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ID Date Author Type Category Subject
7837   Mon Dec 17 11:20:58 2012 JenneUpdateSUSBeam dumps on vertex oplevs removed

I'm not sure when this was done, but there were beam dumps in front of the lasers for BS/PRM oplevs as well as ITMY/SRM oplevs.  MICH wasn't holding lock very nicely, so I poked around, and the Sum values for all of these optics' oplevs seemed too low, so I went to look, and found dumps.  I have removed these, and now BS and ITMY oplevs are back to normal.  (PRM and SRM are still misaligned right now, so I'll check those later, but they should be fine).

BS's oplev has been enabled while non-existant, at least for the whole weekend, since I found it enabled.  ITMY I found misaligned, so it's oplev servos were off.

In other news, we should get back in the habit of restoring all optics before we leave for the night / whenever locking activities are finished.

12066   Thu Apr 7 12:51:24 2016 gautamUpdateendtable upgradeBeam height differences

Steve has finished installing the enclosure on the new endtable. So Eric and I decided to try and lock the X arm and measure the beam height of the transmitted IR beam relative to the endtable. We initially thought of using POX DC as a the LSC trigger but this did not work as there was no significant change in it when the arm was flashing. Eric then tried misaligning the ITM and using AS110 as a trigger - this worked. We then recompiled the ASS model to take AS110 as an input, and ran the dither alignment. After doing so, I measured the beam height at two points on the new endtable.

Bottom line:

• The beam is roughly level across the table (along the North-South direction, within the precision to which I could place the irides and measure the height). The table has also been levelled pretty well...
• The beam height is ~4.7" across the endtable

So the beam is about 0.7" higher relative to the endtable than we'd like it to be. What do we do about this?

• Is it even possible to raise the table by 0.7" so we can have a level beam everywhere? Are there some constraints related to how the enclosure is attached to the window?
• Are we okay with tolerating a solution where we keep the beam level at 4", and use Y10 and Y11 (see layout in elog 12060) to raise the beam by 0.7", and then have slightly higher posts for the optics downstream of this point?

I've also placed two irides extending the cavity axis on the endtable. These should be helpful in aligning the green to the arm eventually.

4222   Fri Jan 28 13:07:31 2011 JenneUpdateIOOBeam is back on the WFS

The MC WFS have had beam dumps in front of them for the past ~2 weeks, until I could find the appropriate optic to put in the WFS path, to avoid melting the WFS' electronics.

Koji noted that Steve had a W2-45S in a secret stash near his desk (which Steve later had put into the regular optics storage shelves down the Yarm), so I used that in front of the black hole beam dump on the AS table.  Now the beam is ~1W reflected from the unlocked mode cleaner, and ~100mW goes to the MC REFL PD.  The other 900mW now goes to this W2, and only ~5mW is reflected toward the MC WFS.  Most of the 900mW is transmitted through the window and dumped in the black hole.  There is a ghost beam which is reflected off the back surface of the wedged window, and I have blocked this beam using a black anodized aluminum dump.  I will likely change this to a razor dump if space on the table allows.  I have aligned the beam onto WFS1 and WFS2, although I did not re-align the mode cleaner first, so this alignment of the WFS will likely need to be redone.

WFS1 has about 2mW incident, and WFS2 has about 3mW incident, when the mode cleaner is unlocked.  I have not yet measured the power incident when the MC is locked, although obviously it will be much smaller.

Except that I might temporarily remove one of the WFS for more quantum efficiency measurements later today, the WFS should be ready to turn back on for alignment stabilization of the mode cleaner.

 Quote: My goal this afternoon was to measure the quantum efficiency of the MC WFS.  In the process of doing this, I discovered that when I reverted a change in the MCWFS path (see elog 4107 re: this change), I had not checked the max power going to the WFS when the MC unlocks. Current status: MC locks (is locked now).  No light going to WFS at all (to prevent MC WFS french-fry action).  Quantum Efficiency measured. The Full Story: Power to WFS: Rana asked me to check out the quantum efficiency of the WFS, so that we can consider using them for aLIGO.  This involves measuring the power incident on the PDs, and while doing so, I noticed that WFS1 had ~160mW incident and WFS2 had ~240mW incident while the mode cleaner was unlocked.  This is bad, since they should have a max of ~10mW ever.  Not that 200mW is going to destroy the PD immediately, but rather the current out, with the 100V bias that the WFS have, is a truckload of power, and the WFS were in fact getting pretty warm to the touch.  Not so good, if things start melting / failing due to extended exposure to too much heat. The reason so much power was going to the WFS is that it looks like Yuta/Koji et. al., when trying to use the WFS as a MC1 oplev, changed out 2 of the beam splitters in the MC WFS / MC Refl path, not just one.  Or, we've just been crispy-frying our WFS for a long time.  Who knows?  If it is option A, then it wasn't elogged.  The elog 3878 re: BS changeout only mentions the change of one BS. Since the MC Refl path has a little more than ~1W of power when the MC is unlocked, and the first BS (which was reverted in elog 4107) is a 10% reflector, so ~100mW goes to the MC Refl PD, and ~900mW goes to the MC WFS path.  In front of a Black Hole beam dump was sitting a BS1-33, so we were getting ~300mW reflected to be split between the 2 WFS, and ~600mW dumped.  The new plan is to put a W2 window in place of this BS1-33, so that we get hopefully something like 0.1% reflected toward the WFS, and everything else will be dumped.  I could not find a W2-45S (everything else is S, so this needs to be S as well).  I found a bunch of W2-0deg, and a few W2-45P.  Does anyone have a secret stash of W2-45S's???  To avoid any more excessive heat just in case, for tonight, I have just left out this mirror entirely, so the whole MC WFS beam is dumped in the Black Hole.  The WFS also have aluminum beam dumps in front of them to prevent light going in.  None of this affects the MC Refl path, so the MC can still lock nice and happily.

10325   Fri Aug 1 22:56:27 2014 KojiUpdateGeneralBeam lost in the chamber???

I was investigating several issues on the IFO. As many of you noticed and not elogged, ITMX had frequent kicking without its oplev servo.
Also I had C1:LSC-TRY_OUT flatted out to zero even though I could see some fringes C1:SUS-ETMY_TRY_OUT.

Restarted all of the realtime models (no machine reboot).

Now I don't find any beam on REFL/AS/POP cameras.

If I look at BS-PRM camera, I can see big scattering, the beam is in the BS chamber.
I jiggled TT1 but cannot find neither a Michelson fringe nor POP beam.

So far I can't figure out what has happened but I'm leaving the lab now.

IMC is locked fine.
I can see some higher order mode of the Yarm green, so the Y arm alignment is no so far from the correct one.

11839   Wed Dec 2 17:14:33 2015 yutaroUpdateLSCBeam on POX11 is possibly not centered well

I checked how POXDC level changes when the angle of ITMX is varied. ETMX was misaligned.

Then I found that in YAW direction the POXDC level is maximized but it doesnt have plateau, and in PIT direction it is not maximized so that it is at the slope and it doesnt have plateau, as shown in attached figures. These results indicate that the beam size on POX11 is not small enough compared to the size of the diode and it is not centered well.

Attachment 1: 47.png
Attachment 2: 41.png
11847   Fri Dec 4 12:33:52 2015 yutaroUpdateLSCBeam on POX11 is possibly not centered well

To focus POX beam on POX11 PD, I added an iris and a lens before POX11 PD as you can see in Attachment 1.

It seemed that the beam is well focused, but the behavior of POXDC has not changed, as shown in Attachments 2 & 3.

Attachment 1: image1-3.JPG
Attachment 2: 07.png
Attachment 3: 47.png
11850   Fri Dec 4 23:02:13 2015 yutaroUpdateLSCBeam on POX11 is possibly not centered well

[yutaro, Koji]

Now, the beam on POX11 PD is well centered and well focused.

We found out why POXDC had behaved as reported in elog 11839. There were a few reasons: the beam was not focused enough, hight of a mirror was not matched to the beam well, path of the light reflected by misaligned SRM was occasionally close to the path of POX beam.

