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ID Date Author Type Category Subjectup
  905   Mon Feb 2 18:38:18 2015 xiaoyueDailyProgressCrackleOSEMs working

[Gabriele, Xiaoyue] For the past week we finished soldering all the OSEM electronic components and cables, and now the 6 OSEMs for damping the two blades are all mounted.

Today we built the model X1KR2 for sensing and controlling,

and the interfacing medm screen kr2_damping,

We fitted the whitening TFs for all 12 circuit boards and uploaded the dewhitening filters. Finally we checked the OSEMs and they are all working properly now after some debuggingsmiley  Our next step is to do calibration, and start to design the damping loop.

  741   Mon Oct 21 18:26:46 2013 ericqDailyProgressCrackleOct 21 Crackle Meeting Minutes

Crackle Meeting Oct 21

Attending: EricG, EricQ, Norna, Koji, Gabriele, Rana

  • Stainless steel blades are installed. Measurements will be done this week, to compare to maraging steel. Carbonsteel is next. 

  • We need to examine the choice of filters, windows, etc. in the signal processing chain, to be sure we're achieving maximum sensitivity. Literature about lock-ins should be helpful. We can empirically test out different choices on undriven data that exists. 

  • Discussed crackle propagation. What paths exist for crackle originating at different locations to move the test mass? Can twisting blade modes excite the fiber violin modes? Scaling of crackle from small to big blade also warrants further investigation. 

  • Discussed hysteresis (and contrasted to anelastic effects). Will measure hysteresis on carbon, stainless and maraging steels to try and connect with crackle measurements. 

  • We want to measure crackle at more drive frequencies, for potentially publishable results. This will be done after the carbon and stainless steel measurements. Should we measure all amplitudes and frequencies in the "bad" steels, or only a few to contrast?

  • Talked about systematic effects. My efforts on the Barkhausen noise calculation will be posted soon. DAC crackle noise can be remeasured with an AD620 instead of AD829, for better common mode rejection. ADC crackle should be measured as well. 

  • Discussed Crackle v2 experiment. How should blades be mounted / aligned in either Fabry-Perot or Michelson topologies? Better seismic common mode rejection is possible with horizontal mounting, but we lose the pre-stress of the masses used when loaded vertically. Eric and Gabriele will make some cartoons and quantitative estimates of noise in different measurement schemes, to be discussed at the next meeting. 

  • Also to be delivered next meeting: outline for Crackle paper. Need to consider what the target audience is: materials crackle people who would want to hear about a measurement of this effect or instrumentation people who want to hear about a new noise source

  1664   Sun Feb 19 14:32:47 2017 GabrieleNoise HuntingCrackleOffline seismic noise subtraction

Following up on my previous elog, here's how we could implement an offline subtraction of seismic noise that still provides us with a time series that can be fed into our standard analysis. It is based on FFT filtering and subtraction:

  1. for each chunk of locked data, compute averaged transfer function and coherence between the z accelerometer and the Michelson signal
  2. then compute the full length FFT of both ACC_Z and MICH signals
  3. interpolate the averaged transfer function and coherence to the full FFT frequency bins
  4. set to zero all bins with low coherence
  5. subtract the ACC_Z FFT tims the transfer function from MICH
  6. go back in time domain with an inverse FFT

This technique provides a time series of the improved MICH signal for each chunk of data. The attached MATLAB code implements it for a set of crackling noise measurement periods from this morning, with common drive ON and OFF.

The plots below shows the coherence of the original MICH signal with the accelerometer and the MICH/ACC_Z transfer functions. Clearly there is more coherence in the OFF periods than in ON periods.

The plot below compares the original MICH spectrum with the one computed from the subtracted time series. The subtraction works quite well: in the off periods the sensitivity at low frequency is improved a lot. There is less improvement in the ON periods.

However, looking at the residual coherence between ACC_Z and the subtracted MICH, it's clear that the subtraction works equally well in the ON and OFF periods. So the reason we don't improve much the sensitivity during the ON periods is that there is some other noise which is larger. 

This offline technique has many advantages:

  • it gives us the best possible subtraction during each lock (even if it's non causal),
  • it provides us with a time series which can be used for our analysis and for further noise hunting
  • it can be applied to old measurements too
  • it's easily extended to any other channel which turns out to be coherent with the improved MICH signal
Attachment 5: crackle_seism_sub.m
%% list of locked periods
clear all
gps = [1171554465, 1171556397, 1171558332, 1171560245, 1171562210, 1171564186, 1171566121, 1171568044]; % COMMON DRIVE: on, off, on, off, on, off, on, off
dt  = 1800;
leg = {'on', 'off', 'on', 'off', 'on', 'off', 'on', 'off'};
lin = {'-',  '--',  '-',  '--',  '-',  '--',  '-',  '--',};

%% get data and downsample to 512 Hz to make things faster later on
for i=1:numel(gps)
    c = nds2.connection('cymac2.caltech.edu', 8088);
... 146 more lines ...
  1563   Sat Jun 18 20:44:54 2016 XiaoyueDailyProgressCrackleOil pump broken

I was trying to resolve the pump issue. After filling in oil until the oil level reaches in between the reference lines I still got oil fume coming out. It's most possibly from the connection part between the filter and the pump. It seems that the oil mist is not going into the filter but diffuse terribly through a leakage at the connection. There is no working pumps left in the lab. I wil borrow the dry pump from 058f for now.

  867   Wed Dec 3 15:13:39 2014 GabrieleDailyProgressCrackleOne photodiode is broken

During the night the plate sagged to one side, so I had to realign everything again. I also found out that the SP photodiode was not working anymore. It output was stably at -11 V, regardless of the laser beam. I removed it and substituted with another space PDA100 we had in the cabinet. Now I see fringes in both photodiodes. The shadow sensor signals are also good.

Next steps:

  • cable the photodiodes, shadow sensors and could drivers to the ADC/DAC 
  • modify the model to cope with the new ADC/DAC channels
  • check if the coil drivers and local damping are working
  • cable the new PZT 
  1349   Thu Dec 10 16:26:54 2015 GabrieleDailyProgressCrackleOne set of OSEM electronics populated

Today I completed the assembly of a complete set of boards for six OSEMs, and integrated them into a box. I'm still missing a couple of connections between the boards, but the electronics is almost ready to be tested.

Attachment 1: IMG_3118.JPG
IMG_3118.JPG
Attachment 2: IMG_3118.JPG
IMG_3118.JPG
Attachment 3: IMG_3118.JPG
IMG_3118.JPG
  1042   Fri Jul 10 12:16:28 2015 GabrieleDailyProgressCrackleOnline calibrated Michelson displacement noise

Since our locking loop has a bandwidth of about 30-40 Hz, it's no more enough to multiply the in-loop Michelson error signal by the inverse of the optical gain to properly reconstruct a calibrated displacement noise, at least at low frequency.

