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ID Date Author Type Category Subjectup
  17424   Thu Jan 26 00:18:54 2023 KojiUpdateCamerasRecording CCD cameras

Connect any video signal supported by the adapter. Use Windows / Mac or any other OS. If it keeps complaining, contact Magwell support.

  4123   Fri Jan 7 00:49:16 2011 JenneUpdateGreen LockingRecovered Xarm Green Lock, Preparation for Beat Note Measurement

[Kiwamu, Jenne]

We went this evening in search of a beat note signal between the Xarm transmitted green light and the PSL doubled green light. 

First, we removed our new ETMX camera (we left the mount so it should be easy to put back) from the other day.   We left the test masses exactly where they had been while flashing for IR, so even though we can no longer confirm, we expect that the IR beam axis hasn't changed.  We used the steering mirror on the end table to align the green beam to the cavity.  We turned on the loop to lock the end laser to the cavity, and achieved green lock of the arm.

Then we went to the PSL table to overlap the arm transmitted light with the PSL doubled light.  We made a few changes to the optics that take the arm transmitted light over to the PD.  We found that the arm transmitted light was very high, so we changed from having one steering mirror to having 3 (for table space / geometry reasons we needed 3, not just 2) in order to lower the beam axis.  We also found that the spot size of the arm transmitted beam was ~2x too small, so we changed the mode matching telescope from a 4x reducer to a 2x reducer by changing the 2nd lens from f=50mm to f=100mm (the first lens is f=200mm).  We made the arm transmitted beam and the PSL green beam overlap, but we saw no peak on the spectrum analyzer. 

We checked the temperature of the PSL and end lasers, and determined that we needed to adjust the set temp of the end laser.  However, we still didn't find any beat note.

We then tweaked the temperature of the doubling oven at the end station, to maximize the power transmitted, since Kiwamu said that that had worked in the past.  Alas, no success tonight.

We're stopping for the evening, with the success of reacquiring green lock of the Xarm.

  12611   Sat Nov 12 01:09:56 2016 gautamUpdateLSCRecovering DRMI locking

Now that we have all Satellite boxes working again, I've been working on trying to recover the DRMI 1f locking over the last couple of days, in preparation for getting back to DRFPMI locking. Given that the AS light levels have changed, I had to change the whitening gains on the AS55 and AS110 channels to take this into account. I found that I also had to tune a number of demod phases to get the lock going. I had some success with the locks tonight, but noticed that the lock would be lost when the MICH/SRCL boosts were triggered ON - when I turned off the triggering for these, the lock would hold for ~1min, but I couldn't get a loop shape measurement in tonight.


As an aside, we have noticed in the last couple of months glitchy behaviour in the ITMY UL shadow sensor PD output - qualitatively, these were similar to what was seen in the PRM sat. box, and since I was able to get that working again, I did a similar analysis on the ITMY sat. box today with the help of Ben's tester box. However, I found nothing obviously wrong, as I did for the PRM sat. box. Looking back at the trend, the glitchy behaviour seems to have stopped some days ago, the UL channel has been well behaved over the last week. Not sure what has changed, but we should keep an eye on this...

  16924   Thu Jun 16 18:23:15 2022 PacoConfigurationBHDRecovering LO beam in BHD DCPDs

[Paco, Yuta]

We recovered the LO beam on the BHD port. To do this, we first tried reverting to a previously "good" alignment but couldn't see LO beam hit the sensor. Then we checked the ITMY table and couldn't see LO beam either, even though the AS beam was coming out fine. The misalignment is likely due to recent changes in both injection alignment on TT1, TT2, PR2, PR3, as well as ITMX, ITMY. We remembered that LO path is quite constrained in the YAW direction, so we started a random search by steering LO1 YAW around by ~ 1000 counts in the negative direction at which point we saw the beam come out of the ITMY chamber yes


We proceeded to walk the LO1-LO2 in PIT mostly to try and offload the huge alignment offset from LO2 to LO1 but this resulted in the LO beam disappearing or become dimmer (from some clipping somewhere). This is WiP and we shall continue this alignment offload task at least tomorrow, but if we can't offload significantly we will have to move forward with this alignment. Attachment #1 shows the end result of today's alignment.

Attachment 1: Screenshot_2022-06-16_18-29-14_BHDLObeamISBACK.png
Screenshot_2022-06-16_18-29-14_BHDLObeamISBACK.png
  9755   Wed Mar 26 22:22:43 2014 ManasaUpdateGeneralRecovery

The following that went unnoticed from yesterday were recovered today:

1. ETMX and ETMY 'misalign' scripts weren't running. Troubleshooting showed slow machines c1auxex and c1auxey weren't responding. The machines were reset.

2. PRM oplev gains were zero. Gain values were set looking back at the burt files.

3. X end PZT power supplies were turned ON and set to 100V.

4. X end doubler temperature was reset to the last optimal value on elog (36.35 deg).

 

Some hitches that should be looked into:

1. Check: ASS for X arm seems not quite doing its job. ETMX has to be moved using sliders to obtain maximum TRX and the arm alignment was seen to be drifting.

2. Check: Status of other slow machines and burt restore whichever needs one.

  9756   Thu Mar 27 10:34:39 2014 ericqUpdateGeneralRecovery

Quote:

1. Check: ASS for X arm seems not quite doing its job. ETMX has to be moved using sliders to obtain maximum TRX and the arm alignment was seen to be drifting.

ETMX ASC output was turned off for whatever reason. Switched it on, ASS is fine.

  10326   Sun Aug 3 17:19:32 2014 ericqUpdateGeneralRecovery efforts

[ericq, Jenne]

We both happened to come by today to fix things up.

When I arrived, the PMC was locked to a 01 mode, which I fixed. The PMC transmission is still worryingly low. MC locked happily. 

ETMX was getting odd kicks, the kind where a DC shift would occur suddenly, and then go away a few moments later. I turned off all dynamic coil outputs, and looked at the MON output of the SOS driver with a scope to try and see if the DAC or dewhitening was glitching, but didn't see anything... Meanwhile, Jenne fiddled with the TTs until we got beams on POP and REFL. (EDIT, JCD:  Useful strategies were to put an excitation onto TT2, and move TT1 until the scattered beam in the chamber was moving at the excitation frequency,  Find the edges of TT2 by finding where the scattered light stops seeing the excitation, and center the beam on TT2.  By then, I think I saw the beam on the PRM face camera.  Then, put a temporary camera looking at the face of PR2.  Using TT2 to center here got us the beam on the POP camera.)

