We took a look at the Xend green, and we weren't able to make it lock. We improved the alignment a little bit, and when we looked at the error signal, it looked nice and PDH-y, but for whatever reason, the cavity won't catch lock.
While aligning the green to the arm, Jamie noticed that the reflection from the intracavity power (not the prompt reflection) was not overlapping with the input beam or prompt reflection. This means that the cavity axis and the input green beam were not co-linear. I adjusted the BS and ITMX to get the IR transmitted beam (which had been near clipping on the top edge of the first (2 inch) optic it sees out of the vacuum) back near the input green beam spot on the combining beam splitter. Then we continued tweaking the green alignment until we saw nice TEM00 flashes in the cavity. The SNR of the error signal increased significantly after this work, since the cavity buildup was much higher. But alas, still no lock.
I tweaked the alignment of ITMX and ETMX a teeny bit to get the TEM00 flashes back (the work in the previous elog was pre-dinner, so it had been a few hours), then took a screenshot of the error signal and refl dc power on the photodiode for the green xend setup.
The error signal is certainly noisy, although I think when Jamie and I were looking at it earlier this evening, the SNR was a little better.
I need to look at the modulation depth, to see if it's correct, ... maybe lock the Xarm on IR and scan the green laser PZT to check the sideband heights.
I should also check to make sure that the PD is powered, and the gain is high enough (currently the PD gain is set to 20dB). Earlier today, when I set the gain to 30dB, Jamie said that it was saturating, so I put it back down to the 20dB where we found it.
Still no lock of the green though :(
Edit: realized I was bad and didn't label the traces on the plot: green is refl dc power, blue is demodulated error signal.
The Y end aux laser light leaking after the doubling crystal has been coupled into the 70m long PM fiber.
Input power = 250mW; Output after 70m = 20mW
The poor efficiency is partially due to the ellipticity of the beam itself and partially due to the compromise I had to make using a single lens to couple the light into the fiber (given the limitations in space). But 20mW should be more than sufficient for a beat note setup.
Light propagates as follows after the doubling crystal:
Doubler ---> Harmonic Separator (45deg) ---> Lens (f=12.5cm) --> Steering mirror (Y1) --> Fiber collimator ( Thor labs CFC-2X-C) --> FIber end
I will update photos of the setup shortly.
I have left the 70m fiber in its spool sitting at the Y end and blocked the light before the last Y1 steering mirror in the above setup. So it should be safe.
Through the course of the work, I disabled the ETMY oplev and enabled it before closing the enclosure. I also reduced the AUX laser power and brought it back up after the work.
I did a check to see if the arms are locking in both IR and green and they did.
10% seems like a pretty bad coupling efficiency, even for a single lens. I know that the NPRO itself isn't so elliptical as that. Where is the other 230 mW going? random scattering?
Given that this is such an invasive process and, since its so painful to lose a whole night of locking due to end table business, I suggest that you always measure the out-of-loop ALS noise at the end of the end table work. Just checking that the green laser is locked to the arm is not sufficient to prove that the end table work won't prevent us from locking the interferometer.
We should insist on this anytime someone works on the optics or electronics at EX or EY. Don't have enough time to do an out-of-loop ALS spectrum? Then don't work at the end tables at all that day. We've got PZT alignment and mode matching work to do, as well as the rebuild of the EX table enclosure, so this is a good discipline to pick up now.
Taking into account the ellipticity of the input beam, the available lenses and the space restrictions (lens can be placed only between z= 8 to 28cm), I calculated the best possible coupling efficiency (using 'a la mode').
The maximum possible mode overlap that can be obtained is 58.6% (matlab code and plot attached)
modematching = 0.58632
Optimized Path Component List:
label z (m) type parameters
----- ----- ---- ----------
L1 0.0923 lens focalLength: 0.0750
I used the above configuration and was able to obtain ~52% coupling.
Input power = 250mW
Output power with absorptive ND 1.0 = 13 mW
I used the absorptive ND filter before the lens to keep the coupled output power within the range of fiber power meter and also avoid scattering of enormous amount of uncoupled light all over the table.
