looks like a fu**** up the data aquisition with the short cable, but i don't know how.
When trying to calibrate the data taken i realized that something was totally wrong as i got some khz/rHz but i couldn't find the mistake.
So i thought the gain setting if the preamps must be different than i wrote down but they are not.
So i checked the calibration again, this time in 1MHz steps all the way through the system but everything was/is OK.
Got almost exactly the same mixer response vs frequency tuning and also the dc-coupled signal was what i measured before.
Here the problem:
The VCO feedback signal gives us a rough idea what frequency we have, which i checked over and over again and matches the frequency counters we have.
The problem is that the DC signal (already amplified) for pi in phase change goes from something like 2.3V to -2.4V.
The same signal from end to end of the VCO goes from -1.7V to 1.4V, see here.
I measured this several times, last week and today and this is fact whatever equipment i use (scope, multimeter, DAQ).
So now the funny part:
The DC signal and the VCO monitor signal look almost identical (shape, uncalibrated).
As i checked the calibrated (fitted) VCO monitor signal reflects the beat signal within 100kHz or so, but 1MHz for 100% sure.
Now, taking the recorded DC-signal from the mixer from 35h of data and using the calibration (which i did several times) the beat frequency is out of range of the VCO, totally different !
The recorded signal does not match the VCO signal at all! Using the same coefficient to calibrate the spectrum recorded with the AC-coupled amplifier the noise is way to high, higher than everything we had before.
So i took the VCO monitor signal, assuming it is right and showing me the absolute beat frequency and calculated the right coefficient for the mixer signal, which is something like 5MHZ/V instead of 170MHz/V.
Done that, comparing both time series looks fine.
Taking this coefficient to calibrate the spectrum then gives, at least for a the features around a couple of Hz the right level we measured several times before.
The lower frequencies are dominated by the noise of the preamp, so no comparison possible.
So i don;t know what's wrong because i can't reproduce what we recorded. Everything looks the same as last week, checked over and over again. But the DAQ shows something different.
As we can't trust whatever we recorded we redo the measurement. After bringing back the 4-way splitter to 40m i'm using 2-way splitters now instead, increased the signals where possible and the gain of the preamp. re-calibrated everything and triple checked, taking another measurement over night. We will see tomorrow....
Plots, 32h data stretches.
noise plot shows spectrum of data taken with and without 30mHz high pass correction
Found at least two (minor) problems:
it is not a lot of difference in slope but it is surprisingly linear over a range of 40MHz!
to have real data for comparison i measured the noise of the SR560 for different (high) gain settings.
Will add lower gain settings and line powered measurements later.
test setup: SR560, AC-coupled, low-noise setting, battery powered measured with SR785
datafiles contain raw output noise values, so for input referred noise plz divide by gain given in filename. first col = frequency, second col = noise spectral density
yes, for yesterdays measurements 7.435 MHz/V.
But that number changes from measurement to measurement as i'm changing the setup every time to improve SNR (or bring back stuff i've stolen from 40m)
So, from the first plot, is the calibration factor for cable delay technique ~ 8MHz/Volt?
this afternoon the framebuilder (and/or the NDS server) stopped for unknown reasons. We could see real data with Striptool but the framebuilder only saw old, non-changing EPICS values and was still writing them.
Rebstarting the daqd didn't help so i rebooted fb2 and the problem was gone. For some reason we have to manually start daqd und nds, but not a big deal every 6month or so. I also restarted the SoftIOC still running on fb2 which provides the calibrated VCO feedback signal
I had a closer look into effects caused by power fluctuations. To summarize : It is easy to measure TFs from power fluctuations to any other point in the system. SNR is good to very good.
I mainly focused on the effect of changes in shape of the TF from power fluctuations into changes of the beat frequency when changing the power levels anywhere in the system, especially in the cavity paths.
After playing a couple of hours and getting a feeling for what's going on i finally realized that only if changing the power in the ACAV-path strange things happen. This includes changing the total power going to both cavities as well. As taking TF's is taking too long at low frequencies i decided to switch from swept sine measurements to a simple digital modulation of the laser power with 100ms period. This is slow enough to see thermal effects in the cavity and other strange things which i will show below.
For the first set of measurements i reduced the power to the REFCAV path to about 460uW. The total modulation is very large, about 15.6%. The max power to ACAV was about 5.76mW, which i reduced later.
