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  1304   Fri Aug 21 16:00:05 2015 DmassDailyProgressCOMSOLSeismic transfer function model in COMSOL

This is half of a very nice transfer function plot - See if you can get COMSOL to give you the phase information too and plot that in tandem (e.g. https://en.wikipedia.org/wiki/Bode_plot). 

Also, I'm mildly curious how the frequency shifts with support distance. Are you using material parameters for the supports, or treating them as an area of constrained geometry?

  1306   Tue Aug 25 16:54:15 2015 BrianDailyProgressCOMSOLSeismic transfer function model in COMSOL

After playing around a bit more with Comsol, I realized and fixed some slight mistakes. I added in the phase information. The transitions in phase occur very rapidly and with resolution I was running at I couldn't get any points during the transitions. I used a loss factor of 1e-8 for the silicon (as measured here).

This simulation runs by considering the support points of finite size as fixed constraints.

I'm going to try and check effects of the support positions.

I also uploaded my model to the svn under: /trunk/CryoLab/comsol/cryo_cavity_sagging_transfer_functions/

Attachment 1: TF.pdf
  1307   Tue Aug 25 19:02:54 2015 BrianDailyProgressCOMSOLSeismic transfer function model in COMSOL

I added the ability to loop over the support separation. I checked values from .5in to 3.5in. All of these were with L=4", R=1", and at 60 degrees.

The first plot shows each different support separation plotted together. The remaining plots are the bode plots for each individual support separation. Up until aout 10kHz, the transfer functions are pretty much the same besides the slightly different gains at low frequency.

I also committed these changes to the SVN.

Attachment 1: TF.pdf
Attachment 2: TF_0.5in.pdf
Attachment 3: TF_1.0in.pdf
Attachment 4: TF_1.5in.pdf
Attachment 5: TF_2.0in.pdf
Attachment 6: TF_2.5in.pdf
Attachment 7: TF_3.0in.pdf
Attachment 8: TF_3.5in.pdf
  1322   Thu Sep 10 10:47:46 2015 DmassDailyProgressCOMSOLSeismic transfer function model in COMSOL

Try fitting a function of the form:

G / (1 - i f / Q - f^2 / f_res^2) where you fit G, Q, and f_res. You should almost just be fitting for Q. It will disagree, but I'm curious how much. After this, try adding in a time delay: 
G -> G*exp(i*2*pi*f*tau), where you fit the time constant tau. This is somewhat nonphysical, but is not a terrible initial way to approximate a collection of poles at higher frequencies.
  1323   Thu Sep 10 18:13:37 2015 BrianDailyProgressCOMSOLSeismic transfer function model in COMSOL

I used the matlab function lsqnonlin to do this fitting. This function fits real valued functions so I had it fit both the real and imaginary parts. The first plot is done with tau=0 while in the second I allowed it to solve for tau. I used the DC gain as the initial guess for G, 1e8 for Q, the location of the maximum for f_res and 0 for tau. The version with non-zero tau would not converge on the default settings so I played with the parameters until it converged. Both solvers also ended up giving a warning that the value they converged to might not be a true solution. I tried playing around with the settings of the solver and it ended up giving results that were very clearly off. I might try to play around with it a little bit more later.

The fit with no tau agrees pretty well up until the higher order resonances kick in.

The fit with tau also agrees pretty well in magnitude for the first resonance however the phase is all over the place. As I said, the solver had issues with this so I might try a few things to make it work better.

Between the two fits, G and f_res stayed fairly constant between the 2 as expected. However Q decreased by an order of magnitude in the solution with tau.


Attachment 1: TF_fit.pdf
Attachment 2: TF_fit2.pdf
  1324   Thu Sep 10 21:31:24 2015 KojiDailyProgressCOMSOLSeismic transfer function model in COMSOL

Wrong: G / (1 - i f / Q - f^2 / f_res^2)
Correct: G / (1 - i f / f_res /Q - f^2 / f_res^2)

Relationship between Q and decay time

There is a way to do the complexfitting by converting C into 2xR

i.e. Conver a data set from
a1 + i b1
a2 + i b2
a3 + i b3
an + i bn


1 a1
1 a2
1 a3
1 an
2 b1
2 b2
2 bn

and instead of the complex function f(x)+i g(x), use the 2D fitting function
z(x) = kroneckerDelta(x,1) f(y) +kroneckerDelta(x,2) g(y)

Then the 1D complex fitting is mapped to 2D real fitting.

In any case, your transfer functions might not have sufficient resolution to allow us to do the Q fitting.

  1325   Fri Sep 11 15:39:28 2015 BrianDailyProgressCOMSOLSeismic transfer function model in COMSOL

I tried again with fitting the correct transfer function. The fit is pretty much the same except with a slightly higher gain and resonant frequency. However, I noticed that the solver wasn't changing the value of Q. In the plot below, I initially guessed 5e7. I tried decreasing the initial guess of Q, but the solver wouldn't change it at all until I got to around 1e5. So as Koji said, there's probably not enough resolution to get a good fit for Q.

With the correct transfer function, I couldn't get the solver to converge anymore with a tau parameter I guess because the function it is trying to fit is slightly more complicated now.

The approach with changing a complex valued function to a 2-dimensional real valued function is what I have been doing for the fitting. I guess I didn't make that too clear in my original post.

EDIT: Evan pointed out to me that the complex pole pair should create a phase lag of 180, not a phase lead. I had used a Hermitian transpose (') instead of a standard transpose (.') on the COMSOL data which flipped the sign of the phase. For the fitted data, there was a flipped sign in the transfer function. It should be: G / (1 + i f / f_res /Q - f^2 / f_res^2). I updated the below plot with this in mind. Changing this didn't affect any of the parameters as both the data and the fitting function had reversed phase.

Attachment 1: TF_fit.pdf
  1550   Wed Apr 5 23:27:12 2017 johannesNoise HuntingNoise BudgetSensing noise

I investigated the sensing noise issue a little further. I measured the noise on the error signal without light on the PD, with light but no sideband modulation, and with sidebands. In both cases there was a total of 3.5 mW of optical power on the PDs and I confirmed the cavities to be far off any relevant resonances.

Obviously scattered light is still a HUGE problem. The figures show the 'quiet' state when the floating table has been left alone for a couple minutes. I'm going to change the hose on the vacuum pump to a more flexible one i bought a while ago and see if it gets better. All optics are already on 3/4 inch posts. We should have the new super-windows soon, which will be a nice feature. Using a buzzer might tell us where the scatterers are.

Other observations: 3.5 mW are enough to give the shot noise some clear separation from the PD noise. The noise levels seem to be a little higher in the East path, which is due to the resonant PD having more transimpedance gain.

I added all the sensing noises (ignoring scattered light) together for the two paths, where I assumed ~80% visibility in the cavities for the reduction in light power on the PD for shot noise.

We're not as clearly limited by noise in the LB1005 as I thought. In the next few days I'll measure the beat noise with better resolution and try to make sense of these numbers. I also still have to diagnose if something is up with the PDs and why they seem to be saturated so quickly.

Attachment 1: error_noise_west_20170405b.pdf
Attachment 2: error_noise_east_20170405b.pdf
Attachment 3: error_noises_west_20170405b.pdf
Attachment 4: error_noises_east_20170405b.pdf
  1553   Fri Apr 7 01:44:22 2017 johannesNoise HuntingNoise BudgetSensing noise

I used the test inputs on the resonant PDs to see if the notches are still in the right place. I found that the overall transfer functions have qualitatively quite different shape above their resonances. It seems like the west path doesn't have much of a notch at all, and a secondary peak in its response between 300 and 400 MHz. I haven't worked with these PDs before, is this within the specs or is it likely that someting on the board is broken? Koji told me after today's meeting that care must be taken that the fast gain chip doesn't oscillate, maybe that's what's going on?

The resonance frequencies were fine, but the notch locations were off by ~1MHz in both cases. I opened the PDs and their RF cages and corrected this. It required a little back-and-forth because closing the RG cage seems to change some capacitances, shifting the exact notch locations up by up to 50 kHz. The modulation frequencies are 32.7 MHz (65.4 MHz notch) for West and 33.59 MHz (67.18 MHz notch) for East.

The resonances were pretty much unchanged during the re-tuning.