Then, What we did is following:

- changed orientation of SRM slightly

- changed the hight of the mirror whose hight had not matched well, by changing the pedestal (hight of which mirror was changed is shown in Attachment 1.)

- put a lens with f=250 mm (where the lens is located is shown in Attachment 1.)

- refined alignment for the POX beam to hit on the center of POX11 PD.

As a result, POX DC level behaved as shown in Attachment 2&3 when the orientation of ITMX was varied (Attachment 2: POX DC vs ITMX PIT, Attachment 3: POX DC vs ITMX YAW).

You can see broad plateau when varied in both PIT and YAW directions, and the beam is at the center of the plateau if ITMX is aligned ideally.

Attachment 1: image1.JPG
Attachment 2: 56.png
Attachment 3: 04.png
8331   Fri Mar 22 01:28:56 2013 ManasaUpdateLasersBeam profile of NPRO from ATF

The NPRO from ATF has been installed on the POY table.

I have been making measurements to characterize the beam profile of this laser. I am using an AR coated laser window as a beam sampler at 45deg and the razor blade technique to measure the beam size along z. Details of the procedure along with analysis and results from this will follow.

12081   Mon Apr 18 00:29:00 2016 gautamUpdateGeneralBeam profiling + injection current scan

Summary

I've finished up the remaining characterization of the repaired 1W Innolight NPRO - the beamscan yielded results that are consistent with an earlier beam-profiling and also the numbers in the datasheet. The output power vs diode current plot is mainly for diagnostic purposes in the future - so the plot itself doesn't signify anything, but I'm uploading the data here for future reference. The methodology and analysis framework for the beamscan is the same as was used here.

Attachment #1 - Beam-scan results for X-direction

Attachment #2 - Beam-scan results for Y-direction

Attachment #3 - Beam profile using fitted beam radii

Attachment #4 - Beam-scan data

Attachment #5 - Output power vs Injection current plot

Even though I remember operating at a diode current of 2.1A at some point in the past, while doing this scan, attempting to increase the current above 2.07A resulted in the "Clamp" LED on the front turning on. According to the manual, this means that the internal current limiting circuitry has kicked in. But I don't think this is a problem as we don't really even need 1W of output power. This is probably an indicator of the health of the diode as well?

Attachment #6 - Output power vs Injection current data

It remains to redo the mode-matching into the doubling oven and make slight modifications to the layout to accommodate the new laser + beam profile.

I plan to do these in the morning tomorrow, and unless there are any objections, I will begin installing the repaired 1W Innolight Mephisto on the X endtable tomorrow (18 April 2016) afternoon.

Attachment 1: BeamScan_x.pdf
Attachment 2: BeamScan_y.pdf
Attachment 3: ZScan.pdf
Attachment 4: BeamScan.mat
Attachment 5: Innolight_Current_Scan.pdf
Attachment 6: Innolight_Current_Scan.mat
511   Mon Jun 2 12:20:35 2008 josephbBureaucracyCamerasBeam scan has moved
The beamscan has been moved from the Rana lab back over to the 40m, to be used to calibrate the Prosilica cameras.
7367   Sat Sep 8 00:04:53 2012 JenneUpdateGeneralBeam scan measurement plan - to do Monday morning.

[MikeJ, Jenne]

We have a plan for how we're going to measure the beam after PR3.  Mike is going to write up a nifty program that will spit out the waist of the beam if you give it a bunch of razor blade measurement data.

Since the beam bounced off of the pitched ITMX is coming out of the chamber so high, it's kind of a pain to setup optics to steer the beam down the walkway next to the Yarm.  So, I have a new vision.

I think that we can get the beam right after PR3 onto the PRM/BS oplev table using 3 clean mirrors (of which we have many spares, already clean).  Once on the oplev table, we can put a 2" Y1 mirror to steer the beam down the walkway, after taking off the short east side of the table.  Then we can use the little breadboard on the mobile blue pedestal for the razor blade / power meter setup.

The razor blade on a micrometer translation stage will be the first thing on that table that the beam sees. Then, a 2" lens to get the beam small enough to fit on the power meter.  Then, obviously, the power meter.  We can measure the distance between the oplev table and the razor blade using the laser range finder, which has pretty good accuracy (it's sub-centimeter, but I don't remember the exact number for the precision).

A lens is not okay if we're trying to get the beam directly onto the beam scanner, since it will distort the beam.  However, as long as the razor blade is before the lens, and we're just using the lens to get the full intensity of the non-obscured part of the beam onto the power meter, I think using a lens should be fine.  If we don't / can't use a lens, we're going to run into the same problem we have with the beam scanner, since the power meters all have a fairly small aperture.  Even the big 30W power meter's aperture will be on the order of the size of the beam, so we won't be able to guarantee non-clippage.

The main problem I see with the technique as I have described it, is that the beam is going to hit 4 mirrors (3 in-vac, one outside) before going to the razor/lens/power meter.  We have to make sure that we're not clipping on any of those mirrors.  Also, this measurement version takes the beam after PRM, PR2 and PR3, but not after the BS and ITM.  I don't think we're concerned with either of those 2 optics, (especially since this is refl off the front of the BS, so won't see any potential clipping on the BS cage), but just in case we are, this measurement isn't so useful, and we'd have to come up with a different way of placing the mirrors on the in-vac tables to get a beam bounced off  of  a yaw-ed ITMX.

Perhaps it would be easier to just go with the pitched ITMX version of the measurement, but I could use some ideas / advice on how to mount mirrors and lenses ~4 feet off the ground outside of the chambers, and not have them waving around on skinny sticks.

EDIT: Another idea is to instead use the beam transmitted through the BS, put a single clean steering mirror in the ITMY chamber, and get the beam out of the ITMY door.  This could either be the beam before the ITM, or we could yaw the ITM a little and take the reflected beam.

7369   Mon Sep 10 08:50:35 2012 SteveUpdateGeneralBeam scan measurement plan - to do Monday morning.

I misaligned ITMX pitch on Friday and brought out the beam at 44" height. The beam was bouncing to much. I only realized it this morning why. The OSEM voltages are 1.8, 1.7, 0.2 and 0.9V  Even with a stable 8-9 mm diameter beam you would be clipping

on the beam scanner 9 mm aperture. You can bring out the beam with one mirror right after  PR3, just remove  PRMOP2

14018   Tue Jun 26 10:50:14 2018 poojaUpdateCamerasBeam spot tracking using OpenCV

Aim: To track the motion of beam spot in simulated video.

I simulated a video that moves the beam spot at the centre of the image initially by applying a sinusoidal signal of frequency 0.2Hz and amplitude 1 i.e. it moves maximum by 1 pixel. It can be found in this shared google drive link (https://drive.google.com/file/d/1GYxPbsi3o9W0VXybPfPSigZtWnVn7656/view?usp=sharing). I found a program that uses Kernelized Correlation Filter (KCF) to track object motion from the video. In the program we can initially define the bounding box (rectangle) that encloses the object we want to track in the video or select the bounding box by dragging in GUI platform. Then I saved the bounding box parameters in the program (x & y coordinates of the left corner point, width & height) and plotted the variation in the y coordinates. I have yet to figure out how this tracker works since the program reads 64*64 image frames in video as 480*640 frames with 3 colour channels and frame rate also randomly changes. The plot of the output of this tracking program & the applied signal has been attached below. The output is not exactly sinusoidal because it may not be able to track very slight movement especially at the peaks where the slope = 0.

Attachment 1: cv2_track_fig.pdf
3864   Thu Nov 4 17:57:27 2010 steveUpdateGeneralBeamScanner is working again

Koji, Suresh and Steve,

The beam scanner is back from repair.  Koji and Suresh helped to move the I/O address jumpers on the interface card from default position  300 to 320

It is working again.

Attachment 1: P1070024.JPG
14058   Thu Jul 12 15:15:47 2018 SandrineUpdate Beat Note Measurements for Cavity Scans

(Gautam, Sandrine)

We calculated the expected power of the beat note for Annalisa's Y arm cavity scans.