The following graphs shows the response of the beat signal to the digital modulation, beat signal calibrated to Hz, modulation signal a.u.., but the amplitude didn't change between measurements.
on the left graph (measured at very low power) one can see that the power modulation causes a sudden changes in frequency of the beat signal.
This can be caused by everything producing an offset in the EP which depends on power, e.g. RF-AM, higher-order modes in reflection etc. Nothing unusual so far.
If the optical power going to the ACAV is increased one can find a point where the change in frequency is almost gone, showed in the center graph.
When the power in increased further the sign of the frequency shift changes! Now some thermal effect becomes dominant, most likely the cavity as the period is 10s and after 5s equilibrium isn't reached.
Now this explains everything i saw before: When measuring the TF from power fluctuations to frequency shift and varying the power the shape of the TF changed because we have two different effects which can even cancel at the right power level send to the cavity!
As we know already that the size of the error signals is too tiny this effect will probably be almost gone once we fixed the demodulation for ACAV. I tried to reduce it a little bit by re-aligning the cavity but no luck - it's already aligned very well. I didn't try the EOM asthis changes the alignment to the other cavity as well and i was to lazy to re-align everything
after rebooting both crates i found that the perl script parameters for both loops are inconsistent with what's documented in the elog here.
Tara, can you plz check what the right numbers are. The numbers in the startup script are totally different from the values you posted.
corrected the PMC transPD calibration. Old values for EGUF/EGUL fields in the record were 74.1/-74.1.
The corrected values are 133/-133. Value now reflects what i measure with the Thorlabs power meter.
Changed both entries in the database file.
I re-aligned the PMC, same for the electro-optic amplitude modulator (EOAM). I also adjusted the power levels everywhere up to the PMC for maximum performance.
I also had to re-adjust the PMC phase. Startup-setting was 2.29V but had to be 3.29V! Error signal is 2.46Vpp at mixer mon.
Power is set to 20mW transmitted through the PMC when modulation is off. Power can be adjusted behind the PMC as it was setup before.
Power modulation is +/-1.5mW around 20mW with max on function generator (~15%).
due to the crash temperature of ACAV is 48 Celsius and RCAV 36 Celsius.
after rebooting yesterday the ACAV heater was railing over night.
Today i figured out that the integrater value in the startup skript had the wrong sign!
Changed the entry according to Tara's post last week. Now temp controller is working again.
Tom from the electrical workshop fixed the two sockets at the computer desk.
We also checked the other sockets as they were labeled wrong,so we didn't know which circuit breaker they go to.
Re-labeled the sockets.
All circuit breakers are located in the main panel in the ATF lab.
installed one of Jan's seismometers on the PSL table to measure TF from seismic to beat signal and to try some simple seismic noise cancelation
current heater value is around 4.66. changed crate startup value to current value
right! my hope was as the pendulum frequencies of the cavity also shows up in the seismometer we can use that to feed back to my little piezo shaker and reduce the effect a little bit.
The TF measurement is NOT for the fringe wrapping, it's for all the other stuff we see in the spectrum.
My hope was that we can gain at least little information about that for better noise estimate which does not couple through the cavity. So far our model only takes the cavity into account, not the rest.
The fringe wrapping signal comes from the velocity difference between the table and the cavity. You can calculate the spectrum of this by propagating the seismometer spectrum through the single pendulum approximation for the cavity.
For the feedback, you only have to suppress the seismometer signal. Since the fringe wrapping is nonlinear, its not predicted by the transfer function method.
PID process running on the Sun workstation stopped at lunch time with error "couldn't read process variable ...bla bla bla "
Turned it on again.
HVAC system seems to be broken as the temperature keeps rising. There is no cold air supply.
At the moment the temp changes about 0.5K every hour and we are at 25.5 degC at the moment.
We called 4717 to get it fixed
i've suspended another cavity (very similar to the LIGO refcav) on an old stack which is a bit longer than the one we currently use. That shouldn't matter for the viton damping measurements of the wire suspension.
The suspension itself is not identical to the one we use. The posts are different, i had to make some springs which can handle the load, and i had to cut some new wire. Below pictures of the new one and the last picture is from the existing one for comparison. We don't have any spare parts from the original suspension.