Attachment 1: resPD_west_globl_20170406.pdf
Attachment 2: resPD_east_globl_20170406.pdf
Attachment 3: resPD_west_notch_20170406.pdf
Attachment 4: resPD_east_notch_20170406.pdf
Attachment 5: resPD_west_reson_20170406.pdf
Attachment 6: resPD_east_reson_20170406.pdf
  1554   Fri Apr 7 02:09:31 2017 KojiNoise HuntingNoise BudgetSensing noise

I can't see the important part of the transfer function (f>100MHz)!
But I think it is quite possible. In any case you probably need some modification of the RF preamp.

You can look at the schematics of the same PD circuits at the 40m.

(Click each PD link like REFL33)

This modification at the 40m also involves the correction of the RF output impedance, which needs to be 50Ohm.

  1889   Wed Jan 31 16:14:38 2018 ZachNoise HuntingSiFiSensing noise not improved with higher-level mixers or iLIGO RFPDs

[Johannes, Zach]

I'm still trying hard to understand my current sensing noise floor of about 10-16 m/rtHz (= 300 mHz/rtHz) before I go forward with replacing the test cavities with the dummy cantilever cavities. Long story short, I don't have any more great ideas at the moment, so I think I will move on with the payload recplacement very soon.

After lots of work (see replied-to post), I was able to all but rule out nonlinearities in the beat readout and intensity noise as the culprit. As quoted in the box at the bottom, my next move was to investigate the PDH demod setup and locking RFPDs to see if I could make any improvements there.


Direct beat

One interesting thing to note before moving on to the deliberate changes below is that, for perhaps the first time, the beat had drifted to a very low frequency of around 25 MHz last night. This afforded me a unique opportunity to rule out any weirdness in the downconversion step of the beat readout, where I typically use a Marconi to step the beat frequency from several hundred MHz down to ~10-20 MHz for piping into the RedPitaya. These direct beat measurements from last night showed no appreciable differences from the Marconi "superheterodyne" method. By this morning, the beat had shifted back up to ~55 MHz, near the high end of the RedPitaya range. I made one last direct beat measurement, then switched back to the superhet readout, and verified that---again---there was no change to the beat spectrum.


Switch to level-17 mixers

Last night, I swapped out the level-13 mixers I was using until that point for level-17 units, as Johannes said he saw some noise reduction in the same sensing-noise-limited band when switching to higher-level mixers in the past. As I mentioned in the last post, I was able to circumvent the need for amplifiers on the LO signals by simply switching which OCXO box outputs were used for what. (The "EOM" output spits out +25 dBm, while the "LO" output gives +14 dBm. I am currently only using between -6 dBm and 0 dBm on my fiber EOMs (see aside below), so the large amplitude from the OCXO box is not really necessary. The only difference between these two outputs is that one comes from the "CPL" output of a directional coupler, while the other comes from the main through port, so there's no reason they can't be used for the opposite purpose w.r.t. the output labels on the box.) I am now sending the "LO" output, with 20-dB attenuation (= -6 dBm as a result), to the fiber EOMs, and the "EOM" output, with 8-dB attenuation (= 17 dBm) to the new level-17 mixers.

After sweeping the error signals and retuning the demodulation phases, I re-locked the cavities with no problem, but observed that the new mixers had no significant effect on the beat noise.

Aside: I should note that, prior to the mixer exchange, I was using a -3 dBm drive to the EOMs. During my testing last week, I had reduced the EOM drive to this level from the 0 dBm level I had been using for months prior to this. I didn't notice a difference when I did this, so I left the extra 3 dB of attenuation in. After having switched the mixers out yesterday, I tried going back to 0 dBm, whereupon I found that the noise had marginally increased. In light of this, I went to an even further reduced EOM drive of -6 dBm, and saw that this made the noise marginally lower as compared to the -3 dBm drive state. Reducing the drive further (to -9 dBm or -12 dBm) resulted in a noise level at or higher than that with the larger 0 dBm drive. So, it seems like the noise is minimized with -6 dBm EOM drive---at least with the current level-17 mixers in place.


Locking with gold iLIGO RFPDs

This was actually the first thing I started working on Monday afternoon. I picked up the spare gold iLIGO RFPD that Gautam had spotted for me at the 40m and brought it back for testing. Its label said "? POP(22,110) ?" on it, so it seems there was some question as to what exactly it was. I put it in my optical setup and ran an AM TF on it, and it seemed like it had a decent response at 110 MHz. I then opened it up and attempted to modify the TF, but what I found inside was a complete mess of flying components, cut traces, and other fun features. I tried doing some reverse engineering, but eventually called it quits that night after hours of getting nowhere.

Luckily, I had another idea: simply borrow the PDs from the CryoCav experiment for an afternoon. Even more luckily, Johannes agreed to let me do this ;-)

This morning, I borrowed his PDs and put them into the REFL ports on my table (first in the E path only, then in both). In each case, I again swept the error signals and retuned the demodulation phase with the new PDs. Below is a plot comparing the beat noise in three cases: 1) the usual state with two PDA255s for locking, 2) the hybrid state with a gold PD on the E path but a PDA255 still on the W, and 3) the state with gold PDs in both paths.

As you can see, there is no appreciable difference between the three configurations. (NB: The reduced noise at lower frequencies in the all-PDA255 case is the result of the suspension being more rung up this morning than last night. The features are a combination of more motion showing up linearly at very low frequencies (near 1 Hz), as well as scattering between a few tens of Hz and ~150 Hz.)

For the record---unlike with the PDA255s---the lowest noise was achieved with no optical attenuation in the REFL paths before the gold PDs. Whereas I need significant (OD 1.0-1.5) attenuation to get this minimal noise level with the PDA255s, adding any such attenuation while using the gold PDs resulted in a higher lowest achievable noise. This latter behavior with the gold PDs is more in line with my intuition, since the reduction of the optical gain should increase the beat-referred sensing noise. I'm not sure if this tells me anything new about the weird attenuation effect with the PDA255s.


Conclusion & plan

I'm not sure what else there is to try at the moment that justifies continuing to hold off on the payload swap. One thing to note is that, with the recent (albeit slight) reducion in noise, it is clear that there are some features in the ~few-kHz region that make the picture a little less clear. For example, the noise at 2.5 kHz appears limited by these narrowband features, and not by the broadband floor anymore. Also, above ~4.5 kHz (i.e., above the highest newly resolved feature), the noise is well understood as residual laser frequency noise. Therefore, the only place where the noise remains anomalously high is in the narrow band between 700 Hz and 1.5 kHz. Here, it seems too flat to be the result of narrowband displacement features, and the noise floor mystery persists.

In light of the above, I don't think there's enough left to learn in this configuration to keep waiting on the cavity swap, so my plan is to move forward with that. Hopefully tomorrow, Johannes and I will bond a mirror to the second dummy cantilever (which I need to clean and bring over from the KNI), and then I can open up the chamber and start with the swap.

With the dummy cantilever cavities in place, I can continue with the noise hunting once I see the new displacement noise situation. One "free" thing to try is iterating the shape of the servo TFs to further reduce the laser frequency noise. (In fact, I can still do this some tonight as a last-ditch effort before switching out the cavities.)



More stuff I'm going to try:

  • Johannes says that his mystery noise went down every time he moved to a higher-level mixer. I am using a level 13 right now, so I will bump it to a level 17 to see if I get any improvement. For this I'll need a LO amplifier, so it makes sense to switch to the onboard RF electronics of the PDH box. (Alternatively, since I'm using fiber PMs and need to attenuate the RF signals to them as it is, I can probably just swap which OCXO box outputs I'm using for PM vs. mixer to get a beefier LO drive.)
  • I'm still not able to fully rule out the PDA255 REFL PDs as at least part of the problem, since I have this weird effect where I need to attenuate the REFL beam more than I should to obtain the lowest beat noise. I am planning to commission and swap in a spare gold iLIGO RFPD that Gautam spotted at the 40m and see if that makes any difference. I wish there were two, but hopefully I can gain at least some insight with just the one (either a factor of sqrt(2), or perhaps more if one of the existing PDs is dominant for some reason).
  2427   Wed Jun 19 17:28:48 2019 ShubaHowTo Set up Pleione
Pleione is a remote desktop which has COMSOL MultiPhysics, MATLAB, Solidworks and few other programs installed in it. You might want to use it. I am detailing the procedure to do so.
Firstly, you should contact Michael Pedraza (his office is in East Bridge First Floor) to set you up an account on Pleione. You need to provide your Wifi and Ethernet IP Address.

