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ID Date Authorup Type Category Subject
  1590   Tue Jun 13 12:48:33 2017 VineethNotesDAQADC Effective resolution test

Both the ADC channels were grounded and a time series of 16,384 samples (acquire utility in RP is used to record data) were taken to measure the offset and the effective resolution of the channel.

The distribution of the ADC codes was plotted and a gaussian fit was applied.

Channel 1:

The standard deviation was calculated to be  \sigma_{ADC} = 4.6 $ codes  which gives 

N_{eff} = Log_2(\frac{2^N}{6.6*\sigma_{ADC}}) = 12 $ bits    against the 14-bit resolution specified in the datasheet.

Channel 2:

The standard deviation was calculated to be   \sigma_{ADC} = 4.8 $ codes which gives 

N_{eff} = Log_2(\frac{2^N}{6.6*\sigma_{ADC}}) = 12 $ bits    against the 14-bit resolution specified in the datasheet.

 The deviations from a perfect gaussian distribution here qualitatively suggest the extent of non-linearity while sampling. The second channel seems to have high non-linearity errors compared to the first one.

A detailed analysis of the non-linearity errors is possible, but requires atleast 10M samples for a resolution of 0.1LSB and a 99% confidence bound. The acquire script in Red Pitaya can sample atmost 16,386 points which is very low compared to our requirement. 

The same limitation applies for measuring the maximum SNR of the ADC channels.



Attachment 1: g_fit_adc_ch1_gr.png
Attachment 2: g_fit_adc_ch2_gr.png
  1600   Mon Jun 19 19:39:13 2017 VineethNotesDAQADPLL simulation

I closed the ADPLL with a Low pass filter (FIR) and a PI controller with Kp = 0.008 and KI = 0.01. The  simulation results are attached.  The lock in range was measured to be \pm 1.062 $ MHz (through simulation) about the free running frequency of the NCO, beyond which the PLL does not lock onto RF signal.

Attachment #1 - The ADPLL circuit and the output signal for a step response of 375 kHz.

Attachment #2- Bode plot of the digital FIR filter.


Attachment 1: Screenshot_from_2017-06-16_17-26-42.png
Attachment 2: Screenshot_from_2017-06-19_19-35-10.png
  1601   Mon Jun 19 20:03:41 2017 VineethNotesDAQADPLL testing

The previous ADPLL design was programmed into the FPGA and tested for the lock-in range.

ADPLL Output

Input frequency


Output Voltage


24.375 4
24.500 134
24.600 238
24.700 342
24.800 447
24.900 549
25.000 650
25.335 998
25.350 out-of-lock

The free running frequency ( the initial frequency offset of the NCO) was set at 24.375 MHz.  The lock in range of the ADPLL was measured to be \pm 0.975 $ MHz.

The attachments below show the output voltages for the locked in state of the PLL for different inputs (2nd and 3rd rows of the table).

Attachment 1: IMG_20170616_171953.jpg
Attachment 2: IMG_20170616_172026.jpg
  1610   Thu Jul 6 13:04:18 2017 VineethNotesDAQClosed loop measurements

I made some modifications to the digital IIR filter, by replacing the multipliers with  constant multipliers where the coefficients are hardcoded in the FPGA. Two such filters are put in cascade in the ADPLL and the closed loop measurement was made.

The excitation for the loop was given in the feedback path and the output frequency estimate is monitored to obtain the closed loop transfer function. The input is given at -4dB Full-Scale of the ADC and an additional gain is multiplied to keep the loop in locked state during the measurement.

The plots for different values of the proportional gain is attached. The values of other parameters are, Ki = 0.01 (integral gain) and Ks = 0.004 (source gain).

Later, I'll update the gain calibrated plots and the open loop transfer function plots.


Attachment 1: Closed_loop.pdf
  1618   Tue Jul 11 16:19:28 2017 VineethNotesDigital PLLPLL output format

The frequency estimate in the PLL model is a 26-bit word and has to be truncated to 14-bits (the resolution of the DAC). So, I've added a function, pm_select_slice(chID,slice_number) in the python wrapper to choose the required bits from the complete word.

Let the frequency estimate be stored in a variable f_est, a 26-bit word with f_est[25] being the sign bit. Then, the output is determined as below, ({} implies concatenation)

slice_number Output word
0 f_est[25:12]
1 {f_est[25],f_est[23:11]}
2 {f_est[25],f_est[21:9]}
3 {f_est[25],f_est[19:7]}
4 {f_est[25],f_est[17:5]}
5 {f_est[25],f_est[15:3]}
6 {f_est[25],f_est[13:1]}
7 {f_est[25],f_est[12:0]}



  1619   Tue Jul 11 17:23:30 2017 VineethNotesDigital PLLPLL output calibration

The output format of the PLL was described in ELOG ID 1618.

Today I calibrated the output of the PLL for various slice parameters. The difference in the output voltage was measured for a change in the input signal frequency ( the input is given from the signal generator, so we know the frequency value exactly) and the conversion factor ( in V/Hz) was determined.

The measured values match with the calculated ones with a maximum error of  5.8%.

Slice Measured value (V/Hz) Calculated value (V/Hz) error (%)
0 8.27 x 10-9 8.00 x 10-9 3.37
1 1.65 x 10-9 1.60 x 10-9 2.92
2 6.49 x 10-8 6.40 x 10-8 1.44
3 2.65 x 10-7 2.56 x 10-7 3.58
4 1.04 x 10-6 1.02 x 10-6 1.59
5 4.10 x 10-6 4.09 x 10-6 0.16
6 1.69 x 10-5 1.64 x 10-5 2.95
7 3.09 x 10-5 3.28 x 10-5 5.79


  1620   Tue Jul 11 19:59:54 2017 VineethNotesDAQRed Pitaya ADC-DAC Transfer function

While measuring the transfer function for a signal from ADC to DAC, the plots showed a bump at a frequency of 250kHz which is clearly visible in the closed loop measurements in ELOG entry 1610. The source of this might be from the DAC or both ADC & DAC because if it were only from the ADC, the closed loop transfer function would not have recorded it as the signal goes through heterodyne demodulation.        


Attachment 1: ATF.pdf
  1622   Thu Jul 13 12:46:38 2017 VineethNotesDAQRed Pitaya ADC-DAC Transfer function

In the following link it is explained that the output of the DAC has a offset which is directly derived from the input DC supply voltage. This couples the variations in the input DC to the analog output voltage and might be the reason for the behaviour we saw in the ADC-DAC Transfer function.