Beat Note Measurement

We began by calculating the transmitted power of the PSL and AUX. We assumed that the input power of the PSL was 25 mW and the input power of the AUX was 250 uW. We also assumed a loss of 25 ppm for the ITM and ETM. We used T1 = 0.0138 and T2 = 25 x 10-6.

$P_{t} = \frac{t _{1}^{2}t_{2}^{2}}{1+r_{1}^{2}r_{2}^{2}-2r_{1}r_{2}}$

$t = \sqrt{T}$

$r = \sqrt{1-T-L} = {\sqrt{R}}$

The transmitted power of the PSL is approximately 100 uW, and the transmitted power of the AUX is approximately 0.974 uW.

$P_{t}^{PSL} = 100 uW$                          $P_{t}^{AUX} = 0.974 uW$

The beat note was calculated with the following:

$P_{beat} = 2\sqrt{P_{PSL}P_{AUX}} = 20 uW$

The  expected beat note should be approximately 20 uW.

10333   Tue Aug 5 19:05:41 2014 AkhilUpdateGeneralBeat Note Testing on EPICS Channels

Finally,  the efforts put in the Frequency Counter paid off . I tested the working of both the FC and EPICS channels that I created by displaying the beat note on MEDM screens. EricQ helped me locking the X arm ( Y arm free) thus acquiring only the X arm beat note from the frequency counter. We plotted the beat note on MEDM and clearly could see a stable beat note when the arm was locked. Now it can be said that the FC(two of course) can replace the spectrum analyzer outside and also get the beat-note frequencies  into EPICS channels. The channel names of these two beat note frequencies are:

X Arm:          C1:ALS-XBEAT_FREQ_MHZ

Y Arm:          C1:ALS-YBEAT_FREQ_MHZ

(Note: There are many problems in alignment of the arms and we could have beat note only for some time after putting a lot of effort).

8721   Wed Jun 19 01:45:49 2013 ManasaUpdateGreen LockingBeat frequency sweep for 3FSR

Measurements:

1. Calibrating offset :

I measured the shift in the beat frequency while scanning through the offset. Offset stepped by 50 resulted in 1MHz shift of the beat frequency.

2. Anti-whitening filter for beatbox output:

I made an anti-whitening filter for the beatbox output in the ALS_BEATX_FINE_I module by inverting the whitening filters that Jamie had installed in the beatbox earlier (elog).  I have kept the old anti-whitening filter in the module as well for the time-being because the new anti-whitening filter was not as good as the old one in stabilizing the servo (large error signals and unstable ALS).

3. Beat frequency scan for 3FSR:

With ALS loop enabled, I did an offset sweep corresponding to 3FSR (FSR = c/2L = 3.7MHz). The loop doesn't seem to be stable enough to reduce the arm fluctuation to get a resonance for IR. Time series of scan is shown below:

4. No-loop and in-loop spectrum:

I measured the spectrum of the error signal (C1:ALS-BEATX_FINE_I_IN1) with ALS loop enabled and disabled. To suppress the peaks at 3.2Hz and 16.5Hz, I turned ON the corresponding filters. I have recorded the error signal spectrum with only 16.5Hz res gain filter turned ON. Turning ON res gain 3.2Hz filter kicked ETM.
Spectrum of error signal shown below:

To resolve:

1. What is wrong with the new anti-whiteing filter?

2. Why would the res gain filters kick ETM and show no noise suppression?

11470   Thu Jul 30 15:58:00 2015 ericqUpdateLSCBeat note Alignment fluctuation effects measured
 However, I wonder how much of the low frequency noise can be explained by instability of the beat alignement on the PSL table, and how this might be quantified.

I followed my hunch, and the truth comes out.

I recalled that the aLIGO demod board has a handy DB9 output on the back panel for the detected power at the RF and LO inputs. I hooked this up into the BEATY ADC channels while checking the ALSX spectrum in IR lock.

This is assuredly the limiting factor in our ALS sensitivity.

Note: I'm calling the fluctuations of the beatnote amplitude "RF Amplitude RIN," to put things in reasonble units. I haven't looked up the board's conversion of dBm to V, but the LO should be around 0dBm in this measurement.

The coherence between the phase tracker output and the LO amplitude is significant over a broad range, mostly dipping where real cavity motion peeks up into the spectrum.

Also, the feature from 10-100Hz in the RIN spectrum is one I've often seen directly in ALS spectra when beatnote alignement is bad or the beatnote frequency is high, convincing me further that this is what's to blame.

So: what do we do? Is there anything we can do to make the green alignment more stable?

Attachment 1: RF_RIN.png
Attachment 2: RF_RINspec.png
Attachment 3: RFampCoh.xml.zip
11478   Tue Aug 4 03:02:30 2015 ericqUpdateLSCBeat note Alignment fluctuation effects measured

Notes from tonight's work:

• PMC alignment tweaked. Not much gained
• WFS/MC2 offsets tweaked after recentering beams on WFS and some hand alignment.
• Vertex oplevs realigned for the first time in forever
• With an RF coupler, measured the X green beatnote to be +5dBm into the splitter. This resulted in -33dBm at the control room analyzer.
• Switched the ALS demod board inputs, from piping the delayed signal to the RF input, to sendingit to the LO input. This was motivated by wanting the mixer closer to compression, hopefully to reduce beatnote amplitude fluctuation sensitivity. This won some noise >100Hz.
• This led to record ALS noise levels - X:217Hz, Y:203Hz
• +2dBm into the board still leaves us some headroom for futher amplification. Board schematic lists +10dBm LO as "nominal," but maybe this isn't worth it...
• PRFPMI locking is still stalled at bringing in the RF signals. Debugging continues.
• Some beatnote amplitude fluctuation investigations (see below)
• Note to self: demod board schematics include an unspecified RF lowpass. Check out what got stuffed in there.

I've explored the beatnote fluctuations a bit further.

First, I realized that we already had a channel than functions much like an RF level monitor: the phase tracker Q output. I verified that indeed, the Q signal agrees with the RF monitor signals from the demod board within the phase tracker bandwidth. This simplifies things a little.

I also found that the Y beat suffers a fair bit less from these effects; which isn't too surprising given our experience with the alignment stability.

One possible caveat to my earlier conclusions is that the beatnote amplitude could be fluctuating due to real RIN of the green light transmitted through the cavity. In fact, this effect is indeed present, but can't explain all of the coherence. If it did, we would expect the DC green PDs (ALS-TR[X/Y]) to show the same coherence profile as the RF monitors, which they don't.

The next thing I was interested was whether the noise level predicted via coherence was realistic.

To this end, I implemented a least-squares subtraction of the RF level signal from the phase tracker output. I included a quadratic term of the RF power, but this turned out to be insiginficant.

Indeed, using the right gain, it is possible to subtract some noise, reproducing nearly the same spectrum as the coherence based estimate. The discrepency at 1Hz is possible from 1Hz cavity RIN, as suggested by the presence of some coherence with TRX.

However, this is actually kind of weird. In reality, I would've expected the coupling of RF level fluctuations to be more like a bilinear coupling; changing the gain of the mixer, rather than directly introducing a linearly added noise component. Maybe I just discovered the linear part, and the bilinear coupling is the left over low frequency noise... I need to think this over a little more.

Attachment 1: coherences.png
Attachment 2: linX.png
4502   Thu Apr 7 21:58:57 2011 AidanSummaryGreen LockingBeat note amplitude

Having convinced myself that the green Hartmut PD is giving an acceptable response at RF frequencies I decided to double-check the beatnote at IR (fiber transmission from the X-end beating with the PSL). This took a while because I had to realign the beam into the fiber at the X-end (I had a PD monitoring the output from the fiber on the PSL table and 40m of BNC cable giving me the signal from it at the X-end).

Eventually, I managed to get a beatnote on the PD. At first there was no signal at the temperature calculated using Koji and Suresh's calibration, but it turned out that the mode-overlap wasn't good enough on the PD. Now I can clearly see beats between a couple of modes, one of which is much stronger than the other. I think we should use a frequency discriminator on the output from the IR PD to servo the end laser and keep the strong beat note within <100MHz of DC.

