Finished calculations for harmonic distortion at each of the 10 outputs of the RF distribution box. The diagram can be found on Suresh's post  http://nodus.ligo.caltech.edu:8080/40m/4342

THD calculation consisted of gather data on the dBm at harmonics of the fundamental frequency. These dBm values were converted into units of power and plugged into the appropriate THD equation pulled from Wikipedia: 

On the table, the number 1-6 correspond to the harmonic number of the input frequency used. For example, the first five PD's listed used an 11MHz source, while the second set of five PD's listed used a 55MHz source. Values listed under certain harmonics are dBm measurements at the corresponding frequency. The P-subscript values are essentially the dBm measurements converted to units of power (Watts) for ease of calculation in the equation above. THD is then calculated using these power units; I have converted the ratios to percentages.

It should be noted that as with all THD calculations, the more data points collected, the more precise the THD % will be.

By the way, the outputs on the physical RF distribution box for REFL165 and AS165 are actually labeled as REFL166 and AS166. 

 
Fast work indeed! It would be nice if we could have the following details filled in as well
a) A short title and caption for the table, saying what we are measuring
b) the units in which this physical quantity is being measured.

It is good to keep in mind that people from other parts of the group, who are not directly involved in this work, may also read this elog.

The SRM bounce peak 18.33 Hz. Suresh helped me to retune filter through Foton, but we failed to remove it.

ITMX_OPLEV_PERROR shows high coherence with oplev laser. This is our lousiest set up. I will work on it next week.

Generally speaking we can say that JDSU-Uniphase 1103P and 1125P laser intensity noise do not limit oplev servo performance.

However, the overall well being of filters, gain settings, beam sizes on qpds are poor.

Liyuan is measuring the He/Ne telescopes in the Y arm between the tube and CES wall. He'll be here till 1pm

 Liyuan is continuing his measurement in the Y arm till noon today.

 Steve had me measure the RIN of a JDSU HeNe laser. I used a PDA520, and measured the RIN after the laser had been running for about an hour, which let the laser "settle" (I saw the low frequency RIN fall after this period).

Here's the plot and zipped data.

Steve: brand new laser with JDSU 1201 PS

TEST QPD sn 222 was calibrated with 1103P directly looking into it from 1 m. ND2 filter was on the qpd.

 The room temp drops 1 degree C on the 4th day. The weather has changed.

The SRM qpd was moved to accommodate the HeNe laser qualification test for LIGO Oplev use.

The qpd was saturating at 65,000 counts of 3 mW

ND1 filter lowering the power by 10 got rid of saturation. I epoxied an adapter ring to the qpd.

Atm3 was taken before saturation was realized with Koji's help.

Atm4 ND1 on SRM qpd. Now it is working and everything is moving.

 SRM as set up in Atm4  26,000 count compared with ETMY oplev servo in operation 7,500 counts for 3 days

Next steps: measure beam size at qpd,

                  place qpd on translation stage for calibration,

                  change 1103P mount to single one

 SRM qpd is installed on translation stage and the shims removed from laser V mounts.

The ETMY oplev servo is on.

SRM oplev servo:  100 microrad/count is an estimate, not calibrated one.

 SRM qpd is back to its normal position. The mount base is still on delrin base. SRM and ITMY need centering.

Tomorrow I will set up the HeNe laser test at the SP table with Ontrack qpd

ETMY oplev servo on. SRM qpd with ND1 ------no component------- 1103P

 QPDsn222 is on translation stage with ND2 filter on SP table. The 1103P is mounted with two large V mounts 1 m away.

This qpd will be calibrated Monday. It has only slow outputs.

I took off the silicon rubber heaters which were used by a SURF last year for heating the enclosure. The heater data sheet has mentioned the power dentsities, but I doubted the values. So I wanted to measure the actual power density by these heaters. I think the rubber heaters are broken somewhere within, the surface is not heated evenly. Although I don't have a good quantative reason to use, I was thinking to use a thermoelectric cooling module for the enclosure. 

From the data I collected few days back, I am trying to obtain a transfer function of temperature inside the enclosure to that of outside. My aim is to measure the pole frequency of temperature fluctuations inside the enclosure relative to the outside fluctuations.

[Sandrine, Koji, Terra]

Summary: We completed multiple scans at different heating powers for the reflector set up, observing unique HOM peak shifts of tens of kHz. We also observed HOM5 shifts with the cylinder set up. Initial Lorentzian fittings of the magnitude give tens of Hz resolution. I summarize the main week's work below. 

