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ID Date Author Typeup Category Subject
  1064   Tue Apr 15 22:12:39 2014 DmassDailyProgressLab WorkProgress

[Dmass, Evan]

I spent a day trying to find the body modes in-air, and was not successful; the room temperature beat noise is ~100x worse than the in-vac noise. I spent most of my time trying to figure out why the beat was so noisy in air, and decided to just pump down.

We closed both seals on the vacuum chamber (Indium + Rubber), and are trying to recover alignment.


Alignment progress:


I've got about 40% transmission through the west cavity, judging by the refl DC signal. I'm not sure what the mode is because I can't see it on the camera. It does show up on the trans diode though.

However, the pitch is extremely low. I had to lower the two input waveplates by 0.25" in order to get the beam to clear them. It's very low on the window as well.
I suspect this is a low-order mode, and the transmission is too low in pitch to make it onto the camera. I haven't touched any of the transmission optics.
This seems odd, because if anything the beam should be higher, right?

I locked to the mode you were talking about and also couldn't see it on the Camera (I didn't check with a card+viewer) - this means it's somewhere above 7th order, or the new beam is off the camera but on the PD (possible)

The alignment could have changed - either the springs could have moved a tiny bit (the Cu wool inside them can give them something of a slip/stick behavior when stretching out, so their length under load can change a little bit), or the balancing could be off (I moved one of the cavities a bit to get the East and West cavities similar *close* distances from the ESDs, I may have borked the balancing.
Let's see how close to window edges we are once we find the 00 to decide if we need to open it and re-balance or not - I think a 40% transmission indicates the West trans beam is clearing just fine
I expect our problem to be: new pitch is pointed up (input beam low, output beam high) based on the slight cavity movement I made when installing ESDs
  1065   Wed Apr 16 16:38:14 2014 DmassDailyProgressLab WorkProgress: aligned and pumping down

[Dmass, Evan]

We aligned the West cavity today.

Mode matching was only ~35% transmission, we're unsure why. (size of dip is 33 mV, off resonance voltage is 94 mV, and we kept in mind the dark voltage of the PD and the cavity fill time) Problem solved; we were clipping. See below.

We played a little bit with the mode matching mirrors, but weren't able to get the mode matching to go up.

The beam goes roughly through the middle of the windows (I gently shook the cryostat a little bit to *unstick* the platform suspension - this may or may not have done anything)

The beam looks sketchy (nongaussian) after the EOM; we're unsure why. The AOM is a possible culprit based on changes to the spot shape on a card with AOM translational position.

Pumping down as it seems we do not need to re-balance the platform.

[Evan - Addendum: I aligned into the east cavity. The coupling here is OK: 75%, as inferred from refl DC east (size of dip is 60 mV, off resonance voltage is 78 mV, and I kept in mind the dark voltage of the PD and the cavity fill time). As with west, I had to lower the heights of the input waveplates in order to get the beam to clear.

Returning to west, I found we were clipping on the edge of the last (kinematically mounted) steering mirror. I lowered the post height by 0.25", translated the mirror a bit, and recovered good alignment. The power on the cavity (as inferred from trans and refl PDs) was increased compared to before.

From west refl DC, the size of the resonance dip is 133 mV, and the off-resonance voltage is 178 mV, again accounting for the dark voltage and the cavity fill time. That's 75% coupling. —Evan]

  1066   Thu Apr 17 16:32:37 2014 DmassDailyProgressLab WorkCryo beat prep

Ordered LN2 refill (delivery by Monday)

Picked up diamond pattern beam dumps from 40m:

  • 8 x beam dump holders
  • 24 x 1" green glass plates
  • Cut 24 x teflon shims (the baked PEEK shims were very expensive, and only needed for vacuum work, which we do not require)
  • Need to clean these
  • Ordered 2 x (2"x2") square ND filters to cut into pieces (trans = ~10% and ~30% @ 1200nm, and looks flat above 900. Thorlabs didn't give data above 1200nm)
  • Ordered 3" posts
  • Still need: box of 3/16" 4-40 screws to clean and use

Beam dump info: http://nodus.ligo.caltech.edu:8080/40m/9419


I clicked around but couldn't find data on the transmission of the green glass @ 1550nm - do we know anything about this?




  1067   Tue Apr 22 13:56:08 2014 steveDailyProgressLab Workgreen glass dumps


 Green glass transmittance                    is a bit higher at 1550 nm (~few ppm) 

Reflectivity was measured at 1064 nm


  1069   Thu Apr 24 11:55:51 2014 EvanDailyProgressPDHRefl PD transfer functions


We have taken transfer functions of the gold RFPDs. We used the Zurich box to drive the laser bias tee near from 5 MHz to 80 MHz, thereby exploiting the current-to-power coupling of the laser. We then measured the transfer function that takes the broadband ThorLabs RFPDs (PDA10CF) to the gold RFPDs.

The RF transimpedances at the PDH frequencies are 730 V/A for west at 32.7 MHz and 1380 V/A for east at 33.59 MHz.

Data and plots are on the SVN at Measurements/Diodes.










The quoted transimpedance of the PDA10CF is 5 kΩ. We assume the responsivity is 1 A/W at 1500 nm.

Below are the dc powers and voltages of the PDs. For the gold PDs, we measured both the power incident on the diode and the power reflected off, then subtracted the two. The PDA10CFs were loaded with 50 Ω, and the gold DC outputs were loaded with high impedance. Voltages were measured on an oscilloscope.

  West PDA10CF West Gold East PDA10CF East Gold
Power (µW) 272(5) 1160(10) 290(5) 900(10)
Voltage (V) 1.23(1) 0.134(2) 1.41(1) 54(2)

Using the transimpedance and responsivity for the PDA10CFs, we infer a quantum efficiency of 0.90(2) for the west PDA10CF and 0.97(2) for the east PDA10CF. The QE for west is not great, but maybe this means the beam is not small enough when it reaches the diode.

If we assume that the responsivity of all these diodes is 1 A/W, the efficiency of the gold PDs is 1, and the PDA10CF response is flat over the frequencies of interest, then the calibration factor we need to apply to the above transfer functions in order to refer them to volts per amp in the gold PDs is 1.1 kΩ for west and 1.6 kΩ for east.


We use vectfit4 to fit six poles.

Complex s-plane poles and zeroes are as follows:

  West East
Poles (× 106 rad/s) –25.2 ± 204.1j −40.1 ± 187.1j
  –120.4 ± 179.7j −14.9 ± 211.6j
  –7.6 ± 272.7j −49.5 ± 233.3j
Zeroes (× 106 rad/s) −480.6 −299.3
  162.7 ± 181.8j 169.7 ± 209.1j
  −5.8 ± 274.2j −37.9 ± 206.9j

The RF transimpedances at the PDH frequencies are 730 V/A for west at 32.7 MHz and 1380 V/A for east at 33.59 MHz.



Audio-band TF estimate

Given a PDH modulation frequency (32.70 MHz for west and 33.59 MHz for east), we can use the above RF transfer functions to estimate the post-demodulation response of the RFPDs when illuminated with two audio-frequency amplitude sidebands on either side of the PDH carrier frequency. The mathematical derivation is given in the PDF attachment. The plots below show the estimates for the gold RFPDs. The quoted magnitudes (in V/mA) are a naive application of the above calibration factors plus the demodulation factor of 1/2; there will be additional loss in the mixer.

For each plot, the demodulation phase has been chosen to coincide with the phase of the RF transimpedance at the PDH frequency. Varying the demodulation phase by ±10 degrees appears to affect the overall magnitude of the transfer functions, but not their shape or phase.




To produce amplitude modulation, we used the source on the Zurich box to modulate the laser current via a bias tee attached to the laser. This tee has a series impedance of 27 Ω + 3 nF + 20 Ω into the laser diode's cathode. We detuned each cavity so that no HOMs were resonant, and hence each cavity looks like a high reflector. We then took the transfer function which takes PDA10CF RF voltage to gold PD RF voltage.

Attachment 9: rfpd_demod_derivation.pdf
  1071   Fri Apr 25 17:22:27 2014 nicolas, dmassDailyProgressBody ModeBeat to ESD feedback is not enough gain

Spoiler alert: One of the body modes has been found, and will be logged in another log.

We tried feeding the beat signal back to the ESD to find the body mode of the second cavity.

We could not get enough gain in our feedback loop to change the ring down/up time of the known body mode before the beat noise was saturating our SR560.

This means we can't get enough gain to make the body mode unstable and we will have to search for the second body mode directly.


