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  SUS Lab eLog, Page 1 of 38  Not logged in ELOG logo
ID Dateup Author Type Category Subject
  5   Fri Oct 19 16:11:38 2007 pkpOtherOMCOMC PZT response
Sam and I locked the laser to the OMC cavity and looked at the error signal as a function of the voltage applied to the OMC PZT.
Here are two plots showing the response as a function of frequency from 1 kHz to 100 kHz and another high-res response in the region of 4.5 kHz to 10 kHz.
Attachment 1: fullspec.jpg
Attachment 2: 4.5to10.jpg
Attachment 3: 4.5to10.pdf
Attachment 4: fullres.pdf
  6   Sat Oct 20 11:54:13 2007 waldmanOtherOMCOMC and OMC-SUS work
[Rich, Chub, Pinkesh, Chris, Sam]

Friday the 18th was a busy day in OMC land. Both DCPDs were mounted to the glass breadboard and the OMC-SUS structure was rebuilt to the point that an aluminum dummy mass is hanging, unbalanced. The OSEMs have not be put on the table cloth yet, but everything is hanging free. As for the DCPDs, if you recall one beam is 3mm off center from the DCPD tombstone. Fortunately, one DCPD is nearly 3mm offcenter from the case in the right direction, so the errors nearly cancel. The DCPD is too high, so the beam isn't quite centered, but they're close. We'll get photos of the beam positions in someday. Also, the DC gain between the two PDs is, at first glance, different by 15%. DCPD1, the one seen in transmission has 315 mV of signal while DCPD2 has 280 mV. Not sure why, could be because of beam alignment or tolerances in the Preamp or the angle incident on the diode or the QE of the diodes. The glass cans have *not* been removed.
  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
Attachment 2: calibpzt2.pdf
Attachment 3: all2.pdf
Attachment 4: 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.
  14   Thu Oct 25 17:52:45 2007 waldmanOtherOMCOMCs with QPDs
[Rich, Chub, Pinkesh, Sam]

Yesterday we got the QPD, OTAS, and PZT cabling harness integrated with the OMC. We found a few things out, not all of them good. The QPDs went on no problem and could be fairly well aligned by hand. We "aligned" them by looking at all four channels of the QPD on the scope and seeing that there is signal. Since the beam is omega = 0.5 mm, this is a reasonable adjustment. We then connected the OTAS connector to the OTAS and found that the heater on the OTAS was bonded on about 30 degrees rotated from its intended position. This rotated the connector into the beam and caused a visible amount of scattering. This wasn't really a disaster until I removed the connector from the heater and broke the heater off of the aluminum parts of the OTAS. Two steps backwards, one step forward. After the OMC, OMC-SUS integration test we will re-bond the heater to the aluminum using VacSeal. In the meantime, the OMC has been moved to Bridge 056 for integration with the OMC-SUS. More on that as we make progress.
  16   Thu Oct 25 23:35:36 2007 waldmanOtherOMCHang the OMC!
[Pinkesh, Sam]

We tried, convicted and hung the OMC today. The OMC was found guilty of being overweight, and unsymmetrically balanced. The unsymmetry was kind of expected and was corrected with a hefty stack of counterweights positioned over the counterweighting holes. The stacks will be measured at some future date and correctly sized objects machined. The overweightness showed up when the level hanging breadboard was about 5 mm low. This showed up in the board height above the table as well as the OSEM flag positions within their holes. The problem was remedied with a liposuction of the intermediate mass. We removed both small vertical cylinder weights that Chris added, and then we removed the heavy steel transverse weight that can be used to adjust the tip around the long axis (I forgot what its called).

The top of the breadboard ended up about 154 mm off the table. The breadboard is 39 mm thick, and the optics are centered (30 - 12.7) = 17.3 mm below the surface for a as hanging beams height of 154 -39 - 17.3 = 97.7 mm or about an 0.150 inches lower than we were aiming for. Can I get a refund?

We screwed up in multiple ways:
  • The slotted disks that capture the wires do not have the alignment bore used to center the wire in the hole
  • We didn't correctly route the far field QPD cable so it runs funny
  • We didn't have a tool which could be used to get two of the DCPD preamp box mounting screws (which are M3's chub!)
  • We don't have the cable clamps to tie off the electrical cables to the intermediate mass
  • We don't have any of the cabling from the OMC-SUS top to the rack so we can't test anything
  • We haven't uploaded pretty pictures for all to see

We left the OMC partially suspended by the OMC-SUS and partly resting on the installation lab jacks which are currently acting as EQ stops. After we fix the cabling we will more permanently hang it. PS, It looks like the REFL beam extraction will be tricky so we need to get on that....
Attachment 1: IMG_1483.jpg
Attachment 2: IMG_1481.jpg
  19   Fri Oct 26 17:34:43 2007 waldmanOtherOMCOMC + earthquake stops

[Chub, chris, Pinkesh, Sam]

Last night we hugn the OMC for the first time and came up with a bunch of pictures and some problems. Today we address some of the problems and, of course, make new problems. We replaced the flat slotted disks with the fitted slotted disks that are made to fit into the counterbore of the breadboard. This changed the balance slightly and required a more symmetric distribution of mass. It probably did not change the total mass very much. We did find that the amount of cable hanging down strongly affected the breadboard balance and may also have contributed to the changing balance.

We also attached earthquake stops and ran into a few problems:

  • The bottom plate of the EQ stops is too thick so that it bumps into the tombstones
  • The vertical member on the "waist" EQ stops is too close to the breadboard, possibly interfering with the REFL beam
  • The "waist" EQ stops are made from a thin plate that doesn't have enough thickness to mount helicoils in
  • Helicoil weren't loaded in the correct bottom EQ stops
  • The DCPD cable loops over the end EQ stop looking nasty but not actually making contact

However, with a little bit of jimmying, the EQ stops are arrayed at all points within a few mm of the breadboard. Meanwhile, Chub has cabled up all the satellite modules and DCPD modules and Pinkesh is working on getting data into the digital system so we can start playing games. Tonight, I intend to mount a laser in Rana's lab and fiber couple a beam into the 056 room so we can start testing the suspended OMC.
  20   Fri Oct 26 21:48:40 2007 waldmanConfigurationOMCFiber to 056
I set up a 700 mW NPRO in Rana's lab and launched it onto a 50m fiber. I got a few mW onto the fiber, enough to see with a card before disabling the laser. The fiber now runs along the hallway and terminates in rm 056. Its taped down everywhere someone might trip on it, but don't go out of your way to trip on it or pull on it because you are curious. Tomorrow I will co-run a BNC cable and attenuate the NPRO output so it can only send a few mW and so be laser safe. Then we can try to develop a procedure to align the beam to a suspended OMC and lock our suspended cavity goodness.

