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ID Dateup Author Type Category Subject
  1659   Sat Jul 2 16:12:49 2016 AntonioNotesFSSFSS : THINGS TO BE DONE (some notes)

In order to have a frequency stabilization system (FSS) working with a new frequency modulation (FM)

I collect some notes of things that need to be done;

In vacuum cavities

  1. Choose the FM for both South and North cavities;
    1. Currently we are using ~14MHz;
    2. PMC are not into the path;
  2. Buy the oscillators;
  3. Modify the photodetectors to the right frequencies or build new ones;
  4. Buy one BB EOM
    1. Currently we have a resonant ~14MHz and one BB with an external resonant circuit;
  5. Build two resonant circuits for the BB EOMs: one applied to what we already have just replacing the external circuit and one to new BB EOM that we will buy;
  6. We need to buy (perhaps) a HV driver;
  7. Later we may need to modify the FSS servo board;
  8. (...please add if something is missing...)

 

PMCs

In parallel and/or later in order to implement both the PMCs in the North and South path we need:

  1. Buy one BB EOM;
    1. We currently have a resonant 21.5MHz;
  2. Build a resonant circuit at 21.5MHz;
  3. Build a resonant photodetector at 21.5MHz;
    1. We currently have one at that frequency;
  4. Build two servo boards for both PMCs;
    1. We may have one available;
  1660   Mon Jul 4 16:38:24 2016 AntonioDailyProgressMode matchingMode matching plan to the North Cavity: few options

Summary

A couple of mode matching solutions for the north cavity that look promising have been found.
I'm thinking the following plan 2c to be implemented.

introduction

- We'll use the PMC in each path
- We also need to insert a FI in each path (to use the reflected light from the cavities)
- EOMs after the PMC will be inserted for the in-vacuum cavities PDH

These constrains the mode macthing to be done with the use of three lenses. One to focus the beam
into the BB EOM and the other two to mode match the North cavity.

Approach

Question1: Can we place the FI without changing the mode matching solution?

Answer: In Plan1 the profile caluclation is shown. As you can find there, considering that
the maximum distance from the waist of the PMC for the FI is ~ 1.32m (FI length ~ 10cm)
we have a diameter around ~2mm.

Question2: OK. We need to use some focusing lenses. How strong will they be?

Answer: In Plan2-a/b/c the profile caluclations with the sensitivity to the displacement of the lenses
 are shown. As you can find there, we have at the FI a beam radius that is around ~750um at most.
However the three setups show a different sensitivity with Plan2a and 2c being lens sensitive to lens 
displacement compared to 2b.

Conclusion

Along with the discussion above, I will use 2c. (or 2a looks applicable too)



For all the profiles the reference point is the waist at the PMCc (330um), zero point in the plots
and the cavity waist is located at 1.96m far from the PMC waist.

Plan 1: FI after the two MM lenses

Setup

In this case we have 3 lenses of focal "fi"(mm) located at "li"(m):

l1 = 0.16;

l2 = 0.762;

l3 = 1.107;

f1 = 0.1432;

f2 = 0.688;

f3 = 0.572;

Beam Profile

If we want to "risk" a 2 mm diameter beam into the FI.

The following plans 2(a,b,...) have all the FI in the between the two MM lenses. I just report

one figure with the setup:

Plan 2a:

Setup

l1 = 0.183;

l2 = 0.773;

l3 = 1.32;

f1 = 0.229;

f2 = 0.143;

f3 = 0.229;

Beam Profile

We can place the FI around 1.2m or further than the North cavity. The further we go and the smaller is the beam

until we reach the waist at 0.9m.

Sensitivity of lenses displacement

Dipending on which directions we move the lenses we may have 20% mismatch with a displacement of ~4cm for lens 2

while for Lens 3 only few percent.

Plan 2b:

Setup:

l1 = 0.117;

l2 = 0.951;

l3 = 1.313;

f1 = 0.229;

f2 = 0.143;

f3 = 0.143;

Beam Profile

Sensitivity

Plan 2c:

Setup

l1 = 0.148;

l2 = 0.683;

l3 = 1.325;

f1 = 0.1432;

f2 = 0.1719;

f3 = 0.229;

Beam Profile

Sensitivity

This shows to be less sensitive to the lens displacement, compared to the others. Furthermore we notice that the two lenses are somehow indipendent as the curve are more circular;

There is something not ok with the Gouy phase, I need to cross check what is wrong there. I should fix this doubt later, for now it is not important.

 

I would propose plane 2c

 

Data:

Data are currently stored in a shared Box Inc folder, but we may wanto create an svn folder for all our data.

  1661   Thu Jul 7 10:37:46 2016 AntonioDailyProgressMode matchingMode matching plan to the North Cavity: CORRECTION

Motivation
I have realized that I made a mistake measuring the distance of the North cavity waist from the PMC (waist).
I have done the same analysis presented in ID 1660 and adjusted the plan.

Conclusion
A new set of lenses has been found respecting all the constraints given in ID 1660
 



 

Setup
The waist of the North cavity is located at 1.701m far from the PMC waist. The size is 210um.
The chosen lenses focal lens f"i" (m) located at l"i"(m) are:

l1 = 0.136;

l2 = 0.489;

l3 = 1.041;

f1 = 0.114;

f2 = 0.114;

f3 = 0.229;



 

1. Beam profile



2. Sensitivity to lenses displacement






 

 

  1662   Thu Jul 7 17:59:34 2016 awadeDailyProgressMode matchingSouth path laser beam profile (again)

I have put down and pulled up the first section of the south path a few times.  I've developed a habbit of packing optics in at close range and we decided that this was not so great when we had plenty of space.  Starting at the begining again I had another go at measuring the mode strait out of the laser.  Data is attached below and z0 = 0 is referenced to the front of the laser head.

I changed the power at point 8 and there seems to be a distict change in slope there.  There are two waveplates, a PBS (which I think is UV silica) and a W2 coated window that is definitly fused silica.  

This data is much cleaner than earlier measurment and I'm not sure if the change in waist (for the measurment) is enought to be worried about.  I will use these values for subsqutent modematching.  

Horz. beam waist = 193.4873 um
Horz. beam waist position = 27.7636 mm
Vert. beam waist = 139.5525 um
Vert. beam waist position = 34.6003 mm

 

 

  1663   Fri Jul 8 22:09:33 2016 AntonioDailyProgressopticNorth path construction progress 2

North Path

Summary
Following entry ID 1649, additional work along the North Path has been done

Conclusion
We have the light into the North cavity mode mateched and aligned to TEM00 
with a visibility {(Vis  = Vmax-Vmin) / Vmax} of ~71%.

Next Step
If FOR THE MOMENT we accept this mode matching I will hook up the electronics
and work on the North cavity locking, leaving modematching improvments for later.




Things done
1. PMC has been placed in the path and aligned. Visibility was about ~90%;
2. PMC has been removed;
3. A PLCX-51.5-UV lens has been cleaned and placed in the optical path soon after the PMC;
4. A resonant EOM (14.5MHz) for PDH sidebandshas been placed and aligned:

  • Power at the input was ~3mW, at the output ~2.95mW

5. Two steering mirrors have been cleaned and placed in the path;
6. Two mode matching lenses have been placed:

  • A PLCX-51.5-UV lens has been placed in a location that few inches above the nominal
    found in the design (ID 1661) ;
     
  • A PLCX-103.0-UV lens has been placed in its nominal location;

7.  In between the two lenses a FI (IO-5-1064-HP) has been aligned:

8. The TEM00 has been found by scanning the PZT of the laser with a triangular wave (+-1V) at 10Hz through an HV amplifier set at ~40V;
    Additionally a voltage calibrator has been used to actuate on the crystal temperature of the laser in order to localize the are to be scanned with the PZT.
 

