Continuing with the previous alignment that we stoped on friday, we re set up my heavily cleaned iPhone on FaceTime. The Phone alowed us to see the laser on the ITMX and center it on that optic.
We did a few quick XARM oltf measurements. We excited C1:LSC-ETMX_EXC with a broadband white noise upto 4 kHz. The timestamps for the measurements are: 1318199043 (start) - 1318199427 (end).
We will process the measurement to compute the cavity pole and analog filter poles & zeros later.
Here is a demonstration of the methods leading to the single (X)arm calibration with its budget uncertainty. The steps towards this measurement are the following:
** Note: We ran the same procedure using dtt (diaggui) to validate our estimates at every point, as well as check our SNR in b and d before taking the ~3.5 hours of data.
We repeated the same procedure as before, but with 3 different lines at 55.511, 154.11, and 1071.11 Hz. We overlay the OLTF magnitudes and phases with our latest model (which we have updated with Koji's help) and include the rms uncertainties as errorbars in Attachment #1.
We also plot the noise ASDs of calibrated OLTF magnitudes at the line frequencies in Attachment #2. These curves are created by calculating power spectral density of timeseries of OLTF values at the line frequencies generated by demodulated XARM_IN and ETMX_LSC_OUT signals. We have overlayed the TRX noise spectrum here as an attempt to see if we can budget the noise measured in values of G to the fluctuation in optical gain due to changing power in the arms. We multiplied the the transmission ASD with the value of OLTF at those frequencies as the transfger function from normalized optical gain to the total transfer function value.
It is weird that the fluctuations in transmission power at 1 mHz always crosses the total noise in the OLTF value in all calibration lines. This could be an artificat of our data analysis though.
Even if the contribution of the fluctuating power is correct, there is remaining excess noise in the OLTF to be budgeted.
Here is the open loop gain of the XARM loop.
The reference is from the pre-upgrade era. We get the extra phase delay because we have two anti-aliasing filters. One is the hardware filter at about 7kHz for 16kHz sampling. This filter should have been replaced to the one for 64kHz sampling but it has not yet happened. The second one is the software anti-aliasing filter applied when down sampling from 64kHz to 16kHz. So we have double AA filters, which are the culprits for the extra phase delay.
We should either replace the hardware AA filter to the 64kHz one (preferred way), or change the software AA filter to a less aggressive one (easier temporary fix).
Used diaggui to get OLTF in preparation for optimal system identification / calibration. The excitation was injected at the control point of the XARM loop C1:LSC-XARM_EXC. Attachment 1 shows the TF (red scatter) taken from 35 Hz to 2.3 kHz with 201 points. The swept sine excitation had an envelope amplitude of 50 counts at 35 Hz, 0.2 counts at 100 Hz, and 0.2 at 200 Hz. In purple continous line, the model for the OLTF using all the digital control filters as well as a simple 1 degree of freedom plant (single pole at 0.99 Hz) is overlaid. Note the disagreement of the OLTF "model" at higher frequencies which we may be able to improve upon using vector fitting.
Attachment 2 shows the coherence (part of this initial measurement was to identify an appropriately large frequency range where the coherence is good before we script it).
this model doesn't seem to include the analog AA, analog AI, digital AA, digital AI, or data transfer delays in the system. I think if you include those you will get more accuracy at high frequencies. Probably Anchal has those included in his DARM loop model?
I've updated the c1LSC simulink model to add the so-called UGF servos in the XARM and YARM single arm loops as well. These were earlier present in DARM, CARM, MICH and PRCL loops only. The UGF servo themselves serves a larger purpose but we won't be using that. What we have access to now is to add an oscillator in the single arm and get realtime demodulated signal before and after the addition of the oscillator. This would allow us to get the open loop transfer function and its uncertaintiy at particular frequencies (set by the oscillator) and would allow us to create a noise budget on the calibration error of these transfer functions.
The new model has been committed locally in the 40m/RTCDSmodels git repo. I do not have rights to push to the remote in git.ligo. The model builds, installs and starts correctly.
To reduce burden on c1lsc, I've shifted the added UGF block to to c1oaf model. c1lsc had to be modified to allow addition of an oscillator in the XARm and YARM control loops and take out test points before and after the addition to c1oaf through shared memory IPC to do realtime demodulation in c1oaf model.
The new models built and installed successfully and I've been able to recover both single arm locks after restarting the computers.
We aligned the BS, ITMY, and ETMY PIT and YAW to get the flashing on X-arm whilst also keeping the flashing of Y-arm. From attachment 1, it is clear that POXDC photodiode is not receiveing any light, so our next task is to work on POX alignment.
Here are some plots from analyzing the C1:LSC-XARM calibration. The experiment is done with the XARM (POX) locked, a single line is injected at C1:LSC-XARM_EXC at f0 with some amplitude determined empirically using diaggui and awggui tools. For the analysis detailed in this post, f0 = 19 Hz, amp = 1 count, and gain = 300 (anything larger in amplitude would break the lock, and anything lower in frequency would not show up because of loop supression). Clearly, from Attachment #3 below, the calibration line can be detected with SNR > 1.