While measuring the transfer function for a signal from ADC to DAC, the plots showed a bump at a frequency of 250kHz which is clearly visible in the closed loop measurements in ELOG entry 1610. The source of this might be from the DAC or both ADC & DAC because if it were only from the ADC, the closed loop transfer function would not have recorded it as the signal goes through heterodyne demodulation.        



  1623   Thu Jul 13 13:36:21 2017 VineethNotesDigital PLLLaser transfer function measurement

The phasemeter developed in the Red-Pitaya was used to measure the transfer function of the diode lasers.

The diode current of one of the free running lasers (not locked to the cavities) was modulated with a source of -13dBm. The beatnote obtained was around 20 MHz with a Vpp = 1.5V. The modulation changes the frequency of the beatnote, which in turn translates into the voltage change in the output of the DAC. The transfer function measured would then be the product of Laser TF, ADC->DAC TF, and the closed loop TF of the phasemeter. So, to obtain the Laser TF we divide the other factors out (which were measured with the same parameters).

The gains used for the measurement are Kp = 0.03 and Ki = 0.01


Attachment 1: PTF.pdf
Attachment 2: LTF.pdf
Attachment 3: CTF.pdf
Attachment 4: drawing.svg
  1631   Fri Jul 21 16:17:14 2017 VineethNotesDigital PLLLaser transfer function measurement

The updated diagram for the transfer function measurement is attached below.

Attachment 1: drawing.svg
  84   Sat Feb 19 11:46:49 2011 WarrenCryostatDrawingsCryo cavity support proposal

Attached below is the version 1 of a support for the cryo cavity. 

The basic idea is to mimic the lens-mounts that slip through a set of round "rails", actually shafts.

Not shown is a 1/8" copper plate "wrapped" around three sides, and a pressed against the frames by six large screws.

The only detail to be worked out is the precise way to shape and size the two "fins" that actually touch the silicon cylinder.

Now need feedback from y'all.

Attachment 1: cavity_mount_v1.png
  88   Mon Feb 21 10:10:29 2011 WarrenCryostatDrawingscryostat support


I like the idea of interchangable fins!  Thinking about good ways to do this.   And the CAD drawing looks great!  I am envious; maybe I need to learn how to do that.

There is at least one major mistake in the design shown: the two support frames need to be slide on the rails, and the rails need separate support brackets to attach to a rigid mounting plate above. 


Do Rana and David have comments?

How about we skype some further conferring today.   Let's set it up by email.



  90   Fri Feb 25 11:42:46 2011 WarrenCryostatDrawingscavity support version 2

Here is version 2.    Complete in all mechanical parts, if you can use a little imagination and read my mind on a few of the details.  Only part left out, that I know of, is the thermometers and heaters, but they are quite easy to add, probably on outside.

As drawn, maximum distance between knife edges is 2.5".   Enough?


Attachment 1: cavity_mount_v2_pg1.png
Attachment 2: cavity_mount_v2_pg2.png
  122   Fri Apr 1 14:17:26 2011 WarrenThings to Buy cryogenic o-rings

So Creavey Seal Company says they can make teflon coated orings that work down to -450 degrees fahernheit.  

(sic) I will try to attach their catalog.  Look at page 6 for the temperature range of the Astra Seal 

Attachment 1: Creavey-Brochure.pdf
Creavey-Brochure.pdf Creavey-Brochure.pdf Creavey-Brochure.pdf Creavey-Brochure.pdf Creavey-Brochure.pdf Creavey-Brochure.pdf Creavey-Brochure.pdf Creavey-Brochure.pdf
  208   Thu Jun 23 19:33:27 2011 Warren JohnsonNotesmaterial propertiesspecific heats of 3 materials

For thermal design, we need to know specific heats of materials.   I looked up the basics in the cryogenics book by Guy White, 3rd edition.  You need to do some simple calculation to get the needed numbers for the entire temperature range.   I did this for the three dominate materials in the cryostat: copper, aluminum, and silicon, and plot the results below.   The programs that do it are attached.   



Attachment 2: specfic_heats.m
% specific_heats.m
%    to get the specific heat of a solid, we can use Debye's model 
% for the specific heat due to lattice vibrations.
%   Only three ingredients, 1) the universal debye function, 2) the debye 
% temperature for the solid, and 3) the atomic mass (to convert from moles
% to grams.   Results are plotted for three elements.  The results checked 
% against wikipedia values for copper and aluminum.  


... 44 more lines ...
Attachment 3: debye_func.m
function c_v = debye_func(T_D)
% given normalized temperature T_D = abs temp T divided by debye temp
%     (valid  for .033 < T_D < 10 ) 
% return the specific heat c_v, in units of J/mol K

% load table of debye function, 
% copied from Experimental Techniques in Low Temp Physics, 3rd ed.,
%   by Guy K. White, Table D, page 314.
DT = [
    .0333 .072
... 29 more lines ...
Attachment 4: test_debye_func.m
% test_debye_func
%    check that interpolation routine is working.

% load in table of debye func 
DT = [
    .0333 .072
    .04 .124
    .05 .243
    .0625   .474
... 37 more lines ...
  237   Wed Jul 13 15:30:20 2011 Warren JohnsonCryostatDrawingstwo cavity support structure

Here is a .pdf drawing of a proposed support structure for the thermal shields and the cavities.   It is an aluminum frame which bolts to the bottom of the top plate.  The VectorWorks drawing file is appended.


Attachment 2: cavity_mount_7_10.vwx
  253   Wed Jul 20 15:03:03 2011 Warren JohnsonHowToCavityshield support

I have read the "link" above, and when I really understand it, I hpe will understand how the Airy point suspension works.   So far, I don't get the fundamental point, which is how to make the length independent of acceleration, or really, independent of weight.  

Regardless, in the mean time, seems to be no reason not to put the suspension of each cylinder at it's Airy point, which will not overlap, because the lengths are different.

  254   Thu Jul 21 11:41:13 2011 Warren JohnsonCryostatDrawingsdual cavity support frame drawings

Here are the proposed general arrangement drawings. 

This is version 2, or the version of July 20, hence the file name  cavity_mount_7_20.vwx. 

The main purpose of these drawings is to determine the configuration of the two "frames" which provide the mechanical connection between the

cold plate and the outer thermal shield, labeled shield A.  David should check that the cavities line up correctly with the windows and the cold plate of the cryostat. 

The main changes from the earlier version is the addition of a number of "struts" to stiffen up the frames in all directions.

The details needed for actual construction are in drawings yet to come, hopefully very soon. 