We then walked PRM and the TTs around to keep those two camera beams and get the PRM oplev beam back on its QPD. At this point, ITMX was misaligned (by us), and ITMY aligned to get some recycled flashes into the Y-arm. Y-arm was locked to green, and we poked TTs to get better IR flashes. Misaligning PRM, we had Y-Arm flashes of ~0.7. From there, the michelson and then X-arm were roughly aligned. Both arms were seeing flashes of about 0.7, and the MICH fringes on the AS port look nice.

Frustratingly, the SUS->LSC communication for TRY and TRX isn't working, and could not be fixed by any combination of model or front-end restarting... Thus we haven't been able to actually lock the arms and run ASS. THIS IS VERY FRUSTRATING. 

Additionally, at the point where we were getting light back into the Yarm, the ITMX that were seen on Friday were happening again, tripping the watchdog. Also, something in the Yarm cavity is getting intermittently pushed around, as can be seen by the green lock suddenly wandering off. All of these suspension shenanigans seem to be independent of oplev damping. 

It troubles me that this whole situation is fairly similar to the last time we lost the input pointing (ELOG 10088)

In any case, we feel that we have gotten the IFO alignment to a lockable state.

  10327   Sun Aug 3 23:47:56 2014 KojiUpdateGeneralRecovery efforts

It's great that you guys found the beam.
Yes, ITMX kick and lost communication for TRY were the motivation of my CDS rebooting.

  17443   Thu Feb 2 19:41:49 2023 AnchalSummaryPowerShutdownRecovery from power outage events

[Anchal, JC, Radhika, Paco]

JC reported that power outage happened twice in 40m today at around 4:17 pm.

We followed instructions from this page mostly to recover today. Following are some highlights:

Main laser controller fan broke

Paco reported that the adhoc fan in the back of main laser controller slid down and broke. Their might be contamination on the table from broken fan parts. Paco replaced this fan with another fan which is larger. I think it is time to fix this fan on the controller for good.

Main volume valve V1 shutdown

The main volume valve shut down because c1vac turned off. We restored the vacuum state by simply opening this valve again. Everything else was same as until the final step in vacuum resetting steps.

Mode cleaner locking issues

The burt restore for mode cleaner board settings do not bring back the state of channels C1:IOO-MC_FASTSW and C1:IOO-MC_POL. This has been an issue which has puzzled us in the past too as we try to get the mode cleaner to lock after power outage recovery. I have now added these channels and their required state in autolocker settings so that autolocker scan in the correct state always. It seems like I added with Yuta's name in the commit author.

 

  5592   Sat Oct 1 17:47:21 2011 KojiSummaryGeneralRecovery from the power shutdown

[Steve Koji Kiwamu]

- The storage on linux1 and linux1 itself (with this order) were turned on at 10:30am

- Kiwamu restored the vacuum system

   => opened V4, started TP1 (maglev) and opened V1.

      The pressure went downfrom 2.5 mTorr to the normal level in about 20 minutes.

- A regular fsck of linux1 was completed at 5pm

- Nodus was turned on. Mounting /cvs/cds succeeded

- The control room computers were turned on

- The rack power for FB turned on, FB and megatron started.

- Kiwamu and Koji went through all of the rack powers and started the slow and fast machines.
As we found Solensen for -15V at ETMX was current limited. +/-15V were turned off for now.
==> Kiwamu turned on Solensen again for investigation and found it became okay.
It's now ON.
 
- c1sus and c1ioo had many red lamps on the FE status screen.
Ran killc1XXX for all of the FE codes on c1lsc and c1ioo.
Then, made software reboot (sudo shutdown -r now)
All of the FEs shows green (after some randome clicking of DIAGRESETs)

- HVs on 1X1 were turned on. The are not vacuum HV, but used only in the air

- Turned on the RF generation box and the RF distribution box

- burtrestore slow machines (c1psl, c1susaux, c1iool0, c1iscaux, c1iscaux2, c1auxex, c1auxey)

  5593   Sat Oct 1 22:53:49 2011 kiwamuSummaryGeneralRecovery from the power shutdown : lockable

Found the Marconi for the 11 MHz source had been reset to its default.

 => reset the parameters. f = 11.065910 MHz (see #5530) amp = 13 dBm.

Interferometer became lockable. I checked the X/Y arm lock, and MICH lock, they all are fine.

  5599   Mon Oct 3 08:38:21 2011 steveSummaryVACRecovery from the power shutdown is completed

 

I failed to start Rana's favorit anciant desktop computer at the vac rack. He has to baby this old beast just a little bit more.

Vacuum status: Vac Normal was reached through Farfalla: Rga was switched back to IFO and and Annuloses are beeing pumped now.

V1 was closed for about a day and the pressure reached  ~2.8 mTorr in the IFO.  This leakrate plus outgassing is normal

The ref cavity 5000V was turned on.

The only thing that has to be done is to restart the RGA. I forgat to turn it off on Friday.

 

Attachment 1: poweroutage.jpg
poweroutage.jpg
  5662   Thu Oct 13 21:40:59 2011 ranaSummaryVACRecovery from the power shutdown is completed

 As it turns out Steve is not crazy in this particular instance: the vacuum computer, linux3 , has some issues. I can login just fine, but trying to open a terminal makes the CPU rail and the RAM max out and eventually the machine freezes.

Under KDE, I can open a terminal OK as root, but if I then try a 'su controls', the same issue happens. Let's wait for Jamie to fix this.

  5596   Sun Oct 2 13:45:13 2011 ranaSummaryGeneralRecovery from the power shutdown: apache / svn

Restarted Apache on nodus using Yoichi's wiki instructions. SVN is back up.

  4432   Wed Mar 23 12:40:22 2011 Chief Recycling OfficerHowToEnvironmentRecycle stuff!

The following is a message from the LIGO 40m Chief Recycling Officer:

Please get up off your (Alignment Stabilization Servo)es and recycle your bottles and cans!  There is a recycling bin in the control room.  Recent weeks have seen an increase in number of bottles/cans thrown away in the regular garbage.  This is not cool. 

Thank you,

---L4mCRO

  10449   Thu Sep 4 01:28:32 2014 ericqUpdateLSCRecycling cavity lengths

 Going off some discussion we had at lunch today, here is my current knowledge of the state of cavity lengths. 

Acknowledging that Koji changed the sideband modulation frequency recently, the ideal cavity lengths are (to the nearest mm):

  • Lprc = c / ( 4 * fmod) = 6.773 m
  • Lsrc = c / ( 5 * fmod) = 5.418 m

We when last hand measured distances, after moving PR2, we found:

  • Lprc  = 6.752 m = 2.1 cm short
  • Lsrc  = 5.474 m = 5.6 cm long. 