We read the test point right after the excitation C1:LSC-XARM_IN2 which, in a simplified loop will carry the excitation suppressed by 1 - OLTF, the open loop transfer function. The line is on for 5 minutes, and then we read for another 5 minutes but with the excitation off to have a reference. Both the calibration and reference signal time series are shown in Attachment #1 (decimated by 8). The corresponding ASDs are shown in Attachment #2. Then, we demodulate at 19 Hz and a 30 Hz, 4th-order butterworth LPF, and get an I and Q timeseries (shown in Attachment #3). Even though they look similar, the Q is centered about 0.2 counts, while the I is centered about 0.0. From this time series, we can of course show the noise ASDs in Attachment #3.
The ASD uncertainty bands in the last plot are statistical estimates and depend on the number of segments used in estimating the PSD. A thing to note is that the noise features surrounding the signal ASD around f0 are translated into the ASD in the demodulated signals, but now around dc. I guess from Attachment #3 there is no difference in the noise spectra around the calibration line with and without the excitation. This is what I would have expected from a linear system. If there was a systematic contribution, I would expect it to show at very low frequencies.
I would expect to see some lower frequency effects. i.e. we should look at the timeseries of the demod with the excitation on and off.
I would guess tat the exc on should show us the variations in the optical gain below 3 Hz, whereas the exc off would not show it.
Maybe you should do some low pass filtering on the time series you have to see the ~DC effects? Also, reconsider your AA filter design: how do you quantitatively choose the cutoff frequency and stopband depth?
[Paco, Anchal, Radhika]
We tried to debug why the XARM green laser isn't catching lock with the arm cavity. First I tried to improve alignment:
- Aligned the arm cavity axes by maximizing IR transmission.
- Adjusted M1 and M2 steering mirrors to align the X green beam into the arm. GTRX reached ~0.3.
- At the vertex table, I adjusted the lens in the GTRX path to focus the beam onto the DCPD. This increased GTRX to ~0.7.
- Visually I confirmed that TEM00 of the green laser was flashing in the arm cavity, fairly centered. But it was not catching lock.
We suspected the XARM AUX PZT might be damaged/unresponsive. Paco, Anchal, and I fed several frequency signals to the PZT and looked for a peak in the AUX-PSL beatnote spectra at the expected frequency. We confirmed that the X-arm AUX PZT is responsive up to 12 kHz (limited by ADC samping rate). We have no reason to suspect the PZT wouldn't be responsive at the PDH modulation frequency of 231 kHz.
- Investigate PDH servo box / error signal.
I tested the mixer by feeding it a 300 kHz signal sourced from a Moku:Go. I kept the LO input the same - 231.25 kHz from the signal generator. The mixer output was a ~70 kHz waveform as expected, so demodulation is not the issue in green locking.
Next I'll align the arm cavities with IR and check to see if the green REFL signal looks as expected. If not, we'll have to invesitage the REFL PD. If the signal looks fine, and we now know it's being properly demodulated, the issue must lie further downstream.
I took a transfer function measurement of the XEND PDH servo box, from servo input to piezo output [Attachment 1]. The servo gain knob was set to 10. The swept sine input was 50 mVpp, as to not saturate the servo components. I toggled the local boost on/off for these measurements. With the boost on, coherence was lost from ~100Hz-10kHz, and the saturation light indicators were flashing. I will retake this measurement shortly.
Atachment 2 is from a previous measurement of this PDH servo TF, found here. For this measurement, boost was off and the gain knob was set to 2.0. (If there is a more recent measurement than 2010, please point me to it.)
We retook transfer function measurements of the XEND PDH servo box, this time setting the gain knob to 3.5 to avoid saturation. Once again I toggled the boost on/off. Attachment 1 shows the resulting bode plots, which now resemble the previous measurements circa 2010. This measurement along with the previous one suggest that setting the gain knob too high might affect the loop shape in an unpredictable way. With this accounted for, it seems the PDH servo box is functioning as expected.
Paco suggested that alignment could still be the primary reason why the XEND green laser is not catching lock. With the xarm cavity aligned with IR, I adjusted the M1 and M2 steering mirrors for the green laser, looking at the REFL PD output in an oscilloscope. Paco joined and was able to achieve better mode matching by adjusting mirrors and rotating the half-wave plate. At this point, we could see TEM00 consistently flashing. Green transmission also reached a value of 3, from around 0.5 that I was able to achieve previously (this channel is not normalized).
We broke the loop to make sure the demodulated signal looked as expected, and indeed it resembled a PDH error signal. After reconnecting the loop (with the gain knob set to 3.5), Paco lowered the REFL PD gain by 3 stages and I was able to raise the gain knob to 8 without the servo saturating. I turned boost on and toggled the servo inversion until the laser started to hold lock for a few seconds. The piezo output signal looked reasonable at this point, without clipping on either end.
