I made a short stop at the 40m on Sunday night and found that hundreds ants are in the coffee maker.
I removed ants around the sink and washed the coffee maker.

It looked the ants were everywhere in the lab tonight. They seemed to prefer warm places like in the coffee maker and below the coffee mill.
So, I recommend that Steve should confirm there is no ants in the coffee maker again before the first coffee of the week is made.
Othewise they will add some more acidity to your cup.

 We still had some ants visiting the sink area this morning. These ants seem to be addicted to our our Peet's coffe

Spectracide: Bug Stop insect killer was sprayed. Please wash your eating dishes well ! and keep area clean.

I updated my bandpass filter and have included the bode plot below in Figure 1. It is a fourth order elliptic bandpass filter with a passband ripple of 1dB and a stopband attenuation of 30 dB. It emphasizes the area between 3 and 40 Hz.

Below, I applied this filter to the huddle test data. The results from this were only slightly better in the targeted region than when no pre-filter was applied. 

When I pre-filtered the mode cleaner data and then used an IIR wiener filter, I found that the results did not differ much from the data that was not pre-filtered. I'm not sure yet if I'm targeting the right region of this data with my bandpass filter, and will be looking more into choosing a better region. Also, I am only using certain regions of ff when calculating the transfer function, and need to optimize that region also. I uploaded the code I used to make these plots to github.

After asking Alex specifically what he did yesterday after I left, he indicated he copied  a bunch of stuff from Hanford, including the latest gds, fftw, libframe, root.  We also now have the new dtt code as well.  But those apparently were for the Gentoo build   After asking Alex about the ezca tools this morning, he discovered they weren't complied in the gds code he brought over.  We are in the process of getting the source over here and compiling the ezca tools. 

Alex is indicating to me that the currently compiled new gds code may not run on the Centos 5.5 since it was compiled Gentoo (which is what our new fb is running and apparently what they're using for the front ends at Hanford).  We may need to recompile the source on our local Centos 5.5 control machines to get some working gds code.  We're in the process of transferring the source code from Hanford.  Apparently this latest code is not in SVN yet, because at some point he needs to merge it with some other work other people have been doing in parallel and he hasn't had the time yet to do the work necessary for the merge.

For the moment, Alex is undoing the soft link changes he did pointing gds at the latest gds code he copied, and pointing back at the original install we had.

I have modified Arbcav to be way cooler than it used to be.

Main modifications:

• Can now truly model an arbitrary cavity geometry
  • The previous version could only handle a few different topologies. In each case, it would unfold the cavity into the equivalent linear cavity and use the g-parameter method to calculate gouy phases, etc.
  • The new model uses the closed cavity propagation matrix to find the supported mode, and then explicitly calculates the accumulated gouy phase by propagating the beam through the full cavity. This is done analytically with z_R, so there is negligible slow-down.
• Now plots a diagram of the cavity geometry, both to help you and for you to verify that it is calculating the right thing (<-- this is the cool part)
  • Plots the beam path and mirror locations
  • Specifies whether mirrors are curved or flat
  • Prints mirror parameters next to them
  • Finds all intracavity waist locations and plots them
  • Gives waist information (size in X, Y)

Since the information is already there, I will have the output structure include things like the input beam q parameter, which could then be fed directly to mode matching tools like ModeMatchr.

The function takes as input the same arguments as before. Example for a square cavity:

out = arbcav([200e-6 50e-6 200e-6 50e-6],[0.75 0.75 0.75 0.75],[1e10 9 1e10 9],[45 45 45 45],29.189e6,10e-6,1064e-9,1000);

i.e.,

out = arbcav(transmissivity_list, length_list, RoC_list, angle_list, modulation_freq, loss_list_or_loss_per_mirror, wavelength, num_pts_for_plot);

If you don't give it a modulation frequency, it will just plot carrier HOMs. If you don't give it RoCs and angles, it will just plot the transmission spectrum.

I'm still fine-tuning some functionality, but I should have it up on the SVN relatively soon. Comments or suggestions are welcome!

Some screenshots:

Cavity geometry plots (linear, triangular, square, bowtie):

Transmission and HOM spectra (these correspond to the square cavity at lower left, above):

I haven't finished debugging the scripts so that the measurement is fully automatic, like the MC, but I did measure the arm spot positions just now. 

