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  40m Log, Page 190 of 341  Not logged in ELOG logo
ID Date Author Type Categoryup Subject
  240   Wed Jan 16 14:06:24 2008 robUpdateLSCmonday's locking
rob, tobin, johnnie

We did some locking work monday night, with decent progress. Working in the PRFPMI style, we managed to get through the part of the script that hands off the offset-CARM DOF to the MCL, but were not successful in engaging the AO path.

We also confirmed the problem with tdsread which prevent it from reading from multiple TLS (Three Lettered Subsystems) at the same time. Tobin traced this to a problem with the ezca library which tds uses, but it's not clear how to fix it. For now we just split the tdsread calls so that there are no multiple TLS calls. Tobin will report further on this.
  241   Wed Jan 16 14:09:45 2008 robUpdateLSCtuesday's locking

I got a little further with the locking (PRFPMI) last night, after discovering that the cable going from the CM board to the MC board was unplugged at the MC side. This explains why we weren't able to engage the AO path last night. Tonight, I got up to the point where DARM is handed off to OMC transmission, a step which repeatedly failed.
Eventually I realized that although all the lights are the green, the OMC Trans signal was not being updated in the LSC's memory. I suspect this is because the c1ass machine was powered down. Work continues.
  244   Thu Jan 17 14:13:20 2008 robUpdateLSCWednesday's locking
Incremental progress on locking yet again. This time the handoff of DARM to the OMC worked, and progress halted at handing off control of the common mode to REFL166.
  249   Fri Jan 18 15:31:47 2008 robUpdateLSCThursday's locking

rob, johnnie, andrey

On Thursday night we got the intereferometer fully locked in a power-recycled FPMI state. The obstacles included the REFL166 phase being wrong by 180 deg (because that's the correct phase for DRMI locking) and getting confused (again) by the "manual" mode dewhite switching at the ETMs. After turning on the dewhites and the MICH correction, we took the noise spectrum below.
Attachment 1: DARMnoise080118.png
DARMnoise080118.png
  252   Tue Jan 22 02:33:45 2008 robUpdateLSCDRMI work

0) The ETMY oplev needs work/centering

1) recentered DRMI oplevs

2) Did some light DRMI locking. Looked at the loops and the DD signals. The PODD signals look flaky; the beam may not be on the diode. MICH and PRC handoffs to DD signals were spotty, but not a total disaster. Changed the PD9 phase by 115 degs. Work continues on the DD_handoff subscript.

3) John says "There are ants everywhere."

4) Andrey is now versed in the arts of decimation.
  272   Sat Jan 26 02:08:53 2008 JohnOmnistructureLSCFibres
There is now a fibre running from the SP table to the ISCT at the Y-end. In the coming days I will try to mode match the beam from this fibre into the arm through ETMY. To achieve this I will be altering the optical layout of this table.
  296   Mon Feb 4 22:01:57 2008 JohnSummaryLSCFibres auxiliary locking - Fibers
I managed to couple ~75% of the light transmitted from the y arm, through the fibre, back to the SP table. I hoped that this would be a good way to match the beam from the fibre into the arm. Still no flashes. It looks like the cameras just aren't sensitive enough.
  315   Wed Feb 13 20:37:11 2008 JohnUpdateLSCFibre locking - Fiber
Sam and I observed fringes in the light reflected from the Y arm. These fringes are due to the sidebands and not the carrier. To improve matters we plan to reduce the RF AM and increase our modulation index.
  342   Wed Feb 27 22:05:03 2008 JohnUpdateLSCAuxiliary locking
A summary of the status of the auxiliary arm locking effort.

To help with lock acquisition we are attempting to independently lock the Y arm using light injected through ETMY. At present this secondary light source is an NPRO laser situated on the SP table. The laser light is transported to the ETM using a single mode optical fibre. In the future we might pick off some PSL light and apply a frequency shift.

We have been able to successfully mode match the fibre beam into the cavity and have been attempting lock the cavity using standard PDH signals (phase modulation sidebands are added to the light before it enters the fibre).

As yet no acceptable error signals have been produced. The demodulated RF signal is showing a time varying, bipolar dc offset.

We have minimised the residual amplitude modulation of the EOM but we expect small signals due to the undercoupled nature of the system, it could be that whatever RFAM still present is varying with time and causing this behaviour. We are also able to produce similar offsets by stressing (i.e. bending, shaking) the fibre. Could it be that the fibre is somehow converting PM into AM? Are we seeing etalon effects in the fibre or elsewhere?

If we cannot make any further progress with the existing setup we shall move the NPRO to the ETM table and try again. We are also looking into purchasing some other types of fibre.

Other things to consider are injecting through POY or using some other wavelength - neither seems obviously better.

Fiber, behavior
  348   Fri Feb 29 13:51:17 2008 JohnSummaryLSCPD6 response
I checked the response of PD6 using the AM laser. It looks happy enough.

16 averages
-10dBm source power
77.3mV dc on the diode
  349   Sun Mar 2 23:43:45 2008 ranaHowToLSCPD6 response
John's PD plotting script is superior to all of the ones we had before; lets make him post the script so we can all use it.