For Windows: Go to run and type in "mstsc"
Pleione Account:
Computer: pleione.ligo.caltech.edu
Username and Temporary password will be sent out by Michael.
Once you have successfully logged into Pleione you need to change your password.
Here is the keyboard command to change your password.
Windows: Ctrl + Alt + Delete
"Choose change password"

E-Drive Home account:
There will be a shortcut on your Desktop "E:\wxyz" please use this directory to save all your work to. Try not to save your data to the default folders "Documents and Settings" or your Desktop.

To log-off Pleione:
Right Click on the Start button lower left bottom > Shutdown or sign out > Signout "This logs you off"
If you are working on something then choose the option "Disconnect", or click on the “X” upper right corner locks your session, this way you can pick up where you left at.
The server gets rebooted sometimes to install some updates. You might be contacted if you are logged in when the server gets rebooted.

If you’re not working on anything please "Sign out".

Caltech VPN
Also, it will be useful to have Caltech VPN on your computer. Then, you will be able to access all the facilities off campus as well. Log on to vpn.caltech.edu/ and register. You will need your UID number (also mentioned on your ID card) for registering on Caltech VPN. Once permitted, you can login to VPN with your access.caltech.edu credentials.
  1608   Wed Jul 5 08:41:54 2017 Jordan KempCryostatTemperature SensingSet up diode calibration

MONDAY JULY 3rd, 2017

I read notes on how to operate the temperature diode. I took apart an old piece of aluminum equipment to use as a mount for the temperature sensors. On the left is the calibrated temperature diode, and on the right is the diode to be calibrated. Using the drill press, I carved a groove to place the uncalibrated diode. I have a circuit connecting the positive terminal of the uncalibrated diode to the crystats feedthrough, connecting the negative terminal to the positive terminal of the calibrated diode, then from the negative terminal of the calibrated diode to another electrical feedthrough. This configuration provides identical current for both diodes. I have two circuits connected in parallel, detecting the voltage drop across each diode (with negligible loss in current). I could not test this system because I could not find the connector for the electrical feedthroughs. The diodes are secured using clips provided with the calibrated diode. Insulating grease is used to shield the contacts of the diodes from the conductive aluminum base.

  680   Wed Feb 6 00:52:41 2013 DmassDailyProgressLab WorkSetback

I had to replace two of the optics right after the faraday (both were crummy 1064 optics that I was using to pickoff ~1% of the 1550 beam for power monitoring). They had to go.

They were wedged, so this blew the alignment downstream far enough off that I was missing lenses.

I tried to just tweak the things upstream of these optics (which are 50:50 beamsplitters) to realign to the path, and was able to recover alignment in one of the arms.

Sadly, I noticed that I was getting serious distortion of the beam through the faraday. WIthout a 5 axis type mount, I'm afraid I need to realign everything downstream of the Faraday after aligning to it, which means realigning the cavities somewhat from scratch. This is disappointing - I will work at this tomorrow carefully and well rested so that I don't have to completely scrap my previous alignment work and rebuild the table.

  749   Wed May 8 14:48:47 2013 DmassDailyProgressLab WorkSetback

Had some minor setbackage:

Dewar went soft b/c it ran out of cryogens overnight.

Ran out of cryogens because there was too much gas (He) in the insulating (outer) vacuum**

Too much gas in the outer vacuum because of the honking leak between the inner and outer vacuum

**will calc a data point for hold time vs pressure from this.

Once i let He into the outer vac, it takes quite a while to get back to low pressure:=> I either babysit the cryostat constantly filling it up for the duration of this run, or open it up and fix the leak.

After consulting with Warren, I decided to fix the leak rather than brute forcing back down and trying to live with it.

Borrowing the He leak detector from a very generous Schwab lab as soon as they are done with it (possibly as early as tomorrow). Gathering quotes for our own leak detector.


Backfilling with N2 to warm up, and will open the outer seal and start leak checking the inner as soon as I get the leak checker.


The leak is now huge. When backfilling the inner vac with N2, the outer vac came up to room pressure as well.

  2777   Thu Jul 8 14:51:08 2021 shrutiDailyProgressLab WorkSetting changes

Following our discussion earlier when we realized that the AC electronics of the 1811 may be saturating, since our RF power (at the mod freq of 33.59 MHz) is near or over 55 microW, I added an additional 10dB attenuator before the EOM. The total attenuation is now 40 dB. To compensate for this I removed the 10 dB attenuator at the input of the LB1005.

I also added a 50 ohm terminator in parallel to the 20 dB attenuator at the analog input modulation port of the ITC 502 current driver. This is to ensure a more or less accurate 20 dB attenuation of the control signal since the ITC 502 input has an impedance of 10 kOhm.

Everything seemed to lock once again when the gain on the LB1005 was increased from 5 to 6.


  2778   Thu Jul 8 19:10:59 2021 aaronDailyProgressLab WorkSetting changes

We should note that the LB box driving into 50 Ohm is current limited at its output (assuming we don't narrow the voltage window using the trim pots on the back panel). It can supply up to +- 20 mA, so if we see control voltages approaching 1 V we are nearing saturation of the servo controller.


I also added a 50 ohm terminator in parallel to the 20 dB attenuator at the analog input modulation port of the ITC 502 current driver. This is to ensure a more or less accurate 20 dB attenuation of the control signal since the ITC 502 input has an impedance of 10 kOhm.

  1867   Fri Jan 12 11:03:33 2018 johannesHowToDAQSetting up automatic scripts

To run background scripts on cryoaux, e.g. python temperature control servos using systemd: There are several examples one can use as templates in /home/controls/services/. For background tasks it's recommendable to set up a logfile, check the python scripts that are called by the existing service files to see how it's done. When ready do the following:

  1. Copy the new yourscript.service file to /etc/systemd/system/
  2. Start the service with sudo systemctl start yourscript.service
  3. Stop the service with sudo systemctl stop yourscript.service
  4. To start the service automatically on reboot use sudo systemctl enable yourscript.service,
  5. sudo systemctl disable yourscript.service does the opposite, as one might expect.


  1683   Wed Aug 16 17:08:49 2017 Jordan KempCryostatCryo QSetting up temperature dependence of eigenfrequencies

I finished my cryostat setup. I built a suspension for the liquid nitrogen tank of the crystat in order to make setting up the GeNS easier. I glued the RTD to a disk with apoxy, and sealed the chamber. I was able to shoot in a 7mW red laser, and capture the reflecting beam using the below configuration of optics. Once I set up the electronics, this experiment will be ready to be run.

Attachment 1: setup.png
Attachment 2: lasers.png
  1528   Tue Feb 28 22:04:35 2017 AaronDailyProgressCryo QSetting up the ESD, vacuum update, a few others

Brittany and I have been in the lab setting up ESD-driven actuation and moving towards cold Q measurements.

Pele now gets down to ~3e-3 torr, while Nalu (the smaller cryo) gets down to about 3e-5 torr. There is still a lot of Mylar shielding and adhesive inside the cryostat; this shielding was installed by the previous owners, so presumably was vacuum compatible. We are divided on whether the outgassing is what's limiting the vacuum, but one solution could be to add the charcoal to the container, it might help the vacuum especially when we add nitrogen.

We soldered leads to the ESD and a temperature sensor on the heat shield. We were working on feeding the ESD an AC signal in a way that does not send too much current through the ESD--this will be completed Wednesday afternoon. We also tweaked the JaNS mount, so it is slightly more stable.

As noted previously, we have both uploaded changes to the drawings for GeNS on the DCC [D1600472]. We can bring them for comments to the Thursday meeting, or we can find a few people individually. We may need to make a few fixes to the assembly after the modifications before Thursday.

22 of the 25 wafers we ordered from University Wafer will ship Wednesday or Thursday. I asked that they ship the remaining 3 when available.

We also modified the COMSOL model [D1700104] with new materials properties, temperatures, disk parameters, and are getting closer to believing we can predict the forest of modes on the FFT. We think we see the lowest butterfly around 560--does this make sense to people?

Look forward to more to come!