After some final adjustments to the steering mirrors and the half-wave plate, the green laser can hold lock for around 5 seconds. However it's unclear why the loop isn't more stable, and more updates are to come.
- Upon arrival, MC is locked, and we can see light in MON5 (PRM) (usually dark).
# XARM locking
- Read through "XARM POX" script (path='/cvs/cds/rtcds/caltech/c1/burt/c1configure/c1configureXarm')
- Before running the script, we noticed the PRM watchdog is down, so we manually repeat the procedure from last time, but see more swinging even though the watchdog is active.
- Run a reEnablePRMWatchdogs.py script (a copy of reEnableWatchdogs.py with optics=['PRM']), which had the same effect.
- We manually disable the watchdog to recover the state we first encountered, and wait for the beam in MON5 to come to rest.
- The question is; is it fine to lock Xarm with PRM watchdog down?
- To investigate this, we look at the effect of the offset on the unwatchdog-PRM.
- Manually change 'PRM_POS_OFFSET' to 200, and -800 (which is the value used in the script) with no effect on the PRM swinging.
- Moving on, run IFO > CONFIGURE > ! (X Arm) > RESTORE XARM (XARM POX), and ... success.
# MC-POX noise spectra
- With XARM locked, open diaggui and take spectra for C1:LSC-POX11_I_ERR_DQ, C1:LSC-POX11_Q_ERR_DQ, C1:IOO-MC_F_DQ
- Lost XARM lock while we were figuring out unit conversions...
- Assuming 2.631e-13 m/counts (6941) and using 37.79 m (arm length), 1064.1 nm wavelength, we get a calibration factor of 2.631e-13 * c / (2*L*lambda) ~ 0.9809 Hz/count
- (FAQ?, how to find/compute/measure the correct calibration factors?)
- Relock XARM, retake spectra. Attachment 1 has plots for POX11_I/Q_ERR_DQ spectrum (cts/rtHz, we couldn't find relevant calibration) and MC_F_DQ in (Hz/rtHz from referring to 15576, we couldn't get the units to show on y scale.)
# MC-POY noise spectra (attempt)
- Now, run IFO > CONFIGURE > ! (Y Arm) > RESTORE YARM (YARM POY), and XARM locks (why?)
- Could PRM watchdog being down be the cause?
- Try C1ASS > (YARM) ! More Scripts > ON, and looked at YARM PIT/YAW striptool.
- C1ASS > (YARM) ! Freeze Outputs, then OFF
- Go back to IFO > CONFIGURE > ! (Y Arm) > Align YARM (ASS ON: Unfreeze), try running this then Freeze, then OFF Zero Outputs.
- Try RESTORE YARM (POY) again, still not working.
- Try RESTORE YARM ALS, then try again after opening the shutter, but also fail to lock AUX.
- Is the PRM WD behind some evil misalignment? Will move forward with XARM bc it is happy.
# ARM locking
- Attempted the IFO > CONFIGURE > ! (X Arm) > RESTORE Xarm (XARM ALS) but green failed to lock and we lost XARM lock.
- Try to recover XARM lock... success. It's nice to have a (repeatable) checkpoint.
- Attempt YARM lock. Not successful. It just seems like the lock Triggers are not raised (misalignment?)
- From C1SUS_ETMY, try changing the bias "C1:SUS-ETMY_YAW_OFFSET" manually to reduce the OPLEV_YERROR. Changed from -47 to -57.
- Retry YARM lock script... no luck
- From C1SUS_PRM, try changing the bias "C1:SUS-PRM_PIT_OFFSET" manually to reduce OPLEV errors. Changed from 34 to 22 with no effect, then realized the coil outputs are disabled because the WD is down...
- So we do the following BIAS changes "C1:SUS-PRM_PIT_OFFSET" = 34 > 770 and "C1:SUS-PRM_YAW_OFFSET" = 134 > -6
- Enable all Coil Outputs, turn WD to Normal, turn OPLEVs ON, (this time the beam does not swing like crazy).
- Fine tune BIASes "C1:SUS-PRM_PIT_OFFSET" = 770 > 805 and "C1:SUS-PRM_YAW_OFFSET" = -6 > 65
- Saw YARM locking briefly, then unlocking, but we stopped once the OPLEV_ERRs no longer overloaded (from magnitudes > 50 to ~ 40).
- Retry YARM lock... no luck
- From C1SUS_ETMY, try changing the bias "C1:SUS-ETMY_PIT_OFFSET" from -1 to 6.
Stop for the day. Leave XARM locked, MC locked.
To avoid the strange kicking of ETMX, I locked XARM with ITMX actuated instead of ETMX so that I changed elements of C1LSC_OUTPUT_MTRX; before: XARM=ETMX, after: XARM=ITMX.
And I change C1:LSC-XARM_GAIN from 0.007 to 0.022.
With this setup, I ran dither but then error signals of dithering oscillated as shown in the figure below.