Notes to self: items needed from the 40m
  • ND10 and ND20 neutral density filter
  • EOM and mount set for 4 inch beam height
  • Post for fiber launch to get to 4 inch
  • Mode matching lens at 4in
  • 3x steering mirror at 4in
  • RF photodiode at 4in
  • Post for camera to 4in
  • Light sheild for camera
  • Long BNC cable
Some of these exist at 056 already
  21   Sat Oct 27 19:00:44 2007 waldmanConfigurationOMCHanging, locked OMC with REFL extracted.
I got the OMC locked to the fiber output today. It was much more difficult than I expected and I spent about 30 minutes or so flailing before stopping to think. The basic problem is that the initial alignment is a search in 4-dimensional space and there is naturally only one signal, the reflected DC level, to guide the alignment. I tried to eyeball the alignment using the IR card and "centering" the beams on mirrors, but I couldn't get close enough to get any light through. I also tried to put a camera on the high reflector transmission, but with 1.5 mW incident on the cavity, there is only 1.5 microwatts leaking through in the best case scenario, and much, much less during alignment.

I resolved the problem by placing a high reflector on a 3.5 inch tall fixed mount and picking off the OMC transmitted beam before it reaches the DC diodes. I took the pickoff beam to a camera. The alignment still sucked because even though the beam cleanly transmitted the output coupler, it wasn't anywhere close to getting through the OTAS. To resolve this problem, I visually looked through the back of M2 at M1 and used the IR card to align the beam to the centers of each mirror. That was close enough to get me fringes and align the camera. With the camera aligned, the rest was very easy.

I restored the PDH setup we know and love from the construction days and locked the laser to the OMC with no difficulty. The laser is in Rana's lab so I send the +/- 10V control signal from the SR560 down a cable to 058E where it goes into the Battery+resistor box, the Throlabs HV amplifier, and finally the FAST channel of the NPRO. BTW, a simple experiment sows that about 35 +/- 3 V are required to get an FSR out of the NPRO, hence the Thorlabs HV. The EOM, mixer, splitter, etc is on the edge of the table.

With this specific OMC alignment, ie. the particular sitting on EQ stops, it looks like all of the ghost beams have a good chance of coming clear. I can fit a 2 inch optic in a fixed mount in between the end of the breadboard and the leg of the support structure. A picture might or might not be included someday. One of the ghost beams craters directly into the EQ stop vertical member. The other ghost barely misses M2 on its way down the length of the board. In its current configuration, the many REFL beam misses the leg by about 1.5 inches.
  25   Mon Oct 29 11:07:22 2007 waldmanSoftware InstallationOMCSoftware install on OMS
[Alex, Sam]

We spent a little time this morning working on OMS and getting things restarted. A few changes were made. 1) We put openmotif on OMS so that the burtrb doesn't throw that crappy libXm any more. 2) We upgraded OMS to a 32 kHz sampling rate from 2 kHz. All the filters will have to be changed. We also added a PDH filter path to maybe feedback PDH signals cuz that will be cool. Maybe someday I will write up the very cool channel adding procedure.
  26   Mon Oct 29 12:20:15 2007 waldmanConfigurationOMCChanged OMS filters
I changed the OMS configuration so that some of the OMC-SUS LED channels go to a breakout box so that we can input the PDH error signal. After lunch, we will try to lock the cavity with a PDH error signal and digital filters. Then its on to dither locked stuff. Note that this LED business will have to be changed back some day. For now, it should be extremely visible because there are dangling cables and a hack job interface lying around.
  27   Mon Oct 29 23:10:05 2007 waldmanConfigurationOMCLost in DAQspace
[Pinkesh, Sam]

In setting up a Digital based control of the hanging OMC, we naively connect the Anti-Imaging filter output to an Anti-Aliasing input. This led to no end of hell. For one thing, we found the 10 kHz 3rd order butterworth at 10 kHz, where it should be based on the install hardware. One wonders in passing whether we want a 10 kHz butter instead of a 15 kHz something else, but I leave that for a later discussion. Much more bothersome is a linear phase shift between output and input that looks like ~180 microseconds. It screams "What the hell am I!?" and none of us could scream back at it with an answer. I believe this will require the Wilson House Ghost Busters to fully remedy on the morrow.
Attachment 1: SS.pdf
Attachment 2: SS.gif
  30   Tue Oct 30 13:58:07 2007 ajwConfigurationIOOMC Ringdowns
Here's a quick fit-by-eye to the latter part of the data from tek00000.xls.

The prediction (blue) is eqn 41 of

T1 = T2 = 0.002. Loss1 = Loss2 = 150 ppm.
MC3 assumed perfectly reflecting.
Velocity = 320 um/s (assumed constant), 2 usec into the ringdown.

OK, there's one little fudge factor in the prediction:
I multiplied D by 2.
Attachment 1: CavityRingdown.png
Attachment 2: CavityRingdown.m
% CavityRingdown.m
% Eqn 41 of 
% "Doppler-induced dynamics of fields in Fabry–Perot
% cavities with suspended mirrors", Malik Rakhmanov (2000).
% http://www.ligo.caltech.edu/docs/P/P000017-A.pdf

clear all

% read in ringdown timeseries:
at = importdata('tek00000.csv');
... 121 more lines ...
  37   Wed Oct 31 09:45:28 2007 waldmanOtherOMCResolution to DAQland saga
[Jay, Sam]

We did a rough accounting for the linear delay this morning and it comes out more or less correct. The 10 kHz 3rd order butterworth AA/AI filter gives ~90 degrees of phase at 6 kHz, or 42 microseconds. Taken together, the two AA and AI filters are worth 80 microseconds. The 1.5 sample digital delay is worth 1.5/32768 = 45 microseconds. The remaining 160 - 125 = 35 microseconds is most likely taken up by the 64 kHz to 32 kHz decimation routine, assuming this isn't accounted for already in the 1.5 sample digital delay.