These numbers aren't especially precise....I just picked numbers off of a StripTool plot, but they give us a good idea of how very far off we are.  Also, I don't know yet which way the signs go...I have to think about that in terms of the direction I mis-balanced the coils.  It's the same convention as the MC though.  You can see in the attached quad camera image (quadrants match the corners of the table) that these numbers aren't unreasonable.

EDIT: It occurs to me now, a little later, that it had been at least half an hour since I last realigned the cavities, so some of this apparent miscentering is due to the input pointing drift. That doesn't account for all of it though. Even when the cavities have very high transmitted power, the spots are visibly miscentered.

In either .../scripts/XARM or ...../scripts/YARM run either A2L_XARM or A2L_YARM.

The wrapper script will, like the MC script, open a striptool so you can monitor the lockin outputs, setup the measurement, run the measurement, including misbalancing coils on the optics for calibration, and then calculates the spot positions.  It records the measurement in a log file in /data_spotMeasurements under each arm's directory.  The wrapper script then runs the plotting script which reads the logfile, and plots all past measurements.

Here is that plot for the Yarm:

The first two points were measured within a few minutes of eachother, the third set of points was after input pointing adjustment during IFO alignment.  Clearly the pointing that optimized the cavity transmission (trying to leave the test mass mirrors alone, and only moving TT1 and TT2) does not also give the best spot centering.  I claim that this is a result of the arm being aligned to the green beam, which was never locked to the 00 mode when we were at air.  This is a lesson learned....take the time to deal with the green beams.

After Den's work with the ASS model this week, all of the channel names were changed (this wasn't pointed out in his elog....grrr), so none of the A2L scripts worked. 

They are now back, however there is still some problem with the plotting that I'm not sure I understand yet.  So, the measurement works, but I don't think we're saving the results and we certainly aren't plotting them yet. 

I wanted to check where the spots are on the mirrors, to make sure Den's stuff is doing what we think it's doing.  All of the numbers were within ~1.5mm of center, although Rossa keeps crashing (twice this afternoon?!?), so I can't copy and paste the numbers into the elog.

A near-term goal is to copy over Den's work on the Yarm to the Xarm, so that both arms will auto-align.  Also, I need to put the set of alignment scripts in a wrapper, and have that wrapper call-able from the IFO Configure screen.

Also, while thinking about the IFO Configure screen, the "save" scripts weren't working (on Rossa) today, even though I just made them work a week or so ago. Rossa, at least, was unhappy running csh, so I changed the "save" script over to bash.

We had persistent frustration by occasional unlock during ASSing.
Today, I added triggers to the servo gains in order to elliminate this annoyance.

Each ASS servo gain slider is multiplied with the corresponding LSC Trigger EPICS channel (i.e. C1:LSC-iARM_TRIG_MON, where i=X or Y).
This has been done by ezcaread modules in RCG.

The model and screen have been commited to svn.

I'm going to work on the X arm to measure the arm cavity finesse.

The idea is to measure the cavity transfer function to estimate the frequency of its cavity pole. That should be a more accurate measurement than that based on the cavity decay time.

I'm starting now and I'm going to inject a swept sine excitation on the OMC_ISS_EXC input cable laying on the floor nearby the AP table (see pic).

In orderf to do that I disconnected the cable from the OMC breakout box laying on the floor. I'm going to plug the cable back in as soon as I'm done.

 Since I need to measure the transfer function between TRX and MC_TRANS_DC I picked off the beam going to RFAM PD to send it to a PDA255 photodiode (cannibalized from the AbsL's PLL) which I installed on the PSL table.

I centerd the beam on the PD and I was able to see the injected signal.

I think I'm ready to measure the transfer function.

Except for the RFAM PD everything is as before.

I'm gonna go grab dinner and I should be back to keep working on that in about one hour.

 Back from dinner. Taking measurements.

         f_fsr
dfmn =   ----- * (m+n) * acos(sqrt(g1*g2))
           pi

Tomohiro and I performed some tests under Rana's guidance to find cross corelations between WFS1 and WFS2 output signals in both pitch and yaw. We performed this test to further understand the degree to which our output matrices have been diagonolized.