One way to reconstruct the out-of-loop Michelson displacement is to mix the error signal with the control signal

z = invG * error - Act * control

where invG is the inverse optical gain (m/mW) and Act is the actuator response (m / uN). 

This morning I measured the plant transfer function and used it to reconstruct and fit the actuator response, see the first attached plot. The fit is accurate between 7 Hz and 200 Hz. We didn't figure out yet what's the origin of the structure at ~150 Hz, but that's off topic here.

I implemented the actuator transfer function in the calibration filter banks. Now the signal BBOARD_LOCK_MICH_Z is a combination of both control and error signals, and should provide a reasonaly accurate out-of-loop reconstruction of the noise. The second attached plot shows that it's working reasonably well, since in that period of time we were locked on non-whitened signals, so the 1e-13 m/rHz level is the ADC-noise equivalent. The green trace is the calibrated out-of-loop, which shows the right compensation for the locking loop gain peaking between 20 and 100 Hz.

Attachment 1: actuator.png
actuator.png
Attachment 2: online_calibrated_displacement.png
online_calibrated_displacement.png
  1044   Fri Jul 10 17:58:44 2015 GabrieleDailyProgressCrackleOnline calibrated Michelson displacement noise

The reconstructed Michelson displacement noise has very high dynamic range, with a lot of low freqeuncy stuff that can leak into high frequency if we take a too short time to compute a spectrogram. Since in any case the calibration below 7 Hz is not very accurate and we don't care much, I added in the model a filter bank just before the MICH_DZ signal. So we can add a whitening filter. For the moment I just included an elliptic high pass, to compress the low frequency signal. 

Attachment 1: calibrated_with_highpass.png
calibrated_with_highpass.png
  1803   Sat Aug 3 23:19:55 2019 DuoSummaryElectronicsOp amp oscillation caused by capacitive loading and its fix

When we connect the voltage monitor channel of the noisemon board to a long cable (100ft), the op amp (LT1792) oscillates. Usually putting a 50 ohm resistor at the end will fix it. In this post, I studied how the oscillation happens and why putting a 50 ohm resistor will fix it.

We know 1) op amp has a dominant pole, giving a phase shift of 90 degrees 2) op amp oscillates when the loop gain is unity and the phase shift is 180 degrees. 3) Op amp has some non-zero output resistance.

Based on 3), we can see that when the output is capacitively loaded, there will be another pole in the transfer function due to the RC configuration. Since both R and C are small, it will be at high frequency (as op amp oscillations usually are). Thus, beyond the dominant pole, the phase will keep shifting to 180 degree based on 1). When this happens before the loop gain drops to unity, there will be oscillation based on 2).

Fix: insert a resistor at the output. This fixes the problem since it adds a zero with frequency a bit higher than the parasitic pole. This zero pulls the phase up so that when the loop gain reaches unity, the phase is around 90, at least far from 180, preventing oscillation from happening. The transfer function of this is simulated in LISO. From the plot, we can see the effect of the output resistance pulling the phase up to zero (90 in the case of an opamp because of the dominant pole).

Attachment 1: osci.fil
uinput vin 1
gnuterm pdf
r rout 50 vin vout
r rproc 50 vout rproc_1
c cload 1n rproc_1 GND
uoutput vout:db:deg
freq log 1 1G 1000 ### from data file
Attachment 2: osci.pdf
osci.pdf
  752   Thu Nov 21 22:58:51 2013 xiaoyueDailyProgressCrackleOptical Setup

Gabriele and I set up a rough Michelson interferometer alignment.

Matlab model -- Gabriele

crackle_setup.jpg

Side view (left) Top view (right)

side_view.jpgtop_view.jpg

where the maroon beams are reflected from end mirrors. 

We tilted both of the 45 degree mirrors to deviate the reflected beam with an angle from incoming beams to bypass mirror 1(M1), to be detected by photodiode (PD).

Also it should be noted that the two end mirrors (ENDM1, ENDM2) have height difference of 1cm approximately, so we differentiate the optical path lengths of the two arms accordingly.

The alignment procedure is listed briefly as below:

- prepare the first mirror with a horizontal beam using irises.

- prepare a vertical reference using two irises aligned with a home-made plumb bob. 

- Align the 45 degrees inclined mirrors for vertical beams.

- Make sure the end mirrors are horizontal by overlapping incoming and reflected beams

- Roughly align the entire setup with beams superimposed

- Tweak the 45 degrees mirrors to separate the beam with a small angle (but large enough to bypass M1)

- Recenter or decenter mirrors to extract the symmetric port 

- Install photodiodes and beam dumps

(We may want to order more visible range mirrors / D-cut mirror / D-cut mirror mounts.)

  1367   Tue Feb 2 17:45:51 2016 GabrieleDailyProgressCrackleOptical board assembly

Today I assembled back the optical bread board, installing all the components in their rough position, corresponding to the Optocad design.

I also installed and clamped the magnets on the two blocks:

Finally I added a (nominal) 50% beam splitter on the laser board, to create a pick-off beam that we could use for power stabilization of the laser. I also added a photodiode. A lens is needed in front of it to reduce the beam size. I almost installed it, but then the lab red cabinet got stuck: I can't open it anymore and so I can't get any post or clamp out of it. I'll try to fix it tomorrow.

Attachment 3: 2016-02-02_17.26.30.jpg
2016-02-02_17.26.30.jpg
  1717   Thu May 11 11:16:47 2017 GabrieleNoise HuntingCrackleOptical gain variation due to low frequency drive

We can track the optical gain using the amplitude of the calibration line in the Michelson error signal. The plot below compares the spectrum of the calibration line amplitude with and without the low frequency excitation on. We can see a small peak at 1F when the drive is on. The blue triangles show the positions of the higher harmonics of the driving frequency.

Any noise that is actually constant in the error signal (shot noise, electronic noise, etc...) will appear as modulated at 1F due to this optical gain variation.

Attachment 1: michelson_optical_gain.png
michelson_optical_gain.png
  778   Tue Mar 4 19:04:48 2014 GabrieleSummaryCrackleOptical layout for the new crackle interferometer

 Here is the first draft of the optical layout for the next crackle interferometer. The blades have the correct dimensions, but all the rest of the mechanics is just tentative. However, the optical mount footprints have all the correct dimensions.

A 2 inches beam splitter is used, to properly separate the beams in the two directions of propagations. The beam splitter has a wedge to separate the reflections of the secondary face. These spurious beam will be dumped with small black glass baffles.

To equalize the length of the two Michelson arms, the mirror M4 will be mounted on a linear translation stage. 