  1529   Wed Mar 1 17:27:22 2017 brittanyDailyProgressCryo QSetting up the ESD, vacuum update, a few others

I will add a summary of where we are with the set-up to help guide the reader

  • We have a silicon wafer mounted on the JaNS (suspension system) inside the cryostat
  • We have a laser sampling the wafer with an optical lever in to a QPD
  • This QPD is fed in to the spectrum analyzer
  • We have added the ESD in to the cryostat and is mounted in a way that is the over wafer (we are not sure how far away above it is since we can not visually verify it once everything is in there)
  • We have the ESD leads soldered to the previous feedthrough on the cryostat (not rated for high voltage)
  • The ESD leads outside of the cryostat are connect to a BNC connector that we soldered on
  • We are using the 40m driver box to send in voltages to the ESD

Yesterday, we were conseratively ramped up sending in DC voltages from 0-150V. We did not see any response from the disk.

We applied AC signals to the high voltage supply with square waves from a function generator. Similarly, we did not see any response from the disk.

We verified that what we were seeing was the disk by actually knocking it off it's suspension system and completely losing the return beam.


Brittany and I have been in the lab setting up ESD-driven actuation and moving towards cold Q measurements.

Pele now gets down to ~3e-3 torr, while Nalu (the smaller cryo) gets down to about 3e-5 torr. There is still a lot of Mylar shielding and adhesive inside the cryostat; this shielding was installed by the previous owners, so presumably was vacuum compatible. We are divided on whether the outgassing is what's limiting the vacuum, but one solution could be to add the charcoal to the container, it might help the vacuum especially when we add nitrogen.

We soldered leads to the ESD and a temperature sensor on the heat shield. We were working on feeding the ESD an AC signal in a way that does not send too much current through the ESD--this will be completed Wednesday afternoon. We also tweaked the JaNS mount, so it is slightly more stable.

As noted previously, we have both uploaded changes to the drawings for GeNS on the DCC [D1600472]. We can bring them for comments to the Thursday meeting, or we can find a few people individually. We may need to make a few fixes to the assembly after the modifications before Thursday.

22 of the 25 wafers we ordered from University Wafer will ship Wednesday or Thursday. I asked that they ship the remaining 3 when available.

We also modified the COMSOL model [D1700104] with new materials properties, temperatures, disk parameters, and are getting closer to believing we can predict the forest of modes on the FFT. We think we see the lowest butterfly around 560--does this make sense to people?

Look forward to more to come!


  1592   Wed Jun 14 18:33:20 2017 brittanyUpdateCryo QShiny new GeNS Update
Lab update from the interim time between the last update and now 
Aaron and I received the machined version of the GeNS that we were designing in COMSOL. Differences between what we had before (Janky Nodal Suspension) to what we have are :
  • A platform that lowers the disk on to the sphere that slides very smoothly
  • A mounting post and set screws for the lens that (at the center of the GeNS)
  • No more tape and zip ties
We assembled the GeNS + disk lowering mechanism and are making a list of modifications that we will make in the future design. We put the new assembly in the cryostat and ran into a few ridges/lips/things-sticking-out that existed in the inner aluminum shell along with in the cryojacket. We removed them and everything fits in there nicely now. Also, we needed to make the wires to the ESD longer to accommodate our new loading mechanism.
We worked out a new way of loading the whole thing in to the cryojacket that is attached to a stand (that is bolted to the optics table). Originally, we would load the disk into the GeNS/JaNS from the bottom and close up everything from the bottom. Now, we worked out a version to load the disk, attach the aluminum shield + cryotanks then load it in to the cryojacket on the stand. We are going to experiment with ways that we would be able to have the entire GeNS + aluminum shield permanently attached (and the actuator fixed) and load in the cryo tanks after the disk is loaded.
The linear actuator to move the platform up and down from outside of the cryostat is on its way (get here in 3 weeks?). Instead we used took off the rotary part of the rotary feedthrough that we already had to act as a linear actuator. We later learned that this doesn't linearly actuate when we pull a vacuum #ThxPhysics . We were able to set up a way to hold the platform up while we stitch up the cryostat (using shrinky dinky wrap) then use the rotary feedthrough to move the platform up and down after it's all closed up (but not holding vacuum). 
We set up the optical lever, pumped down on the system (it was down to ~5e-3 in the late afternoon today), set the ESD back up to the spectrum analyzer, plugged everything back in to the lock-in amplifier and scope. We were able to see the disk wobbling around from other things going on but couldn’t see our drive signal. It could be due to the fact that we were still pumping down. The plan is to continue along on seeing if we can drive it again tomorrow. Within the next couple weeks, we will be integrating the DAQ to remove the role of the spectrum analyzer and scope.


Attachment 1: GeNS_1.jpg
Attachment 2: GeNS_2.jpg
Attachment 3: TopDown_GeNS.jpg
  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
  405   Mon Jan 9 14:43:09 2012 DmassThings to BuyGeneralShopping List

7 Inch fat posts

Coat hanger shaped thing for crane

 Emory paper?


  1375   Mon Mar 7 21:00:02 2016 JohannesNotesCavityShort cryo cavity design

Cavity length

Nothing new here. The shorter the better for enhancing thermal noise. A laser tuning range of 10-20 GHz sets a lower limit for the cavity length of ~1cm, which has an FSR of 15 GHz.


A design choice needs to be made for the mirror radii of curvature, maximizing the coating noise impact factor. For crystalline coatings the minimum roc is 10cm, while for amorphous coating it can be shorter. The graph shows the combined coating noise impact factor from both cavity mirrors as a function of roc. For the plano-concave curve one mirror is held flat, while for the concave-concave cavity both mirrors are changing. Only for mirrors with roc smaller than ~1.5cm the half-symmetric cavity shows a larger impact factor. Above the symmetric configuration is to be favored.



The short length with its big FSR results in an enlarged cavity linewidth. This affects the frequency discriminator negatively and we need to assert that sensing noise is not keeping us from reaching the coating noise limit. For an impedance matched cavity the optical reflective transfer function close to the resonance is 2*df/FHWM, with df being the frequency difference between cavity and laser, and FWHM the linewidth of the cavity. Using PDH this translates to a demodulated error signal (in terms of light power) of 4P0J0(m)J1(m)df/FWHM [W/Hz]. Assuming a modulation index of m=0.2 and an initial carrier power P0=1mW, the shot noise on the RPD contributes 2.3e-12 W/rt(Hz) of sensing noise. It is suppressed by the optical gain in the cavity, and reaching a frequency stability of df=1e-3 Hz/rt(Hz) (motivated by expected coating brownian noise levels) requires a cavity linewidth of less than about 175kHz. In a 1cm cavity this requires a Finesse of better than 85,000. A high finesse requires better, thicker coatings, which further enhances coating noise.


The round-trip losses that correspond to a finesse of 100,000 are 63 ppm, so we can aim for T=30ppm per mirror. A Ta2O5-SiO2 quarter-wave stack coating requires 17 quarter-wave layers to achieve this according to my calculations. The corresponding coating thickness is about 8.5 microns. The projected coating brownian noise level, again as a function of roc in a L=1cm cavity is shown in the following graph (at room temperature), calculated with the simple CBN noise model from Nakagawa et al. from 2001, gives an idea of the expected level of the frequency noise. The parameters used are stated in the figure.


Cavity geometry

My recommendation would be to fix the mirror rocs and then fine-tune the cavity length for coating noise levels and g-factor. I found that for roc=10cm a cavity length of 1.875cm yields g=0.66, which is a good place to be, with an FSR of 8GHz. Note  that the high finesse relaxes any g-factor constraints, so it is not that essential to stick with values that are considered safe. I think that somewhere around there we can find our cavity configuration.

Attachment 1: weff_vs_roc.pdf
Attachment 2: cbn_vs_roc.pdf
  2026   Tue Apr 24 20:47:02 2018 johannesDailyProgressOpticsShort-lived short cavity

I had an optical bonded cavity for about 2 minutes today before the second mirror plopped off during handling, but I'm confident that I can repeat the bonding attempt tomorrow with more success.