It remains to be seen whether this phase delay is good enough to lock the laser to the OMC cavity
  42   Wed Oct 31 23:55:17 2007 waldmanOtherOMCQPD tests
The 4 QPDs for the OMC have been installed in the 056 at the test setup. All 4 QPDs work and have medm screens located under C2TPT. The breadboard mounted QPDs are not very well centered so their signal is somewhat crappy. But all 4 QPDs definitely see plenty of light. I include light and dark spectra below. QPDs 1-2 are table-mounted and QPD 2 is labeled with a bit of blue tape. QDPs 3-4 are mounted on the OMC. QPD3 is the near field detector and QPD4 is the far field. In other words, QPD3 is closest to the input coupler and QPD4 is farthest.

Included below are some spectra of the QPDs with and without light. For QPDs 1 & 2, the light source is just room lights, while 3&4 have the laser in the nominal OMC configuration with a few mWs as source. The noise at 100 Hz is about 100 microvolts / rtHz. If I recall correctly, the QPDs have 5 kOhm transimpedance (right Rich?) so this is 20 nanoamps / rtHz of current noise at the QPD.
Attachment 1: QPD_SignalSpectrum.pdf
Attachment 2: QPD_SignalSpectrum.gif
  43   Thu Nov 1 01:28:04 2007 waldmanOtherOMCFirst digital lock of OMC
[Pinkesh, Sam]

We locked a fiber based NPRO to the suspended OMC tonight using the TPT digital control system. To control the laser frequency, we took the PZT AI output and ran it on a BNC cable down the hallway to the Thorlabs HV box. The Thorlabs is a singled ended unit so we connected the AI positive terminal only and grounded the BNC to the AI shield. We could get a -6 to 1.5 V throw in this method which fed into the 10 k resisotr + 9 V battery at the input of the HV box. The HV out ran to the NPRO PZT fast input.

We derived our error signal from a PDA255 in reflection with a 29.5 MHz PDH lock. The signal feeds into one of the unused Tip/Tilt AA channels and is passed to the PZT LSC drive through the TPT_PDH1 filter bank. In the PZT_LSC filter we put a single pole at 1 Hz which, together with the phase we mentioned the other night (180 degrees at 3 kHz) should allow a 1 kHz-ish loop. In practice, as shown below, we got a 650 Hz UGF with 45 degrees of phase margin and about 6 dB of gain margin.

The Lower figure shows the error point spectrum with 3 settings. REF0 in blue shows lots of gain peaking at 1.5 kHz-ish, just where its expected - the gain was -40. The REF1 has gain of -20 and shows no gain peaking. The current trace in red shows some gain peaking cuz the alignment is better but it also has included a 1^2:20^2 boost which totally crushes the low frequency noise. We should do a better loop sweep after getting the alignment right so we can see how much boost it will really take.

Just for fun, we are leaving it locked overnight and recording the PZT_LSC data for posterity.
Attachment 1: 071101_PZT_firstLoopSweep.pdf
Attachment 2: 071101_PZT_firstLoopSweep.gif
Attachment 3: 071101_OMC_FirstLock_spectra.pdf
Attachment 4: 071101_OMC_FirstLock_spectra.gif
  58   Fri Nov 2 12:18:47 2007 waldmanSummaryOMCLocked OMC with DCPD
[Rich, Sam]

We locked the OMC and look at the signal on the DCPD. Plots included.
Attachment 1: 071102_OMC_LockedDCPD.gif
Attachment 2: 071102_OMC_LockedDCPD.pdf
  59   Sat Nov 3 16:20:43 2007 waldmanSummaryOMCA good day's work

I followed up yesterday's test of the PZT with a whole mess of characterizations of the PZT control and finished the day by locking the OMC with a PZT dither lock and a 600 Hz loop. I haven't analyzed any of the data yet, so its not calibrated in physical units and etc. etc. etc. Since a lot of the sweeps below are of a "drive the PZT, look at the PDH signal" nature, a proper analysis will require taking out the loop and calibrating the signals, which alas, I haven't done. Nonetheless, I include all the plots because they are pretty. The files included below are:

  • DitherLock_sweep: Sweep of the IN2/IN1 for the dither lock error point showing 600 Hz UGF
  • HiResPZTDither_sweep: Sweep of the PZT dither input compared to the PDH error signal. I restarted the front end before the sweep was finished accounting for the blip.
  • HiResPZTDither_sweep2: Finish of the PZT dither sweep

More will be posted later.
Attachment 1: 071103_DitherLock_sweep.png
Attachment 2: 071103_DitherLock_sweep.pdf
Attachment 3: 071103_HiResPZTDither_sweep.png
Attachment 4: 071103_HiResPZTDither_sweep.pdf
Attachment 5: 071103_HiResPZTDither_sweep2.png
Attachment 6: 071103_HiResPZTDither_sweep2.pdf
  60   Sun Nov 4 23:22:50 2007 waldmanUpdateOMCOMC PZT and driver response functions
I wrote a big long elog and then my browser hung up, so you get a less detailed entry. I used Pinkesh's calibration of the PZT (0.9 V/nm) to calibrate the PDH error signal, then took the following data on the PZT and PZT driver response functions.:

  • FIgure 1: PZT dither path. Most of the features in this plot are understood: There is a 2kHz high pass filter in the PZT drive which is otherwise flat. The resonance features above 5 kHz are believed to be the tombstones. I don't understand the extra motion from 1-2 kHz.
  • Figure 2: PZT dither path zoom in. Since I want to dither the PZT to get an error signal, it helps to know where to dither. The ADC Anti-aliasing filter is a 3rd order butterworth at 10 kHz, so I looked for nice flat places below 10 KHz and settled on 8 kHz as relatively harmless.
  • Figure 3: PZT LSC path. This path has got a 1^2:10^2 de-whitening stage in the hardware which hasn't been digitally compensated for. You can see its effect between 10 and 40 Hz. The LSC path also has a 160 Hz low path which is visible causing a 1/f between 200 and 500 Hz. I have no idea what the 1 kHz resonant feature is, though I am inclined to point to the PDH loop since that is pretty close to the UGF and there is much gain peaking at that frequency.
Attachment 1: 071103DitherShape.png
Attachment 2: 071103DitherZoom.png
Attachment 3: 071103LSCShape.png
Attachment 4: 071103DitherShape.pdf
Attachment 5: 071103DitherZoom.pdf
Attachment 6: 071103LSCShape.pdf
Attachment 7: 071103LoopShape.pdf
  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
Attachment 2: 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
  73   Tue Nov 6 23:45:38 2007 tobinConfigurationComputerstektronix scripts!
I cooked up a little script to fetch the data from the networked Tektronix scope. Example usage:

linux2:scripts>tektronix/tek-dump scope0 ch1 foo.csv

"scope0" is the hostname of the scope, "ch1" is the channel you want to dump, and "foo.csv" is the file you want to dump it to. The script is written in Python since Python's libhttp gave me less trouble than Perl's HTTP::Lite.
  79   Wed Nov 7 14:01:31 2007 waldmanOmnistructureOMCFrequency and Intensity noise
One of the biggest problems I had using the PZT to lock was excessive noise. I did a little noise hunting and found that the problem was the cable running from the rack to the laser fast input. As a reminder, the laser has a 4 MHz / volt fast input. We require about 300 MHz to go one FSR, so there is a Thorlabs HV box between at the NPRO fast input which takes 0-10 V -> 0-150 V. The 150 V HV range is worth about 600 MHz of NPRO frequency.

