Aligning the beam for the PLL of the AbsL Experiement I rotated the polarizer along the path of the MC Input pick off beam (= the pick off coming from the MC periscope).
I've been trying to lock the PLL for the AbsL Experiment but I can't see the beat (between the auxiliary NPRO and the PSL).
I believe the alignment of the PLL is not good. The Farady Isolator is definitely not perfectly aligned (you can see it from the beam spot after it) but still it should be enough to see something at the PLL PD.
it's probably just that the two beams don't overlap well enough on the photodiode. I'll work on that later on.
I'm leaving the lab now. I left the auxiliary NPRO on but I closed its shutter.
All the flipping mirrors are down.
I've opened the AP table and I'm working on it.
I re-aligned the Faraday on the AP table. I also aligned the beam to the periscope on the PSL and all the other optics along the beam path. Now I have a nice NPRO beam at the PLL which overlaps with the PSL beam. The alignment has to be further improved because I see no beat yet.
I wonder if the all the tinkering on the PSL laser done recently to revive the power has changed the PSL NPRO temperature and so its frequency. That could also explain why the beat doesn't show up at the same temperature of the NPRO as I used to operate it. Although I scanned the NPRO temperature +/- 2 deg and didn't see the beat. So maybe the misalignment is the casue.
Not feeling very well right now. I need to go home for a while.
AP table closed at the moment.
NPRO shutter closed
I re-aligned the Faraday on the AP table. I also aligned the beam to the periscope on the PSL and all the other optics along the beam path. Now I have a nice NPRO beam at the PLL which overlaps with the PSL beam. The alignment has to be further improved becasue I see no beat yet.
I wonder if the all the tinkering on the PSL laser done recently to revive the power have changed the PSL NPRO temperature and so its frequency. That could also explain why the beat doesn't show up at the same temperature of the NPRO as I used to operate it. Although I scanned the NPRO temperature +/- 2 deg and didn't see the beat. So maybe the misalignment is the casue.
We definitely changed the PSL NPRO temp while fiddling around, trying to increase the laser power. I think it's noted in the elog both times that it's happened in the last few months (once when Rana, Koji and I worked on it, and then again when it was just Koji), but we opened up the side of the MOPA box so that we could get at (and change) the potentiometer which adjusts the NPRO temp. So you may have to search around for a while.
Yes it did.
For long time, the crystal temperature C1:PSL-126MOPA_LTMP was 43~46deg. Now it is 34deg. Try ~10deg lower temperature.
I wonder if the all the tinkering on the PSL laser done recently to revive the power have changed the PSL NPRO temperature and so its frequency. That could also explain why the beat doesn't show up at the same temperature of the NPRO as I used to operate it. Although I scanned the NPRO temperature +/- 2 deg and didn't see the beat.
Also auxiliary NPRO turned on and mechanical shutter opened.
Beat found at 30MHz with auxiliary NPRO temperature of 37.19 degrees, vs. ~48 deg as it used to be.
The beat is small (-70dBm). PLL alignment has to be improved.
PLL alignment improved. Beat amplitude = -10dBm. Good enough.
DC readouts at the PLL photodiode:
V_NPRO = -4.44V
V_PSL = -3.76V
The NPRO beam is attenuated by a N.D.=1 attenuator just before going to the photodiode.
Something strange happened at the last. Right before -10dBm, the amplitude of the beat was about -33dBm. Then I was checking the two interfering beams with the IR card and saw that they overlapped quite well. I then turned my head back to the spectrum analyzer and suddenly the beat was at -10dBm. Not only, but a bunch of new peaks had appeared on the spectrum. Either I inadvertently hit the PD moving it to a better position or something else happened.
Like if someone was making some other modulation on the beam or the modulation depth of the PSL's sidebands had gone up.
I locked the PLL and made some first measuremtns of the spectrum of the error signal. I'll post them later.
I closed the shutter of the NPRO.
I'm working on the AP table. I also opened the auxiliary NPRO shutter. The auxiliary beam is on its path on the AP table and PSL table.
Closing the AP table and the NPRO shutter now.