Here the original suspension:
while shaking the table we see some loose connections for the temp sensors for RCAV, but not for ACAV.
will post graphs later
Koji and i updated the shaker today. We replaced the short multilayer piezoelectric actuator (10mm) by a longer one (20mm) from the NEC TOKIN’s we have. (datasheet)
The pzt is glued to a brass disk, about 2" diameter and clamped between the side of the table and the steel frame around it using a aluminum base on the other side. (will add photo later).
We use a modified PMC servo card as a HV piezo driver. The modified schematic can be found below.
We added a 1kOhm resistor in the output which forms a ~100Hz low-pass with the 1.5uF capacitance of the PZT.
We get a good SNR ratio for TF measurements even when using white noise as the source. doing some low-frequency TF measurement over night.
WiIl also try a swept-sine measurement if required, but takes too long at low frequencies.
We also tried to build a simple loop using two stanford preamps to suppress the horizontal seismic motion of the table but couldn't see any improvement. Will wait for the measured TF to design the right loop.
HV amplifier schematic (modified PMC servo):
Why i did this: When measuring the beat signal with the PLL and the cable-delay setup simultaneously we didn't see correlation between both techniques.
We realized that we can't measure the current noise level using the (short) cable. However the estimated "noise floor" for the setup was much below.
So something must be wrong:
So i checked the noise floor of the cable setup again - it looks like i don't know how to determine the real noise floor for the setup:
I've set up a simple cable delay measurement setup using a Marconi, a power splitter and a +7dBm mixer. Marconi power is +10dBm, so LO is +7dBm.
Cable is 3 feet or so. Mixer signal is filtered (LP 1.9MHz) and terminated (50Ohm) before amplified with the battery-powered FET preamp.
I measured the noise floor by terminating the RF input of the mixer. Changing the input range and so the phase noise of the LO does not change the my "noise floor".
Then i measured the output for different input range settings of the Marconi starting from 1MHz down to 1kHz.
Here the result (not calibrated to frequency noise, only absolute voltage noise levels i measured, but does not matter for what i wanted to see):
For 1MHz and 500kHz input range everything looks OK at high frequencies. The characteristic bump at high frequencies is highly visible.
The low frequency noise looks different to what we measured before using the PLL, but that's not the problem right now.
Starting with 100kHz the measurement limit is reached but still above the "noise floor" with a terminated input.
So how do i measure the "real" noise floor without using a super low-noise oscillator?
Here the frequency noise of the same Marconi measured using the PLL:
will try tomorrow using the long cable...
piezo is clamped between frame and side of table. More details see here
i called them every day in the past and told them that something is wrong with the HVAC system of the labs but the didn't fix that.
So i called them today again and asked them to come and get in touch with me so that i can show them what's wrong.
So this afternoon a guy showed up and i showed him where the thermostats are, where the cold air is usually coming out and how it was before.
He first suggested that the HEPA filters might be the issue and asked me when we replaced it the last time.
So i said to him that it's pretty unlikely that all of them in three labs are bad from one day to another and that there is no cold air coming out anywhere.
He finally measured the temperature everywhere using one of those infrared thermometers and realized that all of the HVAC system has constant 78F anywhere in the labs.
He finally tracked it down to a hose usually connected to the main cold air valve of the system which was loose hanging around.
He connected it back and now we have cold air again
Temperature is rapidly decreasing from 81F at the moment.
it looks like not only estimating the right noise floor / sensitivity of the setup causes problems, also the absolute values and shape is different comparing both methods.
great, so we can check how sensitive the beat signal scatter bump is to cavity motion if we excite individual eigenmodes of both cavities before we change the suspension. This (hopefully) gives us information about which motion we are most sensitive at the moment so we can optimize the "new" suspension for that.
I lock both cavities, setpoints are.
ACAV 37.0 C
RCAV 34.85 C
nice, much better, i only have two little things at the moment:
we should check the mixer signal, the noise level is too high. We should compare it with the other EP signal (of the other cavity). If there is a huge difference between both we should start searching there. We should also check the RF noise level of the RF-PD. Maybe something is broken there.
Ok, hmm, still doesn't make sense. The first problem is that you don't have 50Ohm termination for the mixer signal when measuring with the scope, but if you connect it to the pdh box you have 50 Ohms. But that can't explain the plot alone.