OLD SETUP: Single side of DAC differential (10 Vpp) -> 9V in series with 10 kOhm -> 10 kOhm input impedance of Thorlabs HV -> NPRO

We used the single side of the DAC differential because we didn't have a differential receiver. This turned out to be a bad idea because the cable picks up every 60 Hz harmonic known to man kind.

NEW SETUP: Digital conditioning -> DAC differential (digitally limited to 0 - 1 V) -> SR560 in A-B mode gain 10 (0 - 10 V output)-> Thorlabs HV -> NPRO.

This has almost no 60 Hz noise and works much, much better. Moral of the story, ALWAYS USE DIFFERENTIAL SIGNALS DIFFERENTIALLY !

Note that I may be saturating the SR560 with 10 V output, Its spec'd for 10 Vpp output with 1 VDC max input. I don't know whether or not it can push 10 V out....
  82   Thu Nov 8 00:55:44 2007 pkpUpdateOMCSuspension tests
[Sam , Pinkesh]

We tried to measure the transfer functions of the 6 degrees of freedom in the OMS SUS. To our chagrin, we found that it was very hard to get the OSEMs to center and get a mean value of around 6000 counts. Somehow the left and top OSEMs were coupled and we tried to see if any of the OSEMs/suspension parts were touching each other. But there is still a significant coupling between the various OSEMs. In theory, the only OSEMS that are supposed to couple are [SIDE] , [LEFT, RIGHT] , [TOP1, TOP2 , TOP3] , since the motion along these 3 sets is orthogonal to the other sets. Thus an excitation along any one OSEM in a set should only couple with another OSEM in the same same set and not with the others. The graphs below were obtained by driving all the OSEMS one by one at 7 Hz and at 500 counts ( I still have to figure out how much that is in units of length). These graphs show that there is some sort of contact somewhere. I cant locate any physical contact at this point, although TOP2 is suspicious and we moved it a bit, but it seems to be hanging free now. This can also be caused by the stiff wire with the peek on it. This wire is very stiff and it can transmit motion from one degree of freedom to another quite easily. I also have a graph showing the transfer function of the longitudnal degree of freedom. I decided to do this first because it was simple and I had to only deal with SIDE, which seems to be decoupled from the other DOFs. This graph is similar to one Norna has for the longitudnal DOF transfer function, with the addition of a peak around 1.8 Hz. This I reckon could very be due to the wire, although it is hard to claim for certain. I am going to stop the measurement at this time and start a fresh high resolution spectrum and leave it running over night.

There is an extra peak in the high res spectrum that is disturbing.
Attachment 1: shakeleft.pdf
Attachment 2: shakeright.pdf
Attachment 3: shakeside.pdf
Attachment 4: shaketop1.pdf
Attachment 5: shaketop2.pdf
Attachment 6: shaketop3.pdf
Attachment 7: LongTransfer.pdf
LongTransfer.pdf LongTransfer.pdf LongTransfer.pdf
Attachment 8: Shakeleft7Nov2007_2.pdf
Attachment 9: Shakeleft7Nov2007_2.png
  86   Fri Nov 9 00:01:24 2007 waldmanOmnistructureOMCOMC mechanical resonances (Tap tap tappy tap)
[Pinkesh, Aidan, Sam]

We did a tap-tap-tappy-tap test of the OMC to try to find its resonances. We looked at some combination of the PDH error signal and the DCPD signal in a couple of different noise configurations. The data included below shows tapping of the major tombstone objects as well the breadboard. I don't see any strong evidence of resonances below the very sharp resonance at 1300 Hz (which I interpret as the diving board mode of the breadboard). If I get free, I 'll post some plots of the different breadboard resonances you can excite by tapping in different places.

(The "normalized" tapping response is abs(tap - reference)./reference.)
Attachment 1: Fig1.png
Attachment 2: Fig2.png
Attachment 3: Fig4.png
Attachment 4: Fig2.pdf
Attachment 5: Fig1.pdf
Attachment 6: Fig4.pdf
Attachment 7: ResonanceData.zip
  87   Fri Nov 9 00:23:12 2007 pkpUpdateOMCX and Z resonances
I got a couple of resonance plots going for now. I am still having trouble getting the Y measurement going for some reason. I will investigate that tommorow. But for tonight and tommorow morning, here is some food for thought. I have attached the X and Z transfer functions below. I compared them to Norna's plots - so just writing out what I was thinking -

Keep in mind that these arent high res scans and have been inconviniently stopped at 0.5 Hz Frown.

Z case --

I see two small resonances and two large ones - the large ones are at 5.5 Hz and 0.55 Hz and the small ones at 9 Hz and 2 Hz respectively. In Norna's resonances, these features arent present. Secondly, the two large peaks in Norna's measurement are at 4.5 Hz and just above 1 Hz. Which was kind of expected, since we shortened the wires a bit, so one of the resonances moved up and I suppose that the other one moved down for the same reason.

X case --

Only one broad peak at about 3 Hz is seen here, whereas in Norna's measurement, there were two large peaks and one dip at 0.75 Hz and 2.5 Hz. I suspect that the lower peak has shifted lower than what I scanned to here and a high res scan going upto 0.2 Hz is taking place overnight. So we will have to wait and watch.

Pitch Roll and Yaw can wait for the morning.
Attachment 1: Xtransferfunc.pdf
Xtransferfunc.pdf Xtransferfunc.pdf Xtransferfunc.pdf
Attachment 2: Ztransferfunc.pdf
Ztransferfunc.pdf Ztransferfunc.pdf Ztransferfunc.pdf
  92   Sun Nov 11 21:21:04 2007 ranaHowToComputersNew DV
To use the new ligoDV (previously GEO DV) to look at 40m data, open up a matlab, set up for mDV as usual,
and then from the /cvs/cds/caltech/apps/ligoDV/ directory, type 'ligoDV'.