 

Attachment 1: crackle_v2.pdf
crackle_v2.pdf
  517   Fri May 4 19:37:22 2012 ZachMiscCoating QOur samples

I was asked to identify our suspects:

  • Initial disc from Gregg Harry
    • Dimensions: 3" diameter x 1/8" thickness, beveled edge
    • Substrate: fused silica
    • Coating: 45-layer Tantala:Alumina

Mug shots:

GHarry_disc.png g_harry_3in_disc_copy.png

 

  • New disc from Steve Penn (there are two, but one is cracked and not shown here)
    • Dimensions: 3" diameter x 1/10" thickness, beveled edge (more than GH's)
    • Substrate: fused silica
    • Coating: None
    • Comments: Broke off of fiber inside chamber during prep and was chipped on the side. Alastair flame polished the chipped portion.

Mug shots: (freshly chipped on left, smoothed on right)

SPenn_disc_chipped.png SPenn_disc_chip_smoothed.png

 

  709   Tue Aug 13 21:09:44 2013 GiorgosSummarySUSOvercoming Saturation: Feedback through DC Coils and Mu-metal

In my previous post, I explained the saturation issues of the ADC and DAC we faced. To prevent saturation of the DAC, we will implement our gain after the feedback filter -we already introduced a gain of 25 at the coil's conditioning stage-and use a gain of approximately 1/25 for the digital filter. In this way, even if the ADC saturates at 10V, the feedback filter will send maximum 400mV (10V/25) to the DAC. However, the possibility that the ADC saturates still exists.

To make matters worse, we cannot even exploit the 10V saturation range of our ADC. The reason is that the undesired cross-coupling between coils and sensors is almost as large as the signal produced by the plate. Although vector fitting was very successful at cancelling cross-coupling, this method can only be implemented inside the digital control, after the ADC. That been said, if we are to stay within the 10V range, only 5V can come from the plate; the rest 5V will inevitably come from cross-coupling. Practically, this means that the signal from the plate is successfully sensed and transferred to the digital control for very small displacements, beyond which the ADC saturates and feedback control is impossible. We use two ways to tackle this obstacle:

  1. We revert back to using DC coils for feedback control. They are located further away from the AC sensors and cross-coupling is smaller, such that a larger proportion of the ADC' 10V range can be dedicated to the signal from the plate.
  2. We use mu-metal to cover all sensors and coils. We hope that mu metal's high permeability will shield off the magnetic field that comes from cross-coupling. It should also leave the magnetic field changes produced by the plate's movement almost intact, since the plate's magnets are located directly below and above the sensors, the only place we did not cover with mu metal.

We measured cross-coupling effects before and after the use of mu metal and noticed a drastic reduction in cross-coupling (about a factor of 3). The figures below show the measured transfer function before and after the use of mu-metal.

Unshielded.bmpShielded_CrossCoupling.bmp

One of the things to also note is that the bottom strain gauge motors readings are damaged and no longer worth looking at to determine when the plate is near equilibrium. This is probably the result of excessive weight on them from the plate. The top strain gauge motors seem to still be working fine.

  1219   Thu Sep 10 22:53:02 2015 XiaoyueDailyProgressCracklePD circuit board wired up

Today I wired up the PD output (both whitened and unwhitened channels) and the power supplies:

Following Federico and Gabriele's suggestion I rearranged the position of PD circuit board so no OSEM or translational stage cables go close by. The cables are traced as in the diagram below,

I also re-soldered the black box for the cabling integration outside vacuum accordingly.

 

  1186   Thu Aug 27 10:32:38 2015 XiaoyueDailyProgressCracklePD circuit integrated to a box

I built a box and relavant cables for the photodiode circuits: 

The anodes and cathodes are fed to a 9 pin connector with the pin connection: 1-GND, 6-K1, 2-A1, 7-GND, 3-K2, 8-A2. Two BNC outputs are for the PD outputs, left for PD1, and the right one for PD2. Power supplies (red +, blue GND, black -) are wired back to the electronic rack using a long 4-wire cable.

 

  1190   Thu Aug 27 13:48:38 2015 GabrieleDailyProgressCracklePD circuit integrated to a box

Everywhere else we use red=positive, black=ground and blue=negative. Please add a note to the bad of the box!

Quote:

Power supplies (red +, blue GND, black -) are wired back to the electronic rack using a long 4-wire cable.

 

  1417   Thu Mar 10 21:44:36 2016 GabrieleNoise HuntingCracklePD clipping?

I think the reason the noise is bad can be traced to the fact that the AP and SP photodiodes see signals that are not completely coherent, and their spectra look quite different:

Maybe one (or both) are not well centered and the beam is clipping.

 

Attachment 1: pd_coherence.png
pd_coherence.png
  1434   Wed Mar 23 14:55:24 2016 GabrieleDailyProgressCracklePD de-whitening tuning

With the chamber open, I blocked one of the two Michelson arms, and injected some intensity noise, such to dominate the normal PD noise at all frequencies (noise shaping: double pole @ 10 Hz, double zero @ 100 Hz).

I then measured the transfer functions SP_OUT/SP_WHITE_IN1 and AP/AP_WHITE_IN1, which assuming a flat response of the non-whitened signals, gives the right compensation to equalize the response to intensity noise of the two PDs. I could fit both TF with a single pole and a single zero, with residuals below 1% at all frequencies.

Then I implemented those filters (dewhite2), and checked the TF AP_WHITE_OUT/SP_WHITE_OUT. This was very close to be flat (not 1, since the two PDs see slighly different powers). I fitted the ratio with a 4th order filter. I had to remove a zero/pole pair at high frequency that fitted the phase rotation above 1 kHz or so. I implemented this filter in the SP bank (dewhiteC) and measured again the ratio of the two PD. I tweaked a bit the gain to match the SP_WHITE/AP_WHITE ratio to SP/AP (change of about 0.1%). The ratio is quite flat, and from the deviation from a flat TF I can esitmate the following mis-cancelation of RIN, which is better than 1/1000 below 200 Hz and better than 1/100 below 2.5 kHz.

  140   Tue Dec 15 09:28:39 2009 ranaMiscSUSPD front end noise

This is the LISO estimate of the PD front end noise. It could be improved somewhat by using a higher value resistor, but there's no point.

The shot noise level for the 20-40 uA of current we have is more than 1 pA/rHz so we should be OK above 30 mHz.

So even the level of the dark noise below seems too high and also the 10,000 V/A statement. The feedback resistor we had used to be 100k...

Attachment 1: dcpd.png
dcpd.png
  141   Tue Dec 15 10:58:56 2009 ZachMiscSUSPD front end noise

10k was a typo--fixed.

Quote:

This is the LISO estimate of the PD front end noise. It could be improved somewhat by using a higher value resistor, but there's no point.

The shot noise level for the 20-40 uA of current we have is more than 1 pA/rHz so we should be OK above 30 mHz.

