The circuit noise improves so much, but many line noises appeared.
Where do these lines (40, 80, 200 Hz...) come from?
These does not change if we changed the microphones...
Anyway, I have to change the circuit (because of the low-pass filter). I can check if the circuit I will remake will give some effects on these lines.
I do not think that 1U rack power supply influenced on the preamp noise level as there is a 12 V regulator inside. Lines that you see might be just acoustic noise produced by cpu fans. Usually, they rotate at ~2500-3000 rpm => frequency is ~40-50 Hz + harmonics. Microphones should be in an isolation box to minimize noise coming from the rack. This test was already done before and described here.
I think we need to build a new box for many channels (32, for example, to match adc). The question is how many microphones do we need to locate around one stack to subtract acoustic noise. Once we know this number, we group microphones, use 1 cable with many twisted pairs for a group and suspend them in an organized way.
I do not think they are acoustic sounds. If so, there should be coherence between three microphones because I placed three at the same place, tied together. However, there are no coherence at lines between them.
I am trying to find what limits the reduction rate with wiener filtering.
I did some calculations below:
Reduction rate estimation by microphone noise
When the instrumental noise (noise in microphone) and noise injected to signal after the acoustic signal is injected exist, the noise cancellation rate is limited. (I will write a short document about it later.) I assumed that there is only instrumental noise and that the other noise in PMC is below enough, and calculated the cancellation rate. The instrumental noise is modeled according to the measurement before (ELOG).
The green line is the original PMC signal, the red one is PMC residual error, and the blue one is PMC residual error estimated by the cancelling rate.
Around 30 - 80 Hz, the wiener filtering seems to be already good enough. However, I do not know what limits the cancellation rate (such as 100 - 200 Hz).
I hypothesized that the wiener filter is not good because of some peaks or other noise. So I filtered the PMC signal and mic signal to see the difference.
The red line is wiener filter with no filters, the blue one is with filters (low pass, high pass, and notch).
The wiener filter seems to get smoother but the PMC residual error did not change at all.
I will attach a document which describes how the noise affect the wiener filter and the noise cancellation ratio.
And I re-estimate the SN ratio in the microphone (but still rough):
The yellow line is modeled signal level, and cyan line is modeled noise level.
Then, the estimated filtered residual noise is:
The noise is already subtracted enough below 80 Hz even though there is still coherence.
Above 300 Hz, the residual error is limited by other noise than acoustic noise since there is no coherence.
I am not sure about the region between 100-300 Hz, but I guess that we cannot subtract the acoustic noise because primary noise (see the document), such as a peak at 180 Hz, is so high.
[Rana, Ayaka, Jenne]
We aligned the REFL beam to the center of PD.
Also we removed the small black parts from mirror holder so that the beam is not clipped. They are originally for holding the mirror, but the mirror should be held by the small screw on the side of the mirror mount. This screw was hidden by the label, so we moved the label on the right hand side of the mirror mount (See a picture below).
Also we removed the half-wave plates and PBS so that laser power is increased.
Then I aligned the beam for PMC, locked MC, and centered the beam spots.
The MC2 pitch is a little bit high but still close enough to the center.
Jenne had also centered the beam spots on QPDs for WFS.
In order to perform acoustic noise cancellation with MCL signal, I am trying to find sweet spots for microphones.
I set microphones at various places around MC chambers, and see how coherent microphones and MC signals are.
I had checked the half part of MC.
The acoustic noise around the MC2 chamber is most critical so far. I could subtract the signal and the sensitivity got 2 times better.
I will see the acoustic coupling from the other side of MC.
I got two seismometers and one microphone back from Tara.
They are now near the Gurlap under the MC.
In order to estimate whether we can see acoustic coupling in arms or not, we have to calibrate signals to phase noise.
I used the same method as Yuta and Jenne did (6834).
I switched from ETM locking to ITM locking since only ITM actuators are calibrated (5583), and measured the open loop transfer function and the transfer function from ITM excitation to POX/POY error signal. Then I can estimate the calibration value H [counts/m] from POY/POX error signal to displacement.
Yarm; H = 9.51 x 1011 counts/m
Xarm; H = 6.68 x 1011 counts/m
Phase noise in arms:
blue; Xarm, green; Yarm
I will calibrate the acoustic signal and see if it is reasonable that we can see the acoustic coupling signal in the arms.
But I guess it is difficult. Actually I have not seen coherence between ETM feedback signals and acoustic sounds yet. (I measured acoustic noise near POX and in PSL table.)
If I find that it is hopeless, I will create some sounds and try to measure transfer function from acoustic sound to arm cavity signals.
I am interested in how the transfer function calculated by wiener filtering is different from the measured transfer function.
I found that we do not have enough phase margin. This is why the arm locking is not so stable.
I uploaded a zip file that contains data files used for the calibration.
OLTF_x/y.txt: the open loop transfer function (measured by IN1/IN2 in arm servo filter bank).
coh_x/y.txt: coherence of OLTF. I used the data where coherence > 0.98.
ext_err_x/y.txt: the transfer function from ITM excitation signal to POX/POY error signal.
coh_x2/y2.txt: coherence of ext_err. I used the data where coherence > 0.98.
The LSC whitening filter was off because the xarm was unlocked when the POX Q whitening filter was turned on. (We have to study what was wrong.)