9. A Lambda/2 has been placed after the FI in order to have one linear polirized light:

  • With light at ~45degree I have seen 2 TEM00 resonance not exactly the same;
  • By rotating the waveplate we can have purely one or the other resonance (not sure yet at which angle);

10. A lens and a steering mirror have been placed in the path of the rejected light from FI to monitor the visibility/resonances with the DC output of the photodiodes;
    For now the visibility is at ~71%

11. This is the current setup:

 

  1664   Wed Jul 13 00:06:17 2016 Antonio, AndrewDailyProgressElectronics EquipmentNorth path construction progress 3

Summary
Following ID 1649 & 1663 electronics for the FSS of the North path has been hooked up.
The North cavity was ready for locking today.

Conclusion
The error-signal of the North PDH does not look right. The cavity can be locked but it is
not an easy operation for now as it was before. We need to investigate on the error-signal
and nail down the reason.



Notes:

  • Initially the err-sig had a weird shape and a low SNR (~2), later it became for unknown reasons a proper
    PDH with still the same SNR.
  • Attempts to lock the North cavity have been done anyway. Without driving the BBEOM 
    cavity stayied locked for half an hour. 
  • The error signal was taken at the monitor of the TTFSS and at the FSS box on the table (out1 and out 2); they were similar

 

 

  1665   Wed Jul 13 10:24:24 2016 ranaNotesFSSFSS : THINGS TO BE DONE (some notes)

I think what we have from Wenzel is 2 OCXO of each of these frequencies:

32.7 MHz

33.59 MHz

36.0 MHz

37.0 MHz

Are there any combinations of these that work? I think perhaps that cryo lab and 40m have some of these.

  1666   Wed Jul 13 13:31:33 2016 EvanNotesFSSFSS : THINGS TO BE DONE (some notes)

I did the same analysis as Antonio: https://nodus.ligo.caltech.edu:8081/PSL_Lab/1394

My recollection is that this was the justification for ordering the 36 and 37 MHz OCXOs.

  1667   Wed Jul 13 15:59:58 2016 awadeDailyProgressMode matchingSouth path path plan

I've mapped out a path for the south laser beam (draft attached). I've left the final stages until I have placed and measured the final modulator to get the most accurate MM solution possible.  z values are referenced to the laser head and quoted waists are the mean of the two axis.  After placing the first lens I took another beam profile. Its not so great for the vertical nearer the waist.  The fitted values are

Horz. beam waist = 250.5078 um
Horz. beam waist position = 1051.8195 mm
Vert. beam waist = 191.9869 um
Vert. beam waist position = 1069.7957 mm

Here I used a W2-PW1-1037-UV-1064-45P wedge and a 1" PBS-1064-100 (BK7 unfortually, but good enough for a quick measurment). 

Data is attached below along with plot.

-awade Wed Jul 13 15:49:10 2016

  1668   Wed Jul 13 20:03:07 2016 AntonioNotesFSSFSS : THINGS TO BE DONE (some notes)

That is perfectly in agreement with my analysis.

 

36MHz and 37MHz are good according with our analysis if 1MHz separation between the two modulation frequencies is good enough;

If 1MHz separation is not enough than we should consider 34.5MHz and 37MHz. Here the risk is to get close to the order 23 (n+m) at

a nominal detuning frequency that is around 2.2MHz.

 

Quote:

I did the same analysis as Antonio: https://nodus.ligo.caltech.edu:8081/PSL_Lab/1394

My recollection is that this was the justification for ordering the 36 and 37 MHz OCXOs.

 

  1669   Wed Jul 13 20:43:03 2016 Antonio, AndrewDailyProgressNorth CavityCavity locked!!!

Motivations
We had problems with the error-signal monitor from the TTFSS board. We thought that something 
was wrong with it because the SNR was very small. Before starting the process of nailing down what was
wrong with the electronics I wanted to put some effort to reach a good lock again because I suspected
that the monitor was wrong. We need to check it (later)!

Conclusions
The cavity now it can be stably locked for half an hour.


 

The cavity was initially easy to lock without the EOM. Few crucial things have been done to make the lock:

  • Lower the power entering the cavity from 3mW to 1mW;
  • Inverter on "-" on the FSS box on the table;

With this changes the lock was possible but the control-signal was very noisy;
 

Thanks to Andrew, he added a lambda/4 before the BB EOM to remove additional circular light (after the PBSO).
The quality improved considerably.

I have then twicked the gain Knobs in order to have a better lock. The lock is satisfying (fo now!). Aidan will
provide us of a feedback on the temperature too. Then the lock will stay longer.


Some settings:

  • It cannot be locked with boost ON.;
  • Inverter on FSS box on "-";
  • Common = 600; Fast = 750; Offset = 550;
    The Offset has been adjusted in a way that after the lock the cavity was on resonance (monitoring the DC channel of the PD in reflection);
     
  1670   Thu Jul 14 20:57:07 2016 AntonioDailyProgressopticSouth path construction progress

Summary
Following ID 1649 we start the implementation of the South cavity path.
Some optics has been placed along the path.

Conclusions 
A quick beam profile measurement and the implementation of the BB EOM
is the next step.
 


Things done:

  • Optics as shown in the setup have been placed;
  • References of the beam along the path have been aligned  (Those should be kept in place)


Notes:

  • The FI is mounted on a base which brings it NOT at 3" height; Two steering mirrors have been used to overcome this issue;
  • There is a second beam coming from the FI; I did not have a chance to play with it, but Andrew did and the origin of that is not totally understood;

Setup

  1671   Fri Jul 15 13:07:46 2016 awadeDailyProgressMode matchingSouth path: EOM1 Placement and beam profile

The first EOM was placed at the waist along with PBS and wave plates in the order lambda/4-lambda/2-PBS-EOM-lambda/2.

The beam was reameasured after this point (as a check). Fit was:

Horz. beam waist = 176.5893 um
Horz. beam waist position = 780.5592 mm
Vert. beam waist = 94.6329 um
Vert. beam waist position = 811.292 mm

Next we place a PLCX-25.4-46.4-UV-1064 (f = 103.20 mm) for next alignment into AEOM.

  1672   Fri Jul 15 14:34:58 2016 awadeDailyProgressMode matchingSouth path: AEOM Placement and beam profile

The second EOM (an amplitude modulator: Newfocus-4104) was placed at the next waist and optics installed were lambda/2->AEOM(4104)->PBS. Location of the AEOM (referenced to laser head) was 1012 mm.

The beam was reameasured after this point (as a check) with a AR coated window pickoff. Fit was:

Horz. beam waist = 183.5091 um
Horz. beam waist position = 957.1859 mm
Vert. beam waist = 226.7887 um
Vert. beam waist position = 856.7048 mm.

Accounting for the AEOM and PBS length and RI this is pretty much right for placement of the AEOM.

 

--> Edit, fixed authour name. Fri Jul 15 17:06:45 2016

  1673   Fri Jul 15 20:42:53 2016 awadeDailyProgressMode matchingSouth path: EOM2 Placement and beam profile

A PLCX-25.4-64.4-UV-1064 (f = 143.23 mm) lens was placed in the path at z = 1247 mm.  The order of components after the AEOM was (Space for waveplate)->PBS->Steering Mirror->Lens->lambda/2->EOM2. The lens placement was very clost to the steering mirror, but it was difficult to find a choice of lens that would accommodate a suitably focused solution.

This final EOM (before the PMC) was placed at z = 1440 mm (ref. to laser head position).  

The beam was profiled after the EOM2 to get better characteristics for MM to the PMC.  The fit was:

Horz. beam waist = 246.0624 um
Horz. beam waist position = 1397.5481 mm
Vert. beam waist = 210.7356 um
Vert. beam waist position = 1252.1215 mm

The waist was set a little bigger to ease the MM placement sensitivity of the first lens for matching into the PMC. Data and plot attached.

Next MM to the PMC (although this wont be put in place until after we have gotten a first beat note).