Then I found that if C1:ASS-XARM_ETM_PIT_L_DEMOD_SIG_GAIN / C1:ASS-XARM_ETM_YAW_L_DEMOD_SIG_GAIN in C1ASS_LOCKINS_XARM.adl are changed as 0.200 -> 0.100 and 0.200 -> 0.100, respectively, the dithering works well.
But the burt file that is loaded when you let dithering "ON" is not changed, because now I don't know how to update a burt file. So, if you let dithering "ON", the dithering will run with the condition that C1:ASS-XARM_ETM_PIT_L_DEMOD_SIG_GAIN / C1:ASS-XARM_ETM_YAW_L_DEMOD_SIG_GAIN are not 0.100 but 0.200.
I changed the snapshot file for ASS, /opt/rtcds/caltech/c1/scripts/ASS_DITHER_ON.snap as follows:
L124 > C1:ASS-XARM_ETM_PIT_GAIN 1 -5.000000000000000e-02
=> C1:ASS-XARM_ETM_PIT_GAIN 1 -1.500000000000000e-02
L128> C1:ASS-XARM_ETM_YAW_GAIN 1 5.000000000000000e-02
=> C1:ASS-XARM_ETM_YAW_GAIN 1 1.500000000000000e-02
The purpose of this change is to avoid the oscillation when the dithering of X arm is running.
XARM lock was achieved by POX11_I
- The whitening gains of POX11_I and Q are 42dB so that POX11_I have the same amplitude as AS55_I
- The demod phase of POX11 was adjusted to eliminate the PDH signal from the Q phase. The phase is -100.5deg.
- In order to lock the XARM with POX11_I_ERR, I had to increase the trigger threshold from 0.1 to 0.2 as the arm was
kicked with the threshold of 0.1.
- Lock the X arm with AS55_I at the XARM configuration.
- Adjusted POX11 demod phase so that POX11_Q is minimized.
- POX I&Q whitening gains were adjusted. When they are 42dB, POX11_I_ERR and AS55_I_ERR have almost the same signal amplitude.
(In reality, POX11_I_ERR has +1dB larger amplitude.)
- Adjusted POX11 demod phase again with better precision.
- Measured transfer function between AS55_I_ERR and POX11_I_ERR. As the sign was opposite, the demod phase was -180deg flipped.
- Tried to lock the arm with POX11_I_ERR. It did not acquire the lock. The arm looked kicked by the servo.
- Increased the trigger threshold from 0.1 to 0.2. Now the arm is locked with POX11_I_ERR.
I started a script on Friday night to collect some data for a reflection armloss measurement of the XARM. Unfortunately there seemed to have been a hickup in some data transfer and some errors were produced, so we couldn't really trust the numbers.
Instead, we took a series of manual measurements today and made sure the interferometer is well behaved during the averaging process. I wrote up the math behind the measurement in the attached pdf.
The numbers we used for the calculations are the following:
While we average about 50 ppm +/-15 ppm for the XARM loss with a handful of samples, in a few instances the calculations actually yielded negative numbers, so there's a flaw in the way I'm collecting the data. There seems to be a ~3% drift in the signal level on the PO port on the order of minutes that does not show in the modecleaner transmission. The signals are somewhat small so we're closing the shutter over night to see if it could be an offset and will investigate further tomorrow. I went back and checked my data for the YARM, but that doesn't seem to be affected by it.
[Anchal, Paco, Rana]
We locked the XARM using POX11 and made a noise budget for the single arm displacement; see Attachment #1. The noise budget is rough in that we use simple calibrations to get it going; for example we calibrate the measured error point C1:LSC-XARM_IN1_DQ using the single cavity pole and some dc gain to match the UGF point. The control point C1:LSC-XARM_OUT_DQ is calibrated using the actuator gain measured recently by Yuta. We also overlay an estimate of the seismic motion using C1:PEM-SEIS_BS_X_OUT_DQ (calibrated using a few poles to account for stack and pendulum), and finally the laser frequency noise as proxied by the mode cleaner C1:IOO-MC_F_DQ.
A couple of points are taken with this noise budget, apart from it needing a better calibration;
We took the long BNC cable that ran from ETMX to ETMY and ran it from ETMX into the control room instead. This way Cici and Deeksha can send small voltage signals to the AUX PZT and read back using the beatnote pickoff that is usually connected to the spectrum analyzer.
We now have two 80-foot, female-to-female XLR cables for our pretty new microphones, one yellow and one purple. They have been tested and appropriately labeled.
Also, here is a very helpful pdf for how to properly attach the XLR connectors to a raw quad cable, as well as one for how to put the actual connectors together (ignore the cable instructions on the connector page... the cable depicted is not a quad cable).
XLR(F)-XLR(M) cable for the blue microphone is missing. Steve ordered one.
We found one in the fibox setup. As we don't use it during the vent, we use this cable for the microphone.
Once we get the new one, it will go to the fibox setup.