So even the level of the dark noise below seems too high and also the 10,000 V/A statement. The feedback resistor we had used to be 100k...

 

  1580   Mon Jul 18 22:23:58 2016 XiaoyueDailyProgressCracklePD saturation

[Seiji, Xiaoyue] We noticed PDs were saturated at maximum fringe with current power so we reduced the laser output beam current (from adj-4 to adj-6), and increased the servo gain to keep ~ 63 Hz UGF. After avoiding saturation we are getting a MICH sensitivity very close to what we had with maraging steel blades before: blue is reference spectrum with maraging steel blades, green is spectrum yesterday with PD saturated, red is spectrum today after reducing laser power.

I pumped down to ~ 500 mTorr, and started another overnight crackling noise measuremnet with driving frequency 0.125 Hz, pk2pk amplitude ~ 30 um from ~ 10 pm 1152939556

 

 

  1132   Wed Aug 5 22:25:15 2015 XiaoyueDailyProgressCracklePD uncorrelated noise, more balancing for intensity noise, and shot noise

[Yingtao, Xiaoyue]

Pressure this morning (11:40 am) went up to 990 mTorr; not too surprise, the alignment is no longer good.

We vent the chamber in order to characterize uncorrelated photodiode noise. We blocked M4 mirror (right arm), and found the polarization very bad (two photodiode readings are very different). We tuned the incoming beam polarization to equalize the two PD power. Then we increased the power to same as we locked (3.54 V from oscilloscope), taking one hour measurement:

Wed Aug  5 14:08:52 PDT 2015
Wed Aug  5 15:10:09 PDT 2015    

With one arm blocked, we also injected white RIN noise to balance the two photodiodes better:
Look at AP(SP)_OUT / RIN, 300~600 Hz, balance SP/AP gain = 1.0671
Look at AP(SP)_WHITE_OUT / RIN, 300~600 Hz, balance SP_WHITE/AP_WHITE gain = 1.0302

After balancing, no distinguishable difference at high frequency can been seen from the all four TF's, measurement saved as RIN-PD_balance.xml.

I also measured power input to the PDs in order to investigate shot noise:
AP = 1.00 mW, new calibr = 1 /(-5615+20) counts = -1.7873e-4 cts2mW, compared to old calibr -1.661e-4
SP = 1.02 mW, new calibr = 1.02 mW /(-5704+20) counts = -1.7936e-4 cts2mW, compared to old calibr -1.736e-4

However since the new calibration is not so far from the old ones -- the difference is within a power meter measurement error. I am leaving it with the old ones. 

Feeding the PD output directly to spectrum analyzer I measured ~ 45 uVpk @ 6.5 kHz. Calibrated from V to mW using the oscilloscope and power meter reading (1.02 mW = 3.54 V) I estimated the high frequency noise floor to be ~1.3e-8 W/sqrt(Hz). However theoretically the shot noise level should be sqrt(2*6.626e-34*1.01e-3*3e8/1064e-9) ~ 2e-11 W/sqrt(Hz).

After talking to Gabriele I realized that what I measured from single PD is intesity noise. Nevertheless the RIN 45e-6 / 3.54 = 1.2e-5, which is strangely ~ one order of magnitude higher than the RIN ~ 2e-6 reported (Entry 1115) from digital measurement of the sum PD reading.

I also updated unwhiten signal to 16 kHz sampling rate.

I left the pump on from 7:25 pm. At the moment of writing the pressure goes down to 153 mTorr.

  1029   Mon Jul 6 15:00:34 2015 GabrieleDailyProgressCracklePD whitening

Following up on the work reported in 1025 and 1028, I connected a parallel path on the photodiodes, providing whitened channels in addition to the standard ones. The new channels are called AP_WHITE and SP_WHITE. I modified the mixing matrix to allow using them in place of the normal diode signals. The MEDM screen includes two buttons that call external scripts to switch with a ramp of 3 seconds (for some reason the ramping is not visible in the screen, but it's happening).

While connecting the new signals, I found out that ADC channel 8 is likely broken.

  1436   Thu Mar 24 15:29:03 2016 GabrieleDailyProgressCracklePD windows removed, PD mounted back

[Xiaoyue, Rich, Gabriele]

Rich cut the PD cans and we removed the protective window. Xiaoyue soldered an extension to the wires, and I installed the PDs on the beadboard, moving them up by few cms. I checked with the beam profiler that the beam radius is about 150-170 um at the current PD positions. I had to remove Koji's beam dump: the plan is to orient the PD so that their reflection is damped on the square baffles with holes.

  560   Wed Aug 15 23:45:13 2012 igalDailyProgressCracklePD-Seismometer coherence

This week I also measured the coherence between the seismometer (GS-13) and the photodiode:

GS13_PD_Coherence.png

Clearly there is little to no coherence between the seismometer and the photodiode except under 300Hz. There may simply be a lot of seismic noise at this frequency. Either way, it is evident that seismic noise will not be a limiting factor in our experiment.

  1903   Thu Feb 18 09:39:01 2021 PacoDailyProgressElectronicsPDH error signal misbehaving

Error signal

Upon closer inspection the error signal seems to vary quite significantly on the scope (scanning @ 2 Hz), sometimes completely flipping its sign even though it always triggers on the same side of the ramp (see attachment for video, along with some neck excersise).

This might be the same behaviour from before, whereby the demodulated signal might still be "riding" a low-freq componen which can't be compensated with the LO (Marconi's carrier resolution = 1 Hz). Using the 10 MHz external Rb reference doesn't change anything. It seems that even with the coupler, reflections may be entering the mixer... 

Adding a LP filter (BLP-1.9+) right at the mixer output solves this for good. Even using 36 MHz LO vs anything else doesn't make a difference so this explains the previous issue. Moving back to lock using stable err signal.

For reference, the LO carrier is set to 36.000 MHz, +7 dBm (so the EOM is driven with an estimated +30 dBm well below the saturation or damage threshold +40 dBm).

DOPO locked

Achieved a good lock for pretty much all of the afternoon today. The laser ran at 937 mA current, the optimal gain on SR560 was found to be 50, with a LP cutoff at 300 Hz (12 dB/oct rolldown). The 300 Hz cutoff supresses most of the nasty 8 kHz noise (and harmonics) which I can hear with enough gain. Source still to be determined.

Attachment 1: misbehaving_pdh.mp4
  403   Tue Feb 7 13:05:12 2012 ZachDailyProgressCoating QPEEK connector is in

I received the 4-terminal PEEK in-vac connector for the high voltage today. It looks like it will work just fine.

I'm glad I didn't spring for the next-day delivery!