  1072   Fri Apr 25 18:58:54 2014 DmassDailyProgressBody ModeBody Mode Measured

[Nic, Dmass]

West Cavity:

  • Body Mode: ~ 34755.6 Hz
  • Ringdown tau ~ 0.2 sec
  • Room temperature Q ~ 2e4

Located West body mode using ESD to ring it up (SNR ~100)

Total beat RMS still big w.r.t. rung up body mode, so used a lock in to take a ring down measurement (locked to ~100 Hz offset from body mode)

Was unable to locate East body mode using same techniques I used for West - was able to ring up West body mode on east drive through crosstalk (East drive -> West body mode had ~1/4 the coupling strength)


Eyeball fitting an exponential curve to this of the form B*exp(-t / tau), we find tau = 0.21


Room temp  Q = pi * tau / period = pi * tau * freq = 2e4

[DYM - added experimental diagram and description of measurement below]

Measurement details:

  1. Lock both cavities and PLL, look at beat on SR785 (~5 averages, ~16 Hz bandwidth, ~35 kHz )
  2. Use function generator to drive modulation input of HV amplifier (connected to ESD)
    • Use triangle wave FM on drive frequency (~100Hz depth) on ~30kHz to 40 kHz drive
  3. Scan drive frequency around and watch for increase in spectrum
    • Saw ~10x increase in RMS on West cavity
    • For East cavity, all we saw was crosstalk - need to search more thoroughly
  4. Tune frequency of drive to find maximum amplitude in spectrum
    • For West Cavity, I found the frequency to be 34755.6 Hz
    • When I tried to drive the the East Cavity, I only found the same frequency, at 1/4 the amplitude (it seems like crosstalk)
  5. Use Lock in amplifier to read out the body mode oscillation in the beat after ringing it up -
  6. Make lock in bandwitdth (1/time constant) longer than offset between lock in freq and body mode freq
  7. Turn off drive and record ringdown on scope (below)
  8. Fit-By-Eye the slope of the ringdown on a log scale (below)

It was easy to find the West body mode, however I did not find the East body mode

Attachment 1: WBodyModeRingdown300K.pdf
Attachment 2: BodyModeRingdown.pdf
  1073   Mon Apr 28 17:48:02 2014 DmassDailyProgressBody ModeEast Mode still not found


  • Did a more thorough search for the East body mode from 28 kHz to 40 kHz, saw nothing
  • Confirmed that the mode we saw in the beat when driving East ESD was from crosstalk via PDH control signals
  • Fixed output polarizations for maximum overlap at beat via waveplate tuning



  • I looked at the beat and ESD drive signals on a SR785 (8 Hz BW, 5 averages, exponential averaging), as described in elog:1072
  • Set DS345 drive to: f_drive~35kHz, 1V offset, 2Vpp amplitude  // Internal modulation: FM, sine, rate = 1 Hz, depth = 120 Hz
  • ESD drive looked like ~120 Hz(ish) wide band limited noise centered around 35 kHz drive frequency
  • Moved f_drive around 34756 kHz in 10 Hz steps, every step that was within 60 Hz of the body mode frequency rung this it up (good, we can't miss)
  • Checked PDH control signals, While driving East ESD, saw mode in beat and West PDH. Consistent with hypothesis of cross talk induced excitation.
  • Scanned from 28 kHz to 40 kHz in 100 Hz steps with drive described above, waiting ~5 seconds / step for response
  • No obvious increase in beat RMS from East cavity => we can't ring up the East body mode for some reason


  • Q exceedingly low, (e.g energy gets pulled out immediately)
  • Q very high (not enough energy into the system from our band limited noise to make the RMS go up on such a short timescale)
  • Electrical malfunction (no signal getting to ESD, short somewhere, open connection close to ESD, etc - short seems unlikely if we can ring up West via crosstalk)
  • Bad coupling strength (screws somehow too loose so ESD is floppy, ESD too far from cavity)


Immediate Future + Misc:

  • I fixed the beat overlap via waveplates - this changes with window stress (pressure/temp)
  • Vacuum seems like it was just wet, it's pumping down lower now
  • I accidentally left pump off over weekend (with chamber sealed) - will pump for 1-2 days and then go cold
  • Will take one beat at room temperature tomorrow at lowish pressure, and look at coherence (PDH control E/W and trans intens E/W - ANY OTHER SUGGESTIONS FROM PEANUT GALLERY?)
  • Will cool down in a day or two, and measure the 120K West Q via ringdown, take beat spectrum, and look at coherences again
  1077   Wed Apr 30 09:08:33 2014 DmassDailyProgressPDHRefl PD transfer functions

[Evan, Dmass]

These transimpedances are much smaller than what was previously measured (cryo:976).

To try to resolve the discrepancy, we used a Marconi to directly modulate the west laser at its PDH frequency (32.7 MHz at −20 dBm, again through the bias tee) and watched the gold PD and ThorLabs PD responses on an oscilloscope.

The gold PD response was 8.8(7) mVpp, and the PDA10CF response was 12(3) mVpp. This ratio is 0.7(2) V/V, i.e. −3(2) dB, consistent with the transfer function taken last week.

Power and dc voltage on the gold PD were 1.4 mW and 153 mV, respectively. On the PDA10CF they were 0.29 mW and 1.24 V, respectively.

Conclusion: the transimpedance numbers from a few months ago appear to be incorrect.

  1080   Fri May 2 02:40:34 2014 DmassDailyProgressBody ModeBody Mode Measured

[Evan, Dmass]


West body mode frequency shifted to 34909 Hz (from 34756 Hz)

Ringdown time ~0.07s (down from 0.2 sec)

Q(120) is smaller than Q(300) by a factor of 0.35

Attachment 1: Ringdown34909.pdf
  1081   Sat May 3 18:16:55 2014 DmassDailyProgressBody ModeBody Mode Measured

I measured the transfer function of the ESD drive to beat frequency deviations in [Hz/Volts]

The first plot is the raw transfer function in blue, and with the cavity pole divided out in green

Calibration is:

[100e3 Hz_rms / 1.41 Vrms] for the beat (100 kHz range on Marconi, PLL 

[10 Vesd / Vdrive ] for the ESD drive

[0.099m / 1.93e14 Hz]

Total calibration applied: [100e3 Hz_rms / 1.41 Vrms] / [10 Vesd / Vdrive ] * [0.099m / 1.93e14 Hz]


From the transfer function looks like there are two modes close to each other, one of which has a ~4x lower Q

Attachment 1: ESD_TF_Raw.pdf
Attachment 2: ESD_TransFun.pdf
  1082   Sat May 3 19:57:56 2014 DmassDailyProgressBody ModeBody Mode Response Seen in RIN Transfer Function

I was able to see what looks like the radiation pressure driven [m/W] response from the body mode when driving RIN and looking at the beat readout.

This is only a *tiny* change in the [pm/RIN] transfer function, which is presumably dominated by the PLL's offset driven Hz/RIN sensitivity


  • I was unable to find some sort of minimum by tuning the PLL offset (using the error point offset knob on the front of the LB1005 box)
  • The transfer function would go down by a few dB when introduced a *significant* offset
  • I could not make the transfer function increase by introducing an offset

looking at elog:1050, when we measured the photothermal transfer function:

  • we see that we *were* able to change the coupling using the offset knob in the past ~5dB for +/- 0.04 setting on knob
  • the coupling level at ~35 kHz was ~900 Hz/RIN with an input power of 522 mW (which is 0.46 pm/RIN), smaller than this measured coupling by ~5x (which is equal to the difference in power ratio - is this what we expect?)

 Is there anything other than photothermal which could have a power dependent RIN to frequency noise (as read by the PLL) coupling?

[edit: the high frequency portion of the beat can look similar regardless of whether or not the PDH loops are oscillating (from gain peaking). The total residual RMS phase noise of the PLL would increase, and cause a larger RMS based offset in the PLL. Since the gain changes depending on input power, and it sets the minimum PLL intensity noise coupling we can attain, it's possible for the transfer function of RIN to meters to look like it is power dependent even where it is dominated by the PLL's (unintentional) Q phase sensitivity if we are not careful about tuning the loops. I need to keep this in mind when I make my more in depth measurement of the rad pressure transfer function]

Attachment 1: RINtoLength_TF_Raw.pdf
Attachment 2: RINtoLength_TF.pdf
  1083   Mon May 5 17:09:06 2014 nicolasDailyProgressBody ModeBody Mode Measured


From the transfer function looks like there are two modes close to each other, one of which has a ~4x lower Q

 I wonder if this is the East body mode, but with bad Q for some reason.

Try taking the ESD transfer function from the east actuator and see if the relative sizes of the bumps change.

  1084   Tue May 6 11:40:19 2014 EvanDailyProgressPDHUpdated noise budget

The RF transimpedances at the PDH frequencies are 730 V/A for west at 32.7 MHz and 1380 V/A for east at 33.59 MHz.

 I have changed these transimpedances in the noise budget, and have removed the fudge factors. The new noise budget plot is attached.

Attachment 1: cryonb_20140506.pdf
Attachment 2: cryonb_20140506.fig
Attachment 3: pdhbudget.pdf
  1086   Tue May 6 17:34:41 2014 DmassDailyProgress West

Refilled cryostat, fixed alignment/clipping, took beat


The portion of the west cavity retroreflection which is transmitted through the ultimate steering mirror (90% reflective beamsplitter) was going through the edge of the optic and hitting the mount.

I moved the offending optic, realigned the west path to 00 (fine tuned by minimizing the 01/10 transmission peaks), and dumped the beam with a razorblade stack.


I took beat spectra at 120K, and looked at the following coherences:

  • Beat with W PDH Control
  • Beat with E PDH Control
  • W PDH Control with E PDH Control
  • Beat with W Trans RIN
  • Beat with E Trans RIN

West PDH control dominated at high frequency, confirming that we are limited by residual laser frequency noise at high frequency

East PDH showed up at high frequency

Neither transmitted RIN was notably coherent** with the beat in the bucket, supporting my claim that the anomalous RIN portion of the noisebudget showed today was not true (it should be lower - Evan and I need to reconcile our versions of the noisebudget perhaps) 

We believe that the noise in the bucket is the input referred PDH noise. Temporary solution: amplify inside the PDH box to preserve our SNR, attenuate before the current driver to unload some of our actuator gain

Data is on the svn in: CryoLab/Measurements/CavityBeat125/May_06_14 

[edit: added beat and coherence - I will process this more before adding it to the noisebudget and doing a subtraction, so don't mind the colors/multiple traces for now]

Attachment 1: ColdBeatMay0614.pdf
  1091   Tue May 13 18:44:16 2014 DmassDailyProgressCryoCooldown summary

We went warm over the weekend (2014-05-09).

The LN2 stick arrived, but it is too big to fit into the LN2 chamber. We will need to cut it down a bit.