Frank Seifert has been a big help, and understands most of the reasoning behind this design.

The set-screws that support shield A from the frame are now planned to be "peek", which is a good thermal insulator.  

Extra thermal conduction between the two elements  can be supplied with metal wires; these wires are not shown. 

None of the other thermal and mechanical connections between shields and cavity are shown.     

These CAD drawings have been done in vectorworks, which gives a free version of their full package to academics.  

Shown is the .pdf generated by printing the CAD format .vwx.  Also appended is the drawing exported to the .dwg format, which may be readable by other CAD packages.



Attachment 2: cavity_mount_7_20.vwx
Attachment 3: cavity_mount_7_20.dwg
  261   Fri Jul 22 13:16:46 2011 Warren JohnsonUpdatePurchasesshould play with your new toys


You should play with your new toys.   You can start with the simplest cryogenic system, a dewar filled with LN2 (liquid nitrogen), attach the temperature sensors to a wood stick

(careful they can't slip off), wire them up to the electronics, and lower them into the the dewar, pausing for a minute or two when they are in the cold vapor, but have not touched

the liquid yet.  Then start to test stuff, like how much temp fluctuation do you see in boiling liquid (probably not small, and how much temp gradient inside the dewar, etc. 

Then borrow a simple test cryostat from somebody, mount two thermometers on the same piece of cold plate, cool down the cold plate with a modest stream of liquid, so

it take 30-60 minutes to reach 77K, and then monitor the temp difference between the two thermometers.   etc.   etc.    Have fun.

  262   Fri Jul 22 14:07:51 2011 Warren JohnsonCryostatDrawingsShop drawings for dual cavity test system

So here are the drawing promised yesterday.   I hope that any decent job shop can turn these into the parts we want.

First, a drawing with the 'holes' specifications and location


and then a specification of the geometery.







Attachment 4: cavity_mount_7_20_frame1C.pdf
Attachment 5: cavity_mount_7_20_frame1D.pdf
  264   Fri Jul 22 14:17:26 2011 Warren JohnsonCryostatDrawingsthe rest of the shop drawings

There is a serious bug in eLog, which accepts an upload in seconds, but then becomes unresponsive for as long as 9 minutes (! timed it for the last drawing in the entry just made! ).    It feels like it wants to take 9 minutes to make the thumbnail that it wants to display on my local machine, before it will let me do any other editing.   Decided to put the last drawing separately, to see if timing is different.

last drawing


Attachment 2: cavity_mount_7_20_frame1E.png
  329   Tue Nov 1 05:40:58 2011 Warren JohnsonCryostatVacuumFirst tests of cryostat

Congratulations!  You are now the proud owner of a custom Precision Cryogenics optical dewar! Enjoy!

My recommendations on acceptance tests:

Check for damage on the crate.  Photo any.

Open it.   Lift dewar by it's 2 lifting eyes. Use a bridle. Or lift it by hands from flange.

Pump out inner chamber, or optics space (OS). See if it holds vacuum; leave it at vacuum.  Pump out outer chamber (insulating vacuum, IV), Ditto. 

Leak check outside shell. Use He leak detector, open wide to IV, and spray He gas systematically over entire surface, especially on the windows and all the welds.  Looking for small cracks that evaded the factory test or opened up enroute.   Find em now, and it is the companies problem.  Find em later, and it is your problem.  Leaks through O-ring should be fixed by opening, moving or replacing O-ring, and repeat test of welds till satisfied.

Vent to air, undo top flange, lift whole inside up and out, from top plate eyebolts. 

The obviously fragile parts of this dewar are the windows, and the thin  G10 shell that connects the OS to the top plate, (especially it's epoxy joints).  A nice sharp blow to any of the fragile parts could cause an expensive break.  The pipes and valves sticking out of top plate can be damaged, but it takes a bigger hammer.

I would avoid stressing the G10 shell that connects the OS to the top plate.  In other words, don't set it down on the bottom of the OS.    Unless manufacturer states, in writing, that you can do this. 

Leak check the inner shell, or outside of OS, with He leak detector open full bore, spraying helium gas systematically over the outside.  Again, want to find cracked welds or window seals now, not later. 

You are done, until your first cooldown.  Before start, pump out both spaces, then backfill 1 Torr of helium gas into OS, put helium leak detector on the IV, and start flowing LN2 slowly into reservoir.  you are looking for leaks that open up at first cold shock.   Slow transfer means that cold N2 gas comes out other pipe at a modest rate. Should only take half hour, or less, to get a some liters of liquid accumulated.   If all is well, should see background on leak detector go down with cooling.     On second cooldown, you can up the transfer rate.     Good to do this soon, before manufacturer's implied (or explicit) warranty runs out. 

Last use of leak detector is a routine check of a new indium seal on OS flange.  You might occasionally find a leak and fix it sooner with a leak detector, rather than finding it after a LN2 transfer. 


  864   Thu Sep 19 16:08:08 2013 Warren JohnsonCryostat sketch of future cryostat

attached is my sketch for the day of a future cryostat for 1-300K work.



Attachment 1: cryostat_9_18_13.jpg
  54   Thu Jan 6 23:32:26 2011 ZachLaserLaserBeat Found

I think your higher-frequency ITC 510 data is just the same low-frequency data vector plotted against the higher frequencies. Might want to double check that.

DYM: Zach Wins.


Here is the RIN of the Diodes, as measured on the HP35670A.

A second plot is included, which shows what we think is the Driver contribution to the RIN...

  1. I took each laser diodes P vs I curve to get dP/dI, then divided by P_laser to get dRin/dI
  2. This gave me 0.138 W/A and 0.26 W/A for the Emcore and Covega Laser Didoes, respectively
  3. This corresponds to a dRIN/dI of 3.5/Amp and 5.2/Amp (if what I described in 1 is OK)
  4. The contributions to RIN in the noisebudget from the driver noise are HIGHER than the actual RIN measured
  5. I think this is fine and it merely means that the transfer function of current noise into diode RIN is not the flat unity gain model implicitly assumed in 1 above
  6. Maybe a good measurement to do at some point is to T a (low noise) buffer onto the input current to the diode, and measure the coherence and transfer function between the driver current noise and the laser diode RIN
  7. Using each laser diode's P(I) curve to get the transfer function of current noise into RIN seems increasingly less bad at low frequencies, which is I would expect (no crazy low frequency features in the transfer function) - and it looks like we might be limited by the driver noise for low frequency RIN.
  8. If RIN and frequency noise are coherent, and we close a loop on frequency noise, this won't be a huge problem, since actuator noise is supressed by our (not yet existent) loop gain, and it should crush the current noise...the question, as ever, is "Can we obtain enough bandwidth to lower the PSD to what we define as an acceptable value"?


