ELOG 40m

Higher 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

1115

Wed Nov 5 12:41:36 2008

Absolute 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

Attachment 2: HOM_resonances_Xarm.png

Attachment 3: HOM_resonances_Yarm.png

1197

Fri Dec 19 16:38:09 2008

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

LSC 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

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

Kakeru

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

Quote:

1233

Fri Jan 16 18:25:32 2009

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

Attachment 2: ErrorSignals.pdf

Attachment 3: coherenceBetweenArms.pdf

Attachment 4: ArmPowersAfterPDwasCentered.pdf

1241

Wed Jan 21 16:18:17 2009

Kakeru

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:

1242

Wed Jan 21 22:53:08 2009

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

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

Locking 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

SRC 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

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.

1304

Sat Feb 14 16:53:26 2009

Quote:

I found a '$<' in the offloadMCF script. I don't know precisely what that construct means, but I think it caused the script to wait for input when it shouldn't. It probably got in there accidentally. We need to be careful when we're opening scripts just to look at how they work that we don't accidentally change them. I like to use the command 'less' for this purpose.

With this gone, the script worked properly, although the lock didn't last long. I don't know if the next stage in the process is failing or if it's just a bit too noisy in the afternoon. I didn't get a chance to do much testing since the sus controller (susvme1) went nuts. In retrospect, this could be due to something in the script, so maybe we should try a burt restore to Friday afternoon next time someone wants to look at it.

1305

Sun Feb 15 09:35:00 2009

Quote:

I found a '$<' in the offloadMCF script. I don't know precisely what that construct means, but I think it caused the script to wait for input when it shouldn't.

'$<' acts like 'read' in csh. I might have put it in the offloadMCF script to debug the behavior of the script.
Sorry I probably forgot to remove it from the script when I left.

1306

Sun Feb 15 15:53:21 2009

Quote:

I didn't get a chance to do much testing since the sus controller (susvme1) went nuts. In retrospect, this could be due to something in the script, so maybe we should try a burt restore to Friday afternoon next time someone wants to look at it.

I tried the burt restore today, it didn't work. Also tried some switching of timing cables, and multiple reboots, to no avail. This will require some more debugging. We might try diagnosing the clock driver and fanout modules, the penteks, and we can also try rebooting the whole FE system.

1308

Mon Feb 16 10:18:13 2009

Quote:

I tried the burtrestore today, it didn't work. Also tried some switching of timing cables, and multiple reboots, to no avail. This will require some more debugging. We might try diagnosing the clock driver and fanout modules, the penteks, and we can also try rebooting the whole FE system.

I rebooted the whole FE system and now c1susvme1 and c1susvme2 are back on.

I can't restart the MC autolocker on c1susvme2 because it doesn't let me ssh in. I tried to reboot it a few times but it didn't work. Once you restart it, it becomes inaccessible and doesn't even respond to pinging. Although the controls for the MC mirrors are on.

The mode cleaner stays unlocked.

1309

Mon Feb 16 14:12:21 2009

Quote:

I can't restart the MC autolocker on c1susvme2 because it doesn't let me ssh in. I tried to reboot it a few times but it didn't work. Once you restart it, it becomes inaccessible and doesn't even respond to pinging. Although the controls for the MC mirrors are on.

The mode cleaner stays unlocked.

MC autolocker runs on op340m, not on c1susvme2.
I restarted it and now MC locks fine.
Before that, I had to reboot c1iool0 and restore the alignment of the MC mirrors (for some reason, burt did not restore the alignment properly, so I used conlog).

1316

Tue Feb 17 05:20:11 2009

Since we excluded *.snap and *.req files from the svn control in the medm directory and these were not restored by the svn co, the burt part of the align/mis-align scripts were not working correctly this evening. So I recreated .req files and cooked up some mis-aligned .snap files.
After some cut-and-try work, I was able to run the dither alignment scripts fine.

Due to the above mentioned delay, the locking work started around midnight.