2014-05-13: filled again with LN2, and the cold plate reached 80 K around 16:00. I then filled with about 2 buckets, bringing the LN2 level to ~8 inches.

2014-05-13, 18:45: 5 inches left. Added another bucket, bringing the level back up to 8 inches.

2014-05-14, 1:30 AM - almost empty - filled and pumped the inner vac pressure (with the exchange gas) down to 5e-3 torr, left the pump on the insulating vac

2014-05-14, 10:30AM 6" left, cold plate at 77 K, sus platform at 98 K, cavities at 125 K. Added 2/3 bucket, now at 8".

2014-05-14, 11PM - filled, still pumping

2014-05-15, 18:50: 5" left, added 1 bucket and now we are at 8".

2014-05-16, 18:00: 7" left, did not fill




  1098   Tue Jun 10 10:11:15 2014 Evan, Dmass, NicDailyProgressPDHDiode driver + laser transfer function measurement

We decided we didn't believe the old diode driver + laser TFs that we took (cryo:1060); they are suspiciously flat and we're not convinced we know what the Zurich box is doing.

So last week we set up the laser/laser PLL again and used the HP4395 to take a transfer function from voltage into the diode driver to optical frequency of the laser. We used a dc-coupled, unity-gain SR560 as a buffer between the PLL control and the 50 Ω input of the HP4395. This undoubtedly influences the shape of the TF above 100 kHz.

Nic implemented a slow temperature control using the cymac in order to keep the PLL control signal less than ±1 V.

The first attachment shows the TF from driver voltage to laser frequency. I have not yet undone the OLTF of the PLL, so this plot should only be trusted below 50 kHz. Calibration is 800 kHzpk/1Vrms, as

The second attachment shows the PLL OLTF, as measured with a HP4395.

The third and fourth attachments are diagrams of the measurement setups. I should redo these on a computer so that they're more readable.

The plots, data, and code are on the SVN at Measurements/DiodeDriver/DiodeDriverPlusLaser/20140604.

[Edit, 2014–06–16: By eye I've found the PLL OLTF consists of a pole at 0 Hz, two poles at 400 kHz, three poles at 700 kHz, and a phase delay of 2 µs. The plot is attached.]

[Edit, 2014–06–23: I've used my estimate of the PLL OLTF to undo the effect of the PLL on the diode driver + laser TF.]

Attachment 1: lasertfs.pdf
Attachment 2: plloltf.pdf
Attachment 3: laserpll1.jpg
Attachment 4: laserpll2.jpg
Attachment 5: pllfit.pdf
Attachment 6: lasertfs.pdf
Attachment 7: tfs_undone.zip
  1107   Wed Jun 25 22:41:37 2014 DmassDailyProgressPDHTRUE Diode driver + laser transfer function measurement

Here is the "Diode Driver + Cabling + Laser" transfer function, calibrated out to 1.2 MHz.

Much calibration was done, VERY mindfully of loading.

We should shorten the cabling between the driver and the laser if we want to buy back some phase.

Previous measurements had not correctly calibrated out the Marconi, and instead treated the 800 kHz/Vrms number as though it were true indefinitely - I measured and calibrated this out, and the transfer function finally makes sense.

Attachment 1: W_DriveCabLasTF.pdf
  1108   Fri Jun 27 14:19:25 2014 DmassDailyProgressPDHMarconi TF

I measured the transfer function of the Marconi (IFR 2032) so that we could properly calibrate it out of our measurements***

Initially, I tried locking one Marconi to the other via the 10 MHz clock signal which they can send to each other.

The phase noise at the freq discriminator was too high once I switched Marconi A to external FM with an 800kHz range, so I had to set up a PLL to lock one Marconi to the other.

Here is the measurement setup:


[20140628: Is this diagram missing a wire into the RF port of the mixer? —Evan]

The 4395 drives the input to the Marconi (50 ohm in line added here so that the power splitter is correctly loaded)

I read at the output of the mixer, before the 5 MHz low pass (the 2f shouldn't matter for a lock in detection / transfer function)

I used the AC coupling module for the differential probe (Agilent 1141A), as well as the 50 ohm load which comes on its test/BNC adapter board 


  1. Calibrated out cabling by taking the input to the Marconi, and plugging it into the input of the differential probe at the driver/attenuator levels to be used in the measurement - confirmed ~0dB/0deg in measurement ban
  2. Measured unlocked Vpp of discriminant with a scope at the output of the low pass filter to get [V/rad] calibration - assumed the mixer is white
  3. Measured [V(rad) / V(input) ] transfer function with 4395
  4. Measured PLL gain with SR785 (UGF @ 4 kHz, 90 degrees of phase)

Offline Calibration:

  1. Divided out PLL response to correct low freq portion of measured transfer function
  2. Multiplied by [rad/V] calibration
  3. Multiplied transfer function by i*f (because fourier transform on derivative of a variable is 2*pi*i*fourier transform of the variable, and 2*pi*f = omega)

The result:



What it means:

  • On these Marconi settings (95 MHz carrier, 800 kHz range), we can trust the magnitude out to 400kHz, but we have already had 200 degrees of phase response there.
  • If we want to measure the laser TF out to higher frequencies, we should use the discriminant output instead of the marconi input - it requires slightly less calibration (have to correct for PLL, and i*f, instead of correct for PLL and correct for Marconi transfer function, the morphology of which may depend on settings)
  1109   Fri Jun 27 15:16:43 2014 DmassDailyProgressPDHLaser TF measurement setup

Here is a detailed diagram of the setup used to measure the Current Driver + Cabling + Laser [Hz/Volt] transfer function in elog:1107


The red and green traces show the two measurement setups

For both setups, the green and red arrows were shorted to each other to perform a through calibration

For the laser PLL, this introduced a 2.21 dB underestimate of the transfer function because of the 500 ohm load at the input of the current driver (easily calculable for both Wye and Delta resistive power splitter topologies) - this is consistent with the low frequency difference we saw between the SR785 measurements and the HP4395 measurements (good.)


I measured the driver + cabling + laser + delay with two different PLL gains:


I measured (and divided out) the PLL calibration:


I added 2.21 dB to the magnitude response (from the 50->500 ohm load impedance mismatch)

I multiplied by the marconi transfer function (see elog:1108 for transfer function):


Finally, I subtract out the phase from time delay after the output of the laser (11.2 ns - calculated from free space from laser to beat PD, and BNC cable from 1811 to the mixer) to get the transfer function of Hz at output of laser vs volts at input of driver:


 Data is on svn in CryoLab/Measurements/DiodeDriver/DiodeDriverPlusLaser/2014-06-25W/DataForFitting.mat, in physics units of Hz/V

  1110   Mon Jun 30 16:35:58 2014 EvanDailyProgressPDHLoops lock with on-board rf electronics

New rf powers

I have transitioned the west PDH loop (resp. east PDH loop) so that it now locks with the CRYO–001 (resp. CRYO–002) on-board rf electronics.

This necessitated some changes to the rf power distribution. The new situation is as follows:

  • West Marconi carrier at 32.70 MHz, and east carrier is at 33.59 MHz. For each, the carrier power is 6 dBm. Each Marconi drives a hybrid splitter.
  • For both west and east, power from one port of the splitter is sent directly to the amplifier for the resonant EOM (ZX60-100VH+). This means I have removed the 16 dB of attenuation that used to precede each amplifier.
  • Power from the other port of the splitter is sent through a 3 dB attenuator and then into "LO in" on CRYO–001 for west (resp. CRYO–002 for east). This means there is 12 dBm into each ERA–5+ onboard amp, and therefore +8 dBm into each mixer (not accounting for the insertion loss of the LO phase shifter).

For some reason, the east LO phase shifter doesn't seem to be shifting. It seems like the AD587 and 10kΩ trimpot are working fine. This will require some more investigation.

West loop vitals

  • Power on cavity, off resonance: 409(1) uW 11(1) uW = 398(2) uW
  • Power on west refl PD, off resonance: 350(1) uW 20(1) uW = 330(2) uW
  • Voltage on west refl PD, off resonance: 370(2) mV (dark voltage has been subtracted)
  • On-resonance voltage dip for west refl PD: 313(8) mV
  • Error signal peak-to-peak: 335(10) mV
  • Carrier power on west trans PD: 1.96(2) V (dark voltage has been subtracted)
  • Single sideband power on west trans PD: 9.1(5) mV (dark voltage has been subtracted)

From this we infer:

  • Modulation depth is 0.136(7) rad
  • Visibility is 85(2) %

East loop vitals

Data not yet taken

  1111   Tue Jul 1 14:42:31 2014 DmassDailyProgressPDHLaser TF measurement setup

Redid the measurement for both the East and West Driver -> Laser with the following changes:

  • Shortened cabling to laser diodes (trimmed about 1.5 - 2m off the length)
  • Mechanically isolated cabling to laser diode from diode
  • Took transfer functions at mixer output instead of marconi input (so I get information out to much higher frequency)
  • Changed PDs / beat frequency: from 100MHz beat on an 1811 (125MHz) to 300 MHz beat on a 1611 (GHz)

Result: Transfer functions out to 5 MHz, less apparent phase delay than before


== Cabling ==

  • Old:
    • 1m BNC cable from driver output to breakout board in rack
    • BNC at breakout to DSUB at Diode with 20.5' (6.24m) of  24 AWG twisted shielded pair with shield connected at diode driver end
      • Foil shield only w/ shielding wire only
      • twisted pair char impedance: 54 Ohms
      • Inductance: 0.19 uH/ft
      • Mutual Capacitance: 35 pF/ft @ 1kHz,
      • Ground capacitance: 63 pF/ft @ 1kHz
  • New
    • BNC at driver to isolated BNC rigidly mounted next to diode with 12.2' (3.7m) of thicker shielded twisted pair wire - shield connected at both ends (is this bad?)
      • Foil shield + shield braid + shielding wire
      • twisted pair char impedance: 120 ohms
      • mutual capacitance: 42 pF/m
      • conductor to conductor+shield capacitance: 75 pF/m
      • speed: 0.66c
      • attenuation: 2dB/100m @ 1MHz 
    • BNC to DSUB via short (2 inches) unshielded twisted pair on table

 I trimmed 3.5m of cabling from the diode driver to diode setup without having to sacrifice cleanliness of the setup. At 0.66c, this corresponds to 17.7ns, or 6.4 degrees at 1MHz

== Mechanical Isolation ==


We used to have large acoustic/mechanical coupling into frequency noise when we touched/flicked the cable near the laser.

