.5 MHz sitting on the shelf and BB EOM drivers that just need stuffing onto boards.  This might take Anchal a week to do, he seems to be good with electronics.

Another unresolved problem is the residual AM from the 14.75 MHz phase modulators.  I was never really able to reduce this down and keep it down.  Thermal or alignment drift seemed to make it really hard to minimize.  It could be bad alignment though the crystal or thermal drift.  They have insulating hats on but they still are less than optimal.  An ISS will do a bit to suppress noise opened up by this effect but we would like to solve it properly.

2232   Wed Sep 12 16:27:19 2018 anchalDailyProgressBEATBack to beat

Attached are transfer function measurements of the North and South Cavity Reflection RFPD (14.75MHz resonant RFPD) along with dark noise around 14.75 MHz.

The transfer functions are measured by injecting into the test in port and reading out from RF port at -15dBm source power. The noise spectra are measured by shorting the test in port and taking spectrum from RF port when the detectors are on. In both measurements, the photodiode is blocked with a beam dump.
These measurements were required because of the conclusions made in PSL:2230. Indeed as suspected, the south path resonant RFPD measuring reflection of the cavity at 14.75 MHz has a ~100 times weaker response than the north counterpart as seen in the attached plots. Since the dark noise of south RFPD is about half of the noise of north RFPD (see plot 2), it suggests that south RFPD circuit itself is not working properly and is not amplifying the signal enough. Andrew mentioned that he and Craig saw this earlier and decided to shift FSS to higher frequencies with crystal oscillators. We have the OCOX preamp for 36 MHz and 37 Mhz ready to go with RFPDs at 35.5 MHz that can be tuned to these frequencies. So future steps are to replace the RFPDs with these 35.5MHz ones and tuning them to 36 MHz and 37 MHz and putting in broadband EOM driven by resonant EOM drivers at these frequencies. See future posts for updates on these steps.

Edit:[09/14/2018, 16:12] Changed plots to physical units. Used 2k Transimpedance for Bode Plot and 2.5kHz bandwidth (801 points in 2MHz) for noise plots.

Edit:[09/22/2018, 10:12] Added how measurements were taken, the reason for them and some conclusions. I'm getting into the third year now!

Attachment 1: NorthandSouth_14750kHz_RFPD_Measurements.pdf
2233   Fri Sep 14 00:57:14 2018 ranaDailyProgressBEATBack to beat

please replace TF and noise plots with ones that have physical units on the y-axis: V/A for the Bode plot and W/rtHz for the noise plot

2237   Fri Sep 21 18:10:53 2018 ranaDailyProgressBEATBack to beat

I don't understand what that means. Please provide 10x more details on how the measurement was made.

Also, clearly one of these traces is not like the other. What does that mean ???

 Quote: Attached are transfer function measurements of the North and South Cavity Reflection RFPD (14.75MHz resonant RFPD) along with dark noise around 14.75 MHz. Edit:[09/14/2018, 16:12] Changed plots to physical units. Used 2k Transimpedance for Bode Plot and 2.5kHz bandwidth (801 points in 2MHz) for noise plots.

2355   Thu Jun 6 18:14:17 2019 anchalDailyProgressBEATMoku Lab Frequency Noise Analysis

I spent some time understanding moku and even though it has some flaws (like no scriptable channel for recording data), it seems like using the Phasemeter instrument in mokulab will get rid of all of our PLL problems.

### Measurement details:

• I connected a low noise Wenzel Crystal Oscillator of frequency 24.483 MHz in the input port of mokulab.
• Using its phase meter app which has a maximum tracking bandwidth of 10kHz, we can directly record time series data of frequency and phase at a maximum acquisition rate of 125kHz.
• If we set acquisition rate lower than 500 Hz, we can also see a real time frequency noise ASD through the iPad. This is good for day to day checking.
• Mokulab records data so fast as its FPGA fills up the RAM directly. I was able to record 100s of data without reaching the limit of RAM memory.
• Then, one has to transfer the data to from RAM to SDCard or Dropbox or iPad. Since I have a laptop with SDcard reader, I'm just using that.
• The files are in binary format and we need to use their executable software to convert it into .csv or .mat. This software is not compiled for linux so we need to use our laptops for this step too.
• This generates a very large file. I have written a python code which uses scipy.signal.welch to calculate PSD of the frequency data. I need comments on if this is a good function to use. The code is int he attached .zip file.
• This code, mokuReadFreqNoise.py also uses this psd calculation multiple times to have 90 datapoints in each frequency space decade. For higher frequencies, the scipy.signal.welch function uses the time series in segments and averages as well.
• I also used the data from CTN:2286 of our present marconi in PLL when instead of beatnote, the same crystal oscillator was at the measurement end.