 

  1674   Sun Jul 17 14:17:27 2016 awadeDailyProgressMode matchingTape fleck in S-EOM2 (NF-4004) aperature and lens move

I was concerned about some of the spacial features I was seeing after the second broadband EOM.  On closer inspection there was a small fleck of what looked like plastic tape in the input aperture (pictured).  

Fleck of plastic, now removed 

I removed the fleck and it looks all clear now.  

Also, I thought maybe the beam was just a the limit of size for one axis. I moved the lens (PLCX-25.4-64.4-UV-1064) forward to z = 1252 mm. The re measured beam had the fitted characteristics:


Horz. beam waist = 194.3474 um
Horz. beam waist position = 1472.415 mm
Vert. beam waist = 140.8421 um
Vert. beam waist position = 1418.9487 mm

 

  1675   Sun Jul 17 19:31:19 2016 awade, AntonioDailyProgressEOMResidual light light after PBS and AEOM

Just a quick noted about some resudual beam spots I noticed after the second PBS in the south path (after the AEOM).  

When I went to minimise transmitted power through the second PBS with a power meter two spots were present and most obvious. I didn't really understand where these would come from.  Checking further up the path, these features are not present before the AEOM and PBS. I swapped out the PBS, then the waveplates: no change.  I then compleatly removed the AEOM from the path (but not the PBS) and the spots were removed. The beam looked well centered (by eye) and had good cleanance from the edges of the aperature.

Antonio thought it might be some alignment issues with the modulator and did some precition alignment.  His method was to look at power at the output and maximise (we measured power of 296 mW and out of 280.7 mW which gave an loss of ~5%).  He also fixed the CCD viewer position and compaired the computed center of the beam with the AEOM removed and then installed. This seemed to improve the non-polarisation filtered beam throughput and improve the shape of the beam.  

The optimal operating point for the AEOM should be at 50% throughput through the PBS so the remianing junk spots should be small. Still not sure what effect is producing the residual remaining spots were. This is something we should keep in mind.

 

These are two spots slightly off diagonal 
  1676   Mon Jul 18 15:19:02 2016 awadeNotesMode matchingRemeasured beam profile after second EOM and after realignment

I made some further alignment adjustments of the modulators and some changes to the polarization inputs and outputs. A lambda/2 wave plate was installed directly after the AEOM (for now) so that the PBS could be manually tuned to the 50% transmission point for s- polarisation into the AEOM.  For now we will ignore the circular polarization generated by the AEOM and the small amount introduced by the previous EOM.  

After checking the field after first EOM I determined that the residual polarization was 108 uW out of 83.4 mW or 0.13 %. Without the EOM installed the extinction was down to ~10 uW.  For now this level of circular polarization should be tolerable.  It is something we can optimize later.  

I also realigned (again) the AEOM and 2nd EOM using Antonio's methods.  This gave a satisfactory looking Gaussian beam. The profile was again remeasured. Its fitted values are:

Horz. beam waist = 259.6243 um
Horz. beam waist position = 1436.9167 mm
Vert. beam waist = 215.7146 um
Vert. beam waist position = 1319.3657 mm

Data and plots attached below.

 

  1677   Thu Jul 21 11:49:29 2016 awadeDailyProgressMode matchingBeam measurement for MM into south cavity

I placed a pair of lenses and a cylindrical lens in the path after the final EOM before the PMC location to provide a MM solution close to that of the PMC when we eventurally impliment this. The goal was 330 um waist.  The PMC base was bolted in position and with a quick alignment the cavity was scanned to see how well it will mode match when we install.  Visablity was found to be 84.4 % (with 1.000 V off resonance and a dip down to 156 mV on reflection).  All this is so that we have a fair idea of the MM solution and placement for later installation.

I took the PMC out today and took a proper beam profile referenced to the steering mirror just before the PMC.  Data and plot of fit are attached the fitted profile values were:

Horz. beam waist = 256.7996 um
Horz. beam waist position = 2635.8799 mm
Vert. beam waist = 211.6388 um
Vert. beam waist position = 2538.9972 mm
--
Mean beam waist = 234.2192 um
Mean beam waist position = 2587.4386 mm

However, looking at the plot it looks like the fit overshoots the actual measurments close to the waist.  It may be that the large distance measurments bias the measurment (and there are more of them). But the waist was definitly located closer to the reference point at which the PMC base was placed yesterday.  I haven't modeled it but I find a visablity of 84 % for a waist of 234 um hard to belive if the PMC cavity is designed for a 330 um.  For now it is probably ok to assume 330 um for this next modematching step.  

Next final MM to the south cavity. We expect that this should take to the end of today.

  1678   Thu Jul 21 12:30:15 2016 awadeDailyProgressMode matchingBeam measurement for MM into south cavity

Correction: Wrong plot (at least the x-scale is wrong).  The updated one is attached.

Also the offset of the data from the laser head position is 2698 mm.

Quote:

I placed a pair of lenses and a cylindrical lens in the path after the final EOM before the PMC location to provide a MM solution close to that of the PMC when we eventurally impliment this. The goal was 330 um waist.  The PMC base was bolted in position and with a quick alignment the cavity was scanned to see how well it will mode match when we install.  Visablity was found to be 84.4 % (with 1.000 V off resonance and a dip down to 156 mV on reflection).  All this is so that we have a fair idea of the MM solution and placement for later installation.

I took the PMC out today and took a proper beam profile referenced to the steering mirror just before the PMC.  Data and plot of fit are attached the fitted profile values were:

Horz. beam waist = 256.7996 um
Horz. beam waist position = 2635.8799 mm
Vert. beam waist = 211.6388 um
Vert. beam waist position = 2538.9972 mm
--
Mean beam waist = 234.2192 um
Mean beam waist position = 2587.4386 mm

However, looking at the plot it looks like the fit overshoots the actual measurments close to the waist.  It may be that the large distance measurments bias the measurment (and there are more of them). But the waist was definitly located closer to the reference point at which the PMC base was placed yesterday.  I haven't modeled it but I find a visablity of 84 % for a waist of 234 um hard to belive if the PMC cavity is designed for a 330 um.  For now it is probably ok to assume 330 um for this next modematching step.  

Next final MM to the south cavity. We expect that this should take to the end of today.

 

  1679   Thu Jul 21 13:08:50 2016 awadeDailyProgressMode matchingBeam measurement for MM into south cavity


I am missing the target here. The size is 330um, but I did not get the waist target location.

 

Quote:

I placed a pair of lenses and a cylindrical lens in the path after the final EOM before the PMC location to provide a MM solution close to that of the PMC when we eventurally impliment this. The goal was 330 um waist.  The PMC base was bolted in position and with a quick alignment the cavity was scanned to see how well it will mode match when we install.  Visablity was found to be 84.4 % (with 1.000 V off resonance and a dip down to 156 mV on reflection).  All this is so that we have a fair idea of the MM solution and placement for later installation.

I took the PMC out today and took a proper beam profile referenced to the steering mirror just before the PMC.  Data and plot of fit are attached the fitted profile values were:

Horz. beam waist = 256.7996 um
Horz. beam waist position = 2635.8799 mm
Vert. beam waist = 211.6388 um
Vert. beam waist position = 2538.9972 mm
--
Mean beam waist = 234.2192 um
Mean beam waist position = 2587.4386 mm

However, looking at the plot it looks like the fit overshoots the actual measurments close to the waist.  It may be that the large distance measurments bias the measurment (and there are more of them). But the waist was definitly located closer to the reference point at which the PMC base was placed yesterday.  I haven't modeled it but I find a visablity of 84 % for a waist of 234 um hard to belive if the PMC cavity is designed for a 330 um.  For now it is probably ok to assume 330 um for this next modematching step.  

Next final MM to the south cavity. We expect that this should take to the end of today.