Clamping the cable made it a lot better, but since we weren't clamped right next to the diode, there was still large response when touching the table.

I do not know if the source is triboelectric, metal on metal rubbing at the DSUB interface at the diode (it was still present even with screws in the DSUB), or mechanical shaking of the diode coupling into frequency noise/current through some other effect. The entire butterfly mount has always been very sensitive to touching (we can hear huge glitches in the PDH error signal when we touch the package).

To mechanically decouple the cabling from the diode I made the following adapter next to the diode:


There is very little response when I touch the twisted pair between the BNC and DSUB, and no response when I whack the cabling upstream of the breakout

== Measurement Setup ==


The beat was ~300MHz on a 1GHz 1811 PD.

I did not calibrate out:

  • The 1811 response (small RF sidebands on its transfer function about 300 MHz)
  • The ZHL-2010 amplifier
  • The ZFM-3 Mixer

== The Data ==

See attached


  1. Raw Data
    • I took the transfer function both before and after the 5MHz low pass filter (mostly out of curiosity about whether or not the increase in RMS from the unfiltered 2f would cause transfer function error)
  2. 5MHz LPF (BLP5)
    • Transfer function of BLP5 taken in the same loading conditions that the BLP5 is used in, though not ideal (50ohms on front and back, so 25 Ohms total load), is identical to how it is plugged in for the PLL
  3. PLL Calibration
    • I measured -G/(1-G), and then did algebra to convert this to -(1-G).
    • Measuring -G/(1-G) instead of (1-G) directly gives us a noisier calibration at low frequency, in the band of the PLL. I do not care so much - we just need these measurements to line up with the control point measurements made previously (at the Marconi actuation input)
  4. Calibrated Data
    • Calibration procedure:
      1. Correct for PLL (mult by -(1-G))
      2. Subtract out time delay
      3. Correct for 500 Ohm input @ driver (+2.21 dB)
      4. Correct for 5 MHz LPF where we take the TF after this
      5. Turn into magnitude 10^(xx/20)
      6. Calibrate into rad/V from discriminant slope 300mVpp: 150mV/rad
      7. Turn into Hz/V - multiply by i*f

== Discussion ==

The pink trace (elog:1109) has much larger phase delay at 1 MHz than the measurements taken in the above described setup

  • Taken at the Marconi error point (hence no information/coherence above ~1.2 MHz)
  • I changed the cabling and trimmed out 3.5m, 17.7ns, or 6.3 degrees of phase - the extra delay seen in the pink trace is much larger than this
  • The beat was at 100 MHz, on a 125MHz PD (1811)
    • When the 2nd derivative of phase is large and negative (concave down) about where we are taking the transfer function, we get an extra apparent phase delay in the transfer function as we increase the drive frequency
    • On a 125 MHz PD, the 2nd derivative of phase is very large around 100 MHz (for a simple pole, the effect is actually zero at the pole frequency / inflection point)
    • For calculating audio (and RF) sideband transfer function about some frequency in a device (e.g. in the gold PDH PDs, or the beat PD), see <this note> by Evan

There was no significant difference in taking the transfer function before or after the 5 MHz low pass filter once we calibrated out the BLP5 transfer function

The West coefficient of [Hz/V] is larger than the East by ~30%

The transfer function of V/Hz seems to go UP after 2-3 MHz, maybe this is gain peaking/oscillation in the diode driver?

Attachment 4: 2014-06-29BLP5cal.pdf
Attachment 5: 2014-06-29PLLcal.pdf
Attachment 6: 2014-06-29CalibratedDriCabLasTF.pdf
  1116   Thu Jul 3 17:05:46 2014 DmassDailyProgressPDHPDHv2 transfer functions

I measured the PDHv2 transfer functions at high and low gain, with the invert switch up and down, for both east and west boards.

Measurements were taken with a 4395, with a 50 ohm in line added to the input - the output of the board is 50 ohms.

Cabling was calibrated out using the 4395 through calibration menu.

Drive level was -50 dBm into a resistive power splitter. No saturation was seen (via distortion of high frequency portion of transfer function) until higher drive levels. Confirmed by adding attenuator and looking at lower drive levels, and seeing noisier transfer function with same morphology. 

I have normalized them against each other so that we can compare non-idealities. If the invert switch was ideal, or the gain knob truly just increased and decreased gain, all the curves would lie atop each other.

Reading plot: Compare red traces with each other, and similarly with blue.

Naming is: XYZ = [X][Y][Z] = [East/West][invertUp/invertDown][gainHigh/gainLow]



Evan will reply with the most up-to-date transfer function model we have.



Attachment 1: PDHv2_TFs_all.pdf
  1117   Fri Jul 4 15:01:00 2014 EvanDailyProgressPDHLTSpice simulation of CRYO-001 TFs


Evan will reply with the most up-to-date transfer function model we have.

I've updated the CRYO–001 Spice model and added it to the SVN under electronics/PDH-box.

Attached is a plot (and the corresponding data) for the CRYO–001 no-boost TF for the two gain states and the two invert states. Notably, the simulation does not predict that the shape of the TF should change at high frequencies.

Attachment 1: cryo001tfs.pdf
Attachment 2: cryo001spice.zip
  1118   Sun Jul 6 22:25:30 2014 DmassDailyProgressPDHComparison of PDH box with spice model

 Here is a comparison of the results of what LTspice thinks the transfer function of the PDHv2 boards is vs the actual transfer functions.

It appears the high gain state is where the measured transfer functions look the most different from the simulations. We lose tons of phase at "high gain" 


We should get those gain knobs out of there asap and replace them with static components if we want to push the UGF, or entirely bypass the 2nd stage, if we don't plan on using it to boost the signal.


The drive level in the "high gain" measurement state was -50 dBm into a resistive power splitter, with a 50 ohm T'ed into the PDHv2 input, so -56dBm total, or 0.5 mVp

Maybe there is some funny capacitance in the potentiometer which causes this?



Attachment 1: CRYO-001_compare.pdf
  1119   Sun Jul 6 22:37:52 2014 EvanDailyProgressPDHComparison of PDH box with spice model


Maybe there is some funny capacitance in the potentiometer which causes this?

 In cryo:872 I found that I needed to add an extra 50 pF of capacitance in the feedback of the variable gain stage in order to make the model agree with the data.

I've done that here (except I've added 50 pF across the pot, not across the entire feedback resistance). It seems to give a similar behavior to what you've measured.

With the gain low, this 50 pF capacitance is shorted by the parallel 0 Ω resistance of the pot, and hence the LTSpice model predicts that this extra capacitance doesn't change the "gain low" TFs.

Attachment 1: cryo001tfs.pdf
Attachment 2: cryo001spice_50.zip
  1121   Mon Jul 7 06:52:34 2014 EvanDailyProgressPDHComparison of PDH box with spice model



Maybe there is some funny capacitance in the potentiometer which causes this?

 In cryo:872 I found that I needed to add an extra 50 pF of capacitance in the feedback of the variable gain stage in order to make the model agree with the data.

I've done that here (except I've added 50 pF across the pot, not across the entire feedback resistance). It seems to give a similar behavior to what you've measured.

With the gain low, this 50 pF capacitance is shorted by the parallel 0 Ω resistance of the pot, and hence the LTSpice model predicts that this extra capacitance doesn't change the "gain low" TFs.

 Likewise for CRYO–002. The Spice model is at electronics/PDH-box/cryo-002_spice.asc.

Attachment 1: cryo002tfs.pdf
Attachment 2: cryo002spice_50.zip
  1123   Wed Jul 9 01:10:42 2014 DmassDailyProgressPDHMade new low pass filters for after mixer

I made some new filters for the IF port of the PDH mixer - pics below


I don't know how to correctly model what appears to be a self resonance due to the series capacitance of the inductors used (3.3uH)

I can raise the SRF by decreasing the inductors and increasing the capacitors, but this costs us phase at low frequency (b/c of the impedance of the inductors w.r.t the 50 ohm output resistor)


Attachment 1: MeasSummary.pdf
  1124   Tue Jul 15 08:36:30 2014 EvanDailyProgressPDHRefl PD transfer functions


Audio-band TF estimate

Given a PDH modulation frequency (32.70 MHz for west and 33.59 MHz for east), we can use the above RF transfer functions to estimate the post-demodulation response of the RFPDs when illuminated with two audio-frequency amplitude sidebands on either side of the PDH carrier frequency. The mathematical derivation is given in the PDF attachment. The plots below show the estimates for the gold RFPDs. The quoted magnitudes (in V/mA) are a naive application of the above calibration factors plus the demodulation factor of 1/2; there will be additional loss in the mixer.

For each plot, the demodulation phase has been chosen to coincide with the phase of the RF transimpedance at the PDH frequency. Varying the demodulation phase by ±10 degrees appears to affect the overall magnitude of the transfer functions, but not their shape or phase.

I've extended the audio-band TF estimate to 10 MHz. Additionally, I've overplotted a simple RC pole TF, with the pole chosen (by eye) to coincide with 45 degrees of phase accumulation in the actual audio TF. Files in Measurements/Diodes have been updated accordingly.