We tweaked the mirror on the AP table to go through the center of the lens in order to get a more circular beam, but it seemed ineffective. So we put an IR card in front of the lens and behind the lens to see if the beam was circular or ovacular, but could not tell. We also moved the camera to see, but still couldn't see a distinct circle or oval. So Mike and Q will do a beam scan tomorrow in both the X and Y directions to see if the beam is circular or not.
Measured frequency noise is ~10Hz/rtHz @100Hz.
Measure the out-of-loop noise of Xarm ALS:
1. The X-arm was locked for IR using PDH error signal.
2. 'CLEAR HISTORY' of the phase tracker filters.
3. Measured the power spectrum of the phase tracker output. I have used the newly created calibrated channel "PHASE_OUT_DQ. So the phase tracker output now reads in Hz.
The measurement was done with beat note frequency at ~40MHz. The flat noise level of 10Hz/rtHz from 20-100Hz (in plot 2) is not good. We should investigate as to what sets this noise level. The spike at 60Hz is because the 60Hz frequency comb filter was not enabled.
I plan to the following to get a clearer outlook
1. Connecting the beat box to an RF source and measure the noise levels for a range of frequency inputs to the beatbox.
2. Measure the noise at C1:ALS-BEATX_FINE_I_IN1 (before the antiwhitening filters) and check whether the new whitening filters has done anything good with respect to minimizing the DAQ noise.
X-Arm ASS was fixed.
ASS_DITHER_ON.snap was updated so that the new setting can be loaded from the ASS screen.
The input and output matrices and the servo gains were adjusted as found in the attached image.
The output matrix was adjusted by looking at the static response of the error signals when a DC offset
was applied to each actuator.
The servo was tested with misalignment of the ITM, ETM, and BS. In fact, the servo restored transmission
from 0.15 to 1.
The resulting contrast after ASSing was ~99% level. (I forgot to record the measurement but the dark fringe level of ASDC was 4~5count.)
Please remember that Xarm ASS needs FM6 (Bounce filters) to be ON in order to work properly.
While Gautam is working the restoration of Yarm ASS, I worked on Xarm.
Basically, I have changed the oscillator freqs and amps so as to have linear signals to the misalignment of the mirrors.
Also reduced the complexity of the input/output matrices to avoid any confusion.
Now the ITM dither takes care of the ITM alignment, and the ETM dither takes care of the ETM alignment.
The cavity alignment servos (4dofs) are running fine although the control band widths are still low (<0.1Hz).
The ETM spot positions should be controlled by the BS alignment, but it seems that these loops have suspicion about the signal quality.
While Gautam wa stouching the input TTs, we occasionally saw anomalously high transmission of the arm cavities (~1.2).
We decided to use this beam as this could have indicated partial clipping of the beam somewhere in the input optics chain.
Then the arm cavity was aligned to have reasonably high transmission for the green beam. i.e. Use the green power mon PD as a part of the alignment reference.
This resulted very stable transmission of both the IR and green beams. We liked them. We decide to use this a reference beam at least for now.
Attachment1: GTRX image at the end of the work.
Attachment2: ASSX screen shot
Attachment3: ASSX servo screen shot
Attachment4: Green ASX servo screen shot
Attachment 5: Screen shot of the ASS X strip tool
Attachment 6: Screen shot of the ASS X input matrix
Attachment 7: Screen shot of the ASS X output matrix
Reasonable amounts of time were spent bending the AG4395 to my will; i.e. figuring out the calibration things Jenne and Rana did, finding the right excitation amplitude and profile that would leave the light steadily locked, and finding the right GPIB incantation for getting spectra in PSD units instead of power units. I'm nearing completion of a newer version of AG4395 scripts that have proper units, and pseudo-log spectra (i.e. logarithmically spaced linear sweeps)
Here is too many traces on one plot showing parts of the OLTF for the x green PDH. One notable omission is the PD response (note to self:check model and bandwidth). The servo oddly seems to have a notch around 100k. My calibration for the CLG injection may not have been perfect, instead of flattening out at 0dB, I had 2dB residual. I tried to correct for it after the fact, assuming that certain regions were truly flat at 0dB, but I want to revisit it to be thorough. I found some old measurements of the Innolight PZT PM response, which claims to be in rad/V, and have included that on the plot.
In the end, the mixer and PZT response make it look like getting over 10kHz bandwidth may be tough. Even finding a good higher modulation frequency to be able to scoot the LP up would leave us with the sharp slope in the PZT phase loss, and could cause bad gain peaking. Maybe it's worth thinking about a faster way of modulating the green light?
Tomorrow morning, I'll calibrate all the noise spectra I have into real units. These include:
However, looking at the floors, it occurs to me that I may have left the attenuation on the input too high, in an effort to protect the input the PDH box, which rails all the time when not locked to a 00 mode, sometimes even with the input terminated or open. It's kind of a pain that the agilent makes it really hard to see the data when you're in V/rtHz mode, because I should've caught this while measuring :/
I used a scope to capture a pdh signal happening, which will let me transform the mixer output into cavity motion. The control signal goes to the innolight PZT with a ~1MHz/V factor. Here are the uncalibrated plots, for now.