I have attached the screenshot of the out of loop ALS noise before opening the table (BLUE) and after closing down (MAGENTA). The beat note frequency and amplitude before and after were (14.4MHz/-9.3dBm) and (20.9MHz/-10 dBm).
Since I obtained a poor coupling efficiency from the earlier setup, I went back to calculate the coupling efficiency of the current setup.
For the current setup, I took the average of the x and y waist of the input beam and calculated the distance at which the input beam diameter would match the (fiber +collimator) beam diameter.
Average waist = 40.2um @-3.3mm from face of doubling crystal
(Fiber PM980 + Collimator f=2.0mm) beam waist = 205um
Distance(z) at which the input beam waist is 205um = 11.9cm
The closest available lens was f = 12.5cm. So I used it to couple the input beam by placing it at z ~12.5cm on a micrometer stage.
Since this gave only 10% coupling, I went back to calculate (using 'a la mode') the best possible coupling that can be obtained taking into consideration the ellipticity of the beam.
The maximum obtainable coupling (mode overlap) is 14.5% which is still poor.
Optimized Path Component List:
As for the X Arm, this the transfer function I measured for the Y arm cavity.
This time I'm using a different photodiode than the PDA255 on the Y end table.
The diode I'm using is the PDA520 from where TRY comes from.
I'm going to repeat the measurement with PDA255.
Measurement repeated with the PDA255 PD at the end but not big changes
I was getting the Y Arm ready for Eric Q's loss measurements and so I looked at the noise and loop shape. The loop shape was strange:
You can see that the gain margin is too low at high frequencies. That's why we have >15 dB of gain peaking. Way too much! I think this is from Masayuki and Manasa increasing the phase margin at some point in the past. I lowered the gain by 3 dB from 0.1 to 0.07 and now the awful gain peaking is less. But what about the low frequency gain? Is there enough?
I calibrated the OUT channel with 14 nm/count (1/f^2) with a Q = 10 pole pair at 1 Hz. The error signal is done to cross over at 180 Hz. It looks like the resonant gain at 25 Hz is a little too much and the in-loop RMS is 10 pm. Jenne says the linewidth is ~1 nm, so this seems sort of OK. Except that the LIGO-I DARM RMS had to be <0.1 pm for ~the same linewidth. Do we need to do better before trying to bring the arms into resonance?
I've remove FM1 and FM8. I put the RollRG of FM8 into the BounceRG and renamed it BounceRoll. Also changed the Y-arm restore so that RollRG and the 5,5:0,0 are no longer triggered automatically since the double integrator was overkill and we already have a 1:0 in FM2. I also lowered the peak gain for the roll mode RG from 30 to 10 dB because it was also overkill. We've gained a few more degrees at the UGF.
Yesterday and today I was in the lab doing many cavity scan.
First I did many measurement with the cavity aligned in order to get the position of the 00 modes, then I misaligned the beam in many different ways to enhance the higher order modes.
In particular, I first misaligned the mode cleaner to make the beam clipping into the Faraday. To do this, I set to 0 the WFS gain, but I left the autolocker still enabled. In this way, the autolocker couldn't bring the mirrors back to the aligned position.
Then I misaligned also the TT2 to get even more HOMs.
Eventually, Rana came and we misaligned TT1 to clip the beam, and using TT2 we aligned back the beam to the arm.
To increase the SNR, we changed the gain of the TRY PD, setting it to 20dB (which corresponds to a factor 100 in digital scale)
I attached one scan that I did with Rana on Sunday night. I could not upload a better resolution image because the file size was too big, but here's the path to find all of the scans:
There are many folders, one per each day I measured. In each folder there are measurements relative to aligned cavity, Pitch and Yaw misalignment.
The PDA520 used for TRY was set to 0 dB analog gain. This corresponds to ~500 counts out of 32768. The change to 20 dB actually increases the gain by 100. This makes the single arm lock saturate at ~25000 counts (obviously in analog before the ADC). The right setting for our usual running is probably 10 dB.
For the IMC WFS, we had disabled the turn on in the autolocker to use the IMC to steer the beam in the FI, but that was a flop (not enough range, not enough lever arm). In the end, I think we didn't get any clipping.
I lined up the Y Arm for locking and then centered the oplevs for ETMY and ITMY.