Set-up

Heater set-up is described in several previous elogs, but attachments #1 and #2 show the full heater set-up and wiring/pinouts in and out of vacuum, since we're all intimately aware of how confusing in-vacuum pinouts can be. We are not using the Sorenson power supply (as described in 14071); we just have the BKPrecision power supply 1735 sitting next to the ETMY rack and are manually going out to turn on/off. 

We've continued to use the scan setup described in elog 14086, which is run using /users/annalisa/postVent/AGfast.py. Step by step notes for setting up the scan, running the scans, and processing the scans are attached in notes.txt.

Inducing/witnessing HOMs

The aux input beam was already clipped and on wednesday (after Trans was centered, 14093) we also clipped the output aux beam with razor blade (angled vertically and horizontally, elog 14103) before PDA255; we clipped ~1/3 of the output beam. Attachment #3 shows before and after clipping output, where orange 'cold' == unclipped, black 'mean' == clipped (all in cold state). Up to HOM5 is visible. 

Measurements

Below is a summary of the available scan data. We also have cold (0A) scans CAR-HOM5 and full FSR scans for most configurations. 

We tried the cylinder set-up again tonight for the first time since inital try and can see shifts of HOM5 - see attachment #5; we haven't looked in detail yet, but it looks like odd modes are more effected, suggesting the ring heat pattern is off centered from the beam axis. 

Scan data is saved in the following format: users/annalisa/postVent/scandata/{reflector,cylinder}/{parsed,unparsed}/{CAR,HOM1,HOM2,HOM3,HOM4,HOM5}{_datetime}{_parsed,_unparsed}.{txt,pdf}

Minimum heating

On 7/26 we increased the power to the elliptical reflector heater in steps to find the minimum heater power required to see frequency shifts with our measurement setup. Lowest we can resolve is a shift in HOM3 with 1.7W (0.5A/3.4V). According to Annalisa's measurements in elog 14050, this would be something like 30-60 mW radiated power hitting the test mass. We only looked at CAR - HOM3 for this investigation; data for scans at 0.4A, 0.5A, 0.6A is available as indicated above.

Lorentizian Fitting

The Lorentzian fitting was done using the equation a + b / sqrt(1+((x-c)/d*2), where a = constant background, b = peak height above background, c = peak frequency, d = full width at half max. 

The fitting is still being edited and optimized. We will crop the data to zoom in around the peak more.

The Lorentzian fit of the magnitude shows ~10Hz of resolution. (See attachment 6 for the carrier at 8A and attachment 7 for HOM 1 at 9A)

We're working on fitting the full complex data.

[Annalisa, Koji]

Today both the heater and the reflector were delivered, and we set down the setup to make some first test.

The schematic is the usual: the rod heater (30mm long, 3.8 mm diameter) is set inside the elliptical reflector, as close as possible to the first focus. In the second focus we put the power meter in order to measure the radiated power. The broadband power meter wavelength calibration has been set at 4µm: indeed, the heater emits all over the spectrum with the Black Body radiation distribution, and the broadband power meter measures all of them, but only starting from 4µm they will be actually absorbed my the mirror, that's why that calibration was chosen.

We measured the cold resistance of the heater, and it was about 3.5 Ohm. The heater was powered with the BK precision DC power supply 1735, and we took measurements at different input current.

We also aimed at measuring the heater temperature at each step, but the Fluke thermal camera is sensitive up to 300°C and also the FLIR seems to have a very limited temperature range (150°C?). We thought about using a thermocouple, but we tested its response and it seems definitely too slow. 

Some pictures of the setup are shown in figures 1 and 6.

Then we put an absorbing screen in the suspension mount to see the heat pattern, in such a way to get an idea of the heat spot position and size on the ETMY. (figure 2)

The projected pattern is shown in figures 3-4-5

The optimal position of the heater which minimizes the heat beam spot seems when the heater inserted by 2/3 in the reflector (1/3 out). However, this is just a qualitative evaluation.

Finally, two more pictures showing the DB connector on the flange and the in-vacuum cables.

Some more considerations about in-vacuum cabling to come.

Steve: how are you going to protect the magnets ?

Summary

We installed two heaters setup on the ETMY bench in order to try inducing some radius of curvature change and therefore HOMs frequency shift.

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

We installed two heaters setup.

Elliptic reflector setup (H1): heater put in the focus of the elliptical reflector: this will make a heat pattern as descirbed in the elogs #14043 and #14050. 

Lenses setup (H2): heater put in a cylndrical reflector (made up with aluminum foil) 1'' diameter, and 2 ZnSe lenses telescope, composed by a 1.5'' and a 1'' diameter respectively, both 3.5'' focal length. The telescope is designed in such a way to focus the heat map on the mirror HR surface. For this latter the schematic was supposed to be the following: 

This setup will project on the mirror a heat pattern like this:

which is very convenient if we want to see a different radius of curvature for different HOMs. However, the power that we are supposed to have absorbed by the mirror with this setup is very low (order of 40-ish mW with 18V, 1.2A) which is probably not enough to see an effect. Moreover, mostly for space reasons (post base too big), the distances were not fully kept, and we ended up with the following setup:

In this configuration we won't probably have a perfect focusing of the heat pattern on the mirror.

