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ID Date Author Type Category Subject
48   Mon Jul 18 14:29:07 2016 GabrieleGeneralDaily ProgressStatus of the test setup

I checked that the QPD electronics works as expected, and that I can acquire the signals using the ADCs. A new model (x3cr1) is up and running. It acquires the four quadrants, convert them from counts to volts, and compensate for the analog whitening filter. The four quadrant signals are X3:CR1-Q1_OUT, X3:CR1-Q2_OUT, X3:CR1-Q3_OUT, X3:CR1-Q4_OUT.

A matrix is used to compute the X and Y signals, defined as X = (Q1+Q4-Q2-Q3) and Y = (Q2+Q4-Q1-Q3). The SUM signal is also computed as SUM = (Q1+Q2+Q3+Q4).

Finally, the X and Y signals are normalized with the sum to produce X3:CR1-X_NORM_OUT and X3:CR2-Y_NORM_OUT.

A filter bank (ESD) is connected to the DAC channel #0 to produce the excitation that will be sent to the high voltage amplifier. I checked that the DAC is working properly (adding offsets). The input to the ESD filter bank is in volts.

The normalized X and Y signals, the sum of all four quadrants and the output of the ESD driver filter bank are saved to frames. The model runs at 65kHz.

49   Mon Jul 18 17:46:04 2016 GabrieleGeneralDaily ProgressFirst look at QPD signals

Here's the first spectrum of the QPD X and Y signals, acquired with the digital system. Roughing and turbo pumps are still on.

The noise floor seems quite non stationary. To be investigated.

51   Mon Jul 18 18:05:46 2016 GabrieleOpticsDaily ProgressOptical setup

Small modifications to the optical setup:

• added a 1m focal lenght lens (AR coated for 1064nm...) right before the beam enters the viewport, to reduce the beam spot size on the QPD
• added a 0.3 ND right after the laser, to avoid saturation of the QPD (about 75 microwatts total on the QPD)

52   Tue Jul 19 17:15:37 2016 GabrieleFacilityDaily ProgressVacuum pressure acquired

[Alena, Gabriele]

We connected the analog output of the vacuum gauge controller to one of the ADC channels. The signal is calibrated so that the pressure is 10^(X3:CR1-PRESSURE_LOGTORR_OUT). Unfortunately the RTG does not know how to compute 10^x...

53   Tue Jul 19 17:21:48 2016 GabrieleGeneralDaily ProgressFirst modes excited!

We can't generate any arbitary signal with the real time model, since awg is not working properly yet. For the moment being I added a uniform random number generator in the model (only option I found for noise) and send it into the ESD filter bank. In this way I can generate band-passed noise.

I plugged in the DAC output to the HV amplifier input, and I could send white noise to the electrostatic drive. Behold: I was able to excite quite a few modes. In the following trace blue is a reference and red is right after I sent white noise (3 V peak to peak) to the disk for a while (less than 1 minute). Excitation stopped at 4:56pm LT.

Using COMSOL and tuning the disk thickness at 1.018 mm I could hit the frequency of the first butterfly mode (1109 Hz) and get a reasobly good estimate of the other modes.

After about half a hour of ring down, most of the modes are gone but the two lowest are still going strong.

Note that the flattish background noise seems to be generated by some sort of glitches. I tried to swap the laser and the power supply, without change. More investigations are needed.

Note that the roughing pump was still on during the test.

56   Thu Jul 21 12:10:14 2016 GabrieleGeneralDaily ProgressGlitch hunting

Yesterday I could clearly see the glitches as jumps in the time domain plot of X and Y signals, and trace them to somewhat harder to see jumps in the quadrant signals.

• I tried to catch them with an oscilloscope looking at the analog signals, but couldn't see any. However, for a brief period I had a persistent square wave oscillation in all transimpedance stages
• I hooked the single ended output of the whitening filter to an ADC, and I could see the jumps there too. So I can exclude it's a problem of the differential drivers

One suspect was an intermittend oscillation of the transimpendance amplifier, so I looked into the schematics (D1600196) to see what could be the optimal value of the compensating capacitor C7. Following some useful notes online I computed the optimal value of C7 to be close to 2pF (instead of 10pF). I used 30-35 pF as the QPD capacitance, and 10 MHz has the gain-bandwidth product of the opamp. I swapped all 10pF capacitors with 2pF. After this I can still see the glitches in the spectra, but I can't find them anymore in the time domain. So things seem to have improved, although I still have annoying glitches.

Rich suggested to test the stability of the transimpendance stage by driving the output with a square wave and looking for the signal ringing. Here's his note:

I tried this for both the TI stage and the whitening stage, using 1k and 1uF and a square wave at 10 kHz. Here are the results, which look reasonable to me (firts is the TI, ringing at about 0.5 MHz, second is the whitening, almost no ringing):

So now I'm quite confident that the electronics is working. In the first trace you can see some intermittend background noise, due to the ambient light leaking into the QPD.