Then select which NDS server you want to look at and then start clicking to get some plots.
Attachment 1: Screenshot-1.png
  93   Mon Nov 12 10:53:58 2007 pkpUpdateOMCVertical Transfer functions
[Norna Sam Pinkesh]

These plots were created by injected white noise into the OSEMs and reading out the response of the shadow sensors ( taking the power spectrum). We suspect that some of the additional structure is due to the wires.
Attachment 1: VerticalTrans.pdf
VerticalTrans.pdf VerticalTrans.pdf VerticalTrans.pdf VerticalTrans.pdf
  99   Wed Nov 14 07:48:38 2007 nornaOmnistructureOMCOMC Cable dressing
[Snipped from an email]

1) Last Friday Pinkesh and I set the OMC up with only the top three OSEMs and took a vertical transfer function. We had removed the other OSEMs due to difficulty of aligning all OSEMs with the weight of the bench etc bringing the top mass lower than the tablecloth can accommodate. See attached TF.Clearly there are extra peaks (we only expect two with a zero in between) and my belief is that at least some of them are coupling of other degrees of freedom caused by the electrical wiring. Pinkesh and I also noticed the difficulty of maintaining alignment if cables got touched and moved around. So.....

2) Yesterday Dennis and I took a look at how much moving a cable bundle around (with the peak shielding) changed the DC alignment. In a not too precise experiment ( using HeNe laser reflecting off the bench onto a surface ~ 1 metre away) we saw that we could reposition the beam one or two mm in yaw and pitch. This corresponds to ~ one or two mrad which is ~ the range of the OSEM DC alignment. We discussed possibility of removing the cabling from the middle mass, removing the peak and taking it from the bench directly to the structure above. I asked Chub if he could make an equivalent bundle of wires as those from the two preamps to see what happens if we repeat the "moving bundle" experiment. So...

3) Today Chub removed the cabling going to the preamps and we replaced it with a mock up of wire bundle going directly from the preamps to the structure above. See attached picture. The wires are only attached to the preamp boxes weighted down with masses but the bundle is clamped at the top. We repeated the "wiggle the bundle" test and couldnt see any apparent movement ( so maybe it is at most sub-mm). The cable bundle feels softer.

The next thing Chub did was to remove the second bundle ( from photodiodes, heater, pzt) from its attachment to the middle mass and strip off the peek. It is now also going to the top of the structure directly. The whole suspension now appears freer. We discussed with Dennis the "dressing " of the wires. There are some minor difficulties about how to take wires from the bright side to the dark side, but in general it looks like that the wires forming the second "bundle" could be brought to the "terminal block" mounted on the dark side and from there looped up to the top of the structure. We would have to try all this of course to see the wiring doesnt get in the way of other things (e.g. the L and R OSEMs). However this might be the way forward. So...

4) Tomorrow Pinkesh and I will check the alignment and then repeat the vertical transfer function measurement with the two bundles as they are going from bench to top of structure. We might even do a horizontal one if the middle mass is now within range of the tablecloth.
We can then remove preamp cables completely and lay the second bundle of cables on the optical bench and repeat the TFs.

The next thing will be to weigh the bench plus cables. This will allow us to
a) work out what counterbalance weights are needed - and then get them manufactured
b) firm up on how to handle the extra mass in terms of getting the masses at the correct height.

And in parallel Chub will work on the revised layout of cabling.

Looking a little further ahead we can also get some stiffness measurements made on the revised bundle design ( using Bob's method which Alejandro also used) and fold into Dennis's model to get some sanity check the isolation.

I think that's it for now. Comments etc are of course welcome.

Attachment 1: OMC-11-13-07_011.jpg
Attachment 2: VerticalTrans.pdf
VerticalTrans.pdf VerticalTrans.pdf VerticalTrans.pdf VerticalTrans.pdf
  102   Wed Nov 14 16:54:54 2007 pkpUpdateOMCMuch better looking vertical transfer functions
[Norna Pinkesh]

So after Chub did his wonderful mini-surgery and removed the peek from the cables and after Norna and I aligned the whole apparatus, the following are the peaks that we see.
It almost exactly matches Norna's simulations and some of the extra peaks are possibly due to us exciting the Roll/longitudnal/yaw and pitch motions. The roll resonance is esp prominent.

We also took another plot with one of the wires removed and will wait on Chub before we remove another wire.
Attachment 1: VerticalTransPreampwireremovedNov142007.pdf
VerticalTransPreampwireremovedNov142007.pdf VerticalTransPreampwireremovedNov142007.pdf VerticalTransPreampwireremovedNov142007.pdf VerticalTransPreampwireremovedNov142007.pdf
Attachment 2: VerticalTranswiresclampedNov142007.pdf
VerticalTranswiresclampedNov142007.pdf VerticalTranswiresclampedNov142007.pdf VerticalTranswiresclampedNov142007.pdf VerticalTranswiresclampedNov142007.pdf
  105   Thu Nov 15 17:09:37 2007 pkpUpdateOMCVertical Transfer functions with no cables attached.
[Norna Pinkesh]

The cables connecting all the electronics ( DCPDs, QPDs etc) have been removed to test for the vertical transfer function. Now the cables are sitting on the OMC bench and it was realigned.
Attachment 1: VerticaltransferfuncnocablesattachedNov152007.pdf
VerticaltransferfuncnocablesattachedNov152007.pdf VerticaltransferfuncnocablesattachedNov152007.pdf VerticaltransferfuncnocablesattachedNov152007.pdf VerticaltransferfuncnocablesattachedNov152007.pdf
  106   Mon Nov 26 16:05:43 2007 Norna RobertsonMiscSUSMass of OMC silca bench (measured 16 Nov 2007)
For the record here is a note sent round OMC-SUS colleagues on 16th November.


A team of Chris, Chub, Helena and me ably led by Sam (who did the scary lifting) got the OMC bench dismounted from its suspension and weighed today. It is now sitting on its custom plate with dark side down (minus the preamp boxes) in the centre of the optics table in room 56. It will remain there for the forseeable future. The metal parts of the suspension will be disassembled by Chris with Ken Mailand's help starting Monday.