Happy New Year CryoLab.



  441   Mon Mar 12 12:56:06 2012 ZachElectronicsLab WorkNeglected Cavity Pole

Don't you want your zero at the HWHM?


I had mistakenly thought that the cavity pole didn't effect me in reflection.

From the sweeps in my picasa album, my cavity pole appears to be:

  • FWHM ~approx 50 kHz (30kHz on paper)
  • I will compensate with a 40kHz zero


 DYM: Correct. I typo'd FWHM when I meant HWHM

  632   Tue Dec 18 13:22:24 2012 ZachNotesSchematicsPDH Loop


I vaguely think there is no effect from this in reflection, but I cannot remember

 There ought to be a pole in the signal chain from the cavity in reflection; if the signal sidebands are too far out, there is no phase shift from the cavity and the contribution from them beating with both control SBs cancels.

  634   Thu Dec 20 11:22:48 2012 ZachDailyProgressCavitycavity locked

I think 1 mW is where it starts to go nonlinear. They say "1 mW max power (linear operation)" at 1300 nm, and the responsivity is a little higher at 1550 nm. That said, I guess it shouldn't be totally wacky until you go a bunch higher... I would definitely expect it to be not so good at 10 mW.


The manual for the 1611 says it can take 10 mW without damage, but should it still work correctly at this power level?


  643   Mon Jan 7 13:22:13 2013 ZachDailyProgressControl SystemRinging in the new B'ak'tun

EDIT (ZK): It looks like 8" is the roundtrip length of your cavity, so your FSR = 1.5 GHz number makes sense. This puts the cavity pole at 37.5 kHz, which is still pretty close. Actually, judging from the green curve below, it looks like the 3-dB point is something like 35 kHz. How did you guys figure 150 kHz for the cavity pole? Nicolas: We already compensate for the 40kHz pole with the PDH box. I estimate the additional pole to be about 15-20kHz because that's where the asymptotes of the two slopes cross, which is at the pole in a log-log plot. ZK: I see. I didn't read the post that I replied to closely enough. So, you moved the zero down to 40 kHz where it should be, but there is now still an extra pole at 20 kHz or so.

How sure are you about the cavity pole?

8" cavity -> FSR = c / (2 * 0.2 m) = 750 kHz

F = 20k -> FWHM = 37.5 kHz

=> fpole = 18.75 kHz.

That seems pretty close to your ghost pole.


Rich and I started off by changing R22 from 10Ohm to 39Ohm. This lowered the zero that compensates the cavity pole to around 40kHz. This straitened out the loop TF quite nicely. However, upon closer inspection, there is still some effective low pass filter at about 20kHz. We were also able to improve the stability of the loop by changing the SR560 from AC to DC coupling! Here are TFs of the loop gain:


The loop doesn't behave so badly, with a UGF around 15kHz it stays locked fairly well, though the noise is still large, causing ~10% power modulation.

The path forward is provided in the following picture:


If we've been nice little boys, Santa will bring us a new resonant gold PD for Christmas.


  650   Tue Jan 8 11:31:08 2013 ZachElectronicsControl SystemPDHv2 box xsfer function

  • We have what looks like a 160kHz pole which I do not understand

 It looks like a GBW issue. Are you using AD829s (especially on high-gain stages)? Otherwise, you could easily have a BW of ~160 kHz for slower parts. Also, since there are many in series, the phase lag adds up.

  669   Tue Jan 29 22:47:49 2013 ZachNotesMeetingsMeeting notes

  • Don't bother with Wenzel Xstals for now - borrow Marconi from Gyro to make it work - can get quote now
    • Email Zach about taking a Marconi from the gyro








Do you need it indefinitely? I am supposed to come back and take some gyro data next month, during which I'll have to steal it back. Other than that, you can have one.

  707   Tue Mar 12 15:57:25 2013 ZachElectronicsControl Systemtrying to get more bandwidth from pdh2

 Are R24, R25, and R27 all the same value? If not, then the invert scheme doesn't work quite right.


Finally I noticed that the invert switch does not do a simple -1 multiplication. It actually multiplies by -2 over most of the band, but also introduces significant delay at high frequency. At 1MHz we're talking 50 degrees. We should probably use it only with the switch in the 'up' state and just set the sign correctly elsewhere, like with the A-B box, which seems to gave pretty good phase performance.

  712   Fri Mar 15 15:46:03 2013 ZachElectronicsControl Systempdh2 gain control knob

How much gain is "high gain"? The GBW product of the AD829 is 120 MHz, so if it's something huge like G = 1000 then you would expect the pole that low. I'd guess it's not that high...


When you turn the knob to high gain, it doesn't work exactly as intended, the gain is not frequency dependent and it seems to act more like a boost then an overall gain stage. The gain increase starts to fall off at about 100kHz.


  1016   Fri Feb 21 04:09:48 2014 ZachElectronicsSensorsomniPD

For our future experiments, I thought it would be good to create a new general-purpose workhorse photodetector. Presenting: the omniPD.

My intention is to design something with excellent performance as either a DC or RF receiver, perhaps simultaneously. To do this, I borrowed elements from several things, including:

  • The old RFPD V2.0 that Alastair and I made a few years back, which itself is essentially a less specialized aLIGO LSC RFPD.
  • The e/aLIGO OMC DCPDs
  • The mevans BBPD
  • The aLIGO PSL ISS electronics
  • Other general Rich/Daniel/etc. magic

See the attached schematic.


Diode and bias

As usual, the diode is a reverse-biased 2-mm Perkin-Elmer InGaAs, though others could be substituted (e.g., 3-mm for larger area or a Si diode for lower capacitance). The bias is provided by an AD587 low-noise reference. It is trimmable before going through a 2-pole active lowpass at 0.1 Hz and a current buffer.

DC path

The DC signal goes through a large (RF-choking) inductor before being fed into a standard transimpedance amplifier. This is drawn as a low-noise FET chip (OPA140) with high impedance in the schematic, but could easily be a BJT part for brighter applications. There is a transimpedance switching option à la OMC DCPD, using a small-signal relay. The output then goes through two switchable generic filter stages (e.g., for whitening, etc.). The first of these stages has an optional offset injection for DC signal subtraction before further amplification, with the offset voltage derived in a similar way to the PD bias. I made sure to add pads on pin 5 of these stages in case we want to use AD829s for speed.