### Implications:

• I'm unable to find datasheet of the model number of the crystal oscillator we have but all of them have less than -165 dBc/Hz Phase Noise at 1 kHz offset from the carrier.
• I'm not very sure how to translate that into frequency jitter noise.
• But assuming (and mostly this assumption is very good) that this OCXO is much better than the synthesized frequencies of Moku or Marconi, what I am measuring in these experiments is actually frequency noise of the devices themselves.
• And anyway, both measurements were taken essentially in the same way. The comparison shows that Moku Lab is doing much better in measuring frequency noise. So if we can keep the frequency drift of BN less than 10kHz/100s which is not too difficult and has been achieved with our modern day not so good temperature control.
• When using external reference 10 MHz Rb clock, the noise in Moku is way lower than our expected coating brownian noise. So if the rest of the experiment works, moku would not give us problems of any noise floor in measurements.

Since it doesn't hurt:

Code and Data

Attachment 1: MokuFrequencyNoiseAnalysis.zip
Attachment 2: MokuFrequencyNoiseAnalysis.pdf
2356   Fri Jun 7 17:42:06 2019 anchalDailyProgressBEATThree Corner Hat Method

A wise man told me to use three corner hat method to extract individual frequency noise information of Marconi, Moku and the Wenzel crystal. I updated my mokuReadFreqNoise.py to support frequency noise calculation for two channels and their difference.

I'm perplexed to see the result actually and I'm not sure if this is what was expected.

### Measurement details:

I did two identical runs (I wasn't sure if I was seeing the truth from my first run) with the following settings:

• Both Marconi and Moku are directly connected to Rb 10 MHz clock with equal length cables.
• Wenzel Crystal 500-13905 oscillating at 24.4835 MHz was connected to input 1 of Moku.
• Marconi with set to same frequency was connected to input 2 of Moku. Note that modulation feature was not on in this experiment so the expected noise is lower than CTN:2286 experiment.
• With two channels recording, the acquisition rate is 15.625 kSa/s only.

### Analysis:

• In the first plot, I just plotted the frequency noise of input 1 (Wenzel), input 2 (Marconi) and (input 2 - input 1) using mokuReadFreqNoise.py. Although I have checked this code multiple times, I really want a new set of eyes to go through it and confirm it is calculating this correctly.
• I assumed that the difference between two channels will have negligible frequency noise of Moku itself and is a good approximation of frequency noise between Wenzel and Marconi.
• In the second plot, I used the 3 corner hat method to calculate individual frequency noise ASDs of the three instruments. Some points are missing as the sum of two contributing PSDs was lower than non-contributing PSD at some points.
• In the third plot, to disregard the assumption I have made above, I used data from CTN:2286 experiment. Remember though that in this experiment, modulation was on at 500 Hz/V actuation slope.
• In the fourth plot, I used 3 corner hat method again but with CTN:2286 experiment data.

### Implications:

• Well clearly the 4th plot is useless. It is comparing two different versions of Marconi data, so it is essentially blurring out all data.
• From 2nd plot, if this experiment was meaningful, even though Moku seems an order of magnitude more noisy than Marcon (which is just freely running), Moku's frequency noise is less than 2 mHz/rtHz upto 400Hz and if we ignore the bump at 500 Hz as some experimental artifact, is less than 3 mHz/rtHz upto 1 kHz. Expected coating brownian noise is between 4-10 mHz/rtHz from 100Hz-400Hz region.
• Ideally, we would like an order of magnitude lower instrument noise than what we are trying to measure. So maybe Moku is not a good choice.
• But I still am not sure if I should take strong inferences from this experiment because when I do low acquisition and use Moku's inbuilt frequency noise ASD plotter, I get results as attachment 3 which is also a check of how good my code computes the ASD.
• This graph shows that the difference spectra is growing above 100 Hz contrary to previous results. This would mean that the frequency noise of moku is least above 100 Hz.  But that is not the case when I do a fast acquisition.
• So overall, after today's efforts, I'm back to square one. I'm not sure if Moku should be used for frequency noise measurement
Attachment 1: MokuFreqAnalysis3CorHat_Run1.pdf
Attachment 2: MokuFreqAnalysis3CorHat_Run2.pdf
Attachment 4: MokuFreqAnalysis3CorHat.zip
Attachment 5: IMG_20190606_192741.jpg
2357   Mon Jun 17 18:05:20 2019 anchalDailyProgressBEATMoku Frequency Noise Measurement