As Chris suggested in a recent meeting, using some 3D-printed parts I made a rig to hold the cavity vertically so I can rest the second mirror to be contacted on top and move it around as I search for the least lossy position. I confirmed the orientation of the mirror wedges by shining a laser through the annulus - the deflection of the transmission is pretty significant and easily identifies the wedge (deflection points to the thickest part of the mirror barrel)


Getting the initial alignment on the input side was tricky because of the significant wedge and silicon's high index of refraction. I looked for dips in the reflected light to make sure I have something to see on the camera in transmission, and then used a lens for better visibility. Once I had visual feedback it was easy to identify the 00-mode and align the input beam to it. As before, I didn't really do any mode-matching, just placed an f=10 cm lens such that the reflected beam had roughly the same size on the viewer card as the incoming beam, which means that the wavefront matched the mirror curvature semi-well. The lens was also producing a waist within ~2cm of where it needed to be. With this I achieved ~70% visibility, which was plenty to lock to the cavity.

Using one of the SiFi fiber-integrated EOAMs I modulated the laser power with a square wave and was able to perform optical ringdown measurements on the short cavity. Due to the systematic error I described in elog 2014 I estimate that there is about a 5 ppm uncertainty on the round-trip loss when using the EOAM to switch the light (a square wave step LARGER than the half-wave voltage sends out-of-phase light into the cavity, which actively depletes the cavity field, driving the loss estimate up). The reverse is unfortunately also true, if the beam is not extinguished entirely the loss will seem lower because the input light still pumps the cavity field. For every ringdown I therefore first minimize the off-state power using a pick-off PD, but doing this several times with no change to cavity alignment in between I found that the loss estimates I obtain have a spread of ~5 ppm. In comparison, for back-to-back ringdowns without changing anything the estimates are well within 1 ppm of each other.

On the first attempt I measured 140 ppm roundtrip loss, which is not far from the best I've seen before (137 ppm) and within the systematic error. I was curious though if tapping on the mirror can move it into a better position. Not really. I tapped several times, which did move the mirror (had to realign the input light) but I didn't see a significant decrease or increase (minimum was 138 ppm, maximum 144 ppm). I previously measured T=65 ppm on the HR coating, so 130 ppm are from mirror transmission.

Using my fingers on the rim of the mirror (AR coating had to be off so I could do interferometry) I pushed the top mirror down in a likable position. Applying slight lateral and rotary forces while pushing there was no slipping, so I thought it was a job well done and took the cavity out of the mount. I shook it a little and turned it upside down, the mirror held, and I placed it back in the storage mount, new mirror facing up. I then though it would be nice to get a picture that shows both mirrors attached, but when I took it back out and tilted it the freshly applied mirror plopped off, fortunately not falling far, and landing on the AR coating side, which I inspected and found no damage.

I was trying to first contact mirror and spacer and do it again, but wasn't patient enough to let the FC dry sufficiently - and it ripped while peeling. I spent the rest of the day re-applying first contact, but couldn't get it all off in one piece. In the end I succeeded, but then saw some residue on the bevel sad I will continue this quest tomorrow with some fresh FC from Downs.

I also heat-sonicated all the parts for the cavity-baking today using the new bath in the PSL lab. I confirmed that the complete jig holding a cavity will fit into the little baking oven, so I'll do a baking dry-run with only the jig parts tomorrow as I re-assemble the botched cavity.

  696   Fri Mar 1 14:48:52 2013 nicolasNoise HuntingNoise BudgetShot noise, how does that work?

The photocurrent in terms of the phase of the beat signal is:

I_PD = I_DC + I_RF sin(\phi)

where \phi is (omega1-omega2)*t.

The discriminator, current per radian, is

dI/d\phi = I_RF cos(\phi) ⇒ I_RF for \phi = 0

So a fluctuation in photocurrent causes a interpreted fluctuation in phase as:

D\phi = (d\phi/dI) DI.

Phase fluctuations are related to frequency fluctuations as f * D\phi = Df, so

Df = f / I_RF * DI

for shot noise, DI = sqrt(2*I_DC*e) (ignoring cyclostationary corrections).

so Df = ( f / I_RF ) * \sqrt( 2 * I_DC * e)

Assuming I_RF = I_DC = 1mA we get:

Df = 1.8e-6 ( f / 100Hz ) Hz/rt(Hz)

Seems low enough!

  2282   Mon Feb 25 20:45:10 2019 KojiMiscmaterial propertiesSi Block stored in Cryo Lab

On Friday cleaning, we vacated the east optical table in QIL. The Si scatterometer was disassembled and the Si block was moved and stored to the cryo lab.


  605   Mon Nov 19 02:26:01 2012 DmassLaserCavitySi Cavity flashes

I managed to align close enough to the Si cavity to see flashes:



More details to follow!

  606   Tue Nov 20 08:10:18 2012 nicolasLaserCavitySi Cavity flashes


  608   Tue Nov 20 14:36:48 2012 DmassLaserCavitySi Cavity flashes

Below is a picture (+ beams) of what I put on the table to do cavity scans. The mode matching lenses are what Nic called out in elog:603.

Cavity flashes can be seen in the video in elog:605


I spent a little while trying to get a clean cavity scan in reflection (I want to see the dip in power from the 00), but got nothing which resembled a lorentzish dip. (Noisy crap)


I scanned frequency via the laser current, using the ITC510 unit to control the diode.


  • Triangle wave ~10 mA from 1 Hz -> 1 kHz
  • 40 MHz / mA at diode
  • => ~400MHz sweep
  • Tuned temperature so that 00 flashes on camera were near middle of scan
  • Super Duper noisy, with the refl PD oscillating / spiking as I sweep through 00

I will post sweeps when I can get them off this fine floppy disk format onto my computer and matlab them into readable plots.


The relevant time constant for filling the cavity with light is:

  • Finesse ~20k
  • Length = 8"
  • FSR = 1.5GHz
  • LW = 1.5GHz/20k = 75kHz
  • 20cm * 20e3 / 3e10 cm/s  = 13 us
  • 75kHz/13us = 5MHz/ms

so if we pass through the cavity at 5MHz/ms, we are sweeping through the cavity on the order of its filling time

Sweeping at 400MHz:   400MHz * ms / 5MHz = 80 ms. Since it's a triangle wave, period = 160 ms, or f _sweep > 6Hz

I do not understand the behavior, I am sure there is something simple I am missing.

Attachment 1: cav_scans.png
  613   Wed Nov 28 16:22:27 2012 DmassLaserCavitySi Cavity flashes

I am naming the cavities by their mirror serial numbers. We have:

  • Cavity1934 (mirrors 0019 and 0034)
  • Cavity1621 (mirrors 0016 and 0021)

I have convinced myself that Cavity1934 is good, but think that 1621 might need a redo. The story:


Assuming zero loss on the mirrors, I recall we designed the cavities to have ~20k finesse. As mentioned before, this corresponds to a ~13us fill time (just effective path length).

  1. Aligned to Cavity1934 so that I saw mostly 00 and 01 modes on the camera
  2. Put a beamsplitter on the transmission (I borrow an unused 1064 nm one from the ATF and checked that it transmitted a reasonable amount of 1550, ~25%)
  3. Aligned onto a PD (PDA50B from thorlabs - 400kHz BW // ~800ns rise time)
  4. I played around with sweep speed and amplitude
    • SR DS335 function generator to make a triangle wave
    • this into the "analog sweep" input of the ITC510 with a voltage divider on the input
    • Calibration of the setup is ~6mA / Vpp
  5. I tuned the alignment by making the crappy forest of transmitted 00 flashes "higher" on average
  6. I couldn't get a clean transmitted peak with slow sweeps (frequency+length noise presumably a bit high), so I cranked the sweep frequency until I saw something smooth
  7. I got a smooth (repeatable) asymmetric tranmission by sweeping through the 00 a bit faster than the decay time of the cavity.
    1. 2Vpp // 500 Hz triangle wave (12 mA @ 500 Hz)
    2. ~7us time constant on the smooth part of the ramp up
    3. ~15 us tau for cavity decay
  8. I tested at a slowed sweep speed (200 Hz), and saw the same dynamics
  9. For some (possibly naive) reason, I think that the transient decay of the cavity can give me a number for the cavity time constant
    1. This number was 15.3 +/- 1.1us
  10. If this is stupid for some reason, I think that I can *at least* average the ramp up and ramp down to get a rough idea of finesse
    1. ramp up tau ~ 7us => 7us < tau_cav < 15 us
    2. If this IS just the time constant of the cavity, we get finesse by:
      • 1/tau = f_pole = 90 kHz ==> Finesse ~ 1.5GHz / (2 x 90kHz) = 8300
    3. This does not seem like a completely ridiculous number.