Looks like PD6 is not too happy after all; the 199 MHz response is not much higher than the 166 MHz response. I thought we were supposed to have them balanced to within 6 dB or so?
  360   Wed Mar 5 12:51:48 2008 JohnSummaryLSCInitial Ligo Arm finesse versus lambda
I've taken the coating recipes for the initial ligo arm cavity from Rana's web page (ligo.caltech/edu/~rana/mat/)
and plotted the finesse as a function of wavelength. There is some uncertainty over the indices of refraction but
the main conclusion remains unchanged - i.e. it appears that using other wavelengths will be difficult.
Stefan is looking at how to tune the layers of any new mirrors to make dichroic optics.
Attachment 1: FofLambdaLIGOI.jpg
FofLambdaLIGOI.jpg
  367   Mon Mar 10 20:46:41 2008 JohnConfigurationLSCETMY Trans PD & QPD
I've placed a 10% reflector in the path from ETMY to the trans and quadrant photodiodes.
  373   Thu Mar 13 02:52:06 2008 LisaConfigurationLSCLocking with 3f
Today we have tried to use the reflected signal demodulated at 3*f1 ~ 99 MHz (REFL31) for length control.
This signal is cool because it is generated by the beating of sidebands, so it is not very sensitive to what the carrier does inside the IFO.
In particular, its gain and the demodulation phase shouldn't change much while changing the CARM offset during the locking sequence.
The idea is therefore to use REFL31_I and REFL31_Q for controlling MICH and PRCL, with the goal of making the lock acquisition sequence more robust.

We minimized hardware changes by using the 199MHz demodulation board, changing the local oscillator to 99.586317 MHz, with an amplitude of +10 dbm (the 3f signals are therefore acquired as LSC-PD6_I and LSC-PD6-Q).

We locked both the PRM and the DRM in a stable way using the REFL31_I and REFL31_Q, after tuning the demodulation phase (50) and removing their offsets.
On the other hand, we weren't able to acquire the lock in the DRM configuration directly by using the 3f signals. We needed instead to use the f signals first, and switch to the 3f signals once the lock was already acquired, otherwise ending up locking DRM at a different working point.
One explanation for that might be the fact that the beam impinging upon the 3f diode is too big compared with the diode size (only 1 mm, half of the size of the f1 diode).
For these reason, in presence of misalignments, some of the reflected light goes in high order modes, which can be partially (or all) off the diode, thereby generating multi-zero crossing in the demodulated error signal.

The next step before making the test with the whole IFO is therefore to modify the telescope in front of the 3f diode in order to reduce the beam size and repeat the tests we did tonight in DRM configuration.


P.S.: We made a test by changing the frequency of the local oscillator by a little bit and then coming back to the original value. We observed that the phase of the signal can change, so every time this frequency is moved the 3f demod phase need to be retuned.

John, Rob, Rana, Lisa
  381   Fri Mar 14 15:52:07 2008 robConfigurationLSCLSC code change

I've edited the LSC code to send different signals to the ASS box. Now, instead of the previously selected error signals deemed to be acceptable for the Alignment Sensing and Stabalization system, it sends the LSC control signals for each suspension to the ASS box (in its new incarnation as the Adaptive Susurration Subtraction system). These are the signals after the output matrix, and also after the LSC-[SUS] filter modules.
  386   Thu Mar 20 16:06:27 2008 robConfigurationLSCLSC code change

I changed the LSC code again. I noticed that when turning off the LSC (e.g., going from LA to OFF), the cpu time would jump from ~50 to ~80, and irrevocably de-sync all the SUS controllers. This was because turning off the LSC would suddenly zero the inputs to the decimation filters that send information to the ASS box, which for some reason greatly increases the computation time of the iir filter function call. I changed the code so that these inputs are never zeroed. The ASS receives inputs from the LSC all the time now.

I also noticed that the ASS machine was running in ~2400 usec. Yes, 2,400 microseconds. I don't know how long it's been doing that, but I restarted it. Immediately after restart, it ran at 1700 microseconds. After using the "RESET" field in the adaptOnline code, that dropped to ~100 usec. Now it's not doing any adaptive filtering, as I don't know what the good settings are and no-one has been elogging their IFO work the last few days.
  396   Sun Mar 23 00:56:42 2008 JohnUpdateLSCMore on 3f
We ended our last attempt at 3f locking concerned about the beam size on PD6. I investigated tonight. The beam was not obviously overfilling the diode and a quick tweak of the steering mirror revealed a decent plateaux. Nevertheless we decided to try a different approach to see if we found the same problems as before on a different diode.

This time our 3f diode was Refl 33. I put a splitter on the output of the diode at the LSC rack sending one half into the usual refl 33 board, the other into refl DD 199 (which is demodulating at 99Mhz).

I got as far as handing off PRC to the 3f signal in lock. More work needed.
  407   Mon Mar 31 14:01:40 2008 jamieSummaryLSCSummary of DC readout PD non-linearity measurements
From March 21-26, I conducted some measurements of the response non-linearity of some mock-up DC readout photodetectors. The detectors are simple:
Vbias ---
        |
       PD
        |-------- output
     resistor
        |
       ---
        -
This is a description of the final measurement.

The laser current modulation input was given a 47Hz sine wave at 20mV. A constant small fraction of the beam was shown onto the reference detector, and a beam that was varied in DC power level was incident on the test detector. Spectra were taken from both detectors at the same time, 0.25Hz bandwidth, over 100 averages.

At each incident power level on the test detector, the Vpk in all multiples of the modulation frequency were measured (ie. V[i*w]). The difference between the 2f/1f ratio in the test and reference was then calculated, ie:
V_test[2*w]/V_test[1*w] - V_ref[2*w]/V_ref[1*w]
This is the solid black line in the plot ("t21-r21_v_power.png").