Tonight, the DD hand-off was not robust. I spent sometime to optimize this.
After the optimization, the locking proceeded to the DC CARM/DARM control state stably.
The CARM->MCL hand-off failed because the LSC-MC offset button was off.
I added a line to turn on the button in the ontoMCL script.
Today, the offloadMCF script worked fine.

Next, the cm_step script stumbled on the "ENGAGERIZING" of the AO path.
I got a hunch that the AO path might not be connected to the MC board.
Indeed, OMC_OSC_FM was connected to the IN2 of the MC board. Looks like it was used for the optimization of the modulation frequencies.
Probably I had the hunch because I did it Smile

I was able to increase the arm power up to 3.9.
The script failed when it tried to switch the CARM signal from TR_DC to SPOB_DC.
I haven't tackled on this issue yet.

1317

Wed Feb 18 03:17:40 2009

Yoichi, Kakeru,

Last night, the cm_step script failed at the hand-off of CARM error signal from TR_DC to PO_DC.
This was fixed by reducing the PO_DC gain by a factor of 2.
Currently the script fails when changing C1:LSC-DEMOD_GAIN to zero.
To be honest, I don't fully understand the purpose of this step.

1323

Thu Feb 19 04:16:17 2009

Rob, Yoichi

We checked the CM-MC cross over just before turning off the moving zero.
There was a slight bump in the gain of the MC_L loop at (I believe) the optical spring freq. (~400Hz) just below 0 dB. The phase margin there was very thin.
Removing the moving zero will increase the bump more and make the loop unstable.
Rob suggested to increase the AO gain a bit more.

To see if the AO path is really working, I connected the OUT2 of the MC board to a spare DAQ channel (C1:PEM-OSA_APTEMP).
I confirmed that the PO_DC signal is actually coming to the AO path input of the MC board.
I also hooked up the SR785 to the A excitation channel of the common mode board, so that we can measure the loop gain of the AO path.
After these preparation, the lock acquisition process became somewhat unstable. The ifo loses lock randomly at various places in the lock acquisition steps.
So, as of 4:00 am, I have not gotten a chance to try Rob's suggestion nor the TF measurement with SR785 yet.
I will continue the work tomorrow (i.e. tonight ??).

1330

Fri Feb 20 19:31:16 2009

MICH low gain problem

Last night, we found that MICH UGF was too low. Even after re-aligning the PDs, it was still too low.
Today, I compared the UGFs of MICH and PRC when in the DRMI configuration locked with the single demod. signals.
In this configuration, MICH signal comes from REFL33Q and the PRC signal comes from REFL33I (the same PD).
The PRC UGF was about 100Hz whereas MICH was only ~10Hz.
Since they uses the same PD, the low gain is not caused by the PD.
I checked conlog history and confirmed there is no change in the MICH->BS path in the last few days.
I also checked the svn history of chans directory for changes in filters. Nothing problematic found.

Then I noticed that the susvme computers were overloaded.
This time, I rebooted all the FE computers just in case.

Then the MICH gain was somewhat recovered (by a factor of 3 or so). Don't know why.

I adjusted the DD_handoff script to set the MICH gain to 0.7 before the bounce-roll filter is engaged.

1348

Tue Mar 3 10:39:07 2009

c1lsc discontinued functioning

The c1lsc has been unstable since last night. Its status on the DAQ screen was oscillating from green to red every minute.

Yesterday, I power recycled it. That brought it back but the MC got unclocked and the autolocker could not get engaged. I think it's because the power recycling also turned c1iscaux2 off which occupies the same rack crate.

Killing the autolocker on op340 e restarting didn't work. So I rebooted also c1dcuepis and burt-restored almost all snapshot files. To do that, as usual, I had to edit the snapshot files of c1dcuepics to move the quotes from the last line.

After that I restarted the autolocker that time it worked.

This morning c1lsc was again in the same unstable status as yesterday. This time I just reset it (no power recycling) and then I restarted it. It worked and now everything seems to be fine.