However, when I looked at the sideband splitting interferometrically, I found:

  • Lprc = 6.759m = 1.4 cm short

This is only 5mm from the hand measured value, so we can believe that the SRC length is between 5 and 6 cm too long. I'm building a MIST model to try and see what this may entail. 

  10451   Thu Sep 4 10:10:23 2014 KojiUpdateLSCRecycling cavity lengths

Com'on. This is just a 60ppm change of the mod frequency from the nominal. How can it change the recycling cav length by more than a cm?

https://wiki-40m.ligo.caltech.edu/IFO_Modeling/RC_lengths

This describes how the desirable recycling cavity lengths are affected by the phase of the sidebands at non-resonant reflection of the arms.

If we believe these numbers, L_PRC = 6.7538 [m] and L_SRC = 5.39915 [m].

Compare them with the measured numbers

  • Lprc = 6.752 m
  • Lsrc  = 5.474 m

You should definitely run MIST to see what is the optimal length of the RCs, and what is the effect of the given length deviations.

  10453   Thu Sep 4 18:16:20 2014 ericqUpdateLSCRecycling cavity lengths

Koji correctly points out that I naïvely overlooked various factors. With a similar analysis to the wiki page, I get:

  • Ideal arm length of 37.795 m
  • Ideal PRC length of 6.753 m
  • Ideal SRC length of 5.399 m

This means that:

  • The PRC, measured at 6.759m, is 6mm long. 
  • The SRC, measured at 5.474m, is 7.5 cm long

Next step is to see how this may affect our ability to sense, and thereby control, the SRC when the arms are going. 

MIST simulations and plots are in the attached zip. 

Attachment 1: 2014-09-CavityLengths.zip
  2071   Thu Oct 8 21:32:59 2009 KojiSummaryGeneralRecycling cavity loss

I looked at the data of the day before yesterday (Oct 06) to know how much is the recycling gain.

X arm: (TRX_PRecycled) / (TRX_PRMmisaligned) * T_PRM = 83.1/0.943*0.07 = 6.17
Y arm: (TRX_PRecycled) / (TRX_PRMmisaligned) * T_PRM = 99.2/1.017*0.07 = 6.83

==> G_PR = 6.5 +/- 0.5     (oh...this estimation is so bad...)

From the recycling gain and the arm cavity reflectance, one can get the loss in the recycling cavity.

G_PR = T_PRM  / (1-Sqrt(R_PRM * (1-L_PRC)*R_cav))^2

==> loss in the recycling cavity L_PRC: 0.009+/-0.009
       (About 1% loss is likely in the recycling cavity)

Quote:

<<X arm>>
Measured arm reflectivity R_cav: 0.875 +/- 0.005
Estimated round trip loss L_RT: 157ppm +/- 8ppm
Estimated finesse F: 1213+/-2

<<Y arm>>
Measured arm reflectivity R_cav:
0.869 +/- 0.006
Estimated round trip loss L_RT: 166ppm +/- 8ppm
Estimated finesse F: 1211+/-2

 

  1900   Fri Aug 14 02:57:46 2009 ClaraUpdatePEMRedo of the Huddle Test

I put all three seismometers and all six accelerometers together in the foam box with peanuts. Three of the accelerometers are facing in the x-direction and three are in the y-direction. Both Guralps are aligned on the NS axis and the Ranger is pointing vertically.

**EDIT: The accelerometers are in the x and z directions, not x and y. Sorry, I was sleepy when I wrote this.**

One of the accelerometers was refusing to show anything, and after a few hours of checking connections and swapping cables, I discovered that someone had unplugged the cable from the ADC. A quick glance in the dataviewer shows that the channel has been unplugged since about 3 in the afternoon on August 8th (Saturday). So... obviously all the accelerometer measurements made with that channel since then did not actually get recorded. Yay.

Anyway, as of 2:45, everything is working and taking data. Clearly we're not getting a full night's worth... hopefully that's okay.

  16623   Tue Jan 25 16:42:03 2022 AnchalSummaryBHDReduced filter gains in all damped new SOS

I noticed that our current suspension damping loops for the new SOS were railing the DAC outputs. The reason being that cts2um module has not been updated for most optics and thus teh OSEM signal (with the new Sat Amps) is about 30 times stronger. That means our usual intuition of damping gains is too high without applying correct conversion cts2um filter module. I reduced all these gains today and nothing is overflowing the c1su2 chassis now. I also added two options in the "!" (command running drop down menu) in the sus_single medm screens for opening ndscope for monitoring coil outputs or OSEM inputs of the optic whose sus screen is used.

 

  5096   Tue Aug 2 17:40:04 2011 JennyUpdatePSLReducing beam intensity incident on photodiode

I am using a PDA255 photodiode to measure the power outputted by the NPRO beam on the PSL table. (I'm going to then use a network analyzer to measure the amplitude response of the PZT to being driven at a range of frequencies. I'll detect the variation in in response to changing the driving frequency using this PDA255.)

The PDA255 has an active area of 0.8mm^2 and a maximum intensity for which the response is linear of 10mW/cm^2. This means that a beam I focus on the PD must have a power less than 0.08 mW (and even less if the spot size is smaller than the window size).

I used a power meter to measure the beam power and found it was 0.381 mW.

The second polarizing beam splitter in the setup transmits most of the beam power, but reflects 0.04 mW (according to the power meter). I'm going to place the photodiode there in the path of the reflected beam.

  741   Fri Jul 25 19:57:18 2008 JenneUpdatePSLRef Cav & PMC
"PMC is in, but is still being worked on. Leave it alone." ---Rana

Ref. Cavity is locked again. Still a work in progress. I think we're ready to mode match on Monday. ---Jenne
  1847   Thu Aug 6 18:26:26 2009 JenneUpdatePSLRef Cav and PMC aligned

[Alberto, Jenne]

We aligned both the reference cavity and the PMC, each by looking at their Trans PD on Davaviewer, and adjusting the two steering mirrors to maximize the transmission power.  We got a pretty good amount of improvement for the ref cav, but since the PMC hasn't decayed a whole lot, we got a much smaller amount of improvement.

  1850   Thu Aug 6 23:29:47 2009 JenneUpdatePSLRef cav reflection PD is funky

After Alberto and I worked on aligning the reference cavity, Rob asked the important and useful question: what is the visibility of the reference cavity.  This helps tell us if we're optimally aligned or not even close.