Seen in attachment 1 is our base level with no injected noise source. In each figure, we also have inlcuded the coherence plot which compares each control signal to the overalll YARM power signal.

Attachments 2-5 display our results for injecting noise into each control signal individually.

We found the following corelations for each respective test:

We judged our corelated signals by the peaks seen from out noise injection on the power spectrum as well as by their coherence at the same frequencies of our noise (20Hz-30Hz) compared to the overall power spectrum of YARM.

Performing this measurement was done using diaggui and awggui. The diaggui files for each test are saved at: "users/Templates/singleArmCal/ArmCavityNoise_230317_2_WFS1_PIT"

To properly fix each of the control signals to the same magnitude plotted for YARM output, we callibrated each plot using the settings seen in Attachment 7. First the units were changed on the plots to represent the true scale of each measurement:

We found that the ETMY actuation strength is 10.843e-9 / f^2 (from 17376) and used this to clibrate the plots to the nanometer scale. Next the gain was adjusted such that each plot would align over the YARM output when noise was injected onto it, setting a basis for all four measurements.

Finally, some filtering poles were added to the callibration for each plot such that it resembled that of the filters seen by the YARM ouput signal. (RXA: this is the 28 Hz ELP filter to simulate the dewhitening filters)

The measurements were taken with the settings seen in Attachment 8, and noise injected using the parameters seen in attachment 9.

RXA: Some edits/comments:

The noise was injected as band-limited random noise with a Normal distribution. We used noise rather than lines so as to capture the linear and bilinear noise contributions. In the case where the coupling is mostly bilinear, we would not expect to see much coherence.

The first attachment is a ASC noise budget for the single arm - in the high gain mode, the noise does not limit the noise as seen by the arm. Next is to see if its due to the MC dewhitening filters being on/off?

Last week we vented and we cleaned the main optics of the arm cavities.

I measured the frequency of the cavity poles for both the arm cavities to see how they changed (see previous elog entry 2226). These the results:

fp_X = 1616 +/- 14 Hz

fp_Y = 1590 +/- 4 Hz

 The Y arm cavity pole moved down by 130 Hz, whereas the X arm moved by only 34 Hz. I wonder if that is because Kiwamu spent much more time on cleaning ITMY than on any other optic.

[Rana, Manasa]

ALS noise suppressed to 1KHz/rtHz. 1kHz RMS.

Plot 1: Scan of X arm by changing offset into Phase Tracker -> Xarm loop. Filter bank ramp time set to 120 s + using a 30 mHz low pass filter. IR beam is aligned to x arm, but not well.

Plot 2: ALS error signal with loop open (BLUE), closed with old filters (PURPLE), and with new, better boost (RED).

Plot 3: Bode plot of new boost (FM10), v. old, sad boost (1:50 pole:zero). RMS is now less than 1 kHz or ~50 pm. (in your face, Kiwamu!)

Changes made to the ALS servo:

1. C1ALS-TRX 

ALS-TRX has been calibrated to read from 0-1 instead of counts in 1000 s. Calibration factor = 1/4500 = 0.00022

2. C1ALS_BEATX_FINE

Old antiwhitening filter has been removed. Added LPF at 1000Hz to remove glitches at high frequencies.

3. C1ALS-BEATX_FINE_PHASE

No changes made.

4. C1ALS-XARM

FIlter FM5 modified. 1000:1 changed to 3000:1

5. Offset for ALS scan were given through C1ALS_OFFSETTER1 with LPF50m enabled.

The filter modules of the servo were:

 Next:

Check PZT out range for ALS. Figure out what the deal is with ALS SLOW servos.

Add DQ channels for ALS.

Automatic ALS up script (enable and disable phase tracker included).

 Isn't this still a factor of 2 away from the limit in the paper?

My understanding is that that number is an in-loop evaluation of the loop so far (as the first step of the loop evaluation).
This is not what we can directly compare with the number in the paper.

Basically the entry 8741 is telling us that the new filter suppresses the error signal better than before.
That's clearly shown in the attachment 2.

The interferometer is warming up!

I had some issues locking the IMC at first. It turned out that the MC3 side OSEM signal wasn't getting to the ADC. A satellite box sqush fixed it. 

I touched up the PMC alignment; the best I could do is 0.75V, probably due to the AOM being in place. 