1401

Fri Mar 13 20:23:37 2009

AO path transfer function with X-arm locked

I measured the AO path transfer function while the X-arm is locked with the POX PDH signal.
The POX-I signal was already connected to the input 1 of the CM board. So I injected a signal from the EXC-B channel of the board and measured the transfer function from TP2B to TP1A. To open the loop, I disabled the switch befor the EXC-B.
The attached plot shows the measured transfer function.
There is a bump around 2kHz, which can also be seen in the AO path TF posted in elog:1399, but not the large structure at around 3.8kHz.
The 3.8kHz structure is probably created by the feedback.

Attachment 1: AOPath-Xarm.png

1415

Sun Mar 22 22:39:24 2009

Calibration of the ITM and ETM Actuation

I used the following procedure to calibrate the ITMX actuation signal.

Free Swinging Michelson:
------------------------
- Restore Michelson
- Align Michelson: Minimize AS_DC (PD3_DC_OUT) by tweaking BS alignment.
- Enable Whitening filters for PD1_Q and PD3_DC.
- Run offsets script to get rid of DC and RF offsets.
- Use DTT Triggered Time Series to get time series and measure peak-peak
amplitude of PD1_Q using DTT horizontal cursors. (Templates/Calibration/090322/FreeSwing.xml)

Michelson Sweeps:
-----------------
- Lock Michelson
- Drive ITMX_LSC_EXC using ITMX-MI-Sweep.xml template.
- (Next time remember to turn on a low pass in the MICH loop so that its an open loop measurement below 50 Hz).
- Fit and so some math.

Arm Sweeps for the ETMs:
------------------------
- Lock a single arm
- Sweep ITM & ETM.
- Then sweep MC2 and record drive signal from MC board to the VCO driver.
- Compare and contrast.

Attachment 1: free.png

1418

Mon Mar 23 15:50:44 2009

Configuration

filters deleted, lsc rebooted

The LSC time had gone too high. I deleted ~20 filters and rebooted. CPU time came down to 50 usec.

The filters all looked like old trash to me, but its possible they were used.

I didn't delete anything from the DARM, CARM, etc. banks but did from the PD and TM filter banks. You can always go back in time by using the

filter_archive/

1423

Tue Mar 24 19:55:24 2009

[Rana, Jamie, Jenne]

SPOB DC hasn't been so good lately, so we installed a new PO DC PD on the PO table. We used a 30% reflecting beam splitter (BS1-1064-30-1025-someotherstuff). We didn't check with a power meter that it's a 30% BS, but it seems like that's about right. The beamsplitter is as close as we could get to the shutter immediately in front of the regular POB/SPOB PD's, since that's where the beam gets narrow. The new picked-off-pickoff beam goes to a Thorlabs 100A PD. We haven't yet checked for reflected beams off the PD, but there is a spare razor blade beam dump on the table which can be used for this purpose. The output of this PD goes to the LSC rack via a BNC cable. (This BNC cable was appropriated from it's previous "use" connecting a photodiode from the AP table to a bit of air just next to the LSC rack.) Our new cable is now connected where the old SPOB DC cable used to be, at the input of a crazy Pomona Box tee.

For reference, the new levels of POB DC and SPOB DC, as measured by their BNC DC out connections is ~4mV each. Since the beamsplitter is 70% transmissive, we used to be getting about 5.7mV on each PD.

The new photodiode puts out about 40mV, but it has an ND1.0 filter on, so if more gain is needed, we can take it off to get more volts.

1424

Tue Mar 24 23:23:05 2009

We also found that the SPOB RF cable was going through a splitter before going into the SPOB demod board. The other
input of the splitter was open (not terminated). Using 50m Ohm devices without terminated inputs is illegal. It
makes there be standing waves in the cables and makes the RF phase very dependent on cable lengths. We took away
the splitter and ran the cable straight. So expect some change in the SPOB gain and phase plus some shame.