I did a scan of the ref cav temperature, using /scripts/PSL/FSS/SLOWscan, but there seems to be no real signal is C1:PSL-FSS_RFPDDC.  As shown in Alberto's 200-day plot, it does change sometimes, but if you zoom in on the flat parts, it seems like it's not really reading anything meaningful.  I did a cursory check-out of it, but I'm not 100% sure where to go from here:  There are (as with all of these gold-box PDs) 3 outputs:  a ribbon cable (for ADC purposes I think), an SMA for the RF signal, and a BNC for the DC signal.  The photodiode is clearly working, since if you stick the Lollypop in front of the PD, the cavity unlocks.  I plugged a 'scope into the DC BNC, and it also behaves as expected: block the beam and the signal goes down; unblock the beam and the signal goes up.  Something of note is that this readout gives a positive voltage, which decreases when the beam is blocked.  However, looking at the dataviewer channel, nothing at all seems to happen when the beam is blocked/unblocked.  So the problem lies somewhere in the get-signal-to-DAQ path.  I unplugged and replugged in the ribbon cable, and the value at which the channel has been stuck changed.  Many days ago, the value was -0.5, for the last few days it's been -1.5, and after my unplug/replug, it's now back to ~ -0.5 . The other day Alberto mentioned, and made the point again today that it's a little weird that the PD reads out a negative voltage.  Hmm.

 

Do we have a tester-cable, so that instead of the ribbon cable, I can plug that connector (or pins thereof) into a 'scope?

  1851   Fri Aug 7 00:10:14 2009 ranaUpdatePSLRef cav reflection PD is funky

we have a tester cable, but you don't want it. Instead the problem is probably at the cross-connect. The D-cable goes to a cross-connect and you can probe there with a voltmeter. If the signal is good there, trace it to the ADC. Also trend for several years to see when this happened - Yoichi may know the history better.

Also, we still need to complete the FSS RFPD task list from last year.

 

  1860   Fri Aug 7 17:05:34 2009 JenneUpdatePSLRef cav reflection PD is funky

Quote:

we have a tester cable, but you don't want it. Instead the problem is probably at the cross-connect. The D-cable goes to a cross-connect and you can probe there with a voltmeter. If the signal is good there, trace it to the ADC. Also trend for several years to see when this happened - Yoichi may know the history better.

Also, we still need to complete the FSS RFPD task list from last year.

 

 [Jenne, Ben]

I called in the reinforcements today.  Ben came over and we looked all around at all of the cross-connects and cables relating to the FSS.  Everything looks pretty much okey-dokey, except that we still weren't getting signal in the DataViewer channels.  Finally we looked at the psl.db file, which indicates that the C1:PSL-FSS_RFPDDC channel looks at channel 21 of the ADC cross connect thing.  We followed the cable which was plugged into this, and it led to a cable which was disconnected, but laying right next to the Ref Cav refl PD.  We plugged this into the DC out SMA connection of the photodiode (which had not been connected to anything), and suddenly everything was mostly golden again in dataviewer land.  RFPDDC_F now has a signal, but RFPDDC is still flat. 

 

Even though this seems to be working now, it's still not perfect.  Rob suggested that instead of having this SMA cable going from the photodiode's DC out, we should take the signal from the ribbon cable.  So I'm going to figure out which pin of the D-connector is the DC out, and take that from the cross connect to the ADC cross connect.  This will help avoid some persnickity ground loops. 

  5072   Sat Jul 30 20:41:50 2011 ranaUpdatePSLRefCav Stabilization back on

 Untitled.png

I turned the RefCav heater and servo back on a couple days ago. At first it was stabilizing again at a low setpoint, but in reality the right temperature (~40 C). After fixing the in-loop signal offsets, the setpoint now correctly reflects the actual temperature.

Jenny is going to calibrate the sensors using some kind of dunking cannister next week.

  12573   Wed Oct 19 18:32:25 2016 rana, yinziUpdatePSLRefCav thermal control: heater is dead

We wanted to re-activate the Heater for the reference cavity today to use it as a testbed for PID autotuning and the new heater driver circuit that Andrew is working on for the coating thermal noise experiment.

Unfortunately, it seems that the large power supply which is used for the heater is dead.sad Or maybe I don't remember how to use it?

The AC power cord was plugged in to a power strip which seems to work for IO chassis. We also tried swapping power strip ports.

We checked the front panel fuses. The power one was 3 Ohms and the 'bias' one was 55 Ohms. We also checked that the EPICS slider did, in fact, make voltage changes at the bias control input.

Non of the front panel lights come onfrown, but I also don't remember if that is normal.

Have those lights been dead a long time? We also reconnected the heater cable at the reference cavity side.

  1001   Fri Sep 26 19:08:43 2008 ranaConfigurationPSLRefcav Trans: PD + Camera + Dumps
I went out to improve the Refcav trans path.

I removed all ND filters to get rid of the fringing.

I removed the anodized Al dump that was there. Black anodized Aluminum dumps are forbidden for use as
dumps in any low phase noise setup (such as our frequency stabilization cavity). The scatter was going
directly back into the cavity and making noise. For now its undumped, but Steve will find the
reflections and dump them on unblemished razor blade dumps mounted stiffly.

I will post a photo of the new setup later - the new setup is sketched on the control room markerboard.

The transPD level is now 8 V, up from its previous 3-4 V. We will probably have to also put a lens
in front of it to get the beam size down.
  1923   Tue Aug 18 14:24:43 2009 YoichiSummaryPSLReference Cavity Inspection
Rana, Koji, Yoichi

To see why the reflected beam from the RC is distorted, we took out
the periscope and the iris in front of the RC. The periscope mirrors
had some gunk and dusts on them. We blew nitrogen air onto them to
remove the dust. Since the gunk did not come off with the air, we
wrapped a Q-tip with lens cleaning paper soaked in Methanol, and wiped
the surface of the mirrors. We did this because it was hard to remove
the mirrors from the periscope (they were in a spring loaded mirror
holders. The springs were too strong to safely remove them without
damaging the mirrors).

Looking into the RC from the front mirror revealed nothing obstructing
in the path.

After the cleaning, we put the periscope back and observed the direct
reflection from the cavity (not locked). It was still distorted
exactly like before.

So we did some tests.
First we injected He-Ne to the RC. It turned out that multiple
reflections from the optical window (not AR coated for He-Ne) made it
almost impossible to investigate anything with He-Ne. But this
observation made us to suspect maybe one side of the window is not AR
coated.

We placed the periscope about 50cm away from the RC and injected the
beam from an angle, so that we can observe the direct reflection.