I haven't touched the WFS offsets, but the current ones seem to be doing ok. I'll touch them up tonight when the seismic activity has calmed. 

I made some changes to the state of the PZT/PC crossover gain in the mcdown script, resulting in the IMC catching lock quicker. 

Thankfully, the tip tilt pointing stayed good during the upgrade. I barely had to touch the ETM alignment to lock the arms. ETMX is showing some errant motion, though... 

Steps:

1) Turn off feedback to ETMY (the ETMY button on the LSC screen).

2) Put a 1 into the YARM->MC2 output matrix element on the LSC screen.

3) Turn off FM6 (comb), FM7 (0.1:10) on the MC2_MCL filter bank. This is to make the IOO-MCL loop more stable and to reduce the IOO-MCL low frequency gain.

4) Set the MC2-LSC gain to 0.5, turn the output ON, turn ON FM4 & FM5 & FM6 of the MC2-LSC filter bank.

5) Turn on the input of MC2-LSC and the arm should now lock.

6) After locking, set the MC2-MCL gain to zero. Hopefully with a few second ramp time.

Voila!

(A comment by KA - c.f. this entry )

 [ericq, Gabriele] 

Summary: After today's meeting, Gabriele and I looked into the arm loss situation, to see if we should really believe the losses that had been suggested by my previous measurements. We made some observations that we're not sure how to explain, and we're thinking about other ways to try and estimate the losses to corroborate previous findings. 

We first looked to see if the ASS had some effective offset, leaving the alignment not quite right. Once ASS'd, we twiddled each arm cavity mirror in pitch and yaw to see if we could achieve higher transmission. We could not, so this suggested that ASS works properly. 

We then looked at potential offsets in the Xarm loop. We found that an input offset of 25 counts increased the transmission, but only very slightly. With this offset adjusted, we confirmed the qualitative observation that locking/unlocking the xarm causes a much bigger change in ASDC than doing the same with the harm.

However, we noted that the ASDC data (which is the DC value of the AS55 RFPD) was quite noisy, hovering around 50 counts. Looking at the c1lsc model, we found that we were looking at direct ADC counts, so the signal conditioning was not so great. We went to the LSC rack and stole the SR560 that had been hooked up as a REFLDC offsetter, and used it to give ASDC a gain of 100, and a LP at 100Hz, since we only care about DC values. We then undid the gain in the input FM; and this calmed the trace down a fair bit. The effects due to each arm locking/unlocking was still consistent with previous observations. 

At this point, we looked at the arm transmission and ASDC signals simultaneously. Normally, when misaligning a cavity, one would expect the reflected power to rise and the transmission to fall.

However, we saw that when misalignment the Yarm in yaw in either direction, or the Xarm in one direction, both the IR transmission and ASDC would fall. This initially made us think of clipping effects. 

So, we checked out the AS beam situation on the AP table. On a card, the beam looks round as we could tell, and the beam spot on AS55 was nice and small. (We tweaked its steering a little bit in pitch to put it at the center of the "falling-off" points) The reflection and transmission falling effect remained. 

At this point, we're not really sure what could be causing this effect. After the reflected beams recombine at the BS, the output path is common, so it's strange that this odd effect would be the same for both arms. 

Lastly, we discussed other ways that we may be able to see if the Xarm really has ~500ppm loss. Since its transmission is ~1.4%, Gabriele estimated that we may be able to see a ~300Hz difference in the arm cavity pole frequency between the two arms, based on the modification of the cavity finesse due to loss. Since we don't currently have the AOM set up to inject intensity noise, we talked about using frequency noise injection to measure the arm cavity poles, though this would be coupled with the IMC pole, but this could hopefully be accounted for.

[Katrin, Jenne]

We took the data for the new absolute length measurement of both arms, after the latest vent and move.  We will analyze soonly.  We had done a round of analysis,  but then Koji pointed out that our data wasn't so clean because the whitening filters were on (and saturated the ADC).  We now have the data (but not the analysis) for the better data with the WF off.

So our dirty-data preliminary number for the X arm is 37.73meters, which is 14cm different from our old length.  We were supposed to move by ~20cm, so....either this measurement is bad because the data sucked (which it did), or we are 6cm off.  Or both.