The SUS whitening filters were on.
The all digital filters except +6dB filter were on.
[Rana, Jamie, Ayaka]
We could not lock the arms with MC_L loop on. Therefore we measured the change in YARM error signal when the MC_L is turned on.
(data; POYerr_MCF.xml in zip file)
Green line; POY error signal when MCL loop was on and the YARM loop gain (0.5) was so high that the saturated control signal made funny peak around 250 Hz.
Blue line; POY error signal when MCL loop was off and the YARM loop gain was low (0.2).
Pink line; POY error signal when MCL loop was on (the gain was -300) and the YARM loop gain was low (0.2).
Red line; POY error signal when MCL loop was on, another low pass filter (2nd order, cut off frequency of 55Hz) was added to MCL loop and the YARM loop gain was low (0.2).
We also changed the filter trigger in order to lock YARM. The FM7 and 8 trigger was turned off. It means that spectrum above was took with FM2,3,4,5,6,9,10 on. Whitening filters were also on.
MCL control signal makes the arm spectrum bad because the MCL control signal moves MC2 mirror additionally and adds extra frequency noise.
Ideally, error signal should be the same at higher frequencies and go down at the lower frequencies when the MCL loop is on because MCL signal should suppress the seismic noise.
Before we added the LPF, MCF/MCL loop cross over (which was taken with the template /users/Templates/MC/MCL-MCF_xover-2012-8-23.xml) is below;
(MCL-MCF_xover.xml in zip file)
After the LPF is added, the cross over has been changed as below;
(MCL-MCF_xover2.xml in zip file)
For now, I will just turn off the MCL loop for the acoustic noise experiments.
Last Thursday, I put the speaker and my laptop in the PSL table, and make triangular wave sound with the basic frequency of 40Hz, and Gaussian distributed sound.
(I create the sounds from my laptop using the software 'NHC Tone Generator' because I could not find the connector from BNC to speaker plug.)
And I measured the acoustic coupling in MCF signal. The all the 6 microphones were set in PSL table around PMC and PSL output optics.
The performance of the offline noise cancellation with wiener filter is below.
(The target signal is MCF and the witness signals are 6 microphones.)
I can see some effects on MCF due to the sound on PSL table. Though I can subtract some acoustic signal and there are no coherence between MCF signal and mic signals, still some acoustic noise remains.
This is maybe because of some non-linearity effects or maybe because we have other effective places for acoustic coupling measurement. More investigations are needed.
Also, I compared the wiener filter and the transfer function from microphones signal to MCF signal. They should be the same ideally.
(Left: Wiener filter, Right: Transfer function estimated by the spectrum. They are measured when the Gaussian sound is on.)
These are different especially lower frequencies than 50 Hz. The wiener filter is bigger at lower frequencies. I guess this adds extra noise on the MCF signal. (see the 1st figure.)
The wiener filter can be improved by filterings. But if so, I want to know how can we determine the filters. It is interesting if we have some algorithms to determine the filters and taps and so on.
The more investigations are also needed.
I have been searching for the way we can subtract signal better since I could see the acoustic coupling signal remains in the target signal even though there are no coherence between them.
I changed the training time which is used to decide wiener filter.
I have total 10 minutes data, and the wiener filter was decided using the whole data before.
(Right: the performance with the data when the triangular sound was created. Left: the performance with the data when the gaussian sound was created.)
I found that the acoustic signal can be fully subtracted above 40 Hz when the training time is short. This means the transfer functions between the acoustic signals and MCF signal change.
However, if the wiener filter is decided with short-time training, the performances at lower frequencies get worse. This is because wiener filter do not have enough low-frequency information.
So, I would like to find the way to combine the short-time training merit and long-time training merit. It should be useful to subtract the broad-band coupling noise.
The attached plots display RMS noise from various accelerometers and seismometers over the past 90 days. One can see how after the reinstallation of the seismometers in November, RMS from the GUR1Z and GUR1X channels decreases by a factor of about 100 from data in August. Additionally, the RMS over the course of the last 90 days has notably decreased in all instruments. In many cases, the RMS is only the result of inherent electronics noise, rather than from a signal.
We found that seismometer was working and the calibration in the filter banks should have been wrong.
We turned off the all FM2 filter in RMS filter banks.
We also installed STS seismometer. It is under the BS. Now we have spectrum of three seismometers.
RA: the above plot is kind of unreadable and useless. Please replace with something legible and put in some words about why there is a wrong filter, what exactly it is, etc., etc. etc. And why would you leave in a filter which is not supposed to be on? We might as well leave a few secretly broken chairs in the control room...
In order to calibrate MC_F signal, I need to know the calibration value from thorlab's PZT driver to laser frequency.
The calibration value should be ~ 15MHz/V (the PZT driver has 15 gain, and the laser has the calibration value of ~ 1MHz/V according to the laser spec sheet), but I want to confirm it.
This can be measured by sweeping the input voltage of the PZT driver and see the transmission signal from unlocked PMC.
1. Response of PMC transmission when the signal is inputted to laser PZT
I inputted 0.2 Hz triangular wave with 5Vamp and 2.5V offset into the PZT driver and see the transmission signal from PMC. After the PZT driver and before the laser, there is an analog low pass filter but its cut off frequency is 1 Hz so I did not take it into consideration.