4534   Fri Apr 15 22:54:20 2011 Aidan, BryanUpdateGreen LockingBeat note amplitude on Vertex PD

I was investigating the beat note amplitude on the vertex PD again yesterday. The incident power on the PD was 150uW in the PSL green beam and 700uW in the X-ARM green beam. With perfect overlap and a transimpedance of 240, I expected to get a beat note signal of around 25mV or -19dBm. Instead, the size was -57dBm. Bryan and I adjusted the alignment of the green PSL beam to try and improve the mode overlap but we couldn't do much better than about -50dBm. (The noise floor of the PD is around -65dBm).

When we projected the beams to the wall of the enclosure, the xarm beam was 2 to 3x as large as the PSL green beam, indicating that the beam size and/or curvatures on the PD were less than ideal. There is a telescope that the XARM beam goes through just before it gets to the PD. I mounted the second lens in this telescope on a longitudinal translation stage. With some finagling of the position of that lens we were able to improve the beatnote signal strength to -41dBm.

Obviously the ideal solution would be to measure the beam size and RoC of the PSL beam and XARM beams and then design a telescope that would match them as precisely as possible because there's still another 20dB signal strength to be gained.

8386   Mon Apr 1 23:22:17 2013 AnnalisaUpdateAuxiliary lockingBeat note between "Alberto" NPRO laser and PSL laser

I measured the beat note between the "Alberto" NPRO laser and the PSL varying the PSL temperature and find the matching NPRO temperature that gave the beat.

I first switched off the FSS loop for the PSL, then I varied its temperature and switched on the loop back.

PSL temperature has been varied starting from 31.88 °C (its starting temperature) down to 23.88 by 1°C step, and then from 31.88 °C up to 36.92 °C, always with a 1°C step.

For each PSL temperature, the NPRO temperature was varied as well, in way to find the temperature to have a beat note between the two.

The trend of the NPRO laser temperature reminds the frequency change of the laser as a function of the crystal temperature continuous tuning.

I made measurements only for the first temperature of the NPRO laser which gave me the beat note. Tomorrow I'm going to find the beat note also for higher frequencies of the NPRO laser.

Attachment 1: Beat_Note.jpg
8396   Tue Apr 2 22:39:17 2013 AnnalisaUpdateAuxiliary lockingBeat note between "Alberto" NPRO laser and PSL laser

The beat note between the PSL laser and the "Alberto" NPRO laser has been measured. In particular, for each PSL temperature, more than one Aux laser frequency has been found.

The second of the three curves seems to be more stable than the other two, even if a "step" trend can be found in all of them (maybe due to the frequency change of the NPRO laser as a function of the crystal temperature continuous tuning, as mentioned in the previous elog). This is the reason why the points are not perfectly aligned, and the errors on the fit parameters are so big.

Attachment 1: Beat_note_3col.jpg
8345   Mon Mar 25 23:20:57 2013 AnnalisaSummaryAuxiliary lockingBeat note found!

[Annalisa, Manasa]

The beat note between the main PSL and the auxiliarly NPRO has been found!

The setup didn't change with respect to the one described on the previous note on the elog. A multimeter has been connected to the laser controller diagnostic pin to read out the voltage that indicated the laser crystal temperature.

The connector has been taken from the Yend table laser controller.

The voltage on the multimeter gave the same temperature shown by "Laser temperature" on the display of the controller, while "set temperature" was wrong.

The temperature has been varied using the laser controller with reference to the voltage read on the multimeter display.

Starting from 35.2 °C, the temperature has been first lowered until 20 °C and no beat note has been found, then temperature has been increased up to 35.2 °C and the first beat note has been found at 38.0 °C.

It has been detected at a frequency of about 80 MHz with an RF power of -27 dBm and a frequency fluctuation of about  +/- 4 MHz.

To do:

I made more measurements slowly varying the laser temperature, to see how the beat note frequency changes with it. I'll make the plot and post it as soon.

8368   Thu Mar 28 19:32:22 2013 AnnalisaSummaryAuxiliary lockingBeat note found!

I plot the variation of the beat note frequency as a function of "Alberto" NPRO laser's temperature.

After some discussion, now I'm going to vary the PSL temperature and find the auxiliary NPRO temperature matching to have the beat note between the two.

Attachment 1: BeatFreq.jpg
10918   Sat Jan 17 15:07:28 2015 manasaUpdateGeneralBeat note frequency discrepancy

I was around the PSL table and the X end table today.

X end table:

I was at the X end table making some distance measurements for the telescope to the fiber coupler. I have NOT moved anything as yet.

PSL table:

I wanted to get some data to look at the beat note frequency discrepancy between the green and IR.

I tried making measurements using the spectrum analyzer at the PSL table but the MC was getting unlocked quite often with PSL enclosure open and HEPA on high.

So I switched off the power supplies to the RFPDs (3 of them) on the PSL table and disconnected the Xmon input to the adder (at the IOO rack) which brings the green beat note signals to the conrol room. I connected the fiber RFPD output to the adder and took a bunch of measurements of the green and IR beat notes from the spectrum analyzer in the control room. I have NOT undone this setup assuming there are no locking plans for tonight and I will come back tomorrow.

If anyone is planning to do some locking in the meantime, you can undo the connections keeping in mind to turn off the power to the RFPDs before you do so.

P.S. I walked in this morning to PSL FSS slow actuator at 0.89 and brought it down to zero before making measurements.
I also touched the steering mirrors that are part of the Y green beat note alignment while looking at the amplitude of the RF signal on the spectrum analyzer.

10956   Thu Jan 29 11:59:48 2015 manasaUpdateGeneralBeat note frequency discrepancy

Below is the set of plots comparing measurements for the green and IR beat notes frequencies. The measurements were made on the spectrum analyzer at the same time. So I have not taken measurement error into account.
From the plots, the discrepancy is not very large.

Attachment 1:
Shows the two sets of measurements scaterred along y=x.

Attachment 2:
Since plot 1 shows the points tightly scattered to y=x, I plotted the difference between the two measurements against their mean to blow out the deviations.

I will do the same comparison using the frequency counter readout once we have RF amplifiers installed.

Attachment 1: Freq_compare1.png
Attachment 2: Freq_compare2.png
8361   Wed Mar 27 21:53:21 2013 AnnalisaUpdateABSLBeat note of ATF auxiliary laser

After measuring the beat note, the "Alberto" NPRO auxiliary laser has been moved from the PSL table to the POY table. Its beam profile is going to be measured. It's going to be used as green laser on the END table, in place of the broken one.

The auxiliary laser borrowed form ATF lab (which will be used for the ABSL measurement) has been set on the PSL table to make a measurement of the beat note between it and the main laser.

The setup is mostly the same of the previous beat note measurement . In this case, laser input power is 326 mW, so I needed to replace one of the mirrors of the steering optics with a BS 50% reflecting in order to have less than 1 mW on the PD.

Now, the total power on the PD is less than 0.5 mW.

I didn't measure the beat note yet to leave the PSL table as quite as possible for the locking procedures.

To do:

Measure the beat note, fiber coupling the NPRO laser to bring it to the POY table.

8369   Thu Mar 28 23:00:30 2013 AnnalisaUpdateABSLBeat note of ATF auxiliary laser found

The beat note for the ATF lab laser has been found.

The measurement has been carried out in the same way as described in elog 8368.

The only difference is that in this case I started from a temperature of 35.2 degC, and I reduced it until the minimum which was 30.71 degC. No beat note in this range.

Then I rised on the temperature and I found the first beat note at 41.46 degC. It has been detected at a frequency of about 120 MHz with an RF power of -53 dBm and a frequency fluctuation of about  +/- 5 MHz.

I tried to improve the alignment to have a stronger beat, but it was the maximum I could reach. Maybe I could increase the power hitting the photodiode, which was 0.453 mW.

8333   Fri Mar 22 23:23:38 2013 AnnalisaUpdateLockingBeat note still missing

[Annalisa, Manasa, Koji]

I updated the setup for the beat note. The main reason is that I needed to keep the ADJ to 0 in way to operate at the nominal laser power.