I'll do another analysis with the clean data for both arms later today/tomorrow.

After analyzing the cleaner data, I get the following:

Y_Length_long  =  37.757 meters

X_Length_long  =  37.772 meters
 

As stated in the wiki, the goal arm length was  L = 37.7974 m for each arm. 

So we're within 2cm for X, and within 4cm for Y.

According to Kiwamu's awesome tolerance calculation, we need to be within 2cm for each arm.  Given that we started out 20cm wrong for X and 25cm wrong for Y, we're a lot closer now, even though we aren't meeting our Yarm requirement yet.

Probably some Optickle action is in order, to see what these new lengths give us in terms of sideband phase and other stuff.

If you want more digits on my calculated numbers (which are probably meaningless, but I haven't done a careful error analysis), in my directory ...../users/jenne/Xarm and ..../users/jenne/Yarm run Xarm_find_peaks_and_length.m and Yarm_find_peaks_and_length.m  respectively.  These will output the lengths.

The second sideband is resonant in the arms for a cavity length of 37.9299m.

The nearest antiresonant arm lengths for f2 (55MHz) are 36.5753m and 39.2845m.

If we don't touch the ITMs, and we use the room we still have now on the end tables, we can get to 37.5m.

This is how the power spectrum at REFL would look like for perfect antiresonance:

And this is how it looks like for 37.5m:

Or, god forbid, we change the modulation frequencies...

Last night (Oct 07), I ran armLoss script in order to obtain the latest numbers for the arm cavity loss.
Here is the summary

Parameters:

TE=10ppm, LE=L_RT/2, RE=1-TE-LE
tE=Sqrt(TE), rE=Sqrt(RE)

TF=0.005, LF=L_RT/2, RF=1-TF-LF
tF=Sqrt(TF), rF=Sqrt(RF)

rcav = -rF +(tF^2 rE)/(1-rF rE)
R_cav = rcav^2

F = pi Sqrt(rF rE)/(1-rF rE)

I like to ask someone to review the calculation on the wiki.

Arm cavity loss measurement (Dec. 14, 2009)

• Arm visibility (given): 0.897 +/- 0.005 (20 pts) (2.5%UP!!)
• Cut off freq (given): 1616 +/- 14　[Hz] (2.1%UP!!)
• Finesse (derived): 1206 +/- 10 (2.1%UP!!)
• Round Trip loss (estimated): 127 +/- 6 [ppm] (28%DOWN!!)
• Front Mirror T (estimated): 0.00506 +/- 0.00004

• Arm visibility (given): 0.893 +/- 0.004 (20 pts) (2.1%UP!!)
• Cut off freq (given): 1590 +/- 4 [Hz] (8.2%UP!!)
• Finesse (derived): 1220 +/- 3 (8.2%UP!!)
• Round Trip loss (estimated): 131 +/- 6 [ppm] (37%DOWN!!)
• Front Mirror T (estimated): 0.00500 +/- 0.00001 

Previous measurement (Oct 07, 2009 & Nov 10, 2009)

X Arm:  

• Arm visibility (given): 0.875 +/- 0.005 (34 pts)
• Cut off freq (given): 1650 +/- 70 [Hz]
   
• Finesse (derived): 1181 +/- 50
• Round Trip loss (estimated): 162 +/- 10 [ppm]
• Front Mirror T (estimated): 0.0051 +/- 0.0002

Y Arm: 

• Arm visibility (given): 0.869 +/- 0.006 (26 pts)
• Cut off freq (given): 1720 +/- 70 [Hz]
   
• Finesse (derived): 1128 +/- 46
• Round Trip loss (estimated): 179 +/- 12 [ppm]
• Front Mirror T (estimated): 0.0054 +/- 0.0002

Yesterday evening Nic and me were in the lab. The Mode Cleaner was unlocked, but after many attempt we could fix it and we did many scans of the Y arm cavity.

Today I was not able to keep the MC locked. Koji helped me remotely, and eventually the MC locked back, but after half an hour of measurements I had to stop.

I made some more scan of the Y arm though. I also tried to do the same for the X arm, but the MC unlocked before the measurement was finished. I'll try to come back in the night.

