Measured frequency fluctuation of the beat between PSL and YARM lasers.
Yesterday, it was very tricky to adjust the voltage offset to the slow YARM laser input to achieve the appropriate beat frequency. Today, it was much easier. During measurement beat around 25 MHz. Calibration factor 40 mV per 10 MHz.
Red and blue curves: frequency fluctuation of the beat node between PSL and YARM laser.
Green and broen curves: Actuation on ETMY. In ALS_CONTROL.adl ETMY filter bank 4 and 5 were switched on. Gain was 0.3
Nice reduction of the frequency fluctuation.
Y axis is in volts^2 per counts. In order to go to MHz/sqrt(Hz) you have to take the square root and then times [20Volts/(2^16)counts]*[10Hz/0.04V].
Started to scan the cavity, but this didn't work. Green light all out of lock. IR beam was badly aligned to cavity. Now, my time is over and I have to leave you.
Thanks, for your help and the nice time.
Leaving a note on the ALS feedback before I forget:
The MC2 suspension needs to have an input for the ALS feedback in the realtime model like ETMs.
[Tomotada / Kiwamu]
The open loop transfer function of the Y end PDH loop was remeasured : the UGF was found to be at 17 kHz.
The phase margin at the UGF was about 27 deg.
While the measurement we noticed that the modulation onto the laser PZT was too big
and it was creating a big AM on the reflected light with an amplitude of a few mV.
So we put a 20 dB attenuator to decrease the modulations and the reflected light became much quitter.
Also the servo shape formed by Newfocus LB1005 looks too simple : we should have a more sophisticated servo filter (i.e. PDH box!!).
As promised, I will get on this this week.
Locking activity last night :
The free run beat-note in 532 nm has been measured.
However I couldn't close the ALS loop somehow.
Every time I tried closing the loop it broke the Y end PDH lock in a couple of minutes.
(Things to be done)
1. Optimization of the Y end PDH servo loop
2. Refinement of the broadband RFPD setup
Some updates on the Y end green PDH lock
(Measurement of the Y arm fluctuation)
(Temporary servo setup)
I found that the temperature controller of the PSL doubling oven had been disabled.
The Y arm green PDH servo is working fine with a sufficient amount of suppression.
And the servo configuration looks like this :
(the Error signal)
I took a spectrum of the error signal when the laser was locked to the Y arm and found that it meets the requirement.
The noise budget on the Y arm ALS has begun.
Right now the fluctuation of the green beat-note seems mostly covered by unknown noise which is relatively white.
(Though I feel I made a wrong calibration ... I have to check it again)
I modified the Y-Arm PDH box (S/N 17) to have the same TF as the one of the temporary setup described in Kiwamu's earlier entry. Note that the TF below was taken with the gain knob set to 0, so that the proper DC gain is achieved with a setting of ~4. This is desirable because it gives us wiggle room.
The changes were:
Below is the TF along with the LISO model. They are different at low frequencies because the box must have been railing internally (though the phase shows that the result is as expected), and there is a feature around 60 kHz that probably arises from some op amp instability. I will see if adding a small cap somewhere does the trick, and then take a new TF with a lower source voltage.
I'll try to lock the arm with the box tomorrow.
Another way to make a 1:100 pole:zero boost is to use resistors and capacitors in a Pomona box
mixer -> LB box -> Pomona box -> PZT
Pomona Box = R1 = 7.2 kOhm, C2 = 22 uF, R2 = 72 Ohms (sr560 = $2400, pomona ~ $50)
For the RMS calculation, it would be good to notch out the harmonics. They don't matter since our ALS feedback will have notches at those frequencies.
I wouldn't bother...
I installed the newly modified PDH box #17 and locked the Y-Arm.
I wasn't able to bring the REFL level down to the 30% that Kiwamu claimed to get, despite readjusting the alignment---I got ~40-45%. I attained a UGF of ~8 kHz, lower than the 20 kHz that Kiwamu said he got with the temporary setup, probably because the PDH box just isn't as fast. Despite that, it looks like the error suppression is actually better than before...
Here is an error spectrum:
I have to admit that this calibration is worthy of suspicion and should be done more rigorously. I simply used the measured UGF frequency and known servo TF and PZT actuator gain to estimate the optical response. I am pretty confident that it's accurate to within a factor of 3 or so.
The 2nd trial of the Y arm ALS noise budgeting :
(Removal of broad band noise)
I will add the dark noise of the broad-band beat-note PD and the MFD read out noise on the budget.
(The broad-band noise vs. gain of the Y end green PDH)
Last night I was trying to identify the broad band noise which is white and dominant above 20 Hz (#5970).
I found that the level of the noise depended on the servo gain of the Y end green PDH loop.