(TEK00000.CSV, TEK00001.CSV in the zip file)
I could not the side-band resonances. I guess it was because the generated signal is not big enough (but still the maximum range of the signal generator.)
Therefore, in order to calibrate the input voltage to the frequency, I need to know finesse or FWHM frequency.
2. Responce of PMC transmission when the voltage of PZT on the PMC is swept
In order to measure the finesse and FWHM frequency, I also swept the PMC PZT voltage with the DC offset slider at the FSS.adl and tried to measure the finesse of PMC. (reference: elog #904)
(PMC-PZTcal_121203.xml in the zip file)
The result of fitting:
V_FSR (the PZT voltage difference between the 2 resonances) ~ 63 +/- 7 V (= 731MHz (given))
V_FWHM (the PZT voltage to sweep FWHM) ~ 0.32 +/- 0.04 V (~ 3.7 MHz)
Finesse ~ 200 +/- 30
However, this finesse value is much smaller than the value on the Wiki, 800. (Manasa showed me.)
V_FSR is comparable to the result Rana got at the referenced elog. But I am not sure about the V_FWHM because it is hard to figure out how large the PZT voltage changed from the template file (PMC-PZTcal.xml).
Are those mode wrong? But if so, where is the correct mode resonances? I think they should be visible...
3. Calibration value
When I know the FWHM frequency, I can calibrate the input on the PZT driver into laser frequency.
The results are:
if I take the finesse of 800 and FSR of 731 MHz (the values on the Wiki): ~5.0 MHz/V
if I take the finesse of 200 and FSR of 731 MHz (the measured value): ~20.0 MHz/V
Actually, the measured value is closer to the value calculated from the spec sheet.
Hmm... Does anyone find falses in my measurement?
If not, the finesse can be 4 times smaller than the value which was 5 years ago?
BS chamber seemed to be kicked again around 10:00 am today.
I moved PZT mainly in YAW and locked both arms. I adjusted the beam to be almost on the center of both ETM by sights.
In order to see the acoustic coupling on arm signals, I set 6 microphones and the speaker on the AP table. The microphones are not seismically isolated for now.
I have a signal generator under the AP table.
When I played the 43 Hz triangular wave sound, I could see some coherence between POY error signal and microphones even though there is no peak in POY.
I will just leave the picture of spectrum that shows the injected acoustic sound effects due to the oplevs.
red line: POY error without oplev feedback nor acoustic noise
blue line: POY error without oplev feedback but with acoustic noise
brown line: POY error with oplev feedback but without acoustic noise
green line: POY error with oplev and acoustic noise
You can see there is noise only at green line around 70 - 100 Hz. And it does not look like the acoustic signal is injected directly to the arms but the acoustic sound couples to the original noise source.
We observed that oplev servos affect the arm spectra badly (elog #7798). Some of them are fixed, but still they inject noise into the arms.
So I tried to turn the oplevs off and to see the acoustic noise effect. However, the mirrors moves so much that the signal does not seem to be linear any more, and the noise spectrum of arms changes especially around 60 - 100 Hz as you can see the spectrogram of YARM error signal below. This makes it difficult to find acoustic coupling noise. Therefore, I tried to fix the oplev servos so that the noise spectra do not get worse when the oplev servos are on.
Checking oplev UGFs
I checked the oplev open loop transfer functions. The UGFs of oplevs are all around 1-3Hz and phase margin looks enough except the BS oplev.
The gain of the BS oplev OLTF has so low that the signal is not fed back. Moreover, there is much phase delay in the BS feedback loop than the others'.
The counts of BS oplev sum is not changed so much for this 4 months, so the oplev beam seems to hit the BS correctly.
I am not sure what makes difference.
Clipped oplev beam fixed
Den and I found the output beam of ETMY oplev was clipped the other day. Also I found the scattered beam of ITMY oplev was on the edge of the mirror inside the vacuum and it made more scattered lights.
(before) -> (after)
I fixed both of the clipped beam. But still the oplev feedback inject the noise into the arm. (red: oplev off, blue: oplev on)
How NOT to:
The janitor can not clean in areas like this. He may only steps on these cables accidentally as he dust wiping our chambers.
Sorry for the mess. I fixed it.
The BS oplev pitch feedback came back.
The problem was that 300^2:0 filter was off. And I turned on all the low pass filters (ELP35), then the oplev servo does not seem to inject big noise into the arms as long as I see the spectra of POY and POX. These low-pass filters will be modified tomorrow so that the acoustic coupling noise is minimized.
Last night, I injected acoustic noise at POX table and AS table with oplev controls on (LPF is on).
1. acoustic noise at the POX table
I set the microphones and speakers at the POX table and see the acoustic coupling.
I could see slight change around 40 Hz. This can be caused by the oplev feedback loop because the speaker was on the same table as the ITMX oplev.
2. acoustic noise at the AS table
I controlled XARM with AS error signal and set the microphones and speaker on the AS table.
The resonance a 200 Hz seemed to be enhanced. But still we are not sure that it is caused by acoustic noise. Because this resonance is enhanced when the OL gain is high, and the gain adjustment was so critical that this resonance was easily enhanced even when the acoustic noise is not injected. And sometimes it has gone away.
I calibrated MC_F signal into Hz/rtHz unit using the transfer function from MC_F to PMC feedback signal.