The response of a simulated non-linear detector was also calculated based on the Vpk measured at each harmonic in the reference detector, assuming that the reference detector had a purely linear response, ie:
V_nl[beta,2*w]/V_nl[beta,1*w] - V_l[2*w]/V_l[1*w]
these are the dashed colored lines in the plot ("t21-r21_v_power.png").

The result of the measurement seems to indicate that the non-linearity in the test detector is less than beta=-1.

The setup that was on the big optics table south of the laser, adjacent to the mode cleaner, is no longer needed.
Attachment 1: t21-r21_v_power.png
t21-r21_v_power.png
  429   Sun Apr 20 18:23:27 2008 ranaSummaryLSClocking attempts
I noticed that the adaptive FF for the MC had stopped doing anything; this turned out
to be that the MC lost lock and the mcdown script turned off the FF path to MC1.

Although there's no elog, it looks like there was ~60 attempts at locking the IFO
between 12:38 and 4:27 on Saturday afternoon. I'm attaching here a plot showing
lock attempt durations and a histogram of lock times.
Attachment 1: quix.png
quix.png
  466   Tue May 6 17:28:39 2008 robConfigurationLSCAP33 -> POX33

I am in the process of switching the POX166 and AP33 photodetectors, so that they become POX33 and AP166. The IFO_CONFIGURE buttons won't work until I finish.
  467   Wed May 7 15:25:41 2008 robConfigurationLSCAP33 -> POX33

Quote:

I am in the process of switching the POX166 and AP33 photodetectors, so that they become POX33 and AP166. The IFO_CONFIGURE buttons won't work until I finish.


Done. We're now in the 40m CDD configuration.
  468   Thu May 8 01:07:24 2008 ranaSummaryLSCFrequency Noise test: MC Trans Backscatter
There is a wandering hump in the MC_F spectrum. It can move around on the time
scale of seconds between 40 and 200 Hz. It has an amplitude ~5-50x above the background spectrum. This seems new; I don't remember it
from a year ago. It is there in the IFO unlocked as well as the IFO locked as well as the locked + CM mode.

Tapping the AS table and/or the PSL table enclosures produces a broadband increase in the MC_F spectrum but doesn't
selectively effect the hump.

We thought it might be backscatter from the MC TRANS path and so we stuck in one of Steve's cool black glass V's into
this space. No effect. We should design a black glass V dump which we can replicate in large quantities for us and for
the sites. Something like the one on the LSC PDs, but with a 1 sq. inch opening area and a 2 inch depth.


We have also done this on the MC2 - TRANS beam before. No noise reduced there either.

The noise hump is appearing in MC_F but not in CARM_IN1 (after the CM handoff) so it seems like the MC has enough gain
to squash it. This also exonerates the MC REFL path since anything there would not be effected by the MC servo gain and
so would be visible in CARM.

My best guess is that there is something really, really scattery on the PSL table. But for now it looks like this is not a
big factor in the locking
issues.
  557   Tue Jun 24 15:15:09 2008 JohnSummaryLSCLocking efforts
Rob, Rana, John

In the past week or so we've been working on reducing the CARM offset using a DC signal (SPOB DC).
We were able to get up to arm powers of around 30 (where a single arm cavity lock is a power of 1)
before instability set in and we would lose lock for, as yet, unknown reasons.

In recent nights locking efforts have taken a few backward steps.

Since last Thursday engaging the AO path has proved troublesome, i.e. engaging it would instantly
cause loss of lock. This seems to be related to problems with the mode cleaner servo. For the past
few nights it has been behaving strangely and could not be operated with the usual super boost stages.
Last night the situation was improved. MC boost stages could be used and the AO path engaged. The
cause of this problem and its spontaneous resolution are not understood.

Last night we were unable to switch CARM to SPOB DC. I've attached a spectrum of the MC2 length signal.
This path is being used for CARM and so gives an indication of the frequency noise after the mode
cleaner. At the moment the plot is calibrated in units of Rana's gut feeling. We already tested to see if
any of the excess noise was introduced by the WFS. No evidence was found. We'll try to make a useful
calibration soon and see if our problems are related to excess frequency noise.

Another realisation from last night was the effect of arm detuning on the analogue CARM path. When CARM is detuned
the coupled cavity pole removes an extra 90 degrees of phase. The digital path has the `moving zero' to compensate
for this. The analogue path has no such compensation and can therefore become unstable at moderate detunings.
We propose trying to reduce the CARM offset further before engaging the analogue path. This will give higher
gain and move the UGF to a region of increased phase margin.
Attachment 1: mcl080623.png
mcl080623.png
  583   Fri Jun 27 15:20:52 2008 robDAQLSC.ini file change

I removed C1:LSC-XARM_CTRL from the frames and added C1:LSC-CARM_ERR
  690   Thu Jul 17 13:08:37 2008 JohnSummaryLSCHOM resonances in the arms
On Tuesday night we attempted to lock the full DRFPMI in the optical spring configuration with the +f2 sideband resonant in the SRC.
Despite having no problems locking on the +f2 in a DRM we couldn't lock the full IFO.

There was some discussion about whether a +f2 higher order mode resonance in the arms could cause this problem.

I calculated the positions of the first six higher order modes for the carrier and all sidebands (using Siegman p 762 (23) with a plus sign).
Plot attached. Colors indicate different frequency components, numbers are the mode index (m+n). Thick lines are fundamental modes of
the sidebands. Heights of HOM indicators have been scaled by 1/(m+n)^2.

It appears that the first order transverse mode of the +166 is indeed partially resonant. We might try to tweak the sideband frequencies a
little to see if this helps us. It would probably be prudent to measure the MC length first.