RF path

The RF path nearly identical to the aLIGO LSC RFPD design, but with only two rejection notches and a single readout for simplicity. As with that design, the components can be chosen so that it is a more traditional resonant circuit (rather than notch readout), and I have added a feature where the entire RF path is switchable via relay to a straight 50-ohm resistor for broadband readout. This, again, is using a typical small-signal relay (I know that "RF relays" exist, but I could find no reason this part wouldn't work up to the target of 100 MHz given things like insertion loss and through resistance---in any case, we can change it to an RF relay if we need to).


Since this is supposed to be a generic design, I have made most features optional. For example:

  • The DC Z switch relay can be omitted entirely and one can use a resistor hard-wired into the feedback path instead.
  • Likewise, the RF-path relay can be omitted and there are pads to short the anode to the notch/resonant group.
  • The offset generation and addition need not be stuffed either

I plan to have a 5x3 header array on the board, with one row all at +5V and the opposite row connected to a BIO connector (with the middle row connected to the individual switches), so that the user can choose---option by option---to have parameters hard-set or controlled externally (e.g., by CDS). A nice thing to add would be a threshold-based encoder board using an ADC chip (either external or perhaps as a daughterboard) so that we can control each PD's state with a single CDS DAC output.


My dream is to fit this in the same housing as the BBPD, since it seems to be a nice design and someone already did it. That said, it's a lot to fit in such a small package, so we'll see. One thing I am very fond of is the ThorLabs-style 3-pin power connector.


Based on LISO modeling, it seems feasible to have an ultra-low-noise ~MHz DC path while simultaneously having excellent RF performance. We don't always require this, but there are at least a few examples I can think of where this would come in handy:

  • Combination input RIN/RAM monitor PD: Monitor input power fluctuations and RAM compactly.
  • Combination beat/TRANS RIN PD: Make the usual low-noise beat measurement and the transmitted light RIN compactly (and without sacrificing power). Also, one can use the ~100-MHz broadband function to locate a beat before pushing it into the sweet spot and engaging resonant detection.
  • Combination REFL intensity sensing and RF locking: This is something we need for the noise-suppressed dilution technique, since we will not want to have a beam splitter that will introduce vacuum fluctuations into the otherwise closed system.

Besides, apart from this nice dual-banding, the PD should be generic enough to be useful for all traditional purposes individually.

I would love it if anyone had any suggestions before I start making the PCB layout.

Attachment 1: omniPD.pdf
  1019   Fri Feb 21 13:47:36 2014 ZachElectronicsSensorsomniPD

Right-o. This is the kind of switching that existed in, e.g., the eLIGO OMC whitening box, but grounded-input is definitely better. I modified it to use the switching that's used for the VGA in the common-mode servo board. This avoids both floating inputs and also the potential loading that would come from leaving the inputs connected.



I would love it if anyone had any suggestions before I start making the PCB layout.

The switchable generic filters right now leave the inputs floating when the filter is disabled. Would it be better to ground the inputs or just leave the inputs connected and only switch the outputs?


Attachment 1: omniPD.pdf
  1022   Tue Feb 25 17:03:01 2014 ZachElectronicsSensorsomniPD



1) I am suspicious of the DC part being truly low noise at low frequencies with the switching and extra components. Excess 1/f noise?

2) For the application of balanced ISS PDs, can we really make the package small enough so that they're close together?

3) I wonder about the low noise reference. Its good at DC, but don't we spoil it a little if we pass the DC signal through too many components before it gets to the PD?

4) Would it be possible to not stuff the RF parts and just use this as a DC PD?

1) I think the relays ought not be a problem, judging by their performance on the OMC Z switch. I was under the impression that the MAX333 was an acceptable switch for low-noise signals (it is used immediately after the unity-gain input of the common-mode board, so it is presumed to have a noise floor much lower than an AD829's). That said, the CMB does show anomalously high low-frequency noise. I can do some testing to measure this, but what is the way around it? Use relays for everything? That would be OK, but it takes up a lot more space.

2) From my interrogation by EKG, I imagined that the balanced ISS PD would be its own specialized thing in a custom box. So, I didn't plan for that to be within the scope of this workhorse. Since we already know almost exactly what we will want from a balanced ISS setup, shouldn't we just make another design that is compact and best suited for that purpose?

3) I'm a bit confused here. Do you mean for the bias or for the offset? In either case, all we're doing is adding some extra active low-passing, right? The AD587 has a floor of ~100 nV/rtHz, so the filter brings this (and any junk from the potentiometer) down to the OPA140's floor of 5 nV/rtHz. As for drift, the AD587 is rated to ~10 ppm/K = 100 uV/K, while the OPA140's drift is well under 1 uV/K.

4) Yes.

  1023   Tue Feb 25 23:52:18 2014 ZachElectronicsGeneralMAX333 is quiet

Since we are thinking of using the MAX333 quad analog switch in a low-noise environment for the omniPD (see thread at CRYO:1016), we want to make sure it is not noisy.

I measured its noise tonight and found that the shorted-input noise of both NC and NO channels was limited by thermal noise from the ~130-ohm closed-circuit resistance, to the level measurable using a LT1128 preamp:


Here are a photo and sketch of the measurement setup:

20140225_214609.jpg 20140225_234647.jpg


  1025   Thu Feb 27 03:34:58 2014 ZachElectronicsControl SystemmicroBIO

In CRYO:1016, I alluded to the idea that it would be cool to have a concentrated binary input/output system with which to control our various electronics (with the omniPD as a first example). I thought about this some more and came up with the following idea, which I'm calling "microBIO".

It is an 8-bit BIO controller that takes as an input a single differential analog signal. An IC ADC (ADC0804) is used to convert this analog signal into 8 parallel digital outputs, which are then buffered (LMV324 --- low-voltage, single-supply, rail-to-rail quads) and used to drive switches, relays, or whatever. In a sense, the device under control is therefore effectively a digital storage medium. In this way, each device can have up to 8 independent binary control parameters all controlled with a single CDS output.