PEEK_connector.png

  1874   Fri Dec 11 16:04:36 2020 PacoLab InfrastructureOpticsPPKTP crystals

Two crystals from Raicol arrived. Picked them up from Downs today and inspected them (see photos below). The lengths are nominal (20 mm), they are serialized as 123 and 124, and the ends look like they have the specified (AR) coating. I reached out for Covesion two days ago to track the ovens so we can mount these guys, but have yet to hear back from them.

Attachment 1: raicol_124.jpg
raicol_124.jpg
Attachment 2: raicol_123.jpg
raicol_123.jpg
  1111   Thu Jul 30 13:52:41 2015 ericqDailyProgressCracklePSD calculation methods for high dynamic range data

When I saw the spectrum from this lock (Jul 30 02:00 UTC), it struck me how steep the seismic rolloff is the calibrated MICH_DZ channel. As I recall, for such spectra, the seismic folks sometimes use an alternate PSD calculation method, since windowing effects can spoil such sharp rolloffs in data with high dynamic range. 

The basic idea is that instead of doing multiple windowed FFTs and averaging (i.e. the normal pwelch method, which I believe DTT uses), they window the whole data segment and take a single FFT to minimize the window spectral leakage. Then, the PSD is rebinned to something more suitable for the human eye. 

I gave this a shot, and there is a little, but a nontrivial, difference in the resultant spectra in the frequency regieme we care about. The times I looked at were the 600 seconds leading up to 1122256817. 

NB: I haven't actually inspected the seismic code (asd2 I believe its called, and linked somewhere in the LHO alog), so I may not have done the rebinning in the best way. I chose to do it by preserving the RMS as integrated from the nyquist frequency down to zero, but this has the problem of biasing some features to lower frequency, as can be seen in some of the peaks around 10Hz. I don't think this invalidates the reduction at 20Hz, however.

In practice, maybe the calibrated PSDs should be produced with DTT set for long segments, and some rebinning. Or done by some other means.

Attachment 1: krk_asd2.png
krk_asd2.png
  8   Mon Oct 22 19:27:14 2007 pkpOtherOMCPZT calibration/ transfer function.
We measured the PZT transfer function by comparing the PZT response of the circuit with the cavity in the loop, with that of the circuit without the cavity in the loop. Basically measure the transfer function of the whole loop with the laser/PZT and Op-amps in it. Then take another measurement of the transfer function of everything else besides the PZT and from both these functions, we can calculate the PZT response.

The calibration was done by using the error signal response to a triangular wave of volts applied to the PZT. A measurement of the slope of the error signal , which has three zero-crossings as the cavity sweeps through the sidebands, gives us the Volts/Hz response. In order to derive a frequency calibration of the x axis, we assume that the first zero crossing corresponds to the first side band (-29.5 MHz) and the third one corresponds with the other sideband (+29.5 MHz). And then by using the fact that we know the response of the cavity to a constant frequency shift, we can use the Volts/Hz measurement to calculate the Volts/nm calibration. The slope that was calculated was 3.2e-6 V/Hz and using the fact that the cavity is 1 m in length and the frequency is 1064 nm, we get a calibration of 0.9022 V/nm.

Attachment 1: calib.pdf
calib.pdf
Attachment 2: calibpzt2.pdf
calibpzt2.pdf
Attachment 3: all2.pdf
all2.pdf
Attachment 4: noPZT2.pdf
noPZT2.pdf
  9   Tue Oct 23 09:01:00 2007 ranaOtherOMCPZT calibration/ transfer function.
Are you sure that the error signal sweep is not saturated on the top ends? This is usually the downfall
of this calibration method.
  166   Thu Jan 6 01:03:24 2011 ZachDailyProgressCreakPZT installed/tested

  I rigged up a way to use the small ThorLabs PZTs we took from the 40m yesterday. After an hour or so of going back and forth from the ATF to the SUS lab with random optical hardware to find something suitable, I finally found a solution using one of the fancy translation stages we have for the eventual gyro modematching. Here's a shot of the whole assembly:

 
2011-01-05_17.21.29.jpg
 
That was just to find a way to mount to the magnetic base we are using; I still needed a way to actually hold the PZT and connect it to the mirror on the shim "blade". We knew we wanted to have something give-y like rubber between the PZT and the blade itself to suppress high-frequency noise in the actuator, so I found a piece of rubber grommet to do the trick. The grommet had a hole in it, of course, so I wrapped it in a piece of shrinkwrap so that I could glue it along the flatter surface to the PZT. On the other end, I needed something firm attached to the PZT with which to hold it (gripping the PZT itself might damage it and in any case would reduce the range of motion). I chose to use a polyester film capacitor---with the leads trimmed---and glued it to the other side. Here is a closeup:
 
2011-01-05_16.30.40.jpg
 
This thing is supposed to put out 4.5 um with 150 V applied, so I figured I could get a decent signal using a drive on the order of 5-10 V (since we are using 633-nm light, this is on the order of a fringe). I installed a PDA100A at the AS port of the interferometer and realigned the beams from both arms to overlap. The manufacturer warns never to reverse the polarity of the PZT leads, so I applied a ~6 Vpp drive with an offset of +5V. I could clearly see an output coherent with the drive on the scope over a wide range of frequencies. I decided to plug it into the Agilent and look at the spectrum. Here is an example of one with a 3-Hz drive signal. There is a lot of upconversion because the mirror is swinging through a couple of fringes. I was able to change the overtone structure by adjusting the drive amplitude and offset (so that it stayed roughly linear).
 
3Hz_spect.png
For the heck of it, I thought I might try and measure a transfer function from the PZT to the PD signal. It can be seen below. Even with maximum integration, the ambient noise is very high at the moment, and turning up the drive doesn't help since the thing quickly loses linearity, but to the naked eye the TF looks roughly like what one would expect from a driven pendulum with a resonance somewhere around 100-200 Hz. Rana and I noticed that the simple system with the shim clamped to the base and the mirror glued to its top had a fairly high Q, but the thing is now damped by the rubber contact, so the resonance is not very evident in the TF.
 
PZT-AS_TF.png
From these very simple trials, I would guess that these PZTs will work quite nicely once we can close the loop and operate at the dark fringe. I have unfastened the second unit from the mirror on which we found them, and I will try and put a new wire on the ground lead tomorrow so that we can test it.
  854   Thu Oct 23 19:01:53 2014 ericqSummaryCracklePZT measurement

 I set up a quick Michelson, with PDs at both ports, as in the crackle1 Michelson, to test the PZT we bought from Holofringe. (I had to borrow the beamsplitter from crackle1, since I needed a non-polarized one. I also borrowed one if its PDs). 

I was actually even able to get a loose lock with just a SR560 as a servo filter and no HV driver, since mounts on the table are pretty stiff. With a thorlabs MDT694 and the SR560 set to a single pole at 30Hz and G=50 and PD100A gains at 0dB, I locked the michelson at half fringe with ~30k bandwidth, and took the OLTF, to back out the SR560 and optical gains, and get the PZT+Driver TF. 