* The ITMY OL has still got the old style laser. Steve, pleaes swap this one for a HeNe. Also the optical layout seems strange: there are two copies of the laser beam going into the chamber (??). Also, the QPD transimpedance needs to be increase by a factor of ~10. We're only getting ~500 counts per quadrant. Its worth it for someone to re-examine the whole ITMY OL beam layout.
* The ETMY OL beam was coming out but clipping on the mount for the ETMY OL HeNe. This indicates a failure on our part to do the ETMY closeout alignment properly. In fact, I get the feeling from looking around that we overlooked aligning the OL and IPPOS/ANG beams this time. If we're unlucky this could cause us to vent again. I undid part of the laser mount and changed the height on the receiving mirror to get the beam back onto the QPD.
I noticed that there is significant green light now getting into some of the IR PDs; beacuse of this there are weird offsets in the TRY QPD and perhaps elsewhere. We had better purchase some filters to tape over the front of the sensitive IR sensors to prevent the couplling from the green laser.
* There is a beam on IPPOS, but its too big for the detector (this has always been the case). We need to put a 2" lens with a weak focusing power on this path so as to halve the beam size on the detector. Right now its clipping and misleading. There is also a 0.9V offset on the SUM signal. I'm not sure if this readout is working at all.
* I couldn't find any beam on IPANG at all. Not sure what's changed since Kiwamu saw it.
* ITMY OL: Also the optical layout seems strange: there are two copies of the laser beam going into the chamber (??).
* The ETMY OL beam was coming out but clipping on the mount for the ETMY OL HeNe. This indicates a failure on our part to do the ETMY closeout alignment properly.
The 2nd beam from this laser is for the SRM's OpLev, so that shouldn't be changed.
For better or worse, we didn't do anything to the ETM OpLevs, because they don't have any in-vac steering optics. We did however go through and check on all the corner OpLevs.
I went to the Y end. The AUX laser was on Standby. I pushed the Standby button. The laser turned on and there was some green light. However, the controller displayed the message "CABLE?" which according to the manual means that the laser head is powered but there is no control over the laser (e.g. the control cable is disconnected). I turned off the controller and disconnected both the power and control cables. I put them back and turned the controller back on.
I pushed the Standby button, the laser turned on and this time the controller displayed the laserhead's state. I was able to change the current/temperature. The problem seems to be resolved.
The first Faraday isolater rejected beam path from the NPRO is fixed.
I borrowed the GTRY BBPD for the REFL165 trial before.
Now the PD is back on the PSL table.
The PD is intentionally misaligned so that anyone can find it is not aligned.
I did a calibration measurement for the Y part of the BeatBox using a Marconi. This is in order to get a more accurate calibration for the arm cavity scan measurement.
The calibration factor I found is:
C1:ALS-BEATX_FINE_PHASE_OUT 50.801 +/- 0.009 deg/MHz
During my cavity scan measurement, I had recorded the beat frequency and amplitude from the Spectrum Analyzer at each zero crossing.
I connected the Marconi to the RF in of the Y part of the BeatBox, and I set the Marconi carrier frequency at one of this zero-crossing frequency that I had recorded, while I set the amplitude in way to have on the spectrum analyzer the same beat amplitude that I read during the measurements or, equivalently, in order to have C1:ALS_BEATY_FINE_Q of the order of 1200 (which is the same value I had during my measurements).
I started with
Then I monitored the C1:ALS_BEATY_FINE_I on the oscilloscope and I adjusted the carrier frequency so that I had zero signal on the oscilloscope. Eventually the frequency corresponding to the zero crossing was 79.989 MHz.
I resetted the phase (clear history in the BEATY_FINE_PHASE panel) and I started changing the frequency by steps of 0.2 MHz, and I spanned about 70 MHz (from 32 to 102 MHz).
The calibration coefficient I found is not so different from the one that Yuta measured (elog 8199).
Here are the fit parameters:
y = a + bx
a = -4239.7 +/- 0.6 deg
b = 50.801 +/- 0.009 deg/MHz
I tried the "Yarm + PRMI" configuration to see what happens.
The Y arm was locked at a resonance and held with the ALS technique.
On the other hand, the X arm was freely swinging.