Numbers used:

L = 38.5750; %average of Alberto's recent measurements elog #556
Retm = 57.375;
f166 = 165.977195e6;
f33 = 33.195439e6;
Attachment 1: HOMresonances.png
HOMresonances.png
  717   Tue Jul 22 22:11:58 2008 YoichiUpdateLSCX-arm g factor measurement
Alberto, Yoichi

We measured the g factor of the X-arm by slightly shifting the 166MHz sideband frequency:

We first locked the X-arm to TEM00 mode. Then misaligned the ETMX in yaw a little bit until the transmitted power is a half of the normal value.
In this way, we can expect that TEM01 mode will be resonated in the arm if a sideband with a suitable frequency is introduced.
To add such a sideband, we used the 166MHz EOM. According to John's calculation (ELog entry 690), the TEM01 mode of the 166MHz upper sideband is only about 100kHz away from the resonance. So by changing the 166MHz modulation frequency, we should be able to see the 166MHz upper sideband resonating in the X-arm.
We used the 166MHz PD at the AS to find the resonance.
When we modulated the 166MHz RF frequency by +/- 100kHz, we could see spikes in the AS166_I signal.
Then we fine tuned the RF frequency slowly by hand to find the exact resonant frequency. At that time, the X-arm PDH servo was oscillating at ~480Hz.
So the resonance was determined by maximizing this signal in the AS166_I.
The 166MHz signal was originally at 165.977195 MHz. I found the resonance around 165.985MHz. It is surprisingly close to the original modulation frequency (only 7.3kHz away). This number yields the g factor of 0.626 and the transverse mode interval of 0.285*FSR. I used the arm length of 38.5750m in this calculation. Because of the 480Hz oscillation, it was difficult to precisely determine the resonant frequency. We will try this again tomorrow after mitigating the oscillation.
Although the resonance of the 166MHz upper sideband is located at a lower frequency in John's prediction, we found a resonance at a higher frequency.
This can be interpreted as the discrepancy between the actual g-factor and the designed g-factor.

To confirm what we saw was really an arm cavity resonance, we will try to do the same thing with the arm cavities all mis-aligned.
(We expect no signal in this configuration.)

Appendix: the expected signal from AS166 port when the 166MHz upper sideband passes by a resonance of the arm cavity.
Since the carrier is resonating in the cavity and kept there by the PDH feedback using 33MHz sideband, its phase is virtually fixed at the AS166 port. The lower sideband's phase also does not change much because it is off resonance. The upper sideband get a large phase change when approaching to the resonance. This effectively rotates the modulation angle of the 166MHz sidebands, which was orthogonal to the carrier when off resonance (i.e. phase modulation), to create 166MHz amplitude modulation. Because the sideband axis is rotated, the signal should appear both in I and Q phases.
  718   Tue Jul 22 22:25:31 2008 ranaUpdateLSCLooptickle for existing 40m
John and I have adapted the Stefan-Looptickle model of the 40m upgrade to have the parameters of the old one.
(old one = what we have had for the last 4 years).

Its in the /cvs/cds/caltech/iscmodeling directory on the CDS computers but you can also check it out from the
MIT CVS repo; its part of the whole shebang.

It makes the attached theoretical NB. Feel free to modify it.
Attachment 1: nb.png
nb.png
  728   Wed Jul 23 22:34:07 2008 YoichiUpdateLSCArm cavity g-factor measurement
I tried the same thing as the X-arm to the Y-arm.
I'm puzzled. I found exactly the same behavior as the X-arm in the AS166 demodulated signals, whereas I expected different resonance frequency because of the arm length difference.

Here is more detailed account of the measurement today.

I locked the Y-arm and mis-aligned the end mirror in Yaw until the transmission power gets half.
Then I injected a 30Hz sinusoid into the error point of the Y-arm servo to shake the ETMY.
I observed AS166_I and AS166_Q as I changed the 166MHz frequency.

At 165.977MHz, both AS166_I and AS166_Q showed the 30Hz signal (15cnt p-p).
At 165.981MHz, Only I phase showed the 30Hz signal (40cnt p-p). No signal in Q.
At 165.984MHz, I and Q became the same amplitude again (20cnt p-p).
At 165.987MHz, Only Q phase showed the 30Hz signal (40cnt p-p). No signal in I.

Outside the above range, the signal decreases as the frequency go away. I think this is (at least partly) because the 166MHz sidebands no longer go through the MC at those frequencies.

I then locked the X-arm to the TEM01 mode. I saw exactly the same behavior as described above. This could be the resonance of TEM02 mode. I was expecting to see the resonance of TEM00 mode at the opposite side, but nothing there.

I unlocked the arm cavities and tried the same frequency scan of the 166MHz with one of the end mirrors shaken at 30Hz. I saw no signal at the AS166 port.
I also tried locking Y-arm and shaking the ETMX. No signal.
So it has to be something to do with the cavity resonance.

Since the MC transmission curve for 166MHz is folded in the measurement, it makes the interpretation of the results harder.
  729   Thu Jul 24 01:04:01 2008 robConfigurationLSCIFR2023A (aka MARCONI) settings

Quote:


P.S.: We made a test by changing the frequency of the local oscillator by a little bit and then coming back to the original value. We observed that the phase of the signal can change, so every time this frequency is moved the 3f demod phase need to be retuned.



We discovered this little tidbit in March, and remembered it tonight. Basically we found that whenever you change the frequency on one of these signal generators (and maybe any other setting as well), the phase of the signal can change (it's probably just the sign, but still...), meaning that you when you return settings to their intial value, not everything is exactly as it once was. For most applications, this doesn't matter. For us, where we use one Marconi to demodulate the product of two other Marconis, it means we can easily cause a great deal of grief for ourselves, as the demod phase for the double demod signals can appear to change.