First, we checked the shape of the beam leaving the periscope. It was good.
We then observed the reflected beam from the RC. It was also good, no distortion.
We made sure that it was really the reflection from the mirror, not from the window
as follows.
We measured the separation between the in coming beam to the cavity and the reflected beam
at two locations. From this, we can guess where the two beams intersect (the reflection point).
The estimated reflection point was far inside the RC enclosure, indicating that it was really
reflection from the front mirror of the RC.
Since we did not see any other reflection beam, we concluded that the AR coating of the window
is good.

We checked the direct reflection beam shape with several different incident angles, but the
beam shape was always good.

We put back the periscope to the original position. This time, we put a high reflectivity mirror
after the output mirror of the periscope. The beam coming out of the circulator (PBS) had a good
circular shape. But still if we let the beam reflected by the cavity, the beam shape is distorted.
Something must be happening in the RC. Unfortunately, we could not figure out what it is.

We put everything back to the original configuration, except for the iris, and the RC alignment
was already good (surprise). After Koji's final tweak, the FSS is now doing fine, but still
the reflected beam is ugly.
  3240   Fri Jul 16 20:25:52 2010 MeganUpdatePSLReference Cavity Insulation

Rana and I

1) took the temperature sensors off the reference cavity;

2) wrapped copper foil around the cavity (during which I learned it is REALLY easy to cut hands with the foil);

3) wrapped electrical tape around the power terminals of the temperature sensors (color-coded, too! Red for the out of loop sensor, Blue for the first one, Brown for the second, Gray for the third, and Violet for the fourth. Yes, we went with an alphabetical coding system, excluding the out of loop sensor);

4) re-wrapped the thermal blanket heater;

5) covered the ends of the cavities with copper, ensuring that the beam can enter and exit;

6) took pretty pictures for your enjoyment!

We will see if this helps the temperature stabilization of the reference cavity.

 

DSC_2271.JPG

The end of the reference cavity, with a lovely square around the beam.

 

DSC_2266.JPG

The entire, well-wrapped reference cavity!

  3241   Fri Jul 16 23:53:27 2010 RanaUpdatePSLReference Cavity Insulation

From the trend, it seems that the Reference Cavity's temperature servo is working fine with the new copper foil. I was unable to find the insulating foam anywhere, but that's OK. We'll just get Frank to make us a new insulation with his special yellow stuff.

The copper foil that Steve got is just the right thickness for making it easy to form around the vacuum can, but we just have to have the patience to wrap ~5-10 more layers on there. We also have to get a new heater jacket; this one barely fits around the outside of the copper wrap. The one we have now seems to have a good heating wire pattern, but I don't know where we can buy these.

I also turned the HEPA's Variac back down to the nominal value of 20. Please remember to turn it back up to 100 before working on the PSL.

  3280   Fri Jul 23 16:02:16 2010 RanaUpdatePSLReference Cavity Insulation

This is the trend so far with the copper foil wrapping. According to Megan's calculation, we need ~1 mm of foil and the thickness of each layer is 0.002" (1/20th of a mm), so we need ~20 layers. We have ~5 layers so far.

As you can see the out-of-loop temperature sensor (RCTEMP) is much better than before. We need another week to tell how well the frequency is doing -

the recent spate of power cycles / reboots of the PSL have interrupted the trend smoothness so far.

Attachment 1: Untitled.png
Untitled.png
  3282   Fri Jul 23 21:14:29 2010 RanaUpdatePSLReference Cavity Insulation

I wrapped another ~3 layers onto there. It occurs to me now that we could just get some 2mm thick copper plates made to fit over the stainless steel can.

They don't have to completely cover it, just mostly. I also took the copper circles that Steve had made and marked them with the correct beam height

and put them on Steve's desk. We need a 1" dia. hole cut into these on Monday.

To compensate for the cooling during my work, I've set the heater for max heating for 1 hour and then to engage the temperature servo.

I also noticed the HEPA VARIAC on the PSL was set to 100. Please set it back to 20 after completing your PSL work so that it doesn't disturb the RC temperature..

  2732   Mon Mar 29 21:43:27 2010 AlbertoConfigurationPSLReference Cavity PD Noise Spectrum

[Rana, Alberto]

This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.

This plot shows the spectrum noise from the RF output of the photodetector.
 
 (here you should be able to see an attached figure, if not it's probably becasue imagemagic has having problems with displaying png files)
2010-03-29_FSS_PD_shotnoise_and_darknoise.png
 
The tall peak at 21.5 MHz is the AM modulation introduced by the EOM. It seems to be caused by a misalignment of the EOM which might be somehow modulating the polarization.
The mount in which the EOM sits is not very solid. We should change it with something similar to that of the other two EOMs in the Mach Zehnder.
By tightening down the plastic screws of the mount Rana reduced the amplitude of the AM modulation by 20dB.
 
The bump in both the dark and shot noise are in corrispondence of the resonance of the PD's electronics. As it appears, the electronics is not well tuned: the bump should coincide with the AM peak.
 
In the case of the dark noise spectrum, the bump is due to the thermal noise of the electronics. It's a good sign: it means that the electronics is good enough to be sensitive to it.
 
Transimpedance Estimate
As a "sanity check" we made an approximate estimate of the transimpedance to make sure that the PD is dominated by shot noise rather than other noises, ie electronic's noise.
 
  1. Supposing that the laser beam hitting the PD was shot noise limited, we measured 1.1V at the DC output. That let us estimate the photocurrent at DC of 20mA, for a 50Ohm output impedance.
  2. The shot noise for 20mA is 80 pA/rtHz
  3. From the nosie spectrum, we measured 3e-7 v/rtHz at 21.5MHz
  4. The impedance at RF is then Z_rf = 3e-7 V/rtHz / 80e-12 pA ~ 4000 Ohm
  5. Since the RF path inside the PD has a gain of 10, the transimpedance is ~400Ohm, which is about as we (ie Rana) remembered it to be.
  6. The PD seems to be working fine.
Attachment 2: 2010-03-29_FSS_PD_shotnoise_and_darknoise.png
2010-03-29_FSS_PD_shotnoise_and_darknoise.png
  2733   Tue Mar 30 06:37:32 2010 ranaConfigurationPSLReference Cavity PD Noise Spectrum

Some more words about the RFAM: I noticed that there was an excess RFAM by unlocking the RC and just looking at the RF out with the 50 Ohm input of the scope. It was ~100 mVp-p! In the end our method to minimize the AM was not so sensible - we aligned the waveplate before the EOM so as to minimize the p-pol light transmitted by the PBS cube just ahead of the AOM. At first, this did not minimize the RFAM. But after I got angry at the bad plastic mounting of the EOM and re-aligned it, the AM seemed to be small with the polarization aligned to the cube. It was too small to measure on the scope and on the spectrum analyzer, the peak was hopping around by ~10-20 dB on a few second timescale. Further reduction would require some kind of active temperature stabilization of the EOM housing (maybe a good SURF project!).