I thought, why not just measure Moku's own synthesized frequency output with itself. Attached plot shows the frequency noise measured this way. The measured noise is divided by the square root of 2 to get individual input referred noise of the phase meter.

Then I thought, maybe the phase noises in the synthesized frequency and phase noise of phase meter could be potentially canceling each other as they are coming through the same source. So I inserted a very long cable between output of Moku and it's phasemeter input so that the two things are time separated by at least 1 cycle of 27.34 MHz. I didn't want to open up and measure the length of these neatly packed SMA cables but they are surely more than 6.58 m (wavelength of 27.34 MHz in these cables) long together.

### Conclusions:

• I do not see any reason why this measurement would not be correct unless the correlation length of phase noise in time is really long and I need a cable which separates the input and output of Moku by many cycles of 27.34 MHz.
• So under the above assumption, Moku's phasemeter input referred frequency noise is less than 1mHz/rtHz up to 4 kHz and is less than 0.1 mHz/rtHz up to ~250 Hz.
• If these numbers are correct, we should be good to go with using Moku as our phasemeter instead of our clunky PLL.
Attachment 2: MokuSelfFreqNoiseAnalysis.pdf
2399   Fri Aug 23 18:15:49 2019 anchalDailyProgressBEATBeatnote after a while

The cavity temperature control (aftter the last fixes by Andrew) seem to be working good actually now that the Vacuum Can temperature is stabilized nicely. SO I didn't want to interfere with the PID's job which it seems is trying to reach to the set point almost critically. However, today, the beatnote came below 125 MHz, so we were in range with New Focus 1811 to take the spectrum. So I did it.

### Two measurements

I used the coupled output from 20 dB coupler to feed the moku and use it's phase meter along with SR785 witht he previous PLL setup. Since the beatnote was still drifitng by around 10 kHz/24 sec, I took spectrum with linewidth of 1 Hz and used 20 averages to catch the PLL frequency noise in between its jumps. Simultaneously (almost), I took measurements with moku also to see if we can reliably switch over to moku. Good thing about moku is that it is faster in adjusting it's carrier frequency to lock to the signal and hence the jumps are unnoticeable. The attached plots are the measurements.

### Uncertainty in moku's ASD plots!

Scott and I have written a modified PSD calculation function, which does everything same as a normal weltch function would do, but on top of it, it provides 15.865% and 84.135% percentile of all the individual segments the function used to calculate PSD. Also, the reported value is median and not mean. Further, this function implements welch function with different sizes of npersegment to ensure more averaging at higher frequencies and equal number of points in each decade. All this is done in mokuReadFreqNoise.py which uses modeifiedPSD.py. Linear detrending of data is also used before calculating the PSDs from the timeseries data provided by moku.

### Conclusions

• I think we can safely switch over to using Moku for measuring beatnote frequency noise, given it is available.
• Beatnote obviously doesn't look so good. But at the point, the FSS aren't working as expected.
• There is a weird peak at around 500 Hz which wasn't there before.
• I'll add the noisebudget with new calculations using different values of Shear and Bulk Loss Angles soon. It is kind of difficult to get these values though.
• My plan is to keep this functioning state all the time. I'll make sure the cavities are locked with good mode matching and near the desired beatnote frequency.
• Then I'll focus on the known issues and sources of error and will keep monitoring the beatnote changes every 2-3 days.

Code and Data

Attachment 1: ComparisonOfMokuandSR785BeatnoteSpectrum.pdf
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