Programmatically, what this means is that every time you touch a Marconi you must elog it. Especially if you change a setting and then put it back.
  730   Thu Jul 24 01:27:00 2008 KojiUpdateLSCArm cavity g-factor measurement

Quote:
I locked the Y-arm and mis-aligned the end mirror in Yaw until the transmission power gets half.
Then I injected a 30Hz sinusoid into the error point of the Y-arm servo to shake the ETMY.
I observed AS166_I and AS166_Q as I changed the 166MHz frequency.


A-ha! Do you always expect the 30Hz signal, don't you?
Because this is the PDH technique.

---------------
Recipe:
You have a carrier and phase modulation sidebands at 166MHz this time.
Inject them into a cavity. Detect the reflection by a photo detector.
Demodulate the photocurrent at 166MHz.

This is the PDH technique.

A 30Hz sinusoid was injected to the error point of the cavity lock.
This means that the cavity length was fluctuated at 30Hz.

We should see the 30Hz signal at the error signal of the 166MHz demodulation, regardless of the tuning of the modulation frequency!
In other words, the 30Hz signal in the demod signal at the 166MHz is also understandable as the beating between the 30Hz sidebands and the 166MHz sidebands.

---------------

So, now I feel that the method for the TEM01 quest should be reconsidered.

If we have any unbalanced resonance for the phase modulation sidebands, the offset of the error signal is to be observed even with the carrier exactly at the resonance. We don't need to shake or move the cavity mirrors.

Presence of the MC makes the things more complicated. Changing the frequency of the modulation that should go throgh the MC is a bit tricky as the detuning produces FM-AM conversion. i.e. The beam incident on the arm cavity may be not only phase modulated but also amplitude modulated. This makes the measurement of the offset described above difficult.

The setup of the abs length measurement (FSR measurement) will be easily used for the measurement of the transverse mode spacings. But it needs some more time to be realized.
  731   Thu Jul 24 02:57:26 2008 robUpdateLSCArm cavity g-factor measurement

Quote:

So, now I feel that the method for the TEM01 quest should be reconsidered.

If we have any unbalanced resonance for the phase modulation sidebands, the offset of the error signal is to be observed even with the carrier exactly at the resonance. We don't need to shake or move the cavity mirrors.

Presence of the MC makes the things more complicated. Changing the frequency of the modulation that should go throgh the MC is a bit tricky as the detuning produces FM-AM conversion. i.e. The beam incident on the arm cavity may be not only phase modulated but also amplitude modulated. This makes the measurement of the offset described above difficult.

The setup of the abs length measurement (FSR measurement) will be easily used for the measurement of the transverse mode spacings. But it needs some more time to be realized.


We should be able to see 166MHz sideband resonances using the double demodulated photodetectors. With these, the 33MHz sidebands will be acting as LO when the 166MHz sideband (or mode) resonates. Some modeling may be necessary to determine if the SNR will be good enough to make this worthwhile, however.
  733   Thu Jul 24 08:09:26 2008 YoichiUpdateLSCArm cavity g-factor measurement

Quote:

A-ha! Do you always expect the 30Hz signal, don't you?
Because this is the PDH technique.


Yes you are right. I realized this when I was thinking about it in the bed Smile
The 30Hz signal should always be present because the carrier is phase shifted at 30Hz by the cavity length change.
I think the change in the signal ratio between I and Q happened because as the 166MHz sidebands get phase change when they move around the MC transmission peak due to the cavity pole of the MC. It changes the optimal demodulation phase for the 166MHz PDH signal at the AS port.


Quote:

We should be able to see 166MHz sideband resonances using the double demodulated photodetectors. With these, the 33MHz sidebands will be acting as LO when the 166MHz sideband (or mode) resonates. Some modeling may be necessary to determine if the SNR will be good enough to make this worthwhile, however.


I will try, but at 100kHz away from the MC FSR (the number predicted by John's calculation), the transmission of the 166MHz sidebands is very weak. I did not see any signal when I swept it +/- 500kHz. Unfortunately, the Marconi's output level is almost at its maximum. So we don't have much room for increasing the sideband power.
  771   Wed Jul 30 15:28:08 2008 robUpdateLSCY arm locked

By using a combination of the SUS-DRIFT mon screen and the optical levers (which turned out pretty well) I steered the BS, ITMY, and ETMY back to their previous positions, and was able to lock the Y arm. The "Restore Y Arm" script on the IFO_CONFIGURE screen works. I couldn't test the alignment script, as a dump truck/construction vehicle showed up and started unlocking the MC.
  830   Tue Aug 12 21:38:19 2008 JohnUpdateLSCAccidental higher order mode resonances in the arms
Recently we had been having some trouble locking the full IFO in the spring configuration (SRC on +166).
It was thought that an accidental higher order mode resonance in the arms may have been causing problems.

I previously calculated the locations of the resonances using rough arm cavity parameters(Elog #690). Thanks to Koji
and Alberto I have been able to update this work with measured arm length and g factors for the y arm (Elog #801,#802).
I have also included the splitting of the modes caused by the astigmatic ETM. Code is attached.

I don't see any evidence of +166MHz resonances in the y arm.