I repeated the process for cavity1621, and was able to get flashes / some sort of "ok" alignment, and when I sweep VERY FAST and average like the Dickens, I get a cavity pole of ~22 MHz, or a finesse of 33.

I tried:

  1. Realigning
  2. Using the same sweep parameters from cavity1934's measurement - was unable to get anything resembling that measurement
  3. Slow and fast sweeps (up to 12mA at 4kHz with a triangle wave)
  4. Restarting the laser multiple times in hopes that I had found a "bad operating region" and mode hopped accidentally

Either I am missing something, or I need pop off the mirrors, clean everything, and reassemble cavity1621. I will crowdsource ideas for what I could be doing wrong shortly.

[EDIT: I no longer trust any of the red text - I was using the ITC510 to do the sweeps, and now believe that it was responsible for the crappy ungrokkable transient behavior I saw. I moved to using the ITC510 *just* for temperature control, and Rich's nice current driver to do the current supply / sweeps, and was rewarded with things that looked like transmission peaks]

  614   Wed Nov 28 20:33:21 2012 ranaLaserCavitySi Cavity flashes

  IF we have measured the reflectivity of the mirrors, there's no reason for the Finesse to be anomolous; the amount of unforseen losses that we get from dirty surfaces is not large compared to the transmission of the mirrors in a F=20k cavity.

Take a look at the 40m measurements of the PMC finesse or the measurements of the RefCav finesse which Yoichi did a while ago.

  616   Fri Nov 30 03:16:41 2012 DmassLaserCavitySi Cavity scans

I redid the scans for both cavities today with a brief break for the crazy air leak.

They look non-flaky now: I blame the ITC510 sweep mechanism / current noise for the previous mickeymousery. I will fit and post them tomorrow.

  617   Sun Dec 2 23:51:51 2012 DmassLaserCavitySi Cavity scans

Attached are the cavity scans.

I used a function generator to make a 100Hz 1Vpp triangle wave, and drive the modulation input of the current drivers.

The calibration of the sweep is:

(Sweep speed) x (current modulation input) x (laser diode current to frequency)

(1 Vpp / 5e-3 s) x (1 mA / V) x (0.31 pm / mA) x (1.94e14 Hz / 1550nm) = 7.76e9 Hz / second

I took a few traces for each cavity, and fit an Airy function to each one using fminsearch. Relevant MATLAB code:

load('TEK00003.CSV'); %y-values for scope trace
 xvalz=linspace(0,200e-6,1e4); %time
[XXX2,FVAL2]=fminsearch(@(X) sum(abs(...
    [3 1e8 1.1e-4*16]);

 The X(2) in the above is the coefficient of Finesse, its just 1/sqrt(F)*FSR to get the cavity HWHM.

The 16.2 is: pi x 7.76e9 Hz / sec x 1 / (FSR = 1.5GHz)

For cavity 1621, the four measurements of HWHM that this gives us are [1.5519e5  1.5570e5  1.6110e5  1.5525e5] Hz

I will use the mean of these as my 1st measurement of the cavity pole.

For cavity1621: f_pole = 1.57e5 +/- 2.9e3 Hz


I repeated the process with cavity1934

The five HWHM measurements for cavity1934 are  [1.6388e5  1.4913e5  1.4889e5  1.5713e5  1.5277e5] Hz

For cavity1934: f_pole = 1.54e5 +/- 6.3e3 Hz

The variance in the 2nd set of measurements was a bit bigger.

I have no idea what the systematics are here, or why the sweeps are asymmetric. I do not believe that these are actually 2-6% numbers, but I think "good enough" is the word of the day.

I will put them in the cryostat and close up today

Attachment 1: sweepz.png
Attachment 2: sweep1934a.png
Attachment 3: sweep1934b.png
  2586   Tue Nov 24 13:23:09 2020 aaronPhotosSi fabSi cantilever photos

Entered lab around Tue Nov 24 13:24:57 2020 to finish photographing Zach's cantilevers.

some things about cameras, and in particular the FinePix F300 EXR

  • ISO -- the camera's sensitivity to light. More sensitive means more noise, but also more signal (useful when exposure time must remain short). 
  • aperture -- size of the opening before the lens. A wide aperture yields a shallow depth of field and lets in more light, but can cause blurriness in the foreground and background. Narrow aperture lets in less light and widens the depth of field, but can lead to diffraction effects or not enough exposure time depending on the application.
  • shutter speed -- how long with the shutter remain open? All the usual tradeoffs of integrating.
For these shots, I have the camera mounted on a tripod and close enough to the cantilevers that the subject takes up the full field. I've turned off the overhead lights and oriented the bright, fluorescent desk lamp away from the camera and slightly up. I'm reflecting some diffuse light back to the cantilevers with a large kim wipe (and my white face / lab coat). I've set minimum ISO, long exposure time, and large aperture. Since I'm handling nominally clean Si, I'm wearing gloves, lab coat, mask, and hair net. I covered the wall behind the shot with paper towel to provide a dark, uniform surface that isn't visible in reflections off the wafer containers and doesn't backlight the shot. The photos are not taken from above because doing so resulted in reflection from the top cover, which I wanted to keep on to avoid dust.
Looks like 5-7 cantilevers could be high Q (no visible contaminants, pitting, cracks, etc). I think there's also a pair of high Q cantilevers in Zach's cantilever cavities cryostat, and maybe one in the cantilever op lev cryostat (QIL). If we measure the Q of the most promising bare cantilevers, we can identify the 3ish best candidates for aSi coating.
Photos can be found in this album. I've pulled a representative good-looking cantilever and attach them here, along with the photo booth setup.
attachment 1: the photo booth
attachment 2: the lines near the top are reflections from the edge of the container. The long sides are parallel, and can give you a sense of the angle of the photograph. 
While doing this, I entered EE to retrieve a spare battery for the camera, and later again to return the camera to its place by the sink.
left around Tue Nov 24 17:50:48 2020
Attachment 1: DSCF3580.JPG
Attachment 2: DSCF3572.JPG
  2590   Sat Nov 28 21:53:12 2020 ranaPhotosSi fabSi cantilever photos

For storing lab photos in W Bridge, you can use our shared google acct instead so that we all have access to it (see chat for secrets)

  2592   Mon Nov 30 10:26:17 2020 ranaPhotosSi fabSi cantilever photos

Thanks, the photos are now on the shared drive.

  1992   Thu Mar 29 22:53:01 2018 johannesSummaryOpticsSi cavity assembly

I prepared the cavities for bonding: All relevant surfaces were prepared with clear first contact. I 3D-printed some parts to help me identify the orientation of the wedge and center the first mirror on the spacer, but I made the tolerances a little too tight. I'm having new versions printing overnight and will attempt the first round of bonding tomorrow.


Based on the pictures shown in elog 1985, I have identified mirrors 1,2,5, and 6 as the ones I want to make into cavities. I will bond mirrors 1 and 5 first, and then figure out a good way to make a cavity and bond the second mirror in place while monitoring the cavity loss. I have as basic idea but need to print out some more parts to see if it works that way.

Attachment 1: IMG_20180329_161459581.jpg
  319   Mon Oct 24 11:56:37 2011 FrankUpdateCavitySi cavity dimensions

measured the Si cavity dimensions. All the mechanics was/is designed for a 2"x4" cavity, but the cavity is smaller and we might have to modify things...

hole at optical axis: 0.53"
venting hole: 0.25"

  1318   Sun Sep 6 15:43:40 2015 ZachSummarySi fabSi fab process photo walkthrough

I wanted to give a visual outline of the process that we have been using to do silicon fab at the KNI.

Until now, we have been performing this process on individual rectangular sections of Si (recall that we sent two 6" wafers out to be diced a while back---see CRYO:1250). The first and second prototypes (CRYO:1260 and CRYO:1264, respectively) showed some imperfections in both 1) the etch pattern (i.e., the 2D pattern that defines the end blocks of the cantilever) and 2) the etch depth uniformity (i.e., the final surface roughness of the etched region). We suspected that either one or both of these are the result of cleanliness issues while handling the individual rectangles. After talking with Shiuh Chao from Taiwan, it seems that a better option is to perform almost all of the process on an entire wafer, then mechanically scribe out the individual cantilevers just before the main etch. There will be more details on that at the end of this entry.