Here is the diagram:
n_mcf is MC_F signal we can get at dtt. I measured n_pmc/n'_mcf using SR.
Other information I used:
G_out = 2.49/123.49 (see the document D980352-E01-C)
Fout has 1 pole at 10 Hz (see the document D980352-E01-C)
A_pzt = 371e+6/63 [Hz/V] (see elog)
F_wt has 1 pole at 100 Hz and 1 zero at 10 Hz.
Then, calibration transfer function of H is fitted as 1e+9/f [Hz/V]:
Then, the calibrated spectrum of MC_F is below:
This calibration have about 20 % error.
Compared to the spectrum in Jenne's paper (elog), above 20 Hz it seems to be laser frequency noise. But now we have extra unknown noise below 10 Hz.
Note: calibration value of laser's PZT is ~ 1MHz/V. This is reasonable compared to the data sheet of the laser. (This is calculated by combining result of H and transfer function of the circuit box1 and FSS.)
Yesterday, I made new mounts for microphones.
I glued a microphone on a pedestal. The cables are attached loosely so that its tension does not make any noise.
At the bottom of the mount, I attached the surgical tube forming a ring by double-side tape so that it damps the seismic vibration.
I made 6 mounts and these are all on the AS table now.
I took some data of XARM signal controlled by AS.
My plan is to find/set an upper limit on acoustic coupling noise in AS signal.
The acoustic noise can be estimated by the Wiener filter, but it is not accurate because it may see residual correlation between AS and microphone signals that should be 0 when the data is long enough.
I will find/set an upper limit by the analysis based on Neuman-Pearson criterion, that is analog of a stochastic GW background search.
If I can find the acoustic coupling noise should be below the shot noise, I am happy. If not, some improvements may be needed someday.
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.
There was an earthquake around 2:30 am. Now all the mirrors except SRM are damped.
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).
We aligned and locked xarm for green.
We aligned and locked x and y arms.
MCL loop makes arms lock unstable, adds a lot of noise at frequencies 60-100 Hz. We'll fix it.
At some point we were not able to lock because of ADC overflows of PO signals. They happened if whitening filters were enabled. So we reduced the gain of POX whitening filters down to 36 dB and POY - to 39 dB. Now cavities can be locked with whitening filters.
Also we changed the pedestal of the lens in the beam path to the POX because the beam was too high.
Global damping screens are in progress for the new global damping infrastructure Jamie discussed in log #8159. The main overview screen is /opt/rtcds/caltech/c1/medm/c1sus/master/C1SUS_GLOBAL.adl. The overview screen links to a few sub-screens in the same directory called C1SUS_GLOBAL_DAMPFILTERS.adl, C1SUS_GLOBAL_GLOBALTOLOCAL.adl, and C1SUS_GLOBAL_LOCALTOGLOBAL.adl.
This global damping is in intended to damp the 4 test masses along global interferometer degrees of freedom that are orthogonal to the cavity signals. Ideally the result will be that OSEM sensor noise from the damping loops is invisible to the cavity signals. Mismatches in the suspensions' dynamics and gains will cause some noise to leak through anyway, but we should be able to tune some of this out by carefully scaling the drives to each suspension.
I made a minor modification to install some output filters in the new global damping GLOBAL box in c1sus.mdl. These will be needed for tuning the suspension drives to compensate for mismatches in the pendulums.
I recompiled and installed the model, but did not start it. Basically same as Jamie left it in 8159. Interestingly, I did not see the new POSOUT that was put in before the SUSPOS DOF filter. I made sure to reopen the .mdl file fresh before making more mods, but for some reason I do not see that update...
While doing initial measurements for the new global damping infrastructure I discovered that the ETMY loop between the OSEM actuation and the OSEM sensors has a gain that is 2.5 times greater than the ITMY. The result is that to get the same damping on both, the damping gain on the ETMY must be 2.5 times less than the ITMY. I do not know where this is coming from, but I could not find any obvious differences between the MEDM matrices and gains.
I uploaded a screenshot of measured transfer functions of the damped ITMY and ETMY sus's. Notice that the ETMY measurement is 2.5 times higher than the ITMY. The peak also has a lower Q, despite having the same damping filters running because of this mysterious gain difference. Lowering the damping gain of the ETMY loop by this 2.5 factor results in similar Q's.
The global damping input and output matrices were installed to run for the Y-arm. Since we are using just one arm for now, only the DARM and CARM DOFs were entered into the matrices.
The input matrix was set to have elements with magnitudes of 0.5 while the output matrix was set to have elements with magnitudes of 1. The input matrix gets the 0.5 because the sensor signals must be avergaed for each global DOF, to make an 'equivalent sensor' with the same gain. The output matrix gets magnitudes of 1 so that the overall gain of the global loops is the same as the local loops. A transfer function was measured on the CARM loop to check that the overall gain is in fact the same as the measured ITMY and ETMY loops.
Simple damping filters were installed for the ITMY and ETMY as well as the global y arm CARM and DARM loops.
The ETMY output tuning filter ETMY_GLOBPOS was set to have a gain of 0.4 because there is an extra gain of 2.5 relative to ITMY in some mysterious place as discussed in log 8172.
New excitation points were added after the global damping loops for more testing options. The updated c1sus.mdl model was re-committed to the svn. Two interesting simulink 'requirements' were found during this minor modification. First, excitation points must be placed on the top level of the diagram. If they are in a subsystem you will get compiling errors. Second, the excitation name must end in _EXC. It will compile OK if you don't do this, but the excitation points will not put out any excitations.