In the attached plot different colours denote different frequencies +33, -33, +166, -166 & CR.
The numbers above each line are the mn of TEMmn.
Solid black line is the carrier resonance.
Attachment 1: HOMresonances.png
HOMresonances.png
Attachment 2: HOMarms2.m
%Check for accidental resonances of HOM in the arms (maybe due to
%sidebands). At the moment there is only data for the y arm.

clear all
close all
clc


%Stuff one might change often
modeorder = 0:5;          % Look for TEMmn modes where m,n run over modeorder
... 157 more lines ...
  883   Mon Aug 25 21:15:23 2008 ranaConfigurationLSCaux NPRO off
Looks like no one has used the Lightwave NPRO on the AS table after Koji left, so I turned it off so that it can rest until Alberto does the X-arm measurements.
  1028   Mon Oct 6 12:45:41 2008 AlbertoDAQLSCC1LSC in coma
Alberto, Joe,

The C1LSC medm screen is frozen and the C1LSC computer is down. We tried to reboot and to restart it first turning off the power and then just rebooting remotely. None of them worked. We check whether any of the cable was unplugged but they were ok. Also all the led turned on to green after rebooting.
Trying to reboot we get the following error message: init_module: device or resource busy.

We called Alex who first suggested to check all the connection and then to swap the timing cable between two Pentek boards but the computer was still down.
It is possible that the board is dead. Alex and Rolf are going to look into this problem and for any spare board.

by now we can't lock any DOF of the IFO.
  1029   Mon Oct 6 16:41:33 2008 AlbertoDAQLSCC1LSC in coma

Quote:
Alberto, Joe,

The C1LSC medm screen is frozen and the C1LSC computer is down. We tried to reboot and to restart it first turning off the power and then just rebooting remotely. None of them worked. We check whether any of the cable was unplugged but they were ok. Also all the led turned on to green after rebooting.
Trying to reboot we get the following error message: init_module: device or resource busy.

We called Alex who first suggested to check all the connection and then to swap the timing cable between two Pentek boards but the computer was still down.
It is possible that the board is dead. Alex and Rolf are going to look into this problem and for any spare board.

by now we can't lock any DOF of the IFO.


Alex, Rob, Alberto,

Alex replaced the Pentek board used by C1LSC with a spare one that they had at the Wilson house. That fixed the DMA failure but since the board had a channel broken, one of the channels (POY) was still not available.

Looking at the wiring diagram of the ASC crate, we found that one of the Pentek boards in it was actually not used by anything and thus available to replace the bad one in LSC. We switched the two boards so that now the one that Alex installed is mounted in the ASC crate and it is connected to the cable labeled 1x2-ASC 6.

C1LSC is up again and all the channels in the C1LSC screen, including POY, now seem to be working properly.
  1097   Tue Oct 28 11:10:18 2008 AlbertoUpdateLSCHigher Order Mode resonances in the X arms

Quote:
Recently we had been having some trouble locking the full IFO in the spring configuration (SRC on +166).
It was thought that an accidental higher order mode resonance in the arms may have been causing problems.

I previously calculated the locations of the resonances using rough arm cavity parameters(Elog #690). Thanks to Koji
and Alberto I have been able to update this work with measured arm length and g factors for the y arm (Elog #801,#802).
I have also included the splitting of the modes caused by the astigmatic ETM. Code is attached.

I don't see any evidence of +166MHz resonances in the y arm.


In the attached plot different colours denote different frequencies +33, -33, +166, -166 & CR.
The numbers above each line are the mn of TEMmn.
Solid black line is the carrier resonance.


I plugged the measures of the length of the X arm and radius of curvature of ETMX I made in to John's code to estimate the position of the resonances of the HOM for the sidebands in the X arm. Here's the resulting plot.
Attachment 1: HOM_resonances_Xarm.png
HOM_resonances_Xarm.png
  1115   Wed Nov 5 12:41:36 2008 AlbertoUpdateLSCAbsolute Length and g-factor measurements conclusions
Absolute Length and g-Factor Measurement for the 40m Arm Cavities, Summary of Results

MOTIVATION OF THE EXPERIMENT
Lately locking the interferometer in the so called spring configuration (SRC on +166 MHz sideband) has been difficult and a possible resonance of an higher order mode of the +166 MHz sideband in the arms was
hypothesized as the cause. We wanted to know the frequencies of the HOMs of the sidebands and see where they are, relatively to the carrier resonance.

THE EXPERIMENTAL TECHNIQUE IN BRIEF
A second laser beam from an NPRO is injected into the interferometer through the AS port. The beam is mode matched to the arm cavities so that it can resonate inside of these. The secondary beam interferes with
the PSL beam and the incident intensity on one end mirror, excluding by now any higher mode, is I(t)=I1+I2+(interference terms)*exp[-i*(f1-f2)*t]. The last term comes from the beat between the two fields at the
relative frequency of the two lasers. For beating frequencies multiple of the FSR of the cavity, the beat gets transmitted and appears at the trans PD.
Whereas the PSL has a constant frequency, the NPRO frequency fluctuates, so that the relative phase between the two is not constant. To prevent that, a PLL servo locks the phase of the NPRO to that of the PSL.
The result is a beat frequency at the steady and tunable value set by the local oscillator of the PLL.