Some features and notes:

  • The input to the controller will be one differential signal via BNC, and the output will be fed to the device via DB15. Power and ground for the microBIO will be provided over the same DB15, so it can be connected inline without any external power. Also, there is no ground connection between the CDS rack and the controller, so there is no potential for ground loops.
  • The ADC is self-clocked, but it is triggered by a 555 timer rigged for very low duty cycle and a frequency of ~2 Hz. This way, the requested device state is only read twice a second or so, and not at the full rate of the ADC (> tens of kHz). This makes it highly unlikely that an incorrect state is written while the analog control signal is slewing.
  • Setting it up this way gives an input range of 0-5V and a step size of 5V/255 ~ 20 mV. This seems OK, but I am doing some testing.
  • I have not explicitly drawn it this way, but in cases where fewer than 8 bits are needed (e.g., the omniPD, which only needs 5), the bits should be addressed beginning with the MSB to ensure greatest immunity to digitization noise. In this case, the effective step size increases by a factor of 28-N.
  • A preliminary current budget suggests that even if all 8 outputs are driving relays and they are all engaged, this should still not overload the 5-V regulator in the device.
  • As shown on the omniPD schematic, there should be jumper switches on each microBIO-compatible device that allow the user to either hard-set any parameter or tie it to BIO control. So, this is a completely optional plug-and-play control scheme that may be added or removed at any time.

Here is a simplified diagram and a preliminary schematic:

microBIO.png microBIO.pdf

I built a test circuit to make sure it all worked as expected. In the video below, I've wired up LEDs to indicate the 8 digital outputs (sorry---couldn't find enough bare LEDs so there are 3 red ones and 5 stacked green pairs, with the pairs tied together as one) and I'm driving the analog input with a triangle wave. At the start of the video, I'm not triggering, and you can see the digital outputs changing wildly. At 0:09, I engage the 555 triggering and you can see it starts sampling twice a second or so, with no apparent chattering. I plan to do a more detailed test of this with the real test stand in the ATF.



microBIO test from Zach Korth on Vimeo.

  1029   Mon Mar 3 00:20:24 2014 ZachElectronicsControl SystemTesting microBIO with digital system

On Friday, I tested the prototype microBIO with the test stand in the ATF. To do this, I built a library part into the ATF model:


This takes EPICS binary inputs corresponding to each of the 8 controller bits and combines them into the appropriate analog signal to be decoded by the uBIO.

Everything seems to work as expected, but there is some sort of problem with the DAC not being able to maintain voltages above around 4 V (with the output gain [V/ct] being compressed before that point, as well). I do not think that there should be a problem with driving the ADC0804 with a differential signal, and that was the plan to avoid ground loops in the final scheme. I isolated the signal with a floating SR560, and, while it made it a little better, the problem didn't go away. More head scratching and testing required.

Here is a video showing where I switch bits 1-5 on and off with the MEDM screen. I didn't mess with 6-8 because the effect mentioned above leads to errors with them. Note that I turn them both on and off sequentially from 1-5, so different analog signal ranges are explored in each case.


microBIO test 2 from Zach Korth on Vimeo.

  1033   Mon Mar 10 21:14:05 2014 ZachDailyProgressstuff happensBeam jitter to modematching unlikely (re: PDH2)


When I plugged anything into the DAQ_EXC input on CRYO-001, from open cables to 50 Ohm terminators, the loop started oscillating. I could make the oscillation go away by changing the gain knob. This implies that plugging anything into DAQ_ECX changes the loop gain. This is baffling at first glance.

This is a shitty effect that Nic has found and mentioned before. The output stage of the PDH2 is a fancy-pants thing that was copied from the Rich uPDH design, and then we thought it would be fine to add the DAC_EXC injection without having any problems. We were wrong, in general, because adding a path to ground from this point through a resistance comparable to or smaller than the others in the stage's network changes the gain in the non-inverting configuration. This can be fixed by making R26 >> R24,R25 at the expense of lower excitation range.

There also appears to be a parasitic high-frequency path when the port is left open, which leads to a bump in the TF at 10 MHz, regardless of the INV setting. Therefore, it also creates a notch when out of phase with the main path. We don't know exactly how this happens, but clearly it's affected by what is connected to the DAC EXC input, so we terminate it.

Note that the former effect is what caused the oscillation when you plugged in the 50 ohm terminator (by changing the gain), while the latter is what caused it when you plugged in an open cable (by opening the parasitic path or by affecting the notch via the capacitance).

So, I would replace R26 with something bigger than 2.2k and terminate DAC_EXC when not in use.

  1103   Mon Jun 23 23:49:49 2014 ZachLaserSiFiCavity axis angle shift vs g-factor

To decide whether or not we can go with 1" windows (easier and cheaper than 2"), here is a conservative calculation of the expected cavity axis shift as a function of the (symmetric) g-factor we choose.

The mirror deflection angle is chosen to be a (rather high) 10 mrad, and the displacement is calculated at 20 cm from the cavity center, which is probably farther than the windows will be.

The calculation is made with one line from the formula in Siegman p. 769.


As you can see, the displacement for even this large angle should be on the 1-2 mm level for us, so we can use 1" windows.

  1104   Tue Jun 24 05:09:35 2014 ZachCryostatSiFiCavity construction ideas

I spent some time learning SolidWorks tonight by generating the following ideas for how to hold the wafers, etc., within the cryostat.

There are e-drawings attached to this log, but here are two screenshots from them:

symmetric_screenshot.png macromirror_screenshot.png

The first one is our usual idea of pinning two wafers using steel rods. The second one has a single wafer pinned this way, but uses standard mirrors as the input couplers for the cavities, as this is an alternative we are considering. The large mounts take up a lot of real estate, so we would likely use smaller-than-standard ones to avoid making the cavities too short (they are 3" and ~2" here, respectively). Obviously, this is all upside down...

The clamp bases are just a first-pass idea I had for them. Our idea is that a long 1/4-20 cap screw would go through and compress the whole system.

Attachment 3: SiFi_Assembly_symmetric.easm
Attachment 4: SiFi_Assembly_macromirror.easm
  1181   Wed Dec 17 13:24:00 2014 ZachLab InfrastructureSiFiCryostat unpacked (x-post from SUS elog)

(I realized that we should probably use the CRYO elog rather than the SUS one, so I've reposted this here).

[Nic, Zach]

Today, we unpacked the IR Labs cryostat that will be the centerpiece of the Cryo SUS experiment. 

Everything was more or less in order, except that the baseplate does not have any outward extensions with which to mount the cryostat to the table. Also, the holes for the screws holding the baseplate to the barrel are not countersunk. So, as of right now, the entire cryostat sits on these screws' caps, which is not ideal. We need to either a.) get a new baseplate made up with some wings on it and countersinking for the screws, or b.) work out another way to hold and mount the cryostat (for example, we might want some soft isolating material there anyway, though it will come at the expense of alignment drift).