The following is a photo walkthrough of a test run I did on a practice wafer on Friday. This wafer is just a single-side polish 3" wafer that was given to me by a KNI staff member, and the mask I used (see below) was the one Justin and I made for the individual rectangles. I did not cut the sections out before etching, so this run was never going to make usable cantilevers; instead, I just wanted to test out the "whole wafer at a time" scheme and see how it affected the etch quality. For this, I only chose to etch from the top (polished) side, as opposed to our standard, double-side etch.


Spin and bake

This is the step during which the photoresist is applied to the wafer. Beforehand, the wafer is cleaned using a 1:1:40 solution of ammonium hydroxide, hydrogen peroxide and water.

To date, we have been using a photoresist called ProTEK, which is actually a new-ish product that was designed to eliminate some steps with traditional wet etching (more on that later). To use this, a special primer must first be applied before the actual photoresist. In each case, the wafer is placed on a "spinner", which holds the center of the wafer via vacuum seal, the primer or photoresist is pipetted onto the central region, and then the wafer is spun at 2000 rpm for 1 minute, in our case:


After each application of either the primer or the photoresist on a side, the wafer must be baked on a hot plate. The primer requires a 1-min bake at 110 C followed by a 5-min bake at 220 C; the photoresist requires a 2-min bake only at 110 C. Since, per the standard procedure, the wafer must physically be placed on a hotplate (with a sacrificial wafer in between if so desired), I chose to apply the coatings in the following order:

  1. Top (polished) side primer
  2. Bottom side primer
  3. Bottom side photoresist
  4. Top side photoresist

I felt that was the best way to protect the more crucial top surface. Here are a couple photos of baking. On the left, the bottom side is up with the primer baking (top side is face down on the sacrificial wafer with its already-baked primer protecting it); on the right, the top side photoresist is baking. You can see some thin-film interference from the minute thickness variations.




After both sides are coated with photoresist, the wafer is ready for photolithography. This is accomplished on a Karl Suss MA6 mask aligner machine. This device aligns a photomask to the wafer (with sub-micron precision, if necessary), and then performs controlled UV exposure on the aligned system to define the etch profile.

So far, we have been using a high-resolution transparency print as our mask. This is a common technique in prototyping, and there is a glass blank in the lab that is used to hold the transparency (with scotch tape). The mask is loaded onto a holder and held with vacuum:

The holder is then slid into the machine, and then a drawer underneath holds the wafer, also via vacuum (there are different holders for each wafer size, and there are also adapters to hold smaller chips). The wafer, which is not yet installed, is held within the orange circular region on the plate below:

After loading, various parameters are set, including the contact type (in our case, the mask and wafer are brought into brief contact, then backed off 30 um) and the exposure type (we use 40 seconds of 365 nm exposure at the standard luminosity, which is fixed). Then, a microscope is used to adjust the position of the wafer under the mask via micrometers with 3 degrees of freedom (i.e., X, Y and theta). This is usually a very precise process, but in our case we can actually get by just looking at the alignment by eye. After everything is aligned, the exposure is run. For this, the microscope caddy moves out of the way and the UV source slides in and does its business. Here's a shot of the whole machine:

After exposure, the lithography pattern is only barely visible to the eye under certain light.



To complete the process, the exposed wafer must then be developed to remove the photoresist from the regions where etching is desired. This is done by swashing the wafer in ethyl lactate repeatedly for 10 seconds at a time, blow drying with an N2 gun in between. Four or five cycles is usually enough, and after this is done the pattern is clearly visible:



It is not clear to me what the gunk is in the regions to be etched (these are the central rectangles here). Repeated washing in the developer does not remove it any further, and it may somehow be related to the final etch quality. More on that later.



Now the wafer is ready for etching in KOH. When I did this with Justin, we set up a beaker on a hotplate with a magnetic stirrer, but the KNI has a full immersion etch bench that is much more convenient. The wafer is held in a wafer holder and dipped into the bath, which is heated to around 80 C and pumped to keep flow:


The etch proceeds at a rate of around 50 um/hour, perhaps faster if the solution is at a higher temperature. For this test, I let it run for about 2.5 hours before coming back to pull it out.



After etching, the photoresist must be removed using a piranha etch, which is just a concentrated solution of sulfuric acid and hydrogen peroxide. I think I didn't perform this at a high enough temperature, so in the following pictures there is still some photoresist on the wafer.


As you can see, the 2D etch definition is much better than in the first two prototypes I made with Justin (see the links at the top of this post). That is, the rectangular regions for the blocks at the end and for the thin cantilever section are very well defined, as compared to those previous trials with the individual rectangular pieces of silicon. So, I am concluding that those imperfections were the result of cleanliness/handling issues with the small pieces, and that they will not be present if we work with a whole wafer at a time.

On the other hand, the etch uniformity (final surface roughness) is still very bad. It is about as bad as the first prototype, and actually much worse than the second one (which itself was bad). From the 2nd pair of photos above, you can see that the unetched regions---though discolored from the remaining photoresist that still needs to be removed---have maintained their optical polish, while the etched regions are dull, even in areas that seem macroscopically uniform. This leads me to think that this is a problem with the actual etch process. It's interesting to note that the very-nonuniform-etch areas don't really correspond to the gunky areas I saw immediately post-development (see above).

In the last pair of photos, you can see some regions of the backside that were unintentionally etched. This is probably due to some photoresist having clung to a surface on which it was resting during any part of this process.



The conclusions are thus: 1) Working with a whole wafer solves a lot of problems, and 2) our etch process is not good.

A major caveat is that, apparently, the ProTEK photoresist was expired. It could therefore be that, with fresh ProTEK, we'll be in good shape. However, we may be better off cutting our losses and just going with the traditional oxide/nitride mask. This is the method used by Shiuh Chao's group in Taiwan, which has produced nice cantilevers, and is a straightforward process that I can develop quickly at the KNI. This process is summarized in this PDF:

I have discussed this with Melissa Melendes, the KNI staff member who has been helping me, and we can begin this on Tuesday. There are only two pieces of machinery that I need to get accustomed with to perform this process:

  1. The low-pressure chemical vapor deposition (LPCVD) machine, in order to deposit the oxide and nitride layers for masking, and
  2. The mechanical scriber, to cut out the individual cantilevers just prior to etching.

In addition, I'll need to make a new mask (one that has an array of identical cantilever patterns on a single wafer). I can make another transparency, which involves making another image and going to a printshop to have a high-resolution transparency printed, or I can just go ahead and make a nice glass chromium mask, if I know what I want. I'm told this can be done relatively easily using a mask maker at the KNI, though I will need to get some training on it.

For now, I will probably use another practice wafer and test out the oxide/nitride scheme to see how the etch improves. On the plus side, I performed the entire process you see here in a single afternoon by myself, so my proficiency is increasing. It will take a little time to learn to use the machines mentioned above, and having to use them in the process will make the whole fab take a little longer, but not by too much. Stay tuned...


Attachment 21: Si_cantilever_fab_Taiwan_processonly.pdf
  579   Wed Sep 12 16:13:36 2012 ranaThings to BuyOpticsSi optics

 Edmund Optics has some Si optics (lenses and windows) which we can buy for doing some simple testing of polished surfaces (~120$ ea.):


  580   Thu Sep 13 16:15:02 2012 DmassThings to BuyOpticsSi optics

I saw these, but had been avoiding them b/c of the cost. I'll go ahead and buy a handful (6) for testing. If I don't use them, we can probably find a use down the road.


  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.

  1262   Wed Jun 3 19:39:18 2015 ZachNotesSiFiSi structure fabrication process

I went into the lab with Justin today to make our second prototype resonator. I am writing the procedure down here from memory. I will correct/update this as we continue.

Note: of all the steps below, only the photolithography is done at KNI. The rest are done within the Painter labs.

Initial prep

  1. Peel specimen from the adhesive packaging it is attached to by the dicing company.
  2. Rinse with acetone and isopropanol.