The PZT resonance is around 92k, which is close to the quoted 88k. Oddly, the DC response seems to be a factor of three too small, ~13nm/V instead of the quoted 36nm/V. I don't see where it could be coming from at this point... If this is true, we would have ~2.5 micron of range through the -100->+100 V swing of the PZT. I can't remember if this is enough. 

Anyways, here's the HV driver + PZT response plot. (Gain of the driver is 15, so DC response of the PZT is the DC value of the plot over 15)

PZTresp.pdf

 

  855   Fri Oct 24 05:25:22 2014 GabrieleSummaryCracklePZT measurement

Quote:

 I set up a quick Michelson, with PDs at both ports, as in the crackle1 Michelson, to test the PZT we bought from Holofringe. (I had to borrow the beamsplitter from crackle1, since I needed a non-polarized one. I also borrowed one if its PDs). 

I was actually even able to get a loose lock with just a SR560 as a servo filter and no HV driver, since mounts on the table are pretty stiff. With a thorlabs MDT694 and the SR560 set to a single pole at 30Hz and G=50 and PD100A gains at 0dB, I locked the michelson at half fringe with ~30k bandwidth, and took the OLTF, to back out the SR560 and optical gains, and get the PZT+Driver TF. 

The PZT resonance is around 92k, which is close to the quoted 88k. Oddly, the DC response seems to be a factor of three too small, ~13nm/V instead of the quoted 36nm/V. I don't see where it could be coming from at this point... If this is true, we would have ~2.5 micron of range through the -100->+100 V swing of the PZT. I can't remember if this is enough. 

Anyways, here's the HV driver + PZT response plot. (Gain of the driver is 15, so DC response of the PZT is the DC value of the plot over 15)

 

 

Very nice measurement! The bandwidth is much larger than I expected, which is good. I also guess that 2.5 microns should be more than enough, at least to acquire the lock, since the fringe is about 0.3 um. We can always reallocate the low frequency component to the blades.

  856   Tue Nov 4 15:43:58 2014 ericqSummaryCracklePZT measurement

A few updates about the PZT:

  • I misreported the bandwidth I achieved with the Michelson PZT loop. Unfortunately, the UGF was 3kHz, not 30k  
  • I remeasured the loop via swept sine, instead of noise injection. All that really changes was the disappearance of the notch-like features below 1k. 
  • I fixed my goof in the loop compensation. The low frequency response is now totally in phase, as it should be...
  • I measured the PZT capacitance, driver output impedance, and driver TF into the SR785 to try and calibrate the driver out of the driver+PZT response

PZTresp.pdf

  • I also took a few pictures of the PZT in the current setup

2014-11-04_13.35.55.jpg2014-11-04_13.36.11.jpg

  893   Thu Jan 22 10:09:35 2015 GabrieleSummaryCracklePZT noise level

To estimate the level of displacement noise introduced by the PZT servo and driver, I temporarely removed the low pass filter and locked with the digital control loop. In this way I could see a large increase of displacement noise. I measured the open loop TF of the digital lock and used it to compensate for the loop shape.

The PZT noise is 2.4e-13 m/rHz at 100 Hz, shaped like 1/sqrt(f), and flattening at 7e-14 m/rHz.

Then I put back the filter, and measured no excess of noise due to the PZT. I know that the low pass is a single pole at 12 Hz, so I can use this and the previous measurement of the noise to get an estimate of the PZT noise in normal operations. This is below the measured displacement noise, so it's not important for the moment being.

  887   Thu Jan 15 16:48:41 2015 GabrieleDailyProgressCracklePZT noise?

The following plot compares the in-loop displacement noise for different configurations:

  • PZT unplugged and Michelson locked with the usual digital loop, bandwidth of about 70 Hz.
  • PZT plugged in with a 76 V offset sent by the Thorlabs driver, no input to the driver. Digital lock as above
  • PZT plugged in with the same offset coming from the servo board (no PZT locking). Digital lock as above
  • PZT servo loop engaged, with a bandwidth of about 2 kHz

It's apparent that the Thorlabs driver is adding a broadband noise that corresponds roughly to a level of 2e-13 m/rHz. My simple servo board is adding even more noise on top of that, at a level of 8e-13 m/rHz.

Using Eric's calibration of the PZT of 13 nm/V, this noise level would correspond to about 10 uV/rHz at the output of the PZT driver. The Thorlabs user manual quotes a ouptut noise lower than 1.5 mV RMS. Assuming a 10 kHz bandwidth as measured by Eric and somehow flat noise, we get 15 uV/rHz, which is compatible with what we see.

I added a pomona box at the output of the PZT driver, with a low pass filter (1Mohm, 1uF) and this is good to reduce the noise of the driver. The following picture compares the displacement noise with the digital loop, and only the PZT driver offset. The blue curve is without low pass and the red with low pass:

However, as expected, in this configuration the driver doesn't have enough range to lock with the PZT (after removing the integrator from the control circuit). Tomorrow I'll try a less aggressive filtering and I'll redesign the control electronics accordingly.

  898   Tue Jan 27 09:11:12 2015 GabrieleDailyProgressCracklePZT offload path

We don't really need it, but nevertheless I added a PZT offload path to the real time model. The PZT control signal can be added to the digital locking loop error signal, using the filter bank LOCK_PZTOFFLOAD. If you switch on the output of this filter bank and switch off the photodiode filter bank outputs, you can use the PZT correction as the input of the digital loop.

  63   Mon Nov 5 14:44:39 2007 waldmanUpdateOMCPZT response functions and De-whitening
The PZT has two control paths: a DC coupled path with gain of 20, range of 0 to 300 V, and a pair of 1:10 whitening filters, and an AC path capacitively coupled to the PZT via a 0.1 uF cap through a 2nd order, 2 kHz high pass filter. There are two monitors for the PZT, a DC monitor which sniffs the DC directly with a gain of 0.02 and one which sniffs the dither input with a gain of 10.

There are two plots included below. The first measures the transfer function of the AC monitor / AC drive. It shows the expected 2 kHz 2d order filter and an AC gain of 100 dB, which seems a bit high but may be because of a filter I am forgetting. The high frequency rolloff is the AA and AI filters kicking in which are 3rd order butters at 10 kHz.

The second plot is the DC path. The two traces show the transfer function of DC monitor / DC drive with and with an Anti-dewhitening filter engaged in the DC drive. I fit the antidewhite using a least squares routine in matlab constrained to match 2 poles, 2 zeros, and a delay to the measured complex filter response. The resulting filter is (1.21, 0.72) : (12.61, 8.67) and the delay was f_pi = 912 Hz. The delay is a bit lower than expected for the f_pi = 3 kHz delay of the AA, AI, decimate combination, but not totally unreasonable. Without the delay, the filter is (1.3, 0.7) : (8.2, 13.2) - basically the same - so I use the results of the fit with delay. As you can see, the response of the combined digital AntiDW, analog DW path is flat to +/- 0.3 dB and +/- 3 degrees of phase.