More investigations will follow.

66   Wed Jul 27 15:43:02 2016 GabrieleFacilityDaily ProgressVacuum chamber dismantled and ready for cleaning

[Alena, Gabriele]

As decided at out Red Door meeting, we're going to clean the vacuum chamber and move it to the large table, which will be enclosed in a clean room.

So today we disassembled and packaged the vacuum chamber, which is now ready to be trasnspoted to be cleaned and baked.

67   Thu Jul 28 13:58:48 2016 Alena, GabrieleCleanDaily ProgressThe old chamber is now being cleaned and baked

Cleaned the chamber in the washing machine at 40m and started 48 baking at 120 C

Attachment 1: 20160728_092350.jpg
68   Thu Jul 28 14:01:27 2016 AlenaGeneralDaily ProgressClean room

Ordered the clean room (hardware+hepa filters) and vacuum gauges

Installed the ADC and DAC boards into a proper box. Also, swapped the temporary DAC board (with cale hack) with the final one. Schematics and PCB are in the DCC: D1600196 and D1600301

The box is sitting on top of the cymac computer, on the back, since I don't have any long cable to connect the ADC.

70   Fri Jul 29 14:08:31 2016 GabrieleGeneralDaily ProgressPeak tracking code for real time model

## Summary

I wrote a C function to reconstruct the amlplitude and frequency of a line. It can be added as a block into a real time monitor. The idea is to use it to track in real time the frequency and amplitude of the disk modes, during the ring down. I did some tests and finally managed to get the function to compile and run on the cymac2 (the crackling lab cymac).

The following plot shows a simulation, since I can't run the code on the new cymac and I don't have the disk installed anymore. The top panel gives the amplitude of a decaying line, and the bottom panel the frequency offset from a reference local oscillator (more below). The nominal values are an initial amplitude of 1, frequency of 1109.375 Hz to be compared to a 1109.0 local oscillator. The fitted decay time is 10.005 seconds, to be comapred with 10 seconds nominal. There is some additive gaussian noise, that causes the ring down to be unmesurable after about 70 seconds of data.

This code will be used for real time estimation of the disk modes, once theot frequency has been roughly estimated with FFTs. The estimation of the frequency work remarkably well. In the first 20 seconds the mean value is 0.3747 Hz, with a standard deviation of 1.5 mHz. When the SNR gets worse (between t=30s and t=50s) the mean value is 0.3745 with a standard deviation of 20 mHz.

Because of the way it's built (see below) the code is sensitive to DC offsets, so the input signal must be high-passed.

## Details on the implementation

The code is based on demodulation of the input signal with a reference local oscillator thta must have a frequency as close as possible to the line we want to track.  The inputs to the block are: the signal to be monitored, sine of the local oscillator, cosine of the local oscillator. The outputs are: amplitude squared of the peak, frequency offset in Hz from the local oscillator.

Here's the math. Let's assume that the signal is

$x(t)=A \sin (2\pi f t + \phi)$

and the local oscillator has a frequency f0:

$s(t)= \sin (2\pi f_0 t) \qquad c(t)= \cos (2\pi f_0 t)$

The code multiply the signal by the two local oscillators and average the result over 65kHz / 8 Hz samples. Therefore we get two output streams at 8 Hz which are

$\left< x(t) s(t) \right> = -\frac{A}{2} \sin\left[2\pi(f-f_0)t + \phi \right]$

$\left< x(t) c(t) \right> = \frac{A}{2} \sin\left[2\pi(f-f_0)t + \phi \right]$

Then the sum of the two squared 8 Hz streams give an estimate of the amplitude squared. The code computes this every second

$\hat A = 4 \left[ \left^2+\left^2\right]$

while the arctangent of the ratio gives a phase that varies linearly with time.

$\arctan \frac{\left< x(t) c(t)\right>}{\left< x(t) s(t)\right>} = \phi + 2\pi(f-f_0)t$

For each of the 8Hz samples the code computes the arctangent (using a home-brewed function based on a lookup table, since we can't import math.h in the RCG). It unwraps it, and then every second fit a line to the unwrapped arctangent, to estimate the frequency offset with respect to the local oscillator.

The C function has some parameters hard coded: the main sampling frequency (65536 Hz), the number of points per second to use for the frequency estimation (8 Hz), the fact that the output is computed every second. The first two parameters can be changed, the third one cannot for the moment being.

Attachment 2: Screen_Shot_2016-07-29_at_2.02.24_PM.png
71   Mon Aug 1 16:56:57 2016 GabrieleElectronicsDaily ProgressPopulated four new QPD boards

Today I populated four more QPD boards. All components are installed, except for the QPD themselves.