In vacuum connections

See Koji's elog #14077 for the final pin connection details. In summary, in vacuum the pins are:

13 to 8 --> cable bunch 0

7 to 2 --> cable bunch 2

25 to 20 --> cable bunch 1

19 to 14 --> cable bunch 3

where Elliptic reflector setup (H1) is connected to cables 0 and 1, and the lenses setup is connected to cables 2 and 3.

Installed setup

This is the installed setup as seen from above:

[ericq, lydia, steve, gautam]

• We aligned the arms, and centered the in-air AS beam onto the PDs and camera
• Misaligned the ITMs in a controlled ramp, observed ASDC level, didn't see any strange features
• We can misalign the ITMs by +/- 100urad in yaw and not see any change in the ASDC level (i.e. no clipping). We think this is reasonable and it is unlikely that we will have to deal with such large misalignments. We also scanned a much larger range of ITM misalignments (approximately +/-1mrad), and saw no strange features in the ASDC levels as was noted in this elog - we used both the signal from the AS110 PD which had better SNR and also the AS55 PD. We take this to be a good sign, and will conduct further diagnostics once we are back at high power.
• Opened up all light doors, checked centering on all 6 OM mirrors again, these were deemed to be satisfactory 
• To solve the green scattering issue, we installed a 1in wide glass piece (~7inches tall) mounted on the edge of the OMC table to catch the reflection off the window (see Attachment #1) - this catches most of the ghost beams on the PSL table, there is one that remains directly above the beam which originates at the periscope in the BS/PRM chamber (see Attachment #2) but we decided to deal with this ghost on the PSL table rather than fiddle around in the vacuum and possibly make something else worse
  Link to IMG_2332.JPG
  Link to IMG_2364.JPG
• Re-aligned arms, ran the dither, and then aligned the PRM and SRM - we saw nice round DRMI flashes on the cameras
• Took lots of pictures in the chamber, put heavy doors back on. Test mass Oplev spots looked reasonably well centered, I re-centerd PRM and SRM spots in their aligned states, and then misaligned both
• The window from the OMC chamber to the AS table looked clean enough to not warrant a cleaning..
• PSL shutter is closed for now. I will check beam alignment, center Oplevs, and realign the green in the evening. Plan is to pump down first thing tomorrow morning

AS beam on OM1

Link to IMG_2337.JPG

AS beam on OM2

I didn't manage to get a picture of the beam on OM5 because it is difficult to hold a card in front of it and simultaneously take a photo, but I did verify the centering...

It remains to update the CAD diagram to reflect the new AS beam path - there are also a number of optics/other in-vacuum pieces I noticed in the BS/PRM and OMC chambers which are not in the drawings, but I should have enough photos handy to fix this.  

Here is the link to the Picasa album with a bunch of photos from the OMC, BS/PRM and ITMY chambers prior to putting the heavy doors back on...

SRM satellite box has been removed for diagnostics by Rana. I centered the SRM Oplev prior to removing this, and I also turned off the watchdog and set the OSEM bias voltages to 0 before pulling the box out (the PIT and YAW bias values in the save files were accurate). Other Oplevs were centered after dither-aligning the arms (see Attachment #8, ignore SRM). Green was aligned to the arms in order to maximize green transmission (GTRX ~0.45, GTRY ~0.5, but transmission isn't centered on cameras).

I don't think I have missed out on any further checks, so unless anyone thinks otherwise, I think we are ready for Steve to start the pumpdown tomorrow morning.

[Chub, Koji, Gautam]

We replaced the EY and IOO chamber heavy doors by 10:10 am PST. Torquing was done first oen round at 25 ft-lb, next at 45 ft-lb (we trust the calibration on the torque wrench, but how reliable is this? And how important are these numbers in ensuring a smooth pumpdown?). All went smooth. The interior of the IOO chamber was found to be dirty when Koji ran a wipe along some surfaces.

For this pumpdown, we aren't so concerned with having the IFO in an operating state as we will certainly vent it again early next year. So we didn't follow the full close-up checklist.

Jon and Chub and Koji are working on starting the pumpdown now... In order to not have to wear laser safety goggles while we closed doors and pumped down, I turned off all the 1064nm lasers in the lab.