Note the -44 dB of DC mon / DC drive is because the DC mon is calibrated in PZT Volts so the TF is PZT Volts / DAC cts. To calculate this value: there are (20 DAC V / 65536 DAC cts)* ( 20 PZT V / 1 DAC V) = -44.2 dB. Perfect!

I measured the high frequency response of the loop DC monitor / DC drive to be flat.
Attachment 1: 07110_DithertoVmonAC_sweep2-0.png
07110_DithertoVmonAC_sweep2-0.png
Attachment 2: 071105_LSCtoVmonDC_sweep4-0.png
071105_LSCtoVmonDC_sweep4-0.png
Attachment 3: 07110_DithertoVmonAC_sweep2.pdf
07110_DithertoVmonAC_sweep2.pdf 07110_DithertoVmonAC_sweep2.pdf
Attachment 4: 071105_LSCtoVmonDC_sweep4.pdf
071105_LSCtoVmonDC_sweep4.pdf 071105_LSCtoVmonDC_sweep4.pdf
  885   Wed Jan 14 11:54:48 2015 GabrieleDailyProgressCracklePZT servo for crackle1

I designed, built and characterized an analog electronic servo loop for the PZT locking of crackle1. An AD620 computes the difference of the two PD signals, then an OP27 is used for a variable gain 0-100, then a low pass filter is built with another OP27, with corner frequency of 100 mHz and unity gain at 1 Hz. I measured the transfer function and it is as expected.

I cabled the PZT BNC connector to the additional BNC feedthroguh that we installed some months ago. Even though the cable is rigid and touching the chamber wall, I couldn't see any worsening in seismic isolation. The following plot compares the damping loop spectra (error signals and correction) when the BNC is disconnected (blue) or connected (red).

Here is a picture of the connection:

I haven't tried to lock with the PZT yet.

 

  1363   Wed Jan 27 01:42:30 2016 XiaoyueDailyProgressCracklePair of thinner highC blades test

I pre-curved two ~0.88 mm thickness high carbon steel blades using the same cold-rolling procedures to check the reproducibility of the work-hardening method. The dimension before tests are very close to each: radius R ~ 90 mm, initial deflection d0 ~ 72 mm (from tip) ~ 66 mm (from holes). 

I prescribed the extension to be 50 : 5 : 75 mm, which is 6 incremental cycles for each blade. The load vs. displacement results are plot as below,

The two tests almost overlap each other, which means the cold-rolling + work-hardening method is fairly reliable. Now at the flat position ~ 60 mm, the load is ~ 10 N, which is ~ 70% of the yielding point at ~ 14 N for both blades. Note there is a load drift after the first large deformation. Most possibly this is due to the non-zero stiffness of the pulling wires. After a huge deformation, the less deflection will push the wire upward and hence exert a negative force to the load cell at zero position.

If we compare the B2, B3 results with C2 ~ 1.06 mm thickness (previous log), we see their deformation curves are very different. In order to have a larger flat-position load, we might want to go with thicker blades. From the available results, a good sanity check can be drawn by comparing the elastic unloading part, where we can extract an approximate stiffness of the blade by doing a linear fit. The stiffness of B3 is measured to be k1 ~ 0.2717 N/mm while the stiffness of C2 is k2 ~ 0.4761 N/mm. Apply the simple equation for stiffness k of a cantilever beam with one load applied at end: k = 3*w*t^3/12*Es/sf/L^3, where w is width, t is thickness, L is length, Es is Young’s modulus, and sf is the shape factor. Considering the only difference between B3, C2 blades is their thickness, we would obtain: k1 / k2 = t1^3 / t2^3. Check that 0.2717 / 0.4761 = 0.5707, and 0.88^3 / 1.06^3 = 0.5722 are in good agreement.

  1014   Wed Jun 24 23:45:21 2015 SaikanthDailyProgressCrackleParallel DTT TF measurements - parameters

As mentioned in the previous update, I have been unsuccessful so far in automating parallel DTT measurements. However, it is essential to have measurements running parallel to some extent to save time. One way, obviously, is to do setup and run measurements manually.

The parallel measurements that I had started left last night had taken a lot of time and didn't finish even after 12 hrs. Meanwhile, independent of this, it has also been noticed that there is a lot of unnecessary data that we are collecting: the data above 5Hz where the coherence is almost always bad. Also, the number of averages, number of measurement cycles etc. were never taken into consideration while running measurements - since we were never worried about time. But then, time is now of interest. Therefore, it would be good to see if any of these parameters can be played around with and if the measurement time can be reduced without affecting the measurements itself.

 

For this, I have repeated measurements of the same type - with just about 10 points each time (for the sake of making a quick comparison) - by varying the number of cycles and averages. Attached are some results. Notice the coherence plot (on top).

Note: 1) The figure names tell about parameters in each case. 2) Not all plots start at the same frequency. I ran measurements from high to low frequency, and I've aborted some measurements which were taking a while because they were taking too much time at low frequencies, and the area of concern really was not the lowest frequencies.

 

Clearly, the coherence is exceptionally bad only for the case with 5 averages and 5 cycles - especially in the 0.3-0.8 Hz region - while it's above 0.7 in the rest of the cases. To decide among which combination to go with among the rest of them, I have looked at the product of averages and cycles, since it determines how long the measurement would go for. The current decision is to go ahead with 8 cycles and 5 averages.

 

Example: For a case with 200 log-spaced points between 0.1Hz to 5hz, I have estimated the time to be about 5.6hrs, which is reasonable enough compared to the earlier time durations. The bigger question, though, is to ask if it remains the same for parallel tests. The measurements will testify this.

Attachment 1: 5av_5cyc.png
5av_5cyc.png
Attachment 2: 5av_8cyc.png
5av_8cyc.png
Attachment 3: 5av_10cyc.png
5av_10cyc.png
Attachment 4: 7av_8cyc.png
7av_8cyc.png
  1018   Mon Jun 29 20:29:19 2015 SaikanthSummaryCrackleParallel TF measurements (automation) now possible

Pending post: one that I was supposed to write last week.

Earlier, I had tried running parallel TF measurements through Python-MATLAB. However, I was unsuccessful and I couldn't make much sense of the problem I was facing. I then tried the other method that was suggested - using the Diagnostic Tools "diag". The process can be summarized as follows.

First, a DTT template is prepared and saved as an XML file. The desired parameters are set; 'start time' is to be chosen as 'Now'.