72   Tue Aug 2 15:58:12 2016 GabrieleElectronicsDaily ProgressSix QPD functional and tested

I finished populating the new four QPD boards, and fixed the first one I populated weeks ago. I tested all five new boards: the output of the transimpendance respond correctly to the ambient light; the output of the whitening also respond correctly and has increased high frequency noise; the differential driver stages are all functional and balanced.

In summary, we have six QPD circuits ready: serial 02 is installed into the box and it has been used for the previous tests. Serial number 01, 03, 04, 05, 06 are not yet into a box, but fully functional. Boxes are ready.

For testing purposed, I also built another ADC interface board: it's complete with the exception of the connector that goes to the ADC.

75   Wed Aug 10 09:39:38 2016 GabrieleElectronicsDaily ProgressAnnuntio vobis gaudium magnum: habemus Cymac

Yesterday I cloned the cymac2 disk and installed it into the cymac3.

Jamie tweaked a few things (I can't really give more details) and now cymac3 is up an running with the same software as cymac2.

I compiled and installed the CR1 model, to readout the QPD. No more jumps in the signals!

To be able to access testpoints and have AWG working I had to follow the hack explained here: Cryo_Lab/781

I tested the following features:

• the model is running and signals look good
• I can access test points and DQ channels in real time with both dataviewer and DTT
• I can access old data with dataviewer, using NDS
• awggui is working and I can inject noise, that is properly going to the DAC channel (I check with a scope in the analog world)
• I can upload filters using foton

The plot below shows the spectrum of the QPD. For the moment being I'm just sending a straight HeNe beam into the QPD, since the test setup with the disk is no more available. Units are arbitrary

76   Wed Aug 10 10:04:35 2016 GabrieleMechanicsDaily ProgressThe prototype of the disk retain system is here

Yesterday we received the prototype of the disk suspension and retain system. Everything looks good. I checked that the disk fits in the holder, and all dimensions are good. The coil holders are out for winding, so I couldn't test the movimentation yet.

Attachment 3: 2016-08-09_14.18.55.jpg
Attachment 4: 2016-08-09_14.19.02.jpg
81   Sat Aug 13 11:17:17 2016 GabrieleFacilityDaily ProgressSpot the differences

82   Sat Aug 13 14:05:02 2016 GabrieleGeneralDaily ProgressFirst work on the measurement user interface

Today I started programming part of the user interface that will be used to perform the measurements. Not much implemented so far, but you can get an idea of the look:

Buttons on the left sidebar will allow the user to perform some basic tasks. The main panel has a plot (which will show spectra or ring down measurements) and a log section.

Attachment 1: Screenshot-C.Ri.Me..png
83   Sun Aug 14 11:40:29 2016 GabrieleElectronicsDaily ProgressSome python code

Today I wrote some auxiliary functions that will be useful for the measurement system:

• noise.py: set of functions to generate band-limited noise (using inverse FFT) and multiple band noise. Using the awg python interface I can also start and stop the noise injection. Some examples of the result:

• readdata.py: read data online, compute a spectrum, and additionally a function that find peaks in a whitened spectrum. All peaks above a minimum SNR are returned: the central frequency is computed by an average of the bin SNR:

• diskmodel.py: reads a list of mode frequencies from txt files. In each file, the first row is the disk diameter in mm, the second is the disk thickness in mm, all other rows are the modes are computed by COMSOL. I also produced a whole bunch of such files, with diameters within 75 +- 0.1 mm and thickness within 1 +- 0.1 mm
84   Sun Aug 14 18:13:52 2016 GabrieleElectronicsDaily ProgressProgress on the user interface

Some progress on writing the user interface:

Now the user can open a file that defines the nominal mode frequencies (from COMSOL simulations) and select which modes to search for:

The plan is that the automation will then inject band limited noise around each nominal frequency, to excite the mode, and then find the exact frequency. The user can set some additional parameters like

• the duration of the excitation (I plan to use this as a maximum: if the mode rings up with large enough SNR after a shorter time, the injection will stop earlier)
• the amplitude of the excitation (units to be determined)
• the bandwitdh, otr in other words how many Hz the noise will be spread around the nominla frequency to search for the mode.
• the minimum SNR to detect a peak (here the automation will get a spectrum before any excitation, to be used as the baseline to whiten all further spectra)
86   Tue Aug 16 11:53:30 2016 Alena, GabrieleCleanDaily ProgressClean room progress

Some progress on the cleam room: bar fixed to the wall, some more structure built, filters in place. We had to (literally) work around a corner of the low ceiling that we haven't noticed before. More contruction will follow tomorrow. We also had to order some additional parts (more extrusions, brackets, screws, etc...)

88   Wed Aug 17 16:41:05 2016 Gabriele, AlenaCleanDaily ProgressClean room construction progress

The clean room frame is built and secured to the floor and wall. Panels are being installed on the ceiling and back. Also, the optical table has been leveled.