Now the input power of the laser is increased (about 315 mW) and needs to be dumped so as not to exceed the PD threshold of 1mW.

Moreover, a lens has been added to match the two beams size.

A BS has been removed from the PSL pick-off beam path, so the PSL power hitting the BS is now about 100 uW, and the total power on the PD is 0.7mW.

I also verified that both the beams are S polarized.

To find the beat note, the laser temperature has been varied  through the laser controller and not adding a Voltage with the power supply.

A range of temperature of 30 degC has been spanned, but we suppose there should be some calibration problem with the controller, since set temperature is not the same as Laser temperature on the display.

Anyway, no beat note has been found up to now.

An external monitor has to be added to check the real temperature of the crystal.

The next possible plan is to vary the PSL temperature and try to find the beat note.

P.S.: The HEWELETT PACKARD 8591E spectrum analyzer works! The monitor only took some time to turn on!

Attachment 1: Beat_note_-_new_setup.jpg
8819   Wed Jul 10 02:28:04 2013 AnnalisaUpdateGreen LockingBeat notes lost!

[Manasa, Jenne, Annalisa]

I was going to find the beat note to start the cavity scan, but I couldn't.

These are the steps I followed:

• locked the arm with IR to reduce the arm swinging
• locked the green on the arm
• started changing the green temperature setting the offset from the slow servo2 in the ALS. The PSL slow actuator ADJ was always set approximately to zero, and the PSL temperature was checked in order to set the auxiliary laser temperature where the beat was expected (as in the plot)

After spanning the temperature by approximately 4degC, we started be suspicious that I couldn't find the beat in the range of temperature where it was supposed to be found, and we started making several trials:

• PD output disconnected from the beatbox and connected to the cable running to the Control Room
• Checked that the cable going to the Control Room was working by sending a signal with the Marconi (the cable was working)
• Put back the amplifier that had been previously removed
• PD DC output checked with the oscilloscope
• Spectrum analyzer connected to the PD output without passing trough the cable

The same trials were done also for the X arm, but we didn't succeed in finding the beat for the X neither.

13566   Mon Jan 22 12:48:48 2018 KojiSummaryGeneralBeat setup for aLIGO EOM test

I'm planning to construct a beat setup between the PSL and AUX beams. I am going to make it in the area shown in a blue square in the attached photo. This does not disturb Johannes' and PSL setups. The beams are obtained from the PBS reflection of the PSL and the dumped beam of the aux path (0th or 1st order beam of the AOM).

Attachment 1: IMG_3048.JPG
5860   Thu Nov 10 05:54:23 2011 kiwamuUpdateGreen LockingBeat-note detected : PSL vs Y arm

[Katrin / Kiwamu]
The beat-note between the PSL green laser and the Y end green laser was successfully detected.
The detection was done by the new broad-band RFPD.
The next step will be an extraction of the frequency fluctuation signal using the delay-line-mixer frequency discriminator.

Here is a picture of the RF spectrum analyzer displaying the direct output signal from the broad-band RFPD.
The beat-note was moving around 100 MHz with an RF power of -36 dBm. The frequency fluctuation was about +/- 7MHz in a time scale of 1 sec or so.

(What we did)
+ Connected a BNC cable which goes from the c1iscey's DAC to the laser slow input
=> this enables a remote control of the laser frequency via the temeperature actuation
+ Realigned the beam pointing of the Y end green laser
+ Installed all the necessary optics on the PSL table
=> currently the PSL green light is adjusted to completely S-polarization
+ readjusted the mode matching telescopes
=> the Y green beam becomes the one with a long Rayleigh range
+ Health check on the broad-band RFPD to see if it is working
+ Installed the BB-RFPD with a +/-15V power supply
+ Fine alignment of the beam combining path
+ Fine tuning of the Y end laser temperature
=> T_PSL = 31.72 deg when the slow FSS feedback is zero.
=> Based on Bryan's measurement (see #elog) the Y end laser temperature was adjusted to 34.0 deg by applying an offset to the slow input.
+ Found the beat note at 100 MHz or so.
=> optimizing the alignment of the beam combining path by maximizing the peak height of the beat-note.
=> maximum peak height observed with an RF spectrum analyzer was about -36 dBm.

11361   Mon Jun 15 22:36:40 2015 rana, kojiUpdateGreen LockingBeatBox Assay: not looking good

Because the ALS beatbox schematic is out-of-date and misleading, we pulled the box to photograph the current implementation and figure out how to proceed. The box is out on the EE bench right now. Schematic Doc added to 40m Document tree: https://dcc.ligo.org/LIGO-D1102241. Some notes:

1. The soldering on this board is pretty messy and there are a lot of flying wire and flying component hacks. I wouldn't trust all of the connections.
2. The GV-81 RF amps in the front end are both stuffed. The 1 dB compression point is 19 dBm, so we want to use them below 10 dBm output. They have a gain of +10.5 dB, so that means they should not be used with and input to the beatbox of more than -10 dBm. Otherwise there will be nonlinear noise generation.
3. Not stuffed: U1-Comparator, A1-attenuator, U2-splitter.
4. Why is the filter after the mixer only 2nd order?? That's not a valid filter choice in any RF world. How much do we want to cut off the 2f mixer output before sending into our low noise, audio frequency (and prone to downconversion) amplifier? The Mini-Circuits amplifiers would have given us >60 dB attenuation in the stop band. This one is only going to give us 20-30 dB when the beat frequency is low. Get rid of diplexer. The schematic claims that its just one pole?? Seems like a 2nd order LP filter to me.
5. The modified schematic (see Koji elog 8855) shows that an OP27 is used for the whitening stage. The current noise of the OP27 with the 3k resistor makes the OP27 current noise dominate below 1 Hz. And what is going on with that filter capacitor choice? We never want to use these tiny things for sensitive filter applications. (cf. Sigg doc on resistor and capacitor choice, the noise reduction book by Ott, H&H, etc.). That's why we have the larger metal-poly, paper, mylar, etc. caps sitting around.

Probably we ought to install a little daughter board to avoid having to keep hacking this dead horse. Koji has some of Haixing'g maglev filter boards. Meanwhile Koji is going to make us a new beatbox circuit in Altium and we can start fresh later this summer.

Interesting link on new SMD cap technology.

Photos of circuit as found

11363   Fri Jun 19 01:24:26 2015 rana, kojiUpdateGreen LockingBeatBox Assay: not looking good

We had decided a few days ago, to bypass the IF part of the BeatBox board and put some of the Haixing Maglev generic filter boards in there so that we could get more whitening and also have it be low noise.

Tonight we wondered if we can ditch the whole BeatBox and just use the quad aLIGO demod box (D0902745) that Rich gave us a few years ago. Seems like it can.

But, it has no whitening. Can we do the whitening part externally? Perhaps we can run the RF signals from the output of the beat RF Amps over to the LSC rack and then put the outputs into the LSC Whitening board and acquire the signals in the LSC ?

11364   Fri Jun 19 01:55:35 2015 ericqUpdateGreen LockingBeatBox Assay: not looking good
 Quote: But, it has no whitening. Can we do the whitening part externally? Perhaps we can run the RF signals from the output of the beat RF Amps over to the LSC rack and then put the outputs into the LSC Whitening board and acquire the signals in the LSC ?

I like this idea; it gives us more control over the whitening, and saves the IPC delay. We could use the currently vacant AS165 and POP55 channels.

We'd only have to move the phase trackers to c1lsc, which means 12 more FMs total. This is really the only part of the c1als model our current system uses, the rest is from before the ALS->LSC integration.

13957   Wed Jun 13 22:07:31 2018 gautamUpdateALSBeatMouth PDFR measurement

Summary:

Neither of the Menlo FPD310 fiber coupled PDs in the beat mouth have an optoelectronic response (V/W) as advertised. This possibly indicates a damaged RF amplification stage inside the PD.

Motivation:

I have never been able to make the numbers work out for the amount of DC light I put on these PDs, and how much RF beat power I get out. Today, I decided to measure the PD response directly.