Decreasing the servo gain lowers the noise level by a factor of 2 or so. This was quite repeatable.
(I changed the gain knob of the PDH box from the minimum to a point where the servo starts oscillating)
(Malfunction in the comparator)
However I had to give up further investigations because the comparator signal suddenly became funny: sometimes it outputs signals and sometimes not.
It seems the comparator circuit became broken for some reason. I will fix it.
[Rana / Kiwamu]
As a part of the ALS noise budgeting we took a look at the Y end PDH setup to see if we are limited by an effect from the Amplitude Modulation (AM).
(AM transfer function)
(Y table setup needs more improvements)
What is meant by the "average response of 50 dB"? Is this dB[ RIN / Hz ] or something? Also, do you mean the average over a broad band or the average response at the chosen modulation frequency over several trials? I don't really understand what measurement was done.
The in-loop Y-Arm error signal looks equal to the beat note noise divided by the Y-Arm OL gain in the broadband-noise region (>20 Hz), which would be the case if the loop was dominated by sensor noise here.
I would re-check the Y-Arm dark noise, or at least check for coherence between the Y-Arm error signal and the beat signal above 20 Hz. The input-referred PDH box noise should not be flat there according to the LISO model, but that might be worth checking, too.
As a part of the ALS noise budgeting we took a look at the Y end PDH setup to see if we are limited by an effect from the RF Amplitude Modulation (AM).
The AM transfer function of the Y end laser has been measured again, but using the frequency-doubled laser this time.
Here is the latest plot of the AM transfer function. The Y-axis is calibrated to RIN (Relative Intensity Noise) / V.
IFBW (which corresponds to a frequency resolution) was set to 100 Hz and the data was averaged about 40 times in a frequency range of 100 kHz - 400 kHz.
Also the zipped data is attached.
It is obvious that out current modulation frequency of 179 kHz (178850 Hz) is not at any of the notches.
It could potentially introduce some amount of the offset to the PDH signal, which allows the audio frequency AM noise to couple into the PDH signal.
Currently I am measuring how much offset we have had because of the mismatched modulation frequency and how much the offset can be reduced by tuning the modulation frequency.
However I couldn't close the ALS loop somehow.
Locking activity last night:
It became able to close the ALS loop (beat-note signal was fed back to ETMY).
The UGF was about 60 Hz, but somehow I couldn't bring the UGF higher than that.
Every time when I increased the UGF more than 60 Hz, the Y end PDH was unlocked (or maybe ETMY became crazy at first).
Perhaps it could be a too much noise injection above 60 Hz, since I was using the coarse frequency discriminator.
Anyway I will try a cavity sweep and the successive noise budgeting while holding the arm length by the beat-note signal.
Another thing : I need a temperature feedback in the Y end green PDH loop, so that the PZT voltage will be offloaded to the laser temperature.
I succeeded in handing off the servo from that of the ALS to IR-PDH.
However the handing off was done by the coarse sensor instead of the fine sensor because I somehow kept failing to hand off the sensor from the coarse to the fine one.
The resultant rms in the IR-PDH signal was about a few 100 pm, which was fully dominated by the ADC noise of the coarse sensor.
Tomorrow I will try :
(1) Using the fine sensor.
(2) Noise budgeting with the fine sensor.
Here is the actual time series of the handing off.
No real progress.
Probably I spent a bit too much time realigning the beat-note optical path.
(what I did)
Status update of the Y arm green lock:
+ Recent goal : automation of the single arm green lock
(Things to be done)
+ Recent goal : automation of the single arm green lock
The existingly used used Pasternack Enterprices RG58 C/U cable lenght ~ 140 ft and the specs are here at Atm1
Atm2 The performance grade RG58-P coaxial cable specs.
About Noise Budget
How to improve it ?
The 60 Hz line noise has gone away.
Here are the latest plots that I have obtained from the Friday night:
The residual motion in the arm displacements reached 70 pm in rms.
Scripting of the single arm automated lock script is 80% done.
The remaining 20 % is not something immediately needed and I start decreasing the priority on the Y arm ALS.
I made the first trial of locking a Power-recycled single arm.
This is NOT a work in the main stream,
but it gives us some prospects towards the full lock and perhaps some useful thoughts.
Lock Acquisition Steps
Actual Time Series
Below is a plot of the actual lock acquisition sequence in time series.
Assumptions on the parameter estimations
After I did a fine alignment of the X green beam path on the PSL table, the X arm beat-note was also obtained.
Here is a picture of the latest setup. The blue lines represent S-polarizing green beams.
During I was working on the PSL table HEPA was at 80 %, and after the work I brought it to 20 %.
I added an ALS feedback path on the MC2 suspension and this path will enable us to stabilise the MC length using the ALS scheme.