Hmm, the amplifier might be a problem, it's a low-noise one but i didn't check the RF noise after installation. So remove the amplifier and redo the measurements. Be careful as the size of the error signal changes quite a lot and you have to turn the gain of the box back to 8 or so. So you have to re-measure everything.
Be very careful comparing the plain numbers! You have different error signal slopes so the corresponding frequency noise level for the sensing might be totally different !
Anyway it is good to compare individual parts which should be equal, e.g. the RFPDs at RF frequencies (or mixed down using the same(!) LO power (or better: mixing/conversion gain)
So what you say in the first sentence is right, but don't forget the gain of the amplifier! You can't compare those without taking that into account.
The increase from 30nV/rt Hz to 1uV/rt Hz is a bit higher as i would expect it but still makes sense. The minimum gain for the ZFL-500LN amplifier is 24dB, the difference between those numbers is ~30dB.
We should measure the mixer output with the amplifier but without the photodiode (input terminated) to see where the noise floor of the amplifier is. Then you know how far that is below the RFPD noise.(and can calculate the RFPD noise level from that i\f you want)
You say that the noise at the mixer output is the same for both loops, but the setup is completely different (different mixer, different LO power (23dBm and 7dBm or so). So be careful. It shouldn't make a big difference but you have to measure the RF noise around 35.5MHz or use the same mixer setup, e.g. plug in the ACAV RFPD in the RCAV mixer and compare then. Then you have real good numbers for comparison of the PDs.
main valve on vac chamber closed, turbo still running (just in case)
ion pump current: 7.3mA
ion pump voltage: 720V
tried to add voltage/current monitor signals to DAQ but analog outputs seem to be broken.
voltage monitor should be 1V/kV but is about 12V
current monitor is railing at 15V as it might only work in low current regime. Manual is wrong, does say signal should be 1V/kV , so don't know what i should expect>
But ususally mA are too much so i expect it to work below 1mA.
pressure for RCAV is OK. Ion pump current is currently at 52uA and falling. Turned turbo off.
ordered new shoe covers. Will try those below as they seem to be better than the ones we had but are much cheaper. Come in packs of 300, so 150 pairs for <$30.
at which modulation frequency? The text doesn't say.
The measurement of the RIN -> frequency coupling of the 40m Reference Cavity is here. Its an upper limit of 1 Hz/uW.
What are the equivalent numbers for the ACAV and RCAV here?
did you try to optimie/minimze the coupling before the measurement? If not you should check how you can make it worse/better (e.g. alignment to cavity, RFPD, polarization, RF-AM etc) and then measure it. It might be that you are sitting on kind of a maximum right now, who knows.
How about measuring a complete TF? Your signal looks large enough to do that.
I measured RIN -> frequency noise coupling coefficient at 10Hz from RCAV. The result is 0.02 Hz/uW (frequency shift/power fluctuation built up in the cavity*)
*by power fluctuation built up in the cavity, I mean [RIN x power input as measured at the input periscope x Finesse/ pi ]
Following the list I wrote down here,
1) TF between FASTOUT/FASTMON is a lowpass with a pole at 10Hz. FASTOUT is fed to the laser for controlling its frequency. FASTMON is a channel for monitoring. This is as expected from the RC low pass filter at FASTOUT.
2) I modulated the beam's polarization and used a PBS so the polarized transmitted beam is amplitude modulated. I used sine wave out from SR785, 2Vpp @ 10Hz connected to the EAOM. I measured the RCTRANSPD signal with an oscilloscope to check the signal is 211 mV +/- 7mV. The power is modualted by ~ 4%.
Then measure the peak at the modulating frequency (10Hz) from RCTRANSPD and FASTMON spectral density with SR785. The linewidth is 0.976 mHz. FASTMON was connected to chA, RCTRANSPD to chB. The data were averaged over 4 samples.
The DC level of RCTRANSPD was 220 mV.
The peak from FASTMON and RCTRANSPD are 6.75 mV/rtHz and 55.2 mV/rtHz. To convert this to the coupling coefficient we have to:
1) convert the voltage output from FASTMON to FASTOUT, the TF is measured and shown above.