Details:

In the end, I decided that slightly modifying the Jenner laser setup was the way to go, instead of futzing around with the PDFR laser. These PDs have a switchable gain setting - for this measurement, both were set to the lower gain such that the expected optoelectronic response is 409 V/W.

[Attachment #1] - Sketch of the experimental setup.

[Attachment #2] - Measured TF responses, the RF modulation was -20dBm for all curves. I varied the diode laser DC current a little to ensure I recovered identical transfer functions. Assumptions used in making these plots:

1. NF1611 and FPD310 have equal amounts of power incident on them.
2. The NF1611 transimpedance is 700V/A.

[Attachment #3] - Tarball of data + script used to make Attachment #2.

Conclusions:

• The FPD310 does not have a DC monitor port.
• So the dominant uncertainty in these plots is that I don't know how much power was incident on the PD under test.
• The NF1611 DC power level could be measured though, and seemed to scale with DC pump current linearly (I had only 3 datapoints though so this doesn't mean much).
• Neither PD has transimpedance gain as per the specs.
• The X PD shows levels ~x10 lower than expected.
• The Y PD shows levels ~x3 lower than expected.
• I will repeat the measurement tomorrow by eliminating some un-necessary patch fiber cables, and also calibrating out the cable delays.
• The setup shown in Attachment #1 was used because I didn't want to open up the BeatMouth.
• But I can pipe the port of the BS not going to the FPD310 directly to the collimator, and that should reduce the systematic uncertainty w.r.t. power distribution between FPD310 and NF1611.
Attachment 1: IMG_7056.JPG
Attachment 2: BeatMouthPDFR.pdf
Attachment 3: BeatMouth_PDFRdata.tgz
13973   Fri Jun 15 14:22:05 2018 gautamUpdateALSBeatMouth PDFR measurement

I did the measurement with the BeatMouth open today. Main changes:

• Directly pipe the RF output of the Menlo PDs to the Agilent, bypassing the 20dB coupler inside the BeatMouth.
• Directly pipe the unused port of the Fiber Beamsplitter used to send light to the Menlo PD to an in-air collimator, which then sends the beam to the NF1611 reference detector.

So neglecting asymmetry in the branching ratio of the fiber beamsplitter, the asymmetry between the test PD optical path and the reference PD optical path is a single fiber mating sleeve in the former vs a collimator in the latter. In order to recover the expected number of 409 V/W for the Menlo PDs, we have to argue that the optical loss in the test PD path (fiber mating sleeve) are ~3x higher than in the NF1611 path (free space coupler). But at least the X and Y PDs show identical responses now. The error I made in the previously attached plot was that I was using the 20dB coupled output for the X PD measurement .

Revised conclusion: The measured optoelectronic response of the Menlo PDs at 10s of MHz, of ~130 V/W, is completely consistent with the numbers I reported in this elog. So rogue polarization is no longer the culprit for the discrepancy between expected and measured RF beatnote power, it was just that the expectation, based on Menlo PD specs, were not accurate.#2 of the linked elog seems to be the most likely, although "broken" should actually be "not matching spec".

While killing time b/w measurements, I looked on the ITMY optical table and found that the NF1611 I mentioned in this elog still exists. It is fiber coupled. Could be a better substitute as a Reference PD for this particular measurement.

 Quote: I will repeat the measurement tomorrow by eliminating some un-necessary patch fiber cables, and also calibrating out the cable delays. The setup shown in Attachment #1 was used because I didn't want to open up the BeatMouth. But I can pipe the port of the BS not going to the FPD310 directly to the collimator, and that should reduce the systematic uncertainty w.r.t. power distribution between FPD310 and NF1611.
Attachment 1: BeatMouthPDFR.pdf
Attachment 2: BeatMouth_PDFRdata.tgz
14498   Thu Mar 28 19:40:02 2019 gautamUpdateALSBeatMouth with NF1611s assembled

Summary:

The parts I was waiting for arrived. I finished the beat mouth assembly, and did some characterization. Everything looks to be working as expected.

Details:

Attachment #1: Photo of the front panel. I am short of two fiber mating sleeves that are compatible with PM fibers, but those are just for monitoring, so not critical to the assembly at this stage. I'll ask Chub to procure these.

Attachment #2: Photo of the inside of the BeatMouth. I opted to use the flexible RG-316 cables for all the RF interconnects. Rana said these aren't the best option, remains to be seen if interference between cables is an issue. If so, we can replace them with RG-58. I took the opportunity to give each fiber beam splitter its own spool, and cleaned all the fiber tips.

Attachment #3: Transfer function measurement. The PDFR setup behind 1X5/1X6 was used. I set the DC current to the laser to 30.0 mA (as read off the display of the current source), which produced ~400uW of light at the fiber coupled output of the diode laser. This was injected into the "PSL" input coupler of the BeatMouth, and so gets divided down to ~100 uW by the time it reaches the PDs. From the DC monitor values (~430mV), the light hitting the PDs is actually more consistent with 60uW, which is in agreement with the insertion loss of the fiber beamsplitters, and the mating sleeves.

The two responses seem reasonably well balanced (to within 20% - do we expect this to be better?). Even though judging by the DC monitor, there was more light incident on the Y PD than on the X PD, the X response was actually stronger than the Y.

I also took the chance to do some other tests:

• Inject light into the "X(Y)-ARM" input coupler of the Beat Mouth - confirmed that only the X(Y) NF1611's DC monitor output showed any change. The DC light level was ~1V in this condition, which again is consistent with expected insertion losses as compared to the "PSL" input case, there is 1 less fiber beamsplitter and mating sleeve.
• Injected light into each of the input couplers, looked at the interior of the BeatMouth with an IR viewer for evidence of fiber damage, and saw none. Note that we are not doing anything special to dump the light at the unused leg of the fiber beamsplitter (which will eventually be a monitor port). Perhaps, nominally, this port should be dumped in some appropriate way.

Attachment #4: Dark Noise analysis. I used a ZHL-500-HLN+ to boost the PD's dark noise above the AG4395's measurement noise floor. The measured noise level seems to suggest either (i) the input-referred current noise of the PD circuitry is a little lower than the spec of 16 pA/rtHz (more like 13 pA/rtHz) or (ii) the transimpedance is lower than the spec of 700 V/A (more like 600 V/A). Probably some combination of the two. Seems reasonable to me.

Next steps:

The optical part of the ALS detection setup is now complete. The next step is to measure the ALS noise with this sysytem. I will use the X arm for this purpose (I'd like to make the minor change of switching the existing resistive power splitter at the delay line to the newly acquired splitters which have 3dB lower insertion loss).

Attachment 1: IMG_7381.JPG
Attachment 2: IMG_7382.JPG
Attachment 3: relTF_schem.pdf
Attachment 4: darkNoise.pdf
14502   Fri Mar 29 21:00:06 2019 gautamUpdateALSBeatMouth with NF1611s installed
• Newfocus 15V current limited supply was taken from bottom NE corner of the ITMY Oplev table to power the BeatMouth on the PSL table
• BeatMouth was installed on top shelf on PSL table [Attachment #1].
• Light levels in fibers were checked:
• PSL: initially, only ~200uW / 4mW was coupled in. This was improved to 2.6mW/4mW (~65% MM) which was deemed sufficient for a first test), by tweaking the alignment of, and into the collimator.
• EX: ~900uW measured at the PSL table. I remember the incident power being ~1mW. So this is pretty good.
• Fibers hooked up to BeatMouth:
• EX light only, DC mon of X PD reads -2.1V.
• With PSL light, I get -4.6 V.
• For these numbers, with the DC transimpedance of 10kohm and the RF transimpedance of 700 ohm, I expect a beat of ~0dBm
• DC light level stability is being monitored by a temporarily hijacked PSL NPRO diagnostic Acromag channel. Main motivation is to confirm that the alignment to the special axes of the PM fibers is still good and we aren't seeing large tempreature-driven waveplate effects.
• RF part of the circuit is terminated into 50ohms for now -
• there is still a quesiton as to what is the correct RF amplifier to use in sending the signal to the 1Y3 rack.
• An initial RF beat power level measurement yielded -5dBm, which is inconsistent with the DC monitor voltages, but I'm not sure what frequency the beat was at, will make a more careful measurement with a scope or the network analyzer.
• We want the RF pre-amp to be:
• Low noise, keeping this in mind
• High enough gain to boost the V/Hz discriminant of the electronic delay line
• Not too high gain that we run into compression / saturate some of the delay line electronics - specifically, the LO input of the LSC demod board has a Teledyne amp in the signal chain, and so we need to ensure the signal level there is <16dBm (nominal level is 10dBm).
• I'm evaluating options...
• At 1Y3:
• I pulled out the delay-line enclosure, and removed the (superglued) resistive power splitters with the help of some acetone
• The newly acquired power splitters (ZAPD-2-252-S+) were affixed to the front panel, in which I made some mounting holes.
• The new look setup, re-installed at 1Y3, is shown in Attachment #2.
Attachment 1: IMG_7384.JPG
Attachment 2: IMG_7385.JPG
8807   Mon Jul 8 21:46:31 2013 manasaUpdateGreen LockingBeatbox

