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.
NPRO shutter closed
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.
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?
oops, my bad. I cranked the 33MHz modulation depth and forgot to put it back. The slider should go back to around 3.
I was actually hoping that the alignment got better.
I might have found the problem with the PLL that was preventing me from scanning the frequencies by 100KHz steps. A dumb flimsy soldering in the circuit was making the PLL unstable.
After I fixed that problem and also after writing a cleverer data acquisition script in Python, I was able to scan continuosly the range 10-200MHz in about 20min (versus the almost 1.5-2 hrs that I could do previously). I'm attaching the results to this entry.
The 'smears' on the right side of the resonance at ~33MHz, are due to the PSL's sideband. I think I know how to fix that.
As you can see, the problem is that the model for the cavity transmission still does not match very well the data. As a result, the error on the cavity length is too big (~> 10 cm - I'd like to have 1mm).
Anyway, that was only my first attempt of scanning. I'm going to repeat the measurement today too and see if I can come out better. If not, than I have to rethink the model I've been using to fit.
I want to try to do the measurement with the network analyzer used as local oscillator, instead of the Marconis that I'm using now. Tha could give me better noise rejection. It would also give me information about the phase.
Also I wouldn't dislike abandoning the GPIB interfaces to acquire data.
Last night (Mar 17) I checked the PLL setup as Mott have had some difficulty to get a clean lock of the PLL setting.
Now the beating signal is much cleaner and behave straight forward. I will add some numbers such as the PD DC output, RF levels, SR560 settings...
Last night (Mar 17) I checked the PLL setup as Mott had some difficulty to get a clean lock of the PLL setting.
I also had noticed the progressive change of the aux NPRO alignment to the Farady.
I strongly agree about the need of a good and robust PLL.
By modifying the old PDH box (version 2008) eventually I was able to get a PLL robust enough for my purposes. At some point that wasn't good enough for me either.
I then decided to redisign it from scratch. I'm going to work on it. Also because of my other commitments, I'd need a few days/1 week for that. But I'd still like to take care of it. Is it more urgent than that?
We use the current PLL just now, but the renewal of the components are not immediate as it will take some time. Even so we need steady steps towards the better PLL. I appreciate your taking care of it.
I checked the setup further more.
Now I have significant fraction of beating (30%) and have huge amplitude (~9dBm).
The PLL can be much more stable now.
It looks like the PLL drifted alot over the weekend, and we couldn't get it back to 9 dBm. We switched back to the new focus wideband PD to make it easier to find the beat signal. I replaced all the electronics with the newly fixed UPDH box (#17) and we aligned it to the biggest beat frequency we could get, which ended up being -27 dBm with a -6.3V DC signal from the PD.
Locking was still elusive, so we calculated the loop gain and found the UGF is about 45 kHz, which is too high. We added a 20 dB attenuator to the RF input to suppress the gain and we think we may have locked at 0 gain. I am going to add another attenuator (~6 dB) so that we can tune the gain using the gain knob on the UPDH box.
Finally, attached is a picture of the cable that served as the smb - BNC adaptor for the DC output of the PD. The signal was dependent on the angle of the cable into the scope or multimeter. It has been destroyed so that it can never harm another innocent experiment again!
We have managed to lock the PLL to reasonable stability. The RF input is attenuated by 26 dBm and the beat signal locks very close to the carrier of the marconi (the steps on the markers of the spectrum analyzer are coarse). We can use the marconi and the local boost of the pdh box to catch the lock at 0 gain. Once the lock is on, the gain can be increased to stabilize the lock. The locked signals are shown in the first photo (the yellow is the output of the mixer and the blue is the output to the fast input of the laser. If the gain is increased too high, the error signal enters an oscillatory regime, which probably indicates we are overloading the piezo. This is shown in the second photo, the gain is being increased in time and we enter the non-constant regime around mid-way through.
Tomorrow I will use this locked system to measure the PZT response (finally!).
After realigning and getting the lock today, I tried to add in the SR785 to measure the transfer function. As soon as I turn on the piezo input on the PDH box, however, the lock breaks and I cannot reacquire it. We are using an SR650 to add in the signal from the network analyzer and that has worked. We also swapped the 20 dB attenuator for a box which mimics the boost functionality (-20 dB above 100 Hz, 0 dB below 6Hz). I took some spectra with the SR750, and will get some more with the network analyzer once Alberto has finished with it.
The SR750 spectra is posted below. The SR750 only goes up to 100 kHz, so I will have to use the network analyzer to get the full range.
[Suresh / Kiwamu]
We did the following things :
- Took the LightWave NPRO out from the MOPA box
- Temporarily took out the laser controller which has been connected to the Y end laser.
- Put the LightWave on AP table and plugged the laser controller and confirmed that it still emits a beam
[Things to be done]
- measure the beam profiles and power
- get a laser controller, which will be dedicated for this laser, from Peter King
[Background and Motivation]
The PRC and SRC length have to be precisely measured before the vent.
In order to measure those absolute length we are going to use the Stochino technique, which requires another laser to scan the cavity profiles.
The LightWave NPRO laser in the MOPA box was chosen for the Stochino laser because it has a large PZT range of 5 MHz/V and hence allows us to measure a wider frequency range.
The laser in the MOPA box had been connected to home-made circuits, which are not handy to play with. So we decided to use the laser with the usual laser controller.
Peter King said he has a LightWave laser controller and he can hand it to us.
Until we get the controller from him we do some preparations with temporary use of the Y end laser controller.
The I-P curve of the LightWave NPRO (M126-1064-700), which was taken out from the MOPA box, was measured. It looks healthy.
The output power can go up to about 1 W, but I guess we don't want it to run at a high power to avoid any further degradation since the laser is old.
X-axis is the current read from the display of the controller. Y-axis is the output power, directly measured by Coherent PM10.
The measurement was done by changing the current from the controller.
[Things to be done]
Hmm. Was the current within the operating range? (i.e. Is it a 700mW laser or a 1W one?)
You can obtain the nominal operating current from the old EPICS values (or some elog entries).
Note that NPROs are designed to be healthy only at around the nominal pumping power
(i.e. thermal gradient, and thermal lensing of the crystal, etc.)
Be aware that this laser should be used under the old SOP. So the appropriate interlocking is mandatory.
And probably we need to modify the SOP such that it reflects the latest situation.
The I-P curve of the LightWave NPRO, which was taken out from the MOPA box, was measured. It looks healthy.
Put the serial numbers into the elog. So we can identify the laser and controller in the future.
The old days the NPRO ( inside the MOPA ) was running ~1.7A 500 mW
Peter King came over to the 40m with a laser controller and gave it to us.
We will test it out with the LightWave NPRO, which was used for MOPA.