Length Measurement
One arm at a time, the cavity is locked to the TEM00 mode of the main laser. The beat frequency is then scanned for a few cavity FSRs and the transmitted power is measured. A linear fit of the resonant frequencies gives
us the FSR of the cavity.

g-factor Measurement
For non-planar Fabry-Perot cavities, the HOMs of the laser are not degenerate and resonate in the cavity at frequencies different from the correspondent fundamental mode. The shift in frequency is measured by the
Transverse Mode Spacing (TMS) and it is a function of the g-factors of the cavity:

TMS=FSR*acos[sqrt(g1*g2)]/pi

with g1=1-L/R1, where L is the cavity absolute length and R1 the radius of curvature of the input mirror, and similarly for g2 for the end mirror.
We measured the TMS by means of the beat between an HOM of the main laser and the TEM00 of the secondary beam. To do that we locked the cavity to either TEM01/10 and looked at the transmitted power for frequencies
of the beat around the TMS expected from the design parameters of the cavity.
Since the phase of the intensity of the beat between TEM01/10 and TEM00 has only DC components if measured across a symmetric portion of the spot, it is necessary to brake the symmetry of the incident beam on the
PD by chopping it just before it hits the sensor.
We approximated g1=1 for the ITMs. The effect of an astigmatic ETM is to brake the degeneracy of the TEM10 and TEM01 modes and split their resonant frequencies. By measuring that shift, we can evaluate the radius
of curvature of the mirror for the axis of the two transverse modes.

EXPERIMENTAL RESULTS
X Arm
FSR     =  (3897627 +/- 5 )   Hz
L       = (38.45833  +/- 0.00005) m
g2x     =   0.31197  +/- 0.00004
g2y     =   0.32283  +/- 0.00004
R-ETM_x = (55.8957   +/- 0.0045) m
R-ETM_y = (56.7937   +/- 0.0038) m

Y Arm
FSR     = ( 3879252 +/- 30 )  Hz
L       = (38.6462   +/- 0.0003) m
g2x     =   0.31188  +/- 0.00004
g2y     =   0.32601  +/- 0.00004
R-ETM_x = (56.1620   +/- 0.0013) m
R-ETM_y = (57.3395   +/- 0.0011) m


CONCLUSIONS
The attached graphs,one for the X arm and the other for the Y arm, plot the distributions of the first HOMs of the sidebands near the carrier resonance in the arm cavities. As it appears, the resonances of
the +166 sideband are far enough for not resonating in the arm cavities if the arms are locked to the carrier.
We have to look for something else to explain the locking problem of the interferometer in the spring configuration.
Attachment 1: 2008-11-04_file_02-05.png
2008-11-04_file_02-05.png
Attachment 2: HOM_resonances_Xarm.png
HOM_resonances_Xarm.png
Attachment 3: HOM_resonances_Yarm.png
HOM_resonances_Yarm.png
  1197   Fri Dec 19 16:38:09 2008 steveUpdateLSCall optlevs centered
All optlevs were centered after full alignment.

Qpd sums are:
ETMX 12,229 counts
ITMX 9,932
ETMY 12,043
ITMY 4,362
BS 1,880
PRM 1,423
SRM 11,641
  1214   Fri Jan 2 18:49:54 2009 YoichiUpdateLSCLSC modulation frequencies adjusted
I noticed that the IFO did not lock in the MICH configuration.
This was because AS166Q signal was too small.
The demodulation phase seemed not right, i.e. the I-phase signal was larger than Q.
I suspected that the 166MHz modulation frequency was not exactly on the MC FSR, since I just
recovered the number written on the Marconi after the power failure.
I measured the optimal frequency by the method explained in elog:752.
It was 165981500Hz, which is pretty close to the number Rob measured in elog:952, but significantly different from
the label on the Marconi.
I set the frequencies of all the MARCONIs accordingly and updated the labels.

After this, the AS166 demodulation phase was still not good enough (the Q and I signals were about the same).
So I rotated the phase by 45deg. In principle, this should set the demod-phase right for DARM too. Is it correct, Rob ?
I also adjusted the PD offsets. After those adjustments, MICH locks stably with a slightly increased gain (20 as compared to 10 before).
  1223   Mon Jan 12 18:53:03 2009 YoichiUpdateLSCAS CCD centering and ASDD demod phase
After Rob's AS beam work, I centered the beam on the AS CCD.
I also optimized the ASDD demod-phase for the MICH signal.
Rob suggested to me that whenever we restart or change the frequency of the DD Marconis, we have to re-optimize the demod-phase
because the initial phase of the Marconi is random. We had the power failure, so it was time to do so.
I confirmed that MICH hand-off from REFL33Q to AS133DDQ is ok.
I will do the same thing for the PRCL, SRCL hand-offs.
  1225   Tue Jan 13 18:59:09 2009 KakeruUpdateLSCAS CCD centering and ASDD demod phase
I tuned the demod-phase for PRCL and SRCL hand-off, but it have not been optimized enoughly.
I continue this work tomorrow.


Quote:
After Rob's AS beam work, I centered the beam on the AS CCD.
I also optimized the ASDD demod-phase for the MICH signal.
Rob suggested to me that whenever we restart or change the frequency of the DD Marconis, we have to re-optimize the demod-phase
because the initial phase of the Marconi is random. We had the power failure, so it was time to do so.
I confirmed that MICH hand-off from REFL33Q to AS133DDQ is ok.
I will do the same thing for the PRCL, SRCL hand-offs.
  1233   Fri Jan 16 18:25:32 2009 Yoichi, Kakeru, RanaUpdateLSCArms were unstable
The single arm lock had been unstable for both arms in the past few days.

Symptoms:
When an arm was locked by itself, the transmitted power showed a lot of fluctuations (sharp drops).
The first attachment shows the arm power fluctuations in power spectrum and time series.
References are when the boost filters are off for the arm feedback.
You can see that when the boosts are off, the power fluctuates a lot.
Also it is obvious that X-arm is a lot worse than Y.