89   Thu Aug 18 18:15:18 2016 Gabriele, AlenaCleanDaily ProgressClean room construction progress

Ceiling, back and side panels are installed. The air filters have been cabled and connected to the power supply.

90   Fri Aug 19 13:46:55 2016 Alena, Calum, Jon, Liz, Steve, GabrieleFacilityDaily ProgressClean room and vacuum complete

This morning we installed the clean room curtains and washed them. It turns out that the air filters are supposed to be powered at 277V (?) instead of 115V. So right now the flux is quite low. We are looking into the problem: either replace them with 115V modules or install a small transformer.

We also installed the vacuum chamber on the table and connected all the pumps and gauges. There are no leaks and we could pump down easily the empty chamber. We left for lunch when the pressure was at a few 1e-6 Tor and still going down.

91   Fri Aug 19 19:05:01 2016 GabrieleOpticsDaily ProgressDisk installed into the chamber

I assembled the disk suspension sytem and installed into the chamber. Although I don't have the magnets and coils, I installed the movable retaining disk, and used it to center the disk.

I first aligned the input laser using the reflection off the black glass, which turns out to be quite bright and very well visible. Tomorrow I'm going to measure how much power we have in the black glass.

The reflection from the disk is slighlty separated from the reflection from the black glass, so I can block it using an iris.

At 6:50pm I closed the chamber and started the roughing pump. At 7:05pm pressure was below 1 Tor so I started the turbo pump. When leaving pressure is about 1.6e-5 Tor.

92   Sat Aug 20 17:01:10 2016 GabrieleGeneralDaily ProgressFirst ring down measurements

Everything is working pretty well. This morning the pressure was about 1.2e-6 Torr. I connected the high voltage amplifier and I could drive the disk without problems.

I measured the beam shape and size at the QPD. We have about 50 uW, we see a TEM01-like mode due to the interference of the two disk surfaces (this is normal). The beam is about 3 mm in diameter. using this information and the estimated optical lever length of 1.2 m, I calbrated the QPD NORM signals in units of angular motion of the disk surface. The computation posted in CRIME_Lab/60 is actually wrong. I'll post the correct one later.

Injecting broadband white noise I could excite all the modes that are visible up to about 30kHz. I tuned the COMSOL model, by changing the thickess of the disk to 1.017 mm, to fit the frequency of the first few modes. Here are the modes I could measure:

Nominal frequency [Hz] Actual frequency [Hz]
1108 1109
2543 2543
4427 4424
6748 6739
6792 6781
9494 9475
10239 10216
12656 12623
14209 14168
16150 16107
16226 16173
18673 18606
20195 20116
21414 21338
23611 23510
24559 24708
27203 27084
29006 28859
29180 29053
29309 29151

Take a look at the attached PDF file for the shape of all the modes, including all that are not visible. We see all the modes we expect to be able to excite with the central suspension of the disk.

The roughing pump is making a lot of non stationary low frequency noise. I turned it off, and the pressure stayed constant at 1.2e-6 Torr over about 1.5 hours. Here's the difference in the QPD spectrum:

It turned out that I have enough excitation authority to knock the disk out of the right place. So I had to vent to recover the situation. I'll open the chamber tomorrow and see what happened.

Here's a first bird eye look at the ring downs. We see beating of the two almost denegerate modes in some cases. Fits will follow, using the procedure I used for the LMA measurements.

Attachment 1: modes.pdf
93   Sat Aug 20 22:18:26 2016 GabrieleGeneralDaily ProgressFirst measured Qs

Using the first ring down of the day (GPS 1155754513 + 3600 seconds), I computed the amplitude of each of the modes already identified, using a short FFT spectrogram (each FFT is 1 second long, overlap of 0.5 s).

Then I used the same code I developed at LMA to fit the ring down, including the beat between the unresolved mode pairs. The fit is versy sensitive to the initial conditions, so I had to fine tune them for each of the 20 modes. Still, all fits were successful with 30 minutes of work.

Here are all the fits:

And in summary all the measured Qs, which turned out to be larger than what I was expecting, considering that the disk is not annealed.

The analysis code in MATLAB is attached.

Attachment 22: first_ringdown.m
%% Compare data right before and right after the excitation
gps0 = 1155754000;
gps1 = 1155754513;
dt = 180;

c = nds2.connection('cymac3.ligo.caltech.edu', 8088);
data0 = c.fetch(gps0, gps0+dt, {'X3:CR1-X_NORM_OUT_DQ', 'X3:CR1-Y_NORM_OUT_DQ', 'X3:CR1-ESD_OUT_DQ'});
data1 = c.fetch(gps1, gps1+dt, {'X3:CR1-X_NORM_OUT_DQ', 'X3:CR1-Y_NORM_OUT_DQ', 'X3:CR1-ESD_OUT_DQ'});

x0 = data0(1).getData();

... 606 more lines ...
94   Sun Aug 21 08:51:22 2016 GabrieleGeneralDaily ProgressNew disk installed

Vented the chamber. Installed a new disk (MO 03). The one I measured yesterday is now named MO 02 and it is the one with the "burnt mark" from the previous experiment (due to the electrostatic drive).

