Right now the fluctuation of the green beat-note seems mostly covered by unknown noise which is relatively white.
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.
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.
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.
The actual temperature of the Xend laser is 0.02 C higher than anticipated based on the formula in elog 3759. Both the PSL and the Xend laser are at their nominal diode currents (2.100 A for the PSL, 2.003 A for Xend), so the curves should be used as they are. The PSL temp (when the slow servo offset is ~0) is 31.71 C. Using curve 2 from elog3759, the Xend laser should be 37.78, which I found was +10 counts on the Xgreen slow servo offset.
Right now the Xend laser is at 37.80 C, and the beat is around 30 MHz. This is +80 counts on the Xgreen slow servo. +60 counts gave me ~80 MHz. When (a few minutes ago) the MC unlocked and relocked, it came back to a slightly different place, so the temp of the Xend laser had to go up a few 10's of counts to get the same beat freq. Right now the PSL slow servo offset is 0.076 V.
The HP8591E is set with ResBW=100kHz, Ref Level= -39dBm (so I'm not attenuating my input signal!). The largest peak I see for the beatnote is -66dBm. The nose floor around the peak is -83dBm. Trace (trace button!) A is set to MaxHoldA, and Trace B is set to ClearWriteB, so B is giving me the actual current spectrum, while A is remembering the peak value measured, so it's easier to see if I went past the peak, and just didn't see it on the analyzer.
Also, I went back and realigned the beams earlier, to ensure that there was good overlap both near the BS which combines the PSLgreen and Xgreen beams, and at the PD. The overlap I had been looking at was okay, but not stellar. Now it's way better, which made the peak easier to see. Also, also, the waveplate after the doubling oven on the PSL table is still rotated so that I get max power on the Xgreen side of things, and not much at all on the Ygreen side. I'll need to rebalance the powers, probably after we make sure we are seeing the beatnote with the BeatBox.
Lay a cable from the BBPD to the BeatBox in 1X2, make the BeatBox do its thing.
Use the dichroic locking to do a sweep of the Xarm.
The Xgreen PD now has a cable going over to the beatbox. Once beatbox characterization is done I can re-find the beat, and we can do some stuff with the beatbox.
Over-sized local laser emergency switch was held by large C clamp at the south end. This was replaced by a smaller one and it is mounted with magnets.
The Innolight laser was turned off, while the interlock was wired.
Dichroic mirror quotes are in the wiki.
ATF is pricy.
We got a good price from Laseroptik, but the wedges are 5 arcminutes. The fused silica grade is 0F, meaning the homogeneity is 5 ppm instead of 1ppm. I requested an other large wedge quote on the substrates.We may have to get substrates from somebody else and ship it to Germany
MLT quote is outrageously high
REO is not interested in this low volume job.
The Laseroptik quote is here.The 2 degrees wedge cost is $40 on each optics! See wiki
While walking past the PSL, I noticed that the PSL's doubling oven's heater was still disabled from the power outage. As with the ETMY heater, I hit the Enable button, and it started warming up (according to the front panel at least).
We aligned the Y arm for IR (C1:LSC-TRY_OUT is now ~ 0.9), and aligned the green beam from the ETMY table. The Y arm green is now resonating in TEM00 mode, but we need some monitors (green trans or green refl) to maximize the coupling.
We noticed that the MC beam spot are oscillating at ~ 1 Hz, mostly in YAW. This wasn't observable before the PMC realignment (elog #6708). We should find out why and fix it.
We aligned the PSL green optics so that the PSL green beam and Y arm green beam interfere. 2 beams are now hitting the Y arm beat PD. The DC level from the beat PD is about 13 mV.
We didn't try to see the beat signal for today, because the temperature of the doubling crystal seemed funny. We need to look into it tommorow.
Currently, the temperature control is enabled and the set point is 36.9 deg C, but the temperature is stuck at 33.0 deg C.
I did the cabling for monitoring DC transmission of the green beam from the end table.
The two PDs are called GREEN TRX and GREEN TRY, and the channel names are C1:GCV-GREEN_TRX and C1:GCV-GREEN_TRY.
The two signal from the PDs go to the ADC_0 card of the c1ioo computer.
Now, C1:GCV-GREEN_TRX/Y are actually connected to the respective PDs, but green beams are not hitting on the PD. We need two pickoff mirrors.
I fixed the temperature control of the oven for the PSL doubling crystal.
The PID settings were not good, and also, TC200 was beging DETUNED. So, I activated TUNE function and adjusted PID settings.
I'm not sure what the DETUNE function is for. The manual can be found here;
Current settings for Thorlabs TC200 are (Red ones are what I changed from the previous setting);
Because I keep taking a long time to search for these, since I can't remember the keywords in the different entries, here are the links:
elog 3759 : Green X end aux laser temperature setting vs. PSL laser temperature setting
elog 4439 : Green Y end aux laser temperature setting vs. PSL laser temperature setting
More words: beat note, doubling, second harmonic.
T_Xend = 8.31 + 0.9293*T_PSL
T_Yend = 6.9825 + 0.87326*T_PSL
Also, C1:GCY-SLOW_SERVO2_OFFSET was 29725 (twenty nine thousand seven hundred twenty five) cts when we sat down to start today.
C1:GCX-SLOW_SERVO2_OFFSET was 80 (eighty) cts when we sat down to start today. Why the offsets are so different, I don't know. But I was able to find the X green beatnote with this small number offset, so it is approximately correct.