V_fastout = V_fastmon x sqrt( 1 / 1 + (f/10Hz)^2 ), so for 10Hz,
V_fastout = V_fastmon x sqrt (1/2)
2) convert the Voltage out to frequency change by 3.09 MHz/V factor.
3) convert RCTRANSPD to RIN by dividing the PSD by DC level (220mV)
4) convert RIN to power fluctuation by x power input (1mW) x Finesse (9710) / pi
Freq/Hz = Vmon x TF x 3.09 [MHz/V]
V_RCtranspd / rctranspd_DC x Pin x Finesse/pi
For 10Hz, 1mW power input, rctranspd_dc = 220 mV, we get
RIN coupling coeff at 10Hz = 0.0188 Hz/uW = 18.8 mHz/uW.
I repeated the measurement with:
1)half modulating power, 1Vpp. the coupling coefficient decreases to 0.015 Hz/uW, so it seems that the modulation range I chose might be too large so the effect was not linear. The coefficient should remain constant!!
2) 20Hz modulating power, 2Vpp (but the line width was 7.8 mHz, not 1mHz for quick measurement)
, the coupling coefficient is 9.35 mHz/uW. The result is smaller than that of 10Hz, which is expected from 1/f effect, but I think I should have used the same line width and look at a few more frequencies.
I'll have to check the coupling coefficient from ACAV, and check the beat signal to see if they are canceled or not.
Comparison to the calculation I did
The calculation I did here gives the noise at 10Hz from 10mW input, RIN = 10^-4, Finesse 10^4, to be 1.4 [mHz/rt Hz] which is already lower than the coating noise (10mHz/rtHz at 10Hz)
I can convert it to the estimated coupling coefficient to compare with what I measured.
1.4 mHz/rtHz = coupling coeff [Hz/W] x Pin [W] x RIN [1/rtHz] xFinesse/pi, or
coupling [Hz/W] = 1.4 mHz/rtHz / Pin [W] / RIN [1/rtHz] / (Finesse/pi)
= 0.44 Hz/W
This is 5 order of magnitude lower than what I measured !!! and the calculated noise is only one order of magnitude lower than the coating. It means that the calculation is not correct and we cannot ignore the effect. we will run into it before reaching coating thermal noise if the effects on both cavities aren't canceled.
we have to design our own. The 40m one has 2" mirrors (too large, we don't have the space), wrong height for incoming/outgoing beam and is clamped to the table, which i think is bad in terms of stability.
The design principle does not look much different compared to the original refcav periscope design, except for the mirror holder itself. That was bad designed for the old one.
They are found in DCC. Some comments
- You can not steer the beam. The beam should be steered before or after the periscope.
- The side plates were too thick. It can be 1/2" thickness to reduce the total weight.
I looked up 40m elog and found Daisuke's design for periscope. I'll make a sketch FSS' periscopes.
The design for 40m pericopes by Daisuke can be found here .
The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.
the lens and mirror are in the ATF, a second VCO is in the left cabinet.
yesterday i got the new top stack plate from the machine shop. They couldn't finish the entire job on time so i had to tap the holes and do the surface cleaning (sand blasting, grinding and polishing to clean the surface and reduce surface area) myself.
Getting a machined surface would have taken until mid next week (minimum, no guarantee) which would have delayed things too much to get a measurements done before the LVC meeting. Looks pretty nice, has kind of a mirror finish.
I started cleaning it this afternoon, the final sonic cleaning is underway right now and i will start air baking tonight (talked to Bob and he said that this is enough). Also boiled the viton in isoprop and it's getting baked in vacuum at the moment (for 24h). The teflon parts and screws i already baked.
We already removed the insulation from acav and vented it. So we are ready to install the new stack once the plate and the teflon is ready. As i don't know how bad the new parts are (outgassing) thought about assembling the new stack and putting it in the acav chamber to pump on it over the weekend. Then we transfer the (cleaner) stack to the final chamber next week. We can then align both beams into the cavities and set up the beat parts while pumping on it with the turbo before we switch over to the ion pump. Or should we put everything together and start pumping Friday? Any suggestions? I tend to do it in two steps as i don't want to ruin the cavities and and if you ask Bob we should have gone through the whole class A cleaning process (which would take a month or more at the moment because they are somewhat of behind)
please be careful near the HEPA bench in 058E (PSL lab). I started the baking of the plate. The plate is sitting on the bench and is hot. It's powered by a HP power supply with 70V sitting on the floor. Warning signs are posted. The heater is insulated but anyways, be careful when near.that area. Setup will be removed Friday around noon. Same for the baking of the viton in Vladimir's lab. Heater and vacuum parts are hot, also the pump itself! Everything is hot, not just warm! Don't touch it.