Strictly speaking, these TFs are only valid below ≈ 5 MHz, because I only did the vector fitting to within ±5 MHz of the diodes' resonant peaks.

Attachment 1: westAudTF.pdf
Attachment 2: eastAudTF.pdf
  1126   Tue Jul 15 20:03:48 2014 DmassDailyProgressPDHTotal PDH Open Loop Transfer Function Budget

I have been spending time understanding the current PDH and making missing measurements.

I combined measurements of every part of the PDH loop into a full TF budget.

Interpolation between the transfer functions was done using MATLAB's interp1 (linear interpolation) on the amplitude and phase data, and is overplotted on each measurement

Things to fix / finalize when I compare to a measurement:

  • I used the East modulation depth for the west error signal calibration - this might be overly optimistic
  • I divided both signals by 10 to reflect what we want to do to the actuator gain (in the current driver / between the current driver and the PDH board output) - this is rough and flexible, and can be tweaked with input power as well
  • PDH measurements were made with the 60kHz boost stage off - this will effect phase and UGF.

Assumptions made:

  • 1mW input power
  • 0.47 modulation depth for both cavities
  • 1 A/W responsivity at PD with 100% coupling
  • 80% mode matching

Fun facts about the plots:

  1. Driver + Laser TF
    • There are no on purpose features here
    • Time delay is 19 ns
    • Current driver has been mesaured to be ~flat (up to a couple dB and 5 degrees below 1MHz)
  2. Cavity Pole +Table Delay + PDH error signal
    • Error signal taken to be: 2 / f_cav * sqrt(coupling * Pcar) * sqrt(P_sb)  = 2 / fcav * sqrt(coup) * gamma * Pinput [Watts/Hz]
  3. Gold PD + Mixer
    • Coupling assumed to be 100%
    • Responsivity assumed to be 1A/W (i.e. 100% QE)
    • 30ns free space delay
    • Mixer loss = 0.63 (measured by input and output power in dBm)
    • Looks close to 2.6 MHz pole below 1MHz
  4. Low pass filter
    • Extended measurement to 1kHz based on measurement down to 500kHz - this seems super reasonable given plot
  5. PDHv2 boards:
    • East (CRYO-002) poles and zeros according to inkscape document
      • Poles: 160 Hz, 720kHz
      • Zeros: 28 kHz, 28 kHz (same)
      • Switch 1: Pole at 0Hz, zero at 160 Hz (integrator)
      • Switch 2: Pole at 0 Hz, zero at 60 kHz (boost) 
    • West (CRYO-001) poles and zeros according to inkscape document
      • Poles: 160 Hz, 780 kHz
      • Zeros: 28 kHz, 87 kHz (see elog:1089 for why this is different from East)
      • Switch 1: Pole at 0Hz, zero at 160 Hz (integrator)
      • Switch 2: Pole at 0 Hz, zero at 60 kHz (boost)
Attachment 1: PDH_OLTF_allsubplots.pdf
PDH_OLTF_allsubplots.pdf PDH_OLTF_allsubplots.pdf PDH_OLTF_allsubplots.pdf PDH_OLTF_allsubplots.pdf PDH_OLTF_allsubplots.pdf PDH_OLTF_allsubplots.pdf
  1127   Wed Jul 16 19:42:25 2014 DmassDailyProgressPDHTotal PDH Open Loop Transfer Function Budget

Nic and I have been trying to get an OLTF of the PDH board, but are running into a funny problem:

When we lock the cavity and take an OLTF, there is a HUGE (~1/f^3) starting around 250 kHz, which makes it so we can only lock with ~50 kHz bandwidth. When we turn up the laser power to increase the gain, we get huge gain peaking at this frequency.

Playing around last Thursday, we were changing things (gain, which FSR we were locking to via laser temp, attenuation between PDH board and current driver, plugging and unplugging the board), and at some point I was able to get this mystery feature to go away and lock with 150 kHz bandwidth, and the transfer function resembled what I put in elog:1126

I took the board out of the rack and powered it with a lab power supply to be able to poke it with a scope probe while locked / taking transfer functions, and I noticed that TP9 and TP10 both had huge peak-to-peak oscillations on them while locked (they were going rail to rail) - TP11 did not show this oscillation. They were oscillating at the frequency where the 1/f^3 behavior started. When I turned down the gain, the oscillations would quiet, but I would be restricted to ~tens of kHz UGF. 

At some point while debugging this, something unknown changed and I was unable to lock again (checked sidebands, power levels, error signal, servo sign, overall gain - all seemed fine). We will investigate more tomorrow.

  1137   Thu Aug 7 13:19:05 2014 DmassDailyProgressLab WorkOverdue Elogs

I have been delinquent elogging progress over the last couple weeks, here is (some of) what has been done:


  • Put sidebands on the laser with direct current modulation
  • Made vibration isolation / mechanical strain relief feed through at laser to reduce vibration induced frequency noise
  • Measured RAM levels - Worst case RAM levels low enough to not limit our noisebudget anywhere (we did not know this, and Mike Martin did not know this when I talked to him)
  • Added phase corrector path to East laser path - was able to see increase in UGF / decrease in noise (not yet cleanly measured, but maybe a ~400-500 kHz UGF total) - currently range limited by the AD829 pushing the EOM
  1138   Thu Aug 7 14:01:38 2014 DmassDailyProgressLab WorkLaser current sidebands

In order to free up the EOMs for a phase corrector path (and not have to figure out how to make a DC path on the resonant step up circuit without ruining the Q of said circuit) I decided to put sidebands on with laser current (again) and actually measure the RAM this time.

The input to the laser is as described in elog:676

There were three reasons we previously abandoned this scheme:

  1. The AM/PM ratio (at DC) was very high and made us nervous (AM/PM ~ 0.5: see elog:827)
    • Thinking slightly harder, it is clear that what we really care about is the changing RAM. Since the unknown source of RAM in the laser is unrelated to the known source of RAM in the EOM (drifting temperature -> drifting axis -> polarization misalignment).
    • It is completely possible to have a large DC RAM level and low RAM noise using the laser current modulation
    • Thus: we should measure RAM
  2. When we touched/stressed the cable connected to the current modulation input, we saw huge (~10MHz) low-frequency frequency shifts (seen while sweeping the cavity)
    • Some strain relief as done for the current modulation cabling (elog:1111)  can mitigate this problem to some degree
  3. In order to use PMCs in our scheme, we would have to put the PDH sidebands on after the PMC
    • This will still be a problem if/when PMCs are put into play. For the time being this can be ignored/tabled

After making a strain relief / mechanical isolation feedthrough for the current modulation path and putting the SMA to butterfly PCB adapter board back on the laser, I put sidebands back on the laser using current modulation

I measured modulation depth by sweeping the cavity and looking at transmission sidebands and was able to achieve gamma = 0.68 @ 33.6 MHz relatively easily (sending +7 dBm at the laser current modulation port)

I demodulated and used a lock in amplifier to monitor the I and Q f the RAM, and recorded it in the cymac - see attached drawing of the setup

I recorded the signals (as well as the measurement noise floor levels) in the Cymac with the following channel names:

  • AUX1: DC Power Level Monitor
  • AUX2: Theta (I/Q angle drift)
  • AUX3: R (total RAM amplitude)

Notable from coherence plot:

AUX3/AUX1 is not appreciably coherent anywhere, so we are not seeing a total power level based fluctuation here

AUX3/AUX2 IS coherent at low frequency (up to ~2 Hz), so whatever is driving the RAM also drives the phase angle of the signal. This is not shocking, just noteworthy.

Calibration needed:

  • AUX1: Counts/(Vdc at output of PD in 50 ohms)
    • [4182 cts] / [272 mV dc into 50 ohms]
  • AUX2: Counts/Deg
    • [32080 cts] / [360 deg]
  • AUX3: Counts/(RAM level measured at output of RAM mon PD into 50 Ohms)
    • [4980 cts] / [3 mVpp at output of PD into 50 ohms]

Applying this calibration we get:

  • Gamma_AM = 1.5/272 = 0.0055 (this is smaller than the old numbers I quoted, but also should be a more accurate representation of what is actually going into the cavity than previous measurements based on where we are measuring).
  • To double check / confirm the calibration: make sure driving 0 dB at the current mod input produces gamma_AM ~ 0.005 (or 3 mVpp RAM on a 272 mVdc signal)

Non DC Gamma:

[ASD in RAM] = [ASD in counts] * [1.5mVpk / 4980cts] * [1Vrms/1.41Vpk] * [1/272mVdc] 

[RAM/Count] = 7.9e-7 RAM/Count

[Freq Noise ASD] = [RAM ASD] * [P_incident] * [Hz/Watt]

P_incident = 1 mW

Plant = 2e-8 W/Hz

Freq noise = RAM * 1e-3 / (2e-8)

At 1 Hz we can see we have: 10 cts/rtHz * 8e-7 RAM/cts * 1e-3W/ (2e-8 W/Hz) = 0.4 Hz/rtHz

At 10 Hz we have 4e-3 Hz/rtHz

Compare to our noisebudget in elog:1099, we can see the RAM noise is above the coating thermal noise but well below the other experimental noises at 1Hz, and well below the coating thermal noise by 10 Hz.