Photoresist "spin and bake" (must be done twice: once per side)

  1. Pre-treatment.
    1. Soak specimen in weak solution of H2O2 and NH4OH for around a minute.
    2. Dry.
  2. Primer.
    1. Attach specimen to spinner (with suction).
    2. Apply primer solution with disposable pipette using designated pump. Apply evenly over surface, enough so that the meniscus nearly reaches the edges.
    3. Spin at 3000 rpm for 1 minute.
    4. Bake at 110 ºC for 1 minute. (All bake cycles are in air on a hot plate.)
    5. Bake at 220 ºC for 5 minutes.
    6. Allow to cool before proceeding to photoresist.
  3. Photoresist
    1. Attach specimen to spinner (with suction).
    2. Apply photoresist solution with disposable pipette using designated pump. Apply evenly over surface, enough so that the meniscus nearly reaches the edges.
    3. Spin at 3000 rpm for 1 minute.
    4. Bake at 110 ºC for 2 minutes.
    5. Allow to cool before proceeding to photolithography.
  4. Until controlled exposure (photolithography) and development, protect the sample from light by wrapping the holder in foil.

Photolithography (this must also be done once for each side)

  1. Mask preparation (only needed when using transparencies, as we are currently)
    1. Place transparency with mask pattern on square glass blank. Center as best as possible to ensure that you'll be within the alignment actuator ranges later.
    2. Slice a piece of scotch tape into thin sections.
    3. Tape the transparency to the glass blank in all four corners, with the transparency as flat against the blank as possible (to avoid diffractive aberrations).
  2. Mounting the mask
    1. Turn on external suction pump.
    2. Install blank with transparency attached to it onto the sliding palette. Transparency should be face up, since the palette will be flipped upon installation into the instrument.
    3. Engage suction.
    4. Flip palette and slide into the instrument.
  3. Mounting the specimen
    1. Use 2" wafer holder.
    2. Install "chip holder" adapter. This blocks most suction holes that would be used for a full 2" wafer, leaving only a few near the center to hold a smaller chip. (This works for our 50-mm-long cantilevers, but we will have to use a different method to hold anything longer.)
    3. Place, align, and center specimen as best as possible onto the holder, then slide into the instrument. Through the window, one should now see the inverted mask held in place above the specimen.
  4. Alignment and exposure
    1. Set exposure type. Some high-resolution lithography requires the mask to be in direct contact with the specimen; we don't need such resolution, so we choose keep a ~50-um gap between the mask and the specimen.
    2. The instrument now fully engages the specimen against the mask (vertically, from below), then backs off the appropriate amount.
    3. Choose an appropriate microscope objective and locate the mask and/or specimen edge on the viewer screen.
    4. Find a good edge for alignment, then align the specimen (movable) to the mask (fixed) in both angle and position with the knobs. Scan the viewer around to ensure good alignment.
    5. When satisfied with alignment, engage exposure. We used 40 seconds at 365 nm, but this depends on the photoresist. The viewer will retract and the illuminator will engage for the designated time, then it retracts and the viewer reengages.
  5. Clean up after yourself and leave instrument as you found it.


  1. Dip the specimen in the developing solution and swash for 10 seconds.
  2. Remove and blow-dry with N2 gun rapidly.
  3. Repeat (1) and (2) until desired pattern is very clear (this is the first point in the process where the pattern should be visible to the eye). The un-exposed regions should be a clean Si surface, while the exposed regions should appear slightly darker.

Etch (to be continued...)

  1452   Fri Oct 7 17:42:41 2016 ZachUpdateSiFiSi substrates sent to Coastline for coating with mini-mirrors

We want our input coupling mirrors to have the same coatings as the mini-mirrors that will be attached to the cantilevers. So, I have sent six blank silicon substrates to John Tardif at Coastline so that he can include them in the coating run.

These are 1-m concave / plano 1" silicon mirrors. They were originally provided by Coastline and they are still in their original packaging.

  1811   Sat Nov 4 16:22:24 2017 ZachUpdateSiFiSiFi table is floating

[Johannes, Zach]

With the necessary parts in hand, we succeeded in getting the SiFi table floating last night.

We followed the instructions in the TMC setup guide. The air supply configuration is as designated there:

Of note is that this system uses special tubes that have restricting orifices inside between each valve and isolator. These short, black tubes are marked with a red rubber band around each, and are located in the setup as prescribed above. There are 4 in all.

I couldn't take a new measurement last night because the cavities were swinging like crazy, but I should have one soon. I'm not sure what to expect, but it would be great if there was some extra isolation of environmental noise in the target (mid-audio) band.


 Johannes and I shook the table somewhat as we investigated what we needed to buy to get the table floated,

The package arrived today, but I forgot to take to the lab. It's in my office. We can set it up tomorrow if you want.


  2501   Fri Jan 10 14:08:57 2020 ChrisLab InfrastructureGeneralSiLabs 5340 timing ticking along

Cryo lab and QIL cymacs have been running without glitches since before the holidays on the SiLabs 5340 timing boards.

QIL was unstable at first, until a DS345 was added to regenerate the 10 MHz reference at the far end of the cable.  (We have plenty of DS345s on the shelf now, but if we ever wish to free this one up, we could apply some of the tips in LT design note 514 at the Si5340 input.)

For future reference, this is what was done to the boards:

  • Short across C15, C17 to DC couple output 0.
  • Set jumper JP1 for I2C, and connect I2C wires on J17.  (The SiLabs boards are normally programmed over USB using ClockBuilder, a Windows application.  It's not documented how to do this from Linux.  However, a documented I2C interface is provided.  Driving that from a python script on a Raspberry Pi lets the boards start up without any Windows dependency.)
  • Short jumpers JP14, JP16 to set power supplies for 3.3V.  (The supplies are USB-configured otherwise.)

The input is a 10 MHz and ~14 dBm sinusoid.  (This is derived from the CMOS clock output of the Jackson Labs LC_XO GPS disciplined oscillator using a tee network suggested by Wenzel, then fanned out by a Symmetricom/Datum 6502 chassis.)  Outputs are two complementary 65536 Hz 3.3V LVCMOS clock signals (sufficient to trigger the TTL inputs of the General Standards ADC and DAC).

The ClockBuilder-generated register configuration is attached and python scripts are in the QIL CDS target directory under qil-timing.

Attachment 1: Si5340-RevD-RTSCLOCK-Registers.txt.gz
  738   Thu Apr 25 11:40:45 2013 dmassDailyProgressLab WorkSideband shenanigans

[dmass, Rich]

We need to get the beat readout rebuilt now that we have changed (fixed) the alignment the alignment in the cryostat.

Nic and myself noticed that one of the paths (laser diode 68 a.k.a. West path) was much harder to lock than the others.

Investigating further, the error signal for this cavity was tiny (~10x) smaller than the error signal for the other cavity.

I followed this for a while, and found that we were back in the state where: WHEN WE STRESS THE BOARD WHICH IS ATTACHED TO THE BUTTERFLY MOUNT, THE MODULATION DEPTH DRASTICALLY CHANGES.

Rich discovered / theorized that the capacitors on the little PC boards (SMA to butterfly pins) which go to the diode were cracked. It was difficult to not stress the board given the construction, so he replaced the 1nF and 3nF SM capacitors with leaded caps. The modulation depth shot through the roof.

  428   Tue Feb 28 13:56:28 2012 DmassElectronicsLab WorkSidebands - resonant EOM circuit

I am trying to take Rana's advice to give myself some sort of manly gamma for my sidebands.

I have a broadband EOM from Thorlabs:

  • V(pi/2) = 350V
  • C=12.8 pF  // Rseries = 1 Ohm (measured with 4195 @ Downs w/ R.Abbs help)

Using the ZHL RF amp (one of the heatsunk ones) from R.abb, I could get ~24 dBm into the EOM, which gave me gamma~0.02.


It looks like I need some sort of resonant circuit to do better, so I am taking Zach's circuit (ATF elog#1248) and cleaning it up so that it works for my application.

Components in hand:


  • ?
  • ?
  • PWB-16-BL

2 x tunable inductors

1 x small inductor (3.3 uH measured on the 878A LCR meter with pokey-probes)

Going hunting for a 1.62 uH inductor from R.Abbot (This would give me 35 MHz resonant frequency)

ELOG V3.1.3-