I followed the instructions and removed the strange anodized heat shield bottom plate that comes with it during shipping, replacing it with the usual one and then resealing the chamber. As directed, I also pumped out the air again---the charcoal getter is not supposed to be exposed to atmosphere for long periods of time.

  1182   Wed Dec 17 13:54:19 2014 ZachLaserSiFiLasers mounted, energized, beat set up

On Monday, after I did some inventory of all the parts we have received from various companies, Dmass helped me mount the RIO lasers into their mounts so that I could get started with the optical setup. We cleaned the surfaces with methanol, applied a small layer of silver thermal compound, and then screwed them in.

I then borrowed the following to run the lasers:

  • The (separate) ThorLabs diode driver and temperature controller from Haixing's maglev setup
  • An integrated ThorLabs diode driver / temperature controller from the TCS lab

After finding the right cables, I powered up the lasers and verified the P-I curve for each as listed on the spec sheets.

I then built a quick (temporary) optical beat setup, combining the two beams on an 1811. I had the temperatures (actually, thermistor resistances) set to what was listed as the testing set point on the datasheet, and as soon as I overlapped the beams and focused them onto the PD, there was already a strong ~50 MHz optical beat.

diagram.jpg setup_with_beat.jpg

I have spent some time since then trying to lock various kinds of PLLs, both to interrogate the free-running frequency noise and to get used to controlling the lasers. Some things I've tried:

  • Locking a Marconi to the free-running beat, which I think might be an exercise in futility due to the relatively small range of the Marconi FM
  • Locking one laser to the other directly using a PLL, which I think might be an exercise in futility due to the bandwidth of the current actuation from the ThorLabs driver
  • With Dmass's help, locking a Zurich PLL to the free-running beat. This appeared to work, and we saw a preliminary frequency noise spectrum that looked about right, but I'm skeptical because the control signal doesn't seem to respond to my slewing one laser's frequency.
  • Briefly, locking one laser to the other at low frequencies using the Zurich PLL control signal as a frequency discriminator. This didn't work, adding to my suspicion.

The first two were not helped by the fairly basic loop shaping afforded by attenuators and an SR560.

I think my next step will be to simply use the I-Q demodulation method (like I did to measure the no-FM Marconi noise in ATF:1877) to measure the frequency noise. I'll compare that to what I get with the Zurich PLL.

  1184   Wed Dec 17 18:11:38 2014 ZachLaserSiFiLasers mounted, energized, beat set up



If the "locked indicator" light is not green on the Zurich (first tab, under "Reference", then what you get out is junk (e.g. you have unlocked the lock in, and i hasn't re-acquired yet) - you can do this by kicking it too hard with a frequency shift, which would be easy to do if you were slewing laser frequency, as the coefficients of the laser [Hz/mA] is so big. When the lock in loses the signal, you have to manually re-lock it (toggle off and on the button which has the mouseover text: "enable the fixed center frequency mode of the PLL"). You can get  something which sort of looks like a PLL signal which has terrible noise and weird glitchy response when the lock in isn't locked in.

Your instinct to look for slewing at the PLL control point is correct, and a sign that the state of the PLL is healthy/unhealthy


 Yes, I noticed this effect. I'm talking about immediately after acquiring---or re-aquiring---PLL lock. I did this several times at different beat frequencies to see what effect it had on the noise (the spectrum changed considerably, which is another bad sign).

  1185   Thu Dec 18 03:39:32 2014 ZachLaserSiFifree-running laser frequency noise

I spent some time tonight measuring the free-running laser beat noise in various ways. Recall that, as of yesterday, I had tried setting up a couple analog PLLs to no avail and I didn't trust the spectrum I was getting from the Zurich PLL. So, I wanted to measure it another way to see if I could corroborate.

First, eye candy:


Now, an explanation of the various measurements.

I-Q demodulation method


This is a method I have used with some success in measuring the Marconi noise in its quietest state (with no modulation and therefore no means of feedback---see ATF:1877). It is done in the following way:

  1. Split the beat PD output and send it to the RF input of two mixers (I used level-7 ZAD-1-1s), using equal path lengths.
  2. Set Marconi to a frequency close to the beat (~50 MHz in this case) and an amplitude of +10 dBm
  3. Split the Marconi output, send one splitter output to each mixer from (1), but with 90º rotation between them.
  4. The outputs of the mixers are now at the difference frequency between the beat and the Marconi, but maintain their I-Q separation. (This is the reason for using the Marconi rather than beating the lasers at a lower frequency in the first place---the I-Q separation is maintained regardless of the differential laser drift, and it also only requires a short cable length.)
  5. Acquire both I and Q signals and perform the I-Q analysis:
    1. Normalize the signals and atan2(I,Q) to get phi, then unwrap(phi) to get continuous phase evolution vs time
    2. diff(detrend(phi))/diff(t)/2/pi to get instantaneous frequency as a function of time
    3. pwelch

The main complication here is that, as you can see in the plot, the high-frequency RMS of the beat is several tens of kHz, which means you still have to sample at a high rate to get what you need. The best acquisition scheme I could think of was the Zurich box, which can do 460 kS/s. Still, to take meaningful data, I had to very carefully tune the laser beat to the Marconi LO and then quickly engage acquisition before the (wildly fluctuating) IF signals drifted above the Nyquist frequency (around one second of data was used to make this trace).

That said, the result doesn't look that crazy, and in fact it agrees very well with the DFD measurement that was carried out in a completely different way (see below).


Delay-line frequency discriminator (DFD) method


This is the usual scheme where one mixes a signal with a time-delayed version of itself to create dispersion. What I did:

  1. Split the PD signal
  2. Using one splitter output, find the appropriate combination of attenuators and amplifiers needed to obtain a LO-worthy +7-dBm signal (I needed -7 dB and then ~+25 from a ZFL-500-LN) and send it to a mixer LO input via a long (several-meter) cable.
  3. Send the other output to the mixer RF input via a short cable (attenuate if necessary---wasn't in my case).
  4. Verify that the DC level of the IF output varies sinusoidally with the beat frequency
  5. Null the output and measure the frequency resolution. I measured 5.5 nV/Hz.
  6. Amplify with SR560 and measure spectrum on spectrum analyzer
  7. Divide spectrum by SR560 gain and the number in (5) to get frequency noise

This method worked swimmingly and reproduced exactly the result I found using the I-Q scheme. The noise floor (cyan in the plot) was measured by sending a quiet Marconi sine wave of the same amplitude and frequency as the beat through the pipeline.