Results of weighing

1) Optical bench including preamps and the wire bundle to the heater, pzt etc = 6.514 kg
2)Preamp cable (mock-up) = 24.4 g (without the connectors to the preamp)
3) The two leaning towers of counterweights (which were situated close to the far corners of the mass - NOT above the holes where they need to be for fixing in place) = 200.2 g and 187.7 g

I remind you all that there are 2 more tombstones ( fused silica 8 mm thick x 30 mm tall x 32? mm wide with 18 mm diam hole- size to be checked from CAD drawing) and two pieces of black glass 1 inch by 1 inch by 3 mm ( I noted at our telecon that these weigh 8.5 gm total) to be added to the bench - situated close to the DC photodiodes which sit on bright side under the preamps.
  107   Mon Dec 17 19:39:52 2007 waldmanLaserOMCFiber seems to be broken
The 50 m fiber running from Rana's lab to 056 seems to be broken. I can't get any light through it to save my life. A 5 meter fiber couples like child's play. I think we should acquire a fiber coupler - then I will couple light into the 5 m fiber that works fine and couple it to the 50 m fiber and prove that its broken. Only then will I go pull the installed fiber from the 40m clean room.

  108   Wed Dec 19 15:29:14 2007 DmassLaserGeneralTip Tilts Being Baked
The Tip Tilt mirrors are now being baked with Bob. They should be ready in time.

The traveler has DCC #E070354-00-X.

As per Bob's know-how, they are cooking at:
80 C for magnets
200 C for steel;
120 C for the Al, Cu, Bronze, and teflon

  109   Wed Dec 19 23:09:43 2007 waldmanLaserOMCOMC relocked
The OMC has been relocked in preparation for final diode alignment, final QPD aligment, and adding the beam blocks. The mode matching is really terrible, so it makes alignment a little difficult because there is a high, high order mode close to the 00 that is making problems for the vertical alignment.

  110   Wed Dec 19 23:10:27 2007 waldmanComputingOMCFramebuilder broken
The framebuilder on OMS isn't doing anything. I can't get data using dataviewer or DTT. Thankfully, I can still use an analog scope to get data.

  111   Thu Dec 20 11:38:46 2007 aivanovComputingOMCframe builder fixed
I put the things back the way they were before we borrowed the ADC board. I can see the TPT data now. Alex
  112   Thu Dec 20 19:11:40 2007 waldmanComputingOMCHoWTO build front ends
For instance, to build the TPT front end code.

  • Save your file /cvs/cds/advLigo/src/epics/simLink/tpt.mdl
  • go to /cvs/cds/advLIGO on the TPT machine
  • do make clean-tpt tpt install-tpt
  • do rm /cvs/cds/caltech/chans/daq/C2TPT.ini (this step is needed because the DAQ install code isn't quite right at the time of this writing.
  • do make install-daq-tpt
  • run starttpt to restart the tpt computer.

  113   Thu Dec 20 19:12:11 2007 waldmanLaserOMCStressful reattachment of heater
Photos may follow eventually, but for now here's the rundown. I scraped the heater clean of the thermal epoxy using a clean razor blade. Then I stuffed a small piece of lint free cloth in the OTAS bore and wrapped the OMC in tin foil. With a vacuum sucking directly from the face of the OTAS, I gently scraped the glue off the OTAS aluminum. I wiped both the OTAS and the heater down with an isoproponal soaked lint-free cloth. I put a thin sheen of VacSeal on the face of the heater, wiping off the excess from the edges with a cloth. Then I clamped the heater to the OTAS using 2" c-clamps from the tombstone back to the heater front, making sure the alignment of the OTAS was correct (connector on the absolute bottom, concentric with the OTAS outer diameter). I added a second clamp, then beaded the outside of the joint with a little bit extra VacSeal, just for kicks. I'll leave it covered at least overnight, and maybe for a day or two.

  114   Sat Dec 22 18:05:07 2007 waldmanLaserOMCHeater bonding (almost) successful
The heater is now (re)bonded to the OTAS using VacSeal instead of the thermal epoxy. Unlike the expensive high-quality epoxy, VacSeal has supported 4 cycles of connector on/off.

Unfortunately, I didn't bond the heater as straight onto the aluminum as I had hoped. The connector is still about 5 degrees from the vertical so that the connector body sticks up close to the beam (but not as close as it might be. The mode matching is sooooo terrible (because I used an unlabeled lens) that it is impossible to identify the little bit of scattered light coming off the connector. Some of it could be (and probably is), junk light. I believe the closest approach of the connector is ~3 mm, but in the event we measure the finesse to be low, we might want to consider reworking the connector to get it down out of the beam.
  115   Sun Dec 23 17:21:57 2007 waldmanLaserOMCQPD3 centering
In the current cabling scheme, QPD3 is the Far field QPD mounted to the breadboard. With the cavity on a "pretty good alignment" I can physically move the QPD and get the signals to go +/- 1. In other words, I can physically move the QPD to zero its signal, which is the whole purpose of this test. The QPD matrix looks like [1 1 1 1; 1 1 -1 -1; 1 -1 -1 1]; Pitch looks right, yaw might be backwards. Note that the QPD is in a "plus" orientation.

I can center QPD4, the "near field" QPD just fine as well. In this configuration.

One caveat going forward - This alignment is going to be a real pain. In order to be in the linear range of the QPD, we need to be +/- omega/10 or something. In other words, with our 500 micron waist size, we need to get within 50 microns. We should get a translation stage made for this.
  116   Sun Dec 23 18:40:12 2007 waldmanLaserOMCDCPD centering
I can use a CCD camera and a macro lens to see the transmitted cavity beam on the DC photodiodes. I don't know which camera b/c there is no name on it, but it is a square edged camera with a 1/4-20 thread on the mount and has a many pinned cable running to a PS-12SU power supply. I also don't know which macro lens, but tis C-mount, about 4 inches long by 1.5 inches diameter with zoom, focus and f-stop adjustments (all unmarked).

When I look at the face of the transmitted PD, I see the three pins behind the diode chip on the left side. I can move the PD such that the beam is centered on the diode, but this is all the way to one side of the radiator clearance holes. When I maximally misalign the diode, the beam overlaps the diode edge on the side of the feed thru pins.

For the reflected DCPD, I cannot center the beam. Viewing the face of the PD with the two visible feedthru pin son the left side of the diode chip, the beam is low to the left and I cannot center it. I will try again tomorrow with the radiator with the oversize thru holes and see if I can center it. I think it may be a good idea to use an oversized radiator on the transmitted DCPD as well.
  117   Wed Dec 26 15:46:22 2007 waldmanLaserOMCDCPD centering
I used the drilled out radiator with both DCPDs and confirmed that I could center the beams. I also found the reflected beams and confirmed that a) there is clearance for the beamdumps and b) the location of the beams. Note that by some freak of fate, the transmitted DCPD ghost beam is only at about +15mm above the breadboard. I should mount the beamdump a bit low to accomodate it.