I briefly tried severl demod signals to calm down the central part, but didn't succeed.
Now I feel I really want to have the X arm locked with the ALS technique too.
Give me the beat-box !
The attached screen shot shows the transmitted light of both arms as a function of time.
TRY is always above 1, since it was kept at a resonance.
Sometimes TRY went to 50 or so.
I calculated how the DC signals should look like in the Y arm PRMI configuration.
The expected signals are overlaid in the same plot as that of shown in #6313.
You can see there are disagreements between the observed and expected signals in the plot below at around the time when the arm is brought to the resonance.
The figure below shows the time series of the Y arm + PRMI trail.
I tried the Yarm + PRMI configuration again.
The PRMI part was locked, but it didn't stay locked during the Y arm was brought to the resonance point.
I will post the time series data later.
(locking of the PRMI part)
Tonight I was able lock the PRMI when the arm was off from the resonance by 10 nm (#6306).
This time I used REFL11Q to lock the MICH instead of the usual AS55Q because the MICH didn't stay locked with AS55Q for some reason.
The PRCL was held by REFL33I as usual.
Also I disabled the power normalization for the error signals because it could do something bad during the Y arm is borough to the resonance.
In order to reduce the number of the glitches, PRM was slightly misaligned because I knew that the lower finesse gives fewer glitches.
(Top plot )
Normalized TRY (intracavity power). It is normalized such that it shows 1 when the arm is locked with the recycling mirrors misaligned.
ASDC and REFLDC in arbitrary unit.
The amount of the arm length detuning observed at the fine frequency discriminator.
At t = 20 sec, the amount of detuning was adjusted so that the cavity power goes to the maximum. At this point the PRM was misaligned.
At t = 30 sec, the cavity length started being slowly detuned to 10 nm. As it is being detuned the intracavity power goes down to almost zero.
At t = 45 sec, the alignment of PRM was restored. Because of that, the REFLDC and ASDC diodes started receiving a large amount of light.
At t = 85 sec, the PRCL and MICH were locked. The REFLDC signal became a high value as the carrier light is mostly reflected. The ASDC goes to a low value as the MICH is kept in the dark condition.
At t = 100 sec, the length started being slowly back to the resonance while the PRMI lock was maintained.
At t = 150 sec, the lock of the PRCL and MICH were destroyed. With the arm fully resonance, I wasn't able to recover the PRMI lock with the same demod signals.
Isn't the point that the 11 and 55 MHz signals have the carrier effect, but the 3f signals are better?
Last night I tried the "Y arm + central part" locking again. Three different configuration were investigated :
In all the configurations I displaced the Y arm by 20 nm from the resonance.
As for the DRMI and PRMI configurations I wasn't able to acquire the locks.
As for the MICH configuration, the MICH could be locked with AS55. But after bringing the Y arm to the resonance point the lock of MICH was destroyed.
With the newly amplified POY signal, locking the mode cleaner to the Y arm at ~30kHz bandwidth was quite straightforward. The offset jumps still happen, and are visible in POY11_I_ERR, but are never big enough to cause much power degradation in TRY (except when turning on CM board boosts, but its still not enough to lose lock). The script which accomplishes this is at scripts/YARM, and is in the svn. The MC2/AO crossover is at about 150Hz with 40deg margin.
For now, I'm using IN1 of the CM board, because I haven't removed the op27s that I put into IN2's gain stages. I believe the slew rate limitations of these prevent them from working completely during the offset jumps. I'll put AD829s back soon.
At first, I had ITMX misalgined to use AS55 as an out of loop sensor, then I aligned and locked the X arm on POX to compare.
Weirdly enough, locking the mode cleaner to the Y arm with 30kHz UGF and two boosts on make no real visible difference in the X arm control signal. This is strange, as the whole point of this affair was to remove the presumably large influence of frequency noise on the X arm signals... Maybe this is injecting too much POY sensor noise?
Below 100 Hz, I suppose this means that the X arm is now limited by the quadrature sum of the X and Y arm seismic noise.
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)
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.
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
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 in this week is : measurement of the current best ALS noise budget.
One of my goals this week is to get people to make plots with physical units:
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.
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.