[Koji, Manasa]

I wanted to investigate on the ALS electronics(in particular the beatbox and the phase tracker) and find out if the beatbox is showing a linear behavior
as we expect it to and as to why we have been seeing sudden jumps at the phase tracker output.

I have been using the Xarm part of the beabox.
I used Marconi as well as signal generator to do frequency sweep/modulation at the RF input of the beatbox and looked at the I_MON output of the beatbox.

We observed sudden jumps in the beatbox output from time to time while we either varied the carrier frequency or the RF amplitude.
Also the beatbox output shows high frequency oscillations at ~95MHz (source unknown). It is for sure that the beatbox is not behaving the way it should
but we could not tell more or troubleshoot with the beatbox mounted on the rack.

I am going to let Annalisa do her Y arm ALS scan tonight and pull out the beatbox tomorrow to fix it.

8855   Tue Jul 16 10:16:23 2013 KojiUpdate Beatbox XARM whitening modified

The X arm whitening filters of the beatbox were modified.
Now we have about 10 times better floor level above 100Hz and ~3 better at 1Hz.

- The previous whitening was zero@1Hz, pole@10Hz, and the DC gain of the unity.
When the Marconi signal (~30MHz -25dBm) was given to the beatbox (via ZFL-1000LN),
the DC output of the beatbox was only 140mV (lame). This corresponded to 220 counts in
the CDS.
(BTW the signals were calibrated by giving frequency deviation of 1kHz is applied at 125Hz.)

- If you compare the analog measurement of the beatbox output and what we see in the I phase signal,
you can see that we were completely dominated by the ADC noise (attachment 2, blue and red).

- The new whitening is zero@5.2Hz, pole@159Hz, and the DC gain of 10.

- This improved the sensing noise by a factor of ten above 100Hz.

- We are stil llimited by the digitizing noise between 3Hz to 100Hz.
We need steeper whitening like 2nd order from 1Hz to 100Hz. (and probably at DC too).
Now the DC amplitude is about 1.4V (and 2200 counts in the CDS).
So, it is interesting to see how the sensing limit changes by increasing
the overall gain by a factor of 3, and have (zeros@1Hz & poles@10Hz)^2.

This can be implemented on a proto-daughter board.

- By the way, the performance below 2Hz is now better than the analog one with the previous whitening.
This improvement might have come from the replacement of the thick film resistors by thin-film resistors.
(See the circuit diagram)

About the nominal power of the beatbox input.

- Marconi (-20dBm 30MHz) was directly connected to the beatbox. The RF output of -15dBm was observed at the delayline output.
- According to the beatbox schematic, the mixer LO and RF inputs were expected to be -9dBm and -19dBm.
- The nominal mixer LO level is supposed to be 7dBm. Therefore the nominal beatbox input should be -4dBm.

- Assuming 23dB gain of the preamp, the PD output is expected to be -27dBm.

- When the PD out is -27dBm, the RF mon is expected to be -5dBm. This is the level of the RF power expected to be seen in the control room.

- The output of the beatbox was measured as the function of the input to the preamp (before the beatbox input).
With the nominal gain, we should have observed amplitude of ~170. And it is now 1700 because of the whitening modification.

Attachment 1: Beatbox_mod.pdf
Attachment 2: ALS_whitening.pdf
Attachment 3: Beatbox_input_dependence.pdf
8818   Wed Jul 10 02:10:41 2013 manasaUpdateGreen LockingBeatbox gets a makeover

 Quote: [Koji, Manasa] I wanted to investigate on the ALS electronics(in particular the beatbox and the phase tracker) and find out if the beatbox is showing a linear behavior as we expect it to and as to why we have been seeing sudden jumps at the phase tracker output. I have been using the Xarm part of the beabox. I used Marconi as well as signal generator to do frequency sweep/modulation at the RF input of the beatbox and looked at the I_MON output of the beatbox. We observed sudden jumps in the beatbox output from time to time while we either varied the carrier frequency or the RF amplitude. Also the beatbox output shows high frequency oscillations at ~95MHz (source unknown). It is for sure that the beatbox is not behaving the way it should but we could not tell more or troubleshoot with the beatbox mounted on the rack. I am going to let Annalisa do her Y arm ALS scan tonight and pull out the beatbox tomorrow to fix it.

The beatbox output showed high frequency oscillations during the troubleshooting process yesterday. I removed the beatbox from the rack. With no RF inputs, just powering the beatbox showed these high frequency oscillations at the beatbox output. This confirms that these oscillations are from the op-amp AD829JR. I replaced these with low noise OP27G. Also I removed the AD829JR that were soldered to the frequency divider and comparator which are not being used. Output buffer U10 was also removed.

After replacing with OP27G, I rechecked the beatbox with and without the RF input. There were no more high frequency contaminations and beatbox seemed to behave as it is supposed to when a frequency modulated RF input is fed. I put the beatbox back on the rack and did  a quick recheck.

Before (top) and after (bottom) pictures

11355   Fri Jun 12 17:09:58 2015 ericqUpdateLSCBeatbox needs whitening gain

The beatbox I and Q signals are too small at the ADC! I was able to reduce the RMS out of loop ALS sensitivity (arm locked on POX) by 300Hz with G=10 SR560s between the beatbox output and the ADC whitening chassis input. Increasing the beat note amplitude via RF amplifier had no positive effect.

There is still a reasonable gap between this and the beatbox's noise levels, as measured with a marconi. There may be additional headroom for whitening gain; the SR560 maximum output range is smaller than the ADC input range.

The high frequency noise has >0.5 coherence with the PDH error signal above a kHz or so, but not much below that.

We should probably either modify the output whitening of the beatbox, or introduce some (variable?) whitening gain in a seperate circuit.

Attachment 1: beatGain.png
11356   Sat Jun 13 18:37:46 2015 ericqUpdateLSCBeatbox needs whitening gain

Here are the promised details!

I was worried about the lack of whitening gain, as I saw that the DC phase tracker Q output (which is the magnitude of the signal in the beatbox's I-Q plane) was no more than 200 ADC counts for X (~120mV), and 800 for Y (~500mV). I.e. this is the largest DC value that either the I or Q ADC channels can see, and the RMS fluctuations are on the order of mV, meaning we're wasting our entire ADC range

However, I also had doubts about this since, even in the nominal state, the ASD of the ADC signals before dewhitening was higher than the expected ADC noise level. However, because of the non-linearity of the conversion of the BEAT_I and Q signals into the phase tracker output, evaluating the contribution of the beatbox output and ADC input voltage noises takes a few more steps.

So, I hooked up a Marconi as the signal source for the beatbox's X channel , with no modulation (presumably the phase noise of the Marconi output is significantly lower than the sensitivity of the beatbox). For all of these measurements, the beat frequency was kept around 50MHz, with an amplitude of -30dBm on the control room analyzer, which is a typical X ALS operating point.