Next, a short 'diag' script to be run from the workstation is prepared. The code itself is simple; here's how it goes

open

restore PATH-TO-DIRECTORY/IN_FILE_NAME.xml

run -w

save PATH-TO-DIRECTORY/OUT_FILE_NAME.xml

exit

Path to directory and file names are to be set appropriately. The above commands are for the Diagnostic Tools "diag". However, since we want to automate this, six such scripts must be run in parallel automatically; this was done through Python using the subprocess module.

 

I have, for the purpose of illustration, attached the diag script, Python script and the input DTT template that I used. Remember, to reproduce what I have talked about in this elog post:

  1. Input template is a set of files such as 'a-exc.xml'
  2. For 6 such files, 6 diag scripts must be appropriately created and saved in the same directory.
  3. The python code, also in the same directory and with appropriate files names, can be executed to begin measurements.
Attachment 1: a-exc
open
restore /home/controls/Saikanth/simultaneous_measurements/a-exc.xml
run -w
save /home/controls/Saikanth/simultaneous_measurements/a-done.xml
exit
Attachment 2: run_multi_tf.py
import subprocess

subprocess.Popen(['gnome-terminal', '-e', "diag -f /home/controls/Saikanth/simultaneous_measurements/a-exc"])
subprocess.Popen(['gnome-terminal', '-e', "diag -f /home/controls/Saikanth/simultaneous_measurements/b-exc"])
subprocess.Popen(['gnome-terminal', '-e', "diag -f /home/controls/Saikanth/simultaneous_measurements/c-exc"])
subprocess.Popen(['gnome-terminal', '-e', "diag -f /home/controls/Saikanth/simultaneous_measurements/d-exc"])
subprocess.Popen(['gnome-terminal', '-e', "diag -f /home/controls/Saikanth/simultaneous_measurements/e-exc"])
subprocess.Popen(['gnome-terminal', '-e', "diag -f /home/controls/Saikanth/simultaneous_measurements/f-exc"])
  1019   Mon Jun 29 21:34:31 2015 KojiSummaryCrackleParallel TF measurements (automation) now possible

Please ZIP a huge text file like XML before posting. I deleted it.

  385   Thu Jan 26 11:49:45 2012 ZachDailyProgressCoating QParts gathered, etc.

[Giordon, Zach]

Yesterday, we went to the 40m and stole most of the parts we will need for the Q-measurement polarimeter. I have asked Giordon to put a list of these parts on the elog, as training for good elog routine.

We brought them to the SUS lab and started placing things on the table. The plan is to build a barebones setup, in air, to get a feel for the readout scheme. As a first pass, we will find any old piece of transparent material and drive it mechanically (read: flick it) to see if we see a low-Q ringdown. In parallel, Giordon is supposed to be working on his SNR calculations (and I am supposed to be answering some questions he has).

We set up a HeNe and some polarizing elements, but were somehow unable to linearize the polarization. This is somewhat puzzling:

  • We are using quarter- and half-wave plates that are ThorLabs-labelled (etched) for 633 nm.
  • We are using a 1-in PBS from the 40m that was unlabeled. There were three such cubes on the SP table near other visible optics, so I assumed that they could be from an old SURF HeNe setup that Rana mentioned. Using a red laser pointer, I verified that I could get reasonable visibility (say ~80-90% by eye) by rotating it with respect to the PBS. The effect was much smaller with a green pointer, so I concluded that it was a 633-nm PBS.
  • Setting the elements up in the usual way, I was only able to get contrast of ~30% or so by adjusting the waveplates. Frank looked up the HeNe head part number and found that its output was circular, but of course this is the point of the QWP.
  • We replaced the head with another one (on the off chance that the head was broken and somehow putting out a time-varying polarization), and this had no effect.
  • Frank, Vladimir and I did some independent testing of the polarization optics. We couldn't find a red laser pointer (which was the only source with which I could get the PBS working), so instead we tested them by eye with the (polarized) LCD computer monitors in the lab.
  • All the optics behaved as expected:
    • Putting the PBS between our eyes and the screen, we could achieve very high visibility by rotating it.
    • Leaving the PBS where it was, and adding the HWP between it and the screen, we could achieve very similar visibility by rotating the HWP alone.
    • Switching out the HWP for the QWP, we could set the QWP such that no orientation of the PBS blocked light from the LCD (i.e., the polarization was circularized).
    • It should be noted that we focused our attention to RED images on the screen, and that these trials did not work equally for all colors
  • So, the question is: if every component seems to be working, and switching the laser has no effect, WHAT THE HELL IS GOING ON?

 

  1146   Mon Aug 17 04:24:24 2015 SaikanthSummaryCracklePending update: Analytical modeling of Crackle2 suspension

Pending update from a couple of weeks ago.

As I mentioned in the first update on Analytical Modeling of Crackle2, I had worked on the Mathematica model built by Gabriele to generate analytical results for the suspension transfer functions. The model could not reproduce the experimentally observed results, and so I had to shift to a different model. I had started using the Mathematica-based SUspension Model CONstructor (SUMCON) built by folks at KAGRA. What one can do with the interface, in brief, is as follows:

  • Define bodies/stages and associate one of them as attached to ground: In our case, the cage is attached to ground, and there's an intermediate suspension stage and the payload (optics breadboard).
  • Give shape information, mass, moment of inertia, initial position values for each of the bodies.
  • Setup wires for suspending one stage from another, and also input properties such as thickness, length, material.
  • Define springs (in our case blade springs) and their properties (Q, spring constant, etc.)
  • Setup other things, which are not relevant to our setup, such as inverted pendulum, heat links, dampers etc.

It is pretty intuitive to use; it can be found here.

Results for Crackle2:

The attachments below show the transfer function predictions for our suspension setup. Unfortunately, even after spending a lot of time, I couldn't recover data points of these plots to plot them on the same axis as experimentally obtained ones. There are a couple of channels available for one to extract data points, but somehow none of them seem to be working for me... Still, one can easily compare the frequency positions of each of these peaks with experimental results and observe the close matching.

 

Attachment 1: x_tf_1.pdf
x_tf_1.pdf
Attachment 2: y_tf_1.pdf
y_tf_1.pdf
Attachment 3: z_tf_1.pdf
z_tf_1.pdf
Attachment 4: p_tf_1.pdf
p_tf_1.pdf
Attachment 5: r_tf_1.pdf
r_tf_1.pdf
Attachment 6: w_tf_1.pdf
w_tf_1.pdf
  1437   Thu Mar 24 16:29:49 2016 GabrieleMiscCracklePhotodiode cabling reference

For future reference, here is the datasheet of the photodiodes we are using: C30665

The wires are:

white = ground
blue = anode
green = cathode

  973   Mon May 11 19:37:51 2015 xiaoyueDailyProgressCracklePhotodiode upgrade

ELOG V3.1.3-