Startep roughing pump at 8:50am. Started turbo pump at 9.00am.

Excited the disk at 9:50:30am with white noise, amplitude 10 V. Pumps are still running, pressure is about 2e-6 Torr

At 11:10am I stopped the roughing pump, pressure is 1.4e-6 Torr. Exciting again the disk at 11:10:45am. At 12:25am I checked again the situation, since both measurements look quite weird, especially for the first couple of modes.

I think the reflection from the balck glass is interfering with the reflection from the disk. Probably I wasn't careful enough when I aligned the disk. At about 12:35 I stopped the turbo pump. I'm going to open the chamber and realign everything again.

95   Sun Aug 21 13:45:56 2016 GabrieleOpticsDaily ProgressWierd behavior of ring downs and Improved setup

The last two ring downs I measured today showed a weird behavior of the lowest modes:

Although I'm not 100% sure, I suspect this is related to the fact that the beam reflected from the black glass was so close to the beam reflected by the disk that I could see interference.

So I broke vacuum and improved the setup, adding a peek washer below one edge of the black glass, to wedge it. In this way the reflection from the black glass is largely separated: it misses the upper periscope mirror and it is dumped on a black panel (together with the viewport reflection).

I realigned everything, installed back the disk and started pumping down at 1:30pm.

96   Sun Aug 21 15:36:16 2016 GabrieleGeneralDaily ProgressNew test

Roughing pump stopped at about 3:30:30pm. HV amplifier on at 3:33:30pm, excitation at 3:35:30pm. Recentered QPD at 3:36pm

97   Sun Aug 21 17:16:03 2016 GabrieleGeneralDaily ProgressRing down measurement with new disk

After fixing the setup, the measurement with the new disk looks great. After more than one hour the first two modes are still ringing down, meaning that the Q's are larger than 10 millions.

Here's the comparison of the spectrum before and after the excitation, with the identified modes:

I used about 4200 s of data to fit the ring downs. Most fits are good. In a couple of cases the peak splitting is large and the algorithm fails to fit the beats:

In summary, here are the Q values for all modes. Despite not being annealed, this disk shows very large Q's

98   Sun Aug 21 17:41:34 2016 GabrieleGeneralDaily ProgressNew test

I checked the status at about 5:20pm, the turbo pump was in error and spinning down, since the roughing pump has been off for about 1.5 hours.

I let the pump switch off.

 Quote: Roughing pump stopped at about 3:30:30pm. HV amplifier on at 3:33:30pm, excitation at 3:35:30pm. Recentered QPD at 3:36pm

99   Mon Aug 22 08:29:24 2016 GabrieleGeneralDaily ProgressNew test

Restarted roughing and turbo pump at about 8:10am.

Quote:

I checked the status at about 5:20pm, the turbo pump was in error and spinning down, since the roughing pump has been off for about 1.5 hours.

I let the pump switch off.

 Quote: Roughing pump stopped at about 3:30:30pm. HV amplifier on at 3:33:30pm, excitation at 3:35:30pm. Recentered QPD at 3:36pm

101   Mon Aug 22 16:59:13 2016 Gabriele, AlastairOpticsDaily ProgressLaser polishing of the disk edges

We set up a test facility for laser polishing the disk edges, using the CO2 laser in the TCS laboratory. We focused the beam with a 10" focal length lens, and installed the disk on a "rotation stage" that we motorized with a hand drill. We used a HeNe optical lever and a small container with water to define the horizontal plane and adjusted the disk as well as we could.

We first tested the procedure on the MO02 disk, which is the one already scared with the electrostatic drive burn mark. This disk is now definitely in bad shape. However, we felt confident in our procedure, so we took out the MO03 disk that was into the measurement system and proceeded to laser polish the edges. Things went quite smothly. Unfortunately we added some small damages to the disk surface in a couple of spots where the CO2 laser went out of alignment and melted the fused silica support of the disk. The edge however looks quite good now.