The DC offset is 3.3mV at the error point from the RAM

The next  question: is the phase of the RAM changing such that, once demodulated, we are limited by the RAM (since what we care about is I, not sqrt(I^2 + Q^2))

The maximum amount of RAM noise from the RAM phase noise is about the same (within a factor of two) as the pure RAM noise (it seems not so surprising that a transformation of variables from I and Q to R and theta and back to I and Q would yield noise which is equal in amplitude for both variables) 


Attachment 1: 0807141425.jpg
Attachment 2: fmodramplot.png
  1140   Tue Aug 19 11:10:48 2014 DmassDailyProgressLab WorkLaser current sidebands

Can't use current modulation based sidebands in "as-is" setup - there were HUGE (5%) line harmonic (60, 120, 180) dips in transmission which got worse as we started gain peaking.

When I switched to using the Pockels cell for PM sidebands, here were no visible dips in transmission visible on the scope when triggered on the AC line.

This means:

If we want an actuator for the phase corrector path, we either need to hunt for and find/mitigate the source of this ground loop, buy another Pockels cell, or add a DC summing path into the resonant sideband circuit


  1191   Thu Jan 15 18:27:02 2015 ZachDailyProgressSiFi - ringdownSapphire washers added, ringdown setup rebuilt, higher Q measured

[Nic, Zach]

Yesterday, we opened up the small cryostat and installed the sapphire washers (SwissJewel SP-175). This is hypothesized to increase the resonator Q by reducing the strain energy leaking into the lower-Q steel clamp.

We found that the inner diameter of the washers is slightly too small to accomodate the inner lip of the lower part of the clamp. We were able to make do just by having the lower sapphire washer sitting on this lip---rather than on the full wider area of the lower clamp section---but it is not ideal.

Nevertheless, we clamped it, resealed and pumped the chamber down. As it pumped, I rebuilt the HeNe optical lever readout. When I finished, I was quickly able to tap the cryostat and see a mode ringing at almost exactly 250 Hz, which is known to be the frequency of this cantilever at room temperature. At a respectable pressure of several x 10-5 Torr, I made a quick-and-dirty ringdown measurement using a scope and a stopwatch. I estimated \tau at roughly 2.5 seconds, giving Q ~ 2000. This was already a few times higher than Marie was able to measure at room temperature (see below).



I went down today and did an actual measurment, using the Zurich box sampling at 7 kHz as DAQ. Fitting the envelope by eye, I found a time constant closer to \tau = 5.55 s, giving Q ~ 4300 (I don't think my stopwatch method was all that wrong yesterday, but I do think the residual gas might have been contributing at the time---the pressure is now at 10-7 Torr). This is not only much better than the previous result, but also within a factor of less than 3 of the expected result for Si, according to Marie's data. Given how cavalier we were with the clamping, I'm fairly confident that the sapphire washer idea (and therefore also the monolithic thicker-clamp idea) works as intended.



  1193   Thu Feb 5 02:04:39 2015 ZachDailyProgressSiFi - ringdownNo big Q increase at low temperature

Dmass helped me solve the Great Funnel Problem of 2015 by fashioning a foil extender to put in the tip of his metal funnel, since my glass funnel has a spout that is too narrow to get enough nitrogren through it. We spent some time yesterday afternoon filling the reservoir, after which I waited and then came back to see if it was still holding liquid. It was, so I added some more and left it overnight, and there still seemed to be some liquid by late this afternoon.

Assuming the cold volume had had enough time to reach low temperature, I made a quick ringdown measurement, only to find that the Q had only increased from ~4000 to ~8000 between room temperature and now. I think this means that the clamp integrity afforded by the sapphire washer sitting on just the lip of the steel clamp is not good.

I'm going to wait for things to warm up and then vent the chamber so that we can:

  1. Improve the clamp
  2. Fix our wiring issues
  1194   Fri Feb 6 04:23:20 2015 ZachDailyProgressSiFi - ringdownNo big Q increase at low temperature

I monitored the reservoir level periodically over the day and night. As of the evening, there appeared to be ~1 cm of LN2 still there. As of around 4am, it appears empty, so it should be OK to open tomorrow. I've sealed the vacuum and shut off the pump in preparation.


I'm going to wait for things to warm up and then vent the chamber so that we can:

  1. Improve the clamp
  2. Fix our wiring issues


  1197   Sat Feb 7 03:50:14 2015 ZachDailyProgressSiFi - ringdownElectrical connections fixed, clamp adjusted, chamber repumped and cooling

I vented the chamber today to redo the clamping and investigate our wiring issues.


Since I observed relatively low Q even at cryogenic temperatures, I assumed there was some jankiness with how we clamped the cantilever when we installed the sapphire washers. Recall that the lower part of the steel clamp has a circular lip near the center around the screw hole, and it was too wide to allow the sapphire washers to fit around it. This meant that the lower washer was only being held by this lip, and not by the full surface area of the clamp. Also, when we installed the washers, we didn't remove the smallest can around the physics package, so we were doing a bit of guesswork as to how well aligned the entire clamp stack was. This meant that there could have been some slight rubbing, for example. Here is a photo of what it looked like in profile when I did remove the can today:

You can see what I mean about the lip, and it's also clear that the stack was not very well aligned. To fix the lip problem, I found a steel washer that was just about the right thickness and drilled the center hole out wide enough that it fit around the lip. This way, the lower sapphire washer will be supported by a larger surface from below (of course, the real solution will be to either design a new clamp or get wide-enough-ID sapphire washers). The picture on the left below shows the washer around the lip.

There was also some dust and other gunk visible to the eye, so I thoroughly cleaned all parts in the stack with methanol and isopropanol. I then carefully restacked the components and reclamped (a little tighter than we did last time, as well). The final stack is shown below at right.



I checked each connection from the feedthrough to the heater or RTD, and found that everything seemed to be in order, so there must have just been a short when we closed up last time. I wrapped some extra kapton around each connector solder joint to provide insulation and extra strain relief, and everything stayed as it should be when I resealed the chamber. I *did* accidentally break a joint on the wire for the ESD while closing up---whoops---but I decided it was more hassle to fix it than necessary for this next run. I'll resolder it when we cycle again.


The chamber is under vacuum now and I filled the reservoir with nitrogen. The clamp was at 200 K when I left around 10pm, so I'm hoping things will be calm and cool when I come in tomorrow.


  1198   Sun Feb 8 02:49:27 2015 ZachDailyProgressSiFi - ringdownQ still low after clamp adjustments, mode cross-coupling suspected

The cantilever was fully cooled by the time I got in this afternoon. I measured some quick ringdowns by looking at the amplitude on the scope, and estimated a Q of 2-2.5 x 104. This is slightly better than what I measured the other day before improving the clamping (see CRYO:1193), but not good---still a few orders of magnitude below what we expect. I heated the system up near 120 K and found a slight reduction in Q.

Unlike before, I noticed a strange sort of sloshing of energy into a higher-frequency mode (~1350 Hz). It was hard to tell, but I got the sense that energy was being dissipated out of the fundamental mode through this higher-order one. I looked at a time-lapse spectrum of the ringdown, and it seemed to confirm this effect. If you look at the movie below (which is just about real time), you can see that the RMS of the two modes between 1-2 kHz pump up and down, while the fundamental mode around 215 Hz monotonically decreases. If you squint, it appears that the full RMS stays constant in most cases while the high-frequency modes ring up, while they all decrease together. This, coupled with the fact that everything rings down to zero if left alone, indicates to me that energy is leaking from the fundamental mode out through these others. As an order-of-magnitude estimate, the amount of energy pumped through these modes as the amplitudes increase and decrease is not inconsistent with the energy lost from the fundamental based on the observed Q.

I did some COMSOLing to try and figure out what is going on, and at first I couldn't explain it; it appeared that even the higher-frequency modes should have too little strain energy density leakage into the steel to explain the effect, especially with the sapphire spacers. In looking a little more carefully, though, I realized that we have not been careful enough in modeling our system: at the bottom of the clamp stack, there is a PEEK platform between the clamp post and the cold plate. This is there by design, to thermally insulate the clamp from the bath (for heating), but it also considerably softens the contact there.

This PEEK piece shouldn't have much of an effect on the fundamental mode, as the energy ratio for that mode is of order 10-4. The second mode at 1350 Hz is nearly as well isolated. However, for the third mode around 1800 Hz, something like 70%(!!) of the energy is expected to reside in the PEEK layer. Since PEEK has very high loss, this is not good. Here are some COMSOL screenshots, with the first 3 showing the first 3 mode shapes, and the fourth showing the (log) strain energy density for the 3rd mode. Note that this model is run at room temperature, so the eigenfrequencies are somewhat higher than in my spectra.


So, my hypothesis is that somehow energy is leaking from the (otherwise well-isolated) fundamental mode into these higher-order ones, where it is immediately lost to friction in the PEEK. One possible step is to get rid of the PEEK piece, but that doesn't address the question of why the cross-coupling exists in the first place. My intuition fails me, so I'm not sure what the right thing to do is.

  1200   Mon Feb 9 18:49:55 2015 ZachDailyProgressSiFi - ringdownQ ~ 6800 at room temp with Si sandwich

As I planned yesterday (CRYO:1199), I tried out a new clamp using spare pieces of broken silicon instead of sapphire washers to sandwich the cantilever (as with the last run, I used the old, stiff rectangular block clamp---the newer cylindrical one is still in the cryostat).

I didn't take a photo, but this was basically just a sandwich consisting of the cantilever (still attached to the central wafer region) as the meat and two scrap broken-off cantilevers on each side as the bread. This was all put near the center of the steel block clamp so that the clamping force was normal, and I made sure that the protruding cantilever had enough room not to be clipped by the block as it swings.

I put it in the new chamber and pumped down, and immediately measured a fairly high Q of ~6800 (ringdown tau ~ 6.4 s, while the mode frequency is ~340 Hz---slightly higher than before due to the clamping being a bit further along the cantilever).