[steve, teng, johannes, lydia, gautam]

• we set about doing some final checks on the Y arm while Johannes and Lydia worked on the X arm alignment
• locked IMC, turned on Oplev HeNes for ITMY, SRM, PRM, BS and ETMY
• I first went into the BS-PRM chamber. Traced Oplev paths for PRM and BS, checked that the beam is approximately centered on all the steering mirrors, and traced the beam with a clean beam card to make sure there was no clipping. The beams make it out of the vacuum onto the PDs, but are not centered
• I also checked the Y arm green - the beam isn't quite centered on the periscope mirrors but I guess this has always been the case and I didn't venture to make any changes
• Checked new PRM foil hats were secure
• Checked the main IR beam out of the IMC, and also the IPANG beam - Steve suggested we keep track of the way this moves during pumpdown. However, I didn't quite think this through and we put the heavy door on the BS-PRM chamber before checking where the IPANG beam was on ETMY table (we later found that the beam was a tad too high. Anyways, this isn't critical, wouldve been nice to have this reference though
• Checked that there were no tools lying around inside the chamber, and proceeded to put the heavy door on
• Moved to ETMY table, and did much of the same as above - Oplev beam makes it successfully out off the ETM, OSEM cables aren't a risk to clipping the green input beam
• Proceeded to put the heavy door on ETMY chamber
• I would have liked to put the heavy door on the ITMY chamber today evening too, but while freeing the SRM from its EQ stops, I noticed that the LL and LR OSEM PD readouts are approximately 60 and 75 % of their saturation values. I think this warrants fixing (I also checked against the frame files from our last DRFPMI lock in march and the PD signals are significantly different) so we should do this before putting the heavy door on. It would also be a good idea to check the table leveling
• The Oplev beams for ITMY and SRM make it cleanly out of the chamber so all looks good on that front
• IR and green beams are well clear of any OSEM cables

Depending on how the X arm situation is, we will finish putting back all the heavy doors on Monday and start the pumpdown

GV Edit 11.30pm: 

• We succeeded in locking the X arm as well, although the transmission peaked at 0.1 (but this is the high gain PD and not the QPD, and also, unlike the Y arm, the 50-50 BS splitting the transmitted light between the QPD and the high gain PD is still in place, so can't really compare with the Y arm value of 0.6)
• To get the lock going, we had to change a bunch of things like the POX DC offset, demod phase, sign of the gain etc. It is unclear whether we are locking on the TEM00 mode, but we judged it is sufficient to close doors and pump down
• Johannes and I centered the ETMX and ITMX OL spots on their respective QPDs. Earlier today, Johannes and Lydia had checked ITMX and ETMX OL paths, everything looks decent
• JE piggyback edit : We also tied the upper ITMX OSEM cables to the suspension cage side using copper wire since particularly UR looked like it could slip and possibly fall down into the beam path
• JE piggyback edit: While leveling the ITMX table, Gautam and I found that some of the screws that secure the weights were not vented. None of these were put in during this vent. We replaced them all with vented screws.
• Rana also checked PRM and SRM alignment, all looks okay on that front - the OSEM problem I had alluded to earlier isn't really a problem, once the SRM is aligned, all the OSEMs are reasonably close to 50% of their saturation value.

Looks like on Monday, we will look to put the heavy doors on ITMY, ITMX and ETMX chambers, and begin the pumpdown

[Jenne, Rana]

We wanted to jump right in and see if we were ready to try the new "ALS fool" loop decoupling scheme, so we spent some time with CARM and DARM at "0" offset, held on ALS, with PRMI locked on REFL33I&Q (no offsets).  Spoiler alert:  we weren't ready for the jump.

The REFL11 and AS55 PDs had 0dB analog whitening, which means that we weren't well-matching our noise levels between the PD noise and the ADC noise.  The photodiodes have something of the order nanovolt level noise, while the ADC has something of the order microvolt level noise.  So, we expect to need an analog gain of 1000 somewhere, to make these match up.  Anyhow, we have set both REFL11 and AS55 to 18dB gain. 

On a related note, it seems not so great for the POX and POY ADC channels to be constantly saturated when we have some recycling gain, so we turned their analog gains down from 45dB to 0dB.  After we finished with full IFO locking, they were returned to their nominal 45dB levels. 

We also checked the REFL33 demod phase at a variety of CARM offsets, and we see that perhaps it changes by one or two degrees for optimal rotation, but it's not changing drastically.  So, we can set the REFL33 demod phase at large CARM offset, and trust it at small CARM offset.