Diagnosis:
The second attachment is the comparison of the error signal spectra between boosts on and off.
(PD3_I is the error signal of X-arm, PD4_I is Y arm). References are boost on.
Since the arm power fluctuation was suppressed by the gain increase, it was suspected that the main
reason for the power fluctuation is not alignment fluctuation. Rather, it is length or frequency fluctuation.

Then I took spectra and coherences of PD3_I, PD4_I and MC_F with both arms locked independently.
You can see broadband coherence between PD3_I (Xarm) and MC_F (frequency noise). In contrast the coherence
between PD4_I and MC_F is smaller. This means X-arm is more susceptible to the frequency noise than Y.
What can make a simple Fabry-Perot cavity more susceptible to frequency noise ? An offset ?
So I canceled the X-arm offset at the X-arm filter bank. Bingo ! The arm power fluctuation of X-arm became as small as Y-arm
in the dataviewer.
But what is making this offset ?
After watching the dataviewer screen for a while, the arm power fluctuation became larger again. I had to re-adjust the artificial offset
to minimize the fluctuation. This made me think that the source of the offset must be something to do with alignment.
In this case, clipping of the beam at the PD was very suspicious.
So I checked the centering of the POX and POY PDs. As expected, POX was terribly off-centered.
POY was also not exactly at the center of the plateau of DC output.
After centering those PDs, the large offset in the arm loops went away.
Now the arm powers are stable without artificial offset in the loop filters.
The last attachment shows the comparison of arm power fluctuation before and after the PD centering.
(references are the measurements before the centering).
Attachment 1: TRXY.pdf
TRXY.pdf
Attachment 2: ErrorSignals.pdf
ErrorSignals.pdf
Attachment 3: coherenceBetweenArms.pdf
coherenceBetweenArms.pdf
Attachment 4: ArmPowersAfterPDwasCentered.pdf
ArmPowersAfterPDwasCentered.pdf
  1241   Wed Jan 21 16:18:17 2009 KakeruUpdateLSCAS CCD centering and ASDD demod phase
I tuned the DD demod-pahse for SRM.
It was tuned as the error singnal is to be 0 when the cavity is locked.

The problem is that the good phase changes if MICH and PRM are handed to DD or not.
This may be a result of the demod-phase of these two signals are tuned to be maximise the error signal, not to be 0-offset.

I will tune these two demod-phases, and write a script to tune.


Quote:
I tuned the demod-phase for PRCL and SRCL hand-off, but it have not been optimized enoughly.
I continue this work tomorrow.


Quote:
After Rob's AS beam work, I centered the beam on the AS CCD.
I also optimized the ASDD demod-phase for the MICH signal.
Rob suggested to me that whenever we restart or change the frequency of the DD Marconis, we have to re-optimize the demod-phase
because the initial phase of the Marconi is random. We had the power failure, so it was time to do so.
I confirmed that MICH hand-off from REFL33Q to AS133DDQ is ok.
I will do the same thing for the PRCL, SRCL hand-offs.
  1242   Wed Jan 21 22:53:08 2009 ranaUpdateLSCAS CCD centering and ASDD demod phase
Just my opinion, but I think all we want out of the DD signals is something to control the DRM
and not be sensitive to the carrier and the CARM offset. So if the handoff can be done so that
the lock point is unchanged from single demod then everything is fine.

A second order concern is how the 133 & 199 MHz signals are mixed in order to minimize the
matrix cross-coupling and the SNR of the diagonal elements.
  1285   Mon Feb 9 16:05:01 2009 YoichiUpdateLSCDRMI OK

After the ISS work, I aligned the IFO and confirmed that DRMI locks with good SPOB and AS166 values.

  1296   Thu Feb 12 11:21:54 2009 YoichiUpdateLSCLocking effort resumed
Last night, I restarted the locking work.
Quite some time was wasted by the disconnected REFL199 by Alberto for the cavity length measurement.
From now on, please put the interferometer back to the original state every day.
If possible, please refrain from changing the IFO settings (cabling, optics, etc).
It is also very important to always restore the full IFO alignment after you are done with your work.

While I was working on the optimization of the DD hand-off, the DRMI alignment got into a strange state.
Even when I did the whole dither alignment procedure from the beginning (from x-arm), the AS166Q did not go above 1000.
PRMI looks ok (SPOB goes above 1100). I could lock the DRMI but the lock position hops to other modes easily.
Manual tweaks of SRM did not help.
After running the whole alignment procedure several times in vain, I was too tired and went home.
I noticed that the single arm lock shows power drops again. There are some offsets in the arm lock loops.
This may have prevented the Michelson alignment from being optimal. I will check this today.
  1298   Thu Feb 12 17:43:33 2009 YoichiUpdateLSCSRC strangeness solved
I found the problem with the DRMI lock I had last night was caused by the zero gain in the PD11_I filter.
I don't know how it happened but putting it back to 1.000 made the DRMI lock far more stable and AS166Q got more than 3000.

I also re-centered POY PD to remove the offset in the y-arm loop. The large power drops while y-arm is locked by itself were eliminated.
  1301   Fri Feb 13 13:35:38 2009 YoichiUpdateLSCLocking status
Yoichi, Jenne, Alberto, Rob

Last night, the locking proceeded until the CARM -> MC_L hand-off.
However, the MC_F gets saturated (as expected) and the IFO loses lock soon after the hand-off.
So we need to offload MC_F.
We ran the offloadMCF script, but it did not work, i.e. just waiting for CARM mode.
Looks like an EPICS flag is not set right.
ELOG V3.1.3-