Handing off the servo from ALS to LSC for one arm is quite easy because servo filters are pretty much same for ALS and LSC. I demonstrated it Y arm during MI is locked.
We need DARM/CARM-kind of handing off in the near future.
What I did:
1. Brought both arms to IR resonance.
2. Brought X arm to off resonance.
3. Locked MI in bright fringe(why can't I lock in dark fringe, when one arm is on resonance?) using AS55_Q and BS.
4. Ran /opt/rtcds/caltech/c1/scripts/ALS/handofftoLSC.py Yarm to handoff. It decreases ALS gain and increases LSC gain in 30 sec ramp time. It also turns on some filters for LSC. Make sure you turn off filter triggers for LSC.
Below is the plot of what I did. You can see LSC feedback signal gradually increasing and TRY getting more stable.
I was dissapointed with ALS not having any DQ channels for feedback signal. I will make them DQ channels tomorrow.
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.
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.
While trying to lock the arms using ALS we found that the locks were not very stable and the in-loop noise was higher than seen before.
I looked into things and checked the out-of loop noise for ALS and found that the Y arm ALS noise (rms) was higher than the X arm.
To troubleshoot, I measured the OLTF of the phase tracking loop. While X arm was healthy, things weren't looking good for the Y arm. Sadly, the Y phase tracking loop gain was set too high with a phase margin of -2 degrees. We brought down the gain from 300 to 150 and set the phase margin close to ~55 degrees.
X arm Phase tracker loop:
UGF = 1.8 K Hz
Phase margin = 50 degrees
Y arm Phase tracker loop:
UGF = 1.6 KHz
Phase margin = 55 degrees
The servos of C1ASS for the Y arm and the beam axis alignments were fixed.
Now we can correctly run the Y arm ASS from the C1IFO_CONFIGURE window as usual.
The sign of some control gains had been flipped for some reasons, so I changed them to the correct signs.
Next : Health-check for the X arm ASS, the loss measurements.
I tried using C1ASS to align the incident beam and suspensions on the Y arm, but it didn't work.
The X arm ASS was also fixed. So both X and Y arm ASS are now back to normal.
Now we can align the arms any time from the buttons on the C1IFO_CONFIGURE window.
The reason why the servo didn't work was that the sign of some control gains had been flipped.
This was exactly the same situation as that in the Y arm ASS (#5067).
[Attachment #1]: ITMY HR face after cleaning. I determined this to be sufficiently clean and re-installed the optic.
[Attachment #2]: ETMY HR face after cleaning. This is what the HR face looks like after 3 rounds of First-Contact application. After the first round, we noticed some arc-shaped lines near the center of the optic's clear aperture. We were worried this was a scratch, but we now believe it to be First-Contact residue, because we were able to remove it after drag wiping with acetone and isopropanol. However, we mistrust the quality of the solvents used - they are not any special dehydrated kind, and we are looking into acquiring some dehydrated solvents for future cleaning efforts.
[Attachment #3]: Top view of ETMY cage meant to show increased clearance between the IFO axis and the elliptical reflector.
Many more photos (including table leveling checks) on the google-photos page for this vent. The estimated time between F.C. peeling and pumpdown is ~24 hours for ITMY and ~15 hours for ETMY, but for the former, the heavy doors were put on ~1 hour after the peeling.
The first task is to fix the damping of ETMY.
The Y arm locks stably for IR PDH now.
The reason for ETMY getting kicked during lock acquisition was finally found to be related to the limiter value set in the Y arm servo. We reduced the limiter value unintentionally and found that the lock acquisition stayed smooth. The limiter value was stepped in 1000s from 7000 and eventually found that the ETMY suspension was kicked when we try to acquire lock with the limiter value was set at 11000.
The limiter for X arm at 11000 is not causing any trouble at the moment.
In the process, we did a bunch of things through the evening to troubleshoot IR locking of the Y arm.
Earlier today running the IFO configure script did not restore the arms to lock and both the ETMs needed to be aligned to lock the arms. The arms stayed locked for 15 minutes and the Y arm lost lock eventually leaving the ETMY in a misaligned state.
The state of the Y arm was similar to what Jenne has explained in ELOG where the ETMY was kicked during lock acquisition and would move to a misaligned state.