Last night something happened on the beat between the PSL beam and the auxiliary NPRO beam, that spoiled the quality of the beating I had before. As a result the PLL has become unable to lock the two lasers.
The amplitude of the beat at the spectrum analyzer has gone down to -40 dBm from -10 that it was earlier. The frequency has also become more unstable so that now it can be seen writhing within tens of KHz.
Meanwhile the power of the single beams at the PLL photodiode hasn't changed, suggesting that the alignment of the two beam didn't change much.
Changes in the efficiency of the beating between the two beams are not unusual. Although that typically affects only the amplitude of the beat and wouldn't explain why also its frequency has become unstable. Tuning the alignment of the PLL optics usually brings the amplitude back, but it was uneffective today.
It looks like something changed in either one of the two beams. In particular the frequency of one of the two lasers has become less stable.
Another strange thing that I've been observing is that the amplitude of the beat goes down (several dBm) as the beat frequency is pushed below 50 MHz. Under 10 MHz it even gets to about -60 dBm.
I noticed the change yesterday evening at about 6pm, while I was taking measurements of the PLL open loop tranfer function and everything was fine. I don't know whether it is just a coincidence or it is somehow related to this, but Jenne and Sanjit had then just rebooted the frame builder.
I confirm what I said earlier. The amplitude of the beat is -10 dBm at 300MHz. It goes down at lower frequencies. In particular it gets to-60 dBm below 20 MHz. For some strange reason that I couldn't explain the beating efficiency has become poorer at low frequencies.
NPRO shutter closed
Problem found. Inspecting with Koji we found that there was a broken SMA-to-BNC connector in the BNC cable from the photodiode.
I measured the open loop gain of the PLL in the AbsL experiment.
I repeated the measurement twice: one with gain knob on the universal PDH box g=3.0; the second measurement with g=6.0
The UGF were 60 KHz and 100 KHz, respectively.
That means that one turn of the knob equals to about +10 dB.
I closed the shutter of the NPRO for the night.
Plots don't really make sense. The second one is inherently unstable - and what's g?
The way the filter's transfer function has been measured is by a swept sine between the "SERVO INPUT" and the "PIEZO DRIVE OUTPUT" connection on the box front panel. The spectrum analyzer used for the measurement is the SR785 and the source amplitude is set at 0.1V.
The two transfer functions are clearly different. In particular the old one looks like a simple integrator, whereas the new one already includes some sort of boost.
That probably explains why the new one is unable to lock the PLL. Indeed what the PLL needs, at least to acquire lock, is an 1/f filter.
I thought the two boxes were almost identical, at least in the filter shapes. Also the two schematics available in the DCC coincide.
Yesterday I measured the Open Loop Gain of the PLL in the absolute length experiment. The servo I used was that of the old Universal PDH box.
The OLG looks like this:
The UGF is at 10 KHz.
To me, they both look stable. I guess that the phase has to go to -180 deg to be unstable.
Why does the magnitude go flat at high frequencies? That doesn't seem like 1/f.
How about a diagram of what inputs and outputs are being measured and what the gain knob and boost switch settings are?
I opened the auxiliary laser's shutter.
I'm currently working on the AP table.
I finished working on the table.
I closed the AUX NPRO's shutter.
I'm working on the AbsL experiment. A measurement which involved the PRC locked is running at the moment.
Please make sure of not interfering with the interferometer until it is done. Thank you.
I'm done for the moement.
I realized that I need to take into account somehow the DC power from the photodiode. By now the measurement of the transmitted beat's power is affected by the total power circulating inside of the PRC and thus it depends on the cavity alignment.
I closed the laser shutter and turned down the flipping mirrors.
I'm planning to restart measuring by 2.30pm today. Till then the interferometer is available.
I'm currently running a measurement on the PRC.
Please don't interfere with the IFO until it is done. Talk with Alberto if you've been intending to work inside the lab.
Leaving for dinner. Back in ~1hr.
I left a measurement running. Please don't interfere with it till I'm back. Thanks.
Per Alberto's instructions, I have closed the shutter on his laser so that the Adaptive Team can play with the Mode Cleaner.