 

  1680   Fri Jul 22 23:28:52 2016 awadeDailyProgressMode matchingSouth path: EOM3 Placement and beam profile (also note on D4σ fit)

Measured beam for third EOM south path

I measured the beam as it would enter the third and final EOM after the PMC (note that the PMC was removed from the path for now).  The first PLCX-25.4-64.4-UV-1064 (f = 143.23 mm) was placed at 2769.5 mm from the laser head (see attached updated map of south path). The beam profile measurements are attached as txt file along with plot. The z = 0 reference point is the first bolt on the second table or 2815 mm referenced from the laser head.

The fit of the data was:

Horz. beam waist = 112.7457 um
Horz. beam waist position = 2908.1548 mm
Vert. beam waist = 115.3321 um
Vert. beam waist position = 2940.4081 mm
--
Mean beam waist = 114.0389 um
Mean beam waist position = 2924.2815 mm

It appears that the actual beam waist is 150 um and 166 um in horizontal and vertical respectively for the data. (At least if we trust the D4σ fit).

 

NOTE ON D4σ METHOD:

It seems that the CCD measurements close to the waist don't fit well with the data at a large distance.   I am using the D4σ definition of beam width.  This should be the same as 1/e^2 clip method when dealing with Gaussian beams. However, it is possible that small beams have biased results from dark noise + offsets in pixels well outside the beam that weight the integral of the beam profile. This may be a deficiency in this method. At some stage I should check how to get the best possible pre calibration for the CCD baseline. 

 

 

 

 

  1681   Sat Jul 23 01:16:06 2016 awade and AntonioDailyProgressMode matchingSouth path: MM solution into south cavity and FI isolation numbers

A mode matching solution was found for the beam to be coupled into the south cavity. 

With lengths referenced to the first hole in the second table (2815 mm from the laser head)

f = 143.23 mm (PLCX-25.4-64.4-UV-1064) @ 0.148 m

f = 103.20 mm (PLCX-25.4-46.4-UV-1064) @ 0.589 m

f = 171.92 mm (PLCX-25.4-77.3-UV-1066) @ 0.974 m

A small alteration was made as we settled on the final solution, the first lens was shifted back to the 0.100 m mark with a commensurate shift in the other lenses.  The effect of this move was move the first waist (and the position of the EOM back a couple of bolt holes.  The change in size of the first waist was small so the solution remained the same with small tweaks to the second and third lens positions. 

In order to fit the components into the vacuum tank and get a beat note as soon as possible we opted to route the beam along a different route to the north path.  The path is shown with positions of components in the attached schematic. Lenses were mounted on linear translations stages for fine alignment and the Faraday isolator polarization axis was slightly altered to give a return path beam that was parallel to the table.  The way the Thorlabs FI come from the manufacture is such that the input polarization is optimized for horizontal polarization input, rather than reflection in the plane of the beam.

With the changes to the end PBS rotation we re measured the through put power and reverse power attenuation. Out of 9.05 mW incident power the forward propagating output was 8.75 mW: this is a 3.3 % loss. The reverse power throughput was 3.062 uW out of 9.01 mW (we rotated through all input polarization and chose the maximum value): this is a 34.7 dB degree of isolation for back reflection.

A quick measurement of the return path efficiency (of back reflected light extracted from the first FI's first PBS port for PDH locking of the cavity) was 8.07 mW from a total of 9.01 mW or 10.4 % loss.  This was a quick measurement, and maybe something we might like to confirm later. 

---

All the components up the periscope have been aligned with their heights and centering checked. I did a quick walk of the lenses to see if I could find a waist of 220 um located at the expected point for the cavity (checked with a flipper mirror on the table). I ran out of range on the second lens (it needs to be moved back more).  However, the solution is pretty close now. 

 

Also, we need to check the beam waist fitting method (see note PSL_Lab/1680), it may be necessary to do a baseline subtraction of the dark CCD detector to be able to use the D4σ method with confidence.  Otherwise double check the the 1/e^2 clipping option in the WinCamD software; you can get the software to display both at once.  We should compare tight waists to beams much further out so see how the two methods scale.

 

First beatnote is close.

 

-awade Sat Jul 23 01:15:45 2016

  1682   Sat Jul 23 20:21:16 2016 AntonioDailyProgressMode matchingSouth cavity Mode matching

Summary
Following the previous entry the South path construction was at the point where mode matching was needed for the South Cavity.
We also needed to install a resonant photodiode for the PDH lock and start to prepare the optic in transmission.

Coonclusions

  • The light is resonating into the South Cavity with a visibility = (Vmax-Vmin)/Vmax = ~ 0.75. 
  • We also have the resonant PD for the light reflected from the cavity.
  • Camera and ISS PD in transmission of the South cavity have been aligned too.
  • All the cabeling are hooked, but they need to be tidied up.

Note:

  1. Lenses have been adjusted and slightly changed compared to the previous entry.
  2. The alignment of the cavity took me a while. The mounts for the steering mirrors on the
    periscope are very sensitive, here the alignment requires a bit of more attention. 
  3. All power supply and cables are connected. I just need to verify the HV power supply settings.

 

  1683   Sun Jul 24 20:03:38 2016 AntonioDailyProgressSouth CavityMode matching and cavities locking

Summary
Today I have aligned the South cavity with the beam prepared and described in the previous ID entry.

Conclusion
The South cavity is aligned and mode matched with a visibility (Vmax-Vmin)/Vmax of ~75%.

 


Things done today:

  • Aligned the beam in order to get the TEM00 mode resonating into the cavity;
    The resulting visibility is of 0.75;
  • Prepared and connected all the power supplies to their components for the south FSS lock;
  • South Cavity locked; The lock happened this morning and it is still locked; (1mW Pin).
  • Realigned (vis = ~ 0.7) and reset all the polarization in the North path, because for some unknown reason
    the North cavity was not anymore in the same place of two days ago, not even close; the waveplates too.
  • Worked on the locking of the North Cavity as it was not showing robust; The North cavity stays locked for few hours now.
  • Started to preapare the Optics for the PLL:
    - Photodiode for the beat note, mirrors and beam splitters alignment;
    - Mirrors and beam splitters alignment;
    - Alignment of the transmitted beams from the cavities into the beat note PD together with Andrews
      The two beams are visually overlapped now;
    - We checked the polarizations at which the two cavities are locked currently:
    North is locked on P;
    South mostly on S (not totally);
    - I and Andrew decided to use 2 lambda/2 at the output in order to be able to change the linear polarization whenever
    we need. The optics at the output are mainly made for s-light;
  • Photodiodes and optics for the ISS are aligned too;
  • I have connected the power supply for the temperature (output=10V);
  • Started some tyding up the table;

 

We can start to look at the beat note!

  1684   Tue Jul 26 11:21:53 2016 awade and AntonioDailyProgress Initial search for beat note refcavs' beatnote

With cavities mode matched and PDH locked on reflection we are now looking to find the beat notes between the two separate cavity-laser systems from the transmitted refcavity beams.

The transmitted beams were realigned through the combining beam splitter using the existing iris positions.  It appears that the the beam alignments, after various adjustments, were coincident on the 125 MHZ beat note detector but at different angles.  One would expect to see a beat note but reduced by the fringe across the beam front. As it is best practice to have overlapping and co-propagating beams, the south path and then the north path were walked to correspond with iris located on the unused port of the PLL (phase locked loop) combining beam splitter. 

We searched for a beat note over a selection of laser temperature offsets.  The South laser was locked to its refcavity at 0.8,1.68,2.53, 3.50 and 4.36 V slow control offset* and then the North cavity was locked at all combinations of 0.475, 1.359,2.252,3.147,4.129,5.042,5.965,6.976 V.  We did not observe a beat note when looking at the PD RF output on a spectrum analyzer. It is very possible that the corresponding FSRs of the two cavities just were not within the bandwidth of the detectors.  We should look around for a RF detector with >=1 GHz bandwidth and place at a pick off somewhere further up the laser path as a way of quickly checking the laser offset without the need to also align the cavity resonance.  This would save us a lot of time and pain carefully walking the refcavity temperature until we had corresponding overlap.  Once we have known operating point this process will be much easier, but a second RF PD would be a very useful diagnostic tool to have; we should generally try to do things the easy way.