To do further investigation on the mysterious gain factor of 2.5 between the ETMY and ITMY POS damping loops, I measured TFs in the POS direction to the locked YARM signal for each. This provides an additional sensor, common to both, so we can see if the gain is coming from the actuation side or sensing side of the damping loops. The difference in these TFs is about
So it seems the majority of the damping gain difference is on the actuation side with some small difference on the sensing side. In order to allow for the later splitting of YARM LSC control between ITMY and ETMY (global damping and the cavity control must be along the same coordinate system), I placed this gain of 2.95 in ITMY_LSC.
To get a first measure of the relative performance of global damping to local damping I measured some TFs between the sensor signal inputs and YARM. So first, while the cavity was still locked with just ETMY, I measured a TF between C1:SUS-ITMY_SUSPOS_EXC and C1:LSC-YARM_IN1. Second, I split the cavity control evenly between the ETMY and ITMY by adjusting C1:LSC-OUTPUT_MTRX. I turned off the local damping and turned on the common DOF global damping (called CARM at this point despite being on just one arm). I then repeated the same TF but driving from C1:SUS-GLOBAL_CARMDAMP_EXC.
The resulting TFs are displayed in the attached figure. The blue curve is then the TF from local damping sensor noise to YARM. The green is global damping sensor noise to YARM. The suppression between local to global is in red. The global damping curve is about 50 to 100 times lower (better) than local damping. This can probably be improved with further tuning to account for remaining differences between the ITMY and ETMY.
Note, the damping loop used in the filter modules for all of these is zpk(0,[15 15],1), with a gain of 30. This purposely has little high frequency filtering so it is easier to see the influence on YARM.
Brett and Kamal
The global damping testing for the week is now complete. The c1sus.mdl simulink diagram settled on the attached screenshot. The top level of c1sus.mdl is shown on the left zoomed in over the new global damping block. The right shows the inside of that block. Also attached in the second screenshot are two of the modal damping MEDM screens. The left shows the main overview screen, the right shows the global damping filters. The overview screen is called C1SUS_GLOBAL.adl and is found in ...medm/c1sus/master/.
We have measured transfer functions and power spectra that show that global damping, with just a moderate amount of tuning (30 minutes of work) reduces the OSEM damping noise seen by YARM_IN1 by a factor between 50 and 80. Log 8193 highlights the transfer function measurements. The power spectra directly measure the noise in the cavity. I am not putting that data here because I have to catch. I will process the data and post it here later.
Overall the global damping tests appear to have been successful, isolating (not removing) the test mass damping noise from the cavity by almost 2 orders of magnitude. Presumably even more isolation is possible with more tuning.
Here is an amplitude spectrum plot of y-arm cavity noise with a 50 Hz cutoff damping filter of the form zpk(0,[50;50],1). The low passing of this filter was intentionally extremely poor in order to see the damping noise in the cavity. The blue trace is the noise with no damping, which may be considered the 'best case' scenario from a noise point of view. The green has regular local damping on the ITMY. The ETMY has no damping for this measurement because the cavity control feedback to the ETMY takes care of its control when the cavity is locked. Notice the the large increase in noise from 40 Hz to 250 Hz, up to 1 order of magnitude. This noise is from the OSEM sensors passing through the damping loops. The red curve shows the y-arm noise with the exact same damping, except it is now applied in the global scheme. In this case, the damping noise falls completely below the baseline level of the cavity and becomes indistinguishable from the 'no damping' case.
If the damping injected enough noise I'd expect we would see a drop of 50 to 80 times switching from local to global. That is, the same factor measured in the transfer functions listed in log entry 8193. However, the damping noise is only at most 1 order of magnitude above the baseline in this measurement. We would have to increase the damping noise by about another order of magnitude before we could expect to see the global damping noise in the cavity measurement.
The units of the cavity displacement in the plot were calculated using the 1.4e12 counts per meter calibration in log 6834. The measured UGF of the LSC loop at the time was 205 Hz. The peak in the plot above 200 Hz appears to be from this unity crossing. Moving the UGF also moves this peak.
Moral of the story: global damping can isolate the damping noise pretty well from the cavity signal.
OK. Today we did the same type of measurement for the Y arm laser as was done for the X arm laser here: http://nodus.ligo.caltech.edu:8080/40m/3759
And attached here is a preliminary plot of the outcome - oddities with adding on the fitted equations, but they go as follows
(Red) T_yarm = 1.4435*T_PSL - 14.6222
(Blue) T_yarm = 1.4223*T_PSL - 10.9818
(Green) T_yarm = 1.3719*T_PSL - 6.3917
It's a bit of a messy plot - should tidy it up later...
I'm going to take the easy question - What are the pink data points??
And I'm going to answer the easy question - they're additional beat frequency temperature pair positions which seem to correspond to additional lines of beat frequencies other than the three highlighted, but that we didn't feel we had enough data points to make it worthwhile fitting a curve.
It's still not entirely clear where the multiple lines come from though - we think they're due to the lasers starting to run multi-mode, but still need a bit of thought on that one to be sure...
Just a quick update... the Lightwave laser has now been moved up to the end of the Y arm. It's also been mounted on the new mounting block and heatsinks attached with indium as the heat transfer medium.