Q measurement is on-going at the timw of writing

102   Tue Aug 23 08:46:04 2016 GabrieleGeneralDaily ProgressDaily tests

MO03 - edge polished:

Turbo off, QPD centered, before excitation (60 seconds)

PDT: 2016-08-23 08:42:54.514987
PDT UTC: 2016-08-23 15:42:54.514987
UTC GPS: 1156002191.514987

Excitation (white uniform noise, amplitude 5 V)

PDT: 2016-08-23 08:45:01.007626 PDT
UTC: 2016-08-23 15:45:01.007626 UTC
GPS: 1156002318.007626

Clean data for ring-down

PDT: 2016-08-23 08:45:46.448949 PDT
UTC: 2016-08-23 15:45:46.448949 UTC
GPS: 1156002363.448949

Restarted roughing pump, QPD got misaligned

PDT: 2016-08-23 10:00:29.259345 PDT
UTC: 2016-08-23 17:00:29.259345 UTC
GPS: 1156006846.259345

MO03 - edge polished:

Band-limited noise, +-10Hz around eahc nominal frequency, amplitude scaled based on the inverse of the peak height obtained with white noise. See attached code and plot

from numpy import * from noise import *

x = loadtxt('/home/controls/Measurements/2016_08_23/mo_02_laserpolished_frequencies.txt')

freqs = x[:,0] ampl = x[:,1] bw = 10

bands = map(lambda x: [x - bw, x + bw], freqs) a = 1 / (ampl/max(ampl)) a[a>50] = 50.

x = multi_band_noise(bands, a, 10, fs=65536) x = x / 30

Ring down after:

PDT: 2016-08-23 11:07:02.661145 PDT
UTC: 2016-08-23 18:07:02.661145 UTC
GPS: 1156010839.661145

106   Sun Sep 11 10:05:33 2016 GabrieleGeneralDaily ProgressNew substrate in the chamber

I installed one of the new substrates (with flats) into the chamber, and started the pumpdown at about 9:45am LT.

Before that, I removed the retaining ring: tomorrow I'm going to glue the magnet to it.

107   Mon Sep 12 16:00:06 2016 GabrieleGeneralDaily ProgressFirst fully automated measurements of ring down

I finished the first version of the automation software to measure the ring down of the disk modes. I tested it with the new substrate that was installed yesterday. Here are some screenshots and a brief explanation of how it works.

It is based on a Python/Tk GUI, that can be launched on the workstation with the command ~/CRIME/crime.py

The main screen is similar to the following. Once a baseline spectrum is acquired, it is shown in the main panel:

The user should specify the folder and prefix of the result files, and other parameters related to the excitation. The when the "Excite and ring down..." button is pressed, here's what happens

1. If a baseline spectrum (before excitation) is not available, one is acquired with the specified parameters
2. A broadband white excitation is applied with the selected amplitude and duration

1. Another spectrum is taken. This is then whitened by dividing it with the baseline. This could be used directly to select the modes that have been excited. However, some parts of the noise floor are non stationary, so a second whitening is performed: the noise background is estimated by removing all lines, and it is then again divided out from the spectrum.
2. All lines above a SNR threshold are then selected and shown in the main window together with the whitened spectrum:

At this point the amplitude of the peaks are continuosly monitored (every second) and thei amplitude shown in a new window. The user can select a subset of the modes for the plotting.

There are some wandering peaks in the spectrum, so some of the peaks aren't actually modes that get excited. This is easily fixed in the post processing of the results.

All peak amplitudes are saved to files in real time, so if you stop the GUI you'll have some partial results.

116   Tue Sep 20 15:34:00 2016 GabrieleOpticsDaily ProgressImproved optical lever layout

## Goal

Improve the optical setup, by increasing the response of the QPD to disk motion.

## The old configuration

In all my previous measurement the optical lever was as simple as possible: no lenses were used, and therefore the beam was free to expand over all its path. The estimated arm lever from the disk to the QPD was 1030 mm.

## QPD response to disk angular motion

The response of the QPD can be characterized with its optical gain in 1/rad, which is how much the normalized signal (difference / sum) changes for one radians of motion of the disk. This is the product of two parts:

1. the gain from angular motion of the disk to beam spot motion on the QPD. In the simple case of free propagation this is 2L, where L is the distance from the disk to the QPD, and the factor 2 is due to the fact that the beam deflection is the double of the disk angular motion. If there is a telescope in between the disk and the QPD, it is easy to compute the total ray transfer matrix:
$\begin{pmatrix} x_{QPD}\\ \theta_{QPD} \end{pmatrix}= \begin{bmatrix} A & B\\ C & D \end{bmatrix} \begin{pmatrix} 0\\ 2\theta_{disk} \end{pmatrix}$
Then the gain is simply the B element of the matrix.
2. the response of the QPD normalized signal to beam motion. This depends only on the beam spot radius w on the QPD. It can be computed by simple gaussian integration, and in the approximation of small beam motion, it is given by the following expression:
$g = \frac{2}{w}\sqrt{\frac{2}{\pi}}$

In the case of the old configuration, the beam spot size on the QPD was measured to be about 1.5 mm in radius, so the optical gain is of the order of 1900 /rad.

## Laser beam profiling

Since I wanted to improve the optical setup, I first needed to measure the beam coming out of the HeNe laser. I used the WinCam beam profile and a Newport rail to measure the beam X and Y sizes at different positions.