I can use a CCD camera and a macro lens to see the transmitted cavity beam on the DC photodiodes. I don't know which camera b/c there is no name on it, but it is a square edged camera with a 1/4-20 thread on the mount and has a many pinned cable running to a PS-12SU power supply. I also don't know which macro lens, but tis C-mount, about 4 inches long by 1.5 inches diameter with zoom, focus and f-stop adjustments (all unmarked).

When I look at the face of the transmitted PD, I see the three pins behind the diode chip on the left side. I can move the PD such that the beam is centered on the diode, but this is all the way to one side of the radiator clearance holes. When I maximally misalign the diode, the beam overlaps the diode edge on the side of the feed thru pins.

For the reflected DCPD, I cannot center the beam. Viewing the face of the PD with the two visible feedthru pin son the left side of the diode chip, the beam is low to the left and I cannot center it. I will try again tomorrow with the radiator with the oversize thru holes and see if I can center it. I think it may be a good idea to use an oversized radiator on the transmitted DCPD as well.
  118   Wed Dec 26 22:39:52 2007 waldmanLaserOMCDCPD beam dumps
I manufactured 2x 1" x 1" black glass beam dumps by vac-sealing the appropriate sized squares to DCPD tombstones. I then locked the cavity and looked at where the DCPD reflections went. The REFL DCPD is no problem, but the choices for the TRANS DCPD aren't so hot. As shown in the enclosed photos, the three options are ugly. I lean towards option 1, but the other two are possibilities as well.

To be clear,

Option 1: beam dump close to the PD at an angle to the beam splitter. Spot is centered on the dump and a little of the tombstone hangs off the edge

Option 2: beam dump close to the PD, parallel to the beam splitter. In order to get the spot on the black glass, the tombstone is very close to the beamsplitter. (This option might be more attractive if the tombstone is rotated 180 degrees.) The tombstone doesn't hang off the edge.

Option 3: We can catch the beam after the beam splitter if we hang the tombstone significantly off the edge, like by half. You'll see in the photos

  119   Wed Jan 23 22:52:21 2008 Norna RobertsonMiscSUSOMC SUS assembly adjustments made and lessons learnt
Calum and I have been working in the 40 m annex re-assembling and testing the OMC SUS after the parts have been cleaned and baked and various parts modified. We have had several problems which needed to be addressed and made several observations of items which need attention for the next OMC. Also we note several points which need to be followed when the OMC is being prepared at LLO for input into the vacuum chamber. The following are in no particular order.

1) Problem with one of the blades
When we got the double pendulum hanging today we noticed that one of the lower blades was interfering with part of the top mass - hitting a block which holds one of the screws for adjusting the position of the clamp for the upper wire ( for adjusting pitch). The blade was 13S - one of those which had an angled clamp of 3.5 degrees (the larger of the angles - the blades at the other end of the bench have 3 degree angles). These large angles are being used to counteract the fact that the bench weighs more than these blades were designed for. The blades thus have a convex curvature (looking from above) and the crown of the bend was where the interference was occuring. The puzzling bit is why only one blade showed this, and not its partner. We tried another 3.5 degree clamp ( in case it was a fault with the clamp) but saw the same effect. We then tried reducing to 3 degrees for that blade - still almost touching. We then reduced the angle for that blade to 2.5 degrees. This worked Ok - and when the masses were hanging again all the blade tips appeared to be at approximately the same height (measured using a steel rule). So the dynamics should be OK. Puzzle remains why only one blade showed this behaviour. Note that for the LHO OMC we will be getting new blades designed for the mass they will be taking, and so we should not need to use angled clamps as large as for the LLO suspension Hence this interference problem should not arise again.
Lesson for the future - need to characterise all blades with the full range of angled clamps to be used. (We did not have time to do this for this suspension).

2) Test of new method of hanging silica bench (using metal bench)
Yesterday with Ken Mailand's help we tried out the new way of suspending the bench using Ken's bench holder with one of the metal benches and the two lab jacks. We learnt that to get the right range of height adjustment to lift the bench to put the discs on the lower wire clamps and then lower the bench again the mating pieces on the lab jacks had to mate with the central portion of the handles at each end rather than the top part of the handles. Also it looks like the lower EQ stop holder (which sits under the bench) should be put in place before the bench is brought in - otherwise difficult to get into place. Finally we note that this job is a three person job - need one on each end to raise and lower the lab jacks and one to fit the discs and then guide the discs and clamps into the holes and check they are seated correctly as the bench is lowered.

3) Height of bench.
We measured the height of the lower surface of the metal bench above the optics table to be (160.5 - 38.75) = 121.75 mm. Requirement is 101.6 + 20 = 121.6 (+/- 2mm) so we are OK. This was without any extra mass added to the top mass. We note that the tabelcloth is close to lower end of its range, but should be OK. However the final alignment needs to be done when the OSEMs are in place.

3) EQ stops and parts which should be removed before putting into vacuum chamber.
For sending the OMC to LLO we put on double nuts on all the EQ stops. This limits the range of the stops and in particular the ones under the bench were not long enough to reach the bench when double nuts are used. So we had to improvise the position of the EQ holder by putting ~ 1/2 inch "shims" to raise the holder ( the shims were three of the masses made for attaching to the metal bench to increase its mass). We also removed all "loose" parts - the adjustment mechanisms for the top blade yaw positions, the magnets and flags, the set screws for locking the larger vertical stops used to hold the top mass, and the screws used to alter the pitch adjustment clamp positions.
When the OMC is reassembled at LLO and made ready for installation, all the second nuts on the EQ stops should be removed. Also once the pitch adjustment using the moveable clamps is set correctly, the screws for doing those adjustments should be removed again. Basically all loose screws should be removed.

4) Parts needing modifications
i) EQ cross pieces holding the plate which goes over the bench need to be reduced in length to fit between the structure legs.
ii) Blind holes in EQ corner brackets.

5) Point to note for next OMC.
i) The slots in the structure which take the dowel pins for alignment need to be lengthened to allow the tableloth the full range of movement which the slots for the attachment screws would allow.
ii) The targets for aligning OSEMS need a hole in the centre for the flag.

More to follow tomorrow
Also we took lots of pictures today. will put relevant ones into installation document.