This is the highest room-temperature Q I've yet measured, beating the ~4300 I measured after we first installed the sapphire washers on the newer cylindrical clamp (see CRYO:1191), and is within a factor of 2 of Marie's prediction in the absence of clamping loss (also shown in that post). This is also by far the cleanest ringdown I've seen: there are a few high-frequency modes present when I first deliver the impulse, but they die away and do not return. The Q also seems far less amplitude-dependent than I've noticed before.

  1201   Tue Feb 10 04:37:05 2015 ZachDailyProgressSiFi - ringdownQ consistently lower in cryostat

A lot of things happened tonight (mostly in the realm of setbacks followed by recovering frome them), but the take-home is that the measured Q of my silicon sandwich clamp seems consistently lower when measured in the cryostat, compared to in the new chamber from the gyro. Here's a rundown of what happened today/tonight:

  • Before dinner, I made a first measurement on the silicon sandwich idea (cantilever sandwiched between a couple spare pieces of silicon on each side --- see CRYO:1200). This gave me the highest room-temperature Q I've measured yet at ~6800.
  • After dinner, I wanted to port this to the cryostat and potentially do a cooling run. Unfortunately, to fit it in the cryo volume, I had to flip the sandwich around so that it was protruding from the clamp in the other direction (for the first run, I had it sticking out over the power resistor to avoid clamping in the region on the other side that has the groove for the Glasgow-style cantilevers, but there wasn't enough room for that orientation in the cryostat, so I had to flip back---I made it work so I didn't clamp over the groove anyhow).
  • Unwittingly, I made the dumb mistake of not first testing this freshly-clamped system again in the simple chamber, and after I closed the whole cryostat again and pumped down, I measured a much lower Q (back down around 3000).
  • So, I opened the cryostat again, and then spaced out and made the further mistake of still not testing this apparently bad clamp job in the simple chamber, just to verify that I got the same low Q. Instead, I went straight to cleaning all the pieces and re-clamping.
  • This time, I put it into the simple chamber and immediately recorded a high Q around 7000 again.
  • This is when some setbacks kicked in:
    • In opening the chamber, one of the RTD wires came loose from the feedthrough.
    • Not realizing that these were just press-fit sockets, I unscrewed the feedthrough to have access so I could reattach the single loose wire, only to have several others fall off.
    • So, I disconnected all the wires, spent some time mapping which one went where, re-soldered some and re-kapton shieled all, then reattached all wires, bunch taped them and taped the bunch to the feedthrough so that none could easily come loose. I also took this time to resolder the ESD wire that I broke the other day.
    • In moving stuff around, I accidentally tugged on the ribbon cable between the QPD and its vectorboard readout circuit, pulling a couple connections.
    • So I spent some time fixing that
  • Now I was ready to do science again, so I transferred the (known good) clamp from the simple chamber back into the cryostat and carefully closed it all up again.
  • After seal and pumpdown, I again measured a low Q around 3000.

So, it seems that the Q is repeatably lower for a particular clamp in the cryostat vs. in the simple chamber. To be sure, I'm going to do the final step of returning the clamp back to the simple chamber tomorrow and see if I again get a higher Q.

I'm not exactly sure why this could be happening. The only mechanical differences from one chamber to the other are:

  1. The clamping block is screwed via holes in the PEEK base to the cold plate in the cryostat, while it is dogclamped to the breadboard in the simple chamber.
  2. In the cryostat, there are wires soldered to the power resistor attached to the clamping block as well as a wire-attached Pt RTD kapton-taped to it. None of this is present in the simple chamber.

I'm tempted to think that (2) could be causing some excess damping, so one thing I will try is simply not connecting these just to see if that makes the probem go away.

  1202   Wed Feb 11 03:15:02 2015 ZachDailyProgressSiFi - ringdownOnly some extra damping is from wires

Following my preliminary conclusion from yesterday (CRYO:1201), I set out to confirm or deny this seeming decrease in Q for a given clamp when going from the simple vacuum chamber to the cryostat.

One potential source of extra damping I considered was the wires attached to the block for the power resistor and RTD, so, while I still had the clamp in the cryostat assembly, I just disconnected these wires and pumped down the cryostat to see if I saw an improvement. I did see an increase in Q from ~3000 to ~5500, but not to the full 7000 I saw before in the standalone chamber. So, I conclude that there is some appreciable damping added by this kapton wiring. We need to use less rigid wire for the last stretch between the coldplate-mounted strain releif and the block.

The last step was to transport the clamp back into the simple chamber and see if I could recover the Q of 7000 that I measured initially. I did, completing the circle of repeatablility. I'm not sure what else could be causing the excess damping in the cryostat.

It is a shame, because I would be very interested to see what this particular silicon sandwich clamp looks like at 120 K, but I seem to have now way of doing so without the extra losses empirically associated with putting it in the cryostat.

  1205   Fri Feb 20 05:22:19 2015 ZachDailyProgressLaserError calibration -> actuation TFs and new laser frequency noise measurement

Using the REFL PDH setup I built the other day (and that was detailed somewhat by Nic and Chris W. in CRYO:1204), I calibrated the error response so that I could make some further measurements. To refresh, this is using 30-MHz sidebands applied using one of our fiber phase modulators, sensing with a 1611 in reflection. The sideband drive was 0 dBm.

Using the sidebands as a reference, I calculated the slope at 6.4 nV/Hz:

Note that the error signal is slightly asymmetric, but there is no large offset.

With this information, I made some measurements:

Actuation transfer functions

I wanted to measure the actuation transfer functions afforded by:

  1. The standalone ThorLabs laser diode driver (LDC201C) that is currently used to drive the west laser
  2. The diode driver in the integrated ThorLabs laser controller (ITC502) that is currently used to drive the east laser
  3. Simply driving into the bias tee on the diode

All measurements were done with the west laser head, driving as described and reading out at the error point, then correcting for the loop gain and calibrating to Hz.

Everything is more or less as expected:

  • The LDC201C only claims 3 kHz bandwidth, which is actually a bit of a stretch as usual
  • The ITC502 claims 500 kHz, and this is also a stretch (there is -45° at 40 kHz), but it's not so bad for some early locking
  • Directly driving into the diode works across this measurement band and likely far, far beyond
  • There are some common features, prominently at low frequencies and somewhat so at higher ones that likely arise from the current -> frequency response of the diode itself


Laser frequency noise

I now have another calibrated measure of laser frequency noise, wherever it dominates over the PMC length noise. I measured the error signal, corrected for the loop gain and calibrated to Hz. For comparison, I've added the measurement using the Zurich PLL on the beat between the two free-running lasers on 12/17/2014 (see CRYO:1185), as well as the RIO spec for this laser.

As you can see, tonight's measurement agrees quite well with the earlier one upt to ~1 kHz, above which the old measurement is probably marred by the relatively low-bandwidth PLL. It seems that the PMC is quiet enough to see the laser noise throughout, and the new measurement now sits closer to the spec up to the highest available point at 10 kHz. Below ~50 Hz, we are probably seeing the well-documented excess noise from the ThorLabs driver. Everything looks as expected.

Locking via laser feedback

Relatively early in the night, after having measured the actuation transfer functions, I sucessfully locked the cavity via feedback to the laser (as opposed to the PMC PZT) for the first time. Below is a comparison of the OLTFs for a 1-kHz loop using the same servo shape (a pole at 1 Hz) using both actuation schemes.

Because of what I had hooked up at the time, I only did this with the (low-bandwidth) LDC201C, so, while the absence of a ~10-kHz resonance is clear, the phase margin is not improved at all (worsened, actually). I only report this as a milestone, and the margin afforded by the ITC502 or by directly driving via the bias tee should be far better.

  1206   Fri Feb 20 05:41:04 2015 ZachDailyProgressLaserSimple ISS test

I wanted to do some intensity feedback testing, for two reasons:

  1. Just to get used to using the fiber amplitude modulators
  2. While I wait for the machined parts for the 1064nm M2 ISS testing on the old gyro table, I might as well use this basically perfect setup to do some initial runs with the M2 at 1550nm

So that I can have as much power as possible, I removed the fiber phase modulator and installed the amplitude modulator in its place. To generate the PDH sidebands, I simply drove into the laser bias tee with the 30 MHz oscillator signal and increased the amplitude until I got the same modulation depth as I measured with the modulator. I also had to readjust the demod phase via cable lengths, but after that the cavity locked just as before (and with an identical OLTF---not shown here). I don't claim that this locking technique is as good as using a phase modulator, in light of possible RFAM effects, but it is likely fine for intensity testing.

I also tried to increase the DC drive current of the laser, but it kept stalling after I tried to increase it above ~115 mA (the output power would increase in accordance with the plot on the datasheet, but then would suddenly crash and not return if the current was lowered until the driver output ON/OFF was cycled---not sure what gives here). So, I set it to 100 mA, where it seemed stable. The output of the laser head at this current is ~12 mW, so the max-transmission output of the amplitude modulator is about 6 mW (due to the 50% insertion loss). Adding a slight DC offset to the modulator, I reduced the output to ~92% to get some linear actuation strength for feedback.

I then tried to create an AC-coupled loop with an SR560, but had problems with stability on the low end. Eventually, I gave up and used the A-B function to subtract the measured DC level of around 4 V from the TRANS PD signal. I then put a pole at 300 Hz and scaled up the gain until I saw oscillations up near 100 kHz, and then slightly back down. Using this offset-subtracted DC-coupled loop, I was able to get solid in-loop performance, obtaining a UGF near 100 kHz and suppressing fluctuations to the dark noise level (consistent with the PDA255's noise) over a wide band.

The next step will be to use my low-noise readout optoelectronics and try out the Chachi servo.


  1210   Tue Feb 24 05:17:04 2015 ZachDailyProgressM2 ISSFirst real M2 test

Tonight I succeeded in using the M2 ISS readout board and the 3-mm diodes to do some real intensity stabilization using the SiFi test setup.