 

*Note that there is a mode hopping region towards the top of the south cavity's range wich prevents going any higher in temperature offset (using the front panel voltage) for now.

  1685   Tue Jul 26 11:33:49 2016 awade and AntonioNotesNorth CavityNorth cavity temperature dropped

The north cavity temperature was dropped this morning. The voltage of the heater power suply was dropped from 10.7 V to 10.4 V.

  1686   Tue Jul 26 11:57:45 2016 awade and AntonioNotesNorth CavityNorth cavity temperature changed at 11:55

Tue Jul 26 11:55:00 2016, north refcavity heater voltage was changed from 10.4 V to 10.6 V. We will leave it to stabilise while we are at group meeting and lunch.

  1687   Tue Jul 26 12:56:13 2016 KojiDailyProgress Initial search for beat note refcavs' beatnote

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

  1688   Tue Jul 26 15:12:53 2016 awadeDailyProgressBEATInitial search for beat note refcavs' beatnote

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

  1689   Wed Jul 27 23:05:45 2016 awade and AntonioDailyProgressBEATInitial search for beat note refcavs' beatnote

We built a temporary beat note detector before the cavities. The configuration is illustrated in the attached schematic.

We used some of the unused light from PBSs to provide a direct beat note measurement on a 1 GHz NewFocus 1611. This was before any EOMs or any other components so there is only a beat between the fundamental frequency of both lasers. Coated window optics were used to pick off a small amount of power and a 50/50 beam splitter was used to combine the light from the two lasers.  Some f = ~200 mm lenses were used to keep the beams reasonably collimated, but no big effort was put into mode matching.  There was plenty of light from both lasers to work with, so even poor spacial overlap was sufficient to give a beat note spectrum.  We achieved approximately -53 dBm beat note signal with a clearance of 10 dB above the dark noise level: this enough for our diagnostic purposes.

Using this diagnostic detector, it was found that the closest pair of refinances between north and south cavities was 300 MHz apart. We began the process of gradually tuning the north cavity temperature but believe it is taking a while to actually settle.  There is no feedback PID control on the cavity shield and then we only have control over one cavity with the other affected by variations in the lab environment.  This is something we will need to work on.  For now it looks like we can maybe dial some constants into a Newport 3040 to get a temperature readout from the existing sensor while electronics are organized to interface with the EPICS/acromag system.

For now we think we will keep the 125 MHz detector. This is convenent and from the data sheets it seems like the 1811's have a slightly better NEP. I couldn't see an actual noise spectrum on the manufacture's website so we might want to actually make a measurement of the dark noise of the two RF detectors for a true comparison (with the same power supply rigged up of course).

We left the north cavity to settle with the voltage slightly increased overnight.

 

 

Quote:

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

 

  1690   Thu Jul 28 19:56:08 2016 Antonio, AndrewDailyProgressBEATSearching for beat note

Summary
In the last two days we have been searching for beat note, in order to get 
our "first" noise measurement of the PLL. While we monitor the beat note of the lasers
at the input side we search the same beat note at the transmission side.

Conclusions
We did not succeded yet. I start to believe that what we see at the input side are
not the beat note we are looking for. Some mode hopping could be happening around
the temperature region we have chosen.
 


Things done

1. We built a Mach Zehnder interferometer at the input side of the lasers, in order to fined the beat
directly from the two lasers; The beat note were found around 450MHz.

2. Once we found them we changed the temperature of the vacuum chamber and we locked the cavities
whit laser beat note of 78MHz.

3. Trasmitted beam have been checked for their spatial overlap and polarizations;

4. Photodiodes have been changed in order to see if some problem was coming from there;

5. Network analizer have been changed in order to check if there were wrong settings.

6. PLL rialigned from beginning;


NO RESULTS

PLAN 
We may want to know where mode hopping happen; We need to scan the temperature and  first measure
the power and see where they are. 

Looking for other beat note at the lasers.

  1691   Fri Jul 29 16:29:53 2016 awadeDailyProgressBEATInitial search for beat note refcavs' beatnote

I forgot the schematic.  Attached to this post.

Quote:

We built a temporary beat note detector before the cavities. The configuration is illustrated in the attached schematic.

We used some of the unused light from PBSs to provide a direct beat note measurement on a 1 GHz NewFocus 1611. This was before any EOMs or any other components so there is only a beat between the fundamental frequency of both lasers. Coated window optics were used to pick off a small amount of power and a 50/50 beam splitter was used to combine the light from the two lasers.  Some f = ~200 mm lenses were used to keep the beams reasonably collimated, but no big effort was put into mode matching.  There was plenty of light from both lasers to work with, so even poor spacial overlap was sufficient to give a beat note spectrum.  We achieved approximately -53 dBm beat note signal with a clearance of 10 dB above the dark noise level: this enough for our diagnostic purposes.

Using this diagnostic detector, it was found that the closest pair of refinances between north and south cavities was 300 MHz apart. We began the process of gradually tuning the north cavity temperature but believe it is taking a while to actually settle.  There is no feedback PID control on the cavity shield and then we only have control over one cavity with the other affected by variations in the lab environment.  This is something we will need to work on.  For now it looks like we can maybe dial some constants into a Newport 3040 to get a temperature readout from the existing sensor while electronics are organized to interface with the EPICS/acromag system.

For now we think we will keep the 125 MHz detector. This is convenent and from the data sheets it seems like the 1811's have a slightly better NEP. I couldn't see an actual noise spectrum on the manufacture's website so we might want to actually make a measurement of the dark noise of the two RF detectors for a true comparison (with the same power supply rigged up of course).

We left the north cavity to settle with the voltage slightly increased overnight.

 

 

Quote:

 

Quote:

How about this procedure?

- Build a temporary beat setup before the cavities. Use 1GHz New Focus InGaAs PD.

- Find the beat note with no cavity locked. Adjust the alignment of the beat setup.

- Lock one of the cavities.

- Scan the laser temp of the other laser to find the beat again.

- Lock the second cavity with several temp settings.

- Figure out how much heating of which cavity you need.

I have just borrowed a 1 GHz InGaAs and HP spectrum analyzer from the 40 m.  While I was doing some other things I have set one cavity to lock and the other path's laser on a ~1 mHz temp ramp with the spectrum analyzer on max hold.  Its still doubtful this will catch a beat note.

I will configure pick off from both paths so at least we can know that we are within the bandwidth of the detector without having to deal with the tiny frequency windows of transmission of the two reference cavities.

You make a good point of figuring out the appropriate heating needed, at least we will know if we are even in the ballpark of getting a beat note.

 

 

  1692   Fri Jul 29 16:52:06 2016 awade and AntonioDailyProgressBEATBeat note signal

We have a beat-note.

For the last few days we have been able to bring the two lasers to within the 125 MHz bandwidth of the Newfocus 1811 detector at the output of the vacuum tank. By tuning the length of the North cavity (turning the temperature up) we were able to also bring the resonance of the cavities to within this bandwidth.  

Using the beat note detector setup we assembled at the start of the laser path (PSL_Lab/1689), we can see that the frequency offset of the north and south cavity resonance settled at 70 MHz for a north heater voltage set point of 11.5 V (74 mA). We still don't know what temperature that is because we have no thermistor monitor setup yet. However, it seems that with enough settling time it is stable enough for these initial diagnostic tests.