A couple of nice piccies...
The good news is that we seem to be running in a linear region of the PSL laser with a degree or so of range before the PSL Innolight laser starts to run multi-mode. On the attached graph we are currently running the PSL at 32.26degrees (measured) which puts us in the lower left corner of the plot. The blue data is the Lightwave set temperature (taken from the display on the laser controller) and the red data is the Lightwave laser crystal measured temperature (taken from the 10V/degC calibrated diagnostic output on the back of the laser controller - between pins 2 and 4).
The other good news is that we can see the transition between the PSL laser running in one mode and running in the next mode along. The transition region has no data points because the PMC has trouble locking on the multi-mode laser output - you can tell when this is happening because, as we approach the transition the PMC transmitted power starts to drop off and comes back up again once we're into the next mode region (top left portion of the plot).
The fitted lines for the region we're operating in are:
Y_arm_Temp_meas = 0.95152*T_PSL + 3.8672
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
X_arm and Y_arm vs PSL comparison.
Just a quick check of the performance of the X arm and Y arm lasers in comparison to the PSL. Plotting the data from the X arm vs PSL and Y arm vs PSL on the same plot shows that the X arm vs the PSL has no observable trending of mode-hopping in the laser, while the Y arm vs the PSL does. Suspect this is due to the fact that the X arm and PSL are both Innolight lasers with essentially identical geometry and crystals and they'll tend to mode-hop at roughly the same temperatures - note that the Xarm data is rough grained resolution so it's likely that any mode-hop transitions have been skipped over. The Lightwave on the other hand is a very different beast and has a different response, so won't hop modes at the same temperatures.
Given how close the PSL is to one of the mode-transition regions where it's currently operating (32.26 degC) it might be worth considering shifting the operating temperature down one degree or so to around 31 degC? Just to give a bit more headroom. Certainly worth bearing in mind if problems are noticed in the future.
Right. I've got a whole load of info and data and assorted musings I've been saving up and cogitating upon before dumping it into these hallowed e-pages. there's so much I'll probably turn it into a threaded entry rather than put everything in one massive page.
An overview of what's coming:
I started out using http://lhocds.ligo-wa.caltech.edu:8000/40m/Advanced_Techniques/Green_Locking?action=AttachFile&do=get&target=modematch_END.png as a reference for roughly what we want to achieve... and from http://nodus.ligo.caltech.edu:8080/40m/100730_093643/efficiency_waist_edit.png we need a waist of about 50um at the green oven. Everything else up to this point is pretty much negotiable and the only defining things that matter are getting the right waist at the doubling oven with enough available power and (after that point) having enough space on the bench to separate off the green beam and match it into the Y arm.
Step 1: Measure the properties of the beam out of the laser. Really just need this for reference later because we'll be using more easily measurable points on the bench.
Step 2: Insert a lens a few cm from the laser to produce a waist of about of a few 100um around the Faraday. Note that there's really quite a lot of freedom here as to where the FI has to be - on the X arm it's around columns 29/30 on the bench, but as long as we get something that works we can get it closer to the laser if we need to.
Step 3: After inserting the FI need to measure the beam after it (there *will* be some distortion and the beam is non-circular to begin with)
Step 3b: If beam is non-circular, make it circular.
Step 4: Insert a lens to produce a 50um waist at the doubling oven position. This is around holes 7/8 on the X arm but again, we're free to change the position of the oven if we find a better solution. The optical set-up is a little bit tight near that side of the bench on the X end so we might want to try aiming for something a bit closer to the middle of the bench? Depends how the lenses work out, but if it fits on the X end it will fit on the Y end.
RIght! Overview out of the way - now comes the trivial first bit
Step 1: Beam out of the laser - this will be tricky, but we'll see what we can actually measure in this set-up. Can't get the Beamscan head any closer to the laser and using a lambda/2 plate + polariser to control power until the Faraday isolator is in place. Using 1 inch separation holes as reference points for now - need better resolution later, but this is fine for now and gives an idea of where things need to go on the bench. The beam is aligned to the 3rd row up (T) for all measurements, the Beamscan spits out diameters (measuring only the 13.5% values) so convert as required to beam radius and the beam is checked to ensure a reasonable Gaussian profile throughout.
Position A1_13.5%_width A2_13.5%_width
(bench) (um mean) (um mean)
32 2166.1 1612.5
31 2283.4 1708.3
30 2416.1 1803.2
29 2547.5 1891.4
27 2860.1 2070.3
26 2930.2 2154.4
25 3074.4 2254.0
24 3207.0 2339.4
OK. As expected, this measurement is in the linear region of the beampath - i.e. not close to the waist position and beyond the Rayleigh length) so it pretty much looks like two straight lines. There's no easy way to get into the path closer to the laser, so reckon we'll just need to infer back from the waist after we get a lens in there. Attached the plot, but about all you really need to get from this is that the beam out of the laser is very astigmatic and that the vertical axis expands faster than the horizontal.
Not terribly exciting, but have to start somewhere.
Step 2: Getting the beam through the Faraday isolator (FI).