For the EOM mount we (meaning Steve) should replace the lame 2-post system that's in there with one of the mounts of the type that is used in the Mach-Zucker EOMs. I think we have spare in the cabinet next to one of the arms.

After the RFAM monkeying, I aligned the beam to the RC using the standard, 2-mirror, beam-walking approach. You can see from the attached plot that the transmission went up by ~20% ! And the reflection went down by ~30%. I doubt that I have developed any new alignment technique beyond what Yoichi and I already did last time. Most likely there was some beam shape corruption in the EOM, or the RFAM was causing us to lock far off the fringe. Now the reflected beam from the reference cavity is a nice donut shape and we could even make it better by doing some mode matching! This finally solves the eternal mystery of the bad REFL beam (or at least sweeps it under the rug).

At the end, I also fixed the alignment of the RFPD. It should be set so the incident angle of the beam is ~20-40 deg, but it was instead set to be near normal incidence ?! Its also on flimsy plastic legs. Steve, can you please replace this with the new brass ones?

Attachment 1: rc.png
rc.png
  2759   Sat Apr 3 11:35:47 2010 ranaConfigurationPSLReference Cavity PD Noise Spectrum

The units on this plot are completely bogus - we know that the thermal noise from the resonant part of the circuit is just V = sqrt(4*k*T*Z) ~ 3nV/rHz. Then the gain of the MAX4107 stage is 10. The output resistor is 50 Ohms, which forms a divide by 2 with the input impedance of the spectrum analyzer and so the bump in the dark noise should only be 15 nV/rHz and not microVolts.

Quote:

[Rana, Alberto]

This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.

This plot shows the spectrum noise from the RF output of the photodetector.

  2760   Sat Apr 3 16:07:40 2010 AlbertoConfigurationPSLReference Cavity PD Noise Spectrum

 I was aware of a problem on those units since I acquired the data. Then it wasn't totally clear to me which were the units of the data as downloaded from the Agilent 4395A, and, in part, still isn't.

It's clear that the data was in units of spectrum, an not spectral density: in between the two there is a division by the bandwidth (100KHz, in this case). Correcting for that, one gets the following plot for the FSS PD:

2010-03-29_FSS_PD_shotnoise_and_darknoise.png

Although the reason why I was hesitating to elog this other plot is that it looks like there's still a discrepancy of about 0.5dBm between what one reads on the display of the spectrum analyzer and the data values downloaded from it.

However I well know that, I should have just posted it, including my reserves about that possible offset (as I'm doing now).

Quote:

The units on this plot are completely bogus - we know that the thermal noise from the resonant part of the circuit is just V = sqrt(4*k*T*Z) ~ 3nV/rHz. Then the gain of the MAX4107 stage is 10. The output resistor is 50 Ohms, which forms a divide by 2 with the input impedance of the spectrum analyzer and so the bump in the dark noise should only be 15 nV/rHz and not microVolts.

Quote:

[Rana, Alberto]

This evening we measured the noise spectrum of the reference cavity PD used in the FSS loop. From that we estimated the transimpedance and found that the PD is shot-noise limited. We also found a big AM oscillation in correspondence of the FSS modulation sideband which we later attenuated at least in part.

This plot shows the spectrum noise from the RF output of the photodetector.

  2742   Wed Mar 31 15:31:53 2010 steveUpdatePSLReference Cavity RF PD base upgraded

Quote:

Some more words about the RFAM: I noticed that there was an excess RFAM by unlocking the RC and just looking at the RF out with the 50 Ohm input of the scope. It was ~100 mVp-p! In the end our method to minimize the AM was not so sensible - we aligned the waveplate before the EOM so as to minimize the p-pol light transmitted by the PBS cube just ahead of the AOM. At first, this did not minimize the RFAM. But after I got angry at the bad plastic mounting of the EOM and re-aligned it, the AM seemed to be small with the polarization aligned to the cube. It was too small to measure on the scope and on the spectrum analyzer, the peak was hopping around by ~10-20 dB on a few second timescale. Further reduction would require some kind of active temperature stabilization of the EOM housing (maybe a good SURF project!).

For the EOM mount we (meaning Steve) should replace the lame 2-post system that's in there with one of the mounts of the type that is used in the Mach-Zucker EOMs. I think we have spare in the cabinet next to one of the arms.

After the RFAM monkeying, I aligned the beam to the RC using the standard, 2-mirror, beam-walking approach. You can see from the attached plot that the transmission went up by ~20% ! And the reflection went down by ~30%. I doubt that I have developed any new alignment technique beyond what Yoichi and I already did last time. Most likely there was some beam shape corruption in the EOM, or the RFAM was causing us to lock far off the fringe. Now the reflected beam from the reference cavity is a nice donut shape and we could even make it better by doing some mode matching! This finally solves the eternal mystery of the bad REFL beam (or at least sweeps it under the rug).

At the end, I also fixed the alignment of the RFPD. It should be set so the incident angle of the beam is ~20-40 deg, but it was instead set to be near normal incidence ?! Its also on flimsy plastic legs. Steve, can you please replace this with the new brass ones?

 Teflon feet removed and heavy brass-delrin pd base installed. Ref-cavity reflected light remains to be beautiful doughnut shape on camera.

Attachment 1: brspdbs.JPG
brspdbs.JPG
  1954   Wed Aug 26 19:58:14 2009 Rana, AlbertoUpdatePSLReference Cavity Temperature Control: MINCO PID removed

Summary: This afternoon we managed to get the temperature control of the reference cavity working again.

We bypassed the MINCO PID by connecting the temperature box error signal directly into EPICS.

We couldn't configure the PID so that it worked with the modified temperature box so we decided to just avoid using it.

Now the temperature control is done by a software servo by using the channel C1:PSL-FSS_MINCOMEAS as error signal and driving C1:PSL-FSS_TIDALSET (which we have clip-doodle wired directly to the heater input).

 

We 'successfully' used ezcaservo to stabilize the temperature:

ezcaservo -r C1:PSL-FSS_MINCOMEAS -s 26.6 -g -0.00003 C1:PSL-FSS_TIDALSET

 

We also recalibrated the channels:

C1:PSL-FSS_RMTEMP

C1:PSL-FSS_RCTEMP

C1:PSL-FSS_MINCOMEAS

with Peter King on the phone by using ezcawrite (EGUF and EGUL) but we didn't change the database yet. So please do not reboot the PSL computer until we update the database.

 

More details will follow.