I see no evidence of anything radically different from my PSL table optical characterization in the IMC transmitted beam, see Attachment #1. The lines are just a quick indicator of what's what and no sophisticated peak fitting has been done yet (so the apparent offset between the transmission peaks and some of the vertical lines are just artefacts of my rough calibration I believe). The modulation depths recovered from this scan are in good agreement with what I report in the linked elog, ~0.19 for f1 and ~0.24 for f2. On the bright side, the ALS just worked and didn't require any electronics fudgery from me. So the mystery continues.

Summary:

I managed to execute the first few transitions of locking the arm lengths to the laser frequency in the CARM/DARM basis using the IR ALS system 🎉 🎊 . The performance is not quite optimized yet, but at the very least, we are back where we were in the green days.

Details:

1. Locking laser frequency to Y arm cavity length using MC2 as a frequency actuator
   • This is the usual diagnostic done to check the single-arm ALS noise using POY as an out of loop sensor.
   • The procedure is now scripted - I had to guess the sign and optimize the gains a few times, but this works deterministically now. 
   • Script lives at /opt/rtcds/caltech/c1/scripts/YARM/Lock_ALS_YARM.py.
   • Attachment #1 shows the result. If we believe the POY sensor calibration, the RMS displacement noise is ~6 pm
2. Encouraged by the good performance of the Y arm, I decided to try the overall transition from the POX/POY basis to the CARM/DARM basis using ALS error signals.
   • The procedure starts with the arm cavities locked with POX/POY, and the respective green frequencies locked to the arm cavity length by the end PDH servos.
   • The DFD outputs serve as the ALS error signals - the PSL frequency is adjusted to the average value of DFD_X_OUT and DFD_Y_OUT.
   • I changed the LSC output matrix element for DARM-->ETMX from -1 to -5, to make it symmetric in actuation force w.r.t. ETMY (since the series resistane on ETMX is x5 that on ETMY).
   • After some guesswork, I fould the right signs for the gains. After enabling the boosts etc, I was able to keep both arms (approximately) on resonance for several minutes. See Attachment #2 for the time series of the transition process - the whole thing takes ~ 1 minute. 
   • A script to automate this procedure lives at /opt/rtcds/caltech/c1/scripts/ALS/Transition_IR_ALS.py.
   • The transition isn't entirely robust when executed by script - the main problem seems to be that in the few seconds between ramping off the IR servos and enabling the CARM/DARM integrators/boosts, the DARM error-point offset can become rather large. Consequently, when the integrator is engaged, ETMX/ETMY get a large kick that misalign the cavity substantially, degrade the green lock, and destroy the CARM lock as well. The problem doesn't seem to exist for the CARM loop. 
   • Anyways, I think this is easily fixed, just need to optimize sleep times and handoff gains etc a bit. For now, I just engage the DARM boosts by hand, putting in a DARM offset if necessary to avoid any kicking of the optic.
   • Attachment #3 shows the length noise witnessed by POX/POY when the arm cavities are under ALS control. If we believe the sensor calibration, the RMS displacement noise is ~15 (20) pm for the Y (X) arm.
   • This is rather larger than I was hoping would be the case, and the RMS is dominated by the <1 Hz "mystery noise".
   • Nevertheless, for a first pass, it's good to know that we can achieve this sort of ALS performance with the new IR ALS system.

Over the week, I'll try some noise budgeting, to improve the performance. The next step in the larger scheme of things is to see if we can lock the PRMI/DRMI with CARM detuned off resonance.

I made some attempts to measure the current length of the arm cavities by using the mass-kicking technique.

However unfortunately I am running out my energy to complete the measurement,

so I will finish the measurement at some time today.

I still have to set an appropriate kick amplitude. Right now I am injecting AWG into ETMY_LSC_EXC at 0.2 Hz with amplutde of 400 cnts.

I guess it needs a little bit more amplitude to get more psuedo-constant velocity.

Volunteers are always welcome !

(some notes)

The procedure was well-described in entry #555 by Dr.Stochino.

Here is just an example of the time series that I took today showing how the time series looks like.

 Why not just scan the Green laser to measure the arm lengths instead? The FSR of the arm is ~3.75 MHz and so all you have to do is lock the arm green and then sweep the PZT until the resonance is found at 3.75 MHz.