To trouble shoot, there were several things that were done. A few of them might not have any direct correlation to the locking issue but could just be a coincidence.
1. The trigger time for the filters in the arm filter modules were set such that they switch on after the SUS violin filters. Arm FM trigger time = 3 s (previously set at 0.1s) and SUS violin trigger time = 1s. This reduced the number of lock loss events.
2. There was some drop in transmission when the bounce filter of Y arm (FM6) turned ON. This was fixed by changing its ramp time (initially set at 1s). The filter has been modified to turn on immediately upon arm lock acquisition before the other triggered filters in the filter module turn on.
3. The QPD and SUS signal cables running to the rack were checked to be intact. Koji found some of them to be loose. But this had no evident correlation with the arm locking problem.
4.The oplev and PD alignment was checked at the Y end. The high gain trans PD for Y arm was checked for good alignment by looking at TRY. It was found that the EXIT light at the Y end is injecting some noise to the transmission PD.
5. The ETMY was given offsets in PIT, YAW and POS and the OSEM sensor values were checked to see if the suspension is behaving well. It was behaving well.
The qpd was removed from the east end table and threaded adaptor ring epoxied on it's shell.
This tube will cut down the amount of emergency exit light getting into the qpd.
[Jamie / Valera / Kiwamu]
The incident beam pointing was improved a lot by using C1ASS realtime code.
Some more details will be posted later. The below is the list of the highlights today.
- The Y arm cavity was aligned to have good beam centering on the mirrors.
- The input PZTs were also aligned to the aligned Y arm by hand.
- Automation of the Y arm alignment using C1ASS_LOCKIN got partially functional with two loops closed. C1ASS correctly servos the centering on ETMY
- The amount of the off-centering on ITMY and ETMY look roughly within 1 mm.
- As a result the intracavity power got bigger by a factor of about 3.5
I started doing a scan of the Y arm cavity with IR with ALS enabled.
ALS servo tuning:
The servo tuning procedure is basically the same as described in elog 8831.
This time I had a stronger beat note(-14 dBm instead of -24 dBm of the last measurement) thanks to a better alignment.
Plot1 shows the Power spectrum of the BEATY_PHASE_OUT. The RMS is smaller by a factor of 2 (400Hz), corresponding to a residual motion of about 25 pm.
Offset setting avity scan
In order to give an offset linearly growing in time, I used the ezcastep script instead of giving the offset in OFFSETTER2. If the ramp time is long enough, it is not necessary to enable the 30mHz filter.
To span 2 FSR, I started from an offset of 450 and I gave a maximum value of 1600 with a delay of 0.2s between two consecutive steps.
I did a first scan with the cavity well aligned, basically to know the position of the 00 peaks and choose the best offset range (Plot2)
Then I misaligned the TT2, first in PITCH and yhen in YAW, in order to enhance the HOMs. (Plot3 and Plot4)
More investigation and measurements needed.
Yesterday I did a cavity scan with IR while holding the Yarm with green.
The gain of the loop is set such that BEATY_FINE_Q_ERR x GAIN = 120k. This is a kind of "empirical low" in order to have the UGF around 1kHz.
Start with FM5 [1000:1] enabled, determine the sign of the gain increasing it in small steps and making sure that the mirror doesn't get a kick. Then gradually raise it while looking at the BEATY_PHASE_OUT power spectrum.
Enable FM7 [RG16.5], FM6 [RG3.2], FM3 [1:5], FM2[0:1], FM10 [40:7].
Plot 1 shows the power spectrum of BEATY_PHASE_OUT (calibrated in Hz).
Offset setting and cavity scan
The C1ALS_OFFSETTER2 was used to set an offset for ALS scan.
Many scans have been done to find the optimal offset conditions, I only attached one (Plot 2).
I also misaligned the END mirror in pitch to enhance the HOMs peaks, but it turned out that it was not enough, because I didn't see a very big difference between the "aligned" and the "slightly misaligned" measurements (Plot 3).
Increase the cavity misalignment both in pitch and in yaw and repeat the measurement.
[Annalisa, Terra, Koji, Gautam]
Summary: We find a configuration for arm scans which significantly reduces phase noise. We run several arm scans and we were able to resolve several HOM peaks; analysis to come.