Attachment 1: rc.png
rc.png
  1956   Thu Aug 27 13:42:08 2009 ranaSummaryPSLReference Cavity Temperature Control: psl.db changes

I made the changes to the psl.db to handle the new Temperature box hardware. The calibrations (EGUF/EGUL) are just copied directly from the LHO .db file (I have rsync'd their entire target area to here).

allegra:c1psl>diff psl.db~ psl.db
341,353d340
< grecord(ai,"C1:PSL-FSS_TIDALOUT")
< {
<       field(DESC,"TIDALOUT- drive to the reference cavity heater")
<       field(DISV,"1")
<         field(SCAN,".5 second")
<       field(DTYP,"VMIVME-3113")
<       field(INP,"#C0 S28 @")
<       field(EGUF,"10")
<       field(EGUL,"-10")
<       field(EGU,"volts")
<       field(LOPR,"-10")
<       field(AOFF,"0")
< }
493,494c480,481
<         field(EGUF,"285.675")
<         field(EGUL,"-214.325")
---
>         field(EGUF,"67.02")
>         field(EGUL,"7.96")
508,509c495,496
<         field(EGUF,"726.85")
<         field(EGUL,"-1273.15")
---
>         field(EGUF,"75.57")
>         field(EGUL,"12.31")
531,532c518,519
<         field(EGUF,"726.85")
<         field(EGUL,"-1273.15")
---
>         field(EGUF,"75.57")
>         field(EGUL,"12.31")
605,617d591
< grecord(ai,"C1:PSL-FSS_TIDALINPUT")
< {
<       field(DESC,"TIDALINPUT- tidal actuator input")
<       field(DISV,"1")
<         field(SCAN,".5 second")
<       field(DTYP,"VMIVME-3123")
<       field(INP,"#C0 S3 @")
<       field(EGUF,"10")
<       field(EGUL,"-10")
<       field(EGU,"volts")
<       field(LOPR,"-10")
<       field(AOFF,"0")
< }
1130a1105,1130
> grecord(ai,"C1:PSL-FSS_TIDALINPUT")
> {
>       field(DESC,"TIDALINPUT- tidal actuator input")
>       field(DISV,"1")
>         field(SCAN,".5 second")
>       field(DTYP,"VMIVME-3123")
>       field(INP,"#C0 S3 @")
>       field(EGUF,"10")
>       field(EGUL,"-10")
>       field(EGU,"volts")
>       field(LOPR,"-10")
>       field(AOFF,"0")
> }
> grecord(ai,"C1:PSL-FSS_TIDALOUT")
> {
>       field(DESC,"TIDALOUT- drive to the reference cavity heater")
>       field(DISV,"1")
>         field(SCAN,".5 second")
>       field(DTYP,"VMIVME-3113")
>       field(INP,"#C0 S28 @")
>       field(EGUF,"10")
>       field(EGUL,"-10")
>       field(EGU,"volts")
>       field(LOPR,"-10")
>       field(AOFF,"0")
> }
1143,1144c1143,1144
<         field(HOPR,"0.010")
<         field(LOPR,"-0.010")
---
>         field(HOPR,"2")
>         field(LOPR,"0")

  1018   Wed Oct 1 23:21:03 2008 YoichiConfigurationPSLReference cavity reflection camera
I re-centered the reference cavity reflection camera, which has been mis-aligned for a while.
I also tweaked an input steering mirror to make the alignment better. This resulted in the increase of the transmission PD voltage
from 8V to 9V.
  1915   Mon Aug 17 02:05:49 2009 Yoichi,ranaUpdatePSLReference cavity reflection looks bad
Rana, Yoichi

It has been a well known fact that the reference cavity reflection beam looks ugly.

We measured the visibility of the RC by locking and unlocking it.
Comparing the reflected beam powers, we got the visibility of 0.46,
which is pretty bad.

The beam going into the RC looks fine (circular on a sensor card).
However, the beam reflected back from the RC is distorted into a
horizontal ellipse, even when the RC is not locked.

We took a picture of the reflected beam hitting a white paper with the
infrared camera (see the attachment). It looks like two overlapping
circles horizontally separated. Could it be a badly coated optics
producing a secondary reflection ?

We looked into the RC's front mirror with an inspection mirror, but we
could not identify any obstructing object.

Rana is now touching the RC alignment.

We plan to remove the periscope before the RC to have a better look
into the cavity for inspection.


Late breaking update:
- We also moved the Refcav reflection camera to look at the leakage through a reflection steering mirror so that there's less chance of distortion. There was previously a W1 window in there as a pickofff. Also changed the camera to autogain so that we can see something.

- Re-aligned onto the refl pd.

- Tweaked alignment into RC. Mainly in yaw. Transmission went from 5V to 7V. In your face, Aso!
Attachment 1: P8170113.JPG
P8170113.JPG
Attachment 2: Untitled.png
Untitled.png
  1917   Mon Aug 17 04:16:13 2009 YoichiUpdatePSLReference cavity reflection looks bad

Quote:
Rana, Yoichi
- We also moved the Refcav reflection camera to look at the leakage through a reflection steering mirror so that there's less chance of distortion. There was previously a W1 window in there as a pickofff. Also changed the camera to autogain so that we can see something.

- Re-aligned onto the refl pd.

- Tweaked alignment into RC. Mainly in yaw. Transmission went from 5V to 7V. In your face, Aso!


After our removal of the pick off window and Rana's re-alignment of the beam into the RC, the RC optical gain increased.
FSS was complaining about it by driving the PC feedback crazy.
I reduced the nominal common gain from 12.5dB to 11dB.
  1136   Fri Nov 14 19:20:42 2008 YoichiUpdatePSLReference cavity ring down
Thanks to Bob making the high-voltage BNC cables for the HV pulse generator, I was able to operate the EOM in front of
the reference cavity.

The conceptual setup is the following:
[HV pulse] ----+           +-->-- [PD2]
               V           |
->--[HWP]->-- [EOM] -->-- [PBS] --<->-- [QWP] --<->-- [Reference Cavity] -->-- [PD1]
                           |
                [PD3] --<--+

The high voltage pulse rotates the polarization of the light after the EOM. When the HV is applied, the PBS reflects most of the light
into PD2 (Thorlabs PDA255), shutting down the incident light into the cavity.
The transmitted light power of the reference cavity is monitored by PD1 (PDA255). The reflected light from the reference cavity
is monitored by the DC output of the RF PD (PD3). PD3 is low-passed so the response is not fast.
Thorlabs says PDA255 has 50MHz bandwidth.