As discussed at today's meeting, we would like to (re)measure the Arm cavity lengths to ~mm precision, and their g-factors. Any arm length mismatch affects the reflection phase of the sidebands in the PRMI, which might be one source of our woes. Also, as I mentioned in a previous elog, the g-factors influence whether our 2f sidebands are getting pulled into the interferometer or not.

These both can be done by scanning the arm on ALS and measuring the green beat frequency at each IR resonance. (Misaligning the input beam will enhance the TM10 Mode content, and let us measure its guoy phase shift)

I started working on this today, but I have measurements to do, since at the time of today's measurements, I was fooled by the limits of the ALS offset sliders that I could only scan through two FSRs. Looking back at Manasa's previous measurment (ELOG 9804), I see now that more FSRs are possible.

Ways I will try to improve the measurement:

• Jenne claims that the main limitation on ALS scanning range is the length to pitch coupling of the ETMs. If so, I should be able to get even more FSRs by scanning with MC2, as I did today, since the IMC cavity length is shorter, meaning more arm FSRs/unit length. More FSRs mean better statistics on the FSR slope fitting.
• FSR error:
  • I am measuring the out-of-loop PDH signal of the arm at the same time as the beat spectrum is being measured, to know the magnitude of displacement fluctuations and any overall offset from the PDH zero crossing.
• Beat frequency error:
  • I updated the HP8591E gpib scripts to be able to set the bandwidth and averaging settings in order to really nail down observed beat frequency.
  • I've written some code to fit the spectrum to a lorentzian profile, for evaluation of the linewidth/frequency uncertainty
  • I am also considering beating the analyzer with a rubidium clock to compensate for systematic errors, since ELOG 9837 says the analyzer is off by 140Hz/10MHz, i.e. 10ppm. Since we're trying to measure 1mm/40m~25ppm, this can matter.

Just for kicks, here are scans from today.

This has been done before:

http://nodus.ligo.caltech.edu:8080/40m/6938

Arm length measurements and g-factor estimates in 2012, but only with an accuracy of ~30 cm. However, Yuta was able to get many FSRs somehow.

For both sidebands to be antiresonant in the arms, the first modulation frequency has to be:

f1 = (n + 1/2) c / (2*L)

where L is the arm length and c the speed of light.  For L=38m, we pick to cases: n=3,  then f1a = 13.806231 MHz;  n=2, then f1b = 9.861594 MHz.

If we go for f1a, then the mode cleaner half length has to change to 10.857m.  If we go for f1b, the MC length goes to 15.200m. A 2 meter change from the current length either way.

And the mode cleaner would only be the first of a long list of things that would have to change. Then it would be the turn of the recycling cavities.

Kind of a big deal.

As per Ignacio's request, I restored the arm locking.

- MC WFS relief

- Slow DC restored to ~0V

- Turned off DARM/CARM

- XARM/YARM turned on

- XARM/YARM ASS& Offset offloading

[ericq,gautam]

There are multiple methods by which the arm loss can be measured, including, but not limited to:

1. Cavity ringdown measurement
2. Monitoring IR arm transmission using ALS to scan the arm through multiple FSRs
3. Monitoring the reflected light from the ITM with and without a cavity (Johannes has posted the algebra here)

We found that the second method is extremely sensitive to errors in the ITM transmissivity. The first method was not an option for a while because the AOM (which serves as a fast shutter to cut the light to the cavity and thereby allow measurement of the cavity ringdown) was not installed. Johannes and Shubham have re-installed this so we may want to consider this method.

Most of the recent efforts have relied on the 3rd method, which itself is susceptible to many problems. As Yutaro found, there is something weird going on with ASDC which makes it perhaps not so reliable a sensor for this measurement (unfortunately, no one remembered to follow up on this during the vent, something we may come to regret...). He performed some checks and found that for the Y arm, POY is a suitable alternative sensor. However, the whitening gain was at 0dB for the measurements that Johannes recently performed (Yutaro does not mention what whitening gain he used, but presumably it was not 0). As a result, the standard deviation during the 10s averaging was such that the locked and misaligned readings had their 'fuzz' overlapping significantly. The situation is worse for POX DC - today, Eric checked that the POX DC and POY DC channels are indeed reporting what they claim, but we found little to no change in the POX DC level while misaligning the ITM - even after cranking the whitening gain up to 40!