A MIST simulation tells me that the green pdh horn-to-horn displacement is about 1.2nm, or ~18kHz. I used this, along with the scope trace attached to the previous post, to calibrate the mixer output at 193419 Hz per V. (EDIT: I was a little too hasty here. What I'm really after is the slope of the zero crossing, which turns out to be almost exactly twice my earlier naïve estimate. See later post for correct spectra)
For the control signal, I assumed a flat Innolight PZT PM response of 1MHz/V. ( Under 10kHz, it is indeed flat, and this is the region where the control signal is above the servo output noise in yesterday's measurements)
Here are all of the same spectra from last night, with the above calibrations.
Going off Jenne's earlier plot, it looks like the in-loop error signal RMS is ten times bigger than the CARM linewidth.
I remeasured all of the noise spectra again today, making sure the input attenuation was as low as it could safely be. I also got a snap of the y green PDH signal; it's fairly larger than I saw the other day, which is good. I used this to calibrate the error signal voltage spectra.
Here are the noise traces for each arm. During these measurements GTRX was about .6, GTRY about 1.0 The Yarm noise doesn't look so good: the error signal is just barely above the mixer+lowpass output noise, and the RMS is plauged by 60Hz lines. (Is this related to what we see in IR TRY sometimes?)
Here are the arms error signals compared directly:
Nick and I did the upgrade for the green steering mirror today. We locked in the TEM00 mode.
We placed the shutter and everything. We move the OL, but we placed it back. Tonight, I'll be doing a more complete elog with more details.
That was super fast! Great job, Andres and Nic!
[Rana, Jenne, EricQ]
We did several things tonight. First, a list (so I can remember them all), and then some details.
(1) Jiggled ETMY SUS cables, removed kicks.
(2) Locked X and Y ALS, looked at POX, POY as out of loop sensors.
(3) Measured stuff (?) at the Yend.
(4) Reconnected REFL DC to SR560.
(5) Attempted CARM offset reduction.
When Rana and I started locking this evening, we saw (as Q has been witnessing for a while now) the ETMY kick a lot. However, it seemed to be kicking even more than usual. Since Q had been down at the end station recabling things, we wondered if a SUS-related cable got bumped. Rana went down to the end and pushed all the cables into their receptacles. One of the last sets that he pushed was the satellite box. We didn't have walkie-talkie communication, but the DC offset of the ETMY oplevs changed just a minute or two before he returned to the control room. So, we guess that it was the satellite box cables that were loose. Unfortunately, there is no clear way to strain relieve them, which is why they can so often be troublesome. Anyhow, the ETMY hasn't kicked since.
We locked the arms with ALS. We saw that the POX signal was about 20% of the full pk-pk height of the PDH signal, so it's mostly within the linear range, but not entirely. It is what it is, however, and we took measurements assuming that it's okay. I calibrated POX by putting an excitation onto ETMX, and matching the height of the peak in POX and BEATX_FINE_PHASE_OUT_HZ.
Q and Rana had also [remembered / put in / something] a digital readback for the end green PDH error point. Q went down to the end and gave me a number of 2600 Hz/V for the err mon port of the PDH board, which is what is connected to the ADC. With that and 20/2^16 V/cts, I had a calibration of 0.8 Hz/ct.
What we see in this plot is that the green end PDH is not the limiting noise for the POX out of loop measurement of the residual arm motion. Also, in the multi-color metrology paper, Fig 7 (which is posted in the control room), we see at about a little over 1 Hz a ratio of about 4.5 between the residual motion and the AUX PDH error signal. In today's plot, I see a ratio of about 20. I infer from this that the green PDH for the Xarm is fine, and that we may want to re-look at the ALS digital loop, but we should leave the X PDH alone.
Here is the Xarm plot:
Q took the data for the Yarm plot, so hopefully he can give it to us in the morning. What we did notice was that the noise was much worse for the Yarm. This prompted Item 3, measuring the loop.
Q and Rana went down to the Yend and measured some things. They came back, and said that they hadn't changed anything in analog while they were down there. One thing that Q did note was that we have almost 90 degrees of phase margin (since it's a 1/f loop), and about 10 dB of gain margin, above the UGF. So, we're in good shape for being able to try triggering the boost on the PDH box. Q will give us more notes on this work, as well as plots, in the morning.
At some point, I remembered that Q and Gabriele had repurposed the SR560 that we had been using for the REFLDC input to the common mode board. So, Q went and put it back, so that REFL DC goes into the SR560, and so does a DAC channel so that we can remotely set the offset. The A-B output goes to the REFL11I whitening channel, since real REFL11I goes into the input of the CM board. I think that today, the SR 560 was left at a gain of 1.
We decided to carry on and try to reduce the CARM offset some. An annoyance is that the Yarm still has pretty significant low-frequency noise, but the idea is that if we can get over to the sqrtInvTrans signals, it will be fine.