The attached plot shows the time series of the above PD signals when the HV was applied.
Input Pulse (blue curve) is the input to the HV pulse generator. When it is high, the HV is applied.
"PBS reflection" (red) is PD2. "Reflection" (green) is PD3. "Transmission" (light blue) is PD1.

The red curve shows huge ringing. At first I thought this was caused by the bad response of the PD.
However, the same ringing can be seen in the PD3 and the peaks match very well.
When red curve goes down the green curve goes up, which is consistent with the energy conservation.
So it looks like the light power is actually exhibiting this ringing.
May be the HV pulse is distorted and the voltage across the EOM is showing this ringing.
I will check the input voltage shape to the EOM using a high impedance probe, if possible.

The green curve shows a slow decay because it has a long time constant. It is not an actual
trend of the reflected light power.

The RC transmission power shows some peaks, probably due to the ringing in the input power.
So just fitting with an exponential would not give a good estimate of the cavity pole.
Even though, we should be able to de-convolute the frequency response of the reference cavity
from the input (red curve) and output (light blue curve) signals.
Attachment 1: RingDown.png
RingDown.png
  1137   Fri Nov 14 20:35:47 2008 ranaUpdatePSLReference cavity ring down
To make the DEI pulser make a fast pulse on the EO shutter EOMs, we had to make sure:

1) the cable had a high voltage rated dielectric. cheap dielectrics show the 'corona'
effect, especially when there is a bend in the cable.

2) the EO has to have a resistor on it to prevent ringing due to the impedance mismatch.

3) We needed ~3.5 kV to get the EO shutter crystal to flip the light by 90 deg.
  1138   Fri Nov 14 22:40:51 2008 YoichiUpdatePSLReference cavity ring down

Quote:

To make the DEI pulser make a fast pulse on the EO shutter EOMs, we had to make sure:

1) the cable had a high voltage rated dielectric. cheap dielectrics show the 'corona'
effect, especially when there is a bend in the cable.


I'll check it with Bob.


Quote:

2) the EO has to have a resistor on it to prevent ringing due to the impedance mismatch.


Did you use a shunt or series resistor ?
If shunt, I guess it has to have a huge heat sink.
Actually, DEI says the pulser does not require any external shunt/series resistors or impedance-matching network.
Looks like it is not true ...


Quote:

3) We needed ~3.5 kV to get the EO shutter crystal to flip the light by 90 deg.


Yes, I adjusted the voltage to maximize the power change and it was about 3.5kV.
  1140   Mon Nov 17 15:07:06 2008 YoichiUpdatePSLReference cavity ring down
I used MATLAB's system identification tool box to estimate the response of the reference cavity, i.e. cavity pole.
What I did was basically to estimate a model of the RC using the time series of the measured input and output power.

First, I prepared the input and output time series for model estimation.
The input is the input power to the RC, which I produced by inverting the PBS reflected light power and adding an offset
so that the signal is zero at t=0. Offset removal was necessary to make sure that the input time series does not give an
unintentional step at t=0.
The output time series is the transmission power of the RC. I also added an offset to make it zero at t=0.
Then I commanded MATLAB to compute the response of a first order low-pass filter to the input and try to fit
the computed response to the measured output by iteratively changing the gain and the cut-off frequency.
("pem" is the name of the command to use if you are interested in).

The result is shown in the attachment.
Blue curve is the input signal (I added a vertical offset to show it separately from the output).
The green curve is the measured output (RC transmission). The red curve is the response of the estimated model.
The estimated cut-off frequency was about 45kHz.

You can see that the red curve deviates a lot from the green curve after t=15usec.
By looking at this, I realized that the bandwidth of the RC cavity servo was too high.
The time scale we are looking at is about 50kHz whereas the FSS bandwidth is about 400kHz.
So when the input light was cut off, the error signal of the FSS becomes meaning less and the
input laser frequency was quickly moved away from the resonance. This is why the green curve does not
respond to the large peaks in the blue curve (input). The cavity was already off-resonance when the input power
showed bumps.

Since the red curve matches nicely with the green curve at the very beginning of the ring down, the estimated 45kHz
cavity pole is probably not that a bad estimate.

To make a better measurement, I will try to reduce the bandwidth of the RC servo by using only the PZT actuator.
If there were no ringing in the input light power, we wouldn't have to worry about the bandwidth of the servo because our
feedback is all made to the laser, not the cavity length.
In order to reduce the ringing in the input power, I asked Bob to make new HV cables using HV grade coax cables.
Attachment 1: Fit.png
Fit.png
  1190   Fri Dec 12 22:51:23 2008 YoichiUpdatePSLReference cavity ring down measurement again
Bob made new HV-cables with HV compatible coaxes. The coax cable is rated for 2kV, which was as high as Bob
could found. I used it with 3kV hoping it was ok.
I also put a series resistor to the pockels cell to tame down the ripples I saw in elog:1136.

Despite those efforts, I still observed large ringings.
I tried several resistor values (2.5k, 1k, 330ohm), and found that 330ohm gives a slightly better result.
(When the resistance is larger, the edge of the PBS Refl. becomes dull).
Since the shape of the ringing does not change at all even when the pulse voltage is lowered to less than 1kV,
I'm now suspicious of the DEI pulser.

Anyway, I estimated the cavity pole using the MATLAB's system identification toolbox again.
This time, I locked the reference cavity using only the PZT feedback, which makes the UGF about a few kHz.
So, within the time scale shown in the plot below, the servo does not have enough time to respond, thus the laser
frequency stays tuned with the cavity. This was necessary to avoid non-linear behavior of the transmitted power
caused by the servo disturbing the laser frequency. With this treatment, I was able to approximate the response of
the cavity with a simple linear model (one pole low-pass filter).

MATLAB estimated the cavity pole to be 47.5kHz.
The blue curve in the plot is the measured RC transmitted power.
The incident power to the cavity can be inferred from the inverse of the red curve (the PBS reflection power).
The brown curve is the response of the first order low-pass filter with fc=47.5kHz to the input power variation.
The blue and brown curves match well for the first 10usec. Even after that the phases match well.
So the estimated 47.5kHz is probably a reasonable number. I don't know yet how to estimate the error of this measurement.

According to http://www.ligo.caltech.edu/~ajw/PSLFRC.png the designed transmission of the reference cavity mirrors is 300ppm (i.e.
the round trip loss (RTL) is 600ppm).
This number yields fc=35kHz. In the same picture, it was stated that fc=38.74kHz (I guess this is a measured number at some point).
The current fc=47.5kHz means, the RTL has increased by 200ppm from the design and 150ppm from the time fc=38.74kHz was measured.
Attachment 1: RC-Ringdown.png
RC-Ringdown.png
ELOG V3.1.3-