Eric then suggested deriving ASDC from the AS110 photodiode, where there is more light. This increased the SNR significantly - in a 10s averaging window, the fuzz is now about 10 ADC counts out of ~1500 (~<1%) as opposed to ~2counts out of 30 previously. We also set the gains of POX DC, POY DC and ASDC to 1 (they were 0.001,0.001 and 0.5 respectively, for reasons unknown).

I ran a quick measurement of the X arm loss with the new ASDC configuration, and got a number of 80 +/- 10 ppm (7 datapoints), which is wildly different from the ~250ppm number I got from last night's measurement with 70 datapoints. I was simultaneously recording the POX DC value, which yielded 40 +/- 10 ppm.

We also discovered another possible problem today - the spot on the AS camera has been looking rather square (clearly not round) since, I presume, closing up and realigning everything. By looking at the beam near the viewport on the AS table for various configurations of the ITM, we were able to confirm that whatever is causing this distortion is in the vacuum. By misaligning the ITM, we are able to recover a nice round spot on the AS camera. But after running the dither align script, we revert to this weirdly distorted state. While closing up, no checks were done to see how well centered we are on the OMs, and moreover, the DRMI has been locked since the vent I believe. It is not clear how much of an impact this will have on locking the IFO (we will know more after tonight). There is also the possibility of using the PZT mounted OMs to mitigate this problem, which would be ideal.

Long story short -

1. Some more thought needs to be put into the arm loss measurement. If we are successful in locking the IFO, the PRG would be a good indicator of the average arm loss.
2. There is some clipping, in vacuum, of the AS beam. It may be that we can fix this without venting, to be investigated.

GV Edit 8 Oct 2016: Going through some old elogs, I came across this useful reference for loss measurement. It doesn't talk about the reflection method (Method 3 in the list at the top of this elog), but suggests that cavity ringdown with the Trans PD yields the most precise numbers, and also allows for measuring T_ITM

Using these numbers for both arms (Modulation takes away .2*.25 = 5% power, mode matching takes away 7% after that), I get the following from my data from March:

Xarm loss is 561.19 +/- 14.57 ppm

Yarm loss is 130.67 +/- 18.97 ppm

Obviously, the Xarm number looks very fishy, but its behavior was qualitatively very different when I took the data. ASDC would change from ~0.298 to ~0.306 when the Yarm was locked vs. misaligned, whereas the xarm numbers were .240 to .275. 

In any case, I'll do the measurement again tomorrow, being careful with offsets and alignment; it won't take too long. 

I want to measure the transfer function of the arm cavities to extract the pole frequencies and get more insight into what is going on with the DC loss measurements.

The idea is to modulate the light using the PSL AOM. Measure the light transmitted from the arm cavities and use the light transferred from the IMC as a reference.

I tried to start measuring the X arm but the transmission PD is connected to the QPD whitening filter board with a 4 pin Lemo for which I couldn't find an adapter.

• I switch to the Y arm where the transmission PD - Thorlabs PDA520 (250KHz Bandwidth) - is BNC all the way.
• I lay an 82ft BNC cable from the Y Arm 1Y4 to 1Y1 where the BNC from the IMC Trans PD and an SR785 are found. 
• I lock the Arm cavities.
• I connect the AOM cable to the source, the TRY PD (Teed off from the QPD whitening filter) to CH1 and IMC_TRANS to CH2 and measure the transfer function using a swept sine with an offset of 300mV and amplitude of 100mV.
• I fit it to a low pass filter function - see attachment 1 - but it seems like the fit rails off at 10KHz. 

Could this be because of the PDA520 limited BWs? I tried playing with the PD gain/bandwidth switch but it seems like it was already set to high bandwidth/low gain.

In any case, the extracted pole frequency ~ 2.9kHz implies a finesse > 600 (assuming FSR = 3.9MHz) which is way above the maximal finesse (~ 450) for the arm cavities.

I disconnected the source from the AOM. But left the other two BNCs connected to the SR785. Also, TRY PD is still teed off. Long BNC cable is still on the ground.

                               when doing the AM sweeps of cavities

make sure to cross-calibrate the detectors

                       else you'll make of science much frivolities

            much like the U.S. elections electors