Zurich PLL method

This method is incredibly straightforward. Simply plug the beat (ensuring it's < 1 Vrms and under 50 MHz) into the Zurich box and lock the internal PLL by pressing "ON" on the screen. Route the PLL control signal to one of the front panel outputs and choose the scale factor in V/Hz. I chose the same number as I measured for the DFD (including the SR560 gain) for ease of comparison on the spectrum analyzer.



  • All methods agree below ~50 Hz 
  • The I-Q and DFD methods agree everywhere, but they are higher than the PLL result by ~2 from 50 Hz to around 10 kHz, above which they re-converge somewhat
  • All traces (save for the PLL in a narrow band from ~50-500 Hz) are higher than those on the spec sheets sent with the laser (shown in black on the plot---note that the West laser is everywhere noisier than the East one).

I'm not sure what to believe. One would think the Zurich PLL is the most trustworthy, but a) I still am bothered by the time-domain behavior I see in the PLL control signal when I adjust the laser beat while watching it, and b) I've generated two nearly identical spectra that differ from it using completely different schemes from measurement to FFT.

All that said, I think the excess noise (and thanks to Dmass for saving me time by pointing this out) is just coming from the ThorLabs drivers, so this should be redone when we have our low-noise ones.


  1187   Fri Dec 19 21:37:12 2014 ZachLaserSiFiAmplitude modulator characterization

Tonight, I did some characterization of the Photline fiber-coupled amplitude modulators we will use for our experiment (MXAN-LN-10 --- datasheet attached nope google it yourself). These are electro-optic devices that work by using an internal mach-zehnder to convert phase modulation into amplitude modulation.

The test setup for all measurements was the same. I used the exact configuration that I have been using for the beat (see CRYO:1182), but I simply blocked one laser, so that only one beam was hitting the 1811 PD. The amplitude modulators were inserted (one at a time) between the East laser and its output coupler.



Insertion loss

The first thing I did was to investigate the insertion loss of the modulators. We chose the low-loss option, which just meant that the company hand-selected modulators with loss of < 3dB (= 50% power transmission).

I didn't go crazy with precision here, because systematics with fiber coupling can easily prevent a measurement to better than a few percent (an example of this: I installed a 1-meter patch fiber between the laser and the output coupler, instead of the modulator, and I actually saw a slight increase in output power vs. the case with the laser going straight to the output coupler… go figure).

In both cases, I measured very nearly 50% reduction in power (at the top of the MZ fringe---see below) vs. the case with no modulator. So, these things have a loss very close to 3 dB, as advertised. An important thing to point out is that we will need to bias these away from maximum transmission to get a linear PM -> AM coupling, so the real power reduction in our setup will be more than 50%.


DC response

These modulators have an SMA-connectorized "RF" input, as well as two bare pins connected to a separate set of "DC" electrodes (they also have two more pins connected to the cathode and anode of an internal PD, presumably at the other MZ output port, which is kind of cool). As far as I can tell, the RF input is also DC coupled, only it is 50-ohm terminated.

I did a DC sweep of both electrodes from 0-10 V while measuring the output power:


(The RF applied voltage range is lower due to sagging from the 50-ohm load).

Fitting these curves, I determined the following Vpis:

  • S/N 03
    • DC: 6.46 V
    • RF: 4.19 V
  • S/N 17
    • DC: 6.39 V
    • RF: 4.91 V

These are consistent with the numbers listed on the datasheet.


Transfer functions

Next I measured the actuation transfer functions ([RIN/V]) from 1 Hz to 100 MHz, driving the RF input while applying a mid-fringe bias to the DC input, and using

  • Agilent 35670A FFT analyzer and the 1811 DC output for 1 Hz - 50 kHz, and
  • Agilent 4395A RF analyzer and the 1811 AC output for 500 kHz - 100 MHz

Note the dead zone from 50-500 kHz---this was by accident, as I forgot to check the low-frequency resolution of the RF measurement. I will redo this sometime.

Here are the results:



  • The jump from 50 kHz - 500 kHz is from the measurement dead zone and carries no information
  • The lag beginning around 10 kHz is from the stated ~50 kHz bandwidth of the DC output of the 1811. The AC output has a low end at ~25 kHz, so there isn't really a good way to make a measurement in this region with that detector. We could use a DC-coupled version to make a continuous spectrum.
  • The slow rollup at low frequencies is well-sampled and repeatable. I'm not sure what causes it, but it appears to be real. In any case, it's pretty small.
  • The delay at high frequencies is consistent with the optical path length from the modulator to the PD. I calibrated the cables' transfer function out, and what is left is this delay which has a 4.13-m free-space equivalent. There is ~64 cm of free-space travel on the table, plus well over a meter from the output fiber of the modulator.

The response very flat, and roughly what is expected from the DC sweep:

(1/P0) * dP/dV|mid-fringe = pi/Vpi ~ 0.5 ( = -6 dB).

  1190   Wed Jan 14 02:38:43 2015 ZachLaserSiFiPMC set up as test cavity

To continue with the laser/modulator testing, I have added Dmass's old PMC to the temporary characterization setup. I have used the other output of the 50/50 BS that combines the two laser diode outputs, so that we can keep the beat setup intact while also being able to send either of the two beams into the PMC.


To do this, I:

  • Made a cursory razor beam scan of the beam emerging from the BS
  • Calculated a MMT solution to the PMC mode using some of our new lenses
  • Installed the telescope and directed the beam towards the PMC
  • Macropositioned the PMC by hand to rougly center it on the transmission of the single-pass beam, as measured using a power meter
  • Scanned the PZT using a 0-10 V triangle from an SRS function generator, then used the diode temperature as a coarse adjustment to look for modes
  • Maximized the first found mode (a horizontal HOM)
  • Looked for nearby lower-order modes, then maximized them and iterated to get to TEM00
  • Installed HWP upstream and then maximized visibility by rotating polarization

The coupling isn't stellar yet, at roughly ~66%, but the MMT is fairly tight and I'm sure I can improve easily. The laser and cavity are stable to well within a linewidth at high frequencies, and only drift apart over many seconds.

Some things I plan to do with this setup:

  1. Dither lock the PMC to the laser(s)
  2. Characterize the phase modulators
  3. Set up reflection PDH lock and feed back to lasers
  4. More stuff
  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.



  1192   Wed Jan 21 15:21:19 2015 ZachLab InfrastructureCryoNew LN2 dewar delivered

I ordered a new LN2 dewar and it has just arrived. Appropriately, for me, it is #305.



  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
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