So, we didn't get much farther than we had in the past, but it was nice to get there at all again. I ran the carm_cm_up script (many times). One of the times, all I wanted to do was see how much I could reduce the CARM offset. CARM was on sqrtInvTrans, DARM was on ALS diff, and I was able to get the arm powers up to about 2.5. I don't know why I lost lock. The sqrtInv signals should be good until at least arm powers of 20 or so.
I was able to see the REFL DC dip, but only a teensy tiny bit. It went down by maybe 1 count. Q suggested looking at how deep it could get while leaving CARM and DARM both on ALS, and setting both offsets to 0. We were seeing arm flashes of about 50 counts, and REFL DC went from 0 to -800. So, I wasn't seeing much of a REFL dip, but it was definitely there when I went to arm powers of 2ish.
We tried looking at different sqrtInv options for DARM, and haven't come to any real conclusion. In the plot below, we are looking at a swept sine between DARM_IN1 (ALSdiff) and either MC_IN1 0.3*(sqrtInvX - sqrtInvY) or SRCL_IN1 (TRX - TRY / sqrt(TRX + TRY) ):
We have a few things to add to the to-do list:
* Put UGF servos for LSC loops in place.
* Implement UGF "servos" (per Koji's suggested method) for phase trackers.
* Write a lockloss script that is run by the ALS watch scripts - print a PDF of error and control signals for every lockloss, and save it somewhere.
* Fix up Ygreen modematching on the PSL table. The X green spot is quite similar on the camera to the corresponding PSL green spot. However the Y green spot is not at all the same as its PSL green spot.
Current Mode Matching and Gouy Phase Between Steering Mirrors
We found in 40m elog ID 3330 ( http://nodus.ligo.caltech.edu:8080/40m/3330) a documentation done by Kiwamu, where he measured the waist of the green. The waist of the green is about 35µm. Using a la mode, I was able to calculate the current mode matching, and the Gouy phase between the steering mirrors. In a la mode, I used the optical distances,which is just the distance measured times its index of refraction. I contacted someone from ThorLabs (which is the company that bought Optics For Research), and that person told that the Faraday IO-5-532-LP has a Terbium Gallium Garnet crystal of a length of 7mm and its index of refraction is 1.95. The current mode matching is 0.9343, and the current Gouy phase between steering mirrors is 0.2023 degrees. On Monday, Nick and I are planning to measure the actual mode matching. The attached below is the current X-arm optical layout.
Calculation For the New Optical Layout
Since the current Gouy phase between the steering mirror is essentially zero, we need to find a way how to increase the Gouy Phase. We tried to add two more lenses after the second steering mirror, and we found that increasing the Gouy phase result in a dramatically decrease in mode matching. For instance, a Gouy phase of about 50 degrees results in a mode matching of about .2, which is awful. We removed the first lens after the faraday, and we added two more mirrors and two more lenses after the second steering mirror. I modified the photo that I took and I place where the new lenses and new mirrors should go as shown in the second pictures attached below. Using a la mode, we found the following solution:
label z (m) type parameters
----- ----- ---- ----------
lens 1 0.0800 lens focalLength: 0.1000
First mirror 0.1550 flat mirror none:
Second mirror 0.2800 flat mirror none:
lens 2 0.4275 lens focalLength: Inf
lens 3 0.6549 lens focalLength: 0.3000
lens 4 0.8968 lens focalLength: -0.250
Third mirror 1.0675 flat mirror none:
Fourth mirror 1.4183 flat mirror none:
lens 5 1.6384 lens focalLength: -0.100
Fifth mirror 1.7351 flat mirror none:
Sixth mirror 2.0859 flat mirror none:
lens 6 2.1621 lens focalLength: 0.6000
ETM 2.7407 lens focalLength: -129.7
ITM 40.5307 flat mirror none:
The mode matching is 0.9786. The different Gouy phase different between Third Mirror and Fourth Mirror is 69.59 degrees, Gouy Phase between Fourth and Fifth 18.80 degrees, Gouy phase between Fifth and Sixth mirrors is 1.28 degrees, Gouy phase between Third and Fifth 88.38 degrees, and the Gouy phase between Fourth and Sixth is 20.08 degrees. Bellow attached the a la Mode code and the Plots.
Plan for this week
I don't think we have the lenses that we need for this new setup. Mostly, we will need to order the lenses on Monday. As I mention, Nick and I are going to measure the actual mode matching on Monday. If everything look good, then we will move on and do the Upgrade.
% In this code we are using a la mode to optimatize the mode matching and
% to optimatize the Gouy phase between mirror 1 and mirror 2. All the units
% are in meter
w0=(50*1e-6)/sqrt(2); % The Waist of the laser measured after SHG
z0_laser=-0.0083; % position measured where the waist is located
lamb= 532*10^-9; % wavelength of green light in mm
lFaraday=.0638; % Length of the faraday
Towards finding the x-arm beat note:
The green would not lock to a maximum GTRX this morning. In the course of aligning the green stably to the X arm, somewhere down the line, the input pointing got messed up (reasons unknown). To set this right, Koji tried to lock the Yarm with POY DC but it wouldn't work. The transmon for Y had to be set up temporarily and the Y arm was locked with TRY. This restored the input pointing and the arms locked with transmission TRX/TRY > 0.9 counts. The transmon path along the Y arm was then re-configured as mentioned in Annalisa's elog.