We tried to find the Ygreen beat note, with no success yet. We calculate from Bryan's formula that the Yend laser should be ~34.68C. But Katrin has an elog saying that she was looking around 19C. I don't know why the discrepancy, but maybe this is part of our problem? Kiwamu elog-responded that the epics output had to be high (~9V) when the temp was 19C. So maybe we need a smaller offset setting in the slow servo with the 34C temperature?
We set the "T+" on the Ygreen laser controller to 34.68C using the dial, and then tried a few large steps with the offset in the Ygreen slow servo. The idea was to see if we could swing past the beat, so we would know vaguely where it was. But we never saw a resonance on the spectrum analyzer, even with a "hold max" trace.
We confirmed that there is signal going to the SLOW input of the laser controller's front panel. Yuta watched a voltmeter while I changed the epics value, and we successfully changed the signal. However, after plugging the SLOW cable back in, we noticed that no matter what we set the epics value to, we don't see any temperature change reported on the front panel display. There is something in the manual( according to Katrin) that the "LT" display is not accurate when a cable is plugged in. But none of the display values changed. I think there is a measured temp output on the back that Bryan mentioned that we could use to see if something is really changing inside.
Anyhow, no beatnote found yet tonight. We confirmed before starting that the alignment onto the beat PD was good, so that's not the problem.
I tried to find Y arm green beat in order to do the mode scan.
I found a beat peak(see attached picture), but the amplitude seems too small.
It is may be because the alignment/mode matching of the green beams at the PSL table is so bad. Or, the peak I found might be a beat from junk light.
What I did:
1. Aligned Y arm to the IR beam from MC.
2. Re-aligned Y end green beam to the Y arm using steering mirrors on the Y end table.
3. Re-aligned PSL green optics.
# C1:GCV-GREEN_TRY is temporary connected to the DC output of the Y green beat PD.
4. Temperature of the PSL laser was 31.48 deg C, so I set "T+" of the Y end laser to 34.47 deg C, according to Bryan's formula (elog #4439);
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
5. Scanned Y end laser temperature by C1:GCY-SLOW_SERVO2_OFFSET. Starting value was 29725 and I scanned from 27515 to 31805, by 10 or 100. Laser frequency changes ~ 6 MHz / 10 counts, so it means that I scanned ~ 2.5 GHz. During the scan, I toggled C1:AUX-GREEN_Y_Shutter to make sure the green beam resonates in TEM00 mode.
# I made a revolutionary python script for toggling channels(/opt/rtcds/caltech/c1/scripts/general/toggler.py). I made it executable.
6. Found a tiny beat note when C1:GCY-SLOW_SERVO2_OFFSET = 29815. I confirmed it is a beat signal by blocking each PSL and Y arm green beam into the beat PD. I left C1:GCY-SLOW_SERVO2_OFFSET = 29815.
7. I found that Bryan's formula;
Y_arm_Temp_meas = 0.95152*T_PSL + 3.8672
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
Y_arm_Temp_set = 0.95152*T_PSL + 3.8672
Y_arm_Temp_meas = 0.87326*T_PSL + 6.9825
according to his graph(elog #4439). So, I set "T+" of the Y end laser to 33.82 deg C.
8. This time, I scanned PSL laser temperature by C1:PSL-FSS_SLOWDC. I found a tiny beat note when C1:PSL-FSS_SLOWDC = 1.0995. C1:PSL-FSS_SLOWDC has 10 V range, so I scanned ~ 10 GHz, assuming the laser frequency changes 1 GHz/K and the temperature changes 1 K/V.
9. Re-aligned PSL green optics so that the beam hits optics at their center, and checked that the poralization of the two green beams are the same.
10. Checked that amplifier ZFL-100LN+ on the beat PD is working correctly. The power was supplied correctly (+15 V) and measured gain was ~ 25 dBm.
11. Exchanged BNC cable which connects the beat PD to the spectrum analyzer. Previous one we used was too long and it had -15 dB loss(measured). I exchanged to shorter one which has -2 dB loss.
Beat note amplitude estimation:
The amplitude of the beat note observed in the spectrum analyzer was ~ -54 dBm. According to the estimation below, it seems too small.
The measured power of the two green beams are
P_Y = 4 uW
P_PSL = 90 uW
So, the power of the beat signal should be
P_beat ~ 2 sqrt(P_Y * P_PSL) = 37 uW
Responsivity and transimpedance of the beat PD (Broadband PD, LIGO-T0900582) are 0.3 A/W and 2 kOhm. So, the power of the electrical signal is
W = (P_beat * 0.3 A/W * 2 kOhm / sqrt(2))^2 / 50 Ohm = 5 uW
5 uW is -23 dBm. We have +25 dB amplifier after the PD and the loss of the BNC cable is -2 dB. So, if the two beams interfere perfectly, the peak height of the beat signal should be ~ 0 dBm. The measured value -54 dBm seems too small. According to elog #5860, measured value by Kiwamu and Katrin was -36 dBm.
PSL laser temperature: 31.48 deg C (PSL HEPA 100%)
Y end laser "T+": 33.821 deg C
Y end laser "ADJ": 0
C1:GCY-SLOW_SERVO2_OFFSET = 29815 (was 29725)
We're trying to do a yarm measurement....before I forget, I want to write this down...
I changed the gain of each of the top 2 SR560's down, by a factor of 2. This made the overload lights quit coming on.