As first, we made a measurement with the already established setup and, as Jon already pointed out, we found lots of phase noise. We hypothesized that it could either come from the PLL or from the motion of the optics between the AUX injection point (AS port) and the Y arm.
In this configuration, we were able to do arm scans where the phase variation at each peak was pretty clear and well defined. We took several 10MHz scan, we also zoomed around some specific HOM peak, and we were able to resolve some frequency split.
We add some pictures of the setup and of the scan.
The data are saved in users/OLD/annalisa/Yscans. More analysis and plots will follow tomorrow.
The attached photo shows the two optics with FC applied.
My original plan was to attempt to close up tomorrow. However, we are still struggling with Satellite box issues. So rather than rush it, we will attempt to recover the Y arm cavity alignment on Monday, satellite box permitting. The main motivation is to reduce the deadtime between peeling off the F.C and starting the pumpdown. We will start working on recovering the cavity alignment once the Sat box issues are solved.
In an earlier elog, I had claimed that the suspected clipping of the cavity axis in the Y arm was not solved even after shifting the heater. I now think that it is extremely unlikely that there is still clipping due to the heater. Nevertheless, the ASS system is not working well. Some notes:
We have to systematically re-commission the ASS system to get to the bottom of this.
Manasa, can you please estimate what kind of mode matching we have on the PSL table between the arm greens and the PSL green? We *do not* want to touch any optics at this point. Just stick in a power meter to see how much power we're getting from each beam, and then think about the peak height we see, and what that might tell us about our mode overlap. If we determine it is total crap, we can think about measuring the beams that go either toward the camera, or the DC PDs, since neither of those paths require careful alignment, and they are already picked off from the main beatnote path. But first, what is our current efficiency? Yarm is first, then Xarm, since Yarm seems worse (peak height is larger for non-00 modes!)
Estimate loss along the Y arm beat path:
1. Measured the beam powers (before the beam combiner):
Y Arm green = 35 uW
Y PSL green = 90 uW
==> Pbeat ~ 2 * sqrt (35 uW * 90 uW) ~ 112 uW
2. Expected power of RF signal
Assuming the PD to have transimpedance ~ 2kV/A and responsivity ~ 0.3A/W,
the expected power of the RF signal = (Pbeat * Transimpledance* Responsivity)^2 / (2 * 50ohm) ~ 45uW = -13.5 dBm
3. Measured power of Y arm beat signal
Turned OFF the beat PDs and rerouted the RF cables such that the spectrum analyzer was reading the RF signal from the Y beat PD itself (without any amplifiers or the beat box itself in the path).
Turned ON the beat PDs and the Y arm beat signal power on the spectrum analyzer measured -58dBm
Even if we consider for losses along the length of the cables, we are still at a very bad state.
4. Bad mode matching??
I don't think mode matching is our main problem here.
Toggling the shutter several times, even with the non-00 modes, the maximum beat power we can see is -50dBm which is still very far from the actual expected value.
The Y arm green transmission had come down to 0.3 and the green steering mirrors on the Y end table required some minor alignment adjustments to bring back transmission to around 0.75 counts.
The beam axis of the Y green light has been aligned.
Now I can see TEM00 mode is flashing on the ETMY camera.
-- (What I will do tonight)
The next step is to refine some electronics in the PDH loops to get the green light locked to the Y arm cavity.
If the beam isn't locked, I guess the in-vac-work will be so difficult because of the low intensity of the green light.
According to a brief check on the circuits, a low pass filter after the demodulation mixer is in a sad situation.
It doesn't pass any signals and in fact it behaves more like an absorber.
On the other hand, the modulation system looks fine to me because I was able to see the 270 kHz sideband converted into AM due to the fringing.
(not yet) Alignment of the Y green beam (#5066)
I succeeded in locking the green light to the Y arm cavity, but it wasn't so robust. Something is unhealthy in the electronics.
I am leaving the Y green system as it is because I already can see a plenty of the green light flashing in the BS chamber.
So just a flashing of the green light is good enough for the in-vac-work.
The next step is to refine some electronics in the PDH loops to get the green light locked to the Y arm cavity.