The measurements are not the best ever, but I can still get a fit for the evolution of the gaussian beam, as shown in the plot below. The beam waist is 254 um, located 340 mm behind the laser output (inside the laser tube).

## Design of the improved setup

I decided to try a brute force algorithmic optimization for the optical gain. I allow two lenses between the laser and the disk and two lenses between the disk and the QPD. I wrote a MATLAB script that picks the four lenses from a list of all those available (I have a Thorlabs LSB02-A lens kit). For each combination of lenses, MATLAB moves them around into pre-defined ranges, and try to find the maximum value of the QPD total optical gain, which is the product of the factor g above and of the B element of the ray tracing matrix.

It turned out that the best optical gains could almost always be obtained by making the beam huge on the disk (5-10 mm radius) and tiny on the QPD (tens of microns). This is not a good solution. So I decided that the beam on the disk must be smaller than 2mm in radius and the beam on the QPD must be larger than 200 microns. I enforced those limits into the optimization code by weighting the gain with a function which is one in the allowed range, and then quickly drops to zero when either of the beam sizes fall out of the allowed range.

The script ran for about half hour and gave me a lot of possible options. After some inspections, I decided to use the following one, which uses only one lens between laser and disk, and two between the disk and the QPD. Distances and focal lengths are shown below. Note that the first distance (laser to first lens) is from the laser beam waist to the lens, so the actual distance must take into account that the waist is estimated to be 340mm into the laser.

With this configuration the optical gain is computed to be 17000 /rad, or about 9 times larger than the original setup. The beam radius on the disk is 1 mm and on the QPD is 0.23 mm.

## Implementation

First of all I measured some distances:

• from the inner side of the viewport to the disk: 420 mm
• viewport thickness: 12 mm, which is about 18 mm optical length considering n~1.5
• so from the input to the chamber to the disk: 438 mm
• from the viewport to the upper external periscope mirror center: 110 mm
• distance between the periscope mirror centers: 275 mm

Using these distanced I build the designed optical setup. Some remarks on the procedure

• I first aligned the laser beam to be horizontal, then added the first lens and centered it by ensuring no beam shift far away from the lens
• I first aligned the periscope to get the beam roughly centered on the inner 45 degrees mirror, and then roughly centered on the black glass
• Then I put a small container with water inside the chamber, on top of the black glass. I aligned the inner mirror and the periscope so that the beam coming back from the horizontal water surface was perfectly overlapped with the input beam. I used an iris on the input beam path
• Then I removed the water container and installed a test disk. I moved the disk around until I got the same beam position in output. This tells me that the disk is horizontal
• Finally I moved the upper periscope mirror to separate horizontally the beam coming back, at the level of the table. The separation is large enough to allow me to pick up the outgoing beam with a mirror.

Here's a picture of the setup, with the optical path highlighted.

124   Fri Sep 23 08:11:48 2016 GabrieleMechanicsDaily ProgressTest of the disk retaining ring motion

In brief, it doesn't work. The magnets and coils are strong enough to push up the ring with a sample inside, but the friction with the three alignment pins is too large and random, so when the current to the coils is increased slowly, the ring doesn't move up smoothly (see first attached video). On the other hand, if the current is switched on abruptly, the ring shoot to the top and stays there. However, if a disk is placed on the support, it is ejected out (see second video). When the current is cut (smoothly or abruptly) the ring doesn't alway comes back to the bottom, but sometimes it stays stuck inclinded.

On the positive side, we probably don't need such a complicated system:

1. in all the pump down I've done so far (ten or more), the disk never moved
2. the ring is very useful, even when used manually, to find the initial centering of the disk: if we machine three small aluminum wedges that can be put under the ring to keep it raised (or three set screws), it can be used to place down the sample in a roughly centered position, that has always been good enough to get the beam almost back into the QPD.

Video1 Video2

127   Mon Sep 26 15:59:09 2016 GabrieleOpticsDaily ProgressQPD auto centering

We have a few motorized mounts (with New Focus picomotors) and one controller (an old New Focus 8753, six axis total) that I connected with a makeshift null modem cable to the laboratory workstation (better cabling and power supply coming soon).

I wrote a couple of python scripts that can be used to continuosly read out the QPD values and move the picomotors if needed. It's wortking quite well, so we should be able to use it in the future to keep the QPD centered during the measurement.

The scripts are in the ~/CRIME directory. Launch the function center() in the autocenter.py script.

140   Thu Oct 20 14:38:24 2016 GabrieleCleanDaily ProgressCleaning of S1600433 and S1600438

Samples #433 (annealed and cleaned) and #438 (as received from Mark Optics) are now with GariLynn for deep cleaning.

Sample #438 was broken during annealing.

189   Tue Nov 15 10:35:11 2016 GabrieleMechanicsDaily ProgressParts for laser polishing setup are here

The parts all fit as expected. They're mounted on the stages.