OMC is due to be crated tomorrow for pick-up on Friday am.
  120   Fri Jan 25 22:08:45 2008 waldmanLaserOMCOMC ready for shipment
The OMC has been boxed in its spiffy black box and is ready for shipment.

We mounted the OMC in its frame, removing any loose mechanical parts and tightening up the nylon tipped set screws. Then we swaddled each optic in tin foil and lint free cloth before wrapping the whole thing in a double layer of the shiny stuff. Finally we wrapped the whole kaboodle in a double layer of ameristat before shoe-horning (literally) it into the box. The package is about 1/4" too long, so it required some effort to get it into the box, and will doubtless require similar effort to remove it.

The OMC in its box was left on top of its shipping container in 056.
  121   Thu Feb 28 13:46:16 2008 David YMMiscOMCTip Tilts - Susp. Wire shipped
As per Adam Mullavey's request, I have shipped the remainder of the (cleaned) suspension wire for the tip tilt assemblies to LLO for the out of vacuum tip tilts. Unclean the wire as necessary, as Bob says the bake job is trivial for it.
  122   Tue Nov 17 10:33:37 2009 ZachMiscSUSAOSEM test progress

 It's dusty in here...



I was recently commissioned to do some noise measurements on the new  AOSEMS. I set up a humble experiment in the LIGO e-lab to do some preliminary measurements:


I made a simple current-to-voltage converter out of an OP27E (using a 100-kohm feedback resistor) to use as the transimpedance amplifier for the readout. This results in a transimpedance of 0.1 V / uA. A simple schematic of the important elements is attached below.


DC power was provided without regulators directly from the laboratory DC supply in the lab. The value of 1.7 V across the LED was set such that the current through it was ~35 mA.


Rana and I took a few important PSDs (one of the DC supply, one of the OP27E with no supplied current, and two with the setup fully connected--one each with and without the PD covered), all from 250 mHz - 200 Hz, AC coupled. Using a sophisticated estimation method (called, by some, the "pick two points and approximate with a power law" method for lack of something fittingly elegant), we obtained a rough estimate of these spectral densities in order to compare them.


These were all converted into equivalent PD current noise. For all but the "supply" noise, this was done trivially by dividing by the transimpedance of the OP27E. For "supply", LED voltage noise had to be converted to PD current noise in the following way:


Z_LED = 1.7 V / 35 mA ~ 50 ohm


equivalent PD current noise = (I_PD / I_LED) * (measured supply voltage noise / 50 ohm)


where the PD-LED current ratio was found empirically to be (I_PD / I_LED) ~ 1 / 1000 by measuring the voltage out of the amp with full brightness (i.e. I_LED = 35 mA, no obstruction) and dividing by the transimpedance (see 2nd figure).


The third figure below is a plot of these spectral densities in common units. Somewhat expectedly, the noise of the "dark" configuration seems limited by the supply noise. However, the "bright" line seems to be dominated by something else. I'm not sure I see how it could be anything but the LED itself, but it is worthwhile to repeat this "test" with a better setup.


On the to-do list:


1. Voltage regulator/reference

Rana thinks that the AD587LN is a good choice of reference given its performance on some LISA tests. I am in contact with AD, and there is no longer a 'LN' package, but I am trying to get samples of the currently manufactured one that is most similar (AD587KNZ).

In the meantime, I am going to find some simple regulators downstairs or at the 40m.


2. Bandpass filter

I was advised that it is a good idea to build your own high/bandpass filter instead of relying on the spectrum analyzer's AC coupling function. I will be doing just this.


3. Switch to a better op amp

        Like the AD743


4. Calibration

I need to find a good way to hold the OSEM in place while I stick something in there with a micron drive without it being unreliably shaky.







Attachment 1: schematic.jpg
Attachment 2: 2009-11-12_17.34.58.jpg
Attachment 3: noise_comparison.png
  123   Tue Nov 17 21:23:08 2009 KojiMiscSUSAOSEM test progress

We have LT1021-7 at the 40m, next to the Alberto's desk. This is the VREF for 7V.


1. Voltage regulator/reference

Rana thinks that the AD587LN is a good choice of reference given its performance on some LISA tests. I am in contact with AD, and there is no longer a 'LN' package, but I am trying to get samples of the currently manufactured one that is most similar (AD587KNZ).

In the meantime, I am going to find some simple regulators downstairs or at the 40m. 


  124   Thu Nov 19 03:41:12 2009 ZachMiscSUSAOSEM calibration

 Tonight, I calibrated the AOSEM's response in [A/m]. I used a sophisticated rig consisting of:


1. One of those anodized Faraday isolator mounts to hold the OSEM


2. A translation stage with a screw gauge to jam something into it (gracefully)


3. Some DC power supplies and multimeters


4. My simple transimpedance amplifier sketched in the previous post (I did not bother upgrading the readout circuit for this measurement since I was just taking a relatively rough DC measurement)




I used a 9/64 hex key to simulate the shadowmaking magnet. A picture of the setup is attached below.


Some pertinent info:


- The current through the LED was maintained at 35 mA throughout the measurement. The measured voltage across it was 1.62 V, giving Z_LED = 46.3 ohm.


- The op amp supply voltage was +/- 10 V, and the PD bias was +10 V.


- The output voltage of the amplifier with the PD fully lit was 3.04 V (measured before and after the test). Note that this voltage increases slightly as the key is inserted due to reflections. 



The second attachment is a plot of the photocurrent versus the position of the key (the x axis is shifted such that the key is roughly centered at x = 0, and x < 0 corresponds to the key being further inside). The response of the OSEM in the linear region is roughly 0.05 A/m.

Attachment 1: 100_0362.JPG
Attachment 2: calibration_plot_11_18_09.png
  125   Thu Nov 19 12:00:04 2009 ZachMiscSUSbandpass filter

 Attached is the transfer function for the bandpass filter I built for the AOSEM readout. The schematic is primitively outlined below. The corner frequencies are what they should be (HP: 100 mHz, LP: 1 kHz)

The Johnson noise for the 1 k resistor is roughly 4 nV/rt(Hz). Frank says that it doesn't make sense to use much lower than 1 k if I'm putting it into an SR785.



           10 uF                  1 k

      -------| |-------------/\/\/\/\/\/------------------

                        | |

        < |

         160 k  >        _|_  160 nF

        <        ---

        > |

        | |

                V                 V


Attachment 1: BPF_bode_11_18_09.png
ELOG V3.1.3-