the shields/heaters for the two cavities will be delayed by an unknown amount at this point. I'm still waiting for the two polished copper tubes and mounting parts to be finished. Also the Kapton heaters still didn't arrive.Even if we would get everything beginning next week at will take at least a week to get it cleaned, heaters wired, glued, cleaned and baked. Same for temp sensors etc. So we will try to use the cavities without it for a first shot. We might be lucky and the difference in frequency is not that big. (370MHz max possible). If we are lucky it's small (and we can slightly tune it by choosing the absolute temperature if they are not equal in length)). We can still work on the alignment and locking and upgrade next week or so. But even if we have a beat at 300MHz we can check the performance of seismic isolation and scattered light and see if we made a big mistake...
closed the valve of the refcav chamber and turned on the ion pump. Initial current was 7mA and went down to .3mA within an hour, so looks pretty good.
we can borrow the 2GHz PD from the cryo experiment for a quick measurement. Will get the shields/heaters next week and have the sensors. So we might be able to add those before the meeting, but i would do the test at 289MHz first to see what the noise looks like. If it's terrible we might have to work on other things as well. e.g. the cavity support.
I aligned the beam to ACAV and RCAV and measured the frequency different between the two cavities to be 289 MHz.
dL~200nm + n*532nm, so you can't say what the common mode suppression really is. The only thing we know is that the length are not off by 1mm or so (which you would see and easily be able to measure), but it could be anything below that. Next time we open it we should use a good digital caliper and measure the actual lengths. But we don't have one which is long enough.
use df/f = dL/L, the differential length between the two cavities is dL = df * L * lambda / c ~ 0.2 micron.
So common mode suppression for thermo-elastic or cavity sagging due to seismic can be approximated to be ~ dL/L = 0.2 micron / 0.2 m = 1e-6.
thought a little bit about how to create the difference of 127MHz. Here are two ways how we can do this right now:
We only use one AOM (as before). We can't use the second one as we would have to use it way beyond it's operating range (which we already tried Friday) or in second order where we don't get enough light if double passed.
So using only one we have the following options:
found the box with the beam splitters Dmass bought almost 2 years ago but never unpacked or used. They are super-polished 50:50 beam splitters for 532&1064nm but optimized for 1064nm.There are 16pcs total, so i don't see why we can't use 3 of them for our beat setup. We now have only SP optics in the critical beam paths except for the windows of the vacuum can, all lenses and wave plates where required. I hope this will reduce the amount of scattered light a little bit. The new setup only uses a minimum of components.
when playing with the setup i've noticed that we are actually very sensitive to temperature fluctuations (more than i expected), so the absolute length (or CTE?) must be more different than we expected. So i've added the thermal insulation, drilled four new holes in the end caps, connected the temp sensors etc. Thermal control loop is running now. Made some changes to channels (mostly range limiuts, warning levels etc) and added four new software channels which i will use to generate absolute calibrated temp signals. The difference between the sensors is about 1K. I know it does not matter for the stabilization but i hate it as you never know if you have a gradient or degraded sensor or so...
I'ver started heating it up to 26.5C (you may ask why this number: laziness - actually i've started the heater before finishing the insulation and that was the actual temperature at the time i finished it). The LO for the AOM already increased to 70.6MHz (from 64MHz). So my guess is that once we reach 30C we are back in range for the iLIGO VCO.
Will change the temperature over the weekend in a couple of steps to measure the temperature sensitivity (differential length change with temp). This will tell us how good the thermal stability has to be to not be limited by thermal fluctuations. We then can make a decision on further things like second, external box, passive foam or metal box with active control etc..
11:47pm : the current VCO frequency @26.5degC is ~71.294MHz still slightly drifting towards higher frequencies - will wait a few more minutes before i step it up.
11:56pm : now 71.318MHz @26.5degC
12:27am : now 71.373MHz
12:42am : now 71.394MHz
1:54am : now already 71.497MHz
2:39pm : now steady at 71.858MHz