However, until today we had trouble finding a beat note after the two ref cavities.  We checked polarization, co-alignment of the beams as well as switching the detector in and out with the NF-1611 (1 GHz) detector borrowed from the 40-m.  It turns out there are some neutral density filters installed right before the final focusing lens in the post refcav beatnote setup. When I checked the specs for the NF-1811 it looked like we were well below the RF saturation point with a lot of room to increase power.

For the NF-1811 saturation power @ 1550 nm is 55 uW. The responsively at 1064 nm looks like about 0.6 of this.  So saturation at our wavelength is about 91.7 uW. I measured the total power after the combining beamsplitter, it was 227 uW, so with 13 dB worth of ND filters this was brought down to about 11.3 uW.  We first switched the ND filters (ND1.0+ND0.3) for a single ND0.5 and a beat note could be see 3 dB above the dark noise (75 dBm) of the detector. We then removed the ND filter completely at got a beat note (at 70 MHz) that had a clearance of 17 dB above dark noise.  Not sure if this is enough to make a 'good' measurement but at least we can now see something at the combined transmission of the cavities. The damage threshold is of course way above this.

There is quite a bit of work to be done improving on this and getting the PLL loop setup.  We also need to start work on getting a calibration together. The PDH loops also need some optimizing.

 

On another side note:

We can see the 14.75 MHz  side bands at the beat note output (and one 2x harmonic of this). I'm not sure if this is ok as I would have through the ref cavities would filter this out and I'm not sure if maybe HOM or other would let this to pass the cavity. 

  1693   Sat Jul 30 19:08:08 2016 Antonio and AndrewDailyProgressBEATbeat note work and PLL

Summary

  • Today we have worked reying to improve the beat note signal in its amplitude.
  • We also connected all the parts for the PLL loop and tryied to lock the PLL but we did not succeded;
  • We noticed at some pint a noise at the input of the sr560 which pheraps prevents the PLL lock


    Conclusions

           We need to improve the lock of the cavities in order to get a more stable signal and solve the origin
           of the noise at the input of the sr560 in order to lock the PLL.

 

  1694   Mon Aug 1 18:47:58 2016 awadeSummaryPLLPresent electronic configuration of the PLL loop

Post documenting the set out of the PLL electronics and cable routing.

We were having trouble with the PLL electronics in obtaining a beat note error signal and lock. We can see a beat note of 17 dB clearance above the DN level on the spectrum analyzer.   This post is to document the present configuration and the powers present at the mixer before I start to troubleshoot (aka pull it apart).

Schematic of the PLL electronics stage attached below.  Settings for the Marconi and SR560 are also below. I would noted that there is a low pass filter on the output of the phase detector (which I assume is a mixer) that extracts the (omega_LO - omega_sig) but has nowhere for the dumped (omega_LO + omega_sig) signals to go.  I'm guessing the non-DC signals are getting reflected directly back into the ZRPD-1+. I can't imagine that is great if back reflections start to form parasitic competing signals with the beat note signal.

PLL Loop electronics

Marconi 2023A
Carrier Frequency = 75.526 MHz (close to PD beatnote)
RF Level = +13 dBm
FM Devn 10.0 kHz (set by Ext DC)

SR560
Filter cutoff = DC
Coupling = DC
Source  = A (B port terminated with 50 ohm)
Inv = On
Gain Mode = Low noise
Gain  = 50
 

The spec sheet for the ZRPD-1+ indicates that it is a Level 7 mixer with a damage threshold at 50 mW, so the +13 dBm probably isn't a risk but may be saturating the mixer.  I don't have a calibrated power level for the RF PD input, but it is low (definitely not +7 dBm).  At this stage we either need to improve the beat note or implement some kind of amplification so that both the PD-RF input and LO inputs into the phase detector are matched at the ideal +7dBm level. 

At this stage both PDH control loops are ringing with DC signals that are very angular (and about 10% of the total DC signal). The angluarness would indicate that they are probably both railing somewhere or converting an control-error signal to amplitude modulations.  Previously we have diagnosed this back to imperfect polarization input into the fast control BB-EOMs.  I checked both North and South path and the inputs to the all EOMs (the 14 MHz and fast control) are very well aligned to 's': modulator polarization is not the issue. Antonio had this very well optimized form yesterday.

Tuning laser temperature around on the north path, it is apparent that there are some new/stronger HOMs around the region of our TEM00 modes. This is only a qualitative observation. It is possible that over time our mounts have drifted and the alignment is off.  This is now on my list of things check and to tune up.  

At this stage the PDH loop issues need to be resolved as they are impeding efforts to reliably form an observable beat note.

The process of unlocking and re-configuring for alignment and other types of diagnostic measurements is cumbersome.  There are BNCs popping out of the experiment everywhere that are also mostly unlabeled.  It would be much nicer to have all the useful control signals and PD signals coming out at a central point, preferably next to the computer interface and electronics rack where we lock and tune gains/temperatures.  I will gradually start migrating the cabling over to that side of the table and then install labeled 'patch lines' that are routed to mounted BNC panels around the table. This way there is a central location close to where we perform locking etc but it is relatively easy to patch through signals and oscilloscope trigger lines for temporary oscilloscopes situated around the table. This also means that consistent labeling can be applied.

It is not a great idea to label the cables with label-writer tape or color code them with electrical tape.  From experience the glues in these breakdown in 2-5 year timeframes either flaking off or leaving a sticky residue that is almost impossible to remove with solvents that don't also dissolve the BNC cable plastic. Heatshink can be good as it can be cut off and doesn't have glue. Otherwise a cable tied labels work well. We have some of these.

It also distresses me a little that BNCs cables are routing HV around the table without a clear system for distinguishing these cables. By Murphy's law these will one day be plugged into something sensitive. Some of them are labeled by are not marked clearly as HV. If I can find some red heat-shrink that fits over the ends of the BNC cables I might start labeling HV lines with a consistent markings to lower the risk of mix ups. 

 

 

  1695   Mon Aug 1 19:05:15 2016 awadeNotesElectronics EquipmentSouth path voltages and box move

Notes on voltage settings of the two PDH control loops.

I was examining how I would move the south path control front-end box over to the rack (it is presently under the south side of the table). I noticed that the voltage on the 17 V lines was set to +/- 23.2 V and that the 24V input lines were set to +24.0V and -27.6V for their positive and negative lines.  I'm not sure if there is a reason for this, i.e. whether the south setup requires a greater voltage overhead.  I have turned the voltages down 17 V and 24V for the respective south path power supplies for now to be safe. Also, the HV lines for the south PDH loop were set to 139.4 V and -133.4 V, this sounds OK to me, as long as there are no PZTs being drive to that negative voltage.

Also, the PD on the south path is powered at +14.96/-15.53 V (pulling +39 mA/-18 mA), the north path is set to +16.81V/-16.71 (pulling +78 mA/-50 mA). I'll have a look back through the eLog to see what their speced voltages are and whether they should be the same.

I have not moved the box yet. 

  1696   Wed Aug 3 10:46:46 2016 awadeNotesSafetyOne set of roof lights blown, cracked fluorescent tubes

When I turned the lights on this morning I noticed that the middle set of lights on the south side of the PSL lab table were not working.  This was the case for both circuits (middle bulb is one switch and two outer bulbs are another switch).

On closer inspection it looks like the glass of the fluorescent tubes are actually cracked at the ends.  I didn't want to remove myself given the risk of it cracking and there being broken glass. Also some older bulbs have mercury in them.

I will log a job with maintenance one I work out who to call.

 

  1697   Wed Aug 3 21:45:19 2016 awadeDailyProgressOtherCable reorganisation, north cavity misalignment, PLL questions

Cable reorganization

I have re routed DC coupled cables from various transmission and reflection PDs to a single BNC feed-through on the north side of the table.  The HV amp and FGs have also been moved there so that there is now a central point at which signals can be observed.  This should make some minor diagnostic tests much quicker, especially as the slow controls are operated from this side of the lab. Eventually we can migrate the South path PDH control over to the rack as well, the only issue will be finding a ribbon cable of the appropriate length.  Most of the key points of alignment an polarization tuning before entering the cavity can be reached from that side, so it makes sense not to have various control loops activated from every corner of the bench.