We then had a look at the transmon QPD inputs (before the dewhitening) for each quadrant.  They are super-duper saturating, which is not so excellent. 
QPDsaturation_12Feb2015.pdf

We think that we want to undo the permanently-on whitening situation.  We want to make the second stage of whitening back to being switchable.  This means taking out the little u-shaped wires that are pulling the logic input of the switches to ground.  We think that we should be okay with one always on, and one switchable.  After the modification, we must check to make sure that the switching behaves as expected.  Also, I need to figure out what the current situation is for the end QPDs, and make sure that the DCC document tree matches reality.  In particular, the Yend DCC leaf doesn't include the gain changes, and the Xend leaf which does show those changes has the wrong value for the gain resistor.

After this, we started re-looking at the single arm cancellation, as Rana elogged about separately.

ALSfool_12Feb2015.pdf

My observation wasn't accurate enough.

The looseness came from the fact that the N-SMA bulk heads were slipping on the black plate.

This is actually what Suresh pointed out (see here). So the thickness of the black plate doesn't matter in this case.

Somehow I was able to tighten the bulk heads using two wrenches and I think they are now tight enough so that the heliax's heads don't move any more.

 One day I'll get to be part of the krew

Summary:

The Y arm cavity length was locked to the PSL frequency with ~26kHz UGF, and 25 degrees phase margin. Slow actuation was done on ETMY using CM_Slow as an error signal, while fast actuation was done on the IMC error point via the IN2 input of the IMC servo board. Attachment #1 shows the comparison of the in-loop error signal spectra with only slow actuation and with the full CM loop engaged.

Details:

1. LSC enable OFF.
2. Configure the CM board for locking:
   • CM board IN1 gain = 25dB.
   • CM_Slow whitening gain = +18dB, make sure the offsets are correctly set. CM_Slow filter bank = -0.015.
   • CM_Slow-->YARM matrix element in LSC input matrix is -2.5.
   • YARM-->ETMY matrix element in LSC Output matrix is 1.
   • AO gain set to +5dB. IMC Servo board IN2 gain starts at -32dB, the path is disabled. The polarity is Plus.
   • Usual YARM FM triggers are set (FM1, FM2, FM3, FM6, FM8), usual YARM servo gain is used (0.01), usual triggering conditions (ON @ TRY>0.3, OFF @ TRY < 0.1), usual power normalization by TRY.
3. Enable LSC mode, wait for the arm to acquire lock.
4. Once the digital boosts are engaged, enable the IMC IN2 path, ramp up the gain to -2 dB. Note that this IN2 path is AC coupled, according to this elog. The corner frequency is 1/2/pi/2e3/6.8uF ~11 Hz. This was confirmed by measurement, see Attachment #3. I couldn't find a 2-pin LEMO-->BNC adaptor so I measured at the BNC connector for the IN2 input, which according to the schematic is shorted to the LEMO (which is what we use for the AO path).
5. Enable the CM board boost.
6. Ramp up the CM board IN1 gain to +31dB. In this config, the CM_Slow signal is ~18,000 cts pk (with the +18dB whitening gain), so not saturating the ADC.
7. Ramp up the IMC IN2 gain to 3dB, engage 2 Super Boosts (can't turn on the third). Limiter is always ON.
8. Use the CM board error point offset adjust to zero the POY11_I error signal average value - there seems to be some offsets when engaging the boosts. The value I used was 0.9 V (this is internally divided by 40 on the CM board).
9. Whiten the CM_Slow signal - this doesn't seem to have any impact on the noise anywhere.

I hypothesize that the high-frequency noise (>100 Hz) is higher for POY than POX in Attachment #1 because I am using the "MON" port of the demod board - this has a gain of 2, and there could also be some flaky components in this path, hence the high frequency noise is a factor of a few greater in the POY spectrum relative to the POX spectrum (which is using the main demodulated output). For REFL11, we have a low noise preamp generating the input signal so I don't think we need to worry about this too much.

The PC Drive RMS didn't look any stranger than it usually does for the duration of the lock.

Attachment #2 shows the OLTF of the locking servo with the final gains / settings, which are in bold. The loop is maybe a bit marginal, could possibly benefit from backing off one of the super boosts. But the arm has stayed locked for >1 hour.

The purpose of this test was to verify the functionality of the CM board and also the IN2 of the IMC servo board in a low-pressure environment. Once I confirm that the modelled OLTF lines up with the measured, I will call this test a success, and move on to looking at REFL11 in the arms on ALS, PRMI on 3f config. I am returning the REFL11 signal to the input of the CM board, but the SR785 remains hooked up.

Unrelated to this work - PMC alignment was tweaked to improve input power to IMC by ~5%.

I'll put more detail in the morning, but I was able to get the PRM/ITMY/ETMY coupled cavities locked with 32kHz bandwidth using the AO path. (However, this is a pretty low-finesse situation, since the BS is dumping so much power out of the PRC. Full buildup is only 3 or 4 times the single arm power)

Since our ALS is better than it was a month ago when I last played with this, I was able to hop straight from ALS to REFL11 I on resonance, with the PRY locked on 3f.