The high frequency noise, which has been a dominant noise above 30 Hz in the Y arm ALS (#6133), decreased by a factor of 5.
This reduction was done by increasing the modulation depth at the Y end PDH locking. Now the noise floor at 100 Hz went to 0.2 pm/sqrtHz.
However the noise source is not yet identified and hence it needs a further investigation.
(Increasing the modulation depth)
One of my goals in this week is : measurement of the current best ALS noise budget.
Last night I took a new noise spectra of the Y arm ALS, which is shown in the attached figure below.
The displacement of the arm cavity observed from the IR PDH is at 66 pm in rms. In the measurement the arm length was stabilized with the ALS technique.
Here is a new time series plot showing how stably ALS can control the arm length.
In the middle of the plot the cavity length was held at the resonance point for ~ 2 min. and then it passed through the resonance point to show the full shape of the PDH signal.
Apparently the PDH signal is now quieter than before (#6133)
One of my goals this week is to get people to make plots with physical units:
I did some more stuff for the Y arm ALS and updated the noise budget:
After the works, the rms displacement improved a little bit, so it is now at 24 pm in rms.
Though, it turned out that the MFD's ADC is now limiting the noise in a frequency band of 200 mHz - 5 Hz.
So tonight I will increase the gain of the whitening filter to push down the ADC noise more.
(What I did)
+ added the DAC noise and comparator noise based on measurements.
+ redesigned the servo filter shape to suppress the seismic noise below 10 Hz.
The attached plot below shows the newly designed open loop transfer function together with the old one for a comparison.
UGF is at 120 Hz and the phase margin is about 27 deg.
Surprisingly increasing the gain of the whitening filter didn't improve the noise curve.
It suggests that the ADC noise is not the limiting factor below 10 Hz.
I placed an other Y2-LW-1-2050-UV-45P/AR steering mirror into the beam path of the green beam launching in order to avoid the ~30 degrees use of the 45 degrees mirror. The job is not finished.
During the Y arm ALS I found that the noise of the AS55 demod signal was worse than that of POY11 in terms of the Y arm displacement.
There is a bump from 500 mHz to 100 Hz in the AS55 signal while POY11 didn't show such a structure in the spectrum.
The plot below is the noise spectra of the Y arm ALS. The arm length was stabilized by using the green beat-note fedback to ETMY.
In this measurement, POY11 and AS55 were served as out-of-loop sensors, and they were supposed to show the same noise spectra.
In the plot It is obvious that the AS55 curve is louder than the POY curve.
The alignment is finished after the realization that the 3rd steering mirror had to be adjusted too.
The input power increased from 1.2 to 1.4 mW
I did a fine alignment on the Y end green setup. The green light became able to be locked again.
The Yarm green laser really wanted to lock on a 01/10 mode, so Kiwamu suggested I go inside and realign the green beam to the arm. I did so, and now it's much happier locked on 00 (the Yarm is resonating both green and IR right now).
Two extender plates ready for cleaning. The existing optical table tops have 38" OD. Using two of these the OD will be 44"
Two new BBPDs have been installed on the PSL table.
The first one was installed by Koji a few days ago, and I stalled the second one today.
They will serve as beat-note detectors for the green locking.
Next step : I have to lay down a long SMA cable which goes from the BBPD to the IOO rack.
Xarm is aligned for both IR and green.
Here is a photo of the beam paths of the PSL beat setup. I want to make sure that the X-green BBPD sees a nice beam from both the PSL and the Xarm, without disturbing the currently working Y setup. I keep getting confused with all the beamsplitters, especially the green PBSes, which operate at ~56deg, not 45deg, so I made a diagram.
Meh. I've searched in steps of 20 counts in C1:GCX-SLOW_SERVO2_OFFSET units (16 bit +\- 10V DAC, and 1GHz/V coeffecient for the Xgreen aux laser means this is ~0.6MHz per 20 count step). I went from -400cts to +800 cts and haven't found the beatnote yet. Meh.
Both PSL green and Xgreen beams are going to the Xgreen BBPD. Both beams are easily visible, so while I didn't actually measure the power, it should be sufficient. The arm is being re-locked in green for each step, but it's not locked in IR, but that doesn't matter for just finding the beatnote.
I've got the output of the BBPD directly connected to the 50 ohm input of the HP8591E spectrum analyzer, with the freq span from 10MHz to 120MHz. The BBPD is supposed to be good up to ~100MHz, so I should catch any beatnote that's there. I have to head out, so I guess I'll continue the search tomorrow.
One of Kiwamu's suggestions was that, since no one is using the Ygreen concurrent with my fiddling, I rotate the waveplate after the PSL doubling oven so that max power goes to the Xgreen path, thus giving myself a bigger signal. I'll try that tomorrow. Today, I didn't ever touch the waveplate.