Started out with an f=100mm lens at position 32,T on the bench which gave a decent looking waist of order 100 um in the right sort of position for the FI, but after checking the FI specs, it's limited to 500W/cm^2. In other words, if we have full power from the laser passing into it we'd need a beam width of more than 211 um. Solution? Use an f=150mm lens instead and don't put the FI at the waist. I normally don't put a FI at a waist anyway, for assorted reasons - scattering, thermal lensing, non-linear magnetic fields, the sharp changing of the field components in an area where you want as constant a beam as possible. Checked with others to make sure they don't do things differently around these parts… Koji says it doesn't matter as long as it passes cleanly through the aperture. So… next step is inserting the Faraday.
The beam profiles in vertical and horizontal around the FI position with the f=150mm lens in place are attached. Note that the FI will be going in at around 0.56m.
I fired up some old waistplotter routines, and set the input conditions as the measured waist after the lens and used that to work out what the input waist is at the laser. It may not be entirely accurate, but it /will/ be self consistent later on.
Vertical waist = 105.00 um at 6.282 cm after laser output (approx)
Horizontal waist = 144.63 um at 5.842 cm after laser output (approx)
Step 3: Inserting FI and un-eliptical-ification of the beam
The FI set up on it's mount and the beam passes through it - centrally through the apertures on each side. Need to make sure it doesn't clip and also make sure we get 93% through (datasheet specs say this is what we should achieve). We will not achieve this, but anything close should be acceptable.
Setting up for minimum power through the FI is HWP @125deg.
Max is therefore @ 80deg
Power before FI = 544 mW
Power after FI = 496 mW (after optimising input polarisation)
Power dumped at input crystal = 8.6mW
Power dumped at input crystal from internal reflections etc = 3.5mW
Power dumped at output crystal on 1st pass = approx 8mW
OK. that gives us a 90.625% transmission and a 20.1mW absorption/unexplained loss.
Well - OK. The important part about isolators isn't their transmission, it's about how well they isolate. Let's see how much power gets ejected on returning through the isolator…
Using a beam splitter to pick off light going into and returning from the FI. A 50/50 BS1-1064-50-1025-45P. And using a mirror near the waist after the FI to send the beam back through. There are better ways to test the isolation performance of FI's but this will suffice for now - really only want to know if there's any reasonable isolation at all or if all of the beam is passing backwards through the device.
Power before BS = 536 mW (hmmn - it's gone down a bit)
Power through BS = (can't access ejected on first pass)
Power through FI = 164 mW (BS at odd angle to minimise refractive effect so less power gets through)
Power lost through mirror = 8.3mW (mirror is at normal incidence so a bit transmissive)
Using earlier 90.6% measurement as reference, power into FI = 170.83 mW
So BS transmission = 170.83/536 = 0.3187
BS reflectivity therefore = 1 - 0.3187 = 0.6813
Power back into FI = Thru FI - Thru mirror = 155.7 mW
Power reflected at BS after returning through FI = 2.2mW
Baseline power at BS reflection from assorted internal reflections in FI (blocked return beam) = 1.9mW
Note - these reflections don't appear to be back along the input beam, but they *are* detectable on the power meter.
Actual power returning into FI that gets reflected by BS = 0.3 mW
(note that this is in the fluctuating noise level of measurement so treat as an upper limit)
Accounting for BS reflectivity at this angle, this gives a return power = 0.3/0.6813 = 0.4403 mW
Reduction ratio (extinction ratio) of FI = 0.4403/155.7 = 0.00282
Again - note that this upper limit measurement is as rough and ready as it gets. It's easy to optimise this sort of thing later, preferable on a nice open bench with plenty of space and a well-calibrated photodiode. It's just to give an idea that the isolator is actually isolating at all and not spewing light back into the NPRO.
Next up… checking the mode-matching again now that the FI is in place. The beam profile was scanned after the FI and the vertical and horizontal waists are different...
Step 3b: Non-circular? We can fix that...
A quick Beamscan sweep of the beam after the Faraday:
25.8 503.9 478.8
25 477.5 489.0
24 447.1 512.4
21 441.6 604.5
20 476.3 645.4
19 545.4 704.1
18 620.3 762.8
OK. It looks not too bad - doesn't look too different from what we had. Note that the x axis is in local table units - I found this useful for working out where things were relative to other things (like lenses and the FI) - but it means the beam propagates from right to left in the plot. in other words, the horizontal waist occurs first and is larger than the vertical waist. Also - they're not fitted curves - they're by-eye, best guesses and there's no solution for the vertical that doesn't involve offsets... discussion in a later part of the thread.
Anyway! The wonderful thing about this plot is that the horizontal and vertical widths cross and the horizontal focussing at this crossing point is shallower than the vertical. This means that we can put a lens in at the crossing point and rotate it such that the lens is stronger in the horizontal plane. The lens can be rotated until the effective horizontal focal length is right to fix the astigmatism.
I used a 200mm lens I had handy - a rough check sweeping the Beamscan quickly indicated should be about right though. Adjusting the angle until the beam size at a distant point is approx circular - I then move the profiler and adjust again. Repeat as required. Now… taking some data. with just that lens in:
24 371.7 366.1
21 360.3 342.7
20 447.8 427.8
19 552.4 519.0
18 656.4 599.2
17 780.1 709.9
16 885.9 831.1
Well now. That looks quite OK. Fit's a bit rubbish on vertical but looks like a slight offset on the measurement again.