1456

Mon Apr 6 21:50:43 2009

Arm Locking via pushing MC2

Inspired by our 'No Refcav' scheme here, I was inspired to re-explore the idea of locking the
CARM DOF using only feedback to the MC/laser. Last week I got this to work on the single arm and
full IFO at Livingston.
I also estimate the MC noise there.

Today I found the settings to allow X-arm locking here without any feedback to the ETM or ITM:

- Set the LSC Output Matrix to feed the XARM signal to MC2.
- Turn OFF the input of the LSC-ETMX filter bank (this does not disable tickling).
- Turn OFF FM7 (0.1:10) in MC2-MCL.
- Turn ON MC2-LSC with a gain of 0.2 and FM3 FM4 FM5.

That's enough to lock the arm - its pretty stable. This also assumes that the LSC-MC2 bank has its nominal gain of -0.178.

To determine the gain of +0.2 in the MC2-LSC filter bank, I measured the TF from MC2->PD3_I and from ETMX->PD3_I. I adjusted
the gain to be equal at 150 Hz for acquisition and the sign to be opposite to account for the (-) in LSC-MC2. The TF is
attached.

After locking, I type a zero into the MC2-MCL filter bank and that shuts off the feedback from the MC servo to MC2. This is
now topologically similar to the standard CM servo configuration.

The second attachment has the trends of this locking. You can see that the MC_F goes off into the weeds, but the MCL signal
does not so much. I think maybe the MC length is drifting a lot - not the arm.

The third attachment shows the spectra.

Attachment 1: mc2-xarm.pdf

Attachment 2: Untitled.png

Attachment 3: nohands.pdf

1549

Tue May 5 14:02:16 2009

DARM DC response varies with DARM offset

Note the effect of quadrature rotation for small offsets.

Attachment 1: DARM_DARM_AS_DC_2.png

Attachment 2: DARM_DARM_AS_DC_3.png

Attachment 3: DARM_DARM_AS_DC_2.pdf

Attachment 4: DARM_DARM_AS_DC_3.pdf

1575

Tue May 12 01:11:55 2009

DARM response (DC Readout)

I measured the DARM response with DC readout.

This time, I first measured the open loop transfer function of the X single arm lock.
The open loop gain (Gx) can be represented as a product of the optical gain (Cx), the filter (Fx), and the suspension response (S), i.e. Gx = Cx*Fx*S.
We know Fx because this is the transfer function of the digital filters. Cx can be modeled as a simple cavity pole, but we need to know the finesse to calculate it.
In order to estimate the current finesse of the XARM cavity, I ran the armLoss script, which measures the ratio of the reflected light power between the locked and the unlocked state. Using this ratio and the designed transmissivity of the ITMX (0.005), I estimated the round trip loss in the XARM, which was 170 ppm. From this number, the cavity pole was estimated to be 1608Hz.
Using the measured Gx, the knowledge of Fx and the estimated Cx, I estimated the ETMX suspension response S, which is shown in the first attachment.
Note that this is not a pure suspension response. It includes the effects of the digital system time delay, the anti-imaging and anti-aliasing filters and so on.

Now the DARM open loop gain (Gd) can also be represented as a product of the optical gain (Cd), the filter (Fd) and the suspension response (S).
Since the actuations are applied again to the ETMs and we think ETMX and ETMY are quite similar, we should be able to use the same suspension response as XARM for DARM. Therefore, using the knowledge of the digital filter shape and the measured open loop gain, we can compute the DARM optical gain Cd.
The second attachment shows the estimated DARM response along with an Optickle prediction.
The DARM loop gain was measured with darm_offset_dc = 350. Since we haven't calibrated the DARM signal, I don't know how many meters of offset does this number correspond to. The Optickle prediction was calculated using a 20pm DARM offset. I chose this to make the prediction look similar to the measured one, though they look quite different around the RSE peak. The input power was set to 1.7W in the Optickle model (again this is just my guess).

It looks as if the measured DARM response is skewed by an extra low pass filter at high frequencies. I don't know why is it so.