192   Wed Nov 16 09:43:25 2016 Gabriele, AlastairGeneralDaily ProgressCO2 laser polishing setup

Yesterday we assembled the lase polishing system. The Co2 laser power can be controlled using a waveplate, so we can turn on the laser at maximum power and let it stabilize, before actually turning up the power sent to the disk.

The beam is focused with a 10" focal length lens, and sent to the disk edge, poiting slightly upward to avoid hitting any other part of the disk.

The disk is moved with a combination of a linear and rotation stage, controlled with a MATLAB script. We tuned the translation and rotation speed so that the edge always moves at about 0.5 mm/s. Some refinement of the movimentation procedure will follow.

We tried the setup with one of the damaged samples, and the results are quite good.

More work this afternoon

Attachment 2: 2016-11-15_16.35.14.jpg
256   Fri Jan 6 12:19:22 2017 Alena, Bob, Stephen and CalumGeneralDaily ProgressSecond vacuum chamber

Cleaning and baking (200 C air foe SS and 120 for Al) parts for the new vacuum chamber

270   Fri Jan 20 17:20:20 2017 GabrieleOpticsDaily ProgressInput part of the optical lever setup installed and aligned

Today I installed and aligned part of the optical components for the optical lever of the new setup. For the moment being I installed only the input components, and aligned the beams into the vacuum chamber. Since I don't have any in-vacuum optics yet, there's nothing more that can be done now.

272   Mon Jan 23 16:29:52 2017 GabrieleOpticsDaily ProgressInput part of the optical lever setup installed and aligned

Today I cabled and installed the four QPD, in a temporary position. I also assembled four picomotor mounts that will be used for the auto-centering.

 Quote: Today I installed and aligned part of the optical components for the optical lever of the new setup. For the moment being I installed only the input components, and aligned the beams into the vacuum chamber. Since I don't have any in-vacuum optics yet, there's nothing more that can be done now.

276   Wed Jan 25 10:02:23 2017 GabrieleElectronicsDaily ProgressHigh voltage system in place

This morning I installed in the clean room the high voltage amplifier and the high voltage relays. Everything is cabled as planned. No way to test it yet, since there's nothing in the chamber!

277   Wed Jan 25 16:12:05 2017 GabrieleGeneralDaily ProgressSome assembly
• assembled the four in-vacuum periscopes (no mirrors yet)
• soldered kapton wires to the four ESD board; the vacuum side HV connectors are installed
• installed the picomotor into the traslation stage, and soldered the two picomotor wires to a twisted kapton pair. Note: the white wire is connected to the kapton wire which is twisted on the other side.
278   Thu Jan 26 17:06:59 2017 GabrieleGeneralDaily ProgressFour fold measurement system installation

Today I installed the four GeNS systems into the chamber, connected the ESD to the HV feedthrough and put the periscopes in place.

Note 1: the lens holders have been remachined to solve a dimension problem, by removing completely the two lips against which the lens was supposed to be sitting. To recover a reasonable centering of the lens, I added two small shims inside each holder. They are made out of wrapped aluminum foil.

Note 2: I added some small shims (again made of wrapped aluminum foil) below the base plate, to make it as close to horizontal as possible

Finally, I managed to align the optical level beam for the QPD1, using as usual a small container with water to get the horizontal reference.

Some pictures below.

Attachment 3: 2017-01-26_14.04.06.jpg
280   Fri Jan 27 14:56:55 2017 GabrieleOpticsDaily ProgressOptical lever setup completed

The four optical levers are completely installed and aligned to a horizontal reference.

282   Mon Jan 30 15:16:54 2017 GabrieleGeneralDaily ProgressFour samples in chamber
• checked that the high voltage paths are working:
• the DAC signal is properly received and amplified by the Trek 2220 HV amplifier
• the HV switch box is working as expected: I can switch on and off each path individiually using the DAC signals
• installed four samples (refer to the optical drawing for the numbering)
• S1600472 in bay 1
• S1600478 in bay 2
• S1600473 in bay 3
• S1600480 in bay 4

• balanced all samples and centered all optical levers. There is a problem with the fifth connector in the ADC interface board (or maybe with one ADC channel). So one of the quadrants of QPD4 was not working. I moved the connector to number 6 and updated the model. Everything looks fine now.
• the power in the four QPD is quite diverse: this is not unexpected, since it depends on the interference of the reflection from the two surfaces of the disk

• installed the picomotor controllers on the bench, connected them to the net using the switch on the top of the clean room. For the moment being they are using DHCP.
• connected all picomotors to the controllers.
• Bottom controller, from left to right: QPD1x, QPD1y, QPD2x, QPD2y
• Top controller, from left to right: QPD3x, QPD3y, QPD4x, QPD4y
• tested the remote control and motion of all picomotors

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