The angle of the lens looks awful, but if it's stupid and it works then it isn't stupid. If necessary, the lens can be tweaked a bit more, but there's always more tweaking possible further down the line and most of the astigmatic behaviour has been removed. It's now just a case of finding a lens that works to give us a 50 um beam at the oven position...
Step 4: Matching into the oven
Now that the astigmatism is substantially reduced, we can work out a lens solution to obtain a 50um waist *anywhere* on the bench as long as there's enough room to work with the beam afterwards. The waist after the Faraday and lens is at position 22.5 on the bench. A 50 mm lens placed 18 cm after this position (position 14.92 on the bench) should give a waist of 50 um at 24.57 cm after the waist (position 12.83 on the bench). This doesn't give much room to measure the beam waist in though - the Beamscan head has a fairly large finite size… wonder if there's a slightly less strong lens I could use…
OK. With a 66 mm lens at 23 cm (position 13.45 on the bench) after the waist we get a 50 um waist at 31.37 cm after the waist (position 10.15 on the bench).
Closest lens I found was 62.9mm which will put the 50um point a bit further towards the wall, but on the X-arm the oven is at position 8.75 ish. So anything around there is fine.
Using this lens and after a bit of manual fiddling and checking with the Beamscan, I figured we needed a close in, fine-grained measurement so set the Beamscan head up on a micrometer stage Took a whoie bunch of data around position 9 on the bench:
(mm) (um mean) (um mean)
-15 226.8 221.9
-14 210.9 208.3
-13 195.5 196.7
-12 181.0 183.2
-11 166.0 168.4
-10 154.0 153.1
-9 139.5 141.0
-8 127.5 130.0
-7 118.0 121.7
-6 110.2 111.6
-5 105.0 104.8
-4 103.1 103.0
-3 105.2 104.7
-2 110.9 110.8
-1 116.8 117.0
0 125.6 125.6
0 125.6 125.1
1 134.8 135.3
2 145.1 145.6
3 155.7 157.2
4 168.0 168.1
5 180.5 180.6
6 197.7 198.6
7 211.4 209.7
8 224.0 222.7
9 238.5 233.7
10 250.9 245.8
11 261.5 256.4
12 274.0 270.4
13 291.3 283.6
14 304.2 296.5
15 317.9 309.5
And at this point the maximum power available at the oven-waist is 298mW. With 663mW available from the laser with a desired power setting of 700mW on the supply. Should make sure we understand where the power is being lost. The beam coming through the FI looks clean and unclipped, but there is some stray light around.
7 868.5 739.9
6 1324 1130
5 1765 1492
4 2214 1862
The plot looks pretty good, but again, there looks to be an offset on the 'fitted' curve. Taking a couple of additional points further on to make sure it all works out as the beam propagates. I took a few extra points at the suggestion of Kiwamu and Koji - see the zoomed out plot. The zoomed in plot has by-eye fit lines - again, because to get the right shape to fit the points there appears to be an offset. Where is that coming from? My suspicion is that the Beamscan doesn't take account of the any background zero offsets when calculating the 13.5% and we've been using low power when doing these measurements - very small focussed beams and didn't want to risk damage to the profiler head.
Decided to take a few measurements to test this theory. Trying different power settings and seeing if it gives different offset and/or a changed width size
7 984.9 824.0 very low power
7 931.9 730.3 low power
7 821.6 730.6 higher power
7 816.4 729.5 as high as I'm comfortable going
Trying this near the waist…
8.75 130.09 132.04 low power
8.75 106.58 105.46 higher power
8.75 102.44 103.20 as high as it can go without making it's saturated
So it looks like offset *is* significant and the Beamscan measurements are more accurate with more power to make the offsets less significant. Additionally, if this is the case then we can do a fit to the previous data (which was all taken with the same power setting) and simply allow the offset to be a free parameter without affecting the accuracy of the waist calculation. This fit and data coming to an e-log near you soon.
Of course, it looks from the plots above (well... the code that produces the plots above) that the waist is actually a little bit small (around 46um) so some adjustment of the last lens back along the beam by about half a cm or so might be required.
I went through the entries.
1. Give us a photo of the day. i.e. Faraday, tilted lens, etc...
2. After all, where did you put the faraday in the plot of the entry 4466?
3. Zoomed-in plot for the SHG crystal show no astigmatism. However, the zoomed out plot shows some astigmatism.
How consistent are they? ==> Interested in seeing the fit including the zoomed out measurements.
OK. Taking these completely out of order in the easiest first...
2. The FI is between positions 27.75 and 32 on the bench - i.e. this is where the input and output apertures are. (corresponds to between 0.58 and 0.46 on the scale of those two plotsand just before both the vertical and horizontal waists) At these points the beam radius is around 400um and below, and the aperture of the Faraday is 4.8mm (diameter).
Laser set up - note the odd angles of the mirrors. This is where we're losing a goodly chunk of the light. If need be we could set it up with an extra mirror and send the light round a square to provide alignment control AND reduce optical power loss...
Faraday and angled lens - note that the lens angle is close to 45 degrees. In principle this could be replaced with an appropriate cylindrical lens, but as long as there's enough light passing through to the oven I think we're OK.
3. Fitting... coming soon once I work out what it's actually telling me. Though I hasten to point out that the latter points were taken with a different laser power setting and might well be larger than the actual beam width which would lead to astigmatic behaviour.