Attachment 1: SUS_Resp.png

Attachment 2: DARM_Resp.png

1576

Tue May 12 01:22:51 2009

Using the armLoss script (/cvs/cds/caltech/scripts/LSC/armLoss), I measured the round trip loss (RTL) of the arms.

The results are:
XARM: RTL= 171 (+/-2) ppm
YARM: RTL = 181 (+/-2) ppm

To get the results above, I assumed that the transmissivity of the ITMs are the same as the designed value (0.005).
This may not be true though.

1577

Tue May 12 15:22:09 2009

Quote:

It looks as if the measured DARM response is skewed by an extra low pass filter at high frequencies. I don't know why is it so.

One large uncertainty in the above estimate is the cavity pole of X-arm because I simply assumed that the ITMX reflectivity to be the designed value.
I think we can directly measure the X-arm finesse from Alberto's absolute length measurements (i.e. from the width of the resonant peaks in his scans).
By looking at Alberto and Koji's posts (elog:1244 elog:838), it looks like the FWHM of the peaks are around 3kHz. With the FSR ~ 3.8MHz, it gives a finesse of about 1300, which is reasonable.
Alberto, can you check your data and measure the FWHM more precisely ?
Note that we want to measure the FWHM of the peak in the *power* of the beat signal. The beat amplitude is proportional to the electric field *amplitude* of the transmitted auxiliary laser. What we need to get a finesse is the FWHM of the transmitted laser *power*. Thus we need to take the power of the beat signal.

1591

Fri May 15 17:30:00 2009

This is the two arms locked, for an hour. No integrator in either loop, but from this it looks like ETMY may have a bigger length2angle problem than ETMX. I'll put some true integrators in the loops and do this again.

Attachment 1: armslock_no_int.png

1592

Sat May 16 16:20:33 2009

arms, coils, locks, #2

Quote:

There appear to be at least two independent problems: the coil balancing for ETMY is bad, and something about ITMX is broken (maybe a coil driver).

The Y-arm becomes significantly misaligned during long locks, causing the arm power to drop. This misalignment tracks directly with the DC drive on ETMY. Power returns to the maximum after breaking and re-establishing lock.

ITMX alignment wanders around sporadically, as indicated by the oplevs and the X-arm transmitted power. Power returns to previous value (not max) after breaking and re-establishing lock.

Both loops have integrators.

Attachment 1: twoproblems.png

Attachment 2: coil_imbalanceETMY.png

Attachment 3: ITMXalignment.png

1650

Thu Jun 4 01:32:20 2009

Here is a set of mode scans of the AS port, using the OMC as a mode scanner. The plot overlays various configurations of the IFO.

To remove PZT nonlinearity, each scan was individually flattened in fsr-space by polynomial (3rd order) fitting to some known peak locations (the carrier and RF sidebands).

Attachment 1: modes.png

1661

Mon Jun 8 18:22:27 2009

DAQ

Added PD11 I amd Q slow channels

I just added two slow channels to C0EDCUEPICS to monitor the input of PD11. The names are:

[C1:LSC-PD11_I_INMON]
[C1:LSC-PD11_Q_INMON]

1667

Thu Jun 11 03:15:15 2009

Pete, Alberto,

tonight we worked on the tuning of the double demod phases for the handoff of the short DOFs control signals.

Only MICH can now undergo the handoff. PRC can't make it.

Basically, we tuned the PD6 demod phase and reduced the offset in PD6_I. Then we tuned the relative gain of PD6_I and PD2_I so that the two open loop transfer function of the control loops would match. We tried that in several ways and several times but without success.

I guess we're missing to do/check something.

1669

Thu Jun 11 22:14:10 2009

DD Handoff for the Short DOFs completed

This afternoon I tuned the handoff script for the SRC, after that Rob eralier during the day had already adjusted that for PRC. To do that, I followed the procedure in the Wiki.

I measured the OL transfer function of the single demod path and of the double demod path and tuned thier gains so that they matched
I tuned the double demod pahses of PD_7 and PD_8 in order to reduce the offset in the PD_x_I signals

After that the SRC could get locked with the double demod signals. the open loop transfer function emasurement on the PRC loop showed that it was nearly unstable. Rob reduced a little its gain to improve the stability.