North Cavity Misalignment

I misalignment the north cavity alignment. The visablity was down to 13%, so it needed fixing. I was tweaking this up by tuning the laser crystal temperature close to the refcav resonance and then fast scanning over the resonance.  Unfortunately the temperature of the cavity moved slightly and I ended up walking off without realising that it wasn't spacial mode match but frequency mismatch.  I hope this won't take too long tomorrow to fix, but should be doable; the main issue will be finding the correct frequency to get at least some resonant action. The beat upstream beat note detector can be helpful with this, giving some absolute reference of frequency to work with.

PLL loop

While I am optimizing the PDH loops to hopefully improve the quality of the PLL beat note there is another test measurement that might be good to do. We can use the RF signal from the newly setup upstream beat-note detector to perform a measurement of free running laser frequency noise of the two lasers. I'd imagine we can just calibrate to Hz/sqrt[Hz] and adjust by a factor of sqrt[2] OR opt to lock one laser to a reference cavity and leave the other free running.  This is a measurement that we can't do with the other beat note detector. It is also a good way to test the PLL loop electronics with a sizable beat note signal. 

I'm not sure why the PLL loop didn't have an RF amplifying stage before the phase detector for the beatnote signal.  The mini circuits ZRPD-1+1 data sheet seems to suggest that it should ideally be run with +7 dBm at both RF ports for a ~1V signal. All the references I could find on the eLog suggested a previous beat note magnitude on order of -20 dBm to -30 dBm.  I don't know if there is an advantage to placing all the amplification after the mixer and LPF stage or whether an low noise RF amplifier would improve the signal to noise. This might be old ground people have gone over, I haven't come across it yet in the elog. Important to note is that the isolation between RF ports is about 58 dB at the 70 MHz we have been operating at. This is enough that with the +13 dBm LO (that the present setup was configured to), that a leakage couple back into the PD would be at the same level as our actual beat note.  This would be a concern if back reflections were to create parasitic circuits.

The best situation would be to obtain a better, cleaner beat note and then think about ideal RF powers for our circuitry. 

 

 

  1698   Thu Aug 4 13:33:39 2016 awadeNotesInventoryBorrowed SRS DB64 delayer box from Crackle Lab

Borrowed SRS DB64 delayer box from Crackle Lab.  

Has a set of binary switches in increments of 0.5,1,2,4,8,16,32 ns.  This should be useful in varifying if we have the optimal phase delay for the error signal.  Distance from cavities to PDs has changed but the cabling is the same.  We might as well know that we are opimised rather than trust that it is just OK.

  1699   Thu Aug 4 13:46:08 2016 awadeNotesTempCtrlChecked pins on D9 temp control feedthrough

The original feed-through pin connections into the refcav tank can be found in PSL_Lab/1111.  I checked the resistance across various pins to confirm if the south cavity was really shorted and if the thermistors (I presume that are thermistors) are accessible and working.

Resistance measured across pines (as of Thu Aug 4 13:40:13 2016) are as follows:

1-6: heater on cavity (south): 85.6 Ohm

3-7: Temp sensor on cavity (south) 19 MOhm

4-8: heater on cavity (north) 156.8 Ohm

5-9: Temp sensor on cavity (north) 19 MOhm

It appears that the south cavity does have heating actuation (this resistance value also matches up with the PSL_Lab/1111 post).  Perhaps the short people are talking about is intermittent.  Or maybe nobody actually knows what is going on here. The above is what I measured.

  1700   Thu Aug 4 15:15:32 2016 AidanSummaryTempCtrlRTDs used in aLIGO TCS RHs

These are the RTDs we use in aLIGO TCS

http://www.omega.com/Temperature/pdf/1PT100KN_2PT100KN.pdf

 

  1701   Thu Aug 4 18:01:03 2016 ranaSummaryTempCtrlRTDs used in aLIGO TCS RHs

The heater driver in the old design (iLIGO PSL) is this horrible programmable power supply with high output noise (see measurement of 40m PSL heater).

What we want instead is something that takes in +/- 10 V and drives a DC current (not PWM) into our heaters (which are ~50-100 Ohms). A BUF634 is almost good enough; it can do 200 mA at 10 V. Is there a BUF634 equivalent which can do more like 500 mA? Otherwise we can just use a opamp + transistor.

What is being used for the ring heaters at the LIGO sites?

  1702   Fri Aug 5 00:08:16 2016 awadeSummaryTempCtrlRTDs used in aLIGO TCS RHs

I assume the key parameters are the noise characteristics here.  They seem to commonly quote the input noise voltage which I'm guessing gives a measure of the equivalent noise injected by the buffer itself. 

Data sheets list the input noise voltage of the BUF634 as 4  nV/√Hz @ 10 kHz . There is another buffer (300 mA), the LMH6321, its datasheet claims  input voltage noise 2.8 for >=10 kHz. I don't know what this means for noise at mHz.

Is there a reason we can't just buffer with more general purpose high current OP Amp? Is the the noise just a killer for that whole catalog of devices? Looking at a random selection, all the ones I've looked at have higher input noise voltage values but the digi-key product selector doesn't let me filter for noise characteristics so maybe there are low noise ones. 

 

 

 

Quote:

The heater driver in the old design (iLIGO PSL) is this horrible programmable power supply with high output noise (see measurement of 40m PSL heater).

What we want instead is something that takes in +/- 10 V and drives a DC current (not PWM) into our heaters (which are ~50-100 Ohms). A BUF634 is almost good enough; it can do 200 mA at 10 V. Is there a BUF634 equivalent which can do more like 500 mA? Otherwise we can just use a opamp + transistor.

What is being used for the ring heaters at the LIGO sites?

 

  1703   Sun Aug 7 23:27:01 2016 awadeDailyProgressOtherRecovering alignment of north refcav and optimisation of PDH error signal

After misalignment the north path (PSL:1697) today I installed some irises and painstakingly walking the beam back into alignment.  Then, scanning the laser PZT and stepping through crystal temperature, I found the TEM00 mode and realigned.  The visibility attained was 20.2 % (2.41 V off res., 1.60 V on res.): this is marginally better than the 13% earlier reported.

Power going into the north refcav now is measured to be 1.679 mW with 1.489 mW returning when off resonance, this is a 12 % round trip loss.  Not sure what is happening there, something to check on later.

Transmission signal on the Thorlabs PDA10CS transmission PD is 3.00V on resonance and 9.40 mV off resonance (there is a dark offset of 9.40 mV).  Side bands (the two 14.75 MHz PM side bands) were 37.8 mV, this is 28.4 mV above the dark voltage. Thus an estimate of the modulation conversion to sidebands is 0.9% of the carrier.

I found the error signal at the monitor port of PDH control box to be -346 mV to +432 mV (~Vpp=770 mV).  It was also not very symmetrical, especially for the side band components. (sorry no picture, I don't have a working floppy disk).

As the RF PD for the PDH loop have been move in distance from the refcavs since the last implementation by Tara and Evan, we should expect the effective delay between RF and LO to have changed. However, the cabling has been kept the same. Using a SRS DB64 delayer box in the LO electrical path, the error signal could be improved to -832 mV to +684 by delaying the signal by 20 ns: this effectively doubled the error signal peak-to-peak. This may explain some lost gain in the control loops.  It is good practice to start with the best possible error signal anyway, so I made some adjustments. There was a 3 m SMA cable in the PD RF line that I replaced with a 1 m cable. I then swapped out a short 20 cm SMA in the LO line for a 2.2 m BNC (not ideal changing types, by all I had for a single length for now).  Thus I affected a change of 4 m between the paths, increasing the phase delay by ~20 ns (waves travel~2/3 c in coaxial cables). The resulting error signal was -800 mV to 552 mV. This is not as much as I'd hopped, by I think the mounts may have relaxed in the time it took me to optimize cable length.