Here are some quick OLTF plots I took along the way.

I'm using this configuration to validate my loop modelling for the full double arm case. Right off the bat, this tells me that the "minus" polarity on the CM servo is the correct one. I didn't use REFLDC at all tonight, but I figure I can check it out by doing the transition backwards, so to speak.

Wait. It is not so clear.

Do you mean that the IFO was locked with REFL11I for the first time?

Why is it still in the "low finesse" situation? Is it because of misalignment or the non-zero CARM offset?

Sorry, the X arm is completely misaligned. This is the configuration I first tried in ELOG 9859, that is: a PRM->ITMY recycling cavity and ITMY->ETMY arm cavity. ITMX is completely misaligned, so the BS is dumping much of the recycling cavity light out, which is why I wrote "low finesse." This is the first time I've used REFL11 to control any of our cavities, though.

We added the Thorlabs HV Driver in between the FSS and the NPRO today. The FSS is locking with it, but we haven't taken any loop gain measurements.

This box takes 0-10 V and puts out 0-150 V. I set up the FSS SLOW loop so that it now servos the output of FAST ot be at +5V instead of 0V. This is an OK

temporary solution. In the future, we should add an offset into the output of the FSS board so that the natural output is 0-10 V.

I am suspicious that the Thorlabs box has not got enough zip to give us a nice crossover and so we should make sure to measure its frequency response with a capacitive load.

 We measured the Thorlabs HV Driver's TF today. It is quite flat from 1k to 10k before going up to 25 dB at 100k,

and the response does not change with the DC offset input.

The driver is used for driving the NPRO's PZT which requires higher voltage than that of the previous setup.

We need to understand how the driver might effect the FSS loop TF, and we want to make sure that the driver

will have the same response with DC input offset.

Setup

We used SR785 to measure the TF. Source ch was split by a T, one connected to Driver's input, another one connected to the reference (ch A). See fig2.

The driver output was split by another T. One output was connected to NPRO,

another was connected to a 1nF capacitor in a Pomona box, as a high pass filer (for high voltage), then to the response (ch B)

 The source input is  DC offset by 2V which corresponds to 38 V DC offset on the driver's output.

The capacitance of the PZT on the NPRO is 2.36 nF, as measured by LC meter.

 The result shows that the driver's TF is flat from 1k to 10k, and goes up at higher frequency, see fig1.

The next step is trying to roll of the gain at high frequency for PZT. A capacitor connected to ground might be used to roll off the frequency of the driver's output.

We will inspect the TF at higher frequency (above 100 kHz) as well.

Inside the FSS box, the FAST path has a ~10 Hz pole made up from the 15k resistor and the 1 uF cap before the output connector.

This should be moved over to the output of the driver to make the driver happy - without yet measuring the high frequency response,

it looks like to me that its becoming unhappy with the purely capacitive load of the NPRO's PZT. This will require a little surgery inside

the FSS box, but its probably justified now that we know the Thorlabs box isn't completely horrible.

I repeated the high bandwidth POY locking experiment.

1. The "Q" demod output (SMA) was routed to the common mode board (it appears in the past I used the LEMO "MON" output instead but that shouldn't be a meaningful change).
2. As usual, slow actuation --> ETMY, fast actuation --> IMC error point.
3. Loop UGF measurement suggests that bandwidth ~25kHz, with ~25 degrees phase margin. Anyway the lock was pretty stable.

One thing I am not sure is - when looking at the in-loop error point spectra, the Y-arm error point did not get suppressed to the CM board's sensing noise floor - I would have thought that with the huge amount of gain at ~16 Hz, the usual structure we see in the spectra between 10-30Hz would be completely squished. Need to think about if this is signalling something wrong, because the loop TF measurements seemed as expected to me.

1020pm: plots uploaded. As I made the plot of the spectrum, I realized that I don't have the calibration for the Y-arm error point into displacement noise units, so it's in unphysical units for now. But I think the comment about the hump around 16 Hz not being crushed to some sort of flat electronics noise floor. For the TF plots, when the loop gain is high, this IN1/IN2 technique isn't the best (due to saturation issues) but I don't think there's anything controversial about getting the UGF this way, and the fact that the phase evolves as expected when the various gains are cranked up / boosts enabled makes me think that the CM board is itself just fine.