I still had trouble getting the X-green locked in TEM00 (similar situation mentioned by Jenne in elog). The arm cavity mirrors were tweaked to get the green to resonate in TEM00 but it wouldn't stay locked when the temperature of the x-end NPRO was changed. Koji helped recover missing links to filters for the ALS_X_SLOW servo from the archives. Enabling the filters helped keep the green locking stable for laser temperature changes (which corresponds to 'offset' change in ALS_X_SLOW servo screen).
PSL green alignment was checked once again and the X-end laser temperature was scanned trying to find the beatnote. RFMON from the beatbox was connected to the spectrum analyzer. I have scanned through the whole range of offset but have not been able to find the beat note yet.
The search will continue tomorrow
We should consider to hook up the temperature monitors of the NPROs to the ADCs.
Squishing cables at the ITMX satellite box seems to have fixed the wandering ITM that I observed yesterday - the sooner we are rid of these evil connectors the better.
I had changed the input pointing of the green injection from EX to mark a "good" alignment of the cavity axis, so I used the green beam to try and recover the X arm alignment. After some tweaking of the ITM and ETM angle bias voltages, I was able to get good GTRX values [Attachment #1], and also see clear evidence of (admittedly weak) IR resonances in TRX [Attachment #2]. I can't see the reflection from ITMX on the AS camera, but I suspect this is because the ITMY cage is in the way. This will likely have to be redone tomorrow after setting the input pointing for the Y arm cavity axis, but hopefully things will converge faster and we can close up sooner. Closing the PSL shutter for now...
I also rebooted the unresponsive c1susaux to facilitate the alignment work tomorrow.
I don't know who left the X arm locked, but I just ran the Align Full IFO script, so everything is good in case Yoichi/someone comes in to lock the IFO this weekend.
There is an oscillation in the Xarm at 631Hz, which is not in the Yarm. There is a small peak in POY11_I at this frequency, but only when the Xarm is locked. If the Xarm unlocks, the peak disappears from POY. The peak is 3 orders of magnitude larger in POX than in POY, and 4 orders of magnitude larger than the POY noise when this peak is not present. In the plot, I have turned off the POY whitening, so that its situation is the same as POX (we still need to fix POX whitening switching). Dark noise (MC unlocked) is the same for both PDs.
POX11 oscillation at 630 Hz was stopped by installing 630 Hz resonant gain to LSC_XARM.
After few hours, oscillation stopped. So I removed the resonant gain.
Our guess is that 630 Hz peak is some violin mode or something, and it was excited somehow, and didn't stopped for very long time because of its high Q. It coupled into POY11 somehow (scattering, electronics, etc).
Don't use resonant gain - it can lead to a loop instability since it makes the loop have 3 UGFs.
Just use a elliptic bandstop filter at this harmonic frequency separately for each test mass. There are many detailed examples of this in elog entries from Rob and I over the past ~10 years. This bandstop should get clicked on automatically after lock acquisition.
I aligned the Xgreen and PSL green to overlap on the X beat PD, and reconnected the splitter which combines the X and Y beat signals and sends them to the control room.
I've been stepping the Xend laser temperature offset in steps of 20 counts, making sure the cavity unlocks and relocks on TEM00. So far I have not seen any beat signals for the Xarm. I've gone from 0 to 840.
I'll be back in a few hours to keep trying, although interested parties are invited to give it a whirl.
We measured the Xend green laser PDH Open loop transfer function by following method:
Attachment 1 shows the closed loop of Xend Green laser Arm PDH lock loop. Free running laser noise gets injected at laser head after the PZT actuation as . The PDH error signal at output of miser is fed to a gain 1 SR560 used as summing junction here. Used in 'A-B mode', the B port is used for sending in excitation where .
We have access to three ports for measurement, marked at output of mixer, at output of SR560, and at PZT out monitor port in uPDH box. From loop algebra, we get following:
, where is the open loop transfer function of the loop.
So measurement of can be done in following two ways (not a complete set):
Attachment 1 shows the case when excitation is sent at control point i.e. the PZT output. As before, free running laser noise in units of Hz/rtHz is added after the actuator and I've also shown shot noise being added just before the detector.
Again, we have a access to three output points for measurement. right at the output of mixer (the PDH error signal), the feedback signal to be applied by uPDH box (PZT Mon) and the output of the summing box SR560.
Doing loop algebra as before, we get:
So measurement of can be done by
This means at 100 Hz, with 10 integration cycles, we should have needed only 3 mV of excitation signal to get an SNR of 100. However, we have been unable to get good measurements with even 25 mV of excitation. We tried increasing the cycles, that did not work either.
This post is to summarize this analysis. We need more tests to get any conclusions.