Tomorrow I will try to look at south path PDH optimization.

 

-awade, Sun Aug 7 23:26:49 2016

 

 

  1704   Thu Aug 11 16:25:44 2016 awadeMiscDocumentationCertificates PO 75-S290726 and 75-S291757

Edit: I have moved the compliance docs to a private location.

 

Items for PO 75-S290726 and 75-S291757 have arrived.  

The following labels were generated and stuck to the box for future mounting
PBSO-1064-050
PLCX-25.4-1030.2-UV-1064
QWPO-1064-05-2-R10 x4
 

Items have been put in the lab and their certificates of compliance are attached here, on the wiki and their hard copies are in the lab.

 

  1705   Sat Aug 13 20:41:34 2016 awadeDailyProgressOtherSouth cavity error signal troubleshooting and cavity alignment

The south path cavity was realigned and I fixed the control box error signal monitor issues.

The south path alignment had drifted over time and in the course of aligning the north cavity I had touched the south periscope knobs in error.  After realignment the reflection signal dip on resonance was V_min,V_max = (304.0,80.0) mV on reflection => Visibility of 58%. This was about the same as before*. 

In the past we found that that there was no error signal coming out of the mixer monitor BNC on the front panel box.  In the course of checking the power supply configuration, before moving it to the rack on the north side of the table, I noticed that the common banana plug pins on the +/-17 V and +/-24 V were very wobbly.  In fact they were barely making contact.  Banana plugs are terrible. This can be a big problem in general if the 24V lines disconnect as HV amps can often fiz without their lower voltage supplies connected. 

Also, none of the power supplies were grounded to a common ground, which might open us up to ground loop noise, depending on the details of the grounding arraignments inside the FSS boxes themselves.  We also don't want them floating all over the place at low frequencies as this may have unintended consequences for noise. I have no experience of this being a problem as I have always just grounded things properly, better to do it and be safe than sorry.

On another note, one of the HV power supplies (the +ve one) was grounded with what turned out to be a piece of solder linking the front panel pins: bad.

HV amp grounded with piece of solder

I have now removed a lot of the thin wires grounding the common pins on each power supply replaced them with thick wire (better to go with super thick wire) and grounded the front panels together using brand new solid banana plug patch cables (good to many amps).  I snipped off the old green banana plugs a soldered on new ones and added two diameters of heat shrink to provide some strain relieve for the joint.  Still don't think this is good enough, but it will do for now.

After doing all this and powering back up I saw a decent error signal at the error signal monitor port. There might have been an issue with one of the low voltage supplies not having been referenced properly to a common ground. We now see an error signal of order 744 mV peak to peak.  

Also, I got a delayer box and checked the phase delay of the LO relative to the reflection PD RF signal to see if we had the best possible error signal. In the south path the demodulation quadrature was correct and no cable changes were needed.  I didn't get to locking or measuring a transfer function, but this should be done to characterize performance and give us an idea of what to optimize next. All of this is leading to a better quality beat note and is good to have on record for future design iterations.

- awade Sat Aug 13 20:41:28 2016

*Note I have used a different methadology FringVis = (V_max-Vmin)/(V_max+V_min), this should give the actual contrast rather than ratio of peak to total power.

  1706   Sun Aug 14 21:24:11 2016 awadeMiscTempCtrlKapton stick on resistive heaters

 

Edit: Datasheet was removed.

We have some kapton heaters, I found them in a draw when cleaning. These should be useful for our new shield temperature control.

These are from the kit KH-KIT-EFH-15001.  I don't know how old they are or if Kapton stickiness goes off, but here is a list of remaining available sick on resistive heaters and their Ohmage:​

Kapton resistive heaters in stock
Quantity Dimensions  Resistance [Ohm] Shape
1 0.75"x2.5" 23.5 Rectangular
1 0.75"x3.25" 33.2 Rectangular
1 1"x3" 40 Rectangular
2 2"x3" 103.7 Rectangular
2 2.5"x2.5" 108.6 Rectangular
2 1.5"x8" 252.0 Rectangular
3 3"x3" 153.7 Rectangular
1 1.5"x4.5" 120.3 Rectangular
1 6"x1" ? Rectangular
2 1.5"D 32.8 Circular
1 2"D 59.4 Circular
1 3"D 169.0 Circular

 

Data sheet is attached. Note that heating ranges from 3-8 W per square inch at the 12 V supply level for various different choices of shape and wire pattern.

 

  1707   Thu Aug 25 07:36:06 2016 ranaMiscTempCtrlKapton stick on resistive heaters

I think we'll need a sketch of the physical layout in order to do the plant modeling for the heating loop. OzGraffle.

  1708   Sat Aug 27 17:42:40 2016 awadeSummaryTempCtrlHeat load of vacuum can held above room temperature.

Here are some numbers on the heat radiated away from the vacuum can by leakage through the foam insulation and from the exposed metallic parts. The heat loss is dominated by the foam as it has an larger surface area. However, these numbers are maybe a little rough as they don't account for the Al foil cladding on the outside and ignore the details of the cylindrical geometry.  

I've been working on documenting the thermal aspects of the vacuum can.  Info is spread across the elog in various places but not in one place.  This stuff is gradually getting added to the wiki which will be the central collecting point for information to avoid this iterative amnesia.  I am also almost finished on a graphic that summarizes the setout of the vacuum can and its sensors. This is just the heat loading calculations.

The tank is 22" long with 8.3" tube diameter with two 10" flanges on the ends.  This apparatus is clad all around by 2" (average) thick CertiFoam 25 see post PSL:178 for characteristics (note that Frank's values are for 1" thickness only, this must be scaled to the thickness used). Total dimensions of the foam box are 12x12x36" giving a surface area of ~1.3 m^2.  For a tank held at 35 C above room temperature 20 C this is 11.3W of heat loss.

At the top of the vacuum can there are three half nipples welded along the top, these hang out above the insulation as access is needed for the turbo/roughing pump connection, the ion pump and the sub-D 9 feedthrough. The exposed surface is a mix of shiny stainless steel and matt/sandblasted bits.  Shiny and matt exposed areas are respectively 0.0296 m^2 and 0.0168 m^2 (not including the ion pump) which is not big.  With emmisivities of 0.09 and 0.18 for these two surface types we get a total of 0.5265 W radiative heat dissipation for a tank held at 35 C above 20 C room temperature.  

Thus total estimated heat for a 35 C tank is 11.78 W. We don't need to run it at this temperature but I use it as a reference value.  See attached graph for heat loss as a function of vacuum can temperature.

A summary of these numbers and details is in the attached matlab file.

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For reference the tank has 4 resistive heating mats wrapped around it. The small ones near the ends are 30 Ohm and the two larger ones near the center are 70 ohms each. These have been connected in parallel+ series network that gives a total resistance of 50 Ohms and can be driven with up to 115 V.  To just maintain the tank at 35 C we would need 24.5 V with ~0.5 A. This seems like a lot but is almost doable with available OPAMP buffers.

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The next step is to work out heat capacity. I can't find design drawings for the tank itself on the elog/wiki, maybe its too far back in ancient history. There is a solidworks drawing on the SVN but was made in the student version of solidworks so is very buggy.  I will try an extract numbers to get an idea of the mass of metal in the tank. 

Also in progress is a step function measurement of tank cooling.  I spent some of this week working out how to integrate a new RTD acromag card into our existing EPICS setup so we can log the temperature drop after heating is turned off.  This took a while as I was unfamiliar with this kind of setup and also the power supply turned out to be not doing what I thought it was. The voltage current source is now hooked up and a thermister fitted for logging temperature. This measurement should give us some more grounded numbers on the real characteristics of the tank thermal decay rates.

 

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