10am 12 March: i realized that the "Y-arm error point" plotted below is not the true error point - that would be the input to the CM board (before boosts etc), which we don't monitor digitally. The spectra are plotted for the CM_SLOW input which already has some transfer function applied to it. In the past, I routed the LEMO "MON" connector on the demod board to the CM board input, and hence, had the usual SMA outputs from the demod board going to the digital system. I hypothesize that plotting the spectra for that signal would have showed this expected suppression to the electronics noise floor.

In summary, on the basis of this test, I don't see any red flags with the CM board.

[Paco, Mayank, Yuta]
CARM Common Mode Board works for YARM locking

For using it for FPMI and PRFPMI, we tested the CARM Common Mode Board by implementing the high bandwidth YARM lock in a way similar to what Paco did in 2021 (40m/16248).
(YARM locking work was done yesterday and modeling was done today,)

What we did:
  - Connected POY11_I MON to IN1 of CARM Common Mode Board. (POY11_I MON is basically similar to POY11_I_ERR as C1:LSC-POY11_PHASE_R=-9.033 deg is almost zero).
  - Locked YARM at UGF of around 200 Hz using POY11_I_ERR.
  - Turned on CARM Common Mode Board with C1:LSC-CM_REFL1_GAIN=+25 dB, C1:IOO-MC_AO_GAIN=-2dB, C1:LSC-CM_REFL_OFFSET=2.972 V to remove the offset. (BOOST OFF, SUPER BOOST 0, POLARITY PLUS, OPTIONs Disabled). Increasing the gains unlocked the lock (+30 dB, +4dB is probably the maximum we could get).
  - Measured OLTF of CARM loop at TP1A and TP2A of CARM Common Mode Board using Moku Pro.
  - Modeled YARM loop by fitting the measured OLTF data (G_YARM; plotted in blue curve in Attachment #1).
  - Modeled IMC loop by fitting the measured OLTF data (G_IMC; plotted in green curve in Attachment #1; OLTF data is from 40m/17009).
  - Measured CARM Common Mode Board transfer function from IN1 to AO output. This was basically flat upto 1 MHz in 0dB setting for all (Attachment #2).
  - Using CARM OLTF can be calculated as

G_CARM = G_IMC / O_IMC * O_YARM * C_YARM * F_CMB / (1+G_YARM) = r * G_IMC * C_YARM * F_CMB / (1+G_YARM)

  where C_YARM is YARM cavity pole (~4 kHz), O_IMC and O_YARM are IMC REFL and POY11_I optical gains. r is some gain used to fit the data. F_CMB is a CARM Common Mode Board transfer function, which is basically flat.
 - OLTF of CARM loop measured at CARM Common Mode Board can be calculated as

G_meas = G_CARM / (1 + G_IMC)

Result:
 - Attachment #1 gives modeled G_meas (brown line) and measured G_meas (pink dots). r was tuned to match the overall gain. The measurement and the model matches well.
 - G_CARM (purple line) also looks stable.

Next:
  - Try high bandwidth CARM loop in FPMI

I calibrated the AS error signal into the displacement of the YARM cavity in the same way as I did before (elog).

The open loop transfer function is:

The transfer function from ITMX excitation to AS error signal is:

Then I have got the calibration value : 5.08e+11 [counts/m]

The calibrated spectrum in unit of m/rtHz is

REF0: arm displacement
REF1: dark noise + demodulation circuit noise + WT filter noise + ADC noise (PSL shutter on)
REF2: demodulation circuit noise + WT filter noise + ADC noise (PD input of the circuit (at 1Y2) is connected to the 50 Ohm terminator)
(The circuit and WT filter seem to be connected at back side of the rack. Actually there is a connector labelled 'I MON' but it is not related to C1:LSC-ASS55_I_ERR)

Also we changed the AS gain so that ADC noise does not affect:

However, this did not make big change in sensitivity. I guess this means that circuit noise limits the sensitivity at higher frequencies than 400 Hz.
I tried to adjust the AS gain carefully but I could not do that because of the earthquake. Further investigation is needed.

This is NOT calibrated. Its sort of calibrated in the 500-1000 Hz area, but does not correctly use the loop TF or the cavity pole.

As for the noise, remember that the whole point of changing the AS whitening gain was to turn on the whitening filter AFTER locking. With the WF OFF, there's no way that you can surpass the ADC noise limit.

No, I did not apply open loop TF to it (actually I could not measure the open loop TF because of the earthquake last night). So I should not have said it was the displacement.

Also I changed the AS gain with whitening filter on and xarm locked. Still it does not make any change.

Since I found that the the AS sensitivity seems to be limited by circuit noise, I inserted a RF amplifier just after the AS RF output.
Now, the sensitivity is improved and limited by the dark noise of the PD.