First, I built a foam box to use as a temporary enclosure for the PMC and diodes until we get our real box finished:

There are holes for the input and REFL beams, and the diodes are held with makeshift mounts that clamp down on the sockets. Clearly, these aren't as stiff or stable as what we're having built, but they do the job for now. There is a steering mirror before the 50/50 BS so that the position on each diode can be adjusted separately with ease.

I didn't want to open the Chachi ISS box can of worms yet, so I just built my own temporary breadboard circuit. I had done some preliminary SR560 locking, so I knew roughly what I wanted, and I measured the modulator -> PD transfer function again today and verified that it was flat well above 100 kHz. I made a 2-stage pole/zero-style circuit, with a double (removable) pole at 300 Hz and a zero at 10 kHz to bring phase back to -90° around the target UGF of 100 kHz. It looks like this:

I wanted to DC couple, so I came up with an idea to pick off the stable 5-V bias supply from the M2 board and sum it with the (negative) output of the in-loop PD in the first stage of the servo. I had some current-related issues with the summer at first, but these went away when I increased the input resistors a bit (n.b., to fix the gain I had to change some other components, and as a result the controller TF is actually slightly different than shown above, but not much).

Hooking it up, it locked right away with the expected UGF of near 100 kHz (not yet measured, but inferred from the transfer functions and spectra). Here is the stabilization result:

As you can see, the out-of-loop signal is stabilized to the shot noise level (which is \sqrt{2} higher than the bare shot noise for half the beam due to the well-understood correlated noise imprinted by the loop) from about 5 kHz down to just below 100 Hz. Below this, there is clearly some differential environmental noise between the PDs. I did some beam scanning to try and minimize with some success, but not much. I'm not sure what the coherence below 20 Hz indicates---the in-loop signal is suppressed to below the measurement noise level, while the OOL signal exhibits excess differential noise, so I don't see why there should be any coherence.

In any case, this is a nice verification that:

  • The M2 readout board works with real optical signals
  • The intensity feedback system for the SiFi experiment works
  • The 3-mm diodes (save for the one bad one---see CRYO:1207) behave nicely, at least for these relatively low powers
  • The PMC -> PD scheme shows promise for our future tests with nicer hardware
  1211   Wed Feb 25 04:29:49 2015 ZachDailyProgressSiFi - ringdownTaiwan cantilever has higher Q, going for cryo cycle now

[Nic, Zach]

Nic got a Glasgow-style cantilever from a group in Taiwan, and a quick test in the rapid cycle chamber showed that it had pretty low loss, so we are running it in the cryostat now. As a reminder, these are the rough dimensions of this style cantilever:

Below is a photo of the box it came in, showing the actual 92-um thickness of this sample, as well as a shot of it in the vacuum chamber. For some reason, this particular sample's clamping tab did not fit in the groove that Nic had built into the clamping block for the other Glasgow cantilevers, so I had to mount it to the side against the flat faces of the clamp (as I've been doing with our larger samples).


This evening, we transferred it over to the cryostat and restored all the electrical connections for what will hopefully be a fruitful cryo run. Here is a ringdown of the fundamental mode (~106 Hz) at room temperature:

The measured decay time of 41 seconds corresponds to a Q of around 14,000, which is about as good as we expect at room temperature. This sample is probably better than our other ones for at least 2 reasons:

  1. It is made from a better-quality (FZ) wafer, and
  2. It has been manufactured monolithically with a thicker clamping tab, which our modeling suggests is a very effective way to evade clamping losses by keeping strain energy within the silicon.

Given that we didn't see much improvement at all with our other samples when going to low temperature, I believe (2) is by far the biggest effect. The Glasgow wafers only have the clamp-region thickness extended to one side, which is modelled to be worse than if you go both ways, but it is still much better than we can do with our discrete sandwiching.

I filled the LN2 reservoir and the volume is cooling overnight. I did some rough ringdowns at a point when the steel block was registering around 160 K and found greatly improved Qs already (approaching and perhaps exceeding 105). We will continue to make measurements tomorrow.

  1213   Fri Feb 27 05:47:27 2015 ZachDailyProgressSiFi - ringdownTaiwan cantilever fundamental mode Q

[Nic, Zach]

We measured the Q of the fundamental (~106 Hz) mode of the Taiwan cantilever in two ways. First, we used Nic's active steady-state method, and then we did a traditional ringdown. The results seem to agree, but the precision of the first method is much better due to the dynamic range of the readout for this mode: the motion becomes nonlinear at an amplitude only a few times greater than the background excitation level. Over a ~4-hr average, the loss is measured to be 1.45 x 10-6 ± 2.9 x 10-7, giving a Q of ~6.9 x 105.

Here is a plot of the instantaneous phi from the calibrated control signal. This data has already been fed through a ~1-hr lowpass, and then the data from the initial settling time has been truncated away. The mean and standard deviation of the rest of the points are what is reported.

After this measurement was made, we shut off the servo and allowed the mode to ring down. Here is that ringdown, along with a predicted range of theoretical curves using the result from above. As you can see, they are fairly consistent with what is measured, considering that the system quickly reaches a regime where it is excited by the environment (that is, only the initial part of the ringdown, where the agreement is good, is very trustworthy).

This Q is a couple orders of magnitude lower than what is expected for this mode at this temperature, but it is also only a factor of 2-3 worse than the best measurements using a similar apparatus at Glasgow (to my knowledge).

It bugs me that we don't seem to have any information about what steel looks like at low temperatures. Given my COMSOL strain energy modeling, the energy ratio for this mode is about 3 x 10-4, so this could be explained by clamp loss if the steel Q is as low as a few hundred. I'm looking into other modes to try and support or refute this hypothesis; since different modes have different energy ratios, we may be able to see what's going on. In parallel, I'm asking Matt and others to find out what is really known about cryogenic steel.


  1214   Wed Mar 4 02:32:45 2015 ZachDailyProgressSiFi - ringdownSi spacer added to clamp holding Taiwan cantilever

The most recent measurements on the Taiwan-sourced Glasgow-style cantilver (see CRYO:1213) are encouraging, but the best Q measurement at low temperature is still a couple orders of magnitude worse than what is theoretically achievable, and about one order of magnitude worse than our conservative clamp loss estimates. Also, I've done some measurements on other modes (that have different expected clamp loss contributions due to the relative strain energy ratios) to try and sort out what is going on, with little success. Finally, some modes---including the 2nd bending mode at ~650 Hz---exhibited very low Q for no known reason.

One thing I thought about is that, since the Taiwan cantilever did not fit in the groove that was built into the block for the Glasgow-style cantilevers and therefore is just sandwiched between the two large pieces making up the clamp (see CRYO:1211), the clamp is likely pushing down at somewhat of an angle, which could lead to all sorts of non-idealities. Since the other Si samples we have lying around are roughly the size of the clamping region of this cantilever (~300-500 um), I opened up the cryostat today and reclamped the cantilever using a spare broken-off 300-um-thick cantilever piece as a spacer on the other side:

Pumping it all back down, I immediately measured Qs a bit higher than what we saw last time around at room temperature. The last measurement I made before leaving was tau ~ 135 s ==> Q ~ 46000, though it had been increasing up to that point, likely from the residual pressure, which was at ~10-3 Torr when I left. Compare this with the Q of ~14000 from the last time around, though admittedly I did not record the pressure at which this was measured.

  1216   Thu Mar 5 22:35:43 2015 ZachDailyProgressSiFi - ringdownTaiwan room-temp Q > 10^5, cooling now

On Tuesday night, when I added the Si spacer to the clamp, I measured a Q of ~46000, but I noted that it had been increasing up to that point, likely due to the residual gas damping (see CRYO:1214). Last night, I made another measurement and found it to be much higher, at ~1.2 x 105 (tau ~ 350 s). This is much better than we have seen at room temperature thus far, so it looks like my spacer addition has helped.

I remeasured this an hour or so later and saw no appreciable increase. I checked again today and it appears as though it may have increased slightly, but it was hard to say for sure due to higher environmental noise. Really, we need the steady-state ringdown to make a good measurement at this level.

The LN2 dewar was refilled today, so I filled the cryostat and we'll see how it looks at low temperature tomorrow.

  1217   Fri Mar 6 19:02:28 2015 ZachDailyProgressSiFi - ringdownQ at low temperature after reclamp… worse?

The cantilever had cooled to around 100 K by this morning, so I set up the mode ringer and began an active measurement on the fundamental mode. The online loss angle measurement for a 3-hr period beginning around an hour after lock is shown below (this is the control signal filtered by a 2nd-order low pass at 0.2 mHz.

As you can see, the loss is hovering around 3 x 10-6, giving a Q around 3 x 105, which is slightly but significantly lower than what we measured before I added the Si spacer to avoid skewing the clamp (CRYO:1213). I would chalk this up to the spacer actually making the clamp worse, but we did in fact see a huge improvement at room temperature (CRYO:1216). So, like, what the hell man?

I've left it running to collect more data over the weekend. I haven't gone over the temperature readout/control system with Nic, so I set up a simple temperature readout in the meantime so that we can have at least a coarse Q(T) measurement as it warms. To do this, I simply put a 1k resistor inline with the RTD and put 5V across with a lab supply. The second set of RTD leads goes to the temperature readout input in the digital system, so this is now just a DC readout of the voltage across the RTD. The lockin input channel X1:SCQ-TEMPERATURE_LOCKIN_DEMOD_SIG_OUT is calibrated to volts, and is equal to 5 V * RRTD/(1k + RRTD).


The LN2 dewar was refilled today, so I filled the cryostat and we'll see how it looks at low temperature tomorrow.




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