I finished measuring the AbsL for this morning. The IFO is again available.
Please don't mess up with the interferometer though. I'll be back in a couple of ours
At the moment I'm running a measurement on the PRC and I'm planning to leave it going for the time we'll be at the 40m meeting.
Please be careful in the lab. Thank you.
I started a long measurement of the PRC's transmissivity. I'm leaving the lab and I'm going to be back at about 8 tonight. Please do not disturb the interferometer. it is important that the MC and the PRC stay locked all the time.
That measurement is finished. I'm now going to start another one that will take another hour or so. I'm leaving it running for the night. If you want to work on the IFO, it should be definitely done by 11pm.
A measurement will be running for the next hour. Please be careful.
This afternoon the watchdogs stopped working: they didn't trip when the suspension positions crossed the threshold values.
I rebooted c1susaux (aka c1dscl1epics0 in the 1Y5 rack), which is the computer that runs the watchdog processes.
The reboot fixed the problem.
Nice and interesting plot.
I suppose slight decrease of the Schnupp asymmetry (in your model) adjusts the discrepancy in the high freq region.
At the same time, it will make the resonance narrower. So you need to put some loss at the recombination (=on the BS)?
...of course these depends on the flatness of the calibration.
I'm leaving a measurement running overnight. It should be done in about one hour.
Tomorrow morning, If you need to use the interferometer, and you don't want to have the auxiliary beam going onto the dark port, you can turn down the flipping mirror and close the NPRO's mechanical shutter.
This is what I measured last night:
This is not a fit. It's just a comparison of the model with the data.
I turned off the modulation at 166MHZ becasue I don't need it if I'm only locking the PRC.
It was introducing extra amplitude modulations of the beam which interfered with the AbsL's PLL photodiode.
I'm going to turn it back on later on.
I turned back on the 166MHz modulation just a bit. I set the slider at 4.156.
When it was totally off the MZ seemd quite unhappy.
You can turn the 166 off if you want. MZ is unhappy after its turned off, but that's just the thermal transient from removing the RF heat. After a several minutes, the heat goes away and the MZ can be relocked.
One of these days we should evaluate the beam distortion we get in EOMs because of the RF heat induced dn/dT. Beam steering, beam size, etc.
I just started a measuremtn that will be running for the next hour or so. Please be careful with the interferometer.
Done. IFO available
Lately I've been trying to improve the PLL for the AbsL experiment so that it could handle larger frequency steps and thus speed up the cavity scan.
The maximum frequency step that the PLL could handle withouth losing lock is given by the DC gain of the PLL. This is the product of the mixer's gain factor K [rad/V ], of the laser's calibration C [Hz/V] and of the PLL filter DC gain F(0).
I measured these quantities: K=0.226 V/rad; C=8.3e6 Hz/V and F(0)=28.7dB=21.5. The max frequency step should be Delta_f_max = 6.4MHz.
Although in reality the PLL can't handle more than a 10 KHz step. There's probably some other effect that I'm not.
I'm attaching here plots of the PLL Open Loop Gain, of the PLL filter and of a spectra of the error point measured in different circumstances.
I don't have much time to explain here how I took all those measurements. After I fix the problem, I'm going to go go through those details in an elog entry.
Does anyone have any suggestion about what, in principle, might be limiting the frequency step?
I already made sure that both cables going to the mixer (the cable with the beat signal coming from the photodiode and the cable with the LO signal coming from the Marconi) had the same length. Although ideally, for phase locking, I would still need 90 degrees of phase shift between the mixing signals, over the entire frequency range for which I do the cavity scan. By now the 90 degrees are not guaranteed.
Also, I have a boost that adds another 20 dB at DC to the PLL's filter. Although it doesn't change anything. In fact, as said above calculating the frequency step, the PLL should be able to handle 100KHz steps, as I would want the PLL to do.
I just aligned PRM and locked PRC and I noticed that SPOB is much higehr than it used to be. It's now about 1800, vs 1200 than it used to be last week.
Isn't anyone related to that? If so, may I please know how he/she did it?