(Note: I did not apply the open loop TF on this xml file.)
REF3: dark noise + circuit noise + WT filter noise + ADC noise
REF4: circuit noise + WT filter noise + ADC

With this situation, I injected the acoustic noise:

REF5, 6, 7: with acoustic excitation
no reference: without acoustic excitation

We could see the coherence only at the same frequencies, around 200 Hz as we saw before (elog).

As part of our CESAR testing last night, we had a look at the noise of the 1/sqrt(TR) signal. 

Looking at the time series data, while we were slowly sweeping through IR resonance (using the ALS), Rana noted that the linear range of the 1/sqrt(TR) signal was not as wide as it should be, and that this is likely because our SNR is really poor. 

When a single arm is at a normalized transmission power of 1, we are getting about 300 ADC counts.  We want this to be more like 3000 ADC counts, to be taking advantage of the full range of the ADC.

This means that we want to increase our analog gain by a factor of 10 for the low gain Thorlabs PDs. 

Looking at the photos from November when I pulled out the Xend transmission whitening board (elog 9367), we want to change "Rgain" of the AD620 on the daughter board.  While we're at it, we should also change the noisy black thick film resistors to the green thin film resistors in the signal path. 

The daughter board is D04060, S/N 101.  The main whitening board for the low gain trans QPD is D990399, RevB, S/N 104.

We should also check whether we're saturating somewhere in the whitening board by putting in a function generator signal via BNC cable into the input of the Thorlabs whitening path, and seeing where (in Dataviewer) we start to see saturation.  Is it the full 32,000 counts, or somewhere lower, like 28,000? 

Actually the gain was changed. From gain of 2 (Rgain = 49.4kOhm) to 20 (Rgain = 2.10kOhm), Corresponding calibration in CDS was also changed by locking the Xarm, running ASS, then setting the average arm power to be 1. Confirmed Xarm is locking. And now the signal is used for CESAR.  We see emperically that the noise has improved by a factor of approximately 10ish.

Speaking of the whitening board, I had neglected to post details showing the the whitening was at least having a positive effect on the transmon QPD noise. So, here is a spectrum showing the effects that the whitening stages have on a QPD dark noise measurement like I did in ELOG 9660, at a simulated transmission level of 40 counts. 

The first whitening stages gives us a full 20dB of noise reduction, while the second stage brings us down to either the dark noise of the QPD or the noise of the whitening board. We should figure out which it is, and fix up the board if necessary. 

The DTT xml file is attached in a zip, if anyone wants it.

 I have made the same modification to the Yarm trans PD whitening board as was done for the xend, to increase our SNR.  I put in a 2.1kOhm thin film resistor in the Rgain place.

When I was pulling the board, the ribbon cable that goes to the ADC had its connector break.  I redid the ribbon connector before putting the board back. 

I see signals coming into the digital system for both the high gain and low gain Y transmission PDs, so I think we're back.  I will re-do the normalization after Jamie is finished working on the computers for the day.

[Rich, Jay, Koji]

We blasted the aLIGO RF PD with a 1W IR beam. We did not find any obvious damage.
Rich and Jay brought the PD back to Downs to find any deterioration of the performance with careful tests.

The power modulation setup is at the rejection side of the PBS in front of the laser source.
I checked the beams are nicely damped.
As they may come back here tomorrow, a power supply and a scope is still at the MC side of the PSL enclosure.

We (rana, yehonathan and i) briefly talked about having high power going into the IFO. I worked on some calcs a couple of years ago, that are summarized here. There is some discussion in the linked page about how much power we even need. In summary, if we can have

• T_PMC ~85% which is what I measured it to be back in 2019
• T_IMC * T_inputFaraday ~60% which is what I estimate it to be now
• 98% mode matching into the IMC
• power recycling gain of 40-45 once we improve the folding mirror situation in the recycling cavities
• and a gain of 270-280 in the arm cavities (20-30ppm round trip loss)

then we can have an overall gain of ~2400 from laser to each arm cavity (since the BS divides the power equally between the two arms). The easiest place to get some improvement is to improve T_IMC * T_inputFaraday. If we can get that up to ~90%, then we can have an overall gain of ~4000, which is I think the limit of what is possible with what we have.

We also talked about the EOM. At the same time, I had also looked into the damage threshold as well as clipping losses associated with the finite aperture of our EOM, which is a NewFocus 4064 (KTP is the Pockel medium). The results are summarized in Attachments #1 and #2 respectively. Rana thinks the EOM can handle factor of ~3 greater power than the rated damage threshold of 20W/mm^2.

Jenne made the 40m tour for the annual visit of 30-40 students.

c.f.

Tour 2009 http://nodus.ligo.caltech.edu:8080/40m/1717

Tour 2008 http://nodus.ligo.caltech.edu:8080/40m/737