The DD handoff is now working and we can get back to locking the interferometer.

1685

Thu Jun 18 23:20:40 2009

I-Q ratio with RF detected DARM

This is a ratio of PD1_I to PD1_Q (so a ratio of the two quadratures of AS166), measured in an anti-spring state. It's not flat because our set up has single sideband RF heterodyne detection, and using a single RF sideband as a local oscillator allows one to detect different quadratures by using different RF demodulation phases. So the variation in frequency is actually a measure of how the frequency response of DARM changes when you vary the detection quadrature. This measure is imperfect because it doesn't account for the effect of the DARM loop.

Even though you can choose your detection quadrature with this setup, you can't get squeezed quantum noise with a single RF sideband. The quantum noise around the other (zero-amplitude) RF sideband still gets mixed in, and negates any squeezing benefits.

Attachment 1: IQratio.jpg

1878

Mon Aug 10 17:27:47 2009

These are the settings which determine the transmon (eg, TRX) amplitude, and which are updated by the matchTransMon scripts.

For the X arm

op440m:AutoDither>tdsread C1:LSC-TRX_GAIN C1:LSC-LA_PARAM_FLT_01 C1:LSC-LA_PARAM_FLT_00
0.0023
0.155
119.775

For the Y arm

op440m:AutoDither>tdsread C1:LSC-TRY_GAIN C1:LSC-LA_PARAM_FLT_04 C1:LSC-LA_PARAM_FLT_03
0.00237196
-0.116
19.9809

1929

Wed Aug 19 18:02:22 2009

All of the LSC RF PDs have been aligned. I didn't really change much of anything, since for all of them, the beam was already pretty close to center. But they all got the treatment of attaching a Voltmeter to the DC out, and adjusting the steering mirror in both pitch and yaw, finding where you fall off the PD in each direction, and then leave the optic in the middle of the two 'edges'.

Before aligning each set (PO, Refl, AS), I followed the procedure in Rob's new RF photodiode Wiki Page.

Also, for superstitious reasons, and in case I actually bumped them, I squished all of the ribbon cable connectors into the PDs, just in case.

1966

Thu Sep 3 23:41:32 2009

Today I aligned the beam to PD3 (POX) since Steve had moved it.

The DC power read 1.3mV when the beam was on the PD.

1999

Thu Sep 24 20:17:05 2009

Comparison of Material Properties for the new RFPD Mounts

	Steel	Brass	Aluminum	Delrin
Density (kg/m^3)	7850	8500	2700	1420
CTE (ppm/C)	12	19	23	100
Young's Modulus (GPa)	200	110	69	2
Hardness
Color	grey	gold	light silver	any

2016

Tue Sep 29 01:50:10 2009

Configuration

new modulation frequencies

Mode cleaner length measured tonight.

33196198

132784792

165980990

199177188

[Tag by KA: modulation frequency, MC length]

2101

Fri Oct 16 03:16:50 2009

rana, rob

funny timing setup on the LSC

While measuring the Piezo Jena noise tonight we noticed that the LSC timing is setup strangely.

Instead of using the Fiber Optic Sander Liu Timing board, we are just using a long 4-pin LEMO cable which comes from somewhere in the cable tray. This is apparent in the rack pictures (1X3) that Kiwamu has recently posted in the Electronics Wiki. I think all of our front ends are supposed to use the fiber card for this. I will ask Jay and Alex what the deal is here - seems like to me that this can be a cause for timing noise on the LSC.

We should be able to diagnose timing noise between the OMC and the LSC by putting in a signal in the OMC and looking at the signal on the LSC side. Should be a matlab script that we can run whenever we are suspicious of this. This is an excellent task for a new visiting grad student to help learn how to debug the digital control system.

Attachment 1: 1X3_1.JPG

2104

Fri Oct 16 13:25:18 2009

Koji