At this point, the beatbox output noise was below the ADC noise (as measured by an SR785). Nevertheless, I found that the beat spectrum driven by the Marconi lined up to be very close to the ALS beat spectrum across a wide band, explaining much of the noise.

At this point, I inserted SR560s in between the beatbox I and Q outputs, and the AA chassis leading to the ADC. A gain of 10 reduced the resultant phase tracker noise by that same factor at nearly all frequencies. A further increase in gain did not lead to the noise changing appreciably, probably because the real beatbox noise was now contributing, as is suggested by some common peaks in the direct beatbox output phase tracker spectra.

Going back to the real green beat signal with the SR560s still at G=10, I obtained the result shown in ELOG 11355. I will soon repeat this process with the Y ALS.

As I mentioned in the previous ELOG, we may be further helped by more whitening gain than can be provided by the SR560s (and we obviously need a robust long term circuit for this gain). If it then turns out we are limited by beatbox noise to a degree we are not happy with, we could perhaps look into reintroducing some RF gain into the X beat. As Koji mentions in ELOG 8855, the beatbox operates best at an RF input of around -4dBm.

11357   Sat Jun 13 23:52:14 2015 ericqUpdateLSCBeatbox needs whitening gain

Nice find!

We ought to use our noise model of the ALS signal chain to determine what the right gains are, rather than hunt and peck. More likely we'll start from the right gains.​

Once the gains and/or whitening filters make sense, maybe we'll see some effect from fixing the green PDH loops.

8245   Wed Mar 6 20:21:34 2013 JamieUpdateGeneralBeatbox pulled from rack

I pulled the beatbox from the 1X2 rack so that I could try to hack in some output whitening filters.  These are shamefully absent because of my mis-manufacturing of the power on the board.

Right now we're just using the MON output.  The MON output buffer (U10) is the only chip in the output section that's stuffed:

The power problem is that all the AD829s were drawn with their power lines reversed.  We fixed this by flipping the +15 and -15 power planes and not stuffing the differential output drivers (AD8672).

It's possible to hack in some resistors/capacitors around U10 to get us some filtering there.  It's also possible to just stuff U9, which is where the whitening is supposed to be, then just jump it's output over to the MON output jack.  That might be the cleanest solution, with the least amount of hacking on the board.

In any event, we really need to make a v2 of these boards ASAP.  Before we do that, though, we need to figure out what we're going to do with the "disco comparator" stage back near the RF input.  (There are also a bunch of other improvements that will be incorporated into v2).

8292   Thu Mar 14 11:51:14 2013 JamieUpdateGeneralBeatbox upgraded with output whitening, reinstalled

 Quote: I pulled the beatbox from the 1X2 rack so that I could try to hack in some output whitening filters.  These are shamefully absent because of my mis-manufacturing of the power on the board. Right now we're just using the MON output.  The MON output buffer (U10) is the only chip in the output section that's stuffed: The power problem is that all the AD829s were drawn with their power lines reversed.  We fixed this by flipping the +15 and -15 power planes and not stuffing the differential output drivers (AD8672). It's possible to hack in some resistors/capacitors around U10 to get us some filtering there.  It's also possible to just stuff U9, which is where the whitening is supposed to be, then just jump it's output over to the MON output jack.  That might be the cleanest solution, with the least amount of hacking on the board.

I modified the beatbox according to this plan.  I stuffed the whitening filter stage (U9) as indicated in the schematic (I left out the C26 compensation cap which, according to the AD829 datasheet, is not actually needed for our application).  I also didn't have any 301 ohm resistors so I stuffed R18 with 332 ohm, which I think should be fine.

Instead of messing with the working monitor output that we have in place, I stuffed the J5 SMA connector and wired U9 output to it in a single-ended fashion (ie. I grounded the shield pins of J5 to the board since we're not driving it differentially).  I then connected J5 to the I/Q MON outputs on the front panel.  If there's a problem we can just rewire those back to the J4 MON outputs and recover exactly where we were last week.

It all checks out: 0 dB of gain at DC, 1 Hz zero, 10 Hz pole, with 20 dB of gain at high frequencies.

I installed it back in the rack, and reconnected X/Y ARM ALS beatnote inputs and the delay lines.  The I/Q outputs are now connected directly to the DAQ without going through any SR560s (so we recover four SR560s).

13817   Fri May 4 21:17:57 2018 gautamConfigurationALSBeathMouth pulled out of PSL table

I have been puzzled about the beat note level we get out of the BeatMouth for some time.

• The beat PD used is the Menlo FPD310.
• But the version we have is an obsolete version of the product, for which a manual is hard to find.
• Hence, I don't know the transimpedance/electrical characteristics of this PD.
• The optical damage threshold of the PD is quoted as 2mW, which is a number I have been careful not to exceed.
• But I've felt that the beat amplitude level we get out of this PD is far too low considering the amount of DC optical power (as measured with a fiber power meter) incident on the PD.
• After some emailing and online hunting, I've gathered some numbers for the PD which are now on the wiki.
• The fiber beam splitters we use inside the BeatMouth don't have PM fibers. There are 3 such splitters inside the BeatMouth. So the overlap efficiency on the PD is unknown.
• But even so, the beat levels I was seeing were too low.

I have pulled the box out in order to re-characterize the DC power levels incident on the PD, and also to change the gain setting on the PD to the lower gain which is more compatible with the level of optical power we have going into the BeatMouth. The modern catalog for the FPD310 (see wiki) suggests that the maximum output voltage swing of the PD is 1Vpp driving a 50ohm load. With 50% overlapping efficiency between the PSL and AUX beams, and 400 uW of optical power from each beam, I expect an output of 0.5Vpp. Even with perfect overlap, I expect 0.8Vpp. So these numbers seem reasonable.

I also plan to check the scaling of electrical beat amplitude to optical power for a couple of levels to see that these scale as expected...

13824   Tue May 8 00:40:51 2018 gautamConfigurationALSBeathMouth pulled out of PSL table

Summary:

I did some more BeatMouth characterization. My primary objective was to do a power budget. Specifically, to measure the insertion loss of the mating sleeves and of the fiber beam splitters. All power numbers quoted in this elog are measured with the fiber power meter. Main takeaways:

• Measured insertion loss of all mating sleeves, which are ADAFCPMB2, are in agreement with the < 1dB spec. 1 dB in power is ~20%.
• But there is large variance in the above number, between different supposedly identical connectors.
• Measured insertion loss from input port to coupled ports of the fiber beamsplitters are slightly larger than spec (~3.5dB), when measured after mating the fiber beamsplitter (which has Hi1060 flex fiber) and PM980 fiber (which is what brings light to the BeatMouth).
• But measured insertion loss when mating is between Hi1060 flex fiber ends is more in line with the expected value of ~3.5dB.
• Isolation of fiber beam splitters seems to match the spec of >55dB.

Results:

• I used the fiber bringing 416uW of IR light from EY for this test.
• Insertion loss was measured by injecting the fiber light at one port and measuring the transmitted power at various other ports.
• In order to couple the fiber power meter across a single mating sleeve, I used a short (~1m) patch cable from newport (F-SY-C-1FCA). Technically, this is single mode fiber with the correct type of connector, FC/APC, but is not PM.
• See Attachment #2 for the labeling of the connectors which is how I refer to them in the table below.
• Lest there be confusion, I use the definition of insertion loss  $\mathrm{Insertion ~loss [dB] }=10\mathrm{log_{10}}(\frac{P_{in}}{P_{out}})$.
Mating Sleeve # Insertion loss [dB]
1 0.38
2 0.65
3 0.71
4 0.43
5 0.95
6 0.79
7 0.5

Remarks / Questions:

1. Is there any way to systematically reduce the insertion loss? Like getting a better mating part?
2. Question for the fiber experts: How do we deal with the unused port of the beam-splitters? Right now, they are just capped. But as you can see in Attachment #1, the caps certainly don't block all the light. What are the implications of back-scattered light into the fiber on the ALS noise? I guess the beamsplitter itself has the spec'd 55dB directivity, so do we not care about this?
Attachment 1: IMG_6986.JPG
Attachment 2: IMG_6987.JPG
ELOG V3.1.3-